Atmospherically transported volcanic glass in deep-sea sediments: Theoretical considerations

Abstract

Measurement of long-term fluctuation in the volcanically derived atmospheric dust load requires isolation of the regional accumulations caused by relatively local eruptions. Only then will it be possible to evaluate the contribution of volcanic dust to past global climatic changes and to changes in marine fauna and chemistry. As a means of delineating the regional effects of such eruptions, a series of experiments have been initiated involving the separation and counting of volcanic dust particles deposited in deep-sea sediments downwind from known eruptions during the past 2 m.y. To provide working limits for the series of analyses and to facilitate the design of efficient associated piston coring programs, a simple model has been developed relating distance from source to volcanic dust particle size for volcanic eruptions of various explosive intensities and for various upper atmospheric transporting powers (wind strengths). It is shown that paleowind velocity determinations can be made by measuring the downwind increase in the width of the dust cloud fallout pattern. By using the measured paleowind it is then possible to employ measurable critical distances of selected size fractions from known eruptive sources to derive the height of the associated volcanic cloud top and base, from which the net paleoexplosivity can be estimated. Less precise estimates can be made if it is necessary to assume paleowind velocities. Given a traverse of cores taken at unknown distances from the parent volcanos, it is possible to derive the distances to the sources. For a single sampling locality, relative eruptive intensities of a known source over a long period can be estimated. It is shown that the most efficient use of ship time would be the taking of two coring traverses broadside to the fallout direction at distances up to 3000 km from the source (to facilitate paleowind velocity estimates) and a single coring traverse along the fallout axis (to derive the height of the volcanic cloud top and base).