The Younger Dryas (YD) event, which began approximately 12,900 years ago, was a period of abrupt and rapid cooling in the Northern Hemisphere whose primary cause remains unclear. The cooling event is of ~1400 year duration and its onset coincides with abrupt Pleistocene megafaunal extinctions and the disappearance of the Clovis people in North America. The prevalent postulated mechanism has been a temporary shutdown of the thermohaline circulation following the breakup of an ice dam in North America. However, a recent paper proposes that the cooling was triggered by multiple cometary airbursts and/or impacts that engendered enormous environmental changes. The evidence in support of this hypothesis is a black layer marking the Younger Dryas boundary in sediments from North America and Europe. This boundary is enriched in magnetic grains high in iridium, magnetic microspherules, nanodiamonds, and fullerenes containing extra-terrestrial He-3. The impact hypothesis is being intensely debated for while the YD horizon shows evidence of extensive forest fires, it lacks traditional impact markers. Moreover, the origin of the magnetic spherules and iridium bearing magnetic grains remains unclear. It is yet to be demonstrated conclusively if these magnetic grains are meteorite debris or micrometeorites or just terrestrial grains singed in a forest fire. The purpose of this pilot study is to test the impact hypothesis by examining the osmium (Os) isotopic composition and osmium and iridium concentrations in magnetic objects separated from the appropriate sections at Gainey (Michigan), Murray Springs (Arizona) and Lommel (Belgium). Additionally, they will also look for evidence of meteorite debris in high resolution sediment cores from Crystal and Mastodon Lakes in Illinois.
One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ~12,900 years ago. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB), which is a layer of sediments deposited 12,900 years ago. Microspherules are marble-shaped objects (see Fig. 1). A lot of charcoal is also found at the YDB suggesting extensive forest fires at that time. The key question that we addressed was whether the observed objects were produced during an impact or they had resulted from soil singed in forest fire/coal fire. To evaluate this we made use of the geochemistry of the sediment and magnetic grains/spherules recovered from the boundary. In these samples we looked at the tell-tale enrichment of meteorite derived osmium. This element is abundant in meteorites but scarce in rocks and soils exposed on the continents. We also examined different types of minerals present in the spherules. We found no evidence of meteorite derived osmium in any of the samples. But curiously some of the spherules found in the village of Melrose, Susquehanna Co., Pennsylvania show the presence of minerals that can form only at extremely high temepratures (more than 2000 °C). Such tempeartures cannot be achieved by forest or coal burning. So while demonstrating that the spherules are likely quenched silicate melts produced following the impact at the YDB we find that they exhibit no evidence of meteorite derived osmium. The most exciting result is that we were able to trace the orgin of hese spherules and some others found in New Jersey to a region in Quebec about 1400 km away (Fig. 2). Publication: Wu and others (2013) Origin and provenance of spherules and magneticgrains at the Younger Dryas boundary. Published in: Proceedings of National Academy URL: www.pnas.org/cgi/doi/10.1073/pnas.1304059110
