Ancient atmospheric triple oxygen isotope signatures preserved in fossilized cosmic spherules constrain paleo-CO₂ levels

Iron-rich I-type cosmic spherules — micrometeorites formed by the complete melting and oxidation of extraterrestrial Fe, Ni metal particles — incorporate oxygen from the Earth’s atmosphere. As a result, they can be used to assess the triple oxygen isotope composition of atmospheric O₂, offering insights into CO₂ levels and global primary production [1,2]. When recovered from sedimentary rocks, these spherules can preserve a record of atmospheric conditions dating back billions of years. To date, no published triple oxygen (and triple iron) isotope data exist for fossil I-type cosmic spherules. This study aims to address this gap as we establish using fossil I-type cosmic spherules as an archive of Earth's atmospheric oxygen isotope composition and by quantifying associated CO₂ levels during key geologic periods. We analyzed a collection of fossil I-type cosmic spherules extracted from Phanerozoic sedimentary rocks, focusing on both triple oxygen and triple iron isotope compositions. This approach allowed us to reconstruct the triple oxygen isotope anomalies in past atmospheric O₂ and to assess the potential terrestrial alteration of these spherules. Our data reveal moderate ancient CO₂ levels during the Miocene (~8.5 Ma) and late Cretaceous (~87 Ma). We also demonstrate the competitive precision of using I-type cosmic spherules for paleo-CO₂ determination. Additionally, our work indicates that morphologically intact spherules can be isotopically altered by terrestrial processes, underscoring the need for rigorous sample screening.

[1] Pack et al. (2017), Nat. Commun. 8, 15702.

[2] Fischer et al. (2021), Paleoceanogr. Paleoclimatol. 36, e2020PA004159.