WAS TROPICAL CLIMATE CONSTANT IN THE PENNSYLVANIAN?

Paleotropical climate change associated with Pennsylvanian Milankovitch cyclicity is controversial. Paleoclimatic interpretations of glacial-eustatic sedimentary cycles (cyclothems) using sedimentological data suggest seasonally dry conditions before, and ever-wet conditions during coal accumulation. The constancy of plant communities within Pennsylvanian coal swamps from cycle to cycle, particularly in the mid-to-late Desmoinesian, has led paleoecologists to stress the climatic constancy of the Pennsylvanian tropics. A better understanding of Pennsylvanian swamp ecology may help to resolve this question. In the late Atokan - early Desmoinesian, a cordaite – seed fern – tree fern community flourished in the peat swamps of the Western Interior Basin. Cordaites have been interpreted both as marine swamp trees (mangroves) based on root morphology and pyrite distributions, and as freshwater plants adapted to drained peat substrates. Mangrove peat rarely occurs in siliciclastic depositional settings. However, most paralic peat swamps experience salt water influx during the dry season. Growth in predominantly freshwater swamps that experienced seasonal saltwater influx may account for the paleogeographic distribution of cordaite-dominated swamps. In the late Atokan – early Desmoinesian coals of Iowa , the succession of plant communities indicates changing rainfall conditions during peat accumulation. In the late Atokan Blackoak coal, the presence of a cordaite – tree fern – seed fern community at the base of the seam suggests seasonally dry climates at the onset of peat accumulation. Conditions became gradually wetter during peat accumulation, culminating in a Lepidophloios community, adapted to growth on deeply flooded peat substrates. A similar community progression occurs in the early Desmoinesian Cliffland coal. The amount of rainfall varied consistently during coal accumulation in late Atokan – early Desmoinesian glacial-eustatic cycles