A Glimpse into the DIC Lab: An Interview with Dana Greeley

Author: Kathryn Williams

The sea was angry that day, my friends – like an old man trying to send back soup in a deli. So I went to talk to Dana Greeley about DIC… Here is that “conversation”.

Kathryn: What is DIC?

Dana: DIC is an acronym that stands for Dissolved Inorganic Carbon. It is also referred to as Total CO2 (carbon dioxide). It is the sum of the dissolved carbonate species in the seas: carbon dioxide, carbonic acid, bicarbonate, and carbonate. DIC is a key parameter when making measurements related to pH and carbon dioxide flux estimates.  But do most of the readers of this blog really want to hear about this? Can’t we talk about the nice sunsets we’ve had recently? Here give them the link to DIC from Wikipedia. Then we can use your blog post to talk about something more entertaining.

Sunset on the Indian Ocean from the NOAA Ship Ronald H. Brown

Kathryn: OK then, when did you start sailing and why do you still go out to sea to measure DIC?

Dana:  A few of my favorite literary quotes might explain my motivation: “Of all the things that drive men to sea, the most common disaster, I’ve come to learn, is women.1” But that was not the case with me. “If you really want to hear about it, the first thing you’ll probably want to know is where I was born, and what my lousy childhood was like, and how my parents were occupied and all before they had me, and all that David Copperfield kind of crap, but I don’t feel like going into it, if you want to know the truth.2” “Some years ago – never mind how long precisely – having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world.3

Kathryn: OK, alright, hold it right there! Back to the interview… How do you measure DIC?

Dana: Aarrgh, alright. But you’re going to lose your readers if we go into this full on. Send the scientists to read The Handbook and I’ll give you the layman’s version here. We collect seawater from each niskin and take those back into our 20’ shipping container that has been modified as a sea-going laboratory. Inside that lab we hook up the seawater sample to our equipment and withdraw a measured volume. That volume then drops down into a test tube shaped piece of glass (stripper) where we add a small amount of dilute acid and bubble CO2 free air through the seawater so that it resembles a nice freshly poured glass of Fresca. You know what a Fresca is don’t you Mr. Scholarship winner? The (stripped) gas from that Fresca is then sent to a cell (picture a glass beaker with side arm) which contains a blue pH reactive solution that turns clear with the addition of CO2. The cell sits in an analyzer (coulometer) which sends a light path through this (blue) solution and on the other side sits a detector which collects the light and counts the coulombs and with some additional bells and whistles we determine the total CO2 (aka DIC) within that seawater sample. You’re losing your readers here. Seriously, if you want to know more, read the Handbook.

DICE system
DICE: Dissolved Inorganic Carbon Extraction. System used by the DIC Lab to extract DIC out of water samples

Hey, did you know the captain spotted a falcon back a week ago? Turns out it was an Amur Falcon, late returning migrant to its breeding grounds.

Amur Falcon
Amur Falcon on the NOAA Ship Ronald H. Brown

Kathryn: OK, back to the interview… Why are you out here measuring DIC; what is it used for?

Dana: Knowledge is Power! More data means more knowledge which yields a greater understanding.  These GO-SHIP cruises are a continuation of the CLIVAR/WOCE lines and this repeat hydrography helps to improve our understanding of the ocean carbon cycle and how it is changing over time. Data from those previous hydrographic cruises show that the ocean is not evolving with smooth decadal trends. Therefore we need to continue to go to sea to make these carbon measurements until an Argo type float can replace us humans. The old saying still holds true, “Don’t send a buoy to do a man’s job.”  Our DIC analysis helps climate scientists study climate change and predict future climate states with different climate scenarios. Speaking of, can you believe how hot it is outside today? I wish we could package up this heat and save it for the next time someone back home says, “It’s really cold outside, they are calling it a major freeze, weeks ahead of normal!”

Any last comments?

Yes, did you know we have now crossed into the Arabian Sea? It will be interesting to see what the Oxygen Minimum Zone holds in store for us as we continue north. I expect it will be a CO2 maximum, but that’s why we come to measure.

1 Charles Johnson, Middle Passage

2 J. D. Salinger, The Catcher in the Rye

3 Herman Melville, Moby Dick

A Peek into the Life of a CTD Watch Stander

Author: Yashwant Meghare

“DON’T PANIC” are the words I have written in big letters on my notes for how to operate a CTD. My excellent teacher, Kristene McTaggart (Kristy) laughs and agrees that it is indeed a very good thing to keep in mind.

Two days later, I have a dream that I let the CTD hit the bottom while going on an unapproved bathroom break. I woke up, very disappointed with my-dream-self.

May 5th, 2018

CTD: stands for conductivity, temperature and depth, and refers to a package of electronic instruments that measure these properties. Often, CTD is attached to a rosette that holds Niskin bottles for water sampling.

Somewhere in the middle of the Indian Ocean, at the so called “Station 22” I am sitting next to the person who operates CTD when I am off my watch- which goes from midnight to noon.

– Survey. Computer. Deploy the CTD.

– Winch. Computer. Down at 30 m/min. Target depth … (well whatever the depth is at that station)

The communication is very short and limited in order to avoid miscommunication or any kind of confusion. But as the CTD goes down to the unexplored abysmal depths of the ocean, it’s not the just the CTD that’s under pressure. The operator experiences the same feeling of increasing pressure.

– Quick instrument check for any signs of technical error. All good. (Well, at least that’s what we like to see.)

As the CTD goes deeper and deeper and things start to get more consistent, the operator is relieved. (or are they?)



FAST FORWARD >> 3 hours

The process of driving a CTD down to the depth can seem very uneventful. You will have to power through with a cup of coffee or some music that will bang you awake. (Or in my case, once when Kristy hid behind the door to scare me. I didn’t need much coffee after that.)

You keep increasing the speed from 30 meters per minute to 45 and then to 60. While it does seem like an uneventful job, there are spurts of times when there’s a bunch of tasks. Keeping a log of things at the beginning is important to have the final file tell you what data is important and what can be discarded. Keeping an eye for any system errors to mark them in the log is necessary. This works much like a checkpoint system in race. Anytime there’s an issue, flag it to check it later. So, even though there’s no consistent activity, it does require your attention all the time for quality data. This includes several markers in the data file to mark the beginning of the activity, the moment when CTD is underwater, when CTD is at maximum depth, and when it is going up.

The rosette hosting the CTD is not just working to measure the salinity, temperature and depth. It is an assorted set of instruments. It has a transmissometer to measure the clarity of the water, fluorometer, LADCP, oxygen sensors, altimeter that measures the height from bottom.

The altimeter activates when the CTD is within 100 meters of the seafloor. The goal is to keep the CTD off the bottom, but just close enough that the sediments don’t get sucked up into the pump (which can cause trouble).

– All stations, CTD is at maximum depth.



FAST FORWARD >> 3 hours

The journey back up involves collecting water samples at every few hundred meters.

– Winch, standby. Winch, slowdown. Winch stop. Collect water sample.

– Winch, up.

Once the CTD is back on deck, the bottles are checked for any leaks and made sure they had been fired. The real-time data collected is copied and saved on the system. The CTD operator helps with any sampling if needed.

Once all the samples have been collected, there are other simple tasks to take care of such as setting up the CTD for the next cast. There are small details that one must pay attention to for a successful cast. Andrew Stefanick (Andy), who seems like a pro with the CTD instruments helps with and teaches me all the fine details. Uncorking the bottles and cleaning the optical sensors needs to be done in a certain manner so that the equipment is not damaged.

The wording makes it sound like the CTD is some small fragile object, but in reality, it can’t be moved without the help of electrical pulley.

The purpose of GO-SHIP cruises is to observe changes in our oceans over time and this can be done only if we have quality data. From the Niskin bottles, the water is collected to do various types of analysis. Dissolved organic radiocarbon analysis, CO­2 levels, radiometric dating, nutrient concentrations, particulate organic matter, pH and alkalinity levels, dissolved oxygen concentration, CFC gas analysis, calcium analysis and density profiling are all performed on the water collected in the bottles on the CTD. So, this makes CTD a crucial part of the cruise and puts a lot of responsibility on CTD operators. With 132 CTD casts planned between Durban and Goa, this cruise will explore the Western Indian Ocean after a very long time and look for any changes that have occurred over the decades.



Behind the Science: Dissolved Oxygen

Author: Leah Chomiak

Hello from the middle of the Indian Ocean! The skies are blue, the seas are calm, the air is fresh and full of oxygen (O2)… and so is the water column! My name is Leah, one of the scientists onboard I07N running dissolved oxygen analyses along with my counterpart, Sam. Together, we are in charge of sampling, assessing, and quantifying the amount of gaseous oxygen dissolved in the seawater from every bottle of every station… coming to a grand total of 3,432 samples once we close out the transect in India. Wowza!


Dissolved oxygen, the same stuff you and I breathe each second of the day, is crucial to the marine environment; whether that be oceans, lakes, rivers, or ponds, for obvious reasons. Oxygen is essential for life and is taken up by organisms through a process termed cellular respiration where oxygen is taken in and carbon dioxide is spit out in return. The reverse process, photosynthesis, results in the generation of oxygen through the uptake of carbon dioxide. Just like on land, plant cells fill the oceanic environment in the form of plankton, where oxygen is produced. All remaining organisms of the sea, from fish and sharks to corals and giant squids, rely on the presence of dissolved oxygen in the water column just as we rely on oxygen in our atmosphere – to survive! We measure the dissolved oxygen to understand not only the biological and chemical aspects of seawater, but also for understanding the physical movement of currents and water masses too.

o2 titrationWe start by first collecting a sample of seawater out of a Niskin bottle that is closed at a certain depth in the water column from a CTD rosette, yielding samples from 24 different depths spanning the surface to the ocean floor. Immediately after collecting the seawater sample, the sample is ‘fixed’ with two reagents that react to form a precipitate – in simplest terms, all of the dissolved oxygen reacts to form a solid and is suspended in solution. By performing this reaction, we essentially have the power to “freeze” all oxygen in the sample at that given moment, meaning photosynthesis and respiration are halted and no further O2 is produced or consumed. Once the entire CTD has been sampled, the O2 samples are brought back to our lab inside the ship, where we then give them a shot of strong acid to dissolve the precipitate (turning the sample a cool yellow color) and at last perform a titration to determine the amount of oxygen present in the sample.

Since the surface ocean and the atmosphere are in constant exchange with each other, oxygen from the atmosphere is assimilated into the surface waters, and we tend to see high oxygen near the surface for this reason. Due to the availability of sunlight near the surface, plankton aggregate and help to contribute to the surface oxygen levels through photosynthesis. Samples taken near the bottom of the ocean also tend to have high oxygen levels, one being because the extreme pressure and cold temperatures facilitate the dissolution of gas in seawater, and two, certain water masses, such as the cold and dense Antarctic Bottom Water are known to have high O2 content due to recent exposure with the atmosphere. Central in the water column is where the oldest water lies, and oxygen tends to be lacking due to uptake by biological organisms. Not only is oxygen useful in assessing biological productivity, its also a great parameter to use when observing global ocean circulation!

The outflow of the Arabian Sea/northern Indian Ocean is observed as one of the largest and most distinctive oxygen minimum zones (OMZ) in the world, where oxygen concentrations are extremely low due to high biologic consumption and other chemical factors. As we continue to head north along the I07N transit line, it will be very interesting to see how our oxygen profiles change as we enter the OMZ. It has been 23 years since this hydrographic line was last sampled; I wonder what we will observe this time around!


How do oceanographers measure and sample from the surface to the bottom?

Author: Denis Volkov

Figure 1. A rosette onboard the Research Vessel “Roger Revelle” during a research cruise in the Pacific Ocean in September 2016 (photo by D. Volkov). Note that the gray sampling bottles are open prior to the deployment.

The primary “workhorse” of sea-going oceanographers is a so called rosette – a framework with 12 to 36 sampling bottles (in our project we have 24 bottles with a volume of 12 liters each) clustered around a central cylinder, where a CTD and/or other sensor package can be attached. A CTD is an instrument that measures the conductivity, temperature, and pressure of seawater (the D stands for “depth,” which is closely related to pressure). The conductivity measurements are used to determine salinity. These are essential physical properties of seawater that determine its density and to a large extent ocean circulation. Usually, a rosette also houses Acoustic Doppler Current Profilers (ADCP) that measure the horizontal velocity, and oxygen sensors that measure the dissolved oxygen content of the water.

In order to take measurements, the ship stops and a CTD cast is carried out. The location where measurements are taken is called an oceanographic station. The rosette is lowered on a cable down to just above the seafloor with the sampling bottles opened at both ends, so that water can freely circulate through them. The CTD is connected to a computer onboard the ship, and scientists can monitor changing water properties in real time. When the instrument ascends, the sampling bottles are closed selectively at predefined depths by a remotely operated device.

During the I07N project, we are planning to complete 132 stations and collect water samples that will be analyzed for oxygen, nutrients, salinity, dissolved inorganic carbon, alkalinity, pH, chlorofluorocarbon, dissolved organic matter, dissolved organic radiocarbon, particulate organic matter, and some other parameters.

Figure 2. A rosette being lowered into the water by science technicians on board the Research Vessel “Ronald H. Brown” during the first I07N test station in the Indian Ocean on Apr. 25, 2018 (photo by D. Volkov).