Shields, Graham, Kimura, Hiroto, Yang, Jiedong, and Gammon, Paul (2004) Sulphur isotopic evolution of Neoproterozoic-Cambrian seawater: new francolite-bound sulphate δ34S data and a critical appraisal of the existing record. Chemical Geology, 204 (1-2). pp. 163-182.

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Abstract

Francolite-bound sulphate sulphur isotope compositions from the 570±20 Ma Doushantuo Formation are used to constrain seawater δ34S during the late Neoproterozoic. Phosphorites of the lower ore have δ34S compositions between 32.3‰ and 37.8‰, averaging 36.1‰, δ13C values between −4‰ and −1‰, and do not exhibit anomalous cerium depletion. By contrast, phosphorites of the upper ore, which contains some of the earliest animal fossils known, have δ34S between 31.6‰ and 34.5‰, averaging 33.3‰, δ13C values between −1‰ and +2‰, and exhibit a distinct negative cerium anomaly. The retention of a cerium anomaly in these upper Doushantuo phosphorites implies that oxidising conditions prevailed during early diagenetic REE scavenging, indicating that these samples are more likely to retain a seawater sulphur isotope composition. On this basis we argue that seawater δ34S was 32‰ (±2‰) during the deposition of the upper Doushantuo unit. The slightly higher values of the lower Doushantuo unit may reflect either slightly higher ambient seawater δ34S or isotopic fractionation due to bacterial sulphate reduction. Almost identical constraints on seawater δ34S have been reported from lowermost Cambrian phosphorites across South China, indicating that seawater δ34S was similar at ~570 and ~540 Ma. A critical examination of all published data provides no firm evidence for major fluctuations in seawater sulphate δ34S during the Neoproterozoic–Phanerozoic transition interval. This indicates that the onset of pervasive bioturbation had only a negligible effect on global pyritisation rates and questions recent reports of massive fluctuations in seawater sulphate δ34S in response to the “Snowball Earth” glaciations. The existing δ34S record is consistent with a sustained increase in seawater δ34S between 750 and 570 Ma. Shorter-term fluctuations cannot be ruled out entirely, but will require more detailed study, incorporating rigorous methodologies to constrain the effects of non-marine influences.

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