NZ study pinpoints the final cold snap of the last ice age - 27/09/2010
A geological study of glaciers in the South Island has finally resolved a long-standing debate about the demise of the last ice age in New Zealand.
In unison with a cold snap across Antarctica, known as the Antarctic Cold Reversal, large glaciers in the Southern Alps grew and pushed down-valley, before suddenly pulling back about 13,000 years ago.
Previous studies of core samples from Arctic and Antarctica ice sheets revealed a climate see-saw at the end of the last ice age, with warming in the north matching cooling in the south, and vice versa.
But scientific debate has raged over whether the Arctic or the Antarctic conditions were the more important influence worldwide.
The Southern Alps glacier movements matched the Antarctic temperature patterns. This shows that Antarctica and New Zealand began warming up 13,000 years ago at the same time as Europe sank back into a 1,300-year-long ice-age freeze.
The cold snap in Europe is known as the ‘Younger Dryas’, after a cold-tolerant species of rose that became widespread at that time.
The study involved 10 scientists led by Aaron Putnam from the University of Maine in the United States. The group, including two New Zealand-based scientists, studied glacier moraines in the Lake Pukaki and Lake Tekapo catchments on the eastern side of the Southern Alps.
The findings are published this week in the international science journal Nature Geoscience. By using improved methods for dating glacier moraines, the Southern Alps study has confirmed that New Zealand glaciers responded to the Antarctic Cold Reversal, rather than to the northern Younger Dryas cold snap.
Flowing glaciers carry rocks and dirt which build up mounds and ridges, called moraines, at the downhill end of the glacier. When glaciers retreat, cosmic rays bombard these moraines, producing concentrations of distinctive isotopes in the glacial rocks.
Scientists can work out when a glacier retreated by measuring the amount of the cosmogenic isotope beryllium-10 in moraines.
The beryllium-10 dating method enabled the scientists to measure when the Southern Alps glacier advance came to an end, marking the final event of the last ice age in New Zealand. .
The scientists showed that a large glacier in the Tasman River valley near Lake Pukaki began shrinking about 13,000 ago, and within a few centuries the ice level had dropped by at least 150m, signifying a major retreat of the glacier.
David Barrell of GNS Science, one of the authors of the paper, said “The Birch Hill moraines of the Tasman valley near Mt Cook are one of the classic glacier moraine sets of New Zealand. Until now, no-one knew exactly how old they are.
“This study has shown that they were produced by glacier expansion that culminated at the end of the Antarctic Cold Reversal, and that the glacier then shrank rapidly.”
“A strength of this study is that we replicated these findings using similar moraines in the Macaulay valley, near Lake Tekapo. These new results also coincide closely with an advance of Franz Josef Glacier identified during previous studies at Canavans Knob on the western side of the Southern Alps.
“There had been a suspicion that the Franz Josef Glacier advance about 13,000 years ago was an isolated event. Indeed, a 2008 study hypothesised that the Franz Josef Glacier advance was caused by a landslide onto the glacier.
“The new study, and a similar one published in the journal Nature in early September, highlights a widespread climatic event in the Southern Alps that culminated about 13,000 years ago. As a result, the landslide hypothesis proposed for Franz Josef Glacier should be re-examined.”
The scientists suggest that variations in the positions of the global wind belts and oceanic water masses provide an explanation for these climate changes. A sustained northward migration of the ‘roaring forties’ westerly wind belt may have bathed the Southern Alps in cooler air from the Southern Ocean, causing glacier advance. A southward migration of the westerlies about 13,000 years ago could have led to the ice recession.
According to study lead author, Aaron Putnam, a glacial geologist at the University of Maine, “Applying a precise and accurate dating method to glacier landforms has confirmed that the effects of Antarctic Cold Reversal extended to New Zealand. It brings us closer to understanding the intricacies of the global climate system.”