Paleoceanography of the North American Western Interior Seaway based on geochemical analysis of carbonate shell material

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Paleoceanography of the North American Western Interior Seaway based on geochemical analysis of carbonate shell material

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Paleoceanography of the North American Western Interior Seaway based on geochemical analysis of carbonate shell material
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Schmidt, Matthew William.
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Tampa, Florida
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University of South Florida
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English
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viii, 81 leaves : ill. ; 29 cm.

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Paleoceanography -- South Dakota ( lcsh )
Paleoceanography -- Cretaceous ( lcsh )
Dissertations, Academic -- Geology -- Masters -- USF ( FTS )

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Thesis (M.S.)--University of South Florida, 1997. Includes bibliographical references (leaves 76-81).

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University of South Florida
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Universtity of South Florida
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F51-00131 ( USFLDC DOI )
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PALEOCEANOGRAPHY OF THE NORTH AMERICAN WESTERN INTERIOR SEAWAY BASED ON GEOCHEMICAL ANALYSIS OF CARBONATE SHELL MATERIAL by MATTHEW WILLIAM SCHMIDT A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology University of South Florida August 1997 Major Professor : Peter J. Harries, Ph.D.

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Graduate School University of South Florida Tampa Florida CERTIFICATE OF APPROVAL Master's Thesis This i s to certify that the Master's Thesis of MATTHEW WILLIAM SCHMIDT w i th a major in Geology has been approved by the Examining Committee on July 23, 1997 as satisfactory for the thesis requirement for the Master of Science degree E x amin ing Comm i ttee : Professor: Peter J. Harries, Ph. D. r l S. JoneS?Pt;h. I ' \ ' E Member: Terrence M Quinn Ph D '--'Memt?er : Lisa L Robbu;s Ph D :

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DEDICATION This work is dedicated to those special professors and teachers over the years who have fueled my imagination, sparked my curiosity, and inspired me to reach for the stars

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ACKNOWLEDGEMENTS I would like to thank Dr. Peter Harries m y advisor and mentor while at USF, for his unyielding support, inspiration, and guidance on this project. I also wish to thank Dr. Lisa Robbins for her inspiration, input and for generously allowing me the use of her lab ; Dr. Jeffrey Ryan for his many hours of help and advice on using the DCP-ES ; and Drs. Douglas Jones and Terry Quinn for their valuable contribution as a members of my committee In addition, I would like to acknowledge Richard Hammond of the South Dakota Geologic Survey and his family for their wonderful hospitality while in South Dakota, and for making the cores used in this study available. I also owe a great deal of thanks to Samantha Andrews, Michael Blornme, and Billy Shrewsbury for their friendship, encouragement and support throughout the past two years I would also like to thank Sigma Xi, the Geological Society of America, the Museum ofNatural History in New York, and the Tampa Bay Fossil Club for their generous grants in aid of research This project would not have been possible without their financial support.

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TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ABSTRACT INTRODUCTION GEOLOGIC SETTING PREVIOUS WORK :METHODS Field Methods and Selection of Samples Stable Isotope and Trace-Element Analysis RESULTS DISCUSSION 11 lll V1 1 3 8 22 23 26 Isotope and Trace-Element Paleontology 38 Diagenet i c Effects 3 8 Postmortem Transportation 49 Vital Effects 50 Estimation of Temperature Salinity and Density of Water 53 Masses Trace-Element Interpretation 61 Paleoceanographic Implications 63 CONCLUSIONS 74 LIST OF REFERENCES 76

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LIST OF TABLES Table 1. Summary of oceanographic conditions as predicted by various circulation models for the Late Cretaceous WIS 19 Table 2 Summary of isotopic analysis (reported in %o PDB) and traceand minor-elemental concentrations (reported in ppm) 32 Table 3 Range of possible bottom-water paleotemperatures at different Ow values for the eastern portion of the WIS 55 Table 4 Range of possible intermediate-water paleotemperatures at different Ow values for the eastern portion of the WIS 56 Table 5 Bottom-water paleotemperatures and paleosalinities from the eastern portion of the WIS as calculted from Mg/Ca and Sr/Ca ratios in the prismatic layer of inoceramids. 59 ii

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LIST OF FIGURES Figure 1. Generalized map of the Western Interior Seaway during the Claggett and Bearpaw cycles. 4 Figure 2 Time-Stratigraphic Ammonite Zones of the Upper Pierre Shale and stratigraphic column showing various members of the Pierre Shale in eastern South Dakota 6 Figure 3 Location ofUpper Cretaceous coring sites in eastern South Dakota used in this study 7 Figure 4 Schematic diagram of a west to east transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns as proposed by Wright (1987) 10 Figure 5 Schematic diagram of a north to south transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns as proposed in a model by Hay et al (1993) 13 Figure 6 Schematic diagram of a north to south transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns as proposed in a model by Hay et al (1993) 14 Figure 7A. Schematic diagram of a north to south transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns during the winter months as proposed by Glancy et al. (1993). 16 Figure 7B Schematic diagram of a north to south transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns during the summer months as proposed by Glancy et al. (1993) 17 Figure 8 Diagram of estuarine circulation pattern in the Western Interior Seaway as proposed by Slingerland et al. (1996) 18 iii

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Figure 9A. SEM photomicrograph showing excellent preservation of prismatic layer in inoceramid sample MC 296 0 28 Figure 9B SEM photomicrograph showing excellent preservation of calcite prism in inoceramid sample MC 296 0 28 Figure 9C SEM photomicrograph showing preservation of calcite prisms in inoceramid sample SW-8 29 Figure 9D. SEM photomicrograph showing preservation of calcite prisms in inoceramid sample SW-8. 29 Figure lOA. SEM photomicrograph showing preservation of nacreous layer in ammonoid sample MC 289.6 30 Figure lOB. SEM photomicrograph showing preservation of nacreous layer in ammonoid sample MC 289.6. 30 Figure lOC. SEM photomicrograph showing preservation of nacreous layer in ammonoid sample from Coon Creek. 31 Figure 11. Stable isotope summary of shell material from Miner Co. core 35 Figure 12 Stable isotope summary of shell material from Brown Co. core 37 Figure 13. Stable isotope summary of shell material from Gregory Co. core 38 Figure 14. Sr verses Mg in prismatic layer of inoceramids collected from cores along the eastern portion of the WIS. 41 Figure 15. Sr verses Mg in nacreous layer of inoceramid fossils collected from cores along the eastern portion of the WIS 42 Figure 16 Mn verses Mg in prismatic layer of inoceramids collected from cores along the eastern portion of the WIS. 44 Figure 17 Mn verses Mg in Upper Cretaceous aragonitic samples from cores along the eastern portion of the WIS 45 Figure 18. Fe verses Na in Upper Cretaceous inoceramid samples from cores taken along the eastern margin of the WIS 47 Figure 19. o180 and o13C for altered and unaltered shell material based on traceand minor-elemental concentrations 48 iv

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Figure 20 Summary of stable isotope values from epifauna of the WIS and Coon Creek 52 Figure 21. Estimated temperature salinity and density ranges as predicted by 8180 paleothermometry and Mg/Ca paleothermometry. 57 Figure 22 Probable density ranges for the intermediateand bottom-waters along the eastern portion of the WIS based on salinity and temperature calculations 62 Figure 23. Average 8180 of epifauna from across the WIS and the Tethyan Sea 65 Figure 24 Summary of stable isotope values from the nektic fauna of the WIS and Coon Creek. 67 Figure 25. Average 8180 of nektic fauna from across the WIS and Tethyan Sea. 68 Figure 26 Comparison of stable isotope values from the epifauna and nektic fauna of the WIS and Coon Creek 69 v

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PALEOCEANOGRAPHY OF THE NORTH AMERICAN WESTERN INTERIOR SEAWAY BASED ON GEOCHE:rvfiCAL ANALYSIS OF CARBONATE SHELL MATERIAL by MATTHEW WILLIAM SCHMIDT An Abstract Of a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology University of South Florida August 1997 Major Professor : Peter J. Harries, Ph.D vi

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ABSTRACT Paleoceanographic conditions for the Creta c eous Bearpaw Seaway (Middle Campanian to Early Maastrichtian) were reconstructed through geochemical analysis of well preserved carbonate shell material collected from the Upper Pierre Shale Samples were collected from outcrops and from cores drilled by the South Dakota State Survey at three locations in eastern South Dakota. Shell material from epifaunal Inoceramus bivalves and from nektic ammonites were used as a proxy for the bottomand intermediate water masses, respectively Stable oxygen and carbon isotopic signatures as well as minorand traceelement abundances, from both calcitic and aragonitic skeletons were used to determine likely temperature and salinity ranges for the epifaunal and nektic habitat groups 8180 values and Mg/Ca and Sr / Ca ratios from the epifauna suggest that conditions along the bottom were warmer and more saline than in the overlying waters Water densities for each temperature-salinity comb i nation within each habitat group were then calculated, and these results provide evidence for the existence of two distinct water masses along the eastern portion of the seaway In order to gain a more complete understanding of the stratification, circulation, and origin of water masses within the basin, the geochemical data from this study were then combined with previous data on approximately coeval shell material from the central and western portions of the seaway The geochemical predict i ons of several paleoceanographic models were compared to the measured 8180 and 813C values from across the seaway This comparison suggests that Glancy et al.'s (1993) model best explains the water-mass dynamics within the basin. Warm, saline waters formed in the vii

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INTRODUCTION The geochemistry of fossilized carbonate shell material from siliciclastic deposits is a focus of research because the chemical signals preserved in the biogenic material have the potential to record information on both the original depositional environment and later diagenetic environments ofthe surrounding sediments (e g ., Whittaker et al., 1987 ; Carpenter et al ., 1988; Morrison and Brand, 1988; Pirrie and Marshall, 1990; Ludvigson et al. 1994) Within the central, northern, and western portions ofthe Western Interior Seaway (WIS) rock successions from the CenomanianTuronian Greenhorn cycle through the Campanian-Maastrichtian Claggett and Bearpaw cycles have been the subject of several stable isotopic and traceand minor-elemental investigations In particular, abundant molluscan fossils from marine sequences of the WIS have constituted the basis for paleoceanographic reconstructions (Wright, 1987; Whittaker et al. 1987; Kyser et al. 1993 ; Ludvigson et al., 1994; Hay et al 1993 ; Glancy et al 1994; Fatherree, 1995 ; and Slingerland et al 1996) and provided information on the diagenetic history of the fossils themselves (Carpenter et al. 1988 ; Ludvigson et al 1989). During the Late Cretaceous, a broad epicontinental seaway oriented north-south flooded the interior ofNorth America and connected the tropical waters of the Tethys with the Boreal waters of the Arctic It is likely that the presence of this flooded interior 1

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basin had a profound effect on the climate ofNorth America at this time However it is d i fficult to predict the nature of paleoceanographic circulation within the former seaway because a modem analog for an epicontinental seaway connected with latitudinally distinct oceans does not exist today The WIS differs from all modem marginal seas in that it was open at both ends, spanned 45 oflatitude, and connected a tropical and polar sea Therefore, many workers have suggested various paleoceanographic models for the seaway However, a careful examination of all physical and chemical data on the WIS is necessary in order to gain an accurate understanding of its circulation patterns Samples for this study come from the upper Pierre Shale of South Dakota and correspond to the Late Campanian to Early Maastrichtian Bearpaw sea While past paleoceanographic and paleoclimatic reconstructions have been based on data from western and central locations in the WIS, this is one of the first studies conducted on the geochemistry of corresponding strata and shell material from the eastern portion of the seaway. This study uses a combined approach of both isotopic and traceand minor elemental geochemistry of shell material to more accurately reconstruct paleoenvironmental conditions The data collected as part of this investigation are then used in conjunction with previously published data to gain a more complete picture of the WIS, and to further constrain paleoceanographic interpretations of the former epicontinental sea 2

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GEOLOGIC SETTING An interior seaway oriented in a north-south direction extended across the North American continent from the Late Albian until the late Early Maastrichtian Its history records several transgressive/regressive cycles, each ofwhich resulted from the interplay of tectonic activity eustatic sea-level change, and subsidence due to sediment loading This epicontinental sea connected the tropical Tethys Sea to the south with the Boreal Sea to the north In the United States, the seaway occupied a foreland basin that extended from the mid-western states on the east to the active Cordillera consisting of the Late Jurassic to Late Cretaceous Sevier orogenic belt on the west (Figure 1) (Armstrong, 1968) Throughout the Cretaceous, continued eastward thrusting from the western orogenic belt resulted in increased subsidence along this margin and produced an asymmetrical basin that was generally shallower to the east. The basin and seaway can be divided into four distinct tectonic and water-depth zones (Kauffinan, 1977, 1984) The western "foreland" zone was characterized by the highest subsidence rates caused by the thrusting of the growing orogenic belt to the west. High siliclastic sedimentation rates filled most of the available accommodation space in this zone As a result, water depth was probably less than 50 m in this zone The west-central "axial" zone also had high subsidence rates, but water depths ranged from 200-300 m and possibly even up to 500 m during maximum transgressions. 3

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70 Figure 1 Generalized map of the Western Interior Seaway during the Claggett and Bearpaw Cycles Dotted lines represent the extent of preserved sediments Map also shows locations of this and previous studies on WIS fossils : x This Study; +Fatherree, 1995; *Wright, 1987;. Tourtelot and Rye, 1969 ; Lowenstam and Epstein, 1954 ; o Whittaker et al. 1987 40 30

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Th e ea s t-central hinge zone experienced much lower subsidence rates and water depths ranged from 100-2 0 0 m Finally the easternmost stable cratonic zone wa s a broad platform that experienced the least subsidence Sedimentat ion rates we re lowe s t in this zone and consisted mainly of fine clastics and calcarenites representative of sediment starvation Water depths were less than 100 m. The Pierre Shale represents sedimentation during the last two transgressive regressive c y cles which flooded the basin They are termed the Claggett ( l at e Early Campanian ) and the Bearpaw (latest Middle Campanian to Late Maastrichtian) C y clothems In eastern South Dakota, the Pierre Shale can be divided into eight lithologic members: the Sharon Springs the Gregory, the Crow Creek the DeGrey the Verendrye the Virgin Creek, the Mobridge and the Elk Butte Members (Figure 2) These members have been classified into time-stratigraphic zones based on ammonite bios t ratigraphy and radiometric dating of numerous bentonites (Cobban and Reeside 1952 ; Obradovich and Cobban, 1975 ; McNeil and Caldwell, 1981 ; Cobban, 1984) Specimens used in this study come from the DeGrey member of the Bearpaw C y cle whi c h i s equivalent to the ammonite biozones Exite/oceras jenneyi Didymoceras c hey e nnense, and Baculites compressus, spanning approximately 1. 5 Ma. They were collected from cores drilled by the South Dakota Survey in Miner Brown and Gregory Counties South Dakota (Figure 3) Results from this study are augmented by the data from previous studies as outlined in Figure 2 5

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Eastern South .:i Otherstudie1 used Dakota Units :ll .,; in this paper Ma r/) B. cJ/nobatlsl Elk Butte grandis Mobridge 1--Wright 1987 &. 70 1 B Baculusl II-" s Fatheme 1995 eliasi u u. uu ., 1hi$ s1114y a LVirgin Creek ., ., 1 954 B. jensen/ p.. s I.) c u... 0. >. '0 B reesid
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-..J II Sioux Ridge SDGS Coring locations a BHP Coring locat i ons Figure 3 Location of Upper Cretaceous coring sites in eastern South Dakota used in this study Brown Miner Co.

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PREVIOUS WORK Many workers have investigated the fractionation of stable isotopes into mollusc s hells as a proxy for paleoceanographic and paleoclimatic conditions. In general it is assumed that molluscs secrete their shells in chemical and isotopic equilibrium with seawater (e. g., Urey et al ., 1951 ; Saltzman and Barron, 1982; Krantz et al. 1 9 87 ; Landman et al 1989 ; Krantz 1990 ; Wefer and Berger, 1991 ; Klein et al. 1996a and b) In addition, the trace-element concentrations in molluscan shells are influenced by the trace-element composition ofthe ambient seawater and can thus be used as a proxy indicator of ancient seawater chemistries (Buchardt and Fritz, 1978 ; Lorens and Bender 1980 ; Whittaker et al. 1987) Other studies have shown that factors such as temperature and growth rate have an insignificant effect on the trace-element compositions of most biogenic carbonates (Lorens and Bender, 1977; Buchardt and Fritz, 1978) Therefore stable isotope and geochemical analysis of unaltered shell material can provide an estimate of the environments in which ancient molluscs lived. A plethora of studies exist which document the stable isotopic signals from a range of different regions, geologic periods and taxonomic groups Of these, several studies have recorded the isotopic ratios ofo180 and o13C in biologically produced calcite and aragonite from the WIS (e.g., Lowenstam and Epstein, 1954 ; Tourtelot and Rye, 1969; Rye and Sommer, 1980 ; Whittaker et al., 1987; Wright 1987 ; Carpenter et 8

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al., 1988; Fatherree, 1995) These workers noted significant differences in the isotopic composition of epifaunal bivalves (used as a proxy for bottom-water chemistry) and ammonite shell material (used as a proxy to detennine the chemistry of intermediate waters) Lowenstam and Epstein (1954) and Tourtelot and Rye (1969) found that the benthic fauna within the seaway have more negative 8180 values and more positive o13C values than the nektic fauna. This indicates that the bottom-waters were either fresher, warmer, and may have had a different carbon source than the intermediate-waters. These results were interpreted as evidence for a temperature-inverted and vertically stratified water column that experienced sluggish circulation. Since then, several other workers have also noted offsets in stable isotope values between the benthic and nektic molluscs from the WIS (Rye and Sommer, 1980; Whittaker et al., 1987; Wright, 1987; Carpenter et al. 1988; Pratt et al., 1993; Fatherree, 1995). Based on differing isotopic signatures between nektic and benthic molluscs, Wright (1987) proposed a paleoceanographic circulation model for the Late Cretaceous Bearpaw Seaway (Figure 4) She inferred decoupled water masses consisting of a slightly brackish upper layer, a more nonnal marine intennediate layer, and a warm, saline bottom layer The existence of a freshened lid across the seaway has also been suggested by several other authors (Kauffinan, 1975; Kyser et al., 1993; Pratt et al., 1993). This freshened layer acted to stratify the seaway, creating oxygen-deficient bottom-waters which controlled the distribution of faunas and organic carbon burial. Although disputed by Slingerland et al. (1996), paleo-rainfall reconstructions of Parrish et al (1982) indicate that fresh-water run-off was greatest along the western 9

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.... 0 !Wright (1987) Modell !Western Margin I I wind-Driven Surface Currents I Reduced Salinity Lid ............. Warm, Saline Bottom Layer !Eastern Margin I !Evaporation I j j ,_,0-own--w-ell-in--.gj Figure 4 Schematic diagram of a west to east transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns as proposed by Wri g ht (1987) Considerable vertical exaggeration

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margin of the basin and that evaporation exceeded fresh-water input along the eastern margin According to Wright (1987) the upper layer that formed along the western margin o f the seaway was then driven eastward by prevailing wind patterns as it gradually increased in salinity due to evaporation into a humid air mass above the shallow eastern margin Wright (1987) calculated that disequilibrium evaporation along the eastern margin could have preserved the originally low 5180 signature of the once brackish water As evaporation continued, the density of the upper water layer increased When its density exceeded that of the intermediate water mass it sank in the eas t ern port i o n ofthe WIS. Thus Wright's (1987) model points to the seawa y's eastern margin as the site of bottom-water formation. Once formed the bottom water then returned to the center of the basin in a western counter-current that was c onfined to the bottom few meters of the seaway. IfWright's (1987) model is correct, then the isotopic composition of benthic fauna across the entire seaway should show similar depleted 0180 values In addition, it should be possible to identify the site of bottom-water formation along the eastern portion of the seaway If this is indeed the area where surface-waters downwelled to form the bottom-waters for the rest of the seaway, then there should be an area along the eastern margin where the isotopic values of the surfaceand intermediate-waters match those of the bottom-waters Any nektic fauna living in these waters along the eastern portion of the seaway should, therefore record depleted 0180 values as well Thus, the i sotopic composition of ammonite shell material collected from the eastern portion of the seaway should converge on that of the benthic fauna 11

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Hay et al (1993) proposed two likely models for the circulation and format i on of water masses in the WIS Model I suggests that warm waters from the Tethys mixed with cooler polar waters of equal density from the Boreal Sea to form a denser, third water mass on the bottom ofthe seaway (Figure 5) As oxygen-depleted water in the center of the basin downwelled, it flow e d along the bottom and was finally exported to the southern Tethys Sea The formation of such waters could have been the cause ofthe oceanic anoxic events" during the Cretaceous The second scenario suggests that dense warm, saline waters from the Tethys entered from the south, spread across the seaway and formed the lowest water mass i n all but the northern portion of the WIS (Figure 6) This model also requires that the cooler, less saline waters from the Boreal Sea flowed into the WIS to form a freshened lid on the seaway Finally, a narrow zone of intermediate-waters formed from the mixing of these two distinct water masses, and then exited to the north If these two scenarios are correct, then the stable isotopic and trace-element signals recorded in the benthic fauna of the WIS should be uniform In addition, the bottom-water signal of the benthic fauna within the seaway should also be recorded in the benthic fauna of the Tethys. IfModel I is correct, the nektic fauna living in the intermediate-water masses should record warmer temperatures in the southern part of the seaway and cooler temperatures in the northern portion of the seaway. If Model II is valid, then the nektic fauna should record more uniform temperatures and indicate a less than normal salinity for the intermediate-water mass Glancy et al (1993) suggested an oceanographic model for the WIS that is similar to the model by Hay et al. (1993) Their model calls for the incursion of warm Tethyan waters from the south to displace the less saline waters of the upper water 1 2

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I.H I Hay et al. ( 1993) Model I I Tethys Sea I !Boreal Seal Evapora tion I i i Saline Warm J Mixing l IntermediateWaters 1 I Fre shened, Cool +--+---40N D e n s e BottomWater Formation & Downwellin g / Int erm e d 1 ateWater s 50 N 60" N Figure 5. Schematic diagram of a north to south transect through the centra l portion of the Western lnterior Seaway with water stratification a nd circulation p atterns as proposed in a mod e l by Hay et al. ( 1 993 ).

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!Hay et al ( t993) Model II I ] !Tethys Sea I !Boreal Seaj Evaporat ion I i i -Mi xed---Intermediat e Layer lwarrn, Saline Bottom-W a ters I 40N 50"N 60 N Figure 6. Schematic diagram of a north to south transect through the central portion of the Western Interior Seaway with water stratification and circulation patterns as proposed in a model by Hay et al. (1993).

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layers ofthe seaway (Figure 7A & 7B). Their computer-generated climate model suggests that under maximum insolation forcing, conditions would favor the import of oxygen-poor waters from the Tethys and the export of surface-waters during the winter months alternating with the export of intermediate-waters during the summer If this model is correct, then the benthic fauna should record uniform oxygen isot o pe values across most ofthe bottom ofthe seaway Because the surfaceand intermediate-waters would originate within the seaway from evaporation, the nekt i c fauna should record distinct isotopic and geochemical signals and indicate reduced salinity waters As these intermediate-waters are then exported to the Tethys to the south (Figure 7B) the nektic fauna of the Tethys should contain the same species as the WIS and the geochemical signals between the two areas should be similar. In addition, this model predicts that upwelling along the eastern margin was possible during part of the year If upwelling was intense enough to cause the oxygen-poor bottom waters to mix with intermediate waters in this region, then this should result in the exclusion of fauna not adapted to low oxygen conditions Slingerland et al ( 1996) have recently proposed a circulation pattern that contrasts somewhat from those above (Figure 8 and Table 1 ). Although Slingerland et al. (1996) constructed their model with data from the earlier Greenhorn Cycle, similar isotopic patterns existed between the Greenhorn Seaway and the later Claggett and Bearpaw Seaways (Pratt et al 1993; Ludvigson et al., 1994) A computer-generated atmospheric model was used to approximate the Turonian climate over North America The model estimated high amounts of freshwater run-off for both the western and eastern margins of the seaway. Overall, the computed freshwater input 15

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0\ I Glancy et al. ( 1993) Modeq Winter !Tethy s Sea I Bor ea l S eal 4----4---4---------. ----+ ----+ ----+ -----. ----+ F r es h wa t e r Ex p ort !Mix ed Wat e r s I ....................... W ate r s 40N 50 N 60 N Fi gure 7 A. Schematic diagram of a n o rth t o s outh tra n s e ct thr o u g h th e ce ntr a l p o rti o n of the W e s t ern Inter ior Seaway with w a ter s tratific a ti o n a nd c ir c ula t i o n p atterns d urin g th e wi nter m o nth s as proposed by Glancy et a!. ( 1993 )

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..... -....1 IGiancy et al. Summer !Tethys Sea I !Evaporation., t t jBoreal Seaj -+ -+ -+ +-------+-----+----+---"' .-/ '----'-----+ -----.... . ... .. ...... .... ........ ___ ......... Water s BottomWater s 40 N 50 N 60 N Figure 78. Schematic diagram of a north to south transect through the central portion of the Western interior Seaway with water stratification and circulation patterns during the summer months as proposed by Glancy et al.( 1993).

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Slingerland et al. (1996) Model Marine Conditions ..,s Figure 8 Diagram of estuarine circulation pattern in the Western Interior Seaway as proposed by Slingerland et al. (1996). Brackish waters flank the margins of the seaway, and more open marine conditions exist in the center of the seaway. 18

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Table 1. Summary of oceanographic conditions as predicted by various circulation models for the Late Cretaceous WIS BottomIntermediate Temp and Temp. and B110 Variation: B110 water water Variation in Variation in Bottom-waters Variation : Formation Formation BottomIntermediate Interm ed iatewaters waters waters Wright Along Normal Normal, but Little o r no Depleti o n in (1987) eastern marine waters or no warmer on variation across B180 along margin variation eastern seaway eastern margin; margin open marine values in othe r areas Hay et al. Mixing of Waters from Intermediate Wanner in Uniform across Saline, warm (1993) I waters Tethys and temps., south, cooler basin signal to the from Boreal Seas uniform in the south; fresh, Tethys and across northern cool signal to Boreal bottom regions the north Seas Hay et al. Evapora Evaporation Uniform Warmer in Uniform across Saline, warm (1993) II tion from from across all the south, basin signal to the Tethyan freshwater but the very cooler in the south; fresh, waters runoff and northern northern cool signal to Boreal waters section of regions the north WIS Glancy et EvaporaEvapora tion Uniform Uniform Uniform across Fresh to al. (1993) tion from from across all across basin, basin normal marine T ethyan freshwater but the very similar to s ignal waters runoff and northern Tethyan precipitation; section of temps possible WIS upwelling along eastern margin Slingerland Mixing of Mixing o f Poss ibly Cooler on More negative More negative et al waters in brackish cooler on western on eastern and on eastern and (1996) center of waters from western margin and western western basin runoff with margin and warmer on margins; more margins; more WIS marine warmer on eastern normal marine n ormal marine waters eastern margin in center of in center of margin basin basin 19

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for the Turonian is almost 20 times the amount of evaporation, ind i cating that the seaway was an exporter of freshwater Fresh and therefore less dense river waters entered the seaway from the eastern and western margins and created offshore-dipping water-surface slopes, down which the fresh waters subsequently flowed In the process, they were deflected by the Coriolis force and piled up along each coast until the offshore pressure force arising from each surface slope balanced the Coriolis force The wat e rs then moved isobathyally alongshore as geostrophically confined jets (Figure 8)(Slingerland et al., 1996). The shear couple arising from the coastal jets and water surface slopes arising from the contraction of the water column as it densified toward the center of the basin drew Tethyan and Boreal surface waters into the seawa y where the y mixed The mixed waters formed a denser third water mass which downwelled, split into two flows and returned to the global ocean. The mean annual wind field also contributed to the counterclockwise gyre The southwesterlies across the southern half of the WIS were sufficiently northward directed to accelerate water toward the seaway's southeastern shore, from which it was deflected northward, contributing to the northward-directed geostrophic flow Easterly winds dominated across the northern half of the WIS, contributing to the westward migration of waters across the northern portion of the basin. If conditions hypothesized in this model are valid, there should be more variati o n in the geochemical signals between the fauna on the eastern and western margins of the seaway The model predicts a high influx of fresh waters along both margins. However the eastern fauna should indicate higher temperatures because of the influence of Tethyan waters from the south The western fauna should indicate cooler temperatures 20

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as northern waters formed the gyre along this margin (Figure 8) This model also predicts a salinity contrast between the marginal waters and the waters in the center of the seaway If such a contrast did exist in the WIS, then it should be recorded isotopically by more depleted 8180 values in the fauna along the margins The estuarine waters would have affected the faunal distribution as well by favoring a brackish fauna population. 21

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METHODS Field Methods and Selection of Samples Specimens used in this study were collected from cores stored in Vermillion, SD by the South Dakota State Survey. Although it was impossible to identify ammonite biozones from the fragments of ammonites found in the cores, it was possible to identify the members of the Pierre in each core based on depth ranges identified by workers at the State Survey. The Crow Creek is also texturally and lithologically unique among the other shale members of the Pierre and served as a useful marker bed. While some of the upper members of the Pierre were missing, the DeGrey member was included in each of the cores selected for study. Therefore, it was decided to limit the collection of shell material to the DeGrey member of each core. The shell material was visually inspected in the lab under a microscope for any obvious dissolution or diagenetic effects. Choice of shell material for geochemical and isotopic analysis was based on abundance, preservation of original mineralogy, and ease with which the shell material could be separated from the matrix. Select samples were examined and photographed using a Scanning Electron Microscope (SEM) in order to determine the preservation of original shell microstructure. Based on these initial observations, several specimens were selected for further isotopic and geochemical analysis. In selecting specimens, special attention was given to 22

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zones in the cores which contained both inoceramid and ammonitic shell fragments occurring over a short depth range. This was done to ensure a virtually coe v al comparison between the isotopic and chemical compositions of different water masses along the eastern margin If the shell material is not pristine, then the isotopic and geochemical signals could record a diagenetic history instead of the environmental conditions in which the animals lived In order to get accurate paleoceanographic and paleoenvironmental interpretations from these analyses it was important to first establish that the conclusions are based on geochemical signals from unaltered shell material Many authors have established a series of criteria to select the best shell material to use in such studies While Wright (1987) believes that it is possible to determine the best preserved shell material by SEM photomicrographs and standard petrographic analysis, others have shown that these methods are unable to detect subtler forms of chemical alteration (Buchardt and Weiner, Pirrie and Marshall Elorza and Garcia-Garmilla, 1996) Instead, these authors made use ofboth SEM photographs as well as detailed geochemical analysis to determine the extent of diagenesis Stable Isotope and Trace Element Analysis Samples for analysis were obtained either by flaking off fragments of the shell with a scalpel or by using a microscope-mounted dental drill In the inoceramids, samples of calcite were removed from aragonitic nacre with a dissecting needle The samples were then washed with triple distilled water in an ultrasonic bath three times for 10 minutes each to remove organic contaminants and any remaining matrix Next, the 23

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samples were placed in an oven at 30 C and left to dry overnight. Finally th e dried carbonates were separated into splits for stable isotopic and trace element analysis Stable isotopic analyses were performed at the University ofMichigan s Stable Isotope Laboratory and at Mountain Mass Spectrometry located in Evergreen, Colorado. The samples sent to the University of Michigan were roasted in a vacuum for one hour. The aragonite samples were roasted at 200C and the calcite samples were roasted at 380 C to remove any organic material within the shell structure Samples were then reacted with phosphoric acid at 72C in a Finnigan MAT automatic carbonate preparation device The 180/1 6 0 and 13C/12C ratios of the evolved C02 were determined using a MAT 251 triple-collector isotope ratio mass spectrometer. Oxygen and carbon values are reported in standard delta (8) notation relative to PDB. Precision (lcr) was determined using NBS standards, and was better than 0.06%o for 8180 and 0.03%o for 813C values Samples sent to Mountain Mass Spectrometry were analyzed on an Optima Mass Spectrometer by Micromass Carbonate samples were prepared on a MultiPrep system, and then reacted with phosphoric aci d in individual reaction vessels The gas generated was then passed through a water trap frozen into a 'cold finger' and finally analyzed on the mass spectrometer Precision was determined to be better than 0.04%o for 8180 and 0.08%o for 813C values. Direct current plasma-emission spectroscopy (DCP-ES) conducted at the University of South Florida was used to determine the trace element concentration of the shell material About 100 mg of dried sample was digested in 15 m1 of 1 M HCl. This solution was then diluted by a factor of three with a solution of 1 N HN03 spiked with LiC03 The diluted samples were then analyzed for Ca, Mg, Sr, Mn, Fe, Ba, and Na 24

PAGE 37

. concentrations and compared to standards of known concentration Each sample was analyzed two times by the instrument. Precision for the 'Mean' standard analyzed three times during each run' often samples was determined to be better than 0 .16 ppm Because it is not possible to determine the sample precision from two data points, the total range of each value is plotted on Figures 14-18 25

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RESULTS Under SEM, the calcite prismatic layer of the inoceramid shell material (Figures 9A-D) appears well preserved The prismatic layer typically shows distinct inter-prism boundaries (Figures 9A-D). Figures 9A and 9C show the contact between the prismat ic layer and the inner nacreous layer. While the sample from South Wheeler (Figures 9C and 9D) has a blocky surface texture possibly due to recrystallization, the sample from the Miner County Core (Figures 9A and 9B) has a smoother surface texture and may represent better preservation. In both examples the original structure of the prismatic layer is preserved. However, the Miner County sample contains small conduits between some of the calcite prisms, possibly due to the loss of organic filaments that were originally present. Pirrie and Marshall (1990) noted that calcitic surface cements up to 10 11m thick can form in these void spaces This may explain the presence of distinct crystals located between the prisms in the upper left hand comer of Figure 9 A. SEM analysis of the aragonite from the ammonitic shell material shows the preservation of individual nacre platelets (Figures 1 OA-C). The random arrangement of loose plates is probably due to the loss of the organic matrix that originally held them together However Figure 1 OB from Miner County shows the edge of a shell with the plates still intact probably due to exceptional preservation (Figure lOB) Buchardt and 26

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N -...) Figure 9A. SEM photomicrograph showing exce llent preservation of prismatic layer in inoceramid sample MC 296 0 Note contact of calcite prisms with inner nacreous layer in upper right hand comer of photograph Magnification is approximately 500x, scale bar is 10 J.Lm. Figure 9B SEM photomicrograph showing excellent preservation of calcite prism in inoceramid sample MC 296. 0 Note void space at contact with adjacent prism probably th e former location of organic matrix Magnification is approximately 2000x scale bar i s 10 !lffi.

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N 00 Figure 9C. SEM photomicrograph showing preservation of calcite prisms in inoceramjd sample SW-8 Note contact of calcite prisms with inner nacreous layer in upper left had comer of photograph. Blocky texture of crystal swfaces are probably the result of recrystallization Magnification is approximately 500x, scale bar is 10 Figure 9D SEM photomicrograph showing preservation of calcite prisms in inoceramid sample SW-8 Blocky texture of crystal swfaces are probably the result of recrystallization. Magnification is approximately 500x, scale bar is l 0

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N \0 Figu r e 1 OA. SEM photonilcrograph s howing preservation of nacreou s lay e r in arrunonoid sample MC 289 6. Note preservation of origina l aragonitic plates Magnification is approximately 2000x, scale bar i s 10 Figure lOB SEM photonilcrograph showing preservation of nacreou s layer in ammonoid samp le MC 289.6. Note preservation of original aragonitic plates Plates in bottom of photograph are still intact. Magnification is approximately 2000x scale bar is 10

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w 0 Figure 1 OC. SEM photomi c r ogra ph showing pre se rvation of nacreou s layer in ammon o id sam ple from Coon Creek. Note preserv ation of miginal aragonitic plat es Di ssolutio n ma y be the ca use of the many hole s in the plat es. Magnification i s approximately 2000x, sc ale bar i s 10 jlm.

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Weiner ( 1981) observed local rounding of edges and corners of individual plates resulting from modest internal dissolution However both the Miner County sample (Figures lOA and lOB) and the Coon Creek sample (Figure lOC) show plates with relatively sharp edges The small holes in some of the plates (Figures lOA and lOC) are either original pores within the shells, or they represent dissolution of shell material during diagenesis If the latter is the case, then the sample from Coon Creek is less well preserved than the Miner County sample because it contains a higher percentage of dissolution In order to assess the effects of diagenesis on the shell material that appeared diagenetically unaltered under visual examination, selected shell material was analyzed for its trace-element concentrations (Table 2) Based on the criteria set forth by previous workers on the trace-element concentration in fossil shell material (Burchardt and Weiner, 1981; Whittaker et al 1987; Morrison and Brand 1988; Woo et al., 1993 ; Elorza and Garcia-Garmilla, 1996) and on a comparison with trace-element analyses on modern molluscs (Dodd, 1965; Lorens and Bender, 1980; Klein et al ., 1996a and b), the shell material from this study ranges from well-preserved to poorly preserved. The results of the isotopic analysis are also listed in Table 2 Values for Miner Co (Figure 11) show that 8180 and 813C values are grouped in distinct taxonomic clusters. The epifaunal inoceramid bivalves have the lowest 8180 and the highest 813C values with averages of8180 calcite= -3 77 %o and 8180 aragonite= -3 50 %o; 813C calcite= 3 .26 %o and 813C aragonite= 3 32 %o. The nektic cephalopods have higher 31

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Table 2 Summary oflsotopic Analysis (Reported in %o PDB) and Trace-and Minor-Elemental Concentrations (Reported in ppm). Mg/Ca and Sr/Ca Ratios Are Multiplied by 1000 Sample 1.0 material 813C 8180 Mg Sr Mn Fe Na Ba Mg/Ca Sr/Ca MC 276 2 ino calcite 3 08 -3. 24 7290 1324 72089 1268 3823 34 8 MC 279 0 ino calcite 2.55 -3.65 4968 1103 18080 6976 3472 12 8.43 1 87 MC 282.2 ino? 3 52 -3.97 MC 288.0 ino calcite 1 .81 -3 86 8236 1386 19767 354 3177 20 12 65 2 13 MC 291.0 ino calcite 4 .25 -5.13 7563 1933 597 387 5263 10 MC 295 5 ino calcite 4 06 -4.35 MC 296 0 ino calcite 3 78 -4.05 6072 1855 1007 591 5167 15 MC 300 0 ino calcite 2.36 -3 85 MC 300 1 ino calcite MC-8 276 3 ino arag 4 58 -3 32 MC 285 5 ino arag 2.98 -4 09 9054 13955 6119 12099 6838 6537 15.89 24.49 MC 285 6 ino arag 0 .61 -3 80 MC 288 0 ino arag 3 48 -3 42 MC 296 0 ino arag 4 97 -2.86 MC 278.0 baculite 0 .23 0 24 MC 279 2 baculite -0 58 -0.40 MC 283.8 baculite 0 88 0 08 267 939 134 1E+05 11506 27 MC 289 6 baculite 1 82 -1.22 566 2131 1241 2356 4556 38 MC 295 5 baculite 0 35 -1.09 MC 308 9 baculite -1.19 -1. 32 793 2857 68 474 4111 156 MC 288.0 Matrix -2.76 -0. 95 Brown Co material 813C 8180 Mg Sr Mn Fe Na Ba Mg/Ca Sr/Ca BC 184 2 ino calcite 3 77 -1.58 5591 1376 43662 777 5042 75 BC 205.4 ino calcite 1.65 -5.21 7453 1345 5531 1786 3979 45 10. 58 1.92 BC 220 0 ino calcite 4.43 -4.49 7034 1495 1468 356 4814 54 9 72 2 07 BC 221. 5 ino calcite 5 78 -4 07 62M 1620 1836 360 4760 54 8.67 2 24 BC 227.0 ino calcite 6793 1501 1510 413 4152 83 9 .31 2 06 BC 227.8 ino calcite 2.1 -1.41 5861 1239 1385 415 5107 287 8 .36 1 77 BC 250 1 ino calcite 3.62 -2. 79 1591 1190 130 575 3232 16 BC 294. 5 ino arag 4.67 -3.76 BC 281. 8 bacul ite -0.16 -0.58 BC 283.0 baculite 3 99 -1.26 (Table 2 Continued on Next Page) 32

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Table 2 (continued) Sample 1.0. material 813C 8180 Mg Sr Mn Fe Na Ba Mg/Ca Sr/Ca Gregory GC 188 0 ino calcite 7090 1416 17725 733 3797 40 10 25 2 05 GC 190 0 ino calcite 7562 1366 66886 643 3807 78 12.49 2 26 GC 191.7 ino calcite 3.59 -3 18 4710 1486 1167 487 3593 42 6 68 2 .11 GC 192 5 ino calcite 3.91 -4 15 7226 1627 1022 324 4221 19 10.19 2 29 GC 193.0 ino calcite 6536 1325 46161 470 3787 31 10 52 2 13 GC 191. 5 ino arag 3 .38 -3 .01 1488 1921 4375 413 5617 71 1.92 2 .48 GC 191. 5 ino arag GC 191.7 matrix -4 54 -12.2 South Wheeler SW81no ino calcite 1 63 -2.31 6246 1673 237 257 4490 12 8 55 2 29 SW Degrey ino calcite 4 72 -3 03 mb White River WR9 ino calcite 2.47 -1.86 WR7 calcite 1.05 -2.97 WR9 Matrix -5 27 -0 .31 DeGrey Mb. ino arag 3.74 -2.98 1019 2326 1094 1171 5279 234 DeGrey Mb. baculite -3.12 -1. 17 Oral SO ino arag 5502 1321 3205 479 3911 13 GameR. ino arag 5 24 -4 08 91 2523 57 238 5355 158 0.12 3 .47 GameR. ino calcite 3.08 -3 Cheyanne ino arag 454 2016 172 1429 4352 99 0 66 2 93 River 3 3

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-p .. -5 -4 3 -2 5180 ,.. 5 4 3 2 1 n -1 -1 -2 (.) .., ... c.o lno Calcite lno Aragonite eAmmonites Figure 11. Stable isotope summary of shell material from Miner Co core. Ino =Inoceramid shell. 34

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8180 values and lower 813C values with averages of8180 = -0 .62o/oo and 813C = 0 25o/oo. The results from the Brown core (Figure 12) show a similar pattern The inocerarnids also have the lowest 8180 and the highest 813C values with averages of 8180 calcite = -3.26 %o and 813C calcite= 3.56 o/oo. The cephalopods from Brown Co. have average 8180 = -0 92 o/oo and 813C = 2 08 %o. The inocerarnids from Gregory Co. (Figure 13) also have low 8180 and the high 813C values, with averages of 8180 calcite= -3. 67 o/oo and 813C calcite= 3.75 %o. The one inoceramid aragonite sample from the Gregory Co. core had a 8180 value of -3.01 %o and a 813C value of3. 38 o/oo. No ammonites were found in this core. 35

PAGE 48

0180 Figure 12 Stable isotope summary of shell material from Brown Co core lno = Inoceramid shell 36

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c 4 .. (.) 3 2 c.o 1 1\ 5 -10 -5 -1 -2 -3 -4 ,.. Figure 13. Stable isotope summary of shell material from Gregory Co core 37 lno Calcite I no Aragonite A Matrix

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DISCUSSION Isotope and Trace-Element Paleontology In order to use the data from this study to reconstruct paleoenviromental conditions along the eastern margin of the WIS, it must first be assumed that the geochemical signals preserved are original However, several factors could influence the distribution and the preservation of stable isotopes and trace elements in fossil molluscan shell material These factors include 1) diagenetic effects, 2) postmortem transportation, and 3) vital effects ." These three possibilities must be carefully examined and eliminated before a paleoenvironmental reconstruction can be attempted Diagenetic Effects Elorza and Garcia-Garmilla (1996) provide geochemical criteria for determining diagenetic alteration in fossil Inoceramus shell material from Upper Cretaceous sediments of the Basque-Cantabrian Region of northern Spain They found that the altered material was enriched in Ba, Sr, Mg, Mn, and Fe coupled with a depletion in Na with increasing diagenesis Whittaker et al. (1987) and Morrison and Brand (1988) evaluated the extent of diagenesis in molluscs (including both inoceramids and ammonites) from the Upper Cretaceous WIS Results showed a trend of decreasing Sr/Ca ratios and increasing Mg/Ca ratios with increasing diagenesis 38

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Inoceramid and Ammonite Shell Microstructure Because mineralogy partially controls the partitioning of certain elements into the lattice structure of carbonate minerals (Chave, 1954), an effort was made to compare carbonate shell material from this study to equivalent mineralogies in modem and fossil examples. The microstructure of inoceramid shells originally consisted of a thick, lowMg calcitic (LMC) prismatic outer ostracum and an aragonitic, nacreous inner layer. The prismatic structure consists of parallel, adjacent structural units with columnar calcite prisms whose c-axes are arranged perpendicular to the outer shell surface (Elorza and Garcia-Garmilla, 1996). Although each calcite prism is not a single crystal, each prism does lack diverging arrangements of second-order subunits and each prism is surrounded and bounded by a sclerotized organic matrix. The nacreous layer is composed of numerous horizontal lamellae secreted on top of each other. Each lamella is an aggregate of tabular aragonite deposited on an interlamellar organic matrix. Individual tablets may be up to 10 J..Lm in diameter (Carteret al 1991; Ubukata, 1994) Studies on fossil ammonoids and modem Nautilus shells reveal that most consist of three aragonitic regions: an outer prismatic layer, a nacreous layer and an inner prismatic layer. The outer prismatic layer consists of regular prisms, each with needle like crystallites 0.2-0.5 J.Lffi in diameter (Kulicki, 1996). These prisms are oriented perpendicular to the outer shell surface. While the prismatic layer is thicker in early postembryonic stages, in later development it comprises only a small fraction of the total shell thickness The nacreous layer constitutes the majority of the shell It is constructed of aragonitic hexagonal plates which are arranged in layers separated by interlamellar 39

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membranes The plates are secreted in vertical stacks and are separated from each other by conchiolin (Kulicki, 1996). Lamella thickness in the nacreous layer varies in different parts ofthe shell, but is generally about 0.25 thick. Many ammonoids also have pores located in their nacreous layer The inner prismatic layer forms later in the development of the animal, usually after the second or third septum It varies in thickness, and it is generally used to smooth out any unevenness or hollows on the interior of the shell It generally consists of regular prisms with visible needle-like crystallites (Kulicki, 1996). Minor-and Trace-element Analysis The values ofMg (average= 6100 ppm) and Sr (average= 1437 ppm) from inoceramid calcite of this study (Figure 14) are higher than modem calcitic bivalve averages 2800 ppm Mg 950 ppm Sr (e. g. Woo et al., 1993) A similar trend of high Mg and Sr concentrations is also apparent in the aragonite fraction of the inoceramid samples (Figure 15) Fossil aragonite samples have a mean concentration of 2381 ppm for Mg and 3436 ppm for Sr, while modem aragonitic molluscs have a range of 10-850 ppm for Mg and a range of 1900-2400 ppm (mean= 2000 ppm) for Sr (Milliman, 1974; Morrison and Brand, 1986). As predicted by partitioning coefficients, the concentration ofMg is much lower in the aragonitic samples, while the concentration of Sr is slightly higher. After studying the behavior of several trace elements in diagenetic systems, Woo et al ( 1993) showed that Mg and Sr are the most susceptible trace-elements to diagenetic alteration According to their results, original Sr concentrations in either 40

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Sr vs. Mg in Inoceramid Calcite from the WIS 8000 7000 6000 e 5ooo Q, 4000 C) ::E 3000 2000 1000 0 0 500 I T Range of Modem Calcitic Bivalves 1000 Sr (ppm) *tl!i 1500 2000 Figure 14 Sr verses Mg in prismatic layer ofinoceramid s collected from cores along the eastern portion of the WIS Error bars represent the total range of values measured The solid outlined field represents the chemical range of unaltered Recent calcitic bivalves Cretaceous bivalves from this study are enriched in both Mg and Sr with respect to their modem counterparts. 41

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-E a. Sr vs. Mg in Inoceramid Aragonite from the WIS 6000 5000 4000 E; 3000 en ::! 2000 1000 Range of Modem Molluscan Aragonite >I. 0 500 1000 1500 2000 2500 3000 Sr (ppm) Figure 15. Sr verses Mg in nacreous layer of inoceramid fossils collected from cores along the eastern portion of the WIS Error bars represent the total range of values measured The solid oultined field represents the chemical range ofunaltered Recent aragonitic molluscs Unaltered Cretaceous bivalves from this study have reasonable Mg and Sr concentrations 42

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calcite or aragonite specimens should become depleted during diagenesis However, diagenetic alteration is predicted to increase the Mg concentration in original aragonitic samples, and to decrease the Mg concentration in original high magnesium calcite samples. Because inocerarnids secrete low magnesium calcite, the Mg concentrations in their shells should not be affected by diagenesis. Based on these diagenetic predictions, the elevated concentrations of Sr in all the samples from this study are evidence for limited diagenesis, and probably are the result of originally high Sr concentrations in the WIS The effects of diagenesis on Mg are more difficult to predict, but the high Mg concentrations in the diagenetically resistant calcitic inoceramid samples suggest that the WIS may have also experienced elevated concentrations of Mg. Morrison and Brand (1988) and Woo et al. (1993) also analyzed the Mn content in mollusc shells from the Late Cretaceous ofNorth America. They found that Mn generally increases with diagenesis Morrison and Brand ( 1988) concluded that minor dissolution of aragonitic shell material results in Mn concentrations of ppm, and that at ppm the original aragonite is completely replaced by calcite Analysis of the Mn concentration in the calcitic samples from this study (Table 2) show elevated Mn levels in some of the samples. Calvert and Pedersen (1993) noted that anoxic marine sediments tend to have elevated concentrations ofMn and other trace-elements. The high levels ofMn could therefore be the result of anoxic bottom conditions which are also suggested by high organic content of the black shales. The samples with high concentrations ofMn (MC 288.0, MC 276.2, MC 279 0, BC 184 2, G 188.0, G 190.0, and G 193 0) on Figure 16 may also indicate diagenetic alteration. Analysis of the Mn concentration in the aragonitic samples from this study (Table 2 and Figure 17) show a 43

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8000 7000 6000 e 5ooo c. .e: 4000 3000 2000 1000 0 0 Mn vs. Mg in Inoceramid Calcite from the WIS I I 1000 2000 3000 4000 5000 6000 Mn (ppm) Figure 16. Mn verses Mg in prismatic layer ofinoceramids collected from cores along the eastern portion of the WIS. Error bars represent the total range of values measured 44

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-E Cl. Cl. C) ::!! 1200 1000 800 .----600 400 200 fL 0 0 Mn vs. Mg in Aragonitic Samples from the WIS + Range of Unaltered Cretaceous Aragonitic Molluscs Range of Unaltered Recent V Aragonitic Molluscs 0 . . 200 400 600 800 1000 1200 Mn (ppm) 1400 Figure 17 Mn vs. Mg in Upper Cretaceous aragonitic samples from cores along the eastern margin of the WIS. Error bars represent the total range of measured samples. Open diamonds are Inoceramus samples and solid circles are ammonite samples 45

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similar trend of increasing Mn concentrations with diagenesis. According to these data, MC 285 5, 5D, and G 191.5 are the most altered, 19B, 7C, and MC 289 6 are partially altered, and 15B, 6D, 8D, MC 283.8, and MC 308 9 are unaltered (mean Mn concentration of95 ppm). A comparison of Fe with respect to Na in inoceramid shell material (Figure 18) shows a clustering of Fe concentrations around 300-800 ppm with a mean of 464 ppm for the calcitic samples and a mean of 93 7 ppm for the aragonitic samples (excluding the two exceptionally high values). According to this plot, calcitic samples 20B MC 276 2 MC 279 0 and BC 205.4 are altered because they contain Fe concentrations above 900 ppm. Data points of Fe verses Na for the aragonitic samples show that sample MC 285 5 and the ammonite sample MC 289.6 have Fe concentrations that are too high to be considered diagenetically unaltered. The Na concentration in the calcitic part of the inoceramid fossils has a mean value of 4234 ppm, while the aragonitic inoceramid fossils have a mean of 5404 ppm The three ammonite samples measured have a mean N a concentration of 6724 ppm. Figure 19 compares the stable isotope values of the unaltered and the altered shell samples and shows a trend of decreasing 813C values with diagenesis However, there does not seem to be a corresponding trend of decreasing 8180 values as one would expect during meteoric diagenesis; instead there seems to be a slight increase in the 81 80 values with diagenesis Based on this observation, the 8180 values do not seem to be affected as easily by diagenesis as do the 813C values. However, if diagenesis is responsible for the depletion in 813C values, it probably occurred early in the history of these sediments; the originally warm, saline waters trapped in the pore spaces (before 46

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Fe vs. Na in Inoceramus Sam pies 7000 6000 5000 -E 4ooo Q. Q. cu z 3000 2000 1000 0 0 500 1000 Fe (ppm) 1500 2000 Figure 18. Fe verses Na in Upper Cretaceous inoceramid samples from cores taken along the eastern margin of the WIS Solid diamonds are calcite samples and open triangles are aragonite samples Error bars represent the total range of measured samples. Altered specimens have Fe concentrations above 900 ppm 47

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<> Altered Shell Material vs. Unaltered Shell Material <> 6 <> <>. <> <> Unaltered lnos Altered lnos 0 Altered Ammonites 0 Unalte"ted Ammonites -2 Figure 19. 81 80 and o13C for altered and unaltered shell material based on traceand minor-elemental concentrations. 48

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compaction) could have re-equilibrated with the abundant 12C-rich organic carbon contained in the black shales. This would result in porewater with the same relative 81 80 values as the original bottom-water but with depleted 813C values. However compaction of the shales, the fine-grained material develops low porosity and premeability so that the remaining chemical signature is basically preserved (Quinby Hunt and Wilde, 1996). Based on results plotted in Figure 19, it seems reasonable to assume that most of the measured 8180 values in the shell material from this study are close to their original ratios. However, the o13C signatures may be somewhat altered. In addition, Saltzman and Barron (1982) analyzed the 8180 of inoceramid shells from Cretaceous deep-sea cores and found the values produced reasonable paleotemperatures for the ocean basins They also found that inoceramid samples with depleted o13C values were the most diagenetically altered among their specimens. A comparison of the best preserved shell material with the diagenetically altered shell material from this study (Figure 19) also shows a trend of decreasing o13C values with increasing diagenesis. This evidence suggests that the depleted 8180 and the elevated 813C values recorded in the epifauna across the bottom of the WIS are probably original Postmortem Transportation In order to make paleoenvironmental interpretations based on the fossils found along the eastern margin of the WIS in this study, it must be assumed that the shells are preserved in a location close to the regions they inhabited when alive Inoceramid shell fragments are the most common type of fossils found in all three cores and in the surrounding sediment examined in outcrop in central South Dakota. The inoceramid 49

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fragments are randomly distributed throughout the cores However, the y are so metimes grouped into clusters of multiple s hells which may reflect a gregarious life hab it. Most s how little signs of abrasion or dissolution Anunonoid shell fragments were s omewhat more disarticulated and much rarer In addition to the excellent state of preservation of most of the fossils themsel v es there is no sedimentological evidence to suggest these fossils have undergone postmortem transportation No evidence of turbidites such as graded beds and plane parallel rippled and convoluted lamina was observed in the cores used for this stud y Instead, the sediments consisted of finely laminated black shales with very little v ariat ion in color or composition The enclosing siliclastic sediments probabl y originated from the western margin of the seaway, being deposited in a low-energy environment on the bottom of a sluggishly circulating seaway However, some workers ha v e suggested that B aculit e s shells were originally filled with gas by the animal to regulate bou y anc y in th e water After death, this gas would have allowed them to float considerable distances This hypothesis seems unlikely because the conical shape of their gas-filled shells would cause them to float in a body chamber -up position after death, allowing the gas to quickly escape Therefore, it seems unlikely that the fossils found in the c o res were transported far from their original habitat, and they are therefore taken to represent a valid sampling of the fauna along the eastern portion of the WIS Vital Effects In order to explain the low 8180 v alues of inoceramids Tourtelot and R ye ( 1 969) proposed that these b i valves did not precipitate their shells in isotopic equilibrium with 50

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seawater by suggesting that metabolic fractionation occurred Although other organism s s uch as hermatypic corals, some echinoderms and algae exhibit vital effects due to the incorporation of metabolically ligh t" C02 into their skeletons (Keith and Weber, 196 5 ; Weber and Raup 1966 ; Swart, 1 983), most molluscs are thought not to exhibit vital effects" (Urey et al., 1951; Epstein et al., 1953) The incorporation of metabolically light C02 results in carbonate shells that are depleted in both 8180 and 813C. However, the isotopic signatures of epifauna from the WIS are not typical of the metabolic fractionation mentioned above Instead, analysis of inoceramids from this study consistently show a high 813C value as compared to the ammonites (Figures 11-13 ). In addition multiple epifaunal taxa from the WIS show a similar trend of depleted 8180 and enriched 813C values (Wright, 1987 ; Fatherree, 1995). For these reasons metabolic fractionation in inoceramids is unlikely and cannot explain the abnormally low 8180 values recorded in the epifauna from the WIS In order to further rule out the possibility of "vital effects" in inoceramids samples from the Late Campanian Coon Creek Formation in Tennessee were also geochernically analyzed Because there is a greater faunal diversity at Coon Creek it is generally believed to represent an open-marine environment (Lowenstam and Epstein, 1954) Both well-preserved aragonitic and calcitic shell material from several molluscan classes are preserved in a carbonate-rich siltstone (Weiner et al. 1976) A comparison of the stable isotope values between WIS inoceramids from this study and Coon Creek inoceramids shows that the samples from Coon Creek have more open marine 8180 values and similar 813C values (Figure 20) The 8180 values from Coon Creek inoceramids support the hypothesis that conditions were fully marine during the 51

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-5 Stable Isotope Values for Epifauna 0 U) 0 0 0 tO 0 0 o 0 [J co )[]0 eo [J [J tJ 0 + X -5 This Study [J -4 Glendive, MT Rapid Creek, SO Oral SO X Redbird SO o Saskatchwan, Canada Coon Creek, TN -3 -2 ... :! n -1 Figure 20 Summary of stable isotope values from epifauna of the WIS and Coon Creek 52 (..) .., ..... c.o

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deposition of these sediments Because Coon Creek inoceramids record reasonable environmental conditions, inoceramids can be considered free of vital effects If the inoceramids from the WIS are diagenetically unaltered, then they probably record original seawater chemistry as well Estimation of Temperature, Salinity, and Density of Water Masses Paleothermometry equations based on the o 180 of carbonate shell material were first developed by Urey et al. (1951), and later corrected by Craig (1965) for calcite specimens : rec) = 16 .914 2(8c-ow)+ 0 13(8c-owi (1) and Grossman and Ku ( 1986) for aragonite specimens : rec) = 21.8-4.69(ocOw) (2) This method of calculating paleotemperatures is based on the assumption that water with the heavier isotope of oxygen (H2180) is less reactive than 'li ghter' water (H2160), and that this difference in reactivity increases with an increase in reaction rates Because reaction rate is proportional to temperature, the o180 value of shell material precipitated at higher temperatures is more depleted in 180 than shell material secreted at lower temperatures. However, in order to get a temperature value from equations (1) and (2), the oxygen isotopic value of the ancient seawater (ow) must be known or assumed. Although the Ow of modem seawater is known to be very close to -0.22 %o on the PDB scale (SMOW = 0), it is difficult to detennine the 8w of restricted waters in the Cretaceous WIS. Most researchers agree that the Cretaceous seas had a Ow less than modem marine values because of the addition of 180 depleted water now stored in the 53

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polar icecaps. Shackleton and Kennett (1975) estimate that the glaciation effect on Ow would have made an ice-free world like the Cretaceous as much as -1.0%o lighter than the oceans of today. However, local effects such as the addition of 180 depleted freshwater runoff from the continental margins and evaporation rates over the seaway are more difficult to predict. These factors make i t difficult to estimate paleotemperatures from the oxygen isotope equations Paleotemperatures based on the 81 80 of fossil shell material from this study are tabulated in Tables 3 and 4 for a range of8w values from -1 to -3%o SMOW Assuming a SMOW of -1 o/oo, the average temperature calculated for the epifauna is 30 3C and 20. 8 C for the nekton. A Cw of -2%o SMOW produces an average epifauna temperature of25. 8 C and an average nekton temperature of 16. 1 C. A plot of intermediateand bottomwater conditions based on these calculations is illustrated on Figure 20. This combination of temperatures and salinities for the two water masses is impossible because it places less dense waters on the bottom of the seaway. Nevertheless it is likely that the different taxonomic groups lived in water masses with different Bw values, making direct comparison between bottom-water and intermediate-water conditions difficult. Because of the uncertainty inherent in estimating the Dw of ancient seawater from the WIS, the ()180 paleotemperatures were compared to another paleothermometry technique developed by Klein et al (1996a). They found that skeletal Mg/Ca ratios in the modem epifaunal bivalve Mytilus trossulus can also be used as a paleotemperature proxy. Mytilus shares some similarities with inocerarnids in that both are epifaunal benthic bivalves and both secrete shells made of a calcitic prismatic layer and an inner 54

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T a ble 3 R a n g e of po s sible bottom-water pale o temperature s at diff e rent O w v alues f o r the e as tern port i on ofthe WIS Temperatures are calculated fro m the 81 80 o f inoceramid c alc ite and ar ag oni t e Temperatu res in 'Calcite col u mn were computed wi th the equation of C rai g (196 5 ) and temperatures in th e Aragonite' w ere computed with the e quation of Grossman and Ku, (19 86 ) Temperatur e s are giv e n i n C material 8180 ow= 1 2198 Calcite Arag ow=-2 2196 Calcite Arag o w=-3 .2196 Calcite Arag ino calcite -3 .24 SMOW 1 25 92 SMOW 2 21. 32 SMOW=-3 17 ino calcite -3 65 27.87 23 17 18.7 ino arag -4 09 35 26 30.57 25 88 ino arag 3.8 33.9 29 .21 24.52 ino calcite -3 86 28 9 24.14 19.6 ino arag -3.42 32.12 27.43 22.74 ino calcite -8.28 53 .03 47 13 41.5 ino calc ite 6 .01 40 34 .69 29 6 ino calcite -4 05 29 83 25.02 20 5 ino calcite -3 85 28.85 24.09 19 6 ino calcite -5 96 39 73 34.43 29 4 in o calcite 3 29 26 15 21.54 17 2 ino calc i te 2 .31 21. 63 17.28 13.2 ino calcite 3 03 24 93 20 39 16 1 ino calcite -1.86 19 64 15.41 11. 4 calcite -2.97 24.65 20 12 ino arag -2 98 30.06 25.37 15.9 20.68 i no cal cite -1. 58 18 43 14 27 10 4 ino cal cite -5 .21 35.73 30.62 25 8 ino ca l cite -4 49 32 03 27.11 22 4 ino calcite -4.07 29 93 25 12 20 6 ino calcite -1.41 17 7 13 58 9 73 ino calc ite -2 79 23 82 19 34 15 1 ino arag -3.01 30.2 25 .51 20 82 ino calcite -3 18 25 63 21. 05 16 7 in o calcite -4 15 30 32 25 49 20 9 Averages, degrees Cels i us : 28.8 32 .31 24 06 27 62 19.6 22.93 55

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Table 4. Range of possible intermediate-water paleotemperatures at different&. values for the eastern portion of the WIS Temperatures are calculated from the 8180 of ammonoid aragonite according to the equation of Grossman and Ku (1986). Temperatures are given in C and are listed under the' Arag column. material 8180 li'W"-1.2198 A rag liw=-2.2196 Arag liw=-3.2196 Arag baculite -3.32 SMOW=-1 31.64994 SMOW=-2 26 96088 SMOW=-3 22.27088 baculite 0.24 14.95354 10.26448 5 574476 baculite -0 4 17 95514 13 26608 8.576076 baculite 0 08 15.70394 11. 01488 6.324876 baculite -1.22 21.80094 17.11188 12 42188 baculite -1.09 21.19124 16.50218 11.81218 baculite -1.32 22.26994 17.58088 12. 89088 baculite -1.17 21. 56644 16 87738 12.18738 baculite -0.58 18 79934 14.11028 9.420276 baculite -1.26 21.98854 17 29948 12.60948 Average,Degrees C 20.7 16.1 11. 4 56

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30 SJ 25 20 I 15 10 0 g., 5 0 0 q -19 20 DINSriY -M .... \0 00 -q q q q q q --21 22 23 24 25 26 21 28 29 30 31 32 33 34 35 36 37 31 39 40 41 SALINITY Figure 21. Estimated temperature, salinity, and density ranges as predicted by 8180 paleothermometry and Mg/Ca paleothermometry Note that both methods produce impossible conditions in which bottom-waters are less dense than inferred intermediate-waters Density is given in kglm3 and FP = freezing point. 57

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aragonitic nacreous layer. Klein et al (1996a) determined that the Mg/Ca ratio is related to the temperature of precipitation according to the following equation : TCC) = 2 50cto .36) [(Mg/Ca) 1000] 2 07ct2 .3s> (3) Because Mg/Ca ratios in modem seawater remain more constant over a wide range of salinities than do Ow values (Klein et al., 1996a), this method eliminates the need to estimate the chemistry of ancient seawater. Assuming a similar relationship as in Mytilu s trossulus between temperature and the Mg/Ca ratios of the inoceramids the Mg/Ca ratios in the calcitic samples from this study indicate a range of values from 29 C to 19 C, with an average temperature of 22 .2C for the bottom-waters along the eastern portion of the WIS (Table 5) Part of the fluctuation could be due to seasonal variation or small-scale climate change over a few tens ofthousands of years. Assuming a Ow of-1, the corresponding o180 paleothermometry temperatures are all (with the exception of sample BC 227.8) higher by about 6 to l2 C This difference can be reconciled only if the actual 0... of the seaway was considerably lower than the Cretaceous open-marine values The lower Ow for the WIS may have resulted from dilution by freshwater While the WIS did experience high rates of freshwater runoff as evidenced by the large deltas along its western margin, it is unlikely that these lower density waters could have undergone enough evaporation within the seaway to form the warm saline bottom waters. Instead, evaporation from the freshwater lid probably contributed to the formation of the intermediate-water masses lf22C is taken as the 'average' bottom-water temperature, then this value can be substituted into the o180 temperature equation along with the average measured O c (8180 = -3 64 %o) of the benthic fossils to solve for the This indicates a Ow 58

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Table 5. Bottom-water paleotemperatures and paleosalinities from the eastern portion of the WIS as calculated from Mg/Ca and Sr / Ca ratios in the prismatic layer of inoceramids. Temperatures calculated according to the equation ofKlein et al. (1996a) and salinities calculated according to the equation ofKlein et al ( 1996b ). o 180 paleotemperatures are also given for comparison Temperatures are in o c and salinities are in %o. Calcite Mg/Ca 8180 Sr/ Ca Sample Mg/Ca *1000 Temps Temps Sr/Ca *1000 Salinity 59 10.578101 24 37525 35 7 1.908969 42 95679 69 9.724642 22.2416 32 2.06716 46 32256 7B 8.6714754 19.60869 29.9 2 242623 50 05581 89 9.3119572 21.20989 2 057596 46.11906 99 8.3645431 18 84136 17 .7 1 768242 39.9626 209 8.5515432 19 30886 24 9 2.290543 51.07538 1C 10.251342 23.55835 2.047377 45.90163 2C 12.494599 29 1665 2 .2 57025 50 36223 4C 6 6847882 14.64197 25 6 2.109044 47 21369 sc 10.188383 23.40096 30.3 2 294008 51. 1491 6C 10.520136 24.23034 2.132677 47 71654 1D 8.4325628 19.01141 27.9 1 872205 42 17458 3D 12 650496 29 55624 2.128896 47 63609 Average Temps: 22.24242 28 Salinity: 46 81893 59

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(suway) :::: -2.17 Assuming that the freshwater influx into the seaway had a o180 somewhere between -10 and -20o/oo (SMOW) (Rye and Sommer, 1980; Glancy eta!., 1993 ) the salinity of the seaway can be calculated according to the following equation : S = [1-(ow + 1.22) / (or + 1.22)] x 34 .3%o (4) where Ow is -2 17 Or is the value of fresh water and S is the resulting salinity Nonnal salinity for the Cretaceous is assumed to 34 .3%o because of the dilut ion effect from the melting of icecaps, corresponding to a Ow (ocean) of -1.22%o (equivalent to a SMOW of -1) This set of calculations suggests a salinity of approximately 3 1 to 34 o/oo for the bottom-waters of the WIS A water mass with a temperature of22C and a salinity of 32%o would have a density around 1.022 to 1.023 kg m3 (Figure 20)(Hay eta!. 1993 ), probably not dense enough to fonn the bottom-waters of a stratified seaway Although the relationship between salinity and Sr/Ca ratios in modern Mytilus trossulus is less direct than the temperature relationship with Mg/Ca ratios discussed above, Klein et al. (1996b) did observe a trend of increasing Sr/ Ca ratios with increasing salinities according to the following equation : Sr/Ca = 0 047 X S -0 .11 (5) Assuming a similar relationship existed in the inoceramids, then the high Sr concentration in the prismatic portion of their shells could be an indication of elevated salinities. According to Equation (5), the salinities of the bottom-waters 47%o, indicating a very saline bottom-water mass However, it is possible that the elevated Sr levels are the result of multiple factors, only one of which may have been hypersaline conditions Equations (3) and (5) suggest that the bottom-waters were wann ( ::::22 25C) greater than nonnal salinity (up to 46%o), and had a low 0... -2%o). These 60

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temperatures and salinities would result in a density of>l.0271.029 kg m-3 (Figure 21 ) dense enough to form a mass of waters along the bottom of the seaway that would not mix well with the upper, more oxygenated water-masses Trace-Element Interpretation Lorens and Bender (1980) found that the Sr/Ca ratios in both the calcitic and aragonitic layers of Mytilus increase linearly with an increase in the Sr/Ca ratios of the solution in which the bivalve is growing. They also determined that the Mg!Ca ratio in the calcitic part of the shell increases exponentially with an increase in the Mg/Ca ratio of the ambient solution, but that the Mg/Ca ratios in the aragonitic layers of the shells only increase linearly with an increase in the Mg/Ca ratios of the solution Therefore, Mg and Sr levels in the WIS waters must be considered as a contributing factor in an explanation of the high Mg and Sr concentrations found in the shell material from this study The weathering ofMg-rich volcanic rocks that were forming along the tectonically active western margin of the seaway probably served as a source for the increased Mg concentrations (Whittaker et al. 1987) A trend of increasing Mg values in the late Cretaceous seaway in Canada was also noted by Whittaker et al (1987). Dissolved Sr could have come from the weathering of older Paleozoic carbonates abundant on the eastern margin of the seaway The Mn and Fe content of fossil invertebrates is controlled in part, by envirorunental conditions The chemistry of oxic and anoxic waters differs vastly, resulting in wide variations in dissolved metal content and speciation (Quinby-Hunt and Wilde, 1996) The Mn and Fe values of the fossil specimens in this study are higher than 61

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30 :; 25 20 I 15 10 0 a. nrnsrrr g N (T'I .... ..... q q q N N q q q q --19 20 21 22 23 24 25 26 21 28 29 30 31 32 33 34 l5 36 37 31 39 40 41 SALINITY '7'00 1 026 1 027 1 028 1.029 1 030 1.031 Figure 22. Probable density ranges for the intermediateand bottom waters along the eastern portion of the WIS based on salinity and temperature calculations Intermediate-water conditions inferred from o 1 80 values of ammonoids Bottom-water conditions inferred from combination of 81 80 values and minor-element concentrations ofMg and Sr in inoceramid shell material Density is given in kg/m3 and FP = freezing point. 62

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those ofRecent and ancient fossil shells precipitated in open marine waters ( Figures 1 6 18) Brand ( 1987) attributed the high Mn concentrations in their epifaunal foss i ls from the WIS to reduced oxygen levels and Quinby-Hunt and Wilde (1996) found that Mn becomes more mobilized as conditions become more reducing Thus more Mn i s available in anoxic waters to combine with calcium mineral lattices (Qu i nby-Hunt and Wilde 1 9 96) Morrison and Brand (1988) also found that high levels ofFe (mean of 790 ppm) can be an indicator of reduced oxygen levels in seawater Therefore the elevated Mn and Fe values found in both the epifauna and the nektic fauna support the assumption that dysaerobic conditions persisted through most of the water column along this part of the eastern margin. The low abundance of anunonite fossils found along the eastern portion of the seaway contrasts to the large concentrations of these fossils found in the central and western portions of the basin. The presence of dysaerobic condit i ons extending from the bottom and into the intermediate-water mass may represent an environment unique to the eastern margin of the seaway excluding many types of nektic fauna adapted to better oxygenated waters Paleoceanographic Implications Data from the least altered shell material was used to test the accuracy of various paleoceanographic models for the WIS The data collected as part of this study were then used in conjunction with the data collected from others (Lowenstam and Epstein, 1954, Tourtelot and Rye, Whittaker et al., 1987 ; Wright, 1987 ; Fatherree, 1995) to gain a more complete picture of water masses and circulation within the 63

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seaway Figures 1 and 2 show the locations and chronostratigraphy of this study as well as several others from which data have been collected on the oxygen and carbon isotopic composition of shell material from the Late Cretaceous ofNorth America. All of the isotopic studies used in this paper include measurements on shell material from Inoceramus bivalves and various species of Bacu/ites and other ammonites from the WIS The epifaunal data from across the seaway have been compiled in Figure 22. Because there is limited variation in the measured 8180 and 813C values in the epifaunal shell material (considering that these data come from chrono-stratigraphic horizons spanning about 10 Ma), the bottom-waters across most ofthe seaway must have maintained a stable isotopic composition on the order of 1.0 x 104 years The average 8180 values for the epifauna from each study site were summarized in Figure 23. These data indicate consistent bottom-water conditions across the entire bottom of the Claggett and Bearpaw Seas. The cores from this study contained no infaunal skeletal debris and very little evidence ofbioturbation Therefore, the bottom-waters along the eastern portion of the basin are inferred to have ranged from dysoxic to anoxic during the deposition of the DeGrey member of the Pierre Shale. These shales also have a high organic content, suggesting an oxygen-depleted depositional environment that restricted decay by aerobic bacteria. Without this bacteria present to recycle the 12C-rich organic carbon, most of the light carbon became buried in the sediments This 12C sequestering may have been the mechanism that allowed the bottom-waters to become increasingly enriched in 13C as 64

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0\ v. 400 oo Bottom Water Circulation within the WIS 40 BearpawSea -:!.43 -2.&4 -ln Dysoxic/Anoxic Bottom Waters Figure 23. Average o180 of epifauna from across the WIS and the Tethyan Sea. Data taken from this and previous studies from across the WIS. Values represent the average o180 of epifaunal shell material from each study site.

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the fauna living there preferentially incorporated the lighter carbon in their organic tissues The molluscs living in the int erm ediate water mass of the WIS show a variation in o180 ranging from +0. 6 to -4. 2 %o and a variation in o13C ranging from +4. 0 to -3 .9o/oo (Figure 24 and 25). However, the values for the nektic fauna are shifted to more positive o 180 values and to more negative o 13C values as compared to the epifauna (Figure 26). While the distribution of values in Figure 24 could be the result variation over a span of approximately 10 Ma, seasonal and latitudinal variation, or insufficient sample numbers, the data could also indicate true isotopic variation between water masses Differences between water masses in which the two groups of fauna lived suggests the persistence of a vertically stratified water column. The more positive o 180 values of the intermediate-waters in the WIS resulted from either cooler temperatures, the presence of more normal marine waters, or a combination ofboth. The averages for the nektic fauna show a wider range in o180, probably the result of intermediate-waters fonning from the mixing of several sources (including freshwater runoff, e v aporated freshwater, and Tethyan waters) These data suggest a more variable conditions within the intermediate-water mass that could change throughout the year or over larger scale climate cycles. The persistence of stable isotopic variation in shell material from the epifauna and the nekton along the eastern margin of the WIS (Figure 11) is not consistent with Wright's (1987) model. The ammonitic shell material found in the cores does not show depleted o180 values that would indicate the downwelling of warm, saline waters along 66

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5 Stable Isotope Values for Ammonites o0t$J q, coo 0 0 5 cq -fu [] 0180 [] <> This Study c Glend i ve, MT .t. Rap i d Creek SO Oral SO [] [] 0 Saskatchwan Canada Coonfbreek TN IIJ [] [] "' () 4 3 (.) C") 2 .... (() c.() 0 1 ooO <>Cb a n 0 I -2 os:f'a 1 c:bo co f-0 [] t -2o [] -4 "' .., F i gure 24. Summary of stable i sotope values from the nektic fauna of the WIS and Coon C r eek. 67

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0\ 00 IntermediateWater Circulation Patterns 400 Figure 25. Average 8180 ofnektic fauna from across the WIS and Tethyan Sea Data taken from this and previous studies. Values represent the average 8180 of ammonoids from each study site.

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Stable Isotopes for Epifaunal and Nektonic Shell Material [] WIS Epifauna c WIS Nekton Coon Creek Nekton o Coon Creek Epifauna 6 -6 CC Epifauna .., ..... \ I (J M ..... c..o Figure 26 Comparison of stable isotope values from the epifauna and nektic fauna of the WIS and Coon Creek (CC). 69

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the eastern margin ofthe WIS. Instead, the 6180 values of the nektic fauna continue to be more enriched than the epifaunal6180 values (Figures 11 and 12). The oceanographic model produced by Slingerland et al. (1996) is also inconsistent with the oxygen isotopic data of fossil molluscan shell material. There is no evidence to suggest the circulation of brackish waters in the bottom-waters or in the intermediate-waters along the eastern margin of the seaway. Instead, the 6180 values of the epifauna along the eastern margin closely match the values of the epifauna in the central and western portions of the seaway (Figure 23). While it is possible that the marginal circulation currents did not affect the bottom of the seaway, the ammonites living in the upper 100 m of the seaway should have been affected by the presence ofthe proposed freshened gyres. However, the 8180 values of the nektic fauna suggest a more open-marine signal rather than a brackish-water signal in the eastern portion ofthe seaway (Figure 25). While Slingerland et al. 's ( 1996) model predicts reduced salinity waters along both margins, it suggests that the eastern current should be warmer because it was derived from the tropical Tethyan waters. However, if it is assumed that both margins experienced similar estuarine conditions the 6180 values from nektic shell material on the western margin are more depleted, indicating warmer temperatures. Based on these observations, this model does not fit the geochemical data from the Bearpaw Sea. While Model I ofHay et al. (1993) does correctly predict the uniform bottom water conditions within the WIS, it fails to explain how the mixing of two distinct water masses can result in the formation of oxygen-depleted bottom-waters with a low Ow. In addition, the nekton living in the southern part of the WIS should indicate warmer water 70

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conditions as compared to the nekton living at higher latitudes. However, this trend is not evident in the 8180 of the ammonites from the WIS (F i gure 2 5) Based on the available data the oceanographic models of Glancy et al. ( 1 9 93) (Figures 7A and 7B) and Model II ofHay et al. (1993) (Figure 6) seem the best for the Late Cretaceous Bearpaw Sea. These models suggest the i ncursion of warm, saline poorly oxygenated Tethyan waters from the south formed the bottom-wate r s across all but the most northern parts ofthe WIS The poleward transport of warm, saline bottom-waters derived from evaporation in the Tethys was also suggested by Johnson et al. (1996) in their model ofCretaceous reef collapse Johnson et al. (1996) suggested that the transport of warm, saline, oxygen depleted waters from the tropics provides a plausible mechanism for subsurface heat export and Oceanic Anoxic Events during times of elevated atmospheric C02 levels during the Middle and Late Cretaceous Brasset al. (1982) and Barron and Washington (1985) also suggested that high rates of evaporation and warm sea surface temperatures resulted in saline Tethyan waters As evaporation continued, the Tethyan surface-waters became increasingly saline, sank, and flowed northward along the bottom of the WIS. The benthic fauna consumed the limited suppl y of oxygen in these warm bottom-waters, and they became increasingly depleted in oxygen with age (Figure 23). This explains the uniform dysoxic to anoxic conditions that dominated the bottom-waters of the seaway and allowed for the preservation of organic-rich shales across the basin. However, Glancy et al.'s (1993) model differs from Model II ofHay et al (1993) in that it calls for direct exchange between Tethyan and WIS waters. As dense Tethyan waters were imported into the bottom of the WIS, upper and intermediate WIS waters 71

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were exported back into the Tethys, resulting in a net balance between the two water bodies Thus, there should be similarities in both the geochemical signatures and in the types ofnektic fauna found in the northern part ofthe Tethys and in the WIS Figures 23 and 25 suggest that the former statement is true and fossil evidence suggests that there were several pulses ofnektic Tethyan fauna into the WIS (Kauffman 1984) Glancy et al. s (1993) model also predicts multiple sources for the formation of intermediate waters. During the summer months (Figure 7B), this model indicates that intermediate waters formed from the evaporation ofbrackish surface waters. If these waters increased in salinity due to evaporation, they would have sank to form areas of warm intermed i ate-waters This might explain the relatively light 8180 values found in the nektic fauna from the center ofthe seaway (Figure 25) Locations farthest from the downwelling areas might be expected to have had cooler waters. However, these intermediate-waters never became dense enough to displace the older, more saline, low oxygen waters on the bottom of the seaway. During the winter months (Figure 7A), this model predicts high precipitation over the southern end of the WIS and the export of a reduced salinity 'lid' to the Tethys. Over time, the intermed i ate-waters experienced more isotopic variation in Ow, a fact that is demonstrated by the greater variability of 81 80 values recorded in the nektic fauna (Figure 25) Another fact that is difficult to reconcile with the proposed oceanographic models was noted by Wright (1987) when she measured the enriched 8180 values ofthe infauna found at her study site in Glendive, Montana. She found that the infaunal 8180 values are closer to those of the nektic fauna rather than the epifauna in the WIS While the model of Glancy et al. ( 1993) may best explain the dominant mode of circulation 72

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within the WIS, there may have been occasional shifts to a temporary circulatory mode resulting in cooler, better-oxygenated waters on the bottom of the seaway Although there are sparse indications of infaunal activity from the cores used in this study most evidence suggests that there was only a sporadic infaunal population in the WIS during the Late Cretaceous. The conditions that allowed for infauna to thrive in the central and western portions of the seaway may represent short-lived incursions of better oxygenated waters into the deeper portions of the seaway. As part of this study concretion beds occurring in the Pierre Shale near Osage, Wyoming were also examined for their fossil content. The concretionary layers are many times more fossiliferous than the surrounding black shales However, this temporary breakdown of the vertically stratified water column did not have as great an effect on the eastern portion of the seaway, or it was too short-lived in the shallower, more restricted waters on the eastern margin to become recorded in the sediments. These oscillations could have been driven by certain forcing mechanisms of the Late Cretaceous climate cycle. Nevertheless, the vertically stratified water column resulting in warm, saline, poorly-oxygenated waters on the bottom of the seaway was the dominant oceanographic factor for the WIS 73

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CONCLUSIONS 1 Fossil specimens representing strata from the eastern portion ofthe Western Interior Seaway ofNorth America are abundant in cores and in outcrop from sites in eastern South Dakota. While the majority of these fossils are well-preserved, it is possible to identify the diagenetically altered specimens by minorand trace-element analysis Although mild diagenesis of some shell material has resulted in a decrease in o 13C values, these fossils maintain their original 8180 values 2. Most of the fossil specimens contain elevated concentrations ofMg, Sr, Mn, and Fe when compared to modem molluscan analogs. The high Mg!Ca ratios in the epifaunal inoceramid calcite may be an indication of warm bottom-waters, averaging over 22C. The elevated concentrations of Sr are also probably original and reflect a combination of saline bottom-waters and an increased Sr concentration of the WIS Mn and Fe concentrations may be due to: I) anoxic bottom-water conditions 2) diagenetic alteration, or 3) a combination ofboth. 3. Calculations indicate the bottom-waters had an average temperature of were greater than normal salinity (up to 46%o), had a low Ow -2%o), and had a density of > 1.0271.029 kg -m-3 74

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4. Intermediate-waters formed as a result of evaporation from a freshwater lid and from mixing with the intermediate-waters of the Tethys. As a result the intermediate-waters were well oxygenated with less than normal marine salinities Temperatures for the intermediate waters from the eastern portion of the WIS indicate a range from 20 8 C with a SMOW = -l%o or 16 .1C with a SMOW = -2%o. 5 The distribution of 8180 values in both the nektic and epifauna from the WIS can best be explained by the paleoceanographic model of Glancy et al. (1993). This model suggests the flow of dysoxic bottom-waters from the Tethys into the bottom layer of th e WIS. In addition, this model predicts a more dynamic formation of upperand intermediate-waters within the WIS before they were exported to the Tethys The presence of concretionary horizons in the center of the basin may represent temporary breakdowns in the dominant circulation pattern for the WIS. These concretions contain abundant infauna with more normal-marine 8180 values It was during these events that cooler, oxygenated waters penetrated the bottom of the basin, disrupting the vertical stratification usually present. 75

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LIST OF REFERENCES Armstrong, R. L., 1968 Sevier Orogenic Belt in Nevada and Utah: Geological Society of America Bulletin, v. 79, p 429-458. Barron, E J., and Washington, W M., 1985, Warm Cretaceous cl i mates : high atmospheric C02 as a plausible mechanism, in Sundquist, E and Broecker, W. S eds., The Carbon Cycle and Atmospheric C02 : Natural Variations Archean to Present : Geophysical Monogram 32, p. 546-553 Brass, G. W., Southam, J. R., and Peterson, W. H., 1982, Warm saline bottom water in the ancient ocean : Nature, v. 296, p. 620-623. Buchardt, B., and Fritz, P., 1978, Strontium uptake in shell aragonite from the freshwater gastropod Limnaea stagnalis: Science, v. 199, p 291-292 Buchardt, B., and Weiner, S 1981, Diagenesis of aragonite from Upper Cretaceous ammonites : a geochemical case-study : Sedimentology, v 28, p 423-438. Brand, U 1987, Depositional Analysis of the Breathitt Formations's marine horizons Kentucky, U.S.A.: Trace elements and stable isotopes : Chemical Geology (Isotope Geosciences Section), v. 65, p. 117-136. Calvert, S. E., and Pedersen, T. F., 1993, Geochemistry ofRecent oxic and anoxic marine sediments: Implications for the geological record : Marine Geology, v 113, p 67-88 Carpenter, J. S., Erickson, J. M Lohmann, K. C., and Owen, M. R 1988, Diagenesis of fossiliferous concretions from the Upper Cretaceous Fox Hills Formation, North Dakota: Journal of Sedimentary Petrology, v 58, p. 706-723 Carter, J. G., et al ( 16 others), 1991, Glossary of skeletal biomineralization in Carter J. G ed., Skeletal Biomineralization: Patterns, Processes, and Evolutionary Trends: Van Nostrand, p. 337-399. Chave, K. E., 1954, Aspects of the biogeochemistry of magnesium 1. Calcareous marine organisms : Journal of Geology v. 62, p. 266-283 76

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