Search

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.

Global oceans might have completely frozen during the Cryogenian Sturtian glaciation (717–660 Ma), which would have resulted in marine anoxia due to the oceans' prolonged separation from the atmosphere. Sedimentological evidence reveals that the ice sheets were dynamic throughout this glaciation. The ice sheets' retreat increased the availability of nutrients and led to active marine biogeochemical cycles. However, direct geochemical constraints on the synglacial marine chemistry, such as data from carbonates, are rare. We conduct detailed sedimentological and geochemical analyses on the Cryogenian Fulu formation in the Yangtze Block's southeastern margin. The Fulu Formation mainly consists of sandstones with rare carbonates. Facies analysis shows that these sandstones were deposited in deep water environment during the waning Sturtian glaciation. Three facies are recognized, including the sand lobe, lobe fringe and basin plain facies. According to the geochemical results, the Fulu Formation has a large variation in organic carbon isotope (δ13C org) and pyrite sulfur isotope (δ34S py) (δ13C org : −29.5‰ ∼ −22.0‰, mean = −25.7‰, n = 143; δ34S py : −19.4‰ ∼ −29.0‰, mean = 1.7‰, n = 88), but shows low values of TOC (<0.1 wt%) and pyrite content (<0.4 wt%). Dolostone layers were mainly deposited in the basin environment and are characterized by: conspicuous Fe and Mn content (Fe carb : 15746–64,428 ppm, mean = 43,457 ppm; Mn carb : 4436–8576 ppm, mean = 6078 ppm; n = 13), stable inorganic carbon isotope (δ13C carb : −3.7‰ ∼ −2.5‰, mean = −3.1‰, n = 23), and absence of Ce anomaly (0.93–1.07, mean = 0.99, n = 13). These stable δ13C carb , low TOC and pyrite content suggest that sustaining primary productivity and low seawater sulfate level. Turbidity deposition, rather than marine chemistry controls the oscillations of δ13C org and δ34S py records under these conditions. The release of adsorbed REE during the Mn-oxide reduction causes a coupled fluctuation of REE and Mn carb content. Mn-oxides in turbidity deposits from shallow water indicate a oxic/suboxic shallow ocean. The absence of the Ce anomaly, on the other hand, suggests that synglacial deep ocean remained anoxic. Thus, despite low primary productivity and seawater sulfate concentration, sustaining turbidity deposition maintained the ocean redox stratification during the waning Sturtian glaciation. • Turbidity deposition controls carbon and sulfur isotope variation. • Active biogeochemical cycles during the Sturtian glaciation. • Mn and REE data suggest oxic-suboxic condition in shallow ocean. • Absence of Ce anomaly suggests anoxic condition in deep ocean. [ABSTRACT FROM AUTHOR]

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δMg), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δMg. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δMg: (1) micrite samples ranged from −2.86‰ to −2.97‰; (2) pure marine cements varied from −3.40‰ to −3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (−3.27‰ to −3.75‰); (3) values for the mixture component were −3.17‰ and −3.49‰; and (4) brachiopod shells ranged from −2.20‰ to −3.07‰, with the thickened hinge area enriched in Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δMg. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δMg. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δMg of the Huangjin limestone makes it difficult to reconstruct δMg using bulk carbonate/calcareous sediments. Finally, δMg was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰-4.8‰) of fractionation during biogenic carbonate formation. [ABSTRACT FROM AUTHOR]

Precipitation of cap carbonate lithologies is a key feature of Cryogenian global glaciations. Negative carbonate carbon isotopic compositions (δ 13 C carb ) of these cap carbonates have been variably interpreted as massive drawdown of atmospheric CO 2 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 δ 13 C carb 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 δ 13 C carb . In the open shelf, δ 13 C carb profile of the Songlin section remains almost constant (−3 to −4‰), while the δ 13 C carb of the Jiulongwan section records a negative excursion, decreasing from −3.5‰ to −7‰. δ 13 C carb of cap carbonates deposited in the slope environment does not show stable stratigraphic trend. In the basin environment, δ 13 C carb demonstrates a sharp decline in the middle part of cap carbonates to the nadir value of ∼−11‰. The negative δ 13 C carb 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 δ 13 C carb 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. [ABSTRACT FROM AUTHOR]

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 (δC) and organic carbon (δC) isotopes, as well as the concurrent positive shifts in sulfur isotopes of carbonate associated sulfate (CAS, δS) and pyrite (δS), unequivocally indicating the presence of SPICE in the Jiangnan Basin. A 4‰ increase in δC of the Qingxi limestone implies the increase of the relative flux of organic carbon burial by a factor of two. Concurrent positive excursions in δS and δS 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 S-enriched VOSC in atmosphere, followed by lateral transportation and aerobic oxidation in atmosphere, and precipitation in open oceans result in a net flux of S from continental margins to open oceans, elevating δS 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. [ABSTRACT FROM AUTHOR]

• The stratigraphic Mg isotopic profiles and the depositional cycles are coupled in the massive dolostone succession. • Mg isotope could be used to sequence the dolomitization events. • Massive dolostone could be generated by the temporal and spatial stacking of multiple dolomitization events. • Sea level fluctuation results in the stacking of multiple dolomitization events. The origin of ancient massive dolostones, i.e. continuous dolostone sequence with a thickness >100 m and a platform-wide distribution, is the key issue of the 'Dolomite Problem' that cannot be clearly demonstrated by any existing dolomitization model individually or sequentially. It has been proposed that the massive dolostone could be generated by the stacking of multistage dolomitization events linked to the sea-level fluctuation, which results in repeatedly occurring of limestone precipitation-dolomitization cycles. However, the sequence of dolomitization events cannot be differentiated by any sedimentological or traditional geochemical techniques. Here we report Mg isotopic compositions of the massive dolostone (δ 26 Mg dol) from the middle Cambrian Qinjiamiao Formation (QJM) in the Yangtze Platform, South China, which consists of cyclic depositions of shoaling upward sequences. The stratigraphic variation of δ 26 Mg dol is coincident with the depositional cycles, suggesting the dolomitization might be periodic and be coupled with the sea-level oscillation. As dolomitization fluids experience changes in δ 26 Mg values during dolomitization processes, the intra-cycle stratigraphic δ 26 Mg dol profile reflects the processes of dolomitization. Our study indicates that the massive dolostone could be generated by the temporal and spatial stacking of multiple dolomitization events that are associated with sea-level fluctuation. If this model can be verified by other massive dolostone successions, the origin of massive dolostone may be resolved. [ABSTRACT FROM AUTHOR]

Rapid diversification of eukaryotes immediately after the Marinoan Snowball Earth glaciation suggests the possible linkage between the extreme icehouse climate and biological evolution. To unravel how the severe ice-age had triggered biological evolution, it is essential to retrieve the postglacial environmental background. Cap carbonate that directly overlies the glacial deposit represents the earliest chemical deposition after the Snowball Earth glaciation, and thus records the postglacial seawater compositions. In the Yangtze Block, South China, the Nantuo Formation represents the deposition of the Marinoan Snowball Earth glaciation, and the overlying cap carbonates of the basal Doushantuo Formation have nearly identical thickness and similar lithological sequence and widely outcropped from the shelf to basin environments. In this study, we report carbonate associated Fe (Fe carb) and carbonate associated Mn (Mn carb) contents of the Doushantuo (Marinoan) cap carbonate samples collected from 13 outcrop sections and 2 drill cores, spanning from the shelf to basin environments of the Yangtze Block. In addition, we also analyzed Fe and Mn contents in siliciclastic deposits (Fe clast and Mn clast) of the upper Nantuo Formation. The Doushantuo cap dolostone have significantly higher Mn carb but comparable Fe carb values, and thus show extremely high Mn carb /Fe carb ratios, compared with dolostone of other ages. This observation suggests precipitation of the Doushantuo cap dolostone from Mn-enriched yet Fe-lean seawater after the Marinoan Snowball Earth glaciation. The Mn-enrichment in the postglacial ocean might be attributed to a continuous hydrothermal Mn2+ flux for ~15 million years during the Snowball Earth. The hydrothermal-derived Mn2+ could be accumulated in the Snowball Earth ocean due to the absence of synglacial carbonate precipitation and the persistent reduced condition during the Snowball Earth glaciation. In addition, the postglacial remobilization of MnO 2 from glacial sediments might have been fueled by the recovery of surface ocean productivity, contributing to the postglacial seawater Mn2+ inventory. MnO 2 remobilization is supported by the lower Mn clast values of the top of Nantuo Formation. Our study supports the delayed cap carbonate precipitation after the melting of the Marinoan Snowball Earth glaciation, and the postglacial seawater was suboxic and enriched in Mn2+ but not Fe2+ throughout the Yangtze Block. Thus, the Ediacaran ocean oxidation might have predated the evolution of eukaryotes or the marine redox condition during cap carbonate precipitation might have been oxic enough for the evolution of eukaryotes, though there is a slightly delayed appearance of Doushantuo-type fossils in the stratigraphic record. • The Doushantuo (Marinoan) cap carbonates have extremely high Mn2+ contents throughout the Yangtze Block. • The top of Nantuo Formation has lower Mn clast contents. • The Doushantuo cap carbonate was precipitated from Mn-enriched seawater after the Marinoan Snowball Earth glaciation. • Seawater Mn2+ might be derived from synglacial hydrothermal input as well as remobilization of MnO 2 during deglacial. [ABSTRACT FROM AUTHOR]

The Neoproterozoic global glaciations (720–635 Ma) represent the most severe icehouse climatic conditions in Earth's history, collectively known as the 'Snowball Earth events'. The 'Snowball Earth' hypothesis (Hoffman et al. 1998) proposed that the termination of the global glaciation was driven by the extremely high atmospheric p CO 2 level (300 times of present atmospheric level, PAL) that was accumulated during the global glaciation by volcanic degassing for tens of million years. Because of extremely high temperature, the deglaciation might associate with intense continental weathering that brought abundant bicarbonate and alkaline earth metals (Mg, Ca) to the ocean, resulting in the rapid precipitation of cap carbonate all over the world. Thus, cap carbonate precipitation marks the termination of global glaciation. The deglacial intense continental weathering has been recently supported by Mg isotopes. However, the Mg isotope data also indicate that the deglaciation might have significantly predated the cap carbonate precipitation, and limited data suggest that the weathering intensity was already low during the cap carbonate precipitation. In order to further test whether the cap carbonate precipitation was associated with low weathering intensity, here we analyzed Mg isotopic composition of siliciclastic component (δ26Mg sil) of the Doushantuo cap carbonate in the Yangtze Block, South China. Four cap carbonate sections (the Tongle (TL) drill core, the Fanyang (FY) section, the Wuluo (WL) drill core, and the Jiulongwan (JLW) section), spanning from shelf to basin environments, were analyzed. δ26Mg sil ranges from −0.43‰ to +0.39‰ (n = 10), from −0.31‰ to +0.48‰ (n = 9), from −0.06‰ to +0.10‰ (n = 7), and from −1.42‰ to −0.03‰ (n = 8) for the JLW, FY, WL, and TL samples, respectively. There are negative correlations between Mg/Al and δ26Mg sil in JLW, WL, and TL. The geochemical data indicate that the variation of δ26Mg sil might be attributed to authigenic clay formation during the cap carbonate precipitation. The binary mixing model indicates that the isotopic compositions of terrestrial clays range from 0‰ to +0.3‰, comparable to the values of modern soil clays but significantly lower than the values of the upper Nantuo Formation (up to ~ + 0.90‰), suggesting the moderate to low weathering intensity during the cap carbonate deposition. Our study suggests low atmospheric p CO 2 level during cap carbonate precipitation probably due to the consumption by intense continental weathering • δ26Mg sil of the basal Doushantuo cap carbonate is reported. • Variation in δ26Mg sil may be attributed to authigenic clay formation in the ocean. • Mg isotopes indicate moderate to low intensity of continental weathering during cap carbonate precipitation. [ABSTRACT FROM AUTHOR]