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

Shields, Graham and Kimura, Hiroto and Yang, Jiedong and Gammon, Paul (2004) Sulphur isotopic evolution of Neoproterozoic-Cambrian seawater: new francolite-bound sulphate δ^(34)S data and a critical appraisal of the existing record. Chemical Geology, 204 (1-2). pp. 163-182. ISSN 0009-2541. https://resolver.caltech.edu/CaltechAUTHORS:20170412-081128693

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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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://doi.org/10.1016/j.chemgeo.2003.12.001DOIArticle
http://www.sciencedirect.com/science/article/pii/S0009254103003826PublisherArticle
Additional Information:© 2004 Elsevier B.V. Received 15 August 2003, Accepted 9 December 2003, Available online 22 January 2004. The authors gratefully acknowledge the help of J.V. Cizdzel and K. Azmy (Univ. Nevada, LasVegas) and Y. Hu (James Cook Univ., Australia) for REE analyses, and W. Abdi, P. Middlestead and R. Hartree for technical support during laboratory work at Univ. Ottawa, Canada. We also acknowledge the help of Z.-Zh. Wang and X.-C. Tao for their assistance during sample collection and the helpful advice of reviewers Paul Gorjan and Harald Strauss. [LW]
Subject Keywords:Neoproterozoic; Phosphorite; Sulphur isotopes; China; Rare earth elements; Carbon isotopes
Issue or Number:1-2
Record Number:CaltechAUTHORS:20170412-081128693
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170412-081128693
Official Citation:Graham Shields, Hiroto Kimura, Jiedong Yang, Paul Gammon, Sulphur isotopic evolution of Neoproterozoic-Cambrian seawater: new francolite-bound sulphate δ34S data and a critical appraisal of the existing record, Chemical Geology, Volume 204, Issues 1–2, 15 March 2004, Pages 163-182, ISSN 0009-2541, http://doi.org/10.1016/j.chemgeo.2003.12.001. (http://www.sciencedirect.com/science/article/pii/S0009254103003826)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76530
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:12 Apr 2017 15:44
Last Modified:03 Oct 2019 17:02

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