The variations of the cosmic dust flux falling onto Earth since 40 Ma have been estimated using the noble gas composition of marine sediments
The influx of extraterrestrial material onto Earth is well known through the fall of meteorites, such as the one which may have caused the dinosaurs’ extinction 65 Myr ago. However, the main amount of extraterrestrial material reaching the Earth (~40,000 tons/yr) comes from sub-millimetric particles, called interplanetary dust particles and forming the cosmic dust, which generally originate from asteroid collisions or comet tails. The variation of the cosmic dust flux falling onto Earth could be related to the climate cycles of Earth. Thus, it is important to constrain the variations of the cosmic dust flux during geological timescales.
After their fall, the particles accumulate on ocean floors, to be further incorporated into sediments. Consequently, scientists have access to million of years of cosmic dust trapped in sediments recovered from drill cores into the oceanic floors. The particles have 3He and 20Ne abundancies distinct from that of terrestrial materials. Thus, the measurements of 3He and 20Ne abundancies in marine sediments of various ages can be used to study the variations of the cosmic dust flux falling onto Earth.
However, He is an element that can partially be lost by a diffusion process when the particle is heated during its entry in the atmosphere or during its stay on the seafloor for million of years. The measurement of its abundance has thus to be coupled to another elements to assess the He loss. Ne diffuses less rapidly that He and is less likely to be lost by the particles. However, the low abundancy of 20Ne in the interplanetary dust particles makes its measurement in laboratory challenging. Chavrit, Moreira and Moynier from the Institut de Physique du Globe de Paris coupled the measurements of He and Ne abundancies to estimate and correct the He loss. After analytical development to allow these analyses to be made on such samples, Chavrit and collaborators measured the composition of marine sediments to follow the evolution of the cosmic dust flux from today to 40 Myr ago.
The researchers showed that the elemental and isotopic composition of the interplanetary dust particles contained in the sediments is similar to that of the implanted solar wind onto the surface of the particles. This signature acquired into space is in favour of the He retention in the cosmic dust particles deposited in the terrestrial sediments. Consequently, these results show that the flux of cosmic dust stayed relatively constant during geological timescales with two sudden increases during the late Eocene (35 Myr ago) and late Miocene (8.2 Myr ago).
Chavrit D., Moreira M.A. and Moynier F. (2016) Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments. Earth Planet. Sci. Lett. 436, 10-18.
Institut de Physique du Globe de Paris
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