The Glacial History of the Upper Clarence Valley, North Canterbury, New Zealand

New Zealand mountain glaciers are sensitive to atmospheric variability and they etch their length changes into the geological record as glacial deposits in response to paleoclimate events. The end of the last major glaciation (~65-18 ka) involved large readjustments of Earth's climate and cryosphere between ~19-11.5 ka that culminated in ice recession due to warming of 4–6°C. Constraining deglaciation in New Zealand helps to constrain potential drivers of past glacier-climate interactions in the Southern Hemisphere, which supports evaluations of future cryosphere response to modern climate change.

This study employs geomorphological mapping and cosmogenic 10Be surface exposure dating to produce a high-precision moraine chronology that constrains the timing and magnitude of glacier length changes in the upper Clarence valley, North Canterbury, New Zealand. We present 42 new surface-exposure ages from greywacke boulders embedded in well-preserved moraines that document the catchment's deglaciation. Exposure ages from moraine boulders successfully constrain the timing of nine distinct moraine formation episodes (glacier readvance or standstills) to show a distinct pattern of overall retreat since Marine Isotope Stage-4, succeeded by rapid and sustained retreat at the end of the Last Glacial Maximum. The oldest preserved glacial event in the upper Clarence valley occurred at 64594 ± 847 yrs BP during Marine Isotope Stage-4. The successive eight landform assemblages produced ages of 17484 ± 200 yrs BP, 18149 ± 198 yrs BP, 16511 ± 190 yrs BP, 18558 ± 166 yrs BP, 18265 ± 149 yrs BP, 19090 ± 514 yrs BP, 18549 ± 248 yrs BP, 18599 ± 123 yrs BP, during Marine Isotope Stage-2. Though not precisely temporally consecutive, six of these eight landform ages are statistically indistinguishable from one another, indicating that deglacial warming and glacial retreat was underway from ~18 ka.

Net retreat of the former Clarence valley glacier is consistent with similar moraine chronologies elsewhere in the Southern Alps, which supports the regional pattern and implies that Southern Alps glaciers achieved glacial maxima prior to the global cold nadir of the last glacial cycle and responded to common climatic forcings between ~19-17 ka.