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Elusive trigger for cooling 13,000 years ago might have been found

(Source: arstechnica.com)

Sometimes the plucky investigator in a mystery story isn’t baffled by a “whodunnit.” Sometimes they are pretty sure about the who, but the evidence ain’t where it ought to be. Studying past climate events can be like that, with likely explanations waiting in limbo for years until good evidence turns up—or points to another explanation.

About 13,000 years ago, the warming out of the last ice age temporarily reversed course around the North Atlantic. This cold “Younger Dryas” period lasted almost 2,000 years. Like most climate events that primarily affect the North Atlantic region, ocean circulation is the prime suspect.

Jamming the conveyor

Global ocean circulation is a bit like a branching conveyor belt, with currents pushing water one way at the surface and allowing it to return along the bottom. In the Atlantic, surface currents move north until they grow salty and cold, at which point they stop being less dense than the underlying deep water. In several areas around Greenland, surface and deep waters can mix while a deepwater current heads off to the south.

This conveyor belt can be jammed by preventing the mixing around Greenland. If a large volume of fresh water is rapidly dumped into the salty North Atlantic, for example, the surface water layer loses density and can’t mix downward.

For thousands of years before the Younger Dryas, the ice sheet covering much of North America drained its meltwater through the Mississippi and into the Gulf of Mexico. We know that because seafloor sediment cores record the presence of all that freshwater in isotopes of oxygen. But right at the start of the Younger Dryas cooling, that isotope signal ends—as if the firehose of fresh glacial meltwater was suddenly turned off.

The ice sheet hadn’t stopped shrinking—the world was still warming—so where did all that water go? The obvious explanation is that it ended up in the North Atlantic, jamming up the conveyor belt of ocean circulation and plunging the surrounding region into a prolonged cold snap. It’s a great explanation with just one problem—we haven’t found convincing evidence of meltwater between the ice sheet and the North Atlantic.

Where’s the water?

Attention has mostly focused on the Gulf of St. Lawrence in eastern Canada, based on an assumption that the shrinking of the ice sheet had uncovered an easier route for meltwater to flow east rather than south. While there have been hints of evidence to support this, seafloor sediment cores haven’t shown the appearance of that oxygen isotope signal that shut off so abruptly in the Gulf of Mexico.

Now, a team led by Lloyd Keigwin at the Woods Hole Oceanographic Institute think it’s found that signal—2,500 miles away in the Beaufort Sea along northwest Canada. Other research had shown this was a possibility, so the team retrieved sediment cores just off the coast, near the mouth of the Mackenzie River.

The sediment in the core includes a layer of coarse sand and gravel, indicating stronger flow from the Mackenzie. That layer lines up nicely with the start of the Younger Dryas, and the tiny shells of single-celled critters called foraminifera show a sharp change in their oxygen isotope signature. That is what you’d expect from a prolonged increase in glacial meltwater.

This would mean that rather than being dumped out the St. Lawrence and south of Greenland, the meltwater would flow north around the Canadian Archipelago to reach the seas between Greenland and Scandinavia. It’s a more circuitous route, but one that has worked in ocean simulations.

The researchers can’t really guess at the volume of glacial water based on these sediment cores alone. The change in oxygen isotopes lasts almost 700 years, although the peak difference is closer to a century long. Interpretation is complicated by the fact that a persistent flow of draining meltwater could also have been punctuated by the sudden emptying of a massive lake called Glacial Lake Agassiz.

And although it’s tempting to simplify historical events in your mind, it could be that there was more at work than just a glacial firehose coming down the Mackenzie River. The researchers note that many other smaller sources of glacial meltwater could have trickled into the North Atlantic, nudging it toward a threshold that this firehose finally pushed it across.

Nevertheless, the researchers conclude that their evidence shows that a rush of water from the Mackenzie River was “most probably the trigger” for the Younger Dryas cooling. This may not yet be enough to persuade those who have been scouring the St. Lawrence for clues, but it’s a significant development in this long-running game of climate Clue. Maybe the ice sheet did it in the Mackenzie, with the pipe wrench.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0169-6  (About DOIs).

More Info: arstechnica.com

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