Qirui Zhang

A New Age Constraint on the Onset of the Neoproterozoic Glaciations in the Yangtze Platform, South China

A sample from the top of the Banxi Group (early Neoproterozoic) in Hunan Province (South China) was dated by the SHRIMP zircon U-Pb method. The resulting age of 725 ± 10 Ma probably solves the debate between the synglacial and the preglacial correlation of the Liantuo Formation and indicates that the Liantuo Formation most likely correlates with the preglacial Banxi Group. Since the Banxi (Danzhou) Group is upwardly transitional to the glacial Jiangkou Group, the age becomes a new maximum age constraint on the onset of the Jiangkou glaciation, which therefore most likely correlates with the Sturtian glaciation.

Nonglacial origin for low-δ18O Neoproterozoic magmas in the South China Block: Evidence from new in-situ oxygen isotope analyses using SIMS

Low-δ 18 O signatures in supracrustal rocks have been used as geochemical proxies for cold paleoclimates, e.g., glaciations. Unusual low-δ 18 O values found in Neoproterozoic igneous rocks in parts of the South China Block have thus been genetically linked to Neoproterozoic glaciation events. However, we report here new oxygen isotope compositions from Neoproterozoic magmatic zircons in central southern China using in-situ techniques that argue against such an interpretation. Our results show that (1) low-δ 18 O magmatic zircons started to appear in the South China Block from ca. 870 Ma, coinciding with the tectonic switching from Sibao orogenesis to postorogenic extension, which occurred more than 150 m.y. prior to the first glaciation event. The most abundant low-δ 18 O magmatic zircons have ages of 800–700 Ma. (2) The 830–700 Ma magmatic zircons are characterized by their bimodal nature of oxygen isotope compositions, i.e., mantle-like δ 18 O values (+4.4‰ to +5.8‰) and high-δ 18 O values (+9.3‰ to +10.8‰). (3) A sharp temporal change in maximum zircon δ 18 O values in the South China Block coincided with the onset of continental rifting and the possible arrival of a plume head. (4) No negative δ 18 O zircons have been identified in this study, contrary to previous studies. These features strongly argue against a glaciation origin for low to negative δ 18 O values in Neoproterozoic magmatic zircons from southern China. We propose that two stages of high-temperature water-magma interaction during plume-driven magmatism and continental rifting best explain the low-δ 18 O magmas. The most important implication of this study is that formation of such low-δ 18 O magmatic zircons was not necessarily related to glacial events and should not be used as a geochemical proxy for a cold paleoclimate.

Geochemical climate proxies applied to the Neoproterozoic glacial succession on the Yangtze platform, South China

A Neoproterozoic succession of glaciomarine deposits of probably Sturtian age is preserved on the Yangtze Platform in South China. At that time, the South China block was located in intermediate to low paleolatitudes at ca. 40°. The snowball Earth hypothesis offers one possible explanation for the occurrence of low latitude tillites. The hypothesis is largely based on geological and geochemical observation made in deposits underlying or overlying such tillites on several continents. In contrast our study focuses on evidence offered by the tillites themselves. We use major, trace and rare earth geochemistry to evaluate the environmental conditions prevailing during the glaciation. Of particular interest are the intensity of chemical weathering and the relative degree of oxygenation of Neoproterozoic (Nanhuan-Sinian) marine bottom waters. CIA values were obtained from preglacial sand- and siltstones, the matrix of the glacial deposits, fine-grained clastic sediments of a unit intercalated in the glacial succession, and postglacial siltstones and black shales. The data indicate relatively low degrees of chemical weathering for the glacial deposits. In contrast, pre- and postglacial deposits display comparatively elevated levels. This is also true for the intercalated unit, which we interpret as the product of a warmer and more humid interglacial period. Data for S/TOC, U/Th, Cd, Mo, and the Ce anom of the glaciomarine samples indicate the presence of oxic bottom waters during the glaciation. The snowball Earth hypothesis predicts the shutdown of chemical weathering on the continents and complete anoxia of the global ocean largely covered by sea ice for several million years. The geochemical record of the Neoproterozoic tillites on the Yangtze Platform is difficult to reconcile with the snowball Earth hypothesis.

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...

Elemental composition in surface snow from the ultra-high elevation area of Mt. Qomolangma （Everest）

A total of 14 surface snow (0–10 cm) samples were collected along the climbing route (6500–8844 m a.s.l.) on the northern slope of Mt. Qomolangma in May, 2005. Analysis of elemental concentrations in these samples showed that there are no clear trends for element variations with elevation due to redistribution of surface snow by strong winds during spring. In addition, local crustal aerosol inputs also have an influence on elemental composition of surface snow. Comparison between elemental concentration datasets of 2005 and 1997 indicated that data from 2005 were of higher quality. Elemental concentrations (especially for heavy metals) at Mt. Qomolangma are comparable with polar sites, and far lower than large cities. This indicates that anthropogenic activities and heavy metal pollution have little effect on the Mt. Qomolangma atmospheric environment, which can be representative of the background atmospheric environment.

Multiple climate cooling prior to Sturtian glaciations: Evidence from chemical index of alteration of sediments in South China

Investigation of climatic conditions prior to the Sturtian glaciations is critical to understanding the trigger mechanism for the series of Neoproterozoic global glaciations. In this study, we report high-resolution chemical index of alteration (CIA) records in the sediments of South China prior to the Sturtian glaciation (820~720 Ma). Our results showed there occurred multiple climate cooling before the Sturtian glaciations in South China: (1) a series of episodic and possibly global climate cooling periods from ca. 750 Ma to 725 Ma, which also caused some diachronous regional glaciations; (2) a permanent climate cooling period between ca. 800 Ma and 770 Ma, probably contemporaneous to the global “Bitter Springs stage” δ13C negative excursion; (3) a climate cooling period between ca. 815 Ma and 810 Ma. The three stages of climate cooling are also supported by their correspondence to previously reported extremely low δ18O records of igneous/metamorphic minerals from South China. These climate cooling periods also coincide with the magmatism and rifting events in South China. We argue that tectonic movements were the primary control on the climate cooling before the Neoproterozoic global glaciations.

Major and trace element geochemistry of the Neoproterozoic syn-glacial Fulu iron formation, South China

The Fulu iron formation (IF) is an iron-rich unit in the Neoproterozoic glacial successions, South China. The major element data suggest that the iron sources of the Fulu IF are derived from binary mixing from hydrothermal and detrital loads. The Fulu IF is characterized by slightly positive Eu anomalies similar to other Neoproterozoic IFs, indicating that a high-temperature hydrothermal input may contribute little to Neoproterozoic IFs. A shift from non-existent to slightly negative Ce anomalies of the Fulu IF indicates that the IF precipitated across an iron chemocline separating a weakly oxic surface ocean from an oxygen-depleted deep ocean.