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K-T iridium (through a Holocene perspective)

In case this paper hasn't been mentioned yet (it should go a long way to 
resolving the volcanic contribution vs. meteoric contribution debate):

Gabrielli,P., C. Barbante, J. M. C. Plane, A. Varga, S. Hong, G. Cozzi, V. 
Gaspari, F. A. M. Planchon, W. Cairns, C. Ferrari, P. Crutzen, P. Cescon, and 
C. F. Boutron. 2004. Meteoric smoke fallout over the Holocene epoch revealed by 
iridium and platinum in Greenland ice. Nature 432:1011-1014.


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 
meteroids 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 atmospher of the last glacial age.  We suggest that 
nanometre-sized meteoric smoke particles, formed from the recondensation of 
ablated met!
 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.