Stable Isotope, Major and Trace Element Chemistry of Modern Snow from Evans Piedmont Glacier, Antarctica: Insights into Potential Source Regions and Relationship of Glaciochemistry to Atmospheric Circulation and Vigour
Authors:   , Bull Julia Ruth 
Published: November 2021

<p>This thesis presents a sub-seasonally resolved, decade long record of snow pack chemistry from Evans Piedmont Glacier (EPG), southern Victoria Land coast, Antarctica. Snow chemistry measurements were made at ca. 20 analyses per year for stable isotope ratios [delta to the power of 18]O and [delta]D, major ions Ca+, Cl-, K+, Mg+, MS-, Na+, NO3-, SO42- by ion chromatography (IC), and major and trace element chemistry by inductively coupled plasma mass spectrometry (ICP-MS). Na, Mg, Al, Fe, Mn and Ba were measured by ICP-MS using a hydrogen flushed collision cell to reduce the formation of polyatomic ion interferences, whereas Ti, V, Cr, Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Sb, Cs, Ba, La, Ce, Pb, Bi, Th and U were measured in non-collision cell mode to increase count sensitivity. ICP-MS analytical precision is typically 5 to 10 % (2 rsd) that is two orders of magnitude at minimum below natural variability (e.g. samples range between Na = 10 to 18031 ppb and Al = 5 to 3856 ppb). The presence of undigested mineral dusts in weakly acidified samples, however, complicates the measurement of elemental concentrations in snow samples by randomly entering the ICP-MS. Despite this, the range of sample concentrations (Zr = 3.0 to 5630 ppb) is still orders of magnitude higher than sample reproducibility. The dominant source regions of element chemistry transported to EPG snow are identified as marine (Na, Mg, SO4, Cl, K, As and Sr) and terrestrial derived aerosol (Al, Mn, Fe, Ba, Ti, V, Ni, Cr, Zn, Rb, Y, Zr, Cd, Sb, Cs, Ba, La, Ce, Pb, Th and U), with minor contributions from anthropogenic (V, Cr, Ni, Cu, Zn, As, Sb and Pb) and volcanic emissions (Bi, SO4 and K). This is based on both elemental ratio modelling and ICP-MS time resolved analysis that identifies elements present in particulate form (mineral dusts). EPG snow chemistry is related to measured meteorological conditions at nearby Cape Ross. Winter maxima of elemental concentrations is consistent with maximum winter wind speed and low precipitation rates. Furthermore, winter snow samples that are depleted in SO42- relative to other marine derived elements (e.g. Na), indicate the sea ice surface is an important source of marine aerosol transported to EPG in addition to an open ocean source. Annual maximum chemistry concentrations of terrestrial derived elements (e.g. Zr) are significantly correlated to maximum annual wind speed measured at Cape Ross (r2 = 0.68, p< 0.01). Lower correlation of marine derived chemistry (e.g. Na) and maximum wind strength reflects additional controls of source region and other meteorological parameters such as storm duration on marine derived chemistry. In contrast to elemental concentrations, elemental ratios are less sensitive to extreme wind conditions. Rather elemental ratios provide a more robust signature of changes in mean atmospheric circulation related to delivery of aerosol from different source regions and via different transport fractionation processes. Al/Na is controlled by variable delivery of terrestrial (Al) and marine (Na) aerosol to EPG, although the longer term trend is driven primarily by changes in Na. Al/Na is significantly higher between winter 2000 and summer 2006/07 with a mean value of Al/Na = 0.15 compared to Al/Na = 0.02 prior to 2000. Although sea ice extent was highly variable over this time period, there is no clear relationship between Al/Na and sea ice. Rather, Al/Na is significantly correlated to mean summer wind speed measured at Cape Ross (r2 = -0.51, p<0.01). This demonstrates the sensitivity of Al/Na to changes in the average transport of marine aerosol to EPG during summer, when an open ocean source is most proximal. The shift in Al/Na is also concurrent with a shift in the relationship between [delta]18O and d excess, indicative of a changing precipitation source region to EPG. The observed changes in EPG chemistry are concurrent with shifts in mean Southern Oscillation Index (SOI), a measure of the El Nino Southern Oscillation (ENSO) strength and polarity. Al/Na is low when SOI is predominantly negative (El Nino), associated with increased summer wind strength. This is in accordance with a strong Amundsen Sea Low, positioned directly north of the Ross Sea as previously reported during El Nino years. Although the establishment of a statistically significant relationship between SOI and EPG Al/Na ratios is inhibited by the brevity of this record, this study highlights the potential for the 180 m firn core also extracted from EPG to track long-term changes in SOI. Elemental chemistry of EPG provides a high resolution tool to reconstruct atmospheric circulation changes within the southern Ross Sea region.</p>