ANALYSIS OF CENOZOIC CLIMATE AND BIODIVERSITY GRADIENTS ON BOTH SIDES OF THE NORTH ATLANTIC – IMPACT OF GULF STREAM INTENSITY ON CONTINENTAL PATTERN

At present, the Gulf Stream is a main factor causing milder, maritime climates in Western and Northwestern Europe when compared to conditions in North America at the same latitude. Actually, the topic is very active, and a slowing-down or cessation of the thermohaline circulation in the North Atlantic and its consequences for Western Europe are discussed within the context of future climate change. A reduced intensity of the Gulf Stream is also assumed for most of the Cenozoic, prior to the closure of the Central American Strait. Here we use the paleobotanical record available on both sides of the North Atlantic to study the impact of Gulf Stream intensity on past continental climate and biodiversity in various Cenozoic time slices. To reconstruct past patterns we employ quantitative techniques of paleoclimate reconstruction (Coexistence Approach, CLAMP) and analyse diversity of Plant Functional Types (PFTs), supplemented by model sensitivity studies. First results based on ca. 250 Miocene floras indicate that in both continental transects studied, latitudinal temperature gradients are overall shallower than today and do not show the pointed difference in steepness presently observed and primarily related to northward heat transport by the Gulf Stream. Furthermore, biomes existing at that time at about the same latitude on both sides of the Atlantic, appear less diversified than today. However, cooler than present conditions suggested by the model for the mid-latitudes of Western Eurasia under a reduced Gulf Stream are not supported by proxy data. Thermophilous pfts such as broadleaved evergreen woody plants even tend to have a diversity centre in the latter region. Being most striking in the Eocene, this persisting pattern, still discernible in the late Miocene, might be partly explained by the warming effect of the Tethyan Ocean. Currently Pliocene sites are included to study the direct impact of the closure of the Central American Strait. [grant: DFG MI 926 / 8-1]