Tonggang Zhang

On the coevolution of Ediacaran oceans and animals

Fe speciation and S-isotope of pyrite data from the terminal Proterozoic Sheepbed Formation in Canada and Doushantuo Formation in China reveal that ocean deep waters were anoxic after the global glaciations (snowball Earth) ending 635 million years ago, but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations. This supports a long-postulated link between oxygen levels and the emergence of eumetazoa. Subsequent ventilation of the deep ocean, inferred from shifts in Fe speciation in Newfoundland (previously published data) and western Canada (this report), paved the way for Ediacaran macrobiota to colonize the deep seafloors.

Sulfur and carbon isotopic variations in Neoproterozoic sedimentary rocks from southern China

Abstract A new set of δ34Ssulfide δ34Ssulfate and δ13Ccarbonate values has been reported from Neoproterozoic sedimentary rocks in southern China. The interglacial blackshales of the Datangpo Fm. display higher δ34Ssulfied values with > + 20‰. average, but the postglacial black shales from the Doushantuo Fm. show negative δ34Ccarbonate values. However, the Jinjiadong Fm., the same post-glaciation 3 as the Doushantuo Fm., has positive δ34Ssulfide values, implying that the δ34S value of sedimentary sulfides wouldbe controlled by lithofacies and paleogeographic environments. The δ34Ssulfate values relative to δ13Ccarbonate were obtained by extraction of trace sulfate from the successive carbonate sequences in the Yangtze Gorges sections. A preliminary interpretation suggests that the oceanic environment may fluctuate dramatically at the post-glacial Doushantuo stage and, then, recover its stability at the Dengying stage on the basis of the high resolution δ34S and δ13C corves of seawater.

Variations of sulfur and carbon isotopes in seawater during the Doushantuo stage in late Neoproterozoic

Successive analyses of sulfur and carbon isotopic compositions of carbonates strata in the Doushantuo Formation in the Yangtze area were accomplished through a method of extracting trace sulfate from carbonates. Sulfur and carbon isotopic compositions of coeval seawater were estimated from the samples that show the least diagenetic alteration. A high-resolution age curve of sulfur isotopes in seawater sulfates was obtained in the Doushantuo stage, which reflects the trend of variation in seawater sulfur isotopes after the Neoproterozoic snowball Earth event. Similar characteristics of variation in carbon isotopes were observed in the coeval carbonates. A large positive δ34S excursion over +20‰ occurs in ancient seawater sulfates in the early Doushantuo stage. Simultaneously, the δ13 C values in ancient seawater carbonates exhibit a positive excursion up to 10‰ The maximum δ34S and δ13C values are +46.4‰ and +6.9‰, respectively. In the middle Doushantuo stage, the range of variation in δ34 S values of seawater is relatively narrow, but δ13C values are quite high. Then, δ34S values of seawater become oscillating, and the same occurs in δ13C values. Negative excursions in δ34 S and δ13 C values occur simultaneously at the end of the Doushantuo stage, and the minimum δ34S and δ13C values dropped down to −10.1‰ and –5.7‰, respectively. The characteristics of variations in the sulfur and carbon isotopes of ancient seawater imply strong changes in oceanic environment that became beneficial to inhabitation and propagation of organism. The organic productivity and burial rate of organic carbon once reached a quite high level during the Doushantuo stage. However, the state of environment became unstable after the global glaciation. The global climate and environment possibly were fluctuating and reiterating. The negative excursions in δ34S and δ13C values occurring at the end of the Doushantuo stage may represent a global event, which might be related to oxidation of deep seawater.

Stratigraphic architecture of the Neoproterozoic glacial rocks in the “Xiang-Qian-Gui” region of the central Yangtze Block, South China

Abstract The Yangtze Block in South China is one of the important regions where Neoproterozoic glacial rocks are well developed and studied. However, the classification and correlation of the Neoproterozoic glacial sequences in the central Yangtze Block stillremain controversial among Chinese geologists. The original Sinian sections around the Yangtze Gorges Region became an official standard for classification and correlation since the 1950s. Subsequent regional geologic studies, however, resulted in different classification and correlation, because of its incompleteness. We select one of the complete sections in the bordering areas of Xiang (Hunan). Qian (Guizhou) and Gui (Guangxi). as a standard of classification and correlation. The temporal and spatial distribution, i.e. the stratigraphic architecture, of the glacial rocks in the central Yangtze Block is suggested. Our results indicate that the glacial sequence on the Yangtze Block was deposited during the Nantuo Ice-age, the Datangpo Interglacial-ag...