| Summary: | An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65million years ago1. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate2 or the nano- phase material of the vaporized impactor3. But the fate of platinum-group elements (such as iridium) that regularly enter the atmosphere via ablating meteoroids remains largely unknown. Here we report a record of iridium and platinum fluxes on a climatic-cycle timescale, back to 128,000 years ago, from a Greenland ice core4. We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age. We suggest that nanometre- sized meteoric smoke particles5,6, formed from the recondensa- tion of ablated meteoroids in the atmosphere at altitudes >70kilometres, are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps. This implies an average global fallout of 14 6 5 kilotons per year of meteoric smoke during the Holocene.
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