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

Journal Article · Fri May 13 00:00:00 EDT 2022 · Quaternary Science Reviews

DOI:https://doi.org/10.1016/j.quascirev.2022.107548· OSTI ID:1868162

Tielidze, Levan G.  ^[1]; Eaves, Shaun R. ^[1]; Norton, Kevin P. ^[1]; Mackintosh, Andrew N. ^[2]; Hidy, Alan J. ^[3]

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1. Victoria Univ. of Wellington (New Zealand)
2. Monash Univ., Melbourne, VIC (Australia)
3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

We report 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. Ōhau, 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°23'54"S, 169°39'48"E) based on 38 beryllium-10 (¹⁰Be) 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 ~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 ~880 m (~1120 m a.s.l.) at 19.8 ± 0.3 ka, and ~770 m lower (~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 °C colder than present (1981–2010) at 19.8 ± 0.3 ka, while it was 4.4 ± 0.9 °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 °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.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); Victoria University of Wellington; Royal Society of New Zealand
• Grant/Contract Number: AC52-07NA27344; AC52- 07NA27344; LLNL-JRNL-820919
• OSTI ID: 1868162
• Alternate ID(s): OSTI ID: 1868120
• Report Number(s): LLNL-JRNL-820919; 1032635
• Journal Information: Quaternary Science Reviews, Vol. 286, Issue na; ISSN 0277-3791
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
last glacial maximum
last deglaciation
equilibrium line altitude
past climate
Southern 15 Alps
New Zealand