EXTENSION OF LGM ALPINE ICE LIMITS IN NORTHWEST NELSON, NEW ZEALAND: IMPLICATIONS FOR PRECIPITATION INFLUENCES ON MID-LATITUDE GLACIERS

Glacial landforms and sediments in the lower Takaka valley, in the northwest corner of the South Island, New Zealand, document the expansion of Late Pleistocene alpine glaciers ca. 20-25 km beyond previously recognized limits. The ice expansion bears implications for Southern Hemisphere mid-latitude climatic processes during the last glacial maximum (LGM).

Evidence of Late Pleistocene glaciation in the lower Takaka valley includes a variety of landforms and sediments at the mouths of two alpine valleys. These features had been mapped previously as Middle Pleistocene alluvium. At the mouth of the Cobb-Takaka valley, till lies behind and upon several valley-side bedrock obstructions. In close association with these obstructions lie at least two end moraines, associated outwash terraces, and a series of prominent kame terraces. Similarly, the mouth of the Anatoki valley features till, end-moraine remnants, and possible kame terraces. Lateral moraine remnants and kame terraces in the nearby Go-Ahead Creek valley document that alpine ice in the Anatoki valley breached a low divide and filled Go-Ahead Creek with an outlet ice lobe. All of these features are mildly weathered and minimally modified by erosion, suggesting Late Pleistocene age. A series of cosmogenic isotope exposure ages from bedrock exposures in the Cobb valley, collected during a previous study, indicate that the valley-mouth landforms likely correlate with the global LGM.

The expansion of LGM ice beyond previously recognized limits was of large magnitude, but was likely short-lived. Cross profiles of both glaciated valleys were modified minimally by Late Pleistocene glacial erosion. The topography of the upland ice accumulation areas suggests that the expansion of alpine ice into the lower Takaka valley required an ice cap over most of the northwest Nelson ranges, increasing the inferred LGM ice extent in New Zealand by ca. 7%. The expansion correlates with a mid-latitude insolation maximum, suggesting that enhanced westerly airflow and precipitation delivery, rather than dramatic thermal decline, drove the expansion.