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

Abstract

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.

Grain-size distribution of volcaniclastic rocks 2: Characterizing grain size and hydraulic sorting

“…The mechanisms of transport for subaqueous volcaniclastic density currents are poorly understood in comparison to their siliciclastic homologues (Manville et al, 1998;White, 2000;Manville et al, 2002;Freundt, 2003;Manville and Wilson, 2004;Allen and Freundt, 2006;Talling et al, 2012), as they commonly contain coarse clasts of varying density (pumice or scoria), which are much less dense than conventional dense (i.e. non to poorly-vesicular) clasts.…”

Construction of volcanic records from marine sediment cores: A review and case study (Montserrat, West Indies)

“…Tephra deposited in the ocean typically sinks at ~1-3 orders of magnitude faster than Stokes-Law settling due to the generation of vertical density currents (Carey, 1997;Manville and Wilson, 2004;Kandlbauer et al, 2013). This process leads to tephra deposits commonly exhibiting sharp bases in sediment cores, with bioturbated or gradational upper contacts.…”

“…For example, tephra fallout deposits from the Campanian Ignimbrite, in the Mediterranean Sea, suggests tephra thickness varies according to the surrounding bathymetry (Engwell et al, 2014), and because submarine flows are commonly channelized in depressions, fallout tephras are more likely to survive erosion when they are emplaced on bathymetric highs. In less dynamic environments tephra fallout deposits in marine sediments are commonly normally-graded as they reflect the different sinking velocities of the different eruptive particles through the water column until they reach the ocean floor (Manville and Wilson, 2004). Additionally, akin to subaerial deposits, structures within tephra fallout Page | 11 deposits reflect the duration of an explosive eruption.…”

“…In areas of high burial rate, deposits thicker than 0.5 cm may be widely preserved, but elsewhere even deposits several centimetres in thickness may become reworked and dispersed within the sedimentary sequence (Wetzel, 2009), forming diffuse cryptotephras. Bioturbation by benthic organisms or reworking by currents can result in patchy preservation (Hunt and Najman, 2003;Manville and Wilson, 2004) and the formation of tephra lenses or 'pods'. Cores may thus fail to sample an intact portion of the tephra deposit.…”

“…Cores may thus fail to sample an intact portion of the tephra deposit. Moderate bioturbation can be detected by a blotchy appearance of tephra in sediment cores, but reworking may be so extensive as to visually homogenise the tephra with the background sediments (Hunt and Najman, 2003;Manville and Wilson, 2004).…”