Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

Abstract

Abstract. Runoff from small glacier systems contains dissolved organic carbon (DOC) rich in protein-like, low molecular weight (LMW) compounds, designating glaciers as an important source of bioavailable carbon for downstream heterotrophic activity. Fluxes of DOC and particulate organic carbon (POC) exported from large Greenland catchments, however, remain unquantified, despite the Greenland Ice Sheet (GrIS) being the largest source of global glacial runoff (ca. 400 km3 yr−1). We report high and episodic fluxes of POC and DOC from a large (>600 km2) GrIS catchment during contrasting melt seasons. POC dominates organic carbon (OC) export (70–89% on average), is sourced from the ice sheet bed, and contains a significant bioreactive component (9% carbohydrates). A major source of the "bioavailable" (free carbohydrate) LMW–DOC fraction is microbial activity on the ice sheet surface, with some further addition of LMW–DOC to meltwaters by biogeochemical processes at the ice sheet bed. The bioavailability of the exported DOC (26–53%) to downstream marine microorganisms is similar to that reported from other glacial watersheds. Annual fluxes of DOC and free carbohydrates during two melt seasons were similar, despite the approximately two-fold difference in runoff fluxes, suggesting production-limited DOC sources. POC fluxes were also insensitive to an increase in seasonal runoff volumes, indicating a supply limitation in suspended sediment in runoff. Scaled to the GrIS, the combined DOC (0.13–0.17 Tg C yr−1 (±13%)) and POC fluxes (mean = 0.36–1.52 Tg C yr−1 (±14%)) are of a similar order of magnitude to a large Arctic river system, and hence may represent an important OC source to the near-coastal North Atlantic, Greenland and Labrador seas.

Organic carbon flux and particulate organic matter composition in Arctic valley glaciers: examples from the Bayelva River and adjacent Kongsfjorden

“…Assuming that AA and phytoplankton carbon represent the labile POM pool, the labile proportion in the total POM flux will be ∼ 10 % of the total POM flux (i.e., for POC flux, 9.5 %; for PN flux, 11 %). This proportion is comparable to that of the Greenland Ice Sheet POM, in which the labile component is estimated at 9 % using a carbohydrate approach (Lawson et al, 2014). Due to the rapid removal process in the estuarine and adjacent fjord, most glacier meltwater POC is expected to be buried within adjacent fjords (Dittmar and Kattner, 2003).…”

“…Moreover, by incorporating the total surface runoff (25 km 3 year −1 ) from Svalbard's glaciers due to melting of snow and ice (Hagen et al, 2003) and the DOC content of glacier meltwater (Table 1), we estimate the DOC flux for Svalbard to be 0.02 ± 0.01 × 10 6 tons year −1 (Table 5). The POC flux of Svalbard is equivalent to only 6 % of that from the Greenland Ice Sheet (0.9-0.94 × 10 6 tons year −1 ) (Bhatia et al, 2013;Lawson et al, 2014), and is significantly smaller than the POC flux of the Mackenzie River (1.8-2.1 × 10 6 tons year −1 ) (Dittmar and Kattner, 2003). However, in terms of DOC flux, the value from Svalbard is 13-25 % that of the Greenland Ice Sheet (0.08-0.15 × 10 6 tons year −1 ) (Bhatia et al, 2013;Lawson et al, 2014).…”

“…The POC flux of Svalbard is equivalent to only 6 % of that from the Greenland Ice Sheet (0.9-0.94 × 10 6 tons year −1 ) (Bhatia et al, 2013;Lawson et al, 2014), and is significantly smaller than the POC flux of the Mackenzie River (1.8-2.1 × 10 6 tons year −1 ) (Dittmar and Kattner, 2003). However, in terms of DOC flux, the value from Svalbard is 13-25 % that of the Greenland Ice Sheet (0.08-0.15 × 10 6 tons year −1 ) (Bhatia et al, 2013;Lawson et al, 2014). In comparison, DOC fluxes from glaciers in the Gulf of Alaska and from the small Arctic Yana River and the Mackenzie River are 0.13 × 10 6 , 0.09 × 10 6 , and 1.4 × 10 6 tons year −1 , respectively (Dittmar and Kattner, 2003;Holmes et al, 2012).…”

“…Unlike temperate glaciers (e.g., alpine glaciers), suspended sediment in high Arctic glaciers becomes increasingly available to the fluvial system through the melt season and hence the particulate organic carbon (POC) output may remain at an elevated level throughout the melt season. Indeed, the flux of POC in high Arctic glacial meltwater is typically higher than that of DOC (e.g., Bhatia et al, 2013), while the labile proportion is relatively low (9 %) (Lawson et al, 2014). As terrestrial DOC travels much farther away than POC does (Dittmar and Kattner, 2003), the DOC and POC impact of glacier meltwater to the ocean is different.…”

“…To date, most studies of organic matter in Arctic glacial meltwater have focused on the Greenland Ice Sheet (e.g., Bhatia et al, 2013;Lawson et al, 2014), with little attention paid to smaller valley glaciers, such as those on Svalbard (Irvine-Fynn et al, 2012;Kuliñski et al, 2014;Tye and Heaton, 2007). However, a comparison of Alaskan glaciers (Hood et al, 2009) and the Greenland Ice Sheet (Bhatia et al, 2013) reveals that valley glaciers exhibit higher area-weighted fluxes of organic carbon.…”