U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameSources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico
Document AuthorGordon, E.S.
Goni, M.A.
Short DescriptionGEOCHIMICA ET COSMOCHIMICA ACTA 67 (13): 2359-2375
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2003

Abstract: Suspended sediments (SS) from the Atchafalaya River (AR) and the Mississippi River and surficial sediment samples from seven shallow cross-shelf transects west of the AR in the northern Gulf of Mexico were examined using elemental (%OC, C/N), isotopic (delta(13)C, Delta(14)C), and terrigenous biomarker analyses. The organic matter (OM) delivered by the AR is isotopically enriched (similar to-24.5%) and relatively degraded, suggesting that soil-derived OM with a C4 signature is the predominant OM source for these SS. The shelf sediments display OC values that generally decrease seaward within each transect and westward, parallel to the coastline. A strong terrigenous C/N (29) signal is observed in sediments deposited close to the mouth of the river, but values along the remainder of the shelf fall within a narrow range (8-13), with no apparent offshore trends. Depleted stable carbon isotope (delta(13)C) values typical of C3 plant debris (-27parts per thousand) are found near the river mouth and become more enriched (-22 to -21parts per thousand) offshore. The spatial distribution of lignin in shelf sediments mirrors that of OC, with high ligpin yields found inshore relative to that found offshore (water depth > 10 m). The isotopic and biomarker data indicate that at least two types of terrigenous OM are deposited within the study area. Relatively undegraded, C3 plant debris is deposited close to the mouth of the AR, whereas more degraded, isotopically enriched, soil-derived OM appears to be deposited along the remainder of the shelf. An important input from marine carbon is found at the stations offshore from the 10-m isobath. Quantification of the terrigenous component of sedimentary OM is complicated by the heterogeneous composition of the terrigenous end-member. A three-end-member mixing model is therefore required to more accurately evaluate the sources of OM deposited in the study area. The results of the mixing calculation indicate that terrigenous OM (soil-derived OM and vascular plant debris) accounts for similar to79% of the OM deposited as inshore sediments and 66% of OM deposited as offshore sediments. Importantly, the abundance of terrigenous OM is 40% higher in inshore sediments and nearly 85% higher in offshore sediments than indicated by a two-end-member mixing model. Such a result highlights the need to reevaluate the inputs and cycling of soil-derived OM in the coastal ocean.