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

Abstract

The occurrence of snowpack features has been used in the past to classify environmental regimes on the polar ice sheets. Among these features are thin crusts with high density, which contribute to firn stratigraphy and can have significant impact on firn ventilation as well as on remotely inferred properties like accumulation rate or surface mass balance. The importance of crusts in polar snowpack has been acknowledged, but nonetheless little is known about their large-scale distribution. From snow profiles measured by means of microfocus X-ray computer tomography we created a unique dataset showing the spatial distribution of crusts in snow on the East Antarctic Plateau as well as in northern Greenland including a measure for their local variability. With this method, we are able to find also weak and oblique crusts, to count their frequency of occurrence and to measure the high-resolution density. Crusts are local features with a small spatial extent in the range of tens of meters. From several profiles per sampling site we are able to show a decreasing number of crusts in surface snow along a traverse on the East Antarctic Plateau. Combining samples from Antarctica and Greenland with a wide range of annual accumulation rate, we find a positive correlation (R2 = 0.89) between the logarithmic accumulation rate and crusts per annual layer in surface snow. By counting crusts in two Antarctic firn cores, we can show the preservation of crusts with depth and discuss their temporal variability as well as the sensitivity to accumulation rate. In local applications we test the robustness of crusts as a seasonal proxy in comparison to chemical records like impurities or stable water isotopes. While in regions with high accumulation rates the occurrence of crusts shows signs of seasonality, in low accumulation areas dating of the snowpack should be done using a combination of volumetric and stratigraphic elements. Our data can bring new insights for the study of firn permeability, improving of remote sensing signals or the development of new proxies in snow and firn core research.

Identifying atmospheric processes favouring the formation of bubble free layers in Law Dome ice core, East Antarctica

“…The seasonality of BFLs indicates that at the Law Dome site, the majority of BFLs occur in autumn and winter. This contrasts to the findings of Fegyveresi et al (2018) and Weinhart et al (2021) working at the WAIS Divide and Greenland ice core sites, respectively, where BFLs predominantly occurred in summer (WAIS) or late summer and autumn (Greenland). This seasonality suggests that BFLs may be related to regional climate variability, rather than local surface processes.…”

“…The third type of BFLs are crusts. Crusts in ice are also characterised by thin ice layers (typically 0.5-1 mm) without bubbles and are found in all seasons on both the Greenland and Antarctic ice sheets (Fegyveresi et al, 2018;Fitzpatrick et al, 2014;Weinhart et al, 2021). Most studies suggest that crusts at the snow surface can be generated via snow metamorphism, controlled by more than one factor, including solar radiation, wind, snowfall, humidity and temperature.…”

“…Further, based on the observation of glazed surface formation, Albert Divide site and suggested that the frequency and thickness of BFLs in the WAIS deep ice core could be used as a proxy for the occurrence of temperature inversions which can enhance firn metamorphism (Fitzpatrick et al, 2014;Fegyveresi et al, 2018). In addition, the crusts from three different Greenland and Antarctic sites were analysed by Weinhart et al (2021). They suggest that it is difficult to ascribe a single mechanism for all crust formation, as a number of environmental conditions (solar radiation, humidity, wind, temperature, etc.)…”

Melt in the Greenland EastGRIP ice core reveals Holocene warm events

“…Combining satellite and ice core data to create a melt archive has been done in several studies such as Mote (2007), Keegan et al (2014), or Trusel et al (2018). Melt layers, i.e., bubblefree layers, can easily be confused with wind crusts (see the "Methods" section), which have been studied by Fegyveresi et al (2018) and Weinhart et al (2021).…”

Ice Sheet Surface and Subsurface Melt Water Discrimination Using Multi‐Frequency Microwave Radiometry

“…The parameterization of the energy balance model was calibrated using the firn pit LWC observations. It was noted in this process that the pits used had a significant variability with respect to each other that was taken as an indication of the expected small‐scale lateral variability in ice‐layer stratigraphy and density in polar firn (e.g., Marchenko et al., 2016; Weinhart et al., 2021). Given the coarse multi‐kilometer sampling footprint of the satellite microwave radiometric measurements, we assumed here that the model provides a general landscape representation consistent with the regional meteorology, surface energy balance, and snow conditions.…”