Hydrological control of river and seawater lithium isotopes.

Seawater lithium isotopes (δ 7 Li) 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 δ 7 Li remain debated. Here we report time-series δ 7 Li measurements from Tibetan and Pamir rivers, and combine them with published seasonal data, covering small (<10 2   km 2 ) to large rivers (>10 6  km 2 ). We find seasonal changes in δ 7 Li across all latitudes: dry seasons consistently have higher δ 7 Li than wet seasons, by -0.3‰ to 16.4‰ (mean 5.0 ± 2.5‰). A globally negative correlation between δ 7 Li and annual runoff reflects the hydrological intensity operating in catchments, regulating water residence time and δ 7 Li values. This hydrological control on δ 7 Li is consistent across climate events back to ~445 Ma. We propose that hydrological changes result in shifts in river δ 7 Li and urge reconsideration of its use to examine past weathering intensity and flux, opening a new window to reconstruct hydrological conditions.