ISOTOPIC CONSTRAINTS ON THE LATE ARCHEAN CARBON CYCLE FROM THE WESTERN MARGIN OF THE KAAPVAAL CRATON, SOUTH AFRICA

Few existing studies provide insight into the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 Myr ago. Stable carbon isotopic measurements of shallow stromatolitic carbonates (0‰ PDB) 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 δ¹³C 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. 2590Ma-2500Ma); 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 in water depth (to likely > 1 km). Both shallow and deep-water carbonate precipitates are repeatedly found to be around -0.5‰, inconsistent with a strong Late Archean isotopic depth gradient. A mathematical model suggests that this observation is consistent with a reduced biological pump and/or increased ocean dissolved inorganic carbon due to higher atmospheric pCO₂. Certain horizons do show distinct isotopic depletion. Such areas are often shaly and tend to be organic and/or iron rich. This variability occurs on a cm scale and most likely stems from diagenetic remineralization of isotopically depleted organic matter. Interestingly, in sediment-starved areas where iron formation developed, we find that siderite tends to be ¹³C-depleted, sometimes by as much as -14‰. These observations suggest a carbon cycle in which iron respiration played a conspicuous role. Taken in context with organic carbon isotopic data (-29 to -44‰ PDB), steady-state considerations suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen.