Doushantuo Cap Carbonate Research Articles

Involvement of Hg-bearing methane seeps in forming Ediacaran cap carbonate in South China

During the Cryogenian-Ediacaran transition, an extremely cold climate transitioned into a warm climate which was accompanied by the worldwide precipitation of cap carbonates showing striking negative carbonate carbon isotope (δ13Ccarb) anomalies. High mercury (Hg) contents up to 180 ppb have been reported from the Doushantuo cap carbonates, but the origin of Hg anomalies and its relationship with cap carbonate precipitation remain unclear. To address these issues, we measured Hg concentrations and isotopes along with total organic carbon (TOC) content and δ13Ccarb from the basal Ediacaran carbonates at the Jiulongwan section of western Hubei Province, South China. The results show that in laterally persistent intervals, Hg spikes (as high as 77 ppb) co-occur with decrease of Δ199Hg values (as low as 0.07‰), increase of δ202Hg values (−1.47‰), and conspicuous negative δ13Ccarb shifts (as low as ca. -17‰). Such Hg isotopic signature with high Hg concentration are comparable to those of natural gas hydrates and thus are ascribed to be an involvement of Hg-bearing methane seeps in precipitation of cap carbonate after melting of ice-sheets of the Marinoan glaciation (latest Cryogenian). Anaerobic oxidation of methane from hydrates via sulfate reduction may have produced bicarbonate (HCO3−), which was transformed into cap carbonate when combining with alkaline ions transported from continent to seawater via weathering process. This study provides the first evidence for Hg-bearing methane oxidation during cap carbonate precipitation.

Sedimentary model of the Ediacaran Doushantuo cap carbonates from the southeastern margin of the Yangtze Block: Evidence from carbon and oxygen isotopes and elemental geochemistry

Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post‐glacial warm environment. Some consensuses on the distinctive negative excursion of δ13Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong‐Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong‐Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post‐Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygen‐rich condition with syn‐depositional or pene‐contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow‐ to deep‐water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep‐water basinal condition, rather than within the entire oceanic basin.

Origin of K-bentonites in the Doushantuo cap dolostones from South China and its potential link with the sedimentary model of the Marinoan cap dolostones

A study of the K-bentonites in the Doushantuo cap dolostones on the Yangtze Platform in South China would facilitate understanding the origin of the Marinoan cap dolostones and testing the snowball Earth hypothesis. To date, sporadic work has been conducted on the whole-rock geochemistry of the K-bentonites from the platform and slope of the Yangtze Platform, to preliminarily understand the genesis of the K-bentonites. However, a study of the whole-rock geochemistry of the K-bentonites from the basin and on zircon geochemistry of the K-bentonites from the entire Yangtze Platform has not been carried out. Therefore, the nature of the primary magmas, source and tectonic setting of the source volcanoes, and the episodes of source volcanic eruptions of the K-bentonites are unknown, which hinders unraveling the possible link between the sedimentary model of the Marinoan cap dolostone and the K-bentonites. Here, we perform a systematic study of the whole-rock and zircon geochemistry of the K-bentonites. The major and trace element concentrations of the K-bentonites and trace element contents in their volcanic zircons suggest that the primary magmas of the K-bentonites from the Jiuqunao and Heyu sections are different in nature. The same geochemical results indicate that the source volcanoes of all the K-bentonites may occur in the extensional setting on the northern margin of the Yangtze Block. Hf isotopic compositions of the volcanic zircons in the K-bentonites imply that the source magmas of the K-bentonites are mainly mantle-derived, and may have mixed with the crustal materials from the northern margin of the Yangtze Block. Based on the difference in whole-rock and zircon geochemistry of the K-bentonites from Heyu and Jiuqunao, two episodes of the source volcanic eruptions of the K-bentonites could be identified. The K-bentonite at Heyu most probably represents the record of the old episode, while that at Jiuqunao represents the record of the young one. The here proposed dual episodic deposition of K-bentonites within the Doushantuo cap carbonates holds potential significance to constrain 1) the duration of the cap carbonate deposition and 2) their genesis in potential slushball or snowball Earth environments.

Iodine records from the Ediacaran Doushantuo cap carbonates of the Yangtze Block, South China

Late Neoproterozoic cap carbonates formed following the Cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the Snowball Earth events. Large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post-Marinoan cap carbonates across the globe, but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations. In this study, we present I/(Ca + Mg) profiles of four sections from a shallow-to-deep water transect of the basal Doushantuo cap carbonates, one of the most extensively studied Ediacaran units in South China. Most samples have I/(Ca + Mg) values lower than 1 μmol/mol with a few exceptions in the shelf sections up to ~2.5 μmol/mol. All calcite samples with extremely low δ13C have iodine signal below detection limit, suggesting carbonate precipitation from anoxic seawater or diagenetic fluids. Among the four sections, the slope section has the lowest I/(Ca + Mg) values, suggesting that it may have experienced more extensive fluid-buffered diagenesis and more severe loss of IO3–. In combination with available major, trace, rare earth element and isotope data, the temporal I/(Ca + Mg) variations support freshwater mixing and deep-water upwelling in proximal sites and transient oxidation during deposition of the Doushantuo cap carbonates.

Continental weathering intensity during the termination of the Marinoan Snowball Earth: Mg isotope evidence from the basal Doushantuo cap carbonate in South China

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 pCO2 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 (δ26Mgsil) 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. δ26Mgsil 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 δ26Mgsil in JLW, WL, and TL. The geochemical data indicate that the variation of δ26Mgsil 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 pCO2 level during cap carbonate precipitation probably due to the consumption by intense continental weathering

Germanium/silica ratio and rare earth element composition of silica-filling in sheet cracks of the Doushantuo cap carbonates, South China: Constraining hydrothermal activity during the Marinoan snowball Earth glaciation

The rapid diversification of eukaryotes within 3 million years after the Marinoan global glaciation suggests the potential linkage between the snowball Earth event and biological evolution. To understand how the global glaciation and the biological evolution were linked, it is necessary to reconstruct the sequence of events with high temporal resolution that occurred during the deglaciation. In this study, we focus on silica-filling within sheet cracks. Sheet cracks abundantly occur in the basal part of the Marinoan cap carbonate (635 Ma), and are characterized by the interconnected horizontal and vertical cracks that are sequentially cemented by dolospars, silica and calcite. As the second stage of crack filling, the origin of silica-filling remains controversial. If the seawater origin is confirmed, the silica-filling might archive the signal of ocean chemistry in the aftermath of snowball Earth event. In this study, we measured the Germanium/Silica ratios (Ge/Si) and Rare Earth Elements (REE) compositions of silica-filling from the Doushantuo cap carbonate in the Yangtze Platform, South China. The silica-filling has low Ge/Si ratios (~1 μmol/mol), and displays a light REE (LREE) depleted REE pattern (normalized to PAAS) with positive Eu anomalies (Eu/Eu*). These results indicate the predominant seawater origin of silica with some contribution from hydrothermal fluid, suggesting the penecontemporaneous precipitation of silica-filling with respect to sheet crack formation. In order to quantify the contribution from hydrothermal fluid, we developed a binary mixing model by using Ge/Si ratio and Eu/Eu* as two independent proxies. The modeling results indicate mixing of ~5 vol.% hydrothermal fluid with contemporaneous seawater, or ~20 wt.% of Si in silica-filling from hydrothermal fluid. The modeling results also show that the synglacial hydrothermal flux might be one order of magnitude lower than the modern level, suggesting extremely weak hydrothermal activities during the Marinoan snowball glaciation.

Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation

Oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the Marinoan glaciation. However, the relationship between the oceanic dissolved O2 (DO) levels and early metazoan evolution remains equivocal. To provide a temporal and spatial reconstruction of the redox conditions for this critical interval, this study investigates the early Ediacaran cap carbonate deposits within intra shelf facies at three localities in the Three Gorges area, the Yangtze platform, South China. Trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals. The Member II dolostones of the Duoshantuo Formation, which overlies the cap carbonates, show a negative Ce anomaly. No Ce anomaly is observed in the lower units of the cap carbonates. In contrast, positive Ce anomalies (Ce/Ce* > 1.3) in the uppermost part of the cap carbonates are observed in all studied sections. These positive Ce anomalies are accompanied by high Mn/Fe ratios (ranging from 5.04 to 14.96) and slightly positive MREE anomalies, suggesting that a Fe-Mn-(oxyhydro) oxide coparticipation occurred during the cap carbonate deposition. The positive Ce anomalies may result from the reductive dissolution of the Ce-enriched Mn-(oxyhydro) oxides across a Mn(IV)/Mn(II) redoxcline, in a distinct manganous water wedge that was sandwiched between well-oxygenated and anoxic ferruginous deep water layers. This wedge may have exhibited low oxygen conditions, with approximately 10 µM DO (dissolved oxygen), in comparison to the >90 µM DO expected in an oxic setting and 0 µM DO in an anoxic setting. Therefore, the presence of a positive Ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the Marinoan glaciation. Early Ediacaran metazoans were possibly restricted to fully oxygenated conditions and were absent in the manganous or ferruginous-rich environments.

Raman geothermometry of carbonaceous material in the basal Ediacaran Doushantuo cap dolostone: The thermal history of extremely negative δ13C signatures in the aftermath of the terminal Cryogenian snowball Earth glaciation

Extremely negative δ13C values (as low as −48‰) of blocky calcite cements filling cavities, sheet-cracks, and fractures in the basal Ediacaran cap dolostone of the Doushantuo Formation in South China have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal Cryogenian snowball Earth glaciation. Alternatively, these extremely negative δ13C values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the Doushantuo cap dolostone. The latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13C-depleted calcite occurred at temperatures up to 476°C (mean=378°C, n=4) whereas carbonate fabrics with moderately negative δ13C values (generally>−10‰) were formed at much lower temperatures (mean=112°C, n=4). To assess these competing hypotheses, we systematically investigated the petrography, carbonate carbon and oxygen isotopes, and carbonaceous material (CM) Raman spectroscopy of the Doushantuo cap carbonate. Raman geothermometry suggests that CM associated with different microfabrics and δ13C signatures—including sedimentary fabrics with moderately negative δ13C values (e.g., peloids, stromatolite laminae, microbial mat fragments), diagenetic fabrics with moderately negative δ13C values (e.g., some void-filling or crack-filling cements), and diagenetic fabrics with extremely negative δ13C values (e.g., void-filling or crack-filling blocky and bladed calcites)—experienced temperatures generally <300°C (largely 200–300°C). Contrary to temperature estimates based on carbonate clumped isotopes, CM Raman geothermometry reveals that different microfabrics show largely overlapping paleotemperatures and that there is no apparent relationship between δ13C signatures and paleotemperatures. Whereas the origins of these extremely negative δ13C values remain unresolved, the Raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500°C that only affected the thermal history of the calcite with extremely negative δ13C values. Instead, the new data suggest that the analyzed microfabrics experienced the same peak temperatures <300°C, probably related to the burial history of the host rocks or pervasive hydrothermal activity that affected all microfabrics in the Doushantuo cap dolostone.

Carbon isotope gradient of the Ediacaran cap carbonate in the Shennongjia area and its implications for ocean stratification and palaeogeography

The geochemistry and paleogeography of the Doushantuo succession in the Shennongjia area have been insufficiently studied. Here, we report on the carbon and oxygen isotope compositions of Ediacaran Doushantuo cap carbonates from four sections (Longxi, Muyu, Yazikou and Songluo) in the Shennongjia area. A large C-isotopic gradient (~5‰) between the Longxi and Songluo sections, here identified for the first time, is inferred to have been dynamically maintained by photosynthesis in surface waters and anaerobic oxidation of dissolved organic carbon (DOC) in deep waters. Spatial variation in C-isotope chemostratigraphy among the four sections was related to Marinoan post-glacial sea-level elevation changes. At Longxi, a positive δ 13Ccarb shift below the horizon containing tepee-like structure resulted from intense photosynthesis during early regression. A negative δ 13Ccarb shift within the tepee-horizon was triggered by sulfate reduction and freshwater mixing with 13C-depleted dissolved inorganic carbon (DIC) during late regression. A positive δ 13Ccarb excursion in the uppermost part of the cap carbonate was related to enhance primary productivity and organic matter burial during early transgression. At Muyu, the carbon isotopic variation tendency, which is similar to that at Longxi, may have been mildly influenced by the surface water environment. At Songluo, the positive δ 13Ccarb excursion, up to -4‰ in the lower part of the cap carbonate, was probably associated with methanogenesis in deep waters during late transgression to early regression and subsequently disappeared due to decomposition of methane hydrate during late regression. At Yazikou, the consistently stable δ 13Ccarb values around -4‰ indicate that the cap carbonate may have deposited at intermediate water depths. As evidenced by diagnostic sedimentary characteristics of the study sections, the palaeogeographic framework of the Shennongjia area exhibited deepening from SE to NW during the Early Ediacaran Period in the aftermath of Snowball Earth.

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's 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 (<+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.

Sedimentary Environment of Ediacaran Sequences of South China: Trace Element and Sr-Nd Isotope Constraints

Post-Marinoan sedimentary rocks from the Yangtze block in South China were investigated to seek constraints on their provenance, genetic relationship with paleoclimate, depositional environment, and changes. Sedimentary rocks from the Doushantuo Formation in the Jiulongwan section of the Yangtze block were collected for whole-rock trace elements and Sr-Nd isotopic analyses. Flat-to-slightly middle rare earth element (REE)–enriched REE-plus-yttrium patterns, negative-to-positive Ce anomalies (Ce/Ce* of 0.86–1.40), positive La anomalies (La/La* of 1.02–1.42), and suprachondritic Y/Ho ratios (36.8–49.5) obtained for the Doushantuo cap carbonates indicate their deposition in a fluctuating inner-shelf setting where freshwater and seawater mixed together and the influence of hydrothermal fluid from the seafloor is absent. All Doushantuo carbonates and shales have variable Sr isotopic compositions, with initial Sr isotopic ratios (0.704030–0.712720) that support significant river-water contribution during the deposition of the Doushantuo Formation. Samples of the Doushantuo Formation collected across sedimentary profiles show distinct variations in Nd isotopic compositions and diagnostic trace elemental ratios (Th/Sc). The cap carbonates received significant input from a broad continental area undergoing extensive weathering immediately after the cease of the glaciation era. In contrast, the upper Doushantuo Formation sediments received more mature detritus from the nearby Archean Kongling complex, reflecting receding sea levels. In contrast to the negative Ce anomalies observed in lower Doushantuo rocks, negligible Ce anomalies in upper Doushantuo rocks suggest an anoxic setting, in agreement with the growth of phosphatic concretions and the presence of framboids of pyrite and a higher concentration of organic matter.

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

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.

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

Marinoan cap carbonates have been suggested to be primarily deposited in glacial meltwater and upwelled seawater. However, elemental geochemistry evidence for this depositional model is lacking. Here, we report high-spatial-resolution measurements of major, trace and rare earth elements of the Doushantuo cap carbonates from the Jiulongwan section in the Yangtze Gorges area, South China. Our results show that: 1) the basal cap carbonates display slight MREE enrichment, weak positive La anomalies, near-chondritic Y/Ho ratios, and slight negative Ce anomalies; 2) the lower-middle cap carbonates show slight LREE depletion or MREE enrichment, weak positive La and Eu anomalies, supra-chondritic Y/Ho ratios, and slight negative Ce anomalies; 3) the upper-middle cap carbonates have consistent enrichment of P, Fe, and trace metals, slight LREE depletion, and weak positive Ce, La and Eu anomalies; and 4) the upper cap carbonates exhibit LREE enrichment, weak positive La and Eu anomalies, supra-chondritic Y/Ho ratios, and mild negative Ce anomalies. These findings indicate that the Doushantuo cap carbonates did not precipitate from normal contemporaneous seawater, rather, the basal cap carbonates were deposited in oxygenated, relatively pure deglacial meltwater; the lower-middle cap carbonates in oxygenated brackish water; the upper-middle cap carbonates in upwelled anoxic brine water; and the upper cap carbonates in oxygenated brackish water. Our depositional model is consistent with the proposed sequence of events after the meltdown of Marinoan glaciation by Shields (2005).

Hydrothermal origin of elevated iron, manganese and redox-sensitive trace elements in the c . 635 Ma Doushantuo cap carbonate

Abstract: Major and trace element, including REE, concentrations of the Doushantuo cap carbonate ( c . 635 Ma) in South China show enrichment in Fe, Mn and redox-sensitive elements and slightly negative Ce anomalies, indicating anoxic environments during cap carbonate precipitation. High Fe T /Al ratios but very low concentration of extractable pyrites suggest ferruginous rather than euxinic conditions. The REE + Y patterns of samples show enrichment of heavy REE (HREE), positive Eu anomalies and positive Y anomalies, implying a hydrothermal origin for elevated concentration of Fe, Mn and redox-sensitive elements. The results suggest that ferruginous Ediacaran oceans may have rooted from hydrothermally induced iron accumulation in severely glaciated Cryogenian oceans.

Organic carbon isotope constraints on the dissolved organic carbon (DOC) reservoir at the Cryogenian–Ediacaran transition

Prominent negative carbonate carbon isotope (δ 13C carb) anomalies from some Ediacaran successions are accompanied by invariant or decoupled organic carbon isotope (δ 13C org) values and have been interpreted as resulting from the remineralization of a large dissolved organic carbon (DOC) reservoir capable of buffering carbon isotopes of organic matter. This inferred oceanic DOC reservoir was thought to have initiated with the onset of Cryogenian glaciations (ca. 720 Ma) and lasted for millions of years until the late Ediacaran Period (< 560 Ma). Carbon isotope analyses of the basal Doushantuo Formation (ca. 635 Ma) in south China reveal that (1) the cap carbonate has δ 13C org around −26‰ (VPDB) and relatively low Δδ 13C (22 ± 2‰) and (2) the overlying organic-rich black shale and shaly dolostone have more negative δ 13C org (−28‰ to −35‰) and higher Δδ 13C (28‰–30‰). Both δ 13C carb and δ 13C org show a + 6‰ shift within a 4-m-thick interval overlying the Doushantuo cap carbonate. The δ 13C org values of the cap carbonate are associated with low TOC (mostly < 0.1%); their paleoceanographic significance requires further tests in other Ediacaran basins. The co-varying positive shift in δ 13C carb and δ 13C org following cap carbonate deposition is best interpreted as resulting from a rapid increase in organic carbon burial, which may have resulted in the rise of oxygen and heralded the first appearance of animals a few meters above the Doushantuo cap carbonate. The data suggest that a large oceanic DOC reservoir did not exist in the early Ediacaran ocean. Excess oceanic DOC required to explain the Ediacaran Shuram and upper Doushantuo δ 13C excursions, if it existed, had to be developed during the Ediacaran Period after cap carbonate deposition.

Trace element and rare earth element of cap carbonate in Ediacaran Doushantuo Formation in Yangtze Gorges

For the Doushantuo cap carbonate at the Jiulongwan section in the Yangtze Gorges, its concentrations of redox sensitive elements suggest two distinct enrichments in stratigraphy. These enrichments occur at about 0.8 m and 3.3 m above the bottom of cap carbonate, respectively. They are interpreted as the temporary anoxic depositional conditions due to the oxidation of seeped methane. REE+Y patterns of the cap carbonate are classified into three types with different styles: (1) from the bottom to 2.45 m, representing the behaviors of freshwater and suggesting that massive meltwater swarmed into surface oceans during the deglaciation; (2) from 2.45 m to 3.3 m, indicating the pattern of ancient seawater possibly due to upwelling of deep water; and (3) from 3.3 m to the top, showing “MREE bulge” pattern with HREE-depletion as a result of diagenesis. The three-stage REE+Y patterns represent the transformations of shallow water in the wake of the Marinoan glaciation in this region: the fresh meltwater was dominant first, and then it interfused into the oceanic basin by the transgression and upwelling. Bloom of plankton further introduced anoxia near the water-sediment interface.

The age of the Nantuo Formation and Nantuo glaciation in South China

AbstractA U–Pb sensitive high‐resolution ion microprobe (SHRIMP) age of 654.5 ± 3.8 Ma from an ash bed immediately below the Nantuo Formation in South China provides the lowest age constraint for the Nantuo glaciation, which has been correlated with the global ‘Marinoan’ glaciation. A U–Pb SHRIMP age of 636.3 ± 4.9 Ma from a fallout tuff within the basal layer of the Nantuo Formation, along with the existing age of 635.2 ± 0.6 Ma from its overlying Doushantuo cap carbonate, suggests that the traditionally defined Nantuo Formation may have been deposited in a short time period at the end of the Nantuo glaciation. In combination with available ages globally, the data support a relatively short duration and rapid termination of ‘Marinoan’ glaciations.

Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China

Distinctive sedimentary structures, textures, and extremely negative δ 13 C carb values from the Ediacaran Doushantuo cap carbonate (ca. 635 Ma) in south China were taken as evidence for a methane hydrate destabilization event, but existing data for a methane-derived δ 13 C carb signature were reported from a single locality in the Yangtze Gorges area. Here we report negative δ 13 C carb values as low as −48‰ (Vienna Peedee belemnite) from two additional sections ~6 km and 55 km from the original locality. These negative δ 13 C carb values were obtained from isopachous cements that fill stromatactis-like cavities, sheet cracks, and fractures, and from partially recrystallized carbonate crusts, consistent with carbonate precipitation at cold methane seeps. The new data support a widespread methane release event following the Nantuo glaciation in south China (ca. 635 Ma), which may have contributed to the origin of unusual sedimentary and isotope features of cap carbonates.

Carbon isotope variation through the Neoproterozoic Doushantuo and Dengying Formations, South China: Implications for chemostratigraphy and paleoenvironmental change

Primary carbon isotope trends in marine carbonate rocks are particularly useful for chemostratigraphic correlation and paleoenvironmental reconstruction of the Neoproterozoic Earth System. Differences, however, exist between the published carbonate C-isotope profiles from the Neoproterozoic Yangtze platform and the global C-isotope record through the equivalent time interval. More C-isotope profiles are required for the Yangtze platform from where excellently preserved early animal fossils have been discovered. We studied C, O and Sr isotope compositions and Mn–Sr concentrations of two successions of the Neoproterozoic Doushantuo and Dengying Formations in Guizhou and Zhejiang Provinces, South China. Most of the 87Sr/ 86Sr ratios lie above 0.709, higher than the contemporaneous seawater values, and are likely to represent diagenetic alteration and possible leaching of clay detritus. The δ 18O profiles exhibit more fluctuation than the δ 13C profiles, indicating that oxygen isotopes are more sensitive to diagenetic effects. Both δ 13C profiles in Zhejiang and Guizhou show negative excursions to − 3 to − 4‰ in the Doushantuo cap carbonate overlying the Nantuo (Marinoan) diamictite and at the Doushantuo–Dengying boundary, and positive shifts to + 3 to + 4‰ through the rest of the Doushantuo and Dengying Formations. We suggest that oceanic overturn was responsible for both the negative δ 13C excursion within the Doushantuo cap carbonate and that at the Doushantuo–Dengying boundary. However, the mechanisms for the two overturn events are different. The former overturn occurred under conditions of high temperature and high evaporation rates after thawing of the Nantuo (end-Cryogenian) Snowball Earth that had resulted in the stagnant and stratified ocean under sea ice. The Doushantuo interval witnessed sustained high bioproductivity, elevated rates of organic matter accumulation in deep water and sediment, and progressive δ 13C stratification in the ocean, CO 2 drawdown to sediments and thus cooling, which led eventually to high latitudinal glaciation, vigorous thermohaline circulation and oceanic overturn at the end of the Doushantuo period. This latter overturn released CO 2 back to the atmosphere, thus raising the temperature and preventing further cooling. High productivity revived subsequently during the Dengying interval. More work will be needed to further constrain and interpret C-isotope evolution across the Precambrian–Cambrian boundary on the Yangtze platform.

Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China

Abstract The 3- to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of δ13C values as low as −41‰ in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.