Uniform summer cooling drove glacier re-advance across New Zealand during the late-glacial climate reversal

Abstract:

Rapid, millennial-scale climate events characterised the last global glacial-interglacial transition (18-11 ka). In New Zealand, the timing and magnitude of climatic events during this period are poorly understood. Improving our understanding of these events will help to identify the mechanisms via which rapid shifts in climate occur. In this study, we report results from geomorphological mapping, cosmogenic ³He exposure dating and numerical glacier modelling, which show evidence for re-advance of mountain glaciers on Mt Ruapehu in central North Island, New Zealand (39°S) during the late glacial chron (15-11 ka). Using a distributed energy balance model, coupled with a 2D ice flow model, we perform a range of experiments and sensitivity analyses to constrain estimates of past temperature associated with the mapped and dated former ice limits. We find that glaciers in North Island re-advanced early in the late glacial period in response to a likely temperature cooling of 2.5 - 3.4 °C relative to present day, assuming precipitation remained within ± 20% of present. This reconstructed cooling is greater than recorded in nearby pollen archives, which may reflect a seasonal bias between climate proxies. Using our glacier model, we quantify the length sensitivity of glaciers on Mt. Ruapehu to seasonal climate changes. We find that a 3 °C cooling relative to present causes a c. 80% increase in glacier length when applied to the austral summer months (Dec-Feb), compared to c. 20% in winter (June-August). Thus, glaciers in North Island, New Zealand are most sensitive to temperature changes during summer. Strong agreement between our late-glacial reconstructions and other summer temperature proxy records (e.g. mountain glaciers, chironomids) from the Southern Alps, suggest New Zealand experienced uniform summertime cooling during the late-glacial climate reversal.