N-C27 Predominance and 13C enrichment by bicarbonate assimilation of race A of Botryococcus braunii in Chinese Maoming oilshales

Abstract

In Maoming oilshales, the presence of races B and L of Botryococcus braunii (B. braunii) was confirmed by the detection of botryococcanes and C40 lycopanes and their derivatives. However, there were no specific biomarkers for race A of B. braunii. The contribution of race A of B. braunii has been implied by the distribution of non-specific n-alkanes with an odd carbon-number preference and a predominance of n-C27. However, confirmation of its contribution to oilshales has been lacking until now. We found that n-C27 is a dominant n-alkane in most Maoming samples which exhibits a positive δ13C excursion (approximately 2–3‰) compared to other homologues. In addition, the extremely heavy δ13C values of botryococcanes (−6.0‰ to −8.5‰) are rationalized to reflect bicarbonate assimilation in race B of B. braunii. The positive excursion of the δ13C values of n-C27 extracted from the same samples could be attributed to bicarbonate assimilation by race A in a similar manner. Bicarbonate assimilation by race A would also explain the isotopic enrichment of n-C14 ~ n-C33 alkanes in the range of −20.1‰ to −26.1‰. They are close to the values of some higher plant sources (C4 plant) and largely different than that of the reported algal n-alkanes (<−30.6‰). The bicarbonate consumption mechanism utilized by both races A and B of B. braunii in Maoming oilshales was triggered by the sharp decrease in pCO2 during the Late Eocene period, recorded globally at the Eocene-Oligocene transition. The presence of race A of B. braunii in most of Maoming sediments indicates that it was wide spread and populated in Maoming Basin over a long period of time, serving as the main biological source material of oilshales on a large scale.