Extending the varve record for Central Patagonia: Assessing glacier dynamics and responses during the last deglaciation

Understanding the mechanisms controlling ice-sheet deglaciation requires detailed reconstructions of glacier retreat, incorporating glacial landforms, sedimentology, and geochronology. Patagonia, in southern South America, has a suite of exceptionally preserved glacial landforms and is directly affected by the precipitation-bearing Southern Westerly Winds, making it one of the most climatically sensitive areas in the world. However, due to combination of a lack of high-resolution geomorphological mapping, and the use of geochronological dating techniques with low precision (̈±10%), reconstructions have been precluded from adequately testing models of regional environmental change as
well as changes in the behaviour of the Patagonian Ice sheet at millennial, centennial, and decadal timescales. High resolution sedimentary archives, such as glaciolacustrine varve sequences, offer the potential for detailed reconstructions of past ice-sheet dynamics. Specifically, glaciolacustrine varved sediments can be utilised to estimate at an annual resolution, the rates and/or duration of change during deglaciation within a specific basin.
This thesis presents detailed geomorphological mapping of the Los Antiguos sector and the Jeinemeni valley improving the spatio-temporal reconstruction of ice-lobe retreat and palaeolake evolution. This was then used to target two key sites, Chakra el Pedregoso and Chile Chico, which contained glaciolacustrine varve sediments. Through detailed sedimentological analysis two independent, site specific, master varve chronologies were developed, creating a ~3100-varve year composite chronology. The identification of tephra within these sites anchors the records to the calendar-year timescale between 16.3-13.2 ka cal BP. Using a Bayesian framework to integrate varve counts, tephra ages and published geochronologies, a ~5200-year master varve chronology for Lago General Carrera/Buenos Aires is presented between 18486 ± 328-13243 ± 162 cal yr BP. This record details the onset of deglaciation and critically provides a revised age for drainage of the Deseado to Bayo lake level and a major Atlantic-Pacific drainage reversal between 14419-14359 ± 114 cal yr BP. This extended record also allows the assessment of palaeoclimatic forcing mechanisms of glacier dynamics, identifying an initial phased-locked, synchronous warming between Antarctica and the LGCBA basin is shown to disintegrate when the LGCBA ice-lobe becomes dominantly lake-terminating with an active calving ice margin.