How climate change strangled a Jurassic ocean ecosystem


When a warming world depleted the ocean’s supplies of oxygen, extinctions followed and recovery took hundreds of thousands of years.


At left: before oxygen levels dropped, bivalves were large and numerous. At right: afterward, they were small and comparatively rare.
The University of Texas at Austin / Rowan Martindale

183 million years ago, oceans around the world started running low on oxygen. Though the cause of what is called the Toarcian Oceanic Anoxic Event (T-OAE) is uncertain – most likely global warming triggered by huge volcanic eruptions – scientists do know it lasted for several hundred thousand years and caused mass extinctions.

In a paper published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, researchers have traced the effects of the T-OAE in detail on a marine ecosystem at what is now the Ya Ha Tinda Ranch fossil site in Alberta, Canada. It is of interest to scientists because it demonstrates how different ecosystems react to severe climate change.

Study authors Rowan Martindale from the University of Texas at Austin and Martin Aberhan of the Leibniz Institute for Evolution and Biodiversity Science in Berlin used geochemical data from the fossil site that had been previously collected by Benjamin Gill and Theodore Them of Virgina Tech University to establish a timeline against which they could examine paleontological evidence.

Prior to the T-OAE, the Ya Ha Tinda marine community was diverse and included a variety of species of fish, ammonites, ichthyosaurs (dolphin-looking reptiles), sea lilies, clams, coleoids (squid-like octopods) and lobsters. During the T-OAE, however, this underwater ecosystem shrank and restructured.

In particular, a large species of clam that had been the most abundant biological feature was completely wiped out by the reduction of oxygen in the environment. Only smaller species of clams survived.

Sites in Europe have been found that suffered similar effects to their ecosystems during the T-OAE, but the Ya Ha Tinda community appears to have had a much more difficult time recovering from the impact.

The exact reason for this struggle to rebound is unknown, but the researchers state that the environment was undergoing local stresses that kept oxygen levels low.

The authors say that this study should serve as a warning about the potential impacts of ongoing and future climate change to marine communities, since the T-OAE was the side-effect of a similar environmental shift.

“On a human time scale,” notes Aberhan, “climate-related stresses can have very long-lasting effects, with no signs of recovery for hundreds of thousands of years.”

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Angus Bezzina is a writer from Sydney, Australia.
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