SPATIAL DECREASE OF LITHIUM ISOTOPES ALONG THE GHEZ RIVER AT THE TECTONICALLY ACTIVE NORTHEASTERN PAMIR PLATEAU

The ~9‰ increase of seawater lithium isotopes (δ⁷Li) during the Cenozoic was proposed to reflect an enhanced continental erosion and weathering, associated with tectonic uplift. However, this interpretation contradicts with modern observations that instead showed high δ⁷Li values in floodplains, and low values in upstream mountains. Whether mountain uplift or the floodplain formation controlling the Cenozoic seawater δ⁷Li rise remains controversy. Here, we investigate spatially riverine δ⁷Li in the tectonically active Ghez catchment with contrasting topography and lithology in the NE Pamir Plateau. Our results show a significant decrease of δ⁷Li from the upper mountain to the lower floodplain, by ~15‰. The high δ⁷Li values (up to 20‰) in the upstream reflect relatively flat topography with long fluid residence time facilitating incongruent weathering and thus large Li isotopic fractionation, while the subsequent decline of δ⁷Li values to ~10‰ in the midstream is consistent with sharp topography. On the contrary, the rock-like low δ⁷Li values in the downstream (~5‰) reflect congruent weathering by ancient sedimentary rocks with clay dissolution at the floodplain. These observations, contrary to previous studies, indicate that weathering in floodplains associated with tectonic uplift is not necessary producing high riverine δ⁷Li, and thus renewed our understanding of the role of floodplains on deciding riverine δ⁷Li from the land to the oceans.