Archival of the water stable isotope signal in East Antarctic ice cores

Abstract

Abstract. The oldest ice core records are obtained from the East Antarctic plateau. Water stable isotopes records are key for reconstructions of past climatic conditions both over the ice sheet and at the evaporation source. The accuracy of such climate reconstructions crucially depends on the knowledge of all the processes affecting the water vapour, precipitation and snow isotopic composition. Atmospheric fractionation processes are well understood and can be integrated in Rayleigh distillation and complex isotope enabled climate models. However, a comprehensive quantitative understanding of processes potentially 5 altering the snow isotopic composition after the deposition is still missing, especially for exchanges between vapour and snow.In low accumulation sites such as found on the East Antarctic Plateau, these poorly constrained processes are especially likely to play a significant role. This limits the interpretation of isotopic composition from ice core records, specifically at short time scales. 10Here, we combine observations of isotopic composition in the vapour, the precipitation, the surface snow and the buried snow from various sites of the East Antarctic Plateau. At the seasonal scale, we highlight a significant impact of metamorphism on surface snow isotopic signal compared to the initial precipitation isotopic signal. In particular, in summer, exchanges of water molecules between vapour and snow are driven by the sublimation/condensation cycles at the diurnal scale. Using highly resolved isotopic composition profiles from pits in five East Antarctic sites, we identify a common 20 cm cycle which 15 cannot be attributed to the seasonal variability of precipitation. Altogether, the smaller range of isotopic compositions observed in the buried and in the surface snow compared to the precipitation, and also the reduced slope between surface snow isotopic composition and temperature compared to precipitation, constitute evidences of post-deposition processes affecting the variability of the isotopic composition in the snow pack. To reproduce these processes in snow-models is crucial to understand the link between snow isotopic composition and climatic conditions and to improve the interpretation of isotopic composition as a 20 paleoclimate proxy.

Constraints on post-depositional isotope modifications in East Antarctic firn from analysing temporal changes of isotope profiles

“…(2) Isotope modification occurring directly at or above the surface is the key driver for shaping the interannual isotope variations. Such processes might be acting on falling, loose or drifting snow, or on the top layer (first few centimetres) of deposited snow (Ritter et al, 2016;Casado et al, 2016). The fact that our trench records are reproducible on spatial scales of at least 500 m implies that the atmospheric parameters and conditions controlling potential processes would also need to be spatially coherent.…”

“…Particularly in low-accumulation areas for which the snow surface is exposed to the atmosphere for a substantial time, a variety of processes are thought to considerably modify the original atmospheric isotope signal during or after deposition of the snow, thus from seasonal to interannual timescales (e.g. Ekaykin et al, 2014Ekaykin et al, , 2016Hoshina et al, 2014;Touzeau et al, 2016;Casado et al, 2016).…”

Atmospheric moisture supersaturation in the near-surface atmosphere at Dome C, Antarctic Plateau

“…Here the wind speed and specific humidity are measured at z = ∼ 3 m above the surface, and the snow surface temperature is obtained from measurement of the upwelling infrared radiation (Vignon et al, 2016) considering a snow emissivity of 0.99 (Brun et al, 2011). C Q is a bulk transfer coefficient which is written as follows:…”

Numerical experiments on isotopic diffusion in polar snow and firn using a multi-layer energy balance model

“…This is particularly timely since many recent studies have studied the parallel evolution of isotopic composition of water vapor and surface snow during summer both in Greenland and Antarctica (Steen-Larsen et al, 2014;Ritter et al, 2016;Casado et al, 2016a;2016b (Touzeau et al, 2016). Indeed, 17 O-excess is strongly influenced by kinetic diffusion driven fractionation which may be quantified by the implementation of 17 O-excess in our Crocus-iso model.…”