C-13 ISOTOPE DEPLETION IN IKAITE CRYSTALS - EVIDENCE FOR METHANE RELEASE FROM THE SIBERIAN SHELVES

Ikaite crystals (CaCO3 x 6H(2)O) have been found at 232- to 238-cm sed iment depth in R/V Polarstern core PS2460-4 from the Laptev Sea contin ental margin in a water depth of 204 m. delta(13)C values of this phas e average -36.3 +/- 0.4%, PDB (N = 2), which is significantly outside the range of normal marine carbonates. The CO2 involved in the precipi tation of the ikaite is most probably derived from methane, which has extremely depleted C-13 isotope values. Two possible sources of methan e in these sediments are: (1) methanogenesis (decomposition of organic matter under anaerobic conditions); and (2) gas hydrates, which are k nown to occur in the Siberian shelf regions.