VARIABILITY OF EQUATORIAL CLIMATE DURING THE PENNSYLVANIAN: EVIDENCE FROM LATE ATOKAN - EARLIEST DESMOINESIAN WETLAND FLORAS

Wetland floras provide a sensitive indicator of equatorial rainfall, and suggest that Pennsylvanian equatorial climates varied from stage to stage, within cyclothems (4th or 5th order transgressive-regressive cycles), and during the accumulation of individual coal seams. The dominant plants within Pennsylvanian wetlands reflect stage-level variability in equatorial rainfall. Gymnosperms (cordaites and medullosan seed ferns) and tree ferns predominated in late Atokan – earliest Desmoinesian swamps, suggesting relatively dry conditions. Lycopsids predominated in mid-to-late Desmoinesian swamps, indicating extremely wet conditions. Tree ferns and seed ferns predominated in Missourian swamps, indicating a return to relatively dry conditions.

Each Pennsylvanian stage contains a number of cyclothems, which may record the advance and retreat of continental glaciers (i. e. Pennsylvanian Milankovitch cycles). The constancy of equatorial climate within and between cyclothems is controversial. Sedimentary data suggest relatively dry climates before coal deposition and wet climates during coal deposition. The constancy of plant communities within coal swamps from cyclothem to cyclothem suggests constancy of equatorial climate, particularly rainfall.

Late Atokan – earliest Desmoinesian wetland floras from Iowa support rainfall variability during cyclothem deposition, and further suggest that rainfall varied consistently during coal deposition in some cycles. Late Atokan – earliest Desmoinesian coals record a transition from ‘relatively dry’ to ‘relatively wet’ conditions, with cordaitean gymnosperms at the base of the seam, medullosan seed ferns and tree ferns in the middle of the seam and lycopsids, at the top. High percentages of conifer pollen in these coals suggest even drier conditions in terra firma habitats adjacent to the wetland. This result implies that wet-loving persisted in refugia during times of less rainfall. These Iowa coals accumulated on the northern edge of the equatorial rainy belt. Coals of similar age from the Illinois Basin, in the center of the equatorial rainy zone, experienced wetter conditions and may not show such a strong ‘dry to wet’ transition.