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Glacier Fluctuations During Termination 1 in Mackenzie Stream Valley, Southern Alps, New Zealand

By Colin William Dowey


Massive changes in Earth’s cryosphere, atmosphere, and oceans marked the end of the last glaciation (Termination 1: ~18,000-11,000 yrs ago). But the underlying causes are not fully understood. Here, I present a 10Be surface-exposure chronology of glacial landforms in Mackenzie Stream valley, located in the Ben Ohau Range of New Zealand’s Southern Alps, that spans Termination 1. This valley affords a clean climate signal because it is devoid of complicating factors such as ice- marginal lakes and tributary valleys. Thus the Mackenzie record is suitable for testing proposed climate drivers, including shifts in atmospheric CO2 (Broecker, 2013; Parrenin et al., 2013) and global atmospheric circulation (Denton et al., 2010; Toggweiler, 2009), as well as flips of the earth\u27s atmosphere-ocean system between multiple states (Lorenz, 1976; Broecker and Denton, 1989), perhaps paced by orbital variations (Saltzman et al., 1984). The Mackenzie record documents a Last Glacial Maximum (LGM) position of a glacier terminus at the valley mouth 18,397 ± 333 yrs ago, followed by rapid and extensive deglaciation of the valley by 16,696 ± 183 yrs ago. This deglaciation reflects a ~3.75°C increase in mean annual temperature from full-glacial conditions (~70% of the total Termination 1 temperature rise in the Southern Alps). Five late- glacial readvances confined to the valley head are recorded by moraine ridges in the eastern upper catchment with ages of 14,112 ± 175 yrs, 13,316 ± 107 yrs, 13,296 ± 293 yrs, 13,135 ± 344 yrs and 12,011 ± 234 yrs. This moraine sequence indicates that the former glacier achieved its most extensive late-glacial position 14,112 ± 175 yrs ago and that four periods of moraine ridge construction punctuated the following ~2000 yrs of net glacier retreat. Glacier fluctuations during Termination 1 in Mackenzie Stream valley are not easily explained. A comparison with the WAIS Divide atmospheric CO2 reconstruction (Marcott et al., 2014) shows that the extensive glacier retreat between 18,397 ± 333 and 16,696 ± 183 yrs ago corresponded to only a ~20 ppm increase in atmospheric CO2 concentrations (25% of the total glacial/interglacial CO2 rise). In addition, the glacier terminus stood at nearly the same position 16,696 ± 183 yrs ago and 14,112 ± 175 yrs ago despite a ~28 ppm increase in atmospheric CO2 (35% of the total glacial/interglacial CO2 rise). The Mackenzie record also does not permit definitive conclusions regarding the phasing of atmospheric wind-belt shifts and glacier recession, although it is possible that the onset of early Termination 1 glacier retreat preceded a poleward shift of the southern westerly wind belt. The possible mismatch between the Mackenzie glacier record and these proposed drivers leaves open the possibility that rapid recession early in Termination 1 reflects a flip between stable modes of operation of the global ocean- atmosphere system (Broecker and Denton, 1989). This rapid warming from a glacial to interglacial state is reminiscent of an almost intransitive” dynamic climate system in which a long-lived stable regime can abruptly shift to another stable regime (Lorenz, 1975)

Topics: Glacial landforms, Climate changes, New Zealand, Southern Alps, Climate, Glaciology
Publisher: DigitalCommons@UMaine
Year: 2015
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Provided by: University of Maine
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