TEMPERATURES OF CHICXULUB CARBONATE ACCRETIONARY LAPILLI FORMATION FROM CLUMPED ISOTOPES

The petrography and geochemistry of carbonate accretionary particles (lapilli) from Cretaceous-Paleogene (K-Pg) boundary sediments exposed along the Brazos River, TX suggest a unique calcite formation environment in the ejecta blanket immediately following the Chicxulub impact event. Clumped isotope thermometry applied to these small lapilli (~0.05-0.3 cm) should help further constrain the conditions and timing of their formation. Our initial hypothesis was that these calcites formed as the extremely hot (>1000 °C) post-impact ejecta cloud expanded and cooled. Because the kinetics of carbonate clumped isotope bond ordering are predicted to be nearly instantaneous at such high temperatures, we expected clumped isotope temperatures (T(Δ₄₇)) from quickly cooled lapilli to reflect similarly high temperatures. Our observations, however, are more nuanced. Lapilli T(Δ₄₇) range from 60-195 °C despite little variability in measured carbonate δ¹⁸O (-6.2 ± 1.21‰), which points to lower temperature closed-system processes dominating the clumped isotope signal. We believe this is consistent with calcite formation via portlandite (Ca(OH)₂) carbonation reactions in the cooling post-impact atmosphere, though other formation mechanisms are plausible. Indeed, low measured δ¹³C values (-10 to -2‰) values are consistent with an atmospheric carbon source, not marine. Finally, to partially rule out thermal and isotopic diagenetic overprinting of lapilli, we measured benthic foraminifera and a mudstone from the same locality. Low T(Δ₄₇) from these samples suggests the Brazos River section carbonates were not effected by clumped isotope bond reordering during deep burial, which is supported by independent burial history estimates. Benthic foraminifera clumped isotopes may also be a new window into the ancestral Gulf of Mexico shelf paleoenvironments immediately before and after this important geological boundary and event.