Origin and provenance of magnetic spherules at the Younger Dryas boundary

The Younger Dryas impact hypothesis proposed by Firestone et al (2007) suggests that a comet or meteorite impacted or exploded over North America 12,900 BP and caused the extinction of North American megafauna, major cultural changes and population decline among the Clovis Paleoindians, and the Younger Dryas cooling. This hypothesis has been based on discovery of high concentrations of magnetic grains and spherules at the Allerod-Younger Dryas boundary, which also contains nanodiamonds and elevated Ir concentrations in bulk sediments and in magnetic grains/spherules. The hypothesis suggests that the magnetic grains/spherules were created during the meteorite impact and that they contain meteorite derived Ir. This hypothesis is under critical scrutiny as there have been no impact craters found and independent workers have found no evidence of elevated Ir concentrations in bulk sediments at the Younger Dryas boundary (YDB). Osmium isotope analyses of the YDB bulk sediments also indicate terrestrial origin. In this study, we examined the key issue of the origin and provenance of magnetic grains/spherules recovered from the YDB. We have examined individual magnetic objects from two well-dated YDB sites, which were claimed to be enriched in Ir. Although these objects have high Os concentration, they have terrestrial Os isotope signatures. We have also investigated spherules recovered from newly discovered sites in Pennsylvania and New Jersey, which are associated with a black mat similar to that found at the YDB. The timing of the black mat formation of the two sites has not been established. Detailed mineralogical and chemical studies of these spherules attest to their formation temperatures to be ∼1200 °C to > 2200°C. The Sr-Nd isotopes indicate that the parent of melts generating the spherules was an ancient shale. Although the Os isotope data of single objects/clusters provide us no indication of a meteoritic signal, the spherules could be generated in a meteorite impact. However, as the time of formation of these objects is not known their association with YDB needs to established. If these spherules are found to be less than 150 years old we would further need to establish that they are not created by industrial activity.