Dissolved potassium isotopic composition of major world rivers

Abstract

High-precision potassium (K) isotope ratios have recently been proposed as a new tool for tracing continental weathering and reconstructing Earth’s past climates. The premise is that the K isotopic composition of seawater is sensitive to terrestrial weathering changes. Modern seawater (δ41KNIST SRM3141a = +0.12 ± 0.07‰) is significantly enriched in heavy K isotopes compared to the Bulk Silicate Earth (BSE) and the Upper Continental Crust (UCC). However, the controls causing such a large isotopic fractionation between these two major reservoirs are not well understood. Dissolved K in river water is one of the major inputs of K to seawater. To constrain the poorly defined K isotopic composition of riverine input to the global ocean and to understand the controlling factors of the K isotope composition of seawater, we analyzed the K isotopic composition of 32 river samples from 24 major rivers globally. These rivers drain all continents except Antarctica and collectively account for 40% of the annual global river discharge and 39% of the total K flux into the ocean. We observed a large range in K isotopic composition across all the rivers analyzed, ranging from δ41K = −0.59 ± 0.04‰ to −0.08 ± 0.04‰, but found no significant K isotopic variations among samples collected from the same river under differing flow conditions. We attribute the dissolved K isotopic composition of global rivers to the fraction of K retained in clay minerals during chemical weathering. Isotopically light K is retained with the clay fraction during weathering leading to heavy isotope enrichment in the dissolved K load relative to the BSE and UCC. The flux-weighted and regionally-adjusted mean composition of all rivers studied here (−0.38 ± 0.04‰) serves as a global estimate of the riverine δ41K value. The seawater K isotopic composition (i.e., +0.12 ± 0.07‰) cannot be explained solely by the riverine input. Other mechanisms (hydrothermal input, reverse weathering, biological fractionation) are needed to explain seawater K isotopic composition.