Chemical weathering drives the cycling of elements from continents to sedimentary basins and consequently modulating the composition of the crust. Silicate weathering represents a major sink to atmospheric CO₂ over geological timescales, regulating the geological carbon cycle and the long-term evolution of the climate system. 

Li isotopes are considered as the most promising tracer of silicate weathering processes currently. However, most time-series data do not have high-enough resolution to decipher the role of climate and extreme hydrological events (e.g. storms) in Li isotopic fractionation, and to eliminate the associated uncertainties. 

A research group led by Prof. JIN Zhangdong from the Institute of Earth Environment (IEE), Chinese Academy of Sciences, collected high-resolution samples in the middle reaches of the Yellow River (Fig. 1) and elucidated the significance and utility of riverine Li isotopic fractionation in the context of conservative mixing and related weathering processes. 

They also conducted a comprehensive investigation on how riverine δ⁷Li was affected by water sources and weathering processes in semi-arid regions, and how and why weathering processes varied over a hydrological year. 

The results showed that the dissolved Li was mainly derived from silicates and evaporites in the arid to semi-arid Yellow River basin. Silicate weathering of loess during the monsoonal season dominated the Li flux in the middle reaches of the Yellow River, with a positive relationship between dissolved Li flux and physical erosion rate. 

Evaporite contribution for riverine Li was relatively constant in the middle reaches of the Yellow River but slightly increased after the storm event, with an average proportion of ~25%, which might represent the proportion of evaporite contribution to global oceans. 

Seasonal variations in the riverine Li isotopic compositions were dominantly controlled by temperature with a fractionation gradient as -0.182‰ per degree centigrade over the full year with deviations likely driven by re-dissolution of suspended particulate matter, extreme hydrological events, and groundwater contribution (Fig. 2).  

Temperature dependent δ⁷Li value variation of river water inputted into oceans indicated that Cenozoic climate cooling itself may be able to explain ~2‰ of the 9‰ rise of Cenozoic seawater δ⁷Li value. The seasonal variation in riverine Li isotopes highlighted that erosion and weathering of loess may provide valuable clues on secular chemical weathering and seawater δ⁷Li variation spanning a range of time scales. 

This work presented the first temperature-dependent seasonal δ⁷Li on the continental scale to better understand Cenozoic Li isotopic evolution and qualified evaporates contribution of Li flux continents to oceans. 

The study, published in Geochemica et Cosmochimica Acta, was supported by the Key Research Program of the CAS, the NSFC Program, and ERC Consolidator grant. 

Fig.1 Sketch map of the Yellow River drainage basin, with major tributaries and sampling site (Longmen hydrological station). Lithologically, loess and desert dominate within the upper and middle reaches of the Yellow River basin. Inset map shows the Yellow River drainage basin. (Image by GOU Longfei et.al) 

 
Fig. 2 The suspend particulate matter (SPM) concentrations of the Toudaoguai (TDG) and Longmen (LM) hydrological stations over 2013, showing that SPM was mainly derived from loess between TDG and LM during the monsoon season. (Image by IEE)