Geochemical and petrographic evidence for the co-occurrence of races B and L of Botryococcus braunii in the Maoming oil shales, China

Around 30 Maoming oil shale samples were collected and analyzed geochemically to investigate their biomarker compositions. C31 and C33 botryococcanes were abundant in some samples whilst high contents of lycopanes (C39 and C40) and a monoaromatic lycopane derivative with concomitant occurrence of botryococcanes were also detected in other samples. These indicate that the fossils of the races B and L of Botryococcus braunii co-exist in the Maoming sediments thus providing an excellent opportunity to explore the morphological differences of two races of B. braunii. Different preparation and observation methods were carried out for better and detailed morphological observations enabling the fossil characters between race B and L of B. braunii to be recognized. Better observations were obtained using white fluorescence transmitted light under UV-induced fluorescence microscopy because yellow fluorescence was avoided. Race B of B. braunii has polygonal cup mouths and is often covered by an envelope. However, race L has a relatively smaller colony size and round cup mouth. The co-occurrence of race B and L of B. braunii in Maoming sediments possibly implies a tropical to subtropical climate during the Late Eocene-Oligocene period in the study area.