Oral

IAMAS

JCM04 - Recent Advances in Ice Core Science

Impacts and Corrections of Post-Depositional Processes on Ice Core Trapped Gases: Insights from the Tibetan Plateau and Antarctic Blue Ice Areas

Ice cores serve as crucial archives for glacial-interglacial paleoclimate reconstructions. However, post-depositional alterations of glaciers can significantly alter the isotopic composition and gas ratios of trapped air, impacting the accuracy of ice core dating and paleoclimate interpretations. Key post-depositional alteration processes?such as gas loss, melting, and respiration?induce variable degrees of gas fractionations on ice core trapped air. Despite their importance, research on these processes remains limited. Different types of ice cores, such as the Tibetan ice core, blue ice, and the basal sections of deep ice cores, are subject to various post-depositional processes due to their formation conditions and local climate. Our investigations of the Guliya and Chongce ice cores from the Tibetan Plateau identified seasonal melting and respiration as the most influential post-depositional processes altering trapped gas composition. Melting disrupts firn porosity, forming bubble-free melt layers with dissolved air. This process induces significant isotopic fractionation, generating uncertainties in ���?O of O�� values. Depending on equilibrium or kinetic dissolution, oxygen isotopic fractionation can yield positive (+0.7��) or negative (-2.2��) offsets, substantially impacting ���?O accuracy. To ensure dating reliability, samples with melt layers were identified and excluded based on anomalously high ��Ar/N�� ratios (>30��). Respiration, prevalent not only in basal sections of deep ice cores but throughout Tibetan Plateau ice cores due to their high organic content. This process could be corrected using respiratory isotopic fractionation factors and ��O��/N�� ratios under a Rayleigh fractionation framework. Additionally, we analyzed shallow ice samples from the Grove Mountain blue ice area, Antarctica. Laboratory pumping experiments revealed that fractured samples were prone to gas loss, prompting a recommendation to minimize pumping duration during gas extraction for blue ice samples. Visible cracks in samples correlated with anomalously negative ���?N values, indicating preferential diffusion and sealing of lighter isotopes into ice cracks. Overall, it is important to evaluate post-depositional isotopic effects on ice core trapped gases, and to develop proper correction method for these processes. It is also essential to adopt rigorous sample selection protocols to avoid misleading of paleoclimate interpretations. This study is especially crucial in regions like the Tibetan Plateau and Blue Ice Area where complex interactions between glacier dynamics, local climate, and biogeochemistry impact ice core trapped air preservations.