Final Cold Snap of the Last Ice Age – Putnam et al – Nature Geoscience

An international science journal this week published a paper, authored by a UMaine Ph.D. candidate, which claims to resolve a long-standing debate about the end of the last ice age in New Zealand.

Aaron Putnam, a glacial geologist in UMaine’s Department of Earth Sciences and Climate Change Institute, was the primary author of the paper that appeared in “Nature Geoscience.” The paper cited improved methods for dating mounds of dirt and rocks left behind by glaciers in order to confirm the New Zealand glaciers responded to climatic events in Antarctica, rather than events that originated at the Arctic.

“Applying a precise and accurate dating method to glacier landforms has confirmed that the effects of Antarctic Cold Reversal extended to New Zealand,” Putnam said in a press release issued by the Institute of Geological and Nuclear Sciences, which is based in New Zealand. “It brings us closer to understanding the intricacies of the global climate system.”

Putnam is a Chapman native who has a master’s degree from UMaine.

George Denton, a glacial geologist and Climate Change Institute faculty member, was also an author on the paper. The study involved a total of 10 scientists.

According to the paper, scientists have debated whether conditions at the Antarctic or Arctic had more important influence worldwide. Previous studies of core samples from Arctic and Antarctica ice sheets revealed warming in the north matched cooling in the south, and vice versa, at the end of the last ice age.

In unison with a cold snap across Antarctica, known as the Antarctic Cold Reversal, large glaciers in the Southern Alps area of New Zealand grew and pushed down-valley, before suddenly pulling back about 13,000 years ago.

The Southern Alps glacier movements matched 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. The cold snap in Europe is known as the Younger Dryas.

The scientists involved in the study looked at glacier moraines, which are mounds and ridges of rocks and dirt left behind at the downhill end of a flowing glacier after its retreat. When the glacier retreats, cosmic rays bombard the moraines, producing concentrations of distinctive isotopes in the glacial rocks.

By measuring the amount of one specific isotope, beryllium-10, found in the moraine, researchers worked out when a glacier retreated. That technique enabled the scientists to measure when the Southern Alps glacier advance came to an end, which marked the final event of the last ice age in New Zealand.

Studies of the moraines showed that a large glacier in the Tasman River Valley in the Southern Alps began shrinking about 13,000 ago, and within a few centuries the ice level had dropped by at least 150 meters, signifying a major retreat of the glacier.

This is the first time anyone has determined the age of the moraines, according to the paper.

The scientists suggest that variations in the positions of the global wind belts and oceanic water masses provide an explanation for the climate changes that were happening at the time. A sustained northward migration of the roaring forties westerly wind belt 05 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.

Hodgson, D. & Sime, L.  Southern Westerlies & CO2.  Nature Geoscience, Perspective, 10 . 2010.