Scorelines on the seafloor along North America's east coast testify to past upsurges of immense icebergs that scraped along the bottom. Seafloor mapping has revealed these lines off southern Florida. Astonished geologists have reported their findings in Nature Communications with dates for when a few of these lines were made and a model of how they could be possible.
The idea of sailing icebergs from the poles to water the deserts were once popular but turned out to be impractically expensive. Not only is there the problem that the ice melts fast once it hits warmer waters, but in the North Atlantic, any such project would be pushing against powerful ocean currents, requiring enormous fuel use.
If it would be a challenge for boats tugging icebergs to fight the Gulf Stream all the way to Florida, it's hard to imagine them doing so unpowered. Dr Alan Condron of the Woods Hole Oceanographic Institution and Dr Jenna Hill of the United States Geological Survey is fully aware of the obstacles to such a journey would make Odysseus's adventure the Odyssey look like crossing a quiet pond, but can't ignore what they see.
The evidence is provided by scrapings on the seafloor known as scours, left by icebergs more than 300 meters (1000 feet) thick. These are common in coastal waters in areas where icebergs are common today, but seafloor mapping has revealed more than 700 of them south of North Carolina.
"The appearance of scours at such low latitudes is highly unexpected not only because of the exceptionally high melt rates in this region, but also because the scours lie beneath the northward flowing Gulf Stream." Condron said in a statement.
Iceberg outbursts are known as Heinrich events, and glaciologists are sufficiently familiar with them and so have given them numbers. Condron and Hill collected sediment cores from a number of the south Florida lines and found they were made 31,000 years ago, from a Heinrich Event 3.
“We also expect there are younger and older scours features that stem from other discharge events, given there are hundreds of scours yet to be sampled,” said Hill.
Since icebergs reaching so far south is clearly impossible under modern conditions, Condron explored what could have allowed it. A model of the North Atlantic produced only one scenario, where the icebergs surfed their way south on immense floods of meltwater coming from Hudson Bay.
For the process to occur glaciers in the region must have been melting fast, but some blockage caused cold water to build up behind it. When the dam broke an immense flood was released into the ocean. "These floods create a cold, fast flowing, southward coastal current that carries the icebergs all the way to Florida," said Condron. "The model also produces 'scouring' on the seafloor in the same places as the actual scours."
Eventually, the meltwater would have mixed with the warmer, saltier ocean around it and been carried north again. In the process, it would have disrupted the transfer of heat that keeps Northern Europe habitable, but differently from if it had gone directly into the North Atlantic, a much more studied scenario. There is a possibility such a disruption could happen again, based on more continuous melting of polar ice due to global heating and that remains alarming.
The more we know about Heinrich events, the better we can make our models to assess the current dangers.