The effect of tidal forcing on iron cycling in intertidal salt marsh sediments
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In this study we investigated the effect of tidal forcing on iron cycling in intertidal saltmarsh sediments (ISS). Historically, sulfate has been considered the major terminal electron acceptor involved in organic carbon remineralization in ISS. Although sulfate is a more efficient electron acceptor for organic matter degradation in anoxic ISS, irons rapid recycling at the surface of ISS may allow it also to be an important electron acceptor for the remineralization of organic matter. Bioturbation, macrophyte-mediated irrigation, and semidiurnal tidal forcing in this environment may increase the abundance of O2 in the top few cm of the sediment, rapidly oxidizing iron and inhibiting sulfate reduction. To determine if the cycling of iron may be faster than previously thought in these sediments, we combined sediment core chemical profiles of reduced and oxidized insoluble iron with in-situ electrochemical profiles of O2, Fe2+, soluble organic-Fe3+ complexes, FeS(aq), and hydrogen sulfide in the top few centimeters of unvegetated creek bank sediments over several tidal cycles. We also installed monitoring wells in the tidal creek bank to quantify tidal forcing and to investigate tidal direction in the sediments. We built a transient, reactive transport model to simulate measured geochemical profiles and test our understanding of diagenetic processes. Additional tests were run on the model to investigate the importance of bioirrigation compared to tidally-induced porewater advection. Results indicate that tidal action is a more dominant transport process. It affects the cycling of iron in ISS by flushing reduced species out of the sediment during flood tide, and allowing oxygen and oxidized species deeper into the sediment during ebb tide. As a result, amorphous iron oxides are replenished at the sediment surface, and microbial iron reduction may be the main respiratory process in the first tens of centimeters of creek bank saltmarsh sediments subjected to intense tidal forcing.