Paleoclimatic reconstruction for the Albian–Cenomanian transition based on a dominantly angiosperm flora from the Cedar Mountain Formation, Utah, USA

Abstract

The Soap Wash flora from the Cedar Mountain Formation of Emery County, Utah (Albian–Cenomanian boundary) consists almost entirely of angiosperms and is positioned stratigraphically near the first appearance of flowering plants in this region. We applied both univariate (leaf margin analysis—LMA, and leaf area analysis—LAA) and multivariate (Climate Leaf Analysis Multivariate Program—CLAMP) methods to the 17 morphotypes recognized in the flora to reconstruct palaeoclimate. LMA produced a mean annual temperature (MAT) estimate ranging from 19 °C to 26 °C. CLAMP yielded 16 °C–18 °C. LAA produced a mean annual precipitation (MAP) of 81 cm, while CLAMP generated a mean growing season precipitation of 134–187 cm, within the range of standard error for the LAA estimation. CLAMP also estimated a nine to ten month growing season with some temperature seasonality and substantial seasonality in precipitation. This reconstruction is broadly consistent with sedimentological data that suggested a semiarid to monsoonal precipitation regime for the region and with climate models that predict warm summers and strong winter seasonality in precipitation. Leaf physiognomic parameters of the Soap Wash flora fall within the range of variation circumscribed by the calibration sets of all three methods, so the differences in reconstructed MAT values cannot be explained exclusively by this kind of systematic error. We propose two alternative hypotheses: First, modern leaf form-climate relationships may not yet have evolved. This seems unlikely because CLAMP has been successfully applied to older material elsewhere. Second, the Soap Wash flora may be out of equilibrium with its prevailing climate due to contemporaneous rapid climate flux. If correct, this pattern may suggest a link between mid-Cretaceous environmental perturbations and the rapid diversification and geographic spread of flowering plants.