Abstract: A common problem at Paleo-Indian sites in the Great Lakes region of North America is the recovery of radiocarbon dates as much as 10,000 yr younger than dates obtained with other methods1. Paleo-Indian artifacts are always found at the B-C sediment interface, consistent with great antiquity. The discovery of micrometeorite tracks and embedded micrometeorites in Paleo-Indian artifacts across North America proves they were bombarded during a cosmic event. Measurements of uranium, depleted in 235U by 17-77%, and unexpected concentrations of 239Pu in artifacts, associated chert types, and sediments at depth indicates that they were also bombarded by thermal neutrons produced in cosmic-ray induced nuclear reactions. Radiocarbon dating assumes that there is no substantial change in isotopic composition over time, but a thermal neutron event would convert residual 14N in charcoal to 14C thus resetting the radiocarbon date. Analysis of data from 10 sites across North America indicates there were episodes of cosmic ray bombardments in Late Glacial times. The data are consistent with a recent catastrophic event at 12,500+-100 bp that irradiated the artifacts and associated charcoal with a thermal neutron flux of ~1015 neutrons/cm2. Previous events ~40,000 bp and earlier affected the bedrock chert sources. These data provide the first clear body of terrestrial evidence supporting either one of two prevailing hypotheses for catastrophe in Paleo-Indian times: 1) a giant solar flare during a geomagnetic excursion as explored by Wolfendale2 and Zook3, and 2) a supernova shockwave as forwarded by Brackenridge4, Clarke5, and Dar6. The evidence is reviewed, and implications for Late Glacial mass extinctions and associated plant mutations are explored.
1 Bonnichsen, R. Clovis Origins, in Clovis: Origins and Adaptations (eds Bonnichsen, R. & Turnmire, K.L.) 309-329 (Center for the Study of the First Americans, Corvallis , 1991).
2 Wdowczyk, J. and Wolfendale, A.W., Nature 268, 510-512 (1977).
3 Zook, H.A., Proc. Conf. Ancient Sun, 245-266 (1980).
4 Brackenridge, G.R., Icarus 46, 81-93 (1981).
5 Clark, D.H., McCrea, W.H., and Stephenson, F.R., Nature 265, 318-319 (1977).
6 Dar, A., Laor, A., and Shaviv, N.J., Phys. Rev. Lett. 80, 5813-5816 (1998).
This work was supported by the National Science Foundation under grant PHYS-9986999 and by the Director, Office of Energy research, Division of Nuclear Physics, of the Office of High-Energy and Nuclear Physics, of the U.S. Department of Energy under contract DE-AC03-76SF00098.
Table 1. Data observed for Paleo-Indian artifacts.
Table 2. Relative 235U concentrations in artifacts and control cherts.
Figure 1. Map of Paleo-Indian sites.
Figure 2. Sediment profiles and potassium at the Gainey site.
Figure 3. Photos of chondrules and tracks in artifacts.
Figure 4. Ice and marine core data showing a recent event at ~12.5 kyr bp.
Figure 5. Geographical distribution of event data.
|Figure 1, unetched (no acid) view of surface of flake with apparent embedded particle; angled shatter cone visible.|
|Figure 2, unetched view of "orangish" spot on surface of flake; all of these spots are preferential (on only one side of respective flakes).|
|Figure 3, unetched view of more orangish spots on surface of flake.|
|Figure 4, unetched view of more orangish spots; lower center spot appears to be within an "impact" zone as evidenced by shatter feature primarily visible above spot.|
|Figure 5, etched (flake acid-treated, HF 48%, 66oF, 2 minutes) view of pits revealed "under" the orangish spots; in all cases, the orangish spots were pits that apparently infilled with iron.|
|Figure 6, etched (HCL, 28%, 66oF, 10 minutes) view of angled impact pits on surface of flake.|
|Figure 7, etched (HCL) view of more angled impact pits on surface of flake to right of those featured in Figure 6.|
|Figure 8, etched (HCL) composite view of Figures 6 and 7; "trajectories" drawn in, all parallel, angled pits to immediate right of lines (note: long straight line at right 1/3rd of photo is artifact of tape shadow although small pit is visible to left of micron scale).|
|Figure 9, etched (HCL) view of three (3) large impact pits at a diagonal from lower left to upper right; note smaller pits visible near features.|
|Figure 10, etched (HCL) view of 2 large impact pits that touch, note smaller pits below right.|
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