Mountain Glacier Chronology from Boulder Lake, New Zealand, Indicates MIS 4 and MIS 2 Ice Advances of Similar Extent

Abstract

Dating of past glaciation in New Zealand allows Quaternary climatic events to be identified in areas at a great distance from northern hemisphere ice sheets and associated climatic feedbacks. Moreover, climate reconstruction in New Zealand provides insight into the amount of climate change that occurred in the Southwest Pacific where zonal circulation is an important integrator of the climate signal. Boulder Lake is a relatively low-elevation cirque in a range of moderate-relief (∼1600 m) mountains in South Island of New Zealand, and it experienced cirque and valley glaciation during the Late Quaternary. Geomorphic mapping, 10Be and 26Al exposure, and luminescence dating provide evidence for glacial advances during the Last Glacial Cycle, specifically during Marine Isotope Stage 4 (MIS 4) and Marine Isotope Stage 2 (MIS 2). The MIS 4 advance was fractionally larger and is dated by a former ice-marginal lacustrine deposit (minimum age) with a basal Optically Stimulated Luminescence (OSL) sediment deposition age of 64.9 ± 10 ka. Paired 10Be and 26Al constrain a slightly less extensive MIS 2 glacial advance to 18.2 ± 1.0 and 17.8 ± 0.9 ka, coincident with the Last Glacial Maximum (LGM). Glacial equilibrium-line altitudes during both MIS 4 and MIS 2 phases were ∼960 m lower than the present. This corresponds to a cooling of 5–7°C, taking possible precipitation variability into account. Our findings and a growing number of publications indicate that many temperate valley glaciers reacted differently to the major ice sheets during the Last Glacial Cycle, reaching their maximum extent during MIS 4 rather than during peak global ice volume during MIS 2.