A wetter Cretaceous.
Shrinking ocean basins pushed water (darkest blue) onto the continents.

Image courtesy of R. Dietmar Müller

The Great Flooding From Beneath the Sea

In the age of the dinosaurs, great inland seas kept the climate balmy and redirected the course of evolution by opening huge new niches. But these seas weren't constant, and scientists have long debated how high they rose and why. A new study finds that global sea level was 170 meters higher than today and chalks the dynamism up to an ever-changing sea floor.

When researchers tackle the problem of high Cretaceous seas, they don't think in terms of the volume of seawater. Instead, they consider the capacity of the ocean basins: When the basins shrink, they push water up onto the continents, and when they expand, the inland seas drain back into the ocean. With this in mind, Dietmar Müller of the University of Sydney, Australia, and his colleagues scoured the geologic record to reconstruct what the sea floor was like up to 140 million years ago.

The new reconstruction, reported tomorrow in Science, comes up with sea level 82 million years ago that was 170 meters higher than today. Estimates had ranged from 40 meters to 240 meters. Most of the sea-level fall since then has been due to the loss of Pacific mid-ocean ridges and their accompanying shallow sea floor, according to the reconstruction. The ridges had pushed water onto Cretaceous continents, but plate motions carried many of them down into the deep-sea trenches along the American coasts. That left the Pacific deeper and the inland seas withdrew.

"I think it's progress," says marine geologist Kenneth Miller of Rutgers University in Piscataway, New Jersey. "Long-term sea level was significantly higher [in the Cretaceous]. I would say 150--plus or minus 50--meters" is a good range now. Together with the continent-scale ups and downs driven by the churning of Earth's interior, that amount of sea-level rise would roughly account for the full extent of inland seas like the one that spread into the heart of North America.