Lithium isotopes as a new indicator for change in hydrology

Abstract

Hydrology 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.