From Kevin O. Pope <kpope@starband.net>

Impact dust not the cause of the Cretaceous-Tertiary mass extinction (part 2)
By Kevin O. Pope
Geo Eco Arc Research
16305 St. Mary's Church Road
Aquasco, Maryland
20608
USA

Geology; February 2002; v. 30; no. 2; p. 99-102

GLOBAL PATTERNS AND PROCESS

The characteristics of the clastic debris in the fireball layer show clear geographic patterns that are not readily explained by ballistic transport. Alvarez et al. (1995) noted that the launch velocity required for ballistic transport of shocked quartz to distal K-T boundary sites can only be achieved by ejecta that is subjected to shock pressures that would have annealed or melted the quartz. They explained this apparent anomaly with a velocity boost imparted to moderately shocked ejecta by the vapor plume. Nevertheless, models of Chicxulub ballistic ejecta dispersal, with a velocity assist from the vapor plume (Durda et al., 1998), do not reproduce the mass distributions of clastic debris shown in Figure 1. Durda et al.'s (1998) model predicts that impacts produce a distribution of ballistic ejecta that is largely symmetrical around the crater. The model does not explain why Italy has more than an order of magnitude less debris than Pacific DSDP Site 576, which is at about the same radial distance from Chicxulub as Italy. Likewise, ballistic transport of clastic ejecta cannot explain the size sorting shown in Figure 2. Ballistic transport to distal sites occurs mostly outside the atmosphere, where no sorting would occur. Once in the atmosphere, drag would preferentially reduce the trajectory of smaller particles, producing patterns inverse to what is observed.

The mass and grain-size distributions of clastic debris in the fire-ball layer are better explained by (1) ballistic deposition of moderately shocked ejecta on top of the atmosphere near the crater; (2) subsequent spread of the debris by stratospheric winds; and (3) gravitational settling of debris as the cloud spreads. Such a process was proposed by Toon et al. (1997) and Covey et al. (1990). Covey et al. (1990) modeled the wind dispersal of a cloud of impact dust with an initial loading of 5 x 10^15g centered (1000 km radius) on the Manson crater in Iowa. After five days, dense clouds of debris continued to rain down over North America, the northern Atlantic, and the Pacific; moderate dust loading had spread to central and western Europe; and very little dust had spread to the southern high latitudes. The speed of westward spread of the cloud was ~150 km/h. This pattern of dispersal is similar to the spread of the volcanic plume of the 1982 eruption of El Chichon, located just southwest of the Chicxulub crater, which spread westward at ~70 km/h and encircled the globe with a narrow band of debris (Rampino and Self, 1984).

To examine the wind dispersal of ejecta, grain-size distributions were modeled for three potential dispersal wind speeds: 70, 150, and 400 km/h (Fig. 2), based on the velocities of the Chichon plume, the Covey et al. (1990) impact simulation, and the jet stream, respectively. Note that the particle-size distributions found in the K-T fireball layer follow a power-law relationship similar to the model distributions, and that these distributions are mostly within the range expected for particles dispersed by winds with speeds of between 70 and 400 km/h.

The information on ejecta mass in Figure 1 and ejecta size in Figure 2 can be combined by assuming an initial particle-size distribution. The distribution assumed here is that measured in pyroclastic deposits (Sheridan, 1979). Pyroclastic deposits are a reasonable analogue for clastic ejecta and have been well studied down to the micrometer size range. Figure 3 presents a series of calculations of ejecta dispersion beginning with an initial mass (10^16g and 10^17g) centered on the crater with the size distribution noted. Two models of strato-spheric wind dispersion of ejecta (150 km/h and 400 km/h) were then applied. The calculations assumed that the clastic ejecta were dispersed in a radial fashion, which the data and Covey et al.'s (1990) model suggest is not the true case. This simplification will

underestimate the true clastic ejecta mass in the fireball layer, given that the latitudinal dispersal of ejecta was probably more limited than the radial dispersal used in the calculations. Calculations were based on the distance particles of a given size range would travel before settling (Fig. 2); then the mass represented by that size range (taken from the size distribu-tion) was distributed over the radial distance covered.

There are two conclusions to be drawn from Figure 3. (1) The distribution of mass is not highly sensitive to the wind speed, because the 150 km/s and 400 km/s calculations produced similar results. This insensitivity is because most of the mass is concentrated in the larger size fraction, so that the different wind speeds only greatly affect sedimentation near the crater. (2) The mass distribution is highly sensitive to the initial mass. The pattern of modeled mass dispersal for an initial loading of 10^16g compares well with the measured mass in the fireball layer (Fig. 3). Because this simplified model tends to underestimate the mass, the conclusion to be drawn is that the observed mass in the fireball layer is consistent with an initial mass of ~10^16g. If the initial mass was 10^17g, much more clastic debris would be expected than is observed. Note that the mass loading in this model is that part of the ejecta that was dispersed by winds and does not equate with the total mass ejected into the atmosphere.

Returning to the issue of the distribution of clastic debris in the K-T fireball layer, the anomalous small mass of debris in Italy, and perhaps Walvis Ridge and New Zealand, can be explained by the asymmetrical dispersal patterns of stratospheric winds. If the impact occurred during summer in the Northern Hemisphere, debris would be transported mostly westward; thus debris must travel three times further to Italy than to the western Pacific. Similarly, stratospheric winds are much less effective in transporting debris latitudinally; hence little debris may have reached New Zealand.

DISCUSSION

Implications for the K-T Mass Extinction

Although the submicrometer-size component of the fireball layer cannot be directly examined, it must be very small. Assuming a grain-size distribution typical for distal volcanic-ash deposits (e.g., Carey and Sigurdsson, 1982), the submicrometer-size component of the clastic debris in the fireball layer is probably <1%. The total mass of clastic debris in the fireball layer estimated here is <10^16g. Therefore, the mass of submicrometer-size dust in the fireball layer is <10^14 g, and is perhaps as little as 10^13g. This mass is two to three orders of magnitude less than that needed to shut down photosynthesis. These results shed doubt on the importance of impact dust in the mass extinction that marks the K-T boundary. A global atmospheric loading of <10^14g of submicrometer-size dust would not cause the catastrophic impact winter often proposed (e.g., Covey et al., 1994).

There are, of course, impact hazards other than dust clouds. For the K-T event, the shutdown of photosynthesis and global cooling are more likely to have been caused by the impact production of sulfate aerosols from the target rock (e.g., Pope et al., 1997), and by soot from global wildfires (e.g., Wolbach et al., 1990).

Implications for Impact Hazards

Dust clouds have also been used to estimate the effects of small impacts (Toon et al., 1997). Given that a Chicxulub-size asteroid (10 km diameter) generates only modest amounts of fine dust, the dust effects from smaller impacts are probably negligible. This conclusion has major ramifications for assessments of future impact hazards. Chapman and Morrison (1994) assumed that the impact of an asteroid between 0.6 and 5 km in diameter would produce enough dust to cause global crop failures leading to the death of 25% or more of the world's population. The lower and nominal (0.6-1.5 km) asteroid sizes used in their calculations are much too small to have global consequences from the dust. Other factors such as sulfate aerosols from the asteroid (Kring et al., 1996) and soot from fires set by ejecta reentry (Toon et al., 1997) only become important globally for asteroids $3 km in diameter. Therefore, the often cited 1:20 000 risk of death by impact (Chapman and Morrison, 1994), which assumes mass mortality during relatively small (1.5 km asteroid) impacts, is greatly overstated.

ACKNOWLEDGMENTS

This research was funded by the National Aeronautics and Space Administration Exobiology Program contract NASW-96030.

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Copyright 2002, Geological Society of America

Dan