Wandering Icebergs

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. 

Core-sample analysis sheds new light on icebergs 

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.

Freshwater from the south advanced to the 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.

“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.”