CARBON, NITROGEN, AND SULFUR ISOTOPES FROM THE 2.1 GA FRANCEVILLE BASIN, GABON

We analyzed carbon, nitrogen, and sulfur isotopes archived in shallow marine carbonates of the 2.1 Ga Franceville Basin, Gabon to investigate biogeochemical cycling in the interval shortly after the Great Oxidation Event (GOE). The Franceville Basin preserves a sequence of 2.1 Ga sandstones, shales, and carbonates overlying Archaean crystalline basement rock. The region has been stable since the Paleoproterozoic and regional greenschist-facies metamorphism is thought not to have disturbed primary stable isotope signatures of carbonate. This is supported by previous work documenting the lack of correlation between δ¹³C_carb and δ¹⁸O_carb, as well as the absence of any stratigraphic trend in δ¹⁸O_carb (Préat et al., 2011). Carbon isotopes of bulk organic matter have a mean value of -33.9‰ (n=35, SD=5.65), with no easily discernable stratigraphic trend. There is little covariation between our δ¹³C_org data and the δ¹³C_carb values of the same samples reported by Préat et al. (2011). The difference between δ¹³C_org and δ¹³C_carb of the same sample ranges widely from 26.33 to 50.28‰ VPDB, with a mean value of 38.30‰ (SD=6.23). Our carbon data are consistent with several explanations, including the variable incorporation of organic matter from multiple sources, incorporation of light carbon from methane metabolisms, as well as organic matter recycling. Pyrite sulfur isotope compositions were very positive and variable δ³⁴S_pyr with an average value of +22.03‰ VCDT (n=30; SD=10.55). Such enriched δ³⁴S_pyr values are similar to those in other sections in the early Proterozoic and the late Neoproterozoic. Our results are largely in agreement with other recent studies documenting relatively low oxygen levels soon after the GOE.