Jurassic marine rocks yield clues to globally warmed future


History is repeating itself as global warming threatens a recurrence of extinction and upheaval in the oceans last seen 180 million years ago, scientists have warned.

A study of ancient rocks and fossils along the North Yorkshire coast offers clues about what to expect in the next century if climate change predictions hold true.

During the Early Jurassic era (that is, some 200-145 million years ago), temperatures increased by the same level forecast for the decades to come. As temperatures rose, oxygen levels fell, causing drastic changes to marine communities.

Extinction levels

More than a quarter of ocean genera – families of species – became extinct, as did 5 per cent of biological “families”.

In taxonomy, a “family” is a larger group consisting of several genera. For instance, the family canidae includes domestic dogs, coyotes, foxes, jackals, and wolves.

While the Jurassic seas eventually recovered from the effects of global warming, the ecosystems they harboured were drastically changed.

Prof Richard Twitchett, from the University of Plymouth’s School of Geography, Earth and Environmental Sciences, said: “Our study of fossil marine ecosystems shows that if global warming is severe enough and lasts long enough, it may cause the extinction of marine life, which irreversibly changes the composition of marine ecosystems.”

The researchers learned about environmental conditions on the sea floor during the Early Jurassic by studying sedimentary rocks and marine fossils. They then correlated the findings with published data on changes in temperature, sea level and oxygen concentrations. The results are reported in the online journal Public Library of Science ONE.

Co-author Dr Silvia Danise, also from the University of Plymouth, said: “Back in the laboratory, we broke down the samples and identified all of the fossils, recording their relative abundance, much like a marine biologist would do when sampling a modern environment.

Sea-floor community

“Then we ran the ecological analyses to determine how the marine sea-floor community changed through time.” The team found a “dead zone” in the rock, which showed virtually no signs of life and contained no fossils. This was followed by evidence of a return to life, but with new species recorded.

Prof Twitchett added: “The results show in unprecedented detail how the fossil Jurassic communities changed dramatically in response to a rise in sea level and temperature and a decline in oxygen levels.

“Patterns of change suffered by these Jurassic ecosystems closely mirror the changes that happen when modern marine communities are exposed to declining levels of oxygen. Similar ecological stages can be recognised in the fossil and modern communities despite differences in the species present and the scale of the studies.” – (PA)