Tectonically induced changes in vent fluid compositions and metal concentrations at Main Endeavour Field, northeast Pacific Ocean

• A nine-month time series of seafloor hydrothermal vent fluids is compared with contemporaneous records of vent fluid temperature and local ocean bottom seismicity. • Observed changes in vent fluid chemistry and/or temperature are preceded by swarms of small magnitude, likely shallow-origin, earthquakes. • Changes in vent fluid chemistry include a transient Mg- and sulfate-bearing component attributed to low-temperature hydrothermal fluid, and separately, a ∼20 °C temperature drop in vent fluid temperature that induces a sharp decrease in elements associated with seafloor massive sulfide deposition. Seafloor hydrothermal vents are dynamic systems that respond to changes in magmatic heat fluxes, crustal permeability and connectivity, and mineral precipitation. However, the logistical challenges associated with marine science operations often preclude direct observation of these dynamics. Here, we utilize infrastructure of Ocean Network Canada’s NEPTUNE cabled observatory to compare time-series vent fluid samples with contemporaneous records of vent fluid temperature and ocean bottom seismicity at the Main Endeavour Field, northeast Pacific Ocean. We find that previously reported changes in vent fluid chemistry coincide with episodes of increased earthquake activity. Ratios of Mg to other fluid mobile elements are consistent with influx of a Mg- and sulfate-depleted fluid, which we attribute to active circulation of intermediate-temperature hydrothermal fluids within shallow seabed lavas. Concentrations of chalcophile transition metals and metalloids are strongly correlated and sensitive to vent fluid temperature, which we propose indicates differential precipitation/dissolution of sulfide minerals. Episodes of heightened microearthquake activity associated with changes in vent fluid chemistry and/or temperature are characterized by swarm-like bursts of relatively small magnitude, likely shallow-origin, earthquakes. Together, these data highlight the sensitivity of vent fluid temperature and chemistry to seismic activity that alters near-surface permeability and connectivity between seawater and various hydrothermal reservoirs.