Dissertation Abstract
Atmospheric dust deposition in West Antarctica over the past two millennia
Koffman, Bess G 2013 besskoffman.weebly.com
School of Earth and Climate Sciences, University of Maine (United States), 192 pp.
I develop and interpret a late Holocene record of dust deposition from the West Antarctic Ice Sheet (WAIS) Divide deep ice core in order to reconstruct past changes in atmospheric circulation. The WAIS Divide core was collected from a high-accumulation (0.2 m water equivalent per year) interior site (79.468 deg. S, 112.086 deg. W) with annually resolvable layering through the Holocene. My approach combines continuous and discrete physical and geochemical analyses of surface snow and ice samples from the upper 577 m (2400 years) of the core.
Results from an experiment testing common glaciochemical methods show that acidification strength and time significantly increase trace elemental concentrations leached from impurities in snow/ice. Continuous measurements reveal elevated microparticle concentrations associated with acidity peaks for the Unknown (1258 C.E.), Kuwae (1458 C.E.) and Tambora (1815) volcanic eruptions. Ash particles from explosive tropical eruptions have particle size distributions (PSDs) 0.6 to 1.5 micro-m finer than the background atmospheric dust deposited at this site, and are deposited about 3-6 months earlier than sulfate aerosols. In contrast, particles from the Buckle Island, Antarctica (1839 C.E.) eruption produced a PSD greater than 5 micro-m coarser-than-background. These observations may be used to infer the relative latitude and/or magnitude of unknown eruptions as measured in polar ice cores, and therefore their potential impact on global climate.
Over the past two millennia, the background dust flux remained around ca. 4 mg per square meter per year with a modal particle diameter of 5 to 8 micro-m. West Antarctic dust flux and modal diameter are both higher than in central East Antarctica, but comparable to lower-elevation and coastal sites, suggesting a strong local dust emissions influence. The coarse particle percentage, a measure of the dust PSD, shows significant positive correlations with mid-latitude zonal wind speeds (r equals 0.4 to 0.5; p less than 0.1). Through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, I infer that the SH westerly wind belt occupied a more southerly position during the Medieval Climate Anomaly (ca. 950-1350 C.E.), and shifted to a more northerly position at the onset of the Little Ice Age (ca. 1430 C.E.) due to cooler surface temperatures and a contraction of the SH Hadley cell.
Results from an experiment testing common glaciochemical methods show that acidification strength and time significantly increase trace elemental concentrations leached from impurities in snow/ice. Continuous measurements reveal elevated microparticle concentrations associated with acidity peaks for the Unknown (1258 C.E.), Kuwae (1458 C.E.) and Tambora (1815) volcanic eruptions. Ash particles from explosive tropical eruptions have particle size distributions (PSDs) 0.6 to 1.5 micro-m finer than the background atmospheric dust deposited at this site, and are deposited about 3-6 months earlier than sulfate aerosols. In contrast, particles from the Buckle Island, Antarctica (1839 C.E.) eruption produced a PSD greater than 5 micro-m coarser-than-background. These observations may be used to infer the relative latitude and/or magnitude of unknown eruptions as measured in polar ice cores, and therefore their potential impact on global climate.
Over the past two millennia, the background dust flux remained around ca. 4 mg per square meter per year with a modal particle diameter of 5 to 8 micro-m. West Antarctic dust flux and modal diameter are both higher than in central East Antarctica, but comparable to lower-elevation and coastal sites, suggesting a strong local dust emissions influence. The coarse particle percentage, a measure of the dust PSD, shows significant positive correlations with mid-latitude zonal wind speeds (r equals 0.4 to 0.5; p less than 0.1). Through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, I infer that the SH westerly wind belt occupied a more southerly position during the Medieval Climate Anomaly (ca. 950-1350 C.E.), and shifted to a more northerly position at the onset of the Little Ice Age (ca. 1430 C.E.) due to cooler surface temperatures and a contraction of the SH Hadley cell.