SEA SURFACE TEMPERATURE CHANGE FROM LATE ORDOVICIAN δ18O DATA, BROMIDE AND VIOLA SPRINGS FORMATIONS, OKLAHOMA, USA

Conodonts from the Upper Ordovician Bromide and Viola Springs formations near Fittstown, Oklahoma were analyzed for δ¹⁸O data as a proxy for determining Late Ordovician (Sandbian-Katian, Mohawkian) sea surface temperature change. This interval is of interest because it correlates to a postulated transition form warm water carbonates (Bromide equivalent) to cool water carbonates (Viola Springs equivalent) which were deposited in the subtropical to tropical epicontinental sea of Laurentia. This interval also encompasses the GICE (Guttenberg Carbon Isotope Excursion). Forty-six samples were collected at 1.5 meter intervals across the upper Bromide and lower Viola Springs formations spanning 16 meters below the contact and 54 meters above. Thirteen of the samples have been processed for δ¹⁸O data. δ¹⁸O _V-PDB values show a difference of 0.9‰ ranging from 18.74‰ (1.5m above contact) to 19.64‰ (15.5m below the contact). Inserting these results into the temperature equation described by Longinelli and Nuti (1973, Earth and Planetary Science Letters, 20, pp. 337-340) and assuming a value of -1‰_VSMOW for δ¹⁸O of sea water, these numbers reflect a change in 3.87°C with values ranging from 26.52°C and 22.65°C respectively. The δ¹⁸O results presented here do not show a transition from warmer to cooler waters across the Bromide-Viola Springs contact. Rather, they display the opposite trend with initial cooler temperatures in the upper Bromide and a trend towards warmer waters near the upper Bromide-Viola Springs contact. The trend in strata spanning the GICE is more complicated. There is an initial cooling at the base of the Viola Springs that continues into the base of the GICE. Between 18 and 21 meters there is a calculated warming of nearly 2°C followed by apparently relatively stable temperatures for the remainder of the GICE and through our topmost processed sample at 54 meters above the base of the Viola Springs.