Interpreting the Dispersal Characteristics of Widespread Volcanic Ash Marker Horizons in the Geologic Record

广泛分布的火山灰层在地质记录中形成了重要的年代地层，在火山学、古气候、大气环流和放射性测年等研究中起着至关重要的作用。它们构成的事件视界允许在数万平方公里的空间域中进行精确的时间关联。它们形成的许多方面已经被很好地理解，但关于过去发生的最大规模喷发的火山灰的扩散和沉积的关键问题仍然存在。它们的分布模式通常在明显的逆风方向上显著延伸，它们的宽度相对于历史上较小的沉降物来说是宽的。控制广泛灰层形成的因素将使用拉格朗日粒子跟踪模型进行研究，该模型最初是根据污染扩散规范制定的。该模型将被修改为1)考虑大尺度伞状云的火山灰注入，2)将粒子沉降变化作为雷诺数的函数，以及3)考虑平流层运输，包括典型的风变化作为高度和时间的函数。将对1991年菲律宾皮纳图博火山喷发进行模拟，以评估该模式的预测能力。模型结果将与有关喷发羽流运动和火山灰沉积面积的观测数据进行比较。另外还将对印度尼西亚多巴火山的两次喷发进行模拟（最年轻的多巴凝灰岩和最古老的多巴凝灰岩事件）。这些模拟的结果将用于评估对流层和平流层运输在产生整个印度洋和南中国海沉积物中发现的异常广泛的温层分布中的作用。

Interpreting the dispersal characteristics of widespread volcanic ash markerhorizons in the geologic recordSteven Carey, University of Rhode IslandEAR-0545112Abstract Widespread volcanic ash layers form important chrononstratigraphic horizons in the geologic record and have been critical in studies related to volcanology, paleoclimate, atmospheric circulation, and radiometric dating. They constitute event horizons that allow for precise temporal correlation across spatial domains of several tens of thousands of km2. Many aspects of their formation are well understood but critical questions remain about the dispersal and deposition of volcanic ash from the very largest eruptions that have occurred in the past. Their distribution patterns often extend significantly in an apparent upwind direction and their widths are wide relative to smaller historic examples of fallout. The factors that control the formation of widespread ash horizons will be investigated using a Lagrangian particle tracking model that was originally formulated from a pollution dispersal code. The model will be modified to 1) account for injection of volcanic ash from large-scale umbrella clouds, 2) treat particle settling variations as functions of the Reynolds number, and 3) account for stratospheric transport, including the typical variation of wind as a function of altitude and time. Simulations will be carried out on the 1991 eruption of Mt. Pinatubo in the Philippines in order to evaluate the predictive capability of the model. Model results will be compared with observational data on the movement of the eruption plume and the area of ash deposition. Additional simulations will be carried out on two eruptions of the Toba caldera in Indonesia (Youngest Toba Tuff and Oldest Toba Tuff events). Results of these simulations will be used to assess the role of tropospheric versus stratospheric transport in producing the exceptionally wide distribution of the tephra layers that are found in sediments both throughout the Indian Ocean and South China Sea.