ORGANIC-MATTER SOURCES AND EARLY DIAGENETIC ALTERATIONS IN ARCTIC SURFACE SEDIMENTS (LENA RIVER DELTA AND LAPTEV SEA, EASTERN SIBERIA), II - MOLECULAR AND ISOTOPIC STUDIES OF HYDROCARBONS

The hydrocarbons extracted from three surface sediment samples from th e delta of the Lena River and adjacent areas of the Laptev Sea were ex amined to derive information on their source(s) (natural vs anthropoge nic; terrestrial vs marine). The abundance, nature, distribution and i ndividual stable carbon isotope ratios of these hydrocarbons were dete rmined. It thus appeared that the Laptev samples are characterized by (i) a negligible level of oil pollution due to anthropogenic and/or na tural petrogenic inputs, (ii) an almost exclusive occurrence of long-c hain, predominantly odd-carbon-numbered, n-alkanes and (iii) some diff erences in the level of bacterial alteration with a relatively higher degradation of the n-alkanes in the seaward sediment. A non-terrestria l origin is proposed for these n-alkanes, based on the present results and on previous observations on carboxylic acid moieties and the pyro lysis products of the three surface sediments. Such hydrocarbons are p robably not derived from the epicuticular waxes of higher plants but c hiefly correspond to autochthonous products, likely of algal origin. T hese observations, added to recent results on samples from other types of marine environments, stress that an abundance of long-chain, predo minantly odd, n-alkanes in extracts should not be systematically consi dered, alone, as evidence for a major contribution of higher plant wax es. The relatively low contribution of terrestrial n-alkanes in the La ptev sediments should result from the combination of (i) a major dilut ion of the terrestrial input due to high primary productivity in summe r, promoted both by a large influx of nutrients provided by the Lena R iver and by the specific conditions in such an Arctic environment and (ii) the high level of degradation of the organic material transported by the Lena River; thus the latter probably provided a low input of l ipids from terrestrial sources to the Laptev samples. (C) 1998 Elsevie r Science Ltd. All rights reserved.