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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.Our data are a factor of 80–640 lower than the values expected under our constraints on ISM grain composition from a SN derived steady-state actinide production (the 2σ upper limit of ~840 atoms per cm Pu flux over a time period of 24.5 My (~1/10 of the SS rotation period in the Galaxy) corresponding to a relative travel distance of the SS of 650 pc (taking the mean speed of the measured ISM dust particles of 26 km s (ref.Half of the heavy elements including all actinides are produced in r-process nucleosynthesis, whose sites and history remain a mystery.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.The crust data for all sections and for all three deeper layers are compatible (for details see also Supplementary Table 1).Owing to a higher chemical yield (integral sensitivity, column 5, Table 1) layers 3 and 4 provide lower limits.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.
Each layer was subdivided into three vertical sections (B, C and D) with areas between 70 and 85 cm, totalling 12 individually processed samples. Finally, sample X, the bottom layer of hydrothermal origin (Fig. This Fe Mn crust (with a total thickness of 25 cm) was sampled in 1976 from the Pacific Ocean at 4,830 m water depth: large samples used in this work were taken from one part of the crust (hydrogenous crust, layers 1–4, left in the figure)) and from the bottom (hydrothermal origin, layer X, crust started to grow ~65 My ago The results for the four crust layers and the blank sample, obtained from the AMS measurements on 11 individual crust samples, are listed in Tables 1 and 3 (see also Supplementary Tables 1 and 2; identification spectra are plotted in Fig. We observed one single event in each of the two crust subsamples, namely layer 3, section B (B3), and layer 4, section D (D4).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.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.