RETHINKING CRETACEOUS CLIMATE

New data and ideas are changing our view of conditions during the Cretaceous. Revisions of plate tectonic motions indicate that the paleotopography of the continents was lower than has been thought, especially in western North America and Eastern Asia. These corrections largely eliminate the ‘cold continental interior paradox’ whereby fossils of plants that could not tolerate freezing occurred in regions indicated by climate models to be well below freezing in winter. The lower topographic slopes imply that most of the river courses were meandering, and that much more of the land surface was covered by water in rivers, oxbow lakes and bogs than had been through. These water surface provided much more of the greenhouse gas, water vapor, raising the overall planetary temperature and lowering the meridional gradient. Carbon dioxide was not the only other significant greenhouse gas; methane contributed significantly to the warmer climate, being generated by dinosaurs and bogs. The controversy over the height of Cretaceous sea levels has been resolved by knowledge of the effects of passage of the subducted slab of the Farallon Plate beneath the North American crust. The cause of shorter term sea level changes of the order of 30 to 50 meters is not because of growth and decay of ice sheets, but more likely the filling and release of water from groundwater reservoirs and lakes although there may have been some ice in the Early and latest Cretaceous. Suggestions of very warm tropical ocean temperatures (> 40 °C), often discounted, may be correct. They have major implications for the nature of plant life on land generally limited by Rubisco activase to temperatures below 28°C. A major rethinking of the nature of conditions on a warmer Earth is both needed and underway.