An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate or the nanophase material of the vaporized impactor. 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 core. 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 particles, 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 circulation 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.
Since 1976 in the United States, Canada, and Japan, and later in other countries, the exhaust system of gasoline powered cars has been equipped with catalytic converters containing Pt and/or Pd and/or Rh. This has resulted in a very significant decrease in urban air pollution for various chemical species such as NOx, CO, and hydrocarbons. There has however been concern that their ever increasing use might lead to Platinum Group Metals (PGMs) becoming widely dispersed in the environment. From the analysis of Pt, Pd, and Rh in central Greenland recent snow and ancient ice using the ultrasensitive inductively coupled plasma sector field mass spectrometry technique, we show here that the concentrations of these metals in snow dated from the mid 1990s are indeed 40−120 times higher than in ice dated from 7000 years ago. The fact that such an increase is observed far away from populated areas at a high altitude location indicates there is now a large scale contamination of the troposphere of the Northern Hemisphere for PGMs. Pt/Rh mass ratio in the most recent snow samples is close to the same ratio documented for catalytic converter exhausts in a recent study, which suggests that a large fraction of the recent increase for Pt and Rh might originate from automobile catalytic converters
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