Hydrological control of river and seawater lithium isotopes.

Seawater lithium isotopes (δLi) record changes over Earth history, including a ∼9‰ increase during the Cenozoic interpreted as reflecting either a change in continental silicate weathering rate or weathering feedback strength, associated with tectonic uplift. However, mechanisms controlling the dissolved δLi remain debated. Here we report time-series δLi measurements from Tibetan and Pamir rivers, and combine them with published seasonal data, covering small (<10  km) to large rivers (>10 km). We find seasonal changes in δLi across all latitudes: dry seasons consistently have higher δLi than wet seasons, by -0.3‰ to 16.4‰ (mean 5.0 ± 2.5‰). A globally negative correlation between δLi and annual runoff reflects the hydrological intensity operating in catchments, regulating water residence time and δLi values. This hydrological control on δLi is consistent across climate events back to ~445 Ma. We propose that hydrological changes result in shifts in river δLi and urge reconsideration of its use to examine past weathering intensity and flux, opening a new window to reconstruct hydrological conditions.