Organic carbon export from the Greenland ice sheet

Glacial meltwater exports a unique type of organic carbon to marine systems, distinct from non-glacially derived riverine export, potentially capable of stimulating downstream marine primary productivity. Here, we describe for the first time the bulk-level dissolved organic carbon (DOC) and particulate organic carbon (POC) isotopic composition of glacial meltwater draining the Greenland ice sheet (GrIS). These data, in conjunction with an earlier study that investigated the molecular-level composition of GrIS dissolved organic matter, collectively describe the concentration, radiocarbon content, and lability of organic carbon in subglacial discharge from a land-terminating outlet glacier during a melt season. By scaling up our measurements across the ice sheet, we estimate that the annual DOC flux from the GrIS (0.08Tg/y) is equivalent to that from a small Arctic river (discharge (Q)<50km³/y), and that the annual POC flux from the GrIS (0.9Tg/y) may be comparable to that of a large Arctic river (Q>200km³/y). The DOC flux is derived primarily from beneath the glacier (subglacial) (>75%) in the early season, and from surface ice-melt (up to 100%) transmitting through the subglacial environment at the peak of the meltseason. The POC flux is primarily derived from the subglacial environment throughout the meltseason. The early season (low flow) glacier discharge contains higher DOC concentrations (0.5–4.1mgL⁻¹), and exports more enriched carbon (Δ¹⁴CDOC ∼ −250‰) compared to the peak season (high flow) discharge, when the concentrations are lower (0.1–0.6mgL⁻¹) and the Δ¹⁴C is more depleted (Δ¹⁴CDOC ∼ −400‰). Conversely, the POC export (1.4–13.2mgL⁻¹, Δ¹⁴CPOC ∼ −250‰) shows no temporal variation in either concentration or radiocarbon content throughout the meltseason. Dissolved ion loads in concomitant samples reflected the seasonal evolution of the subglacial drainage system, confirming the influence of subglacial hydrology on the composition of the bulk carbon pools. Based on this work, we conclude that (1) different mechanisms control the DOC and POC flux from glacial systems; (2) chemically-distinct DOC pools are accessed by seasonally-evolving hydrological flow-paths; and (3) the GrIS can deliver labile carbon, which may also be ¹⁴C-depleted, to downstream proglacial and marine environments.