Uptake of dissolved oxygen during marine diagenesis of fresh volcanic material

Convergent plate volcanism typically occurs close to the oceans, hence a high proportion of fresh, highly reactive, volcanogenic material is rapidly deposited onto the seafloor. Previous studies (Haeckel et al., 2001) have shown that dissolved oxygen (O2) is extensively depleted in the pore waters of ash deposited in the South China Sea from the 1991 Pinatubo eruption. Here, we report the results of an extensive field, laboratory and modelling study of dissolved O2 concentrations and ancillary geochemical data (pore water NO2 + NO3 and solid phase FeII/FeIII and organic carbon) in the sediments surrounding the volcanic island of Montserrat, Lesser Antilles. Dissolved O2 is depleted to zero within 0.3 cm of the sediment–water interface in sites containing the thickest layers of volcanogenic material (35 cm), compared to a penetration depth of ∼6 cm in sites with minimal ash loading of <0.5 cm. Experimental studies using volcanogenic sediment in a flow-through cell obtained similar O2 consumption rates to those observed in studies of individual minerals and basalt (White and Yee, 1985). These results, and comparison with other geochemical data, lead us to conclude that the dominant mechanism for dissolved O2 uptake in volcanogenic sediments is oxidation of silicate-bound FeII by a coupled electron transfer reaction. The observation that rapid dissolved O2 uptake by volcanogenic sediments is a ubiquitous feature of deposition of fresh volcanic material in the oceans may have global implications. While the global amount of dissolved O2 consumed by this process is trivial compared to that resulting from oxidation of organic carbon, the widespread deposition of volcanic ash from massive explosive eruptions may lead to enhanced preservation of organic carbon in marine sediments and thus lowering of atmospheric CO2 concentrations during critical periods in Earth history.