Physics
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmpp43a..01f&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #PP43A-01
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6022 Impact Phenomena (5420, 8136), 6240 Meteorites And Tektites (1028, 3662), 9350 North America
Scientific paper
The Younger Dryas event boundary (YDB) is a thin sedimentary layer of 12.9 ka age containing an assemblage of materials formed due to a major ET impact centered over northern North America. The event coincided with the megafaunal extinctions and the Younger Dryas cooling episode. The YDB layer contains peaks in magnetic grains, microspherules, and iridium, in addition to charcoal and soot that resulted from extensive wildfires. Two impact-related carbon-rich markers, glass-like carbon and carbon spherules, have not been reported previously in North America. Vesicular, glass-like carbon, in pieces up to several cm, occurs in the YDB at 22 sites with concentrations ranging up to 16 g/kg. Their glassy texture suggests melting during formation, with some fragments grading into charcoal, and CF-IRMS analysis reveals a composition of >70 percent carbon. One sample exhibited a strong fullerene signature containing ET helium with a ratio of 84× that of air, comparable to the Tagish Lake meteorite (90×). Similar glass-like carbon has been reported from the Azuara crater in Spain (Ernstson, et al. 2001). Carbon spherules (0.15-2.5 mm) are black, highly vesicular, low-density, subspherical-to-spherical objects found in 18 widely distributed sites at varying concentrations up to 1500/kg. SEM analysis shows that the spherules have cracked and patterned surfaces, honeycombed cells, no inclusions, and sometimes display hollow cores. SEM/EDS and microprobe analyses show the carbon spherules to be >75 percent carbon. Similar carbon spherules are reported from the a crater in Germany (Rösler, et al., 2005). The carbon spherules most likely are either ablation products from the impactor or combustion products of the impact. Sediment samples were analyzed for Ir, and YDB samples from 9 sites exhibited elevated Ir values up to 3.75 ppb, while there was no detectable Ir above or below the YDB. Extracted magnetic grains have values up to 117 ppb, which is 25 percent that of typical chondrites and up to 5000× crustal abundance. The YDB layer, representing a major ET impact event at 12.9 ka, appears to coincide with the late Pleistocene megafaunal extinction and the onset of Younger Dryas cooling. We propose that neither this cooling nor the extinction would have occurred in the absence of this impact event.
Becker Lee
Belgya T.
Bunch Ted E.
Darrah Tom
Dickenson O. J.
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