Evidence for enhanced volcanism and weathering during the middle Eocene from mercury and osmium isotope records

Rapid plate reorganization may have influenced global climate during the Eocene, but the linkage between them, especially during the middle Eocene, remains poorly constrained. In this study, we reconstructed high-resolution records of mercury (Hg) and osmium (Os) abundances and isotopes from the Eocene sedimentary sequence of Site U1514, drilled in the Mentelle Basin off southwestern Australia, to gain insights into the interplay between tectonics, volcanism, and continental weathering during the middle Eocene. A comprehensive analysis of these isotopes confirms that the middle Eocene (~45–38 Ma) experienced extensive volcanic activity, evident from notably elevated Hg enrichments as well as near-zero Δ199Hg values. This enhancement of volcanic activity is intricately linked to widespread plate tectonic activity in the Southern Ocean. The prolonged and enhanced volcanic activity is likely to have played a crucial role in triggering global warming incidents, including the Late Lutetian Thermal Maximum (LLTM) and middle Eocene Climate Optimum (MECO). The analysis of seawater 187Os/188Os (Osi) in conjunction with supporting data (i.e., influx in terrigenous materials and elemental indices) underscores that the middle Eocene was a critical period in the intensification of continental weathering response to the CO2 rise and warming. This intensified continental weathering is most prominent during the MECO warming, further supporting the prevalence of warmer and more humid climate conditions.