Late Jurassic paleoclimate of Europe and Africa.

摘要

The Late Jurassic deposits of the informally-named Lourinha formation of Portugal, the Morrison Formation of the Western United States, and the Stanleyville Group of the Democratic Republic of Congo provide a valuable record of paleoclimate for Late Jurassic terrestrial paleoenvironments. The fluvio-deltaic sediments that compose the Lourinha formation were deposited adjacent to an embayment of the proto-North Atlantic. The Lourinha formation represents a warm, sub-humid climate, with high mean annual precipitation distributed seasonally. Isotopic analysis of carbonate cement in groundwater concretions indicates that mean annual surface temperature was 10°C to 20°C warmer than present. Calculated values for the chemical index of alteration minus potassium produce a mean annual precipitation estimate of 1150 mm/yr, and paleosol morphologies show that rainfall occurred seasonally. The Morrison Formation, deposited in the interior of western North America, developed under similar, but slightly more arid conditions. Clay mineral analysis of the Stanleyville Group reveals much more arid conditions than either the Lourinha or Morrison formations. Mean annual temperatures in the Gondwanan interior appear to have been similar to those at higher latitude in Western Europe. The observed climatic differences between these areas are the result of their different paleolatitudinal and paleogeographic positions. Isotopic analysis of pedogenic carbonates from all three areas generates an average estimate of 4700 ppmV for the concentration of carbon dioxide in the paleoatmosphere. This estimate is higher than those of previous studies, and is more than twice as large as some suggested values derived from mass-balance models and the stomatal index proxy. The pedogenic carbonates also yield estimates for primary paleoproductivity, which is compared with estimates of faunal richness for the Lourinha and Morrison formations. Portugal has the highest estimated paleoproductivity and faunal richness. Faunal data is lacking for coeval African paleoenvironments comparable to the Stanleyville Group, but this study area has the lowest paleoproductivity.