Hot atmospheric formation of carbonate accretionary lapilli at the Cretaceous-Paleogene boundary, Brazos River, Texas, from clumped isotope thermometry

The Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five major extinctions. Researchers have examined ejecta from the Chicxulub impact, and most recently a drill core from the crater itself, yet the processes and chemical reactions occurring in the impact vapor plume are poorly constrained. Rounded carbonate particles, identified as accretionary lapilli, have been found thousands of kilometers from the impact crater and may be a unique record of plume conditions. We present carbon (delta C-13), oxygen (delta O-18), and clumped (Delta(47)) isotope ratios of lapilli from the Brazos River, Texas (USA), as well as from foraminifera and a mudstone. Unaltered lapilli delta C-13 and delta O-18 values covary, ranging from -9.38 parts per thousand to -2.10 parts per thousand and from -7.72 parts per thousand to -5.36 parts per thousand, respectively, and they are distinct from mudstones, foraminifera, and secondarily altered lapilli in the same section. Clumped isotope temperatures [T(Delta(47))] from the lapilli range from 66 degrees C to 539 degrees C and average 155 +/- 46 degrees C (1 standard deviation), with sedimentary and fossil carbonates recording clement, shallow ocean-like T(Delta(47)). These data are consistent with petrography and hypothesized vapor plume formation, and we argue that the delta C-13 and delta O-18 values result from target rock decarbonation. Atmospheric temperatures >100 degrees C extending >1800 km from the Chicxulub crater imply an uninhabitable zone within seconds to minutes of the impact that was 10x larger in diameter than the crater itself.