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, low-humidity, 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.

Highlights

  • Visual and thin-section examination of the West Antarctic Ice Sheet (WAIS) Divide deep ice core revealed an annual signal linked to bubble and grain characteristics (Fitzpatrick et al, 2014) and numerous crusts

  • Our results yield an average vertical temperature difference of ∼ 3.6 ± 0.7 ◦C over the ∼ 24 h low-wind glaze period highlighted in Fig. 8, or a gradient of ∼ 18 ◦C m−1. This result is consistent with the PRD string data, which indicate a temperature difference between the near-surface air sensor (AWS) and the shallowest in-snow sensor (S1, 20 cm depth) of ∼ 3.0 ◦C during the peak of the inversion and glazing episode on 24 December 2012; the near-surface air sensor was ∼ 1 m above the surface, and we infer that, as a result, it somewhat underestimated peak T

  • The great abundance of crusts at WAIS Divide compared to other ice cores we have studied may be because conditions are “just right” at WAIS Divide

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Summary

Introduction

Visual and thin-section examination of the West Antarctic Ice Sheet (WAIS) Divide deep ice core revealed an annual signal linked to bubble and grain characteristics (Fitzpatrick et al, 2014) and numerous crusts. Fegyveresi et al.: Surface formation, preservation, and history of low-porosity crusts bly ice (Fig. 1). Their presence in greater abundance than seen in most cores (e.g., Alley, 1988) motivated studies to understand their formation, possible influence on other paleoclimatic data, and potential for recording paleoclimatic conditions themselves

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