Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry

Julie Trotter¹, Ian Williams¹, Chris Barnes², Christophe Lécuyer³ and Robert Nicoll¹

 1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Earth and Ocean Sciences, University of Victoria, BC, Canada
3 Laboratoire CNRS UMR 5125 Paléoenvironnements and Paléobiosphere, Université Claude Bernard Lyon 1, Villeurbanne, France

Figure 1. Portion of a polished epoxy mount showing ion microprobe pits, excavated during in situ oxygen analysis using the SHRIMP II, in conodonts and a Durango apatite grain (centre).

The Ordovician Period, long considered a Supergreenhouse state, saw one of the greatest radiations of life in Earth's history. Previous temperature estimates of up to ~70°C have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time. We present a very different global climate record determined by in situ ion microprobe (SHRIMP) oxygen isotope analyses of Early Ordovician-Silurian conodonts. This record shows a steady cooling trend through the Early Ordovician reaching modern equatorial temperatures that were sustained throughout the Middle and Late Ordovician. This favourable climate regime not only implies that the oxygen isotopic composition of Ordovician seawater was similar to today, but that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period.

Figure 2. Generalized global biodiversity pulses and tropical seawater temperature trend through the Ordovician.

Published in Science, July 2008: Trotter, J., Williams, I., Barnes, C., Lecuyer, C., Nicoll, R. (2008) Did cooling oceans trigger Ordovician Biodiversification? Evidence from conodont thermometry, Science, Vol 321, 25 July, 550-554.