The lipid geochemistry of a Recent sapropel and associated sediments from the Hellenic Outer Ridge, eastern Mediterranean Sea

Abstract

Five sections (0-7, 29-36, 53-60, 78-85 and 104-111 cm), of a 0-2 m sediment core from the Hellenic Outer Ridge, in the eastern Mediterranean Sea, have been examined for lipids. Three of these sections were from a 73 cm thick S₁(ca. 6000—9000 years b.p.) sapropel layer, one from an upper ooze layer and one from a lower marl. The lipids were extracted and the major classes analysed in detail by gas chromatography and computerized gas chromatography—mass spectrometry. In all sections, then-alkanes were dominated by C₂₅—C₃₁components, showing a high odd-over-even predominance, with smaller amounts of lower chain-length components. The acyclic ketone fraction consisted mainly of C₃₇—C₃₉di- and triunsaturated alken-2-ones and alken-3-ones. Alkanols, ranging from C₁₂—C₃₂with a high even-odd preponderance, were present in all sections, maximizing atn-C₂₂orn-C₂₆. The sapropel contained abundant phytol (up to 7000 ng g^-1dry sediment), and considerable amounts of 22 :1, 24:1 and 26 :1n-alkenols; in the non-sapropelic sediment, phytol was only a minor com ponent, and non-alkenols were detected. In addition to these alcohols, the sapropel also contained C₂₈—C₃₂1,13-, 1,14- and 1,15- diols and 15-keto-alkan-l-ols, the 30 :0 compound predominating in both series. In all sections, fatty acids were the most abundant lipid class. These were mainly C₁₂—C₃₀straight-chain compounds, maximizing at 16:0 with a high even—odd predominance; most were saturated, but C₁₆, C₁₈, C₂₀, C₂₂and C₂₄monoenoic acids and small amounts of C₁₆, C₁₈, C₂₀, C₂₂and C₂₄polyenoic acids were present. A range of branched and cyclic acids were also identified. The non-sapropelic upper and lower sediments differed from the sapropel in containing higher levels of branched acids (especially C₁₅and C₁₇iso- and anteiso-compounds) and C₁₈monoenoic acids: these differences could be related to differing inputs, especially in terms of microbial communities. The sterol distributions of the sapropel displayed a wide range of structures (C₂₆—C₃₁), totalling over sixty different components. These included both 4-methyland 4-desmethylnuclei, a variety of C₈—C₁₁side-chains, and encompassed Δ⁵, Δ^5,22, Δ^5,24, Δ^5,24(28), Δ²², Δ²⁴⁽²⁸⁾, Δ⁷and Δ⁸⁽¹⁴⁾unsaturation plus a range of fully saturated stands. Major components were 4α, 23, 24-trimethyl-5α-cholest-22-en-3β-ol (dinosterol), cholest-5-en-3|I-ol (cholesterol), 24-methylcholesta-5,22-dien-3β-ol and 24-ethylcholest-5-en-3β-ol. In contrast, the non-sapropelic sediments contained very low levels of only a few sterols, chiefly cholesterol and dinosterol, probably due to input differences. In addition to sterols, the sapropel also contained small amounts of stanones and sterenes. A significant terrigenous input of lipids is evident throughout the core (especially from then-alkane data), but the sapropel lipid composition appears to be predominantly of marine origin. Individual ‘biological marker’ lipids suggest inputs from Dinophycean and Haptophycean algae to the sapropel. Potential contributions of lipids from organisms such as foraminifera and pteropods, remains of which were observed in the sediment, are difficult to assess due to a paucity of data on the lipid compositions of such organisms. The lipids of the non-sapropelic sediments showed a much less prominent marine signal, especially in terms of the lower levels of phytol and sterols and the higher relative abundance of terrestrialn-alkanes. Two main models have been proposed to explain the formation of organic-rich sapropel facies; (i) stagnation of the water column and the establishment of anoxic conditions in bottom water and sediments, resulting in enhanced preservation of sedimentary organic matter, and (ii) increased biological production providing an increased input of organic matter to the sediments. The lipid composition strongly suggests that this sapropel received a large marine-derived input of organic matter. Since this was less evident in the overlying and underlying sediments, sapropel deposition appears to have been associated with an increased autochthonous input. The anoxic nature of the sapropel, by restricting degradation to anaerobic processes, will also have contributed to the differences in lipid composition between the sediment types. Little diagenesis of lipids in the sapropel was evident. Small amounts of sterenes and 5β(H)-stands were present, probably formed by dehydration and reduction, respectively, of precursor sterols. Diagenetic dehydration of phytol may have contributed to the presence of minor amounts of certain other isoprenoid lipids.