Enhanced volcanic activity and long-term warmth in the middle Eocene revealed by mercury and osmium isotopes from IODP Expedition 369 Site U1514.

• A comprehensive Hg and Os isotopic record was reconstructed for the entire Eocene. • Isotopic signals indicate enhanced volcanism and continental weathering in the mid-Eocene. • A prolonged warmth lasting for ∼5 Myr is identified in the mid-Eocene. • A clear link between continental weathering and climate is evident during the MECO. • The Eocene climate is closely linked to active plate tectonic-driven volcanic activity. Rapid plate reorganization may have influenced global climate during the Eocene; however, its linkage remains poorly constrained, particularly during the middle Eocene. To elucidate this tectonic–climatic relationship, here, we conducted a comprehensive analysis based on high-resolution mercury (Hg) and osmium (Os) abundance and isotope data obtained from the complete Eocene sedimentary sequence of Site U1514, drilled in the Mentelle Basin off southwest Australia. The Hg signals in this sedimentary sequence, which are characterized by significantly high enrichment and insignificant mass-independent fractionation (Δ199Hg) signal, confirm that the middle Eocene (∼45–38 Ma) was a period of persistent, increased volcanism, accompanied by intense tectonic activity. In particular, a remarkable seafloor volcanic eruption persisted for approximately 1.5 million years (∼42.0–40.5 Ma), immediately preceding the Middle Eocene Climate Optimum (MECO). Contemporaneously, the trends toward a slightly more radiogenic seawater 187Os/188Os (Os i) composition denote the prevalence of intensified continental weathering under a warm, humid climate during the middle Eocene, a phenomenon particularly evident during the MECO. Importantly, the Hg and Os records from Site U1514 reveal the occurrence of a multi-million-year warming reversal amid the long-term Eocene cooling trend, which likely contributed to significant CO 2 reduction during the late Eocene. These findings significantly enhance our understanding of Eocene climate dynamics, which are fundamentally linked to intensive tectonic-driven volcanic activity and associated continental chemical weathering. [ABSTRACT FROM AUTHOR]