Andrews, M. Grace; Jacobson, Andrew; Osburn, Magdalena; Flynn, Theodore
Melting of the Greenland Ice Sheet (GrIS) has accelerated in recent decades. Given the close association between the water and carbon © cycles, melting of the GrIS may also drive local and global C cycle feedbacks. However, few studies have quantified such feedbacks, which may have important implications for predicting future climate or understanding linkages between ice sheet destabilization and climate change in the geologic past. Here, we investigate seasonal and interannual dissolved C cycling at the margin of the Russell Glacier, west Greenland. By synthesizing isotopic analyses of water (δ18O) and C (δ13C and Δ14C) with geomicrobiological observations, we present evidence for previously unknown connections between the GrIS’s supra‐ and sub‐glacial dissolved C cycles. Supraglacial streams have variable concentrations of dissolved organic carbon (DOC) and are the dominant source of DOC in subglacial discharge. Supraglacial stream dissolved inorganic carbon (DIC) concentrations are uniform and sourced from a spatially and temporally constant mixture of organic C (~25%) respired by aerobic heterotrophs inhabiting the GrIS surface and dissolved atmospheric C (~75%). Supraglacial inputs account for ~50% of subglacial discharge DIC. The remaining subglacial DIC derives from carbonate weathering and microbial CO2 production, with the latter attributable to abundant anaerobic heterotrophic communities observed in subglacial discharge. Furthermore, we find that supraglacial streams deliver young DOC to the subglacial environment during snowmelt and rain events. These pulses of organic C may drive heterotrophic microbial respiration, with the cumulative effect being a seasonal shift in the source of basal DIC, from microbial‐ to carbonate‐dominated.