This shows the change in carbon 13 concentration, and sea surface temperatures over the entire Ordovician period.
"Carbon isotopic (δ13Ccarb) variations in seawater through the Ordovician alongwith the conodont-apatite based tropical seawater temperature trend from Trotter et al. (2008). Ordovician δ13Ccarb data replotted from Gao et al. (1996), Kump et al. (1999), Saltzman (2005), and Saltzman and Young (2005). Timescale (with new global stage names; H, Hirnantian), major biodiversity and extinction intervals are from Webby et al. (2004)."
Seth A. Young, Matthew R. Saltzman, William I. Ausich, André Desrochers, Dimitri Kaljo
"Did changes in atmopspheric CO2 coincide with latest Ordovician glacial-interglacial cycles"; Palaeogeography, Palaeoclimatology, Palaeoecology; 2010; pg. 337