Reconstruction of the San Lorenzo ice cap during the last glacial cycle: insights into the build-up and demise of the last Patagonian ice sheet

Abstract

Patagonia in southernmost South America is situated in a climatically sensitive region of the Southern Hemisphere. It is the only stretch of land to fully intersect the southern westerly wind belt, an important component of the global climate system. Robust glacial chronologies from this region can help unravel the interhemispheric (a-)synchronicity of climate events and the underlying drivers of glaciation and climate change. Despite exceptional preservation of moraine records in Patagonia, little is known about the build-up and the evolution of the last Patagonian Ice Sheet (PIS) throughout the entire last glacial cycle. The response of glaciers to abrupt climate events during the last deglaciation and glacier change during the Holocene remain poorly resolved in many parts of Patagonia. This thesis reconstructs the extent and timing of glacial advances and the evolution of palaeolakes on the eastern side of Monte San Lorenzo. This site was chosen for its well preserved geomorphological record and the potential to yield new insights in the glacial and climate history of the region. Geomorphological mapping revealed three landform assemblages. In the Belgrano valley, the geomorphology is dominated by three large moraine systems and associated outwash terraces deposited during advances of the former Belgrano glacier. Once the Belgrano glacier retreated to within its trough, the geomorphology is dominated by glaciolacustrine landforms documenting a formation of a higher, palaeolake Belgrano. Nearer the mountains, moraines, flutes and active outwash tracts evidence recent, Holocene mountain glaciation. Cosmogenic nuclide exposure dating reveals that the maximum extent of the Belgrano glacier occurred at ~75 ka, towards the end of Marine Isotope Stage (MIS) 5. The second advance dated at ~25 ka, coincident with the global Last Glacial Maximum (gLGM), was significantly smaller. Third advance of the Belgrano glacier was dated at ~13 ka, coeval with Antarctic Cold Reversal (ACR). Subsequent rapid ice retreat, accompanied by a palaeolake development, was interrupted by a short ice marginal stabilisation during the Northern Hemisphere (NH) Younger Dryas (YD). After this, glaciers withdrew to the mountains and the palaeolake drained, marking the final demise of the PIS. Finally, a Holocene advance/still-stand of the San Lorenzo glacier is constrained to 2.8 ± 0.2 ka. An ELA lowering of 221 - 271 m relative to the present was required to match this limit, which translates to a temperatures decrease of ~ 1.3 - 1.6 ˚C assuming an adiabatic lapse rate of 0.6 ˚C / 100 m and no change in precipitation. The late Holocene advance coincided with a climatic deterioration observed regionally and worldwide, and may have been caused by reduced solar activity at the time.

The main implications of this work are as follows: 1) Glaciation of the Patagonian Andes initiated early in the last glacial cycle. 2) The topography, catchment size and the shifting ice divides played an important role in determining the magnitude of glacier advances. 3) During the last glacial-interglacial transition (LGIT), glaciers in the region responded to a dominant Antarctic climate signal, and to lesser extent to the NH YD signal. 4) During the Holocene, glaciers in Patagonia achieved greater extent prior to the last millennium and the overall pattern of Holocene glacier change contrasts with that observed in New Zealand.