Sulphur isotopic evolution of Neoproterozoic-Cambrian seawater: new francolite-bound sulphate δ^(34)S data and a critical appraisal of the existing record

Abstract

Francolite-bound sulphate sulphur isotope compositions from the 570±20 Ma Doushantuo Formation are used to constrain seawater δ^(34)S during the late Neoproterozoic. Phosphorites of the lower ore have δ^(34)S compositions between 32.3‰ and 37.8‰, averaging 36.1‰, δ^(13)C 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 δ^(34)S between 31.6‰ and 34.5‰, averaging 33.3‰, δ^(13)C 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 δ^(34)S 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 δ^(34)S or isotopic fractionation due to bacterial sulphate reduction. Almost identical constraints on seawater δ^(34)S have been reported from lowermost Cambrian phosphorites across South China, indicating that seawater δ^(34)S was similar at ∼570 and ∼540 Ma. A critical examination of all published data provides no firm evidence for major fluctuations in seawater sulphate δ^(34)S 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 δ^(34)S in response to the "Snowball Earth" glaciations. The existing δ^(34)S record is consistent with a sustained increase in seawater δ^(34)S 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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