Lithium isotopes as a new indicator for change in hydrology

AbstractHydrology is a critical driving force that affects surface erosion and weathering, which regulates the long-term stability of Earth’s climate by removing atmospheric CO₂. However, reconstructing past hydroclimate remains a non-trivial challenge, due to the difficulty in isolating temperature effects. The negative correlation between globally compiled Li isotopes (δ7Li) and runoff suggests the potential of δ7Li as a hydrological proxy, yet confirming its validity still requires more field evidence. Here, we report the first weekly riverine δ7Li from a remote, rapidly eroding Muztag glacial basin (peak elevation of 7546 m) within the largest glacierized region of the mid-latitudes. Our results show remarkable seasonal changes in river δ7Li (up to ∼15‰), from 17.9‰ in the dry season to 2.5‰ in the wet season. The very low δ7Li in this tectonically active basin during the wet season contradicts the long-standing uplift-weathering hypothesis that intensified erosion increased incongruent continental weathering and higher riverine δ7Li. Instead, our findings unexpectedly exhibit a clear hydrological signal, with δ7Li showing a significant relationship with lake water levels but weak correlations with temperature. Changes in lake levels, together with riverine 87Sr/86Sr and Al/Si ratios, suggest that hydrologically driven variations in water–rock interaction time affect secondary mineral formation and seasonal riverine δ7Li. A further comparison of δ18O and δ7Li with temperature and lake level indicates that δ7Li provides a more suitable proxy for local hydrological changes than δ18O. Considering the debates regarding whether δ18O reflects changes in temperature or precipitation at regional or global scales, our findings have important implications for using δ7Li in lacustrine sedimentary records (e.g., authigenic carbonates) as a promising tool to decode past hydrologic state, thereby helping to isolate temperature effect.