University Of Tasmania
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Investigation of meteorological events preserved in high resolution snow pit and fin core records

posted on 2023-05-26, 03:48 authored by McMorrow, AJ
A key problem in ice core palaeclimate studies is the interpretation of the various measurable parameters in ice in terms of climate and environmental conditions. This study is aimed at developing a closer understanding of the\ connection between high resolution snow/firn measurements and meteorological conditions. Ultra high resolution\ snow pit and shallow firn core records of oxygen isotope ratios (del18O) and a suite of trace chemical species including marine biogenic sulphur compounds (methane sulphonate (MSA), non-sea salt sulphate), nitrate and major sea salt\ ions (sodium, chloride, magnesium), were generated at a high accumulation site on Law Dome, East Antarctica.\ Concordance between accumulation events identified in the records up to 7.7 km apart confirms that the observed\ chemical and isotopic variations are the result of regional rather than local surface effects. This allows calibration of\ the snow pit and firn core records with measured meteorological parameters.\ Event scale dating of the records was established using hourly snow accumulation measurements from a co-located\ automatic weather station (AWS). The ultra high resolution nature of this study and independent dating scale provide\ an opportunity to examine exact timings in the seasonality of each chemical species. The traditional summermaximum\ species of del18O and MSA show consistent relative phasing during mid-summer over four annual cycles.\ Nitrate shows an erratic seasonal cycle with a general trend characterised by narrow peaks during spring and early\ summer, preceding the mid-summer peaks in del18O and MSA. Non-sea salt sulphate cycles indicate similar\ characteristics to MSA signals during summer, but are more comparable to nitrate signals during spring, autumn and\ winter. This suggests the summer non-sea salt sulphate signal is driven by biological activity, yet appears to be linked\ with nitrate signals outside the summer season. Finally, the sea salt species indicate a seasonal cycle characterised by\ maximum concentrations during autumn, winter and spring.\ Event scale dating of the snow pit and firn core records allows direct comparisons between the chemical and isotopic\ signals and observed meteorological conditions. Local meteorological conditions recorded by the AWS are combined\ with synoptic scale meteorology derived from Advanced Very High Resolution Radiometer satellite imagery and\ back trajectory analysis to identify potential source regions and transport mechanisms influencing the chemical and\ isotopic signals. Potential source regions and transport mechanisms are examined for the marine biogenic indicators\ (MSA, non-sea salt sulphate). Results indicate that the seasonal variation in marine biogenic activity is reflected in the\ Law Dome records, and the sea ice zone provides an important source region. However, results also indicate that\ lower latitudes, and the Heard Island region (50 degrees S, 70 degrees E) in particular, may provide an important additional source\ region for MSA and non-sea salt sulphate outside the summer season. High sea salt signals are generally associated\ with intense cyclonic systems, yet variations in atmospheric circulation and transport mechanisms also impact on the\ sea salt record. Comparisons between del18O signals and local air temperatures reveal the del18O record is an excellent\ proxy for temperature at Law Dome, although high (warm) del18O events are found to be influenced by atmospheric\ circulation and associated with rapid advection of air from low latitudes. Finally, results suggest that spring nitrate\ signals at Law Dome may be linked to the intrusion of stratospheric air through the breakdown in the polar vortex\ during spring.


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