Nutrient transition metals in a time series of hydrothermal vent fluids from Main Endeavour Field, Juan de Fuca Ridge, Pacific Ocean

Cabled deep-sea observatories can deliver continuous power and communications linkages to seafloor instruments and sampling devices. Building on this capability, a nine-month time series of hydrothermal vent fluids was recently collected from the well-studied Main Endeavour Field on the Juan de Fuca Ridge (northeast Pacific Ocean) using a novel remotely triggered vent fluid sampling system connected to Ocean Network Canada's NEPTUNE observatory (Seyfried et al., 2022). These samples exhibit very low Mg concentrations (Mg = 0.19–3.07 mmol/kg), indicative of little-to-no contamination by ambient seawater, providing excellent insight into sub-seafloor hydrothermal processes. Here, we present analyses of these samples for transition metals identified as biological nutrients (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, and W) and identify possible controlling processes. Overall, nutrient transition metal concentrations in collected samples reflect dynamic responses to subtle deep-seated and near-surface changes in the hydrothermal system. Roughly two months into the deployment, inmixing of a Mg- and sulfate-rich fluid is decoupled from observed changes in vent fluid temperature but coincides with noticeable decreases in fluid Co and Mo concentrations, likely indicating subtle or more deep-seated cooling of the system and subsurface deposition of these temperature-sensitive metals. Several months later, a ∼20 °C drop in vent fluid temperature from 304 °C to 280–285 °C over ∼20 hours is accompanied by ∼90% decreases in Cu, Zn, and Cd concentrations and an additional decrease in Mo attributable to precipitation of metal sulfides, presumably in the shallow subsurface. Relative stability in concentrations of other metals (V, Cr, Mn, Ni, W) suggests more deeply seated higher-temperature controls, though covariations in Cr and Ni concentrations decoupled from vent fluid temperature suggest subtle, temporally variable lithologic controls. Molybdenum concentrations (29–220 nmol/kg) are higher than expected based on previous analyses of seafloor hydrothermal vent fluids and do not reflect contamination by modern Mo-rich seawater. This finding has implications for understandings of hydrothermal Mo delivery to the ocean, relevant to hypotheses about the evolution of Mo-dependent biological pathways among early life forms in anoxic and Mo-poor ocean environments thought to be prevalent throughout the Archean Eon.