Featured Faculty Research - IsoTopics

Chronology of the Last Glaciation in the Southern Hemisphere (figures)

Brad Singer, UW-Madison
Wes Hildreth, USGS-Menlo Park
Yann Vinzce, University of Geneva, Switzerland

Telling geologic time in the Late Pleistocene beyond the range of carbon-14 dating has been a challenge that until recently posed a barrier to addressing many problems in paleoclimate, paleomagnetism, neotectonics, and volcanology. However, remarkable improvements in radioisotopic methods including 40Ar/39Ar dating of lava flows and ash deposits and the development of surface exposure dating using cosmogenic nuclides such as 26Al, 10Be, and 3He are breaking down these barriers and opening up exciting new avenues of research.

It is clear that Earth’s orbital position relative to the sun has modulated global climate in an oscillatory pattern for the past 2 million years. There is growing evidence, though, that deglaciation and warming were not exactly globally synchronous following the last glacial maximum, and perhaps earlier ice ages (e.g., Blunier et al., 1998). Instead oceans, atmosphere, topography, and biota interact in a complex network of feed-backs such that regional responses to shifts in solar radiation may differ in tangible ways between the hemispheres (Alley and Clark, 1999; Gillespie and Molnar, 1995). Because much of our understanding of paleoclimate is heavily biased by interpretations of northern hemisphere records–and very little quality data exist south of the equator–I am interested in the terrestrial responses to climate change in the southern hemisphere.

As an example, my colleagues and I have obtained precise constraints on the timing of deglaciation in the central Andes of Chile by dating several lava flows erupted in the Laguna del Maule basin along the crest of the Andes at 2100 m elevation using the 40Ar/39Ar geochronometer. Steep-sided valleys and unconformaties within older Quaternary volcanic deposits indicate that this region at 36° South latitude (Fig. 1) was glaciated repeatedly during the Pleistocene (for comparison, Yosemite Park is at 36 °N, at the same elevation). One lava flow that was striated and grooved by overriding glacial ice (Fig. 2) after its eruption was dated at 25,600±1,200 yr BP (Fig. 3). Three other lava flows each covering 4 to10 km2 of the basin floor are morphologically pristine and show no sign of burial or erosion by glacial processes. The three lavas yielded 40Ar/39Ar isochron ages of 23,300±600 yr BP, 21,200±3,000 yr BP, and 20,700±1,200 yr BP (Fig. 3), indicating that this corridor of the Andes was glacier covered between 26,000 and 23,000 years ago, but that the glaciers retreated for the last time before 23,000 years ago.

Our results are controversial because carbon-14 dated moraines farther south in New Zealand and at Lago Lanquihue, and Torres del Paine, Chile (Fig. 1) suggest that glacial advances occurred 14,000 and 11,000 years ago (Lowell et al., 1995; Marden, 1997). Our new ages are, however, consistent with evidence that portions of the southern hemisphere, like Canal Whiteside (Anderson and Archer, 1999; Fig. 1) emerged from the last ice age as much as 5,000 years ahead of the northern hemisphere. We postulate that the early ablation of glaciers at 36°S reflects an increase of solar radiation in southern hemisphere several thousand years prior to the north (recall that tilt and precession of the Earth’s rotational axis in its orbital plane mean that the magnitude of incoming solar radiation and seasonality in the two hemispheres evolve according to a complex but opposing periodicity). This accumulated solar energy may also have ablated extensive Antarctic sea ice (Kim et al., 1998), which in turn allowed a rapid poleward contraction of the circumpolar westerly winds (the roaring 40’s) which carry all the precipitation required to sustain Andean glaciers of any size (Heusser, 1989). This hypothesis explains continued glaciations to the south of Laguna del Maule and the present position of the Patagonian ice fields (Fig. 1). I hope to test this hypothesis in the new UW-Madison Rare Gas Geochronology Laboratory through further dating of lavas on more southerly Andean composite volcanoes.

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