Persistent cooling in the Ordovician (Darriwilian-Sandbian) revealed by conodont δ18O records in the Tarim Basin, NW China: Climatic and sedimentary implications

The Great Ordovician Biodiversification Event (GOBE) was considered to be primarily triggered by climate cooling. However, the precise timing of the greenhouse-icehouse transition during the Ordovician Period remains largely unconstrained. In order to reconstruct palaeotemperatures during the Middle-Late Ordovician transition in the Tarim Basin, northwestern China, a high-resolution SIMS-based oxygen isotope ( delta O-18(apatite)) profile was obtained from 65 conodont elements from the Nanyigou section. The oxygen isotope record based on monogeneric samples reveals an increase in delta(18) O-apatite by 1.6 parts per thousand (VSMOW) during the late Darriwilian to Sandbian, translating to tropical sea-surface water cooling of similar to 7 degrees C, probably marking the initiation of polar ice sheet growth and the prelude of the Hirnantian maximum. This delta(18) O apatite shift agrees well with the time-equivalent delta(18) O(apatit)e records from other continents, thus suggestiing a global signal. Furthermore, the positive shift of delta O-18(apatite) overall coincides with inorganic carbon isotope (delta(13) C-carb) positive excursion, indicating that enhanced organic carbon burial may have led to the atmospheric pCO(2) drawdown. Additionally, the temporal coincidence between the delta O-18(apatite) positive excursion and development of the purplish-red nodular limestone of the Tumuxiuke Formation suggests that global cooling may have contributed to the formation of coeval marine red beds.