The cold event started with a catastrophe of epic proportions when an ice dam, left over from the remnants of the melting Ice Sheet over Canada, gave way, releasing a huge volume of cold freshwater into the Labrador Sea in the North Atlantic. The glacial Lake Agassiz finished draining in less than a year but the consequences of the megaflood lasted much longer.
‘We believe that the water from Lake Agassiz likely formed a freshwater cap over the ocean, which slowed down ocean currents that bring warm water northwards in the North Atlantic,’ says lead author Dr Tim Daley, of Swansea University. The consequence was the largest climatic event in the Northern Hemisphere in the last 10,000 years, with a sharp drop in atmospheric temperatures.
While the event had been captured in records from Europe and Greenland, it was yet to be recognised on the western side of the Atlantic. Daley and colleagues looked for clues in Nordans Pond Bog, on the island of Newfoundland. They looked at the oxygen isotopes in Sphagnum mosses that make up most of the peat.
Oxygen atoms are not all the same. Some are heavier than others and the exact proportion between types – called isotopes – depends on the environment. The cellulose found in the leaves of Sphagnum mosses uses oxygen from rain. Because the Sphagnum has a very simple biology, it offers a ‘robust record of oxygen isotopic changes in precipitation that can be directly compared with global climate models,’ says Daley.
The team found that about 8400 years ago the proportion between oxygen isotopes in the rain, as recorded in the Sphagnum leaves, dropped 4.5 parts for every thousand in less than 100 years. ‘This is a significant negative anomaly, much larger than we expected,’ says Daley.
Daley explains what this sharp ratio decrease means: ‘negative oxygen isotope anomalies suggest cooling of atmospheric temperatures. The air temperature during the Canadian summer became similar to that of modern autumn or early spring.’
‘This is the first measurable link between the cold event registered regionally and cooling in north-eastern North America,’ Daley says, adding that current models do not yet correctly simulate what happened – ‘our data from Newfoundland show that the cold event was more severe than we thought.’
The team also argues, in the report published in the September edition of the journal Geology, that the north-east coast of America is highly sensitive to climatic changes triggered by the effects of freshwater discharges on North Atlantic ocean currents.
While the catastrophic drainage of Lake Agassiz was a sudden, one-off event, the ongoing melting of Greenland’s ice sheet is ‘very likely changing the freshwater balance of the North Atlantic,’ says Daley.
‘The situation is very different, but Greenland’s melting has accelerated and we just don’t know what kind of consequences it might bring,’ warns Daley.