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2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 5
Presentation Time: 9:10 AM

REDOX TRACE ELEMENT AND ORGANIC CARBON GEOCHEMISTRY OF THE NEOARCHEAN CAMPBELLRAND CARBONATE PLATFORM (TRANSVAAL SUPERGROUP, SOUTH AFRICA)


SCHROEDER, Stefan1, GOLDBERG, Tanya2, LACASSIE, Juan Pablo1, BEUKES, Nicolas J.1, FISCHER, Woodward3, GUTZMER, Jens1, KNOLL, Andrew H.3 and STRAUSS, Harald4, (1)Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa, (2)School of Earth Sciences, James Cook University, Townsville, 4811, Australia, (3)Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 2138, (4)Geologisch-Paläontologisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, Münster, 48149, Germany, sts@rau.ac.za

Drill cores through the Campbellrand-Malmani carbonate platform (~2.64 to 2.50 Ga, Transvaal Supergroup) provide a platform-to-basin transect to study dynamics of redox-sensitive trace elements, organic matter and organic carbon isotopes before the early Paleoproterozoic increase of atmospheric oxygen levels. Cores represent proximal slope and distal slope/basin environments. The cored sedimentary succession records flooding of the Kaapvaal Craton, establishment and aggradation of an extensive carbonate platform, and platform drowning, followed by deposition of shales, iron formation and chert across former platform environments.

Redox-sensitive trace element concentrations are correlated with concentrations of detrital elements Al and Th. This indicates that detrital input, rather than redox processes, dominated the trace element budget in the studied section. Elements Fe and Mn were effectively separated during deposition and/or diagenesis by incorporation into pyrite, siderite and carbonate, respectively.

Carbonates have organic carbon contents of ≤ 2 wt.% TOC, whereas up to 7 wt.% TOC were observed in shale lithologies. Organic carbon is broadly correlated with total sulfur concentrations, indicating coupled C and S cycles.

Significant spread and little systematic stratigraphic variation characterize δ13Corg and the isotopic difference between organic and carbonate carbon (ε), which could reflect some thermal overprint. However, δ13Corg values between -29 ‰ (PDB) and -44 ‰ (PDB) are considered to be close to the original signature. The isotopic difference ε varies between -28 ‰ (PDB) and -36 ‰ (PDB). These results are consistent with several mechanisms of C fixation. The more 13C-enriched values likely reflect photoautotrophic C fixation as the main process of primary productivity, whereas very depleted values suggest a contribution from bacterial biomass and reworking of organic matter under anoxic conditions.

Session No. 20

T139. Changes in Ocean and Atmospheric Redox State and the Evolution of Life I
Sunday, 22 October 2006: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 38, No. 7, p.56
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