CIRCULATION OF NORTH AMERICAN EPICONTINENTAL SEAS DURING THE CARBONIFEROUS BASED ON STABLE ISOTOPE ANALYSES OF BRACHIOPOD SHELLS

Previous studies have identified δ¹³C events in the Carboniferous that imply major shifts in the carbon cycle. However, inherent in this interpretation is the assumption that epicontinental seas are chemically representative of the global ocean. Our study uses stable isotope analyses of brachiopod shells to examine changes in climate and circulation of the North American epeiric sea. Formations were selected for study to provide shallow marine environments with geographic coverage of North America. These formations include the Grove Church and Shumway (Illinois Basin), Ames (Appalachian Basin), Bird Spring (Bird Spring Basin), and Sausbee and Oread (US Midcontinent). In all, 137 shells were found to be well preserved based on screening with plane light and cathodoluminescence microscopy of thin-sections. Upper Chesterian Grove Church (Illinois Basin) samples have δ¹³C and δ¹⁸O averages of 1.1‰ and -3.1‰ respectively. These low values could be interpreted as a local effect caused by terrestrial runoff and perhaps oxidation of organics. Terrestrial influences are suggested by the depositional environment: near-shore marine and proximal to a delta. Because the basin is semi-restricted and shallow, the potential for organic matter oxidation is similar to modern analogues such as the Bahaman Banks and Florida Bay, which can show a δ¹³C depletion in dissolved inorganic carbon of 4‰ or more relative to the surrounding ocean. Bird Spring samples from below the mid-Carboniferous boundary at Arrow Canyon, NV average 3.7‰ and -1.4‰ for δ¹³C and δ¹⁸O respectively. The higher δ¹³C and δ¹⁸O values, compared with samples from the time equivalent Grove Church, likely reflect the freer exchange with the Panthalassic Ocean at this most western edge of North America, and best represent open-ocean conditions. Samples from the Virgilian Ames-Shumway-Oread cyclothem show a progression of δ¹³C and δ¹⁸O enrichment moving west from near the Appalachians (1.9‰ and -3.8‰) to the Illinois Basin (3.2‰ and -2.4‰) and finally to the US Midcontinent (4.2‰ and -1.5‰). This is interpreted as the transition from nearshore, terrestrial influence with enhanced organic matter oxidation and lower salinity to deeper (but <100 m) well mixed conditions with normal salinities and potential for seafloor ventilation and upwelling.