Isotope excursions and shifting oxidation states recorded in the Paleoproterozoic Franceville Basin

Abstract:

Geochemical studies of Paleoproterozoic rocks have revealed that the initial rise of oxygen was protracted and that Earth’s surface environments fluctuated between oxic and anoxic states over hundreds of millions of years. Marine sediments of the 2.1 Gyr-old Franceville Basin of west central Africa are only lightly metamorphosed, and their geochemistry may thus reveal unique insights into the environmental and metabolic conditions during the history of rising oxygen levels. In the Franceville Basin stratigraphic variation totaling 10‰ in δ¹³C_carb was previously documented. This contribution builds on this work and characterizes changes in C, N, and S cycles using stable isotope values. The results from systematic analysis of several biologically mediated redox proxies preserved in carbonates from the Franceville Basin will be presented, including δ³⁴S values of pyrite and δ¹³C and δ¹⁵N values of bulk organic carbon and kerogens. Consistent with independent reports of an excursion in δ¹³C_org in the Franceville Basin and elsewhere, we find ~20‰ stratigraphic variation in δ¹³C of bulk organic carbon. Initial results for δ¹⁵N 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. I/Ca ratios range from near zero to near Phanerozoic levels and are consistent with the presence of iodate. Chromium reducible sulfide has been extracted from all but one sample, confirming the presence of pyrite. δ³⁴S of pyrite as well as δ¹³C and δ¹⁵N of kerogen will also be presented. The biochemically diverse array of proxy analyses presented here have varying thresholds of sensitivity to oxygen levels and hence will allow detailed reconstruction of the redox history of basin waters. As minimum O₂ thresholds are often needed for certain biochemical processes, the resulting data will also have implications for key steps in the evolution of biochemical pathways.