Estimations of heat transfer from Grotto's North Tower: A NEPTUNE Observatory case study

The overall heat transfer through an active hydrothermal sulfide edifice, in particular the North Tower of Grotto, has been estimated at 80 MW or greater based on the following in situ measurements and assumptions: (1) The heat transfer by diffuse flow is estimated at 33-380 MW based on extrapolating the acoustically mapped area to all sides of the North Tower (“visible” area=30 m2; extrapolated area=100 m2) and using the range of available spot measurements of temperature (6-23 °C) and vertical velocity (0.07-0.28 m/s). The lower number (33 MW) is more likely, but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain. (2) The heat transfer by focused flow is estimated at 30-70 MW based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges. (3) Conductive heat transfer out of the mound is unknown, but is likely to be much less than the advective heat transfer. Additionally, the plume transport of heat is estimated at 20-40 MW based on the direct measurement of temperature within the plume (at 5-25 m above the top of the edifice). Despite uncertainties, the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow. Furthermore, not all plumes from individual smokers may merge even for so small an area as the North Tower of Grotto.