Date of Award

5-2014

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

Karl J. Kreutz

Abstract

What caused the termination of the last ice age in the Southern Hemisphere? During the last termination, massive glacial collapse occurred in the southern midlatitudes during a period of decreasing local summer insolation intensity. The inability of local changes in summer insolation to explain this observed climate change in the Southern Hemisphere points to other forcing mechanisms for the termination. For example, a new hypothesis for the termination has emerged in which melting and collapse of Northern Hemisphere ice sheets lead to growth of North Atlantic sea ice, southward shift of the intertropical convergence zone (ITCZ), poleward movement of the southern westerly wind belts and associated ocean fronts (Anderson et al., 2009), and abrupt warming in the southern mid-latitudes (Denton et al., 2010). In particular, two pulses of warming that together make up most of the Southern Hemisphere termination have been linked to Northern Hemisphere cold events, Heinrich Stadial 1 (HS-1) and the Younger Dryas. Carbon dioxide emitted from the Southern Ocean during these two events may have been instrumental in driving the end of the ice age to completion. If the hypothesis stated above holds true, then warming in the Southern Hemisphere (Termination 1) would occur at the same time as cooling in the Northern Hemisphere (Heinrich Stadial 1). Testing this hypothesis and others of the last termination requires detailed and well-dated glacial and paleoclimate reconstructions from both hemispheres in order to compare the timing and spatial pattern of climate events with potential forcing mechanisms.

The goal of this project was to reconstruct the glacial history of Bahía Inútil, in the Strait of Magellan region of Patagonia, in order to constrain the timing of the onset of the termination in the middle latitudes of South America. Field work consisted of mapping the glacial geomorphology of prominent moraine belts on the south side of Bahía Inútil at two field sites, Estancia Tres Hermanos (ETH) and Estancia Rosa Irene (ERI). Chronology for these moraines came from 10Be surface-exposure ages of thirty- three erratic boulders embedded within the crests.

Twenty dates of boulders from the inner moraine at ERI yielded a mean age of 18,304 ± 607 years BP. This moraine belt was the last terrestrial expression of the Bahía Inútil Lobe (BIL) before it retreated back to Cordillera Darwin, 135 km to the southwest, and thus dates the end of the Last Glacial Maximum (LGM). It also is a close minimum- limiting age for the start of the termination. Outboard of the ERI belt, the moraine system at ETH produced six exposure ages ranging from 22,069 ± 643 to 39,519 ± 1,314 years BP. Any robust interpretation of these dates proves impossible. This range of ages may be the result of previous exposure of the boulders to cosmic rays, of exhumation of boulders from the moraine, or from reworking of old boulders due to multiple overriding glaciations.

These data indicate that the end of the LGM and the start of deglaciation in Tierra del Fuego began just after -18,300 years BP when ice receded from the ERI moraines. Ages of basal organic material from bogs in Cordillera Darwin indicate that ice had retreated from Bahía Inútil back into the heart of the mountain range by ~17,000 years BP (Hall et al., 2013), implying an average rate of retreat of ~100 m/yr. This rapid collapse marks the onset of the Southern Hemisphere termination and is similar in timing to that found in northern Patagonia, as well as in New Zealand (Denton et al., 1999; Putnam et al., 2013).

These data from southernmost South America indicate that the termination in the southern mid-latitudes to at least 54 °S occurred at the same time as Heinrich Stadial 1 in the North Atlantic region. Thus, while it was cold in the Northern Hemisphere, the southern mid-latitudes as far south as the Drake Passage underwent rapid warming and glacier collapse. This asynchronous behavior between hemispheres is consistent with the hypothesis that southward shift of the westerlies and oceanic fronts in the southern midlatitudes, two phenomena which can be linked to northern stadials (Denton et al., 2010), were instrumental in initiating the termination in the Southern Hemisphere.

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