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Abundant sulphate in the Neoproterozoic ocean: implications of constant δ34S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian

By
Norman R. Moles
Norman R. Moles
School of Environment and Technology, Cockcroft Building, University of Brighton, Lewes Road, Brighton BN2 4GJ, UK
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Adrian J. Boyce
Adrian J. Boyce
Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QU, UK
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Anthony E. Fallick
Anthony E. Fallick
Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QU, UK
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Published:
January 01, 2015

Abstract

The timing and extent of ocean oxygenation is controversial. Proterozoic sulphur isotope datasets often show marked fluctuations over small stratigraphic intervals, suggesting that oceanic sulphate concentrations were much lower than modern values. A large accumulation of Neoproterozoic sulphate (>8 million tonnes preserved), as stratiform barite rock, is located in the Grampian Highlands near Aberfeldy. Diagenetic/metamorphic alteration has caused pronounced δ34S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in δ34S, mean 36±1.5‰. We infer that this is representative of contemporaneous seawater sulphate, and that δ34Sseawater was constant during deposition of a stratigraphical thickness >250 m of mostly fine-grained clastic sediments. Uniformity of δ34Sseawater during barite precipitation, even in thick (>10 m) beds and with the co-occurrence of abundant sulphides incorporating bacteriogenically reduced sulphur, implies no limit to availability of seawater sulphate during hydrothermal exhalative events. Our data, combined with previous δ34S research on Dalradian metasediments, suggest a stability, abundance and constancy of ocean sulphate in the Neoproterozoic. This contrasts with isotopic data using trace sulphate in limestones. It appears that, around the time of the Marinoan glaciation (c. 635 Ma), the ocean, although stratified at least locally, comprised a substantial reservoir of sulphate-bearing oxygenated seawater.

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Contents

Geological Society, London, Special Publications

Ore Deposits in an Evolving Earth

G. R. T. Jenkin
G. R. T. Jenkin
University of Leicester, UK
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P. A. J. Lusty
P. A. J. Lusty
British Geological Survey, UK
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I. Mcdonald
I. Mcdonald
Cardiff University, UK
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M. P. Smith
M. P. Smith
University of Brighton, UK
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A. J. Boyce
A. J. Boyce
Scottish Universities Environmental Research Centre, UK
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J. J. Wilkinson
J. J. Wilkinson
Natural History Museum and Imperial College London, UK
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Geological Society of London
Volume
393
ISBN electronic:
9781862396692
Publication date:
January 01, 2015

GeoRef

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