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

Abstract

Abstract. Stable isotope ratios δ 18 O and δD in polar ice provide a wealth of information about past climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured isotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are formed and stored in firn and ice. As δ 18 O and δD in the snowfall are strongly correlated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given site. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of snow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in the δ 18 O and δD profiles with typical wavelengths of ∼ 20 cm. These observations are unexpected as the accumulation rates strongly differ between the sites, ranging from 20 to 80 mm w.e. yr −1 (∼ 6-21 cm of snow per year). Various mechanisms have been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with the similarity of the different profiles independent of the local accumulation conditions.Here, we systematically analyse the properties and origins of δ 18 O and δD variations in high-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance between peaks) of ∼ 20 cm by counting the isotopic maxima. Spectral analysis further shows a strong similarity between the sites but indicates no dominant periodic features. Furthermore, the apparent cycle length increases with depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal cycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of the isotopic variations. This in turn implies a similar distance between isotopic maxima in the firn profiles.Our results explain a large set of observations discussed in the literature, providing a simple explanation for the interpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results underline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small signal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations.

Ice wedges as archives of winter paleoclimate: A review

“…High‐resolution mapping of the spatial variability of stable isotopes and statistical comparisons of the temporal variability recorded by the ice wedges and the driving climate signal, similar to studies performed for ice cores, should provide insight into the signal formation and preservation. Both questions could be supported by numerically modeling signal formation . Very low d values may indicate that the original climate signal has been overprinted due to disequilibrium fractionation, limiting the paleoclimatic significance.…”

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.…”

Assessing the robustness of Antarctic temperature reconstructions over the past 2 millennia using pseudoproxy and data assimilation experiments

“…Most individual records have a data resolution ranging from 0.025 to 5 years. In order to limit the influence of nonclimatic noise induced by postdepositional processes (e.g., Münch et al, 2017;Jones et al, 2017;Laepple et al, 2018), they were all 5-year averaged for reconstructing the last 2 centuries and 10-year averaged for reconstructing the last 2 millennia. This lower temporal resolution also limits the potential influence of small age uncertainties.…”

“…Using such a controlled framework allows us to precisely assess the performance of the different reconstruction methods via a series of diagnostics including the root mean square error (RMSE) and the correlation coefficients between the reconstructions and the model target (model simulations from which the pseudoproxies are derived), the coefficient of efficiency (CE) of the reconstructions, and the standard deviation of the model truth and the reconstructions. The CE (Lorenz, 1956), which is classically used to measure the skill of reconstructions (e.g., Steiger et al, 2014;Klein and Goosse, 2018), is defined for a time series including n samples as follows:…”