A study of the late Ordovician Period has demonstrated that for part of the era the world was very cold, rather than very hot, as previously thought. The report suggests the deep ocean currents known as the thermohaline triggered a huge flowering of biodiversity. The findings suggest that these currents may be similarly important today.

The Ordovician Period lasted from 488 to 443 million years ago. The era is known for its marine life such as trilobites and early vertebrates. The late Ordovician saw an intense Ice Age accompanied by a powerful mass extinction, the cause of which has been much debated.

A 15-year study of, among other things, 45,000 fossilized trilobites and brachiopods, enabled a team led by Dr. Christian Rasmussen of the National History Museum Denmark to produce what they call “the first unambiguous evidence for a sudden Mid Ordovician icehouse.” Moreover, the authors claim that the start of this Ice Age coincided with the beginnings of the Great Ordovician Biodiversification Event (GOBE).

The causes and timing have been somewhat disputed, but paleontologists agree that the GOBE saw a dramatic increase in marine species, following a slower recovery from extinctions at the end of the Cambrian period.

Starting in the early Ordovician, a continent called Baltica, consisting of what is now much of Scandinavia, the Baltic states and part of Russia, drifted from 50° South to 40° South. This movement should have heated it up. Instead in Nature, Rasmussen has presented evidence from western Russia of increasing cool water species and carbon and oxygen isotopes indicative of cold water around 467 million years ago. Meanwhile, sea levels fell by at least 150 meters (465 feet), indicating that polar regions were becoming heavily glaciated.

The trends were confirmed from Scandinavian deposits laid down at the same time. The fall is too rapid to be explained by factors such as continental movements or the spread of plants onto the land. The authors acknowledge they do not know what caused the cooling, but have more confidence regarding its effects.

“Cooler ocean currents increased thermohaline circulation in the oceans,” they write, “Creating upwelling zones where plankton evolved and radiated. Moreover, oxygen was more accessible for organisms, once ocean temperatures fell.”

^{A reconstruction of the thermohaline currents before and after the mid-Ordovician cooling. Rasmussen et al/Nature}

“Our findings demonstrates the onset of icehouse conditions to occur some 30 million years prior to what was previously believed. The Late Ordovician Ice Age is thus no longer an enigmatic paradox in an otherwise prolonged super-greenhouse interval,” the authors write in a statement. 

Consequently, “The greatest increase in marine biodiversity in Earth history is now associated with icehouse conditions, not as previously believed, extreme greenhouse conditions.”

The importance of strong, deep ocean currents to flourishing marine life makes recent signs that these currents are currently weakening disturbing. The evidence for long-term cooling also discredits the claim that the more extreme Ice Age at the end of the Ordovician was unprecedented and indicative of unknown climatic drivers.