Near-Real-Time Geochemical Constraints on the April 2025 Mid-Ocean Ridge Eruption at the East Pacific Rise 9°N

Most of Earth’s eruptive volcanism occurs along mid-ocean ridges (MORs), yet direct observations of eruptions and their immediate hydrothermal consequences remain rare. On 29 April 2025, the science party of R/V Atlantis Expedition AT50-36 directly observed a long-anticipated eruption at the East Pacific Rise (EPR) 9°N only hours after its onset, representing the most rapidly detected and documented deep-sea MOR eruption to date. The expedition occupied the area from 10 April to 3 May 2025, allowing characterization of hydrothermal and water-column conditions from ~14 days before the eruption to ~96 hours afterward.  In this communication we present on-board fluid geochemistry measurements documenting both pre-eruption vent fluid chemistry and post-eruption water-column responses. High-temperature and diffuse-flow fluids were sampled at the Bio9, P Vent, Tica, BioVent, and YBW vent fields during DSV Alvin dives conducted in the days preceding the eruption. Post-eruption bottom waters were investigated using a CTD–rosette system equipped with an in situ electrochemical analyzer. Pre-eruption measurements of dissolved Fe (dFe), H₂S, dissolved Mn (dMn), and pH show elevated H₂S:T and H₂S:dFe ratios relative to previous years at EPR 9°N, consistent with subsurface phase separation and volatile-enriched hydrothermal fluids prior to eruption. Immediately following the eruption, high-temperature vent sources could not be accessed due to aborted Alvin dives; however, CTD profiles revealed pronounced bottom-water anomalies in at least one of pH (up to 0.8 units), H₂S (up to 70 µM), or dFe (up to 841 nM) at CTD stations conducted over Bio9, P Vent, Tica, BioVent vent fields. These geochemical anomalies were spatially widespread along the ridge axis and extended to at least 10 m above the seafloor, with pH and temperature perturbations closely coupled to elevated H₂S concentrations. Although temperature anomalies in bottom waters decayed within four days of the eruption, pH, H₂S, and dFe anomalies persisted. A CTD cast conducted four days post-eruption revealed, via the rosette-mounted electrochemical analyzer, H₂S concentrations of up to 40 µM coincident with turbidity and redox potential anomalies extending to at least 600 m above the seafloor, indicating the development of a vertically extensive hydrothermal megaplume. Such concentration ranges are typically confined to the immediate vicinity of black smoker orifices, highlighting the exceptional spatial scale of hydrothermal discharge following this eruptive event. The near-real-time observation of this MOR eruption provides new constraints on eruption-triggered hydrothermal fluxes, plume formation, and the role of episodic volcanic events in modulating ridge-axis hydrothermal systems.