Nitrogen isotope evidence for redox variations at the Ediacaran-Cambrian transition in South China

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Date: Sept. 2013
From: Journal of Geology(Vol. 121, Issue 5)
Publisher: University of Chicago Press
Document Type: Author abstract; Report
Length: 234 words

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Abstract :

Nitrogen isotope ([[delta].sup.15]N) analyses of the early Cambrian (ca. 542-520 Ma) strata in the Yangtze Gorges area, south China, reveal a prominent positive [[delta].sup.15]N excursion ([greater than or equal to] + 8%), concurrent with a negative shift in organic carbon isotopes ([[delta].sup.13][]) at the early Nemakit-Daldynian or early Fortunian stage. This positive [[delta].sup.15]N excursion, in combination with the previously reported paired [[delta].sup.13][C.sub.carb]-[[delta].sup.13][] at this interval, can be interpreted as resulting from denitrification with large isotopic fractionation. Subsequent [[delta].sup.15]N from early Nemakit-Daldynian to early Tommotian (stage 2) are dominated by stable values fluctuating between -2[per thousand] and +2[per thousand], close to that of atmosphere [N.sub.2] (~0[per thousand]) accompanied by the low and stable [[delta].sup.13][] (~-33[per thousand]) and decoupled [[delta].sup.13][C.sub.carb]- [[delta].sup.13][]. We argue that the relatively low and invariant [[delta].sup.15]N values from this interval likely resulted from enhanced N fixation with limited isotopic fractionation. Enhanced N fixation is generally associated with significant nutrient N loss mainly through denitrification and anammox, indicating the expansion of an anoxic environment. The [[delta].sup.15]N records of the study sections show large difference from that of potential time-equivalent Zhujiaqing Formation in the Xiaotan section. This discrepancy can be plausibly attributed to different redox conditions in a stratified ocean inherited from the Ediacaran. Anoxic ocean conditions may have lasted for millions of years until the early Atdabanian (stage 3), when trilobites started to dominate the Cambrian ocean. Online enhancement: appendix table. DOI: 10.1086/671396

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Gale Document Number: GALE|A345276421