Early Palaeocene palaeoclimatic inferences from fossil floras of the western interior, USA

Abstract

Twenty one early Palaeocene floral assemblages were collected from a variety of alluvial depositional environments from sites in the Williston, Powder River and Raton basins of the western interior of the USA. Leaf fossils were assigned to morphotypes and their physiognomy was quantified for palaeoclimatic analysis. Two methods were used, Climate Leaf Analysis Multi variate Program (CLAMP) and Leaf Margin Analysis (LMA). Temperature estimates and trends derived from CLAMP and LMA indicate that early Palaeocene palaeotemperatures decreased from the latest Cretaceous, particularly in northern areas of North America. Climatic zones shifted southwards from their latest Cretaceous positions, resulting in a steepened north-south temperature gradient and decreased equability in the early Palaeocene. Sub-tropical conditions prevailed as far north as the Raton Basin with the 20°C mean annual temperature isotherm (megathermal-mesothermal boundary) located at approximately 45°N palaeolatitude. The 13°C mean annual temperature isotherm (mesothermal-microthermal boundary) was located at approximately 55°N palaeolatitude to the south of the Williston Basin. CLAMP estimates indicate that precipitation increased significantly in all areas sampled directly after the K–T boundary event, especially in the southern Rockies. Precipitation levels decreased somewhat after this initial peak but remained high throughout the early Palaeocene. Evidence from this and other studies suggests that seasonality increased from the latest Cretaceous into the Palaeocene and was most pronounced in central and northerly areas of the North American continent.