Particulate dominance of organic carbon mobilization from thaw slumps: implications for stream systems and permafrost carbon release

Abstract:

Climate change is increasing the frequency and intensity of thermokarst, and accelerating the delivery of terrestrial organic material from previously sequestered sources to aquatic systems, where it is subject to further biogeochemical alteration. Rapid climate change in glacially conditioned ice-rich and ice-marginal terrain of the Peel Plateau, western Canada, is accelerating thaw-driven mass wasting in the form of retrogressive thaw slumps. Despite major perturbation of downstream sedimentary and geochemical fluxes, few studies have examined changes in flux and composition of particulate organic carbon (POC) in streams and rivers as a result of thermokarst. Here we show that the orders of magnitude increase in total organic carbon mobilized to streams from thaw slumps on the Peel Plateau is almost entirely due to POC. Slump-mobilized particulate OC originates from different sources and is compositionally distinct from dissolved OC. Furthermore slump-mobilized POC appears to contain relatively greater amounts of degraded organic matter than POC present in non-slump affected stream networks, as inferred from base-extracted fluorescence of particulate organic matter, and thus is potentially more recalcitrant. Permafrost PO¹⁴C ages >44,000 ¹⁴C yrs BP and regional geology suggest that petrogenic organic carbon is likely an important source to slump-mobilized POC. Furthermore a substantial portion of slump-mobilized POC will be contained within primary sediment stores in valley bottoms, where net accumulation is currently exceeding net erosion, resulting in century to millennial scale sequestration of thermokarst-mobilized POC. Thus this study highlights the pressing need for better knowledge of sedimentary cascades, mobilization, and storage reservoirs in slump-affected streams, and baseline assessments of the biodegradability of POC within a sedimentary cascade framework. This will be critical in constraining the balance between biospheric carbon burial and petrogenic organic carbon oxidation as Earth surface processes fundamentally alter many regions of the Arctic . Explicit incorporation of POC dynamics in land-water carbon mobilization in the face of permafrost thaw is critical for understanding implications of thermokarst for regional carbon cycling and fluvial ecosystems.