Chemostratigraphy of the Mid-Carboniferous boundary global stratotype section and point (GSSP), Bird Spring Formation, Arrow Canyon, Nevada, USA

The IUGS in 1998 decided that the Mid-Carboniferous (Mississippian–Pennsylvanian) global boundary stratotype section and point (GSSP) be assigned to a sequence of sedimentary units within the lower Bird Spring Formation in Arrow Canyon, Nevada, USA, with the boundary defined by the first appearance of Declinognathodus noduliferus. This study represents the geochemical work on brachiopods, matrix and cement covering a 35 m interval of the GSSP about the Mid-Carboniferous boundary. Samples were collected wherever brachiopods were noted in the sequence interval and their stratigraphic position was carefully measured using the ‘A’ markers left by Amoco geologists from their detailed biostratigraphic work. an chemostratigraphic 1000 Sr/Ca value of unaltered brachiopods from the Mississippian side of the boundary (1.928±0.575) are similar to their Pennsylvanian counterparts (2.141±0.427), and both are within the range of values obtained for Recent low-latitude brachiopods (1.50 to 5.50). Manganese concentrations are similar, with mean values of 33 mg/kg and 27 mg/kg for Mississippian and Pennsylvanian boundary brachiopods respectively, and fall well within the manganese range of Recent low-latitude representatives (4 to 58 mg/kg). In contrast, Mississippian unaltered boundary brachiopods (244±125 mg/kg) contain significantly more Fe (P=0.001) than Pennsylvanian ones (141±97 mg/kg). Although both of these averages are above, there is some overlap in their ranges with that recorded for Fe in Recent counterparts (10 to 127 mg/kg). Oxygen isotope values of unaltered brachiopods from the Mississippian side of the boundary (−2.38±0.82‰, PDB) are similar to the Pennsylvanian ones (−2.19±0.72‰, PDB) and well within the range recorded in Recent low-latitude specimens (−3.04 to −0.40‰, PDB). Carbon isotope values for the Mississippian boundary brachiopods (+2.01±0.81‰, PDB) are dissimilar to those from the Pennsylvanian (+2.42±0.83‰, PDB), but both are within the range observed in Recent low-latitude brachiopods (−0.94 to +3.11‰, PDB). The 1000 Sr/Ca, Mn, Fe, δ13C and δ18O values/contents of the unaltered brachiopods reflect oceanographic conditions of the Arrow Canyon sea, and as such are valuable proxies for local, and possibly regional, basin correlation. s time, the 87Sr/86Sr values of unaltered brachiopods provide the most useful geochemical tool in recognizing and correlating the Mid-Carboniferous boundary on a global scale. The frequency and variations of the Mississippian 87Sr/86Sr trend (0.708 082±0.000 040) are pronounced and significantly different (P=0.045) from the essentially invariant trend for the post-boundary Pennsylvanian (0.708 123±0.000 031) brachiopods from the GSSP. The Mid-Carboniferous boundary 87Sr/86Sr value based on unaltered brachiopod samples from within 1 m of the boundary is 0.708 056±0.000 021.