DETECTING PENNSYLVANIAN MILLENNIAL-SCALE CLIMATE CHANGES USING POLLEN ANALYSIS IN DEEP-MARINE LIMESTONE-SHALE RHYTHMITES

Deep-water “rhythmites” are characterized by thin, rhythmically interbedded limestone layers alternating with shale layers. They are common in Precambrian through Cenozoic poorly oxygenated offshore marine deposits. Previous studies report that individual limestone-shale couplets of various Paleozoic rhythmites represent ~1000-3000 years and are interpreted as the result of millennial-scale wetter to drier climate changes with shale layers representing wetter climates and limestone layers representing drier climates. To test this climatic model, we compared the characteristics of eolian/fluvial-derived pollen assemblages in marine limestone versus shale interbeds from two different Middle Pennsylvanian rhythmite successions in central New Mexico. If the pollen-producing terrestrial plant community changed in response to wetter versus drier climate changes, then the pollen diversity and abundance might be reflected in offshore deposits and different pollen morphotypes and abundances would occur in the limestone versus shale layers. Preliminary results from two different sections in central New Mexico show four statistically significant pollen morphotypes including bissicate, triporate echinate, periporate, and trilete. Of these morphotypes, bissicates are the most common in shale layers, whereas limestone layers contain greater abundances of the other three morphotypes. The observation that limestone and shale layers in Pennsylvanian rhythmites record different pollen morphotypes suggests that (1) these deep-water rhythmites are primary depositional features rather than products of diagenetic unmixing during shallow burial, and (2) millennial-scale climate changes controlled terrestrial vegetation patterns and/or regional wind directions/ocean circulation patterns.