Extraterrestrial Helium in Seafloor Sediments: Identification, Characteristics, and Accretion Rate Over Geologic Time

Abstract

Almost 40 years after the discovery of extraterrestrial helium in seafloor sediments, renewed attention is being focused on using helium as a proxy for the sedimentary abundance of extraterrestrial debris. Extraterrestrial He is carried to the seafloor by the finest fraction of interplanetary dust and is retained in at least some sediments for hundreds of millions of years. Helium isotope systematics uniquely identify the extraterrestrial component, which is apparently hosted within magnetite and silicate grains. In some sediments 3He is completely derived from this source, in others the extraterrestrial fraction can be computed from the measured 3He/4He ratio. Variations in the sedimentary concentration of extraterrestrial 3He must reflect both changes in sedimentation rate and fluctuations in the accretion rate of 3He from space. When changes in sedimentation rate can be controlled for, variations in extraterrestrial 3He can be related to changes in the accretion rate of IDPs arising from major solar system events including asteroid collisions and enhanced cometary activity. A 3He record in sediments spanning the last 70 Myr provides insights to such events, including the first compelling evidence for the occurrence of a shower of long-period comets, 35 Ma.