Increased primary mineral dissolution control on a terrestrial silicate lithium isotope record during the middle Miocene Climate Optimum

Lithium isotopes have great potential to fractionate at subaerial environment, so they are promising proxies for tracing chemical weathering history. However, it remains uncertain how to relate lithium iso-topic compositions (87Li) in sediments to silicate weathering and past climate in terrestrial setting. To tackle this question, we choose the warm and wet middle Miocene climate optimum (MMCO, 17- 14 Ma) with large climate variation amplitude and the following cold and dry middle Miocene climate cooling period (MMCC, 14-11 Ma) with damped climatic variation amplitude, as two climate contrasting intervals to study lithium isotopes and content ([Li]) variations of terrestrial mudrocks. Our study site is located at the eastern Qaidam Basin of the northeastern Tibetan Plateau and the precipitation pattern of this part of the basin is similar to the Chinese Loess Plateau over the two studied intervals. The study site is suitable for clarifying link between climate and 87Li variations because previous studies have demon-strated no provenance shift over the studied interval and short transport distance from source to the study site suggests little chemical weathering over the transportation process. Both 87Li and [Li] values show larger amplitude over the MMCO (spanning from-1.2 to 3.2 % and from 32.8 to 93.9 ug/g, respec-tively) than the MMCC interval (-1.8 to 1.2 % and 41.7-80.8 ug/g, respectively), consistent with general feature of global and Chinese Loess Plateau climate. However, 87Li data are higher over the MMCO in comparison with the MMCC, contrary to expectation of more lithium isotope fractionation at warm inter -val. We interpret the more positive 87Li values over the MMCO using increased ratio of primary mineral dissolution over secondary mineral precipitation. Qaidam mudrock and benthic Li records show comple-mentary changes, suggesting their possible link. Furthermore, past research reveals lack of focused uplift of the northeastern Tibetan Plateau over the studied 18-11 Ma. So we conclude that global climate, rather than Tibetan uplift, played an important role in modulating inland Qaidam Basin climate variations over the middle Miocene and that increased chemical weathering over warm climate can lead to a small mag-nitude of lithium isotope fractionation. Therefore, interpreting chemical weathering history, and hence climate, using lithium isotope data in terrestrial setting requires considering relative importance of pri-mary mineral dissolution versus secondary mineral precipitation.(c) 2023 Elsevier Ltd. All rights reserved.