Chasing plumes over the Rainbow vent

News date: 
April 17, 2015

The first leg of 64PE398 cruise with RV Pelagia has just come to its end, bringing us back to the harbor of the island of Faial (photo, right) in the Azores, after ten intense days of sailing and sampling above the Rainbow hydrothermal vent field. The cruise brought together scientists from the NIOZ (physicists, biologists, and geologists) and from the University of the Azores (biologists), combining the scientific goals of the MIDAS project and of the Dutch TREASURE project, both devoted to the investigation of the impact of deep sea mining in complex areas such as the Mid Atlantic Ridge.

Despite a rough start, the weather rapidly improved, allowing us to follow our initial plan. We were able to smoothly recover all three MIDAS moorings, that will provide us about 5 months of continuous observations of current velocities in the bottom 600 m, in the near and far field of the Rainbow chimneys. Unfortunately, not all instruments worked as planned, and our hydrographic measurements will not be as complete as we have wished. We will have another chance next year, since one of the MIDAS mooring will be redeployed in the second leg of the cruise, to be picked up in May 2016.

During the cruise, especially during the nights, we also had the occasion to continue the hydrodynamic and hydrographic survey started last year, during the previous MIDAS-TREASURE cruise, sampling the area around the vent with surface to bottom CTD (conductivity, temperature and depth measurements, equipped also with oxygen and turbidity sensors) and lowered ADCP, to assess instantaneous velocities.

 

While last year no plume signature was detected south-southwest of the vent field, this year we have observed a strong signature in turbidity as far as 4.2 km southwest of the plume source. Moreover, the plume has been also detected at about 3 km right north of the source, showing that the strong topographic control observed last year is not always present around the northern tip of the Rainbow hill, and that the area affected by the plume might be larger than observed last year. Most importantly, these observations show that the plume dispersal pattern is highly variable in time. The time series of current velocities that we just recovered will certainly help in shedding some light on the time scales of these variability, and on the main responsible mechanisms.

Left: The MIDAS-TREASURE team and the crew of RV Pelagia during the first leg of 64PE398 cruise. Photo by M. Lavaleye.

 

For the MIDAS team, it is now the time to start the data analysis, while we wish a safe and successful second part of the cruise to our TREASURE colleagues that are still at sea!