MAASTRICHTIAN LATITUDINAL TEMPERATURE GRADIENTS AND TROPICAL TEMPERATURES: PALEOBOTANICAL EVIDENCE FROM NORTH AMERICA

What evidence does paleobotany provide on Cretaceous tropical temperatures? Although leaf megafloras from the Cretaceous tropics are still poorly known, more abundant megafloral assemblages from the middle latitudes can place important constraints on tropical temperatures. Horrell (1990) argued that paleobotanical evidence for increased warmth at middle latitudes during the Late Cretaceous and early Tertiary indicates warm tropical temperatures, because energy budget principles imply that tropical temperatures must be higher than those of middle latitudes. For the Late Cretaceous, Horrell based his arguments on the work of Wolfe and Upchurch (1987), which analyzed North American paleofloras and used Leaf Margin Analysis to estimate Mean Annual Temperature (MAT).

For the Maastrichtian, Horrell’s hypothesis was tested by: 1) analyzing a larger number of North American leaf megafossil assemblages than Wolfe and Upchurch (1987), 2) calculating MAT with alternative equations and a combination of Leaf Margin Analysis and multiple regression methods, and 3) using MAT to derive latitudinal temperature gradients. Both early and late Maastrichtian floras were analyzed to improve geographic coverage and to average out the effects of climatic fluctuations. Latitudinal temperature gradients within North America were calculated by simple linear regression of estimated MAT against paleolatitude, based on the approximately linear latitudinal temperature gradient that occurs today at middle latitudes. Leaf Margin Analysis gives an average latitudinal temperature gradient for North America of 0.3-0.5°C per degree latitude during the Maastrichtian, with the exact value depending on the choice of leaf/temperature equation and paleogeographic reconstruction. Calculated latitudinal temperature gradients predict an average MAT of 24-27°C at 30 degrees latitude, 3-6° warmer than today. This implies that equatorial MAT during the Maastrichtian was no cooler than that of the Recent and probably was in the range of 30-33°C, based on the temperature gradient seen today between the tropics and middle latitudes.