EXPLORING THE STABLE ISOTOPE RECORD OF GLOBAL CHANGE AND PALEOCLIMATE: THE MID-CARBONIFEROUS GSSP (ARROW CANYON, NEVADA) AND THE URAL MOUNTAINS, RUSSIA

The mid-Carboniferous was a time of growth of continental ice sheets, increased burial of organic carbon, decreasing pCO₂ and closing of an equatorial gateway. Evidence for these global changes include positive shifts in d¹³C and d¹⁸O across the mid-Carboniferous boundary and emergence of a ~2‰ d¹³C divergence between North American and European sections. To refine the timing and magnitude of these isotopic shifts and the events they signify, we analyzed brachiopod and micritic carbonate d¹³C and d¹⁸O from the mid-Carboniferous boundary (MCB) GSSP at Arrow Canyon and micritic carbonate d¹³C and d¹⁸O from four Uralian sections. At Arrow Canyon we found a negative excursion in micritic carbonate d¹³C (d¹³C_carb) from +1‰ in the Late Chesterian to -2‰ at the MCB, then an increase to +3‰ in the Early Morrowan. Well preserved, non-luminescent brachiopods sampled above the MCB agree well with d¹³C_carb data; however, brachiopods do not exhibit this negative d¹³C excursion at the MCB. More poorly preserved luminescent and slightly-luminescent brachiopods do exhibit decreased d¹³C coincident with the d¹³C_carb negative excursion. This suggests that the negative d¹³C_carb shift at the MCB is indicative of diagenesis. Subaerially exposed strata typically show ¹³C depletions, with values < –1‰ indicating the influence of meteoric water on calcite precipitation. Non-luminescent brachiopod d¹⁸O values average –1.4‰ below the MCB and –1.2‰ above the boundary. These high values below the boundary may indicate that glacial onset occurred slightly before the MCB. Uralian sections record a 2-3‰ shift in d¹³C_carb across the MCB similar to the GSSP, though the range is from +2‰ in the Late Serpukhovian to +5‰ in the Bashkirian. Uralian d¹³C_carb data agree both with U.S. mid-continent and Moscow Syneclise data in the magnitude of the d¹³C shift. This trend in carbon isotopes from both Panthallassic and paleo-Tethyan regions indicates a global cause, perhaps due to increased burial of organic carbon. The ~2‰ offset in d¹³C values between regions could be due to differences in paleoceanography, such as a nutrient-rich, upwelled western Pangean water mass versus more restricted, nutrient-depleted paleo-Tethyan waters. Such findings need to be tested with a coupled ocean-atmosphere GCM.