Origin of silty basal ice in Greenland

Stratigraphically disturbed and debris-rich, the basal silty layers of Greenland ice sheet bear unique information on the environment prior to the ice sheet build up [1] and on the subglacial ecosystem [2]. Previous studies have shown that processes such as mixing, diffusion, melting and refreezing, gas expulsion, and in-situ production of CO2 and CH4 have disturbed the paleoclimatic signals within the ice silty layers of Greenland [2, 3, 4]. Analytical techniques commonly used in deep ice core studies encounter limitations when applied to the ice-bedrock interface layers due to embedded debris. As part of the DEEPICE project, we have revisited the paleoclimatic information preserved within the basal layers of Greenland updating gases measurement techniques suitable for analysing silty ice. We analysed the gas composition (N2, O2, Ar, CO2, CH4, N2O) of samples retrieved from the basal ice and underneath sediments of Camp Century (new data set) and from the basal ice of GRIP (new data set for N2O and improving previous resolution for other gases species). Our results show a large accumulation of greenhouse gases (up to 12% and 3% of CO2 and CH4) associated with O2 depletion (down to 5%), a relative accumulation of Ar (up to 1.5%) and a total gas loss (down to 5mL of gas per kg) within the last meters of ice above the bedrock. As the greenhouse gases probably have a biological origin, we performed genomic sequencing at different locations of the silty sequence to determine whether the genes required for potential biogeochemical cycling were present and whether they differed with depth. Differences are reported between Camp Century and GRIP.  In the light of these new measurements, we will discuss the processes at play in the silty layers.   [1] Christ et al., 2021, Proc. Nat. Acad. Sci. [2] Souchez et al., 2006, Geophys. Res. Lett., 33, L24503. [3] Verbeke et al., 2002, Annals of Glaciology 35, 231-236. [4] Goossens et al., 2016, The Cryosphere, 10(2), 553-567.