6. Considering the Paleontological Evidence

     Problem 18: The comet explosion theory does not account for the two millennium duration of the Pleistocene extinction. Firestone and West propose that their comet explosion catastrophe occurred at the Alleröd/Younger Dryas boundary. Hence according to their theory most of the casualties should have occurred at this time within a matter of a day or so. However, the mass extinction did not transpire just in a matter of days as their theory suggests. It had already been underway for at least two thousand years prior to that date. For example, Donald Grayson and David Meltzer (2003) note that of the 35 genera involved in that extinction, only 15 can be shown to have lasted beyond 12,000 C-14 years ago (13,900 calendar years b2k) with the implication that the majority became extinct over a millennium prior to the AL/YD transition. While these authors advance this as an argument against the Prehistoric overkill hypothesis, it is equally valid against a terminal comet event. In a May 2007 news interview, Dr. Meltzer noted that the earlier extinction dates for certain genera give reason for doubting the Firestone-West comet theory (Roach, 2007).
 Evidence of the extended character of the Pleistocene extinction may be seen in figure 10 which has been adapted from a "death rate" histogram published in 1983 by Drs. Meltzer and Mead. It charts the temporal distribution of dates on the remains of extinct megafauna taken from various North American sites and shows the number of remains found in each of a consecutive series of time slots, plotted here in terms of calendar years instead of radiocarbon years. The dark histogram bars represent dates occurring prior to the Allerod/Younger Dryas boundary date while the light bars represent dates occurring more recently than that boundary date. The lower pixilated bars represent the subset of more reliable radiocarbon dates. As can be seen here, the megafaunal death rate rose precipitously around 14,350 calendar years b2k, and for 1400 years maintained a relatively high mortality level up to the 12,950 years b2k Younger Dryas boundary date. This poses a problem for the Firestone-West comet theory which attempts to explain the extinction with a short-term comet explosion event or events occurring at the AL/YD boundary.

(click to enlarge)

Figure 10. Upper profile: chronological distribution of calendar dates on remains of extinct land mammals from 163 North American locations. Black bars indicate dates earlier than 12.9 kyrs b2k. Pixilated bars indicate the subset of more reliable dates. Adapted from radiocarbon histograms published by Meltzer and Mead (1985) and Martin (1987).  Lower profile: The rate of meltwater discharge from the North American ice sheet as indicated by the oxygen isotope profile for Gulf of Mexico core EN32-PC4 (after Broecker et al., 1989).

Geophysical correlations with the death rate histogram. In viewing the Meltzer-Mead histogram, one notes a striking correlation between the onset of increased mortality and climate. The peak spanning the interval 14,350 to 13,800 years b2k coincides with a time of peak warmth during the Bolling Interstad, which coincided with a sharp rise in meltwater discharge from the ice sheets, evident in Gulf of Mexico salinity records. The Bölling/Allerod warming was not restricted just to the Northern Hemisphere, but was a period of global warming that is not easily explained as arising due to endogenous fluctuations of the Earth's climatic system (LaViolette, 2005c). In other words, some external effect must have been perturbing the Earth to cause temperatures during this period to rise to levels typical of interglacial periods at a time when the ice sheets were near their maximum extent. It is difficult to pass off the association of this warm period with the rise in megafaunal death rate as a coincidence. The superwave theory proposes that the megafaunal extinction arose because of hazardous effects from a highly active flaring Sun. It also suggests that the glacial meltwater flooding which occurred during this period of global warming posed an additional hazard. As noted earlier, this also was related to the superwave cosmic dust incursion.
 Evidence that supersized solar flares played an important role in the megafaunal extinction is indicated by the highly elevated radiocarbon production rate that occurred from 13,500 to 12,800 years b2k and which is evident in figure 1. As seen in figure 10, this period spanned the highest peak in the megafaunal extinction histogram. Spurts of C-14 production are also found to coincide with earlier peaks in the histogram, including one close to the time of the Gothenburg geomagnetic excursion. We also have evidence from the lunar data of Zook et al (1977) that the deglacial period was marked by highly elevated solar flare activity.
 Problem 19: The progressive rise in atmospheric radiocarbon and the progressive YD cooling does not fit the abrupt signature of a comet explosion. As mentioned earlier, it is unlikely that a comet explosion or impact would itself produce such a large increase in radiocarbon. Besides if this were the case, there should have been an abrupt spike in C-14 of a few years duration, a time comparable to the atmospheric residence time of dust from a comet explosion.. Instead we see a progressive rise with most occurring over a 200 year period following the beginning of the YD cooling. Also we see that an early phase of the rise actually began around 13,500 years b2k, well before the time proposed for the comet explosion. The short duration of an impact event also has difficulty explaining why climate progressively cooled for two centuries. Dust from an impacting comet would have settled out within a few years. So why did climate continue to cool and why was this cooling accompanied by a progressive increase in C-14? The comet theory has no answer.
 Problem 20: A firestorm induction of the YD climatic cooling does not conform to ice core evidence.  According to the supernova/comet theory, fires were ignited at the time of the YD boundary comet explosion (or impact), aggravating and likely extending the cooling caused by the aerial dispersal of cometary dust. The theory's backers claim that evidence of this firestorm is to be found in the "black mat" also termed the "Usselo Horizon," a sooty layer formed both in North America and Europe during this AL/YD transition and attributed to the widespread occurrence of forest fires. However, ice core evidence suggests that this conflagration occurred over a century after the YD cooling had already commenced. Ion data from the GRIP and GISP2 Greenland ice core records register this burning as a many fold increase in formate, oxylate, and ammonium ions, all indicators of biomass combustion. But, it shows that this rise occurred about a century after the date when climate first began to cool, as indicated by the date when the oxygen isotope ratio began its progressive decrease to cooler temperatures.
 Atmospheric dust, and possibly soot, also increased about the time of this increase in oxylate and formate, as indicated by a decrease in the light transparency of the GISP2 Greenland ice core. Ice core transparency declined about 125 ± 15 years after the isotope decline that marked the beginning of the YD cooling. This would be around 12,885 years b2k relative to a YD boundary date of 13,010 years b2k based on the Cariaco Basin varve chronology. Examining the ammonium and nitrate ion concentration data for this portion of the GISP2 ice core record (Mayewski, 1994) and high-resolution ammonium ion data spanning the corresponding section of the GRIP ice core record (Fuhrer, et al.,1996), we find that these indicators attained very high values from 12,884 to 12,853 years b2k, marking an event whose magnitude was unique and that has not recurrred since. This ice core evidence is discussed in detail in the latest update of my dissertation Galactic Superwaves and Their Impact on the Earth Environment (LaViolette, 2008). Based on this polar ice evidence, we may determine that this conflagration began 12,884 years b2k and lasted for three decades. Interestingly, this falls close to the time of one of the two largest C-14 production spurts in the entire Cariaco Basin record which dates at 12,887 ± 10 years b2k. This implicates a super-sized solar storm as being the cause of this ignition, and not necessarily a comet impact. A study of the high-resolution oxygen isotope profile for the GRIP ice core obtained by Johnsen, et al., (1997) shows that climate in Greenland underwent a dramatic an abrupt warming following the occurrence of this solar event, changing from glacial to interglacial temperatures within two years. This warming was likely due to the immense amount of energy imparted to the Earth's atmosphere by the solar proton event. It was quickly followed by a dramatic cooling which may be attributed to the screening of sunlight by the smoke laden air.
 This global conflagration, then, cannot be blamed as the cause of the YD cooling since by that time climate had already cooled almost to the YD temperature low. The cooling, which had already been in progress for 125 years prior to that time, must be attributed to a cause other than a comet impact and its proposed firestorm. Solar flare cosmic rays appear to be the most likely choice. Solar cosmic rays striking the atmosphere would have ionized the atmosphere, producing large quantities of condensation nuclei which would have seeded cloud formation. The resulting global increase in cloud cover would have resulted in a global climatic cooling. In support of this, we find that periods when atmospheric radiocarbon was increasing, i.e., periods of high solar activity, coincide with times of climatic cooling. Furthermore such a solar flare-climate connection explains why dust peaks in the polar ice record recur with the solar cycle period (Ram and Stolz, 1999; Donarummo, 2002). Solar cosmic rays would have induced the formation of clouds having droplets too small to produce rain, resulting in a period of greater aridity and atmospheric dustiness.
 Problem 21: Proposed thermal neutron flux intensities way too high. When Firestone first discussed the YD boundary extinction in 2001, he had suggested that the C-14 anomaly was produced by cosmic rays from a supernova explosion. At that time he did not give a date for the supernova explosion, but four years later he and West (LBNL, 2005) suggested that the event took place 41 kyrs b2k with cosmic rays and high-velocity extraterrestrial material from the outburst arriving around 34,000 and 12,950 years ago. Firestone and Topping (2001) had proposed that on the AL/YD boundary date thermal neutron fluxes reached as high as 1017 neutrons/cm2, and Firestone (2002) later quoted fluxes as high as 1020 neutrons/cm2. Based on the calculations of Southon and Taylor (2002), these levels would translate into the equivalent absorption of between 106 and 109 rads per hour, or to an energy influx of 105 to 108 ergs/cm2/s. This is between 10% and one hundred times the level of insolation that the Earth receives from the Sun. If this were the case, it is difficult to understand how their proposed four kilometer diameter frozen cometary mass would have made its 250 light-year journey to the solar system intact. For if it were accompanied on its journey by such high cosmic radiation intensities, it should have been entirely vaporized. The Earth, then, should have been struck by a hot nebular wind, not by a frozen cometary mass.
 Moreover, in their critique of Firestone and Topping's article, Southon and Taylor have pointed out that if the Earth's atmosphere had been exposed to neutron fluxes even as high as 1017 neutrons/cm2, over a 1000 km diameter region, radiocarbon produced from neutron bombardment of Earth's nitrogen atmosphere should have increased atmospheric C-14 levels globally by a millionfold, leaving current C-14 levels thousands of times higher than are actually observed. This is not to say that cosmic ray radiation did not play a critical role in the terminal Pleistocene megafaunal extinction, but not quite in the fashion that Firestone, Topping, and West have suggested.


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