Seasonal influences on firn layering and bubble trapping, WAIS Divide, West Antarctica

Deposition and near-surface diagenesis on ice sheets create layering in firn that may influence the climatic record and associated proxy data preserved in ice cores, including the number-density and volume of trapped bubbles. Diurnal and seasonal variations, as well as the timing of accumulation in polar regions, affects the near-surface firn through changes in the exposure to various meteorological, atmospheric, and turbulent fluxes. These changes can result in greater surface metamorphism, evaporation, sublimation, convection in the snow pack, surface crusting, and more pronounced grain growth. At sites with relatively high accumulation rates and numerous individual depositional events per year, such as WAIS Divide, these near-surface processes can result in highly layered, irregularly-stratified firn with numerous internal features and high density contrast. If preserved at depth, this high degree of layering may affect bubble trapping at pore close-off and the resulting ice-core proxy data. Here we have been investigating recently obtained data from the WDC06A ice core, in order to discern evidence of these possible seasonal biases. Real-time snowpit, surface, meteorological and net-solar energy data from three concurrent WAIS Divide field seasons have been analyzed in order to better quantify the conditions and net-surface energy flux surrounding near-surface metamorphism. Distinct and pronounced surface crusts, or "glazes", are observed frequently at the site. These noteworthy features, and the processes which lead to their formation, are not yet well understood. Observations indicate that these glazes and their associated surface cracks and polygons are more likely to form during relative low-wind, low-humidity, high-temperature episodes. During each documented case, these episodes were brought about by clear-sky days with maximum diurnal variability of incoming solar energy. Furthermore, distinct hoar frost growth was observed on crusts that were exposed to multiple clear-sky days. Observations in the ice core suggest that the pronounced surface crusts also form in wintertime, but are more common in summer. Measurements to assess possible effects of these features on bubble number-density, total trapped air, and grain size and orientation in the core are ongoing.