Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the ∼2.2–2.1 Gyr shallow-marine Lucknow Formation, South Africa

Autor: Andrey Bekker, S. Schröder, H.S. Van Niekerk, N.J. Beukes, Harald Strauss
Rok vydání: 2008
Předmět:
Zdroj: Terra Nova. 20:108-117
ISSN: 1365-3121
0954-4879
DOI: 10.1111/j.1365-3121.2008.00795.x
Popis: Past oceanic sulphate concentration is important for under- standing how the oceansredox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before 1.2 Gyr probably reflects low seawater sulphate and ⁄ or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22-2.06 Gyr Lomagundi positive d 13 C excursion. However, the 2.2-2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca-sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires ‡2.5 mM L )1 sulphate concentration, higher than cur- rent estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of £200 lM L )1 , but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.
Databáze: OpenAIRE
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