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Dispersal of the 1991 Pinatubo Tephra in the South China Sea

By Martin G. Wiesner¹ and Yubo Wang¹

¹Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Bundesstraße 55, D-20146 Hamburg, Germany.

ABSTRACT

Sea-bottom cores taken across the central South China Sea have revealed that the 1991 Pinatubo tephra blankets an area of at least 37 x 10⁴ square kilometers extending from 10° to 16°N. and 111° to 120°E. The ash is dispersed in a westerly elongated lobe reflecting the prevailing direction of the upper-level winds. Thickness varied from 6.2 centimeters close to Luzon Island to <0.1 centimeter at the continental slope off southeastern Vietnam. The total bulk volume of the airfall ash is calculated at 2.7 cubic kilometers, with the submarine ash volume approximating 0.9 cubic kilometers. The core data underestimate the proximal volume of the tephra deposit because the extrapolated maximum thickness at the source (19.6 cm) is only about 50 percent of the actual value published previously. Bimodal grain-size distributions in the medial part of the tephra layer suggest that both fine- and coarse-mode constituents were transferred to the deep sea in an aggregated form at high sinking speeds. This phenomenon explains the lack of any significant lateral advection of the pyroclasts despite strong ocean surface currents. The bimodality further indicates that secondary thickening may have taken place, but to date no such thickness 'bulge' has been found.

INTRODUCTION

Atmospherically transported volcanic ash is one of the major lithogenic constituents of the unconsolidated sediments in the South China Sea (Niino and Emery, 1961; Chen, 1978; Chen and Zhou, 1992). On the basis of the dispersal patterns of pumice and glass shards in the uppermost sediment sections, Wang and others (1992) have shown that in historic times pyroclasts were mainly sourced from the volcanoes on the Philippine island arc, where the total eruption frequency varies between 4 and 10 per decade (Simkin and Siebert, 1994). Large-magnitude prehistoric eruptions are recorded by numerous discrete ash layers, but their areal extent and volumes are still largely unknown (Wiesner and others, 1994). One of these layers has recently been dated at 33-40 ka B.P. (Chen and Zhou, 1992) and could be traced across the central and southwestern parts of the basin at considerable thicknesses (>4 cm; Wiesner and others, 1994). This tephra may have originated from the largest eruption in the history of Mount Pinatubo, the Inararo episode (>35 ka B.P.; Newhall and others, this volume).

Because most downwind ash plumes derived from the Philippines appear to have been carried across the sea (Kennett, 1981), reconstructions of past eruption intensities require extensions of land-based studies in the marine realm. However, oceanic currents, subaqueous gravity mass flows, or benthic activity may significantly affect the ultimate depositional position and thickness of tephra on the sea floor. This short note reports preliminary data on the thickness and dispersal of volcanic ash in the deep South China Sea injected by the June 1991 cataclysmic eruption of Mount Pinatubo and presents tentative estimates of the total and submarine airfall ash volumes.

PROCEDURES

In May 1994, R/V Sonne conducted research operations in the South China Sea to investigate the sedimentary record of short-term Quaternary variabilities of the monsoonal climate in Southeast Asia (Sarnthein and others, 1994). Bottom sediments were sampled by a deep-sea spade box corer assembly to obtain undisturbed sediment surfaces. The core box is 50 x 50 x 60 cm in size and fitted beneath a gimbal-mounted sliding ram over a trapezoidal frame that can be variously weighted to facilitate penetration in sediments of varying character. The corer is also equipped with a pinger system through which lowering, bottom contact, and heaving operations are directly monitored on the ship's 3.5-kHz echo sounder, thereby allowing controlled sampling. Upon penetration, two lids automatically close the top of the box and thus seal the sample against resuspended bottom sediment ('bow wave' effect). After retrieval of the device, the box's front panel is removed to immediately determine the thickness of individual layers and to enable sampling of the sediments in a vertical section.

Of the 65 surface sediments cored, only 10 samples contained the discrete Pinatubo ash layer (a more detailed sampling of the tephra will be carried out in 1996). The ash was megascopically clearly distinguishable from the underlying and overlying dark olive-green or brownish pelagic sediments by its pale-gray or dark-gray color. Thickness was measured prior to subcoring with a slide caliper and averaged across several points of the core section to account for irregularities in the surface morphology of the hemipelagic sediments (small-scale sediment ridges or microdepressions). Core locations and thickness data are listed in table 1.

First data on the pyroclast properties of the ash bed were obtained for core nos.1, 2, 5, and 6. The samples were subjected to grain-size fractionation carried out by standard dry sieving (<4 phi) and settling (>4 phi) techniques at 0.5-phi intervals down to 2 m. Component abundances were determined by particle counting in each of the grain-size intervals greater than 6 m, using a standard petrographic microscope. Electron microprobe analyses were performed on a CAMEBAX 724 at 15 keV with 15 nA sample current (spot size 20 m on phenocrysts and point beam on glass bubble walls).

Table 1. Locations, water depths, and thickness of the Pinatubo ash layer recovered by sea-bottom cores in the South China Sea (SCS-C = sediment trap array).

[Coring locations are shown in figure 1]

ATMOSPHERIC INJECTION AND DOWNWIND PROGRESS OF THE PINATUBO ASH

Magmatic explosions at Mount Pinatubo commenced on June 12, 1991, with a series of vertical and lateral blasts that culminated in a paroxysmal explosion on June 15 at 1342 (Koyaguchi and Tokuno, 1993; Wolfe and Hoblitt, this volume). Ash was ejected to a maximum altitude of 35-40 km, and at about 1420 the cloud spread out laterally into a giant umbrella region between 25 and 30 km altitude. Radial expansion velocity was 125 m/s until 1440, and subsequently the downwind part of the cloud was advected across the South China Sea by high-velocity stratospheric winds (Global Volcanism Network Bulletin, 1991; Koyaguchi and Tokuno, 1993). About 21 h after the onset of strongest activity, the ash cloud covered the major part of the South China Sea, horizontally extending from 5° to 20°N. and 105° to 120°E. (Global Volcanism Network Bulletin, 1991). By June 18, the plume had already passed the South China Sea and was centered over the Bay of Bengal (Bluth and others, 1992; Stowe and others, 1992).

AREAL EXTENT, THICKNESS, AND TEXTURE OF THE ASH IN THE DEEP SEA

The cores containing the visible Pinatubo ash layer define a tephra blanket that covers an area of at least 37 x 10⁴ km², extending from 10° to 16° N. and 111° to 120°E. (fig. 1). Maximum thickness recovered was 6.2 cm to the east of the Manila Trench, thinning rapidly to 0.8 cm over a distance of about 590 km in a westerly direction (fig. 1). Farther downwind beyond 116°E., ash thicknesses were 0.1 cm except for core no.7 at the northwestern flank of the ash lobe, where the tephra was found to occur in thin scattered patches on the pelagic sediment surface (fig. 1 and table 1).

Figure 1. Thickness of Pinatubo fallout tephra (in centimeters) determined from sea-bottom cores (core number in parentheses) and location of the sediment trap system (SCS­C). Dashed-dotted line marks the limit of the discrete ash layer; open circles are cores that lack visible tephra (a detailed description of these cores is given in Sarnthein and others, 1994). Depth contour is 200 m.

The progressive decrease in thickness with distance from source is accompanied by significant changes in grain-size and phenocryst abundance. Close to the coast of Luzon (core nos.1 and 2), the deposits consisted of coarse- to medium-sand-sized pyroclasts grading upward into silty ash (fig. 2). The finer grained section was approximately 1 cm in thickness, and the overall textural appearance of both ash beds is quite similar to the June 15 and postclimactic onshore deposits (layers C and D of Paladio-Melosantos and others, this volume). Total phenocryst abundance (plagioclase, hornblende, biotite, quartz, and heavy minerals) averaged 45 percent by volume, and the maximum clast size was 2,000 m. Toward the medial parts of the ash lobe, normal grading becomes less distinct and the mean grain-size decreases. At core nos. 5 and 6, the bulk of the tephra was silt- to clay-sized, with the phenocrysts comprising about 26 percent of the ash. This is nearly two times lower than noted for the airfall deposits on Luzon Island (Pallister and others, 1992), indicating a loss of crystals during transport. The crystal fraction was dominated by plagioclase (average composition An_32-35) and hornblende (Mg# = 67), the latter being occasionally rimmed by clear cummingtonite. Glass shards were rhyolitic (77% SiO₂) in composition with an average refractive index of 1.482. Maximum clast size was 400 m.

Figure 2. A, Surface view of core no. 9 showing the distal pale-gray Pinatubo ash layer (1 mm in thickness) covered by thin patches of brownish pelagic sediment (length of box edge is 50 cm). Note the presence of scattered burrows and the sharp basal contact of the ash in the lower left region of the core. B, Archive box of core no. 2 showing the proximal ash layer in a vertical section. The tephra consists of a normally graded, coarse-grained basal layer 5 cm in thickness topped by 1 cm of fine pale-gray ash. Note the sharp contacts of the tephra layer to the underlying and overlying brownish hemipelagic sediment. Distortion at the upper left side of the section is due to pushing of the archive box into the core.

In general, the ash beds were characterized by a sharp basal contact (fig. 2). At thicknesses of less than about 0.1 cm, small-scale burrows and other structures induced by bottom-dwelling organisms were frequent (fig. 2A), but for the massive ash layers (>2 cm) the tephra surface was undisturbed and did not appear to have been significantly reworked by benthic organisms (fig. 2B). The tephra was usually capped by a film of fluffy pelagic material (fig. 2A), an observation that is in agreement with the relatively low fluxes of particulate matter in the central South China Sea, which averaged 82 mg/m²/day for the years 1992-94 (Wiesner and others, in press). Similar flux rates have been reported for the northern South China Sea (Jennerjahn and others, 1992). Assuming a packing density of 2 g/cm³ for particulate matter (Honjo, 1986), the vertical accumulation rate would be 0.015 mm/yr. Because not all of this material reaches the sea floor, due to organic matter mineralization and carbonate dissolution (Wiesner and others, in press), the total thickness of pelagic sediment deposited since the cessation of the eruption should be less than 0.045 mm, which is too thin to measure directly. Higher rates of sediment accumulation during the period following the fallout until May 1994 were obvious for core nos. 1 and 2, where the ash beds were topped by up to about 1 cm of pelagic sediment (fig. 2B). We suggest that this is probably related to lateral advection of suspended riverine material from the nearby Philippines and enhanced in-situ particle production in the surface waters due to upwelling off western Luzon (Pohlmann, 1987).

VOLUME ESTIMATES

On the basis of observed thickness, we have constructed three isopachs (1, 2, and 6 cm; fig. 1) and, by using a planimeter, we calculated the areas contoured by these isopleths at 61,500 km² (1-cm segment) and 36,100 km² (2-cm segment). The limited number and geographic extent of cored ash beds, however, permit excursions of the 1-cm and 2-cm isopach contours. Considering exponential thinning of the ash layer and interpolation of the thickness between the various core sites along downwind traverses, we calculate the precision to be less than +-5,000 km² per segment. Plotting the above data as thickness (log) versus area^1/2 (fig. 3) and using the equations given by Fierstein and Nathenson (1992), we calculate the slope of the straight line between the two points to be 0.0120. Extrapolating the straight line to area = 0, the maximum thickness (T₀) of airfall ash deposited at the source would be 19.6 cm (fig. 3), and for the total ash volume (V) being defined as 2T₀ x k^-2 (Fierstein and Nathenson, 1992), we arrive at about 2.7 km³. However, the value for T₀ is much lower than the actual maximum thickness on land (39 cm; Paladio-Melosantos and others, this volume), suggesting an inflection of the straight line to occur closer to the source. Our data therefore underestimate the proximal ash volume. Replacing T₀ by the maximum thickness recovered by the cores (6.2 cm) yields a volume of the submarine tephra that approximates 0.9 km³.

Figure 3. Log thickness versus area^1/2 plot for the 1- and 2-cm isopachs. Open circle is derived from the assumption that the box core nos. 8, 9, and 10 mark the western limit of the 0.1-cm isopleth. T₀, maximum thickness of airfall ash deposited at the source.

At this stage, it is not possible to give a better estimate for the ash volume deposited in the South China Sea because of insufficient data on the distal and southern parts of the tephra lobe. If it is assumed that core nos. 8, 9, and 10 mark the western boundary of a 0.1-cm thickness contour (fig. 1) and these sites are connected by a straight line, the square root of the area covered by the 0.1-cm segment would be 339 km. Since this value plots below the regression line in figure 2, we conclude that the 0.1-cm isopach must extend much farther to the west.

INTERPRETATION OF DATA

The westerly elongated lobe of the Pinatubo tephra appears to reflect the upper-level wind field that prevailed during the eruption. Upper tropospheric and stratospheric winds were directed to the west and southwest at speeds of 18-24 m/s (H. Houben, NASA, written commun., 1993). Below the 6-km altitude across the South China Sea, winds were constantly blowing to the northeast (H. Houben, NASA, written commun., 1993). The onset of these southwest monsoonal winds took place by the end of May, with speeds increasing from 4 to 6 m/s in June and reaching a maximum of around 8 m/s in August (Wiesner and others, in press). Long-term records of the oceanic circulation in the South China Sea (see Shaw and Chao, 1994, for summary) have shown that in response to the southwest-monsoon wind vectors, surface currents flow to the east and northeast from June through August at relatively high velocities of 0.4-0.8 m/s; undercurrents flow into the same direction but at greatly reduced speeds (<0.05 m/s). Therefore, ash particles arriving at the sea surface would have been transported back towards the Philippines or into the direction of the Bashi Strait between Taiwan and Luzon Island at 35-70 km/day. The lack of any shift or bending of the axis of the sea-floor ash lobe to northerly directions suggests, however, that if any lateral advection did occur, the quantities of ash transported must have been very low.

Further evidence for the ash deposits being hardly distorted by the current system is provided by data obtained from fully automated collection devices (sediment traps) that were operating during the Pinatubo eruption at 14°36' N., 115°06' E. (fig. 1). The traps were placed along a fixed mooring array at depths of 1,190 and 3,730 m and are designed to collect particulate matter settling through the water column for a designated period in a preprogrammed sequence (Honjo and Doherty, 1988). Within less than 3 days after the release of the major eruption plume, each of these traps simultaneously intercepted a total amount of 9 kg/m² of ash (Wiesner and others, in press). On the basis of the mean grain density (2.34 g/cm³) and average porosity (52.5%) of this material, Wang (1994) calculated a depositional thickness of 0.8 cm. This value is compatible with the data derived from the cores taken close to the trap site (table 1) and indicates that, at least in the medial parts of the lobe, the ash was not redistributed by bottom currents. Furthermore, the ash intercepted by the traps was strongly bimodal in grain size, peaking at 11 and 88 m (Wang, 1994). Such a distribution is usually indicative of particle aggregation (Sorem, 1982; Cornell and others, 1983; Brazier and others, 1983), and this process may have brought about sufficiently high settling velocities to rapidly carry both fine- and coarse-grained pyroclasts through the strong surficial water currents. The fact that it took only about 58 h from the initial atmospheric injection of ash at Mount Pinatubo to the first registration of the pyroclasts by the traps (Wiesner and others, in press) reveals that subaqueous settling rates at the trap site must have been greater than 1,550 m/day.

The bimodality of the medial ash adds further uncertainty to the volume estimate. The formation of ash clusters may produce secondary thickening of deposits, as has been recognized, for example, in the medial parts of the ash lobe from the 1980 eruption of Mount St. Helens (Carey and Sigurdsson, 1982). This will tend to increase the dispersal of intermediate-thickness isopachs and correspondingly reduce the areal extent of thinner isopachs, leading to steeper slopes in the log of thickness versus area^1/2 plot. At present, however, the spacing of the sea-bottom cores is not close enough to detect the existence of such a phenomenon in the South China Sea.

The large discrepancy that exists between the horizontal extension of the subaerial ash plume and the deep-sea tephra needs further investigation. We suppose that only small amounts of ash were released from the northern and southern margins of the ash cloud and were easily incorporated into the pelagic sediments by the benthic fauna or diluted by the background sedimentation. Preliminary microscopic inspection of cores adjacent to the ash lobe proved the presence of finely dispersed pumice fragments, glass shards, and phenocrysts largely less than 20 m in size. On the basis of the data of Ledbetter and Sparks (1979) it roughly takes about 1 year for a 20-m sized pyroclast with a density of 2.5 g/cm³ to settle individually through a 4,000-m water column. During this period, pelagic sedimentation in the South China Sea would produce a 0.0015-cm-thick layer (see above); consequently, ash beds of this thickness or less would be hardly recognizable in the cores. In reality, however, vertical settling of fine particles occurs by co-aggregation with larger, faster settling particles, thereby accelerating the sinking speed of fines by many orders of magnitude (Honjo, 1986). This implies that if pelagic dilution has affected the formation of the discrete Pinatubo tephra it should have been effective at much lower thicknesses than indicated by the above value. We believe, therefore, that the northern and southwestern boundaries of the tephra blanket are relatively well defined and that the quantities of ash deposited in the northern and southern South China Sea may not significantly add to the total tephra volume. Our 2.7-km³ estimate is slightly lower than the 3.4-4.4 km³ range favored by Paladio-Melosantos and others (this volume).

Within the proximal part of the ash lobe, the massive and 'instantaneous' sedimentation of pyroclasts caused a mass mortality of benthic biota followed by a stepwise recolonization of the ash substrate (Hess and Kuhnt, in press). Even 3 years after the eruption, the benthic community structure was still far from its background levels (Hess and Kuhnt, in press) and, as a consequence, the ash layer was hardly affected by benthic reworking.

ACKNOWLEDGMENTS

We are indebted to the officers and crew of the research vessel R/V Sonne for their assistance in coring operations. M. Holmes, C.G. Newhall, K.S. Rodolfo, and W.E. Scott are thanked for their constructive reviews that helped to improve the manuscript. Financial support by the Bundesministerium für Forschung und Technologie (Bonn) is gratefully acknowledged.

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