Authigenesis of carbonates in the deposits of the gas-hydrate-bearing structure of the CHAOS (Sea of Okhotsk)

Abstract

The paper presents the results of studies of authigenic carbonates sampled in the near-surface sediments of the gas-hydrate-bearing CHAOS structure located on the western slope of the Deryugin Basin in the Sea of Okhotsk. Carbonates were at different stages of formation, the most common morphological forms of which were concretions. Microscopic studies have shown that the concretions are formed by pelitomorphic carbonate cementing the terrigenous clay-clastic matrix and organic detritus. According to X-ray phase analysis, the dominant mineral composing nodules is cryptocrystalline high-Mg calcite (14.2–16.9 mol % Mg). Based on the geochemical modeling the intervals of the sediments with favorable conditions for the formation of basic carbonate minerals (aragonite, calcite, and dolomite) were identified. Almost over the entire area of the structure, the formation of carbonates occurs at more than 1m and up to 5m sub-bottom depths. In the central part of the structure (station LV31–27GC) the formation of carbonates is also possible in the upper part of the sedimentary section — from 0 to 1.5 m. On the basis of balance calculations of the δ13C isotopes content in nodules, the contribution of the main carbon sources involved in their formation was estimated. These sources turned out to be organic matter and microbial methane in a ratio of 67.5 : 32.5. It was found that the conversion of carbon dioxide into bicarbonate ion occurs under conditions of early diagenesis due to the interaction of the fluid with aluminosilicates. Calculations of the isotopic composition of pore waters (the main source of oxygen in carbonates) and paleotemperatures indicate crystallization of carbonates at low (close to 0 °C) temperatures. Based on 230Th/U-dating of carbonates, the age of the studied unloading source was determined as ~3.5 thousand years. It was found that the CHAOS structure is one of the “youngest” on the northeastern slope of the Sakhalin Island.