Coupled sulfur, iron and molybdenum isotope data from black shales of the Teplá-Barrandian unit argue against deep ocean oxygenation during the Ediacaran

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• 作者：Florian Kurzweil^a ; ^{flokurzweil@hotmail.de" class="auth_mail" title="E-mail the corresponding author} ; Kerstin Drost^a ; Jan Pa&scaron ; ava^b ; ^{jan.pasava@geology.cz" class="auth_mail" title="E-mail the corresponding author} ; Martin Wille^a ; Heinrich Taubald^a ; Daniel Schoeckle^a ; Ronny Schoenberg^a
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2015
• 出版时间：15 December 2015
• 年：2015
• 卷：171
• 期：Complete
• 页码：121-142
• 全文大小：2880 K

文摘

The Earth’s atmosphere and hydrosphere changed from an Archean anoxic to a modern oxygenated world in two major steps, the Paleoproterozoic Great Oxidation Event (2.4–2.3 billion years ago) and the Neoproterozoic Oxidation Event (0.8–0.5 billion years ago). Both events had a strong influence on the availability of redox sensitive and bio-essential metals within the ocean and are, thus, strongly linked to fundamental biological innovations and diversification. Biological diversification during the Precambrian–Cambrian transition between 555 and 540 million years ago may have been driven by ocean–atmosphere oxygenation. The exact timing and the extent of (deep) ocean oxygenation within this time period remains unresolved though.

Here we present major and trace element compositions as well as Mo, S and Fe isotopic data of organic-rich black shales from the Teplá-Barrandian unit, Czech Republic. New in situ zircon U–Pb ages provide a maximum depositional age of 559.8 ± 3.8 million years. Black shales with strong metal enrichment show low δ⁵⁶Fe values due to the dominance of authigenic pyrite-Fe with δ⁵⁶Fe values around −0.6‰ over detrital Fe with δ⁵⁶Fe values around 0.1‰. Samples with lower authigenic metal enrichment show relatively low Mo/TOC ratios and increasing δ³⁴S values, which is interpreted to reflect basinal restriction and longer seawater renewal times. In analogy to the modern Black Sea, the accompanied depletion of basinal Mo_aq due to near quantitative Mo removal might have led to the preservation of the seawater δ⁹⁸Mo in the respective black shales. Our best estimate for this seawater Mo isotopic composition <560 million years ago is +1.2‰ in δ⁹⁸Mo, which is nearly identical to seawater δ⁹⁸Mo values inferred from Mid-Proterozoic black shales. The lack of higher δ⁹⁸Mo values in black shales (and seawater) argues against contemporaneous Mn oxide formation in well oxygenated deep sea settings, which would preferentially adsorb isotopically light Mo leaving behind an isotopically heavy ocean. By contrast, the deep ocean might have remained ferruginous with the hydrothermal Fe still outbalancing surficial oxygen production. Our results therefore contribute to a growing data set, which suggests limited deep water oxygenation during major biological innovations in the late Ediacaran period.