The discovery of ikaite from sediment cores in the eastern margin of the Japan Sea and its significance

Abstract

The low-temperature carbonate mineral ikaite was recovered from sediment cores from the Sado West area, in the eastern margin of the Japan Sea. The ikaite-bearing sediments consist of dark-colored, bioturbated or burrow-mottled mud with sand and silt laminae. There are rare occurrences of ikaite within mud containing thin, parallel laminations indicative of anoxic sedimentary environments. Ikaite occurs either as veins of granular aggregates or as large crystals, some of which exceed 10 cm in length. The nature of the ikaite occurrences indicate that it is unlikely that ikaite formed directly on the seafloor. The ikaite deposits are found at depths shallower than 10 meters below the sea floor (mbsf), but pore-water analysis suggests that the ikaite crystals formed within the sulfate–methane transition (SMT) zone, likely at 2–5 mbsf. The ikaite stable carbon isotopic ratios are low, -33 to -34 per mil, indicating that carbon derived from microbial oxidation of methane gas was involved in ikaite formation, consistent with the occurrence of ikaite within the SMT. The most recent formation of ikaite is calculated to be more than 9,700 years ago. All ikaite occurrences are in cores that do not contain methane hydrates. Hydrate-bearing cores are usually associated with calcite nodules. Further investigation is required to determine the controlling factors in the formation of either ikaite or ordinary calcite. Ikaite becomes unstable at depths greater than 70 mbsf as the methane flux or the temperature increases during burial. Large crystals of ikaite are replaced by stable calcite to form Gennou Ishi.