London, Oct 5: Researchers have for the first time found that changes in the salinity level of the North Atlantic Ocean had a direct bearing on the temperature over Greenland and tropical patterns during the last Ice Age.

Using chemical traces in fossil shells of microscopic planktonic life forms, called formanifera, in deep-sea sediment cores, scientists reconstructed a 45000-60000 year old record of ocean temperature and salinity.

Comparing the results to the record of abrupt climate change recorded in ice cores from Greenland, they found that the Atlantic got saltier during cold periods, and fresher during warm intervals.

“The freshening likely reflects shifts in rainfall patterns, mostly in the tropics. Suddenly, we're looking at a record that links moisture balance in the tropics to climate change. And the most striking thing is that a measurable transition is happening over decades,” Nature quoted Howard Spero of the University of California at Davis as saying.

He said though a sheet of ice covered much of North America and Europe during the Ice Age, the ice records reconstructed showed repeated patterns of sudden warming called Dansgaard-Oeschger Cycles, when temperatures in Greenland rose by 5 to 10 degrees Celsius over a few decades.

“Close to the tropics, warm, moist air forms a zone of heavy tropical rainfall, called the Intertropical Convergence Zone, which dilutes the salty ocean with fresh water. Today, the tropical rainfall zone reaches into the northern Caribbean, but during the colder periods of the Ice Age it was pushed much further south, towards Brazil. That kept fresh water out of the northern Atlantic, so it became more salty,” Spero said.

He said the findings also revealed that the deep ocean circulation was very sensitive to the saltiness of north Atlantic surface waters.

“Warming climate, higher rainfall and fresher conditions can alter the circulation. During glacial times, reduced circulation caused climate to cool,” he added.

“The new results show that as the climate cooled in Greenland, salinity rapidly increased in the North Atlantic subtropical gyre. The build-up of salt during these cold intervals when the conveyor circulation was reduced would have primed the system to quickly restart on transitions into warm intervals. However, the actual trigger that caused Atlantic circulation to restart during the Ice Age is still unknown,” said lead author Matthew Schmidt of the Georgia Institute of Technology.

“Once warming began, melting ice sheets would have contributed fresh water to the Atlantic, but this would have been partly buffered by the elevated saltiness of the Atlantic,” he added. (ANI)