Getting Down With the CTD
Today is a great day for science on the Southern Ocean: the weather is overcast but (so far) dry, and the seas are relatively calm with wave heights of 9-11 feet (at the time of this writing). One of the first things many of us do in the morning is check the weather map, which also indicates the likely wave height we’ll experience during the day. Wave height (and the sea state in general) is one of the main factors that affects our daily schedule. If the weather becomes too rough, the waves too large, operations shut down for safety reasons, both for the people and the equipment.
Providing the weather holds, we hope to get through our entire range of water, particle, and sediment captures today and hopefully even send down the JPC (jumbo piston corer), which will be used by one of our other two small science parties aboard. Many of us are looking forward to seeing the JPC in action. It’s a huge piece of equipment that can penetrate deeper into the seafloor and capture sediment from several to tens of meters below the sediment water interface. We will watch from a safe vantage point, as it’s also one of the most dangerous pieces of equipment used on a research vessel such as ours.
To give you an idea of all we hope to achieve today, here is our schedule*:
0800: Arrive onsite and survey
1000: Send down the CTD.
1230: Deploy McLane pumps
1830: Deploy Kastencore
2000: Jumbo piston corer
*Subject to weather delays and order changes
So far, we are right on schedule, and the McLane pumps are currently being deployed as I write this. After a couple of frustrating days with coring problems and the delays they cause, I’m certain everyone feels relief to be right on track today.
This morning, I am visiting with Janice and Sheldon, our ET (electrical technician), as they monitor the CTD’s descent. A couple of hours before, we arrived at our next sampling site around the latitude of 61.88˚ S. The first step, as always, was the survey to find a flattish area of seafloor that appears to have sediment.
The area is surveyed in two directions (at 90 degrees to one another). The red arrows show the same site from these two directions, and it looks very suitable for coring: flat and with thick sediment (as evidenced by the depth of the black and grey.)
When the ship is directly over the site, Sheldon confers with the marine technicians and the winch operator via radio.
The CTD sensors sit below the rosette of Niskin bottles.
There is one last check to make sure all 24 Niskin bottles attached to the CTD, which will collect water samples during the return to the surface, are cocked (open) and that the lanyards which trigger them to close are all secure and functioning.
Our onscreen view of the Baltic Room, showing the winch cable in the foreground and the CTD about to be lowered outside. A marine technician steadies the equipment and watches for problems.
Then the CTD is lowered into the water, which we observe on the monitor.
The CTD is lowered out of the Baltic Room, as seen on the monitor. (The red boat in front is one of our two lifeboats. The smaller boat behind it is the launch for shore excursion or small boat sampling when the NBP can’t dock or get to a shallow spot to sample). As well as the two lifeboats, we also have a number of large life rafts.)
The CTD has a pump that draws water past its sensors. The first step is to briefly soak the rig at 10m (the pump turns on automatically in seawater) to ensure the sensors are fully flushed with seawater so they’ll work properly. Then the CTD is brought back up to a depth of about 5 m before steadily being lowered, taking readings along the way. Later, as it returns, Sheldon will “fire” or trigger the Niskin bottles to close, collecting water at pre-chosen depths in the water column.
Janice and Sheldon watch the CTD’s descent on the computer screen. Along the way, the sensors track the following, which are color-coded:
Green: fluorescence (an indicator of the amount of chlorophyll in the water)
As the depth increases, the temperature first decreases also, while the change in oxygen and salinity (at these shallow depths) are minimal. You can see the spike in the fluorescence (the green line) at around 60 m. This is where the chlorophyll maximum is—which indicates we will find the greatest density of phytoplankton here.
Pat uses this information when calculating where to deploy the McLane pumps, as he wants to collect particles from the chlorophyll maximum. We also collect water from the chlorophyll maximum specifically. While two bottles were supposed to be fired at 50 m and two at 75 m, it is normal to adjust one of these depths to capture water from the chlorophyll maximum. Today, the consensus is that Sheldon will fire the two bottles assigned to 50 m at 65 m instead.
The CTD also measures the clarity of the water and its density. Another window shows the reading of the heading sensor, which is basically a compass that determines whether the CTD is spinning. We want any rotating to be minimal, otherwise the winch cable can be damaged. The motion of the ship affects this, too, so the crew are very aware of keeping the boat steady. A usual, each operation is a coordinated team effort.
Janice and Sheldon are jolly as they sit side by side for the long wait. Like so many people who work in Antarctica, they have known each for many years and have worked together in different settings. Their conversation makes me chuckle as they reminisce about old times and joke together.
Finally, the CTD is approaching its destination depth of 1500 meters. Sheldon is ready with the radio. Right on 1500 meters, he says, “All stop.”
“All stop,” repeats the winch operator and halts the winch.
Sheldon fires two Niskin bottles to collect water and instructs the winch operator to retract the cable and the speed at which he should do so. They’ll repeat this process until they have taken water samples from all twelve predetermined depths.
Once the equipment returns to the surface, there’ll be a flurry of activity as Mark’s team transfers water from the Niskin bottles to up to eight different sample bottles, but that is a story for another day…