Ordovician-Silurian fossil micrometeorites from Gotland (Sweden) and Anticosti Island (Canada): implications for the post-Ordovician cosmic dust flux

Earth has continuously accreted extraterrestrial material, ranging from kilometer-scale impactors to microscopic dust particles, providing a long-term record of the Solar System debris flux across Earth’s geological time. An estimated 40,000 ± 20,000 metric tons of extraterrestrial material reaches Earth yearly, primarily in the form of micrometeorites (microscopic dust particles between 10 µm and 2 mm) [1], [2]. They provide a sensitive archive of Solar System dynamics, as temporal variations in micrometeorite flux may reflect asteroid family-forming events and highlight broader dynamical processes shaping the evolution of the Solar System. These records are crucial for contextualizing sample return data from current and future space missions.

While the extraterrestrial flux during the Ordovician period has been well documented through fossil chromite analysis and associated with the L-chondrite parent body breakup, Silurian micrometeorite records remain restricted to a single study from 1963 [3]. The Silurian Period (443.1 – 419.0 Ma [4]) forms a critical time interval, as it bridges the Ordovician breakup event (~466 Ma) [5] and established micrometeorite records from the Meso- and Cenozoic [6, 7]. Here, we present one of the first Silurian micrometeorite collections, recovered from Wenlock-aged (~433-427 Ma) limestones on Gotland (Sweden). In parallel, complementary material from one section on Anticosti Island (Canada) targets Hirnantian strata near the Ordovician-Silurian boundary (~445-444 Ma), approximately 20 Myr after the L-chondrite parent body breakup at  ~466 Ma, whereas the Wenlock material from Gotland tests whether a residual micrometeorite signal persisted into the Silurian.

Micrometeorites were extracted after acid dissolution (15% HCl) of limestone samples, followed by magnetic separation and optical microscopy. To date, 7.5 kg of Gotland material from three locations have been processed. Scanning electron microscopy (SEM) identified six I-type cosmic spherules, including four recovered from marly limestone. This observation challenges the common assumption that purer carbonate lithologies preferentially preserve micrometeorites. Electron microprobe analyses (EPMA) indicate typical I-type texture and compositions, consistent with magnetite-dominated mineralogy. Secondary Ion Mass Spectrometry (SIMS) triple oxygen isotope analyses of three particles yield variable results: one plots within the ordinary chondrite field (δ¹⁸O = 6.30‰, ∆¹⁷O = 0.51‰), while two (δ¹⁸O = 4.22‰, ∆¹⁷O = 0.44‰ and δ¹⁸O = 8.04‰, ∆¹⁷O = 0.17‰) fall near the terrestrial fractionation line (~ δ¹⁷O = 0.52 x δ¹⁸O, [8]), potentially reflecting terrestrial overprinting or isotopic exchange during diagenesis. From the Anticosti Island rock samples, 5.2 kg have been dissolved and are currently undergoing magnetic separation and optical picking for candidate micrometeorites. 

These preliminary results demonstrate that Silurian sediments can preserve micrometeorites and may indicate a continued, but potentially reduced extraterrestrial flux following the Ordovician L-chondrite breakup. Ongoing geochemical, mineralogical and isotopic analyses will further constrain preservation effects and parent body affinities. Comparisons between Baltic and Laurentian paleoenvironments, alongside existing Paleozoic records [9-11], will help refine reconstructions of the mid-Paleozoic extraterrestrial flux. Such reconstructions are critical for linking Solar System dynamics and planetary processes, and for improving interpretative frameworks used in planetary geology and sample return missions.

References. [1] Genge et al. (2008) Meteorit. Planet. Sci. 43(3)497-515. [2] Love & Brownlee (1993) Science 262:550-553. [3] Mutch (1964) Ann. N.Y. Acad. Sci. 119:166-185. [4] Melchin et al. (2020) Geologic Time Scale 2020, Elsevier 695-732. [5] Terfelt and Schmitz (2021) PNAS 118(24), e2020977118. [6] Suttle and Genge (2017) Earth Planet. Sci. Lett. 476:132–142. [7] Suttle et al. (2023) Geochim. Cosmochim. Acta 355:75-88. [8] Clayton (1993) Annu. Rev. Earth. Planet. Sci. 21:115-149. [9] Krämer Ruggiu et al. (2025) Geochim. Cosmochim. Acta 405:114-131. [10] Dredge et al. (2010) Scottish J. Geol. 46(1)7. [11] Voldman et al. (2013) Geol. J. 48(2-3)222-235.