by Bob Kobres
One thing that can be stated with high confidence about impact events on Eartheach will be different. This seems rather obvious, yet there is a tendency of academic procedure to try to classify phenomena into previously determined definitions. This may in fact be too constraining given our meager knowledge of the physical makeup of objects that have violently joined our planet.
Short of finding an impact site, the best means of identifying accretion events that have affected the biosphere of Earth is to focus on determining the rate and extent of change that have been preserved in the sedimentary record. Quick changes are suspicious in that the status quo of any period tends to be stable over a long time interval. Of natural phenomena, there is nothing as capable of producing conditions that can disrupt an existing order as the interaction of the biosphere with extraterrestrial objects.
A major recent quick-change marks the transition from the Pleistocene to the present or Holocene time period, however no true impact crater thus far located dates to the ~11,500 BP rapid change time. What else might we search for to determine if this change was caused by external input?
In my opinion it is necessary to focus on all evidence that can be dated to this transitional time so that a rate of change gauge can be formulated. It is important to appreciate that an impacting object may consist of nothing more distinguishing than water and dust. Also of importance is noting unusual features of this time period such as thick, widespread, relatively sterile, layers of sediment that could have been deposited by flood waters and wind. Large changes in fauna and flora are supportive of externally induced change as well. But even if the tree-ring people get a good wood sequence across this time of change that proves the transition to be abrupt there is still apt to be doubt as to cause without the proverbial smoking guna crater or so.
As I have indicated elsewhere I do not view the Carolina Bays as true impact craters but I do think that they are impact related structures. The way that I suspect these features came about has to do with some generally ignored aspects of weak material impacting Earth. First there is the tendency for friable conglomerates to pull apart as they encounter Earths gravity-well and further disrupt and spread apart as they enter our planets atmosphere. This will allow a great deal more energy to be exchanged in the atmosphere than would be the case for an object of similar mass but made of tougher stuff. The result of this is a lot of radiation (mostly in the infrared) roasting the surface of our planet for several seconds prior to the arrival of any blast or over-pressure wave. Obviously this radiation will not be absorbed equally by all materials that happen to be exposed to it and because of the fairly short duration of the exchange even a tree canopy would likely shield much of the ground beneath the vegetation. What would suck in the energy quite readily would be water exposed to the sky.
Now at this point it is also important to realize that the geometry of any non-shaded body of water will play a major role in how the fluid will respond to the energy exchange as it evolves. With the same exposed surface area, a deep pool will not experience as great of a water temperature change as a shallow pool. Also, due to a greater volume to surface area ratio, a deeper body of water will not experience as rapid of a pressure change when the compressed atmosphere pushes upon it as a shallow pond with the same surface area. To illustrate the influence of geometry it is useful to think of two containers that are each capable of holding eight cubic meters of water. Container B has a surface area of one square meter and is eight meters deep. Container C has a surface area of eight square meters and is one meter deep. For simplicity the radiant energy will be constant at 100 kilo-calories (kcal) per square meter per second for ten seconds which will result in container B absorbing 1,000 kcal, and C seething with 8,000 kcal. Obviously container C is a much more efficient heat collector than the other example due to its larger surface area. It would require eight container Bs to collect as much energy and that heat would be distributed throughout eight times the volume of water resulting in a much smaller temperature increase than would be the case for the water in container C. As for the effect of a compressed atmosphere on the two containers it is necessary to keep in mind the duration of the pressure change and how the contained fluid would respond mechanically to that change. In general container B will respond sluggishly as compared to container C which will rise or fall to applied pressure almost immediately. From this reasoning it could be anticipated that a large footprint impact event might leave the fish in a large deep lake relatively unscathed while it blasted boiled beavers out of their shallow ponds and into the beyond by means of violent steam explosions.
The point that I am attempting to emphasize here is that, with regard to potential influence on the biosphere, an accretion event begins when the extraterrestrial stuff gets trapped by Earths gravity not when it slams into earth or water on the planets surface. As with nuclear air bursts a violent liberation of energy in the atmosphere can affect a much greater area than would the same release at ground level; features such as the Carolina Bays may well be surviving signatures of this type of impact event. Steam explosions induced by the dynamics of an impact event do not require a one to one relationship with pieces of the impacting object, one passing fireball could cause a number of suitable ponds to violently flash into vapor.
The Atlantic coastal plain had experienced the handy-work of beavers for thousands of years prior to whatever caused the rapid change, which included a rise of sea level, around 11,500 years ago. It is very likely that this region of North America, where Carolina Bays are found today, was an extensive wetland area created by members of this keystone species. Unfortunately for the buck-toothed rodents, the thousands of shallow ponds that they had engineered would be ideal reactors to a radiant-heat/ over-pressure/ under-pressure event such as outlined above. In the wake of such an encounter the quilt of irregularly shaped ponds formed by the beavers, or the lay of the land, would have a geometry imposed upon them by the steam explosions, giving the still shallow basins an unusually uniform appearance.
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