Computer simulation of transport and deposition of the campanian Y-5 ash

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Analyses of grain-size and modal composition of the Campanian tuff ash layer (Y-5) from 11 deep-sea cores have been carried out. This layer represents ash fall that has been correlated with the 38,000 y.b.p. Campanian ignimbrite (Thunell et al., 1979), a deposit formed by the largest eruption documented in the Mediterranean region during the late Pleistocene (Barberi et al., 1978). The bulk deposit is bimodal in grain-size and dominated by glass shards. The calculated mean grain-size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near-constant fine mode of approximately 13 microns. Distal samples are unimodal in grain-size. These data combined with a set of vertical profiles of wind (10 year average) have been used as input to a computer model that simulates fallout of tephra. Modelling indicates that the downwind variation of grain-size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km. The observed fine mode of the deposit cannot, however, be generated by transport of ash as individual particles at these elevations. Such transport would result in deposition of virtually all of the fine ash beyond the studied area. Deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co-ignimbrite ash clouds with a maximum elevation of 3 km. The large volume of ash in the fine mode (>70 wt.%) and the irregularity in azimuth of low-level winds argue against major low-level transport of co-ignimbrite ash. Rather, the ash may have been derived from both a plinian eruption column and high-altitude clouds of co-ignimbrite ash, with settling of fine ash as particle aggregates. © 1983.

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Journal of Volcanology and Geothermal Research