δ^(13)C_(org) perturbations preserved by the interglacial Datangpo Formation in South China with implications for stratigraphic correlation and carbon cycle

Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated“Snowball Earth”hypothesis and the early evolution of metazoan life.The carbon cycle and redox conditions of the Sturtian-Marinoan non-glacial interval have been subjected to much controversy in the past decades because of the lack of a high-resolution stratigraphic correlation scheme.As one of the typical Sturtian-Marinoan interglacial deposits,the Datangpo Formation was widely distributed in South China with shales continuously deposited.The previous zircon dating data of the Datangpo Formation provide important ages for global constrain of the Sturtian-Marinoan non-glacial interval.Here we present a high-resolution straitigraphic study of the organic carbon isotopes of the Datangpo Formation from a drill core section in northern Guizhou Province.Based on measured episodicδ^(13)C_(org) perturbations,three positive shifts and three negative excursions are identified.Aδ^(13)C_(org)-based chemostratigraphic correlation scheme is proposed herein that works well for the Datangpo Formation regionally.Meanwhile,theδ^(13)C_(org) vertical gradients changed dynamically throughout the formation.This discovery implies that a significant ocean circulation overturn might have occurred in the upper Datangpo Formation,coinciding with the potential oxygenation.

Oxygen and hydrogen isotope ratios of chert from the Sixtymile Formation in Grand Canyon National Park,USA：a warm palaeoclimate,freshwater deposit

New oxygen and hydrogen isotope ratios of chert from middle, intraformational breccias, and upper breccia members of the Sixtymile Formation（SMF） in eastern Grand Canyon National Park（AZ） yield palaeoclimate estimates between 27 and 33℃. The isotopic compositions of cherts define a domain approximately parallel to the meteoric water line when plotted on a δD–δ-（18）O diagram; these data indicate that meteoric water was involved during formation of the chert. In thin section, the absence of interlocking mega quartz（〉35 lm） and silicafilled fractures and veins, along with preserved micromorphological silica fabrics, suggest that the chert has not been permeated by later hydrothermal fluids. Petrographic observations in thin section such as cyclic silica precipitation phases and glaebular micromorphologic fabrics lend support to the interpretation that meteoric waters were involved during chert precipitation. The post 742 Ma SMF has been correlated with diamictite（transition） beds of the Kingston Peak Formation（CA）, which in turn have been interpreted to have been deposited during the Sturtian Ice Age（-750–700 Ma）. Absence of facetted and striated clasts and other diagnostic glaciogenic features in the SMF,an unconformable contact with the stratigraphically older Chuar Group, coupled with warm palaeotemperature data inferred from stable isotope values of chert, tentatively suggest that deposition of sediment in the SMF likely did not take place during the Sturtian Ice Age.

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

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of ~13C and 6180 values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ3C chemostratigraphy possesses some unique characteristics： （1） stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, （2） increase of the δ13C values from -3 to ＋5‰ across the C2/C3 boundary, （3） no δ13C anomaly between the CI and C2 boundary, and （4） presence of an anomalous high δ13C value （＋2.3%0） and a faint positive correlation between δ13C and δ18O values in the C1 unit. Evidence of quite low δ13C anomalies （with a nadir of -41‰）, ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high 613C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units in- dicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.

Multiple cycles of glacier advance and retreat during the Nantuo （Marinoan） glacial termination in the Three Gorges area

The Snowball Earth hypothesis suggests that the sea water had totally been kept frozen for millions of years in Neoproterozoic glaciation, followed by a rapid and catastrophic deglaciation resulting from the elevated concentration of atmospheric CO〉 However, the sedimen- tary records are not consistent with the Snowball Earth hypothesis. The Nantuo Formation in Three Gorges area is composed of diamictites, sandstones and siltstones. The geochronology and the unconformity with underlying Liantuo Formation indicate that the Nantuo Formation in Three Gorges area may be the partial record （i.e., the final stage） of the Nantuo glaciation. Our studies on sedimentary successions of the Nantuo Formation convince the stepwise transition from the Cryogenia icehouse to the Ediacaran greenhouse, in which multiple glacier advance- retreat cycles rather than a catastrophic termination could be identified.