Ice sheet expansion in the Cretaceous greenhouse world.

Globally elevated temperatures during the Cretaceous extreme greenhouse climate interval were punctuated by the Valanginian cooling event, which was characterized by a positive carbon isotope excursion, global cooling, and a glacial event approximately at 135 Ma. Disentangling ocean temperature and continental ice volume trends enables us to better understand climate fluctuations over deep time. We investigated the ocean temperature-ice sheet dynamics of glaciation events that occurred in the Cretaceous greenhouse world. New clumped isotope and O data from sites in the Tethyan Ocean show that seawater temperatures decreased by 5-6 °C, consistent with the development of glacial periods, and maximum ice volumes about half the size of present-day Antarctica. This cooling event provides a counter-example to other Mesozoic climate transitions driven by changes in atmospheric greenhouse gas contents. Our results emphasize the importance of quantitatively reconstructing continental ice volume, providing further support for exploring deep-time Earth climate dynamics.