Pyrite Sulfur Isotope Systematics Suggest Low Marine Sulfate Levels across the Ediacaran–Cambrian Transition

The Ediacaran–Cambrian (E–C) transition was a critical period in Earth’s history, and it was characterized by variable but potentially significant increases in atmospheric oxygen levels (pO2). Sulfate is an essential oxidant and plays a key role in regulating Earth’s surface redox conditions. However, there is uncertainty regarding seawater sulfate levels during the E–C transition. To address this issue, organic carbon (δ13Corg) and pyrite sulfur isotope (δ34Spy) analyses of the Heyu section (the Dengying Formation) in the Sichuan Basin, South China, were carried out. The δ13Corg varies between −36.4‰ and −27.4‰, and δ34Spy ranges from −6.6‰ to 27.3‰. The TOC and pyrite content ranges from 0.13 wt.% to 4.28 wt.% and from 0.01 wt.% to 0.94 wt.%, respectively. The marine sulfate concentration was modeled using a one-dimensional diffusion–advection–reaction model (1D-DAR). The modeling results indicate that the seawater sulfate level remained at a relatively low level of ~2–6 mM. The increased pyrite burial at extensive marine anoxia during the terminal Ediacaran period was likely a driver of such a low sulfate level

Pyrite Sulfur Isotope Systematics Suggest Low Marine Sulfate Levels across the Ediacaran–Cambrian Transition

The Ediacaran–Cambrian (E–C) transition was a critical period in Earth’s history, and it was characterized by variable but potentially significant increases in atmospheric oxygen levels (pO2). Sulfate is an essential oxidant and plays a key role in regulating Earth’s surface redox conditions. However, there is uncertainty regarding seawater sulfate levels during the E–C transition. To address this issue, organic carbon (δ13Corg) and pyrite sulfur isotope (δ34Spy) analyses of the Heyu section (the Dengying Formation) in the Sichuan Basin, South China, were carried out. The δ13Corg varies between −36.4‰ and −27.4‰, and δ34Spy ranges from −6.6‰ to 27.3‰. The TOC and pyrite content ranges from 0.13 wt.% to 4.28 wt.% and from 0.01 wt.% to 0.94 wt.%, respectively. The marine sulfate concentration was modeled using a one-dimensional diffusion–advection–reaction model (1D-DAR). The modeling results indicate that the seawater sulfate level remained at a relatively low level of ~2–6 mM. The increased pyrite burial at extensive marine anoxia during the terminal Ediacaran period was likely a driver of such a low sulfate level

Ocean oxidation during the deposition of basal Ediacaran Doushantuo cap carbonates in the Yangtze Platform, South China

© 2016 Elsevier B.V.. Precipitation of cap carbonate lithologies is a key feature of Cryogenian global glaciations. Negative carbonate carbon isotopic compositions (δ13Ccarb) of these cap carbonates have been variably interpreted as massive drawdown of atmospheric CO2 via extensive continental chemical weathering, methane oxidation, or postglacial upwelling. Each of these interpretations argues a non-steady state of carbon cycle in the aftermath of Marinoan global glaciation. To further explore the postglacial marine carbon cycle, we measured δ13Ccarb of cap carbonates from six localities in the Yangtze Platform, South China. The studied cap carbonates were deposited in a variety of sedimentary environments, ranging from the open shelf, slope, to basin facies. Cap carbonates deposited in different environments show distinct stratigraphic trends of δ13Ccarb. In the open shelf, δ13Ccarb profile of the Songlin section remains almost constant (-3 to -4‰), while the δ13Ccarb of the Jiulongwan section records a negative excursion, decreasing from -3.5‰ to -7‰. δ13Ccarb of cap carbonates deposited in the slope environment does not show stable stratigraphic trend. In the basin environment, δ13Ccarb demonstrates a sharp decline in the middle part of cap carbonates to the nadir value of ~-11‰. The negative δ13Ccarb excursion is best interpreted in terms of oxidation of dissolved organic carbon (DOC), thus recording a pulse of ocean oxidation during cap carbonate precipitation. Clearly absence of negative δ13Ccarb excursion in all slope and most open shelf sections may imply that such oxidation event was not ubiquitous in the Yangtze Platform. We speculate that the renewed thermohaline circulation during deglaciation brought oxic surface water into ocean interior, which oxidized the basin environment of the Yangtze Platform. However, the deglacial thermohaline circulation was not strong enough to cause complete oxidation of the ocean. The sporadic oxidation in the open shelf, on the other hand, might result from the terrestrial influx of oxidant from postglacial continental weathering. Our study suggests that ocean oxidation, though sporadic, might have occurred during cap carbonate precipitation, and predated the first appearance of putative animal embryos

Sulfur and oxygen isotopes of sulfate extracted from Early Cambrian phosphorite nodules: Implications for marine redox evolution in the Yangtze Platform

© 2016, China University of Geosciences and Springer-Verlag Berlin Heidelberg. Phosphorite nodule beds are discovered in the black shale of basal Niutitang Formation throughout the Yangtze Platform in South China, recording an important phosphorite-generation event. Platform-wide phosphorite precipitation requires special oceanographic and geochemical conditions, thus the origin of the Niutitang phosphorite nodules may provide valuable information about the ocean chemistry in the Early Cambrian. In this study, we measured sulfur and oxygen isotopic compositions of sulfate extracted from phosphorite nodules collected from the basal Niutitang Formation. Phosphorite associated sulfate (PAS) is a trace amount of sulfate that incorporates into crystal lattice during phosphorite precipitation, accordingly PAS records the geochemical signals during phosphorite nodule formation. Sulfur isotopic composition of PAS (δ34SPAS) ranges from -1.16‰ to +24.48‰ (mean=+8.19‰, n=11), and oxygen isotopic value (δ18OPAS) varies between -5.3‰ and +26.3‰ (mean=+7.0‰, n=8). Most phosphorite nodules have low δ34SPAS and low δ18OPAS values, suggesting PAS mainly derived from anaerobic oxidation of H2S within suboxic sediment porewater. We propose that phosphate was delivered to the Yangtze Platform by a series of upwelling events, and was scavenged from seawater with the precipitation of FeOOH. The absorbed phosphate was released into suboxic porewater by the reduction of FeOOH at the oxic-suboxic redox boundary in sediments, and phosphorite nodule precipitated by the reaction of phosphate with Ca2+ diffused from the overlying seawater. The platform-wide deposition of phosphorite nodules in the basal Niutitang Formation implies the bottom water might be suboxic or even oxic, at least sporadically, in Early Cambrian. We speculate that the intensified ocean circulation as evident with frequent occurrences of upwelling events might be the primary reason for the episodic oxidation of the Yangtze Platform in Early Cambrian

Marine Carbon-Sulfur Biogeochemical Cycles during the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Jiangnan Basin, South China

© 2016, China University of Geosciences and Springer-Verlag Berlin Heidelberg. Global occurrences of Steptoean Positive Carbon Isotope Excursion (SPICE) during Late Cambrian recorded a significant perturbation in marine carbon cycle, and might have had profound impacts on the biological evolution. In previous studies, SPICE has been reported from the Jiangnan slope belt in South China. To evaluate the bathymetric extent of SPICE, we investigate the limestone samples from the upper Qingxi Formation in the Shaijiang Section in the Jiangnan Basin. Our results show the positive excursions for both carbonate carbon (δ13C) and organic carbon (δ13Corg) isotopes, as well as the concurrent positive shifts in sulfur isotopes of carbonate associated sulfate (CAS, δ34SCAS) and pyrite (δ34Spyrite), unequivocally indicating the presence of SPICE in the Jiangnan Basin. A 4‰ increase in δ13Ccarb of the Qingxi limestone implies the increase of the relative flux of organic carbon burial by a factor of two. Concurrent positive excursions in δ34SCAS and δ34Spyrite have been attributed to the enhanced pyrite burial in oceans with extremely low concentration and spatially heterogeneous isotopic composition of seawater sulfate. Here, we propose that the seawater sulfur isotopic heterogeneity can be generated by volatile organic sulfur compound (VOSC, such as methanethiol and dimethyl sulfide) formation in sulfidic continental margins that were widespread during SPICE. Emission of 32S-enriched VOSC in atmosphere, followed by lateral transportation and aerobic oxidation in atmosphere, and precipitation in open oceans result in a net flux of 32S from continental margins to open oceans, elevating δ34S of seawater sulfate in continental margins. A simple box model indicates that about 35% to 75% of seawater sulfate in continental margins needs to be transported to open oceans via VOSC formation

Can crystal morphology indicate different generations of dolomites? Evidence from magnesium isotopes

© 2019 Elsevier B.V. Among various geochemical and petrographic approaches, dolomite crystal morphology and dolostone fabric have been widely applied in the study of ancient dolostones. It is proposed that dolomite crystal morphologies and the rock fabric may reflect the formation temperature, and thus can be used to distinguish different generations of dolomite. However, this scenario has also been challenged by some researchers. In order to test whether the dolomite crystal morphology can be used to differentiate different generations of dolomite, in this study, we measured the Mg isotopic compositions (δ26Mg) of dolomite with different crystal morphologies. δ26Mg of dolomite is controlled by a variety of factors, including temperature, magnesium isotopic composition of dolomitization fluids, and the flow rate of dolomitization fluids. If dolomite with distinct crystal morphologies were derived from different dolomitization processes, it is highly plausible that they would have different δ26Mg. Five types of dolomite with distinct crystal morphologies and rock fabric were recognized from three sampling intervals (S1, S2, and S3) in the middle Ordovician Majiagou Formation in North China. Different types of dolomite in the same sampling interval have similar δ26Mg values, suggesting that these dolomites might have derived during the same dolomitization event. Our study indicates that the crystal morphology alone may not unambiguously differentiate the generations of dolomites. We propose the following reasons: (1) the dolomite crystal morphology might be controlled by various factors rather than the formation temperature alone, or (2) the dolomite crystal morphology might be modified in diagenesis, but δ26Mg remains unchanged

Surface ocean nitrate-limitation in the aftermath of Marinoan snowball Earth: Evidence from the Ediacaran Doushantuo Formation in the western margin of the Yangtze Block, South China

© 2020 Elsevier B.V. Geochemical and paleontological studies suggest that a rapid rise in atmospheric O2 level in the termination of the Marinoan global glaciation (635 Ma) might have triggered the diversification of eukaryotes and caused the deep ocean oxygenation. It is proposed that the Earth\u27s surface redox condition was directly linked to the marine primary productivity, which represented the only major O2 source before the evolution of land plants. The marine primary productivity was mainly controlled by the bioavailability of macronutrients, phosphorus (P) and nitrogen (N). Thus, reconstruction of ancient P and N cycles can provide direct constraints on marine organic matter production and the redox landscape of the ocean. Previous studies of the Ediacaran Doushantuo Formation in the Yangtze Block, South China, suggested that the Ediacaran marine N cycle was highly dynamic and was characterized by dramatic fluctuations in the intensity of denitrification. However, these studies were mainly focused on sections deposited in the offshore marine environment, and little is known about the N cycle in the nearshore regions. In this study, we report high-resolution organic carbon (δ13Corg) and nitrogen (δ15NTN) isotopes of the Doushantuo Formation at the E-Shan section in the western margin of the Yangtze Block. The Doushantuo Formation at the E-Shan section is composed of alternating deposition of thin bedded sandstone and mudstone, and was deposited in a nearshore delta environment. The δ13Corg profile displays two prominent negative excursions, which are coincident with two positive excursions in δ15NTN. The positive excursion in δ15NTN implies the enhanced denitrification, resulting in the loss of N (nitrate and ammonium). In addition, there are negative correlations between δ15NTN and TOC content and between δ15NTN and δ13Corg. Such relationships suggest that the reduction of N supply would lower primary productivity and accordingly decrease the carbon isotope of dissolved inorganic carbon (δ13CDIC) in the surface ocean. In addition, this interpretation also implies that N-fixation was not active in the nearshore region. We speculate that the muted N-fixation in the nearshore regions could be attributed to the Fe limitation in oxic shallow seawater and general Mo deficiency of the Ediacaran ocean. Thus, our study indicates that the surface ocean productivity and marine redox landscape were coupled in the Ediacaran ocean

Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans

Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (â 1/42,500-750 million years ago, Ma) subtidal successions, but extremely rare in rocks 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 H 2 S-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 CH 4, 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

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Cryogenian (∼720-635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth\u27s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)-indicative of an episode of intense chemical weathering-occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (\u3c+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition