Vertical flux of particulate organic matter in a High Arctic fjord: Relative importance of terrestrial and marine sources

Abstract

Vertical flux of particulate matter was recorded using a moored sediment trap during 2002-03 in the outer region of the 90 km long NE Greenland fjord Young Sound (74°18’N, 20°18’W). Sea ice covered the fjord for c. 9 months during the deployment. At 65 m depth total flux of material was 1420 g dry weight m-2 and annual fluxes of carbonate (g m^-2), chlorophyll (mg m-2), particulate organic carbon (POC, g C m^-2) and nitrogen (PON, g N m-2), were, 9, 53, 17 and 1.2 respectively. A steep increase in fluxes was observed during the summer thaw when sea ice broke up and water discharge from land began. Within the two months (July and August), >90% of the total annual vertical flux occurred.

Isotopic (δ13C & δ15N) analysis of particulate organic material (POM) in the sediment trap, in phytoplankton and in the river material indicated that a maximum of c. 50% of the POM material originated from land. This is supported by the high C:N ratio (by atoms) of up to 22 found in the trapped organic material during the summer thaw as compared with 7–9 during winter and spring, when no discharge from land occurred. Seasonal measurements of the POC discharge from rivers to the outer region of the fjord corresponded to c. 40% of the vertical POC flux measured in the sediment trap, which further indicates a significant terrestrial contribution to the settling material in the outer fjord area.

Besides POC, dissolved organic carbon (DOC) is discharged in an equal amount to the fjord from rivers, resulting in a total organic carbon (TOC) input from land to the outer region of Young Sound of 13 g C m^-2 yr^-1. This corresponds to c. 40% of the net TOC input from the Greenland Sea and to the outer part of the fjord during the ice-free productive period underlining the significant terrestrial contribution to sedimentation in the outer part of the fjord.

Mean permanent accumulation rates based on the depth distributions of 210Pb, 137Cs and TOC in sediments at 60 m water depth in the outer fjord area revealed a burial of carbon within the sediment of 7.9 g C m^-2 yr^-1. In agreement with the sediment trap measurements, δ13C values within the sediment suggest that a substantial amount (c. 40%) of the POC in the sediment was of terrestrial origin. At the same sites, previous studies have reported an annual release of dissolved inorganic carbon (DIC) due to mineralization from the sediment of 12.6 g C m^-2 yr^-1. The sum of the annual DIC release and the burial within the sediment

represents an expected total input to the sediment of 20.5 g C m^-2 yr^-1and compares well with the vertical flux measurement from the sediment trap of 17.0 g C m^-2 yr^-1 during the present study.