Reconstructing vanished oceans

7 March 2008

Scientists from the University and the Centre for Geodynamics in Norway have made the first comprehensive model of the Earth's sea level rising and falling over the last 140 million years, resolving a long-standing controversy over sea level fluctuations through geological history.

The group, led by Associate Professor Dietmar Müller from the School of Geosciences at the University of Sydney, has reconstructed the volumes of ancient ocean basins from the Cretaceous period until the present, in an article published in the journal Science on Friday.

"A global sea level rise of a metre, driven by slowly melting ice sheets, would have disastrous effects on at least 60 million people in coastal areas worldwide. But even larger sea level fluctuations have occurred in the ancient past, in 'hothouse' climates, when neither humans nor inland ice caps existed," explains Associate Professor Müller.

"Our goal was to understand these changes, as sea level fluctuations have been a driving force in the evolution of animals and plants, in climate change and biogeography," says Associate Professor Müller.

"By creating a detailed set of digital maps of ancient ocean basins we were able to show that cycles of mid-ocean ridge creation, evolution and destruction have profoundly effected shifting coastlines and inland seas through time."

The model is the first to comprehensively map the planet's oceans from the Cretaceous period, when Gondwana - the supercontinent that later broke up into Australia, Antarctica, South America and Africa - was intact. The Indian and Atlantic oceans were formed in this period.

"To give an idea of the time scale that our ocean basin model covers, it starts at the time when dinosaurs were at their most diverse, mammals were small and a minor component of animal life on Earth, and flowering plants were just spreading over the Earth," says Associate Professor Müller.

"If we project our model 80 million years into the future, we can predict that the sea level will continue falling by about 120 metres in the long run, through mid-ocean ridge destruction and the continuing ageing and deepening of the ocean basins," says Associate Professor Müller.

For the majority of Earth's recorded geological history, there were no inland ice sheets and it is quite likely that 80 million years into the future the world will be ice free again. The future looks likely to have a hothouse climate like in the Cretaceous 80 million years ago, when polar mean temperatures were over 14 degrees higher than today, compared to our currently frozen poles.

"Even if all present ice caps melt, causing a sea level rise of about 50 metres, the net result 80 million years from now would still be a 70 metre sea level fall, due to the unstoppable increase in ocean basin depths. It is a powerful reminder that, unlike greenhouse gas emissions, we can't control the planets geodynamics," concludes Associate Professor Müller.

Contact: Jake O'Shaughnessy

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