Contrasting behavior of K isotopes in modern and fossil groundwater: Implications for K cycle and subsurface weathering

•K isotopes of groundwater with different ages are reported for the first time.•Extremely high δ41K (0.69‰) indicates intensive clay formation in modern groundwater.•Clay formation followed by long-time adsorption causes low K/Na in fossil groundwater.•K isotopes are a robust indicator of the degree of subsurface chemical weathering. Chemical weathering of rocks largely determines sources of groundwater hydrochemical components, but the superposition of processes removing cations leads to a wide variability of elemental ratios. In this study, potassium (K) isotopes are used for the first time to quantify the roles of silicate dissolution, clay incorporation and clay adsorption in subsurface K cycle. We investigate the behavior of K isotopes in groundwater with contrasting ages from a sandstone aquifer in the Ordos Basin, NW China. For modern groundwater (< 50 yrs) in the recharge area with abundant CO2, the silicate-derived K has been largely removed by 10-year-scale clay incorporation, generating low [K]/[Na] ranging between 0.026 and 0.048 and extremely high δ41K varying between 0.57‰ and 0.69‰; our calculations reveal that 92% of total K released from silicate dissolution has been removed by clay incorporation, suggesting that using measured [K] alone would underestimate the degree of silicate dissolution. For fossil groundwater (∼104 yrs) in the discharge area, clay adsorption during long-time circulation leads to extremely low [K]/[Na] ranging from 0.001 to 0.011 and much lower δ41K varying from -0.32‰ to 0.37‰; as a result of the combined effect of clay incorporation and clay adsorption, up to 99.2% of silicate-derived K has been removed from the dissolved load. This study provides novel insights into the complex processes controlling the K cycle in groundwater and highlights the potential of K isotopes in understanding subsurface weathering and associated elemental cycles. [Display omitted]