What Happened to the Coal Forests During Pennsylvanian Glacial Phases?

Abstract

Sequence stratigraphic analysis of Pennsylvanian coal-bearing strata suggests that glacial-interglacial fluctuations at high latitudes drove cyclic changes in tropical biomes. A literature review of plant assemblages in this paleoclimatic context suggests that coal forests dominated during humid interglacial phases, but were replaced by seasonally dry vegetation during glacial phases. After each glacial event, coal forests reassembled with largely the same species composition. This remarkable stasis implies that coal-forest refugia existed across the equatorial landscape during glacial phases, expanding to repopulate lowlands during and following deglaciation. One possibility is that refugia comprised small pockets of wetland forest strung out along valleys at some sites, but data are currently insufficient to test this hypothesis. The model presented here, if accepted, dramatically alters our understanding of the coal forests and helps explain aspects of their dynamics.

Wetland-Dryland Vegetational Dynamics in the Pennsylvanian Ice Age Tropics

“…The other flora consisted of plants that were tolerant of moisture deficits of varying degrees (Leary and Pfefferkorn 1977;Galtier et al 1992;Falcon-Lang et al 2009;Plotnick et al 2009;Bashforth et al 2014). These floras oscillated in dominance through time on glacial-interglacial temporal scales (Falcon-Lang 2003cFalcon-Lang and DiMichele 2010), reflecting the climatic variations that occurred in concert with glacial-interglacial cycles and their global effects on climate, sea level, and other factors.…”

When horsetails became giants

“…As the Euramerican climate increasingly became more seasonal and dry through the late Palaeozoic in response to tectonic activity, these rainforests collapsed, eventually being replaced by seasonally dry Permian biomes [18,19]. Although the causes of this aridification remain a matter of controversy, the consensus is that this climate shift led to the fragmentation of the coal-forming forests into isolated rainforest islands surrounded by xerophytic scrubs [9,[20][21][22][23].…”

On the fundamental difference between coal rank and coal type

“…The cool climate mode that prevailed throughout much of the Pennsylvanian was similar to that in the Pleistocene and Holocene, and was dominated by glacial-interglacial cycles and their effects on sea level (Wanless and Shepard, 1936), sedimentary dynamics (Cecil and Dulong, 2003;Cecil et al, 2003a), and biotic patterns Falcon-Lang and DiMichele, 2010). However, in representing a much longer time interval of nearly 20 million years, the Pennsylvanian "coal age" encompassed far greater ranges of variability in climate state and environmental fluctuation than has the Quaternary age (Bishop et al, 2010;Cecil, 1990;Eros et al, 2012;Fielding et al, 2008a,b;Montañez and Poulsen, 2013), including periods of nearly ice-free poles as well as intervals of intense glaciation.…”