Paleoceanography http://doi.org/k8n (2013)

In the modern ocean, the Western Boundary Undercurrent transports deep water from the North Atlantic Ocean into the subtropics. Marine sediment analyses indicate millennial-scale variations in the structure and speed of this current during the transition from interglacial to glacial conditions 70,000 years ago.

David Thornalley of Cardiff University, UK, and colleagues examined the grain size of sediments collected from the Blake Outer Ridge, off the mid-Atlantic coast of North America, to assess the behaviour of the Western Boundary Undercurrent from about 90,000 to 50,000 years ago. According to their analysis, the velocity of the undercurrent varied in concert with several millennial-scale climate oscillations at the end of the last interglacial. Cool events were marked by increased current speeds below 4.5 km and above 3 km depth, whereas the warmer periods showed a flow pattern reminiscent of that seen in the modern ocean.

The transition from interglacial to glacial conditions, however, was marked by the onset of a completely different pattern of flow: currents were substantially weaker between depths of 2.5 and 3.5 km, but stronger above 2.5 km. The researchers attribute this shift in circulation to atmospheric and oceanographic changes associated with the growth of Northern Hemisphere ice sheets at this time.