Isotopically heavy carbon in the C 21 to C 25 regular isoprenoids in halite-rich deposits from the Sdom Formation, Dead Sea Basin, Israel

Abstract

A series of Miocene/Pliocene halite deposits (with extremely low organic carbon contents) from the Sdom Formation (Dead Sea Basin, Israel) have been studied. Distributions and δ 13C contents of biomarkers have been determined using GC–MS and irm-GCMS analyses, respectively. The hydrocarbon fractions consist mainly of pristane, phytane and C 21 to C 25 regular isoprenoids. The predominance of C 21+ regular isoprenoids in the Dead Sea halites and other hypersaline deposits indicate that these components could be derived from ether-bound membrane lipids of halophilic archaea. The lack of intact ether-bound lipids in the polar fractions indirectly infers that such components have already been released at early stages of diagenesis. Their δ 13C contents are enriched in 13C, by up to 7‰, compared to the biomarkers of presumed phytoplanktonic origin (i.e. steranes and hopanes) within the same sediment sample, in agreement with a source other than algae and cyanobacteria, thus, tentatively assigned as halophilic archaea. Based on biomarker distributions, δ 13C contents and mineral compositions, these sediments appear to have been deposited in a salinity stratified water body with a bottom water brine. Continual evaporation and deposition of the higher salts (i.e. carnallites) are favourable conditions for the growth of halophilic archaeal communities.