Two large pingos from ice cores of different origin in the continuous permafrost zone of northwest Siberia, the Messoyaha-1 pingo (10.5 m in height) and the Pestsovoe pingo (17 m in height), have been studied. Obtained distribution of stable oxygen and hydrogen isotopes in pingo ice cores allowed to understand the ice core formation. Ice formation was estimated according to the Rayleigh fractionation in a closed-system versus an open-system framework. For the Pestsovoe pingo the decrease in δ¹⁸O values with corresponding increase in dexc with depth indicates a closed system upon freezing of the lake talik from the top down. For the Messoyakha-1 pingo, the values of δ¹⁸O and δ²Н showed a weak tendency to decrease with depth, with values of dexc varying randomly. Ice that was segregated in the overlying and underlying sediments had similar values of δ¹⁸O and δ²Н and a low slope. Isotopically nonequilibrium ice formation was established for ice which had been segregated in a closed system and for ice cores formed in an open to semiclosed system. The vacuum mechanism of water suction from the surrounding lake or lake talik may have played a significant role during the formation of the upper ice core of the Messoyakha-1 pingo and its additional growth.

Drilling was performed with continuous sampling of an undisturbed frozen core. Undisturbed ice samples were taken for isotope analysis. The isotope compositions of oxygen and hydrogen were determined with a Delta-V continuous flow isotope ratio mass spectrometer (CF-IRMS) with the GasBench device at the Isotope Laboratory of the Geographical Faculty of Lomonosov Moscow State University. International standards V-SMOW and V-SLAP were used for calibration; the error of determination was ±0.6‰ for δ²Н and ±0.1‰ for δ¹⁸O.