No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Sulfur and oxygen isotopic compositions of carbonate associated sulfate (CAS) of Cambrian ribbon rocks: Implications for the constraints on using CAS to reconstruct seawater sulfate sulfur isotopic compositions
Abstract
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.
... Given the uneven occurrence and the poor age control typifying sedimentary barite and evaporites, CAS rocks has been used more widely to reconstruct the high-resolution sulfur isotope composition on the temporal evolution of the marine sulfur cycle throughout Earth's history (Burdett et al., 1989;Kampschulte and Strauss, 2004;Gill et al., 2007;Hurtgen et al., 2009;Ries et al., 2009;Turchyn et al., 2009;Thompson and Kah, 2012;Jones and Fike, 2013;Crowe et al., 2014;Fike et al., 2015;Osburn et al., 2015), favored by the wide and continuous distribution of carbonate rocks throughout the geological record (Fichtner et al., 2017;Richardson et al., 2019). Over the past decades, numerous studies have reported δ 34 S CAS values of bulk carbonate rocks, used as a proxy for the sulfur isotope composition of ancient seawater sulfate (Hurtgen et al., 2002;Kah et al., 2004;Kampschulte and Strauss, 2004;Newton et al., 2004;Gellatly and Lyons, 2005;Gill et al., 2005Gill et al., , 2007Fike and Grotzinger, 2008;Luo et al., 2010;Balan et al., 2014;Song et al., 2014;Wu et al., 2014;Shi et al., 2018;He et al., 2020;Ma et al., 2021) based on the similarity of δ 34 S CAS values with values of sedimentary sulfate minerals in evaporites and barite (Balan et al., 2014;Present et al., 2015) and no apparent or only minor isotope fractionation for sulfate sulfur incorporation into carbonate minerals (Newton et al., 2004;Kampschulte et al., 2001;Kampschulte and Strauss, 2004;Paris et al., 2014). CAS is therefore regarded as a reliable archive for δ 34 S and δ 18 O stratigraphy and the reconstruction of the evolution of the marine sulfur cycle during Earth's history (Kampschulte and Strauss, 2004;Newton et al., 2004;Fike and Grotzinger, 2008;Song et al., 2014;Wu et al., 2014;Schobben et al., 2015;Rose et al., 2019). ...
... Three possible processes were identified to influence the sulfur and oxygen isotope composition of CAS in reef carbonates, including contamination by (1) pyrite oxidation or deposition of secondary atmospheric sulfate (Marenco et al., 2008a;Peng et al., 2014), (2) watersoluble sulfate (WSS; Fichtner et al., 2017;N. Li et al., 2022), and (3) alteration during diagenesis (Berner, 2001;Emerson et al., 2003;Schrag et al., 2013;Ma et al., 2021). The contamination of CAS by pyrite oxidation or WSS can be minimized using appropriate extraction processes (Wotte et al., 2012;Peng et al., 2014;Theiling and Coleman, 2015;N. ...
... Several factors affecting the sulfur and oxygen isotope compositions of CAS in carbonate rocks include the amount of pore fluid sulfate that is incorporated into carbonate minerals, the rate and extent of carbonate recrystallization, microbial sulfate reduction, sulfide oxidation or secondary atmospheric sulfate, the original CAS contents, and the differences in sulfur and oxygen isotope compositions of pore fluid sulfate (Burdett et al., 1989;Berner, 2001;Marenco et al., 2008a;Emerson et al., 2003;Peng et al., 2014;Schrag et al., 2013;Ma et al., 2021). Because coral reefs grow mainly in oxidized, tropical shallow marine environments (Yu, 2012;Kuang et al., 2014;Shao et al., 2017a;Shao et al., 2017b;Guo et al., 2021), reef carbonates with their dominance of skeletal carbonate are different from other types of carbonates such as authigenic methane-seep carbonates (e.g. ...
Carbonate-associated sulfate (CAS) is a useful proxy for the reconstruction of the isotopic composition of ancient seawater sulfate, archiving information on the global sulfur cycle, Earth's surface redox evolution, and biological activity. The CAS proxy is possibly compromised by early diagenetic alteration, which may cause secondary inhomogeneities in the isotopic composition of pore water sulfate achieved in the form of CAS. However, the effects of early diagenesis on the retention of primary δ34S signals of CAS in reef carbonates are not well understood, and it is unknown whether reef carbonate deposits can faithfully record the coeval isotopic composition of marine sulfate. To provide new constraints on the resilience of CAS isotopic signatures of reef carbonate, we have measured and analyzed CAS content and CAS isotopic composition of reef carbonates from the Meiji Atoll (well NK-1) of the South China Sea, representing carbonates affected by meteoric and marine diagenesis including dolomitization. CAS contents were found to decrease rapidly during meteoric and marine diagenesis, with minimum CAS contents caused by prolonged subaerial exposure. Despite low contents, the corresponding δ34SCAS values were found to vary within a narrow range from 21.5 to 24.1‰, compared to values of 20.9 to 22.7‰ of coeval barite, suggesting that the original δ34SCAS values are preserved and largely unaffected by recrystallization. However, δ18OCAS values were found to vary widely, ranging from 7.1‰ to 15.0‰, only recording the same oxygen isotope signal as coeval marine barite for reef carbonates younger than 5 Ma. Our data reveal that reef carbonates can be used as an archive of paleoceanic change, faithfully tracking δ34S changes in coeval seawater regardless of whether reef carbonates underwent recrystallization, including dolomitization. In contrast to sulfur isotopes, the oxygen isotope composition of CAS is more susceptible to alteration during diagenesis. Overall, this study provides new insight for the future reconstruction of the sulfate sulfur and oxygen isotope record of ancient seawater.
... Given the uneven occurrence and the poor age control typifying sedimentary barite and evaporites, CAS rocks has been used more widely to reconstruct the high-resolution sulfur isotope composition on the temporal evolution of the marine sulfur cycle throughout Earth's history (Burdett et al., 1989;Kampschulte and Strauss, 2004;Gill et al., 2007;Hurtgen et al., 2009;Ries et al., 2009;Turchyn et al., 2009;Thompson and Kah, 2012;Jones and Fike, 2013;Crowe et al., 2014;Fike et al., 2015;Osburn et al., 2015), favored by the wide and continuous distribution of carbonate rocks throughout the geological record (Fichtner et al., 2017;Richardson et al., 2019). Over the past decades, numerous studies have reported δ 34 S CAS values of bulk carbonate rocks, used as a proxy for the sulfur isotope composition of ancient seawater sulfate (Hurtgen et al., 2002;Kah et al., 2004;Kampschulte and Strauss, 2004;Newton et al., 2004;Gellatly and Lyons, 2005;Gill et al., 2005Gill et al., , 2007Fike and Grotzinger, 2008;Luo et al., 2010;Balan et al., 2014;Song et al., 2014;Wu et al., 2014;Shi et al., 2018;He et al., 2020;Ma et al., 2021) based on the similarity of δ 34 S CAS values with values of sedimentary sulfate minerals in evaporites and barite (Balan et al., 2014;Present et al., 2015) and no apparent or only minor isotope fractionation for sulfate sulfur incorporation into carbonate minerals (Newton et al., 2004;Kampschulte et al., 2001;Kampschulte and Strauss, 2004;Paris et al., 2014). CAS is therefore regarded as a reliable archive for δ 34 S and δ 18 O stratigraphy and the reconstruction of the evolution of the marine sulfur cycle during Earth's history (Kampschulte and Strauss, 2004;Newton et al., 2004;Fike and Grotzinger, 2008;Song et al., 2014;Wu et al., 2014;Schobben et al., 2015;Rose et al., 2019). ...
... Three possible processes were identified to influence the sulfur and oxygen isotope composition of CAS in reef carbonates, including contamination by (1) pyrite oxidation or deposition of secondary atmospheric sulfate (Marenco et al., 2008a;Peng et al., 2014), (2) watersoluble sulfate (WSS; Fichtner et al., 2017;N. Li et al., 2022), and (3) alteration during diagenesis (Berner, 2001;Emerson et al., 2003;Schrag et al., 2013;Ma et al., 2021). The contamination of CAS by pyrite oxidation or WSS can be minimized using appropriate extraction processes (Wotte et al., 2012;Peng et al., 2014;Theiling and Coleman, 2015;N. ...
... Several factors affecting the sulfur and oxygen isotope compositions of CAS in carbonate rocks include the amount of pore fluid sulfate that is incorporated into carbonate minerals, the rate and extent of carbonate recrystallization, microbial sulfate reduction, sulfide oxidation or secondary atmospheric sulfate, the original CAS contents, and the differences in sulfur and oxygen isotope compositions of pore fluid sulfate (Burdett et al., 1989;Berner, 2001;Marenco et al., 2008a;Emerson et al., 2003;Peng et al., 2014;Schrag et al., 2013;Ma et al., 2021). Because coral reefs grow mainly in oxidized, tropical shallow marine environments (Yu, 2012;Kuang et al., 2014;Shao et al., 2017a;Shao et al., 2017b;Guo et al., 2021), reef carbonates with their dominance of skeletal carbonate are different from other types of carbonates such as authigenic methane-seep carbonates (e.g. ...
... Carbonate-associated sulfate (CAS) and pyrite have been recognized as valuable sulfur isotopic archives of the past sulfur cycle at global and regional scales, respectively (Burdett et al., 1989;Sim et al., 2015;Pasquier et al., 2017), but recent studies have also highlighted the potential impact of depositional environments on the CAS sulfur isotope values (Rennie and Turchyn, 2014;Richardson et al., 2019a;Ma et al., 2021). Here we examined the high-resolution CAS and pyrite sulfur isotope data from the late Paleozoic carbonate sequence in Svalbard, deposited near the western end of the Ural Seaway (Fig. 1), and the Early Permian δ 34 S CAS increase superimposed by rapid oscillations reflects both global-and regionalscale changes over the course of the closure of the Seaway. ...
... 5D-E). Finally, while a positive correlation between δ 34 S CAS and 1/CAS can imply the mixing of carbonate minerals with different origins (Present et al., 2015;Richardson et al., 2019a;Ma et al., 2021), the cross-plot of our data does not show such a trend (Fig. 5F). Overall, alteration of δ 34 S CAS during diagenesis or sample preparation is not apparent from simple correlation analysis of geochemical data; in the following sections, we shall further evaluate the preservation of global seawater signatures in the context of the sedimentology of the Gipsdalen Group, i.e. the changes in relative sea-level and depositional environments. ...
... Our new, high-resolution sulfur isotope data bridge this gap in the δ 34 S CAS record, suggesting that the sulfur isotope composition of global seawater sulfate might increase from the late Asselian to the early Artinskian (Fig. 7). Given the potential influence of depositional environment on the whole-rock δ 34 S CAS record (Rennie and Turchyn, 2014;Richardson et al., 2019b;Ma et al., 2021), however, here we consider both regional and global controls on the sulfur isotope records of the Early Permian succession in Svalbard. (Kampschulte and Strauss, 2004;Wu, 2013;Johnson et al., 2020) with other geological records. ...
The late Paleozoic was characterized by a series of continental collisions and ice ages. Despite the drastic environmental changes, sparse sulfur isotope data hinder our understanding of the late Paleozoic biogeochemical sulfur cycle, especially during the Early Permian. To overcome this potential bias, we present a high-resolution sulfur isotope record of carbonate-associated sulfate (CAS) and pyrite from the Carboniferous-Permian successions of the Svalbard archipelago. Throughout the Carboniferous, our results are largely consistent with the global trend, although the development of restricted environments resulted in a regionally observed δ34SCAS peak of +20‰ during the Gzhelian. The Early Permian δ34SCAS data in Svalbard bridge the gap in the existing record, showing a steady increase contemporaneous with the closure of the Ural Seaway and Gondwana glaciation, albeit superimposed by short-term oscillations. The enhanced incorporation of diagenetic sulfate into authigenic carbonates may have caused small-scale oscillations during the regional regression in the Artinskian, but the long-term increasing trend of δ34SCAS and its relation to known geological events can be best explained by the enhanced pyrite burial flux driven by a major shift in the locus of organic carbon burial from the continent to the ocean, with a lesser contribution from the dissolution of epicontinental seaway evaporites. Since the onset of the Middle Carboniferous Bashkirian δ34SCAS excursion also corresponds in timing to the major glaciation event and the closure of the Rheic Seaway, the sulfur isotope record in the course of the consolidation of Pangea is apparently punctuated by the episodes of increased pyrite burial and evaporite sulfate weathering, delineating the links between paleogeography, paleoclimate, and biogeochemical cycles.
... All oxygen and sulfur isotope compositions are reported in per mil (‰) relative to Vienna Standard Mean Ocean Water (VSMOW) and Vienna Canyon Diablo Troilite (VCDT), respectively. All Death Valley's and part of Beijing's sulfate δ 34 S, δ 18 O and Δʹ 17 O data have been published for entirely different stories 18,23 . All data were obtained at Bao laboratory at Louisiana State University, and can be found in Extended Data Table 1. ...
Sulfate aerosols affect climate by scattering radiation and by changing the microphysical properties of water clouds. In much of the continental interiors that are overwhelmed by anthropogenic sulfate today, the nature of pre-industrial atmospheric sulfate remains pure speculation, which hampers our ability to quantify anthropogenic perturbation on climate and uncertainties in global climate models. Here we show that sequential leaching and multiple-isotope measurement enabled us to effectively distinguish sulfate of different origins, including pre-industrial atmospheric sulfate, retained in certain outcropping carbonates. Data from two interior sites in northern China show that one of the sulfate endmembers consistently has an unusually positive ¹⁷O anomaly (Δʹ¹⁷O at ~+1.8‰) and characteristic δ¹⁸O (~1–5‰) and δ³⁴S (~5–10‰) values. We interpret this sulfate endmember to be integrated pre-industrial atmospheric sulfate from at least the last a few thousands of years. A triple oxygen isotope enabled GEOS-Chem chemical transport model revealed a higher Δʹ¹⁷O value in northern China and the south-western United States in the past, consistent with our data. Overall, pre-industrial atmospheric sulfate aerosol chemistry in the interior of northern China and south-western United States had a higher pH in cloud water, which may have led to a less cloud cover due to cleaner air and coarser aerosol sizes than today.
... First of all, treatment of bulk sample by 3 N HCl is the standard protocol for CAS extraction. Previous studies indicate such treatment would not significantly modify CAS isotope values (Chen et al., 2022;He et al., 2020;Ma et al., 2021;Shen et al., 2016;Shen et al., 2011), precluding pyrite oxidation in the dissolution procedure. Secondly, the post-depositional pyrite oxidation occurs when oxic fluids percolate through cracks, resulting in the preferential alignment of Fe-oxides. ...
The Earth’s middle age or the ‘Boring Billion’ (∼1.8 to ∼ 0.8 Ga, billion years ago) represents one of the most enigmatic intervals in Earth’s history, characterized by the absence of significant carbonate carbon isotope excursions and the sluggish evolution of eukaryotes. It is widely accepted that the atmospheric O2 level was low (<1% PAL or < 10% PAL) and the ocean remained predominantly anoxic with the development of sulfidic (H2S rich) continental margins. It is suggested that 2–10% of seafloor euxinia was sufficient to deplete some micro-nutrients in the ocean inventory, such as Mo and Cu, which are essential for nitrogen-fixation for eukaryotes. However, recent studies report multicellular fossils and episodic/sporadic inception of oceanic oxidation in Mesoproterozoic. These findings imply the possible occurrences of oxygen oasis that provided habitable niches for eukaryote evolution. Such oxygen oasis likely developed at shallow marine seafloor covered with microbial mat, where O2 produced by benthic cyanobacteria resulted in the oxygenation of seafloor. Therefore, in order to constrain the habitability of Mesoproterozoic oxygen oasis, it is essential to reconstruct the O2 fugacity in the seafloor. In this study, we analyzed pyrite sulfur isotopes and pyrite contents of the Mesoproterozoic Wumishan Formation (∼1.4 Ga). The Wumishan Formation is composed of cyclic deposition of, in a shoaling upward sequence, the subtidal calcareous shale, massive thrombolitic dolostone, and microbial laminated dolostone. Both microbial laminated dolostone and thrombolytic dolostone precipitation involved with microbial activities, and thus might record the redox condition of putative oxygen oasis in the Mesoproterozoic oceans. We apply the One-Dimensional Diffusion-Advection-Reaction (1D-DAR) model to simulate diagenetic pyrite formation in sediments. Sedimentation rate can be well constrained by the depositional cycles of the Wumishan Formation. The modelling results indicate that more than 60 ∼ 80% of H2S that was generated in microbial sulfate reduction (MSR) was reoxidized, and that organic matter supply, both from surface water and seafloor, was limited. Thus, our study indicates that the seafloor could be substantially oxygenated in Mesoproterozoic, even when the atmospheric O2 level was extremely low. Shallow marine seafloor covered with microbial mat may function as the oxygen oasis, providing habitable niches for the evolution of eukaryotes.
The early Mesoproterozoic (1.6–1.4 Ga) is a critical stage for oxygenation of Earth’s surface and early eukaryote evolution. However, the relationship between both remains uncertain. Here, we provide new carbonate associated sulfate (CAS) sulfur isotopic compositions (δ34SCAS) and trace elements from the 1.6 to 1.55 billion-year old Gaoyuzhuang Formation in multiple sections within the Yanshan Basin, North China, to investigate the temporal and spatial variation of ocean redox conditions. New and collected δ34SCAS data from five sections (Pingquan, Kuancheng, Qianxi, Jixian and Gan’gou sections) reveal similar sulfur isotopic excursions, but the differences in absolute values indicate spatial heterogeneity of sulfur isotopic compositions, which may reflect
marine low-sulfate concentrations and oceanic redox tratification. The most elevated δ34SCAS values appeared at the top of the Second Member of the Gaoyuzhuang Formation, followed by distinct negative excursions at the bottom of the Third Member, suggesting a dynamic sulfur cycle and seawater redox status during this period. Combined with previous reported carbon and sulfur isotopes and some redox proxies (e.g., Ce anomalies,
manganese-rich deposit, I/(Mg + Ca) and Fe speciation), we suggest that at least two possible oxygenation events may have occurred in the Gaoyuzhuang Formation: (1) the bottom of the Second Member and (2) the bottom or middle part of the Third Member. Different sections show asynchronous timing of these events, indicating that the redox status was spatial heterogenous within the Yanshan Basin. The spatial heterogeneity may be controlled by facies and paleogeographic locations. Considering that decimeter-scale multicellular eukaryotes are preserved
in the middle part of the Third Member, we argue that oxic bottom water condition may be a critical factor for early eukaryote evolution.
Carbonate-associated sulfate (CAS) refers to trace amounts of sulfate incorporated into carbonate minerals during precipitation. CAS has been the most commonly used approach to recover the paleo-seawater sulfate sulfur isotope composition (δ34Ssw) as carbonate rocks are more common and occur in less restricted marine environments than alternative sulfate-bearing minerals (such as gypsum and anhydrite). However, uncertainties remain about the reliability of preparation techniques due to the inadvertent inclusion of contaminant sulfurous species. This study applied three oxidative leaching CAS extraction methods and compared their final δ34SCAS values with those following repeated single leaching in 10% NaCl (aq). The final δ34SCAS values after sequential leaching by a combined 12% NaOCl, 1% H2O2, and 10% NaCl approach were systematically higher by between 0.65‰ and 0.90‰ than rival methods. Our experiments indicate that contamination can affect measured δ34SCAS even given short reaction times (<30 min). A single leach with standard oxidizing reagents may not entirely eliminate contamination when handling organic and pyrite-rich carbonate samples.
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.
The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550–541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O2 fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fecarb), to quantify the seafloor oxygenation. Fecarb of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O2 fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO2 levels. The highly oxygenated seafloor might be attributed to O2 production of the microbial mats. Despite the moderate atmospheric pO2 level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O2 barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.
Late Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with sulphur isotopic compositions distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.
This article is protected by copyright. All rights reserved.
The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆17O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO2 and O2 and the photosynthetic O2 flux at this time3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean4-6 and Phanerozoic primary production7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2 over geologic time8.
Marine evaporitic sulfates (gypsum and anhydrite) can record ancient seawater sulfur isotopic data; however, these records are scarce and widely dispersed owing to both restricted environments in which they form and their propensity to be eroded. The late Neoproterozoic–early Cambrian transition was a pivotal timeframe in Earth’s history, witnessing the early evolution of animal life and major environmental changes. Seawater chemistry changed abruptly during this interval, including significant changes to the sulfur cycle as evidenced by unusually high sulfur isotopic values. This positive sulfur excursion, termed the Yudomski Event, has been reported previously from early and middle Cambrian units in Siberia, Iran, Australia, and India. In this study, we provide the first report of the Yudomski Event in early and middle Cambrian evaporites from the Tarim Basin, northwestern China. Our data support this event having been of global significance. We additionally report early and middle Ordovician sulfur data from China, which constrain the decline of the Yudomski Event to the late Cambrian.
Contaminated sediment can release hydrophobic organic contaminants (HOCs) and thereby act as a secondary source of primarily legacy hazardous substances to the water column. There is therefore a need for assessments of the release of HOCs from contaminated sediment for prioritization of management actions. In situ assessment of HOC sediment-to-water flux is currently done with (closed) benthic flux chambers, which have a sampling time exceeding one month. During this time, the water inside the chamber is depleted of oxygen and the effect of bioturbation on the sediment-to-water release of HOCs is largely ignored. Here we present a novel benthic flux chamber, which measures sediment-to-water flux of legacy HOCs within days, and includes the effect of bioturbation since ambient oxygen levels inside the chamber are maintained by continuous pumping of water through the chamber. This chamber design allows for sediment-to-water flux measurements under more natural conditions. The chamber design was tested in a contaminated Baltic Sea bay. Measured fluxes were 62-2300 ng m(-2) d(-1) for individual polycyclic aromatic hydrocarbons (PAHs), and 5.5-150 ng m(-2) d(-1) for polychlorinated biphenyls (PCBs). These fluxes were 3-23 times (PAHs) and 12-74 times (PCBs) higher than fluxes measured with closed benthic chambers deployed in parallel at the same location. We hypothesize that the observed difference in HOC flux between the two chamber designs are partly an effect of bioturbation. This hypothesized effect of bioturbation was in accordance with literature data from experimental studies.
Carbonate Associated Sulfate (CAS) is trace sulfate incorporated into carbonate minerals during their precipitation. Its sulfur isotopic composition is often assumed to track that of seawater sulfate and inform global carbon and oxygen budgets through Earth's history. However, many CAS sulfur isotope records based on bulk-rock samples are noisy. To determine the source of bulk-rock CAS variability, we extracted CAS from different internal sedimentary components micro-drilled from well-preserved Late Ordovician and early Silurian-age limestones from Anticosti Island, Quebec, Canada. Mixtures of these components, whose sulfur isotopic compositions vary by nearly 25‰, can explain the bulk-rock CAS range. Large isotopic variability of sedimentary micrite CAS (34S-depleted from seawater by up to 15‰) is consistent with pore fluid sulfide oxidation during early diagenesis. Specimens recrystallized during burial diagenesis have CAS 34S-enriched by up to 9‰ from Hirnantian seawater, consistent with microbial sulfate reduction in a confined aquifer. In contrast to the other variable components, brachiopods with well-preserved secondary-layer fibrous calcite-a phase independently known to be the best-preserved sedimentary component in these strata-have a more homogeneous isotopic composition. These specimens indicate that seawater sulfate remained close to about 25‰ (V-CDT) through Hirnantian (end-Ordovician) events, including glaciation, mass extinction, carbon isotope excursion, and pyrite-sulfur isotope excursion. The textural relationships between our samples and their CAS isotope ratios highlight the role of diagenetic biogeochemical processes in setting the isotopic composition of CAS.
The 224Ra/228Th disequilibrium that was recently observed in coastal sediments has been proven to be an excellent proxy for tracing the benthic processes that regulate solute transfer across the sediment–water interface. In order to better utilize this proxy, there is a need to understand the reaction kinetics of 224Ra in sediments. In this study, depth profiles of 224Ra and 228Th in bulk sediments were collected along a transect in the Pearl River Estuary (PRE). Together with bulk sediment measurements, dissolved 224Ra, dissolved inorganic carbon (DIC), and nutrients (NO2− + NO3−, NH4+) in pore water and in the overlying waters were also determined. A marked deficit of 224Ra with respect to 228Th with large spatial variations was observed in the PRE sediments. By use of a diagenetic model for the distributions of dissolved and adsorbed 224Ra in sediments, we infer that adsorption removes 224Ra from aqueous phase at a rate of 0.1 ± 1.1–2000 ± 400 d−1. In addition, adsorption of 224Ra exhibits a rate sequence of oxic freshwater > anoxic freshwater > anoxic brackish water, probably reflecting the effect of the redox conditions and ionic strength on the adsorption–desorption kinetics of 224Ra.
Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.
We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO 3 À , NO 2 À , NH 4 +) and oxygen (O 2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O 2 concentrations at the shallowest station were hypoxic (42 lM) and increased to 125 lM at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (À10.3 mmol O 2 m À2 d À1) and decreased quasi-exponentially with water depth to À3.2 mmol O 2 m À2 d À1 . Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2 mmol N m À2 d À1 . Overall, the sediments acted as net sink for DIN. Observed increases in d 15 N NO3 and d 18 O NO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0& (15 e app) and 14.1& (18 e app). Measurements of d 15 N NO2 further demonstrated that the sediments acted as a source of 15 N depleted NO 2 À . These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH 4 + and NO 2 À . The principal findings were that (i) net benthic 14 N/ 15 N fractionation (e DEN) was 12.9 ± 1.7&, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO 2 À (À22 ± 1.9&), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported e DEN for fine-grained sediments are much lower (4–8&). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater–seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle.
A 25 million year record of the sulfur and oxygen isotope composition of marine carbonate-associated sulfate (CAS) was constructed from 55 nannofossil ooze samples at three different locations. We tested the impact of early marine diagenesis on CAS by comparing the CAS extractions to the sulfur and oxygen isotope composition of the associated pore fluid sulfate, to determine the degree of pore fluid incorporation during carbonate recrystallization. Neither the sulfur nor the oxygen isotope composition of CAS are completely overprinted by incorporation of pore fluid sulfate. We compare our record to the sulfur and oxygen isotope records of coeval barite. The sulfur isotope record is in agreement with the barite record within ±2‰±2‰, except where very young (<2 Ma) sediments are in the presence of highly evolved pore fluid sulfate (δS34=70‰). A simple recrystallization model is used to illustrate the sensitivity of CAS δSSO434 to sedimentation rate, and to emphasize that careful sample selection, along with an analysis of early diagenetic environmental conditions is crucial when interpreting CAS sulfur isotopes. Oxygen isotopes in CAS are more complex and do not reproduce similar values to those of coeval barite. We conclude that oxygen isotopes in sulfate may remain a useful proxy but merit closer attention in the future.
The rapid increase of carbon dioxide concentration in Earth's modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.
Sulfate reduction (SR) has been linked to carbonate mineral precipitation and has been invoked as a possible mechanism for microbialite formation ([Lyons et al., 1984][1]; [Kempe, 1990][2]). Although there is evidence to support the contribution of SR in carbonate mineral precipitation from both
We utilized 224Ra/228Th disequilibrium in the sediment to investigate processes that regulate solute transfer across the sediment–water interface. Depth profiles of dissolved and surface-bound 224Ra and 228Th in the upper 0–20 cm sediment column were measured using a delayed coincidence counter during a cruise to the Yangtze estuary from 15 to 24 August 2011. Along with 224Ra and 228Th, depth profiles of 234Th were collected to determine the bioturbation rate in the sediment. At most study sites, a significant deficit of 224Ra relative to 228Th was observed in the upper 0–10 cm. In contrast, 224Ra was in excess with respect to 228Th in the upper 0–5 cm at the river mouth, possibly due to redistribution of 224Ra from the mid-salinity region. By modeling the 224Ra depth profiles in the sediment using the general diagenetic equation, we demonstrated that in most cases molecular diffusion and bioturbation together can account for only ∼20–30% of the measured flux of 224Ra. We concluded that other mechanisms, especially irrigation, must be invoked to explain the remnant 70% of the observed deviation of 224Ra relative to 228Th. On the basis of the 224Ra/228Th disequilibrium in the sediment and a concept of increased surface area for exchange by irrigation as developed by early investigators, we proposed a new approach – the 224Ra/228Th disequilibrium approach to quantify the transfer rate of other dissolved species across the sediment–water interface. We have utilized this new approach to determine the benthic consumption rate of dissolved O2. The result reveals that benthic consumption is an important loss term of dissolved O2 in the Yangtze estuary and must be considered as one of the mechanisms that lead to hypoxia in this area.
Many δ³⁴S records have been produced from carbonate-associated sulfate (CAS) in order to understand the oxidation state of the Neoproterozoic oceans, but interregional correlation is complicated by the absence of robust chronostratigraphic markers. Here, a globally correlatable stratigraphic interval containing the Wonoka–Shuram (W–S) δ¹³C excursion was analyzed to explore variability in the sulfur isotope record. In the excursion-containing units, the local δ³⁴S record from multiple, closely spaced sections in Sonora, Mexico, was examined to explore potential heterogeneities, and then these were compared to more distant sections elsewhere.
We use a global three-dimensional chemical transport model to quantify the influence of anthropogenic emissions on atmospheric sulfate production mechanisms and oxidant concentrations constrained by observations of the oxygen isotopic composition (Delta17O = &delta17O-0.52 × &delta18O) of sulfate in Greenland and Antarctic ice cores and aerosols. The oxygen isotopic composition of non-sea salt sulfate (Delta17O(SO42-)) is a function of the relative importance of each oxidant (e.g. O3, OH, H2O2, and O2) during sulfate formation, and can be used to quantify sulfate production pathways. Due to its dependence on oxidant concentrations, Delta17O(SO42-) has been suggested as a proxy for paleo-oxidant levels. However, the oxygen isotopic composition of sulfate from both Greenland and Antarctic ice cores shows a trend opposite to that expected from the known increase in the concentration of tropospheric O3 since the preindustrial period. The model simulates a significant increase in the fraction of sulfate formed via oxidation by O2 catalyzed by transition metals in the present-day Northern Hemisphere troposphere (from 11% to 22%), offset by decreases in the fractions of sulfate formed by O3 and H2O2. There is little change, globally, in the fraction of tropospheric sulfate produced by gas-phase oxidation (from 23% to 27%). The model-calculated change in Delta17O(SO42-) since preindustrial times (1850 CE) is consistent with Arctic and Antarctic observations. The model simulates a 42% increase in the concentration of global mean tropospheric O3, a 10% decrease in OH, and a 58% increase in H2O2 between the preindustrial period and present. Model results indicate that the observed decrease in the Arctic Delta17O(SO42-) - in spite of increasing tropospheric O3 concentrations - can be explained by the combined effects of increased sulfate formation by O2 catalyzed by anthropogenic transition metals and increased cloud water acidity, rendering Delta17O(SO42-) insensitive to changing oxidant concentrations in the Arctic on this timescale. In Antarctica, the Delta17O(SO42-) is sensitive to relative changes of oxidant concentrations because cloud pH and metal emissions have not varied significantly in the Southern Hemisphere on this timescale, although the response of Delta17O(SO42-) to the modeled changes in oxidants is small. There is little net change in the Delta17O(SO42-) in Antarctica, in spite of increased O3, which can be explained by a compensatory effect from an even larger increase in H2O2. In the model, decreased oxidation by OH (due to lower OH concentrations) and O3 (due to higher H2O2 concentrations) results in little net change in Delta17O(SO42-) due to offsetting effects of Delta17O(OH) and Delta17O(O3). Additional model simulations are conducted to explore the sensitivity of the oxygen isotopic composition of sulfate to uncertainties in the preindustrial emissions of oxidant precursors.
The oxygen isotopic composition (16O, 17O, and 18O) of sulfate formed from different oxidative reactions has been investigated. In the aqueous phase, sulfur oxidation by H2O2, O3, and O2, catalyzed by Fe(III) and Mn(II) were studied. In the gas phase we have investigated the only relevant reaction for the atmosphere: SO2+OH and its chain termination reaction SO3+H2O. The results show that none of these reactions, gas or aqueous phase, produce a mass-independent oxygen isotopic composition in sulfate. Since H2O2 and O3 are known to possess a mass-independent isotopic signature, we have investigated the possible transfer of this anomaly to sulfate. It appears that both these oxidant species transfer their anomaly. Isotopic analysis shows that two oxygen atoms from H2O2 are found in the product H2SO4. This result is in accord with previous work. For O3 we found that only one of the original ozone oxygen transfers to the product sulfate. These isotopic results contradict the free radical reaction mechanism proposed by Penkett et al. [1979] but agree with the nonfree radical mechanism suggested by Erickson et al. [1977]. Therefore, it appears that only aqueous phase oxidation produces a mass-independent oxygen isotopic composition in sulfate. This finding is a response of the origin of the mass-independent oxygen isotopic composition of atmospheric and mineral deposits of sulfate on Earth [Bao et al., 2000; Lee, 1997]. Furthermore, this finding allows us to quantify the relative proportion of sulfate production by OH (gas phase formation) and by H2O2 and O3 (aqueous phase formation). The results can be used to test atmospheric chemical/transport models.
The pairing of calcium and magnesium isotopes (δ 44/40 Ca, δ 26 Mg) has recently emerged as a useful tracer to understand the environmental information preserved in shallow-marine carbonates. Here, we applied a Ca and Mg isotopic framework, along with analyses of carbon and lithium isotopes, to late Tonian dolostones, to infer seawater chemistry across this critical interval of Earth history. We investigated the ca. 735 Ma Coppercap Formation in northwestern Canada, a unit that preserves large shifts in carbonate δ 13 C values that have been utilized in global correlations and have canonically been explained through large shifts in organic carbon burial. Under the backdrop of these δ 13 C shifts, we observed positive excursions in δ 44/40 Ca and δ 7 Li values that are mirrored by a negative excursion in δ 26 Mg values. We argue that this covariation is due to early diagenetic dolomitization of aragonite through interaction with contemporaneous seawater under a continuum of fluid-to sediment-buffered conditions. We then used this framework to show that Tonian seawater was likely characterized by a low δ 7 Li value of ∼13‰ (∼18‰ lower than modern seawater), as a consequence of a different Li cycle than today. In contrast, δ 13 C values across our identified fluid-buffered interval are similar to modern seawater. These observations suggest that factors other than shifts in global seawater chemistry are likely responsible for such isotopic variation.
The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.
In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow‐water carbonate sediments the δ³⁴S, Δ47, δ¹³C, δ¹⁸O, δ44/40Ca and δ²⁶Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ³⁴S values similar to those found in contemporaneous seawater and δ44/40Ca, δ²⁶Mg values, Sr contents and Δ47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluid‐buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ³⁴S values indicating dolomitization in a more sediment‐buffered diagenetic system where bacterial sulphate reduction enriches the residual SO42‐ in ³⁴S, consistent with high sediment Sr concentrations and low δ44/40Ca and high δ²⁶Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ³⁴S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ44/40Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid‐buffered or closed sediment‐buffered diagenetic system have similar δ²⁶Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow‐water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.
Sulfate (SO₄²⁻) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbonate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S isotope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process. However, comparison between different calcifying species reveals both positive and negative S isotope fractionation between CAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on the component measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals, we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concentration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorption near-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionation between CAS and aqueous sulfate (CAS-SO₄²⁻ isotope fractionation) on sulfate concentration and precipitation rate. In aragonite, the CAS-SO₄²⁻ isotope fractionation is 1.0±0.3‰ and independent of the sulfate (and CAS) concentration. In contrast, the CAS-SO₄²⁻ isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipitation rate, between values of 1.3±0.1 and 3.1±0.6‰. We suggest that the correlation between aqueous sulfate concentration and CAS-SO₄²⁻– isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on the concentration of CAS, through the effect of the sulfate impurity on the carbonate mineral’s energetic state.
Significance
Carbonate sediments of Neoproterozoic age exhibit large secular excursions of carbon isotope composition outside the range of modern seawater dissolved inorganic carbon (DIC), but their origins are controversial. We show that in a Neoproterozoic carbonate platform in Namibia, such excursions disappear on the flanks of the platform, where compositions are more compatible with modern seawater. We attribute the observed spatial variation to early fluid-buffered alteration on the flanks of the platform, where seawater invaded the sediment in response to geothermal porewater convection. Accordingly, the isotope excursions in the platform interior are decoupled from open-ocean DIC, which remained close to the modern range. Our interpretation is testable and, if confirmed, has important ramifications for the origins of ancient carbon isotope excursions.
The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆ ¹⁷ O, δ ¹⁸ O) and sulfur (∆ ³³ S, δ ³⁴ S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆ ¹⁷ O signatures across the GOE, late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (GPP) curve for the Proterozoic. Through these results we argue that changes in GPP across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate.
Early Silurian (∼431 Ma) carbonate rocks record a ca. 4.5‰ positive excursion in the stable isotopic composition of carbonate carbon (δ^(13)C_(carb)). Associated with this isotopic shift is a macroevolutionary turnover pulse known as the ‘Ireviken Event’. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (∼0.6‰) in the stable isotopic composition of carbonate oxygen (δ^(18)O_(carb)). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in δ^(18)O_(carb) but show positive excursions of equal magnitude in both carbonate (δ^(13)C_(carb)) and organic (δ^(13)C_(org)) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7‰ positive excursion in carbonate-associated sulfate (δ^(34)S_(CAS)) and a ca. 30‰ positive excursion in pyrite (δ^(34)S_(pyr)). The increase in δ^(34)S_(pyr) values is accompanied by a substantial, concomitant increase in stratigraphic variability of δ^(34)S_(pyr). The relatively constant offset between the δ^(13)C_(carb) and δ^(13)C_(org) excursions throughout the Ireviken Event could be attributed to increased organic carbon burial, or possibly a change in the isotopic composition of CO_2 sources from weathering. However, a positive correlation between carbonate abundance and δ^(13)C_(carb) suggests that local to regional changes in dissolved inorganic carbon (DIC) during the shallowing-upward sequence may have been at least partly responsible for the observed excursion. The positive excursion recorded in δ^(34)S_(CAS) suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir. An inverse correlation between CAS abundance and δ^(34)S_(CAS) supports the notion of decreased sulfate concentrations, at least locally, consistent with a concomitant increase in pyrite burial. A decrease in the offset between δ^(34)S_(CAS) and δ^(34)S_(pyr) values during the Ireviken Event suggests a substantial reduction in the isotopic fractionations (ε_(pyr)) expressed during microbial sulfur cycling and pyrite precipitation through this interval. Decreased ε_(pyr) and the concomitant increase in stratigraphic variation in δ^(34)S_(pyr) are typical of isotope systematics observed in modern shallow-water environments, associated with increased closed-system behavior and/or oxidative sedimentary reworking during early sediment diagenesis. While the isotopic trends associated with the Ireviken Event have been observed in multiple locations around the globe, many sections display different magnitudes of isotopic change, and moreover, are typically associated with local facies changes. Due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the Ireviken Event, and their relationship to changes in local depositional environment, we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes.
The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability within cap dolostones, both between continents, across single continental margins, and within individual stratigraphic sections has been difficult to reconcile with depositional models. Using a compilation of new calcium and magnesium isotope measurements in Marinoan cap dolostone successions worldwide, we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid-and sediment-buffered conditions. Dolostones from the outer platform formed under fluid-buffered conditions, whereas dolostones on the inner platform and foreslope environment formed under sediment-buffered conditions. This spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea-level rise and development of a meltwater surface ocean. Using a numerical diagenetic model to evaluate the geochemical differences between sediment-and fluid-buffered cap dolostone units, we constrain the chemical and isotopic composition of both the dolomitizing fluid (glacial seawater [δ 13 C ∼ 0-2‰]), the meltwater lens (δ 13 C ∼ −11‰), and the primary aragonite sediment (δ 13 C ∼ −6 to −3‰). These model end-members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal Ediacaran cap dolostones. Our results provide a novel framework for understanding the geochemical variability of cap dolostone units, including large excursions in carbon isotopes, and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last Snowball Earth.
Shallow-water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of Earth's chemical and climatic history. One of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post-depositional diagenetic alteration. In this study, we use paired measurements of calcium (⁴⁴Ca/⁴⁰Ca or δ⁴⁴Ca) and magnesium (²⁶Mg/²⁴Mg or δ²⁶Mg) isotope ratios in sedimentary carbonates and associated pore-fluids as a tool to understand the mineralogical and diagenetic history of Neogene shallow-water carbonate sediments from the Bahamas and southwest Australia. We find that the Ca and Mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis. The observed variability in bulk sediment Ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid (fluid-buffered) to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment (sediment-buffered). Our results indicate that this process, together with variations in carbonate mineralogy (aragonite, calcite, and dolomite), plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers (δ¹³C, δ¹⁸O, major, minor, and trace elements) in shallow-water carbonate sediments in the geologic record. Our results also provide evidence that a large shallow-water carbonate sink that is enriched in ⁴⁴Ca can explain the mismatch between the δ44/40Ca value of rivers and deep-sea carbonate sediments and call into question the hypothesis that the δ44/40Ca value of seawater depends on the mineralogy of primary carbonate precipitations (e.g. ‘aragonite seas’ and ‘calcite seas’). Finally, our results for sedimentary dolomites suggest that paired measurements of Ca and Mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo-environmental information preserved in these enigmatic but widespread carbonate minerals.
Carbonate associated sulfate (CAS) is a proxy for the seawater redox conditions of ancient oceans. Despite its frequent utilization from altered carbonate archives, the impact of diagenetic and anchimetamorphic overprint on CAS itself has not yet been calibrated in a systematic manner. In the present study, CAS abundances and sulfur and oxygen isotopic compositions from early diagenetic sabkha type dolomicrites (Triassic Dolomia Principale Formation) were compared with geochemical (⁸⁷Sr/⁸⁶Sr, δ¹³Ccarb, δ¹⁸Ocarb), optical (cathodoluminescence), and crystallographical data to assess the impact of burial alteration. Data shown here document that δ³⁴SCAS withstands burial diagenesis or anchimetamorphosis (350 °C) and reliably preserves a record of ambient seawater sulfate and early diagenetic redox processes. In contrast, δ¹⁸OCAS ratios of early diagenetic dolomites exposed to burial temperatures of 200 °C and more are enriched in ¹⁸O likely due to exchange between δ¹⁸OCAS and δ¹⁸Ocarb. Carbonate associated sulfate concentrations are directly affected by burial conditions: Under increasing burial depth, temperature, and cation order in the increasingly stoichiometric dolomite crystal lattice, CAS concentration decreases linearly from a mean value of 470 ppm in samples that experienced a burial temperature of 100 °C to values below analytical detection in samples exposed to burial fluids with temperatures in excess of 350 °C. Significant variations of δ³⁴SCAS in samples from the northern Alps are further attributed to local sulfide oxidation triggered by the influence of meteoric water as based on the δ³⁴S and δ¹⁸O values of the water soluble sulfate (WSS). The data shown here are of significance for those concerned with the chemical parameters of ancient oceans and shed light on processes during dolomite formation and diagenetic pathways.
Modern dolomite formation is conditional and restricted in specific environmental and geochemical conditions, accordingly cannot be used as the analog of ancient dolostones, which normally have platform-wide distribution. The key to understand ancient dolostone formation is to find a dolomitization process that is independent of physical/geochemical environments. It is well-known that ribbon rock (one type of calcareous rhythmites, sensu stricto referring to the alternating limestone and marlstone layers or the limestone-marl alternations) that are deposited in nearly all marine environments throughout the Earth's history, are exclusively, though normally partially, dolomitized. Thus, understanding ribbon rock dolomitization may provide valuable insight into the ‘dolomite problem’. In this study, we measured Mg isotopic compositions (δ26Mg) of ribbon rock samples collected from the Ediacaran Doushantuo Formation in South China. The marlstone layers contain higher dolomite contents and are characterized by higher δ26Mg (ranging from − 2.68‰ to − 1.84‰) than the limestone layers (varying between − 3.21‰ and − 2.53‰). A numerical model calculation indicates that clay minerals cannot provide enough Mg for dolomitization, arguing against the traditional interpretation that dolomitization is resulted from diagenetic Mg release from clays. Instead, contemporaneous seawater might provide sufficient Mg for dolomitization, because both Mg isotopes and dolomite content can be simulated by the Diffusion-Advection-Reaction model. Contemporaneous seawater dolomitization requires kinetic barrier be overcome at shallow depth of sediments. We propose the ribbon rock dolomitization is attributed to differential diagenesis of the clay-rich and clay-poor sediment layers, in which Ca2 +-Mg2 + exchange elevates porewater Mg/Ca in clay-rich layers.
The "dolomite problem" refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth-surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the "dolomite problem". Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions ( δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between -0.55‰ and -3.18‰, and shows a negative linear correlation with 1[Mg], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component ( δ26Mgdol) for the lower sample set that is collected from a 4 m stratigraphic interval containing three high-frequency ribbon rock-packstone cycles is -1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (-0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and the Diffusion-Advection-Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of -0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization.
Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (~2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.
Herringbone calcite is a previously undescribed carbonate cement and sea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks. It is abundant in the ∼ 2520 Ma Campbellrand-Malmani platform, South Africa, where field relationships, such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones, demonstrate that it precipitated from ambient marine water. This interpretation is supported by depositional relationships in the ≥ 2.6 Ga Huntsman Limestone of the Bulawayo greenstone belt, Zimbabwe; the 2.6 Ga Carawine Dolomite, Australia; the 1.90 Ga Rocknest Formation and the 1.8-1.2 Ga Dismal Lakes Group, Canada; the Ordovician Porterfield carbonate buildup, Virginia; and various Silurian carbonate buildups in the Midcontinent, United States. Each of these occurrences is associated with anaerobic depositional environments or organic-rich sediments. Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick. Microscopically, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement. The bases of the crystals are optically unoriented, but upwards in each crystal, the optical c axis rotates until it is perpendicular to crystal elongation. The tops of the elongate crystal are thus optically aligned and length slow. The light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals, whereas the dark bands correspond to the optically unoriented, lower parts of the elongate crystals. Microspar crystals are also present in some dark bands. A Mg-calcite precursor for herringbone calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of microdolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low-Mg calcite. Precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface. Since herringbone calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water. Thus, the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time.
Thinly interbedded limestones and dolostones are common in Paleozoic and Proterozoic carbonate sequences. Such rocks, called ribbon rocks, are characteristic of the Conococheague limestone, a platform deposit exposed in western Maryland. Observations suggest that the limestone-dolostone alternations of ribbon rocks reflect original carbonate sand-mud alternations. This interpretation is strongly amplified by comparing Conococheague ribbon rocks to examples of lenticular and wavy bedding from mixed siliciclastic sand and mud flats from the modern North Sea.-from Author
The 34S isotope ratios were determined in gypsum (and/or anhydrite) samples from the Trias (40), Ordovician (38), Cambrian (10), and Sinian (1). Contemporaneous marine gypsums or anhydrites in sedimentary basins are shown to have similar S-isotope compositions, with a variation of = or <1.5per mille. Lower and Middle Triassic marine anhydrites have a range of delta 34S from 14.9 to 32.4per mille with a variation of 28.3-32.4per mille and 14.9-20.9per mille respectively. The Lower Triassic marine gypsum is enriched in 34S by approx 13per mille relative to the Middle Triassic. The delta 34S value for the Sinian gypsum (600-640 m.y.) is 18.8per mille; for the Lower Cambrian 25.3- 29.8per mille and for the Middle Ordovican 21.2-27.7per mille. P.Br.
Pelagic to hemipelagic limestone-marl rhythms are described as minor cycles with time information (periodites). They develop and can be recognized in the field only, if (a) the original sediment had a carbonate/clay ratio on the order of 4 and, under oxygenated conditions, if (b) the thickness of somewhat alternating beds was sufficient (5 to 10 cm) to prevent complete mixing by bioturbation. Simple models demonstrate the effect of fluctuating productivity and dissolution of (chiefly biogenic) carbonate as well as its dilution by “clay”. However, small primary differences in carbonate content or texture of alternating beds may be enhanced by diagenesis, including pressure solution, which strongly affect the final field aspect and the faunal content of the rhythmic sequence. Yet there are several criteria to prove the primary origin of such successions.
The exact correlation and timing of limestone-marl sequences is difficult in many cases, but it can hardly be doubted that the time periods of Quaternary and older carbonate cycles coincide in their order of magnitude (20,000 to 100,000 years). This is also true for their sedimentation rate (0.5 to 3 cm/1000 years) which is controlled by pelagic carbonate production.
Limestone-marl rhythms reflect a depositional area below storm-wave base. In the case of continental margins or epicontinental seas, accumulation approximately balances subsidence, and alternating beds appear to be generated mainly by dilution. In the deep sea, cyclic carbonate deposition is found on rises and plateaus, where it is chiefly controlled by dissolution in conjunction with a fluctuating CCD and/or changing bottom currents.
Rhythms of this type occur world-wide since Paleozoic time. They are probably caused by climatic variations, but simultaneous (generally small) sea level fluctuations may also play a part. During periods of widely extented and intensive plant growth on land, the CO2, content of surface waters may have been diminished for some time, whereas the river supply of dissolved old carbonate was increased. Both processes tend to promote the production and preservation of carbonate in shallow and deeper water.
The sulfur biogeochemical cycle integrates the metabolic activity of multiple microbial pathways (e.g., sulfate reduction, disproportionation, and sulfide oxidation) along with abiotic reactions and geological processes that cycle sulfur through various reservoirs. The sulfur cycle impacts the global carbon cycle and climate primarily through the remineralization of organic carbon. Over geological timescales, cycling of sulfur is closely tied to the redox state of Earth's exosphere through the burial of oxidized (sulfate) and reduced (sulfide) sulfur species in marine sediments. Biological sulfur cycling is associated with isotopic fractionations that can be used to trace the fluxes through various metabolic pathways. The resulting isotopic data provide insights into sulfur cycling in both modern and ancient environments via isotopic signatures in sedimentary sulfate and sulfide phases. Here, we review the deep-time δ34S record of marine sulfates and sulfides in light of recent advances in understanding how isotopic signatures are generated by microbial activity, how these signatures are encoded in marine sediments, and how they may be altered following deposition. The resulting picture shows a sulfur cycle intimately coupled to ambient carbon cycling, where sulfur isotopic records preserved in sedimentary rocks are critically dependent on sedimentological and geochemical conditions (e.g., iron availability) during deposition.
Long-term secular variation in seawater sulfate concentrations ([SO42−]SW) is of interest owing to its relationship to the oxygenation history of Earth's surface environment. In this study, we develop two complementary approaches for quantification of sulfate concentrations in ancient seawater and test their application to late Neoproterozoic (635 Ma) to Recent marine units. The "rate method" is based on two measurable parameters of paleomarine systems: (1) the S-isotope fractionation associated with microbial sulfate reduction (MSR), as proxied by Δ34SCAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by &partial; δ 34SCAS/∂ t(max). The "MSR-trend method" is based on the empirical relationship of Δ34SCAS-PY to aqueous sulfate concentrations in 81 modern depositional systems. For a given paleomarine system, the rate method yields an estimate of maximum possible [SO42−]SW (although results are dependent on assumptions regarding the pyrite burial flux, FPY), and the MSR-trend method yields an estimate of mean [SO42−]SW. An analysis of seawater sulfate concentrations since 635 Ma suggests that [SO42−]SW was low during the late Neoproterozoic (
Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (delta C-13(carb), delta C-13(org)), and sulfur isotopic compositions of carbonate associated sulfate (delta S-34(sulfate)) across the Guadalupian-Lopingian (G-L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The delta C-13(carb) values show a positive excursion from -0.45 parts per thousand to the peak of 3.80 parts per thousand in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to -2.60 parts per thousand in the lowest Heshan formation, then returned to about 1.58 parts per thousand. Similar to the trends of the delta C-13(carb), values, Delta C-13(carb-org) values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean-atmosphere carbon cycle. The first biostratigraphic delta S-34(sulfate) values during the G-L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from -36.88 parts per thousand to -37.41 parts per thousand. These sulfate isotopic records suggest that the ocean during the G-L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G-L biotic crisis.
The sedimentary record reveals first-order changes in the locus of carbonate precipitation through time, documented in the decreasing abundance of carbonate precipitation on the seafloor. This pattern is most clearly recorded by the occurrence of seafloor carbonate crystal fans (bladed aragonite pseudomorphs neomorphosed to calcite or dolomite), which have a distinct temporal distribution, ubiquitous in Archean carbonate platforms, but declining through Proterozoic time and extremely rare in Phanerozoic basins. To understand better the potential influences on this pattern, we built a mathematical framework detailing the effects of organic matter delivery and microbial respiratory metabolisms on the carbonate chemistry of shallow sediments. Two nonunique end-member solutions emerge in which seafloor precipitation is favorable: enhanced anaerobic respiration of organic matter, and low organic matter delivery to the sediment-water interface. This analysis suggests that not all crystal fans reflect a unique set of circumstances; rather there may have been several different geobiological and sedimentary mechanisms that led to their deposition. We then applied this logical framework to better understand the petrogenesis of two distinct crystal fan occurrences the Paleoproterozoic Beechey Formation, Northwest Territories, Canada, and the middle Ediacaran Rainstorm Member of the Johnnie Formation, Basin and Range, United States using a combination of high-resolution petrography, micro X-ray fluorescence and wavelength dispersive spectroscopy, C isotopes, and sedimentary context to provide information on geobiological processes occurring at the sediment-water interface. Interestingly, both of these Proterozoic examples are associated with iron-rich secondary mineral assemblages, have elevated trace metal signatures, and sit within maximum flooding intervals, highlighting key commonalities in synsedimentary geobiological processes that led to seafloor carbonate precipitation.
Limestone-marl alternations are widespread and typical sediments of epeiric basins and are present in varying abundance throughout the Phanerozoic. In many cases, their rhythmic appearance has been interpreted as a direct response to orbital forcing. However, it is a challenge to unequivocally prove a sedimentary origin of the rhythmic intercalation of the two lithologies. This difficulty arises from differential diagenesis that alters calcareous beds in different ways than interlayers (marls), causing a loss of comparability between the final lithologies. Differential diagenesis, between other effects, causes passive enrichment of the inert non-carbonate fraction in interlayers, where calcium carbonate is dissolved, as well as passive dilution in limestone beds, which get cemented by imported calcium carbonate. Therefore, reliable information about primary systematic differences in the precursor sediments is preserved only in constituents that are not modified during diagenesis. Such diagenetically inert components include the spectra of organic-walled microfossils (but not their absolute concentration in the bulk sediment) and the ratios of certain trace elements (also not their absolute concentrations). Systematic differences in diagenetically inert components can provide unequivocal proof of primary differences. In the limestone-marl alternations, we have studied however, such aspects do not directly reflect the lithological rhythm, shedding doubt on limestone-marl alternations as direct archives of environmental change. Box-model computer simulations visualize possible effects of early diagenetic change acting on the precursors of limestone-marl alternations, independent of the presence or absence of a primary sedimentary rhythm. The simulations demonstrate that diagenesis has the potential to seriously distort any primary rhythm present in the pristine sediment. In particular, differential compaction acting mainly on the marl interlayers induces distortions of the ratios of the original frequencies. These simulations emphasize the difficulties in conducting frequency analyses on relative carbonate contents in real-world successions.
Understanding the coupling of oxygen, carbon and sulfur cycles in the past is critical for reconstructing the history of biogeochemical cycles, paleoclimatic variations and oceanic chemistry. The abundance of sulfur isotopes (δ34S) in sulfate from ancient marine carbonates, or carbonate-associated sulfate (CAS), is commonly used, along with other archives (mainly evaporites and barite), to estimate the δ34S of seawater throughout Earth history. Analyses of CAS from hand-picked foraminifera are potentially valuable because this group of organisms is used in numerous paleoceanographic studies. They could provide coupled, high-resolution records of δ13C, δ18O, and δ34S isotopic changes directly linked to orbitally tuned records of climate change through the Cenozoic. Such measurements have not previously been possible due to limitations of sensitivity in conventional IRMS-based techniques. However, the recent development of CAS analysis by multicollector inductively-couple plasma mass spectrometry (MC-ICP-MS; Paris et al, 2013) now allows us to work on samples containing just a few nmol of sulfur with accuracy for δ34S values approaching 0.1‰, and – consequently – to analyze hand-picked samples of foraminifer shells. Here, we report the results of culture experiments with the planktic species Orbulina universa, that establish a shell:seawater δ34S calibration for future applications to the fossil record. Our new method uses <650 μg of carbonate (~15 shells) per analysis. Results show that S isotopes are fractionated consistently by -1‰ between seawater and O. universa tests. We also demonstrate that O. universa faithfully records the [SO42-]/[Ca2+] ratio of the seawater in which it grew.
The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth’s deep past. Carbonate-associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the waterleachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude
that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower d34S value than paleo-seawater sulfate, it could shift the d34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the “super-heavy pyrite” enigma reported in geological records. Our findings call for a re-evaluation of
many published, outcrop-based CAS data and conclusions.
Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ34SCAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation.
The aim of this study was to establish a protocol for the extraction of carbonate-associated sulfate (CAS) for the purpose of tracing the sulfur isotope composition of seawater. Existing CAS extraction methods were evaluated for their efficacy in eliminating non-CAS sulfur from the final CAS isotopic analysis. Five leaching methods were tested on three carbonate samples: (1) 10% NaCl (aq); (2) 10% NaCl (aq) followed by 10% NaOCl (aq); (3) 10% NaOCl (aq); (4) 10% NaCl (aq) followed by 10% H2O2 (aq); and (5) pure water only. All leaching steps were performed until no dissolved sulfate was seen to precipitate on addition of BaCl2 (aq). CAS was then liberated from the carbonate lattice by adding HCl from a dropping filter. All leachates, CAS fractions, and insoluble residues after CAS extraction (chromium-reducible sulfur or CRS) were analyzed for their isotopic composition. These experiments demonstrate that the leachable non-CAS sulfate fraction in carbonates can be proportionately far greater than, and isotopically distinct from the lattice-bound carbonate sulfate fraction. Here we show that some form of pre-leaching, other than with pure water, is necessary to isolate the CAS fraction in carbonates. However, even in cases of repeated pre-leaching and testing for non-CAS sulfate, measured δ34SCAS values may still be significantly influenced by the non-CAS sulfate fraction if δ34SNaCl and δ34SCAS values are sufficiently different. Pre-leaching once or twice with NaOCl and/or H2O2 is shown to be insufficient to ensure elimination of reduced sulfur, e.g. in the form of pyrite, while partial oxidation of reduced sulfur during pre-leaching with these powerful oxidants extends pre-leaching times, and can thus contaminate the final CAS value. Both of these leaching methods are shown to alter final δ34SCRS values by partial oxidation of reduced sulfur, and so need to be applied with care. For a secure CAS extraction from carbonate rocks we recommend repeated leaching with NaCl solution as a standard protocol in future studies, with complementary analyses of pre-leach sulfate concentrations and δ34SNaCl, and CRS concentrations and δ34SCRS as routine checks on possible contamination as well as tools for interpretation. Analyzing δ13Ccarb, δ18Ocarb, and elemental concentrations (Ca, Fe, Mg, Mn, Sr) of the carbonate host rock may help to constrain diagenetic alteration of the measured δ34SCAS. Published interpretations of rapidly changing seawater δ34S and sulfate concentrations need to be reconsidered in the light of these data.
Until now, our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites. Owing to the sporadic occurrence of evaporites through time, the secular sulfur isotope age curve contains many gaps with little or no data. In order to fill in some of these gaps, particularly the Neogene, we have analyzed the sulfur isotopic composition of carbonate-associated sulfate in carbonate tests of planktonic foraminifera. Other investigators have shown that sulfate may occur in biogenic calcites either lattice-bound, as micro-fluid inclusions, in adsorbed phases, or as protein polysaccharides. Whatever the origin, the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived, as shown by results on recent calcareous foraminifera and macrofossils. Using this approach for study of Miocene to Recent pelagic marine sediments supplemented by new data for Miocene marine evaporites from the Gulf of Suez, we have found that theδ34S of seawater has decreased about 2.5‰ over the past 25 m.y. and that most of the decrease has occurred over the past 5 m.y., parallelling a decrease in theδ13C of dissolved oceanic bicarbonate from the same interval.Sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m.y. Alternatively, an increase in weathering rates over the past 5 m.y. would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species. In either case, the net result would be a decrease in atmosphericpO2.