Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal Craton, South Africa

Abstract

Few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago. Stable carbon isotopic measurements of shallow stromatolitic carbonates (~0‰ VPDB) and basinal carbonate minerals (−6‰) in iron formation have been used to infer a strong isotopic depth gradient in Archean ocean basins. From new diamond drill cores obtained by the Agouron Drilling Project from the Griqualand West structural basin in the Northern Cape Province, South Africa, we present δ^(13)C data from carbonates and organic matter that offer fresh insights into the Late Archean carbon cycle. Three drill cores cover the development, progradation, and ultimate demise (by drowning) of the Campbellrand carbonate platform (ca. 2590–2500 Ma); one captures the platform top shallow marine and intertidal paleoenvironments, the other two run through slope and basinal sections deposited adjacent to the platform margin, increasing inwater depth (likely to >1 km). Both shallowand deep-water carbonates precipitated on the seafloor consistently show δ^(13)C values around −0.5‰, incompatible with a strong Late Archean isotopic depth gradient. A mathematical model suggests that these isotopic data are consistent with a reduced biological pump, increased dissolved inorganic carbon in seawater due to higher atmospheric P_(CO2), or both. Certain horizons do show distinct isotopic variability. Such areas are commonly shaly, and they tend to be organic and/or iron rich. Strong C-isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter. In sediment-starved areas where iron formation developed, siderite tends to be ^(13)C-depleted, sometimes by as much as −14‰. These observations suggest a carbon cycle in which iron respiration played a conspicuous role. Carbon isotope ratios from organic matter in shales are commonly >1‰ lighter than stratigraphically contiguous carbonates, but there is no clear water depth trend in the organic carbon isotopic data. Taken as a whole, the δ^(13)C of organic matter can be explained by several non-unique sets of processes, including different autotrophic mechanisms of carbon fixation, heterotrophic recycling (including fermentation and methanotrophy), and post-depositional diagenesis. The most striking feature is the occurrence of organic δ^(13)C values <−40‰, a feature that appears to be commonplace in Late Archean successions. Framed in the context of carbon cycle isotopic mass balance, both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen.

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