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	Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China
	Rodler A. S.; Hohl, S. V.; Guo, Q.; Frei, R.
	2016
关键词	Yangtze Platform Detrital contamination Al-corrected Cr signals Diagenetic alteration Coeval oxygenated seawater Cr composition rare-earth-elements yangtze platform hexavalent chromium snowball earth calcium-carbonate fractionation factors atmospheric oxygen hydrothermal vents cr(vi) reduction ultramafic rocks
英文摘要	The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O- and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated delta Cr-53 signals due to (i) detrital contamination consistent with Cr signatures of other post-Marinoan cap dolostones, (ii) diagenetic carbonate alteration and post-depositional remobilization of Cr. Al concentrations were used as indicator of detrital contamination and to calculate authigenic Cr concentration and isotope signatures when the detrital contribution of Cr was <40%, which generally resulted in slightly lower Cr concentrations and slightly higher Cr isotopic signals in these samples. Diagenetic alteration of a majority of the studied cap dolostones is evident from low Sr concentrations and light delta O-18(carb) signals that correlate with low Cr concentrations and negatively fractionated delta Cr-53 values. Post-depositional remobilization of Cr results in the loss of heavy Cr isotopes and consequently light Cr isotopes are retained during precipitation of secondary carbonates. However, some samples might still show pristine Ediacaran seawater signals. Two samples pertaining to the deep water cap dolostone section at Yanwutan and one sample pertaining to the shallow water section at Jijiawan yield slightly positively fractionated delta Cr-53 signals, particularly when corrected to presumably authigenic signals (delta Cr-53(auth)). These values are consistent with those from other Neoproterozoic carbonate sections deposited in an oxygenated environment. This short sequence of positively fractionated delta-53Cr(auth) is concurrent with a cap dolostone Sr-87/Sr-86 signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few least-altered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in post-glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration. (C) 2016 Elsevier B.V. All rights reserved.
出处	Chemical Geology
卷	436
页	24-34
语种	英语
ISSN号	0009-2541
DOI标识	10.1016/j.chemgeo.2016.05.001
内容类型	SCI/SSCI论文
源URL	[http://ir.igsnrr.ac.cn/handle/311030/43182]
专题	地理科学与资源研究所_历年回溯文献
推荐引用方式 GB/T 7714	Rodler A. S.,Hohl, S. V.,Guo, Q.,et al. Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China. 2016.

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