Constraints on the origin of sedimentary organic carbon in the Beaufort Sea from coupled molecular 13C and 14C measurements

The type and flux of organic carbon (OC) delivered from the continents to the sea can both influence, and be influenced by, climate change on regional and global scales. In order to develop a more complete view of OC delivery in the climatically sensitive Arctic region, we measured the stable carbon and radiocarbon isotopic signatures of individual lipid biomarkers and products of kerogen pyrolysis from the surficial sediments of several sites on the Mackenzie Shelf and adjacent slope of the Beaufort Sea. Even carbon numbered fatty acids exhibit a trend of increasing 14C age with increasing chain length, from modern values for the nC14–nC18 homologues to several thousand years old for those ≥ nC24. Such depleted 14C values for longer-chain fatty acids likely reflect supply of vascular plant OC that has been pre-aged on the continents for several millennia prior to delivery to the Beaufort Sea. Their corresponding δ13C values support a C3 land plant source. The molecular distributions and 14C and 13C signatures of solvent-extractable alkanes point to at least two sources: higher plant leaf waxes and a 14C-‘dead’ component likely derived from erosion of organic-rich sedimentary rocks exposed within the Mackenzie River drainage basin. The δ13C and Δ14C values of n-hydrocarbon pyrolysis products from the corresponding demineralized sediments also suggest a mixed supply of ancient kerogen and pre-aged vascular plant-derived precursors to the Beaufort Sea. On a bulk level, the trend in sedimentary OC contents, C/N ratios, and δ13C values point to an overall decrease in the terrigenous input (mainly from the Mackenzie River) with increasing distance offshore, whereas bulk Δ14C measurements exhibit no trend suggesting a somewhat constant pre-aged component. A coupled molecular isotopic mass balance approach based on the lipid δ13C and Δ14C signatures is used to construct a budget of terrestrial, marine, and petrogenic OC burial on the shelf. Results indicate that roughly 40–50% of the carbon currently being buried in the Beaufort Sea is derived from the weathering of ancient sedimentary rock. The balance is composed of marine and terrestrial input, supporting the qualitative description of OC sources given by the bulk and molecular patterns above. This suggests that mass balances utilizing the δ13C and Δ14C signatures of biomarkers as endmembers can be used to semi-quantitatively deconvolve multiple sources of organic carbon in marine sediments.