Tertiary climatic changes at middle latitudes of western North America

Abstract

A new multivariate analysis of leaf physiognomy provides the basis for estimating various climatic parameters for angiosperm leaf assemblages. Estimates from this Climate-Leaf Analysis Multivariate Program ( clamp) are both accurate (valid) and precise (repeatable). Application of clamp to fossil-leaf assemblages from western North America confirms that Eocene mean annual temperature ( mat) was generally high (early Eocene mat ≈ 19°C at paleolat. 70°N and >27°C at paleolat. 45°N). Significant fluctuations (≈5–8°) in mat occurred during the Eocene. Each successive warm period was cooler than the preceding warm period. Mean annual range of temperature ( mart) was generally low during the Eocene, especially at middle latitudes, but then markedly increased by ≈ 12–13°C at ≈33 Ma as mat decreased by ≈ 7–8°C. These changes at ≈ 33 Ma largely involved a decrease in cold-month mean temperature, which continued to decline into the early Miocene but then increased significantly into the mid-Miocene thermal optimum. Warm-month mean temperature changed little from the Eocene through the early Miocene; this parameter, however, markedly decreased at ≈ 13 Ma, which coincides with the growth of the east Antarctic ice sheet. clamp also indicates that the coastal region of western North America had significantly higher mat than interior regions such as the Great Basin and Rocky Mountains. These differences in mat appeared by, or during, the Eocene and indicate that much of the interior had high altitudes in the Paleogene. These inferences have a significant bearing on global circulation models proposed for Tertiary climates.