Geochemical Perturbations in the Ocean/Atmosphere System During the Middle Devonian to Visean: The State of the Art

Abstract

The Middle Devonian to Lower Carboniferous represents one of the four time windows selected by IGCP Project 386 to quantify variations in the geochemical fluxes as consequence of global change. The Middle Devonian to Visean is an intriguing period since it represents the transition of the Devonian greenhouse into the Carboniferous coldhouse mode. The intense colonization of the continents by vascular land plants started during the Middle to Late Devonian. As a consequence continental weathering may have increased due to developing soils and higher nutrient fluxes to the oceans may have initiated enhanced primary productivity and the deposition of black shales. The burial of organic carbon in the oceans and the growth of the terrestrial organic carbon reservoir may have contributed to a drawdown of atmospheric Pc% and to climatic cooling. Finally, a global mass extinction event occurred in the late Frasnian. The currently available geochemical data base is reviewed with respect to the possibility to quantify short-term or long-term variations in the geochemical fluxes during this time period. Geochemical data set Strontium, oxygen and carbon isotope data were measured on well preserved brachiopod shell calcite, micrites or whole rock samples. The time resolution of the brachiopod isotope records is mostly not