Mobilization of terrestrial organic matter from thawing Arctic permafrost regions: Insights from lignin-derived phenols and their compound-specific radiocarbon ages

Abstract

Global climate change is expected to have a huge impact on Arctic warming, leading to an increased remobilization of permafrost organic carbon. More than twice as much carbon as in the atmosphere is contained in permafrost soils, and may upon destabilization expose large amounts of organic matter to microbial degradation and release climate-forcing greenhouse gases. As an important link in the land-ocean continuum, rivers are important pathways for permafrost OC remobilization. Arctic ocean sediments are thus receptors of terrestrial OC remobilization for a large part of the circum-Arctic drainage basin and offer an archive to study past terrestrial OC remobilization as during the last deglaciation. This thesis studies terrestrial OC in sub-Arctic ocean sediments to study OC remobilization from permafrost regions across temporal and spatial scales. A detailed study of two sediment cores from the Bering Sea and Okhotsk Sea, allowed the reconstruction of vegetation development, permafrost OC mobilization, and transport pathways of permafrost OC remobilization during the last deglaciation. Vegetation development and permafrost OC remobilization have been investigated by analyses of the mass accumulation rate of terrestrial biomarkers (lignin phenols and lipids). To study the influence of climatic conditions on the stability of inland permafrost soils, the sea surface temperature of the Bering Sea was investigated (TEXL86). For the study of terrestrial OC remobilization from different carbon pools, compound-specific radiocarbon analysis (CSRA) was applied to terrestrial compounds (lignin-derived phenols). A modified method for the purification of lignin-derived phenols was used for CSRA. Downcore records of lignin flux from the Yukon and Amur basins covering the early deglaciation to the Holocene are discussed in the first study. It was found that vegetation change and permafrost remobilization occurred earlier in the Yukon than in the Amur basin. The retreat of sea ice coupled with increased sea surface temperatures in the Bering Sea and adjacent ocean areas during the early deglaciation (19.0-14.6 kyr) might have promoted early permafrost mobilization. The results reveal that lipids and lignin might have been delivered to the ocean by identical processes, i.e., runoff and erosion, which is not consistent with previous studies of modern Arctic river systems. A simplified procedure for the purification of lignin phenols based on the published method by Feng et al., (2013b) and an evaluation of the fraction of modern carbon (F14C) and the mass of the associated procedural blank is described in the second study. Lignin phenols were purified only by one HLB SPE cartridge (Waters Oasis, 200 mg, 6 mL). Samples were eluted from the HLB cartridge with 60 mL ethyl acetate to achieve a clean-up of lignin-derived phenols. Single compound separation and collection was achieved by HPLC on a Phenomenex Synergi Polar-RP column at 40 °C. The blank carbon contribution of the entire method, as determined with reference compounds, was found to be 4.17 ± 0.35 mgC with an F14C of 0.55 ± 0.04. In the third study, the revised method of lignin purification was applied to obtain first downcore CSRA records of lignin-derived phenols from sediment cores retrieved off the Yukon and Amur Basins. The radiocarbon ages of phenols from sediments of the Bering and Okhotsk Seas are expected to reflect inputs from at least two different terrigenous sources that are characterized by different ages and degradation degrees during the last deglaciation. The age offsets between lignin phenols and fatty acids were controlled by surface discharge in the Yukon and Amur Basins. Deep OC enriched in long-chain lipids can be mobilized both through coastal erosion and thawing of inland thermokarst which can be transported to marine sediments by surface runoff in the last deglaciation. The radiocarbon ages of lignin phenols and long-chain fatty acids are controlled by processes that are affected by climate change, and the 14C age offset between lipids and lignin phenols can be used as palaeo-proxy for these changing processes. The fourth study investigates lignin phenols flux and radiocarbon ages in short cores off the Lena River, covering the last century. The vegetation in the Lena Basin is a mixture of woody gymnosperm and non-woody angiosperm sources and has not changed significantly in the past century. Young and old terrigenous carbon pools both affect the D14C values of phenols in the two sediment cores. The D14C values of lignin phenols in shallow Laptev Sea sediments may be treated as an indicator of runoff. This thesis highlights the vulnerability of permafrost OC to Arctic warming over time and space, thus contributing to a better understanding of climate-carbon couplings in the Earth system.