Extraterrestrial Accretion From the GISP2 Ice Core

Ablated extraterrestrial dust appears to be a primary particle source for the upper atmosphere, but whether these particles affect climate is uncertain. Ablated dust could govern the rate of stratospheric ozone scavenging or high-altitude cloud formation, and so varying accretion rates could contribute significantly to global climate change. Dust concentrates filtered from glacial ice from the GISP2 core in Greenland were used to quantify the accretion rate of extraterrestrial dust to the Earth during the termination of the last ice age. Using neutron activation analysis, we have analyzed the iridium (Ir) content of six ice samples from the last glacial maximum and the Holocene period. Using 1-2 kg ice samples, ultra-clean sample preparation procedures and long, high flux neutron irradiations, these measurements have precisions that are one to two orders of magnitude better than obtained from previous ice core studies. The Ir/Fe abundance ratio of the GISP2 terrestrial dust source is different than measured in Mississippi mud- the conventional standard used to correct for terrestrial Ir. Following this core-specific terrestrial dust correction and assuming chondritic Ir abundances, we calculate an accretion rate of less than one gigagram per year (1 kton/yr) of chondritic material for the time interval 6,000-12,000 years before present. Our calculated accretion rate is at the lower end of previous estimates published on ice cores, and is lower than estimates of accretion from satellite measurements and analysis of deep-sea sediment. In this context, we will discuss the possible meaning of our data.