Grain size, areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea
The June 15, 1991 climactic eruption of Mt. Pinatubo produced an extensive, largely co-ignimbrite-derived airfall ash layer on Luzon Island and across the central South China Sea. The layer covers an area of ~4×105 km2 with a volume of 5.5 km3. Near the coast of Luzon, the deposit consists of two units: a normally graded basal ash bed, unimodal in grain size, and a finer-grained, internally structureless upper ash bed showing grain size bimodality. With increasing distance from the source, the coarse particle populations of the two units merge and migrate towards a near-constant fine population (~11 μm); the distal region is covered by a fine-mode dominated, virtually ungraded single ash layer. The reversal of the winds from easterly directions at upper-tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse- and fine-mode components fell out from high-altitude eruption clouds. The high-velocity upper-level winds, however, would have transported fine-grained ash particles far beyond the South China Sea, which suggests that their settling was accelerated by aggregation. The boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling, coarse-grained particles. The particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper-level winds. At lower elevations, the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward, producing a thickness maximum in the medial region (at about 160 km from source). The strong displacement of fines, possibly aided by wind turbulence, led to a break in bulk tephra thinning rates close to the coastline. In the distal region, outside the influence of the typhoon, southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry. Within this region, at about 420 km from source, fallout of particle aggregates created a second thickness maximum. Comparison of the field data with previous experimental observations and tephra flux records in the deep sea (Wiesner et al. 1995; Carey 1997; McCool 2002) implies that the transport of ash in the water column was largely determined by vertical density currents. Differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface. Considering typical fall rates of convective plumes, modifications of the initial fallout position of the particles by the South China Sea current system are on the order of only a few kilometers. The results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions, distances, and fallout modes in the deep sea.