Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

E C Lawson,J Laybourn-Parry,P Nienow,D Chandler,M Tranter,M Stibal,P Dewsbury,G P Lis,J L Wadham,C E H Butler

doi:10.5194/bg-11-4015-2014

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.

Highlights

Glaciers and ice sheets are known to harbour diverse and active populations of microorganisms (Anesio et al, 2009; Cheng and Foght, 2007; Lanoil et al, 2009; Sharp et al, 1999; Stibal et al, 2012a)
We focused on a subset of low molecular weight dissolved organic carbon (DOC) (LMW–DOC) compounds, in addition to the presence of protein-like compounds in florescence spectra (Fellman et al, 2009; Hood et al, 2009), in order to assess the magnitude and possible origins of the bioavailable fractions
We show that supraglacially derived DOC may display high concentrations of LMW–DOC, which supports recent work in the Gulf of Alaska suggesting that the labile DOC fraction in glacial runoff originates on the glacier surface (Stubbins et al, 2012)

Summary

Introduction

Glaciers and ice sheets are known to harbour diverse and active populations of microorganisms (Anesio et al, 2009; Cheng and Foght, 2007; Lanoil et al, 2009; Sharp et al, 1999; Stibal et al, 2012a). These drive subglacial chemical weathering (Montross et al, 2012; Sharp et al, 1999) and are responsible for nutrient export in glacial runoff (Barker et al, 2006).

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Conclusion