Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria

We report benthic flux estimates of Fe from China coastal seas based on a recently developed 224Ra/228Th disequilibrium approach. There were considerable temporal and spatial variabilities in benthic Fe fluxes, which spanned over 4–5 orders of magnitude, from <10 µmol m−2 d−1 up to ∼100 mmol m−2 d−1. Nonetheless, we have identified a prominent trend in China coastal seas showing that benthic Fe fluxes tended to decline exponentially with bottom depth. This trend is probably a result of kinetic energy and chemically bound energy gradients that culminate in the coastal seas. In the meantime, large benthic Fe fluxes were more frequently observed within a narrow redox window of 100–150 μM dissolved oxygen (DO) in the bottom water. This observation contradicts an early assumption that iron release from continental margin sediments increases with decreasing DO concentration in the bottom water. It possibly reflects a compromise of oxygen-demanded irrigation that acts to enhance Fe release from bottom sediments, and re-oxidation of dissolved Fe(II) that results in more efficient Fe retention near the sediment-water interface. Notably, benthic fluxes of Fe based on 224Ra/228Th disequilibria were one order of magnitude larger than those derived from the porewater concentration gradient method. Moreover, the isotopic estimates were >30 times higher than the historical measurements based on the traditional incubation method (i.e., the benthic chamber method) in other coastal seas. However, our estimates of benthic Fe flux were in general agreement with the reported flux value based on a new two dimensional imaging technique.