The sequestration of terrestrial organic carbon in Arctic Ocean sediments: A comparison of methods and implications for regional carbon budgets

A variety of approaches have previously been developed to estimate the fraction of terrestrial or marine organic carbon present in aquatic sediments. The task of quantifying each component is especially important for the Arctic due to the regions' sensitivity to global climate change and the potential for enhanced terrestrial organic carbon inputs with continued Arctic warming to alter carbon sequestration. Yet it is unclear how each approach compares in de. ning organic carbon sources in sediments as well as their impact on regional or pan-Arctic carbon budgets. Here, we investigated multiple methods: (1) two end-member mixing models utilizing bulk stable carbon isotopes; (2) the relationship between long-chain n-alkanes and organic carbon (ALKOC); (3) principal components analysis (PCA) combined with scaling of a large suite of lipid biomarkers; and (4) ratios of branched and isoprenoid glycerol dialkyl glycerol tetraether lipids (the BIT index) to calculate the fraction of terrestrial organic matter components preserved in Arctic marine sediments. Estimated terrestrial organic carbon content among approaches showed considerable variation for identical sediment samples. For a majority of the samples, the BIT index resulted in the lowest estimates for terrestrial organic carbon, corroborating recent suggestions that this proxy may represent a distinct fraction of terrestrial organic matter; i.e., peat or soil organic matter, as opposed to markers such as n-alkanes or long-chain fatty acids which measure higher plant wax inputs. Because of the patchy inputs of n-alkanes to this region from coastal erosion in the western Arctic, the ALKOC approach was not as effective as when applied to river-dominated margins found in the eastern Arctic. The difficulties in constraining a marine delta C-13 end-member limit the applicability of stable isotope mixing models in polar regions. Estimates of terrestrial organic carbon using the lipid-based PCA method and the bulk delta C-13 mixing model approach varied drastically at each site, suggesting that organic matter fractions such as amino acids or carbohydrates may affect bulk organic matter composition in a manner that is not captured in the lipid-based analysis. Overall, terrestrial organic matter inputs to the Chukchi and western Beaufort Seas using the average of the methods at each site ranged from 11% to 44%, indicating that land-derived organic matter plays a substantial role in carbon dynamics in the western Arctic Ocean. (C) 2009 Elsevier Ltd. All rights reserved.