The Influence of Reactive Iron on Organic Carbon Preservation in Sediment of the Mississippi River-Influenced Shelf

Reactive iron is a key driver of organic carbon preservation in marine sediments, but its participation in organic carbon remineralization complicates efforts to mechanistically constrain its role in preservation. To address this, we investigated the dual role of iron in the Mississippi River-influenced shelf sediment during low discharge (August 2016) and high discharge (May 2017). Duplicate sediment cores (30 cm depth) were collected from two stations; one core served as a natural reference, while the other was used for an incubation experiment. In the natural cores, reactive iron concentrations in the upper 9 cm were lower in August 2016 than in May 2017, whereas iron-bound organic carbon exhibited the opposite temporal pattern. Post incubation, approximately 10% of iron-bound organic carbon was lost at the offshore stations compared to a substantially greater loss (~59%) at the near-shore station. These results suggest that offshore regions may sustain more efficient organic carbon preservation via reactive iron, whereas the mechanism is considerably less effective in near-shore settings. Such spatial heterogeneity introduces significant uncertainty into current assessments of iron-mediated long-term organic carbon preservation on a global scale and underscores the need for more comprehensive investigations of iron–organic carbon interactions in continental shelve sediments.