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 D ead Sea (Israel) with low total organic carbon contents of 0.4-0.8% were st udied by molecular and isotopic organic geochemical techniques to determine the origin of the extractable lipid components. The molecular investigatio n showed most of the material in this extremely hypersaline environment to be of terrestrial origin. This was indicated by a dominance of 24-ethylchol est-5-en-3 beta-ol and 24-ethylcholesta-5,22-dien-3 beta-ol in the sterol d istribution as well as an abundance of angiosperm triterpenoids like beta-a myrin, alpha-amyrin, lupeol and their oxidized derivatives. The n-alkane di stribution patterns are very similar in all samples studied and typical of an origin from epicuticular waxes of higher land plants. This is corroborat ed by delta(13)C values of the n-alkanes between -28.1 and -33.6 parts per thousand. The even-over-odd carbon number predominance of the long-chain fa tty acids (C-20-C-30) and their range of delta(13)C values (-27.3 to -31.3 parts per thousand) are also in accordance with an origin from C-3 terrestr ial plants. The pronounced C-13 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 bi omass in the Dead Sea water. The n-alcohol distribution patterns show a str ong even-over-odd carbon number preference and, compared to the n-alkanes, are enriched in C-13 by 2-5 parts per thousand, which suggests a small cont ribution of aquatic organic matter particularly to the short-chain homologu es. indications for a supply from autochthonous organisms to the sedimentar y organic matter were derived from the high amounts of phytol with delta(13 )C values between -22.8 and -19.7 parts per thousand, the isotopic composit ion of cholesterol (-23.9 to -21.9 parts per thousand) and low concentratio ns of 24-methyl-5 alpha-cholest-7-en-3 beta-ol and 24-ethyl-5 alpha-cholest -7-en-3 beta-ol. They are attributed to the only primary producer in the De ad Sea described so far, i.e. the unicellular green alga Dunaliella parva. In addition, the archaean cell walls of halophilic bacterial communities li ke Halorubrum sodomense are represented by significant amounts of bis-O-phy tanylglycerol (-22.3 to -23.0 parts per thousand). (C) 2000 Elsevier Scienc e Ltd. All rights reserved.