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Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary.

Title	Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary.
Authors	Yingzhe Wu, Mukul Sharma, Malcolm A LeCompte, Mark N Demitroff, Joshua D Landis
Magazine	Proceedings of the National Academy of Sciences of the United States of America
Date	09/17/2013
DOI	10.1073/pnas.1304059110
Introduction	The Younger Dryas cooling event, approximately 12,900 years ago, is hypothesised to have been initiated by one or more bolide impacts. Supporting this idea, studies have documented varying peak concentrations of magnetic grains containing iridium, alongside magnetic microspherules, at the Younger Dryas boundary (YDB). This study evaluates the origin of osmium (Os) in these materials. Bulk sediment and magnetic grains/microspherules from YDB locations in Arizona, Michigan, New Mexico, New Jersey, and Ohio exhibit (187)Os/(188)Os ratios close to terrestrial values, indicating a continental crust source for Os in these samples. Conversely, bulk sediments from YDB sites in Belgium and Pennsylvania show lower (187)Os/(188)Os ratios, which initially suggested an extraterrestrial Os component. However, the significantly lower Os concentrations in Belgian samples compared to average upper continental crust rule out a meteoritic contribution. Notably, the Pennsylvania YDB site contains 2- to 5-mm spherules featuring suessite (Fe-Ni silicide), a mineral formed at temperatures above 2000 °C. The texture, mineralogy, and age of these spherules are consistent with their formation as ejecta from an impact 12,900 years ago. Although the (187)Os/(188)Os ratios in these spherules are often low, their Os content is also likely terrestrial. Furthermore, analyses of rare earth element patterns and strontium (Sr) and neodymium (Nd) isotopes within the spherules point to their source region as the 1.5-billion-year-old Quebecia terrain within the Grenville Province of northeastern North America.
Quote	Yingzhe Wu, Mukul Sharma and Malcolm A LeCompte et al. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary. Proceedings of the National Academy of Sciences of the United States of America. 2013. Vol. 110(38). DOI: 10.1073/pnas.1304059110
Element	Iridium (Ir) , Osmium (Os) , Nickel (Ni) , Strontium (Sr) , Neodymium (Nd)
Materials	Rare Earth Metals
Industry	Research & Laboratory

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