Using Salt Accumulations and Luminescence Dating to Study the Glacial History of Taylor Valley, Antarctica
Toner, Jonathan Daniel
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Taylor Valley, Antarctica, preserves a record of late Cenozoic Antarctic glaciations. The incursion of the Sea Ice Sheet (RSIS) into lower Taylor Valley during the Last Glacial Maximum is believed to have dammed large proglacial paleolakes. These inundation events are studied by analyzing soluble salt distributions along elevation transects. To better interpret the salt record, factors controlling salt composition and concentration are studied in detail in a soil sampled to 2.1 m depth. In addition, the deposition age of fluvial sediments and possible deltas are dated using luminescence of quartz and feldspar minerals. Soluble salts in Taylor Valley vary with distance from McMurdo Sound. In western Taylor Valley, soluble salts are high and are similar to seawater. In eastern Taylor Valley, soluble salts are low and are comprised primarily of Na-HCO<sub>3</sub>. Soluble salt distributions indicate that during the Last Glacial Maximum, the RSIS filled eastern Taylor Valley, thereby damming paleolakes in western Taylor Valley to approximately 300 m elevation. As the RSIS retreated, smaller paleolakes formed in eastern Taylor Valley up to about 120 m elevation. Na-HCO<sub>3</sub> salts in eastern Taylor Valley derive from calcite dissolution and cation exchange reactions as water percolates into soils. This mechanism of Na-HCO<sub>3</sub> formation predicts that Ca-Cl will evolve as a byproduct. Reactive transport at freezing temperatures was modeled using the modified versions of the geochemical speciation programs PHREEQC and FREZCHEM. This model reveals that cation exchange reactions may account for the Ca-Cl enrichment often found in Dry Valley groundwaters, including the enigmatic Don Juan Pond. To complement the soluble salt record, fluvial terraces in Taylor Valley were dated using both quartz and feldspar luminescence. Quartz and feldspar luminescence results are consistent and indicate that terraces in eastern Taylor Valley are between 4 to 10 Ka old. These luminescence ages are 5 to 12 Ka younger than published <super>14</super>C dates. It is suggested that the <super>14</super>C ages date older glaciolacustrine sediments underlying fluvial terrace sediments and reflect a deglaciation record as the RSIS retreated from Taylor Valley. The luminescence ages indicate that terraces were deposited during warmer climatic conditions that followed deglaciation.