Geomorphic and climatic controls on moraine building and preservation in the Southern Alps Kā Tiritiri o te Moana

Glacier margins fluctuate in response to climate change and often record these changes in the landscape by building ice-marginal (terminal and lateral) moraines. Glacial landscapes are therefore a potentially valuable archive of terrestrial palaeoclimate change. Typically a cooling climate causes glaciers to expand and warming causes glaciers to shrink. However, the dynamic glacier response time and the influence of high-relief mountainous topography on glacier dynamics complicates this behaviour, such that ice-marginal moraines are not always a straightforward record of palaeoglacier or palaeoclimate change. In tectonically active landscapes, such as the Southern Alps Kā Tiritiri o te Moana of Aotearoa New Zealand, high rates of hillslope erosion deliver large volumes of sediment to glaciers, leading to the formation of supraglacial debris layers that further decouple glacier behaviour from climate change.

We use the higher-order ice-flow model iSOSIA to simulate changes in erosion, ice extent and thickness in the response to Late Quaternary climate change and the resulting formation and preservation of moraines in a synthetic mountainous landscape. Our results show that the rate of palaeoclimate change relative to a glacier’s response time determines the geometry, number, and position of ice-marginal moraines, that glaciers can build distinct moraines in the absence of climate change, and that the distance from the glacial maximum may not represent the chronological order of moraine formation. While moraines can be preserved despite erosion by various surface processes and by being overrun during subsequent glaciations, moraine sequences frequently contain gaps that could be misinterpreted as representing periods of climate stability. We apply this model to Franz Josef Glacier Ka Roimata o Hine Hukatere to reconstruct glacier evolution and moraine building in the Southern Alps Kā Tiritiri o te Moana during the Last Glacial Maximum and subsequent deglaciation.