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The onset of the Younger Dryas (YD) climactic event at ~12.9 ka coincides with the extinction of Pleistocene megafauna and drastic changes in human subsistence patterns in the Americas, changes whose cause/s remain a geologic mystery. Firestone et al. (2007) proposed a bolide impact to explain these dramatic environmental changes, citing elevated concentrations of magnetic spherules (MSp) picked from magnetic extracts as key evidence. This study uses a comprehensive suite of highly sensitive rock magnetic measurements to evaluate continuously sampled soil profiles from two well-dated early human archeological sites that span the YD boundary and investigate magnetic evidence for an impact.
Rock magnetic techniques are one of the most sensitive means for detecting subtle changes in grain size, sedimentation rate, or pedogenic development. We expect that the environmental changes (wildfires, vegetation loss, erosion/deposition rates) associated with an impact or airburst event of this magnitude would be preserved in the magnetic record. Hysteresis loops and frequency dependence of susceptibility measurements are used to evaluate the concentration, grain size, and composition of magnetic material, while low-temperature magnetic properties, SEM images and EDS spectra speak to magnetic mineralogy.
We observe gradual decreases in the amount of magnetic material and shifts towards smaller, more magnetically hard (high-coercivity) minerals with depth at both sites. These trends are consistent with a history of uninterrupted soil development rather than a catastrophic impact event. Magnetic extracts were also prepared from bulk soil collected at one site, following the methods of previous investigators in order to explore the possibility of sampling bias during magnetic extraction. Extracted magnetic grains are larger and more magnetically hard than in-situ magnetic material, and are predominantly detrital ilmenites with varying degrees of alteration to high-titanium products. Many grains exhibit hematite exolution lamellae, which gives rise to lamellar magnetic coupling. Pedogenesis dominates the magnetic properties at both sites, producing profiles with near-surface enrichment of fine-grained magnetically hard material that is progressively oxidized with depth.
Nadel, Marcy, "The Rock Magnetic Record Across the 12.9 ka Younger Dryas Boundary: Evidence for Impact?" (2013). Geology Honors Projects. 12.
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