A conifer-dominated palynological assemblage from Pennsylvanian (late Moscovian) alluvial drylands in Atlantic Canada: implications for the vegetation of tropical lowlands during glacial phases

Abstract

Abstract: New research suggests that Pennsylvanian Coal Forests were commonly replaced by conifer-dominated vegetation during seasonally dry glacial phases. Here we describe palynological assemblages from stacked sequences of the Middle Pennsylvanian (late Moscovian) Sydney Mines Formation of Nova Scotia, Canada, which lends some support to this hypothesis. One critical sample from a widespread redbed succession below the Harbour Coal is dominated by conifer pollen (55%), together with abundant representatives of cordaitaleans, other coniferopsids, and rare pteridosperms, ferns and lepidodendrids. It differs markedly from 13 other samples obtained from coals, their roof shale–limestone and associated channel bodies, which are dominated by lepidodendrids, marattialean tree-ferns, cordaitaleans and/or calamiteans. The latter beds represent coastal wetlands, and are attributed to late transgressive and highstand systems tracts, whereas the conifer-bearing sample represents a dryland alluvial plain and may be attributed to lowstand or earliest transgressive systems tracts. Three additional samples from grey shale below a sequence boundary (late highstand) and between a redbed succession and major coal seam (early to mid-transgressive deposits) contain transitional palynofloras with a high proportion of herbaceous gleicheniaceous and sphenopteroid ferns, herbaceous lycopsids, and sphenophylls interspersed with cordaitaleans and/or lepidodendrids. The dominance of conifer pollen within the dryland components of well-defined sequences, which have been linked to relative sea-level change promoted by Gondwanan glaciation, supports the view that Pennsylvanian tropical biomes responded to the beat of glacial–interglacial cycles.