TERRESTRIAL EVIDENCE OF A NUCLEAR CATASTROPHE IN PALEOINDIAN TIMES
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[updated response to post publication questions][.doc version]
by Richard B. Firestone, Lawrence
Berkeley National Laboratory,
and William Topping, Consultant,
THE PALEOINDIAN OCCUPATION of North America, theoretically the point of entry of the first people to the Americas, is traditionally assumed to have occurred within a short time span beginning at about 12,000 yr B.P. This is inconsistent with much older South American dates of around 32,000 yr B.P.1 and the similarity of the Paleoindian toolkit to Mousterian traditions that disappeared about 30,000 years ago.2 A pattern of unusually young radiocarbon dates in the Northeast has been noted by Bonnichsen and Will.3,4 Our research indicates that the entire Great Lakes region (and beyond) was subjected to particle bombardment and a catastrophic nuclear irradiation that produced secondary thermal neutrons from cosmic ray interactions. The neutrons produced unusually large quantities of 239 Pu and substantially altered the natural uranium abundance ratios ( 235 U/238 U) in artifacts and in other exposed materials including cherts, sediments, and the entire landscape. These neutrons necessarily transmuted residual nitrogen ( 14 N) in the dated charcoals to radiocarbon, thus explaining anomalous dates.
The evidence from dated materials
We investigated a cluster of especially young radiocarbon dates concentrated in the north-central area of North America. For example, at the Gainey site in Michigan a 2880 yr B.P. radio-carbon date was reported, while the thermoluminescence date for that site is 12,400 yr B.P.5 Other anomalous dates found at Leavitt in Michigan, 6 Zander and Thedford in Ontario,7 Potts in New York,8 Alton in Indiana, 9 and Grant Lake in Nunavut 10 are summarized in Table 1. The Grant Lake Paleoindian site is most remarkable because its 160 [rc] yr B.P. age is nearly contemporary, while adjacent and deeper samples give ages of 1480–3620 [rc] yr B.P.
Stratigraphic associations place Paleoindian occupations at depth on the pre-historic North American landscape on sediments that form the old C horizon composed of parent material, Wisconsinan deposits that predate Holocene sediment buildup.11,12,13 The young Paleoindian dates cannot be correct, particularly since there are no patterned anomalies noted in later-period prehistoric assem-blages relating to higher stratigraphic positions. In a pioneering study of the Paleoindian site at Barnes, Michigan, Wright and Roosa observed that Paleoindian artifacts were deposited before the formation of spodosols ceased in this area about 10,000 yr B.P.14 This conclusion was based on observing that cemented sediments on artifacts, found outside their original context, defines their original stratigraphic position.
The evidence from particle bombardment
Sediment profiles were taken at Paleoindian sites and at numerous widely separated control locations in Michigan. The C sediment horizon is clearly recognized by its transitional color and confirmed by elevated concentrations of potassium and other isotopes. Color and chemistry are key indicators of this very old soil 11,12,13 derived from parent materials and associated postglacial runoff.15 At Gainey, large quantities of micrometeorite-like particles appear to be concentrated near the boundary between the B and C sediment horizons. They can be separated with a magnet and are identified by the presence of chondrules and by visual evidence of sintering and partial melting. These particles, dissimilar to common magnetites, are found in association with a high frequency of "spherules." The depth profiles for potassium and particles at the Gainey site are compared in Fig. 1. Minor vertical sorting of particles is apparent, with a shallow spike of particles near the surface probably resulting from modern agricultural or industrial activity. Total gamma-ray counting of sediment profiles in the various locations invariably showed increased radioactivity at the B-C boundary consistent with enhanced potassium ( 40 K) and possibly other activities.
Microscopic examination of chert artifacts from several widely separated Paleoindian locations in North America revealed a high density of entrance wounds and particles at depths that are evidence of high-velocity particle bombardment. Chondrules were identified visually; their presence necessarily indicates heating during high-speed entry into the atmosphere. The depth of penetration into the artifacts implies that the particles entered with substantial energy.16 Field simulations with control cherts for large particles (100–200 microns) suggest an entrance velocity greater than 0.4 km/s, and experiments at the National Superconducting Cyclotron Laboratory indicate that the smaller particles left tracks comparable to about 526 MeV iron ions ( 56 Fe) in Gainey artifacts. Similar features are not observed in later-period prehistoric artifacts or in bedrock chert sources. Track angles were estimated visually; track densities were measured with a stage micrometer; track depths were found by adjusting the microscope focus through the track. These data are summarized in Table 1.
Track and particle data in Table 1 suggest that the total track volume (density times depth) is highest at the Michigan, Illinois, and Indiana sites and decreases in all directions from this region, consistent with a widespread catastrophe concentrated over the Great Lakes region. The nearly vertical direction of the tracks left by particle impacts at most sites suggests they came from a distant source.
The evidence from uranium and plutonium
Natural uranium, which is ubiquitous in cherts, has a 235 U/238 U isotopic ratio of 0.72 percent, which varies by less than 0.1 percent in natural sources.17 Significant variations in the isotopic ratio do not occur because of chemical processes; however, a thermal neutron bombardment depletes 235 U and thus alters the ratio. Solar or galactic cosmic rays interacting with matter produce fast secondary neutrons that become thermalized by scattering from surrounding materials. Thermal neutrons see a target of large cross section (681 barns)A for destroying 235 U, compared with a target of only 2.68 barns for neutron capture on 238 U. Therefore, despite the low abundance of 235 U, about 1.8 times as many 235 U atoms are destroyed as 238 U atoms by thermal neutrons.
If a large cosmic-ray bombardment impacted the earth and irradiated the prehistoric landscape with thermal neutrons, the 235 U/238 U ratio would be changed; 239 Pu would be produced from neutron capture on 238 U, followed by the decay of 239 U. Neutrons colliding with nitrogen (1.83 barns) would create 14 C in exactly the same way 14 C is normally produced in the upper atmosphere, necessarily resetting the radiocarbon dates of any organic materials lying near the surface on the North American prehistoric landscape—including charcoals at Paleoindian sites—to younger values. 239 Pu produced during the bombardment will also be partly destroyed by thermal neutrons with 1017 barn cross section. Assuming 239 Pu doesn’t mobilize, it will decay back to 235 U (half-life 24,110 yr), partially restoring the normal abundance.