Geochemical comparison of ikaite and methane-derived authigenic carbonates recovered from Echigo Bank in the Sea of Japan

Abstract

This study compares the geochemistry of ikaite (CaCO3·6H2O) with that of nearby (< 500 m in distance) recovered methane-derived authigenic carbonates (MDACs) of ~10 mol% Mg-calcite. The carbon isotopic composition (δ13C) of MDACs (lower than −49‰ Vienna Pee Dee Belemnite or VPDB) suggests a more prominent influence of anaerobic oxidation of methane (AOM) than that of ikaite (approximately −35‰ VPDB). The higher δ13C value of ikaite is attributed to its growth at sediment depths closer to the seafloor, where the proportion of seawater HCO3− relative to methane-derived HCO3− was higher. Assuming that seawater and AOM are the major sources of HCO3− for ikaite growth, the actual 14C age was estimated to be ~14 cal ky before present, close to the age of the sediments observed around the ikaite. For alkalinity increase at shallower sediment depths by the AOM, a higher methane flux is necessary. However, attributing the ikaite growth at shallower sediment depths to higher methane flux alone posed an issue, as no traces of gas hydrate growth were found in the laminated sediments. The oxygen isotopic composition (δ18O) of the freeze-dried ikaite (ikaite-CO3) was ~2‐3‰ VPDB, which is lower than the corresponding value of ~4.0‰ VPDB for the Mg-calibrated MDACs. It is characteristic of ikaite-CO3 to exhibit such low δ18O values, indicating a unique oxygen isotope fractionation factor for ikaite. The preferential H218O uptake of ikaite for growth also influenced the δ18O value of ikaite-CO3. Hence, owing to H218O depletion in the ambient interstitial water, the δ18O value of the ikaite-CO3 tended to decrease from the interior to the margin. The δ18O values of the ikaite-CO3 were archived in yellowish calcite grains that were formed in the pores after patchy dissociation of the ikaite. The δ18O values of the grains were close to those of ikaite-CO3. Therefore, the δ18O-temperature equation for ikaite is necessary for geochemical interpretation of the calcite δ18O values.