The formation of widespread volcanic ash-fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra (20–180 µm diameter) as it travels from the atmosphere into water. Using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of Mount St. Helens (0.2 g/cm2/hr), the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents. The currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large. At the air-water interface, the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface. The implication of the experiments is that deposition of distal tephra fall layers in deep-sea sediments may be dominantly controlled by diffuse vertical gravity currents, as opposed to passive settling of individual particles through the water column. This process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers. Support for this mechanism comes from observations of greatly accelerated tephra settling rates in the South China Sea following the 1991 eruption of Mount Pinatubo in the Philippines.

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