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Using 34-S as a tracer of dissolved sulfur species from springs to cave sulfate deposits in the Cerna Valley, Romania

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Using 34-S as a tracer of dissolved sulfur species from springs to cave sulfate deposits in the Cerna Valley, Romania
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Sumrall, Jonathan
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Stable isotope
Karst
Geochemistry
Mineralogy
Sulfuric acid speleogenesis
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ABSTRACT: Baile Herculane, located in southwestern Romania, is a unique city that exploits its thermal waters. The geology consists of a granitic basement covered by 200 meters of limestone, marl, and flysch deposits. Extreme faulting carries heat ascending from the mantle, which intercepts percolating meteoric waters. Local springs have high concentrations of dissolved sulfide gas (H₂S) and dissolved sulfate (SO₄²⁻).These dissolved species indicate the progression of sulfate reduction in the aquifer. Water samples were collected in polyethylene syringes to prevent oxidation of sulfide. Then, sulfide and sulfate were quantitatively reacted for stable isotope analysis. Total sulfur isotopic composition was calculated to determine the source of the dissolved sulfur. The source of the sulfur is a sulfate of marine origin (δ³⁴approx. equal20⁴%₀, which I found to come from impurities in the limestone since the Cerna Valley does not possess marine evaporites. The limestones of the Cerna Valley are host to a number of caves, which possess relatively large deposits of sulfates and exotic morphologic features that suggest speleogenesis by sulfuric acid. δ³⁴S of the sulfates relates to sulfide isotopic values from the springs, showing that the dissolved sulfide (upon oxidation) forms sulfuric acid s that reacts with limestone to produce sulfate minerals. A wide range of cave sulfate δ³⁴S values exist indicating that isotopic values of these deposits depend on several factors such as sulfur source, extent of sulfate reduction, and completeness of sulfide oxidation. This also implies that a single, narrow range of sulfur isotopic values does not represent sulfuric acid speleogenesis.
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Thesis (M.S.)--University of South Florida, 2009.
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by Jonathan Sumrall.
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Using 34 S a s a Tracer of Dissolved Sulfur Species from Springs to Cave Sulfate Deposits i n t he Cerna Valley, Romania by Jonathan Sumrall A thesis submitted in partial fulfillment of the requirements for the degree of Department of Geology College of Arts and Sciences University of South Florida Major Professor: Bogdan Onac, Ph.D. Jonathan Wynn, Ph.D. Henry L. Vacher, Ph.D. Date of Approval: March 23 2009 Keywords: stable isotope, karst, geochemistry mineralogy, sulfuric acid speleogenesis Copyright 2009 Jonathan Sumrall

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Dedication To my best friend, Christina, for the love and support to get me through my best and worst days; my parents, Kenny and Kathy, for instilling a work ethic of banging my head against a wall until I find a solution; my grandmother, Betty, for continual suppor t regardless of why, when, or how; and all of my other friends who are constant reminders of happiness to me.

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Acknowledgements Special thanks go out to my major advisor, Dr. Bogdan Onac, and my committee members, Dr. Jonathan Wynn and Dr. Len Vacher for t heir guidance and support throughout this project. Thank you to Dr. Wynn for the guidance and patience in dealing with the many problems that arose during this project. The Domogled Valea Cernei National Park generously allowed access to the field area an d also granted approval to remove specimens for analysis. Dr. I. Povara from the "Emil Racovita" Institute of Speleology, Vera Darmiceanu ("Babes Bolyai" University in in dispensable assistance during our field campaigns. The Romanian National University Research Council (grant ID_544 to Onac ) contributed funds for this research. Many people have helped me along my path to reach USF ; mostly I would like to thank Dr. John My lroie and his wife Joan Mylroie for continual guidance and support from my undergraduate days at MSU to the present.

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Note To Reader Note to Reader: The original manuscript of this document contains color that is necessary for understanding the data. The original thesis is on file with the USF library in Tampa, Florida, USA

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iv T able of Contents LIST OF FIGURES ................................ ................................ ................................ ........... vi LIST OF TABLES ................................ ................................ ................................ ........... v iii ABSTRACT ................................ ................................ ................................ ....................... ix Chapter 1: INTRODUCTION ................................ ................................ ............................. 1 Chapter 2: DESCRIPTION OF STUDY AREA ................................ ................................ 3 2.1 Geogr aphic Setting ................................ ................................ ............................ 3 2.2 Geologic Setting ................................ ................................ ................................ 5 2.3 Hydrogeology ................................ ................................ ................................ ... 7 2. 4 Caves and Karst ................................ ................................ .............................. 11 Chapter 3: BACKGROUND INFORMATION ................................ ............................... 14 3.1 Stable Sulfur Isotopes ................................ ................................ ..................... 14 3.2 Natural Range of Sulfur Isotopes ................................ ................................ .... 15 3.3 Redox Reactions ................................ ................................ ............................. 16 3. 4 Bacterial Sulfate Reduction and Thermochemical Sulfate Reduction ............ 18 3. 5 Sulfuric Acid Speleogenesis: History ................................ ............................. 18 3. 6 Gypsum in Caves ................................ ................................ ............................ 20 3. 7 Previous Sulfur Isotope Studies: Caves ................................ .......................... 22 Chapter 4 : METHODOLOGY ................................ ................................ .......................... 2 5 4.1 Sample and Data Collection ................................ ................................ ............ 25 4. 2 Mass Spectrometry ................................ ................................ .......................... 2 7

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v 4. 3 Total Sulfur Isotope Calculations ................................ ................................ ... 29 4. 4 Dissolved Inorganic Carbon Samples ................................ ............................. 29 Chapter 5 : RESULTS ................................ ................................ ................................ ....... 3 0 5.1 Spring Data: General Field Data ................................ ................................ ..... 3 0 5.2 Spring Data: Stable Sulfur Isotope Measurements ................................ ......... 32 5.3 Springs: Total Sulfur Isotopic Composition of Spring Water ........................ 33 5.4 DIC of Spring Waters ................................ ................................ ..................... 35 5. 5 Cave Sulfate Isotope Data ................................ ................................ ............... 35 Chapter 6 : DISCUSSION ................................ ................................ ................................ 3 8 6.1 Sulfur Source and Fractionation ................................ ................................ .. 3 8 6.2 Sulfur Source ................................ ................................ ................................ .. 39 6.3 Development of a Theoretical Model of Rayleigh Distillation ...................... 42 6.4 Cerna Springs: Dissolved Sulfur Species ................................ ....................... 4 5 6.5 TDS and its relationship to total dissolved sulfur ................................ ........... 51 6.6 Caves: Sulfur Isotopes ................................ ................................ .................... 5 5 Brzoni Cave (gypsum) ................................ ................................ ............. 5 5 Great Cave ................................ ................................ ................... 5 5 Adam and Aburi Caves ................................ ................................ ............. 5 8 Diana and Despicatura Caves ................................ ................................ ... 59 Chapter 7 : CONCLUSIONS ................................ ................................ ............................. 6 0 REFERENCES ................................ ................................ ................................ ................ 6 3 APPENDI CES ................................ ................................ ................................ .................. 6 7 APPENDI X A: Cave Maps and Sample Locations ................................ .......................... 6 8

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vi List of Figures Figure 1. Physiographical map of Romania showing the study area. ................................ 3 Figure 2. Blue white, H 2 S rich spring water from the Neptun springs. .............................. 4 Figure 3. Stability field of aqueous sulfur species. ................................ ............................. 5 Figure 4. A generalized cross section of the Cerna Valley. ................................ ............... 6 Figure 5. The major tectonic features that control water flow. ................................ ........... 8 Figure 6. Generalized cross section showing source of thermomineral waters. ............... 10 Figure 7. Picture showing the limestone in the Cerna Valley. ................................ .......... 11 Figure 8. Picture showing morphology and gypsum deposits. ................................ ......... 13 Figure 9. Sulfur isotopic value of various sources. ................................ ........................... 16 Figure 10 A redox tower showing the half reaction s ................................ ....................... 17 Figure 1 1 A generalized schematic of the reduction of sulfate. ................................ ...... 20 Figure 1 2 Massive gypsum deposits (10 12 cm) in Ion Brzoni Cave ............................ 22 Figure 13 Diagram of gypsum precipitation in Frasassi caves. ................................ ...... 24 Figure 14 Picture of Dr. Bogdan Onac sampling from the Neptun 3 .............................. 2 6 Figure 15 Picture of Dr. Jonathan Wynn measuring dissolved sulfide. ........................... 27 Figure 16. Sprin g and well locations. ................................ ................................ ............... 31 Figure 1 7 Location of caves that were sampled. ................................ .............................. 36 Figure 18 Generalized geologic cross section of the Cerna Valley. ................................ 40 Figure 19 Plot of total sulfur isotopic signature. ................................ ............................... 41 Figure 20. Theoretical model of sulfate reduction ................................ ............................ 43

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vii Figure 21 Plot showing the change in sulfide and sulfate moving upstream ................... 4 6 Figure 22. Plot showing the isotopic composition of dissolved sulfur species. ............... 48 Figure 2 3 Plot showing percent composition of methane in spring water. ...................... 5 0 Figure 2 4 13 C of DIC in spring waters ................................ .............................. 5 1 Figure 2 5 Plot showing two populations of springs based on TDS and total sulfur ........ 5 2 Figure 2 6 Plot showing the gradual 34 S enrichment downstream ................................ ... 5 3 Figure 2 7 A mixing diagram showing the isotopic value of a mixed component .......... 5 4 Figure ................................ ................................ .... 57 Figure 29. Rayleigh Distillation Model of sulfide oxidation. ................................ ........... 58

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v iii List of Tables Table 1. Spring data for the springs of the study area. ................................ ..................... 32 Table 2. Sulfur isotope values reported for the springs in the Cerna Valley. .................. 33 Table 3. Total sulfur values calculated for all springs. ................................ ..................... 34 Table 13 C values from springs/wells. ................................ ................................ .. 35 Table 5 Sulfur isotope values for sulfate minerals. ................................ .......................... 37

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ix Using 34 S a s a Tracer of Dissolved Sulfur Species from Springs to Cave Sulfate Deposits i n t he Cerna Valley, Romania Jonathan Sumrall ABSTRACT Baile Herculane, located in southwestern Romania, is a unique city that exploits its thermal waters. The geology consists of a granitic basement covered by 200 meters of limestone, marl, and flisch deposits. Extreme faulting carries heat ascending from the mantle, which intercept s percolating meteoric water s Local s prings have high concentration s of dissolved sulfide gas (H 2 S) and dissolved sulfate (SO 4 2 ). These dissolved species indicate the progression of sulfate reduction in the aquifer. Water samples were collected in polyethylene syringes to prevent oxidation of sulfide. Then, sulfide and sulfate were quantitatively reacted for stable isotope analysis. Total sulfur isotopic composition was calculated to determine the source of the dissolved sulfur. Th e source of the sulfur is a sulfate of marine origin ( 34 S 20 I found to come from impurities in the limestone since the Cerna Valley does not possess marine evaporites The limestones of the Cer na Valley are host to a number of caves, which possess relatively large deposits of sulfates and exotic morphologic features that suggest speleogenesis by sulfuric acid. 34 S of the sulfates relates to sulfide isotopic val ues from the springs, showing that the dissolved sulfide ( upon oxidation) forms sulfuric acid s that reacts with limestone to produce sulfate minerals 34 S values exist indicating that isotopic values of these deposits depend on several factors such as sulfur source, extent of sulfate reduction, and completeness of sulfide oxidation. This also implies that a single, narrow range of sulfur isotopic values does not represent sulfuric acid speleogenesis.

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1 Chapter 1 INTRODUCTION N umerous researchers have studied the Romania (Popescu Oncescu (1953) Papiu (1960), Pascu (19 68 ), and Marin (1984 ) ( 1992), Simion et al. (1985)) with the aim of und erstanding the hydrogeology and source of the mineralization of the springs. Building on these hydrogeochemical studies, this thesis project aimed to use the sulfur isotopic composition of dissolved sulfur in thermal waters to determine sulfur source. The hypothesis for this work is that isotopic composition of the dissolved sulfur in springs and sulfate minerals from the caves of the Cerna Valley will allow for tracking of the progression of sulfur chemical reactions leading to cave sulfate mineral precip itation. Data collected from dissolved sulfur species and from cave sulfate minerals will be interpreted using a theoretical model of reduction of dissolved sulfate. A scending thermal waters in the aquifers of the Cerna Valley transport large concentrations of dissolved sulf ur towards the surface. The waters discharge as springs or are intercepted by wells and brought to the surface Some of these springs and wells are highly influenced by mixing with meteoric water. The aerobic oxidatio n of hydrogen sulfide gas produces an acidic solution : H 2 S + O 2 H 2 SO 4 2H + + SO 4 2

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2 that has the potential to dissolve limestone and form caves (the so called sulfuric acid speleogenesis hereafter SAS ; Egemeier, 1971; Jagnow et al. 2000) SAS can also be responsible for generation of large amounts of sulfate minerals once a cave is formed The isotopic analysis of these minerals allow for interpretation of the process leading to their formation.

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3 Chapter 2 DESCRIPTION OF STUDY AREA 2.1 Geogr aphic Setting The Cerna Valley i n southwestern Romania (Figure 1) is famous for its thermal was originally used by the Dacians (a group of people that inhabited the area long before the Roman conquest ; 106 A.D.; Cristescu, 1978). Currently the springs and wells around are used by hotels to heat their rooms and supply thermal spas and by the public for healing and medicinal p urposes Figure 1 Physiographical map of Romania showing the study area.

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4 B ile Herculane lies above a significant positive geothermal anomaly. Vertical thermal gradients of this anomaly are 2 6 times higher than the surrounding thermal gradients (gradients above 90 C/km are considered to be an anomaly 992). The anomaly in the Cerna Valley is actually a collection of smaller anomalies; the B ile Herculane area and the 7 Springs area are the largest of these anomalies with gradients reaching 200 C/km The anomalies mark heat transport along faults on the flanks of the Cerna Graben. Temperature of springs in the Cerna Valley reache s 58C. Furthermore, many of the springs have high concentrations of dissolved sulfide ( S 2 HS and H 2 S) and sulfate ( SO 4 2 ) and native sulfur is pre cipitated around some (Figure 2). Figure 2 Blue white, H 2 S rich spring water from the Neptun springs mixing with the Cerna River.

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5 It is important to understand the stability of aqueous sulfur species in terms of dissolved oxyg en fugacity, and pH (Figure 3). A t low oxygen concentrations and moderate pH, HS is an important species (in terms of concen tration). This speciation can produce error when quantifying dissolved sulfide concentrations, as S 2 is commonly us ed to measure dissolved sulfide despite the fact that it is not the dominant species in these conditions. This potential error is not of concern in this study because the waters of interest are in a pH range where the S 2 is the dominant sulfur species. Figure 3 Stability field of aqueous sulfur species as a function of pH and O 2 (after Sharp, 2007). 2.2 Geologic Setting Both geologically and tectonically, the Cerna Valley is a complex region. The basement of the Cerna Valley consists of highly fractured and slightly altered granitoids.

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6 granitic basement at various depths, with all boreholes providing discharge of thermal waters (Figure 4). This proves that the basement has at least a moderate hydraulic conductivity (ability to transmit water). Figure 4 A generalized cross section showing the geology of the Cerna Valley ( from Overlying the basement is the Mesozoic cover, which consists of 5 units (from basement to surface; Oncescu, 1953 ; et al. 1996; Bojar et al. 1998; Krutner & Krstic 2002 ): Lower to Middle Jurassic arko se and carbonate sandstones 10 25 meters Upper Jurassic to Barriasian massif limestones 180 200 meters Berriasian to Hauterivian densely layered limestones and marly limestones 15 40 meters Barremian to Aptian marly limestones known as the Iuta layers and the cap Wildfl y sch formation (Turonian to Senonian) 200 meters The entire sedimentary succession is folded into an asymmetrical syncline structure with its western limb more steeply inclined. The Cerna River flows along a C D

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7 major tectonic feature, which divides the Cerna Mountains (to the west) from the 2.3 Hydrogeology The Cerna Syncline is approximately 25 km long and intersect s the Cerna Graben near B ile Herculane. The Cerna Anticline karstic rocks act as a hydrogeologic barrier, which causes north south drainage of water from the syncline, along the Graben, which is bordered on its eastern a nd western flanks by deep faults The main longitudinal fault is intersected by a number of transverse faults Significantly, t hese faults may allow migration of methane and mineralized water from adjacent valleys ( et al., 2008).

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8 Figure 5 T ectonic features that control water flow are shown in relation to the location of the major springs

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9 Hydrochemical data and investigations in this area date back to the 17 th century; however, the present understanding of the thermomineral waters dates only from the beginning of 20 th hydrothermal and balneotherapeutics (water healing) qualities (Vasilescu an d Liteanu, 1973; Cristescu, 1978; Marin, 1984; Simion et al., Negrea, 2002). In the Cerna River Basin there are three categories of waters: karstic sources, thermomineral waters and the Cerna River (Marin, 1984). The three m ajor tectonic structures that control the flow of water in the basin are the Cerna Syncline, Cerna Anticline, and Cerna Graben (Figure 5). Two major structures are involved in the functioning of the thermomineral aquifer the Cerna Syncline (developed on the right bank of the river) and the Cerna Graben (formed between two deep fractures, NNE SSW oriented, having a displacement of more than 1000 m), respectively (Figure 5) The most important transversal fractures in the Neptun, and Vicol faults. On the intersections with the western fault of the Cerna Graben or immediately nearby, thermomineral waters emerge either directly to the surface or into natural karst cavit ies. Hercules Spring is the main outlet for the Cerna syncline aquifer structure, where mixing of the vadose (surface) karstic and thermomineral (ascending) waters occurs tem perature, and water chemistry, with an inverse relationship between discharge and both water temperature and chemistry A s discharge increases, temperature decreases and

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10 the amount of dissolved solids (and other chemical parameters) decrease. This fluctuat ion is caused by vadose water entering the karst system after significant rainfall events, which flushes the system and dilutes the thermomineral water. Four hypotheses have been developed to explain the phenomenon of the Hercules Spring First, Popescu V fumaroles. Next, Papiu (1960) proposed a vadose origin of the water. In this scenario, water is carried along faults deep into the major tectonic structures, heated according to depth, and return ed to the surface spring. Third, Pascu (19 68 ) proposed that the fluctuations in discharge and gas emissions represent magmatic activity. Last, Simion (1987) proposed a mixed origin of a small component of ascending heated waters (and possibly ascending gas separately) mixing with a larger cold (meteoric) component. Figure 6 Generalized cross section (from A to B on Figure 5) showing the source of thermomineral waters

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11 2.4 Caves and Karst Caves are formed on the southern part of the Cerna Syncline in over 200 m of Upper Jurassic to Lower Cretaceous limestone ending with an Upper Cretaceous impermeable (Figure 7). Carbonic acid speleogenesis, where limestone is dissolved by carbo nic acid produced by the dissolution of CO 2 in water, is responsible for most of the caves in the Cerna region Sulfate minerals, such as gypsum, can occur in these caves from two main sources: (1) o xidation of sulfide minerals (pyrite, FeS 2 ), or (2) preci pitation of the sulfate ion (dissolved from sulfate containing deposits) due to oversaturation. The volume of sulfate minerals in these types of cave is usually low (although there are certainly exceptions). Figure 7 Picture showing the limestone in the Cerna Valley. A number of caves (containing significant gypsum deposits, exotic mineralogy, and unique geomorphology ; Figure 8 ) cluster in the walls along the Cerna Gorge (Onac et

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12 al 2009) These caves formed by ascending thermal waters and the reaction of sulfuric acid ( derived from oxidation of H 2 S ) with limestone, vi a sulfuric acid speleogenesis ( Hill, 1987; Galdenzi and Menichetti, 1995; Jagnow et al., 2000 ). Massive gypsum deposits, corrosion features, anast omotic passages, and exotic mineralogy are typical for cavities developed in the mixture zone between ascending H 2 S rich solutions and descending oxygen rich, meteoric waters that produce limestone dissolution These caves also contain features such as fee der tubes, cupolas, and blind passages; which according to Klimchouk (2007) are evident features of dissolution via ascending solutions. Generally, 34 S of cave sulfates resulting from SAS have been shown to be negative prior to this study. The thermal wa ters in Cerna region are characterized as having high concentrations of dissolved sulfide. Oxidation of sulfide (H 2 S (aq) or H 2 S (g) ) forms sulfuric acid that reacts with limestone to produce sulfate minerals leaving the w alls and roof of the cave almost co mpletely encrusted. The gypsum crust enlarges until it fails under its own weight. This exposes fresh limestone, enlarging the passage. After falling, the crusts are either dissolved by springs or form thick floor deposits. The volume of gypsum in these ty pes of caves is much greater than most carbonic acid caves.

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13 Figure 8 Picture showing morphology and gypsum deposits of a passage in Ion Brzoni Cave (Credit: Bogdan Onac).

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14 Chapter 3 BACKGROUND INFORMATION 3.1 Stable Sulfur Isotopes Sulfur has four stable isotopes with the following masses and relative abundances: 32 S (95.02%), 33 S (0.75%), 34 S (4.21%), and 36 S (0.02%). Stable isotope measurements difference in the ratio of two isotopes between a sample (unknown) and a standard sulfur is the relative abundance of 34 S to 32 34 S, which is calculated by the following equation: 34 S = sulfur is otope analysis is ca lled the Ca on Diablo Troilite (CDT). Since primary international reference standards are quite expensive and in limited supply, secondary international or lab standards are used on a daily basis, and regularly calibrated against international reference ma terials to give an economical, yet precise data. (for sulfur, 34 S) with respect to the standard, and isotope enriched sample The fr equation (here between sulfate and sulfide)

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15 sulfide sulfate = sulfide sulfate = sulfate sulfide which is called the per mil fractionation (Sharp, 2007). This quantity is well approximated by the sulfate sulfide sulfate sulfide sulfate sulfide 34 S values for a sample containing multiple sulfur species stable isotope mass balance can b e applied. Assuming a closed system, the total dissolved sulfur will be composed of dissolved sulfide and dissolved sulfate. Using the following expression, the total dissolved sulfur is otopic value can be calculated, where X is the molar concentration of sulfur. 3.2 Natural Range of Sulfur Isotopes Dissolved s ulfate can be derived from various sources. Oxidation of sedimentary pyrite can produce sulfate in solution. Direct dissolution of sedimentary evaporites (marine origin) either as a geologic unit or as impurities in limestone allow sulfate to enter solution. Oxidation of igneous derived reduced sulfur can also be a source of sulfate. Anthropogenic sources, such as burning coal or other fossil fuels, release sulfur dioxide and other sulfur compounds into the atmosphere where it is oxidized which can be dissolved to form sulfate. Each source of sulfate has a unique isotopic signature (pyrite

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16 ~ 20 to marine evaporites and atmospheric = variable; Figur e 9 ) Figure 9 Sulfur Iso topic value of various sources (After Krouse, 2000 and Sharp, 2007). 3.3 Redox Reactions Oxidation reduction (redox) reactions control concentrations of redox sensitive species. The reduction of sulfate to produce sulfide is an example of one half of a redox reaction (reduction electrons are gained by S). The complete reaction must include a pair of such half reactions, in which one element is reduced (S in sulfates reduced to sulfides) and one element is oxidized (for example, C in methane to form carbon dioxide). In order to understand which redox pairs can occur spontaneously it is convenient to examine a redox tower, which shows changes in Gibbs free energy ( Figure 10 ). For the reaction involving reduction of sulfate to go forward, there must be an oxidation half

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17 reaction that provides a net negative Gibbs free energy change, and thus a spontaneous, or energy producing reaction. In terms of the redox tower shown in Figure 9 this accompanying oxidation half reaction must be above the reduction half reaction on the left. So, for the example of the reduction of sulfate, only the oxidation of CH 4 NH 4 + H 2 or CH 2 O can provide the ox idation half reaction for the reaction to occur spontaneously, i.e. without the addition of free energy. Figure 10 A redox tower showing the half reaction s that occur as redox pairs in terms of electron activity Oxidation on the left and reduction on the right (after Kirschvink and Kopp, 2008). Net change in Gibbs free energy is negative (spontaneous) for pairs that step down to the right.

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18 3.4 Bacterial (BSR) and Thermochemical Sulfate Reduction (TSR) The redox reaction between sulfate and hydrocarbons can occur either by microbial mediat ion or inorganically. Bacterial Sulfate Reduction ( BSR ) usually occurs at low temperatures (0 < T < 60 80C), while Thermoch emical Sulfate Reduction (TSR) occurs at higher temperatures. The source of the sulfur in H 2 S can be dissolved sulfate that is derived from seawater, buried seawater brines, evaporite brines, and/or dissolution of sulfate deposits. The sources of the hydro carbons can be crude oil, microbial methane, thermogenic gas, and/or gas condensate. Sulfur i sotopic fractionation factors of BSR range from 15 to factors during TSR range from 20 (at 100 C) to Machel et al., 1995 ). If a system involved in sulfate reduction is energy limited, the microbial action ceases leaving excess sulfate and any additional fractionation must be a result of TSR In a system limited by sulfate ( either due to low amounts in the rock to dissolve or by insolubility due to factors such as pH), the BSR and TSR reactions occur faster than the accumulation of sulfate. This results in the complete reduction of the sulfate, which gives the sulf ide the isotopic composition of the original sulfate. 3.5 Sulfuric Acid Speleogenesis: History Jagnow et al. (2000) supply a complete review of the history of SAS theory. Egemeier was the first to present the model in a 1971 report to Carlsbad National Park suggesting that some of the large rooms of Carlsbad Caverns were dissolved by sulfuric acid that was based on his work in Kane Caves, Wyoming. The model suggested that H 2 S is volatilized from groundwater to the cave atmosphere (Figure 1 1 ). This H 2 S is oxid ized to sulfuric aci d, which reacts with limestone, to precipitate gypsum.

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19 If the first reaction of sulfide oxidation is quantitative (i.e. no H 2 S escape s ) the sulfur isotopic composition of the sulfuric acid produced during oxidation must reflect the isotopic composition of H 2 S from which it is produced. This is due to quantitative conversion of the S in H 2 S to produce S in sulfuric acid. If the reaction is complete, little or no fractionation is evident between reactants and products, which gives the sulfuric acid the isotopic composition of the initial sulfide gas. SAS is now recognized in caves in the United States, Italy, Romania, and Mexico (Hubbard et al., 1990; Galdenzi and Menichetti, 1995; Srbu et al., 1996; Hose et al., 2000 ; Menichetti et al., 2008 ). In addition to the processes acting above the water table, SAS has been attributed to dissolution reactions that take place at or just below the water table (Hill, 1990).

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20 Figure 11 A generalized schematic of the reduction of sulfate that leads to SAS and gypsum precipitation in caves in the Cerna Valley 3.6 Gypsum in Caves There are several mechanisms which may introduce the sulfate ion into a cave: dissolved sedimentary sulfate entering with percolating meteoric water, oxidation of pyrite or other sulfide minerals or ions (present in the host rock and fluid) to produce sulfuric acid, ascending H 2 S rich geothermal waters oxidized to produce sulfuric acid, decomposition of sulfur compounds in organic matter from soil or guano, and magmatic or geothermal activity introducing sulfate with steam via fumaroles into nearby karst areas. There are three processes which may cause the precipitation of sulfate as gypsum in a cave ( Hill and Forti, 19 97 ) : reaction progresses to oversaturation with respect to

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21 gypsum precipitation due to evaporative concentration, or reaction of sulfuric acid with limestone (SAS). Gy psum speleothems produced by SAS are much different in morphology compared to gypsum sp eleothems in normal carbonic acid caves. In carbonic acid caves, dissolved sulfate enters as water seeps into the cave Evaporation causes an oversaturation of sulfate that result in gypsum precipitat ion This produces gypsum speleothems that appear to gro w from the cave wall. These speleothems can collapse under their own weight or be broken by expansive forces as a new crust forms behind it. Gypsum flowers represent the main type of speleothems; however, other morphological forms exist: balls, balloons, b listers, and powders (Hill and Forti, 1997) In sulfuric acid caves, sulfide gas is released into the cave atmosphere where it is oxidized to form sulfuric acid. This sulfuric acid react s with the limestone walls to form replacement crust. Instead of bein g precipitated from sulfate saturated solutions percolating into the cave these crusts represent the reaction of sulfuric acid (produced inside the cave) with limestone Gypsum crusts on the walls and ceiling represent the main type of speleothems; howeve r other morphological forms exist: rafts in pools, helictites, rims, carpets, chandeliers, fibers, gypsum spar, and gypsum alteration calcite crusts (Hill and Forti, 1997)

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22 Figure 12 Massive gypsum deposits (10 12 cm) in Ion Brzoni Cave (Credit: Bogdan Onac). 3.7 Previous Sulfur Isotope Studies: Caves Hill (1980) had the first sulfur isotope determination performed on a sample of gypsum collected from Carlsbad Caverns. The presence o f 34 S 34 S = ~ 34 without isotopic fractionation (as would be the case for simple preciptation), but originated from th e incomplete bacterial reduction of sulfates from the Castile Formation (energy limited case discussed above ) to produce sulfides and oxidation of sulfide to sulfuric acid (Hill, 1995). In 1986, one of the largest caves in the world (Lechuguilla) was disco vered (Jagnow et al., 2000). Tens of kilometers of its galleries are covered by white, thick gypsum crusts and host spectacular gypsum crystals, up to 5 6 m in length. Along with

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23 these, the cavers discovered blocks of native sulfur throughout the cave that could have not been deposited unless by H 2 S reduction (Davis, 2000). The S isotope ratios measured within the sulfur and gypsum samples suggested a similar pathway for the origin of sulfur the incomple te reduction of sulfate from the Castile Formation (en ergy limited) to produce H 2 S in the Delaware Basin, and oxidation to sulfuric acid (Hill, 1995). A study of Frasassi caves in Italy by Galdenzi and Maruoka (2003) showed the caves were formed via SAS. Stable isotope analyses were performed on sulfates from caves as well as sulfides and sulfates from nearby springs. This identified the source of the sulfide as incomplete bacterial reducti on of sulfate (energy limited) from dissolution of an anhydrite layer at depth. This is confirmed by the presence of sulfates in the springs 34 existing sul fides range between 13.3 and which corresponds to marine evaporites as the source of the diss olved sulfur. As the H 2 S ascends through the groundwater sulfuric acid is produced by bacterial oxidation of H 2 S (Figure 1 3 ). However, not all of the dissolved H 2 S is oxidized, allowing the excess H 2 S gas to diffuse into the cave atmosphere. In the highl y aerobic atmosphere of the cave, the H 2 S is completely oxidized and forms sulfuric acid that dissolves limestone, and produces replacement gypsum. These gypsum deposits are depleted in 34 34 S range from r source is the sulfide that is produced by incomplete bacterial sulfate reduction (under energy limited conditions).

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24 Figure 13 Diagram showing the mechanisms leading to gypsum precipitation in Frasassi caves (after Galdenzi and Maruoka, 2003). Onac et al. (2007) suggested that the isotopically depleted ( 7 to and barite samples from Corkscrew Cave, Arizona were precipitated from hydrothermal sulfidic solutions from depth mixing with oxygen rich meteoric waters These sulfates fall within the same range as other cave sulfates reported from the Delaware Basin (Hill, 1995), suggesting a common sulfate source and process.

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25 Chapter 4 METHODOLOGY 4.1 Sample and Data Collection Two types of samples were collected: water fro m springs for the precipitation of dissolved sulfides and sulfates and sulfate minerals from caves. Temperature, pH, total dissolved solids (TDS), and electrical conductivity were measured using a Hanna Instruments HI 9828 Multiparameter meter. Water coll ection was performed using a Polyethylene syringe and luer lock valve to prevent oxidation of sulfides (Figure 1 4 ). One liter samples were collected in order to ensure the precipitation of an excess amount of both sulfate and sulfide. To determine t he concentration of dissolved sulfides and sulfates a Hach portab le spectrophotometer was used (Figure 1 5 ) Method s 8051 and 8131 (Hach Company, 2002) w ere used and for sulfate and sulfide, respectively

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26 Figure 14 Dr. Bogdan Onac sampling from a spring using a syringe to prevent oxidation of sulfides. Once the concentration of dissolved sulfur species was known, dissolved sulfide was precipitated by the quantitative reaction with z inc a cetate, Zn(CH 3 CO 2 ) 2 to form ZnS. This reaction was carried out at a pH where sulfide is stable, which is 10 11 (the pH was adjusted with NaOH ). The precipitate was then filtered using a Buchner funnel. The ZnS was then reacted with silver chloride (AgCl) to form AgS, a more stable c ompound that is more amenable to combustion analysis This silver sulfide would be used for stable isotope analysis by combustion elemental analysis (EA) coupled to isotope ratio mass spectrometry (IRMS).

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27 Figure 15 Dr. Jonathan Wynn measuring dissolved sulfide on the spring site. Dissolved sulfate was prepared by quantitative precipitation of BaSO 4 with BaCl 2 (Groot, 2004). This reaction was p er formed at a pH between 3 and 4 to prevent the precipitation of b arium carbonate. The p recipitate formed was then filtered using a Buchner funnel and washed with distilled water. The BaSO 4 would be used for stable isotope analysis by combustion EA IRMS (Groot, 2004). 4.2 Mass Spectrometry A Thermo Delta V Isotope Ratio Mass Spectrometer (IRMS) at University of South Florida Stable Isotope Lab 34 S values ( 34 S/ 32 S ratio notation) of total S in mineral samples, both natural, and precipitated from

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28 dissolved sulfur. Isotope ratios were measured by coupled Elementa l Analysis (EA) IRMS, whereby there is a quantitative conversion of S in the mineral sample to SO 2 and the isotope ratios are measured on SO 2 gas eluting from the chromatographic column of the EA The method that this study used for continuous flow sulfur isotope analysis was after Grassineau et al. (2001). Samples were weighed on a microbalance in a tin capsule (which promotes combustion) p rior to placing sam ples in the EA IRMS Inside the EA auto sampler the tin capsules were sequentially combusted in th e presence of Cr(III) oxide, Co oxide, and V 2 O 5 A coordinated burst of O 2 was injected into the combustion chamber at the time of capsule arrival to allow the complete combustion of sulfur to form SO 2 gas. Helium carrier gas carries SO 2 gas from the EA in to the source of the IRMS where it was ionized and then passed through an electromagnetic field separating the ions based on the relative masses. Faraday cup d etectors simultaneously record the number of counts of the appropriate mass es Finally, the numbe rs of counts were converted into isotope ratios by comparison to the count ratios of a standard In order to ensure precision, proper intensity peak shapes were achieved by preparing samples to have a similar ratio of sulfur as the standards. M ass of ea ch sample w as weighed between 0.5 mg to 0.8 mg. The standards used for the analysis were IAEA S 2 34 S = and IAEA S 3 34 = S for sulfides and IAEA SO 5 34 S = and IAEA SO 6 34 S = to bracket our expected range of values. The reproducibility between

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29 4.3 Total Sulfur Isotope Calculations Once isotope measurements were made for the sulfides and sulfates dissolved in spring waters, the total sulfur isotopic value (TS V) can be calculated using a mass balance equation: where X represents the molar concentration of sulfur. 4.4 Dissolved Inorganic Carbon Samples A water sample (800mL) fixed with 4 mg CuSO 4 5H 2 O (to prevent further microbial production and preserve the dissolved CO 2 in the sample ) w as collected from 13 C of dissolved inorganic carbon (DIC) using the IRMS, combining the methods of Torres et al. (2005) and Assayag et al. (2006). Fiv e drops of 103% H 3 PO 4 was added to wate r samples to acidify and drive DIC species (HCO 3 CO 3 2 and CO 2 ) out of solution as CO 2 (gas) After s amples were acidified, they were placed in the auto sampler of the Gasbench II that is coupled to the IRMS for an alysis The standards used for analysis were NBS 13 C = 5.014 ) and NBS 13 C = +1.95 )

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30 Chapter 5 RESULTS 5.1 Spring Data: General Field Data Data from field measurements taken at each spring during the sampling period of 7 to 11 July 2008 are reported in Table 1. Figure 1 6 gives the relative location of each spring within the Cerna Valley Using Venera spring as a datum (being the most downstream sampling location), distance upstream is recor ded as kilometers upstream from Venera spring. Traian Well is located downstream, which is indicated by a negative value for km upstream. No samples for sulfur isotope analysis were collected from Traian Well The mean temperature of the recorded springs in the Cerna Valley was 41.6 C (n=12 s=6.7 ). The max temperature recorded was 52.4 C. The mean pH for the springs was 7.4 ( minimum pH=6.6; maximum pH=8.1). The conductivity ranged from 0.375 mS to 14.23 mS with a mean value of 6.25 mS. Sulfide conentrat ion (S 2 ) ranged from below detection limit to 49 mg/L, while sulfate (SO 4 2 ) concentration ranged from 5 mg/L to 223 mg/L.

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31 Figure 16 Spring and well l ocation s

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32 Table 1 Spring data for the springs of the study area. Spring / Well Name Temp ( C ) pH TDS (ppm) Cond (mS) Total Sulide ( mg/L S 2 ) SO 4 2 (mg/L) km Upstream Traian Well 43.6 7. 2 6720 13.45 N/C N/C 0.7 Venera 42. 6 7.0 5340 14.23 39 16 0 Neptun 3 39.8 6.9 5475 10.91 3 2 113 0.2 Neptun 2 34.8 7 .0 6313 12.62 49 111 0.3 Neptun 1 + 4 44.1 7.2 4326 8.6 3 44 1 23 0.3 Diana 1 + 2 52.4 6.6 1247 2.1 0 3 7 44 0.8 Diana 3 Well 43.2 7.3 3175 6.33 41 9 0.9 Hercules I 36.3 7. 8 2375 5.6 8 N/D 125 1.6 Hercules II 7. 3 1681 3.36 N/D 124 1.6 Scorillo Well 44.0 8.1 642 1.2 8 5 76 4.5 7 Warm Springs Left 35. 2 7. 9 549 1. 30 4 76 4.9 7 Warm Spring Right 52. 4 7. 9 506 1.01 1 223 5 Crucea G. Well 30.6 7. 9 170 0.3 8 N/D 5 5.5 Note: N/C represents no sample collected and N/D represents no species detected. No samples were collected from Traian Well due to the inability to sample from a non meteoric contaminated water source. 5.2 Spring Data: Sulfur Stable Isotope Measurements Sulfur s table isotope ratio measurements for the spring sulfide and sulfate are given in Table 2 The sulfide 34 S values range from 21.9 to 24.0 with a mean value had concentrations of sulfide below detection

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33 limit of our instrument. The sulfate 34 with a mean value adequate stable isotope analysis. Table 2 Sulfur isotope values reported for the springs in the Cerna Valley. Sample 34 S Sulfide ( 34 S Sulfate ( Venera 25.3 N/P Neptun 1 + 4 23.5 35.6 Neptun 2 24.0 32.9 Neptun 3 18.3 32.5 Diana 1 + 2 23.7 27.2 Diana 3 19.0 71.3 Hercules I N/C 17.7 Hercules II N/C 16.6 Scorillo 19.5 25.1 7 Warm Springs (left) 21.9 22.9 7 Warm Springs (right) 14.1 18.7 Note: N/P represents a sample that did not yield adequate precipitate to analyze and N/C represents no sample collected. 5.3 Springs: Total Sulfur Isotopic Composition of Spring Water The total sulfur value (TSV) can be calc ulated by isotope mass balance For example, 7 Warm Springs (right), with sulfide concentration of 0.65 mg/L (1 mmol S/L) and 34 S of 34 S of (1 mmol S/L + ( ) = (74 mmol S/L + 1 mmol S/L) TSV For the example of 7 Springs (Right), the mass balance equation gives a TSV of T able 3

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34 Table 3 Total sulfur values calculated for all springs Note: N/C represents no sample collected. 5.4 DIC of Spring Waters Stable isotope ratio measurements for the total dissolved inorganic carbon (the sum of dissolved HCO 3 CO 3 2 and CO 2 ) in the springs are given in Table 4. The values range from 30.5 to 17 .0 s=7.4 ). Field measurements of each species that makes up DIC were not conducted. Spring Sulfate 34 S ( Sulfide 34 S ( S 2 ( mg/l ) SO 4 2 ( mg/l ) mM S 2 mM SO 4 2 TSV 34 S ( Neptun 3 32.5 18.3 32 113 32 38 26.0 Neptun 1 + 4 35.6 23.5 44 123 44 41 29.3 Neptun 2 32.9 24.0 49 111 49 37 27.8 Diana 3 71.3 19.0 41.2 9 41 3 22.6 Diana 1 + 2 27.2 23.7 36.9 44 37 15 24.7 Hercules I 17.7 N/C N/C 125 0 42 17.7 Hercules II 16.6 N/C N/C 124 0 41 16.6 Scorillo 25.1 19.5 4.9 76 5 25 17.8 7 Warm Springs (right) 18.7 14.1 0.7 223 1 74 18.5 7 Warm Springs (left) 22.9 21.9 4.3 76 4 25 16.4

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35 Table 4 13 C values from springs /wells Sample 13 C ( Traian 23. 8 N eptun 1 + 4 24.4 N eptun 2 24 N eptun 3 23. 7 V enera 16.6 D iana 1+2 16.3 Diana 3 Well 30.5 H ercules II 9.4 Hercules I 8. 6 Scorillo Well 14.1 7 Warm Springs (right) 12.9 7 Warm Springs (left) 10.6 Crucea G. Well 7. 9 5.5 Cave Sulfate Isotope Data Cave locations are shown in Figure 1 7 Cave maps with sample locations are presented in Appendix A. Cave minerals were collected from total of eight cav ities with 74 samples total. These samples included sulfates, phosphates, nitrates, and other unidentifiable mineral phases in the form of crusts, nodules, blocks, carpets, blisters, and other types of speleothems The sulfate samples are the main interest of t his study. Sulfur s table isotopic values for the sulfates from caves are given in Table 4 A number of data points were discarded due to poor chromatography (double peaks, extremely long tails, etc., factors that indicate incomplete combustion, combustion temperatures above the temperature of the EA reactor 1200C with V 2 O 5 ). The remaining values range from 27.7 to +20.3 in 34 S value.

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36 Figure 17 Location of caves that were sampled.

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37 Table 5 Sulfur isotope values for sulfate minerals Sample 34 S Sulfate Minerals Location 1699 27.2 Brzoni Cave 1700 26.4 Brzoni 1701 26.3 Brzoni 1702 27.7 Brzoni 1703 27.5 Brzoni 1704 27.9 Brzoni 1705 23 .0 Brzoni 1770 19.4 Diana Cave 1771 18 .0 Diana 1772 18.6 Diana 1773 19.1 Diana 1774 19.3 Diana 1775 19.5 Diana 1776 19.2 Diana 1777 18.8 Despicatura Cave 1779 18.5 Despicatura 1782 17 .0 Despicatura 1783 11.6 Despicatura 1784 14.3 Despicatura 1786 3 .0 Hercules Mining Gallery 1787 14.1 Hercules Cave 1788 19.8 Cave 1799 6.5 1802 2.6 1806 6.5 Aburi (Steam) Cave 1808 0.5 Aburi Cave 1811 6.5 Aburi Cave 1834 20.1 Neptun 2 Spring 1835 16.7 Neptun 1+4 Spring 1841 20.3 Hercules I Spring

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38 Chapter 6 DISCUSSION 6.1 Sulfur Source and Fractionation: Guadalupe Mountains vs. Cerna Valley Results from this study will be compared to t he isotopic composition of cave deposits in the Guadalupe Mountains in order to determine the similarities and/or differences between the process (and completeness) of the reduction of sulfate in the Guadalupe Mountains and the Cerna Valley. According to Hill (1995), the sulfates in caves of the Guadalupe Mountains are 34 S depleted values compared to the anhydrite of the Castile 34 S = or more This formation is the only source of sulfur in the region, which means the total disso lved sulfur or TSV must be equal to the isotopic value of this sulfate deposit ( ). Oil from the Delaware B asin provides the energy source for the bacterial reduction of dissolved sulfate from the Castile Formation. BSR occurs by the following equation at temperatures less than 80 C (Hill, 1995). Ca 2+ + 2SO 4 2 + Hydrocarbons +2H + = 2H 2 S + CaCO 3 + 3H 2 O + CO 2 A 34 S/ 32 S kinetic fractionation factor causes the H 2 34 S values as low as ate in the Castile Formation). This kinetic fractionation occurs during incomplete BSR of sulfate to sulfide during energy limited conditions, and the sulfide and instantaneous sulfate produced are considered to be in instantaneous equilibrium. Hill (1995) emphasizes that this reaction and not later oxidation reactions is ultimately responsible

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39 for the 34 S depleted gypsum and sulfur in the caves of the Guadalupe Mountains. This is because the latter oxidation reactions are considered to be quantitative; however, if a lack of oxygen or an oxidizing agent were present oxidation would not be quantitative leaving the resulting sulfates 34 S fractionated. Similar to the sulfate deposits of the caves in the Guadalupe Mountains, some of sul fate deposits in the caves of the Cerna Valley have highly 34 S depleted isotopic values. Unlike the Guadalupe Sulfates, Cerna Valley contains cave sulfates which are 34 S enriched. These values represent a difference in the completeness of sulfate reduction and show that SAS is not limited to a single range of sulfur isotopic values The fundamental differences between sulfate reduction the Guadalupe Mountains and the Cerna Valley are the sulfate source ( controlling the amount of sulfate available), the typ e and amount of hydrocarbons present (representing potential energy for reduction), thermal gradients, geologic structure, and the hydrology. These differences all contribute to sulfate reduction 34 S in the Cerna Valley being drastically different than in the Guadalupe Mountains. 6.2 Sulfur Source Total dissolved sulfur concentrations are relatively low compared to the Guadalupe Mountains lacking a sulfate source. Sulfate can be derived from a number of sources before reaching the aquifer which gives a unique sulfur isotopic value for source determination In order to determine the source of the sulfur in Cerna waters (sedimentary pyrite, sedimentary evaporites, igneous sulfur, or anthrop ogenic sulfur), mass balance is used to 34 S value (TSV). This was calculated using isotope

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40 mass balance similar to Rajchel et al. (2002) where TSV is calculated based on the isotopic sum of its parts Since stable iso topes cannot be created or destroyed, in a closed system TSV is indicative of its source. Figure 18 1) Generalized geologic cross section of the Cerna Valley with spring locations marked. 2) A model showing the flow of water and heat in the aquifer complexes and the granite sill In the springs of the Cerna Valley, t he average total sulfur 34 S is approximately 19 ). According to Sharp ( 2007 ) the 34 S of sulfate in the oceans means that the source of the sulfur must be of marine sedimentary origin ( Krouse and Mayer, 2000 ).

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41 Figure 19 Plot of total sulfur isotopic signature (Top) Generalized cross section related to spring/well data (Bottom). Although the source of the Cerna sulfates is marine sedimentary deposits t he geology of the Cerna Valley and its surrounding area lacks the massive bedded gypsum deposits and evaporite deposits producing dome structures that typify the Miocene of the Southern Carpathians which might otherwise be a source of sulfur in the Southern Carpathians ( 1980 ). This clearly identified source can be interpreted as one

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42 of two possibilities : 1) th ere are marine evaporite minerals within the sedimentary layers which have been overlooked or are present as impurities in the limestones or 2) water first encounters marine evaporite layer s outside the Cerna Valley Basin and then migrates into the aquifer s of the basin. The first case is more likely due to the relatively low concentration of sulfur dissolved in the spring s/wells It is highly probable that small amounts of marine sulfate mineral s exist in the limestone of the Cerna Valley Region This is s upported by the cold water well upstream (Crucea G. Well) which has no influence from thermomineral sources, containing 5mg/L sulfate (Table 1), indicated the sulfate is derived from impurities in the limestone. 6.3 Development of a theoretical model of Ra yleigh d istillation In order to 34 S during sulfate reduction, a theoretical model of Rayleigh distillation is developed (Figure 20) Rayleigh distillation occurs when a shrinking reservoir (F represents fraction reservoir remaining) with an initial isotopic composition, R i produces a product (such as sulfate reduction producing sulfide) that differs in isotopic composition from the res the reservoir with a different isotopic composition, R x Knowing that t he total stable isotopic composition of a closed chemical system must remain constant a mass and energy balance m odel is created During sulfate reduction, the isotopic composition of the sulfide depends on 34 S, (2) the amount of sulfate that has been reduced and (3) the equilibrium fractionation factor between the two phases.

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43 Figure 20 Theoretical model of sulfate reduction. The initial conditions of the model are a constant 34 n apparent fractionation, of Zone 1 represents energy limited conditions while zone 2 represents sulfate limited conditions. Initially, a small amount of total sulfate is reduced to form sulfide. This produces a sulfide with a negative 34 S value (Figure 20 zone 1) Sulfate reductio n will continue to occur until available energy is used The isotopic composition of the instantaneous sulfide produced follows the trend:

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44 where R x is the ratio of 34 S/ 32 S of the sulfide at some fraction sulfate remaining, F, R i is the ratio of 34 S/ 32 S of the initial total sulfate ; The total sulfide curve (representing bulk sulfide in the system) is calculated by integrating the instantaneous sulfide curve. S ulfate becomes increasingly 34 S enri ched and instantaneous sulfide follows the same trend, only offset by a constant fractionation factor. As most of the sulfate is reduced, the total sulfide takes on the isotopic composition of the initial source, until reaching the scenario of quantitative sulfate reduc tion (Figure 20 zone 2) T he sulfur isotope fractionation described by this model of Rayleigh Distillation can occur abiotically (thermochemical sulfate reduction, TSR) or may be bacterially mediated (bacterial sulfate reduction, BSR). TSR and BSR occur at two mutually exclusive temperature regimes, with TSR occurring at high temperature environments (60 80 C) and BSR occurring at low temperature (<60 80 C) environments (Machel et al., 1995). Depending on the concentration o f dissolved sulfate and the amount and type of reducing agent, sulfate reduction can proceed to different levels of completeness but will always be limited by either sulfate or by energy In systems where the concentration of sulfate exceeds available energy the resulting sulfide produced will be extremely 34 S depleted (Figure 20 zone 1) Alternatively, a system can be sulfate limited when the concentration of sulfate is relatively low compared to the concentration of reducing agent, or if an extremely strong reducing agent exists. Complete or near complete sulfate reduction can be achieved, leaving the sulfide produced with an isotopi c value of the initial sulfate (Figure 20 zone 2).

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45 The oxidation reaction, which provides energy for reduction, can also be a factor Certain reducing agents (oxidation reactions) such as organic matter or methane, have more potential energy and therefo re more reducing potential than weaker ones such as Fe 2+ or NH 4 + (Figure 10 ) With less reducing potential, sulfate reduction proceeds to an energy limited scenario that leaves the sulfide produced with a 34 S depleted isotopic value (Figure 20 zone 1) Taking this as a model, we can use the TSV in a closed system to track the sulfur source, and use the isotopic composition of the sulfur containing phases (including sulfate minerals) to track the reaction progress of sulfate reduction, oxidation, and reac tion with carbonates to precipitate sulfate minerals 6.4 Cerna Springs: Dissolved Sulfur Species When examining the dissolved sulfur species of spring waters in the Cerna region (Figure 21 ) one can see that sulfate and sulfide follow different trends. Sulfa te concentrations have a larger range of values and tend to scatter downstream. These relatively higher concentrations likely indicate additional sources of water, high in sulfate

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46 Figure 21 Plot showing the change in concentration in sulfide and sulfate moving upstream (Top) The large gap between the two populations of springs represents the granite sill (Bottom) Sulfide concentrations generally increase downstream, and the mixing waters do not change the concentrat ion of sulfide, indicating that sulfate reduction has occurred. Sulfide concentrations measured in the Hercules group of springs were below the detection limit of the Hach Spectrophotometer It has been well documented that these

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47 springs receive a large am ount of their input from meteoric waters, causing fluctuations in discharge, temperature, and dissolved ions ( ). For the purposes of our model, the Hercules group will represent completely oxidized springs, giving no information about reaction progress but useful for total dissolved sulfur interpretations. If sulfate in a closed system is quantitatively reduced to sulfide, the sulfur isotopic composition of the resulting sulfide depends solely on the initial sulfur isotopic compositi on of the source sulfate. Likewise, in a closed chemical system at equilibrium, the stable isotopic composition of the total sulfur in the chemical system remains constant. W hen examining the isotope values for the diss olved sulfur species, t wo population s of water chemistries become evident (Figure 22 ). Spring s (located 4.5 5km, upstream of the granite sill) upstream show a large fractionation factor between sulfate and sulfide (average = Streams ( located 0 1km downstream of the granite sill) upstream show smaller fractionation factors between the two species of sulfur (average = Diana 3 located 0.9 km upstream is the only exception to this second population ( apparent

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48 Figure 22 Plot showing the isotopic composition of the dissolved sulfur species (Top) and how this relates to the underlying geology (Bottom). Partial reduction of dissolved sulfate upstream produces a low concentration of highly 34 S fractionated dissolved sulfid e that is as 34 S depleted as We can interpret this as the energy limited conditions described by our theoretical model. This energy source can be attributed to low concentrations of methane in the upstream springs ( Figure 22 ).

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49 A second group of water chemistries exists after waters move through the Granite Sill after which point it has been shown they encounter methane at high temperatures (70 100 C; et al., 2008). With an abundant supply of methane as an energy source in this group of waters sulfate reduction is able to approach completion This is supported by 34 S values of dissolved sulfur species at Diana 3 ( Figure 22 ). Here, the dissolved sulfate reaches its 34 S values of the sulfa te are extremely 34 S 34 S values of dissolved sulfide take on the approximate isotopic signature of the initial dissolved sulfate. As mentioned above, t he large fractionation factor betwee n sulfide and sulfate at Diana 3 occurs at a point where dissolved methane concentration increases dramatically ( Figure 2 3 ) The presence of methane could be due to the thermal maturation of coal deposit s in Mehadia (to the west) producing methane from wh ich water containing dissolved methane migrates in to th e graben along transversal faults ( 2008).

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50 Figure 23 Plot showing percent composition of methane in spring water (data from 13 C of DIC of spring water was determined in order to confirm this flux of carbon 13 C values of methane are invariably 13 C of the parent material, in this case organic matter from coal deposits Assuming that there are two main DIC sources in the Cerna Valley (dissolved methane and dissolution of limestone) an influx of methane would be apparent by a more negative 13 C value. Figure 2 4 shows 13 C of DIC around 10 Carbon from (Sharp, 2007) are 13 C of DIC is most 13 C depleted at Diana 3 spring ( This indicates that the methane mixed is more (<

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51 13 C depleted. Generally, t he more negative 13 C values of the downstream springs indicate a greater percentage of the DIC is derived from methane oxidation, while upstream DIC is controlled mainly by the dissolution and aerobic respiration. Figure 24 13 C of DIC in spring waters. 6.5 TDS and its relationship to total dissolved sulfur Downstream of the granite sill, a n interesting observation of the TDS data is a general trend of correlation of mineralization to the total dissolved sulfur concentration When examining a plot of Total Sulfur (dissolved) vs. TDS (Total Dissolved Solids) (Figure 2 5 ) t w o populations 34 S values (Figure 17) emerge The l ower springs have a less steep slope when compared to the upper springs.

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52 The correlation of TDS and total sulfur shows that there is 1) a shift in the dissolved specie s controlling TDS or 2) progressive mixing with another source of water containing a different proportion of dissolved sulfur and other ions. Figure 25 Plot showing two populations of springs based on TDS and total sulfur. To support the pattern in TDS indicating mixing with another source of water, a trend emerges downstream of Diana 3 where the TSV increases from ~ ~ In this series of springs, t otal sulfur concentration increases gr adually, which indicates an addition of (in this case) 34 S enriched sulfur (more 34 S enriched than 22 Figure 2 6 ) from an unidentified source

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53 Figure 26 Plot showing the gradual 34 S enrichment from Diana 3 downstream. In order to determine the isotopic composition of the mixed component, a mixing diagram is used (Figure 2 7 ). This is achieved by plotting the isotopic value versus the inverse of the concentration of total sulfur. As t he inverse of the total sulfur concentration approaches zero, i.e., the concentration of the mixing component approaches infinity, the 34 S of the mixing component can be determined A regression line and equation calculates t he y intercept to be the appro ximate v alue of the mixed component (~37

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54 Figure 27 A mixing diagram showing the isotopic value of the mixed component. When determining the source of t he mixed component two sources can be hypothesized First, waters carrying methane along transverse faults from adjacent regions could be reduced by BSR due to lower temperatures away from the thermal source. These r educed su lfates would be sligh tly enriched. In order to prove this, wa ter samples from along this fault would have to be collected and analyzed for dissolved sulfur species isotopic values Secondly, this may reflect mixing within the aquifer itself. If a small concentration of highly reduced sulfate remaining in the aquife r mixes with ~ sulfate, it may account for the 34 S enrichment.

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55 6.6 Caves: Sulfur Isotopes Once a solution oversaturated with dissolved sulfide reaches a cave passage, H 2 S gas begins to effervesce into the cave atmosphere. Oxidation of H 2 S gas occurs under these conditions (assuming O 2 in the atmosphere) Due to quantitative conversion of the S in H 2 S to produce S in sulfuric acid (i.e. no H 2 S escapes) the sulfur isotopic composition of the sulfuric acid produced during oxidation must reflect the isotopic composition of H 2 S from which it is produced. If H 2 S gas escape s this would give the sulfuric acid a 34 S enriched value (since diffusion of H 2 34 S is slower than H 2 32 S). In addition, the reaction of sulfuric acid produce s a suite of sulfate minerals which does not have a fractionation factor if it is a quantitative conversion of the sulfur in sulfuric acid to the sulfur in sulfate minerals Brzoni Cave (gypsum) Sulfate samples from Brzoni Cave (located farthest upstream of any passage or spring investigated ) are extremely depleted ( 2 3 to 28 Since these deposits reflect the 34 S of the original sulfide produced via sulfate reduction, these minerals indicate incomplete sulfate reduction due to energy limited conditions (Figure 20 ) Brzoni Cav e represents a fossil cave (with respect to thermal activity), so these expansive sulfate deposits represent a migration or shrinking of the thermal activity in the Cerna Valley S ulfate minerals in S litrari C ave had 34 S values that range d from 19.8 to A number of sulfate minerals were identified in association with guano and clay deposits. Sulfur isotopes of these minerals reflect different steps in the completeness of the reduction of sulfate. This may be attributed to separate inputs of thermal waters (that

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56 contain different steps in sulfate reduction) to distant passages in the laterally extensive cave (Figure 2 8 ) Positive values represent more complete sulfate reduction while negative values which occur in a differe nt passage than 34 S enriched samples, represent partial reduction. The cave is separated into two main zones shown by the northwestern passage of the cave are 34 S 34 S = 19.8 ), while sulfates from the entrance passage are slightly 34 S depleted to slightly 34 S enriched ( M inerals from the northwestern passage repr esent partial sulfate reduction, while minerals from the entrance passage represent partial oxidation of sulfate. The 34 S enriched minerals of t he entrance passage occur in the presence of guano and clay deposits 34 S values is not due to separate inputs of thermal waters, but to the partial oxidation due to limited oxygen in the guano and clay deposits (Figure 29) Thermal waters would saturate the guano clay profile, and decay of organic matter would sequester most of the oxygen. Under these conditions, 34 S would be preferentially deposited in sulfate minerals resulting in a relatively 34 S enriched dep osit. In figure 29, Rayleigh distillation is shown for the oxidation of sulfide. If this reaction is quantitative, then the resulting final sulfate will have the isotopic composition of the original sulfide ( Figure 2 9). If oxidation is incomplet e (to any degree), sulfate produced is 34 S enriched. This could occur if oxygen (or other oxidizing agents) is limited or if sulfide (H 2 S (g) ) escapes from the cave atmosphere.

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57 Figure 28 Plan map of the Great Cave. Picture in the center of the figure shows gypsum crusts from this part of the cave. Picture at the bottom center of the figure shows unusual suits of sulfate minerals which have reacted with guano and/or clay. Picture in the bottom right shows the entrance to the cave.

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58 Figure 29 Rayleigh Distillation Model of sulfide oxidation. The initial conditions of the model are a constant temperature of 100C, a sulfide 34 S = 22.7 Adam and Aburi Caves Adam and Aburi caves represent caves on high cliff faces affected by thermal activity. Notably, Adam Shaft is host to a large community of bats, impressive accumulations of bat guano, and vigorous steam emissions. Sulfate samples from Adam did not g ive peak shapes ideal for calculating an isotopic ratio (possibly due to column problems in the EA coupled to the IRMS low sulfur content of certain sulfate minerals, higher combustion temperature of certain sulfate species ). Sulfate samples collected from Aburi (Steam) Cave showed slight 34 S enrichment (0.5 medial amount of sulfate reduction or incomplete oxidation described above

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59 Diana and Desp ic tur Caves Caves farthest downstream, especially Despic tur and Diana have enriched sulfur isotope values (+11.6 which correspond well to the sulfide values of nearby springs. Despic tur 34 S values ( + 11.6 + an increase (compared to previously discussed caves) in the completeness of sulfate reduction. Diana Cave (from which Diana 1+2 spring flows from) has a n average sulfate 34 S = +19 .0 which is similar to the isotopic composition of the sulfide fr om Diana 1+2 spring ( 23.7 These sulfate deposits represent the most complete sulfate 34 S is controlled by the 34 S value of sulfide produced by sulfate reduction. This sulfide is quantitatively oxi dized to form sulfuric acid, and the sulfuric acid reacts to precipitate the suite of sulfate minerals. The 34 S values from the entire Cerna Valley show that the resulting cave sulfate isotope values from SAS not only depend on the source of the sulfur, but also depend on the completeness of sulfate reductio n related to the amount of energy and sulfate available for sulfate reduction. The completeness in sulfate reduction controls the isotopic composition of all phases (from sulfide productio n to sulfate precipitation) of sulfur bearing substances and may be variable from one cave setting to another

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60 Chapter 7 CONCLUSIONS 34 S of various sulfur species and phases to identify sulfur source and subsequent reactions. Total sulfur wa s computed using a mass balance equation. This value indicates that the dissolved sulfur species are derived from a marine evaporite source. Two hypotheses exist; however, the most plausible is the presence of minor constituents in the limestones of the Ce rna Region This is supported by low concentrations of total dissolved sulfur. As sulfate waters enter the Northern Aquifer Complex of the Cerna (upstream) a relatively small fraction of the sulfate is reduced to sulfide ( r eduction is limited by the amou nt of energy present) This is supported by relatively low concentrations of dissolved sulfide in the springs upstream (Scorillo Well, 7 Warm Springs Left and 7 Warm Springs Right) and by the isotopic value of the sulfides (extremely 34 S depleted) and sulf ates ( 34 S enriched) As waters move through the Granite Sill and encounter methane at high temperatures (70 100 C ) an increase in sulfate reduction occurs The 13 C values of d issolved inorganic carbon samples confirm an influx of carbon that has extremely 13 C values It is hypothesized that these negative values are due to waters (carrying dissolved methane) that move into the Cerna aquifers along transverse faults from adjacent regions. With an abundant supply of methane in th ese waters sulfate reduction (which is now limited by the amount of sulfate) is able to approach completion.

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61 This occurs by the time the waters reach Diana 3 Here the dissolved sulfate reaches its 34 S values of the sulfate are extr emely 34 S enriched, dissolved 34 S values of dissolved sulfide take on the isotopic signature of the initial dissolved sulfate. Springs downstream of Diana 3 (Diana 1+2 and Neptun Group) show an addition of sulfate 34 S of total sulfur. Several hypotheses can explain this mixing; however, another set of thorough sampling from adjacent valleys is required to determine which hypothesis is most plausible. Once waters enter the cave s of the Cerna Val ley sulfide (either gas or dissolved) is oxidized to form sulfuric acid. If oxidation is quantitative, t his sulfuric acid will have 34 S of the sulfide This acid then reacts to form the suite of sulfate minerals 34 S values of the initial sulfate reduction. Cave sulfate deposits were collected and analyzed in order to understand the mechanism of deposition. Cave passages upstream, where thermal activity is lacking today (such as Brzoni and S litrari caves) possess large am ounts of 34 S depleted sulfate deposits, which indicate energy limited conditions of sulfate reduction similar to the process occurring in the upstream springs. Cave passages downstream (Diana and Despic tur ) have sulfate deposits with positive 34 S valu es. These deposits represent near complete sulfate reduction and 34 S value near the paren 34 S values may represent incomplete oxidation during the formation of sulfuric acid from sulfide,

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62 leaving these deposits slightly 34 S enriched with respect to the sulfide from which it was formed. SAS has been shown in other karst regions of the world to produce sulfate deposits with 34 S isotopic signatures. The Cerna 34 S values show that the resulting cave sulfate isotope values from SAS depend not only on the source of the sulfur, but also on the completeness of sulfate reduction and sulfide oxidation These reactions control the isotopic composition of all phases (from sulfide production to sulfate precipitation) of sulfur bearing substances. As shown by the caves of the Ce rna 34 S values is not indicative of SAS

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63 REFERENCES Assayag, N., Rive, K., Ader, M., Jezequel, D., Agrinier, P., 2006, Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water sa mples, Rapid Communications in Mass Spectrometry 20: 2243 2251. Bojar, A.V., Neubauer, F. & Fritz, H., 1998 Cretaceous to Cenozoic thermal evolution of the Southwestern South Carpathians: evidence from fission track thermochronology. Tectonophysics 297: 229 249. Cristescu, I., 1978, Ed. Sport Turism, Bucuresti, 176 pages Davis, D., 2000, Extraordinary features of Lechuguilla Cave, Guadalupe Mountains, New Mexico, Journal of Cave and Karst Studies, 62(2): 147 157. Diaconu, G., 1987, thermomineral Theoretical and Applied Karstology 3: 109 116. Egemeier, S.J., 1981, Cavern development by thermal waters: National Speleological Society Bulletin 43: 31 51. Gal denzi, S. and Maruoka, T., 2003, Gypsum deposits in the Frasassi Caves, Central Italy. Journal of Cave and Karst Studies, 65 (2): 111 125. Galdenzi, S., and Menichetti, M., 1995, Occurrence of hypogenic cav es in a karst region: Examples from central Italy. Environmental Geology 26: 39 47. Grassineau, N., Mattey, D ., and Lowry, D., 2001, Sulfur i sotope a nalysis of s ulfide and s ulfate m inerals by c ontinuous f low i sotope r atio m ass s pectrometry. Analytical Chemistry 73: 220 225. Groot, P. Ed. 2004, Handbook of stable isotope analytical techniques First edition, Elsevier Publishing, 1258 pages. Hach Company. 2002. The Handbook for DR/2400 Portable Spectrophotometer. Loveland, Colorado. Hill, C.A. 1980, Speleogenesis of Carlsbad Caverns and other caves in the Guadalupe Mountains. Unpubl. Report to the National Park Service, Forest Service, and Bureau of Land Management 201 pages

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64 Hill, C.A. 1987, Speleogenesis of Carlsbad Caverns and other caves i n the Guadalupe Mountains, New Mexico and Texas New Mexico Bureau of Mines and Mineral Resources Bull etin 117, pp. 150. Hill, C.A. 1990, Sulfuric acid speleogenesis of Carlsbad Cavern and its relationship to hydrocarbons, Delaware Basin, New Mexico an d Texas. American Association of Petroleum Geologists Bulletin 74: 1685 1694. Hill, C.A., 1995, Sulfur Redox Reactions: Hydrocarbons, native sulfur, Mississippi Valley type deposits, and sulfuric a cid karst in the Delaware Basin New Mexico and Texas. Environmental Geology 25: 16 23. Hill, C.A. and Forti, P., 1997, Cave Minerals of the World Second Edition, National Speleological Society, 463 pages. Hose, L.D., Palmer, A.N., Palmer, M.V., Northup, D.E., Boston, P.J., and DuChene, H .R., 2000, Microbiology and geochemistry in a hydrogen sulphide rich karst environment. Chemical Geology, 169: 399 423. Hubbard, D.A., Herman, J.S., and Bell P.E., 1990, Speleogenesis in a travertine scarp: Observations of sulfide oxidation in the subsurface in Travertine marl: Stream deposits in Virginia ed. by Herman, J.S., and Hubbard, D.A., Charlottesville, Virginia. Department of Mines, Minerals and Energy, Division of Mineral Resources, p p 177 184. Jagnow, D.H., Hill, C.A., Davis, D.G., DuChene, H.R., Cunningham, K.I., Northup, D.E., and Queen, J.M., 2000, History of the sulfuric acid theory of speleogenesis in the Guadalupe Mountains, New Mexico. Journal of Cave and Karst Studies 62: 54 59. Kirschvink, J.L. and Koppe, R.E., 2008, Palaeoproterozoic ice houses and the evolution of oxygen mediating enzymes: the case for a late origin of photosystem II. Philosophical Transactions of t he Royal Society 363: 2755 2765. Klimchouk, A.B., 2007, Hypogene Speleogenesis : Hydrogeological and Morphogenetic Persp ective. Special Paper no.1, National cave and Karst Research Institute, Carlsbad, NM, 106 pages. Krutner, H. G. and Krstic, B., 2002 Alpine and pre Alpine structural units within the Southern Carpathians and Eastern Balkanides. Proceedings 27 th Cong ress of the Carpatho Balkan Geological Assoc ation Bratislava Krouse, H.R. and B. Mayer 2000 Sulfur and oxygen isotopes in sulfate, in Environmental Tracers in Subsurface Hydrology, ed. by P. Cook and A.L. Herczeg, Kluwer Academic Publishers, Norwell, MA.

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65 Ligeois, J.P., Berza, T., Tatu, M. & Duchesne, J.C., 1996: The Neoproterozoic Pan African basement from the Alp ine lower Danubian nappe system (South Carpathians, Romania ). Precambrian Research 80: 281 301. Machel, H., Krous, H. and Sassen, R., 1995, Products and distinguishing criteria of bacterial and t hermochemical sulfate reduction. Applied Geochemistry, 10: 373 389. Marin, C., 1984, Hydrochemical considerations in the lower Cerna river basin. Theoretical and Applied Karstology 1 : 173 18 2 Menichetti M., Chirenco M.I., Ona c B. P., Bottrell S., 2008, Depositi di gesso nelle grotte del Monte Cucco e della Gola di Frasassi Considerazioni sulla speleogenesi. s. II, vol. XXI: 484 501. LIV, pp. 137. Negrea, S. and Negrea, A., 2002, Cernei Editura Timpul Resita, 255 pages. Northup, D.E., Ca rr, D.L., Crocker, M.T., Hawkins, L.K., Leonard, P., and Welbourne, W.C., 1995, Biological investigations in Lechuguilla Cave. National Speleological Society Bulletin, 56(2): 54 63. Onac, B.P., Hess, J., and White, W.B., 2007, The relationship between the mineral composition of speleothems and mineralization of breccia pipes: evidence from Corkscrew Cave, Arizona, USA. Canadian Mineralogist, 45: 1177 1188. V., Veres, D., Lascu, C., 2009, The relationship etween cave minerals and hypogene activity along the Cerna Valley (SW Romania). Acta Carsologica (accepted). Oncescu, N. RPR. Natura, 1: 15 27. Palmer, A.N., 2007, Cave Geology Dayton, OH, Cave Books, 454 pages. Papiu, V. C. 1960, Studii si Cercatari de Geol ogie Pascu, M., 1968, ul termal Hercules I. 14 : 33 37. Popescu 1921, tude gologique sur les sources minrales des bains d Hercule. Annuaire Minier Roum ain

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66 1992, New Data on the Hercule Thermal Aquifer Obtained by Temperature Theoretical and Applied Karstology 5: 127 138. ., 1984, Hercules thermomineral spring. Hydrogeological and hydrochemical considerations. Theoretical and Applied Karstology 1: 183 19 4 Marin, M., 2008 Thermomineral waters from the Cerna Valley Basin (Romania). Bolyai, Geologia 53 (2): 41 54. 1972, 296 pages. Rajchel, L., Rajchel, J., Szaran, J., and Halas, S., 2002, Sulfur i sotopic c omposition o f H 2 S and SO 4 2 from the mineral springs in the Polish Carpathians. Isotopes in En vironmental Health Studies 38: 277 284 Robinson, R., 1973, Sulfur isotopic equilibrium during sulfur hydrolysis at high temperature. Earth and Planetary Sci ence Letters 18 : 443 450. Rosenfeld, W.D., and Silverman, S.R., 1959, Carbon isotope fractionation in bacterial production of methande. Science 130: 1658 1659. Srbu, S.M., Kan e, T.C., and Kinkle, B.K., 1996, A chemoautotrophically based cave ecosystem: Science 272: 1953 1955. Sharp, Z., 2007, Principles of stable isotope geochemistry Upper Saddle River, NJ, Pearson Education, Inc, 360 pages. Simion, G. 1987, Geological project concerning the redevelopmentof the thermomineral Seve rin County. IPGG Archive, Bucharest. Simion, G., Po The dynamics of underground waters Annales de la Societe Geol ogique de Belg ique 108: 245 249. Torres, M.E., Mix, A.C., and Rugh, W.D., 2005, Precise 13 C analysis of dissolved inorganic carbon in natural waters using automated headspace sampling and continuous flow mass spectrometry, Limnology and Oceanography: Methods 3: 349 360. Vasilescu, Gh. and Liteanu, E., 1973, Noi surse de ape termominerale din zona Studii de Hidrog eologie,

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67 Appendices

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68 Ap pendix A: Cave Maps and Sample Locations

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69 Ap pendix A : ( C ontinu ed)

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70 A p pendix A : ( C ontinued)

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71 Ap pendix A : ( C ontinued)

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72 Ap pendix A : ( C ontinued)

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Using 34-S as a tracer of dissolved sulfur species from springs to cave sulfate deposits in the Cerna Valley, Romania
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ABSTRACT: Baile Herculane, located in southwestern Romania, is a unique city that exploits its thermal waters. The geology consists of a granitic basement covered by 200 meters of limestone, marl, and flysch deposits. Extreme faulting carries heat ascending from the mantle, which intercepts percolating meteoric waters. Local springs have high concentrations of dissolved sulfide gas (HS) and dissolved sulfate (SO).These dissolved species indicate the progression of sulfate reduction in the aquifer. Water samples were collected in polyethylene syringes to prevent oxidation of sulfide. Then, sulfide and sulfate were quantitatively reacted for stable isotope analysis. Total sulfur isotopic composition was calculated to determine the source of the dissolved sulfur. The source of the sulfur is a sulfate of marine origin ([approx. equal]20%, which I found to come from impurities in the limestone since the Cerna Valley does not possess marine evaporites. The limestones of the Cerna Valley are host to a number of caves, which possess relatively large deposits of sulfates and exotic morphologic features that suggest speleogenesis by sulfuric acid. S of the sulfates relates to sulfide isotopic values from the springs, showing that the dissolved sulfide (upon oxidation) forms sulfuric acid s that reacts with limestone to produce sulfate minerals. A wide range of cave sulfate S values exist indicating that isotopic values of these deposits depend on several factors such as sulfur source, extent of sulfate reduction, and completeness of sulfide oxidation. This also implies that a single, narrow range of sulfur isotopic values does not represent sulfuric acid speleogenesis.
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