Hydrological control of river and seawater lithium isotopes

Seawater lithium isotopes (δ⁷Li) 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 δ⁷Li remain debated. Here we report time-series δ⁷Li measurements from Tibetan and Pamir rivers, and combine them with published seasonal data, covering small (<10²  km²) to large rivers (>10⁶ km²). We find seasonal changes in δ⁷Li across all latitudes: dry seasons consistently have higher δ⁷Li than wet seasons, by -0.3‰ to 16.4‰ (mean 5.0 ± 2.5‰). A globally negative correlation between δ⁷Li and annual runoff reflects the hydrological intensity operating in catchments, regulating water residence time and δ⁷Li values. This hydrological control on δ⁷Li is consistent across climate events back to ~445 Ma. We propose that hydrological changes result in shifts in river δ⁷Li and urge reconsideration of its use to examine past weathering intensity and flux, opening a new window to reconstruct hydrological conditions.