Guest Post by Dan Johnston
I have been following the proposed collision theory for the onset of the Younger Dryas for a number of years with considerable interest as it explains so much in a relatively straightforward fashion, if true. The academic response to the hypothesis has been, predictably, harsh and unforgiving with accusations flying back and forth as to procedures and interpretations. I did, however, enjoy one researcher’s claim that he could not reproduce the results reported in one peer-reviewed paper on the evidence for an impact. The impact researchers countered with the valid observation that the scornful researcher did not bother to use the same time-consuming methods they employed to extract extremely small microtectite spherules and nanodiamonds from sediments.
Carl Sagan’s “Extraordinary claims require extraordinary evidence” comes to mind with regard to the Younger Dryas claims but I have to admit, based on the global warming “consensus,” that this fundamental principle of science seems to have been trumped by politics and media attention in the past. I do remember the long struggle involved in getting the Alvarez’s theory of a catastrophic asteroid impact causing the end of the Cretaceous and extinction of the dinosaurs accepted. No matter how much physical evidence was gathered from around the world, acceptance (a much stronger word to me than consensus) didn’t happen until the smoking gun of the Cretaceous impact was located at Chicxulub in the Yucatan. Once the crater had been found, resistance to the asteroid theory rapidly crumbled and it is generally accepted as fact by most scientists today, though there is still a small minority of “deniers.”
The Chicxulub impact was a relatively straightforward exercise in science compared to the extraordinary claims being made about the cause of the Younger Dryas. WUWT has been following the Younger Dryas debate closely for a number of years and I won’t belabor the background information on its evolution except to say that resistance to the idea seems overwhelming despite repeated attempts to present more extensive, rigorous and compelling evidence. Just as with the end of the dinosaurs the extinction of mammoths and other North American megafauna requires an air-tight case and this will only happen when a crater or other irrefutable evidence of an impact is discovered and, preferably, dated.
When confronted with the fiery question of where evidence lies of an impact that only occurred some 12,800 years ago, barely time for a deep breath geologically, the Younger Dryas Impact proponents respond with an extremely weak mumbling about “air blasts,” asteroid clusters and ice sheet impacts that left no evidence of a large chunk(s) of space debris hitting the Earth. They point out that the Tunguska Impact (called an impact even though it left no crater) was an air-blast at an altitude of 3-6 miles (5-10 km) above Siberia that yielded the equivalent of 3-30 megatons of TNT in explosive force. The skeptics counter with Meteor Crater in Arizona, a beautiful little 1,200 ft (4,000 m) diameter hole 570 ft (170 m) deep made by a meteorite a mere 54 yds (50 m) in diameter that released 10 megatons of energy upon impact. They reasonably expect that a piece or pieces of space debris able to spread ejecta over half of the northern hemisphere, cause a 1,300 year halt in global warming and lead to the extinction of numerous species that had survived for hundreds of thousands of years had to leave some evidence of its arrival behind. The YD proponents then countered with the fact that the northern half of North America was covered with ice sheets and an impact on a 1 to 2 mile thick layer of ice may have mitigated the expected formation of a crater.
An impact on an ice sheet does seem reasonable as the evidence from the ongoing investigation points at the impact occurring somewhere in north-central North America near what is now the Great Lakes. Unfortunately, there is no crater evident and the projected impact should have left some imprint on the earth, even if it was a cluster of meteorites or an airburst. Let’s take the researchers at their word and look where the evidence points-the Great Lakes.
Water is capable of hiding almost everything beneath its waves but bathymetric maps showing the depths to lakebed of the Great Lakes can give us some idea of what lies beneath. A number of years ago, I looked at bathymetric maps of the Great Lakes because I was curious about alarmists reports of the five Great Lakes drying up due to global warming. I didn’t see any reason for alarm for the foreseeable future but did note what I considered an anomaly in the map for Lake Michigan in the central part of the southern lake. The map can be viewed at http://www.glerl.noaa.gov/data/bathy/ and is shown below.
Figure 1 Bathymetry of Lake Michigan (Great Lakes Environmental Research Laboratory)
The almost circular feature has a diameter of almost 40 miles (64.5 km) and the topography seems to indicate relatively steep slopes down to a central depression. A higher resolution view of the anomaly is shown in Figure 2 and comes from the Great Lakes Rescue Project at http://www.ngdc.noaa.gov/mgg/greatlakes/lakemich_cdrom/html/images.htm .
Figure 2 High Resolution Bathymetry of Southern Lake Michigan (GLRP)
I am not a geologist but this image definitely suggests the possibility of an impact crater, probably modulated by a considerable thickness of ice lying above the point of impact. A quick check of the assumed conditions at the proposed onset of the Younger Dryas (12900-12800 YBP) shows that at this time Lake Chicago existed as a melt lake with an outlet near Chicago and covered the southernmost tip of the current Lake Michigan. At this time, just north of Lake Chicago, the Michigan Lobe of the Wisconsin Glaciation was in rapid retreat. There is no reason to dismiss the likelihood that the edge of the ice sheet may have lain to the south of the Southern Chippewa Basin (SCB), the location of the anomalous terrain. After the onset of the Younger Dryas, the glacial front advanced for a millennium before the retreat resumed.
A check of the underlying geological explanation for the formation of the SCB is classical geological speculation with no real data. The belief is that easily eroded Devonian shale overlay a more resistant Silurian dolomite (the same or a similar formation to the one that forms Niagara Falls). The glacial front scooped out this almost circular feature by differentially eroding the shale while in doing so leaving behind steep escarpment walls into the basin and a shallow plateau just to the north. The story seems somewhat weak to me and, as far as I can tell, no samples of what lies beneath have ever been obtained from the SCB. I really feel it would be a wonderful idea to find out just what happened to form the SCB.
The Younger Dryas Impact proponents would be well-served by promoting an exploration of the SCB as a starting point while looking for other anomalies further north in the deeper Chippewa Basin and in other Great Lakes. If a smoking gun does exist for their theory, it probably lies under these waters.
As a footnote, an impact with the ice sheet and its catastrophic effects could explain the other primary explanation of the source for the Younger Dryas. An asteroid strike leaving one or more craters in the Great Lakes area could have resulted in a rush of fresh water flowing either north into the Arctic or out the St Lawrence Seaway and shutting down the North Atlantic conveyor transporting heat from the equator. Thus, the two theories would merge into a progressive explanation of the sudden cooling, its long duration and the deaths of the megafauna.