High resolution Osmium isotopes in deep-sea ferromanganese crusts reveal a large meteorite impact in the Central Pacific at 12 ± 4 ka (Invited)

Ferromanganese crusts grow by incorporating metals from ambient seawater and are shown to be faithful recorders of ocean paleochemistry. We studied a well-dated crust (VA 13/2) from the Central Pacific with the aim to provide a high resolution record of glacial-interglacial variations in the seawater Os isotope composition. The uppermost 1.5 mm section of the crust was sampled at every 50 μ m with an 8 mm diameter drill-bit that was mounted on a high precision lathe. This corresponds to a time resolution of 4 ka for the first 550 μ m and of 8 ka for the rest on the basis of ²³⁰Th_excess dating. Surprisingly, the Os contents and isotope ratios in the crust appear to be compromised between 50 and 100 μ m by contact with a source that is highly enriched in Os but with low ¹⁸⁷Os\¹⁸⁸Os ratio. The depth corresponds to an age of 12 ± 4 ka and in comparison to a background seawater isotope ratio of 0.93-1.02, at this time the crust shows an isotope ratio of 0.24 suggesting input of meteorite derived Os. We prepared a thick-section of the crust to investigate whether the Os signal is associated with the presence of extraterrestrial (ET) particles. We found using optical microscopy and LA-ICP-MS that there are no large ET particles and that the Os enrichment of the crust is confined to within 100 μ m of the surface. This suggests that the crust received meteorite-derived Os as extremely small particles. Investigations of surface scrapes of other crusts in the vicinity of VA 13/2 helped us define a large area in the Pacific with ¹⁸⁷Os\¹⁸⁸Os ratios lower than that estimated for the ambient seawater. We infer that the Central Pacific was a site of deposition of Os resulting from dust cloud following a meteorite impact at 12 ± 4 ka that suppressed the ¹⁸⁷Os\¹⁸⁸Os ratios of the crusts. Using the distribution of the Fe-Mn crusts whose surface-scrapes display ¹⁸⁷Os\¹⁸⁸Os ratios much less than ambient seawater (0.9 or less) we find that the meteorite could be up to 100 m in diameter. By examining a number of sites in North America and a site in Europe Firestone et al. (2007) proposed that the Younger Dryas cooling event, which began at 12,900 years ago, was triggered by multiple cometary airbursts and/or impacts. If so, it is possible that a fragment of the impactor may have blown up over the Pacific. Higher resolution studies of marine sediments would be needed to confirm this observation.