Abstract <p>Organic carbon content combined with organic carbon isotope composition have been applied for the study of organic matter transformation in marine sediments during upward gas migration at seep areas of the Laptev Sea. Organic matter extracted from marine sediments was separated into five fractions (hexane, hexane-benzene, benzene, benzene-methanol, asphaltenes) using solvents of increasing polarity. It has been shown that in the seep the destruction of asphaltenes fractions leads to enrichment of benzene-methanol fraction by isotope-light components. δ¹³C values of benzene-methanol fractions were much lower than δ¹³C values of asphaltenes fractions and were associated with the accumulation of bacterial biomass in the sediment core through which the upward methane flow was passed. The organic matter of seep area sediment cores can be classified by two clusters, according to δ¹³C values of benzene fractions of organic matter. The first cluster center was in the surface layer (about 10 cm) of marine sediments. The second cluster center was in a deeper sediment layer. The difference in carbon isotope composition between the cluster centers was 2–3‰. The use of carbon isotope type-curves for different horizons of a sediment core has enabled a better understanding of the biological effects related to upward gas migration in seep areas of the Arctic Seas.</p>