Cosmogenic 10Be constraints on deglacial snowline rise in the Southern Alps, New Zealand

Geochronological dating of glacial landforms, such as terminal and lateral moraines, is useful for determining the extent and timing of past glaciation and for reconstructing the magnitude and rate of past climate changes. In the Southern Alps of New Zealand, well-dated glacial geomorphological records constrain the last glacial cycle across much of the Waitaki River basin (e.g. Ohau, Pukaki, Tekapo) but its southern sector such as the Ahuriri River valley remains comparatively unconstrained. Recently, there has been debate on the scale and rapidity of mountain glacier retreat during the last glacial termination, particularly the 20-17 ka period in New Zealand. Missing from this debate is well-constrained equilibrium-line altitude (ELA) and associated temperature reconstructions, particularly over the period around 17 ka, which can help us to develop a more complete picture of how past temperature changes drove glacier retreat. Here we report the first glacial chronology dataset from the Last Glacial Maximum (LGM) and subsequent deglaciation from the Ahuriri River valley, Southern Alps, New Zealand (44 degrees 23'54 '' S, 169 degrees 39'48 '' E) based on 38 beryllium-10 (Be-10) surface-exposure ages from terminal moraine systems and glaciated bedrock situated at the lower and middle sections of the valley. Our results show that the former Ahuriri Glacier reached its maximum extent at 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 suggesting at least similar to 43% glacier-length loss relative to its full LGM extent. This deglaciation was accompanied by the formation of a shallow proglacial lake. Using the accumulation area ratio (AAR) method, we estimate that the ELA was lower than present by similar to 880 m (similar to 1120 m a.s.l.) at 19.8 +/- 0.3 ka, and similar to 770 m lower (similar to 1230 m a.s.l.) at 16.7 +/- 0.3 ka. Applying an estimate for temperature lapse rate, this ELA anomaly implies that local air temperature was 5 +/- 1 degrees C colder than present (1981-2010) at 19.8 +/- 0.3 ka, while it was 4.4 +/- 0.9 degrees C colder at 16.7 +/- 0.3 ka, assuming no change in precipitation. The substantial glacier retreat in response to a relatively small accompanying increases in ELA (110 m) and temperature (0.6 degrees C) may have been a result of the high glacier-length sensitivity of this glacier system due to its low gradient of former ice surface. Our low warming estimate differs markedly from other deglaciation studies, specifically from Rakaia River valley, which reports a much larger temperature increase at the onset of the last deglaciation. This precisely-dated moraine record along with reconstructed ELA as proxies for atmospheric conditions, provides new insight into post LGM glacier behaviour and climate conditions in New Zealand. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study presents the first glacial chronology dataset from the Ahuriri River valley in the Southern Alps, New Zealand, based on beryllium-10 surface-exposure ages. The results reveal that the former Ahuriri Glacier reached its maximum extent at 19.8 ka and retreated 18 km up-valley by 16.7 ka. The estimated equilibrium-line altitude suggests colder temperatures during these periods compared to the present, with a temperature difference of 5°C and 4.4°C, respectively. This research provides new insights into the glacier behavior and climate conditions in New Zealand during the Last Glacial Maximum.