Late Quaternary glacier-based climate reconstruction from the Southern Alps, New Zealand

Geochronological dating of glacial landforms, such as terminal and lateral moraines, are useful for determining the extent and timing of past glaciation and for reconstructing the magnitude and rate of past climate changes. Here we report the first dataset of Late Quaternary glacial maximum extent and its deglaciation from the Ahuriri River valley, Southern Alps, New Zealand (44°23'54''S, 169°39'48''E) based on 66 beryllium-10 (¹⁰Be) surface-exposure ages from terminal-lateral moraine systems and glaciated bedrock surfaces situated at different sites of the valley. Our results show that the former Ahuriri Glacier reached its maximum extent 19.8±0.3 ka, which coincides with the global Last Glacial Maximum. By 16.7±0.3 ka, the glacier had retreat ~18 km up-valley and this deglaciation was accompanied by the formation of a shallow proglacial lake. Our surface-exposure chronology from the moraines situated upper right tributary of the Ahuriri River valley also indicates that other subsequent advance of the palaeo glacier culminated at 14.5±0.3 ka ago, while the next re-advance or still stand phases occurred at 13.6±0.3 ka. About 1000 yr later (12.6±0.2 ka), the former glacier built another prominent terminal-lateral moraine ridge in the lower section of the upper right tributary valley. In overall, our result supports the hypothesis that climate was ~5°C colder (ELA depression ~880 m) than present at 19.8±0.3 ka, while it was ~4.4°C colder (ELA depression ~770 m) at 16.7±0.3 ka. Furthermore, local air temperature was lower by 3.6°C (ELA depression ~630 m) during the 14.5-13.6 ka and by 2.0°C (ELA depression ~360 m) at 12.6 ka respectively relative to present. Our results clearly demonstrate the structure of last glacial termination in New Zealand such as strong glacier recession during this time-period in accordance of at least five glacier re advances or still stand phases. This new ¹⁰Be surface exposure dataset will help us in better understanding of past glacier-climate interactions in the Southern Alps and in the Southern Hemisphere in general.