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Seawater Sulfur Isotope Fluctuations in the Cretaceous
Abstract
The exogenic sulfur cycle is tightly coupled with the carbon and oxygen cycles, and therefore a central component of Earth's
biogeochemistry. Here we present a high-resolution record of the sulfur isotopic composition of seawater sulfate for the Cretaceous.
The general enrichment of isotopically light sulfur that prevailed during the Cretaceous may have been due to increased volcanic
and hydrothermal activity. Two excursions toward isotopically lighter sulfur represent periods of lower rates of pyrite burial,
implying a shift in the location of organic carbon burial to terrestrial or open-ocean settings. The concurrent changes in
seawater sulfur and inorganic carbon isotopic compositions imply short-term variability in atmospheric oxygen partial pressure.
... Sedimentary biological pyrite sulfur isotope ratios (δ 34 S py ) have been extensively employed to study ancient oceanic and basinal sulfur cycles (e.g., Paytan et al., 2004;Hurtgen et al., 2005Hurtgen et al., , 2009; Thompson and Kah, 2012;Song et al., 2014;Algeo et al., 2015;Fike et al., 2015;Kah et al., 2016;Shi et al., 2018;Stebbins et al., 2019a;Thomazo et al., 2019). Variations in bulk sedimentary δ 34 S py signatures have been plausibly linked to oceanic sulfur cycle perturbations (e.g., Hurtgen et al., 2009;Gill et al., 2011;Halevy et al., 2012;Algeo et al., 2015;Sim et al., 2015;Schobben et al., 2017;Stebbins et al., 2019aStebbins et al., , 2019bYoung et al., 2020) or basin isolation effect (Gomes and Hurtgen, 2013;Kurzweil et al., 2015;Paiste et al., 2020). ...
... Seawater sulfate can precipitate as sulfate minerals (e.g., gypsum, anhydrite, and barite) or be incorporated into the carbonate lattice as carbonate-associated sulfate (CAS) without significant Sisotope fractionations (Thode and Monster, 1965;Takano, 1985;Raab and Spiro, 1991;Staudt et al., 1994;Kampschulte and Strauss, 2004;Barkan et al., 2020;Toyama et al., 2020). Therefore, marine sulfate evaporite and carbonate phases have been extensively utilized to estimate ancient seawater δ 34 S (Claypool et al., 1980;Paytan and Kastner, 1998;Kah et al., 2004;Kampschulte and Strauss, 2004;Paytan et al., 2004;Mazumdar and Strauss, 2006;Gill et al., 2008;Rennie et al., 2018;Toyama et al., 2020). However, the δ 34 S of CAS in bulk carbonates, such as limestone, is susceptible to diagenesis (e.g., meteoric, early, and burial digenesis) and can be influenced by depositional environments (e. g., Rennie and Turchyn, 2014;Present et al., 2015Present et al., , 2019Richardson et al., 2019aRichardson et al., , 2019bRichardson et al., , 2021Toyama et al., 2020). ...
... Our observations reveal statistically significant inverse correlations between the δ 34 S py values and the abundance of terrigenous elements (Al, Th, and ∑ REE, Fig. 7a-c). Given that the terrestrial input has been recognized as the primary source for oceanic sulfate pool (e.g., Paytan et al., 2004;Canfield and Farquhar, 2009), and that the sulfate level plays an important factor affecting the Δ 34 S (δ 34 S SW -δ 34 S py ) in semi-restricted systems (e.g., Algeo et al., 2015;Gomes and Hurtgen, 2015;Kurzweil et al., 2015;Paiste et al., 2020), these inverse correlations strongly suggest that riverine fluxes may have played a role in regulating the local seawater sulfate levels, thereby influencing the pyrite δ 34 S signatures. A decrease in terrestrial sulfate input could have facilitated porewater sulfate distillation, leading to the suppression of Δ 34 S and precipitation of 34 S-enricehd pyrite. ...
The Green Point Formation in western Newfoundland, GSSP of the Cambrian-Ordovician (Є-O) boundary, is dominated by slope rhythmites of alternating lime mudstone and shale interbeds. This formation was deposited in a semi-restricted basin with varying connectivity to the open ocean. In the current study, we investigate textures and bulk δ34S signatures of pyrite (δ34Spy) in the shale to better understand factors influencing the sedimentary δ34Spy fluctuation. Petrographic and SEM examinations reveal two major types of pyrite: (1) framboidal pyrite and (2) anhedral to euhedral pyrite. The latter is further categorized into two subtypes: type 2a anhedral to subhedral pyrite characterized by relict framboidal textures and larger sizes (~10 to 300 μm), and type 2b smaller (typically <10 μm) subhedral to euhedral pyrite. Type 1 pyrite was precipitated near the sediment-water interface (SWI), whereas type 2b pyrite was formed in sediments below the SWI with limited access to the overlying seawater sulfate. Type 2a pyrite was evolved from framboids during early and burial diagenesis. The bulk δ34Spy values, marked by a significant scatter (1σ =10.62‰), range broadly from −17.6 to +22.4‰ (VCDT) and exhibit a pronounced positive excursion of ~20‰ near the Є-O boundary. The abundance of type 2b pyrite generally mimics the changes in δ34Spy, suggesting that the substantial δ34Spy dispersion could be partially attributed to differing proportions of type 2b pyrite within the samples. Moreover, notable negative correlations exist between the δ34Spy values and the abundances of Al, Th, ∑REE, and Fe, indicating that riverine fluxes might have influenced the Δ34Sseawater ‒ pyrite by modulating the regional seawater sulfate and iron reservoir sizes. Therefore, rather than being indicative of oceanic redox oscillations, the positive δ34Spy excursion of ~20‰ of this interval was probably driven by decreased sulfate and iron levels in the local waterbody. The decline in terrestrial input during this δ34Spy shift might have also contributed to a negative δ13Ccarb excursion by reducing nutrient supply and inhibiting primary productivity. Collectively, the bulk sedimentary δ34Spy variability recorded by the Green Point shale may be attributed to a combination of changes in regional terrigenous input and varying amounts of pyrite formed at different diagenetic stages within the samples. The general opposing trends between the δ34Spy signals and the abundances of Al, Th, ∑REE, and Fe, however, imply that fluctuations in riverine influxes might exert a stronger influence on the major δ34Spy trend. These findings suggest that bulk sedimentary δ34Spy variations alone may not be reliable evidence for perturbations of the global sulfur cycle.
... Most of the sulfide produced by MSR is reoxidized to sulfate at redox boundaries, while the rest either precipitates as pyrite when it reacts with reactive iron, or is present as dissolved sulfide within anoxic waters, evolving toward a euxinic ocean (Meyer and Kump, 2008;Yao et al., 2020). Due to the large S-isotope fractionation associated with MSR (up to 72‰; Sim et al., 2011;Wortmann et al., 2001), perturbations of sulfur cycling related to redox changes could be traced by the sulfur-isotope ratio (δ 34 S) of marine sulfate (Adams et al., 2010;Ohkouchi et al., 1999;Paytan et al., 2004Paytan et al., , 1998Yao et al., 2018). ...
... Previous studies trace the marine sulfur cycle across the OAE2 using sulfur-bearing minerals, such as carbonate-associated sulfate (CAS), marine barite, and pyrite, as archives (Adams et al., 2010;Ohkouchi et al., 1999;Paytan et al., 2004). Since pyrite often undergoes diagenetic alteration and most marine barite records are of a low temporal resolution across this interval (Paytan et al., 2004;Yao et al., 2019), high-resolution reconstruction of marine δ 34 S is mainly based on CAS records (Adams et al., 2010;Gomes et al., 2016;Ohkouchi et al., 1999;Owens et al., 2013;Poulton et al., 2015). ...
... Previous studies trace the marine sulfur cycle across the OAE2 using sulfur-bearing minerals, such as carbonate-associated sulfate (CAS), marine barite, and pyrite, as archives (Adams et al., 2010;Ohkouchi et al., 1999;Paytan et al., 2004). Since pyrite often undergoes diagenetic alteration and most marine barite records are of a low temporal resolution across this interval (Paytan et al., 2004;Yao et al., 2019), high-resolution reconstruction of marine δ 34 S is mainly based on CAS records (Adams et al., 2010;Gomes et al., 2016;Ohkouchi et al., 1999;Owens et al., 2013;Poulton et al., 2015). Interestingly, unlike the modern ocean where sulfate is homogeneous in global ocean basins, J o u r n a l P r e -p r o o f Journal Pre-proof the marine δ 34 S shows temporal and spatial differences across the OAE2. ...
The Cretaceous Oceanic Anoxic Event 2 (OAE2) is a greenhouse episode of severe marine anoxia at the Cenomanian-Turonian boundary. This time interval is characterized by rising sea surface temperature, enhanced marine biological productivity, and widespread occurrence of organic-rich black shales. With an export of biological production to the deep ocean, organisms consume vast amounts of oxygen and subsequently utilize nitrate and sulfate as electron acceptors in organic matter degradation, thereby affecting biogeochemical cycles of nitrogen and sulfur. However, due to a lack of comprehensive compilation of global records, the temporal and spatial distribution of oceanic oxygen loss is yet to elucidate. To bridge this gap, this review summarizes the published marine nitrogen and sulfur isotope data sets from worldwide locations across the Cenomanian-Turonian boundary. The compiled records suggest that anoxic and euxinic waters have initially developed on J o u r n a l P r e-p r o o f Journal Pre-proof the southern edge of the proto-North Atlantic before the OAE2 and spread to open oceans of the proto-North Atlantic, epicontinental seas of the Western Tethys and Western Interior Seaway at the onset of the OAE2. Meanwhile, sites above the storm base and in deep seas of the Tethys remain relatively oxic throughout the time. We suggest that distinct nutrient levels and circulation structures primarily control the regional difference in marine redox states. Our findings depict the evolutionary history of ocean deoxygenation and biogeochemical cycles during the past hyperthermal event, with implications for how future global warming may impact seawater chemistry and marine ecosystems.
... Meanwhile, pyrite burial results in a net flux of O 2 to the atmosphere and drawdown of CO 2 over those same timescales. A memory of this recycling is captured in the stable isotope composition of seawater sulfate (5), which, since 130 million years ago (Ma), is most faithfully recorded by marine barite (BaSO 4 ) minerals (6)(7)(8)(9)(10). The sulfur isotope record (δ 34 S) of marine barite (9)(10)(11) has classically been interpreted as reflecting a balance of terrestrial weathering (3,12), microbial reworking (13), and sedimentary export from the ocean (10,14,15), with recent suggestions of an outsized control via sulfur injected through LIP emplacement (16). ...
... A memory of this recycling is captured in the stable isotope composition of seawater sulfate (5), which, since 130 million years ago (Ma), is most faithfully recorded by marine barite (BaSO 4 ) minerals (6)(7)(8)(9)(10). The sulfur isotope record (δ 34 S) of marine barite (9)(10)(11) has classically been interpreted as reflecting a balance of terrestrial weathering (3,12), microbial reworking (13), and sedimentary export from the ocean (10,14,15), with recent suggestions of an outsized control via sulfur injected through LIP emplacement (16). In parallel, the marine barite oxygen isotope record (δ 18 O) is thought to largely reflect changes in microbial recycling, here linking gross metabolism to environmental features like the locus of organic matter oxidation, sea level, and nutrient availability (7,15). ...
... After filtering, samples of similar age yield statistically indistinguishable estimates for marine sulfate oxygen isotope composition; here we use a 1σ uncertainty interval based on the directly measured variability within modern core-top barite (±0.5‰ for δ 18 O and ±0.020‰ for Δ' 17 O) (15,26) as a conservative estimate. Oxygen isotope measurements from this study are compiled along with previous records of δ 34 S (Fig. 1A) (9)(10)(11)15). We apply a previously calibrated offset between barite and seawater sulfate for δ 18 O of −2.7‰ (15,26), whereas the δ 34 S and Δ' 17 O directly record seawater sulfate composition (10,26). ...
The triple oxygen isotope composition (Δ’ ¹⁷ O) of sulfate minerals is widely used to constrain ancient atmospheric p O 2 / p CO 2 and rates of gross primary production. The utility of this tool is based on a model that sulfate oxygen carries an isotope fingerprint of tropospheric O 2 incorporated through oxidative weathering of reduced sulfur minerals, particularly pyrite. Work to date has targeted Proterozoic environments (2.5 billion to 0.542 billion years ago) where large isotope anomalies persist; younger timescale records, which would ground ancient environmental interpretation in what we know from modern Earth, are lacking. Here we present a high-resolution record of the δ 18 O and Δ’ ¹⁷ O in marine sulfate for the last 130 million years of Earth history. This record carries a Δ’ ¹⁷ O close to 0o, suggesting that the marine sulfate reservoir is under strict control by biogeochemical cycling (namely, microbial sulfate reduction), as these reactions follow mass-dependent fractionation. We identify no discernible contribution from atmospheric oxygen on this timescale. We interpret a steady fractional contribution of microbial sulfur cycling (terrestrial and marine) over the last 100 million years, even as global weathering rates are thought to vary considerably.
... The variation and moderately high rates of change of this composition show distinctive excursion curves and characterize specific time intervals in geological time [33], and thus, it is possible to form geological correlations and determine the geological ages of sediments using sulfur isotope analysis [34,35]. In particular, from the Cretaceous to the Cenozoic, high-resolution sulfur isotope excursion was restored based on barite within precisely age-determined core sediments [36,37], and a sulfur isotope curve was obtained from pelagic marine barites of the Cretaceous and Cenozoic periods, resulting in an unprecedented temporal resolution [36,37]. These results generally agree with those obtained from the carbonate-associated sulfates that are present in carbonate sediments [32]. ...
... The variation and moderately high rates of change of this composition show distinctive excursion curves and characterize specific time intervals in geological time [33], and thus, it is possible to form geological correlations and determine the geological ages of sediments using sulfur isotope analysis [34,35]. In particular, from the Cretaceous to the Cenozoic, high-resolution sulfur isotope excursion was restored based on barite within precisely age-determined core sediments [36,37], and a sulfur isotope curve was obtained from pelagic marine barites of the Cretaceous and Cenozoic periods, resulting in an unprecedented temporal resolution [36,37]. These results generally agree with those obtained from the carbonate-associated sulfates that are present in carbonate sediments [32]. ...
... However, as mentioned above, this age should be excluded based on fossil occurrence. The sulfur isotopic compositions (A) were obtained from [37], while the Sr isotopic compositions (B) were obtained from [27,38]. Tangents between the sulfur isotope value obtained in this study and the LOWESS curve allowed for the determination of the geological age (A). ...
Lithostratigraphy and isotopes of Paleogene sequences consisting mainly of terrestrial clastics and limestone were examined in the northern Fergana Basin of Uzbekistan. The studied sections consisted of two facies: the lower sequence coarse-grained terrestrial clastics and the upper sequence limestone clastics characterized by limestone beds. The sulfur isotopic composition of the bivalve obtained from the lower sequence was relatively low, namely, 14.94–16.82‰, which is equivalent to the Early Cretaceous; however, it is possible that the isotopic composition differed from that obtained in open seawater due to the presence of terrestrial clastics and the freshwater effect. In contrast, the sulfur isotopic composition of limestone from the upper sequence was relatively high, namely, 19.37–21.19‰, thereby indicating that they were likely to originate from the Early to Middle Eocene. Furthermore, the strontium isotopic compositions of the lower and upper sequences were 0.707772–0.707875 and 0.707812–0.708063‰, respectively. These values are more similar to those of the Paleogene than the Cretaceous. Finally, lithostratigraphy and age determination allowed us to correlate the upper sequence with representative limestone from the fourth transgression of the proto-Paratethys Sea, whilst the limestone beds were deduced to be remnants of the Eocene marine incursion of the proto-Paratethys Sea.
... The negative δ 13 C org excursion coincides with anoxic conditions and is interpreted to have been induced by increased atmospheric carbon dioxide (CO 2 ) introduced by intense volcanism (Li et al., 2019b). If the negative δ 13 C org excursion resulted from increased volcanic activity, the volcanism must also have made a significant impact on the biogeochemical cycling of sulfur, e.g., through volcanic emissions of sulfur dioxide (SO 2 ) (Paytan et al., 2004;Adams et al., 2010). However, the dearth of sulfur isotope investigations of deposits that accumulated during the Early Silurian has precluded consideration of the relationship of sulfur and carbon isotope trends. ...
... The present study suggests that the two negative δ 34 S pyr excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages reflect increase delivery of sulfate to the global ocean (see Section 5.2). Sulfate is supplied to the oceans mainly through continental weathering and volcanic emissions (dominated by SO 2 ) (Paytan et al., 2004;Bottrell and Newton, 2006;Adams et al., 2010;Gomes et al., 2016). The intense volcanism during the Late Ordovician and Early Silurian on the Yangtze Platform occurred as two episodes based on the stratigraphic distribution of bentonites and dating of zircons from volcanic ashes ( Fig. 2; Yang et al., 2019;Wu et al., 2019;Lu et al., 2020). ...
... Thus, although volcanic eruptions were occurring, they did not perturb the Hg cycle. In addition to emitting SO 2 , volcanism would have introduced substantial quantities of isotopically light carbon (dominated by CO 2 ) into the ocean-atmosphere system (e.g., Ernst and Youbi, 2017), perhaps perturbing the carbon cycle enough to trigger a negative δ 13 C org excursion (Paytan et al., 2004;Payne and Kump, 2007;Wignall et al., 2009) concurrent with the negative δ 34 S py excursion. Therefore, we propose that the coupled negative δ 34 S py and δ 13 C org excursions during Katian to early Hirnantian and late Hirnantian to middle Rhuddanian time were triggered by episodes of heightened volcanism (Fig. 7A). ...
Pronounced excursions of carbon and sulfur isotopes (δ¹³C and δ³⁴S) spanning the Ordovician–Silurian (O–S) transition have been recognized globally, yet the causes of these isotope events and their causal relationships to environmental changes, especially during Early Silurian time, remain the topics of debate. In this study, we present organic carbon and pyrite sulfur isotope (δ¹³Corg and δ³⁴Spyr) records and elemental concentrations from a drill core spanning from the Upper Ordovician (Katian) to Lower Silurian (Aeronian) succession of the Yangtze Platform, China. The newly presented δ¹³Corg profile records previously recognized positive excursions, including the Hirnantian isotopic carbon excursion (HICE), the middle Rhuddanian excursion, and the early Aeronian positive carbon isotope excursion (EACIE). In addition, two negative δ¹³Corg excursions are recognized during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages. The δ³⁴Spyr profile exhibits a great amount of variation (−17.8‰ to +19.6‰) but mimics the δ¹³Corg record. Coupled positive δ¹³Corg and δ³⁴Spyr excursions during the middle Hirnantian and late Rhuddanian to Aeronian are associated with decreased total organic carbon (TOC) content and less reducing water column conditions, whereas the negative δ¹³Corg and δ³⁴Spyr excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian are associated with enhanced TOC values and the establishment of more strongly anoxic to euxinic conditions. The δ¹³C and δ³⁴S excursions during the O–S transition, then, cannot be explained by changes in redox conditions or the burial of organic carbon and pyrite. The heavier and less variable middle Hirnantian and late Rhuddanian to Aeronian δ³⁴Spyr values probably reflect low marine sulfate concentration. In contrast, negative Katian to early Hirnantian and late Hirnantian to middle Rhuddanian δ³⁴Spyr excursions may reflect increased sulfate concentration. The two negative δ¹³Corg and δ³⁴Spyr excursions associated with the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian correspond with episodes of volcanism. We therefore propose that volcanism was the main driver of coupled negative δ¹³Corg and δ³⁴Spyr excursions. As volcanism diminished during the middle Hirnantian and late Rhuddanian to Aeronian, both δ¹³Corg and δ³⁴Spyr returned to more positive values. Our results indicate that volcanism played a crucial role in triggering carbon and sulfur isotope perturbations and environmental changes during the Late Ordovician and Early Silurian.
... FIGURE 9.2 Seawater sulfate S isotope curve from marine barite for 130 Ma to present. Paytan et al., 1998;Paytan et al., 2004;Turchyn et al., 2009;Markovic et al., 2015;Markovic et al., 2016;Yao et al., 2018;Yao et al., 2020. FIGURE 9.3 The Phanerozoic seawater sulfate δ 34 S record. Green circles 5 CAS data (Ueda et al., 1987;Strauss, 1993;Kampschulte and Strauss, 2004;Goldberg et al., 2005;Mazumdar and Strauss, 2006;Gill et al., 2007;Hurtgen et al., 2009;Turchyn et al., 2009;Wu et al., 2010Wu et al., , 2014Thompson and Kah, 2012;Wotte et al., 2012;Present et al., 2015;Sim et al., 2015;Kah et al., 2016;Schobben et al., 2017;Rennie et al., 2018); gray circles 5 evaporites data (Holser and Kaplan, 1966;Sakai, 1972;Claypool et al., 1980;Cortecci et al., 1981;Pierre and Rouchy, 1986;Das et al., 1990;Rick, 1990;Utrilla et al., 1992;Fox and Videtich, 1997;Strauss, 1997;Worden et al., 1997;Kampschulte et al., 1998;Strauss, 1993;Strauss et al., 2001;Longinelli and Flora, 2007;Orti et al., 2010;Peryt et al., 2005;Surakotra et al., 2018;Crockford et al., 2019); blue dash line 5 the modern seawater sulfate δ 34 S value of B21m. ...
... The evidence that the S isotopic composition of seawater sulfate has fluctuated considerably over time, until recently, was based on comprehensive, though not continuous, isotope data sets obtained from marine evaporitic sulfate deposits and pyrite (Claypool et al., 1980;Strauss, 1993). More recently, marine barite has been used to construct a continuous, high-resolution S curve for the last 130 Ma (Paytan et al., 1998(Paytan et al., , 2004Turchyn et al., 2009;Markovic et al., 2015Markovic et al., , 2016Yao et al., 2018Yao et al., , 2020. Methods to analyze the sulfate that is associated with marine carbonate deposits (carbonate-associated sulfate, CAS) have also been developed, and new data sets using these methods are becoming available. ...
... Like evaporites, the δ 34 S of barite is quite similar to that of sulfate in the solution from which it precipitated. Marine barite precipitates in the oceanic water column and is relatively immune to diagenetic alteration after burial thus it records the changes in the sulfur isotopic composition of seawater through time (Paytan et al., 1998(Paytan et al., , 2004Turchyn et al., 2009;Markovic et al., 2015Markovic et al., , 2016Yao et al., 2018Yao et al., , 2020. Moreover, high-resolution, well-dated, and continuous records can be developed as long as barite-containing pelagic marine sediments are available (Paytan et al., 1993). ...
The sulfur isotopic composition of dissolved sulfate in seawater has varied through time. Distinct variations and relatively high rates of change characterize certain time intervals. This allows for dating and correlation of sediments using sulfur isotopes. The variation in sulfur isotopes and the potential stratigraphic resolution of this isotope system is discussed and graphically displayed. New data are used to refine the previously published (Geologic Time Scale 2012) for the Paleocene and Eocene.
... Given the large difference in sulfate reservoirs between lake and marine environments during the Late Cretaceous, it is most likely that a rapid increase in the sulfate reservoir caused by marine intrusion disrupted the balance of the original lacustrine system. Marine transgression at the base of Qingshankou Member I would have flushed marine sulfate (δ 34 S sulfate =~19‰, Paytan et al., 2004) into the basin. The increased sulfate at this time decelerated the MSR rate and amplified the sulfur isotope fractionation between sulfate and hydrogen sulfide, resulting in rapid negative excursion in the pyrite-dominated TS isotopes. ...
... Note that the δ 34 S pyrite peak preserved in the FH-1 core reached ~25‰. Paytan et al. (2004) estimated the Cretaceous open marine δ 34 S sulfte to be approximately 18.4‰ based on marine barites. Although the lacustrine sulfate was totally replaced by marine sulfate (assuming the lacustrine δ 34 S sulfate was +18.4‰, ...
... Paytan et al. (2004) estimated the Cretaceous open marine δ 34 S sulfte to be approximately 18.4‰ based on marine barites. Although the lacustrine sulfate was totally replaced by marine sulfate (assuming the lacustrine δ 34 S sulfate was +18.4‰, Paytan et al., 2004), the peak value δ 34 S pyr formed by closedsystem Rayleigh fractionation should be less than or equal to +18.4‰. ...
During the Late Cretaceous period (~100.5–65 million years ago), two episodes of expansive lake anoxia occurred in the Songliao Basin (~3× the size of Lake Superior), NE China, either lasting less than one million years. These events, known as lacustrine anoxic events (LAEs), were the factors driving the development of the main source rocks for the most productive oilfields in China. The details underpinning the initiation of the LAEs, however, remain problematic. As a test of this scientific issue, we present time series of bulk organic carbon isotopes, pyrite sulfur isotopes, nitrogen isotopes and trace element concentrations from the Turonian–Coniacian Qingshankou Formation in the Songliao Basin. A notable δ³⁴S excursion (~−10‰) and a spike in pyrite sulfur content are recorded in the basal Qingshankou Formation when the basin reached the maximum expansion. We attributed these changes to marine transgression, which would have brought high sulfate concentrations to the lake, thereby promoting microbial sulfate reduction (MSR). Subsequent sulfate limited by MSR and pyrite burial associated with isolation of the lake from the seawater drove a 20‰ positive δ³⁴S shift. Note that the modes of the δ³⁴S that fluctuations we describe from the LAEs have also been recognized in marine black shale during Oceanic Anoxic Event (OAE) 2, implying a common signature of anoxia. The carbon isotope data suggest that the maximal fractionation found in Qingshankou Member I was likely a consequence of the increased [CO2]aq in lake water and inputs from chemoautotrophs. Increased sedimentary δ¹⁵N during the interval of Qingshankou Member I implies intensified denitrification in the oxygen minimum zone, which lowered the water-column N/P ratios, thereby stimulating bacterial nitrogen fixation in Qingshankou Members II-III. By comparing and summarizing the sulfur, carbon and nitrogen cycles of the LAEs during deposition of the Qingshankou and Nenjiang formations, we argue that the forcing function underlying LAEs was enhanced sulfate levels from marine transgression, which might have intensified nutrient recycling and further regulated the lacustrine carbon and nitrogen cycles.
... (a) Least altered and unmetamorphosed pyroxene basalt (M-S1B), (b) metabasalt consisted of zeolite facies mineral assemblage (zeolite and celadonite) with relict of primary minerals such as clinopyroxene and plagioclase (S-S20A), (c) epidotechlorite metabasalt dominated by the greenschist facies minerals and found as lenses embedded in the carbonaceous phyllite (B-S2B), (d) garnet-muscovite orthogreenschist found as a float (S-S9FA), (e) garnet amphibolite showing hornblende-plagioclase assemblage, found as a float (S-S19FA), (f) epidote-glaucophane schist found as a float (S-S11FB), (g) garnet-epidotemagnesiokatophorite schist found as a float (S-S19FC), and (h) eclogite found as a float (B-S11FA). Act, actinolite; C/S, interlayered chlorite/smectite; Cal, calcite; Cel, celadonite; Chl, chlorite; Cpx, clinopyroxene; Ep, epidote; Gln, glaucophane; Grt, garnet; Hem, hematite; Ilm, ilmenite; Mkt, magnesiokatophorite; Ms, muscovite; Mt, magnetite; Omp, omphacite; Pal, palagonite; Ph, phengite; Pl, plagioclase; Py, pyrite; Rt, rutile; Ttn, titanite; Ze, zeolite composition (Paytan et al., 2004) might have been produced by two cases including (1) thermochemical or organic sulfate reduction (TSR) above 80 C (e.g., Hoefs, 1997;Ohmoto & Goldhaber, 1997;Ohmoto & Rye, 1979) and (2) formation of pyrite by sulfate reduction in a closed system with respect to SO 4 2À (e.g., Ohmoto & Goldhaber, 1997). The first case can be caused by reduction of evaporates by the carbonaceous matter in a diagenetic environment resulting in the coexistence of euhedral pyrite, calcite, and relict of sulfate phase (barite) along the foliation of carbonaceous metapelite (Figure 12c). ...
... (a) The δ 34 S values of pyrite from veins, hydrothermally altered rocks, and metapelites, (b) backscattered electron (BSE) image of framboidal pyrite from carbonaceous phyllite (B-S1B), and (c) BSE image of euhedral pyrite from carbonaceous phyllite (B-S5A) coexisting with galena and Sr-rich barite. Symbols; black solid line: reference values from Ohmoto and Rye (1979) showing the distribution of δ 34 S values of evaporites, sulfides of basalt, sulfides of granite, and sulfides of sedimentary rocks; blue dashed line: δ 34 S values of early cretaceous sea water sulfate (Paytan et al., 2004); and red dashed line: median value. Cal, calcite; Cpy, chalcopyrite; Fpy, framboidal pyrite; Gn, galena; Py, pyrite; Qz, quartz; Sr-Bar, Sr-rich barite the altered rocks or ore zones in the orogenic gold deposits (e.g., Clark & Williams-Jones, 2004). ...
The Luk Ulo Metamorphic Complex, Central Java is a product of the Cretaceous subduction and accretion, and includes diverse types of protoliths. Two‐types of primary mineralization have been recognized in this area, namely, (1) seafloor basalt‐hosted massive sulfide mineralization and (2) low‐grade metamorphic rocks‐hosted vein type mineralization. Later erosion of these types of primary mineralization formed placer gold deposits along rivers. However, the source has never been identified. Thus, this study aims at understanding the source of placer gold, the characteristics of the primary mineralization, and the tectonic evolution of the study area on the basis of mineralogy, mineral chemistry, whole‐rock geochemistry, and sulfur isotope analyses. Volcanogenic massive sulfide (VMS)‐type mineralization was identified in the seafloor basalt and few deep‐sea sedimentary rocks, and both the ores and host rocks preserved pre‐metamorphic textures and minerals. The characteristics of this VMS‐type mineralization include (1) crustiform quartz veins with pyrite cutting the host rocks, (2) zonation of local silicification to interlayered chlorite/smectite‐chlorite‐laumontite‐calcite‐epidote alteration from central to outer zone, (3) pyrite‐dominated ores with minor amounts of arsenian pyrite, chalcopyrite, and marcasite, (4) unmetamorphosed host rocks and ores, and (5) sulfur isotope signature with a median δ34S of +3.1‰ suggesting sulfur derived from magmatic source and/or sulfur extracted from basaltic rocks with a small contribution of biogenic sulfur. On the other hand, low‐grade metamorphic rocks‐hosted vein type mineralization was identified as orogenic‐type gold mineralization, and the mineralized veins formed after the peak of metamorphism. It is characterized by (1) pyrite‐arsenian pyrite ores with minor amounts of arsenopyrite, galena, tetrahedrite, chalcopyrite, and sphalerite, (2) quartz‐illite‐graphite alteration assemblage, (3) mineralized veins cross‐cutting the foliation of metamorphic host rocks, (4) high antimony contents of pyrite (up to 1.7 wt%) and rutile (up to 160 ppm), (5) relatively high ore‐forming temperature (423 ± 9°C, calculated from arsenopyrite and graphite geothermometers), and (6) remobilized‐sedimentary sulfur signature of the ores with a median δ34S of −9.8‰. Several lines of evidence suggest that placer gold was likely derived from the erosion of orogenic‐type gold ores in the surrounding areas. This evidence includes the presence of gold‐bearing ores hosted by low‐grade metapelites and metagranitoid with characteristics of orogenic‐type gold mineralization, whereas the VMS‐type ores are barren in gold. The occurrence of the mid‐oceanic ridge‐ and accretion zone‐related mineralization in this area reflects the subduction and amalgamation of oceanic and continental crustal blocks during the Cretaceous period. Discovery of gold mineralization hosted in the Cretaceous basement rocks of the Sunda arc indicates the importance to broaden the gold exploration targets to include not only young volcanic rocks, but also relatively old metamorphic basement rocks. The Luk Ulo Metamorphic Complex, situated in Central Java, is a product of the Cretaceous subduction that contains placer gold deposits. Two types of gold mineralization have been recognized in this area, namely, (1) seafloor basalt‐hosted massive sulfide mineralization and (2) low‐grade metamorphic rocks‐hosted vein‐type mineralization. Several lines of evidences from whole‐geochemistry, mineral chemistry, and sulfur isotope of ores indicate that placer gold was derived from the erosion of orogenic gold ores from the surrounding areas. The occurrence of the mid‐oceanic ridge‐ and accretion zone‐related mineralization in this area reflects the subduction and amalgamation of oceanic and continental crustal blocks during the Cretaceous period.
... Diffusive transport of TEs from pyrite to the pore water or meta- Suhendra et al., 2022). The δ 34 S values of reduction processes and Lower Cretaceous seawater sulfate were taken from Ohmoto and Rye (1979) and Paytan et al. (2004), respectively. morphic fluids removed significant amounts of TEs during the recrystallization of subhedral to euhedral pyrite in LGSF (Fig. 15a-c; e.g., Gundersen et al., 2002). ...
... The source rocks of the Qingshankou Formation contain a large amount of pyrite, and thus the δ 34 S values of the whole-rocks can be approximated using the pyrite δ 34 S values (Cao et al., 2021b;Kaufman et al., 2007). The δ 34 S values exhibit only small fluctuations and are relatively uniform from Qingshankou members I to II-III ( Fig. 5; i.e., close to +19‰) and are consistent with the δ 34 S values (19‰) of Cretaceous marine sulfate (Paytan et al., 2004). This shows that marine incursions had a significant effect on the δ 34 S values of the Qingshankou Formation shales. ...
... Paired δ 34 S CAS and δ 18 O CAS records can be used to evaluate the relative fluxes of sulfate reduction versus sulfide reoxidation in modern and ancient settings, as they vary in response to changes in the biogeochemical S cycle: inputs from weathering and volcanism and outputs via pyrite burial and evaporite deposition (e.g., Paytan et al., 2004;Turchyn and Schrag, 2006;Gomes and Johnston, 2017). The δ 18 O CAS profiles of the Three Gorges and Parachilna Gorge study sections show significant negative shifts of differing magnitude during the falling limb of δ 13 C carb and δ 34 S CAS excursions (Fig. 3), which implies widespread reoxidation of H 2 S in shelf areas during the SE but with varying intensity between basins. ...
... To further assess the origin of pyrite, SIMS results are shown in Fig. 4, and indicate δ 34 S values of pyrite ranging from − 23 to 12 ‰. In addition, the δ 34 S values become more positive with increasing depth, and are lower than the δ 34 S values of Early Triassic (Montney deposition time) and Early Cretaceous (potential time of biodegradation event) seawater 23,24 . ...
Some of the parental material for hydrocarbons produced from low-permeability reservoirs in Western Canada corresponds to thermal products from biodegraded oil. This has been proved by the occurrence of framboidal pyrite, which is often formed during microbial sulfate reduction (MSR). In addition, the identified pyrite framboids are associated with the presence of phosphorus (P). Phosphorus (as phosphate) is a key nutrient and energy carrier for sulfate-reducing bacteria. The pyrite-P assemblage occurs embedded in solid bitumen (thermal residue), which confirms that migrated hydrocarbons provided the environment for microbial growth. Molecular products of severe biodegradation such as 17-nortricyclic terpanes were also detected. Biodegradation effects have been masked not only by thermal degradation of biodegraded oil during maximum burial, but also due to hydrocarbon mixing with late gas-condensate charges. Suitable conditions for biodegradation (< 80 °C, basin uplift) occurred during the Early Cretaceous. The confirmation of paleo-biodegradation means that there was a significant hydrocarbon loss that we have not accounted for. Likewise, MSR and Early Cretaceous seawater sulfate might have played an important role in the generation of the hydrogen sulfide (H2S) detected today.
... Page 2 of 12 poorly understood to date (Fu et al. 2016a;Song et al. 2017;Li et al. 2020). The long-term changes in marine redox conditions are thought to be governed by the relative burial proportions of redox-sensitive sulfur and carbon species (Kesler and Jones 1980;Coveney and Shaffer 1988;Worden et al. 1997;Paytan 1998a;Gorjan et al. 2000;Paytan et al. 2004a;Canfield 2013;Kurzweil et al. 2015). Seawater sulfate is one of the main reservoirs of dissolved sulfur (Holser and Kaplan 1966;Jin-Shi and Xue-Lei 1988;Hurtgen et al. 2002). ...
Middle Jurassic marine anhydrite deposits are extensively distributed in the Qiangtang Basin, Northern Tibet, which is significant for a better understanding of the marine environment conditions of eastern Tethys Ocean during the Middle Jurassic. In this study, we present a continuous and high-resolution record of sulfur isotopes (δ³⁴S) composition of seawater sulfate from Middle Jurassic Lower Xiali Formation anhydrite (CaSO4) obtained from a deep core (~ 300 m) in the Qiangtang Basin, eastern Tethys. XRD results show that samples mainly consist of anhydrite. Combined with the microscopy by SEM, we suggest the anhydrite transformed from the Middle Jurassic gypsum in the Qiangtang Basin during burial, which formed under the arid climatic condition in eastern Tethys Ocean. The δ³⁴S values of anhydrite samples range between + 9.1 and + 15.4 ‰ with an average value of + 13.2‰, which is lower than the sulfur isotopic composition of Jurassic seawater previously reported. Combined with the remarkable changes in ⁸⁷Sr/⁸⁶Sr, δ¹³C and δ¹⁸O, we suggest that the Qiangtang basin in the eastern Tethys Ocean was partly influenced by the chemical weathering, and enhancing of evaporite dissolution leading to a large flux of sulfate to the ocean, permitting high rates of pyrite burial, increasing atmospheric and ocean O2. This study contributes to improve understanding of anhydrite formation in modern and ancient playas, and of the environment conditions of the eastern Tethys Ocean.
... With circulation of fluids, a decrease in Sr concentration occurs during gypsum-anhydrite transformation in the open system and during the hydration of anhydrite (Jaworska 2012). Playa et al. (2000) and Paytan et al. (2004) indicated also that the primary gypsum lithofacies have higher Sr contents compared with the secondary gypsum lithofacies. The Sr contents are low for the secondary Bala gypsum and the units intercalated with sulphates-carbonate and siliciclastics observed mainly at the upper part of drill cores. ...
Bala is located in Central Anatolia and is a sub-basin of the Tuzgölü basin, which consists of many basins formed in response to the closure of the Neotethys ocean. The evaporites in Bala sub-basin formed under the influence of regional uplifting during the Middle Eocene period with gradual shallowing in response to the structural compressional regime. The Bala sub-basin comprises four units from bottom to top: the Lower Unit (BU1) consisting of mudstone, claystone with limestone layers; the Evaporites (BU2) containing dominant gypsum with anhydrite; mixed siliciclastic-carbonate-evaporitic deposits (BU3); and the Upper Unit (BU4) containing conglomerate-sandstone-claystone and siltstones. The evaporitic unit, which are the main subject of study consists of primary selenitic gypsum, anhydrite, locally celestite crystals and secondary gypsum formed due to the anhydritization of primary gypsum and then hydration of these anhydrites. The results obtained from mineralogical-petrographic, geochemical and isotopic (δ¹⁸O and δ³⁴S) investigations indicate that these evaporites were deposited in shallow marine to sabkha environments dominated by arid conditions. The presence of primary selenite crystals, the anhydrite with nodular, chicken-wire, and mosaic structures support precipitation occurred in a partially saline shallow water. The excessive thickness of the evaporitic accumulation, chaotic masses including gypsum breccia, salt diapirs and dome structures are attributed to tectonic processes acting during the formation of the Central Anatolian basins.
... The isotope record of marine sedimentary sulfate through time has been used successfully to determine global variations in the composition of seawater sulfate [3]. The variations in the sulfur isotopic composition of marine sulfate reflect changes in the global sulfur cycle and are also closely related to changes in atmospheric oxygen cycles [4][5][6]. A global sulfur isotope record for the Phanerozoic based on the analysis of sulfate minerals in evaporites was first reconstructed by Holser and Kaplan [7], Holser [8], and Claypool [9], and some researchers have given good supplementary datum later [3,5,[10][11][12]. ...
The sulfate isotope record of marine sedimentary sulfate through time has been used successfully to determine global variations in the composition of seawater sulfate. The variations in the sulfur isotope composition of marine sulfate reflect changes in the global sulfur cycle and are also closely related to changes in the atmospheric oxygen cycles. However, data for the Paleocene are very sparse and the stratigraphic evolution of the sulfur isotope composition of seawater is poorly constrained due to the small number of samples analyzed. The Yarkand Basin, as a northeastern part of the eastern Paratethys ocean with the trumpet-shaped bay, in which a suite of evaporitic sequences named the Aertashen Formation was continuously developed in the Paleocene and was principally composed of massive gypsum interbedded fragmental rocks. The values of sulfur isotopic composition are from 12.2‰ to 20.6‰ (δ34 SCDT or δ34 SVCDT) and the mean is 17.7‰ in 97 gypsum samples in the basin. Three gradually increasing trends of sulfur isotopic curves reflect that enrichment of δ34 S occurred in seawater sulfate, and indicate oxidation of seawater sulfide. These may hint to at least three oxidation events or the bacterial reduction of seawater sulfide that occurred in the Paleocene, and that three oxygen-enriched events or the biological sulfur cycle might exist in this epoch. The sulfur isotopic composition (δ34 SCDT or δ34 SVCDT) in the seawater of the northeastern part of the eastern Paratethys ocean was about 15.0‰ to 20.6‰, and averaged 17.9‰ in the Paleocene. Combined with the previous global sulfur isotopic composition of seawater, the final range of global sulfur isotopic composition of seawater might be from 15.0‰ to 21.0‰, with 17.9‰–18.3‰ the average in the Paleocene, so the variations in the sulfur isotope composition of Paleocene seawater sulfate are reconstructed and supplemented.
... Changes to the rate of MSR and/or the associated isotopic fractionation can lead to variations in the S-isotope ratio of seawater and sedimentary sulfide phases. While the isotopic ratios of seawater and sedimentary sulfides are also affected by other processes, the average isotopic offset between seawater and pyrite is a key parameter for biogeochemical models tracing the deep-time evolution of the sulfur cycle (e.g., Claypool et al., 1980;Garrels and Lerman, 1984;Petsch and Berner, 1998;Paytan et al., 2004;Simon et al., 2007;Wortmann and Paytan, 2012;Rennie et al., 2018). These models make assumptions about the fractionation factor between seawater sulfate and porewater sulfide phases, which rely to a large degree on pyrite data. ...
Microbial sulfate reduction in subseafloor sediments regulates a significant portion of the marine organic matter burial flux. Over secular timescales, sulfate reduction is the fundamental process connecting the biogeochemical cycles of sulfur, carbon, oxygen, and phosphorous. Similar to carbon reduction, sulfate reduction is associated with a strong isotope fractionation process that enables us to track this process through time. It depends on a variety of factors and it has been argued previously that systematic differences between shallow and deep-sea environments might explain secular changes in the marine S-isotope ratio. However, observational data of in-situ fractionation from deep-sea areas are scarce. Here we use a reaction-transport model to analyse the S-isotope fractionation during microbial sulfate reduction in the interstitial water of ODP Site 1226 (Leg 201). We find that the upper 100 metres below seafloor are best explained with fractionation around -76‰, whereas below this depth the degree of fractionation drops to around -42‰. We propose that this shift is caused by changes in the ratio of the rate of microbial sulfate reduction relative to the rate of abiotic sulfide oxidation. Since large parts of deep oceans are characterised by exceedingly low sulfate reduction rates, this process may be widespread and possibly explain why pyrite S-isotope data suggests that the average S-isotope fractionation is around -50‰, rather than the theoretically predicted value below -70‰.
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... Together, time-calibrated records of δ 34 S sulfate and Δ pyr offer the potential for semi-quantitative assessment of [SO 4 2-] seawater using simple steady state models that relate the rate of change of rock-derived δ 34 S sulfate to the size of the global sulfate reservoir (Kah et al., 2004;Planavsky et al., 2012;Algeo et al., 2015). However, whereas seawater δ 34 S sulfate is well-constrained through the Mesozoic and Cenozoic Eras by a combination of relatively continuous marine barite (Paytan et al., 1998;Paytan, 2004), taxon-specific carbonateassociated sulfate (CAS) (Burdett et al., 1989;Rennie et al., 2018), and episodic massive sulfate evaporites (Claypool et al., 1980), the Paleozoic and Precambrian seawater δ 34 S sulfate record is comparatively patchy and uncertain (Fike et al., 2015). The patchiness is due to the subduction of most pre-Jurassic oceanic crust (and deep-sea sediments hosting barite and pelagic fossils). ...
The Capitan Reef Complex in West Texas is famous for its high prevalence of early marine cements, unusual for a Phanerozoic platform, leading some to suggest that Precambrian styles of carbonate sedimentation enjoyed a Permian encore. Here, we use patterns of stable Ca, Mg, C and S isotopes to better understand the environmental driver(s) of the enigmatic cementation. We find that calcite that is the most enriched in ⁴⁴Ca has δ³⁴S values that approach the inferred composition of Permian seawater sulfate. Microbial sulfate reduction in pore fluids must have been spatially and temporally coincident with recrystallization of primary carbonate phases, such that substantial ³⁴S-enriched sulfate was incorporated into diagenetic calcite under relatively closed-system conditions. Moreover, the magnitude of ³⁴S-enrichment of carbonates relative to seawater was strongly influenced by local diagenetic conditions, with fluid-buffered early marine cements, shelf, reef, and upper slope preserving more seawater-like S isotope ratios than the more sediment-buffered lower slope. Some samples are far more ³⁴S-enriched relative to seawater than those from modern sites in similar depositional environments, possibly responding to specific combinations of sedimentary parameters (e.g., grain size, porosity, organic matter rain rate). Additionally, the sulfate concentration in the Delaware Basin might have been slightly lower than modern levels, leading to more extensive isotopic evolution of sulfate in pore waters during carbonate recrystallization. Based on the data and a numerical model of carbonate recrystallization, we suggest that one driver of the extensive seafloor cement precipitation in the Capitan Reef Complex was a Permian water column [Ca²⁺]:[SO4²⁻] ratio somewhere between 1 and modern seawater.
... for Utrilla et al. 1992; ~ 17‰ for Eckardt and Spiro 1999;14-16.4‰ for Cendon et al. 2004, Gaillardet et al. 2003 (Fig. 5). Gypsum samples appear to be within values for tertiary marine origin (δ 34 S values ~ 17-24‰ for Thode and Monster 1965;Nielsen and Rambow 1969;Claypool et al. 1980;Stenni and Longinelli 1990;Barkov et al. 1995;Böttcher et al. 1998Böttcher et al. , 2007Paytan et al. 2004) (Fig. 5). The other values are close to each other, sometimes with marine and sometimes continental effect, and are encountered as evidence of an environment where marine and continent are mixed. ...
The study investigates the origin, age, paleo-depositional environment and formation conditions of gypsum lithofacies deposited in northeastern Muş in Eastern Anatolia using element analysis and sulfur, oxygen and strontium isotope combinations. The correlation diagrams of major ions and trace elements plots in the gypsum lithofacies indicate that carbonates and clastics carried by terrestrial waters (streams, rivers, etc.) significantly contributed to the evaporitic phases. Furthermore, the effects of hydrothermal solutions, increased salinity of the basin and presence of biological activity were associated with high values for major ions and trace elements. While the δ 18 O and δ 34 S isotope contents of some gypsum lithofacies samples show that they are of terrestrial or marine origin, others do not indicate either origin because they are affected by other factors such as water salinity variations, bacterial sulfate reduction, new terrestrial water and detrital inputs carried into the basin. However, 87 Sr/ 86 Sr isotope concentrations analyzed in gypsum lithofacies in the study area indicate Rupelian (Lower Oligocene) time, which is compatible with the Cenozoic age curve. Based on all these data, gypsum lithofacies in this basin were deposited in a transition zone (shallow sea-lagoon-inland sabkha-mudflats) under the influence of both marine and terrestrial conditions. Therefore, we claim that this basin formed during the Rupelian (Early Oligocene), at a time when the sea connection between the Indian Ocean and the eastern Mediterranean was restricted; that is, the southern branch of the Neotethys was extremely shallow during this period and almost terminated in the study area.
... Marine barite collected from seafloor sediments is a powerful and widely used proxy for ancient seawater sulfate (Paytan et al., 1998(Paytan et al., , 2004Schrag, 2004, 2006;Johnston et al., 2014). We next determine the oxygen isotope composition of seawater sulfate during the time of Messinian evaporite deposition (from 5.9 to 5.3 Ma (Krijgsman et al., 1999)). ...
The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO2/pCO2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'¹⁷O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'¹⁷O and δ18O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate.
... mentological and tectonic processes are also recorded in the sulfur isotopic composition of past seawater. The seawater δ 34 SSO4 record is controlled by changes in riverine and/or hydrothermal and volcanic input sources and/or by environmentally controlled changes in partitioning of sulfur between the oxidized and reduced sedimentary sulfur sinks.Paytan et al., (2004) used marine barite, a mineral that shows to record seawater δ 34 SSO4 for the establishment of a detailed Cretaceous and Cenozoic sulfur isotope curve. Prominent fluctuations recorded in this curve can be used as stratigraphic marker levels which mirror global perturbations in ocean chemistry and in the global sulfur cycle. New paleocea ...
The wide field of geochemistry includes: (1) Elemental geochemistry; (2) Mineral geochemistry; (3) Isotope geochemistry; (4) Cosmochemistry; (5) Geochemistry of igneous rocks; (6) Geochemistry of metamorphic rocks; (7) Geochemistry of sedimentary rocks; (8) Ore geochemistry; (9) Soil geochemistry; (10) Chemostratigraphy; (11) Biogeochemistry; (12) Photogeochemistry; (13) Hydrochemistry; (14) Atmospheric geochemistry; (15) Climate geochemistry; (16) Organic geochemistry; (17) Source rock geochemistry; (18) Reservoir geochemistry; (19) Coal geochemistry; (20) Environmental geochemistry; (21) Industrial geochemistry; (22) Medical geochemistry; (23) Analytical geochemistry; (24) Experimental geochemistry; (25) Exploration geochemistry; and (26) Geochemical engineering. Geochemistry has applications in many fields such as medicine, climate, environment, water quality, petroleum, mineral deposits, age dating, etc. This book is an explanation of the basics of the geochemical branches mentioned above.
... The δ 34 S values of seawater have changed greatly since the Precambrian, so that the maximum value of + 35‰ was for the Precambrian-Cambrian and the minimum value of + 11‰ was for the Permian-Triassic (Claypool et al., 1980). Paytan et al. (2004) considered δ 34 S values from 15.3 to 20.2‰ (mean 17.7‰) for Lower Cretaceous seawater. Since the age of the host rocks of Shamsabad deposit is from Lower Cretaceous, the isotopic composition of the studied samples can be compared with that of Lower Cretaceous seawater. ...
To determine the origin of the Shamsabad Fe-Mn deposit, LA-ICP-MS geochemistry in pyrolusite and the sulfur isotopes in galena were studied. The ore is lenticular and is hosted in Lower Cretaceous limestones. Hematite, iron hydroxides and pyrolusite are the ore minerals, accompanied by subordinate todorokite, coronadite, calcite, quartz, pyrite, galena, rhodochrosite and malachite. Field and petrographic evidence, including lamination and colloform textures of the ore and the presence of conformable limestone lenses within the ore, suggest a synsedimentary hydrothermal origin to the mineralization. In addition, the enrichment of As, Ba, Cu, Pb and Zn and depletion of Co and Ti in the pyrolusite is similar to hydrothermal Fe-Mn deposits. The LREE/HREE ratio, low ΣREE values and negative Ce anomalies indicate that hydrothermal fluids played an important role in mineralization. The Mn/Fe, Co/Ni, Co/Zn, La/Ce, LaN/NdN, DyN/YbN, and Y/Ho ratios, as well as all discriminative element diagrams for trace and major elements, are consistent with hydrothermal deposits. The δ³⁴S values of the galena samples indicate that the sulfur originated from magmatic fluids, probably partially mixed with Lower Cretaceous seawater. According to the proposed hydrothermal-sedimentary genetic model, the hydrothermal solutions leaked into the sedimentary basin primarily through possible deep-seated faults in the bedrock. Hydrothermal fluids were enriched in ore-forming materials through leaching from the rocks with which they were in contact. The leakage of the metal-bearing hydrothermal solutions into the sedimentary basin and mixing with seawater altered the Eh-pH conditions, resulting in the deposition of ore materials in the Shamsabad deposit.
... Pelagic marine barites record the δ 34 S value of seawater sulfate with a small isotopic deviation (<0.4‰; Paytan et al., 1998), as widely used in the reconstruction of the sulfur isotope curve for seawater sulfate through geologic time (e.g., Paytan et al., 1998Paytan et al., , 2004. In contrast, the δ 18 O values of marine pelagic barite exhibit an offset of 2.0-2.5‰ to lower values compared to that of seawater sulfate, which can be attributed to the incorporation of sulfate from oxidized organic S compounds (Markovic et al., 2016). ...
The Kuh–Ghalagheh barite deposit is located 43 km southwest of the city of Mahallat in Markazi Province, Iran. The barite mineralization occurs as a stratabound, lenticular massive orebody in siliciclastic rocks of Eocene age. Barite is accompanied by calcite, Fe- and Mn-oxides, quartz, and minor malachite. The concentration of total REEs (∑REE) is very low in barite, ranging from 0.38 to 6.26 ppm. Chondrite-normalized REE patterns show enrichment of LREE relative to HREE, and dominantly negative La, Gd, and Ce anomalies. The δ¹⁸O and δ³⁴S isotopic values of the barite samples fall within narrow ranges of +4.0 to +5.6‰ and +26.2 to +27.8‰, respectively, which do not match those of contemporaneous seawater (Eocene). This suggests that isotopically modified seawater sulfate is the source of oxygen and sulfur in barite. Three types of aqueous, aqueous–carbonic, and carbonic fluid inclusions were found in barite, suggesting that two immiscible fluids (aqueous and carbonic) were involved in the deposition of barite. Salinity values in the aqueous fluid inclusions ranged from 1.57 to 13.40 wt% NaCl equivalent. The aqueous–carbonic inclusions have salinity values ranging from 9.43 to 16.84 wt% NaCl equivalent. The salinity ranges of the aqueous and aqueous–carbonic inclusions show that two fluids have mixed, one with moderate salinity (>15 wt% NaCl equivalent) from basinal origin and a second with low salinity (<5 wt% NaCl equivalent) from seawater. The broad homogenization temperature ranges of 130–258 °C for aqueous and 118–290 °C for aqueous–carbonic inclusions do not represent the actual temperatures of the trapped fluids, but were generated by thermal re-equilibration of the inclusions due to deep burial of barite. The available data are consistent with a cold seeps model for barite formation in the Kuh–Ghalagheh deposit. The entry of Ba-rich basinal brines into the sedimentary basin resulted in the reaction of Ba with seawater sulfate, and the deposition of barite in siliciclastic sediments.
... Experimental studies showed that anhydrite in hydrothermal environments forms upon interaction of seawater with basalt and heating of seawater to temperatures above 120°C (Seyfried Jr and Bischoff, 1979;Shanks III et al., 1981). Due to relatively small equilibrium sulfur isotope fractionation between aqueous SO 4 and anhydrite (Balan et al., 2014;Eldridge et al., 2016), anhydrite has often been used to track δ 34 S variations of seawater through geological time (Bernasconi et al., 2017;Paytan et al., 2004;Strauss et al., 2013). ...
At mid-ocean ridges (MORs), seawater carrying dissolved sulfate (SO4) infiltrates the oceanic crust. Hydrothermal fluid emissions from such systems have much lower δ³⁴S and sulfur is mostly present as reduced sulfide, albeit in lower total sulfur concentrations than in seawater. This has been explained by anhydrite formation and sulfate reduction based on petrographic evidence and mass balance considerations. Here, we utilize the chemical and stable isotope (δ³⁴S, δ¹⁸O) systematics in natural anhydrite and pyrite from various locations along the submarine and on-land section of the Mid-Atlantic ridge near Iceland to quantify the key variables that control anhydrite formation and sulfate recycling in the oceanic crust. Hydrothermal anhydrite exhibited δ³⁴S values of +20.6±1.0‰ and δ¹⁸O values between +2.4 to +25.3‰. Volcanogenic anhydrite in encrustations showed δ³⁴S values of -1.7 to +21.4‰ and δ¹⁸O values between +1.4 and +38.0‰. Hydrothermal pyrite exhibited δ³⁴S values ranging from +3.4 and +19.7‰. Comparison of the natural dataset with results from geochemical isotope modelling revealed that δ³⁴S and δ¹⁸O values of anhydrite and pyrite were dependent on the isotope composition of the source fluid, extent of water-rock interaction, temperature, and redox conditions. Departures of δ³⁴S and δ¹⁸O values in anhydrite from the source fluid were caused by progressive fluid-basalt interaction where lower δ³⁴S and δ¹⁸O values reflected a change in sources of S and O from solely fluid to basaltic origin. The δ¹⁸O values of anhydrite were additionally affected by temperature. Quantitative formation of anhydrite mainly occurred at temperatures <150°C, whereas at elevated temperatures (>200 °C) reduction of seawater-sulfate to H2S and subsequent pyrite precipitation were found to limit anhydrite formation. Extending our calculations to the oceanic crust revealed that the majority of seawater-sulfate is sequestered into anhydrite (3-38 Tg S yr⁻¹) in vicinity of MORs at <200 °C at shallow depth (<1500 m), with only a small portion of seawater-derived SO4 discharged by high-temperature hydrothermal vents (0.1-3.4 Tg S yr⁻¹). However, sequestration of sulfur by anhydrite is not long-lasting due to retrograde dissolution of anhydrite. The removal of anhydrite upon cooling and aging of the crust may result in a return back to the oceans of 10-60% of the sulfur originally sequestered in anhydrite upon hydrothermal alteration in vicinity of MORs.
... CAS is a trace amount of sulfate that is incorporated into the mineral lattices during carbonate precipitation (Burdett et al., 1989;Pingitore Jr. et al., 1995). Because CAS extracted from modern foraminifera and marine calcareous sediments yield sulfur isotopic composition (~ + 21‰) close to that of modern seawater sulfate (Burdett et al., 1989;Kampschulte and Strauss, 2004;Paris et al., 2014), CAS is regarded as a reliable proxy for δ 34 S sw , and the sulfur isotopic composition of CAS extracted from ancient carbonate rocks (δ 34 S CAS ) has been used to represent δ 34 S sw in paleoceans (Gill et al., 2011;Kah et al., 2004;Kampschulte and Strauss, 2004;Paytan et al., 2004;Rose et al., 2019). In the past 15 years or so, numerous studies have accumulated a large number of δ 34 S CAS data throughout Earth's history (Crockford et al., 2019;Fike et al., 2015;Present et al., 2020); however, compared with carbonate carbon isotopes (Payne et al., 2004), δ 34 S CAS shows wider variations in both temporal and spatial scales (Crockford et al., 2019;Fike et al., 2015;Present et al., 2019). ...
Carbonate associated sulfate (CAS) is widely used to reconstruct the deep time seawater sulfate sulfur isotopic composition (δ³⁴Ssw) and marine sulfur cycle. However, δ³⁴SCAS normally shows substantial stratigraphic and spatial variations, casting doubt on its validity in recording δ³⁴Ssw. To understand the origin of δ³⁴SCAS oscillation, we measured sulfur isotopic compositions of CAS (δ³⁴SCAS) extracted from ribbon rock samples collected from the middle Cambrian Xuzhuang and Zhangxia formations in North China. The limestone layers have consistently higher δ³⁴SCAS values but lower CAS contents than the marlstone layers. Sulfate oxygen isotope data (δ¹⁸OCAS, Δ¹⁷OCAS) suggest that such isotopic differences can neither be explained by contamination of secondary atmospheric sulfate (SAS) nor oxidation of pyrite. Instead, the different trends of modification on δ³⁴SCAS values in marlstone and limestone layers may reflect two predominant processes. Low δ³⁴SCAS of the marlstone might mainly result from the influence of benthic H2S flux. Oxidation of H2S in the seafloor would generate ³⁴S-depleted sulfate that is subsequently taken in carbonate precipitated in the seafloor. In contrast, the limestone might have precipitated in more oxic seafloor and was mainly sourced from seawater, with less influence of benthic flux. The diagenetic dissolution and reprecipitation of limestone within MSR zone accounts for the higher δ³⁴SCAS and lower CAS contents. Finally, our study suggests that δ³⁴SCAS record δ³⁴Ssw only when the influence of both benthic flux and early diagenesis was insignificant.
... Globally, this warming is explained by high atmospheric CO 2 concentrations triggered by volcanic activity through periods of seafloor spreading (Arthur et al., 1985;Bice et al., 2006;Hofmann et al., 2008;Turgeon and Creaser, 2008). This hydrothermal volcanic influence may have played a role in oceanic water circulation (Hays and Pitman, 1973;Paytan et al., 2004), causing the warming of bottom waters of low circulation rates and less dissolved oxygen (Khalifa et al., 2018). ...
An organic geochemical investigation combined with sequence stratigraphy was performed in the Ceará Basin, an offshore basin located in Northeastern Brazil. The information available from 30 well logs (gamma-ray, resistivity, density), besides geochemical (TOC, and pyrolysis indexes) and isotopic (δ 13 C) data, aided the preparation of a dataset for this study. The application of sequence-stratigraphic methods helped classify and correlate seismic and organic facies. Four key petroleum source-rock units were identified, from the oldest to the youngest: (1) Mundaú Formation-top of the Rift Sequence (Berriasian-Aptian); (2) Paracuru Formation-Breakup Sequence (Aptian-Albian); (3) Itapajé Member of the Ubarana Formation-Continental Drift Sequence (Albian-Turonian), and (4) Uruburetama Member of the Ubarana Formation-Continental Drift Sequence (Turonian-Maastrichtian). The J o u r n a l P r e-p r o o f geochemical characteristics of the Mundaú Formation (high total organic carbon (TOC), hydrogen index (HI), relative hydrocarbon potential (RHP = (S1 + S2)/TOC)) point to a typical transgressive sequence. Six transgressive-regressive (T-R) cycles were recognized in the entire Paracuru Formation. The best geochemical marker is related to the top of the Paracuru Formation. This stratigraphic unit can be correlated to a major anoxic event and is the best source rock of this basin. Evaporitic facies found in this top section, maximum RHP values (anoxic conditions), and maximum flooding surfaces related to transgressive events characterize this interval. Moreover, the wide spatial cover of organic-rich rocks, carbon isotopic data, and the recognition of favorable characteristics for anoxia in other basins of the Equatorial Margin are suggestive of the Aptian-Albian Oceanic Anoxic Event (OAE-1b) occurring in the Ceará Basin. The Ubarana Formation represented by the Uruburetama Member and the Itapajé Member yields the least promising source rocks. However, high TOC values suggest the occurrence of the late Cenomanian-early Turonian Oceanic Anoxic Event (OAE-2), when organic-rich strata started to deposit in deep-water regions. The predominance of a regressive interval in the Uruburetama Member points to oxic or sub-oxic conditions. Additionally, the correlations between the Brazilian Equatorial Margin and its African counterpart, and the organic geochemical characterization allied to the definition of depositional systems for these regions proved to be useful for oil exploration. Legend of the graphical abstract: Flowchart representing the steps of this research. A) Database components. B) Data interpretation. C) Relative hidrocarbom potential (RHP) variations revealing changes in oxic conditions at depth and correlations with gamma-ray (GR) and lithofacies (Facies) stacking patterns. D) Sections showing the results obtained from data interpretation. E) A simplified model for the Paracuru Formation.
... for Utrilla et al. 1992; ~ 17‰ for Eckardt and Spiro 1999;14-16.4‰ for Cendon et al. 2004, Gaillardet et al. 2003 (Fig. 5). Gypsum samples appear to be within values for tertiary marine origin (δ 34 S values ~ 17-24‰ for Thode and Monster 1965;Nielsen and Rambow 1969;Claypool et al. 1980;Stenni and Longinelli 1990;Barkov et al. 1995;Böttcher et al. 1998Böttcher et al. , 2007Paytan et al. 2004) (Fig. 5). The other values are close to each other, sometimes with marine and sometimes continental effect, and are encountered as evidence of an environment where marine and continent are mixed. ...
The study investigates the origin, age, paleo-depositional environment and formation conditions of gypsum lithofacies
deposited in northeastern Muş in Eastern Anatolia using element analysis and sulfur, oxygen and strontium isotope combinations. The correlation diagrams of major ions and trace elements plots in the gypsum lithofacies indicate that carbonates and
clastics carried by terrestrial waters (streams, rivers, etc.) signifcantly contributed to the evaporitic phases. Furthermore,
the efects of hydrothermal solutions, increased salinity of the basin and presence of biological activity were associated with
high values for major ions and trace elements. While the δ18O and δ34S isotope contents of some gypsum lithofacies samples show that they are of terrestrial or marine origin, others do not indicate either origin because they are afected by other
factors such as water salinity variations, bacterial sulfate reduction, new terrestrial water and detrital inputs carried into the
basin. However, 87Sr/86Sr isotope concentrations analyzed in gypsum lithofacies in the study area indicate Rupelian (Lower
Oligocene) time, which is compatible with the Cenozoic age curve. Based on all these data, gypsum lithofacies in this basin
were deposited in a transition zone (shallow sea-lagoon-inland sabkha-mudfats) under the infuence of both marine and
terrestrial conditions. Therefore, we claim that this basin formed during the Rupelian (Early Oligocene), at a time when the
sea connection between the Indian Ocean and the eastern Mediterranean was restricted; that is, the southern branch of the
Neotethys was extremely shallow during this period and almost terminated in the study area
... In addition to long-term trends, Holser (1977) identified shorter fluctuations (5-50 Myr) in the Upper Devonian and lower Triassic evaporite record; these excursions are recorded by CAS as well (Kampschulte & Strauss, 2004). Increased temporal resolution from barite and CAS found additional rapid excursions, notably associated with Jurassic and Cretaceous intervals of widespread organic-rich shale deposition (Gill, Lyons, & Jenkyns, 2011;Paytan et al., 2004) and Paleogene carbon cycle perturbation (Paytan et al., 1998;Rennie et al., 2018). In addition, some δ 34 S records with high temporal resolution, especially derived from CAS, have rapid variability (Kah et al., 2016;Kampschulte et al., 2001), and data from multiple locations containing similarage strata have δ 34 S heterogeneity (Gill, Lyons, Young, et al., 2011;Present et al., 2015). ...
The δ34S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ34S values must be derived from proxy materials that are impacted differently by depositional and post-depositional processes. We produced new timeseries estimates for the δ34S value of seawater sulfate by combining 6710 published data from three sedimentary archives—marine barite, evaporites, and carbonate-associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ34S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high-frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ34S proxies, indicating that post-depositional processes obscure detailed knowledge of seawater sulfate’s δ34S value deeper in time.
Oceanic Anoxic Event 2 (OAE2) has been the focus of considerable research, but biogeochemical dynamics during the recovery from the carbon cycle disturbance largely remain unknown. Here, we present a high-resolution reconstruction of water column redox and nutrient cycling across the final stages of OAE2, in order to assess controls on the termination of widespread ocean anoxia. We focus on calcareous black shales deposited on a subtropical shelf at Tarfaya, Morocco, representing a location prone to water column anoxia beyond the temporal extent of the OAE itself. Our multi-proxy approach combining iron-sulfur systematics with redox-sensitive trace metal (U and Mo) concentrations documents persistent anoxia, with cyclic fluctuations between weakly euxinic (or possibly ferruginous) conditions and more intense euxinia. During the end of the carbon isotope plateau phase, elemental weathering ratios indicate muted variability in chemical weathering intensity, although fluctuating redox conditions may have been driven by changes in continental weathering inputs of sulfate and reactive iron. By contrast, during the recovery phase, changes in chemical weathering intensity appear to have exerted a strong control on redox fluctuations. Overall, the recovery phase documents progressively less reducing conditions and less intense chemical weathering, which resulted in decreased P recycling and intervals of P drawdown, as indicated by P phase partitioning results. These trends were interrupted by the Holywell Event, during which more intense euxinia and enhanced P recycling transiently returned during an interval of particularly low chemical weathering. Nevertheless, the general trend towards lower P bioavailability in the water column, due to both sequestration of P in the sediments and a likely progressive decrease in P supply via continental weathering, appears to have exerted a major control on the recovery from oceanic anoxia in this shelf setting, and potentially on a global scale.
A new matrix‐matched reference material (NWU‐Brt) with sulfur isotope ratios resembling those of natural barites has been developed for in situ S isotope measurements by laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS). A 100 g quantity of natural barite crystal was milled to ultra‐fine particles and sintered to a solid block using a fast hot‐pressing sintering technique (FHPS). We report δ ³⁴ S ratios determined by isotope ratio mass spectrometry (IRMS), solution nebuliser multi‐collector inductively coupled plasma‐mass spectrometry (SN‐MC‐ICP‐MS) and LA‐MC‐ICP‐MS, involving up to six participating laboratories. The homogeneity of δ ³⁴ S ratios of the synthesised barite was tested by LA‐MC‐ICP‐MS with an analytical spot size of 53 μm. The LA‐MC‐ICP‐MS results show that NWU‐Brt demonstrates a satisfactory homogeneous composition and is an appropriate material for calibrating δ ³⁴ S ratios in barite. IRMS and SN‐MC‐ICP‐MS produced mean δ ³⁴ S values of +14.17 ± 0.42‰ (2 s ) and +14.27 ± 0.23‰ (2 s ), respectively. The results of LA‐MC‐ICP‐MS analyses are consistent with the IRMS and SN‐MC‐ICP‐MS ratios within uncertainty. Overall, our results confirm the suitability of NWU‐Brt for the calibration of δ ³⁴ S ratios in barite using LA‐MC‐ICP‐MS.
Reconstructions of past environmental conditions and biological activity are often based on bulk stable isotope proxies, which are inherently open to multiple interpretations. This is particularly true of the sulfur isotopic composition of sedimentary pyrite (δ34Spyr), which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways. We present a microanalytical approach to deconvolving the multiple signals that influence δ34Spyr, yielding both the unambiguous determination of microbial isotopic fractionation (εmic) and new information about depositional conditions. We applied this approach to recent glacial-interglacial sediments, which feature over 70‰ variations in bulk δ34Spyr across these environmental transitions. Despite profound environmental change, εmic remained essentially invariant throughout this interval and the observed range in δ34Spyr was instead driven by climate-induced variations in sedimentation.
The sulfur (S) mass fraction of carbonate minerals can be used to reconstruct the environmental conditions and S sources at the time of precipitation. As S is present in a wide range of host materials, there is an urgent need to develop a method to extract S from a single mineral phase. We have developed a method to extract structurally substituted sulfate, termed carbonate‐associated sulfate (CAS), from geological and biogenic carbonate minerals using weakly acidic cation exchange resins. Two types of weakly acidic cation exchange resin (methacrylic acid and acrylic acid types) were tested to minimise the blank S contents and decompose carbonate. After repeated cleaning of the resins with high‐purity water or HCl, the blank S contents were reduced to < 0.1 μg, which is < 0.1% of the CAS in the samples. The cleaned resin was used to dissolve 10 and 25 mg of the JCp‐1 carbonate certified reference material (CRM; Japanese National Institute of Advanced Industrial Science and Technology, AIST). Samples and resin were added to 8 ml of high‐purity water at resin/sample ratios of 2, 5, 10 and 20, set on a shaking table, and reacted. The supernatant solutions were sampled sequentially from 0.5 h to 87 h after the start of experiments. The results show that the optimal conditions for decomposing 10 mg of carbonate is a resin/sample ratio of ≥ 10 with a reaction time of ≥ 40 h. Carbonate‐associated S mass fractions were measured for six geological and biogenic carbonate CRMs. The coefficient of variation in carbonate‐associated S mass fractions was ≤ 7%, regardless of the type of resins used. The mass fractions determined with this method recover 74–94% of the total S mass fractions reported in previous studies, suggesting that this method dissolved carbonate, and did not leach other S‐bearing fractions that are not resistant to weak acids. Another benefit of this method is that the decomposed solution can be introduced directly into the ion chromatograph, allowing for more sensitive analyses. We emphasise that this method can also be used for S isotopic measurements, as S contamination from other S‐bearing mineral phases is low.
The stratigraphy of the European late Permian-Triassic commonly lacks chronostratigraphic constraint due to the scarcity of diagnostic fossils for biostratigraphy. This is particularly true for the United Kingdom, and as a result, stratigraphic correlation within and between sedimentary basins is primarily reliant on lithostratigraphy. Evaporitic sulphate can be used to develop time series of δ³⁴Sevap data that can be utilised for stratigraphic correlation. However, the availability of continuous drillcore is limited, whilst drill cuttings are commonly acquired but are widely overlooked for stable isotope stratigraphy. We derive a δ³⁴Sevap record from drill cuttings from the southern North Sea Basin, and successfully correlate it with an equivalent published δ³⁴Sevap record from a continuous drillcore in the Cleveland Basin, Yorkshire, United Kingdom. We have chosen seven points in the δ³⁴Sevap records for stratigraphic correlation, defining eight packages of isotopically distinct coeval strata. This is significant, as the ubiquity of drill cuttings presents the opportunity to derive δ³⁴Sevap curves with high geospatial resolution. Equivalent gamma ray logs were used for correlation and compared with the δ³⁴Sevap curves. The correlations agree relatively well, however, the δ³⁴Sevap correlation permits the development of more robust chronostratigraphic constraints. Specifically, the δ³⁴Sevap records constrain the age of the Bunter Shale and Bunter Sandstone in the western Southern North Sea to the latest Permian. This has significant implications for understanding the stratigraphy and palaeogeographic evolution of United Kingdom Permian-Triassic sedimentary basins, and may have economic implications, since the Bunter Sandstone is being considered as a potential reservoir for CO2 storage in the United Kingdom sector.
Mehdiabad is the world’s largest Mississippi Valley-type (MVT) Zn-Pb deposit (394 million tonnes [Mt] of metal ore at 4.2% Zn, 1.6% Pb) and contains significant barite resources (>40 Mt). Such large accumulations of barite are not common in carbonate-hosted Zn-Pb deposits. Therefore, the origin of the barite and its association with the Zn-Pb mineralization is of significant interest for further investigation.
Field work and petrographic studies indicate that the Zn-Pb-Ba orebodies in the Mehdiabad deposit are hosted by Lower Cretaceous carbonate units of the Taft and Abkuh Formations. Fine- to coarse-grained barite with lesser siderite formed in three stages (S1, S2, and S4), along with a quartz-sulfide stage (S3) with minor quartz, sphalerite, galena, chalcopyrite, and pyrite, and the main Zn-Pb sulfide stage (S5) with massive sphalerite and galena.
The barites have δ34S values from 17.7 to 20.6‰, δ18O values from 13.2 to 16.8‰, Δ33SV-CDT values from –0.001 to 0.036‰, and initial 87Sr/86Sr ratios from 0.707327 ± 0.000008 to 0.708593 ± 0.000008 (V-CDT = Vienna-Canyon Diablo Troilite). The siderites have δ13CV-PDB values from –3.8 to –2.7‰, and δ18OV-SMOW values from 18.2 to 20.9‰ (V-PDB = Vienna-Pee Dee Belemnite, V-SMOW = Vienna-standard mean ocean water). These geochemical data, and the barite morphology, point to a diagenetic origin for all stages of barite. We suggest that S1 and S2 barite precipitated from pore fluids at the sulfate-methane transition zone in a methane-diffusion-limited environment with increasing methane content. S4 barite precipitated when the methane- and barium-bearing cold-seep fluid migrated to the shallow carbonate sediments and formed a methane-in-excess setting. For the three stages, the SO42- in barite came from the residual SO42- in pore fluids undergoing sulfate-driven anaerobic oxidation of methane, and the Ba2+ came from dissolved biogenic barite and terrigenous materials in the Taft and Sangestan Formations.
Primary fluid inclusions trapped in S3 quartz have salinities of 5.6 to 8.1 wt % NaCl equiv and homogenization temperatures of 143.8° to 166.1°C. The quartz has δ18OV-SMOW values ranging from 9.8 to 22.5‰ and δ30Si values from –1.3 to –0.9‰. These data indicate hydrothermal fluid flow occurred between the diagenetic S2 and S4 events. Secondary fluid inclusions with salinities of 17.70 to 19.13 wt % NaCl equiv and homogenization temperatures of 123.0° to 134.0°C are found in the S3 quartz, too. They might represent the hydrothermal event formed by basinal brines in S5.
According to the ore textures and the comparison of the sulfur isotopes between S5 Zn-Pb sulfides and the digenetic barites, the barite provided a host and a sulfur source for the later Zn-Pb mineralization. The relationship between barite and the Zn-Pb mineralization indicates that significant accumulations of sulfates may be a critical exploration target for this kind of giant deposit.
The δ³⁴S values of porewaters, carbonate associated sulfate, and celestine were analyzed from International Ocean Discovery Program Leg 359 Sites U1466, U1467, U1468, and U1471 drilled in The Maldives. These analyses reveal a complex diagenetic history in which the nature of the porewater gradients and their δ³⁴S values varied multiple times during deposition and subsequent diagenesis, with changes recorded in the δ³⁴S values of carbonate associated sulfate and celestine. The δ³⁴S values of these components as well the pore fluids have been influenced by bacterial sulfate reduction which elevates the δ³⁴S values while the concentrations of SO4²⁻ in the interstitial porewaters are drawn down. In most instances, the δ³⁴S values of the carbonate associated sulfate and celestine bear little resemblance to the present day porewater δ³⁴S values or those of the contemporaneous seawater. In cases where the δ³⁴S values of the carbonate associated sulfate are similar to those of the porewaters, it suggests that neomorphism and/or recrystallization processes are taking place at the present time. Where δ³⁴S values of the carbonate associated sulfate are similar to the original sediment values it is likely that either little diagenesis has taken place, or that it has taken place in an environment in which the pore fluids had a δ³⁴S value not significantly different than the original sedimentary value. The δ³⁴S values of the celestine at Site U1467 provide a further constraint in that it indicates formation at a time in which the pore fluids had an even more positive δ³⁴S value, and thus a lower SO4²⁻concentration and higher Sr²⁺ concentrations.
Marine barite is widely considered a reliable recorder of sulfur and oxygen isotope compositions of seawater sulfate. The traditional barite extraction method involves multiple oxidative cleaning steps to sequentially remove other minerals from sediments and additional purification steps to remove residual insoluble O-bearing minerals like rutile. During these processes pyrite is likely oxidized, thereby introducing sulfur from pyrite and oxygen from water and/or air to sulfate. We systematically investigate the effects of pyrite oxidation during barite extraction using two sets of synthetic marine sediments spiked with varying amounts of pyrite. One is subjected to acid-based leaching and Na2CO3-based purification, while the other is only treated with Na2CO3. Our results show a negative correlation between the pyrite-to-barite mass ratio and the apparent S- and O-isotope values of extracted barite. The difference in S-isotope ratios between two sample sets suggests that acid leaching with subsequent Na2CO3 purification results in the conversion of approximately 9% of the added pyrite to barite, whereas purification alone converts about 4.8% of the added pyrite. Based on these findings, we estimate that for marine sediments with a pyrite-to-barite ratio lower than 3 wt% and an S-isotope difference smaller than 50‰ between marine barite and sedimentary pyrite, the S-isotope offset imparted by pyrite oxidation during sequential leaching is within a 0.3‰ error.
The sulfur isotopic composition of marine sediments and sedimentary rocks reveals a wealth of information about sulfur cycling on different spatial and temporal scales. Sulfate minerals such as gypsum/anhydrite, barite or carbonate‐associated sulfate provide a temporal record of seawater evolution that reflects secular changes in the global sulfur cycle. Dissolved porewater sulfate and sulfide, but more so sedimentary iron sulfides and/or organic sulfur reveal details about the diagenetic evolution of marine sediments. Mass‐independent sulfur isotope anomalies have proven to be a prime recorder for the atmospheric oxygen abundance in the first half of Earth's history.
The study area is located in northeastern Anatolia and represents a thick sequence (with gypsum, halite, clay, carbonate and siliciclastics) in a Miocene intermontane basin. The purpose of this study is to determine the depositional basin conditions, diagenetic processes and parent brine of the Miocene evaporite deposits (salts and gypsums) in the basin, by using sedimentological properties and geochemical composition. Sedimentological parameters and high positive correlations between some major ion (Fe, Al, and P oxides) and trace element (Zn, Co, Ni, Mo, Ba, and Sr, etc.) concentrations reveal that Tuzluca evaporites accumulated in a shallow terrestrial environment (playa lake-mud flats) exposed to high salinity conditions
(Al2O3-Sr/Ba) under the effects of tectonism, diagenesis processes (replacement, dissolution-recrystallization, etc.), intense wave and biological activity (Zn-Fe2O3-P2O5, Ba-Sr-Fe2O3, etc.). The 87Sr/86Sr, δ34S and δ18O results of the evaporite minerals also support the view that these evaporites may originate from nonmarine water. Also, these values suggest that these may be derived from the dissolution and re-precipitation of ancient marine evaporites. The variable crystal sizes of the evaporites and low δ18O values indicate that the salinity and pH conditions in the environment changed from time to time. The high trace element values, positive Ce and Eu anomalies and low δ34S value reflect the important contribution of diagenetic and hydrothermal solutions to these evaporites. Some important positive correlations between major ions (Si, Al, Mg, and Ti, etc.), trace (Rb, Th, Pb, Zr, Hf, and Ni etc.) and rare earth elements (La, Ce, Er, etc.), the distribution pattern of REE normalized to MUQ and chondrite, and low δ18O values indicate terrigenous detritic input transported into the basin with the contribution of terrestrial waters (river, meteoric, etc.). Also, the presence of fine-grained clay minerals (kaolinite, illite and montmorillonite), which are determined by these relationships and X-Ray Diffractometer (XRD) reveal that the evaporites deposited in the basin were exposed to both humid and hot-dry climates under alkaline conditions.
Seawater sulfate is one of the largest oxidant pools at Earth's surface today and its concentration in the oceans is generally assumed to have varied between 5 and 28 mM since the early Phanerozoic Eon. Intermittent and potentially global Oceanic Anoxic Events (OAEs) are accompanied by changes in seawater sulfate concentrations and signal perturbations in the Earth system associated with major climatic anomalies and biological crises. Ferruginous (Fe-rich) ocean conditions developed transiently during multiple OAEs, implying strong variability in seawater chemistry and global biogeochemical cycles. The precise evolution of seawater sulfate concentrations during OAEs, however, is uncertain and thus models that aim to mechanistically link oceanic anoxia to broad-scale disruptions in the Earth system remain incomplete. We use analyses of Fe-speciation and redox sensitive trace metals in sediments deposited in the Tethys and Pacific oceans to constrain seawater sulfate concentrations and underlying dynamics in marine chemistry during OAE1a, ∼120 Ma. We find that large parts of the global oceans were anoxic and ferruginous for more than 1 million years. Calculations show that the development of ferruginous conditions requires that seawater sulfate concentrations drop below 600 μM and possibly below 100 μM, which is an order of magnitude lower than previous minimum estimates. Such a collapse of the seawater sulfate pool in less than one-hundred thousand years is a key and previously unrecognized feature of Phanerozoic Earth surface redox budgets. This sensitivity of the Earth system to changes in seawater sulfate concentrations illustrates potential for dramatically altered global biogeochemical cycles with corresponding climate impacts on remarkably short timescales.
Porosity formed after compaction has been recognized for many years in carbonate rocks, but there is little consensus as to the origin of the fluids responsible for dissolution, and the processes by which pores are generated. In particular, it is often hard to identify the source of fluids that are below calcite saturation, and in sufficient volumes, to explain the apparent volume of porosity. This study directly addresses this challenge through analysis of core data from a number of oil fields across north Oman, which produce from the lower Cretaceous (Aptian) Shuaiba Formation. The Shuaiba Formation in all these fields exhibits high total porosity, comprising pores that range in size from less than a micron to centimetres in diameter. There is an increase in the effective porosity upwards within the succession; permeability is highest in the upper 15 m of the formation, beneath the top - Shuaiba unconformity. Since this unconformity represents a hiatus of over 5 million years (My), caused by platform emergence, previous studies often interpreted porosity modification to be due to dissolution by groundwater during penecontemporaneous exposure of the platform. Nevertheless, paragenetic relationships indicate that most of the secondary porosity that was formed early in the burial history was either occluded by drusy calcite cements, or destroyed by compaction. Furthermore, many of the drusy calcite cements are etched, indicating that dissolution took place after cementation. Consistent with this, many of the pores formed on the margins of tectonic fractures and stylolites, indicating that some porosity formation post-dates compaction.
The top of the Shuaiba Formation in all fields is between 600 and 1600 m following uplift in the late Cretaceous and early Oligocene by inversion along deep-seated, strike-slip faults. Possible causes of dissolution are cooling of upward-rising brines fluxed along faults and corrosion by organic acids prior to hydrocarbon emplacement. It is unclear, however, as to the extent to which these fluids could dissolve carbonate before reaching equilibrium. A third possible fluid is groundwater. Sulphur isotope data from paragenetically late pyrite, is consistent with bacterial sulphate reduction, indicative of relatively cool temperatures (< 60 °C). Porosity enhancement might, therefore, have been facilitated by mixing of groundwater with highly saline reservoir fluids, resulting in undersaturation with respect to calcium carbonate. Overall, the data reveal a complex history of porosity modification, which relates to rock fabric, stratal architecture and tectonic history and suggests multiple controls on post-compactional porosity modification. Most significantly, the data strongly suggest that dissolution occurred during orogenic uplift and telogenesis, providing a new explanation for how porosity formation can occur after lithification, along stylolites and fractures, from a volumetrically significant source of carbonate-undersaturated fluid.
Extraterrestrial materials consist of samples from the moon, Mars, Vesta and a variety of smaller bodies such as asteroidsAsteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar systemSolar system. These heterogeneities are not observed on Earth, because they have become obliterated during high-temperature processes over geologic time. Nevertheless, isotopes have been used as a genetic link between meteorites and the Earth (i.e. Clayton in Treatise on geochemistry, Elsevier, Amsterdam, 2004).
Management of water resources in hyper-arid areas faces vital challenges in a global climate change context. Consequently, understanding the effects on groundwater sources can help mitigating the problem of water scarcity and the negative impact of human intervention on the environment. A case study area in the hyper-arid climate of the United Arab Emirates, was tackled here with the focus on applying stable isotopes as tools for evaluating groundwater sources and quality assessment. The results of major ions indicate variable increase in groundwater salinity moving away from Al Hajar Mountains recharge areas to the discharge areas (Arabian Gulf coast). The data of stable isotopes (δ¹⁸OH2O, δ²HH2O, δ¹⁸ONO3, δ¹⁵NNO3, δ¹⁸OSO4, δ³⁴SSO4, δ¹¹B) suggest impact of paleo-groundwater in the abstractions of the wells nearest to the coast. Nitrate isotopes indicate farming activities sources that can be masked due to the contribution from the nitrate-poor paleo-groundwater. Nitrate reduction processes are expected near to the recharge front. Sulphate and boron isotopes further suggest that influence of ancient evaporite dissolution in salinization. Management efforts should be focused on the diffuse sources of quality mitigations that can be vital in fingerprinting local and regional (transboundary) effects.
Authigenic carbonate and barite crusts were analyzed from recently discovered cold seeps on the Lofoten-Vesterålen (LV) continental slope, northern Norway. Carbonate phases in these crusts are methane-derived Mg-calcite and aragonite. Scanning electron microscopy (SEM) was used to petrographically characterize cold seep crusts, and secondary ion mass spectrometry (SIMS) was used to measure the microscale sulfur isotope composition (δ34S values) of authigenic barite. The mean δ34SBaSO4 value from SIMS spot analyses is 70‰ (n = 303), significantly elevated with respect to seawater sulfate (∼21‰). The δ34S values can vary more than 40‰ within individual barite aggregates (< 250 μm) and more than 50‰ within cm-scale samples. δ34SBaSO4 values in layered barite aggregates are most variable parallel to growth axes, with minimal variability in the perpendicular direction; fluctuations in δ34SBaSO4 values along growth axes are inferred to record temporal changes in sulfate distillation during barite precipitation. In layered barite aggregates, δ34SBaSO4 values frequently approach ∼90‰, but at these high δ34S values, barite dissolution features become increasingly prevalent and may reflect an upper limit for porewater ³⁴SSO4 enrichment while maintaining barite saturation in this system. We suggest the primary forcing affecting sulfate distillation is varying activity of anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR) due to temporal changes in cold seep methane flux. These findings provide the first semi-continuous geologic proxy for paleo-methane flux on the Lofoten-Vesterålen continental margin and suggest methane advection rates varied considerably over the course of carbonate-barite crust formation. In addition to systematic microscale changes in δ34SBaSO4 values, periodic intra-aggregate dissolution features indicate a dynamic seepage environment with two or more periods of enhanced methane flux in recent geological history.
In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.
Reconstruction of ocean paleoproductivity and paleochemistry is paramount to understanding global biogeochemical cycles such as the carbon, oxygen and sulfur cycles and the responses of these cycles to changes in climate and tectonics. Paleo-reconstruction involves the application of various tracers that record seawater compositions, which in turn may be used to infer oceanic processes. Several important tracers are incorporated into pelagic barite, an authigenic mineral that forms in the water column. Here we summarize the utility of pelagic barite for the reconstruction of export production and as a recorder of seawater S, O, Sr, Ca and Ba.
Ferromanganese crusts have been used as archives for monitoring long-term evolution of seawater chemistry throughout the Cenozoic. Because of their high metal concentrations, ferromanganese crusts are logical targets for analyzing both radiogenic and stable metal isotopes. An extensive dataset of metal isotopes is now available from ferromanganese crusts. However, further work is needed to validate age models of ferromanganese crusts and to develop frameworks to interpret the isotope signatures. This paper is intended to provide an overview of paleoceanographic study using ferromanganese crusts with special attention to their isotope records. We begin by reviewing age-dating methods of ferromanganese crusts using beryllium and osmium isotopes. Subsequent section summarizes strontium, neodymium, lead, and stable metal (such as molybdenum and thallium) isotope records of ferromanganese crusts which have been used to understand the timing of phosphatization, changes in ocean circulation patterns, and evolution of oceanic elemental cycles. Finally, we will discuss possible directions for future paleoceanographic studies using isotopic compositions of ferromanganese crusts.
The major element, rare earth element, and platinum-group element
chemistry of the Margi umbers have been investigated in order to
evaluate these sediments as recorders of past variations in the Os
isotopic composition of seawater. Because of the compositional
similarity of the Margi umbers to metalliferous sediments and
hydrothermal-plume particles from the modern ocean, we conclude that
these supraophiolite sediments record the Os isotope composition of
Cretaceous seawater. This conclusion requires that the Margi umbers have
in large part retained their compositional integrity throughout burial
diagenesis and subsequent uplift. That the
187Os/188Os ratio (0.52 0.55) of ca. 90 Ma
seawater is similar to, or more radiogenic than, seawater throughout the
Paleogene provides evidence of decoupling of the marine Sr and Os
isotope records. Beyond constraining the
187Os/188Os of Cretaceous seawater, these results
are significant in that they provide a set of chemical criteria that can
be used to evaluate the potential of other metalliferous sediment
sequences as recorders of the 187Os/188Os ratio of
ancient seawater. Establishing these criteria represents an important
step toward extending the marine Os isotope record into the Mesozoic and
Paleozoic.
We demonstrate quantitatively that the world-wide Mid to Upper Cretaceous transgression and subsequent regression may have been caused by a contemporaneous pulse of rapid spreading at most of the mid-oceanic ridges between −110 to −85 m.y. The rapid spreading caused the ridges to expand and hence reduced the volumetric capacity of the basins. The subsequent regression was caused by a reduction in spreading rates beginning at −85 m.y.
Pelagic Cretaceous sediments, deposited in a range of palaeotectonic and
palaeogeographic settings, from continents to oceans, are commonly black and bituminous.
3 particular time-envelopes define the major occurrences of such facies: late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and, to a lesser extent, the Coniacian-Santonian. These intervals define the duration of so-called Oceanic Anoxic Events during which globalmarine waters were relatively depleted in oxygen, and deposition of organic matter, derived from both terrestrial and planktonic sources, was widespread. Cretaceous OAEs correlate closely with transgressions, and such a correlation exists throughout the stratigraphical column. Flooding of land-masses is thought to have transported much terrestrial plant material seawards; the progressive increase in shelf-sea area is thought to have stimulated production of marine plankton. Bacterial consumption of this organic matter favoured the development of poorly oxygenated mid- to late Cretaceous waters in which many of the characteristic facies of the Period, including glauconitic sandstones and phosphatic chalks, were deposited.
Geologic records of isotopic changes in ocean composition present the possibility of unraveling the history of secular changes in the geochemical cycles of carbon, sulfur, and strontium. In general, the conventional interpretation of the C and S isotopic age curves, as reflecting changes in burial fluxes, is favored; however, short-term fluctuations may have been caused by these other effects. For Sr there is no clear, conventional interpretation of secular variability in oceanic isotopic composition against which to weigh the alternatives. -from Author
Models describing the evolution of the partial pressure of atmospheric oxygen over Phanerozoic time are constrained by the
mass balances required between the inputs and outputs of carbon and sulfur to the oceans. This constraint has limited the
applicability of proposed negative feedback mechanisms for maintaining levels of atmospheric O2 at biologically permissable levels. Here we describe a modeling approach that incorporates O2-dependent carbon and sulfur isotope fractionation using data obtained from laboratory experiments on carbon-13 discrimination
by vascular land plants and marine plankton. The model allows us to calculate a Phanerozoic O2 history that agrees with independent models and with biological and physical constraints and supports the hypothesis of a
high atmospheric O2 content during the Carboniferous (300 million years ago), a time when insect gigantism was widespread.
More than two hundred aerobic continental margin, aerobic deep sea, dysaerobic, and anaerobic / euxinic sediments have been examined for their variations in different operationally defined iron fractions, each of which represents a different reactivity towards dissolved sulfide. Aerobic continental margin, deep sea, and dysaerobic sediments contain similar contents of highly reactive iron (dithionite-soluble iron plus pyrite iron), poorly reactive iron (iron soluble in HCl less that soluble in dithionite), and unreactive iron (total iron less that soluble in HCl). By contrast non-turbidite euxinic samples from the Black Sea, as well as euxinic samples from the Cariaco Basin and Framvaren are enriched in highly reactive iron. These sediments contain a small lithogenous fraction and a large biogenous, organic C-rich fraction, which decays by sulphate reduction in an iron-rich water column to form pyrite-rich sediment. Other anaerobic / euxinic samples from the Black Sea, Orca Basin, and Kau Bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron. Degrees of Pyritization (DOP) for all the aerobic, dysaerobic, and anaerobic/euxinix samples (except those low in biogenous material) are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction.
Atmospheric carbon dioxide concentrations are believed to drive climate changes from glacial to interglacial modes, although geological and astronomical mechanisms have been invoked as ultimate causes. Additionally, it is unclear whether the changes between cold and warm modes should be regarded as a global phenomenon, affecting tropical and high-latitude temperatures alike, or if they are better described as an expansion and contraction of the latitudinal climate zones, keeping equatorial temperatures approximately constant. Here we present a reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past approximately 550 Myr) from our database of oxygen isotopes in calcite and aragonite shells. The data indicate large oscillations of tropical sea surface temperatures in phase with the cold-warm cycles, thus favouring the idea of climate variability as a global phenomenon. But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric carbon dioxide concentrations. The results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.
Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinction. Most ocean anoxic events are thought to be caused by high productivity and export of carbon from surface waters which is then preserved in organic-rich sediments, known as black shales. But the factors that triggered some of these events remain uncertain. Here we present stable isotope data from a mid-Cretaceous ocean anoxic event that occurred 112 Myr ago, and that point to increased thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff that led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer duration (approximately 46 kyr) of ocean anoxic event 1b suggest that processes leading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.
The evolution of non-photosynthetic sulphide-oxidizing bacteria was contemporaneous with a large shift in the isotopic composition of biogenic sedimentary sulphides between 0.64 and 1.05 billion years ago. Both events were probably driven by a rise in atmospheric oxygen concentrations to greater than 5-18% of present levels--a change that may also have triggered the evolution of animals.
A mathematical model has been constructed that enables calculation of the level of atmospheric O2 over the past 570 my from rates of burial and weathering of organic carbon (C) and pyrite sulfur (S). Burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance, and C and S contents, of the three rock types: marine sandstones and shales, coal basin sediments, and other nonmarine clastics (red beds, arkoses). Results of the modeling indicate that atmospheric O2 probably has varied appreciably over Phanerozoic time. During the Late Carboniferous and Permian periods O2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps. A large decrease in O2 during the Late Permian was due probably to the drying-up of the coal swamps and deposition of a large proportion of total sediment in C and S-free continental red beds. -from Author
Systematic changes in the chemistry of evaporated seawater contained in primary fluid inclusions in marine halites indicate that seawater chemistry has fluctuated during the Phanerozoic. The fluctuations are in phase with oscillations in seafloor spreading rates, volcanism, global sea level, and the primary mineralogies of marine limestones and evaporites. The data suggest that seawater had high Mg2+/Ca2+ ratios (>2.5) and relatively high Na+ concentrations during the Late Precambrian [544 to 543 million years ago (Ma)], Permian (258 to 251 Ma), and Tertiary through the present (40 to 0 Ma), when aragonite and MgSO4 salts were the dominant marine precipitates. Conversely, seawater had low Mg2+/Ca2+ ratios (<2.3) and relatively low Na+ concentrations during the Cambrian (540 to 520 Ma), Silurian (440 to 418 Ma), and Cretaceous (124 to 94 Ma), when calcite was the dominant nonskeletal carbonate and K-, Mg-, and Ca-bearing chloride salts, were the only potash evaporites.
Deals with the use of sulfur, nitrogen and hydrogen isotopes in recognizing sources of organic matter and subsequent organic matter alteration during oxidation, bacterial degradation and thermal maturation and in tracing the geochemical cycles of nitrogen and sulfur. -from Editor
A model that tracks the coupled cycling of carbon, phosphorus, iron, sulfur, and oxygen as well as carbon and sulfur isotope ratios through surficial reservoirs on multi-million year time scales has been constructed. Phosphorus-limited marine productivity, in which surface ocean P availability is coupled to the degree of anoxia in ocean bottom waters, is employed to enhance Po2 stability. In separate trials, parameters controlling continental weathering fluxes, ocean vertical mixing rates, and burial of terrigenous organic matter are adjusted to examine the effects on the model in terms of reservoir masses, fluxes, and isotopic compositions. As expected, the system responds to imposed perturbations with significant changes in the rates of both burial and weathering of organic carbon and pyrite. These changes in turn influence the isotope ratios of carbon and sulfur reservoirs and the mass of atmospheric oxygen. This paper explores the range of parameter values that concurrently generate equable Po2 and realistic reservoir masses and isotopic compositions. The perturbations applied in this model generate isotope variations of up to ±4 permil for dissolved inorganic carbon and up to ±3 permil for dissolved sulfate, generated under Po2 within a factor of 2 of the present atmospheric level. These isotope excursions last a minimum of 30 my for carbon isotope and significantly longer than model run time (150 my) for sulfur isotopes. Thus in both magnitude and duration, these isotope shifts begin to approach those observed in the geologic record of carbonate δ13C and gypsum δ34S, without requiring catastrophic O2 variations. Recognition that geologically realistic isotope excursions can be modelled with concurrent equable Po2 reaffirms the potential of the geologic records of δ13C and δ34S as useful tools to construct a history of Phanerozoic oxygen.
Original δ¹³C values of abiotically precipitated marine cements from a variety of stratigraphic intervals have been used to document secular variations in the δ¹³C values of Phanerozoic oceans. These, together with the °³⁴S values of coeval marine sulfates, are used to examine the global cycling of carbon and sulfur. It is generally accepted that secular variation in δ¹³C and δ³⁴S values of marine carbonates and sulfates is controlled by balanced oxidation-reduction reactions and that their long-term, steady-state variation can be predicted from the present-day isotopic fractionation ratio (Δc/Δs) the ratio of the riverine flux of sulfur and carbon (Fs/Fc). The predicted slope of the linear relation between δ¹³Ccarb and δ³⁴Ssulfate values is approximately −0.10 to −0.14. However, temporal variation observed in marine cement δ¹³C values and the 6345 values of coeval marine sulfates produces a highly significant linear relation (r² = 0.80; α > 95%) with a slope of −0.24; approximately twice the predicted value. This discordance suggests that either the Phanerozoic average riverine Fs/Fc was 1.6–3.3 times greater than today's estimates or that an additional source of ³⁴S-depleted sulfur or ¹³C-enriched carbon, other than continental reservoirs, was active during the Phanerozoic. This new relation between marine δ¹³C and δ³⁴S values suggests that the flux of reduced sulfur, iron, and manganese from seafloor hydrothermal systems affects oceanic O2 levels which, in turn, control the oxidation or burial of organic matter, and thus the δ¹³C value of marine DIC. Therefore, the sulfur system (driven by seafloor hydrothermal systems) controls the carbon system rather than organic carbon burial controlling the response of δ³⁴S values (via formation of sedimentary pyrite).
Past treatments of the variation of S 34 in marine evaporites have either assumed a steady-state ocean or have invoked rather simplified ocean input-output models. This paper derives more completely the relationships between the parameters that influence the time variation of S 34 in ocean water and the relationship between S 34 in ocean water and net gains and losses of atmospheric oxygen due to the operation of the sulfur cycle. The lower and mid-Paleozoic are shown to have been periods of net gain of atmospheric oxygen by the operation of the sulfur cycle; the upper Paleozoic, particularly the Permian, a period of oxygen loss. It is difficult to relate these oxygen gains and losses to variations in the oxygen content of the atmosphere, because the oxygen flux due to the operation of the carbon cycle is approximately twice as large as the flux due to the operation of the sulfur cycle. Data for the organic carbon and sulfide content of sedimentary rocks of the Russian Platform suggest that a decrease in sulfide from the Paleozoic to the Mesozoic and Cenozoic Era was roughly balanced by an increase in the proportion of organic carbon; however, such data are insufficient to define the abundance of atmospheric oxygen during the Phanerozoic. Biologic data and a better understanding of controls on atmospheric P o 2 are more likely to produce convincing evidence regarding variations of atmospheric oxygen in the past.
Statistical evaluation of 3056 δ13C measurements in carbonate rocks and fossils shows that they record a 2‰ 13C depletion from the late Proterozoic to the early Paleozoic, a 2.5‰ enrichment to the Permian, and a 1.5‰ depletion to the Cenozoic. These variations, not controlled primarily by facies or alteration phenomena, correlate negatively with the δ34S sulfate secular trend, as confirmed by collation of 1083 δ34S measurements. The correlation suggests that the biologically mediated redox fluxes of the C and S cycles have been approximately balanced through this long span of geological time, generally levelling available oxygen. Such a redox system is consistent with the controlling mechanism proposed by Garrels and Perry (1974). Consequently, the sedimentary reservoirs of Corganic as well as Sbacteriological'have varied through geological time.
Using a coupled fluid flow–mass transfer model to calculate the Sr flux resulting from the weathering of the young Columbia River basalts, it can be shown that the dissolution of the reactive phases in the extensive continental flood basalt could have caused the significant inflection in the marine Sr isotope record that occurred approximately 15 million years ago. The results of our model demonstrate the important and often overlooked role that the weathering of large igneous provinces can play in geochemical cycles. Furthermore, by approaching the question of continental Sr fluxes to the ocean using a mass transfer model, we are able to establish a quantitative relationship between the marine Sr record, chemical weathering rates and atmospheric CO2 concentrations. The CO2 drawdown rates that we calculate for the weathering of the Columbia River basalt suggest that, on a several million-year timescale, the formation of large igneous provinces represents a net sink for atmospheric CO2. The removal of CO2 via the rapid dissolution of the Columbia River basalts represents an alternative explanation for the glacial period believed to have followed its formation. We also consider the potential role that basalt weathering could play in the evolution of the 87Sr/87Sr ratio of the oceans since the early Cretaceous. Rapid Sr release during basalt dissolution combined with variations in the relative exposure of young vs. old lithologies could explain changes in the 87Sr/86Sr ratio of the oceans without calling on dramatic increases in the continental Sr flux over that time period. Our results demonstrate the importance of accounting for changes in the relative exposure of different lithologies when considering the nature of global geochemical cycles in the geologic past.
Recent studies of the geological record of cyclic chemical changes in
the earth atmosphere, oceans, and biosphere are discussed in reviews
contributed by leading experts. Topics addressed include the history of
geochemical science and the study of cyclic processes, geochemical
cycles in the continental crust and oceans, geochemical cycles of
atmospheric gases, the interactions of geochemical cycles with the
mantle, biogeochemical cycles of C and S, and the evolving exogenic
cycle. Diagrams, drawings, graphs, and tables of numerical data are
provided.
Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ34S, and 60 of these also for δ18O in the sulfate ion. Detailed δ34S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ34S. A summary curve for δ18O also shows definite variations, some at different times than δ34S, and always smaller. The measured δ34S and δ18O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ18O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ34S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ18O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ34S and δ18O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ34S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).
A calculation of Earth's ocean crustal budget for the past 150 m.y. reveals a 50% to 75% increase in ocean crust formation rate between 120 and 80 Ma. This "pulse" in ocean crust production is seen both in spreading-rate increases from ocean ridges and in the age distribution of oceanic plateaus. It is primarily a Pacific Ocean phenomenon with an abrupt onset, and peak production rates occurred between 120 and 100 Ma. The pulse decreased in intensity from 100 to 80 Ma, and at 80 Ma rates dropped significantly. There was a continued decrease from 80 to 30 Ma with a secondary peak near the Cretaceous/Tertiary boundary at 65 Ma. For the past 30 m.y., ocean crust has formed at a nearly steady rate. Because the pulse is seen primarily in Pacific oceanic plateau and ridge production, and coincides with the long Cretaceous interval of normal magnetic polarity, I interpret it as a "superplume" that originated at about 125 Ma near the core/mantle boundary, rose by convection through the entire mantle, and erupted beneath the mid-Cretaceous Pacific basin. The present-day South Pacific "superswell" under Tahiti is probably the nearly exhausted remnant of the original upwelling. How this superplume stopped magnetic field reversals for 41 m.y. is a matter of speculation, but it probably involved significant alteration of the temperature structure at the core/mantle boundary and the convective behavior of the outer core.
Through consideration of the likely controls of atmospheric O2 operative on a time scale of millions of years, a model has been constructed that shows how O2 can be affected by changes in these controls.-from Authors
Isotope fractionation during sulfate reduction was explored for natural populations of sulfate-reducing bacteria. High fractionations of 30‰ to 40‰ were produced when the natural population metabolized with indigenous organic substrate at environmental temperatures of 15°C to 25°C. Fractionations were unaffected by changes in sulfate concentration between 2 mM and 28 mM. After the natural substrate was exhausted, the sulfate-reducing bacterial population metabolized, in turn, with acetate, ethanol, and lactate. The high fractionations encountered with natural substrate were only reproduced when the amended substrate was supplied at concentrations limiting the activity of the sulfate-reducing population. Higher, nonlimiting concentrations of amended substrate produced lower fractionations of 16‰ to 21% at 25°C. The natural sulfate-reducing population, therefore, probably experienced substrate limitation while utilizing the natural substrate. At the low temperature of 5°C fractionations with amended substrate ranged from 8‰ to 14‰ and were lower than expected based on the normal relationship between rates of sulfate reduction and the extent of isotope fractionation. The processes likely acting to control the magnitude of isotope fractionation are discussed.
In present day marine sediments, almost all of which are deposited in normal oxygenated seawater, rates of burial of organic carbon (C) and pyrite sulfur (S) correlate positively and bear a constant ratio to one another (C/S ∼- 3 on a weight basis). By contrast, calculations, based on the isotopic model of Garrels and Lerman (1981), indicate that at various times during the Phanerozoic the worldwide burial ratio must have been considerably different than the present day value. This ratio change is caused by the requirement that, increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur, in order to maintain a roughly constant level of O2 in the atmosphere. Such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments, as at present, to non-marine freshwater, or to euxinic environments, in the geologic past. A shift to predominantly freshwater burial can help explain predicted high C/S ratios in Permo-Carboniferous sediments, and a shift to euxinic environments can help explain predicted low C/S ratios during the early Paleozoic. It is demonstrated that the three environments today exhibit distinguishably different average C/S ratios.
Anhydrite, gypsum and other sulfates were sampled from a wide range of origins, differing in: geological age, mineralogy, stage of evaporation, inland basin or oceanic facies, and primary or replacement crystallization. For sulfates of oceanic origin, variation with geological age is so large that it nearly masks all other factors. Tertiary, Recent and synthetic gypsum have within ±2‰ of the +20‰ of modern sea water. A large number of Late Permian samples from Germany and the United States are all in the range irrespective of mineralogy or other geological factors. Mesozoic samples are intermediate between the Permian and Tertiary levels, and Early Paleozoic sulfates have up to . These results confirm the general curve found by other workers, in which the isotopic composition of oceanic sulfate varies with time, on a worldwide scale.These variations can only be accomplished by large-scale transfer of sulfur to or from the shale reservoir. Inflow of sulfate exceeded sedimentation in the Late Paleozoic, to increase the sulfate content of the sea by 45‰ with a corresponding enrichment in 32S. In the Mesozoic, sedimentation of sulfide exceeded inflow removing the light sulfur preferentially from the sea. Review of the geochemical balance and cycle of sulfur indicates that an excess inflow of sulfate was derived from shale sulfide by an increase of either weathering or metamorphism-volcanism, along with an even larger amount of sulfur from old evaporites.Alternatively, a decrease of sulfide sedimentation is directly related to biological activity, which in turn may have been controlled by one of several geomorphic changes. The isotope evidence thus indicates substantial changes in the sulfate concentration of sea water, but is symptomatic of even more important changes in the tectonic and paleogeographical regime in the Late Paleozoic and Early Mesozoic.
Simple models for the geochemical response of the ocean to periodic forcing are used as a framework to discuss the shortest periods of change resolvable by present analytical precision of elemental or isotopic ratio measurements of the chemical evolution of seawater. For chemical properties whose budget is dominated by fluxes into and out of the ocean, we show that the geochemical response to changing fluxes is strongly attenuated at periods shorter than the residence time, and that the response at these periods is also phase shifted so as to lag the forcing by one quarter cycle. The amplitude response as a function of period is used to estimate the periods of forcing resolvable by present analytical precision in measuring the 87Sr/86Sr and 234U/238U evolution of seawater, and we find that the shortest periods resolvable are of the order of 1 myr. If variations at periods shorter than this are found, we would argue that these are very likely of local, as opposed to global, origin. We illustrate the importance of phase shifts as a function of forcing period by discussing some recently published data that show a high degree of correlation (over the last 300 kyr) between changes in 87Sr/86Sr and δ18O measured in the same sediment core. At first sight this correlation might be taken as good evidence that both 87Sr/86Sr and δ18O are responding to some common climatic forcing, but given the long residence time of Sr in the ocean, Sr and O should in fact have different phase shifts relative to any proposed common forcing. Thus the apparent correlation between 87Sr/86Sr and δ18O over the past several 100 Ma is not necessarily an indication that they are responding to a common cause. Amplitude response curves are also used to place limits on permissible global seawater 234U/238U variations, and these are then compared with published data to identify local (diagenetic) effects. Application to 187Os/186Os and U records in seawater detectors is proposed.
A currently emerging sulfur isotope record for Phanerozoic seawater, based on structurally substituted sulfate in stratigraphically well constrained biogenic carbonates, allows the detailed assessment of secular variations within the global sulfur cycle and the interaction between the sulfur and carbon cycles. It is superior to the evaporite-based dataset because it enables sampling of the entire biostratigraphic column. Discrete biological and environmental signals can be deciphered from a somewhat “noisy” sulfur isotope record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around −51‰ which appears to be constant throughout the entire Phanerozoic. Observable large spreads of δsulfide for any given sedimentary unit are caused by environmental parameters, such as type and availability of organic carbon or availability of sulfate. In particular, the growing importance of land plants and their impact on the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition.
An improved and updated version of the statistical LOWESS fit to the marine 87Sr/86Sr record and a revised look-up table (V3:10/99; available from j.mcarthur@ucl.ac.uk) based upon it enables straightforward conversion of 87Sr/86Sr to numerical age, and vice versa, for use in strontium isotope stratigraphy (SIS). The table includes 95% confidence intervals on predictions of numerical age from 87Sr/86Sr. This version includes the Triassic and Paleozoic record (0509 Ma) omitted from previous versions because of the paucity of adequate data at the time of preparation. We highlight differences between the previous versions of the table and the current version and discuss some aspects of the 87Sr/86Sr record that may have geological significance. We give examples of how the table can be used and where it has proven useful.
A continuous seawater sulfate sulfur isotope curve for the Cenozoic with a resolution of approximately 1 million years was generated using marine barite. The sulfur isotopic composition decreased from 19 to 17 per mil between 65 and 55 million years ago, increased abruptly from 17 to 22 per mil between 55 and 45 million years ago, remained nearly constant from 35 to approximately 2 million years ago, and has decreased by 0.8 per mil during the past 2 million years. A comparison between seawater sulfate and marine carbonate carbon isotope records reveals no clear systematic coupling between the sulfur and carbon cycles over one to several millions of years, indicating that changes in the burial rate of pyrite sulfur and organic carbon did not singularly control the atmospheric oxygen content over short time intervals in the Cenozoic. This finding has implications for the modeling of controls on atmospheric oxygen concentration.
It is quite possible that the level of atmospheric oxygen has varied (roughly between 15 and 30% O2) over the past 550 million years. This variation is suggested by modeling of the carbon and sulfur cycles, by the excessive sediment burial of organic matter that accompanied the advent of large vascular land plants, and by recent physiological studies that relate to biological evolution.
Data from modern and ancient marine sediments demonstrate that burial of the limiting nutrient phosphorus is less efficient when bottom waters are low in oxygen. Mass-balance calculations using a coupled model of the biogeochemical cycles of carbon, phosphorus, oxygen, and iron indicate that the redox dependence of phosphorus burial in the oceans provides a powerful forcing mechanism for balancing production and consumption of atmospheric oxygen over geologic time. The oxygen-phosphorus coupling further guards against runaway ocean anoxia. Phosphorus-mediated redox stabilization of the atmosphere and oceans may have been crucial to the radiation of higher life forms during the Phanerozoic.
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