A team led by William Patterson of the University of Saskatchewan in Canada and colleagues used a scalpel to slice millimetre-thick pieces off a mud core from Lough Monreagh. Each slice represented around three months.

Focusing on a period around 12,800 years ago, when the Northern Hemisphere was plunged into the Younger Dryas mini ice age, they analysed carbon isotopes to determine how productive the lake was, and oxygen isotopes to look at temperature and rainfall. The ice age, also known as the Big Freeze, set in after a glacial lake in northwestern Canada broke its banks, diluting the salinity-driven currents of the North Atlantic, including the Gulf Stream, which ameliorates the climate and Britain and Ireland, and stalling them.

They found that as the ice age took hold, temperatures dropped rapidly and lake productivity stopped within just a few months – a year at most. Slices from the end of the period showed that it took around 200 years for the climate to return to normal.

February 2010