Isotope paleontology of selected molluscs from the upper Pierre Shale (late Campanian-early Maastrichtian) of the Cretaceous Western Interior Seaway of North America

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Isotope paleontology of selected molluscs from the upper Pierre Shale (late Campanian-early Maastrichtian) of the Cretaceous Western Interior Seaway of North America

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Title:
Isotope paleontology of selected molluscs from the upper Pierre Shale (late Campanian-early Maastrichtian) of the Cretaceous Western Interior Seaway of North America
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Fatherree, James Wilson
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Tampa, Florida
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University of South Florida
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English
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vii, 70 leaves : ill. ; 29 cm.

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Mollusks, Fossil -- North Africa ( lcsh )
Geology, Stratigraphic -- Cretaceous ( lcsh )
Isotope geology ( lcsh )
Dissertations, Academic -- Geology -- Masters -- USF ( FTS )

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General Note:
Thesis (M.S.)--University of South Florida, 1995. Includes bibliographical references (leaves 62-67).

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University of South Florida
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Universtity of South Florida
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021584043 ( ALEPH )
34019952 ( OCLC )
F51-00118 ( USFLDC DOI )
f51.118 ( USFLDC Handle )

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Graduate School University of South Florida Tampa, Florida CERTIFICATE OF APPROVAL Master s Thesis This is to certify that the Master's Thes i s of JAMES WILSON FATHERREE with a major in Geology has been approved by the Examining Comm i ttee on July 17 1995 as satisfactory for the thesis requirement for the Master of Science degree Examining Committee ; Major Ff;ofessor : Peter J. Harries, Ph.D. ,......

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ISOTOPE PALEONTOLOGY OF SELECTED MOLLUSCS FROM THE UPPER PIERRE SHALE (LATE CAMPANIAN-EARLY MAASTRICHTIAN) OF THE CRETACEOUS WESTERN INTERIOR SEAWAY OF NORTH AMERICA by JAMES WILSON FATHERREE A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology University of South Florida July 1995 Major Professor : Peter J. Harries, Ph D

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@ ______ All Rights Reserved

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DEDICATION More than anyone else I would like to dedicate this work to my few true friends and thank each of you for your love, camaraderie, and unconditional support when I need you You know who you are I neve r would have made it this far without all of you

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ACKNOWLEDGMENTS I would like to thank my thesis committee, Dr. Peter Harries, Dr. Terry Quinn, and Dr Lisa Robbins for keeping me in line, and providing me with the motivation and advice needed to comple t e this pro j ect. Things were also made much easier by Chris Pissar i in the field Dr. Erie Kauffman and Claudia Johnson's much appreciated hospitality ; Dr William Cobban's sharing of his personal knowledge and specimens belonging to the USGS collect i ons ; and Dr. Eleanor Snow and Dr John Compton's help with XRD techniques. I would also like to thank Sigma X i for grant #GIAR 94/06 21475, the Tampa Bay Fossil Club for the Ben Waller Scholarship and the Geological Society of America and Conchologists of America for the i r grants i n aid of this research This project would not have been possible without the i r financial support.

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TABLE OF CONTENTS LIST OF TABLES i i LIST OF FIGURES iii ABSTRACT vi INTRODUCTION 1 GEOLOGIC SETTING 5 PREVIOUS WORKS 10 METHODS 12 Bivalves 14 Ammonites 16 RESULTS 20 Bivalves 20 Ammonites 24 DISCUSSION 30 Isotope Paleontology 30 Bivalves 30 Ammonites 35 Paleoenvironmental Implications 44 Paleocirculation Models 44 Surface Water Mass 48 Intermediate Water Mass 50 Bottom Water Mass 51 Revised Paleocirculation Model 53 CONCLUSIONS 59 Isotope Paleontology 59 Paleoenvironmental Implications 61 LIST OF REFERENCES 62 APPENDIX 68

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Table 1. LIST OF TABLES Mean Isotopic Values and Standard Deviations of Samples Collected from Each Specimen. ii 22

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LIST OF FIGURES Figure 1 Location Map of Study Area 6 Figure 2. Time-strat i g r aphic Ammonite Zones of the Pierre Shale 9 Figure 3. Specimens Chosen for Use in this Study. 13 Figure 4. Location of Samples Collected from Bivalve Specimens AO and IS. 15 Figu r e 5. Location of Samp l es Collected from Ammonite Specimens BC1 and BC2 17 Figure 6. Location of Samples Collected f r om Ammonite Specimen OS. 19 Figure 7 Preservation Typical of Pierre Shale Fossils as Shown by an SEM Photomicrograph 21 Figure 8. Isotope Values (per mil PDB) of Samples Collected from Bivalve Specimen AO. 23 Figure 9. Isotope Values (per mil PDB) of Samples Collected from Bivalve Specimens IS, UB, AS1, AS2 I NO, and TS 25 Figure 10. Isotope Values (per mil PDB) of Samples Collected from Ammonite Specimen BC1. 26 Figure 11. Isotope Values (per mil PDB) of Samples Collected from Ammon i te Specimen BC1. 28 Figure 12. Isotope Values (per mil PDB) of Samples Collected from Ammonite Specimens BC2 and OS. 29 Figure 13. Schematic Diagram of Decreasing Growth Temperature and ()1BQ Range Through the Ontogeny of a Bivalve. 33 ii i

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Figure 14 Position Correction of Isotopic Values of Samples Collected from the Septa of Ammonite Specimen BC1. 36 Figure 15. Plot of ()1BQ Values of Samples (per mil PDB) Collected from Baculitid Ammonites in Previous Studies 41 Figure 16. Cross-plot of Isotope Values (per mil PDB) of Samples Collected from Ammonite Specimen DS. 43 Figure 17 Schematic Diagrams of Vertical Ocean/Seaway Stratification 45 Figure 18. Schematic Diagrams of Vertical Seaway Stratification. 47 Figure 19 Distribution of Specimens Used in this Study with Inferred Life Habit and Mean Isotopic Values. 49 Figure 20. 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 (1986) 55 Figure 21. Revised Schematic Diagram of a West to East Transect Through the Central Portion of the Western Interior Seaway with Water Stratification and Circulation Patterns 57 iv

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ISOTOPE PALEONTOLOGY OF SELECTED MOLLUSCS FROM THE UPPER PIERRE SHALE (LATE CAMPANIAN-EARLY MAASTRICHTIAN) OF THE CRETACEOUS WESTERN INTERIOR SEAWAY OF NORTH AMERICA by JAMES WILSON FATHERREE An Abstract A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology University of South Florida July 1995 Major Professor: Peter J Harries, Ph.D. v

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A variety of well-preserved aragonitic and calcitic fossils are common throughout the Pierre Shale (Late Santonian-Early Maastrictian) and are suitable for use in stable isotope studies. Stable isotope records were generated from molluscs by collecting bulk samples from several bivalves, and by sampling both perpendicular to and parallel to the growth bands of other bivalves and ammonites. Samples were also collected sequentially through the shell wall and from the septa of a baculitid ammonite. This sampling strategy made it possible to generate temporal records and to test for the integrity of the isotopic record as well. Data from the bivalve and ammonite specimens provide evidence that some of these organisms did not precipitate all shell material in isotopic equilibrium with surrounding waters, and did not live in waters with a "norma l marine" isotopic composition. These observations aid in the generation of paleocirculation models for the Cretaceous Western Interior Seaway of North America Data presented in this study indicate that a normal-marine intermediate water mass was overlain by a low salinity "lid" which was modified by mixing and evaporation to form a relatively dense bottom water mass w i th a salinity slightly higher than that of the intermediate water mass. The composition, stratification, and circulation of this epicontinental seaway sharply contrast with that of the modern ocean. Therefore any paleocirculation model of the Cretaceous Western Interior Seaway must account for these data. vi

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Abstract Approved: Major Professor: Peter J Harries, Ph.D. Assistant Professor, Department of Geology Date Approved: vii

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1 INTRODUCTION The investigation of paleoenvironmental conditions has seen large advancements with the continuing study of stable isotope compositions of carbonate rocks and fossils Beginning with studies by Urey et al. (1951 ) many workers have shown that the stable oxygen isotope compositions of biogenic carbonates can be used as proxy indicators of the temperature and salin i ty of the water from which the carbonates were precipitated. The 18Qf1 6 Q composition of water is basically a function of its salinity, and the 18Qf16Q of a carbonate precipitated from that water is inversely related to the temperature of that water as well. This relationship makes i t poss i ble to generate records of these environmental changes In order to produce accurate records of paleoenvironmental conditions when using carbonate material it must be determined whether the original isotopic composit i on of the carbonate i s preserved i.e., not affected by post depositional alteration. Biogenic calcium carbonate is precip i tated by invertebrates in three forms, aragonite, high-Mg calcite, and low-Mg calcite. Of these high-Mg calcite and aragonite are considered the most soluble and are metastable at surface temperatures and pressures. Because diagenetic aragon i te cements are rare fossils of aragonitic composition are almost certainly unaltered. Low-Mg calcite i s more stable and is more resistant to diagenetic processes. Foss i ls of organisms which precipitated their shells in the form of low-Mg calcite, and are still found to be composed of low-Mg calcite

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2 are likely of original composition as well. This is especially true if low-Mg calcite fossils are found coexisting with well-preserved aragonite Observations of variations in isotopic values which may be interpreted as seasonal temperature and/or salinity changes, may also indicate preservation of the original isotopic composition (Urey, et al., 1951) Whether or not the organism precipitated its shell in isotopic equilibrium with the water it inhabited must be determined as well. Deviations in the isotopic composition of shell material from those of an inorganically precipitated carbonate from the same water are termed "vital effects" (Urey et al., 1951 ; Rye and Sommer, 1980; Wefer and Berger, 1991 ). Organisms which incorporate variable amounts of metabolically derived C02 which may be independent of the isotopic compos i tion of the surrounding waters, into the bicarbonate pool from which they precipitate shell material are said to exert a vital effect. Metabolic C02 is typically depleted in both 180 and 13C. Therefore covariant shifts toward depleted isotopic values are a good indicator of its presence. However because oxygen i sotopes may re-equilibrate with surrounding waters during transport to, and at the site of precipitation vital effect may be seen only in the carbon i sotopic record (Grossman 1987). Carbon i sotopic values may also be affected by the rate of precipitat i on showing different trends in various o r ganisms w i th changes in growth rate through ontogeny (e g. Land, et al., 1975; Turner, 1982; Wefer and Killingley 1980). Finally the isotopic composition of the water in which precipitation took place must be determined. The composition of marine waters can be affected by evaporation wh i ch preferentially removes H20 16, and through dilut i on by isotopically lighter freshwater from precip i tation, continental runoff and glacial meltwater during warm periods The carbon isotopic composition of marine waters may be affected by the input of freshwater which is typically enriched in

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12C derived from terrestrial plant mater i al (Keith et al., 1964). The carbon isotopic composition of waters may also be affected by biologic activity such as plankton blooms, which may preferentially sequester 12C. 3 Many rocks of the Western Interior Basin of North America contain well preserved fossils which are suitable for use as specimens in such paleoenvironmental isotopic studies Molluscan fossils from this area were used in this study. Urey et al. (1951) and Epstein et al. (1953} proposed that molluscs exhibit isotopic values that are not affected by vital effects More recent studies have shown that many extant molluscs do not precipitate their shells in isotopic equilibrium with seawater (e g., Wefer and Berger, 1991 ); and references therein). While vital effects in the extinct ammonites cannot be ruled out either the poss i bility of their influence in the related modern Nautilus has been addressed by Cochran et al. (1981) and Taylor and Ward (1983). These studies suggest that post-embryonic Nautilus shell material is precipitated in equilibrium with its environment. The oceanographic characteristics of the Cretaceous Western Interior Seaway also present difficulties due to the lack of a modern analog for an epicontinental sea Massive fluvial clastic wedges along the western margin the paucity or absence of many "normal-marine" taxa such as sponges, corals, articulate brachiopods and echinoderms provide evidence for considerable freshwater input at the surface of the seaway. This freshwater input may have diluted much of the waters of the seaway creating reduced salinities below the surface as well (Gill and Cobban, 1966; Sohl, 1967; Wall 1967 ; Jeletzky, 1970 ; Scott, 1970 ; Kauffman, 1977). However, other works suggest that subsurface waters in the seaway had higher than normal salinities (Wright, 1986; Hay et al., 1993}.

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4 This study used a variety of molluscan fossils to better assess possible extrinsic and intrinsic influences on isotopic records generated from specimens collected from the Western Interior Basin. The nature of the paleoenvironmental conditions, i.e. temperatures circulation and freshwater input into the seaway, were investigated as well.

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GEOLOGIC SETTING The Western Interior Seaway of North America was initially formed during the late Early Cretaceous by the flooding of the large foreland basin associated with the Sevier orogen i c belt (Armstrong, 1968), and it persisted 5 unt i l the late Early Maastrichtian. The seaway was connected to both the Boreal Sea to the north and the proto-Gulf of Mexico to the south, and it records several transgressive/regressive cycles (Fig 1A): the Kiowa-Skull Creek, Greenhorn, Niobrara, Claggett, and Bearpaw Cyclothems. Each of these cycles resulted from of a combinat i on of tectonic activity, eustatic sea-level change, and subsidence due to variable sediment loading. The asymmetrical basin and seaway can be divided into four basic tectonic and water-depth zones, following Kauffman (1977; 1984). The westernmost "foreland" zone was characterized by extreme subsidence rates caused by tectonic loading and associated h i gh siliciclastic sedimentation rates proximal to the orogenic belt. Water depths in this zone were probably less than 50 m, and sediments were predominantly sands and silts. The west central "axial" zone had h i gh subsidence rates as well, also in response to tectonic activity. Water depths probably ranged from 200-300 m, but may have been locally as deep as 500 m during maximum transgression Sediments of this zone were silts and clays, interbedded with limestones. The east-central "hinge" zone had much lower subsidence rates relative to the foreland and axial zones, and sediments were predominantly silts, clays, and carbonates. Water

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M O N TA NA B 100 km WY OMING A N D A KOTA 1---------I Osag e 1 \ I I S DA KOTA O r a l Ra p id Creek f./ J-------NE B R ASKA Figure 1 Location Map of Study Area (A) The Probable Extent of the Cretaceous Western Interior Seaway During Late Campanian Time Modified from Gill and Cobban (1966) (B) The western strandline during Baculites compressus time is shown (heavy line) with collection localities. The strandline in central Wyoming was shifted app r oximately 50 km to the west during B scotti time, and 1 00 km westward during B. bacu/us time Modified from Gill and Cobban (1973). 6

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7 depths probably ranged from 100-200 m. Finally the easternmost "stable cratonic" zone was a broad platform with little or no subsidence Sedimentation rates were low, and sediments were generally similar to those of the east central zone with the addition of easterly derived f i ne clastics, and an increase of calcarenites representative of sediment starvat i on. Water depths were typically < 1 00 m The Pierre Shale is composed of two transgressive-regressive wedges, which were deposited during the late Early Campanian Claggett and latest Middle Campanian to Late Maastrichtian Bearpaw Cyclothems. The P i erre Shale i s conformably underlain by the Niobrara Formation and confo r mably overlain by the Fox Hills Sandstone. The Pierre Shale has been studied in detail, and its classic reference locality at Red Bird, Wyoming is well described by Gill and Cobban (1966) At Red Bir d the Pierre Shale i s approximately 1 000 meters in thickness, and can be divided into seven lithologic members: the Gammon Ferruginous Member, Sharon Springs Member, Mitten Black Shale Member, Red Bird Silty Member, a lower unnamed shale member, Kara Bentonitic Member, and an upper unnamed shale member. Carbonate concretions, typically in discrete stratigraph i c horizons, are found throughout the Pierre Shale, and r ange in size from a few centimeters to well over a meter in size Many of the concretions contain very well-preserved fossil shells but others are apparently barren of any macroinvertebrate remains. Rarely, con c retions have also been found to contain pieces of fossil wood of such good condition that they actually smolder and char when a flame is applied. Rocks of the Pierre Shale have been classified into time-stratigraphic zones based on ammonite b i ostratigraphy and radiometric dating of the numerous widespread bentonites (Cobban and Rees i de, 1952; Obradovich and

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8 Cobban 1975 ; McNeil and Caldwell 1981; Cobban, 1984 ) Each of these timestrat i graphic zones is approximately 0 5 my in duration Specimens used in this study are from three ammonite zones Baculites bacu/us, B. compressus and B SCOtti (Figure 2)

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"0 QJ .... Ew Baculi te s clinolobatus <11.0 c: E c: QJ Baculites grandis .... QJ QJ_ 0.<11 o...c: ::>(J) Baculites ba culus <11 c E .... w w Ba culites e!iasi Baculites jenser.i Ba c ulites reesidei "0 Baculites cuneatus w .... EGJ <11.0 c:E Baculi tes compressus C:QJ .... QJ QJ_ Didymoceras cheyennense 3: .o ca E ..c: Q_ w Baculites obtusus B aculites sp. (wea!c ribbed) C:O'-0 c: QJ E -.o E g> E co em Baculites sp. (smooth) <.'J L W A I I I I u) Bacuttes bacu l us IQ..ZMa
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10 PREVIOUS WORK Many studies s t arting with Urey et al. ( 1951) have used o1BQ values of samples taken from calcareous fossils to better understand paleoenvironments by producing proxy records of water temperatures and salinities In many later studies o13C values have also helped to constrain water chemistries The range of fossils used has been limited solely to those taxa hav i ng a well preserved, sampleable calcareous skeleton A number of organisms skeletons have been used including: cnidarians, echinoderms, brachiopods, bryozoans, sarcodinians, arthropods, and molluscs Typically the most difficult requirement to meet is the collection of well-preserved specimens from ancient rocks Rocks of the Cretaceous Western Interior Seaway of North America yield many well preserved fossils of pristine aragonite and calcite representing a variety of taxa and life habits Exactly why these fossils are so well preserved is not well understood While a rapid "encasement" of shells by carbonate concretions may seem to likely be the major factor in preservation several of the best preserved specimens used in this study were found weathering from the shale not inside concretions. Apparently bottom-water, and po r e-water conditions may have led to the preservation of fossils and the formation of concretions independently of each other. This preservation of fossils has made the Western Interior Basin a place of study for many workers involved in global research as well as others wanting to study the paleoenv i ronmental characteristics of epicontinental seas

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1 1 Several workers have utilized specimens from the basin beginning with Tourtelot and Rye (1969). That study was the first to use ammonites and bivalves in an effort to better understand the Cretaceous Western Interior Seaway. Tourtelot and Rye (1969) not only generated paleotemperatures, but generated a record of seasonal water temperature changes by sampling down the conch of a baculitid ammonite in the direction of growth (perpendicular to growth bands). Forester et al. (1977) used ammonites of different ages from the seaway to generate a temperature curve for the Late Campanian to the Early Maastrictian. Pratt (1985) and Barron et al. (1985) used data generated from inoceramid bivalves from the basin to study global climatic cycles. Whittaker et al. (1987), Wright (1986) and Kyser et al. (1993) used a variety of specimens from the basin in an effort to better understand the paleoenvironment, geochemistry, and paleocirculation of the seaway.

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12 METHODS All of the specimens collected in the United States are f rom the Pierre Shale in Wyoming and South Dakota while the Canadian specimens were collected in southern Alberta from the Bearpaw Shale, which is equivalent to the upper Pierre Shale Thousands of fossils were collected during the summer of 1994 Specimens were visually inspected for any obvious dissolut i on or diagenetic effects. Shell material from each specimen was analyzed by X-ray diffraction us i ng standard methods (e.g. Davies and Hooper 1963). Pieces of shell material were powdered using an agate mortar and pest l e and analysed using a SCINTAG instrument. In addition scanning-electron photomicrographs were made of selected specimens to examine the preservation of original shell microstructure using a scanning electron m i croscope Ten specimens representing pseudoplanktic, necktie, and benthic habits were selected for use in this study based on the results of these tests (Fig 3). Powdered samples (50-1 00 J..Lg) were generated using a microscope mounted dental drill. For each sample, a 1 mm-diameter bit was used to remove surficial shell material which was possibly contaminated then a 0 5 mm-diameter bit was used to collect samples at a uniform depth that did not vary by more than 0.2 mm. Stable isotope analyses were performed at the University of Michigan's Stable Isotope Laboratory Each sample was roasted in a vacuum for one hour at 200 C then reacted with phosphoric acid at 73 C in a Finn i gan MAT

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SPECIMEN and SYMBOL 8aculites compressus BC1 Inoceramus sagensis INO Anomia sp. AS1 A Anomia sp. AS2 Baculites compressus BC2 Arctica ovata AO Unidenlified bivalve us Oidymoceras sp. OS Teredo sp. T S ldonearca s p. I S B BC1 & BC2 AMM ZONE LOCALITY 8. c ompressus Rapid Creek, SO 8. compressus R apid Creek, SO B. compressus Rapid Creek, SO B. compressus Rapid C r eek, SO B compressus Sl. Mary Ri v e r Alberta USGS 1104131 B. compressus Sl. Mary River Alberta USGS #04131 B. compressus Sl. Mary River, Alberta USGS #04131 B. scorti Oral, SO USGS #01411 B baculus Os a ge, WY B. baculus Osage, WY AS1/AS2 ISO INO CP 13 Figure 3 Specimens Chosen for Use in This Study. (A) Specimens used in this study with respective ammonite zones and the locality where each was collected. (B) Diagram showing the inferred life habit of each specimen

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14 automatic carbonate preparation device. The 180f160 and 13Cf12C ratios of the evolved C02 were determined using a MAT 251 triple-collector isotope ratio mass spectrometer. Oxygen and carbon isotopic compositions are expressed as a ratio between 180f160 and 13Cf12C, respectively, and this ratio is reported in standard delta (<>) notation. Precision ( cr) was determined using NBS standards, and was better than 0.08%o for ()180 and 0 .04%o for ()13C values. All results are reported in ()relative to the Chicago PDB standard. Bivalves Bivalve specimen AO (Fig. 4A) was sampled every 5 mm in the direction of growth, perpendicular to its growth bands from the beak to the ventral margin Specimen AO was also sampled every 5 mm along one growth band. Bivalve specimen IS was sampled every 5 mm in the direction of growth, perpendicular to growth bands, from the beak to the ventral margin (Fig. 4B), and almost an entire valve was powdered from the small bivalve specimen TS. Three samples were taken from inoceramid bivalve specimen INO, which is a broken valve. Two samples were taken from the prismatic calcite layer, approximately 5 em apart, one nearer the beak, one closer to the ventral margin. The third sample was taken from the nacreous aragonitic layer near the ventral margin One bulk sample was taken from the calcitic specimen AS1, and one from the calcitic specimen AS2, which are small bivalves attached to the valve of specimen I NO. One bulk sample was collected from bivalve specimen UB, which is a broken valve trapped inside the body chamber of ammonite specimen BC2. Stable isotope compositions were determined for all samples collected from these bivalve specimens.

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1 em 5" AO 31 6. 30 20 7. 29 8.. 28 8 2122 g 23242526 10" 5 6 11 12. 13 14" IS 1 em Figure 4 Location of Samples Collected from Bivalve Spec i mens AO and IS (A) Location of samples collected from bivalve specimen AO. Samples 1-18 were taken perpendicular to growth bands; 19-31 were taken parallel to growth bands. (B) Location of samples collected from bivalve specimen IS. Samples 1-8 were taken perpendicular to growth bands. Sample interval is to scale at 5 mm. 15

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16 Ammonites Ammonite specimen BC1 is an incomplete conch, measuring approximately 28 em in length, and was sampled at 1 em intervals in the direction of growth, perpendicular to the growth bands along its entire length. At the anterior edge of the specimen, the curve of the last growth band was followed up to a point nearer the dorsal margin, and perpendicular sampling was continued in order to create a longer isotopic record (Fig 5A). Two growth bands were chosen, one from each end of the conch, and sampled every centimenter from the dorsal margin to the ventral margin (Fig. 58). The conch was broken open, and the five septa found inside this portion of the conch were sampled by collecting powder from several spots on individual surfaces (Fig. 5C). Lastly, five samples were taken from one location, each at an increasing depth through the conch wall in order to produce an internal-to-external isotopic record of the wall (Fig. 5C). All samples collected from the growth bands, septa, and cross-section were analysed for stable isotopic composition Initially, samples taken perpendicular to growth bands were analysed at 1 em intervals. Later, a few additional samples were analysed where stable isotope values changed abruptly over the distance of only a few samples. Ammonite specimen BC2 (Fig. 50) is also an incomplete conch which measures 14.5 em in length. However, because specimen BC2 is missing several small portions of the conch, it was sampled every 1 5 em perpendicular to growth bands, starting 4.5 em from the posterior end. Lastly, three samples were taken from the conispiral ammonite specimen OS, which is a cast that is only partially covered with shell material.

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1 A 2 3 .. 2.5 8 4 5 6 . . 4 .5 5.5 51 I I 7 8 BC1 29 30 31 32 1.0 1.1 1.2 1_3 1.41.5 1_6 1_7 1.8 1.9 20 21 22 23 24 25 26 27 2 . 22.5 23.5 24:5 33 34 35 36 D 38 52 I BC1 40 53 54 I I I. I I Scm BC2 1 2 3 4 5 6 Scm 41 42 43 44 45 46 47 48 49 17 Figure 5 Location of Specimens Collected from Ammonite Specimens BC1 and BC2. (A) Location of samples 1 32 which were collected perpendicular to the growth bands of ammonite specimen BC1. Sample interval is 1 em. (B) Samples 33-49 were collected parallel to the growth bands of ammonite specimen BC1. Sample interval is 1 em; (C) Samples 50-54 were collected from the septa (sample interval to scale) and samples 55-59 were collected sequentially through conch wall of ammonite specimen BC1. Sample interval is 0.3 mm (D) Location of samples collected from ammonite specimen BC2. Sample interval is 1.5 em

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18 One large sample was taken from each successive whorl of the conch which if "unrolled" would equal a sample interval of approximately 20 em (Fig. 6).

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1 2 3 "\em Figure 6. Location of Samples Collected from Ammonite Specimen OS Samples t -3 were taken from successive whorls, perpendicular to growth bands. Sample interval is approximately 20 em 19

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20 RESULTS All shell material analysed was determined to be prist i ne aragonite ( <1 wt % calcite) with the exception of the prismatic layers of the inoceramid bivalve and the shells of the bivalve Anomia both of which were originally calcitic and remain so Void filling cements occurring in fossil specimens and in concretions were also determ i ned to be calc i te. SEM photomicrographs of samples were compared to s i milar photomicrographs of pristine molluscan shell material presented in Buchardt and Weiner (1981 ) and McArthur et al. (1994). All photomicrographs show unaltered tablets of aragonite with no evidence that shell materia l of the selected specimens has been altered (Fig. 7) The results of isotopic analyses of all samples analysed are presented in Appendix 1 The mean isotopic value and standard deviation of samples collected from each specimen are presented in Table 1. Bivalves ()18Q values from samples taken perpendi c ular to the growth bands of specimen AO have a mean of -3. 26%o ( .55). ()13 C values from samples taken perpendicular to the growth bands of specimen AO have a mean of +2 96 % o ( 86) (F i gure 8A) ()1 BQ values from samples taken paralle l to one of the growth bands range from -3.50%o to 3.87%o, and have a mean of -3 69%o

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Figure 7. Prese rvation T ypica l of Pierre Shale Fossils as Shown by an SEM Photomicrograph. Note the unaltered tabular microstructure of th i s aragonite sample taken from a baculitid ammonite. Magification is approximately 2000x. 21

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22 Table 1. Mean Isotopic Value and Standard Deviation of Samples Collected from Each Specimen. SPECIMEN MEAN 818Q MEAN 813C BIVALVES: AO (perpendicular to growth bands) -3.36%o0.55 2.96%o0 .86 (parallel to growth bands) -3.69%o0 1 0 3.63%o0 .39 IS (perpendicular to growth bands) -3.4 7%oO .22 -2.84%o0.67 INO (aragonite) -2.77%o 3.91%o (calcite) -3.43%o.06 3 52%o.18 AS1 -3.52%o 3 .07%o AS2 -3.82%o 3.90%o UB -2.95%o 1.59%o TS -1 0.13%o 15.41%o AMMONITES: BC1 (perpendicular to growth bands) -1.63%o.67 -Q.53%o0.42 (septa) -2.30%o.87 -1.11 %o1.05 (parallel to growth band 1) -0.97%o.14 0 .02%o0.21 (parallel to growth band 2) -1.85%o.22 -Q.44%o0 .24 (cross section) -1 .44 %oO .41 -0.73%o0.26 BC2 (perpendicular -1.92%o0.31 to growth bands) -1.39%o.23 OS (perpendicular 0.98%o1.27 to growth bands) -2.87%o0.07

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23 0 2 3 4 5 6 7 8 9 10 -2 A / \ . ............ \,, ... / ....... .,l .... 3 2 4 --o-818Q 0 813C 5 -1 0 2 3 4 5 6 7 8 9 1 0 CENTIMETERS 0 2 3 4 5 6 7 --o-818Q 4 813 C ... . ...... ...-\., .... 3.75 -3.5 0 co ............. 3 5 0 ..... c-o 3.75 8 . ... ...... ... ... ....... 3 25 3 2 75 2.5 0 2 3 4 5 6 7 Figure 8 Isotope Values (per mil PDB) of Samples Collected from Bivalve Specimen AO. (A) Plot of samples taken perpendicular to growth bands starting at the beak. Note the trend toward higher 8180 values through ontogeny, and inverse relationship of 8 1 8 0 to 813C (B) Plot of samples taken parallel to one growth band Note the similar inverse relationship of values C')

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24 (.1 0). However the o13C values from the same samp l es are more variable along this line ranging from +2.78%o to +3.95%o, and having a mean of +3.63%o (.39) (Figure 88). o 18Q values from samples taken perpendicular to the growth bands of specimen IS have a mean of -3.47%o (.22). o13C values from samples taken perpendicular to the growth bands of specimen IS have a mean of -2 84%o (.67) (Figure 9A). The two samples from the calcitic portion of specimen INO have similar o18Q values of -3.47%o and -3 38%o. These two samples also have similar o13C values of +3.64%o and +3 39%o (Figure 98). The sample from the aragonitic portion of specimen INO has an o18Q value of -2 .77%o, and a o13C value of +3 .91%o (Figure 98). The bulk sample taken from specimen AS1 has an o18Q value of -3 52%o, and a o13C value of +3. 07%o (Figure 98). The bulk sample taken from specimen AS2 has an o18Q value of -3 .82%o, and a o13C value of +3 90%o (Figure 98). Specimen U8 has o18Q and o13C values of -2.95%o and+ 1 59%o, respectively (Figure 98). The bulk sample taken from specimen TS has an o18Q value of -10.13%o, and a o13C value of -15.41%o, which is several per mil lower than the o18Q and o13C values f r om the other bivalves (Figure 98}. Ammonites o18Q values from samples taken perpendicular to the growth bands of specimen 8C1 have a mean of -1.63%o ( 67) (Figure 10A). o 18Q values for samples from the first 15 em are greater than values for the remaining length. These posterior (ontogenetically earlier end of the conch with a smaller diameter) have a mean of -1.01 (.26). The anterior (ontogenetically later end of the conch with a greater diameter) values have a mean of -2.32 (.18).

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25 0 2 3 4 5 -3 -2 ......... /-\ ......... -G--01e0 -3.25 ... -2.5 o13c . .. ... 0 ... 3 5 ........... 0 co -3 (") ..-c-o -3.7 5 -3.5 A -4 -4 0 2 3 4 5 CENTIMETERS 6 6 4 A 4 2 12 o,__ 0 -2 ---2 0 v 0 0 0 0 -4 fl. --4 0 -6---6 -8 Open Symbols o 180 --8 -10-Filled Symbolso 13C <> 10 -12 -8 -12 -14I-14 -16 -16 Figure 9 Isotope Values (per mil PDB) of Samples Collected fr om Bivalve Specimens IS, UB, A S1, AS2, I NO and TS (A) Plot of valu es of samples collected from biva lv e specimen IS starting at the beak Note the tre nd toward l ower o13C va l ues through ontogeny (B) Plot of values of samp les (per mil PDB) collected from bivalve specimens UB (square), AS1 (inverted triangle ), AS2 (triangle) INO (circles), and TS (diamond ) Note the simi larity in 8180 and o13C values among all of the biva l ves, with the exception of specimen TS, suggesting that specimen TS precipitated its shell from water of a different i sotopic composition and / or t e mperature.

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8 0 2 4 6 8 10 12 14 16 1 8 20 22 24 26 28 30 32 34 0 ...... .. 0 .,., . !II. : .. . .... I -1 ........ -1 --o-8180 -2 -2 ................. 513C A -3 -3 0 2 4 6 8 10 12 1 4 16 1 8 20 22 24 26 28 30 32 34 CENTIMETERS 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 3 4 0 0 --o-818Q -1 -1 ................. 513C -2 -2 -3 -3 -4 4 0 2 4 6 8 10 12 14 16 1 8 20 22 24 26 28 30 32 34 Figure 10 Isotope Values (per mil PDB) of Samples Collected from Ammonite Specimen BC1. ( A ) Plot of valu es of samples taken perpendicular to growth bands Note the trend toward lower8180 values through ontogeny (B) Plot of values of samples taken from the septa. Note the similar trend of decreasing 8180 values. 26 8

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27 However 813C values of the same samples do not follow th i s trend toward lower values through ontogeny having a mean of -0 53 % o (.42) The 8180 values from samples taken from the septa of 8C1 have a mean of -2.30 % o ( 87). The ()13C values of the same samples have a mean of -1.11 %o (.05) (Figure 1 08) These values also show a trend similar to values of the samples taken perpendicular to growth bands, except for the values of samples taken from the fourth and fifth septa. ()18Q values from samples taken parallel to the growth band near the posterior of specimen 8C1 have a range of 0 34 %o, and a mean of 0.97 % o ( 14) The ()13C values have a range of 0.25 %o, and have a mean of -0 .02%o (.21 ) ()18Q values from samples taken parallel to the growth band near the anterior of specimen 8C1 have a range of 0 .57%o, and a mean of -1. 85 % o ( 22). The ()13C values have a range of 0 53 % o and a mean of -0.44%o (.24) (Figure 11A) The ()18Q values from samples taken sequentially through the conch wall of specimen 8C1 have a range of 0.96 % o and a mean of -1. 44 % o (.41 ) The ()13C values of the same samples have a range of 0.71 % o and a mean of -0.73 % o (.26) (Figure 128) ()18Q values from samples taken perpendicular to the growth bands of specimen 8C2 have a mean of -1.39%o (.23) ()13C values from the same samples have a mean of -1.92 % o ( 31) (Figure 12A) ()18Q values taken from specimen OS have a total range of only 0 14 %o, and have a mean of -2.87%o ( 07) However ()13C values of the same samples follow a trend toward higher values, and have a mean of +0.98%o (.27) (Figure 128).

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(/) a: w tu i= z w (J _J _J w I (/) z I h: w 0 0.5 0 5 0.5 A 8 0.0 .. . 0.5 .. ...... 8 -1.0 . ' ' .... ', : ... ... ..... 0.0 -0.5 -1.0 0.0 -0.5 . ' ........ . -1.0 8 -1.5 -2 .0 -1.5 -2 0 -a--818Q -1.5 -2.0 -2.5 2.5 2.5 (/) a: w tu i= z w (J 2' ,g ..... Q) a. E E C') s _J _J w I (/) z I 1a... w 0 Figure 11. Isotope Values (per mil PDB) of Samples Collected from Ammonite Specimen BC1. (A) Plot of samples taken parallel to two growth bands. Sample interval is 1 em. The difference in number of samples is due to the change in diameter from one end of the specimen to the other Note the similarity in the shapes of the two 8180 lines. (B) Plot of samples taken sequentially through the conch wall. Sample interval is 0 3 mm. Note the trend toward lower values with increasing depth 28

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4 5 6 7 8 9 10 1 1 12 13 818Q 0 0 ......... ....... 813C 1 1 -2 8 -2 3 4 1 3 8 2 If-) 8 1 8 Q f-0 1 8 1 3 C --1 -2 ,_.. -2 -3--o --3 0 10 20 30 40 Figure 12 I s otope Values ( per m i l POB) of Sampl es C ollec ted fro m Amm o nite Specimens BC2 and OS. ( A ) Plo t of sam p l e s ta ken from ammonite specimen BC2 Note the p o siti v e correlat ion of 81 80 a nd 81 3C values after the second sample (B) Plot of samp l es taken f r o m ammonite specimen OS. Note the near inv ariance o f 8180 val ues relative to 81 3C values 29

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30 DISCUSSION Isotope Paleontology In order to use the data generated in this study for paleoenvironmental reconstructions, any extrinsic and intrinsic influences on the values and records must be assessed. The data generated from these fossils have various ranges of values which follow a variety of trends. These values and the trends they follow may be interpreted as changes in water temperature or as changes in the isotopic composition of the water due to freshwater input or evaporation Vital effects, such as changes in the rate of precipitation or the inclusion of metabolic C02 may also play a role in the observed records. Some records may be the product of a combination of two or more of these Bivalves Based on paleoshoreline reconstructions presented in Gill and Cobban (1973), specimen AO lived approximately 90 km from shore, in the foreland zone, at a depth of several tens of meters (Fig. 1 B). The isotopic record generated from samples collected perpendicular to the growth bands of this specimen shows what appears to be 2.5 cycles in 8180 values which have a

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31 decreasing range through ontogeny (Fig. SA). These cycles in i tially appear to be the product of seasonal changes in temperatures and /or sal i nities with i n the seaway. Meteor i c runoff from land has relatively low 8180 values compared to marine waters due to the depletion of 180 during evaporation and the poleward transport of water vapor Runoff typically has relatively low 813C values as well due to carbon derived from the 13C depleted plant material in soi l s and rivers Therefore, as freshwater mixes with mar ine waters lowering salinity the isotopic composition of the marine water should decrease covariantly. However, values from specimen AO do not covary but are v i rtually inverse after sample number four Therefore salinity was probably near constant. The changes in the 8180 values of specimen AO are more likely due to seasonal temperature changes. Changes in temperature are probably also responsible tor the relat i onship of 8 180 to 813C values The 813 C values of i norganic carbonates and some organic carbonates show a direct rela t ionship to the temperature of the environment of precipitation (McCrea 1950 ; Emrich, et al. 1970; Grossman and Ku 1986) However the effect is typically relatively small being only a traction per mil for every degree of change A relationsh i p between 813 C values and the rate of precipitation has been recognized in organic and inorganic carbonates as well. Th r ough experiments with inorganic carbonates Turner (1982) showed that the rate of precipitat i on can be responsible tor up to a 1 7%o variation Analyses of samples collected from some biogenic carbonates showed a similar effec t as well (e.g. Land, et al. 1975; Wefer and Killingley, 1980) Therefore it is likely that specimen AO had changing growth rates through ontogeny. While the absolute values may be interpreted as ind i cating changes of temperatures and precipitation rates the overall trend in values is interpreted as

PAGE 44

32 representing a decreasing period of shell precipitation due to changes in growth temperature through ontogeny. Changes of growth temperature have been observed in other bivalve taxa (e. g., Williams, et al., 1982; Jones, et al., 1983; Krantz, et al., 1987; Harrington, 1989). The results of these previous studies suggest that while many bivalve taxa may grow over a wide range of temperatures or throughout the entire year many grow over a progressively narrower temperature range throughout the years following sexual maturity. In this case, after sexual maturity is reached, the individual expends more energy on reproduction than on shell secretion during the spawning season. As ontogeny continues the spawning season may become progressively longer, while the amount of time spent secreting shell becomes progressively shorter. This change is reflected in the b18Q record of these bivalves (Fig 13A-C) Specimen AO was estimated to be approximately three years old at the time of death, based on the size of the shell (personal comm., Simon 1994), and exhibits an isotopic record that can be interpreted as a growth temperature pattern indicative of a progressively longer spawning season during the warm months of the year. Thus, the lower (warmer) b18Q values are increasingly truncated each year (Fig. 130) This interpretation is also in agreement with how the rate of precipitation affects the b13C values of specimen AO. It could be assumed that if specimen AO precipitated shell only during the cooler months, and not during warm months, that its growth rate was variable, with precipitation rates either accelerating or decelerating towards the growth temperature threshold. In an effort to further examine the isotope record integrity of specimen AO, samples were a l so taken parallel to one growth band to test for variabilities. Because a growth band is formed during one precipitation uevent11, isotopic

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+ 0 "' 1:0 Seasonal t r e n ds of water T + A Time-> No spawning / Shell precipitation i f) 0-_o_ ( --S;wning/No shell precipi t ation + 8 Time > + 0 "' 1:0 Resultant isotopic record T + c Time-> -2 -r-----------------.--2 D -3 3 0 0 "' 1:0 "' 1:0 -4 4 -Time-> -5 5 Figure 13. Schematic Diagram of Decreasing Gro wth Temperat ure and 81BQ Range Through the Ontogeny of a Bivalve. ( A) Is otop i c and temperature characteristics of surrounding water. (B) After the first year of growth, some bivalves reach sexual maturity and precipitate shell material o nly during non-spawning times. As spawning time increases over a broader temperature range each year, precip itation occurs over a progressively shorter time period (C) Isotop ic conditions are reco rded only during t i mes of shell precipitation creating an i sotopic record skewed toward the colder months. This pattern may also be reversed in some bivalves, showing a record skewed toward warmer months. (D) Growth temperature trend as seen in the 81BQ rec ord (per mi l PDB ) of bivalve specimen AO 33

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34 values along one band should reflect the conditions during the time of precipitation, and should be relatively i nvariant along that line ()180 values along the growth band var i ed by only 0.36%o. However while ()13 C va l ues of specimen AO are similar to those of the other benthic bivalves there is variation of 1.76%o i n samples taken from along the growth band (Fig 78). This change in ()13C values may be due to differing amounts of metabolic C02 being incorporated along the bands The var i ation may also be due to the rate of precipitation. The growth bands of specimen AO are very narrow close to the beak, but increase in width farther from the beak Therefore precipitation must be faster at the wider portions of the growth band if each band was precipitated during one precipitation event. Based on paleoshoreline reconstructions presented in Gill and Cobban (1973), specimen IS lived in the axial zone, over 200 km from the shore, with water depths probably from 100 to 200 m (Fig 1 B). The age of specimen I S is unknown but it is one of the largest of dozens of idonearc i d bivalves collected suggesting it was an adult. The ()180 record generated from specimen IS varies relat i vely little and trends towards lower values ()13 C values tend to follow a more pronounced trend toward lower values through ontogeny ( Fig. 9A ). Wh i le specimen IS exhibits ()180 values which are cons i stent with other b i valve data presented in this and other studies ()13C values are noticeably lower While ()180 values may represent an extremely narrow growth-temperature range or represent a short period of time it is more likely an accurate representation of decreased seasonal temperature fluctuations that would be expected in the deeper waters of the seaway The low ()13 C values are likely caused by the increased influx of isotopically-light metabolic C02 into the bicarbonate pool from which shell material is precipitated The effect is commonly more pronouced in ()13 C values probably because oxygen isotopes may re-

PAGE 47

equilibrate with surrounding waters dur i ng the process of precipitation A similar trend is seen in some other bivalve taxa (Krantz e t al. 1987 and references therein). Ammonites 35 In order to generate the longest temporal record possible from specimen BC1, samples were collected perpendicular to the growth bands and from the septa. The apparent offset in values of samples taken perpendicular to growth bands and values of samples taken from the septa is due to the distance between precipitation of the septa and precipitation at the apertural margin. As each septa is precipitated at the rear of the ammonite s body, shell material is being added simultaneously at the apertural margin at the anterior-most portion of the shell (Fig 14A) If values are shifted to their time-respective position, their inferred true relationship can be seen (Fig. 148) Samples taken perpendicular to growth bands and from the first three septa have similar o1BQ and o13C values, which trend toward lower o1BQ values while o13C values do not. The fourth and fifth septa continue to shift toward lower o1BQ values, but o13C drops to values which are relatively similar to the o1BQ values. These trends could be interpreted three ways as well: as a record of changing water temperatures as a migration of the ammonite into waters of a different isotopic composition, or as a combination of one or both of these with vital effects. If the isotopic record of specimen BC1 is dominated by water temperature changes, the seasonal variation in the temperature of the seaway should be recorded. Seasonal trends recorded in o1BQ and o13C would be expected to

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0 -I 8 -2 -3 -4 2 3 A 4 5 BC1 3 I MISSING t t 1/ POSITION OF COEVAL APERTURA L MARGIN Figure 14. Position Correction of Isotopic Values of Samples Collected from the Septa of Ammonite Specimen BC1. (A} Position of the apertural margin of baculite specimen BC 1 with respect to the position of the septa inside the conch (not to scale) (B) Plot of ()1BQ and ()13C values of samples (per mil PDB) collected from ammonite specimen BC1. Values of samples taken perpendicular to growth bands are represented by squares. Values of samples taken from the septa are represented by circles. Note the fit of the values from the first three septa when they are shifted to their time equivalent positions rather than their positions within the conch 36 0 -l 8 -2 -3 -4

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37 oscillate with some yearly consistency through time Isotopic studies of two coiled Jurassic ammonites report growth rates of approximately 5 septa or 0 33 to 0.25 whorls, and 12 septa or 0.4 whorls per year at a constant rate of insertion (Stahl and Jordan, 1969). Baculite specimens collected from the Pierre Shale were found to have septal spacings of less than 1 mm early in ontogeny, and as much as 5.5 em late in ontogeny, making these rates difficult to apply While growth rates of different species were variable it has been estimated that shallow-water ammonoids reached adulthood in approximately five years (Bucher, et al., 1995). The conch of an adult B. compressus is approximately 1 m in length, suggesting a growth rate of approximately 20 em per year if the growth rate was constant. The samples taken perpend i cular to the growth bands of specimen BC1 represent a total length of approximately 32 em, and 55 em when septal values are shifted to their time equivalent positions (Fig 4A and 148). This suggests that the amount of time represented by specimen BC1 is indeed greater than one year. Studies of modern Nautilus have shown that apertural growth rates vary little prior to adulthood. However septal insertion occurs at an exponentially decreasing rate through ontogeny, with as little as a few days early in ontogeny, to as much as a few months late in ontogeny between each insertion. At this rate of septal insertion, it takes 10 to 15 years to reach maturity (Saunders, 1983 ; Landman and Cochran, 1987). If a relatively fast growth rate of approximately one septa every two months during late ontogeny is assumed for B. compressus, the septa of specimen BC1 would represent approximately 8 months of growth, and the rest of the shell would represent approximately 6 more months of growth. Therefore, if the record is dominated by temperature changes a complete isotopic cycle should be seen, but is not. Seasonal

PAGE 50

temperature changes cannot explain the shift to the lower, covariant isotopic values of the last two septa either. 38 The 8180 values of samples taken perpendicular to growth bands and from the septa of ammonite specimen BC1 trend toward lower values sim i lar to values of the benthic bivalves This trend may be indicative of a migration deeper in the water column into the bottom waters of a different isotopic composition occupied by the benthic bivalves If the trend toward lower 81BQ values is the result of a downward migration values should converge on the relatively low values of the biva l ves. However, when an adjustment for shell composition of approximately 0 8 % o per mil is made to the isotopic values of the calcitic bivalve specimens (Rye and Sommer 1980) from the same loca l ity as the aragonitic specimen BC1, some values from specimen BC1 are as much as 0.44%o lower than those of the bivalves Because some 81BQ values of BC1 are lower and 813C values are much lower relative to the the lowest values from the bivalve specimens from the same locality it is diff i cult to attribute the low covariant values of the last two septa solely to migration. The distribution of i sotopic values of samples taken from specimen BC1 are within the ranges of values from baculites in previous works thus there is no initial evidence for any vital effect. However when values are plotted for samples taken along growth bands a variation is observed. Each growth band is added to the apertural margin of the conch in one precip i tation "event" Therefore values along any one band should be ident i cal. The bands sampled have variations in 81BQ values of up to 0 57%o, and in 813C values of as much as 0 73%o. Exactly why values vary along these lines is unclear, but the 81B Q variability along each line follows a very similar pattern, suggesting that this fractionation is b i ologically controlled and consistent, possibly being caused by differing amounts of metabolic C02 be i ng incorporated along the bands (Fig.

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11 A). While, some specimens of Baculites compressus may not have flank ornament (Gill and Cobban, 1973), weak ribs are present on specimen BC1. The position of maximum conch-wall thickness (ribs) correlates to the "spikes" seen in the plot of these values, therefore this vital effect may be caused by a difference in rate of precipitation along growth bands, especially where ornament is added 39 This observation of possible vital effect along the growth bands of specimen BC1, suggesting that baculites did not precipitate all shell material in isotopic equilibrium with seawater leads to an alternate interpretation. The lowest isotopic values, which are covariant, may be the product of physiological changes after reaching maximum size. The distance between the fourth and the fifth septa of specimen BC1 is much less than the distance between the other septa present in this specimen. In modern Nautilus, this septal crowding is an indication that the organism has reached its maximum size. Crowding of the last septa is seen in other baculitid specimens collected from the Pierre Shale as well. It is possible that a change in metabolism occured as shell growth neared its stopping point, possibly affecting the 818Q and 813C values of this specimen. Increased influx of isotopically light metabolic C02 could explain the shift to lighter covariant values This is similar to the changes in metabolism reported in some bivalve taxa (Krantz, et al., 1987, and references therein). If the values of the last two septa are the product of vital effect the remaining values may be treated independently, and may well be indicative of changes in temperature and habitat. Values of samples taken through the conch wall of specimen BC1 follow the same trend as values of samples taken perpendicular to growth bands. The shallower samples have values that are similar to those of the posterior end, while 818Q shifts to lower values with increasing depth to those more like the

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40 anterior values. This change of ()18Q and ()13C values with depth similar to that reported by Stahl and Jordan (1969) ; Forester et al. (1977) suggests that the inside of the conch wall must be thickened by precipitation from a fluid of the same isotopic composition that simultaneously adds shell material to the apertural margin. Specimen BC2 is a body chamber of another adult B. compressus broken well past the last septa. ()18Q values are nearer to those of the posterior end of BC1, and are higher than those of the bivalves from this local i ty. ()13C values are much lower than those of the bivalves as well. While values from specimen BC 1 other than those from the last two septa may indicate a migration to deeper waters during late ontogeny, those of BC2 do not. However, a relationship of ()18Q to ()13C values through ontogeny similar to that of specimen BC1 is seen. ()18Q to ()13C values are relatively dissimilar during the ontogenetic period just prior to adulthood of specimen BC1, and the earlier part of ontogeny in specimen BC2. At a later point in ontogeny in the records of both specimens BC1 and BC2, the relationship of ()18Q to ()13C values become more similar, relative to prior values and begin to covary This suggests that the values after the third sample of specimen BC2 are also affected by the same post-maturity vital effect as specimen BC1. Baculitid specimens from Tourtelot and Rye (1969); Forester et al. (1977) and Whittaker et al. (1987) show a number of variable isotope records as well (Fig. 15}. This suggests either baculites have some variable mechanism of precipitation, or that members of the same and different species have variable migratory habits or both. The ()18Q values of the conispiral specimen DS are nearly invariant, are similar to values of an inoceramid bivalve, and are relatively low compared to the values of a B. scotti from the same locality presented in Tourtelot and Rye

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CENTIMETERS .. '. '. /\ ,. , ;.... : . \, d.... <> 1 I \ .0 .. (/q : \ 0 \ I I I s;p;/.._ 0 -t.o v. /"' v \ v'rl .. oR 0.5 0.0 -0.5 8 -1.5 "" v,f!J l \ / Bacut;tes scoW -2.0 Baculites compressus '\ -2.5 : \ A : \ -3.0 I\ : \ / \ : lS. \I X -3. 5 species of remaining specimens are unknown CENTIMETERS . . . . . . . 6 1.0 0.5 0.0 0 5 -1.0 -1.5 -2 0 -2.5 -3.0 3.5 --D-Whittaker eta/., 1987 ........ <> ........ Whittaker eta/., 1987 ---0---Whittaker eta/. 1987 ----6---Whittaker et a/., 1987 ---EEl--Whittaker eta/., 1987 --Whitta ke r eta/., 1987 ---e--Forester et a/., 1977 -'1\1-Tourtelot & Rye, 1 969 Figure 15. Plot of 8180 Values of Samples (per mil PDB) Collected from Baculitid Ammonites in Previous Studies. Note the variable tr ends, or la ck of trends in values. A l so note the trend toward l ower values through ontogeny seen in the two specimens in the lower -r ight corner, similar to that of spec imen BC1 of this study. 41

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42 (1969). In light of previous discussions, this may suggest that this was a benthic ammonite and did not migrate vertically in the water column, which has been previously suggested by studies of ammonite morphotype distribut i ons in the seaway (Batt, 1989) (Fig. 16). However, ()13C values have a trend towards increasingly higher values throughout ontogeny, which may point to vital effect as well More data are needed from this g r oup of ammonites in order to better constrain the nature of the isotopic record and make sound interpretations

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0 CXl ...... c-o 813C 0 2 4 0 0 -1 1 0 2 2 CXl ...... c-o oo -3 r3 0 -4 I I I 4 4 -2 0 2 4 813C Figure 16. Cross plot of Isotope Values (per m i l POB) of Samples Collected from Ammonite Specimen OS. Values from specimen DS (filled squares) are compa red to those of an Inoceramus sublaevis (circles) and Baculi te s scotti (diamonds) from the same lo ca lity presented in Tourtelot and Rye (1969) The inoceramid and baculite appear to have precipitated their shells in water masses of different isotopic composition or at different temperatures. Note tha t s pecimen OS has 8180 values similar to those of the ino.ceramid suggesting that specimen DS had a benthic life habit. 43

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44 Paleoenvironmental Implications Paleocirculation Models Surface-water temperatures in modern ocean basins are warmer than bottom-water temperatures, and salinities generally decrease only slightly with depth (e.g Thurman, 1991). Therefore organisms secreting shells in these oceans show a general trend of progressively higher 81BQ isotopic values with increasing depth of precipitation (Fig 17A). However, interpretations of the data generated in this and other studies of the Cretaceous Western Interior Seaway of North America must be made in the context of the paleocirculation of this epicontinental seaway for which there is no modern analog Three basic scenarios are presented below, describing hypothetical vertical distribution of water masses based on paleontologic and geochemical data, and the results of computer-generated models The first scenario assumes a small latitudinal temperature gradient under equable climatic conditions. A reduced-salinity surface layer would have capped the seaway. Warm (29C), high-salinity bottom waters (37 .6%o) from Tethys would have entered the seaway from the south, wh ile cooler (17 C}, low salinity surface wate r s (20%o) would have entered from the north (Hay, et al., 1993). The densities of these two water masses would have been d i fferent enough to prevent large-scale mixing, creating a thin mixing zone with an intermediate temperature (23 C) and salinity (28.8%o). Organisms secreting calcareous shells under these conditions would be expected to show a trend of relatively low 81BQ values at shallow depths, which would increase sharply

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+ A + 8 c T + S%. + I L I T + S % + 818Q RS1 RS RS M l -_f'!j N N l N Mr1 Ml--1--M -------------N ----1-N I Sl I Northern Arctic Water Mass -Southern Tethyan ----__ __ Water Mass -_ Figure 17. Schematic Diagrams of Vertical Ocean/Seaway Stratification General trends in temperature (T) ; salinity ( S%o); and isotopic values (818Q) are shown. (A) Modern stratif i cation of ocean waters. (B and C) Scenar i o 1, which is based on stratification with a small latitudinal temperature gradient and a reduced-salinity layer (RS). Warm-saline waters enter from the south (S) and cooler fresher waters enter from the north (N) creating a thin intermediate mixed layer (M). Dashed lines indicate profiles in the southern end of the seaway; solid lines indicate profiles in the northern end of the seaway 45

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46 below the surface layer and continue to increase with increas i ng depth of shell precipitat i on (Fig. 178). The second scenario assumes less equable condi t ions and a greater latitudinal temperature gradient. A reduced-salinity surface layer would have capped the seaway in this scenario as well, while slightly warmer (30 C), more normal-salinity waters (35%o) would have entered from the Tethys sea and much cooler (1 0 C), but slightly less saline waters (30%o) wou ld have entered from the north (Hay et al. 1993). With modifications due to higher evaporation than precipitation over the southern end of the seaway and higher precipitation than evaporation over the northern end of the seaway, the densities of these two water masses would have been similar enough to allow large-scale mixing creating a third water mass of intermediate temperature (22.5 C) and salin ity (32.6%o). This third water mass would have been denser than either of the original water masses and would have occupied the bottom of the seaway. Organisms secreting shells under these conditions would also be expected to have relatively low ()18Q values at shallow depths which would increase with increasing depth of precipitation (Fig. 18A) The third scenario suggests that surface waters would be reduced i n salinity with an underlying water mass of near-normal salinity (32-35%o) and a slightly hypersaline bottom water (40 %o) (Wright, 1987) Near-brack i sh to brackish surface waters would have lower ()18Q values than the subsurface waters due to dilution by meteoric waters with 8180 values from -10%o to -15%o. The difference in density of these two water masses would be sufficient to prevent almost any mixing Wind currents over the seaway would have pushed this lower-salinity water toward the shallow eastern margin of the seaway where it would be subjected to evaporation in a low to moderate rainfall setting (Parrish and Curtis 1982 ; Parrish et al. 1982) As evaporat i on proceeded the

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+ T + S%. + Rs' RS I s l N I ----A M M M So uthern T et hyan M No rt h ern Arctic -----r -:":_ -8 Res ulting Bottom Waters + T + S%. + RS RS NM NM c SBW Figure 18 Schematic Diagrams of Vertical Seaway Stratification General trends in temperature ( T); salinity ( S%o); and isotopic values (o1BQ) are shown. (A and B) Scenario 2 which is based on stratification with a latitudinal temperature gradient and a reduced-salinity surface layer (RS). Warm-saline waters enter from the south (S) and coo l er-fresher waters enter from the north (N) creating an intermed i ate mi xed layer (M) which forms a more dense bottom layer. Dashed lines indicate profiles in the southern end of the seaway ; solid lines indicate profiles i n the northern end of the seaway ( C ) Scenario 3 (west to east transe ct), The reduced-salinity layer (RS) moves eastward over normal marine waters ( NM), then is subjected to high evaporation rates creating a saline bottom water (SBW). 47

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48 waters in the east would become progressively more saline unt i l they reached densities sufficient to cause sinking Sinking dense waters would have formed a bottom counter current with values which would still be low relative to normal marine waters Organisms secreting calcareous shells under these conditions would have low 818Q isotopic values at shallow depths higher 818Q isotopic values at intermediate depths, and intermediate 818Q values at greater depths of precipitation (Fig. 188) Surface Water Mass Bivalve specimen TS was found in a well-preserved piece of d r iftwood, and has lower isotopic values (818Q and 813 C -10.13 % o and -15 .41%o, respectively) than all other specimens used in this study (Fig. 19). When using the aragonite temperature equation of Grossman and Ku (1986) and a value of -1 %o for Cretaceous ocean water due to dilution by the melting of polar ic e (Shackleton and Kennett 1975) the 818 Q value of -1 0 13%o yields a water temperature of 64.6 C. This is obviously well above the lethal threshold of any known bivalve and must represent precipitation of shell material i n water of considerably lower than normal salinity The 813C values of fluvial waters are typically low relative to marine waters due to an abundance of isotopically light plant material carried in rivers and streams These isotopic values suggest that the water that specimen TS lived in was influenced by freshwater i nput. Members of the family Teredinidae inhabit pieces of wood by boring into them and thus most commonly exhibit a pseudoplanktic life habit (Turner 1966). Extant members of the genus Teredo are known to inhabit r i verine and lacustrine freshwaters to normal-marine waters. However the larvae of riverin e

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49 SURFACE WATER MASS INTERMEDIATE VI/ATER MASS BC 1 (early) MEAN : BC2 MEAN: 1 .33r-(.23) BOTTOM WATER MASS St. Mary R1ver L ocal ity AO MEAN: .26%. (.55) UB : 95%. migration? t-------BCI (lale) MEAN : (:0.!8) ..... INO MEAN: 2.67';. (:0. 1 O) ASI: A S2 : 3 .0 2%. Rap1d Creek Locallly -F igure 19. Distribut ion of Specimens Used in thi s Study with Inferred Life Habit and Mean Isotopic Values. Note that two values are given for specimen BC1 following the interpretation that the shift to l owe r values through ontogeny may be due to migration i n to the bottom water mass Also note that the values given for specimens I NO, AS1, and AS2 have b een corrected by 0.8%o to account for their calc itic composition

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50 species are intolerant of non-fresh waters and remain near the bottom of rivers inhabiting sunken wood to avoid being swept out to sea (Turner, 1966}. This suggests that specimen TS did indeed live a pseudoplanktic life in the waters of the seaway and was not transported from a freshwater source Intermediate Water Mass The baculitid ammonites are assumed to be nektic because their distribution in the rock record is not facies dependent and because of hydrodynamic stability considerations (Kennedy and Cobban 1976) (Fig. 19). Modern Nautilus and all other extant cephalopods are also nektic or nektobenthic The i)1BQ values of -0.56 to -3.46%o, when using Grossman and Ku's {1986) temperature equation yield a temperature range of 19.7 to 33. 3 C This maximum temperature is above the 25 C thermal tolerance of Nautilus, and is also above the 25 to 26 C precipitation thresholds of other ammonites (Stahl and Jordan, 1969; Tan, et al., 1970; Saunders and Spinosa, 1979; Ward, 1979) As mentioned, even if this temperature was tolerable to baculites a change in temperature does not explain the sudden shift to lower covariant 813C values of the last two septa which suggests these values are the result of non-equillibrium precipitation. If the anomalous values of samples from the last two septa can be attributed to vital effects, and values from the remaining samples are interpreted independently calculated temperatures range from approximately 20 to 29 C The maximum temperature is still above the thermal limit suggesting specimen BC1 spent at least the latter part of its life in waters that did not reflect isotopic compositions of open-marine waters possibly migrating into such waters late in

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51 ontogeny. However the lower temperature is within the therma l to lerance range of cephalopods suggesting that the earlier part of its life could have been spent in normal marine waters The o180 values from baculitid ammonite specimen BC2 are typical of precipitation under near normal-marine conditions as well. The o 1 8Q values of -1.18 to -1. 72%o, when using the same temperature equation yield a temperature range of 22. 6 to 25. 2 C This maximum temperature is also close to the thermal limit, but is acceptable as representing precip i tation under normal to near-normal marine conditions. However, values after the th ird sample may also be affected by vital effects. Bottom Water Mass Finally, the remaining bivalve specimens which are all benthic have o18Q values which are generally lower than those of the coexisting b a culites (Fig 19). Specimen AO has o1B Q values from -2.53 to -4.34%o which when used in the temperature equation yield a temperature range of 29.0 to 37.4 C. Th i s maximum temperature is well above the 31 to 35 C lethal limit of subtidal bivalves, which typically cannot precipitate shell material above 30 C (Evans 1948). Therefore, the o18Q value of the water in which precipitation took place must have also been lower than that of normal-marine waters. Specimen UB was only sampled once in order to compare va l ues between different bivalve taxa from the same locality. Specimen AO has o 1 8 Q and o 1 3 C average values of 3.43%o and +2.25%o, respect i vely, if only the maximum and minimum values are used (which should bette r represent the full temperature range) instead of the values skewed toward cooler months. The

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52 8180 and 813C values of the sample from specimen UB are only 0.50%o and 0.66%o higher respectively. While the sample from specimen UB was routed across several growth bands, it still represents only a portion of the bivalve's life, and is an average of that period. Therefore it is not unexpected to find that values are not more similar. The 818Q value of this specimen when using the same temperature equation, yields a temperature of 30 9 C which is also above the thermal limit of shell precipitation of most bivalves suggesting that it also lived in a modified water mass Specimens AS1 and AS2 are attached to specimen INO, in their life position. These three specimens precipitated their shell in the form of low magnesium calcite (only the prismatic layer of the inoceramid), which when precipitated under the same conditions as aragonite should have 818Q values approximately 0.8%o lower than that of aragonite at 20 C (Rye and Sommer, 1980). When this correction is made, values from these three samples are similar to values from the aragonitic sample taken from specimen INO and are not dissimilar to those of the remaining benthic bivalves. This similarity between corrected values also provides more evidence that these fossils are unaltered. The corrected 818Q values from these three specimens range from -2.58 to -3.02%o, and when used in the temperature equation yield a temperature range of 29 2 to 31.3 C. These temperatures are also near the thermal limit for shell precipitation and indicate that bottom waters at this locality were probably modified as well. The values from all the bivalve specimens, which are from several localities, are also in agreement with the third paleocirculation model presented above, typically being too low to rep r esent precipitation from seawater of normal isotopic composition. Several other studies present similar data, with benthic taxa having lower 818Q values than nektic taxa in the seaway. While many of the

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53 bivalve specimens in this study yield temperatures that a r e close to acceptable precipitation limits many bivalves in these previous works yielded h i gher temperatures. lnoceramids from the upper Pierre Shale yield temperatures in excess of 35 C (818Q < -4 5 %a) in Tourtelot and Rye (1969) and in excess of 40 C (818Q < 5 %a) in Wr i ght (1987) Revised Paleocirculation Model While each scenario has incorporated a reduced-salinity surface layer with low 818Q values, many of the fossils used in these studies are not from shallows The first two scenarios also fail to provide a mechanism allowing the transport of isotopically-light surface waters to the bottom waters of the seaway. It should also be noted that data from benthic and nektic taxa collected from t he Maastrichtian shallow marine shelf Lopez de Bertodano Formation, Antarctica have similar 818Q values (Pirrie and Marshall, 1990). This suggests that the pattern of relatively low to higher to intermediate 818Q values with increasing depth seen i n fossils from the seaway are exclusive to the seaway and poss i bly other epicontinental seas, and are not yet found in open marine environmen t s with a nnormaln stratification of waters. Evi dence presented in each of the previous descriptions of probable water mass character i stics is in agreement with the third paleocirculation mode l presented by Wright (1987) Bottom waters i n the seaway must have been derived from the reduced salinity surface layer, from inside the seaway In her model, Wright p r oposed that freshwater input with a 8180 value of -10%o to -15 % a created the reduced salinity surface layer with a 8180 value of approximately -4 % o to -8 %o. This water mass would have been driven eastward by wind

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54 currents, and modified by evaporation until it became sufficiently dense to sink below the intermediate water mass of normal salinity (Fig. 20). One problem was how to modify the surface water with a salinity of 18%a to 25%a to a water mass with a salinity greater than the intermediate layer, while still retaining enough 160 to allow for the anomalously light 818Q values of the benthic taxa. Evaporation from a water mass preferentially removes H2016 which changes the 818Q of that water mass by approximately 1 %a for every 5%a change in salinity (Epstein and Mayeda, 1953) If the surface water mass had a salinity of 18%a, enough evaporation to raise the salinity to 38%a would raise the 818Q value of that water from -4%a to O%a, and from -8%a to -4%a. This water would then be carried to the bottom of the seaway due to the inherent increase in density. However, if the bottom waters had 8180 values of O%a to -4%a, the temperatures generated from inoceramids that lived in the bottom waters are unreasonably high as previously discussed. To account for this anomaly, Wright proposed that "equilibrium evaporation" was responsible, which is a mechanism with which the salinity of a water mass can be changed while maintaining nearly the same 8180water value. This would allow a surface water mass with a 8180 of -4%a to -8%a to undergo evaporation and to sink while still retaining a 818Q of -4%a to -8%a. If these 8180water values of -4%a to -8%a are used in the temperature equation, reasonable temperatures can be generated for the benthic bivalves. 8180 values ranging from -7.63%a to -20.59%a were generated from Cretaceous freshwater bivalves found in fluvial and lacustrine environments that existed adjacent to the seaway by (Glancy, et al., 1993). These data suggest that freshwaters had lower values than previously believed. The low

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FRESHWATER INPUT WINO.ORIVE N SURFACE CURRENTS NORMAl-MARINE INTER M EOIA T E WATER M ASS SALINE BOTIOM-W A TER MASS EVAPORAT I ON Figure 20 Schematic Diagram of a West to East Transec t Through the Central Portion of the Western Interior Seaway with Water Stratification and Ci rc ulation Patterns as Prop osed by Wright (1986) (J1 (J1

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8180 value of specimen TS is also consistent with these data suggesting freshwaters had 8180 values of -15 % o to -20%o, which are lowe r than those presented in Wright (1987) 56 The mechanism needed to transport low-salinity surface waters to the eastern portion of the seaway could have been wind driven currents If winds were from the southwest as suggested by Parrish and Curtis ( 1982), the Coriolis effect would c r eate a surface current that would move 45 to the right. The Ekman spiral effect would affect waters to depths of tens of meters creating a subsurface current that would have a general movement go o to the right (Thurman, 1991 ) The combination of these two forces acting on a shallow low density surface layer would indeed push the waters to the east and southeast areas of the seaway. While the m i xing of waters with disparate densities is difficult in subsurface water masses, it is more easily carried out i n shallow waters churned by wind-generated wave activity (Thurman 1991 ) Thus as low-salin i ty waters moved across the surface of the seaway, it i s probab le that they were constantly be mixed with underlying normal-salinity waters. If a lower average 818Q value of -17.5%o for freshwater input and a temperature of 25 C for a shallow, slowly m i xing surface water mass in the seaway are assumed, a bottom water mass can be produced which would have 818Q values low enough to account for the values of the benthic bivalves This revised model can be broken into 4 parts (Fig 21 ). 1) Freshwater with an average 818Q value of -17 5%o was i nfluxed to the western margin of the seaway creating a low-salinity surface layer with a salinity less than 20 % o and an average temperature of approximately 25 C. 2) Wind currents from the southwest created an eastward to southeastward flowing surface current which drove the surface waters toward the shallow eastern margin. This wind-driven transport also promoted mixing of the low-

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Salinity= 17 5 ()1BQwater = -9.25 ()1BQcarb. = -10 MIXING AND EVAPORATION S alinity= 22.5 t o 30 Salinity= 37.5 ()1BQwater = -3.3 to 7 1 ()1BQwater = -1.8 t o -4. 1 ---------------------------------........ Salinity= 37.5 o180water = -1.8 to -4.1 ()1BQcarb = -2.5 to -4 .8 DOWNWELLING? Figure 21. Revised Schematic diagram of a West to East Transect Throught the Central Portion of the Western Interior Seaway with Water Stratification and Circulation Patterns. Freshwater with an average 81BQ value of -17.5 is influxed from the western margin which moves across the surface of the seaway slowly mixing with subsurface waters and undergoing evaporation. As surface waters reach the eastern margin they become dense enough to sink below the intermediate waters. This bottom water mass with a 81BQ of -1.8 to -4.1 would give a carbonate precipitated at 25 C a 81BQ value of -2.5 to -4 8 CJ1 -.....!

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58 salinity surface layer with the underlying normal-marine waters continuously i ncreasing the salinity of the surface layer Evaporative forces also continuously acted on the surface layer which also increased its salinity 3) Upon rea c hing the shallow eastern margin of the seaway evaporation continued until the surface waters had a density (a salinity of approximately 37.5%o) sufficient to cause sinking. This created a westward flow i ng counter current bottom water with i n the seaway Sinking surface waters may also have caused a downwelling effect which would have aided the transport of low salinity surface waters to the eastern portion of the seaway. 4) Normal-marine water, with an intermediate density (a salinity of approximately 35 %o), was confined between these two layers

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59 CONCLUSIONS Isotope Paleontology 1 Many well preserved specimens from the Western Interior Seaway of North America have r etained original oxygen and carbon isotopic values, and are not altered by diagenetic processes 2 In order to assess temporal variations of isotopic values numerous samples representing an ontogenetic series should be collected from specimens. These may help resolve poss i ble growth temperatures or vital effects Samples should also be taken at a consistent depth in the shells of bivalve and ammonite specimens in order to produce cons i stent and comparable isotopic records Also samples must be t aken at the same position along the flanks when using baculitid ammonites 3 The record generated from specimen AO i s not affected by vital effects values are like those of other benthic b i valves from the seaway. However the record is only representative of a port ion of the seasonal temperature variation due to a decrease of its growth temperature through ontogeny. While the values are also like those of other benthic bivalves the record may be influenced by the rate of shell precipitation and the temperature at which

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60 precipitation took place Values are suggestive of shell precipitation i n modified marine waters. 4 The o1BQ records generated from baculitid ammon i te specimens BC1 and BC2 are complicated by several factors. Values of shell material precipitated prior to reaching maturity are like those of other baculites from within and outside of the seaway, and are indicative of precipitation under normal-marine conditions suggesting they are not affected by vital effects However there is a variation of values along growth bands which may be the product of different precipitation rates, or vital effects Values of shell material precipitated after reaching maturity are affected by vital effect and do not reflect paleoenvironmental conditions. The o1BQ record generated from specimen BC2 has values which are also like those of other baculites from within and outside of the seaway, and are ind icative of precipitation under normal-marine conditions However, values of shell material precipitated after reaching maturity appear to be affected by the same vital effect observed in the record of specimen BC1, and do not reflect paleoenvironmental conditions.

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61 Paleoenvironmental Implications 1. The 8180 values of planktic, nektic, and benthic organisms indicate that some water masses within the seaway were not normal-marine in nature. Surface waters probably had low salinities and were less dense than normal-marine waters. Bottom waters probably had high salinities and were more dense than normal marine waters Intermediate waters probably had normal-marine salinities and intermediate densities 2 While several paleocirculation scenarios have been proposed for the Western Interior Seaway, data from this study support only the model presented by Wright, 1986 which is revised in this study Low-salinity surface waters, modified by mixing and evaporation were probably the source for high-salinity bottom waters 3 The mechanism for this transformation of surface waters is a wind-driven eastward moving surface current which undergoes modification in the shallow eastern margin of the seaway creating a westward-flowing counter-current which would sink under a normal-marine intermediate water mass. 4 Th i s mechanism accounts for the anomalously low ()18Q values of the planktic and benthic specimens from the seaway presented in this study and in other studies which utilized specimens from several localities and a variety of biostratigraphic zones This in turn suggests that the stratification and circulation scenario presented by Wright (1986} and revised in this study was temporally and spatially persistent.

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62 LIST OF REFERENCES Armstrong, R. L. 1968. Sevier Orogenic Belt in Nevada and Utah. Geological Society of America Bulletin, 79, p 429-458 Barron, E. J Arthur, M.A., & Kauffman, E. G. 1985. Cretaceous rhythmic bedding sequences: A plausible link between orbital variations and climate Earth and Planetary Sciences Letters, 72, p. 327-340. Batt, R J 1989. Ammonite shell morphotype distributions in the Western Interior Greenhorn Sea and some paleoecological implications. Palaios, 4, p. 32-42. Buchardt, B & Weiner, S. 1981. Diagenesis of aragonite from Upper Cretaceous ammonites: a geochemical case-study Sedimentology, 28 p 423-438. Bucher, H., Landman, N. H., Klofak S M., & Guex, J. 1995. Mode and Rate of Growth in Ammonoids in press 105 p Cobban, W. A. 1984 Mid-Cretaceous ammonite zones Western Interior, United States. Bulletin of the Geological Society of Denmark, 33, p 7189. Cobban, W. A., & Reeside, J. B., Jr. 1952 Correlation of the Cretaceous formations of the Western Interior of the United States Bulletin of the Geological Society of America, 63, p. 1011-1044. Cochran, J. K., Rye, D. M., & Landman, N .H. 1981. Growth rate and habitat of Nautilus pompilius inferred from radioactive and stable isotope studies. Paleobiology, 7 p. 469-480. Davies, T. T., & Hooper, P. R. 1963. The determination of the calcite : aragonite ratio in mollusc shells by X-ray diffraction. Mineralogical Magazine, 33, p 608-612. Emrich, K., Ehhalt, D H., & Vogel, J. C 1970. Carbon isotope fractionation during precipitation of calcium carbonate. Earth and Planetary Science Letters, 8 p. 363-371

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Epstein S., Buchsbaum R Lowenstam H. A., & Urey H. C 1953 Revised carbonate-water isotop i c temperature scale Geological Society of America Bulletin 64 p. 1315 1326 63 Epstein S & Mayeda T 1953 Variat i on of 018 content of waters from natural sources. Geochimica et Cosmochimica Acta, 4 p. 213-224 Evans R. G. 1948. The lethal temperature of some common British littoral molluscs Journal of Animal Ecology 17, p. 165-173. Forester R. W., Caldwell, W. G. E & Oro, F H 1977. Oxygen and carbon isotopic study of ammonites from the Late Cretaceous Bearpaw Formation in southwestern Saskatchewan Canadian Journal of Earth Sciences 14, p. 2086 2100. G i ll J R., & Cobban, W A 1966. The Red Bird section of the Upper Cretaceous Pierre Sha l e in Wyoming with a section on A new echinoid from the Cretaceous P i erre Shale of eastern Wyoming by Porter M. Kier United States Geological Survey, Professional Paper 393-A A 1-A73. Gill, J R., & Cobban, W A 1973 Stratigraphic and geologic history of the Montana Group and equivalent rocks Montana Wyoming and North and South Dakota United States Geological Survey, Professional Paper 776. Glancy, T J. Jr. Arthur, M.A., Barron E J., & Kauffman, E. G 1993 A paleoclimate model for the North American Cretaceous (Cenomanian Turonian) epicontinental sea. In: W. G. E. Caldwell & E. G. Kauffman (eds ) Evolution of the Western Interior Basin Geological Association of Canada Special Paper 39, p 219-242 Grossman, E. L. 1987 Stable isotopes in modern benthi c foramin i fera : a study of vital effect. Journal of Foramin i feral Research, 17, p 48-61 Grossman E. L., & Ku T. L. 1986 Oxygen and carbon fractionation in biogenic aragonite: Temperature effects 59, p. 59-74. Harrington, R. J. 1989. Aspects of growth deceleration in bivalves : clues to understand i ng the seasonal o180 and o13C recorda comment on Krantz et al. (1987). Palaeogeography Palaeoclimatology, Palaeoecology 70, p. 399-407. Hay W. W Eicher, D. L., & Diner R. 1993 Physical oceanography and water masses in the Cretaceous Western Interior Seaway In : W. G E. Caldwell & E. G Kauffman (eds .), Evolution of the Western Inter ior Basin Geological Association of Canada Special Paper 39 p 297-318 Jeletzky J. A. 1970 Cretaceous macrofaunas In: Geology and Economic Minerals of Canada. Geolog i cal Survey of Canada Economic Geology Report 1 p 649-662

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64 Jones, D S., Williams, D. F & Arthur, M. A. 1983. Growth history and ecology of the Atlantic surf clam, Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments Journal of Experimental Marine Biology, 73, p. 225-242 Kauffman, E. G. 1 977. Geological and biological overview: Western Interior Cretaceous Basin. In: E. G. Kauffman (ed .), Field Guide: North American Paleontological Convention II; Cretaceous Facies, Faunas, and Paleoenvironments Across the Western Interior Basin. The Mountain Geologist, 14, p. 75-99. Kauffman, E. G 1984. Paleobiogeography and evolutionary response dynamics in the Cretaceous Western Interior Seaway of North America In: G E. G. Westermann (ed.), Jurassic-Cretaceous Biochronology and Paleogeography of North America. Geological Association of Canada Special Paper 27, p 273-306. Keith, M. L., Anderson, G. M., & Eichler, R. 1964. Carbon and oxygen isotopic composition of mollusk shells from marine and fresh-water environments Geochimica et Cosmochimica Acta, 28, p. 1757-1786 Kennedy, W. J., & Cobban, W. A. 1976. Aspects of ammonite biology, biogeography and biostratigraphy, Special Papers in Palaeontology 17, p. 1-94. Krantz, D. E., Williams, D. F., & Jones, D. S. 1987 Ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs. Palaeogeography, Palaeoclimatology, Palaeoecology, 58, p. 249-266 Kyser, T. K., Caldwell, W. G. E., Whittaker, S. G., & Cadrin, A. J. 1993 Paleoenvironment and geochemistry of the northern portion of the Western Interior Seaway during Late Cretacenus time. In: W G. E. Caldwell & E. G Kauffman (eds.), Evolution of the Western Interior Basin Geological Association of Canada, Special Paper 39, p. 355-378 Land, L. S., Lang, J. C., & Barnes, D. J. 1975. Extension rate: a primary control on the isotopic composition of West Indian (Jamaican) scleractinian reef coral skeletons. Marine Biology, 33, p. 221-233. Landman, N.H., & Cochran, J. K. 1987. Growth and longevity of Nautilus In: W. B. Saunders & N. H Landman (eds.), NautilusThe Biology amd Paleobiology of a Living Fossil. Plenum Press, New York, p. 401-420. McArthur, J M., Kennedy, W. J., Chen, M., Thirlwall, M. F & Gale, A. S. 1994. Strontium isotope stratigraphy for Cretaceous time: Direct numerical calibration of the Sr isotope curve based on the US Western Interior. Palaeogeography, Palaeoclimatology, Palaeoecology, 108, p 95-119

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McCrea, J. M. 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. Journal of Chemical Physics, 18, p 849-857. McNeil, D H., & Caldwell, W. G. E. 1981. Cretaceous Rocks and their Foraminifera in the Manitoba Escarpment. Geological Association of Canada, Special Paper 21, 439 p. 65 Obradovich, J D., & Cobban, W. A 1975. A time-scale for the Late Cretaceous of the Western Interior of North America In: W. G. E. Caldwell (ed.), The Cretaceous System in the Western Interior of North America. The Geological Association of Canada, Special Paper Number 13, p. 31-54 Parrish, J T., & Curtis, R. L. 1982 Atmospheric circulation upwelling, and organic-rich rocks in the Mesozoic and Cenozoic Eras Palaeogeography, Palaeoclimatology, Palaeoecology, 40, p. 31-36. Parrish, J. T. Ziegler, A.M., & Scotese, C. R 1982 Rainfall patterns and distribution of coals and evaporites in the Mesozoic and Cenozoic. Palaeogeography, Palaeoclimatology Palaeoecology, 40, p 67-1 01. Pirrie, D & Marshall, J. D 1990. High-paleolatitude Late Cretaceous paleotemperatures: New data from James Ross Island, Antarctica. Geology, 18, p. 31-34. Pratt, L. M. Kauffman, E. G., & Zeit, F. G. (eds.). 1985. Fine-grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes. Society of Economic Paleontologists and Mineralogists 1985 Mid-year Meeting, Golden, Colorado, Fieldtrip Guidebook No 4, 249 p Rye, D. M & Sommer, M.A. 1980. Reconstructing paleotemperatures and paleosalinity regimes with oxygen isotopes In: D. C. Rhoads & R. A. Lutz (eds .), Skeletal Growth of Aquatic Organisms p 169-202. Saunders, W B. 1983. Natural rates of growth and longevity of Nautilus belauensis. Paleobiology, 9, p. 280-288 Saunders, W B & Spinosa, C. 1979. Nautilus movement and distibution in Palau, Western Caroline Islands. Science 204, p 1199-1201 Scott R W 1970 Stratigraphy and sedimentary environments of Lower Cretaceous rocks, Southern Western Interior. American Association of Petroleum Geologist Bulletin, 54, p. 1225-1244 Shackleton, N.J., & Kennett, J. P. 1975. Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: Oxygen and carbon isotope analysis in DSDP sites 277, 279, and 281. In: J P Kennett (ed ) Initial Reports of the Deep-Sea Drilling Project, 29, p 743-755.

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Whittaker, S. G., Kyser, T K., & Caldwell, W. G. E. 1987. Paleoenvironmental geochemistry of the Claggett marine cyclothem in south-central Saskatchewan. Canadian Journal of Earth Sciences 24, p. 967-984. Williams, D. F., Arthur, M.A., Jones, D. S., & Healy-Williams, N. 1982. Seasonality and mean annual sea surface temperatures from isotopic and sclerochronological records. Nature, 296, p 432-434. Wright, E. K. 1986. Stratification and Paleocirculation Patterns of the Upper Cretaceous Western Interior Seaway of North America. Ph.D. dissertation, Yale University, 126 p. Wright, E K. 1987. Stratification and paleocirculation of the Late Cretaceous Western Interior Seaway of North America. Geological Society of America Bulletin, 99, p. 480-490. 67

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68 APPENDIX

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69 APPENDIX 1 I sotopic Values of All Samples Analysed. SPEC. SAMPLE # 0180 013C SPEC. SAMPLE # 0180 013C AO 1 -4.34 1.4 UB 1 -2.95 1.59 2 -4.12 2.2 3 -3.66 2.53 INO 1 -2.77 3.91 4 -3.92 0.56 2 -3.47* 3.64. 5 2.58 3 3 -3.38. 3.39. 6 -3.61 3.61 7 3.13 3.1 AS1 -3.52. 3.07* 8 -3.32 3.6 9 -3.53 3.93 AS2 1 3.82* 3.9* 19 -2. 55 2.45 11 -2. 53 2.85 TS -10.12 -15.4 1 12 -2.98 3 28 13 -2.96 3.3 I S -3 .4 -2.27 14 -3.19 3.5 2 -3.35 -2.31 15 -3.4 3.57 3 -3 .23 -2.07 1 6 -3.38 3.27 4 -3.28 -2.56 17 -2.53 2.84 5 -3.39 -3.07 18 -2.92 3.73 6 3.61 -2.86 19 7 -3.57 -3.9 20 -3.5 2.78 8 -3.9 1 -3.67 21 -3.59 3.1 22 -3.62 3.36 23 -3 .67 3.69 24 -3.71 3.69 25 -3.75 3.83 26 -3.74 3.75 27 -3.87 3.95 28 -3.79 4 29 3.7 3.95 30 -3.62 4.01 3 1 -3.73 3.45

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7 0 SPEC. SAMP LE # 0180 013C SP E C. SAM P L E # 0180 D 13C BC 1 1 -1.43 -1 .21 36 -0.9 0 .19 2 -1.46 -0.82 37 1.02 0.2 2.5 -1.34 -0.41 38 -1.1 3 0.08 3 -0.84 0.09 39 1 .09 0.03 4 0 .93 -0.03 40 -0.95 -0.09 4.5 -1.06 0.07 4 1 -1 .51 0 .23 5 -0.56 -0.1 8 4 2 -1.48 -0.34 5.5 0.96 -0.85 43 -2 -0.53 6 -0.82 -0.66 4 4 -1.85 -0.6 7 -0.68 -0.52 45 -2.04 -0.63 8 1 .01 -0.65 46 -2.05 0 6 9 -1.06 0 .38 47 -2.02 -0.57 1 0 -0.99 -0.51 48 -1.93 -0.55 1 1 -0.93 -0.68 49 1 .84 0 1 1 2 -0.85 -0.94 50 -1.04 -0.7 1 3 -0.79 -0.93 51 -2.33 -0.49 1 4 -0.92 -0.96 52 -2.09 -0.17 1 5 -1 .44 -0.72 53 -3.46 -2.82 1 6 -1.27 -1.02 5 4 -2.54 -1.39 1 7 -1.8 -0.67 55 -1 1 2 -0.4 1 8 -1.95 -0.74 56 1 0.66 1 9 1 .86 -1.03 57 1 .35 -0.81 20 2. 1 4 1. 18 58 -1.75 -0.68 2 1 -2.06 -0.88 59 1 .96 -1 11 22 -2.03 -1.15 23 -2.35 -0.96 BC2 1 -1. 18 -2.33 23.5 -2.47 -0.81 2 -1.22 -2.11 24 -2.5 -0.41 3 -1.62 1.89 24.5 -2.13 -0.51 4 -1.72 -2 25 -2.56 -0.24 5 -1 .34 1.72 25.5 -2.51 -0.1 7 6 1 .24 -1.45 26 -2.36 -0.36 2 7 2 .24 -0.1 C6 -2.87 -0.39 28 -2.43 0 2 -2.8 1 1 9 29 2.42 0.04 3 -2.94 2.13 30 -2.45 0.24 31 -2.38 0.09 32 -2.12 -0.06 33 -0.78 0.01 34 -1.09 -0.45 35 -0.79 0 1


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