Diffuse venting and background contributions to chemical anomalies in a neutrally buoyant ocean hydrothermal plumeLavelle, J.W., and M.A. Wetzler Journal of Geophysical Research, Oceans, 104, 3201-3209, 1999. Abstract:Plumes originating in high temperature hydrothermal discharges entrain background water and often entrain diffuse source fluids during convective ascent. The mixing of fluid from those three different sources determines the chemical composition of ensuing neutrally buoyant plumes. Here convection models and aluminum data from the Trans-Atlantic Geotraverse (TAG) site are used to examine relative contributions of focused, diffuse, and background sources to Al anomalies in the plume. The analysis shows that anomalously high Al values observed just above the seafloor, which we assume stem from diffuse flows, can contribute as much as 50% to the Al anomaly in the neutrally buoyant plume found >350 meters above bottom at TAG. Background water contributes nearly an equal fraction of Al to the anomaly. Both entrained contributions depend on gradients in the respective Al profiles external to the rising plume. The amount of entrainment depends, in part, on effective source area: distributed high temperature vents entrain more fluid than a single composite high temperature vent. Larger effective source area is particularly significant for entrainment of diffuse source Al or other tracers trapped within a few tens of meters of the seafloor. Downstream, cross-stream, and temporal variabilities exhibited in the three-dimensional model plume also underscore problems of sampling and interpreting actual plume data. |