Abstract:
Isotopic data of unaltered low-Mg calcite brachiopods from type Bashkirian strata of the Askyn River section (southern Urals, Russia) are correlated with those from the Mid-Carboniferous Boundary Global Stratotype Section and Point (GSSP) at Arrow Canyon (southern Nevada, USA). Strontium isotope and conodont data of precisely located samples from the sub- to tropical locations spanning the Mid-Carboniferous boundary facilitate the correlation of the two sequences and support the discontinuous nature of the sequence at Askyn. Carbon isotopic trends of the two sites are generally divergent and demonstrate the influence (overprinting) of local oceanographic conditions on global parameters. Oxygen isotopes show a similar divergence for the latest Mississippian time period, with concurrence of values for the earliest Pennsylvanian at the two sites. In both instances, differences-variations in water temperature, currents, salinity and burial rate/amount of organic matter or combinations of these factors may have played roles of influencing water chemistry at the local level. It is possible to achieve global correlations at a finer scale than biozones with precisely located material from stratotype and ancillary sections, especially if a Global stratotype POINT has been identified and selected according to universally accepted biostratigraphic criteria. Consequently it is possible to resolve oceanographic influences at the local level and reconcile them with truly global oceanographic conditions and ultimately define global causes/effects such as cryospheric and tectonic events. Seawater chemistry at Arrow Canyon probably represents Panthalassan global Mid-Carboniferous oceanic conditions, supported by its similarity with data from other locations and modern global oceanographic parameters. Askyn represents Paleotethys and global oceanographic conditions, but in part, overprinted by local influences. The Mid-Carboniferous seems to be a time of expansion of the cryosphere as documented by a positive shift in seawater oxygen isotope composition of about 0.5‰ (SMOW) from the latest Mississippian (−1.2‰) to earliest Pennsylvanian (−0.7‰).