An Atlantic hydrothermal plume: Trans‐Atlantic geotraverse (TAG) area, Mid‐Atlantic Ridge crest near 26°N
Authors: Rona Peter A. , Speer Kevin G. 
Published: February 2008

The physical characteristics of an Atlantic hydrothermal plume and its seafloor sources are described from the first data set of water column properties associated with a high‐temperature source area at a slow‐spreading oceanic ridge. The observations comprise five near‐bottom tows of a camera(color video and still)‐temperature sensor array through the buoyant portion and 23 conductivity‐temperature‐depth (CTD) profiles through the neutrally buoyant portion of the plume made at the Trans‐Atlantic Geotraverse Hydrothermal Field in the rift valley of the Mid‐Atlantic Ridge at 26°08′N, 44°49′W in July 1985. The source area is a mound up to 250 m wide and 50 m high constructed primarily of massive sulfides between depths of 3620 and 3670 m at the base of the east wall. Flow and discharge regimes systematically changed from the center to the edge of the mound. High‐temperature black smokers vented at fast rates (>1 m/s) from discrete sulfide chimneys and at slow rates from fractures, and clear solutions vented at slow rates from diffuse sources in the inner zone of the mound; intermediate‐temperature blue‐white and white smokers vented at slow rates from discrete sulfide/sulfate chimneys, and clear solutions vented from diffuse sources in the middle zone; patchy, diffuse discharge of clear solutions through sulfide talus occurred in the outer zone. The diffuse discharge of clear solutions and the discrete discharge from the white smokers were observed to turbulently rise up to several meters above the seafloor where the discharge was laterally advected in prevailing laminar flow at several centimeters per second. A convective heat flux of 8.8×106 W for only the black smokers that vented from fractures was estimated by applying a buoyant plume model to temperature anomalies measured with the towed array; this value is intermediate between values estimated for entire vent fields at the fast‐spreading East Pacific Rise. Discharge from the black smokers rose to form the neutrally buoyant portion of the plume that was cold and fresh relative to surrounding seawater, where it attained density equilibrium between 190 and 350 m above the mound. In contrast, hydrothermal plumes observed in the Pacific were warm and salty relative to surrounding seawater. The difference is attributed to the effect on plume density of entrainment during rise through opposite salinity profiles that decrease downward in the Atlantic and increase downward in the Pacific, resulting in different temperature‐salinity compensations at neutral buoyancy. The temperature difference between the neutrally buoyant portion of the plume and surrounding seawater (negative for the Atlantic; positive for the Pacific) is not a unique function of convective heat flux from a source area, as a consequence of both the effect of the entrainment of salinity on plume density and the separation of diffuse and discrete components of discharge. The hydrothermal plume and the hydrothermal mound are inferred to have grown in size and complexity in response to the development of hydrothermal flow and discharge regimes for at least the past 104 years.