Biospheric oxygenation and atmospheric evolution across Earth's earliest glacial episodes

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The cause(s) of Earth s earliest glaciations are poorly understood, but models that relate changes in solar luminosity and greenhouse gas abundance (primarily methane and carbon dioxide) suggest that variations in the oxidation state of the atmosphere may have driven the oscillatory cycle of early Paleoproterozoic (ca. 2.2 to 2.4 Ga) ice ages. The redox state of the oceans and atmosphere is reflected in multiple sulphur isotope data (32S, 33S, 34S, and 36S) of sulphate and sulphide minerals preserved in ancient sediments. In the latest Archean sediments reveal mass-independent sulphur isotope fractionation, suggesting that atmospheric oxygen was less than 10-5 PAL. However, by the time of the earliest Paleoproterozoic ice age in South Africa, the sulphur isotopic record suggests a significant rise in pO2, to perhaps a few percent or higher. This study proposes to investigate the run up to the next youngest ice age in South Africa, through a high resolution study of minor sulphur isotope variations - including organic-bound sulphur, a proxy that has not yet been investigated in any previous minor isotope study - in deep time drill cores through the ca. 2.3 Ga Timeball Hill Formation, which lies immediately beneath a widespread glacial diamictite. Results should provide empirical constraints that changes in the atmospheric redox state drove the earliest glacial cycles.