Anatomy of a CTD Cast

Paige Logan

CTD watch stander, Salinity analysts, Argo float deployment specialist

The primary mission of this cruise is to take CTD casts every 0.5° latitude (~55km) along the 103°W meridian from Baja California to Antarctica. At our current speed, this works out to about 4 casts per 24 hours. Since they are so important to the success of this cruise, and now we have done so many on leg 2, we thought it was time for a blog post detailing how they work.

First, let's go over what exactly makes up a CTD.

A CTD is a set of sensors that we lower down to the seafloor in order to measure conductivity, temperature, and depth. Conductivity is used to estimate the seawater’s salinity, which is important for examining a number of processes in the ocean. However, the instrument package we lower at each station contains much more than just a CTD (though people on the ship often mistakenly/lazily refer to the whole package, known as the rosette, as a CTD). The CTD itself sits in the middle of a large metal frame, connected to a 6,000+ meter long wire that allows two-way communication between the ship and the package. It samples continuously at 24 Hz (24 samples per second) both on the way down to the seafloor and the way back up to the surface, allowing us to monitor the water column structure throughout the entire cast. Around the outside of the frame sit 24 Niskin bottles, which are cocked open at both the top and the bottom by lanyards connected to a carousel of latches in the middle of the frame. As we bring the package back up to the surface, we pause at a number of predetermined depths and tell individual bottles to close their caps (via an electrical signal down the wire to the carousel) to capture 11 liters of seawater at that depth for analysis back on the boat. In addition to the CTD, there are a number of other important sensors in the center of the package that, like the CTD, sample continuously on both the downcast (to the seafloor!) and the upcast (back to the surface!). There are two oxygen sensors (a primary and a secondary), an altimeter (to tell us how close we are to the seafloor), an ADCP (acoustic Doppler current profiler, to measure velocity), Chipods (high resolution temperature probes, to examine small-scale ocean mixing), a transmissometer (to measure how far light can penetrate into the water), a backscatter (to estimate the amount of particular organic matter), and a fluorometer (to estimate chlorophyll concentrations). As you can probably tell, we try to put as many sensors as possible on the rosette in order to get the most bang for our buck from each cast.

So, how does one of these casts work exactly? 

The work begins 30 minutes before we reach the station. In order to be as efficient as possible with our ship time, we try to do all the prepping of the rosette while we're cruising between stations. First, we empty the Niskin bottles of any remaining water from the previous cast (a task best done with boots and waterproof pants on). Next, we connect the lanyards to carousel to cock open lids of the bottles. Lastly, we check that our smorgasbord of sensors is operating correctly on deck (e.g., salinity should be zero, temperature should be the air temperature).

Once on station, the CTD watchstander (that’s me during the day, Conrad during the night) starts up the CTD from inside the computer lab. The survey technician and a lucky volunteer then work with the winch operator to lower the instrument package over the side of the ship. The winch operator controls the cable attached to the CTD, paying out more or less cable to allow it to move up and down in the water. The cable is composed of a two-way communication wire surrounded by an incredibly strong metal casing (this casing enables it to hold the weight of the whole instrument package, ~ 1700 lbs). Once the instruments are in the water, the CTD watchstander is in charge of examining the data coming in from the sensors to verify that they are all operating as they should be. Once we get within 100 meters of the bottom, the altimeter turns on to tell us the distance between the package and the seafloor. From inside the computer lab, we watch this information very carefully in real-time so we can radio the winch operator to stop the package before it hits the bottom. This is always the most nerve-racking part of the cast since no one wants to be responsible for crashing the rosette into the bottom! Typically we can get the package within 10 meters of the bottom. Quite impressive given that it is connected to a rocking and rolling ship! On the way back up, we stop the package at a number of predetermined depths to collect bottle samples. Since there is typically much more structure in the ocean near the surface than at depth, half of our bottle samples are actually taken in the top 1000 meters (of a typical 3000-5000 meter cast). Once we get the instrument package back on deck, usually 3–4 hours after the start of the cast, the bottle sampling begins! Different scientists on board use the water from the Niskin bottles to examine everything from chlorofluorocarbons (CFCs) to rare earth elements to nitrogen isotopes as we speed on to our next station, but you’ll have to hear about that in a later post! Once everyone is done sampling, we start emptying the bottles for the next cast and run through the whole process all over again!

Here is the rosette in all of its glory! Andrew Collins is pictured collecting samples for dissolved inorganic carbon (DIC) analysis.