Sulfur, Iron, and Manganese Speciation in Anoxic Sediments with Methane (Ría de Vigo, NW Spain)

High sedimentation rates and high organic matter contents promote the development of anoxic conditions in the Ria de Vigo (NW Spain). In these anoxic environments, elements such as S, Fe, and Mn are involved in redox processes affecting the CH4 dynamic. In this work, speciation of S, Fe, and Mn was evaluated through a five-step sequential extraction procedure (exchangeable form and bound to carbonate, present in the reductive phase bound to Fe/Mn oxides, weakly bound to organic matter, strongly bound to organic matter, and residual fractions) in order to identify the origin of S, Fe, and Mn, distinguishing between fractions bound to organic compounds from those in the sulfide phase in these anoxic sediments. The study was conducted on samples from three gravity cores retrieved in the inner and outer zones of the ria in November 2012. S was mainly found in the fraction strongly bound to organic matter, showing that S is incorporated in the organic matter. However, the S in this fraction was associated with total organic carbon only in the inner zone (r = 0.833). More than 65% of total Fe and Mn was found in the residual fraction at all zones, being the highest in the outer zone, and showing an intense diagenesis in the ria, where precipitation of these elements as sulfides is favored by anoxic conditions. SO42- reduction, Fe and Mn reduction processes coexist in the same area within the cores.