SEASONALITY AND CIRCULATION DYNAMICS ALONG THE APPALACHIAN MARGIN OF THE LATE PENNSYLVANIAN EPICONTINENTAL SEA OF NORTH AMERICA: BRACHIOPOD GEOCHEMICAL RECORDS AND THEIR IMPLICATIONS FOR MODELS OF SHELF ANOXIA

Several major uncertainties surround models for estuarine-type circulation and anoxia in the Late Pennsylvanian epicontinental sea of North America (LPES), including the potential for seasonal watermass turnover, the effect of monsoonal wet and dry season cyclicity on water column stratification, and the influence of climate change throughout the transgressive-regressive cycle. This study investigates some of these unknowns using stable isotope and trace element analyses of well-preserved brachiopod shells from the Virgilian (latest Pennsylvanian) Ames Member (Glenshaw Formation, Conemaugh Group) in the Appalachian Basin, which was deposited during the most extensive late Carboniferous transgression of the LPES. Thick-shelled Neospirifer dunbari specimens were sectioned longitudinally and serially-sampled for stable isotopes (δ¹⁸O and δ¹³C) and trace elements (Mg, S, Na, Sr) along transects perpendicular to growth banding to obtain a record of seasonality. In all specimens, δ¹⁸O is nearly invariant, usually fluctuating by less than 0.5‰ throughout the life of the organism, while δ¹³C and trace elements show no consistent evidence for seasonal cyclicity. These data demonstrate a lack of seasonal changes in temperature and water chemistry during the Ames highstand, suggesting an absence of major annual turnover or upwelling events. This situation would have been conducive to maintaining year-round, stable water column stratification, as implied by the superestuarine circulation model (Algeo et al., 2008, Geol. Assoc. of Canada Spec. Publ. 48, p. 7-38).

Smaller-shelled Crurithyris planoconvexa specimens were analyzed from multiple stratigraphic horizons at several sites throughout the basin. On average, δ¹⁸O decreases by 0.6‰ approaching the eastern margin of the basin, consistent with the presence of a salinity gradient associated with freshwater runoff from the proto-Appalachian Mountains. However, differences in δ¹⁸O between stratigraphic horizons at a single location slightly exceed the magnitude of the regional gradient, indicating that environmental and climate conditions changed significantly with the rise and fall of sea level. The degree of stratification and seasonality thus likely varied at different points in the T-R cycle.