Day 4 First CTD

Proposed cruise track

CTD rosette on deck

Bosun at the winch controls

The CTD is deployed

Monitoring the progress of the CTD deployment

There seems to be a motto about research cruises: "Hurry up and wait!" so while everyone has been busy on the ship, there has also been a lot of waiting. Unscheduled flight stopovers, refueling delays, unexpected weather etc. The location of the shortfetch work on air-sea gas exchanges to the west of the Falkland Islands was dictated by the weather (we needed enough wind and breaking waves with whitecaps), but
today arrived at our first scheduled site. One picture above shows the cruise plan - the fat dots are for taking sediment cores (for paleoceanography) and the occasional CTD cast, and the regularly spaced blue dots mark the sites fo the CTD transect (for physical oceanography).

The JCR had been steaming all night to arrive at the green dot, and it was time for some action! The physical oceanography team (i.e. incl. me!) was first up with their CTD cast. A CTD measures Conductivity-Temperature-Depth which is used to determine the physical paremeters of seawater, such as its density at various different depths from which pther parameters like the sound speed can be derived.

Why do we need to know the density of seawater? Heavier water sinks and lighter water rises, so slight changes in density can send huge chunks of water on the move and ocean currents develop. And we are here to study the deep ocean currents flowing out of Antarctica, so measuring the exact density is crucial for a complete picture. Density is calculated from temperature, salinity and depth - the salinity is measured with a conductivity probe and the depth comes from a pressure sensor on the instrument. See my previous post  (Day 2 Salinity labwork) on how this is calibrated to very exact values.

Why do we need to know the sound speed? Well, it is e.g. used to calibrate the echo sounder that tells the ship how deep the water is. The echo sounder works by sending out acoustic "pings" of sound and measures how long it takes for them to be reflected off the seafloor. So if you measure the time of the echo and you know how fast sounds travels in water, you know the depth of water under the keel. As sound travels slower in denser water, and faster in lighter water we use the profile of density from the CTD probe to calibrate the various depth sounders on the ship (there are many, and I'll describe some of them at a later
date). Without knowing the precise density distribution of the water, the depth sounders may well be a few metres out, hence the density measurements from the CTD come in handy.

The CTD itself is mounted on a larger frame holding an array of water sampling bottles, known as a rosette. Each bottle can be individually electronically triggered to close at both ends at a given depth. This traps a sample of water inside which can be analysed further. To prepare the CTD before deployment we "cocked" all bottles in the open position, and ran the pre-deployment checks on the control computer. The equipment passed all the tests, and the probe was ready to go. I went up to the winch room to watch the deployment, as the deck work is done by the ship's experienced crew. The bosun operated the winch and dipped the CTD carefully into the ocean (photo above). First it goes to 10 metres where all the instruments are tested again. When all the readings check out the probe comes back up from the surface and the begings its descent to the bottom.

The winch drops the probe at 60 metres per minutes, or 1 metre per second. We were in relatively "shallow" water for this deployment - only 600m, so the descent didn't take too long. During the descent we monitored the readings from the CTD probe (photo above). We also monitor the amount of cable that is spooled off the winch drum. When this suggests that the probe is getting near the bottom the altimeter takes over. This is an instrument which measures the distance down to the sea bed. Its readings came in as a yellow line that was cautiously creeping towards zero. This is a reading you do not ever want to get - hitting the bottom could mean the potential loss or damage of hundreds of thousands of pouinds worth of high precision equipment. A few tense moments later the altimetre reading had stabilised at 10m. Everything was fine - the winch had stopped and the probe now dangled 10m above the sea floor in 600m depth.

Everything was now ready for the descent back up to the surface, but we still hadn't operated any of the water sampling bottles. A few mouse clicks on the control terminal and the first bottle "fired". An electronic pulse is sent down the wire to release a magnetic trigger that has been holding the bottle open like a mouse trap. When the spring loaded mechnism releases the water at the given depth is sealed tight. We fired a couple of bottles at the bottom and several more on the way up until the whole set of bottles has sealed shut. The last bottles went at 15m depth, and not long after the CTD was back on deck.

One by one we opened up the bottles and decanted the water samples from the rosette into smaller glass bottles for analysis. These water samples will, amongst other uses, help us calibrate the conductivity sensor, to make sure that salinity that is measured during the deployment agrees perfectly with the more precise salinity measurements that can be perfomed in the lab. But before we run the salinity tests, the bottles need to go into the lab and sit there for 24h. This makes sure that they are at the same temperature as the lab in which they will be analysed.

After taking the water samples, we downloaded the data from the CTD onto the ship's computers. The data is then processed and stored in multiple backup systems. The echo sounders use that data to get their exact sound speed profile, but the really insteresting use of the data comes when we analyse the the CTD profiles to learn more about the ocean circulation and how it affects our lime system. This analysis will take many more CTD casts and many months of poring over graphs and figures. But we hadn't arrived at the site of the green dot (see map above) just for a CTD, the paleao team still wanted to take a sediment core. While the ship held its position precisely over the designated GPS position, the other team went to work.... (see next post).