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

Abstract

<p><b>One of the outstanding problems in modern geoscience is identifying the cause of past climate changes, particularly the drivers of rapid climate change during Quaternary glacial cycles. Changes in the physical geography of Earth’s surface during the Late Quaternary are mainly dependent on glacial dynamics – periods of rapid warming produced significant amounts of meltwater that reshaped the landscape, changed global sea-level and influenced climate. Identifying the timing of key climate transitions during past warming episodes, such as the last glacial termination, may help to understand the future evolution of Earth’s climate system (e.g. Denton et al., 2021).</b></p> <p>In this thesis, using geomorphological mapping and sixty-six cosmogenic 10Be surface exposure ages obtained from ice sculpted bedrock surfaces and deposited moraine landforms, I constrain the local Last Glacial Maximum and subsequent timing of last glacial termination in the Ahuriri River valley, Southern Alps, New Zealand (44°15′S, 169°36′E). Using the maximum elevation of lateral moraine (MELM) and accumulation area ratio (AAR) methods, along with application of a temperature lapse rate, I estimate the equilibrium-line altitude (ELA) and associated temperatures from the same periods. The largest glacial event in the Ahuriri River valley occurred at 19.8±0.3 ka when the former Ahuriri Glacier reached its maximum extent, which coincides with the global Last Glacial Maximum. By 16.7±0.3 ka, ice had retreated ~18 km up-valley from the LGM position and deglaciation was accompanied by the formation of a shallow proglacial lake. Surface exposure ages from moraines situated in a tributary of the upper Ahuriri River valley indicate that a subsequent advance of the palaeo glacier culminated at 14.5±0.3 ka, while the next readvance or still stand occurred at 13.6±0.3 ka. About 1000 yr later (12.6±0.2 ka), the former glacier built another prominent terminal moraine ridge in the lower section of the upper right tributary valley. </p> <p>Reconstructions of past glacier geometries indicate that the local ELA was depressed by ~880 m and climate was 5±1 °C colder than present (1981–2010) at 19.8±0.3 ka, while ELA was depressed by ~770 m and climate was 4.4±0.9 °C colder at 16.7±0.3 ka. Subsequent estimations suggest ELA elevations at 14.5±0.3 ka, 13.6±0.3 ka, and 12.6±0.2 ka were ≤700 m, ≤630 m, and ~360 m lower than today. This equates to air temperatures of ≤3.9 °C, ≤3.5 °C, and 2.3±0.7 °C colder than today, assuming no changes in past precipitation.</p> <p>The results reported here provide the first dataset of Late Quaternary glacial maximum extent and deglaciation along with quantitative paleoclimate reconstructions from the Ahuriri River valley, Southern Alps, New Zealand. The small amount of warming estimated in this study between 19.8±0.3 and 16.7±0.3 ka differs somewhat from glacial reconstructions in other major valleys in the Southern Alps, specifically from Rakaia River valley (e.g. Putnam et al., 2013a) where a much larger amount of warming may have occurred during the same time. Robust constraints of glacier changes in the Ahuriri valley between 14.5±0.3 and 12.6±0.2 ka confirm that an early glacier readvance occurred in New Zealand at this time, which has been previously recognised with only limited evidence (e.g. Kaplan et al., 2010; Putnam et al., 2010a). The reconstructed ELA suggests that the coldest part of the Late Glacial reversal occurred at 14.5±0.3 ka. </p> <p>The new constraints from glacial records in the Ahuriri River valley presented in this study offer the opportunity to test hypotheses about the climate system, to better understand the processes that drove ice retreat and readvance during the Last Glacial Maximum and subsequent termination.</p>