The June 15, 1991 climactic eruption of Mt. Pinatubo produced an extensive, largely co-ignimbritederived airfall ash layer on Luzon Island and across the central South China Sea. The layer covers an area of 410 5 km 2 with a volume of 5.5 km 3 . Near the coast of Luzon, the deposit consists of two units: a normally graded basal ash bed, unimodal in grain size, and a finergrained, 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 mm); the distal region is covered by a fine-mode dominated, virtually ungraded single ash layer. The reversal of the winds from easterly directions at uppertropospheric 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 upperlevel winds, however, would have transported finegrained 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.

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SUMMARY39 dacitic pumice and lithic samples from the 1991 eruption of Mount Pinatubo were investigated through both magnetic and mineralogical means. As in a previous study, natural remanent magnetization (NRM) is found to be reversed for most of the samples, with respect to the direction of the actual geomagnetic field direction. A few samples, amongst them ancient lithics transported by pyroclastic flows, show scattered NRM directions. From thermal demagnetization of these particular samples it is concluded that their orientation changed after emplacement. The emplacement temperature is estimated to be more than 460°C from thermal demagnetization of lithic samples.Two magnetic minerals with large grain sizes are observed under the optical microscope: titanomagnetite (TM) and haemo-ilmenite ( hem-ilm). Microprobe analyses yield x#0.10 for TM and y#0.52 and y#0.54 for two hem-ilm phases, in agreement with the observed Curie temperatures (~480°C for TM and~250°C for hem-ilm). The hem-ilm particles display chemical zonation, which seems to be correlated with a change of the domain structure: typically, a ferrimagnetic (FM) phase with slightly higher titanium content is observed in the central part whilst the crystal margin, which is weakly ferromagnetic (WF, due to spin-canted antiferromagnetism) is slightly poorer in titanium. Two different mechanisms for the origin and formation of the two observed phases are discussed: (1) chemical zonation of hem-ilm crystals due to a change in conditions in the magma chamber shortly before eruption; (2) similar to the microstructures observed from synthetic samples, this zonation in the large natural hem-ilm could be the result of migration of the WF phase towards the grain boundary during residence below the order-disorder transition temperature in the magmatic chamber. The room temperature hysteresis loop, which seems to be dominated by TM, provides multidomain (MD)-like parameters: J rs /J s =0.01 and H cr /H c =20. The large coercivity of remanence (15-40 mT), which is attributed to hem-ilm, may be intrinsic or due to interactions between WF and FM phases. The field dependence of the magnitude of the thermoremanent magnetization (TRM) is not linear: it increases first, reaches a maximum (negative) value for an applied field H close to 0.5 mT, then decreases steadily. By extrapolation, it is estimated that the TRM should be zero for a field of about 12 mT and become positive beyond. This total TRM is in fact the sum of several components. AF demagnetization of TRMs acquired in different fields shows the presence of at least three components: a self-reversed (SR) component that contains both hard and moderately hard components and a soft normal component. Independently of the value of H, the median destructive field of the SR component is of the order of 159

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