EXAFS analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed (Vavouto Bay, New Caledonia)

Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between massive Fe lateritic deposits and a lagoon partly registered by UNESCO as a World Heritage site. The New Caledonian mangroves are characterized by a botanical gradient composed of three main vegetal stands (i.e., Rhizophora spp., Avicennia marina and salt flat), which relies on the duration of tidal immersion that imposes gradients of pore-water salinity, oxygenation, and organic content in the sediment. In the present study, we have determined the distribution and speciation of Fe in mangrove sediments along this botanical gradient by using X-ray absorption spectroscopy (XAS) at the Fe K-edge. Both XANES and EXAFS results show that iron speciation strongly follows the redox boundaries marking the intertidal and depth zonations. Fe-bearing minerals eroded from lateritic outcrops, mainly goethite (alpha-FeOOH) and phyllosilicates (serpentine and talc), are the major Fe hosts in the upward horizons. These mineral species progressively disappear with increasing depth where pyrite (FeS2) forms, in the hydromorphic Rhizophora and Avicennia zones. Sulfate reduction is not observed in the drier salt flat zone. In addition to these reduction processes, intense re-oxidation of aqueous Fe(II) and pyrite leads to the formation of poorly ordered ferrihydrite, lepidocrocite (gamma-FeOOH) and likely goethite in the upper sediments beneath Avicennia and Rhizophora stands. The relative proportion of the newly formed poorly ordered ferrihydrite and lepidocrocite is found to be higher in the Rhizophora mangrove stand, which is the closest to the shore. Tidal fluctuations may thus be a major cause for continuous Fe reduction-oxidation cycles in the vegetated mangrove stands, which could significantly affect the iron mass balance in mangrove systems