Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica

Abstract

Abstract. Observations at the West Antarctic Ice Sheet (WAIS) Divide site show that near-surface snow is strongly altered by weather-related processes such as strong winds and temperature fluctuations, producing features that are recognizable in the deep ice core. Prominent "glazed" surface crusts develop frequently at the site during summer seasons. Surface, snow pit, and ice core observations made in this study during summer field seasons from 2008-2009 to 2012-2013, supplemented by automated weather station (AWS) data with short-and longwave radiation sensors, revealed that such crusts formed during relatively low-wind, lowhumidity, clear-sky periods with intense daytime sunshine. After formation, such glazed surfaces typically developed cracks in a polygonal pattern likely from thermal contraction at night. Cracking was commonest when several clear days occurred in succession and was generally followed by surface hoar growth; vapor escaping through the cracks during sunny days may have contributed to the high humidity that favored nighttime formation of surface hoar. Temperature and radiation observations show that daytime solar heating often warmed the near-surface snow above the air temperature, contributing to upward mass transfer, favoring crust formation from below, and then surface hoar formation. A simple surface energy calculation supports this observation. Subsequent examination of the WDC06A deep ice core revealed that crusts are preserved through the bubbly ice, and some occur in snow accumulated during winters, although not as commonly as in summertime deposits. Although no one has been on site to observe crust formation during winter, it may be favored by greater wintertime wind packing from stronger peak winds, high temperatures and steep temperature gradients from rapid midwinter warmings reaching as high as −15 • C, and perhaps longer intervals of surface stability. Time variations in crust occurrence in the core may provide paleoclimatic information, although additional studies are required. Discontinuity and cracking of crusts likely explain why crusts do not produce significant anomalies in other paleoclimatic records.

Spatial Distribution of Crusts in Antarctic and Greenland Snowpacks and Implications for Snow and Firn Studies

“…Glazed surfaces have low porosity, can be one to several millimeters thick and form throughout the year, but according to the case study by Fegyveresi et al (2018) primarily in summer. Glazed surfaces are reported to form after an event with high wind speed and eventually accumulation, followed by clear-sky conditions, low wind speed and higher atmospheric pressure enhancing the possibility of water vapor transport vertically upwards and the formation of subsurface hoar (Albert et al, 2004;Fegyveresi et al, 2018). They can have an extent of up to 200 km 2 and are often found in leeward slopes of large-scale dunes formed by katabatic winds (Scambos et al, 2012).…”

“…Directly at the snow surface formation conditions for glazed surfaces on the West Antarctic Ice Sheet Divide (79°27.78′S, 112°7.51′W) have been monitored by Fegyveresi et al (2018). Glazed surfaces have low porosity, can be one to several millimeters thick and form throughout the year, but according to the case study by Fegyveresi et al (2018) primarily in summer.…”

“…Directly at the snow surface formation conditions for glazed surfaces on the West Antarctic Ice Sheet Divide (79°27.78′S, 112°7.51′W) have been monitored by Fegyveresi et al (2018). Glazed surfaces have low porosity, can be one to several millimeters thick and form throughout the year, but according to the case study by Fegyveresi et al (2018) primarily in summer. Glazed surfaces are reported to form after an event with high wind speed and eventually accumulation, followed by clear-sky conditions, low wind speed and higher atmospheric pressure enhancing the possibility of water vapor transport vertically upwards and the formation of subsurface hoar (Albert et al, 2004;Fegyveresi et al, 2018).…”

Microstructure of Snow and Its Link to Trace Elements and Isotopic Composition at Kohnen Station, Dronning Maud Land, Antarctica

“…When snow surfaces are exposed to the atmosphere including wind polishing and insolation, thin crusts and glazed surfaces of high density form (e.g. Koerner, 1971;Fegyveresi et al, 2018).…”

Very old firn air linked to strong density layering at Styx Glacier, coastal Victoria Land, East Antarctica

“…The evolution of CH 4 in the closed porosity may give information on how the snow layers can induce inhomogeneous records and help constrain the gas age distribution in ice (Fourteau et al, 2017). However, the details are beyond the scope of this study and we will focus on the firn air age in the open porosity.…”