Date of Award

8-2015

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

George H. Denton

Second Committee Member

Brenda L. Hall

Third Committee Member

Joerg M. Schaefer

Abstract

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's 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).

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