Freshwater fossils reveal that 40 million years ago what is now the Arctic Ocean was a freshwater lake, protected from the wider oceans by land bridges. Only when the bridge between Greenland and Scotland subsided could salty water from the Atlantic pass through (later supplemented by a small amount of water from the Pacific through the Bering Strait). A new study has revealed the timing and process by which this occurred, and the consequences for the climate.
Water freezes much more easily without salt, so the salinity of polar seas has a big effect on the world's climate. This salinity, in turn, is affected by the location of the continents at any particular time. In the Eocene era (54 to 36 million years ago) a ring of land kept the Arctic Ocean fresh.
Dr Michael Stärz of the Alfred Wegener Institute wanted to know what happened when the bridge between northern Europe and Greenland subsided, allowing passage of the 3,300 cubic kilometers (805 cubic miles) of salty water that currently flows into the Arctic each year. He modeled the changes as the land slowly sank beneath the waves, a process that took about three million years, starting 36 million years ago.
"Interestingly,” Stärz said in a statement, “the greatest changes in the circulation patterns and characteristics of the of the Arctic Ocean only occurred when the land bridge had reached a depth of over 50 meters (160 feet) below the surface." The ocean depth stabilized for a long period at around 200 meters (650 feet) before subsequently sinking to modern values of about 500 meters (1,600 feet).
The top 50 meters of the ocean are known as the surface mixed layer. In the North Atlantic, it is made up of relatively light, fresh water, which sits above saltier water beneath. It was only when the gap between the continents became deep enough to allow the heavier water to flow through that the nature of the Arctic changed, Stärz reports in Nature Communications.
The Arctic's response during the period where the land bridge was sinking depended in part on other factors, such as the carbon dioxide levels at the time. The Eocene era saw quite large swings in greenhouse gas concentrations (although nothing as fast as the last 50 years). Matching the timing of these to that of the opening of the Arctic-Atlantic connection is challenging, so Stärz modeled the effects of the changes using a variety of atmospheric conditions.
The paper concludes that, whatever the atmospheric conditions, the flow of salt water into the Arctic warmed the planet substantially. The extra salt reduced ice formation, leading to less sunlight being reflected back to space, a pattern similar to the one we see today.