Abstract
An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago1. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate2 or the nanophase 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 recondensation of ablated meteoroids in the atmosphere at altitudes >70 kilometres, 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 ± 5 kilotons per year of meteoric smoke during the Holocene.
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Acknowledgements
This work was supported in France by the Institut Universitaire de France, the Ministère de l'Environnement et de l'Amènagement du Territoire, the Agence de l'Environnement et de la Maîtrise de l'Energie, the Institut National des Sciences de l'Univers and the Université Joseph Fourier of Grenoble. In Italy, it was supported by the Antarctic National Research Programme, and in the UK, by the Natural Environment Research Council. We thank G. McFiggans for discussions regarding the aerosol growth model. This research was also supported by two Marie Curie fellowships of the European Community programme (IHP) and by the Alliance for Global Sustainability. This work is a contribution of GRIP organized by the European Science Foundation: we thank all the personnel working in the field in Greenland.
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Supplementary information
Supplementary Method 1
This section illustrates the quantification of the procedural blank related to the pre-concentration procedure of Greenland ice samples. (DOC 31 kb)
Supplementary Table 1
This table shows the Ir and Pt concentrations measured in Greenland ice with the associated instrumental standard deviation from 100 acquisitions. (DOC 111 kb)
Supplementary Figure 1
This figure shows how the procedural blank was determined in the section Supplementary Method 1. The mass (in fg) of Ir (a) and Pt (b) released/adsorbed to/from the solution during the pre-concentration procedure is the intercept of the regression line of the Ir and Pt masses measured vs. different initial masses of pre-concentrated ultra pure water. The error bars given are the instrumental standard deviations from 100 acquisitions. (DOC 39 kb)
Supplementary Figure 2
This figure shows a comparison of different extraterrestrial accretion rate estimates as shown in Table 1. Our direct measurement estimate of 78 kt yr-1 is slightly larger also than the commonly accepted value of around 40 kt yr-1 (ref. 15) whereas our estimation of the corrected average global fallout of 14 kt yr-1(see text) agrees well with other recent different estimates. (DOC 69 kb)
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Gabrielli, P., Barbante, C., Plane, J. et al. Meteoric smoke fallout over the Holocene epoch revealed by iridium and platinum in Greenland ice. Nature 432, 1011–1014 (2004). https://doi.org/10.1038/nature03137
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DOI: https://doi.org/10.1038/nature03137
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