Importance of iron-mediated organic matter preservation in northern Gulf of Mexico shelf sediments

The preservation of organic carbon (OC) in sediments is controlled by a number of processes among which mineral-OC interactions especially between sediment reactive iron species (rFe) and OC is thought to play an important role. The average reactive iron bound OC in the upper 20 cm of the sediments in the Mississippi river influence continental margin represents 30.8 ± 5.6% of the total OC present in the sediments. This is higher than the current global average of 21.5 ± 8 % in marine sediment. However, association of OC with rFe does not necessarily imply preservation and brings up the broader asked question as to whether rFe actively preserves organic matter present in these sediments or are they simply present in association with each other as they are both abundant in this river influences shelf. To evaluate the potential of OC-Fe interactions in preserving organic matter, both short-term and long-term incubations were carried out. Short-term whole core incubation studies lasting ~2 days demonstrate 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 these sediments. Results from long-term slurry incubations of the sediments (150 days) suggests that Fe-OC interactions do facilitate the partitioning of dissolved OM into the sediments under both aerobic and anaerobic conditions, but their long-term preservation potential were found to be not significant and inconclusive.