The Last Glacial Maximum in the central North Island, New Zealand: palaeoclimate inferences from glacier modelling

Andrew N Mackintosh,Graham S Leonard,Andrew T Calvert,Shaun R Eaves,Joerg M Schaefer,Brian M Anderson,Dougal B Townsend,Alice M Doughty,Chris E Conway,Gisela Winckler

doi:10.5194/cp-12-943-2016

Abstract

Abstract. Quantitative palaeoclimate reconstructions provide data for evaluating the mechanisms of past, natural climate variability. Geometries of former mountain glaciers, constrained by moraine mapping, afford the opportunity to reconstruct palaeoclimate, due to the close relationship between ice extent and local climate. In this study, we present results from a series of experiments using a 2-D coupled energy balance–ice flow model that investigate the palaeoclimate significance of Last Glacial Maximum moraines within nine catchments in the central North Island, New Zealand. We find that the former ice limits can be simulated when present-day temperatures are reduced by between 4 and 7 °C, if precipitation remains unchanged from present. The spread in the results between the nine catchments is likely to represent the combination of chronological and model uncertainties. The majority of catchments targeted require temperature decreases of 5.1 to 6.3 °C to simulate the former glaciers, which represents our best estimate of the temperature anomaly in the central North Island, New Zealand, during the Last Glacial Maximum. A decrease in precipitation of up to 25 % from present, as suggested by proxy evidence and climate models, increases the magnitude of the required temperature changes by up to 0.8 °C. Glacier model experiments using reconstructed topographies that exclude the volume of post-glacial ( < 15 ka) volcanism generally increased the magnitude of cooling required to simulate the former ice limits by up to 0.5 °C. Our palaeotemperature estimates expand the spatial coverage of proxy-based quantitative palaeoclimate reconstructions in New Zealand. Our results are also consistent with independent, proximal temperature reconstructions from fossil groundwater and pollen assemblages, as well as similar glacier modelling reconstructions from the central Southern Alps, which suggest air temperatures were ca. 6 °C lower than present across New Zealand during the Last Glacial Maximum.

Highlights

The Last Glacial Maximum (LGM) describes the global sea level low stand at 26–19 ka, when global ice sheets attained their maximum volume of the last glacial cycle (Clark et al, 2009)
The main findings are as follows: (1) the temperature depression required to simulate the LGM glacial geometries of individual catchments varies between −4.0 and −6.8 ◦C, (2) there is a systematic offset of ca. 1 ◦C in the model-derived palaeotemperatures associated with LGM moraines between the two volcanoes, and (3) using geologically constrained reconstructions of LGM topography has relatively little impact (+0.2 to −0.5 ◦C) on the palaeotemperature reconstruction
Simulations of nine glaciers in the central North Island, New Zealand, using a 2-D, coupled energy balance–ice flow model, suggest that local air temperatures were depressed by 4–7 ◦C relative to present during the Last Glacial Maximum

Summary

Introduction

The Last Glacial Maximum (LGM) describes the global sea level low stand at 26–19 ka, when global ice sheets attained their maximum volume of the last glacial cycle (Clark et al, 2009). This signal is dominated by the former Northern Hemisphere ice sheets; ice extent peaked globally during this interval (Schaefer et al, 2006; Clark et al, 2009). Well-distributed, quantitative estimates of LGM air temperature, derived from climate proxy data, provide important data for constraining. Eaves et al.: The Last Glacial Maximum in the central North Island estimates of climate sensitivity and evaluating climate model simulations that seek to determine the drivers and mechanisms of past climate change

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Conclusion