Hydrothermal vents provide a continual supply of fluid that spreads first vertically (h) and then horizontally (Ld). This creates a cyclonic flow near the bottom and then an anticyclonic flow as the fluid spread horizontally.
To better understand how chemical tracers from the hydrothermal vents spreads over the ocean, the physics team’s work is crucial.
On Fridge JC156, the physics team have two goals. The first one is to try to identify, for the first time, mixing and instabilities in a horizontal scale of 100m. Indeed, previous studies focus on the measurement of large processes and now the physics team want to measure small scale processes as the velocity between the anticyclonic and cyclonic flow of the vents. The second one is to calibrate and process all the basic sensors, essential to obtain data such as current measurements, salinity, temperature …To do so, they are using several techniques and sensors that are described below.
- Measuring small scale processes
Prof. Ric Williams from the University of Liverpool and Dr. Carl Spingys, a post doc at the Univerity of Southampton, are in charge on this cruise to measure small instabilities and map the plume with a small resolution. To do so, they are using:
- The buoyancy frequency (N/s) and the PV
The data obtained thanks to the CTD allowed the physic team to calculate variables useful to determine the scale of the plume: the buoyancy frequency (N/s) and the PV. The buoyancy frequency (N/s) measure the background stratification (the vertical density gradient). It depends on the height of the plume, the buoyancy flux and the range.
The PV is a dynamic tracer. When PV< 0, instabilities occur (gravitational, centrifugal, symmetric instabilities and even horizontal propagation). Therefore PV is used to distinguish different mixing regimes.
- The Tow Yo’s
A Tow-yo is where a CTD, Eh and LSS sensor are moved up/down in deep water along a large distance to track the plume. The ship is moving very slowly (0.2 knots) and the sensors measure at a 100m scale. The obtained data helps to find the vents and to map small scale features around the vent site.
- The VMP
The VMP (Vertical Microstructure profiler) is used to study ocean microstructure (caused by turbulence). It measures very small-scale variations (cm scale) of temperature, salinity, and velocity. The VMP is released into the water and then it sinks at 0.6 m/s on its own thanks to ballast weights attached to it. When it reaches the pre-set desired depth, a trigger mechanism is fired, causing the weights to be released, and the instrument to float back to the surface. The instrument is then picked up by the ship. These deployments take about 4 hours from the time we release the instrument until it returns to the surface. The data is collected by an external sensor and recorded to a disk drive inside the instrument giving us a full depth profile to a depth of 4000m on this cruise (the instrument can go until 6000m in theory).
2.Calibrating and measuring sensors datas
Dave Byrne and Kathrine Turner, both PhD students at the University of Liverpool, are on JC156 in charge of the routine calibration and processing of the ADCP, CTD, Eh and LSS sensors.
- The ADCP
The ADCP (Acoustic Doppler Current Profiler) is an instrument measuring small scale water currents using sounds in a water column up to 1000m long. It works by transmitting “pings” of sound at a constant frequency into the water that ricochet off particles suspended in the moving water and reflect back to the instrument with different frequency waves. By measuring the time it takes for the waves to bounce back and the Doppler shift (difference in frequency between the wave and the profiler), the profiler can measure current speed at many different depths with each series of pings. On the James Cook, we have four ADCPs, two mounted to the bottom of the ship to take constant current measurement as the boat moves. And two going down with the rosette called LADCPs (Lowed Acoustic Doppler Currents Profiler).
- The CTD sensors
The CTD sensors (Conductivity Temperature and Depth sensors) are instruments present on both rosettes that measure conductivity, temperature and depth. After treatments, all the variables useful to follow biological activity and water masses are calculated (salinity, pressure, oxygen, fluorescence, potential density, potential temperature, beam transmission…). These sensors have to be calibrated often with real samples, because the data are crucial during the cast to choose where to collect samples and after to interpret the results.
- The Eh sensors
The Eh sensors measures the seawater electropotential relative to a reference electrode (also called oxidation reduction potential sensor). The way this instrument works is described in the previous post “Meet the Stainless-Steel Team: part1”. Two of them are used on JC156, one on both rosettes to detect anomalies possibly due to hydrothermal vents.
- The LSS sensor
The LSS sensor (Light Scatter Sensor) is used to detect particulate matter variations. The output depends on the nature of the particulates matter and will vary with changes in particle size distribution, shape, index of refraction, organic/inorganic content etc… They work measuring the back-scattered light from the particulate matter.
During this cruise, the physics team expectations are:
- To process data and calibrate all the physics sensors.
- To measure, for the first time, small scale instabilities near vent plumes with a small resolution.
- Prof. Ric Williams, Dr. Carl Spingys, PhD students Dave Byrne and Kathrine Turner