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, δ¹³C_DIC 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 ∆¹⁴C_DIC 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. δ¹³C_DIC isotopes of supraglacial streams are highly depleted (-24‰), suggesting that CO₂ is sourced from microbial respiration of surficial organic carbon. Subglacial portal δ¹³C_DIC isotopes are also relatively depleted (-17‰) but are sufficiently different relative to supraglacial streams so as to require an additional δ¹³C_DIC 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 ∆¹⁴C_DOC 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.