Twentieth century climate change influences isotopic fractionation in Victoria Land ice cores, Antarctica

Abstract

In this thesis, correlations amongst oxygen and hydrogen isotope (δ18O and δD) chronologies from coastal Antarctic ice cores and climatic time-series are used to infer climatic influences on isotopic fractionation in Victoria Land precipitation. New δ18O and δD measurements from the Victoria Lower Glacier (VLG) ice core and existing isotopic records from the Newall Glacier, Hercules Neve and Talos Dome ice cores are jointly examined to assess trends in Victoria Land isotopic variability. The isotopic records are also compared with variations in atmospheric greenhouse gas induced temperature change, solar irradiance, volcanic aerosols, the Southern Oscillation Index (SOI) and the Antarctic Oscillation Index to investigate possible climatic influences on isotopic fractionation. During the austral summer of 2001/2002 a 182.4 m ice core was acquired from the VLG of the Dry Valleys. Measuring the δ18O, δD and tritium content of ice samples from the upper 8.5 m of the ice core, tuning δ18O variations to Scott Base mean summer temperature and adopting the decompaction age model of Bertler (2003) enabled the construction of Twentieth Century δ18O, δD and deuterium excess chronologies. δ18O and δD variability in the four Victoria Land ice cores was visually compared by plotting the time-series on a common graph with the opposing δ18O and δD axes aligned in accordance with the VLG meteoric water line (δD = 7.94 x δ18O - 0.38). These data were also compared to climate variability by using a qualitative visual approach, correlation coefficient calculations and spectral analyses. Coherent variability in the isotopic and climatic time-series was generally interpreted as indicating a natural association between the climate-forcing function of interest (e.g. SOI) and the local climate response (e.g. proxy such as δ18O). The coherent associations were then explored in more detail. The El Niño—Southern Oscillation (ENSO) and solar radiation flux and were found to be the main influences on isotopic variability. Volcanic activity and the Antarctic Oscillation may also have been contributors to climate change, but further research is needed for confirmation. Radiative forcing resulting from greenhouse gas emissions was found to have no apparent affect on Victoria Land climate dynamics. This suggests that the influence of global warming associated with greenhouse gas emissions was suppressed during the second half of the Twentieth Century as a result of regional cooling associated with an increase in El Niño phases of the ENSO (i.e. negative SOI values). Solar irradiance appears to influence δ18O and δD variability in Victoria Land ice cores on Schwabe cycle (i.e. 11 yr cycle) and sun melody (i.e. cycles of ≥ 14 yrs) frequencies. The average power of the Dry Valleys δ18O Schwabe cycle harmonics are nearly three times greater than those of northern Victoria Land harmonics indicating regional contrasts in solar forcing amplitude. The low albedo of rocky terrains may amplify solar radiative forcing in the Dry Valleys, while the the high albedo of ice covered terrains may suppress solar radiative forcing in northern Victoria Land. The sun melody appears to covary with VLG δ18O with isotopic diffusion occurring during the interval of heightened solar irradiance (c. 1920 to 1955). The ENSO contributes a high frequency variability (i.e. cycles of 2 to 7 yrs) in the δ18O, δD and deuterium excess records. Precipitation moisture source localities and transport pathways migrate under ENSO's influence, promoting changes in temperature and humidity during transport to and precipitation in Victoria Land. During La Niño phases when the Amundsen Sea Low (ASL) is north of the Ross Ice Shelf, warm and moist air moves across the Ross Sea to the ice core sites producing higher δ18O and δD and lower deuterium excess in the ice core records. In contrast, during some El Niño phases when the ASL is north of Marie Byrd Land above the Amundsen Sea, cold and dry air ascends the West Antarctic Ice Sheet prior to moisture precipitation at the ice core sites. This effectively results in a lower δ18O and δD and a higher deuterium excess content in the glacier ice.