Dissolved oxygen and suspended particles regulate the benthic flux of iron from continental margins

We present ex situ sediment incubation results from the California and Oregon shelves and compare the calculated benthic flux of dissolved Fe with those from in situ incubations and pore water concentration profiles. We also examine the influence of oxygen depletion and sediment re-suspension on benthic Fe exchange. Ex situ incubation of the California and Oregon shelf sites yielded average benthic Fe fluxes of 3.5 and 8.3 mu mol m(-2) day(-1), respectively, compared to 17 and 55 mu mol m(-2) day(-1) from the in situ Lander determinations, and 73 and 103 mu mol m(-2) day(-1) from modeling of pore water concentration profiles. Differences between benthic Fe flux estimates are primarily accounted for by [1] differences in Fe (II) oxidation kinetics, which result from distinct oxygen consumption rates between incubation methods, and the absence of kinetic considerations in the overlying bottom water in pore water flux calculations, and [2] the effects of biological sediment irrigation that are best represented by in situ incubations due to their sampling area and preservation of bottom water conditions. Bottom water oxygen concentrations were higher at the California shelf site than that at the Oregon shelf site, and probably accounted for the greater discrepancy between methods used to determine benthic Fe flux. The comparison of techniques used to determine benthic Fe flux indicates that the concentration of bottom water oxygen exerts a principle control over the fate of dissolved Fe entering the overlying bottom water - supporting the view that the expansion of coastal hypoxia has the potential to enhance the benthic supply of Fe (II) to shelf waters. An episode of surface sediment re-suspension during ex situ incubation led to a rapid removal of 76-89% of dissolved Fe from seawater, followed by a steady return towards initial seawater concentrations during particle settling, indicating that diffusive inputs of dissolved Fe from sediment pore water are rapidly adsorbed and desorbed by particles during periods of benthic re-suspension. The findings suggest that dissolved Fe concentrations in bottom waters may reflect an equilibrium concentration of non-stabilized aqueous Fe and particle-adsorbed Fe phases - where the addition of suspended particles to bottom waters leads to scavenging of dissolved Fe into labile particulate Fe phases. Thus we suggest that suspended particles are a significant buffer of dissolved Fe released from shelf sediments, an important transport mechanism for benthic Fe inputs, and a regulator of dissolved Fe concentrations in seawater. (C) 2012 Elsevier B.V. All rights reserved.