Posteruption Enhancement of Hydrothermal Activity: A 33‐Year, Multieruption Time Series at Axial Seamount (Juan de Fuca Ridge)

Mid-ocean ridge eruptions, initiating or revitalizing hydrothermal discharge and disrupting seafloor ecosystems, occur regularly as a consequence of plate spreading. Evaluating their impact on long-term hydrothermal discharge requires information on the scale and duration of any posteruption enhancement. Here we describe a unique hydrothermal plume time series of annual (or more frequent) observations at Axial Seamount vent fields from 1985 through 2017, missing only 7years. Axial, a hot spot volcano astride the Juan de Fuca Ridge, experienced eruptions in 1998, 2011, and 2015. In 1998 and 2011 lava flooded the SE caldera and south rift zone, but in 2015 most lava was extruded in a series of flows extending 20km down the north rift zone. Response cruises occurred within 18days (1998) to about 4months, followed by regular posteruption observations. All 30 cruises measured plume rise height (a proxy for heat flux) and turbidity (indicative of chemical changes in vent discharge) at several vent sites, yielding an integrated view of vent field activity. Venting in the SE caldera area persisted throughout the time series, consistent with the imaged location of the shallowest portion of the melt-rich magma reservoir. Eruptions produced substantial and diagnostic increases in plume rise and turbidity, and posteruption enhancements lasted 2-5years, totaling 10years over the course of the time series. Estimates of the relative heat flux indicate a sixfold increase during eruption-enhanced periods, implying that generalizations about mid-ocean ridge hydrothermal fluxes may be underestimates if based on non-eruption-enhanced hydrothermal activity alone. Plain Language Summary We use a three-decade-long time series to understand how seafloor volcanic eruptions affect hydrothermal venting. Axial Seamount, 1,400m deep and about 400km off the coast of Washington state, has erupted in 1998, 2011, and 2015. Each eruption caused a substantial increase, lasting 2 to 5years, in the discharge of hot fluids and chemicals from the seafloor. These eruption periods thus contributed more heat and chemicals to the ocean than the 20years with no eruption activity. Seafloor eruptions may thus play a more important role in hydrothermal venting than previously expected.