Hydrological change is one of the most critical forces shaping Earth's surface erosion, weathering and civilizational evolution. However, reconstructing past hydrology remains a major challenge, as reliable hydrological proxies are scarce.

In a new study published in Geochimica et Cosmochimica Acta on March 12, researchers from the Institute of Earth Environment of the Chinese Academy of Sciences reported that lithium isotopes in rivers may provide a promising new indicator for reconstructing past hydrological changes.

Traditional paleoclimate proxies, such as δ¹⁸O, often incorporate temperature effects, making it difficult to isolate signals of hydrological change.

Recent studies have suggested that river water δ⁷Li is primarily controlled by hydrological processes, showing a negative correlation with runoff, implying that δ⁷Li preserved in lake sedimentary records may record past hydrological changes. However, whether δ⁷Li can be reliably used to reconstruct past hydrology has not yet been systematically assessed, as constraints on the relationship between dissolved δ⁷Li and hydrology fingerprints from sedimentary archives (e.g., lake level) are still required. This has hindered the development of Li isotopes as a novel tool for reconstructing past hydrological changes.

To address this gap, the researchers conducted an intensive field investigation in the Muztag Glacier catchment in the tectonically active Pamir Plateau—known as the "Father of Glaciers". Using high-resolution weekly sampling, they systematically investigated seasonal behaviors in riverine δ⁷Li and their relations with hydrological changes.

Their results showed remarkable seasonal variations in riverine δ⁷Li, with values decreasing by up to ~15.4‰ from the dry to wet seasons. Such a large seasonal difference has not been observed previously.

More importantly, the researchers discovered a clear hydrological control on riverine δ⁷Li. The isotope values showed a significant correlation with lake water levels (r² = 0.61): higher δ⁷Li values correspond to lower lake levels, while lower δ⁷Li values occurred when lake levels were higher. In contrast, no clear correlation was observed between δ⁷Li and temperature.

Significant correlations between δ⁷Li and lake levels, 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 (up to ~15‰). In detail, fast surficial runoff with short water–rock interaction time in the wet season facilitates more congruent weathering, resulting in rock-like riverine δ⁷Li. In contrast, slow flow during the dry season prolongs water–rock interaction time, promotes secondary mineral formation and higher δ⁷Li values.

The researchers also compared the performance of the traditional hydrological proxy δ¹⁸O with the emerging proxy δ⁷Li. They found that δ¹⁸O primarily reflects temperature in regions dominated by westerly circulation and precipitation in monsoon regions. By contrast, δ⁷Li consistently tracks hydrological changes across both climate regimes, as well as across other global climatic settings.

These findings highlight several key advances. First, riverine δ⁷Li exhibits exceptionally large seasonal variations—up to ~15‰—representing the largest seasonal amplitude reported so far. Second, even in tectonically active and highly erosive glacial environments, δ⁷Li responds rapidly and sensitively to hydrological fluctuations. Third, the study quantitatively links dissolved δ⁷Li to observed lake-level variations, providing the first field evidence supporting the use of δ⁷Li preserved in lacustrine carbonates as a hydrological archive. Finally, compared with δ¹⁸O, lithium isotopes appear to be a more sensitive indicator of hydrological variability while largely excluding temperature effects.

Taken together, the results suggest that δ⁷Li has strong potential to become a new and reliable proxy for reconstructing past hydrological changes, opening a new window for hydrological reconstruction. Given the close links between hydrological variability and fields such as paleoclimate reconstruction, extreme drought and flood events, the rise and fall of civilizations, vegetation succession, and the weathering–carbon cycle, this new proxy is expected to have broad applications in future research.

This work was supported by the National Natural Science Foundation of China, the International Partnership Program of Chinese Academy of Sciences for Future Network, and other funding sources.