Abstract
Climate change driven release of methane (CH4) from polar and sub-polar sediments could accelerate global warming, and tracking CH4 in cold sediments over geological time helps predict future releases. Isotopic signatures of ikaite (CaCO3•6H2O) and its pseudomorph, glendonite, may be used to identify past CH4 in cold environments, as alkalinity (AT) from anaerobic oxidation of methane (AOM) can induce precipitation of this mineral at low temperatures. However, the suitability of ikaite as a proxy for CH4 near the sediment surface remains uncertain, as ikaite linked to modern seeps has only been retrieved from sediment depths of several metres. We report ikaite crystals in surface sediments (0-40 cm depth) in Reyðarfjörður, Iceland. High AT fluxes from deeper sediment layers and low stable carbon isotope (δ13C) values of the ikaite (-49.8 to -53.8 ‰) suggest formation from AOM, while sub-bottom profiling indicates shallow gas below the sampling site. As such, the recovered ikaite provides indirect evidence that CH4 locally reaches shallow sediment layers in the studied fjord, considerably expanding the environmental range of CH4-derived ikaite and substantiating ikaite and glendonite as proxies for cold environment CH4 seeps.