Excess methane in Greenland ice cores associated with high dust concentrations

Lee, James E.; Edwards, Jon S.; Schmitt, Jochen; Fischer, Hubertus; Bock, Michael; Brook, Edward J. (2020). Excess methane in Greenland ice cores associated with high dust concentrations. Geochimica et cosmochimica acta, 270, pp. 409-430. Elsevier Science 10.1016/j.gca.2019.11.020

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Ice core records of atmospheric methane (CH₄) and its isotopic composition provide important information about biogeo-chemical cycles in the past. Interpreting these data requires that they faithfully record the composition of the atmosphere. Inthis study, we describe anomalies of up to 30–40 ppb CH₄ that are only observed in dust-rich (>~60 ng Ca/g ice), glacial-period 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 thepresence 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 relatedto ice core storage time. Dust concentrations in Antarctic ice cores are an order of magnitude lower than in Greenlandic icecores 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 onthe isotopic record and on inverse models used to reconstruct CH₄ sources are greater. We propose three potential mecha-nisms to explain the presence of excess CH₄: (1) that CH₄ is adsorbed on dust particles prior to deposition on the ice sheetand is slowly desorbed during the melt-extraction step of sample analysis; (2) that dust acts as a micro-environment within theice sheet for methanogenic extremophiles; or (3) that excess CH₄ is a product of abiotic degradation of organic compoundsduring the melt-extraction step of sample analysis

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