ISOTOPIC EVIDENCE CONSISTENT WITH A WILDFIRE-INDUCED, GREENHOUSE WARMING PULSE AFTER THE CHICXULUB FROM THE BRAZOS RIVER TEXAS CRETACEOUS/PALEOGENE BOUNDARY SECTION

Stable isotopic analyses of individual specimens of the benthic foraminifera Lenticulina show an ~1‰ decrease in δ¹⁸O values beginning 170 cm above the K/Pg boundary at the Brazos River ‘River Bank South’ Cretaceous/Paleogene (K/Pg) outcrop (Falls County, Texas, USA). This negative δ¹⁸O shift is most rigorously documented by analyses of visually screened fragments of gently crushed specimens. SEM examination of broken specimens confirms that excellent test preservation is present throughout the section. Parallel δ¹⁸O analyses of specimens infilled with secondary carbonate, of isolated secondary carbonate, and of test fragments that failed visual screening are offset by 1‰ to 3‰ from the clean fragments reinforcing arguments that clean foraminiferal fragments preserve depositional values.

The negative δ¹⁸O excursion begins within the planktonic foraminiferal P0 Zone (170 cm above the K/Pg boundary and 100 cm below the top of the zone based on the lowest occurrence of Parvularugoglobigerina eugubina) and provides evidence for an ~5°C warming pulse beginning within millennia after the Chicxulub impact. In addition, a number of events and excursions documented within the P0 Zone in Brazos River K/Pg sections suggest sedimentation rates could have been exceptionally high (> 10 cm/yr) during deposition of the first 170 cm of the P0 Zone at the River Bank South locality. Such high accumulation rates allow the possibility that the warming pulse began within a decade of the impact as predicted for warming caused by CO₂ released during post-impact wildfires.