Faunal evidence for a cool boundary current and decoupled regional climate cooling in the Permian of western Laurentia
Abstract Deglaciation following the late Paleozoic ice age was a complex climate transition marked by smaller glacial–interglacial transitions superimposed on an overall Permian warming trend. In contrast to this growing recognition of a dynamic climate system, few studies have investigated geographic variations in postglacial climate changes, which may have been as dynamic as the temporal variations. Because temperature and ocean circulation are important controls on the biogeography of marine invertebrates, many Permian taxa were characteristic of either paleotropical or high-latitude faunal realms. This study takes advantage of that temperature sensitivity, using a database of more than 5000 Permian fossil collections from northwestern Pangea to reconstruct spatial variations in Permian climate changes throughout the region. Cluster analysis demonstrates that localities in the Phosphoria basin (Nevada/Idaho/Wyoming) were taxonomically distinct from coeval faunas in Texas and occupied the transitional zone between Paleoequatorial and Boreal realms. Quantitative scoring of the faunal affinity of all taxa in 1 by 1° paleolatitudinal bins suggests that the western margin of Laurentia was affected by a persistent southward-flowing cool boundary current, and that the effects of the current became more pronounced throughout the Permian in conjunction with cooling in the Boreal ocean. The increasingly cool-water nature of Phosphoria basin faunas contrasted with warming in Texas and implies that northern and northwest Pangea were decoupled from the broader global warming trend following the late Paleozoic ice age.