Southern Patagonian glacial chronology for the Last Glacial period and implications for Southern Ocean climate

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Abstract

The Magellan region of southern South America is in a unique setting, at >50°S on the equatorial side of the Antarctic Frontal Zone, to record in detail terrestrial glacial to interglacial events. A 10Be chronology shows growth and millennial fluctuations of a Patagonian Ice Sheet between ∼25 and 17.6–17.0 cal ka. In the Strait of Magellan, the maximum ice margin position is dated to 24.6±0.9 ka, and other moraine ages are 18.5±1.8 and 17.6±0.2 ka (mean±1 standard deviation). In Bahía Inútil, dated moraine ages are 20.4±1.2 and 17.3±0.8 ka. The chronology of the local Last Glacial Maximum (LGM) reveals a record of atmospheric cooling that was broadly in phase with changes in Southern Ocean conditions, such as sea-ice fluctuations and surface water characteristics. Published modeling results indicate that a decline in temperature of ∼6 °C and slight drying over southernmost Patagonia could simulate the growth and sustained presence of an ice sheet to the mapped LGM limit. The terrestrial record in southern Patagonia and marine records in adjacent oceans indicate mean northward movement of the Antarctic Frontal Zone, which caused the last southern South American ice age. The Antarctic Frontal Zone at present lies only 3–5° to the south. Some significant changes in the Magellan region occurred in step with North Atlantic region and the Northern Hemisphere. For example, the overall time span of the last glaciation and the timing of maximum ice extent were similar between the hemispheres, despite maximum local summer insolation intensity in southern South America. Other characteristics of the southern Patagonian glacial history differ from the North Atlantic region, specifically an out-of-phase relationship during deglaciation, which is more similar to that of Southern Ocean and Antarctic records.

Introduction

Resolving the timing and structure of the Last Glacial period around the globe is important because such knowledge can help evaluate whether the tropics, North Atlantic Ocean, or Southern Ocean may drive abrupt regional and global climate changes. In particular, the Southern Ocean, dominated by the Antarctic Circumpolar Current, affects all three major oceans, influences deep water worldwide, is one of the Earth's major heat sinks, and is an important control on atmospheric CO2 (Toggweiler and Samuels, 1995; Francois et al., 1997; Knorr and Lohmann, 2003). A lack of terrestrial data hinders understanding of the southern latitude air–ocean system and its role in climate dynamics during the global Last Glacial Maximum (LGM) (e.g., 23–19 ka, Mix et al., 2001). Marine records, although invaluable, are less common and are susceptible to 14C reservoir effects (Charles et al., 1996; Lamy et al., 2004). Also, they are only an indirect proxy for past atmospheric conditions, and do not characterize spatial–temporal terrestrial climate variability. Moreover, there are few land areas in middle to high southern latitudes and long ice core records are limited to Antarctica.

A terrestrial record spanning the local LGM near the northern boundary of the Antarctic Frontal Zone, where temperature drops considerably by 6 °C or more (Fig. 1b, Belkin and Gordon, 1996; Olbers et al., 2004), is a proxy for former changes in atmospheric conditions and allows linking with middle to high-latitude marine and ice core records. In this paper, we present a glacial chronology during the LGM from just north of the Drake Passage, on the most southerly continental setting outside Antarctica (Fig. 1), <3–5° north of the ‘modern interglacial’ position of the Antarctic Frontal Zone. Southernmost South America is less than 1000 km from the Weddell Sea where deepwater formation occurs (Fig. 1b) and is the only continental land mass between ∼45 and 65°S. Glacial records are one of the best proxies of past atmospheric behavior and snowline change outside the ice-covered regions, and those from southern South America are specifically proxies of the former behavior of the westerlies and air–ocean systems surrounding Antarctica (Fig. 1).

A firm LGM chronology for the southernmost Andes has been lacking largely because of a paucity of material for 14C dating, especially in arid environments such as in the lee of the Andes. The west side of the southernmost Andes was covered by LGM ice and thus the glacial geologic record is primarily one of deglaciation. In addition, available proxy paleoclimate records from middle and low latitude South America have been used to argue for in-phase and out-of-phase behavior with records in both the Southern Ocean and the North Atlantic region during the Last Glacial period and interglacial transition (Denton et al., 1999; Bennett et al., 2000; Gilli et al., 2001; Heusser, 2003; Kaplan et al., 2004; Smith et al., 2005; Sugden, 2005).

We measured in situ cosmogenic 10Be accumulated in erratics that closely date glacial landforms recording former ice margin positions of Andean ice lobes in the Strait of Magellan and Bahía Inútil, ∼53–54°S (Fig. 2). This investigation ties together new and previous data (e.g., Heusser, 2003; McCulloch et al., 2005), which, collectively, allows a better comparison between the cosmogenic nuclide chronology of Patagonian glacial events and other records of southern latitude climate change, including the adjacent Southern Ocean, during the last major glacial period and termination.

Section snippets

Setting

The Strait of Magellan and Bahía Inútil are major marine inlets surrounded by low-lying coastal areas within the southern tip of Patagonia, extending towards the dry steppe of semi arid eastern Patagonia (Fig. 2). Low-gradient outlet glaciers in the southern part of the ice sheet extended northeast to east down the Strait of Magellan and Bahía Inútil towards the South Atlantic Ocean, where glacial landforms that indicate major ice margin stillstands or readvances have been mapped in detail (

Surface exposure dating

Sampling strategies aimed to refine the overall chronology of McCulloch et al. (2005) and the mapping of Bentley et al. (2005) and Benn and Clapperton (2000), which were guides to collecting. They defined at least three major moraine ‘limits’, ‘B–D,’ for the Last Glacial period along Bahía Inútil and the Strait of Magellan, and one Lateglacial event, E, confined to the mountains at the head of the Strait (Fig. 2). In this study, 14 additional dates supplement 10 previously recorded (Table 1;

Prior 14C dating and amino acid data

Radiocarbon and amino acid racimization data on marine shells in the Strait of Magellan provide additional chronometers (cf., 10Be ages) to reconstruct the glacial history, especially at the beginning and end of the LGM (McCulloch et al., 2005). On the western shore of the Strait of Magellan and the eastern shore of Isla Dawson, 14C ages on reworked shell fragments, from diamict inside limit B, range from >40,000 (i.e. infinite) to 27,690 14C yr BP (44,760±460 to 31,250±670 cal yr BP). These ages

Results

Twenty-four 10Be ages on glacial erratics constrain the timing of former ice margin fluctuations in the Magellan region (Fig. 2; Table 1). All ages shown and discussed assume an erosion rate of 0 mm/ka, unless otherwise mentioned. Dates from the B–C limits on Peninsula Juan Mazia range from ca 25.6±1.5 to 15.7±2.6 ka and suggest limited overall net retreat of the ice margin during this time period. The four boulders from the oldest landform provide the ages of ∼26–24 ka (mean and SD of ∼24.6±0.9 

Chronology of the Last Glacial period

Building upon prior studies, the chronology presented here more firmly constrains the glacial history of southern Patagonia and necessitates slight refinement in the ages and mapping of the moraine limits B–D as recognized by Clapperton et al. (1995) and McCulloch et al. (2005). The new ages suggest that much of the Peninsula Juan Mazia deposits are statistically younger than the ∼25 ka ‘B’ landform just to the southeast as they have a mean age of 18.5±1.8 (Fig. 2; Table 2). The deposits that

Conclusions

Southern Patagonian ice was most extensive from ∼25 to ∼18 ka, with a peak around 25–24 ka. This was a period of rising and maximum summer insolation intensity in the Southern Hemisphere largely due to the precession of the equinoxes (Fig. 4). A stronger influence of the Antarctic Frontal Zone and equatorward movement of cold air from the south relative to the present is one way to explain negated local summer insolation intensity, and a glaciation broadly in phase with global changes.

Acknowledgments

We thank C. Clapperton, M. Clapperton, R. McCulloch, F. Lamy, W. Phillips, D. Hughes, and J. Rabassa for feedback and assistance, and Jacqueline Smith, Tom Lowell and Robert Ackert for constructive reviews that improved the manuscript. This work was supported by the Royal Society of London, specifically a Postdoctoral fellowship to MRK, the UK National Environmental Research Council (NERC), and a Comer Science and Educational Research Fellowship (MRK). This is L-DEO Contribution no. #7044.

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