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

Abstract

Understanding the deposition history and signal formation in ice cores from polar ice sheets is fundamental for the interpretation of paleoclimate reconstruction based on climate proxies. Polar surface snow responds to environmental changes on a seasonal time scale by snow metamorphism, displayed in the snow microstructure and archived in the snowpack. However, the seasonality of snow metamorphism and accumulation rate is poorly constrained for low-accumulation regions, such as the East Antarctic Plateau. Here, we apply core-scale microfocus X-ray computer tomography to continuously measure snow microstructure of a 3-m deep snow core from Kohnen Station, Antarctica. We compare the derived microstructural properties to discretely measured trace components and stable water isotopes, commonly used as climate proxies. Temperature and snow height data from an automatic weather station are used for further constraints. Dating of the snow profile by combining non-sea-salt sulfate and density crusts reveals a seasonal pattern in the geometrical anisotropy. Considering seasonally varying temperature-gradient metamorphism in the surface snow and the timing of the anisotropy pattern observed in the snow profile, we propose the anisotropy to display the deposition history of the site. An annually varying fraction of deposition during summer months, ranging from no or negative deposition to large deposition events, leads to the observed microstructure and affects trace components as well as stable water isotopes.

On the Birth of Structural and Crystallographic Fabric Signals in Polar Snow: A Case Study From the EastGRIP Snowpack

“…A key finding of the present study is a clear seasonal signal in the structural fabric (anisotropy) in the first three meters of snow at the EastGRIP site. This is in agreement with observations in the snowpack at Kohnen, Antarctica, that were recently published by Moser et al (2020). The seasonal variations in the structural anisotropy emerge as a characteristic, asymmetric saw-tooth signal ( Figure 7D) where the timing suggests that the steep edge is imprinted into the profile in early summer.…”

“…These variations become more apparent when the data is smoothed (Gaussian filter with σ = 14 cm black line in Figure 7D): the structural anisotropy repeatedly shows abrupt increases followed by areas of gentle decrease with depth. The anisotropy signal can be used to define seasonal markers (Moser et al, 2020). To this end vertical dashed lines have been manually added at the locations of steepest increase of the anisotropy with depth.…”

“…This was attributed to the stronger summer-to-autumn metamorphism causing the development of vertically oriented ice structures layers that are mechanically stiffer and less sensitive to densification in contrast to the less metamorphosed, more isotropic winter layers. Very recently, Moser et al (2020) analyzed the structural anisotropy of snow from Kohnen Station and reported the emergence of a seasonal pattern in the anisotropy that was attributed to the impact of a stronger daily temperature gradient and higher temperatures during the transition from summer to autumn. Comparatively, density signal measured in isolated firn cores is strongly perturbed by stratigraphic noise.…”

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

“…On the plateau of the Antarctic ice sheet, sulphate (SO 4 2− ) as well as Cl − /Na + can be interpreted as seasonal summer to late summer signals (Weller and Wagenbach, 2007) and also NH 4 + , calcium (Ca 2+ ) and Na + have been used successfully as seasonal markers in DML ice cores for the past 2 ka (Sommer et al, 2000). At Kohnen Station (Figure 1), a link of crusts to seasonal anisotropy has been proposed (Moser et al, 2020). But so far, there has been no study backing crusts as a solid summer proxy in low accumulation areas.…”

“…We test whether a correlation between the accumulation rate and crust concentration exists and compare our data with environmental parameters like wind speed. In a second step, we follow pilot studies (Ren et al, 2004;Hoshina et al, 2014;Moser et al, 2020) to investigate the link between crusts and chemical proxies in snow. In this context we introduce a cutting device specifically made for snow profiles.…”