Global Taghanic and Givetian Seawater Records: An Amelioration of Faunal Realms, Climatic Conditions and High Levels of Atmospheric Carbon Dioxide

Unaltered Givetian brachiopods from the Appalachian and Iowa Basins, and from New Mexico, Nevada and China, and supplemented by material from other localities form the framework for establishing high-resolution strontium, carbon and oxygen isotope seawater records during the Taghanic Onlap and Givetian.

Pre-Taghanic time was characterized by faunal/oceanographic/climatic differences between the Eastern Americas and Old World Realms. Isotopic evidence does not support a climatic gradient as a barrier between the two realms, because similar water temperatures were calculated with the ?18O values of brachiopods. Instead, it is postulated that geographic barriers must have been the primary cause for the observed faunal differences. The late Givetian starting with the Taghanic Event marks a significant change in sealevel and climate, and the onset of global, cosmopolitan oceanographic conditions.

The seawater-87Sr record during the varcus Zone is punctuated by four distinct perturbations of low radiogenic values that may be related to the second tectophase of the Acadian Orogeny. Subsequently, the seawater-87Sr record was invariant during the hermanni Zone (chemical balance between JOC-HC and JR-GW fluxes), but stratigraphically upward was followed by increasingly more radiogenic values starting in the lower Klapperina disparilis Zone. The latter trend in seawater –87Sr was probably related to the third tectophase of the Acadian Phase of the Caledonian Orogeny reflecting increased weathering and erosion of the uplifted terrain (increase in JR-GW flux).

The carbon isotopic composition of pre- and Taghanic brachiopods changed from 2.27 ‰ to 1.08 ‰. Taghanic time was characterized by a decrease in the burial of organic matter by about 13%, when burial fraction (ƒorg) of the pre- to Taghanic fell from 0.240 to 0.209. Based on the increased air and water temperature changes determined by proxies, it is postulated that atmospheric levels of carbon dioxide were about 1200 ppm during the late Givetian.