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Terrestrial archives like deep-sea Fe Mn crust and sediment archives extend over the past tens of million years.Large dust grains entering Earth’s atmosphere have also been observed by radar detections.We chose two independent archives: a large piece (1.9 and 0.4 kg samples) from a deep-sea manganese crust (237 KD from cruise VA13/2, collected in 1976) with a growth rate between 2.5 mm per My (refs 29, 37) and 3.57 mm per My (ref. It originates from the equatorial Pacific (location 9°180′N, 146°030′W) at a depth of 4,830 m and covers the last ~25 My (refs 30, 43, 44, 45). Our second sample, also from the Pacific Ocean, is a piston-core deep-sea sediment (7P), extracted during the TRIPOD expedition as part of the Deep-Sea Drilling Project (DSDP) at location 17°30′ N, 113°00′ W at 3,763 m water depth and covers a time period of ~0.5–2.1 My BP (W.Smith, Scripps Geological Collections, USA, personal communication).Their existence in today’s interstellar medium would serve as a radioactive clock and would establish that their production was recent.In particular Pu, archived in Earth’s deep-sea floor during the last 25 My, at abundances lower than expected from continuous production in the Galaxy by about 2 orders of magnitude.This large discrepancy may signal a rarity of actinide r-process nucleosynthesis sites, compatible with neutron-star mergers or with a small subset of actinide-producing supernovae.About half of all nuclides existing in nature and heavier than iron are generated in stellar explosive environments.
If continuously produced, the Interstellar Medium is expected to build-up a quasi-steady state of abundances of short-lived nuclides (with half-lives ≤100 My), including actinides produced in r-process nucleosynthesis.
Hence, the interstellar flux is calculated by multiplying the measured flux into the crust by a factor of 4/0.21=19.
We thus derive a 2σ limit Pu flux from ISM dust particles penetrating the SS, and their incorporation into terrestrial archives.
Our experimental results are then compared with these estimations.
Based on a uniform production model, although of low statistical significance, supports the scenario of penetration of large ISD particles into the inner SS and may be consistent with the satellite data.
It should be noted that Galactic cosmic-rays penetrate the SS and recent observations clearly demonstrate therein the presence of Th and U, and tentatively of Pu concentrations reaching Earth, integrated over a period of 24.5 My.