Optically Stimulated Luminescence (OSL) dating of glacial lake drainage during the deglaciation of the Patagonian Ice Sheet

Lead Research Organisation: Aberystwyth University
Department Name: Inst of Geography and Earth Sciences

Abstract

When glaciers and ice sheets melt, they release large volumes of fresh water into the oceans. Sometimes this release is gradual, for example via rivers, and sometimes this release is catastrophic, for example via large ice-dammed lake drainage events. The rapid release of large quantities of glacial meltwater are very important because recent research has highlighted their role in influencing deep sea circulation and therefore climate. Large glacial lakes formed on the eastern flank of the Patagonian Ice Sheet at various times during ice sheet growth, at the Last Glacial Maximum and during deglaciation. The meltwater periodically discharged from the lakes surrounding the Patagonian Ice Sheet into the Southern Pacific as the ice sheet changed shape and size, but we do not know the precise timing of these drainage events. The aim of this project is to reconstruct the outline and evolution of these large glacial lakes that formed east of the Patagonian Ice Sheet in Chile and Argentina during recession from its Last Glacial Maximum position. Using satellite remote-sensing images and fieldwork surveys we know that we can identify the extent of these former glacial lakes and their principal meltwater drainage routes (e.g. former deltas, shorelines, spillways and drainage channels). This proposal seeks funds (fieldwork travel and subsistence costs, technical support and laboratory analytical expenses) to collect and date samples of glacial lake sediments from around the margins of the former glacial lakes using a dating technique known as single grain Optically Stimulated Luminescence (OSL). We have conducted a proof-of-concept study using two samples that we collected in 2006 from around the shores of the former glacial lakes. These samples indicate that OSL dating provides an excellent means of dating glacial lake sediments on sub-millenial timescales. Obtaining dates for the growth and drainage of the lakes will enable us to determine the timing of periods of glacier expansion and contraction because the lake filled at times when glaciers advanced (damming the lake) and the lake emptied when glaciers receded (allowing the lake to drain freely). Combining geomorphology with OSL dating therefore provides a novel method of reconstructing the recession of the Patagonian Ice Sheet in this area. This is important because the nature and timing of this deglaciation is controversial and because its synchoneity (or otherwise) with deglacial events in the Northern Hemisphere is debated. By dating the lake drainage events we will also be able to calculate the meltwater volumes and fluxes during each of the major drainage events.
 
Description We carried out extensive fieldwork on James Ross Island to map and analyse the changes to a glacier, which is currently 4km long, over the past 10,000 years. We used a combination of glacier and climate modelling, glacial geology and ice-core data. We found that small glaciers that end on land around the Antarctic Peninsula are highly vulnerable to slight changes in air temperature. Over the next few decades, they will be smaller than at any point during the last 10,000 years. Just small increases in air temperature increased melting so much that even large amounts of extra snowfall could not prevent glacier recession. These small glaciers around the Antarctic Peninsula are likely to contribute most to rising sea levels over the coming decades, because they can respond quickly to climate change.

We also analysed the topography, geomorphology and sedimentology of prominent moraines on James Ross Island, Antarctica, to determine geometric changes and to interpret glacier behaviour. The moraines are very likely due to a late-Holocene phase of advance and featured (1) shearing and thrusting within the snout, (2) shearing and deformation of basal sediment, (3) more supraglacial debris than at present and (4) short distances of sediment transport. Retreat of ~100 m and thinning of 15-20 m has produced a loss of 0.1 km3 of ice. The pattern of surface lowering is asymmetric. These geometrical changes are suggested most simply to be due to a net negative mass balance caused by a drier climate. Comparisons of the moraines with the current glaciological surface structure of the glaciers permits speculation of a transition from a polythermal to a cold-based thermal regime. Small land-terminating glaciers in the northern Antarctic Peninsula region could be cooling despite a warming climate.

We also used cosmogenic isotope exposure-age dating (26Al, 10Be and 36Cl) of erratic boulders on ice-free land on James Ross Island, north-eastern Antarctic Peninsula, to define the evolution of Last Glacial Maximum (LGM) ice in the adjacent Prince Gustav Channel. These data include ice-sheet extent, thickness and dynamical behaviour. Prior to ~18 ka, the LGM Antarctic Peninsula Ice Sheet extended to the continental shelf-edge and transported erratic boulders onto high-elevation mesas on James Ross Island. After ~18 ka there was a period of rapid ice-sheet surface-lowering, coincident with the initiation of the Prince Gustav Ice Stream. This timing coincided with rapid increases in atmospheric temperature and eustatic sea-level rise around the Antarctic Peninsula. Collectively, these data provide evidence for a transition from a thick, cold-based LGM Antarctic Peninsula Ice Sheet to a thinner, partially warm-based ice sheet during deglaciation.
Exploitation Route Important information about how glaciers around the Antarctic Peninsula are likely to contribute to rising sea levels over the coming decades. Information will be useful to Government, planners, and policy-makers concerned with predicting future sea-level rise.
Sectors Environment
URL http://www.antarcticglaciers.org/glaciers-and-climate/numerical-ice-sheet-models/glacier-response-to-climate-change/