Excess methane in Greenland ice cores associated with high dust concentrations

Ice core records of atmospheric methane (CH4) and its isotopic composition provide important information about biogeochemical cycles in the past. Interpreting these data requires that they faithfully record the composition of the atmosphere. In this study, we describe anomalies of up to 30-40 ppb CH4 that are only observed in dust-rich (>similar to 60 ng Ca/g ice), glacialperiod ice measured with standard melt-refreeze methods. The stable isotopic composition of CH4 is also significantly affected. Results from the GISP2 and NEEM ice cores from Greenland show that excess CH4 is either released or produced in the presence of liquid water in amounts which are highly correlated with the abundance of Ca2+ and mineral dust in the sample. Additional experiments show that excess CH4 is unaffected by the addition of HgCl2 (a microbial inhibitor) and is not related to ice core storage time. Dust concentrations in Antarctic ice cores are an order of magnitude lower than in Greenlandic ice cores and no excess CH4 was observed in samples from the Antarctic WAIS Divide (WD) and South Pole (SPICE) ice cores. While the overall structure of the ice core atmospheric methane history is minimally impacted by excess CH4, the impacts on the isotopic record and on inverse models used to reconstruct CH4 sources are greater. We propose three potential mechanisms to explain the presence of excess CH4: (1) that CH4 is adsorbed on dust particles prior to deposition on the ice sheet and is slowly desorbed during the melt-extraction step of sample analysis; (2) that dust acts as a micro-environment within the ice sheet for methanogenic extremophiles; or (3) that excess CH4 is a product of abiotic degradation of organic compounds during the melt-extraction step of sample analysis. (C) 2019 Elsevier Ltd. All rights reserved.