Browsing by Author "Evans, M"

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    Detailed surface exposure age chronology for last glacial sequences in the Rangitata and Waimakariri Valleys, South Island, New Zealand
    (GNS Science, 2009-05-15) Rother, H; Fink, D; Schulmeister, J; Evans, M
    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.
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    The early rise and late demise of New Zealand’s last glacial maximum
    (Proceedings of the National Academy of Sciences of the United States of America, 2014-06-13) Rother, H; Fink, D; Shulmeister, J; Mifsud, C; Evans, M; Pugh, J
    Recent debate on records of southern midlatitude glaciation has focused on reconstructing glacier dynamics during the last glacial termination, with different results supporting both in-phase and out-of-phase correlations with Northern Hemisphere glacial signals. A continuing major weakness in this debate is the lack of robust data, particularly from the early and maximum phase of southern midlatitude glaciation (∼30–20 ka), to verify the competing models. Here we present a suite of 58 cosmogenic exposure ages from 17 last-glacial ice limits in the Rangitata Valley of New Zealand, capturing an extensive record of glacial oscillations between 28–16 ka. The sequence shows that the local last glacial maximum in this region occurred shortly before 28 ka, followed by several successively less extensive ice readvances between 26–19 ka. The onset of Termination 1 and the ensuing glacial retreat is preserved in exceptional detail through numerous recessional moraines, indicating that ice retreat between 19–16 ka was very gradual. Extensive valley glaciers survived in the Rangitata catchment until at least 15.8 ka. These findings preclude the previously inferred rapid climate-driven ice retreat in the Southern Alps after the onset of Termination 1. Our record documents an early last glacial maximum, an overall trend of diminishing ice volume in New Zealand between 28–20 ka, and gradual deglaciation until at least 15 ka. © 2014, National Academy of Sciences of the United States of America.