Paleoceanography http://doi.org/hrc (2012)

Unlike today, during the Cretaceous period, deepwater is thought to have formed from relatively local sources. An analysis of the geochemistry of marine sediments in the North Atlantic Ocean indicates that deepwater formation shifted to the poles, beginning about 80 million years ago towards the end of the Cretaceous.

Stuart Robinson of University College London and Derek Vance of the University of Bristol reconstructed Cretaceous deepwater circulation using neodymium isotopes — a tracer of water mass sources — preserved in Cretaceous fish teeth. They found a synchronous shift in neodymium values in the North and South Atlantic and proto-Indian oceans beginning around 80 million years ago.

The authors favour a scenario in which cooling at the poles initiated deepwater formation in the Southern Ocean, with that water flowing northwards into the Atlantic and Indian oceans. However, they caution that a unique interpretation is not yet possible given the scarcity of data from this time period.