An Early Holocene to 'Little Ice Age' glacier chronology for New Zealand's Southern Alps - challenges, improvements, and implications

The New Zealand Alps offers excellent opportunities to investigate the nature and timing of past climate variability during the Holocene. Some glacial geochronological studies have been published focussing on this key site for the mid-latitudinal southern hemisphere, but a considerable degree of uncertainty and differences in individual chronologies have emerged. Specific glacial characteristics of this climatically and geomorphologically dynamic mountain region coupled with different analytical dating methodologies need to be correctly quantified, combined, and evaluated to better understand the complexities of New Zealand’s more recent glacial history.

In this study we applied ¹⁰Be cosmogenic radionuclide dating (CRN; n = 54) and Schmidt-hammer exposure-age dating (SHD; n = 42,000) to obtain surface exposure-ages from moraine boulders on Holocene glacier forelands in the eastern Aoraki/Mt.Cook National Park (Classen Glacier, Godley Glacier, Murchison Glacier and its tributaries) and the Arrowsmith Range (Ashburton Glacier, Cameron Glacier). SHD was performed utilising both regional SHD age-calibration equations based on published independent CRN ages and locally-adjusted equations based on our new CRN data and additional chronological information. This multi-proxy approach was combined with detailed geomorphological mapping and analysis to tackle the regionally specific 'geomorphological uncertainty' potentially interfering with subsequent interpretation of chronological data. We directed considerable effort to carefully define the glacial geomorphological context to guide our sampling strategies to ensure robust and reliable results.

On the foreland of Classen Glacier we establish a new geomorphologically reliable moraine site for a significant Mid-Holocene advance at 5 - 6 ka. At Godley and Murchison Glaciers, new precise ¹⁰Be ages reveal the outermost Holocene moraines date from the 'Little Ice Age'.  Our new SHD and CRN data coupled with geomorphological mapping shows that Late Holocene moraines which pre-date the 'Little Ice Age', occur at two glaciers in this area. In contrast, SHD and CRN data from Ashburton Glacier constrain the outermost moraine ridge to strong glacial activity during the Early Holocene. The number of advances unambiguously represented by moraines is, however, smaller than previously reported from nearby Cameron Glacier. At both glaciers, moraines representing the maximum 'Little Ice Age' glacier advance give SHD-age estimates that predate respective moraines in Aoraki/Mt.Cook National Park by more than 150 years. With no confirmed evidence for multiple advances during the Early Holocene in Aoraki/Mt.Cook National Park, these significant differences indicate that amalgamation of Holocene glacier chronologies from both areas is not justified. The morphostratigraphic configuration and individual morphology of moraines on the glacier forelands is complex and in some cases may be explained by excessive supraglacial debris caused by large mass movement events. This situation seems prevalent across the forelands and probably typical for the entire Southern Alps.

Our new Holocene glacier chronology shows considerable discrepancy to previously published studies, in particular regarding the number and timing of Late Holocene glacier advances. Further refinement of the Holocene glacier history for the Southern Alps constitutes a significant challenge requiring a more detailed understanding in the spatial variability of individual glaciers of the region.