Atmospheric CO₂ controls on the MIS 6 glaciation: ¹⁰Be chronology of moraines in the Haizishan area, southeastern Tibetan Plateau

A unifying theory interpreting the cause of ice-age cycles remains elusive, in view of the near-synchronous interhemispheric major climate shifts but anti-phased summer solar insolation signatures between hemispheres. Determining the timing and magnitude of mountain glaciations helps in shedding new light on this long-standing unsolved puzzle. Yet, robust glacial chronologies are limited for the pre-Last Glaciation time periods, impeding a full understanding of glacial histories and climatic mechanisms behind them in mountainous regions, such as the Tibetan Plateau (TP). Here, we report thirty-five new ¹⁰Be exposure-ages from moraine boulders in the Niqingqu Valley, eastern Haizishan Plateau, which adds to the existing glacial chronologies of the Hengduan Mountains, southeastern TP. The dating results indicate that four centennial to millennial-scale glacial activities occurred around 168.4 ka in the studied valley. Despite being indistinguishable in age, these high-frequency glacial fluctuations allow us to draw a conclusion that glaciers in the studied valley achieved the marine isotope stage 6 (MIS 6) glacial maximum prior to the Penultimate Glacial Maximum (PGM), inconsistent with Northern Hemisphere. We attribute the MIS 6 glacial maximum identified here to the response of glaciers to a combination of low-level atmospheric CO₂ content and low northern high-latitude summer solar insolation/high summer-monsoon precipitation. The consistent local PGM and low CO₂ levels suggest that low atmospheric CO₂ contents are likely a necessary prerequisite for the MIS 6 glaciation.