July 28, 2008 A global change in climate could explain the explosion in marine biodiversity that took place 460 million years ago. Researchers from Lyon (1) and Canberra (Australia) (2) have found evidence of a progressive ocean cooling of about 15°C over a period of 40 million years during the Ordovician (3). Until now, this geologic period had been associated with a "super greenhouse effect" on our planet.
The results from this study were published in the July 25, 2008 issue of the journal Science.
The researchers found that marine water at the beginning of the Ordovician (480 million years ago) was very warm (around 45°C), too warm for complex living organisms to develop. The temperature measurements were obtained from fossils of primitive eels called conodonts, whose geologic age was known by the researchers. They analyzed a mineral found in these eels' skeleton for changes in the ratio of two oxygen isotopes, which is dependent on the temperature of the ocean water in which the animals lived. The early Ordovician was a time when our planet's atmosphere was still very rich in CO2, causing a strong greenhouse effect and therefore very high ocean temperatures.
The progressive ocean cooling coincided with an explosion in marine biomass and biodiversity (the number of genera and families jumped by a factor of three to four). This event took place during the Upper Ordovician, around 460 million years ago, when ocean temperatures became comparable to those of present day equatorial waters. Not only did marine animals diversify, but their range also spread to the seafloor, and the first coral reefs appeared.
The cooling of the oceans was coupled with atmospheric cooling, indicating that a global change in climate took place. This could have played a major role in the unprecedented increase in biodiversity seen in the Ordovician, which opened the modern era of diversity and complexity.
(1) INSU-CNRS Laboratoire PaléoEnvironnements et PaléobioSphère (CNRS, Université Claude Bernard Lyon 1).
(2) Australian National University.
(3) Geologic period extending from 490 to 440 million years ago.
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- Julie A. Trotter, Ian S. Williams, Christopher R. Barnes, Christophe Lécuyer, Robert S. Nicoll. Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry. Science, 2008; 321 (5888): 550 DOI: 10.1126/science.1155814
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