The evidence to support a hypothesis must be comprehensive in order to be able to reach a scientifically valid conclusion.
Cherry Picking the Younger Dryas. This video discusses the selection of data to provide support for a hypothesis. In general, all the relevant data must be considered and any discrepancies between the hypothesis and data must be reconciled so that the hypothesis can be widely accepted. Choosing only the most beneficial items from what is available is called cherry-picking and it invalidates or limits any conclusions that can be drawn from the sample.
After the publication of my paper in the journal Geomorphology in 2017, I have been promoting the Glacier Ice Impact Hypothesis as a mechanism for the formation of the Carolina Bays and the Nebraska Rainwater Basins. The hypothesis states that an extraterrestrial impact on the Laurentide Ice Sheet ejected glacier ice boulders in ballistic trajectories and their oblique secondary impacts liquefied the ground and created inclined conical cavities that became shallow elliptical bays by viscous relaxation.
The Younger Dryas Impact Hypothesis published in 2007 by Richard Firestone and 25 co-authors proposes that one or more extraterrestrial objects exploded over northern North America triggering a cooling event and a megafaunal extinction. The paper presented magnetic microspherules and other proxies as evidence of the extraterrestrial impact. The Glacier Ice Impact Hypothesis fits well within this framework because the secondary impacts of the glacier ice boulders in the ejecta curtain had sufficient energy to cause the extinction from the Rocky Mountains to the East Coast of the United States.
Occasionally, I contact researchers or university professors by email with comments about their research and my perspective on the Younger Dryas Impact Hypothesis. Generally, my notes are ignored, but sometimes I get responses. Recently, I contacted Professor Vance Holliday after having read a paper he co-authored with three other scientists in 2020. Dr. Holliday is a Professor at the School of Anthropology and Department of Geosciences at the University of Arizona and he has published three papers critical of the Younger Dryas Impact Hypothesis.
His paper published in 2014 with four co-authors states that "Geomorphic, stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the Younger Dryas Boundary. Late Pleistocene extinctions varied in time and across space. Archeological data provide no indication of population decline, demographic collapse or major adaptive shifts at or just after 12,900 years ago. The data and the hypotheses generated by Younger Dryas Impact Hypothesis proponents are contradictory, inconsistent and incoherent."
His paper published in 2016 with two co-authors addressed two questions that persist in the debate over the Younger Dryas Impact Hypothesis: Can the results of analyses for purported impact indicators be reproduced? And are the indicators unique to the lower Younger Dryas Boundary at ~12,800 calendar years before the present? The paper concluded that the purported impact proxies are not unique to the Younger Dryas Boundary.
The title of the paper published in 2020 is "Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ~12,800 Years Ago, Parts 1 and 2: A Discussion", and it had several statements that called my attention. In my email to Professor Holliday, I wrote: In your recent publication, the introduction says that "Human population decline in North America is not supported by abundant published archaeological, geochronological, and stratigraphic evidence." and the last paragraph says that "Most purported evidence for a Younger Dryas Boundary disaster is at only the particulate and molecular scales. With the exception of some fauna, field records provide no evidence whatsoever of continent-wide catastrophic changes at ~12,900 calendar years before the present."
My email to Prof. Holliday said: I would like to call your attention to a couple of papers that discuss a human Y-chromosome bottleneck at the end of the Pleistocene. This was something that humans experienced worldwide. The paper by Karmin with 100 authors proposes that the bottleneck was caused by a global change in culture. Zeng, et al. attribute the bottleneck to competition between patrilineal kin groups. However, the Y-chromosome diversity started to decrease in all regions of the world exactly at the Younger Dryas boundary ~12,900 years ago, and it does not seem likely that similar cultural changes would have occurred simultaneously in regions widely separated by oceans, deserts, mountains and jungles. My email included a graph from Karmin of the Y-chromosome bottleneck and the Younger Dryas temperature record.
The reply from Professor Holliday said: Dear Mr. Zamora: Thank you for your email about the Carolina Bays. I am familiar with your 2017 paper. I must take issue with most of what you present in your message and your paper, however. I still stand by the quotes from our recent publication on the impact hypothesis. Regarding humans, the Younger Dryas Chronozone, and bottlenecks, my “gold standard” is the archaeological record and it provides no evidence whatsoever for a population decline during the Younger Dryas (as proposed by the impact proponents) nor during the early Holocene.
In the figure you sent from the paper by Karmin et al, you neglected to send the full illustration (below). The mitochondrial DNA data (which you left off) shows a very different story with no bottleneck but rather a population rise in the Holocene. Sadly, this sort of cherry-picking of data is a fundamental component of the YD Impact hypothesis.
Getting called out for cherry-picking elicits several reactions. First of all, it is insulting. The term implies that I am biased, but the generalization that cherry-picking is a fundamental component of the Younger Dryas impact hypothesis implicates all the scientists who have authored dozens of papers as collaborators of a conspiracy to undermine the scientific method. However, the accusation also causes me to question myself. Am I excluding important information? Am I being thorough? Do I take into account all the cases or am I just selecting specific information to make my case?
The fact that Karmin and 100 co-authors mention the Y-chromosome bottleneck in the title of their publication means that they already selected this as the main and most important result of their research. For this reason, I am not convinced that the image that illustrates what that the authors themselves have emphasized constitutes cherry-picking on my part. Also, a male Y-chromosome bottleneck does not preclude overall population growth. It is possible for the total population to grow as long as the number of females exceeds the number of males eliminated from the population during the bottleneck.
Regarding the archeological record as a "gold standard" for determining population decline, there seems to be a substantial body of archeological work showing that the human population declined during the early Younger Dryas. The article by David Anderson, Albert Goodyear, James Kennett and Allen West presents three approaches that confirm this.
One of the lines of evidence is the number of Clovis points found at 13,000 calendar years before the present, which is just before the onset of the Younger Dryas. One thousand years later there is a decrease in points that implies a decrease in the population of the study area in the dark outline.
The Clovis people disappeared at the onset of the Younger Dryas and they were followed by the Folsom people. The reduction in the number of points is evident when the distribution is compared to Clovis points in the same region. In addition, the number of points of each style shows a decline followed by a increase consistent with the change in population. So, the archeological record clearly shows a decrease of population in the United States following the onset of the Younger Dryas.
But wait! Professor Holliday says that the archaeological record provides no evidence for a population decline during the Younger Dryas and qualifies it by saying "as proposed by the impact proponents". Well… It turns out that three of the four authors, Albert Goodyear, James Kennett and Allen West, are members of the Comet Research Group and they were also co-authors of the 2007 paper by Firestone. Does this mean that their analysis is somehow flawed and cannot be trusted? The only thing of which we can be certain is that the contentious debate over the Younger Dryas Impact Hypothesis and cherry-picking will continue for many years.
The reply from Professor Holliday to my email continues. Regarding the age of the Bays, your discussions of the OSL dating on pages 213 and 215 do not cite any of the OSL literature nor do they deal directly with the dating. If you consult the publications on the OSL dating you will see that dates are not on the basins but on the Bay rims, long known to be composed of eolian sand. They are clearly not the result of overturned beds. Here solid field data is far superior to remote imagery. OSL is ideally suited to dating eolian sand. The dating provides reproducible results showing that the rims formed ~140-120k years BP and ~50-12k years BP and rest on Last Glacial Maximum and older strata
Actually, my paper did mention the OSL dates reported by Brooks, et al. and the dates are consistent with Professor Holliday's reply. But I must object to the statement that the bay rims "are clearly not the result of overturned beds" because this is not ordinarily tested. And if the tests for overturned flaps are not performed, it is not possible to assure that they are not there.
Overturned flaps, overturned beds or inverted stratigraphy are described by Professor Jay Melosh in his book "Impact Cratering: A Geologic Process". Impact cratering displaces material laterally by horizontal compressive forces and ejects debris ballistically to produce uplifted rims. The stratigraphy near the crater rim is inverted as the flanges produced by the penetration of the projectile fold over the existing landscape.
Inverted stratigraphy can only be detected by examining at least three sections of a core sample in the rim. Proceeding from the top down, the surface layer contains the youngest material that accretes by ordinary eolian and sedimentary processes. Immediately below the youngest layer there is a layer of older material that was excavated during the formation of the elevated rim. Below the layer of older material is a layer of young material that was the surface of the terrain before the impact cavity formed. Deeper layers contain progressively older material.
Now, take a look at the OSL dates published by Brooks, et al. Each sample location has only one value, but for each location a minimum of three samples at different depths are necessary to test for inverted stratigraphy. From the dates obtained by Brooks, it is not possible to determine if there are layers that indicate inverted stratigraphy at any of the locations tested.
By contrast, a paper published by Bunch and 17 co-authors in 2012, shows the dates obtained at different depths of a single test location in the rim of a Carolina Bay near Blackville, South Carolina. The samples for optically stimulated luminescence were taken at 107, 152, and 183 centimeters below the surface. The layer at 107 centimeters had an age of 11.5 thousand years. The layer at 152 centimeters had a date of 18.5 thousand years, and the layer at 183 centimeters had a date of 12.9 thousand years. This clearly shows inverted stratigraphy since an older layer is sandwiched between two relatively younger layers. As far as I know, this is the only publication that has dated different layers in the same core sample of a Carolina Bay rim.
Experimental impacts demonstrate how the overturned flanges created by an impact become the raised rims of the craters with inverted stratigraphy. In this image, the red arrow points to a layer of green sand that was originally on the surface and is now covered by older material excavated by the impact. Eventually, a younger layer will be deposited on top of the older layer by eolian or lacustrine mechanisms.
The reply from Professor Holliday continues: Finally, the notion that an impact hit glacial ice over Saginaw Bay, Michigan, at the beginning of the Younger Dryas Chronozone is impossible because that area was ice-free at the time! Some of the other impact proponents such as Firestone have made this same mistake, i.e., not consulting readily available data on the glacial history of the Great Lakes. I attach a recent commentary on the origins of Saginaw Bay. Professor Holliday attached a paper by Schaetzl, et al. which states that Saginaw Bay was ice-free at the onset of the Younger Dryas.
The idea that there was an extraterrestrial impact at Saginaw Bay is derived from the work of Davias and Gilbride who in 2010 calculated the convergence point of the Carolina Bays and the Nebraska Rainwater Basins at Saginaw Bay based on the axial orientations of the bays. The database with the orientations of the bays is freely available to anyone who cares to re-calculate the convergence point.
Saying that an impact at Saginaw Bay is impossible poses a real dilemma. The Carolina Bays and the Nebraska Rainwater Basins have elliptical geomorphology and radial orientation toward Saginaw Bay. This means that the bays originated as inclined conical cavities from impacts of some material originating from the convergence point of the bays. If the impacts were not made by ejected ice, then what kind of material made the bays? Saying that there was no ice at the convergence point of the bays leaves the origin of the bays unsettled because there is no obvious impact crater there, but this is exactly what we would expect if a layer of ice had attenuated the force of an extraterrestrial impact at that location.
The high energy of emplacement of the Carolina Bays requires them to be associated with an extinction event from the Rocky Mountains to the East Coast of the United States. The onset of the Younger Dryas is the most likely time for the emplacement of the Carolina Bays. A future generation of scientists will have to figure out why there does not seem to be evidence of ice at the convergence point of the bays at the time of the extinction event.
My experiments have demonstrated that oblique impacts of ice projectiles on a viscous target produce inclined conical cavities that are elliptical when viewed from above. These experiments are easy to reproduce. I have found that the conical cavities on sloping ground are deformed during the viscous relaxation.
These deformations can be demonstrated by tilting the experimental container. In general, any deviations from elliptical geometry of the Carolina Bays can be explained based on the characteristics of the terrain and the erosive forces of wind and water.
The scientific method is by necessity an adversarial process where different hypotheses are tested against physical evidence to determine which hypothesis provides the best explanation. The objections of the harshest critics provide the best clues for improving and demonstrating the validity of a hypothesis. A new scientific hypothesis has to face and withstand many challenges on its way to becoming part of mainstream science.