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 14 C age with increasing chain length, from modem values for the nC(14)-nC(18) homologues to several thousand years old for those >= nC(24), Such depleted C-14 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 delta C-13 values support a C-3 land plant source. The molecular distributions and C-14 and C-13 signatures of solvent-extractable alkanes point to at least two sources: higher plant leaf waxes and a C-14-'dead' component likely derived from erosion of organic-rich sedimentary rocks exposed within the Mackenzie River drainage basin. The delta C-13 and Delta C-14 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 delta C-13 values point to an overall decrease in the terrigerrous input (mainly from the Mackenzie River) with increasing distance offshore, whereas bulk Delta C-14 measurements exhibit no trend suggesting a somewhat constant pre-aged component. A coupled molecular isotopic mass balance approach based on the lipid VC and Delta C-14 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 delta C-13 and Delta C-14 signatures of biomarkers as endmembers can be used to semi-quantitatively deconvolve multiple sources of organic carbon in marine sediments. (c) 2006 Elsevier B.V. All rights reserved.