Mercury in foreland basin sediments of the central northern Tibet: evidence for non-volcanic controls in a late Eocene – early Miocene Mercury record

The uplift of the Tibetan Plateau profoundly influences Asian climate and surface processes, yet records of Cenozoic volcanic activity and its environmental responses remain sparse. Mercury (Hg) serves as a powerful volatile tracer for volcanic emissions and its sedimentary record often offers a unique opportunity to identify volcanic events where other proxies are lacking. In this study, we carried out a Hg concentration study in fluvial-lacustrine sediments from the Tuotuohe Basin, providing the first continuous record of Late Eocene to Early Miocene mercury concentration changes over the Tibetan Plateau to date. Our data reveal that depositional conditions —not volcanic flux— primarily controls sedimentary Hg preservation. Paradoxically, Hg concentrations significantly increased after ~25 Ma during volcanically quiescent periods, contrasting with absent enrichment during earlier volcanic peaks (37–33 Ma and 28.5–27 Ma). This decoupling stems from contrasting Hg sequestration mechanisms: (1) Pre-25 Ma, oxidizing fluvial sandstones and limited adsorption capacity suppressed preservation despite active volcanism; (2) Post-25 Ma lacustrine mudstones facilitated Hg retention via clay mineral adsorption, Fe S synergism, and reducing conditions. The shift to enhanced Hg preservation coincides with Late Oligocene warming and South Asian Monsoon intensification (~26 Ma), which expanded lakes and intensified chemical weathering—increasing clay flux (Al content) and creating reducing basins. Our findings demonstrate that depositional conditions can overprint primary volcanic Hg signals, necessitating rigorous facies evaluation when applying Hg as a proxy for volcanism in terrestrial archives. • First continuous Hg record in central northern Tibet spanning ca. 37–20 Ma. • Hg peaks anti-correlate with volcanism due to oxidizing conditions. • Monsoon-driven lacustrine expansion boosted Hg capture via clay adsorption post-25 Ma.