DEPLETED AND SEASONAL PRECIPITATION IN THE PENNSYLVANIAN INTERGLACIAL TROPICS INFERRED FROM SERIAL OXYGEN ISOTOPE VALUES OF MOLLUSK ARAGONITE (Invited Presentation)

The North American Pennsylvanian interglacials are characterized by the presence of a broad epicontinental sea over much of the continent connected to open ocean to the southwest (today’s coordinates). The degree to which this sea reflects open marine conditions is a continuing discussion, and bears on the interpretation of carbon and oxygen stable isotope ratios from carbonate shells (mostly brachiopods). The Middle Pennsylvanian Magoffin Shale exposed in the Appalachian Basin of Kentucky records interglacial conditions in the paleotropics at the far eastern end of the epeiric sea. The unit offers a rare opportunity to study mollusk shells that still retain their original aragonite mineralogy after more than 300 million years, and can thereby provide important perspective on the more abundant isotope data from brachiopod calcite. Serial sampling and δ¹⁸O analysis of the shells of bivalves, gastropods, and bellerophonts reveals regular cyclic variation over ontogeny. Isotope values are depleted (averaging ‑4.6‰) and seasonal variation is high (up to 2.2‰) relative to that expected in fully marine tropical settings. Low δ¹⁸O values extend a documented trend of increasing depletion eastward across North America toward the Allegheny Front, suggesting the increasing influence of meteoric waters with distance from Panthalassa. Such values in the tropics are low enough to require significant distillation, similar to that seen with elevation over the tropical Andean orogen today. The high range of seasonal isotopic variation suggests a seasonal precipitation regime driven by migration of the ITCZ. Importantly, isotope values are lower and more variable than those determined from brachiopods in the basin, suggesting that mollusks record conditions in nearshore settings that receive seasonally variable contributions of isotopically depleted runoff, while brachiopods occupied more marine, perhaps deeper-water, habitats. While intraannual variation primarily reflects changes in salinity rather than temperature in this tropical setting, their relative contributions to the low mean values remains speculative. Carbonate clumped isotopes of shell aragonite may help resolve paleotemperatures if solid-state bond reordering can be ruled out based on a shallow Magoffin Shale burial history.