Keywords:Paleoproterozoic, carbon isotope, nitrogen isotope, Gabon
The appearance of eukaryotes in the Paleoproterozoic is one of the most innovative evolutionary events in the history of life on the Earth. The Paleoproterozoic macrofossils (ca. 2.2 Ga) are discovered successively from the Francevillian Basins in Gabon, and are regarded as microbial colony or possible eukaryote based on their complex structures and large sizes. In order to clarify the environmental conditions of the appearance of these Gabon biota, we investigated the litho- and chemo-stratigraphy in the Franceville (siliciclastic) and Lastoursville (carbonate-depositional) Subbasins. The sedimentary sequence in these intracratonic rift basins is subdivided into five lithostratigraphic units, namely FA to FE. We collected rock samples and analyzed δ13C_org and δ15N_TN of black shales and carbonates of the FB units, across the appearance of the fossils, in the Franceville and Lastoursville Subbasins. In the Franceville Subbasin, δ13C_org decreases gradually from -28 ‰ to -35 ‰ within FB; 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, δ13C_org decreases from -28 ‰ to -45 ‰ within the lower half of FB and slightly decreases to -48 ‰ in the upper half; δ15N_TN changes from +2 ‰ at the bottom via +6 ‰ at the middle back to +4 ‰ at the top of FB. The extremely low δ13C_org (less than -45 ‰) in the Lastoursville Subbasin suggests active methanotrophy. The difference of the δ13C_org and δ15N_TN chemostratigraphy in these two subbasins comes from the depositional settings. Through the correlation of litho- and chemo-stratigraphy in each subbasin, we will discuss the environmental conditions for the appearance of the macrofossils in the Francevillian Basins.