NEAR-FIELD VARIABILITY OF THE TAG NONBUOYANT PLUME, 26-DEGREES-N, MID-ATLANTIC RIDGE

We report the initial results of a nine hour monitoring of the non-buo yant plume above the TAG hydrothermal vent field at 26-degrees-N on th e Mid-Atlantic Ridge. A CTD (conductivity, temperature and pressure) a nd nephelometer (particle sensor) were continuously raised and lowered through the non-buoyant plume for nine hours at a site situated appro ximately 100 m south of the hydrothermal mound. This method permits a detailed record of the variability of the non-buoyant plume close to t he vent field to be made. Our results show that the particulate maximu m of the non-buoyant plume appears approximately 150 m lower in the wa ter column at the start and end of the record, compared with profiles recorded towards the middle of the record. A corresponding structure a lso appears in the temperature and salinity records, with the result t hat the plume particle maximum resides on the sigma(theta) almost-equa l-to 27.8035 isopycnal throughout. Modelling of the rise height of an axisymmetric plume can only account for approximately 95 m of the vari ability in the final level of neutral buoyancy. Plume bending due to c urrents is a likely explanation for the recorded temporal variability.