22-EPN3-010: The Cosmic Dust Flux Over Geological Time – How Extraterrestrial Signals Become Preserved in Earth’s Marine Rock Record
Visit by Martin D. Suttle of the Open University (UK) to TA2.10 Stable, Rare Gas and Radiogenic Isotope Facility at CRPG (France). Isabelle Mattia (Imperial College London) also took part in the visit, supported through other funding.
Dates of visit: 18 – 27 October 2023
Report Summary:
Fossil micrometeorites (MMs) recovered from sedimentary rocks are mostly <2mm-sized I-type cosmic spherules (CSs): iron-rich cosmic particles that have experienced high degrees of melting and oxidation during atmospheric entry. Since extraterrestrial 3He is implanted into cosmic dust by solar and cosmic rays in interplanetary space, fossil I-types could play a significant role as carriers of 3He in sedimentary archives. This project aimed to test this hypothesis by systematically examining the 3He/4He ratios of various types of MMs at different stages of preservation.
Measurements of He isotopes using the Helix SFT mass-spectrometer at CRPG, Nancy revealed variable 3He/4He values, with only 8 of the 36 measurements yielding results above the solar energetic particle (SEP) ratio of 2.17E-04 – most of these were, expectedly, scoriaceous Antarctic MMs which experienced the least atmospheric heating. Two urban CSs and a fossil I-type were also enriched in 3He, suggesting preservation of some implanted nobles gases, although rare, is possible for particles that have experienced high degrees of entry heating and/or alteration during diagenesis. A host chalk enriched in fossil I-types returned a low ratio of 1.00E-06, which could imply 3He may be preferentially removed by secondary fluids peculating within the lithifying sediment, or the source of cosmic dust was not sufficiently saturated in extraterrestrial 3He.The main conclusions from this work are that the 3He signal and the abundance of fossil (I-type) micrometeorites in Earth’s sedimentary rocks are uncoupled. This implies that these two proxies (3He and fossil micrometeorite abundance) record separate size distributions of the extraterrestrial dust flux.
