HOLOCENE MOBILIZATION OF PERMAFROST ORGANIC CARBON WITHIN A NORTHEASTERN BAFFIN ISLAND LAKE CATCHMENT MODULATED BY CLIMATE AND POSTGLACIAL SOIL DEVELOPMENT

Recent anthropogenic warming in the Arctic has caused accelerated regional permafrost thaw, leading to the export of relict organic carbon (OC) to both the atmosphere and surrounding landscapes. Past episodes of warmth exceeding pre-industrial temperatures, such as the Early Holocene (11.7 to 8 ka; thousands of years ago), may serve as an analogue for how the Arctic carbon cycle responds to a warmer climate. To test this hypothesis, we used a Bayesian mixing model, MixSIAR, to quantify the contributions of OC endmembers over time to a sediment record from Lake CF8, Baffin Island, Nunavut, Canada spanning the last 12.3 kyr. For this model, we characterized biogeochemical properties of the lake sediment and three OC endmembers, contemporaneous aquatic biomass, postglacial-aged soil, and MIS 5-aged soil, using Ramped Pyrolysis Oxidation (RPO), radiocarbon (¹⁴C) dating of CO₂ produced by RPO, and stable carbon isotopes. RPO of downcore sediments (n = 8), modern aquatic mosses (n = 2), and modern soils (n = 2), reveals similar patterns in OC volatilization vs. pyrolysis temperature between OC endmembers and the lake sediment mixture, indicating minimal OC degradation between source and sink, and that sediment OC represents a mixture of all endmembers across the spectrum of OC thermal stability. Positive age offsets ranging from 135 to 1,470 ¹⁴C yrs, defined as the lake sediment RPO CO₂ age minus the corresponding aquatic plant macrofossil age at depth, show that relict soil OC (modern soil ages = 2,010 and 2,560 ¹⁴C yrs) was always a substantially contributing endmember in our sediment record. We find two intervals in our MixSIAR results when the contribution of soil OC to Lake CF8 was greater than that of aquatic biomass: before peak warmth at 11.8 ka when relatively young postglacial soil OC (age offset = 310 ¹⁴C yrs) was accumulating on poorly consolidated deglacial sediments, and during peak warmth at 9.0 ka when a deeper active layer facilitated the export of more soil OC with older ¹⁴C ages (age offset = 980 ¹⁴C yrs). These results suggest that factors alongside climate, such as the degree of soil OC degradation, timing and degree of post-glacial soil development, and evolution of soil ¹⁴C residence times influence reconstructions of past Arctic permafrost carbon cycling.