| Summary: | Black smokers are the dramatic result of seawater being heated to high temperatures (generally 250° to 350°C) by magmatic systems, then discharging at the seafloor. However, not all seawater that circulates through the oceanic crust is heated to high temperatures. "Diffuse flow" is a catchall term to describe low-temperature (< 0.2° to ~ 100°C) fluids that slowly discharge through sulfide mounds, fractured lava flows, and assemblages of bacterial mats and macrofauna. Diffuse-flow fluids are generally mixtures of cold seawater and either magmatically heated fluids, conductively heated seawater, or both. Although the limited data indicate that 50–90% of the hydrothermal heat loss occurs as diffuse flow at the seafloor, modeling results coupled with geochemical data suggest that nearly 90% of the heat loss ultimately stems from magmatically heated fluids. There is a critical need to obtain more diffuse-flow measurements to improve models of heat and geochemical fluxes, better understand subsurface fluid flow dynamics, and determine the extent of the subsurface biosphere as well as the spatial and temporal variability of diffuse flow. New measurement techniques and diffuse-flow models provide insight into the characteristics of these subsurface fluids and their manifestation at the seafloor.
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