Fallout of volcanic ash to the deep South China Sea induced by the 1991 eruption of Mount Pinatubo (Philippines)

An ongoing sediment trap experiment in the central South China Sea provided the opportunity to investigate directly the mode and rates of a medial fallout of volcanic ash following the June 15, 1991, eruption of Mount Pinatubo (Philippines). Within less than three days after the release of the major eruption plume, ash accumulation (9 kg/m²) was simultaneously recorded by traps positioned in 1190 and 3730 m water depth at a distance of 586 km to the west of the volcano. A numerical simulation of the fallout reveals that the vertical trajectories of the pyroclasts in both the atmosphere and ocean were controlled by particle aggregation. The aggregation process caused a premature subaerial fallout of fine-grained ash. After crossing the air-sea interface, vertical settling of the ash clusters was enhanced by absorption of water leading to settling rates of more than 1670 m/d. Aggregates were observed at all depths, and their rapid settling is reflected in identical pyroclast spectra in intermediate and deep water. This implies that particle sorting must have been complete in the upper water column and that the fallout was not perturbed by oceanic currents. The tephra blankets an area of 37 × 10⁴ km² in a fan-shaped westward distribution, which reflects the prevailing direction of the upper-level winds. Our data show that atmospheric tephra dispersal patterns are recorded almost instantaneously on the deep-sea floor and corroborate the key role of deep-sea ash layers as highly reliable chronostratigraphic markers, and as a geological tool to infer ancient wind fields.