The chronology of the last deglaciation from two New Zealand valleys and some climatic implications

We present Be-10 and Al-26 chronologies from the paleo-Rakaia glacier and the Clearwater lobe of the Rangitata glacier focussing on the transition from the last glaciation maximum (LGM) to the Holocene. From the Rakaia we demonstrate that the local glacial maximum preceded the global LGM by several thousand years at c. 25 ka. Over the succeeding 12,000 years the glacier retreated only about 10 km and although undated, geomorphic evidence suggests a continued steady retreat after this time. The Clearwater lobe of the Rangitata glacier provides the most detailed LGIT record from any New Zealand glacier because it the valley was protected from significant meltwater flow during the deglaciation. Between 16.4 ka and 13.7 ka the Clearwater ice lobe retreated only 12 km producing 23 closely spaced recessional ice positions. The geomorphology and chronology are categorical in demonstrating that no significant re-advance can have occurred during this period. The inboard termination of the record occurs where the Clearwater valley drops into the main Rangitata Valley and should not be interpreted as the start of the more significant retreat. We highlight that uncertainties in isotope production rates and other elements of cosmogenic age determination (e.g. geomagnetic corrections) means that while we have millenial scale precision on ages during the LGIT, the accuracy of the ages is not as high. Nevertheless, our data clearly demonstrate that the evacuation of ice from these east coast valleys in New Zealand was remarkably slow with glaciers extending to ~60% of their LGM extents at 14 kyr (or younger). There is no evidence for either an early LGIT ice collapse or rapid retreat and consequently no late LGIT major readvance. We contrast our findings with the interpretations of chronologies coming from the Mackenzie Basin and we conclude that the records are compatible, with the chronological differences created by the morphometry of the respective ice catchments.Copyright (c) 2011 INQUA 1