Iron-Mediated Organic Matter Preservation in the Mississippi River-Influenced Shelf Sediments

Continental shelves are important land-ocean interfaces where natural and anthropogenic processes as well as sediment geochemistry regulate organic matter sequestration. The major objective of this work is to understand spatial variability in reactive iron (rFe) bound organic carbon (OC) during low- and high-flow conditions in the recent sediments of the Mississippi River-influenced Louisiana shelf in the Northern Gulf of Mexico. Sediment samples were collected from five sites along the river plume during low (August 2016) and high (May 2017) discharge. Reactive iron was significantly higher in May (6.9 +/- 1.7 mg g(dw)(-1)) compared to August (4.3 +/- 0.9 mg g(dw)(-1)), while OC in August (11.9 +/- 3.7 mg g(dw)(-1)) and May (7.8 +/- 4.8 mg g(dw)(-1)) was similar. The fraction of OC bound to rFe (Fe-OC) in the sediment in May (2.4 +/- 1.5 mg g(dw)(-1)) was not significantly different from August (3.7 +/- 2.7 mg g(dw)(-1)). Molar OC:Fe ratios in the rFe-associated OC ranged between 0.9 and 6.2, with higher ratios in the deeper sediments (6-20 cm). Our results show that 31.7% +/- 6.2% of the total OC in the recent sediments is associated with the rFe phase. Sediment mineralogical composition was characterized using X-ray diffractometry and Fe-K edge X-ray absorption near edge structure spectroscopy. Our findings indicate that most of the iron is present as nonreactive iron in clay mineral smectite and the dominant form of rFe associated with OC is goethite.