A volcanic sedimentation model: Implications of processes and responses of deep-sea ashes

Abstract

Deep-sea ash layers and dispersed ashes in over 500 piston cores have been examined. The regions include the sub-Antarctic sector of the south Pacific (S.P.), east of New Zealand (N.Z.), east of the Azores (Az.), east and west of Central America (C.A.), the eastern Mediterranean (E.M.), and east and west of the Lesser Antilles (L.A.). Analysis of over 4,000 samples in 118 selected cores enables one to determine the ash concentration variations in the cores. Dispersed ash is usually present in greater volume than ashes in discrete layers. The records of these variations have helped to formulate a volcanic sedimentation model. Ash deposition is explained by step-like sedimentation which is generally an outcome of Markov-chain and renewal-type processes. Three types of renewal processes are recognized, each with certain characteristics: type I—forming discrete ash layers from single eruptions with little or no vertical mixing between two consecutive ash layers (most L.A. and E.M. cores); type II—producing zones of mixed ashes from frequent, close-spaced eruptions and/or synchronous eruptions from multiple chambers (Az and C.A.); and type III—having the characteristics of both types I and II (N.Z. and S.P.). The ash distributions were governed by volcanic, atmospheric, and marine processes and by submarine topography. The sedimentation rates and density of volcanoes in the source areas play a major role in determining which type is formed. Both submarine air-fall ash and subaqueous pyroclastic flow deposits from a single eruption may co-exist in sediment cores. Both ash types have their distinct textural and compositional characteristics. Regional high-altitude wind conditions controlled the extent and concentration of air-fall ash. Marine processes play a major role in the formation of different ash types. Oceanic circulation and differential settling affect the distributions of certain components such as pumice fragments. Post-depositional processes can lead to further redistribution. Seven major resedimentation processes were recognized: littoral transports, slumping, turbidity currents, debris- and grain-flows, bottom currents, bioturbations, and fragmentation and/or solution. Each produces an ash-type associated with certain textural and compositional characteristics. Vertical mixing of ashes by the resedimentation processes often blurred the core records.