Heat flow through a basaltic outcrop on a sedimented young ridge flank

One hundred seven thermal gradients were measured in shallow surface sediments using the submersible Alvin within a 0.5 km(2) area on and around the 65-m-high, mostly sediment-covered Baby Bare outcrop located on the eastern flank of the Juan de Fuca Ridge. Heat flow values range from 0.35 W m(-2), which is close to the average value (0.27 W m(-2)) for the sediment-buried 3.5-Myr-old crust surrounding the outcrop, to as high as 490 W m(-2). Some measurements are purely conductive, whereas others are consistent with upward fluid seepage through this sediment layer. Highest heat flow values are found roughly 10 m below the summit along a ridge-parallel fault where shimmering water, springs, and communities of clams were found. Heat flow values surrounding a second fault are elevated to a lesser extent (maximum of 9.2 W m(-2)). The total power output from this 0.5 km 2 area is 1.5 MW, about 10 times greater than the conductive power output predicted for a commensurate area of 3.5-Myr crust. Much of this heat loss is conductive (similar to84%), consistent with an independent estimate of the convective heat flux from Rn/heat anomalies in spring fluids and in the water column above the springs. Calculations suggest that the 64degreesC isotherm, which is the temperature in the surrounding upper crust, is <20 m below the summit, corresponding to a height that is 30 m above the surrounding turbidite plain. These elevated fluid temperatures at shallow depths provide thermal buoyance to drive seafloor seepage from the outcrop.