Can the future of the Gulf Stream system be read from sediments?

The Central and North European climate is strongly influenced by a global system of ocean currents. The Gulf Stream carries large amounts of heat from the tropics to the Arctic, thereby providing relatively mild winter weather for Germany. This system is driven by the cooling of saline waters in the high latitudes where it sinks to the bottom of the ocean - a process called "deepwater formation" - and flows back towards the equator. How stable will this heat transport be when the polar areas are strongly warming? What will be the influence of increased freshwater input from glacier melting and increasing precipitation? An international group of researchers investigated previous changes in the current system to better estimate its future sensitivity.

“The extent to which ocean circulation can be impacted by the changing climate is of key importance to society, particularly with current global warming” said Christina Larkin, lead-author of the study published in the scientific journal Nature Geoscience. She was a PhD student at the Department of Earth Sciences of Cambridge University when she conducted the research and works at the University of Southampton now.

To improve previous evaluations of changes in the critical transition between the last ice age and today's warm period, the researchers determined the ratio between certain Neodymium isotopes in hundreds of samples from sediment cores obtained in the Arctic Ocean and the adjacent Nordic Seas. Several of the cores were provided from repositories at GEOMAR Helmholtz Centre for Ocean Research Kiel, the Kiel University and the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven. With the results from these samples, the propagation of deepwater in the Arctic could be traced with unprecedented precision.

The results show that the water mass exchange between the two investigated ocean basins was functioning in a mode that was similar to the modern one during the last glacial maximum, about 23,000 to 18,000 years before present. "Our data show that, despite much lower sea surface temperatures, the Gulf Stream system in the Nordic Seas was fully operating during the peak of the last glacial, however, with some differences on vertical and lateral extents", says Dr. Henning Bauch. He is a marine geologist at AWI and GEOMAR and a co-initiator and co-author of the current study.

During the transition from glacial to interglacial, conditions drastically changed, as demonstrated by highly variable neodymium isotope values in the analyzed samples. "Very likely the freshwater input from melting ice sheets, for example on Scandinavia and Svalbard, played an important role and led to a decrease in deepwater formation, the major driver of the Gulf Stream system", explains Dr. Robert Spielhagen, paleoceanographer at GEOMAR and another co-author of the study. "A similar response must be expected when the ice melt on Greenland proceeds under global warming. This may cause a decrease in oceanic heat transport towards Europe."

The study underscores the crucial role of the northern regions for the global climate. Moreover, because the Polar North react so sensitively to man-made global warming, it constitutes a key region for climate research. A look back into the past not only helps to estimate future changes it also allows to verify climate model results.

Publication:

Larkin, C.S., Ezat, M.M., Roberts, N.L., Bauch, H.A., Spielhagen, R.F., Noormets, R., Polyak, L., Moreton, S., Rasmussen, T.L., Sarnthein, M., Tipper, E.T., Piotrowski, A.M., 2022. Active Nordic Seas deep-water formation during the last glacial maximum. Nature Geoscience, https://doi.org/10.1038/s41561-022-01050-w

Press release by Cambridge University: "Scientists map deep waters in the Nordic Seas, showing ocean circulation during and after last ice age"