Low seasonality in central equatorial Pangea during a late Carboniferous highstand based on high-resolution isotopic records of brachiopod shells

Abstract

The relationship between Permian–Carboniferous glacial cycles and low-latitude climate remains a subject of vigorous debate. This study investigated seasonality and regional environmental variability in a portion of central equatorial Pangea during a late Pennsylvanian highstand using stable isotope and trace-element analyses of brachiopod shells from the Virgilian Ames Member of the Conemaugh Group in the Appalachian Basin, U.S.A. Well-preserved, thick-shelled Neospirifer dunbari specimens were serially sampled across growth bands to elucidate a record of seasonal variability during the life of the organisms. Because Neospirifer only colonized this marginal basin during near-maximum highstands, when stable, near-marine salinities were established, these data are a direct proxy for intra-annual climate fluctuations during interglacial times. Neospirifer specimens show remarkably little internal chemical variability, with δ 18 O generally fluctuating by 0.4‰ or less and δ 13 C fluctuating by less than 1.5‰ within a single specimen. Moreover, total δ 18 O variability between all specimens is only ~1.5‰. This lack of variation reflects a homogeneous, nonseasonal to weakly seasonal climate during the Ames highstand. Both δ 18 O and δ 13 C are ~1.5‰ lower than those of other Virgilian specimens from regions with a more proximal connection to the open ocean, suggesting at least moderate freshwater influence in the Appalachian Basin during this time, although salinities remained close enough to marine levels to maintain stenohaline fauna. Thus, brachiopod seasonal records indicate normal moist tropical conditions during this penultimate Carboniferous interglacial, with no evidence for strong monsoonal variations in temperature or rainfall.