Description: Microbial anaerobic oxidation of methane in sediments is a kinetic process associated with a carbon isotope effect which enriches the remaining methane in 13C. Three, models: % residual methane, higher hydrocarbon enrichment, and CO2-CH4 coexisting pairs are used to independently calculate fractionation factors (αc) in the range of 1.002–1.014, which overlap the range determined by culture studies, αc is smaller than that associated with methanogenesis by CO2 reduction or by acetate-type fermentation, and comparison of the coexisting CO2-CH4 pairs can distinguish between the formation and consumption processes. Methane oxidation in sediments continues to a threshold concentration of ca. 0.2 mM; the residual methane is either unavailable or unattractive to consumption. Minor amounts of methane may also be produced simultaneously in the methane consumption zone, influencing the apparent fractionation factor in this zone.

Description: High organic loading (4-5 wt%) and sedimentation rates (1.4 mm yr(-1)) in Eckernforde Bay sediments lead to anaerobic conditions within the uppermost 15 cm. Intense bacteria] sulphate reduction (0.011-0.15 mM SO4red2- yr(-1)) exhausts dissolved sulphate around 150 cm sediment depth, resulting in methanogenesis at greater sediment depth by carbonate reduction (1.8-8.5 muM CH4 yr(-1)). Extensive regions of Eckernforde Bay are characterized by acoustically turbid sediments (〉 300 cm sediment depth), resulting from deeper sediments supersaturated in methane, forming free gas accumulation zones. Methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3-10 times greater than methanogenesis, i.e. 46-95 muM CH4 (ox) yr(-1). The pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence. Pockmarks, i.e. shallow surface depressions in some Eckernforde Bay sediments, are not associated with gas ebullition; rather, these physical features result from the expulsion of freshwater. These episodic springs or seepages of groundwater from the underlying, Holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations. The geochemical evidence indicates that the commercial accumulation of petroleum in the lower Cretaceous, Dogger-Beta-Hauptsandstein Schwedeneck field in Eckernforde Bay (1500 mbsf) has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmarks

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