Publication Date:
2014-01-05
Description:
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 34 S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in 34 S, mean 36 ± 1.5. We infer that this is representative of contemporaneous seawater sulphate, and that 34 S seawater was constant during deposition of a stratigraphical thickness 〉250 m of mostly fine-grained clastic sediments. Uniformity of 34 S seawater 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 34 S 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.
Print ISSN:
0305-8719
Electronic ISSN:
2041-4927
Topics:
Geosciences
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