New Constraints on the Thermal Power of the TAG Hydrothermal System and the Dynamics of the Water Column Plume

Thermal power is a fundamental physical parameter for characterizing the scale and magnitude of hydrothermal convection at deep-sea vent fields. Despite its importance, this quantity has been difficult to estimate owing to the technical difficulties associated with making direct observations, especially for large fields featuring complex patterns of discharge. The TAG hydrothermal field, a massive sulfide deposit ~200 m in diameter and 80 m high located on the Mid-Atlantic Ridge at 26(°)N, is an example of such a large and complex vent field, with fluids discharging from the seafloor at temperatures ranging from 3(°)C to greater than 360(°)C over large areas of the field. High-temperature discharge is focused through a central chimney that cannot be seen because it is completely obscured by black smoke. The complexity of the fluid discharge at the seafloor has led researchers to use the characteristics of the water column plume as a means to estimate the total thermal power of the TAG hydrothermal system. However, these methods make critical assumptions regarding lateral advection and the entrainment of ambient fluid into the buoyant plume. We use a combination of CTD "tow-yo" transects, numerical models, current meter data, and detailed dive observations (with both DSRV Alvin and ROV Jason II) to place new constraints on the thermal power of the TAG hydrothermal system. Our estimate of ~1.7 GW for the focused high-temperature discharge, compared to previous estimates of 80-450 MW, requires that the buoyant plume entrain considerably more ambient fluid than previously assumed based on laboratory analogs.