GLENDONITES AND METHANE-DERIVED MG-CALCITES IN THE SEA OF OKHOTSK, EASTERN SIBERIA: IMPLICATIONS OF A VENTING-RELATED IKAITE/GLENDONITE FORMATION

Abstract

Mg-calcite-cemented bioturbation traces, glendonite aggregates of idiomorphic, bi-pyramidal crystals and porous, amber-colored carbonate concretions were recovered from a methane-dominated cold vent area in 380 m water depth at the northern Sakhalin Slope, Sea of Okhotsk. Bioturbation traces consist of Mg-calcite cemented sediment with δ13C values between −37 and −46‰ PeeDee Belemnite (PDB), which implicate methane as the carbon source. Glendonite, a calcite pseudomorphosis after ikaite (CaCO3·6H2O), and amber-colored concretions are both composed of varying amounts of calcite and Mg-calcite with δ13C values between −19 and −34‰ PDB. Isotope analyses of an ikaite crystal recovered in the vicinity reveals δ13C values between −20‰ and −22‰ PDB indicating organic matter as the carbon source. Microscopic investigations of glendonites and amber-colored concretions show a porous fabric of a primary calcite phase that is overgrown by a secondary Mg-calcite cement. As ikaite pseudomorphs to porous calcite and as carbon isotope values are the same for the ikaite and the high end member values of glendonite samples, the primary calcite phase is suggested to be a former ikaite phase. Because they share the equal color, fabric, mineral and isotopic composition with glendonites, the amber-colored concretions are also suggested to represent calcite that transformed from ikaite. A mixture of pseudomorph calcite (δ13C −20‰ PDB), which originally formed as ikaite from degraded organic matter, and Mg-calcite (δ13C −43‰ PDB), which crystallized due to the anaerobic oxidation of methane, can explain the varying carbon isotope data of the glendonites and the amber-colored concretions. The growth of ikaite, the transformation of ikaite to calcite, and the crystallization of Mg-calcite indicate changing geochemical conditions within a cold vent environment at different times. Ideal conditions for the ikaite formation are given during the establishment of a cold vent site when upward-migrating, methane-rich fluids enhance the anaerobe decomposition of organic matter, which again increases the phosphate and alkalinity concentrations near the sediment surface. Lower rates of organic matter decomposition during on-going venting decrease these high phosphate but also sulphate concentrations and allows other carbonate phases as Mg-calcite to form. This additional carbonate precipitation and the ikaite formation itself lower the high alkalinity and destabilize ikaite, which pseudomorphs to porous calcite. Triggered by the further upward-shifting SO4/H2S boundary and increasing methane oxidation rates, the typical cold vent methane-derived carbonate genesis takes place, which cements the sediment pore space and induces the secondary Mg-calcite crystallization within the glendonite fabric. Taking this scenario into account, ikaite formation should be a common process in the beginning of methane-dominated vent activity at cold bottom water temperatures; glendonite pseudomorphs can be assumed to represent a typical manifestation at fossil and recent cold vents at high latitudes.