Evidence for deposition of 10 million tonnes of impactspherules across four continents 12,800 y ago
Airbursts/impacts by a fragmented comet or asteroid have beenproposed at the Younger Dryas onset (12.80±0.15 ka) based onidentiﬁcation of an assemblage of impact-related proxies, includ-ing microspherules, nanodiamonds, and iridium. Distributed acrossfour continents at the Younger Dryas boundary (YDB), spherulepeaks have been independently conﬁrmed in eight studies, butunconﬁrmed in two others, resulting in continued dispute abouttheir occurrence, distribution, and origin. To further address thisdispute and better identify YDB spherules, we present results fromone of the largest spherule investigations ever undertaken regard-ing spherule geochemistry, morphologies, origins, and processesof formation. We investigated 18 sites across North America,Europe, and the Middle East, performing nearly 700 analyses onspherules using energy dispersive X-ray spectroscopy for geo-chemical analyses and scanning electron microscopy for surfacemicrostructural characterization.