Iron mediated organic matter cycling in permeable surface sediments

Coastal permeable sediments cover 50-60% of the continental shelves and are important filters and bioreactors that sitting between the land and ocean. In permeable surface sediments, the dynamic porewater advection can lead to frequent redox oscillation, which significantly affects the coupled cycling of organic matter (OM) and iron. In our study, we focused on the most redox active iron fraction (extractable by 0.5 M HCl), and investigated their effects on OM degradation and retention. During the transition of redox conditions, Fe(III) oxyhydroxides were quantitatively found as the dominant electron acceptors for anaerobic OM remineralization. However, the release of reduced Fe was significantly delayed, with most Fe(II) (~96%) remaining in the solid phase either through adsorption or formation of authigenic Fe(II)-bearing minerals. Under frequent redox oscillation as typically observed in natural coastal permeable sediments, Fe(II) in the solid phase can be re-oxidized and repetitively used as electron acceptor for anaerobic OM remineralization (Iron “redox battery”). In addition, based on our field study along near- to offshore transect in the North Sea, we found that the most redox active iron trapped abundant of dissolved OM (54±20 times than DOM in porewater) that enriched in aromatic and oxygen-rich compounds. It indicates that iron may preferentially promote the retention of terrigenous and aromatic DOM in permeable sediments, thus serving as an important temporal storage for terrigenous OM in the coastal ocean. Further investigations of the dynamic Fe-OM interactions in coastal sediments are warranted to better understand carbon cycling in the coastal area.