Hydrological control of river and seawater lithium isotopes

Seawater lithium isotopes (delta Li-7) record changes over Earth history, including a similar to 9 parts per thousand 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 delta Li-7 remain debated. Here we report time-series delta Li-7 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 delta Li-7 across all latitudes: dry seasons consistently have higher delta Li-7 than wet seasons, by -0.3 parts per thousand to 16.4 parts per thousand (mean 5.0 +/- 2.5 parts per thousand). A globally negative correlation between delta Li-7 and annual runoff reflects the hydrological intensity operating in catchments, regulating water residence time and delta Li-7 values. This hydrological control on delta Li-7 is consistent across climate events back to similar to 445 Ma. We propose that hydrological changes result in shifts in river delta Li-7 and urge reconsideration of its use to examine past weathering intensity and flux, opening a new window to reconstruct hydrological conditions.

Automated summary

Seasonal changes in seawater lithium isotopes (ΔLi-7) were observed in Tibetan and Pamir rivers, with higher ΔLi-7 values during dry seasons compared to wet seasons. This global pattern is negatively correlated with annual runoff, suggesting a hydrological control on ΔLi-7 values. The study proposes that hydrological changes can affect river ΔLi-7 and calls for a reconsideration of its use in studying past weathering intensity and flux, offering a new approach to reconstruct hydrological conditions.