Use of δ¹⁸O_atm in dating a Tibetan ice core record of Holocene/Late Glacial climate.

Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O₂ in air bubbles (δ¹⁸O_atm), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ¹⁸O_ice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δ¹⁸O_atm, although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (O₂) by micro-organisms in the ice, which fractionates the δ¹⁸O of O₂ from the atmospheric value. Here, we make corrections for melt and respiration to address these complications. The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δ¹⁸O_ice are complex and temporally variable; however, Guliya δ¹⁸O_ice values since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature.