Bryan, S.E., Cook, A., Evans, J.P., Colls, P.W., Wells, M.G., Lawrence, M.G., Jell, J.S., Greig, A. and Leslie, R. (2004) Pumice rafting and faunal dispersion during 2001-2002 in the southwest Pacific: record of a dacitic submarine explosive eruption from Tonga. Earth and Planetary Science Letters, 227(1-2), pp. 135-154. ISSN (print) 0012-821x

Abstract

A new influx of sea-rafted pumice reached the eastern coast of Australia in October 2002, approximately 1 year after a felsic, shallow-marine explosive eruption at a previously unknown volcano (0403-091) along the Tofua volcanic arc (Tonga). The eruption produced floating pumice rafts that first became stranded in Fiji in November 2001, approximately 1 month after the eruption. Strandings of sea-rafted pumice along shorelines have been the only record of products from this submarine explosive eruption at the remote, submerged volcano. Computed drift trajectories of the sea-rafted pumice using numerical models of southwest Pacific surface wind fields and ocean currents indicate two cyclonic systems disturbed the drift of pumice to eastern Australia, as well as the importance of the combined wave and direct wind effect on pumice trajectory. Pumice became stranded along at least two-thirds (>2000 km) of the coastline of eastern Australia being deposited on beaches during a sustained period of fresh onshore winds. Typical amounts of pumice initially stranded on beaches were 500–4000 individual clasts per m2, and a minimum volume estimate of pumice that arrived to eastern Australia is 1.25×105 m3. Pumice was beached below maximum tidal/storm surge levels and was quickly reworked back into the ocean, such that the concentration of beached pumice rapidly dissipated within weeks of the initial stranding, and little record of this stranding event now exists. Most stranded pumice clasts ranged in size from 2 to 5 cm in diameter; the largest measured clasts were 10 cm in Australia and 20 cm in Fiji. The pumice has a low phenocryst content (<5% modal), containing the assemblage of calcic plagioclase (An88−74), augite (En35Fs29Wo36), pigeonite (En45Fs46Wo9), and titanomagnetite. Examined pumice clasts are compositionally homogenous, although there is considerable variation in clast vesicularity, both within and between clasts. The pumice composition is low-K dacite (65–68 wt.% SiO2) and similar to other pumice-forming eruptions from the Tonga region. Most clasts stranded along eastern Australia were fouled by a variety of organisms, whereas pumice stranded on beaches in Fiji 1 month after the eruption was free of binding organisms. The dominant rafted organisms in order of abundance were algae, goose barnacles (Lepas sp.), serpulid worms, calcareous algae, bryozoans, corals (Pocillopora sp., Porites sp., Cyphastrea sp.), oysters (Saccostrea sp.), and gastropods (Janthina sp.). The size of some rafted corals implied growth of at least 12 months in viable water. The abundance and variety of fouling taxa, coupled with the long dispersal trajectory (>3500 km) and period of pumice floatation (≥1 year), confirm the importance of sea-rafted pumice as a long-distance dispersal mechanism for marine organisms including marine pests and harmful invasive species. Billions of individual rafting pumice clasts can be generated in a single small-volume eruption, such as observed here, and the geological implications for the transport of sessile taxa over large distances are significant. An avenue for future research is to examine whether speciation events and volcanicity are linked; the periodic development of globalism for some taxa (e.g., corals, gastropods, bryozoa) may correlate in time and/or space with voluminous silicic igneous events capable of producing >106 km3 of silicic pumice-rich pyroclastic material and emplaced into ocean basins.

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