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.