The discovery of what caused a widespread cold anomaly is detailed in the paper called “Freshwater Outburst From Lake Superior as a Trigger for the Cold Event 9,300 Years ago.” The paper is posted on the Science Express website, which previews articles that will be published in a future print edition of Science journal.
The scientists propose that a drift dam in the southeastern corner of Lake Superior broke, causing fresh water to surge through the upper Great Lakes and into the ocean via the St. Lawrence Seaway at the end of the last ice age. That rush of fresh water caused the cold event by disrupting the Gulf Stream that pulls warm water north.
The Gulf Stream current brings warm salty water to the North Atlantic, where after cooling, its increased density causes it to sink to the bottom and flow south. The process drives much of the world’s ocean circulation pattern.
But when fresh water covers the top of the salt water, that warm water doesn’t get the chance to cool and the process is essentially stalled. Without warm water coming from the north, the area cooled and stayed that way for a couple hundred years. While the cold event 9,300 years ago has been recognized, a mechanism that triggered it had not been put forward.
Fisher is a lead writer and one of eight researchers involved in the paper published in Science; Shi-Yong Yu, a postdoctoral fellow at Tulane University, led the group.
“As geologists, we study past events in Earth’s history in association with modern processes to develop a historical context, equipping us to make better decisions for the future,” Fisher said.
“While a flood of fresh water such as what happened with Lake Superior at the end of the ice age is a different mechanism than what we see now with global climate change, this information tells us that the North Atlantic Ocean was very sensitive to minor changes in freshwater input,” Fisher explained. “Such information is useful as currently the increase in glacier melting on Greenland and mountain glaciers is not only driving seal level rise, it is also increasing the flux of fresh water to the oceans.”
Fisher contributed to the research by collecting sediment core samples from small lakes around Lake Superior that showed they were once actually a part of a larger lake in the Lake Superior basin, before the dam broke and the lake fell 43 meters in about a year.
The flood also explains the previously unknown cause of the oxygen isotope changes in Lake Huron and Lake Michigan at that time, when water from Lake Superior rushed into the other lakes, Fisher said.