CO2 buildup drove global warming, the Marinoan deglaciation, and the genesis of the Ediacaran cap carbonates

Mercury (Hg), a highly volatile metal, is capable of tracing volcanism through geological history as LIP events transiently emit large amounts of Hg. There are two indicators that make Hg a unique tool for geochemistry, the Hg to total organic carbon ratio (Hg/TOC) and mass-independent fractionation (Hg-MIF, defined as Δ199Hg). Owing to the affinity of Hg to organic matter, anomalous high Hg/TOC ratios in sediments can better reveal large volcanic eruptions. The anomaly of Hg-MIF is mainly observed in Hg photoreactions, providing a fingerprints of specific reaction pathways of Hg. Volcanic Hg usually has Δ199Hg ~ 0, but photochemical processes in the surface environment can alter this signal, resulting in positive Δ199Hg in marine systems (e.g., seawater and marine sediments) and negative Δ199Hg in terrestrial systems (e.g., soil and vegetation). Here, we examined the Hg records in Ediacaran cap carbonates in South China and Upper Cretaceous oceanic red beds (ORBs) in southern Tibet and the North Atlantic, to obtain their sedimentary material sources and the cause of the termination of Marinoan glaciation and Cretaceous oceanic anoxic events. (1) The cap carbonates show higher Hg concentrations (4.9 to 405 ppb), most of which are comparable to that observed in carbonates deposited during non-LIPs periods. The lack of Hg/TOC anomalies in these cap carbonates suggests that background volcanic activity, rather than a short-term large igneous province event, drove the Marinoan deglaciation. The cap carbonates show positive Δ199Hg values (0.18 to 0.34 ‰) in slope settings and slightly negative to slightly positive Δ199Hg values (0.16 to 0.11 ‰) in shelf settings, suggesting a binary mixing of seawater- and terrestrial-derived Hg in early Ediacaran Ocean. We infer that the accumulation of greenhouse gases, due to ongoing volcanic emissions of CO2 and enhanced release of gas hydrates, triggered global warming. This warming led to melting of sea ice cover, enhanced terrestrial inputs, and large-scale dissolution of atmospheric CO2 into seawater, driving widespread deposition of Ediacaran cap carbonates. (2) In southern Tibet and the North Atlantic, black/gray shales (typical deposition of oceanic anoxic events) show much higher Hg concentrations and Hg/TOC values than ORBs, indicating enhanced Hg flux to global oceans during time of black/gray shale deposition. Black/gray shales show lower Fe3+/Fe2+ and positive Δ199Hg, suggesting a significant input of Hg into the anoxic/dysoxic ocean via atmospheric deposition. The isotope values are consistent with a volcanic source for this excess Hg. ORBs show high Fe3+/Fe2+ and negative shifts of Δ199Hg, suggesting that the dominant source of Hg into the oxic oceans was via terrestrial runoff. These results suggest that volcanism was an important driver of the climate/ocean dynamics during the Late Cretaceous. To sum up, in addition to indicating short-strong volcanic activities, Hg can also trace the source of sedimentary materials under weak magmatism. Moreover, Hg offers a more accurate depiction of the interactions and exchanges among the Earth’s atmosphere-ocean-land system.

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

Newly discovered Sturtian cap carbonate in the Nanhua Basin, South China

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

A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China

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.

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

The origin of cap carbonate after the Ediacaran glaciations

Palaeo-environmental significance of fibrous carbonate cement in Marinoan cap carbonates

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

Cryogenian cap carbonate models: a review and critical assessment

The Earth's most severe glaciations are thought to have occurred about 600 million years ago, in the late Neoproterozoic era. A puzzling feature of glacial deposits from this interval is that they are overlain by 1-5-m-thick 'cap carbonates' (particulate deep-water marine carbonate rocks) associated with a prominent negative carbon isotope excursion. Cap carbonates have been controversially ascribed to the aftermath of almost complete shutdown of the ocean ecosystems for millions of years during such ice ages--the 'snowball Earth' hypothesis. Conversely, it has also been suggested that these carbonate rocks were the result of destabilization of methane hydrates during deglaciation and concomitant flooding of continental shelves and interior basins. The most compelling criticism of the latter 'methane hydrate' hypothesis has been the apparent lack of extreme isotopic variation in cap carbonates inferred locally to be associated with methane seeps. Here we report carbon isotopic and petrographic data from a Neoproterozoic postglacial cap carbonate in south China that provide direct evidence for methane-influenced processes during deglaciation. This evidence lends strong support to the hypothesis that methane hydrate destabilization contributed to the enigmatic cap carbonate deposition and strongly negative carbon isotopic anomalies following Neoproterozoic ice ages. This explanation requires less extreme environmental disturbance than that implied by the snowball Earth hypothesis.

Abstract:This article focusses on the bio-monitoring of total Hg (THg), sulfur (TS) and carbon (TC) concentrations and pool sizes in forest vegetation and soil layers within the context of a maritime-to-inland transect study in southwestern New Brunswick. This transect stretches from the Grand Manan Island in the Bay of Fundy to the mainland coast (Little Lepreau to New River Beach) and 100 km northward to Fredericton. Along the Bay, frequent summer fogs are thought to have led to increased THg concentrations in forest vegetation and soils such that island THg &gt; coast THg &gt; inland THg concentrations. Transect sampling was done in two phases: (i) a general vegetation and soil survey, and (ii) focusing on specific soil layers (forest floor, top portion of the mineral soils), and select moss and mushrooms species. By way of multiple regression, it was found that soil, moss and mushroom THg and TS were strongly related to one another, with THg decreasing from the island to the inland locations. The accumulated Hg pool within the mineral soil, however, far exceeded (i) the estimated THg pools of the forest biomass (trees, moss and mushrooms) and the forest floor, and (ii) the literature-reported and case-study inferred net input/output rates for annual atmospheric Hg deposition and sequestration, Hg volatilization, and Hg leaching. Partitioning the total soil Hg pool into geogenically and atmospherically derived portions suggested that mineral soils in temperate to boreal forest regions have accumulated and retained atmospherically derived Hg over thousand years and more. These results are summarized in terms of further guiding forest THg monitoring and modelling efforts in terms of specific vegetation and soil sampling targets.

The formation of Neoproterozoic tillites and overlying cap carbonates is important to understand drastic climatic and paleo-environmental variations during and after the collapse of the Snowball Earth. This rock assemblage has been found at the northwestern margin of the Tarim Craton in China but its depositional processes are still debated. We carried out stratigraphic and carbon isotopic studies on the tillites of the Yuermeinak Formation and overlying cap carbonates in this area. Detailed field observation and stratigraphic comparison demonstrate that these strata formed around the age of Marinoan deglaciation (ca. 635 Ma). Unconformable contacts with bedrocks and the variation of dip directions of the cap carbonates suggest that a mountainous topography developed in northwest Tarim during the Marinoan glaciation. We proposed a four-stage depositional model from glacial to alluvial fan and/or neritic facies systems. The first stage formed the stratified and massive tillites, recording several glacial cycles during the Marinoan deglaciation. The second stage involved the belated transgression at the end of the Marinoan deglaciation, caused the re-cementation of some tillites and the negative &amp;#948;13C of their matrixes. The third stage included the alternating precipitation of calcareous mudstones and cap carbonates after the Marinoan deglaciation, reflecting frequent sea-level changes. The fourth stage was relevant to a terrestrial environment because of a widespread marine regression. Furthermore, we suggest that the timespan of the intense chemical weathering on exposed continents after the Marinoan deglaciation was comparable to the duration of the transgression, lasting for hundreds of thousands of years. This work was financially supported by NSFC projects (grants 42072264, 41730213, 41902229, 41972237) and Hong Kong RGC GRF (17307918).

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