Cryogenic carbon cycling at an Icelandic glacier

Glaciers and ice caps are recognised as an important component of the global carbon cycle. Carbon within glacial systems exists in organic and inorganic forms, across supraglacial, englacial and subglacial realms. It is often difficult to detach cryospheric carbon cycling from hydrology, with the transfer of carbon between glacial inventories relying upon meltwater flows. Classical glacial hydrology consists of distributed drainage delivering delayed flow meltwaters, throughout the accumulation season, superseded by quick flow, aerated channelized drainage during increased ablation. It is upon this template that most existing studies have addressed the dynamics of carbon within glaciated catchments. However, Icelandic glacial systems provide an opportunity to investigate the role of subglacial volcanism in driving carbon dynamics. Hydrochemical properties of Sόlheimajökull bulk meltwaters indicate untraditional redox conditions, with discharge of reduced, anoxic meltwaters in Summer, when expansion of subglacial drainage intersects the Katla geothermal zone. This unique hydrological regime generates profound effects upon the solute flux from the glacier, particularly with regard to the carbon budget. Dissolved inorganic carbon dynamics are dominated by weathering of basaltic bedrocks and accessory hydrothermal calcites, fuelled by subglacial geothermal proton supply. Widespread basal anoxia during summer facilitates methanogenesis resulting in large quantities of methane being discharged from beneath the glacier (flux range between 9,179 to 22,551 tonnes per year). Evidence suggests subglacial microbial acetoclastic methanogenesis is responsible with δ13C and δD CH4 values of ~60‰ and -320‰ respectively, supported by laboratory identification of methanogenesis in Sόlheimajökull subglacial sediments. The organic counterpart to the carbon cycle is invoked to serve as the energy source for microbial metabolism. Such direct measurements of subglacial methane have rarely been achieved at contemporary ice margins. This study therefore provides an exciting opportunity to identify methane sources and carbon cycling in areas subjected to subglacial volcanism and to consider these within the broader context of global carbon dynamics