Controls on the distribution and accumulation of terrigenous organic matter in sediments from the Mississippi and Atchafalaya river margin

Abstract

Sediment samples from 14 box cores across the Mississippi and Atchafalaya River Margin were examined in order to quantify the magnitude and composition of the OM depositional flux in sediments from a river-dominated margin and identify the fate of terrigenous OM in the study area. Elemental, isotopic, mineral surface area, and terrigenous biomarker analyses suggest that physical sorting of particles across this river-dominated margin controls the chemical composition of sedimentary OM. Sediment accumulation rates in the area ranged from 0.012 to 0.68 cm/year and showed an inverse logarithmic relationship with water depth. Highest organic carbon (OC) content (1.3–1.5%) was observed on the inner shelf (<10 m water depth) and slope (>200 m water depth), with lower OC (0.8%) measured on the outer shelf (10–200 m water depth). Mineral surface area (SA) showed a similar spatial distribution as OC, with the highest values on the inner shelf (average 41 m 2/g) and slope (average 54 m 2/g), and low SA values (average 23 m 2/g) on the outer shelf. Association of OM with minerals and its apparent differential transport across the study area results in the greatest proportion of OM accumulating on the inner shelf (37%), although the slope also plays a significant role in OM accumulation (33%) due to its greater areal extent. Stable organic carbon isotope ( δ 13C OC) values ranged between −23‰ and −21‰ across the study area, with a slight seaward enrichment. The spatial trend in lignin yields mirrored that of δ 13C, with a substantial decrease in lignin content from 1.8 mg/100 mg OC at inner shelf locations to 0.31 mg/100 mg OC at slope locations. The OC in all sediment samples exhibited more depleted radiocarbon compositions (Δ 14C OC) than modern values (>0‰), ranging from ca. −200‰ on the inner shelf to ca. −400‰ on the slope. These isotopic and biomarker data indicate that terrigenous OM in the Gulf of Mexico is heterogeneous, composed of at least two sources: lignin-rich, isotopically depleted plant debris and lignin-poor, isotopically enriched soil-derived OM. The plant debris, which is deposited close to shore, composes a small portion of inner shelf sediments. Soil-derived OM is deposited throughout the study area, although it appears to be preferentially transported to deeper regions. The fate of terrigenous OM in the northern Gulf of Mexico appears to be governed by the hydrodynamic sorting of riverine particles of different compositional character rather than by simple dilution with marine OM.