A Molecular and Isotopic Approach to Examine the Role of Terrestrial Organic Matter in the Carbon Cycle of the Arctic Ocean

Abstract

The organic carbon cycle in the Arctic Ocean is complicated by the delivery and redistribution of terrigenous material through rivers, sea-ice, and erosion. This dissertation combines an isotopic and molecular biomarker approach to assess the role that terrestrial organic carbon plays in the Arctic organic carbon cycle, with a focus on a comparison of geochemical proxies for the quantification of organic matter, an analysis of the sources and transformation of organic carbon to particulate organic matter (POM) and sediments, and an experimental investigation on the kinetics of recycling. Estimates for preserved terrestrial organic components varied considerably for identical sediment samples, suggesting that proxies account for different sources of terrestrial material (i.e., soil versus vascular plants). In spite of the variability, an estimated 12-43% of the organic carbon preserved in surface sediments was terrestrial in origin. This contrasted sharply with surface and halocline POM, in which marine inputs dominated despite spatial variability. With depth, POC composition reflected the increasing significance of inputs from secondary production and microbial degradation, as well as continental material. Acid-volatile sulfide (AVS) and redox-sensitive elements coupled with δ13C and lipid biomarkers demonstrated a transition from intense metabolism of labile marine organic matter in shelf sediments to slower sedimentary metabolism from occasional delivery of labile organic matter in the basin. Experimental determinations of the kinetics of microbial recycling revealed striking contrasts in marine and terrestrial organic carbon lability. Marine organic matter was recycled on very short timescales compared to terrestrial organic matter, corresponding to results of sedimentary and particle analyses. A simplified box model of organic carbon cycling in the Chukchi/Alaskan Beaufort Sea region reveals that 0.9 Mt and 0.7 Mt of marine and terrestrial organic matter, respectively, are buried in shelf sediments, while an additional 0.2 Mt marine and 0.1 Mt of terrestrial organic carbon are buried in basin sediments annually, confirming that land-derived organic matter plays a large role in carbon dynamics in Arctic systems, even on non-river dominated margins.