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Stable isotope ratios δ18O and δD in polar ice provide a wealth of information about\udpast climate evolution. Snow-pit studies allow us to relate observed weather and climate conditions to the measured\udisotope variations in the snow. They therefore offer the possibility to test our understanding of how isotope signals are\udformed and stored in firn and ice. As δ18O and δD in the snowfall are strongly\udcorrelated to air temperature, isotopes in the near-surface snow are thought to record the seasonal cycle at a given\udsite. Accordingly, the number of seasonal cycles observed over a given depth should depend on the accumulation rate of\udsnow. However, snow-pit studies from different accumulation conditions in East Antarctica reported similar isotopic\udvariability and comparable apparent cycles in the δ18O and δD profiles with typical\udwavelengths of  ∼  20 cm. These observations are unexpected as the accumulation rates strongly differ between\udthe sites, ranging from 20 to 80 mm w. e.  yr−1 ( ∼  6–21 cm of snow per year). Various\udmechanisms\udhave been proposed to explain the isotopic variations individually at each site; however, none of these are consistent with\udthe similarity of the different profiles independent of the local accumulation conditions. Here, we systematically analyse the properties and origins of δ18O and δD variations in\udhigh-resolution firn profiles from eight East Antarctic sites. First, we confirm the suggested cycle length (mean distance\udbetween peaks) of  ∼  20 cm by counting the isotopic maxima. Spectral analysis further shows a strong\udsimilarity between the sites but indicates no dominant periodic features. Furthermore, the apparent cycle length increases\udwith depth for most East Antarctic sites, which is inconsistent with burial and compression of a regular seasonal\udcycle. We show that these results can be explained by isotopic diffusion acting on a noise-dominated isotope signal. The\udfirn diffusion length is rather stable across the Antarctic Plateau and thus leads to similar power spectral densities of\udthe 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\udinterpretation of apparent cycles in shallow isotope records, without invoking complex mechanisms. Finally, the results\udunderline previous suggestions that isotope signals in single ice cores from low-accumulation regions have a small\udsignal-to-noise ratio and thus likely do not allow the reconstruction of interannual to decadal climate variations

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