HR: 1330h
AN: OS23A-14    [Abstracts]
TI: Sources of Organic Carbon to a River-Dominated Coastal Shelf: the use of Chemical Biomarkers to Track Inputs to Sediments
AU: * Wysocki, L A
EM: lwysock@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: Bianchi, T S
EM: tbianch@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: Filley, T R
EM: filley@purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47907 United States
AB: The oceans act as a global sink for excess carbon dioxide not only via oceanic primary production, but also through sinking organic matter and burial. Coastal sediments, particularly along river-dominated ocean margins, can be important repositories for both allocthonous and autochthonous organic carbon. Using molecular biomarker analyses, this study examines the sources of organic carbon to the Louisiana shelf within the dispersive path of the Mississippi River. Two cruises were conducted near the mouth of the river on the Louisiana shelf, during periods of high and low river discharge. Sediment samples were collected on a grid of 50 sampling sites that spanned the area of the plume and the surrounding waters, and analyzed for bulk carbon and specific molecular biomarkers. Total organic carbon (TOC) in surface sediment ranged from 0.28 to 1.8%OC. The δ ¹³C values generally ranged from -20.6 to -22.9 ‰, with a few stations showing more enriched values. It is not possible, therefore, to precisely distinguish between marine and terrestrial inputs based on the bulk carbon and δ¹³C values alone. Chlorophyll in the sediment (0.1 to 8.9 μ g/g) was used as an indicator of marine organic carbon inputs, while lignin (Λ₈, 0.3 to 4.7 mg/100 mg OC) was used as a biomarker for terrestrial organic carbon. An isotope mixing model incorporating the bulk isotope and the biomarker data suggests ~ 32 to 41% of the TOC in surface sediments is composed of terrestrial material during high discharge, while only 14 to 29% of the TOC is of terrestrial origin during the low discharge period. Compound specific isotope analyses (CSIA) show differences up to 7 ‰ in syringyl compounds and up to 10 ‰ in cinnamyl compounds, indicating both C₃ and C₄ terrestrial inputs. The relative proportion of marine and terrestrial carbon deposition on the Louisiana shelf varies with river flow and the amount of in situ production. However, even with the high productivity during spring months there is still a substantial terrestrial contribution to the surface sediments, likely due to the recalcitrant character, and consequently longer residence time of this material.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 4805 Biogeochemical cycles (1615)
DE: 4850 Organic marine chemistry
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly