Magnitude and variance of near-bottom horizontal heat flux at the Main Endeavour hydrothermal vent field

We characterize hydrothermal plumes and hydrography within the axial valley of the Endeavour segment (Juan de Fuca ridge, northeast Pacific) using data from an autonomous underwater vehicle, a lowered CTD, and two current meter moorings. Our survey of the Main Endeavour hydrothermal vent field (MEF) shows hydrographic variability on scales as short as ∼20 m and ∼10 min. Within the ∼100 m deep axial valley, the mean flow is 1–5 cm/s to the north, and tidal oscillations have ∼5 cm/s amplitude. Combining the mean flow with an estimate of the difference in average temperature observed north and south of MEF, we find that the net along‐axis horizontal heat flux has a mean magnitude of ∼8–42 MW. An advection/diffusion model forced by local current records explains the observed temporal hydrographic variability and shows that mean horizontal flux variance is correspondingly high (standard deviation ∼63 MW). A steady state MEF heat budget, constrained by the observed horizontal flux and measurements of vertical fluxes from other studies, has two implications: the MEF heat flux is partitioned about equally between diffuse and focused vents, and about 85% of the flux from diffuse plumes is entrained by focused plumes.