Formation of ice sheets 34 million years ago changed ocean acidity

Before ice first began to form in Antarctica around 34 million years ago, the Earth was a very different place – but then greenhouse conditions swiftly gave way to an icehouse climate, causing the oceans to become less acidic.

Scientists at the University of Southampton’s School of Ocean and Earth Science, based at the National Oceanography Centre, Southampton UK and Germany’s GKSS Research Centre have been piecing together how Earth’s changing climate affected ocean chemistry during this period of transition. Their work sheds light on the links between glaciation and the ocean carbon cycle.

Their research, published in Nature (24 April 2008), confirms the connection between two separate phenomena that occurred at the same time: a fall in sea-level caused by Antarctic glaciation and a change in ocean acidity – revealed by a change in the depth at which calcium carbonate shells start dissolving on the sea floor.

Dr Toby Tyrrell of the National Oceanography Centre, Southampton said:

“We were keen to discover why the oceans became suddenly less acidic – the reverse of what is happening today. Although the changes took place 34 million years ago, by understanding how the Earth System operated at this time of dramatic change we can gain insights as to how Earth will respond as we modify it by adding carbon dioxide from burning fuels.”

The team used a global biogeochemical ocean model to test different explanations as to what was happening during the transition from the Eocene period – a time of warm greenhouse conditions with higher ocean acidity, to the Oligocene period – characterised by ice, cooler temperatures and lower ocean acidity.

Dr Tyrrell continued:

“This work has advanced our understanding of how the Earth System worked during this critical period. When most explanations were incorporated into our computer model, it produced results in conflict with the available data. Only one scenario was found to be compatible with the data.”

Dr Tyrrell’s colleague, Professor Paul Wilson also of the University of Southampton’s School of Ocean and Earth Science said:

“Our work suggests that a fall in sea-level had the effect of leaving coral reefs stranded above the high-tide level where they were then eroded by wind and rain. Corals are composed of calcium carbonate – chalk – which reduces the acidity when it dissolves in seawater.”

The third member of the team, Dr Agostino Merico, who is a former postdoctoral researcher at the National Oceanography Centre, Southampton and is now with the GKSS Research Centre in Geesthacht, said:

“With this powerful tool we can peer into the deep past to gain insights into arguably the most important climatic transition of the last 100 million years. With this work we have been able to put together different components and complex processes of the Earth System, and to relate them to each other. The whole point of a model is to abstract core ideas or hypothesis in a way that enables us to learn about them.”

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