Vertical density currents: a review of their potential role in the deposition and interpretation of deep-sea ash layers

Marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology. Recent experiments, and field data from the 1991 Pinatubo eruption, challenge tacit assumptions that, following atmospheric fallout on the ocean surface, marine sedimentation of ash-grade material occurs by Stokes Law settling of individual grains. Laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates: further experiments, described herein, involving sustained particle fluxes entering stratified environments, demonstrate that ash particles can decouple from the transporting fluid, whose vertical motions are limited by the stable density gradient. In the ocean, vertical density currents generated by ash-loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1-3 orders of magnitude faster than possible by Stokesian settling, greatly reducing their residence time. Sea-bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column, plus suspension fallout from the polydisperse turbid layer delivered by convective plumes, yielding normal distribution grading. The turbid layer may continue laterally as a (nepheloid) current under the influence of bottom slopes and/or geostrophic forces, either mechanism possibly modifying the grain-size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations. Consequently, and regardless of any subsequent reworking, deep-sea ash isopachs may not directly reflect an eruptive signal, and the ash layers may not directly record the order, rate, or location of arrival of ash particles at the ocean surface.