Importance of Microbial Iron Reduction in River-Dominated Continental Margin Sediments

Martial Taillefert1, Jordon Scott Beckler2,3, Eryn Melissa Eitel1, Shannon Marie Owings4, Joel D Craig5, Ben Fields5, Cecile Cathalot6, Jens Rassmann7, Bruno Bombled7, Rudolph Corvaisier8, Panagiotis Michalopoulos9, Donald B Nuzzio10 and Christophe Rabouille7, (1)Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA, United States, (2)Mote Laboratory, Ocean Technology Research, Sarasota, FL, United States, (3)Mote Marine Laboratory, Ocean Technology group, Sarasota, FL, United States, (4)Georgia Institue of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA, United States, (5)Georgia Institue of Technology, School of Earth and Atmospheric Sciences, Atlanta, United States, (6)IFREMER, REM/GM/LCG, Plouzané, France, (7)Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS-UVSQ et IPSL, Gif sur Yvette, France, (8)CNRS-UBO, Institut Universitaire Europeen de la Mer, Laboratoire des Sciences de l’Environnement Marin, Plousane, France, (9)Hellenic Center for Marine Research, Anavyssos, Greece, (10)Analytical Instrument Systems, Inc., Flemington, NJ, United States
Abstract:
Remineralization of organic carbon in continental margin sediments exposed to fast deposition processes is thought to proceed primarily via aerobic respiration and sulfate reduction because the supply of nitrate and metal oxides is not usually significant in deep-sea sediments. Dissimilatory metal reduction, on the other hand, may represent a dominant pathway in coastal and continental shelf sediments where delivery of terrigenous Fe(III) and Mn(IV/III) oxides is sufficiently high or mixing processes near the sediment-water interface recycle these minerals efficiently. Passive continental margin sediments receiving outflow from large rivers are well-known deposition centers for organic carbon, but may also be hot spots for metal-reducing microbial activity considering the simultaneous high deposition rates of unconsolidated metal oxides of terrigenous origin. Interestingly, only a few studies have examined the role of microbial metal reduction in carbon remineralization processes in these environments. In this study, a combination of in situ depth profiles, benthic flux measurements, and ex situ measurements in the Rhône River Delta (< 80 m), Cape Hatteras slope (< 700 m), Louisiana slope (<1,800 m), and Congo River fan (~5,000 m) sediments are compared to assess the main redox species involved in early diagenesis. Metal reduction dominated carbon remineralization processes in the top ~20 cm of sediment subject to high deposition, while evidence for sulfate reduction was lacking. These findings suggest that dissimilatory Fe(III) reduction may be more significant than previously thought in continental slope sediments, which may have important implications on carbon cycling in marine environments.