Historical Reconstruction of Organic Carbon Inputs to Sediments in the Colville River Delta, Alaska: The Application of Biomarker Proxies

Abstract:

Arctic permafrost represents about 50% of the total belowground global carbon pool, and thus the fate of this pool, as it thaws in the wake of global warming, warrants close attention. Large-river delta-front estuaries (LDEs) have been shown to be important recorders of natural and human-induced changes in watersheds, as they are critical zones for the exchange of organic carbon between the continents and the ocean. The Colville River is the largest North American Arctic River with a continuous permafrost watershed. Simpson’s Lagoon, an eastward distal component of the Colville River Delta is an excellent location for historical reconstruction work since it is an area well protected from intense ice grounding and has minimal bioturbation. Sediment cores were collected from the mouth of the river and the lagoon in August of 2010, and analyzed for bulk organic carbon and nitrogen proxies, biomarkers (including lignin phenols, fatty acids), and compound-specific ¹³C isotope analysis (CSIA) of fatty acids. Downcore sediment data from CSIA of short-chain fatty acids (C₁₄-C₁₈) to the delta over the past ca. 50 years were found to be more depleted and had a wider isotopic range (-17.0~-33.2‰) than long-chain fatty acids (C₂₂-C₃₀, -30.3~-36.8‰). This possibly reflects alterations of inputs of freshwater flow to the delta which could have resulted in isotopic changes that caused corresponding changes in marine versus freshwater phytoplankton inputs. Downcore short-chain saturated and monounsaturated fatty acid profiles reflected differences in the abundance of bacteria and post-depositional decay of algal inputs across different regions of the delta. Ongoing analyses will also focus on compound-specific radiocarbon analyses (CSRA) of fatty acids and lignin phenols to better understand the changes of organic inputs from terrestrially-derived organic-rich horizons in surface soils vs. old deep permafrost-derived organic horizons.