The impact of the 1991 Mount Pinatubo tephra fallout on the geochemical environment of the deep-sea sediments in the South China Sea

Seven years after the 1991 Mount Pinatubo eruption, the current geochemical environment of ash-covered deep-sea sediments in the South China Sea was investigated. The depth distributions of O2, NO3−, Mn2+, Corg and porosity at six representative sites covered with varying thicknesses of tephra are reported. The shapes of the depth profiles reveal that O2 is totally exhausted within ash layers thicker than 3 cm, while thinner layers are penetrated by oxygen followed by linear downcore profiles with depletion between 8 and 12 cm. Hence, the zone between ash layer and total depletion of oxygen represents a zone of negligible oxygen consumption. Various multi-layer models were developed to explain the observed oxygen concentration profiles. These mathematical simulations also serve to assess both the development since the surface sediments were sealed by the ash fallout as well as the future evolution. Whereas thick tephra layers provoke a nearly stationary situation, i.e. are capable of preventing oxygen from penetrating into the underlying sediments, oxygenation of sediments below thin layers increases rapidly. In these sediments, the effect of oxygen diffusion velocity is drastically reduced by the reaction with dissolved manganese, and therefore explains the linear profiles. Besides, this study is suitable as a case study to assess the effects of human interventions in the deep-sea ecosystem, such as mining and waste dumping.