CLIMATE CHANGE ACROSS THE ATOKAN-DESMOINESIAN TRANSITION (MIDDLE PENNSYLVANIAN) IN EQUATORIAL EURAMERICA

Climate changes of large magnitude in the geological past provide a framework for studying the responses of both physical and biotic systems to external climate forcing. A major and largely overlooked climatic change took place during the Atokan-Desmoinesian transition (middle Moscovian). This change can be identified as an interval of spatial and temporal climatic drying across the Euramerican portion of Pangea, from Western Europe to New Mexico and Colorado. It is superimposed on a general E-to-W spatial gradient of decreasing moisture across Pangea, modified in the east by the effects of the Variscan highlands. Temporal changes in climate indicators during the transition include the following: 1) coal chemistry (low-sulfur to high-sulfur coals) indicative of peat-swamp nutrient status; 2) peat genesis from domed to planar swamps; 3) low calcium carbonate content of terrestrial sediments to much increased calcium carbonate, including coal balls within coals; 4) changes in the degree of chemical weathering in provenance areas as indicated by a change in sandstone composition from clean quartz arenites to lithic arenites and wackes, with increased content of feldspars; 5) disappearance of coals in far western Pangea; and 6) appearance of redbeds in some continental successions in Europe. The proximate driver of this climate change appears to have been reduction in Southern Hemisphere ice, accompanied by regional evidence of sea-level rise, including increase in abundance and extent of phosphatic black shales and marine limestones and greater general extent of Desmoinesian than Atokan strata. The climate induced environmental change is accompanied by quantitative and qualitative changes in tropical wetland floras, particularly an increase in the abundance of drought-tolerant plants and tree ferns.