Terrestrial isotopic evidence for a Middle-Maastrichtian warming event from the lower Cantwell Formation, Alaska

Carbon stable isotope data (δ13C) and new radiometric dates indicate that the non-marine Late Cretaceous lower Cantwell Formation, central Alaska, registers the Middle-Maastrichtian Event (MME), a global warming episode. A stratigraphic section measured at the East Fork of the Toklat River in Denali National Park & Preserve includes a bentonite layer that we have dated to 69.5±0.7Ma using U–Pb isotope analysis of zircons. The new depositional age confirms that dinosaur tracks from the lower Cantwell Formation are coeval with dinosaur bones from outcrops of the Prince Creek Formation on Alaska's North Slope. δ13C data from fossil wood recovered from the lower Cantwell Formation records an ~3‰ positive excursion correlative with positive excursions from terrestrial and marine sections in West Texas (USA) and Gubbio (Italy), respectively. The similarity between marine and terrestrial records suggests that this excursion is the result of a major perturbation in the global carbon cycle, known as the Middle-Maastrichtian Event (MME). Mean annual precipitation (MAP) estimates calculated from δ13C data yield a range of 168–470mm/yr during the MME, 353–1050mm/yr before and 475–1451mm/yr after the warming event. The δ13C record from the lower Cantwell Formation provides the first terrestrial high-latitude evidence for the MME, while MAP estimates for the coeval Prince Creek Formation suggest that the arctic coast may have been more humid than southern interior Alaska during the Middle Maastrichtian.