Structural and isotopic analysis of kerogens in sediments rich in free sulfurised Botryococcus braunii biomarkers

Abstract

Type I kerogens of two relatively immature, unusual hypersaline sediments [with extracts rich in sulfurised Botryococcus braunii (B. braunii) biomarkers] of Miocene/Pliocene age from the Sdom Formation (Dead Sea, Israel), have been investigated using a variety of organic geochemical techniques. Py GC MS revealed a dominant homologous series of n-alk-1-enes and n-alkanes. Solid state 13C NMR and FT-IR data are consistent with a large contribution of B. braunii algaenan. SEM of the isolated kerogens showed a high degree of morphological alteration and the B. braunii colonies were not recognisable. The 13C values of the n-alkanes and n-alkenes in the pyrolysates from the kerogens were substantially lower than the 13C values of the bulk kerogens and other well defined B. braunii biomarkers. Saponification of the kerogens released a series of C16 C34 fatty acids with an even-over-odd carbon number predominance (dominated by the C22 and C28 homologues) and in one of the kerogens, similar 13C values as the pyrolysate n-alkanes/n-alkenes of similar carbon number. RuO4 treatment of the saponification residue released C16 to C34 fatty acids with an even/odd carbon number predominance in the high-molecular-weight region (maximising at C16 and C22 homologues), also having comparable 13C values to the pyrolysate n-alkanes of similar carbon number (n−1). Based on these data, it is suggested that the off-line pyrolysis products (n-alkanes and n-alkenes) have no association with either B. braunii or its algeanan but are derived from other unspecified algae substantially more depleted in 13C. The kerogen thus shows no morphological or molecular evidence for the preservation of B. braunii algaenan despite the presence of its sulfurised biomarkers in the solvent extracts of these sediments. It is suggested that B. braunii that thrived during the formation of the Sdom deposits may have exhibited both a high lipid content and a low algaenan content. Thus the lipids were efficiently preserved by sulfurization whereas the contribution of the algaenan to the kerogen was negligible.