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

Abstract

Abstract. The isotopic composition of water in ice sheets is extensively used to infer past climate changes. In lowaccumulation regions their interpretation is, however, challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition of the snow. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a seasonal cycle but also strong interannual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we investigate the importance of post-depositional processes within the open-porous firn ( 10 cm depth) at Kohnen Station by separating spatial from temporal variability. To this end, we analyse 22 isotope profiles obtained from two snow trenches and examine the temporal isotope modifications by comparing the new data with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find further modifications of the original isotope record to be unlikely or small in magnitude ( 1 ‰ RMSD). These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.

On the similarity and apparent cycles of isotopic variations in East Antarctic snow pits

“…The snow trench (T15) isotope data are available in the PANGAEA repository https://doi.org/10.1594/PANGAEA.876639 (Münch et al, 2017b). The snow-pit and firn-core isotope data used are available in the PANGAEA repository https://doi.org/10.1594/PANGAEA.883787 .…”

“…Subsequently, the snow is transported to the surface by precipitation where it is redistributed and mixed by wind, giving rise to the surface isotope signal δ 18 O surface (t). Finally, the surface signal is buried in the firn column which is accompanied by diffusional smoothing of the signal and densification of the layers (Münch et al, 2017a). Analysing a snow pit or firn core during this process then represents a snapshot of the firn isotope signal δ 18 O firn (z).…”

“…2 and Table 1; Münch et al, 2017b;Laepple et al, 2017). We focus our analysis on the upper 4 m of firn within which cycles have been described and interpreted.…”

“…We focus our analysis on the upper 4 m of firn within which cycles have been described and interpreted. For SP and EDML, a combination of snow-pit and shallow firn-core data allows us to extend the analysis down to a snow depth of 18 m. All records have been published (Table 1), except those for EDML for which we use 22 3.4 m deep profiles sampled from snow trenches (T15) as described and partly analysed in Münch et al (2017a), together with new isotope data from the firn cores B41 and B50. These two cores were drilled close to Kohnen Station in 2012-13, approximately 1 km apart (Alfred-Wegener-Institut HelmholtzZentrum für Polar-und Meeresforschung, 2016).…”

Influence of Summer Sublimation on δD, δ18O, and δ17O in Precipitation, East Antarctica, and Implications for Climate Reconstruction From Ice Cores

“…It was suggested that the observed low temporal δ/ T may reflect a strong gradient between condensation and surface temperature in winter (Ekaykin et al, ; Landais, Ekaykin, et al, ) and/or the vanishing inversion layer in summer (Landais et al, ). In the central Antarctic Plateau with very low snow accumulation rates (0.016–0.038 m/w.e.a; Ekaykin et al, ; Hou et al, ; Jouzel et al, ; Masson et al, ; Watanabe et al, ), postdepositional processes could significantly modify the isotopic composition of surface snow (Casado et al, ; Laepple et al, ; Münch et al, ; Ritter et al, ). Recent observations in the summer have revealed that the isotopic composition of surface snow in the absence of precipitation varies with changes of the surface vapor isotopic composition (Casado et al, , ; Ritter et al, ; Steen‐Larsen, Masson‐Delmotte, et al, ; Touzeau et al, ), suggesting possible isotopic exchange between surface snow and atmospheric water vapor in the polar regions.…”