Meltwater and cold pump effects override climate control of grain size in polythermal glacier ice

The microstructural evolution of polythermal glacier ice remains poorly constrained, limiting the reliability of grain size as a paleoclimate proxy. To this end, we analyzed twelve samples from a deep, near-basal ice core (58.7 m length) drilled at the Miaoergou glacier (Eastern Tibetan Plateau) using an Auto Fabric Analyzer G50. Our microstructural data—including grain size/shape, crystallographic orientation, and recrystallization evidence—indicate that ice crystal development is dominated by two key processes: percolation and refreezing of meltwater (PRMW), and rotation recrystallization. We further conceptualize a cold pump effect (CPE), a thermal mechanism driving heat from warmer downstream to colder upstream regions through ice and bedrock, which shapes the glacier’s thermal structure and thereby influences grain growth. Consequently, we conclude that grain size in polythermal mountain glaciers is an unreliable climate proxy, as its variation is overwhelmingly governed by the synergistic effects of PRMW and CPE, which overprint any primary climate signal.