HIGH-RESOLUTION ISOTOPIC RECORDS OF PENNSYLVANIAN APPALACHIAN BASIN MOLLUSK SHELLS

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 of particular importance when considering that δ¹⁸O values from Paleozoic marine carbonates are generally low and show a depletion trend with increasing age. Competing explanations for this trend include increasing alteration with age, evolution of seawater δ¹⁸O through time, and increasing ocean temperatures with age. Successfully proving original mineralogy, thus eliminating diagenesis as a factor, is a primary hurdle in deconvolving potential causes.

Here, we report data from a serially sampled, Middle Pennsylvanian bellerophont from the Appalachian Basin of Kentucky. XRD analyses indicate an aragonitic mineralogy, and δ¹⁸O data reveal regular cyclic variation over ontogeny, suggesting that original shell carbonate is preserved and records primary environmental (presumably seasonal) conditions over ~2.5 years of the life history of the animal. However, like other Paleozoic δ¹⁸O data, the values are depleted, centering around -5‰. Recent studies focusing on the Carboniferous of the U.S. have suggested that negative values reflect regional differences in salinity within the epicontinental sea. If so, values from the Appalachian Basin, farthest east, should exhibit the most depleted values. Indeed, our data extend the trend toward more depleted values with increasing distance from Panthalassa across the paleo-tropics proposed by Roark, Grossman, and colleagues, suggesting greater amounts of freshwater input closer to the Allegheny Front. Values are quite negative for tropical latitudes, requiring distillation of precipitation with elevation in the orogeny. In addition, intraannual variation is significant, spanning 1.7‰. This highly seasonal precipitation and runoff regime differs from that inferred from published data derived from nearby slightly younger brachiopods, suggesting either that mollusks record conditions in more brackish and hence variable settings, or that equatorial seasonality varied significantly between interglacials during the Paleozoic icehouse, perhaps in association with shifting positions of the intertropical convergence.