DELIVERY OF ANCIENT ORGANIC CARBON TO THE BEAUFORT SEA

The Mackenzie River system represents the primary source of sediment to the Arctic Ocean and provides an excellent opportunity to study the delivery of fossil organic carbon (OC) to contemporary marine sediments.  Bulk radiocarbon dating of fluvial suspended matter and surficial sediments in a transect from the river delta to the Beaufort Basin reveals the presence of a high content of older carbon, with conventional ¹⁴C ages ranging from 7690 to 10700 yr BP.  Bulk d¹³C measurements on the same samples indicate a significant terrigenous OC component, with values ranging from –26.0 to –23.8 ‰ and increasing with distance offshore.  These old ¹⁴C ages may reflect OC of vascular plant decent that is pre-aged (possibly through sequestration in permafrost soils), fossil OC derived by erosion of the extensive outcrops of organic rich sedimentary rocks in the drainage basin, or both.  In order to resolve the contributions from kerogen weathering versus sequestration of higher plant debris, molecular level ¹⁴C and ¹³C measurements were performed on free and macromolecularly bound n-alkanes as well as vascular plant derived n-alkanoic (fatty) acids.  Compound-specific d¹³C values of pyrolysis-liberated alkanes from demineralized sediment are homogeneous from C₁₅ to C₃₅ at ca. –30 ‰, suggesting a predominantly fossil origin.  This is supported by the significantly depressed radiocarbon ages of the same compounds.  Free fatty acids, meanwhile, exhibit a trend of modern shorter chain length (C₁₄-C₁₈) to much older higher chain length (C₂₄-C₂₈) homologues, which along with the companion d¹³C values reflect their autochthonous and allochthonous origins, respectively.  Moreover, the ages of individual alkanoic acids appear to increase with distance offshore, possibly reflecting association with fine grain minerals.  However, the fractional abundance of even the oldest homologue at the most distal site cannot account for the depressed bulk OC ages, further demonstrating the presence of ancient carbon.  In order to estimate the magnitude of this component, model results exploiting these paired molecular level isotopic measurements will be presented.  These results will be coupled with elemental abundance data from each site to yield a more complete picture of OM provenance and cycling in this important arctic margin system.