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Thursday, 7 October 2010

Why Waiho?

Waiho Loop moraine, South Island, New Zealand [From teara.govt.nz] 
A little background ahead of our discussion of New Zealand's climate tomorrow. Many discussions of this subject come down to the Waiho Loop, a spectacular moraine ridge formed by the equally famous Franz Josef glacier on the western side of the Southern Alps - in the picture above its the dark green thing that looks like a moustache. Various dating campaigns have placed the age of the Loop at ~13 ka, and the original C14 ages agree closely with C14 ages for Younger Dryas (YD) moraines in the Northern Hemisphere. These were checked in 2007 due to the high variability in dates from Waiho Loop carbon, and placed slightly earlier, at the end of the Antarctic Cold Reversal (ACR). It has also been argued that the Loop is not a climate signal, and formed independently of cooling, with the glacier advancing due to rock fall onto its surface, although modelling would suggest otherwise.

As several recent and not-so-recent papers have pointed out, there is plenty of evidence for a climate origin of the Waiho Loop, so it's age needs to be incorporated into any interpretation of ACR/YD climate. Problems with the C14 age, from both contamination with younger carbon making ages too old to degradation of the dated material making ages too young have been debated. The C14 age also does not fit with marine core and pollen data, although the reliability of these has been questioned. The Waiho Loop is classically attributed to the YD, and so cited as evidence of synchronous inter-hemispheric climate change, but more recent studies suggest it is older, indicating an ACR advance in New Zealand, and so a wide-ranging Southern Hemisphere cooling event. 

Putnam et al (2010)'s recent study of the Pukaki glaciers confirms that eastern South Island glaciers advanced during the ACR then rapidly retreated when the YD started. This is corroborated by Kaplan et al. (2010)'s study at Irishman's Stream, and they attribute the Waiho Loop to the ACR rather than the YD. They imply asynchronous global climate change, potentially due to migration of major wind systems at this time. So is the date of the Waiho Loop correct, and if so, how should it be interpreted in context of other glacial records from New Zealand and climate teleconnections?