Paper No. 225-1
Presentation Time: 9:00 AM-6:30 PM
AFTER THE GREAT OXIDATION EVENT: δ13C, δ15N, δ34S, AND I/CA RECORDS FROM THE FRANCEVILLE BASIN, GABON
Investigation of isotopic signatures in organic matter and pyrite of the Franceville Basin, Gabon (2.1 Ga) was conducted to better constrain changes in redox conditions and biogeochemical cycling after the Great Oxidation Event (GOE). Paleoproterozoic sediments record the protracted history of rising and fluctuating O2 over hundreds of millions of years following the GOE. Marine sediments of the Franceville Basin are only lightly metamorphosed, and their geochemistry may thus reveal unique insights into environmental and metabolic conditions during this transition. We analyzed several biologically mediated redox proxies preserved in sedimentary carbonates from the Franceville Basin, including δ34S values of pyrite, δ13C and δ15N values of bulk organic matter and kerogens, and I/Ca ratios. Consistent with independent reports of an excursion in δ13Corg in the Franceville Basin and elsewhere, we found ~20‰ stratigraphic variation in δ13Corg with depleted values ranging from -25.4 to -45.6 ‰ VPDB. There is little covariation between our δ13Corg data and the δ13Ccarb values previously reported by Préat et al., 2011. The difference between δ13Corg and δ13Ccarb of the same sample ranges from 26 to 50‰. δ15N values of bulk organic matter range from -6 to +6 ‰, a wider distribution of values than previously reported for the Franceville Basin and more negative than values reported for the similarly aged Onega Basin in Fennoscandia. We extracted pyrite using the chromium reduction method. δ34S values of pyrite are highly enriched, ranging from +14.3 to +57.8 ‰ VCDT, consistent with a small marine sulfate reservoir or with Rayleigh-type distillation in a closed system. I/Ca ratios range from near zero to near Phanerozoic levels and are consistent with the presence of iodate. Currently the Franceville Basin rocks are divided into five units according to lithostratigraphic differences rather than time-bound divisions. We hope the work reported here will help strengthen the isotope stratigraphy of the Franceville Basin so that changes in different parts of the basin may be more easily correlated with each other and with geochemical archives in other parts of the world.