Jan
Smit
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KEVIN POPE'S DUST-FREE? K/T IMPACT STIRS UP LOTS OF DUST!
I read with interest Kevin Popes paper, and agree with some of his conclusions, but I don't agree with some of his reasoning behind it. My main disagreement is that he takes model-derived data (see his figures) rather than real world data one can obtain from the geological record at K/T. I think therefore that he underestimates the amount of dust, but agree that the consequences may have been exaggerated. The blocking of dust was the first scenario to explain the extinctions behind the impact by the Alvarez group, and it would be a micracle if this scenario survived for 25 years without modification. Yet, the collapse of the foodchain by temporary shutdown of photosynthesis explains almost all of the stratigraphic and paleontologic data from the oceans on the K/T extinctions.
That sudden collapse is a reality, notwithstanding the incorrect comment by Gerta Keller on Pope's paper, stating: '.....dust cloud scenario as primary cause for the K/T mass extinction...... simply does not fit the paleontological data that show strong declines in populations for at least the last 0.5-1.0 million years prior to the K/T impact' Keller's comment can only apply to some rudist and inoceramid species, occupying rare and presently vacated niches in the Late Cretaceous, while Keller implies that such decline accounts for all ecologically important species. That is simply misleading. The extant data on extinction of e.g. dinosaurs, pollen, shallow carbonate platform biota, and all planktic calcareous species, nannofossil as well as planktic foraminiferal species (Kellers own speciality), representing the calcareous surface plankton at the end of the Cretaceous, do not show such 'strong decline'. On the contrary, these planktic species thrive -almost unchanged- up to the global ejecta layer itself (see agostK/Tdetail.jpg). No preceding decline there.
But as Kevin said, the sudden collapse of the foodchain could be achieved by sulfate aerosol loading as well. The resulting cooling (from sulphate aerosols), has empirically been supported by 1) evidence for hailstone holes in fossil waterlily leaves at KT, (a bit flimsy), but more importantly, 2) by migration patterns of dinoflagellates just after the KT boundary when cold, boreal species move temporarily from Denmark to Tunisia. (Brinkhuis, H., J. P. Bujak, et al,1998).
One of the problems for the impact-extinction theory gaining a wider acceptance, is the lack of strong, positive, evidence for an impact at the other major extinction horizons (P/T, Late Devonian etc). Evidence of extinction at known major impact events, such as Popigai, Chesapeake bay about 34ma ago, (although this next largest crater after Chicxulub is 10x less energetic than Chicxulub) is lacking. This leaves the backdoor open for alternative explanations for the K/T boundary as well, and I think that for this reason we see a recent (see the last episode of Walking with dinosaurs of the BBC!) resurge of volcanic explanations for Dinosaur extinction (Deccan vs Chicxulub). But the plankton extinction record in the oceans does not agree at all with these volcanic scenarios. Therefore, I also agree with Kevin that the effects of lesser impacts on life may have been overstated in the sense that these do not lead to mass-extinctions.
As for the K/T dustload, I include here some of my estimates for this, based on available K/T ejecta-layer data. The maximum amount of dust can be derived from the pore-space filling between the condensate spherules (see attached figures.) within the global K/T ejecta layer. This layer is between 2-3mm thick, >4000km away from the Chicxulub crater.
Thickness global layer(mm) 2-3 mm Surface area earth(cm2) 5.10E+18 cm2 Volume global layer 3mm thick(cm3) 1.53E+18 cc Weight 3mm thick layer, assuming density3 (g) 4.59E+18 gram global number of spherules assuming 200µ diameter, cubic ordening. 1.91274E+23 total weight of these spherules (density3) (g) 2.40362E+18 gram weight of dust in porespace(g) (dens3) 2.19E+18 gram
The maximum amount of dust may thus be about 2x1018gram, about a factor 100-200 more than estimated by Kevin. These figures are rough numbers, but this is the amount present in the global ejecta layer. It does not matter whether it is in the southern of northern hemisphere, the thickness of the layer and amount/size of spherules remains the same. Remains the question of course, whether all this material has been accumulated as dust size particles. The dropoff below 100µ mentioned by Kevin is based on small scale models and nuclear explosions, and it remains to be seen if that works with Chicxulub-sized impacts. Iridium in the ejecta layer is in extremely small, less than 0.1micron particles, because, despite several attempts at locating particulate matter (nuggets) in the ejecta layer, these have not been found unequivocally. Recently a study appears to confirm this small size. http://www.lpi.usra.edu/meetings/impact2000/pdf/3031.pdf Although some fraction of iridium resides in spinel-rich condensate spherules, more than half does not, and it remains likely that also other parts of the vaporized bolide and vaporized target have the same size distribution as the iridium particles, and have landed as dust!
Some other points I place question marks. In Pope's fig 1 and in the text he remarks that the amount of dust in Italy and Walvis ridge is much less than elsewhere, but he leaves out the simple explanation for that. Both in Italy and WR the K/T ejecta layer is severely disrupted by bioturbation, decreasing the amount of dust in the layer itself considerably. But when you recalculate the numbers of spherules and qz back into a 2-3mm thick layer, the amount/cm2 is the same as in Spain and republic of Georgia, locations that straddle Italy, and where the ejecta layer is well preserved. Same for Walvis ridge at site 524. Woodside Creek in New Zealand and ODP site 465a in the Pacific show this same 2-3mm thick layer. So from the thickness of the fireball layer alone, being the same on both hemispheres, one can infer a ballistic emplacement, not transport by stratospheric winds.
Also, his conclusion that there are clear geographic patterns is based on postdepositional burrowing and dispersal. His figure 2 is not as clear as he claims. Coarse shocked quartz is indeed found in and near North America, (<4000km) then there is a gap in the distribution (larger than shown in his fig 2 (see Smit, 1999). Further (>7000km) distal the sizes are within error, the same size. I don't believe the size decrease follows this power decay law. The shocked quartz is generally believed to be entrailed in the vapor plume, mostly on the very outside, where the grains tend to slow down quicker being at the edge, smaller ones being entrailed deeperin the main cloud, and dispersed worldwide (i.e. >7000km). This size distribution does not seem to support his stratospheric wind dispersal. The third argument is the distribution of the amount of iridium. Contary to the shocked quartz, there is no clear increase in amount towards the crater. On the contrary, the amount increases to regions antipodal to Chicxulub, the south Pacific and Woodside Creek being the most enriched (Kyte et al,1996). Wind dispersal would undoubtly show a higher concentration closer to the source.
sincerely
Jan Smit
Brinkhuis, H., J. P. Bujak, et al. (1998). "Dinoflagellate-based sea surface temperature reconstructions across the Cretaceous-Tertiary boundary." Paleogeogr., Paleoclim., Paleoecol. 141: 67-83. Smit, J. (1999). "The global stratigraphy of the Cretaceous Tertiary boundary impact ejecta." Annual Review of Earth and Planetary Sciences 27: 75-91. Kyte, F. T., J. A. Bostwick, et al. (1996). The Cretaceous-Tertiary boundary on the Pacific plate: composition and distribution of impact debris. The Cretaceous-Tertiary Event and Other Catastrophes in Earth History. G. Ryder, D. Fastovski and S. Gartner. Boulder, Geol. Soc. of Amer. Sp. Pap. 307: 389-402.
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