Investigating ophiolite weathering via lithium isotopes in the Indo-Burma range of northeast India

The weathering of silicate minerals regulates climate on million-year timescales. Some silicate bedrock, particularly ophiolites, are more susceptible to enhanced weathering than other lithologies. Lithium isotopes (δ⁷Li) are a proxy that can be used to help track weathering processes due to the fractionation of Li during secondary clay mineral formation. Here we present data collected from tributaries that source the Nagaland-Manipur Ophiolite Complexes in northeastern India, which ultimately flow into the Irrawaddy River in Myanmar, to test the hypothesis that the weathering of ultramafic terrains generated by arc-continent collisions can drive rapid atmospheric carbon drawdown. Major cations, anions, trace elements, δ⁷Li, and clay mineralogical compositions were measured from river water, bedload, and suspended sediment to gain insight into silicate weathering processes in the region. Our results show streams and rivers in this region contain δ⁷Li values that range from 20.6 to 31.2‰, with the ophiolitic-sourced tributaries being heavier than the global riverine average of ~23‰. This indicates that rivers draining ultramafic lithologies in warm humid climates are experiencing higher degrees of weathering intensity than other drainages comprised of more felsic lithologies. Additionally, XRD results show that most river bedload contains smectite clays, which may help promote organic carbon burial. These data combined highlight new pathways toward understanding silicate weathering as it relates to atmospheric CO₂ drawdown.