Southern Patagonian glacial chronology for the Last Glacial period and implications for Southern Ocean climate
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.
References (60)
- et al.
Long-term cosmogenic 3He production rates from 40Ar/39Ar and K–Ar dated Patagonian lava flows at 47°S
Earth and Planetary Science Letters
(2003) - et al.
A northern lead in the orbital band: north-south phasing of Ice-Age events
Quaternary Science Reviews
(2002) - et al.
The Southern Ocean's biological pump during the Last Glacial Maximum
Deep-Sea Research II
(2002) - et al.
Pleistocene glacitectonic landforms and sediments around central Magellan Strait, southernmost Chile: evidence for fast outlet glaciers with cold-based margins
Quaternary Science Reviews
(2000) - et al.
Terrestrial in situ cosmogenic nuclides: theory and application
Quaternary Science Reviews
(2001) - et al.
Climate connections between the hemispheres revealed by deep sea sediment core/ice core correlations
Earth and Planetary Science Letters
(1996) - et al.
The last glaciation in Central Magellan Strait, Southernmost Chile
Quaternary Research
(1995) - et al.
Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in-situ cosmogenic dating
Earth and Planetary Science Letters
(2003) - et al.
Extended scaling factors for in situ cosmogenic nuclides: new measurements at low latitude
Earth and Planetary Science Letters
(2006) - et al.
Cosmogenic nuclide surface exposure dating of boulders on last-glacial and late-glacial moraines, Lago Buenos Aires, Argentina: interpretive strategies and paleoclimate implications
Quaternary Geochronology
(2006)
Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation
Earth and Planetary Science Letters
Influence of secular variation of the magnetic field on production rates of in situ produced cosmogenic nuclides
Earth and Planetary Science Letters
The Laurentide and Innuitian ice sheets during the last glacial maximum
Quaternary Science Reviews
Polar perspective of Late-Quaternary climates in the Southern Hemisphere
Quaternary Research
The Last Glacial maximum and deglaciation in southern South America
Quaternary Science Reviews
Cosmogenic nuclide chronology of pre-last glaciation maximum moraines at Lago Buenos Aires, 46 °S, Argentina
Quaternary Research
Cosmogenic nuclide measurements in southernmost South America and implications for landscape change
Geomorphology
Cosmic ray labeling of erosion surfaces: in-situ nuclide production rates and erosion models
Earth and Planetary Science Letters
The Mobile Polar High: a new concept explaining present mechanisms of meridional air-mass and energy exchanges and global propagation of palaeoclimatic changes
Global and Planetary Change
Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications
Earth and Planetary Science Letters
Reply to C.J. Heusser's “Southern Westerlies during Last Glacial Maximum”
Quaternary Research
Environmental processes of the ice age: land, oceans, glaciers (EPILOG)
Quaternary Science Reviews
Quaternary of Tierra del Fuego, Southernmost South America: an updated review
Quaternary International
Modelling the inception of the Patagonian icesheet
Quaternary International
Effect of Drake Passage on the global thermohaline circulation
Deep Sea Research Part I. Oceanographic Research Papers
The mid-latitudes of North and South America during the Last Glacial Maximum and early Holocene: similar paleoclimatic sequences despite differing large-scale controls
On the position of southern hemisphere westerlies at the Last Glacial Maximum: an outline of AGCM simulation results and evaluation of their implications
Quaternary Science Reviews
Southern Ocean fronts from the Greenwich meridian to Tasmania
Journal of Geophysical Research
The Last Glacial–Holocene transition in southern Chile
Science
The landforms and pattern of deglaciation in the Strait of Magellan and Bahía Inútil, southernmost South America
Geografiska Annaler
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