The age of ice

The Perito Moreno Glacier, fed by the Southern Patagonian Ice Field in Argentina.  Photo credit:  Bill Onorato (Journal of the Cambridge University Mountaineering Club)

The Patagonian Ice Sheet covered all of southern Chile during the last glacial period.  Now the Southern Patagonian Ice Field, one of its remnant parts, is one of the largest non-polar ice fields in the world and provides fun and games for mountaineers and scientists alike.

Geologists have for the first time been able to directly determine the age of early Pleistocene glaciers in the Lake Pueyrredon valley in Patagonia. The extensive sequence of moraines—glacially formed accumulations of debris—in Argentine Patagonia provide a geologic record of the Patagonian Ice Sheet.   The age of sediment on outwash terraces, which are deposits that extend along a valley downstream from a moraine, shows the age of glacial advances, according to a study published in Geology this month.

Researchers at the University of Edinburgh and the Scottish Universities Environmental Research Centre measured surface exposure ages of elements (¹⁰Be and ²⁶Al) in the outwash sediments.  A cobble from the outermost glacial sequence, which marks the greatest extent of the ice sheet, dates to 1.2 million years old.  A cobble from a younger outwash terrace dates to 600 thousand years old.  This is consistent with age constraints obtained elsewhere in Patagonia via argon dating, and indicates that major glacial advances occurred at 1.2 Ma and 600 ka. Combined with existing data, there is evidence for five major advances of the Patagonian ice sheet since the early Pleistocene.

Boulders on a moraine related to the outwash terrace that yielded the 1.2 Ma cobble are significantly younger, suggesting that boulders underestimate deposition age.

How accurate is this sediment outwash dating?  Outwash terraces indicate long-term stability and original surface morphology because they become isolated after deposition when a reverse in drainage direction occurs.  Outwash sediment ages are tightly clustered, suggesting that age determination is fairly accurate.  Accuracy could be improved by increasing sampling density and pairing depth profiles within the terrace sediment.  

If the stability of outwash terraces is common throughout the Lake Pueyrredon valley, this new information would provide insight into how regional climate signal has developed alongside global trends and has implications for understanding terrestrial climate change.

Ref:  Andrew S. Hein, Tibor J. Dunai, Nicholas R.J. Hulton and Sheng Xu. Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations,  Geology, published online 5 January 2011.