The precise timing and character of glaciations in Patagonia from MIS 6 to the Little Ice Age

By only considering records of climate and glaciers over the period that humans have been monitoring them, one might think that climate normally changes quite rapidly, and that glaciers have always been small. But in the not-so-distant past, an ice sheet covered the Southern Andes, flowing across the southern tip of the continent, and in some places, even terminating into the Atlantic Ocean. Glaciers rewrite the surfaces they inhabit, leaving behind indicators of their past behavior. By studying the landforms they create, we can reconstruct climates of the past. Here I present unique and novel glacier-climate reconstructions over southern and central Patagonia using a state-of-the-art dating approach tied to high resolution spatial mapping and glaciological modeling. The main goal of this thesis is to constrain the precise timing and character of the past advances of three glaciers in Patagonia. To this end, I present new precise 10Be surface exposure datasets from two paleo outlet glacier lobes (at 45°S and 53°S), totaling 71 new moraine boulder ages. In these two valleys, I am able to reconstruct the last three major glaciations (MIS 6, 4, 2), as well as provide a constraint for the last two terminations (T1, T2). At a third site, I create a novel dataset to reconstruct the behavior of the Calluqueo glacier (48°S) from ~7,000 years ago to the present. All three sites are eastwards of the main Andes mountain range, spanning from southernmost Patagonia (53°S) to central Patagonia (45°S). To achieve my thesis objectives, I employed recent improvements in the 10Be exposure dating method, and tied the geochronological studies to new, high resolution maps of the glacial geomorphology created by the former glaciers and associated processes. We find in central Patagonia, the Ñirehuao glacier lobe was most extensive potentially during Marine Isotope Stage (MIS) 8, but certainly prior to MIS 6, followed by a major advance during MIS 6. This study presents one of the first directly dated records of a MIS 6 glacier expansion in Patagonia at 153±5.1 ka, where the glacier may have been in retreat at 137±4.2 ka. During the last glacial cycle, the glacier was most extensive during the middle of MIS 2, at 23.6±0.9 ka. The southernmost section of the Ice Sheet, at Estrecho de Magallanes, was more extensive during Marine Isotope Stage 4 (MIS 4) than during MIS 2, representing the first direct dating of the MIS 4 glacier culmination in South America. Similar to the MIS 2 glacial maximum, within MIS 4 there were multiple advances that we date (6 samples) to between 67.5±2.1 and 62.1±2.0 ka. Inboard of the MIS 4 moraine complex, we date a sequence of geomorphically distinct MIS 2 moraines that represent separate major periods of glacial stability. The MIS 2 maximum extent occurred by 27.4±0.8 ka and was followed by at least four more full glacial culminations over a hundred miles beyond the Andes mountains. About 18 km inboard of the main MIS 2 landforms, the sequence is followed by smaller-scale recessional moraine crests that we date to 18.0±0.8 ka, indicating the glacier was in net retreat at this time. In order to estimate the climate conditions necessary to drive the glacier advances that we date and map, we apply the University of Maine Ice Sheet Model to the Estrecho de Magallanes and Ñirehuao records. Tentative results suggest that the Magallanes lobe may have reached mapped inner and outer MIS 2 moraines with a climate that had approximately 4.5°C and 5.5°C cooler summers, respectively, assuming about 25% less annual precipitation relative to modern conditions. A new record at Calluqueo, in central Patagonia, allows us to reconstruct Holocene (interglacial) glacier changes. Using 33 new 10Be ages with unprecedented precision, geomorphic mapping and historical imagery, we find that the Calluqueo glacier sat at its mid-Holocene maximum extent from ~6,900 until ~6,700 years before the present. Major moraine forming advances subsequently culminated at least seven more times, averaging every 500±31 years, between 5,620±203 and 3,120±106 years ago. A hiatus in moraine formation occurred from 3,120±106 until 1,160±50 years ago (860 CE). Major retreat occurred between 1600-1800 CE, followed by stability from 1800-1940 CE, and pronounced ongoing retreat since after 1940 CE. For the Holocene period, this record represents one of the first precise, directly-dated glacier histories from central Patagonia, and one of the few available for all of Patagonia. The timing of advances of the Calluqueo glacier has little in common with the glacial histories from the Northern Hemisphere, suggesting an inter-hemispheric asynchronicity. All together, we reconstruct the timing of glacial maxima at three sites in terrestrial Patagonia from 53°S to 45°S, with unprecedented precision, from pre-MIS 6 to the present day