Explosion in marine biodiversity explained by climate change

28.07.2008

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 CO_2 , 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.