Untitled Document
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.