Manganese diagenesis in different geochemical environments of the ria de Vigo (Galicia, NW Iberian Peninsula)

Manganese is one of the most abundant elements in marine sediments and plays an essential role in sediment redox processes. However, unlike Fe or S, the Mn cycle has not received the same attention. Mn is a ubiquitous redox-active metal in marine systems; however, its influence on C and S cycling is still poorly understood. In particular, we hypothesize that conditions that favor high H2S production can lead to high Mn pyritization. In the Ria de Vigo, large methane fields have been identified at different depths within sediments, ranging from the surface to 2 m below the surface. Four sediment cores from different locations (outermost, middle, and innermost) within the Ria de Vigo were analyzed. Samples were subjected to a general characterization and a five-step Mn sequential extraction procedure, and analyses were complemented with X-ray absorption spectroscopy (EXAS). The results showed that the main geochemical forms of Mn undergo intense spatial and depth-related variations in sediments. Two geochemical scenarios were identified: one corresponding to the innermost section and another one to the middle and outermost sections of the ria. The former was characterized by intense Mn pyritization and by the absence of the Mn-carbonate fraction due to the high production of H2S because of anaerobic oxidation of methane. The formation of Mn-S bonds was only identified by EXAS. Conversely, in the middle and outermost part of the ria, with or without the presence of methanogenesis in deep sediment layers, the Mn-carbonate fraction was dominant even at depth, along with the presence of methane, high concentrations of H2S and, therefore, high degrees of Fe pyritization. These results suggest that, once Mn-carbonate is formed under suboxic conditions (with low or no presence of H2S) at or near the surface, it remains stable after burial, even under conditions of high H2S concentration.