Fine-scale heat flow, shallow heat sources, and decoupled circulation systems at two sea-floor hydrothermal sites, Middle Valley, northern Juan de Fuca Ridge

Fine-scale heat-flow patterns at two areas of active venting in Middle Vall ey, a sedimented rift on the northern Juan de Fuca Ridge, provide thermal e vidence of shallow hydrothermal reservoirs beneath the vent fields. The ext reme variability of heat flow is explained by conductive heating immediatel y adjacent to vents and shallow circulation within sediments above the rese rvoir. This secondary circulation is hydrologically separated from the deep er system feeding the vents by a shallow conductive lid,within the sediment s. A similar separation of shallow and deep circulation may also occur at s ediment-free ridge-crest hydrothermal environments.