Geology

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    Northwest Greenland Active Source Seismic Experiment
    (2021) Schmerr, Nicholas; Maguire, Ross; Pettit, Erin; Riverman, Kiya; Gardner, Christyna; DellaGiustina, Daniella; Avenson, Brad; Wagner, Natalie; Marusiak, Angela; Habib, Namrah; Broadbeck, Juliette; Bray, Veronica; Bailey, Samuel; Carr, Christina; Dahl, Peter; Weber, Renee
    In summer of 2018, the Seismometer to Investigate Ice and Ocean Structure (SIIOS) team conducted a geophysical field investigation on the Greenland ice sheet in northwestern Greenland at a location where a previous airborne radar survey by Palmer et al. (2013) had detected the signatures of a subglacial lake. The field site is located approximately 50 km north of the town of Qaanaaq. This site was chosen for the SIIOS project as it provides an opportunity for studying how a lander station could be used to detect subsurface water at an icy-ocean world. The purpose of the investigation was to confirm the presence of the subglacial lake and to measure its physical properties such as seismic impedance, as well as to estimate its depth and volume. One component of the investigation consisted of an active source seismic survey that was used to create a reflection image of the lake, as well as to measure the ice-bottom reflection coefficient. The survey was conducted along a roughly northeast oriented traverse, which started above the subglacial lake and crossed the lake’s eastern boundary.
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    Using Shear Waves to Characterize a Firn Aquifer on the Helheim Glacier in Southeast Greenland
    (2019) Guandique, Jonathan Alexander; Schmerr, Nicholas C.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Greenland ice sheet is melting at an accelerating rate due to increasing global average temperatures. Ice penetrating radar surveys and ice cores extracted from the southeastern margin of the Greenland ice sheet near Helheim glacier discovered that a liquid water aquifer has formed within the near surface recrystallized and compacted snow (firn). Here we use active source seismology to probe the structure of this aquifer in the firn, and present results from a joint inversion technique that uses S-waves, P-waves, and surface waves to constrain the attenuation and seismic velocities that inform on the liquid water stored within the aquifer. Confirming past studies, we find that the aquifer lies at 27.9 +/- 3.5 m and has an approximate thickness of 10 +/- 4 m. We determine there is 1565 +/- 769 kg m-2 of water within the aquifer, a downward revision from past studies. Our study of S-waves and surface waves identified a complex structure in the aquifer layer and future work must incorporate full waveform modeling to use these waves for understanding firn aquifers.
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    DETERMINATION OF SULFUR ISOTOPE COMPOSITION IN SULFATE FROM TWO HIGH ELEVATION SNOWPITS BY MULTI-COLLECTOR THERMAL IONIZATION MASS SPECTROMETRY USING A DOUBLE SPIKE
    (2005-05-12) Mann, Jacqueline Lorraine; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The variability of stable sulfur isotopes in nature provides a chemical tool for tracing the various sources of sulfur and a useful tool for understanding the sulfur cycle. It is also well established that snow and ice preserve a record of the sources, sinks, and processing of sulfur that reflect changes in this cycle through time. Our ability to sample this record is however limited by the total sample concentration and the analytical requirements for isotopic analysis. A high-resolution double spike technique using multi-collector thermal ionization mass spectrometry was developed for stable sulfur isotope composition measurements of small concentration sulfate samples (ppb level). The capability of this new technique was demonstrated by measuring internationally recognized standards of known isotopic composition and by measuring snowpit samples with low sulfate concentrations collected from the Inilchek Glacier, Kyrgyzstan and Summit, Greenland. The elemental and high resolution sulfur isotope data for the snowpit samples were used to calculate the relative seasonal contributions of anthropogenic and natural sulfur sources to sulfate at these high-elevation Northern Hemisphere sites. The isotope composition results for the standards demonstrate the double spike technique to be competitive in accuracy and precision with the traditional methods but the sample requirement is smaller. The average uncertainties on the individual isotope composition measurements for the Inilchek and Summit samples were approximately ± 0.10 (2s) and ± 1.5 (2s), respectively. The larger uncertainties for the Greenland samples resulted from increased blank and the smaller sample size used for analysis. Decreasing the blank concentrations by an order of magnitude show that a factor of two to three improvement in the uncertainties on small sample sizes is attainable with the double spike technique. The sulfur isotope values in the Inilchek snowpit demonstrate no seasonality; while the values observed in the Greenland snowpit exhibit strong seasonality, where the values are 34S-depleted in the winter months and are 34S-enriched in the summer months. Mass balance calculations indicate that anthropogenic sources are the main contributor (75%) to sulfate during most of the year for both locations.