Seasonal Variation and Controls on Subglacial Riverine CO2 Concentrations From a Small Catchment, West Greenland Ice Sheet
Abstract:Previous research has suggested that subglacial discharge from the Greenland Ice Sheet (GrIS) may have the potential to be a significant source of CO2 to the atmosphere in a warming world (Ryu and Jacobson, 2011). To trace the flux, sources of, and controls on subglacial CO2, we sampled the Akuliarusiarsuup Kuua River subglacial portal, which receives water from the Isunnguata and Russell Glaciers, west GrIS, six times throughout June – August, 2014. Additionally, we sampled two nearby supraglacial streams. We present preliminary data on pCO2 values, DIC and DOC concentrations, major cation and anion concentrations, δ13CDIC isotopes, as well as ∆14C-DIC and -DOCisotopes.
Waters emerging from the subglacial portal are 2 – 2.5x supersaturated in CO2 with respect to atmospheric equilibrium. pCO2 values rise from ~700 to 1000ppm between June and July then return to ~700ppm in August. Although subglacial pCO2 and ∆14CDIC values vary, throughout the summer they exhibit similar trends as contemporaneous supraglacial stream values, suggesting that subglacial CO2 is at least partially derived from supraglacial meltwater which has accessed the ice sheet base through moulins and crevasses. δ13CDIC isotopes of supraglacial streams are highly depleted (-24‰), suggesting that CO2 is sourced from microbial respiration of surficial organic carbon. Subglacial portal δ13CDIC isotopes are also relatively depleted (-17‰) but are sufficiently different relative to supraglacial streams so as to require an additional δ13CDIC enriched source. A strong correlation (R2 = 0.89, n= 6) between subglacial Ca+Mg concentrations and alkalinity (≈ HCO3) suggests that the additional source of DIC to these waters is dissolution of carbonate. Finally, the correlation (R2 = 0.55, n = 6) between subglacial pCO2 and ∆14CDOC values suggest that one control on variable CO2 concentrations throughout the melt season is the age, and presumably, the lability, of organic carbon available to microbes.