Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin

Use of the δ¹⁸O thermometer in deep time investigations is complicated by uncertainty in the oxygen isotopic composition of seawater and an increasing potential for diagenetic alteration with age. These concerns are particularly important when considering that δ¹⁸O values from Paleozoic marine carbonates tend to be low and increasingly depleted with age. Demonstrating the preservation of original chemistry and thus eliminating diagenesis as a factor is a primary hurdle in reconstructing climatic conditions in deep time. Here, we report stable isotope data from serially sampled, Middle Pennsylvanian mollusks from the tropical Appalachian Basin of Kentucky, USA. X-ray diffraction and scanning electron microscopy indicate an aragonite mineralogy with retention of primary microtextures, elemental chemistry is consistent with the shells of living mollusks, and δ¹⁸O data reveal regular cyclic variation over ontogeny, together suggesting that original shell carbonate is preserved and records environmental conditions over the life history of the animal. However, values are depleted, centering around −4.6‰, and intraannual variation is significant, spanning up to 2.2‰, calling for significant summertime runoff of fresh water to the basin in which the shell-producing organisms grew. Our data extend a documented trend toward more depleted carbonate values eastward across the midcontinent sea with increasing distance from Panthalassa, reflecting a salinity gradient driven largely by isotopically depleted and seasonal runoff from the Central Pangaean Mountains. Mollusk data support the primary nature of low δ¹⁸O values from brachiopod shells in the midcontinent, but they also highlight the problems with calculating paleotemperatures from δ¹⁸O values of carbonates precipitated in epicontinental seas by assuming a marine seawater composition. While carbonate clumped isotope paleothermometry is a potential solution to this seawater δ¹⁸O ‘problem’, we demonstrate that such data from these aragonitic samples are related to burial diagenesis and not the Pennsylvanian paleoenvironment. The bias toward samples from such settings in the Paleozoic, because continental margins are more likely to be deformed, suggests that postulated regional and global temperature histories should be treated with caution until epeiric oxygen isotope seawater values can be better constrained.