Iron reactivity in anoxic sediments in the Ría de Vigo (NW Spain)

Abstract The high sedimentation rates and high organic matter contents in the sediments of the Ria de Vigo (NW Spain) promote the development of anoxic conditions, determining the dynamics of elements like Fe and conditioning his speciation and reactivity. Four gravity cores were retrieved in anoxic sediments of the Ria de Vigo in November 2012. In order to understand the behavior of Fe in these complex environments different fractions of reactive iron were analyzed. The decrease in highly reactive iron and sulfide contents with depth showed the relationship between the iron and sulfur cycle in the middle and outer zones of the ria. In the inner zone, the apparition of shallow methane gas may cause the slower decrease of the highly reactive iron contents. In zones without methane, sediment layers enriched in iron −with a reactivity higher than in other sediment samples− were observed. An increase was observed in the dithionite and total reactive iron contents from the inner to the outer zone of the ria, according to the gas depth. Furthermore, a decrease in Fe (III)-bearing minerals contents with depth was observed in the outer and middle zones, but not in the innermost area where the gas is shallow. The high organic matter and sulfide contents, mainly in the inner zone of the ria, indicate that the most of the Fe (II) is FeS. Moreover, the high contents of total reactive iron and pH values (6.86–7.98) could contribute the formation of stable minerals like pyrite along the Ria de Vigo.