PALEOPROTEROZOIC ORGANIC CARBON AND NITROGEN ISOTOPIC STRATIGRAPHY ACROSS THE APPEARANCE OF MACROFOSSILS IN THE FRANCEVILLIAN BASINS, GABON

The Paleoproterozoic is one of the most important interval in the Earth’s history; characterized by rise of oxygen and the emergence of eukaryotes. The Paleoproterozoic macrofossils (ca. 2.2 Ga) are discovered one after another from the Francevillian Basins in Gabon. They are regarded as microbial colony or possible eukaryote because of their complex structures and large sizes. In order to clarify the tectonic settings and environmental conditions of the appearance of these Gabon biota, we investigated the litho- and chemo-stratigraphy in the Franceville (known for the macrofossils and the Oklo nuclear reactor) and Lastoursville (carbonate-depositional) Subbasins. The sedimentary sequence in these intracratonic rift basins is subdivided into five lithostratigraphic units; FA (sandstones with uranium ore at the top), FB (black shales and carbonates, including the fossil horizon at the upper part), FC (cherts), FD (black shales), and FE (sandstones). We collected rock samples and analyzed δ13C_org and δ15N_TN of black shales of the FB and FD units in the Franceville and Lastoursville Subbasins. In the Franceville Subbasin, δ13C_org decreases gradually from -28 ‰ to -35 ‰ within FB and drops to -40 ‰ in FD, and δ15N_TN changes from +1 ‰ at the bottom via +6 ‰ at the middle back to 0 ‰ at the top of FB. In the Lastoursville Subbasin, negative shift of δ13C_org from -28 ‰ to -45 ‰ and positive shift of δ15N_TN from +2 ‰ to +6 ‰ occur within the lower half of FB. The δ13C_org shows largely different behavior between Franceville and Lastoursville likely because of the depositional setting. The shallower Lastoursville Subbasin might have aerobic methanotrophic activity suggested by the low δ13C_org (-45 ‰). The δ15N_TN shows the change of nitrogen cycle from stably anaerobic to aerobic and back to anaerobic again within the FB unit in the Franceville basin. The aerobic condition preceded the appearance of the macrofossils, though the fossil horizon itself belongs to the following anaerobic condition. Through more precise correlation of litho- and chemo-stratigraphy between each subbasin, we are going to elucidate what was special for the rise of the eukaryotic life in the Francevillian Basins.