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

Journal of the Geological Society, Nov 2004 by Manville, V, Wilson, C J N

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.

Keywords: density currents, volcaniclastic sediments, deep-sea sedimentation, tephra, eruptions.

Explosive eruptions, particularly those involving magma-water interaction and/or production of large ignimbrites, can generate huge volumes of mostly ash-grade tephra that is widely transported in the atmosphere (Walker 1981). This material eventually settles out to form distinctive layers that can be correlated across thousands to millions of square kilometres and spanning different depositional environments to form chronostratigraphic markers (e.g. Drexler et al. 1980; Rose & Chesner 1987; Huff et al. 1996). Such tephra layers provide information on eruption dynamics and chronology as deduced from granulometric properties, and thickness and distribution patterns (e.g. Ninkovich et al. 1978; Wilson 2001). Although the atmospheric transport and deposition of subaerial tephra is relatively well understood (e.g. Sparks et al. 1997), less is known about mechanisms of tephra delivery to the deep ocean (see Carey & Sigurdsson 1980; Stow et al. 1998), where tephra layers form detailed long-term records of volcanism (Kennett 1981; Carter et al. 2003).

Early work, noting that deep-sea tephra layers are overwhelmingly composed of ash-grade material, assumed that on crossing the air-water interface, sedimentation of air-fall ash occurred by Stokes Law settling of individual particles (e.g. Fisher 1965; Ledbetter & Sparks 1979). However, this assumption is contradicted by many features of preserved deep-sea tephra layers, and challenged by theoretical and laboratory studies that demonstrate the potential role of vertical density currents in the rapid transfer of paniculate material through water columns (Bradley 1965; Carey 1997). Data from the 1991 Pinatubo eruption demonstrated oceanic ash transport at vertical velocities 1-3 orders of magnitude faster than possible by pure Stokes Law settling (Wiesner et al. 1995), and oceanographie studies have separately recognized rapid vertical water transport during episodic open-ocean convection (e.g. Marshall & Schott 1999). Oceanic transport and deposition of fine-grained volcaniclastic material probably involves a spectrum between the idealized vertical density currents arising from subaerial fallout of atmospherically transported ash on the ocean surface (as outlined in this paper) and the largely lateral motions of thermohaline, nepheloid and turbidity currents. Here we review and analyse the inception and propagation of particle-laden vertical density currents, and assess their role in the deposition, characteristics and interpretation of deep-sea tephra layers.

Deep-sea tephra layers

Characteristics

Deep-sea tephra layers are generally a few to tens of centimetres thick, with sharp bases and bioturbated and/or gradational upper contacts. Most consist of particles between a few and a few hundred microns in diameter: glass shards are the dominant particle type, with minor amounts of crystal and lithic fragments (Sigurdsson et al. 1980). Continuous layers

Normal vertical distribution grading by settling velocity, with coarse or dense particles concentrated near the base, is typical in deep-sea ash layers, but massive centres sometimes occur (Ninkovich el til. 1978; Sigurdsson el al. 1980; Sparks & Huang 1980). Sorting is often poor and bimodal, with a coarser mode that fines with distance from source and a finer mode that remains fairly constant (Sparks & Huang 1980): a pattern variously attributed to two-stage eruptions (Sparks & Walker 1977; Sparks & Huang 1980), or premature scavenging of finer particles from the atmospheric eruption plume (Carey & Sigurdsson 1982; Brazier et al. 1983). Deposit distributions crudely reflect syneruptive atmospheric dispersal patterns (e.g. Carey & Sigurdsson 1980; Cornell et al. 1983). and obvious modification by oceanic currents is limited to areas with strong flows (Ninkovich & Shacklcton 1975; Carter et al. 1995; Wallrabe-Adams & Lackeschwitz 2003).