Stable oxygen and hydrogen isotope compositions of the Messoyakha-1 and Pestsovoe pingo ice cores, Northwest Siberia

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.

本研究针对西西伯利亚西北部连续多年冻土带中两个成因各异的大型冰核丘（pingo）展开，分别为高度10.5米的Messoyaha-1冰核丘与高度17米的Pestsovoe冰核丘。通过测定冰核丘冰芯中稳定氧同位素与氢同位素的分布特征，本研究厘清了冰芯的形成机制。冰的形成过程通过封闭体系与开放体系框架下的瑞利分馏（Rayleigh fractionation）模型进行估算。就Pestsovoe冰核丘而言，随着深度增加，δ¹⁸O值逐渐降低，同时dexc值同步升高，这表明该冰核丘的冰体形成于湖状冻土融区自上而下冻结的封闭体系中。对于Messoyakha-1冰核丘，δ¹⁸O与δ²H值随深度仅呈现微弱的降低趋势，而dexc值则呈现随机波动特征。赋存于上覆与下伏沉积物中的分凝冰，其δ¹⁸O与δ²H值较为相近，且同位素梯度较低。研究证实，封闭体系中形成的分凝冰，以及开放-半封闭体系中形成的冰芯，均属于同位素非平衡形成的冰体。在Messoyakha-1冰核丘上部冰芯的形成与后续生长过程中，从周边湖泊或湖状冻土融区汲取水体的真空机制可能发挥了关键作用。