Icebergs once drifted up to South Africa | Polarjournal
Antarctic icebergs are not only very photogenic, but also impact their immediate environment by releasing the fresh water they are made of. If enough such ice giants drift freely, they could also affect ocean currents. This probably happened on a grand scale during the Pleistocene ice ages, according to AWI researchers. Picture: Michael Wenger

During glacial periods, icebergs from Antarctica drifted much further north than they do today. An international team led by Cardiff University and which included the AWI has now revealed how this was possible, and what consequences it had for the ocean. Their study found that the transport of frozen freshwater had effects in regions as far away as the Northern Hemisphere and the deep Atlantic. The impacts on the evolution of climate at that time are subject of on-going research.

In glacial periods of the Pleistocene during the last 1.5 million years, icebergs from Antarctica drifted significantly further north than they do today – in some cases close to the southern tip of Africa. This allowed larger amounts of frozen freshwater to reach far into the Atlantic. A recent study by Cardiff University, jointly with experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven, has shown that this transport of freshwater into the Atlantic may have altered the stratification of the deep water masses, with major consequences for large-scale ocean circulation and the climate at the time.

With the help of such cores from the seafloor, researchers can determine where the material came from. In the case of the present study, the drilling was carried out off South Africa, but the material investigated came from Antarctica. Picture: Jens Gruetzner / AWI

As the team describes in an article recently published in Nature, these findings are, on the one hand, based on an extensive analysis of core samples from the deep sea in the southern Atlantic, conducted by a team led by Cardiff University; and on the other, on iceberg simulations run at the AWI. “Using a special ship, sediment cores were drilled at various locations – and then investigated layer by layer,” explains Jens Gruetzner from the AWI, who participated in the eight-week expedition. Core samples from the waters surrounding the Antarctic frequently contain material that originated from the Antarctic continent. The huge glaciers grind away at the bedrock as they gradually slide from the mainland. When the glacier tongues finally reach the ocean, they become thinner towards the tip, increase in flow speed and icebergs break off. These icebergs drift away, carrying the enclosed material – referred to as ice-rafted debris – with them. When an iceberg melts during its journey, the ice-rafted debris sinks and is deposited on the seafloor.

The waves and the ocean currents are the main forces that cause the icebergs to break apart. The sun and warmer water then melt the ice and the fresh water released mixes with the seawater. Materials trapped in the ice sink to the sea floor or fertilize the water, resulting in high concentrations of animals. Picture: Michael Wenger

The recent analysis and the dating of the core samples revealed that during glacial periods, the ice-rafted debriswas deposited further north than today. This means that the icebergs have drifted far into the Atlantic. Using a numerical iceberg model, AWI experts Thomas Rackow and Gregor Knorr have investigated how it was physically possible for icebergs to have covered such long distances. “Icebergs don’t usually travel that far because they erode and break apart in rough seas and storms when entering the Antarctic Circumpolar Current, and melt much farther south,” says Rackow. But, as the model shows, during glacial periods the situation was very different: due to colder temperatures, the sea-ice extent around the Antarctic was significantly larger, and extended much farther north. This ice layer shielded the ocean like a protective blanket and smoothed the sea surface. Therefore, the waters in the region were much calmer, and as a result icebergs could drift much farther north.

The team also analysed the impact of the northward mass transport of frozen freshwater on the Atlantic. Apparently, it changed the stratification of the Atlantic water masses. Today, water that originated at the Antarctic coast can be found at the bottom of the Atlantic. Freshwater from melting icebergs is contributing to this ‘Antarctic bottom water’. Above the Antarctic bottom water lies a layer of deep water, which forms in the North Atlantic and then sinks down and flows southward above even denser Antarctic water. The northward drift of the icebergs altered this structure during the last glacial period. The water from the melting icebergs no longer only sank in the southern Antarctic, but instead increasingly became part of the water masses that were transported as far north as the North Atlantic. This altered influx of freshwater, and the weakening effect it had on deep-water formation in the North Atlantic, essentially allowed waters from the south to displace the North Atlantic waters towards shallower depths.

“The evolution of the climate in the past can help us to better understand how the climate might develop in the future.”

Dr. Gregor Knorr, Climate Researcher AWI

“We know that different water mass characteristics of various water layers can have a major effect on how much heat and carbon dioxide the oceans can absorb from the atmosphere,” comments Gregor Knorr. “When the stratification changes, it may have an enormous impact on the evolution of climate.” Researchers are now investigating in detail what consequences the icebergs had e.g. on the transition from glacial to non-glacial periods. According to Gregor Knorr: “The evolution of the climate in the past can help us to better understand how the climate might develop in the future.”

Dr. Thomas Ranckow / Dr. Gregor Knorr / AWI press release

Link to the study: Starr, A., Hall, I.R., Barker, S. et al. Antarctic icebergs reorganize ocean circulation during Pleistocene glacials. Nature 589, 236–241 (2021).

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