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

Impact dust not the cause of the Cretaceous-Tertiary mass extinction (part 1)
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

ABSTRACT

Most of the 3-mm-thick globally distributed Chicxulub ejecta layer found at the Cretaceous-Tertiary (K-T) boundary was deposited as condensation droplets from the impact vapor plume. A small fraction of this layer (<1%) is clastic debris. Theoretical calculations, coupled with observations of the coarse dust fraction, indicate that very little (<10^14g) was submicrometer-size dust. The global mass and grain-size distribution of the clastic debris indicate that stratospheric winds spread the debris from North America, over the Pacific Ocean, to Europe, and little debris reached high southern latitudes. These findings indicate that the original K-T impact extinction hypothesis-the shutdown of photosynthesis by submicrometer-size dust-is not valid, because it requires more than two orders of magnitude more fine dust than is estimated here. Furthermore, estimates of future impact hazards, which rely upon inaccurate impact-dust loadings, are greatly overstated.

INTRODUCTION

Two decades of research have clearly linked the Cretaceous-Tertiary (K-T) mass extinction to the catastrophic meteorite impact that formed the Chicxulub crater in Yucatan, Mexico. Nevertheless, causal factors in this link remain uncertain, and research continues on the mechanisms by which large impacts disrupt the biosphere. This paper examines the evidence for the impact extinction mechanism originally proposed by Alvarez et al. (1980): photosynthesis shutdown by a global cloud of fine dust. Although several other impact extinction mechanisms have been proposed for the K-T boundary, the dust hypothesis is perhaps the most widely recognized.

Furthermore, impact dust is one of the key environmental perturbations used to estimate future hazards from more modest-sized impacts (Chapman and Morrison, 1994).

ALVAREZ DUST EXTINCTION HYPOTHESIS

The original K-T impact extinction hypothesis of Alvarez et al. (1980) stated that there was a collapse of the global food chain due to the shutdown of photosynthesis by sun-blocking silicate dust injected into the stratosphere. The dust-loading threshold for photosynthesis is ~10^16g of submicrometer-size dust (Gerstl and Zardecki, 1982; Toon et al., 1982). Below this mass, light levels remain sufficient for photosynthesis. Thus, the major challenge in evaluating the Alvarez dust hypothesis is estimating the mass of globally distributed submicrometer-size dust.

THEORY AND EXPERIMENT

Silicate Dust

Toon et al. (1997) used theoretical calculations coupled with energy scaling of experimental and atomic bomb data, adapted from O'Keefe and Ahrens (1982), to estimate that ~3 x 10^17g of submicrometer-size dust was lofted into the stratosphere by the K-T impact. Nevertheless, the data on particle-size distributions for impacts and atomic blasts used by O'Keefe and Ahrens (1982) do not cover size ranges below 50 mym. Below 100 mym, these same data show a sharp drop-off in cumulative mass, suggesting that the target rocks resist fragmentation below the crystal domain size of 100 mym (e.g., Melosh, 1989).

O'Keefe and Ahrens (1982) assumed a simple exponential decrease in cumulative mass from 50 cm to 0.5 mym, which indicated that ~0.1% of impact debris would be <1 mym. Given the evidence for a drop-off in the <100 mym size fraction, a better estimate of the sub-micrometer-size dust is <0.1%, perhaps much less.

Vapor Condensation

Most of the mass in the fireball of an impact is vapor. Theoretical studies of a Chicxulub-size asteroid impact indicate that the vapor plume contained 1-3 x 10^18g of silicate vapor from the target rocks (Toon et al., 1997; Pierazzo et al., 1998). The plume also contained vapor from the carbonates and sulfates in the target rock (e.g., Pope et al., 1997). Most of the Ca and Mg in the carbonates and sulfates prob-ably condensed with the silicate vapors. Given the CO2 and SO2 mass estimated by Pope et al. (1997), the mass of vaporized Ca and Mg added an additional ~5 x 10^17g to the plume. Finally, the contribution of the impactor must be considered, which would add ~1-3 x 10^17g to the plume, or perhaps twice this amount if the impact velocity was >20 km/s (Pierazzo and Melosh, 2000). The total mass of the vapor plume was therefore ~2-4 x 10^18g. These vapors, which spread globally and condensed (e.g., Zahnle, 1990), are the primary source of the global ejecta layer.

O'Keefe and Ahrens (1982) calculated that vapor condensation droplets from a Chicxulub-size impact would be in the size range of hundreds of micrometers. Ablation of these spherules upon atmospheric reentry could produce smaller particles (Melosh and Vickery, 1991), although Zahnle (1990) calculated that the velocity of most condensates would be too low for significant ablation. Furthermore, the size of the droplets is close to the 100 mym size limit, below which little ablation occurs (Melosh, 1989). Therefore, the vapor condensates from a Chicxulub-size impact probably produce minimal amounts of submicrometer-size particles.

GEOLOGIC STUDIES

K-T Fireball Condensates

The thickness of the global ejecta layer is ~3 mm (e.g., Smit, 1999). The term "fireball layer" (Hildebrand and Boynton, 1990) is used here for this global layer. The fireball layer, which contains shocked quartz, spherules, and an Ir anomaly, is the only globally distributed K-T ejecta (other ejecta layers have a limited distribution). Mineralogical studies indicate that the bulk of the fireball layer is altered glass (Pollastro and Bohor, 1993). Well-preserved examples of the fireball layer are composed almost entirely of spherules with relict crystalline textures indicative of quenched melt, and are interpreted to be condensation droplets from the vapor plume (Montanari et al., 1983; Montanari, 1991; Smit et al., 1992a; Pollastro and Bohor, 1993; Bohor and Glass, 1995; Kyte and Bohor, 1995). Spherule diameters range from ~20 to 800 mym (Doehne and Margolis, 1990; Montanari, 1991; Kyte et al., 1996); a mean of 250 mym was reported from sites in Europe (Smit, 1999).

The 3-mm-thick fireball layer represents a global mass of ~3.8 x 10^18g (assuming a mean density of 2.5 g/cm^3 ), which matches the estimates of the vapor-plume mass noted here. Likewise, the compo-sition of the fireball layer is consistent with most of the mass being derived from vapor condensation droplets ~200 mym in diameter. Nevertheless, these analyses do not prove that there is not a fraction of a percent of submicrometer-size dust in the fireball layer.

Clastic Debris (''Dust'') in the Fireball Layer

The most complete analysis of clastic (pulverized rock) debris in the fireball layer comes from the Pacific Ocean (Bostwick and Kyte, 1996). Of the quartz grains examined, ~65% show evidence of impact shock, and these grains have a mean size (d)of 50 mym. The mass percentage of impact clastic debris in the Pacific K-T fireball layer can be estimated by assuming (1) all of the clastic quartz grains were originally deposited in the 3-mm-thick fireball layer; (2) the average mass of quartz grains = ¼ d^3 x 2.5 g/cm^3, based on the ~1:2 aspect ratio reported by Izett (1990) and the density of quartz; and (3) the total mass of clastic debris in the fireball layer is equal to two times the mass of quartz, based on the data in Izett (1990) and the complex lithology of the target site (Sharpton et al., 1990). Given these assumptions, and the data reported by Bostwick and Kyte (1996), the mass percentage of clastic debris in five Pacific sites averages ~0.1% (Fig. 1). The same approach can be used to estimate the mass of clastic debris from Beloc in Haiti and Frenchman Valley in Saskatchewan, Canada, based on data reported by Leroux et al. (1995), and from Petriccio, Italy, based on data reported by Montanari (1991). Haiti has nearly 3% clastic debris in the fireball layer, Frenchman Valley ~0.3%, and Italy only 0.001% (Fig. 1).

Izett (1990) found 0.02%-0.7% clastic grains (by weight) in the fireball layer from sites in the Raton basin of Colorado and New Mexico. About half of the clastic grains are quartz, of which ~50% show impact-shock deformation (Izett, 1990). Sharpton et al. (1990) found 1% clastic grains in a 2-5-mm-thick fireball layer from the Raton basin. A clastic mass of 0.5% for the Raton basin is derived by using the methods outlined here and data reported by Leroux et al. (1995). Taking into account that the lower percentages probably represent incomplete recovery, the total amount of clastic debris in the Raton basin fireball layers is estimated to be ~1%.

Izett (1990) found no shocked quartz in an analysis of 15 000 quartz grains from three sites in New Zealand, although a few grains were found in later analyses. Analyses of a core from Deep Sea Drilling Project (DSDP) Site 527 (Walvis Ridge) in the South Atlantic produced only a few shocked-quartz grains, composing ~2% of the quartz grains in the K-T boundary samples (Huffman et al., 1990). These data are insufficient to make estimates of the clastic-debris mass in these two Southern Hemisphere sites, but given the paucity of shocked quartz, the mass is probably less than that found in Italy.

The mass of clastic debris in the fireball layer follows an inverse power-law relationship with distance from Chicxulub (Fig. 1), with the notable exception of Italy (and perhaps Walvis Ridge and New Zea-land). With Italy omitted, a power-law regression of the mass (y in %) with distance (x in km) gives the function y = 208 012.3 +/- 2.5x^-1.636 +0.30; r = 0.91 (95% confidence interval). Another well-known aspect of the shocked quartz in the fireball layer is that the grains become smaller with distance from North America (e.g., Bohor, 1990; Izett, 1990). A compilation of data on maximum (24 sites) and mean (14 sites) shocked-quartz grain sizes is shown in Figure 2. Similar to the mass, there is a clear pattern of decreasing size with distance from the Chicxulub crater (Fig. 2). A power-law regression of the maximum size (y in mm) with distance (x in km) gives the function y = 482.30 +/- 0.70x^-0.87 + 0.08; r = 0.91 (95% confidence interval).