The following was posted on the Cambridge Conference Network (24 September 1999).
No Evidence for Climate-Perturbing Cosmic Impact Around 4000 BC
Doug Keenan <firstname.lastname@example.org>
There is very strong evidence for a climatic upheaval c. 4000 BP. The big question is "what caused the upheaval?" Many seem to believe that the cause was a cosmic impact. I would prefer to not consider the cause right now. Instead, I would like to consider a smaller, related, question: is there evidence specifically for a climate-perturbing cosmic impact c. 4000 BP? (Please note that this is different from asking if a cosmic impact can explain the available palaeodata.) What follows reviews the evidence that I have seen presented.
- The soil analysis of Courty . Courty reports the absence of the materials usually associated with a cosmic impact: this is evidence against such an impact. (M.A. Courty [private comm.] has agreed that a possible explanation for her data--which shows intense burn - is an eruption whose ejecta contained oil/gas; no other credible explanation has been suggested, as far as I know.) Note too that Courty took samples from a different context than Weiss et al. ; so it is not certain that Courty and Weiss et al. are analysing the same soil. Indeed, Courty claims that her soil pre-dates Akkad, whereas Weiss et al. are clear that theirs is from Akkad's terminus; as I understand it, however, the (relative) date from Courty is unconfirmed by an archaeologist--so it could conceivably be in error.
- Seismic disturbances in part of the Ancient Near East. The ANE has four tectonic plate boundaries within it (and many volcanoes); the Anatolian microplate would seem to be particularly susceptible to disturbance. Earthquakes can thus easily occur in the ANE, without cosmic impact.
- Widespread (global?) seismic disturbances. Peiser  presents seven or eight palaeo-seismic events, in addition to those from the ANE. His reference numbers are used here.
 The date is tentatively c. 2900 BC, by thermoluminescence (in Utah).
 (i) Dates made in the 1960s--thus ranging over many centuries; the event was not an earthquake, but a gradual uplift (on E coast of N America). (ii) Date accuracy not reported; the event was a gradual subsidence (Veracruz, Gulf of Mexico). As Peiser notes, these two events appear to be due to a gradual differential warping (of the N American plate).
 The date is ~3400 (14C) BP, and almost certainly after 1900 BC.
 Large and at about the right time (in Fiji).
 The date is almost certainly before 3750 BC.
 The date is almost certainly after 1650 BC.
 The date is roughly 2900 BC; the event was not an earthquake, but a gradual emergence.
(Note - refs 079 and 157 were unavailable to me; I have depended on Peiser.) The above events do not constitute evidence for a cosmic impact c. 4000 BP.
- The apparent synchrony of (i) earthquakes in the ANE and (ii) the onset of the climatic upheaval. As I understand it, no one has proposed how a cosmic impact could force the observed climatic anomalies. If, however, the ANE earthquakes were coeval with a sufficiently-large eruption, then the climatic anomalies are plausibly explainable [Keenan, 1999]. (This does not mean that an eruption-with-earthquakes is the only plausible explanation, merely that the evidence is not specifically for a cosmic impact.)
- Ancient texts. I propose that an ancient text be considered as evidence for a cosmic impact only if either (i) it is supported by scientific data or (ii) it gives some details that ancient people would be unlikely to guess at. So far, no ancient texts have been cited that meet this criterion.
- Craters reported by Schultz & Lianza . This is essentially certain evidence specifically for a cosmic impact. The date is very roughly 5000 years ago (work is ongoing to make this more accurate--P. H. Schultz, private comm., 1999-09-13). The energy release was ~350 Mtons TNT (roughly 20 times Tunguska) and the local soil is less, but the craters are <10 m deep, since the impact angle was <15 deg from the horizontal. The meteorite was ~150 m in diameter: the implied volume is minuscule (only 1% of what Mt. St. Helens erupted in 1980). This and the impact angle imply that virtually all debris would have been placed on the ground or in the troposphere--from where it would be quickly washed out. Hence effects on the global climate would have been negligible. It is not plausible that the impact, which was in Argentina, would have caused earthquakes in the ANE.
This obviously does not mean that there wasn't a climate-perturbing cosmic impact c. 4000 BP. I believe, though, that there is no evidence available specifically for such an event.
Cheers, Doug Keenan
- Courty, M.-A. "The Soil Record of an Exceptional Event at 4000 B.P. in the Middle East", Natural Catastrophes During Bronze Age Civilisations (editors--Peiser, B. J., Palmer, T. & Bailey, M. E.) 93-108 (British Archaeological Reports [Archaeopress], Oxford, 1998).
Keenan, D. J. "The three-century climatic upheaval of c. 2000 BC, and regional radiocarbon disparities", Los Alamos Archives: Physics/9908052 (1999). [Also available at http://freespace.virgin.net/doug.keenan .]
Peiser, B. J. "Comparative Analysis of Late Holocene Environmental and Social Upheaval", Natural Catastrophes During Bronze Age Civilisations (editors--Peiser, B. J., Palmer, T. & Bailey, M. E.) 117-139 (British Archaeological Reports [Archaeopress], Oxford, 1998).
Schultz, P. H. & Lianza, R. E. "Recent grazing impacts on the Earth recorded in the Rio Cuarto crater field, Argentina", Nature 355: 234-237 (1992).
Weiss, H. & six others. "The Genesis and Collapse of Third Millennium North Mesopotamian Civilization", Science 261: 995-1004 (1993).