High ³He/⁴He ratio in ocean sediments

Two decades ago, Merrihue¹ reported ³He/ ⁴He ratios of >10^-4 in ferromagnetic separates from a Pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to ∼1% of the sediment. A decade later Krylov et al.² compiled ³He/⁴He isotopic data on ocean sediments measured in the Soviet Union and observed that the ³He/⁴He ratio is generally higher in pelagic sediments where the sedimentation rate is lower. They suggested that the high ³He/⁴He ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor. However, these important discoveries were almost completely neglected until we re-examined the problem. We have measured 39 sediments from 12 different sites, 10 sites from the western to central Pacific and two sites from the Atlantic Ocean. We find ³He/⁴He ratios >5 × 10 ^-5 for six sites, well above the values generally observed in common terrestrial materials. The very high ³He/⁴He ratio in the sediments is probably due to input of extraterrestrial materials. Input of stratospheric dust of ≲1 p.p.m., which corresponds to a fallout rate of ∼2,000 tons per year, can explain the observation.