Pennsylvanian vegetation and climate in tropical Variscan Euramerica

Abstract

During late Westphalian and early Stephanian (late Moscovian) times there was a progressive westward withdrawal of the coal swamps from across Euramerica, eventually being restricted mainly to the basins of the Interior Province of North America. These changes can be correlated with the northwards migration of the Variscan Front that disrupted landscapes and drainage patterns. The resulting better-drained substrates caused localised change in the swamp vegetation from being lycopsid- to fern-dominated. Compared with the lycopsids, the ferns had non-determinate growth that produced less biomass per unit area, as well as producing vegetation with a more closed canopy, factors which combined to reduce overall evapo-transpiration. His in turn caused localised reductions in rainfall, which further affected the ability of the lycopsids to dominate the swamp vegetation. By Asturian times, most European coal swamps had mixed lycopsid - fern and fern dominated vegetation, eventually to be replaced by predominantly conifer and cordaite dryland vegetation. Lycopsid-dominated coal swamps persisted only in the westernmost parts of Euramerica, in the coalfields of the Interior Province of the USA, but even here they had disappeared by middle-late Cantabrian times. This tectonically-driven decline in the Euramerican coal swamps was probably responsible for an annual increase in atmospheric CO 2 of c. 0.37 ppm, and may have been implicated in the marked increase in global temperatures near the Moscovian - Kasimovian boundary, and the onset of the Late Pennsylvanian nonglacial interval.