Accelerating melting of Greenland Ice Sheet drives widespread retreats of both land- and marine-terminating glaciers, releases substantial subglacial freshwater and terrigenous particles into Greenlandic fjords, and reshapes biogeochemical cycles and the marine biological carbon pump within these systems. Subglacial discharge from land and marine terminating glaciers impact surface productivity differently within fjords, yet direct measurements of water column particulate organic carbon (POC) fluxes in Greenlandic fjords are scarce. Here, we present summer time variations in ²³⁴Th–derived POC fluxes from the Lindenow (land-terminating) and Sermilik fjords (marine-terminating) in the Southeastern Greenland. Water column temperature, salinity, turbidity, current velocity, total and particulate ²³⁴Th activities, and particulate organic carbon and nutrient concentrations were measured along multiple transects to quantify the marine biological carbon pump strength and transfer efficiency. Preliminary results show contrasting vertical ²³⁴Th profiles and spatial POC fluxes at different fjords. In Lindenow, we observed low POC flux at inner fjord and significantly higher POC fluxes in outer fjord. We attribute this spatial POC flux variations to surface runoff from land-terminating glaciers that delivers high loads of terrigenous particles to surface waters, suppressing surface production and lowering carbon export efficiency in the inner fjord. In contrast, data from marine-terminating Sermilik Fjord reveal clear subsurface increases in both total ²³⁴Th activity and POC flux in inner fjord that correlate with enhanced water column turbidity, indicating a significant terrigenous POC contribution from subglacial discharge. Our study demonstrates that modes of subglacial meltwater supply are keys controlling high latitude, nearshore marine carbon cycles, and that water column ²³⁴Th distributions are sensitive to both in situ POC production in the euphotic zone and subsurface terrigenous POC supply in high-latitude fjord systems.

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