Molecular and isotopic characterization of organic matter in recent and sub-recent sediments from the Dead Sea

Near-surface sediments from two sections in the Nahal Zeelim delta of the Dead Sea (Israel) with low total organic carbon contents of 0.4–0.8% were studied by molecular and isotopic organic geochemical techniques to determine the origin of the extractable lipid components. The molecular investigation showed most of the material in this extremely hypersaline environment to be of terrestrial origin. This was indicated by a dominance of 24-ethylcholest-5-en-3β-ol and 24-ethylcholesta-5,22-dien-3β-ol in the sterol distribution as well as an abundance of angiosperm triterpenoids like β-amyrin, α-amyrin, lupeol and their oxidized derivatives. The n-alkane distribution patterns are very similar in all samples studied and typical of an origin from epicuticular waxes of higher land plants. This is corroborated by δ 13C values of the n-alkanes between −28.1 and −33.6‰. The even-over-odd carbon number predominance of the long-chain fatty acids (C 20–C 30) and their range of δ 13C values (−27.3 to −31.3‰) are also in accordance with an origin from C 3 terrestrial plants. The pronounced 13C depletion of the short-chain fatty acids (C 14–C 18) further substantiates the dominance of terrestrial plant material in the sediments and highlights the limited importance of autochthonous biomass in the Dead Sea water. The n-alcohol distribution patterns show a strong even-over-odd carbon number preference and, compared to the n-alkanes, are enriched in 13C by 2–5‰, which suggests a small contribution of aquatic organic matter particularly to the short-chain homologues. Indications for a supply from autochthonous organisms to the sedimentary organic matter were derived from the high amounts of phytol with δ 13C values between −22.8 and −19.7‰, the isotopic composition of cholesterol (−23.9 to −21.9‰) and low concentrations of 24-methyl-5α-cholest-7-en-3β-ol and 24-ethyl-5α-cholest-7-en-3β-ol. They are attributed to the only primary producer in the Dead Sea described so far, i.e. the unicellular green alga Dunaliella parva. In addition, the archaean cell walls of halophilic bacterial communities like Halorubrum sodomense are represented by significant amounts of bis- O-phytanylglycerol (−22.3 to −23.0‰).