Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratification, and circulation patterns
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2017 Elsevier B.V. Stable isotopic analyses of > 100 well-preserved Carboniferous brachiopod shells, representing two time slices from across North America, reveal systematic regional changes in environmental conditions. For both of the time slices studied, the Chesterian (latest Mississippian) and the Virgilian (latest Pennsylvanian), 18 O and 13 C show a stepwise decrease moving across the continent from modern-day west to east. Higher 18 O in the west reflects greater water depths, less freshwater input, and more direct upwelling influence from cool, open-marine waters than in the east. Modelling based on these regional isotopic data indicates salinity reductions on the order of ~ 14 psu in the Appalachian Basin relative to the Midcontinent Basin for the Virgilian time interval over a range of reasonable thermal gradient and 18 O fw assumptions. The magnitude of this gradient is similar to those of modern-day Hudson Bay and the Panama Bight, which exhibit permanent haloclines driven by freshwater discharge. Considering existing paleoecological evidence for freshening, the lack of more dramatic benthic freshening suggests that the depth of a regional halocline was shallower than the depths inhabited by thick-shelled stenotopic brachiopods, although other lines of evidence are necessary to verify the strength of the halocline. Estimated 18 O temperatures for Appalachian Basin brachiopods are ~ 35 C cooler than those of coeval conodonts, due to either slightly warmer, fresher surface waters or uncertainty in the phosphate 18 O paleothermometer. Despite discrepancies in calculated temperatures, the magnitude of the regional isotopic gradient derived from brachiopod and conodont proxies is similar after making appropriate glacioisotopic and water depth corrections. A decreasing 13 C trend toward the east (southern paleolatitude) likely reflects greater restriction and terrestrial influence near the sea's tropical shoreline. Trace element trends are less consistent and are affected by species and metabolism. Erratic variations in Mg/Ca concentrations, even among pristine samples of the same genera with similar 18 O values, and unrealistic Mg/Ca-derived temperatures justify continued caution in applying existing Mg/Ca thermometer equations to Paleozoic brachiopods.
