Iron Mediated Organic Preservation in a River Influenced Continental Margin

Continental margins such as the Mississippi river influenced Louisiana shelf (MIS), are important land-ocean interfaces where excessive water column productivity and rapid burial results in large amounts of organic carbon (OC) being sequestered in the sediments. The preservation of OC in sediments is controlled by a number of processes among which mineral-OC interactions are considered to be more important. Specifically, the interactions between sediment iron (Fe) species and OC have been shown to promote carbon preservation. However, the exact nature of preservation mechanism is not well understood and is based on limited global data, which specifically lacks estimates from large river deltas. In this study, we report that reactive iron bound OC (rFe-bound OC) represents 30.8 ± 5.6 % of total organic carbon (OC) in the upper 20 cm of the MIS, which is higher than the current global average of 21.5 ± 8 %, with the dominant rFe phase associated with OC found to be goethite. The distribution of bulk OC and rFe in the sediments demonstrate a strong spatial and temporal variability, coincident with the river discharge. Short-term incubation studies lasting ~2 days demonstrated that 8 to 69 % of the rFe-bound OC can be destabilized from the upper 10 cm of the sediment during rapid O2 consumption in the overlying water column. Such rapid changes in rFe-bound OC pool undermines the role of Fe in preserving OC in sediments. Under such rapidly declining O2 conditions, the sediment Fe also played a significant role in benthic phosphorus (P) by mobilizing reactive Fe-bound P species. Long-term incubations (150 days) demonstrated that Fe-OC interactions do facilitate the partitioning of dissolved OM into the sediments under both aerobic and anaerobic conditions, but the exact long-term preservation mechanism of OC by Fe remains enigmatic. Further understanding of Fe-OC interactions was carried out using microfluidic flow cell experiments which provided a pore-scale “view” of the Fe-oxidation kinetics under the influence of pH and OM. Results suggest that the mineralogy of the freshly precipitated Fe oxides shifts from a mixture of goethite and ferrihydrite to predominantly ferrihydrite in the absence of dissolved organic carbon