Rapid Permafrost Carbon Degradation at the Land-Ocean Interface

Abstract:

Climate change has a strong impact on permafrost coasts in the Arctic. With increasing air and water temperatures, the ice-rich unlithified permafrost coasts will thaw and erode at a greater pace. Organic carbon that has been stored for thousands of years is mobilized and degrades on its way to the ocean. The objective of this study is to investigate to what extent permafrost carbon degrades after thawing before it enters the ocean in a retrogressive thaw slump. A slump located on Herschel Island (Yukon Territory, Canada) was sampled systematically along transects from the permafrost headwall to the coastline. Concentrations of particulate and dissolved organic carbon (POC and DOC) as well as its stable carbon isotopes (δ¹³C-POC and δ¹³C-DOC) were measured and compared in frozen deposits and in thawed sediments. Ammonium, nitrite and nitrate were also analyzed in order to identify and understand the carbon metabolization mechanisms taking place during slump activity. Our results show that major portions of permafrost carbon are metabolized right after thawing. Ammonium concentrations are highest in areas where thawed permafrost material directly accumulates. We suggest that before entering the nearshore zone permafrost organic carbon and nitrogen is subject to major degradation and metabolization. This makes permafrost coasts and retrogressive thaw slumps degradation hotspots at the land-ocean-interface.