Calcium Carbonate Hexahydrate (Ikaite): History of Mineral Formation as Recorded by Stable Isotopes

Calcium carbonate hexahydrate (ikaite) is a rare mineral that forms as metastable species in the organic-carbon-rich sediments of King George Basin, Bransfield Strait, Antarctica, consequence early diagenetic decomposition organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. Large crystals grow sediment immediately below transition between microbial sulfate reduction methanogenesis at ~320 cm sea floor (bsf). This process reflected dissolved sulfate, total carbon dioxide, methane concentrations, well carbon, hydrogen, oxygen isotope chemistries interstitial fluids gases host sediment. The ikaite crystal faithfully records its zonal structure changing ratio dioxide pool it gradually diminishes during (δ13Cikaite = −17.5 to −21.4‰). These changes crystal’s environment follow general Rayleigh fractionation. isotopes (δ18Oikaite 1.46 4.45‰) also show strong distribution, unrelated temperature formation, but perhaps controlled by degree recrystallization calcite. depleted 11‰ 2H/1H (VSMOW) relative coexisting water, which excellent agreement with fractionation other hydrated minerals. In addition situ pressure, nucleation Basin may be induced alkalinity, phosphate compounds. Intense metabolism generates such compounds; these, aspartic acid glutamic play an important role, they do biological extracellular precipitation. All indications are low temperatures (such polar environments), calcium supersaturation caused methanogenesis, sufficiently large supply amino acids favor formation. conditions, modified recrystallization, preserved calcite glendonites, thinolites, calcitic pseudomorphs derived from found throughout ancient sedimentary record.