The role of sublimation as a driver of climate signals in the water isotope content of surface snow: Laboratory and field experimental results

Abstract

Abstract. Ice core water isotope records from Greenland and Antarctica are a valuable proxy for paleoclimate reconstruction, yet the processes influencing the climate signal stored in the isotopic composition of the snow are being revisited. Apart from precipitation input, post-depositional processes such as wind-driven redistribution and vapor-snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal. Recent field studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor- snow exchange is an important driving mechanism. Here we investigate how vapor-snow exchange and sublimation processes influence the isotopic composition of the snowpack. Controlled laboratory experiments under dry air flow show an increase of snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer, with an attenuated signal down to 3 cm snow depth over the course of 4–6 days. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Using a simple mass-balance and diffusion box model in conjunction with our observed laboratory vapor isotope signals, we are able to reproduce the observed changes in the snow. This confirms that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 day field experiments at the East Greenland Ice Core Project site (Northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions, and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation play a role in creating the climate signal recorded in the water isotopes in surface snow. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such.

Local-scale deposition of surface snow on the Greenland ice sheet

“…The postprocessing time and the effort to generate a DEM are less than 4 h per DEM and are comparable to or even less than that reported by other studies (e.g. James and Robson, 2012).…”

“…Following studies on changes in the isotopic composition of surface snow (e.g. Casado et al, 2018;Hughes et al, 2021), we use theoretical snow profiles based on the accumulation history from our DEM data set (Fig. 11) to demonstrate the variability of snow ages at one location for several days (Fig.…”

“…It is recommended to use at least three GCPs; however, more GCPs provide a better geo-referencing and a reduced sensitivity to a single point (e.g. James and Robson, 2012;Tonkin et al, 2016). We generated DEMs with 5, 8, or 13 GCPs and used the detailed snow height information from the validation sticks as ground control analysis.…”

An Unmanned Aerial Vehicle Sampling Platform for Atmospheric Water Vapor Isotopes in Polar Environments

“…At the ice sheet/atmosphere interface, both radiative (shortwave and 120 longwave) and non-radiative (sensible and latent heat) energy fluxes occur, affecting the energy mass balance of the ice sheet. The summation of these processes leaves a diurnal imprint on the water isotopes in the upper few centimeters of the firn (Ritter et al 2016, Madsen et al 2019, Hughes et al 2021. Within the PBL, turbulent mixing occurs with a magnitude largely dependent on stratification and wind shear.…”

“…For ice sheets, a common assumption that has persisted since early studies is that the isotopic composition of the ice sheets is solely informed by precipitation events. Yet, this assumption is being overturned with clear evidence that the ice sheet and the atmosphere constantly exchange water isotopologues with different rates leading to post-depositional change in the snow isotopic composition (Steen-Larsen et al 2013Ritter et al 2016;Hughes et al 2021). This paradigm shift has not been fully accounted for in models, nor are these findings utilized 45 for constraining ice sheet-atmosphere interactions.…”