The preservation potential of ash layers in the deep-sea : the example of the 1991-Pinatubo ash in the South China Sea

Following the eruption of Mt. Pinatubo on June 15th 1991, volcanic ash was transported westward to the South China Sea in an atmospheric plume, falling out and and settling to the seafloor within days and forming an up to 10 cm thick layer on an area <400 000 km2. Immediately after deposition, surviving deep-burrowing animals re-opened their connection to the seafloor to obtain water for respiration and/or food take-up. Later, small-sized meiofauna and then macrofauna re-colonized the seafloor, mixing newly deposited organic fluff with the underlying ash. Consequently, ash deposits thinner than 1 mm have not been observed often as continuous layer when cored the first time 6 years after the eruption, while ash about 2 mm thick is now patchily bioturbated. In areas covered by ash thicker than 5 mm, mixing by benthic animals is mainly controlled by the adaptation of the burrowing fauna to variations in the grain-size, the rate of background sedimentation, the availability of benthic food on and within the sediment, and pore water oxygen levels. With respect to these factors four provinces can be distinguished: ^