The characteristics of the water masses in the Nordic Seas are expected to change due to the Earth's higher temperatures and melting glaciers. To find out more about the effects of these changes, scientists turn to similar periods in the past. Two recent studies show that the water properties in the Nordic Sea had varied considerably during the different interglacials.
Whether in Norway, Iceland or Greenland: the glaciers are melting in the North Atlantic and European Nordic Seas, releasing large quantities of freshwater. Every year, the summer ice cover of the Arctic shrinks as well, and this also affects the amount of freshwater in the northern sea between Iceland and Spitzbergen. In this region, dense, salty water from the Atlantic sinks into the depths of the sea and flows southwards whilst near the seafloor. Among other things, this recirculation drives the Gulf Stream and its branches, and is essential for the European thermal balance. The presence of more freshwater in the seas may also affect the currents.
To predict the future effects of climate change, scientists are looking into the climate changes of the past. Completed warm spells are considered good models for the warming-up season we are currently in. Two new studies – one by GEOMAR Helmholtz Center for Ocean Research Kiel, the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven and the Hong Kong University, with the support of the Royal Netherlands Institute for Sea Research (NIOZ), Utrecht University (The Netherlands) and the University of Victoria (Canada) – now show independently that the state of the Nordic Seas differs significantly during different interglacials. At the same time, they provide fresh insights into these important processes at different warm periods of Earth's history.
One of the studies, published in the scientific journal Geophysical Research Letters, attempted to reconstruct the surface salinity in the Greenland Sea and the European North Sea during a warm period around 400,000 years ago. For this purpose, the researchers used sediment cores extracted from the seabed south of Spitzbergen.
The new data shows that the ocean surface in the central Nordic Seas was substantially colder 400,000 years ago. In addition, the layers near the surface were very low in salt, probably due to the continuous melting of the Greenland ice sheet and freshwater input from the Arctic. These processes led to the formation of a thicker, low-salt layer at the surface, which pushed the Atlantic surface water flowing in from the south to greater depths.
In the second study, published in the journal Earth and Planetary Science Letters, a research team led by Dr Benoit Thibodeau from the University of Hong Kong investigated the nutrient utilisation in the surface waters of the Greenland Sea and Norwegian Sea during three interglacial periods between 400,000 years ago and the present. “The major changes of nitrate utilisation recorded here thus suggest that a thicker mixed-layer prevailed during past interglacials, probably related to longer freshwater input associated with the preceding glacial termination,” said Dr Thibodeau.
Although the two studies were independent of each other and used different research methods, the results obtained were quite similar, indicating that the thickness of the surface layer directly controlled the depth flow of the Atlantic water layer on its way to the Arctic Ocean, said co-author and paleo-oceanographer Dr Henning Bauch, from GEOMAR and AWI.
“The results not only call for caution when using older interglacials as modern or near-future climate analogues, our findings also help to better understand the effect of freshwater input on climate-sensitive ocean circulation sites like those in the Nordic Seas,” he added.
More information: www.geomar.de
Links to the studies: http://dx.doi.org/10.1016/j.epsl.2016.09.060 and http://dx.doi.org/10.1002/2016GL070294