A global imbalance in potassium and barium river export: the result of biological uptake?

Abstract

The role of biological cycling on the chemistry of rivers remains poorly understood. In an attempt to close this knowledge gap, here we examine the difference between the elemental supply to catchments through rock degradation and the corresponding elemental riverine export, for two non-nutrient elements lithium (Li) and sodium (Na) and two nutrients-like elements potassium (K) and barium (Ba), in 20 of the largest world river catchments. Overall, the riverine export of K and Ba are lower than their estimated release by catchment scale rock degradation, while the two fluxes match for Li and Na. Barium isotope constraints lending support to this observation, we take this difference between these two element groups as a suggestion of the influence of biological uptake of rock-derived nutrients on river chemistry. Nevertheless, the magnitude of riverine K depletion cannot be reconciled with a pervasive growth of the biota on continents, nor with an “ occult ” export of organic material that would go unnoticed by common sampling protocols. One plausible explanation for this conundrum could lie in thecomplex partitioning of elementsamongst soil, biota, and dead organic matter. As a consequence, our study emphasises the need for further work aiming at deciphering the cycle of rock-derived nutrients in the Critical Zone. catchmentscalemass budgets,for X = Na, Li(non-nutrients) and fornutrient/nutrient-likeelements( X = K and Ba). This approach aims to quantify the catchment scale (im)balance between the supply of a rock-derived nutrient X through rock degradation ( D X ) and the summed dissolved and solid riverine export ( W X and E X , respectively). The difference in the “ solubility ” translates into a difference in the size of the W X and E X arrows.