Scientists have discovered more than 700 tracks on the sea floor off the eastern coast of the United States, left over from icebergs that scraped the bottom thousands of years ago.
When you think of Florida, you undoubtedly think of the beaches along the azure sea, the palm trees and the sun shining profusely. It’s hard to imagine that huge icebergs could be enjoyed from the same beaches in the Sunshine State 31,000 years ago. However, this is true, say the researchers in the journal Nature Connections. “The idea that icebergs could reach Florida is amazing,” said study researcher Alan Condron.
Condron and his colleagues mapped the sea floor off the eastern coast of the United States. This results in the discovery of massive plow paths from Cape Hatteras, North Carolina, all the way to the Florida Keys. They were left behind by huge icebergs (more than 300 meters thick) which, while scraping the bottom, moved south along the eastern coast of the United States.
“The discovery of plow tracks on such a low line is very unexpected,” Condon said. “Not only because of the exceptionally high rate of snowmelt in this area, but also because the tracks lie below the northward moving Gulf Stream.” This means that the icebergs have moved against the current. And not just a short distance, but more than 5000 kilometers!
The researchers sampled sediment that filled plow paths left over by icebergs over time. By dating it, they were able to determine when the icebergs were dragged along the bottom. It turns out that this happened about 31,000 years ago, during the so-called Heinrich Event. This is a natural phenomenon where a large number of icebergs are liberated from glaciers and end up in the North Atlantic Ocean.
Using the models, the researchers investigated how the icebergs moved farther south. The model reveals that icebergs can only get this far when huge amounts of meltwater are released into Hudson Bay as a glacial lake penetrates its banks. “Such a tidal wave creates a cool, fast-flowing southerly current that carries icebergs all the way to Florida,” Condron said. And when the researchers simulated such an island wave with their model, the model predicted the formation of plow paths in the places where they were actually found. “What our model suggests is that these icebergs are carried by currents – created by glacial meltwater – and are in fact surfing along the coast. When a large glacial lake breaks through its shores and releases massive amounts of meltwater into the ocean, there is enough water to create such a current. strong along the coast that moves icebergs against the Gulf Stream.” Thus icebergs can reach Florida. In fact, the model suggests that the icebergs could make it further south and hit the Bahamas.
The research is important because it could provide more insights into how meltwater affects ocean currents. It is feared that climate change – and the resulting melting of the ice caps – will weaken these currents (see box).
The Atlantic Meridian Inversion Occurrence (AMOC) is located in the Atlantic Ocean. It is probably best to compare AMOC to a supermarket conveyor belt. At the surface, warm water is brought north from the equator. The water cools down all the way. In addition, more water evaporates along the way than fresh water is added to it, which increases the salt concentration. The further north you go, the colder and saltier the water will be. This also makes the water heavier. The result? It sinks in the north, after which it is pumped – in depth – back to the equator. There it heats up again and the story begins again. However, climate change threatens to disrupt this flow, due to the rapid warming of the Arctic, especially Greenland. Greenland has a massive ice cap that is melting due to global warming and bringing large amounts of (fresh) meltwater into the ocean. This mixes with the salt water, making it less salty and therefore less heavy. This water easily sinks to the depth, and therefore your mother becomes weaker. The weakening of the AMOC could also have severe consequences for surrounding regions, such as Western Europe. The AMOC releases some of the heat it carries north into the atmosphere, causing temperatures in Western Europe to be higher than they would have been in the absence of the AMOC.
What the new research has shown is that meltwater has been able to travel south for thousands of years. While it was eventually carried north again by the Gulf Stream, once it got north it would have been much saltier due to mixing along the way. However, it is likely that the meltwater would have a much smaller impact on the AMOC than if it had already ended up in the current in the far north. According to the researchers, it shows that the effect of meltwater on the flow – and thus also the climate of the surrounding areas – may be more complex than previously thought.
So the traces left by icebergs on the sea floor thousands of years ago have implications for today. “Now that we can create more detailed computer models, we can also more accurately determine how the ocean rotates,” said researcher Gina Hill. This, in turn, determines how fresh meltwater is transported and can affect the AMOC – and the climate.