Scherer, Jacqueline , Upchurch, Garland .
Paleobotanical evidence on latitudinal temperature gradients and mechanisms of climatic warming during the Late Cretaceous.
The mechanism of high-latitude warming during times such as the Late Cretaceous is not well understood. Increased levels of greenhouse gases and increased latitudinal heat transport by the atmosphere/ocean are two widely cited mechanisms. Oxygen isotopic evidence from marine plankton appears to support increased latitudinal heat transport, because estimated high-latitude temperatures are warmer than those of today and estimated low-latitude temperatures are cooler. However, possible chemical alteration of low-latitude samples makes marine isotopic temperature gradients suspect. We calculated latitudinal temperature gradients for the latest Cretaceous (Maastrichtian) of North America using leaf megafloras from 33-83oN paleolatitude. Both Leaf Margin Analysis and multiple regression models of leaf physiognomy were used for the mid-latitude assemblages, with alternative methods used for low-diversity assemblages from Alaska. Early and late Maastrichtian assemblages were analyzed to improve geographic coverage and average out climatic fluctuations. Latitudinal temperature gradients were estimated by simple linear regression of MAT against paleolatitude, based on the approximately linear gradient found today at middle latitudes. MATs derived from Leaf Margin Analysis give a mid-latitudinal temperature gradient of 0.3-0.5oC per degree latitude, while MATs derived from multiple regression models give a gradient of 0.2-0.5oC per degree latitude. The exact result depends on the paleogeographic reconstruction, the modern floras to which the fossil floras are compared, and the temperature estimate for the North Slope of Alaska. All paleobotanical temperature gradients are consistent with high-latitude warming by increased greenhouse gases because they indicate elevated temperatures relative to today for both middle and high latitudes. Many gradients are also consistent with increased latitudinal heat transport because they produce a lower latitudinal temperature gradient than that of today. MATs predicted for 30 degrees latitude are 21-28oC, higher than those of the Recent and higher than many equatorial paleotemperatures estimated from oxygen isotopes in marine plankton.
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1 - Texas State University, Department of Biology, 601 University Drive, San Marcos, Texas, 78666, USA
Presentation Type: Oral Paper
Location: Salon G - Austin Grand Ballroom/Hilton
Date: Tuesday, August 16th, 2005
Time: 8:45 AM