We present detailed isotope stratigraphies for Carboniferous time based on brachiopod shell calcite from the midcontinent region of North America. Evidence for shell calcite preservation includes (1) preservation of shell microstructure, (2) lack of cathodoluminescence, (3) low Si, Al, Fe, and Mn contents, (4) Na, Sr, and S contents comparable to those of modern brachiopod shells, and (5) δ13C and δ18O values higher than those of associated cements and matrix.
The Carboniferous δ13C record for North America is characterized by three isotopic stages. The earliest stage, C1, follows a 2.0‰ increase in Kinderhookian time (early Tournaisian), from 1.5‰ to 3.5‰, and includes a brief and perhaps local late Kinderhookian excursion to 5.4‰. The δ13C values remain stable at 3.5‰ to 4‰ during stage C1, then decrease about 1‰ near the Meramecian-Chesterian boundary (Visean) to 2‰–3‰ (stage C2). Stage C2 ends with a 1‰–2‰ increase (C2-C3 transition) between middle Chesterian and early Morrowan time (Serpukhovian-Bashkirian). Stage C3 values remain mostly between 3‰ and 4.5‰ upsection to Virgilian strata (Gzhelian). Increases in δ13C probably reflect global increases in sedimentary organic carbon burial and suggest that rho CO2 declines in the earliest and middle Carboniferous strata. The middle Carboniferous δ13C shift of B. Popp, T. Anderson, and P. Sandberg, an ∼3‰ increase in European sections, occurs in North America (C2-C3 transition) but is limited to ∼1.5‰. This 1.5‰ increase was probably caused by increased organic carbon burial, whereas the additional ∼1.5‰ shift in European sections likely reflects changes in ocean circulation patterns associated with the closing of the Equatorial seaway. Based on the timing of the δ13C divergence between North America and Europe, the isolation of the Paleotethys began in late Chesterian time (Serpukhovian).
The δ18O record can also be separated into the three stages. There is a 3‰ increase during Kinderhookian-Osagean time (Tournaisian), corresponding to the Devonian to Carboniferous transition to stage C1, a 3‰ decrease during Meramecian–early Chesterian time (Visean; C1-C2 transition), then a 2‰ increase in late Chesterian–early Morrowan time (Serpukhovian-Bashkirian; C2-C3 transition). The δ18O values then fluctuate between −1‰ and −3‰ (C3 stage) upsection to the Virgilian strata (Gzhelian). If global, the 2‰ to 3‰ δ18O shifts are compelling evidence for cooling and glaciation in Early Mississippian time, warming and deglaciation in Late Mississippian time, and a return to cool, glacial conditions in earliest Pennsylvanian time. The general correlation between δ13C and δ18O shifts suggests that cooling is associated with drawdown of atmospheric CO2.