Detailed surface exposure age chronology for last glacial sequences in the Rangitata and Waimakariri Valleys, South Island, New Zealand
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Date
2009-05-15
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GNS Science
Abstract
The response of mid-latitude Southern Hemisphere glaciers to Quaternary climate
forcing has become a prime research focus in the debate on the dynamics of global
climate teleconnections. Of key importance in this research is the investigation of the
timings of late Quaternary mountain glacier fluctuations in New Zealand relating both
to last glacial ice maxima and ice decay signals. To address these questions we
collected 62 rock samples from glacial moraine sequences in two major valley
systems of the central Southern Alps for surface exposure dating (SED). Here we
present geomagnetically corrected ages derived from cosmogenic 10Be isotope
concentrations that provide absolute age control for glacial events in these valleys
from 23.0 ka to 13.7 ka. Results show that recession from extended LGM positions
commenced close to 22 ka followed by a slow ice retreat and ice margin stabilization
at 19-18 ka. This sequence is similar to other New Zealand sites but commences
several ka earlier than in the Northern Hemisphere. Our data also show that the largest
of the LGM advances in the Waimakariri Valley extended much further than
previously recognized and overran the so-called Avoca surface (previously OIS 8).
Further slow ice retreat re-commenced at around 16.5 ka resulting in multiple closely
spaced retreat positions over a ~10 km distance in both valleys that date to 14.5 - 16.0
ka (Blackwater III in Waimakariri; Spider Lake / Lake Emma in Rangitata). The
youngest late glacial moraines date to 14.0 ka (Poulter, Waimakiriri) and 13.7 ka
(Lake Clearwater, Rangitata). In summary our findings document that:
(1) the period 23.0 – 13.7 ka is characterized by a slow and gradual ice retreat
interrupted by stabilization phases but no major ice re-advances
(2) very extensive valley glaciers of 30 – 50 km length survived in New Zealand until
at least 14 ka
(3) as a consequence of (2), either an accelerated retreat rate or a short-lived ice
collapse would necessarily have occurred after 13.7 ka in order to restrict ice limits to
upper valley positions prior to the onset of the Holocene.
Description
Keywords
Glaciers, New Zealand, Southern Hemisphere, Quaternary period, Ice, Alps, Valleys, Isotopes, Beryllium 10
Citation
Rother, H., Fink, D., Schulmeister, J., Evans, M. (2009). Detailed surface exposure age chronology for last glacial sequences in the Rangitata and Waimakariri Valleys, South Island, New Zealand. Paper presented at the Past Climates meeting, Wellington New Zealand, May 15-17, 2009.