Contrasting riverine K and Li isotope signatures during silicate weathering in the southeastern Tibetan Plateau

Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO2, driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (-0.35%o to +0.05%o) and Li (+7.1%o to +20.4%o) relative to suspended sediments (-0.56%o to -0.42%o and -2.3%o to 0%o, respectively), and the element partitioning between dissolved and suspended loads controls dissolved 87Li and 841K values. Interesting, dissolved 87Li and 841K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)* ratios), the dissolved 87Li values increase from 7.4%o to 13.6%o, whereas dissolved 841K values decrease from -0.04%o to -0.26%o. Compared to its headwater with limited weathering, more secondary minerals (e.g., clays) are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved 87Li values downstream. While dissolved 841K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater 87Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater 841K rise, as with 87Li change, weathering at higher elevation may exerts more effect on high riverine and seawater 841K values. This study improves our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time.(c) 2023 Elsevier B.V. All rights reserved.