Nature Study: Is modern Arctic Ocean Circulation exceptional?

Today a pronounced and stable freshwater layer at the surface originating from inputs of the large Russian rivers almost completely prevents any significant deep water formation in the Arctic Ocean itself. The results of Brian Haley and colleagues from the IFM-GEOMAR now show that this was an exception rather than the rule for most of the past 15 million years.

The Kiel team made their discovery when they carried out geochemical analyses on sediments of the Arctic Coring Expedition (ACEX, Leg 302 of the Integrated Ocean Drilling Program (IODP)) and of a RV Polarstern expedition, which had been recovered near the North Pole on the Lomonosov Ridge between 1.000 und 1.200 m water depth. They reconstructed the seawater isotope ratio of the element neodymium (143Nd/144Nd) from the sediments. The Nd, which has characteristic isotope ratios in rocks as a function of their type and age, is transported to the ocean through weathering, where it provides information on the sources of water masses. To their surprise, the geochemists found that the isotope signature of the seawater was strongly different from the present day values, with the exception of the warm periods of the past 400.000 years. “It is even more surprising that this isotope signature indicated a pronounced influence of the weathering of basaltic rocks”, says Brian Haley. On the Circum-arctic landmasses such rocks, however, only exist in the form of the Siberian “Putorana flood basalts”.

From this geologically unique setting and taking into account the evolution of the continental ice sheets of the past 140.000 years, it was then possible to reconstruct the circulation history of the deep Arctic Ocean. The signature of the basalts can only have arrived at 1.000 m water depth in the central Arctic Ocean if vast amounts of new sea ice formed near the basalt areas in the Kara Sea area. How did the signature arrive at the seafloor? “During sea ice formation the salt of the sea water freezes out and is rejected, thereby forming highly saline brines, which were denser than the surrounding sea water. These brines sank and transported the dissolved Nd isotope signature of the basalts to the sea floor where the sediment cores were recovered”, explains Martin Frank, co-author of the study. Further, the obtained Nd isotope variations imply that the inflow of Atlantic waters was significantly reduced during most of the past 15 million years and during the glacial periods of the past 400.000 years. This also suggests that during these periods of time the main area of Atlantic deep water formation was not located in the Norwegian-Greenland Sea, similar to today, but further south.

The arctic IODP ACEX drilling project was coordinated by the European consortium ECORD (European Consortium for Ocean Research Drilling). This organization consists of partners from 17 European nations participating in the „International Ocean Drilling Programme“. ECORD is also responsible for the planning and coordination of special operations, for which normal drilling vessels cannot be used, as was the case for the ACEX project. For such purposes special platforms are used to achieve the scientific goals.