CENOZOIC VARIATIONS IN THE FLUX OF INTERPLANETARY DUST RECORDED BY HE-3 IN A DEEP-SEA SEDIMENT

HELIUM-3 concentrations and He-3/He-4 ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout(1-3). Here I report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 Myr. The remarkable preservation of the extraterrestrial signature arises from high retention of He-3 within interplanetary dust particles, coupled with loss of radiogenic He-4 from terrestrial mineral grains. The core provides a continuous record of the fallout of extraterrestrial helium for the Cenozoic era. This record suggests that there have been significant variations in the influx of interplanetary dust through time, probably related to asteroidal breakup events or the passage of comets through the inner Solar System. The results also show He-3 to be a far more sensitive tracer of the interplanetary dust flux than is iridium,