首页|Geochemical comparison of ikaite and methane-derived authigenic carbonates recovered from Echigo Bank in the Sea of Japan
Geochemical comparison of ikaite and methane-derived authigenic carbonates recovered from Echigo Bank in the Sea of Japan
扫码查看
点击上方二维码区域，可以放大扫码查看
原文链接
NSTL
Elsevier
This study compares the geochemistry of ikaite (CaCO3 center dot 6H(2)O) with that of nearby (< 500 m in distance) recovered methane-derived authigenic carbonates (MDACs) of similar to 10 mol% Mg-calcite. The carbon isotopic composition (delta C-13) of MDACs (lower than -49 parts per thousand Vienna Pee Dee Belemnite or VPDB) suggests a more prominent influence of anaerobic oxidation of methane (AOM) than that of ikaite (approximately -35 parts per thousand VPDB). The higher delta C-13 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 C-14 age was estimated to be similar to 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 (delta O-18) of the freeze-dried ikaite (ikaite-CO3) was -2-3%. VPDB, which is lower than the corresponding value of similar to 4.0 parts per thousand VPDB for the Mg-calibrated MDACs. It is characteristic of ikaite-CO3 to exhibit such low 6 18 0 values, indicating a unique oxygen isotope fractionation factor for ikaite. The preferential (H2O)-O-18 uptake of ikaite for growth also influenced the delta O-18 value of ikaite-CO3. Hence, owing to (H2O)-O-18 depletion in the ambient interstitial water, the delta O-18 value of the ikaite-CO3 tended to decrease from the interior to the margin. The delta O-18 values of the ikaite-CO3 were archived in yellowish calcite grains that were formed in the pores after patchy dissociation of the ikaite. The delta O-18 values of the grains were close to those of ikaite-CO3. Therefore, the delta O-18-temperature equation for ikaite is necessary for geochemical interpretation of the calcite delta O-18 values.
NSTL
Elsevier
