Near-congruent Li isotope signature from weathering and erosion of a tectonically active mountain range

Abstract

The cause of the ∼9‰ increase in seawater lithium isotopes (δ7Li) during the Cenozoic continues to be debated. Changes in continental denudation driven by tectonic uplift have been proposed as a plausible mechanism for the observed increase in seawater δ7Li. However, this interpretation depends on whether uplift and erosion produce runoff with more fractionated dissolved Li, and therefore higher δ7Li. While some rivers draining major orogens follow this expectation, in this study we report new dataset across a tectonically active mountain range in the northeastern Pamir Plateau, which suggest that the effects of uplift and erosion on riverine δ7Li are not systematic. Our results from the Gaizi River basin show a significant decrease in riverine δ7Li from the high-elevation headwaters to the foreland by ∼15‰. High δ7Li values (up to ∼20‰) in the upstream reflect more incongruent silicate weathering, due to intense glacial activity, extensive peri-glacial floodplains traversed by braided alluvial rivers, and low total runoff that together facilitate large Li isotopic fractionation. Subsequent decline of δ7Li values to ∼10‰ in the midstream is associated with steeper catchments draining high relief, with shorter flow paths and higher runoff, yielding low δ7Li values that are similar to other mountainous catchments globally where secondary phase formation is limited and Li isotope fractionation is more muted. Very low, rock-like δ7Li values (∼5‰) in the downstream likely reflect congruent weathering of ancient sedimentary rocks exhumed at the foreland of the Pamir mountains. The remarkable range in riverine δ7Li values from this one watershed draining a single mountain range indicates that geomorphology, hydrology and local lithology affect surface weathering and thus riverine δ7Li in complex mountain environments.