Problem 22: No evidence for a continent-wide thermal gradient.  The theory of Firestone and West suggests that an aerial explosion or impact of a comet in the vicinity of eastern Canada delivered a shock to the North American ice sheet causing the catastrophic release of glacial meltwater producing floods that spread over the continent. Micrometeorite crater depths evident in PaleoIndian chert samples are found to decrease 20 fold when one proceeds from Michigan to Arizona. So one would expect to see a decline in flooding intensity as well with increased distance from the explosion or impact site. However, there is no indication that flooding intensity declined from eastern North America to Siberia on the west or to Europe on the east.
     Also their proposal that the heat of the explosion would have created a ground level firestorm and that this would account for the formation of the black mat must explain why it appears as well in Europe as the Usselo Horizon. In fact, iridium levels in the European horizon were found to be higher than those found in the North American black mat (Kloosterman, 2007; Firestone, et al., 2006, p. 352). The comet explosion theory could circumvent this by proposing simultaneous impacts in various parts of the globe. However, as was demonstrated earlier, comets from such a supernova would have been few and far between. So, it is difficult to imagine a worldwide barrage, at least with the supernova theory.
    The superwave explanation for the mass extinction.  In contrast to the comet theory, the superwave theory predicts an extended hazardous period, one that transpires over thousands of years with varying intensity. Hence it is able to account for the extended duration of the megafaunal die-off evident in Meltzer and Mead's histogram. As seen in figure 11, the rise in cosmic ray intensity coincided with the rise in megafaunal death rate. As a result of its injection of cometary and interstellar dust, and through its effect on the Sun and Earth, a superwave cataclysm would have been able to abruptly alter the Earth's climate (e.g., produce stadials and interstadials). It would also have increased the Sun's luminosity and level of flaring activity. These effects would have adversely affected biological organisms over several millennia and would all have contributed to the demise of the megafauna.      A diagram of the various ways in which a superwave event would have been hazardous to life is presented in chapter 3 of my dissertation (LaViolette, 1983a); download chapter 3 excerpt.  Interestingly, the model I had proposed in 1983 postulated two cosmic ray peaks arriving during the deglacial interval, one beginning around 14,200 years BP and lasting for 500 years and a second beginning around 13,500 years BP and lasting for 1000 years.  The timing of these peaks turned out to be not far off from what the polar ice record shows.

(click to enlarge)

Figure 11. Megafaunal death rate histogram (hatched profile) compared to beryllium-10 flux in the Byrd ice core (lower gray graph). Be-10 peaks are indicated by arrows.

     A superwave cause would explain why unusual climatic, hydrologic, geomagnetic, and radiological events were taking place during this period of mass extinction. In particular, the geomagnetic flip that occurred at the first death-rate peak and the unstable geomagnetic field that persisted throughout this extinction episode would be a direct result of the opposing polarity imposed by the ring-current magnetic field. The ring current field is the magnetic field generated by the equatorial drift of solar cosmic rays trapped in the Earth's storm-time radiation belts and becomes particularly strong during the arrival of an intense solar proton event (LaViolette, 1983a, 1987a, 1990).
     As mentioned earlier, the C-14 rise at the Allerod/Younger Dryas boundary may be attributed to the intense solar flare storms that were occurring at that time. The Be-10 flux is observed to dip during the interval 13,400 to 12,750 years b2k; see arrow markers in figure 2 and figure 3. Since this period coincides with the rise in C-14 concentration during the AL/YD transition, we may infer that the cosmic ray screening effect of the Sun's enhanced solar wind strength was able to overcome the propensity for Galactic cosmic rays to elevate the Be-10 flux during this interval, suggesting that solar flare activity was quite high at that time.
     The termination at the Clovis horizon could have been due to a particularly powerful solar proton event that was sufficiently strong to overpower the geomagnetic field and contact the Earth's atmosphere (LaViolette, 1990). The four largest C-14 production spurts to occur over the extent of the 4,000-year Cariaco Basin C-14 record (figure 1), occurred on 13,023 ± 10, 12,954 ± 10, 12,887 ± 10, and 12,689 ± 10 calendar yrs b2k (Cariaco Basin chronology). They are spaced from one another by intervals of 69 ± 4 years, 67 ± 4 years, and 198 ± 4 years, that is, by three, three, and nine 22.2-year solar cycle intervals. Three of these events occurred during the Alleröd-to-Younger Dryas climatic transition. Of these, the 12,887 years b2k solar proton event was most likely the terminal event ending the Pleistocene extinction since through its association with the biomass combustion episode recorded in the Greenland ice record, we may connect it to the formation of the black mat which overlies the bones of the extinct Pleistocene mammals.
    During the 12,887 and 12,689 years b2k events, C-14 jumped 2 percent. Based on the time interval between successive sediment samples, these jumps each occured within 8 to 12 years. This is 5 times the rise in radiocarbon produced during the course of a typical solar cycle in modern times. By comparison, the solar proton event that impacted during the 1956 solar maximum, which was one of the largest in modern times, produced a C-14 increase of only 0.016 percent (4% of the variation produced over the course of a typical solar cycle) (Usoskin, et al., 2006). So, we may infer that the 12,887 and 12,689 years b2k C-14 spurt events were 125 times more intense than this record breaking 1956 event. Since the 1956 solar proton event was able to produce a 1% decrease in the geomagnetic field, it stands to reason that these ice age events, which were 125 times stronger, would have entirely overpowered the Earth's field, allowing the full intensity of their solar cosmic ray barrage to contact the Earth's atmosphere.
     By carefully examining the high-resolution electrical conductivity measurement (ECM) data obtained for the GISP2 ice core by Taylor (1993), I have located an acidity spike at a depth of 1708.65 meters that dates at 12,883 ±10 years b2k using the Cariaco chronology and that coincides with the sudden rise in ammonium, formate, and nitrate ion, as well as with the sudden climatic warming (LaViolette, 2008). The acidity spike was a very brief event, lasting less than five weeks, with an initial rise that lasted less than two weeks. Hence we may conclude that it is a record of a super sized solar proton event and that the C-14 spurt recorded in the Cariaco Basin record was also an abrupt rise associated with this solar event. All of the above ice core evidence together with the closely correlating Cariaco Basin C-14 evidence makes a very strong case that a very large solar proton event was the terminal event in the Pleistocene extinction.
     Also as I suggested in 1983, the anomalously young dates found in many megafaunal remains could be evidence of exposure to an intense flux of solar cosmic rays.  I had proposed that excess C-14 may have been created in situ when solar flare proton primaries produced thermal neutron secondaries within the plant or animal tissues, these in turn becoming captured by nitrogen atoms present there transmuting them into excess C-14 (LaViolette, 1983a, ch. 10); download chapter 10 excerpt.  Topping subsequently proposed a similar in situ C-14 production mechanism to explain the anomalously young dates found at PaleoIndian sites (Topping, 1998; Firestone and Topping, 2001). However, one difference is that my 1983 model postulates that the majority of the incident particles were protons, not neutrons.  These would have passed through the Earth's nitrogen atmosphere without producing excessive amounts of C-14, thereby avoiding the problem that Southon and Taylor were concerned about in their critique of the Firestone-Topping paper. Topping (2007) has long preferred the giant solar flare alternative over Firestone's supernova theory as the cause of the extinction. Although the article he coauthored with Firestone in 2001 emphasized the supernova alternative, he advocates a ground contacting SPE as the cause of the elevated radioactivity levels he found at the Clovis horizon.  However, he proposes the event occurred 400 years later around 12,500 years ago.
     It is likely that the Earth's surface would have received nonuniform exposure to solar flare cosmic rays during a ground-contacting SPE.  Particles confined by the collapsing geomagnetic field lines could have been dumped at high intensities in some localities leaving others weakly exposed.  This could explain why date anomalies vary in magnitude from one place to another or from one animal remain to another.
     In summary, the superwave theory is compatible with the idea that cometary masses had impacted the Earth during the period of the megafaunal extinction. As mentioned earlier, the theory predicts that such bodies would have been injected into the solar system at an increased rate during that time.  But it is apparent that a comet impact was not the sole cause of the extinction.  The agent causing both the disappearance of large animals and triggering the influx of comets at that time was not a supernova, but more likely a Galactic superwave.

A chronology of discoveries relevant to the extraterrestrial cause of the megafaunal extinction is presented on page 13.