Exploring the role of snow metamorphism on the isotopic composition of the surface snow at EastGRIP

Abstract

Abstract. Stable water isotopes from polar ice cores are invaluable high-resolution climate proxy records. Recent studies have aimed to improve knowledge of how the climate signal is stored in the water isotope record by addressing the influence of post-depositional processes on the surface snow isotopic composition. In this study, the relationship between changes in surface snow microstructure after precipitation/deposition events and water isotopes is explored using measurements of snow specific surface area (SSA). Continuous daily SSA measurements from the East Greenland Ice Core Project site (EastGRIP) situated in the accumulation zone of the Greenland Ice Sheet during the summer seasons of 2017, 2018 and 2019 are used to develop an empirical decay model to describe events of rapid decrease in SSA, driven predominantly by vapour diffusion in the pore space and atmospheric vapour exchange. The SSA decay model is described by the exponential equation SSA(t) = (SSA0 −26.8) e−0.54t + 26.8. The model performance is optimal for daily mean values of surface temperature in the range 0 °C to −25 °C and wind speed < 6 m s−1. The findings from the SSA analysis are used to explore the influence of surface snow metamorphism on altering the isotopic composition of surface snow. It is found that rapid SSA decay events correspond to decreases in d-excess over a 2-day period in 72 % of the samples. Detailed studies using Empirical Orthogonal Function (EOF) analysis revealed a coherence between the dominant mode of variance of SSA and d-excess during periods of low spatial variability of surface snow over the sampling transect, suggesting that processes driving change in SSA also influence d-excess. Our findings highlight the need for future studies to decouple the processes driving surface snow metamorphism in order to quantify the fractionation effect of individual processes on the snow isotopic composition.

A Snapshot on the Buildup of the Stable Water Isotopic Signal in the Upper Snowpack at EastGRIP on the Greenland Ice Sheet

“…For the period between 3 and 17 July, Harris Stuart et al. (2021) analyzed a different isotope data set from the same study site and found a decrease in d‐excess without changes in δ 18 O, similar to our observations (Figure S2 in Supporting Information S1). They also report two SSA decrease events during this period potentially related to post‐depositional processes, such as vapor‐snow exchange.…”

“…Environmental conditions during our observation period were analyzed and described in Harris Stuart et al. (2021), Wahl et al. (2021), and Wahl et al.…”

“…Moreover, Harris Stuart et al. (2021) documented significant changes in the d‐excess signal of the surface snow following precipitation events, effectively masking the initial precipitation isotopic composition. Following these studies, the presented analysis of d‐excess values compared to prevailing environmental conditions suggests that the observed changes in the 2‐week period after 3 July 2019 (Figure 8) is a local signal, likely caused by a sublimation‐induced decrease in d‐excess as shown in numerous laboratory and observational studies (e.g., Casado et al., 2021; Hughes et al., 2021; Madsen et al., 2019; Sokratov & Golubev, 2009).…”

“…Snow stratigraphy (e.g., specific surface area [SSA] and density) and atmosphere‐snow exchange processes have been studied at the same location for the same observation period in Harris Stuart et al. (2021), Hughes et al. (2021), Wahl et al.…”

“…However, in recent years, the basic principle of this paleo‐thermometer has been questioned (e.g., Casado et al., 2021; Town et al., 2008; Steen‐Larsen et al., 2014). Studies show that the original isotopic composition of snow is considerably altered after the initial deposition by molecular exchange processes at the snow‐air interface at and beneath the surface, including wind‐driven ventilation, sublimation, vapor deposition, and snow metamorphism (Harris Stuart et al., 2021; Town et al., 2008; Wahl et al., 2021, 2022). Such modifications have been reported from laboratory experiments (e.g., Ebner et al., 2017; Hughes et al., 2021; Sokratov & Golubev, 2009) and observations from locations in Greenland (Steen‐Larsen et al., 2014) and Antarctica (Casado et al., 2021; Ritter et al., 2016) with different accumulation rates.…”