Variation in Cosmic Dust Flux Across the Cretaceous-Paleogene Boundary: Implications on the K-Pg Impactor

Introduction: The abundance of fossil micrometeorites (MMs) within sedimentary formations provides empirical evidence for changes in flux of cosmic dust throughout geological time. When compared to modern flux estimates, temporal variations in dust input can be reconstructed once terrestrial concentration or reduction factors are shown to be minimal. A high quantity of fossil MMs associated with a change in flux across the Cretaceous-Paleogene (K-Pg) boundary can ultimately be used to predict the sources of the dinosaur-killing asteroid and its orbital dynamics prior to impact.

Methods: Host rocks from sedimentary sequences along the K-Pg boundary exposed at four sites (Gubbio, Italy; Stevns Klint, Denmark; Algorri, Spain, and; Gams, Austria) were processed by a mechanical crusher and the dust fraction was magnetically separated at least three times to extract any potential magnetic fossil MMs. Cosmic spherules (CSs) were optically identified based on their spherical shape, dark color, and vitreous/metallic luster. These were then categorized and analyzed further by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), noble gas mass-spectrometry (MS) and secondary ion mass spectrometry (SIMS) to obtain information on their petrography, chemistry, and He- and O-isotopic compositions. Sediment chemistry was also obtained through bulk rock analyses using inductively coupled plasma optical emission spectrometry (ICP-OES), He-isotopic measurements, and organic analyses using gas chromatography-mass spectrometry (GC-MS).

Results and discussion: From the four K-Pg sampling locations, a total of 602 fossil CSs were identified, including 526 I-types, 33 G-types, and 9 S-types. The average diameters of spherules within the Stevns Klint, Algorri, and Gams sediments are all ~32 µm, but slightly higher at 45 µm in the Gubbio samples, which is similar to other fossil MM studies [1] [2] [3], yet lower than those from modern collections [4] [5] [6]. The highest concentration of CSs/kg were discovered in the sediments directly below the boundary clay (~2 orders of magnitude higher than those above). Despite differences in depositional setting and palaeobathymetry, most I-types exhibit quench-cooling textures comparable to Antarctic (ANSMET) spherules and other fossil MMs. Gubbio preserves the most pristine I-types, with well-defined surface crystallites and low mottling (<4%), yet the lowest abundance of G- and S-types, suggesting differing preservation pathways for Fe-rich and silicate-bearing spherules. From 15 analyzed fossil I-types, one was enriched in ³He, implying conservation of the isotope, although rare, is possible for highly heated and altered spherules. Almost all the fossil I-types plot near the TFL and are generally depleted in δ¹⁸O relative to Antarctic I-types. Additionally, four I-types plot distinctly above the TFL (Δ¹⁷O = 1.88 to 3.49‰) with very low δ¹⁸O (-20 to -30‰), which is unique and not previously reported in literature. Interestingly however, these spherules have no significant differences in their petrographic or bulk chemical properties than those comparatively enriched in d¹⁸O, suggesting that any extensive hydration that may have taken place does not cause mottling or anomalous implantation of lithophile cations. The He-isotopic abundance and concentration of extraterrestrial amino acids within the fossil MM-enriched units are similar to background levels, suggesting chemical signals of the elevated dust flux may have been removed by diagenetic processes, or is influenced by the original properties of the impactor. The concentration of CSs when compared with local sedimentation rates and modern flux rates show a significant enhancement in cosmic dust flux occured prior to the K-Pg. Further investigations into the chemical and isotopic composition of spherules and their entry dynamics (modelled by numerical simulations) could provide insight into the source of the K-Pg impactor.

Acknowledgments: This work would not be possible without the help of project students and lab technicians. Thank you to the research team Mark Boyd, Bogdana Nica, Ally Wong, Stefania Ton, Michael Wilkonson, & Ren Foley, and the analytical assistors Alejandro Calderon, Pierre-Henri Blard, Gabriel Fénisse, Johan Villeneuve, Dani Fuller, & Diptimayee Behera.

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