Redating the advance of the New Zealand Franz Josef Glacier during the Last Termination: evidence for asynchronous climate change

Dating climatic and environmental records with accuracy and precision is critical if we are to robustly test hypotheses of synchronous change between the hemispheres. In the southern hemisphere, the advance of the New Zealand Franz Josef Glacier to the Waiho Loop terminal moraine has been used as evidence of synchronous global change during the Younger Dryas Chronozone. However, radiocarbon ages for wood incorporated into the sediments associated with the advance span an interval of ∼900 years. Here we demonstrate that weathered 'old' wood was incorporated into the deposit prior to, or during, the advance, and that this material is highly susceptible to contamination by young carbon, resulting in the wide range of age estimates reported previously. We have identified material with little evidence of reworking (well-preserved wood with bark attached) that provides a statistically robust mean radiocarbon age of 11,062±30 BP (∼13.1 ka) for the advance of the Franz Josef Glacier. The timing of this event falls within the later part of the Antarctic Cold Reversal. Comparison with local, regional and global records suggests that cold conditions in the Southern Ocean were probably experienced in New Zealand at this time, driving the glacial advance.