Reconciling the onset of deglaciation in the upper Rangitata valley, Southern Alps, New Zealand

Abstract

Enquiry into the detailed timing of paleoclimate events depends on well-resolved and reliable chronologies. A recently published set of 10Be surface-exposure ages for moraine boulders in the upper Rangitata valley was interpreted to indicate a gradual reduction in glacier surface height between c. 21 and c. 17 ka, without rapid glacier recession beginning c. 18 ka as reported from other valleys. The dating results were suggested to be consistent with moraine geomorphology and an interpreted lack of post-glacial lake formation in the valley. In contrast, we argue that the geomorphology is consistent with sustained glacier recession and we highlight evidence for a post-glacial lake. Furthermore, the upper Rangitata moraine chronology was encumbered by inaccurate altitude values assigned to many of the sample sites. Recalculation of the 10Be ages using accurate elevations produces values that are as much as 3000 years younger than originally reported. The recalculated values indicate no significant age difference across the c. 300-m relative altitudinal spread of sample locations, with mean age of all samples c. 17.7 ± 0.3 ka. The previous inference of slow late-glacial ice retreat is not supported by the recalculated chronology. Rather, the revised glacial chronology implies that substantial ice-surface lowering of the Last Glacial Maximum Rangitata glacier was in progress shortly after c. 18 ka. The revised upper Rangitata chronology is compatible with dating from other eastern valleys of the Southern Alps, indicating moraine formation at c. 18 ka, followed by sustained glacier recession associated with regional climatic amelioration.