Stormy seas

Increased wind during Heinrich events resulted in well-mixed surface waters in the North Atlantic

Credit: © Mila Zinkova 2005

A new study has found that the well-mixed upper layer of the North Atlantic Ocean — an area of intense biological activity directly involved in climate change — thickened during cool periods in the past1. Over the past 65,000 years, there have been eight periods of significant cooling, known as Heinrich events, which were characterized by a massive iceberg release into the North Atlantic Ocean. Until now, it was thought that freshwater discharged from the ice created an isolated layer of low salinity surface water in the region, which disrupted ocean circulation, further affecting the climate.

But a study by Harunur Rashid at the University of South Florida and Ed Boyle of Massachusetts Institute of Technology challenges this concept by reconstructing past ocean surface conditions using the oxygen isotope ratios recorded in single-celled foraminifera fossils taken from a sediment core in the North Atlantic. They discovered that throughout three of five Heinrich events studied, foraminifera living at a wide range of depths experienced similar conditions.

The researchers propose that the upper ocean was mixed during these events owing to changes in atmospheric circulation, which led to stronger winds and increased storminess. This suggests that increased freshwater input, such as from the modern melting of the Greenland Ice Sheet, may not disrupt ocean circulation as much as previously predicted.


  1. 1

    Rashid, H. & Boyle, E. A. Mixed-layer deepening during Heinrich events: A multi-planktonic foraminiferal δ18O approach. Science 10.1126/science.1146138 (2007).

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Newton, A. Stormy seas. Nature Geosci (2007).

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