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 i nvestigated. The depth distributions of O-2, NO3-, Mn2+, C-org and porosity at six representative sites covered with varying thicknesses of tephra are reported. The shapes of the depth profiles reveal that O-2 is totally exha usted within ash layers thicker than 3 cm, while thinner layers are penetra ted 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 mo dels were developed to explain the observed oxygen concentration profiles. These mathematical simulations also serve to assess both the development si nce the surface sediments were sealed by the ash fallout as well as the fut ure evolution. Whereas thick tephra layers provoke a nearly stationary situ ation, i.e. are capable of preventing oxygen from penetrating into the unde rlying sediments, oxygenation of sediments below thin layers increases rapi dly. In these sediments, the effect of oxygen diffusion velocity is drastic ally reduced by the reaction with dissolved manganese, and therefore explai ns 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 a s mining and waste dumping. (C) 2001 Elsevier Science B.V. All rights reser ved.