How is ash dispersed in the ocean around volcanoes?

海洋中的岛屿火山爆发记录通常比陆地上保存的更完整。亚表层沉积物更容易受到侵蚀和（或）掩埋，大体积喷发中几乎所有的物质都将被输送到海洋中。因此，最完整的火山爆发年表可以从海洋沉积物岩心中获得，通常是从细灰层中获得。海洋火山灰有三个主要来源：火山喷发柱的沉降物、进入海洋的火山碎屑流以及火山喷发之间河流或火山喷发引发的泥石流（火山泥流）的再加工物质。我们需要能够清楚地区分这三种不同类型的火山灰，以便准确地重建火山爆发的时间表，以及火山灰层记录的火山爆发类型。准确地确定过去火山爆发的顺序对于评估未来的火山灾害和测试火山动力学模型非常重要。了解火山喷发所产生的火山灰层的空间范围，以及这种范围与风或水流方向的关系，也很重要。什么时候在一个特定的核心位置没有火山灰意味着邻近的火山岛没有发生喷发？该项目的目的是记录细火山灰如何在海洋中分散，以便更好地了解海洋沉积序列如何记录具有潜在危险的爆炸性火山爆发。1995-2007年西印度群岛蒙特塞拉特岛苏弗里耶尔火山爆发，为了解火山沉积物在海洋中的分散过程提供了一个出色的自然实验室。现场是突出的，因为陆上喷发序列已被监测，可以说是前所未有的细节。在这些火山爆发期间将沉积物输送到海洋中的陆地事件的特征和年代，以及它们进入海洋的位置，以及风和洋流的方向，都是众所周知的。喷发物质流量的时间和空间变化也受到了很好的限制，据估计，自1995年以来喷发的0.7立方公里岩浆中有90%以上已被输送到海洋中。除细灰成分外，与这些喷发有关的粗粒海洋沉积物的特征和分布也得到了较好的研究。2005年，NERC资助的一次航行绘制了进入海洋的火山碎屑流以及随后产生的海底颗粒流和浊流的粗颗粒沉积物的地图并取芯。然而，2005年巡航期间使用的振动取芯技术的局限性阻止了对最近喷发的细灰进行采样，因为它在海底形成了一个很容易丢失的“糊状”层。我们现在计划使用箱形岩芯系统地对近地表火山灰进行采样，这些岩芯是在2007年12月NERC资助的一次即将到来的巡航中收集的。这将是第一项全面记录爆发性岛屿火山周围不同火山灰类型分布的研究，并将这种分布与独立的非常严格的喷发序列进行比较。该项目还将为正在进行的NERC资助的研究增加价值，目前正在确定火山灰是否会显着改变海水化学，并分析火山灰沉积的生物反应。

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.