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 the delta of the Lena River and adjacent areas of the Laptev Sea were examined to derive information on their source(s) (natural vs anthropogenic; terrestrial vs marine). The abundance, nature, distribution and individual stable carbon isotope ratios of these hydrocarbons were determined. It thus appeared that the Laptev samples are characterized by (i) a negligible level of oil pollution due to anthropogenic and/or natural petrogenic inputs, (ii) an almost exclusive occurrence of long-chain, predominantly odd-carbon-numbered, n-alkanes and (iii) some differences in the level of bacterial alteration with a relatively higher degradation of the n-alkanes in the seaward sediment. A non-terrestrial origin is proposed for these n-alkanes, based on the present results and on previous observations on carboxylic acid moieties and the pyrolysis products of the three surface sediments. Such hydrocarbons are probably not derived from the epicuticular waxes of higher plants but chiefly correspond to autochthonous products, likely of algal origin. These observations, added to recent results on samples from other types of marine environments, stress that an abundance of long-chain, predominantly odd, n-alkanes in extracts should not be systematically considered, alone, as evidence for a major contribution of higher plant waxes. The relatively low contribution of terrestrial n-alkanes in the Laptev sediments should result from the combination of (i) a major dilution of the terrestrial input due to high primary productivity in summer, promoted both by a large influx of nutrients provided by the Lena River 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 lipids from terrestrial sources to the Laptev samples.