How is ash dispersed in the ocean around volcanoes?

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The marine record of explosive eruptions on island volcanoes is typically more complete than that preserved on land. Subaerial deposits are more prone to erosion and (or) burial, and almost all of the material in large volume eruptions will be transported into the ocean. The most complete eruption chronologies can therefore be derived from marine sediment cores, typically from layers of fine ash. There are three main sources for marine ash; fall out from eruption columns, pyroclastic flows that enter the ocean, and reworked material from rivers or rainfall-triggered debris flows (lahars) in periods between eruptions. We need to be able to distinguish clearly between these three different types of ash in order to reconstruct accurately the chronology of eruptions, and the type of eruption that an ash layer records. Accurately defining the sequence of past eruptions is important for assessing future volcanic hazards, and testing models of volcano dynamics. Understanding the spatial extent of ash layers associated with eruptions, and how this extent is related to wind or current directions, is also important. When does the lack of ash in a particular core location imply that no eruption occurred on the adjacent volcanic island? The aim of this project is to document how fine volcanic ash is dispersed within the ocean in order to better understand how marine sedimentary sequences record potentially hazardous explosive volcanic eruptions. The 1995-2007 eruptions of the Soufrière Hills volcano on the island of Montserrat in the West Indies provide an outstanding natural laboratory for understanding processes that disperse volcanogenic sediment in the ocean. The field site is outstanding because the subaerial eruption sequence has been monitored in arguably unprecedented detail. The character and chronology of terrestrial events that transported sediment into the ocean during these eruptions is unusually well known, together with the location of their entry point into the sea, and the direction of wind and ocean currents. Volumes and temporal changes in fluxes of erupted material are also well constrained for individual eruptions, and it is estimated that over 90% of the 0.7 km3 of magma erupted since 1995 has been transported into the ocean. With the exception of the fine ash component, the character and distribution of coarse-grained marine deposits associated with these eruptions is also relatively well studied. A NERC funded cruise in 2005 mapped and cored coarser-grained deposits from pyroclastic flows that entered the ocean, and the submarine granular flows and turbidity currents that were subsequently generated. However, limitations in the vibro-coring technique used during the 2005 cruise prevented sampling of fine ash from the most recent eruptions, as it forms a 'soupy' layer on the sea floor that was easily lost. We now plan to sample systematically this near surface ash using box cores collected during an upcoming cruise already funded by NERC in December 2007. This would be the first study to document comprehensively the distribution of different ash types around an explosive island volcano, and to compare that distribution to an independently very well constrained eruption sequence. This project will also add value to ongoing NERC funded research currently determining whether ash digenesis significantly alters seawater chemistry, and work analysing the biological response to ash deposition.