Dissolved Inorganic Carbon

Author: Joletta Silva

Hello, my name is Joletta Silva, and I am a master’s student at the University of Miami RSMAS studying Marine Conservation. On the GOMECC-3 cruise I have been working alongside Patrick Mears completing dissolved inorganic carbon (DIC) analyses.

DIC is comprised of CO2, and all the nonliving carbonate species (primarily carbonate and bicarbonate) that are dissolved in the seawater. Generally areas which have high concentrations of carbon dioxide will have correspondingly high levels of DIC. When our data is coupled with data from other carbon parameters (alkalinity, pH, etc) it can be used to determine the rates of ocean acidification in the Gulf of Mexico.

The DICE lab on board Ron Brown. Image credit: Joletta Silva

The dissolved inorganic carbon lab is separate from the main lab on the ship and is built into a modified shipping container which is usually stored at the Atlantic Oceanographic and Meteorological Laboratory on Virginia Key, Florida. This makes transport and setup relatively simple and also allowed for me to become familiar with the machine setup before I got to the RV Ronald Brown.

Life on the ship:

I am working the night shift (11:30 pm to 11:30 am), and generally sleep or  nap during the day, and wake up at around 11 to start my shift. When we are on transect lines, the CTD (whom we have affectionately named Barbara) will descend at each station to various depths, and will return to the surface filled with seawater for sampling. We receive a daily schedule so that we know exactly when to be outside prepared to extract water. I generally take one full bottle from each of Barbara’s niskin containers (up to 24 total, each one containing seawater from a different depth). As soon as I fill the bottle and ensure that there are no bubbles in the sample, I poison it with a mercuric chloride solution to kill any living organisms and ensure that they do not breath, thus changing the CO2 concentration in the sample before it is run through the machine. After getting all my samples from Barbara, I return to the lab and begin to analyze them.

CTD going down at a station. Image credit: Joletta Silva

This can be a long process, as each one of our samples takes about 15 minutes to run. The machines also take about 1.5 hours to calibrate and set up, so a lot of time is spent doing this. Sample analysis is completed using a coulometer and a Dissolved Inorganic Carbon Extractor (DICE).  The coulometer is used to help determine the amount of carbon in the cell. The carbon in the sample is converted to carbon dioxide gas, which then reacts with a proprietary reagent causing the formation of OH- and H+ ions which are detected by the machine which uses photogrammetry, or light streams, to detect and quantify the ions. Basically, the DIC in the sample is converted to carbon dioxide, which is then measured through a titration that gives us a total carbon count.

During a normal shift (when we are not in transit between station lines) I will sample between 2-4 CTDs at varying depths, and have to calibrate the machine twice. There are several other scientists who work night shifts measuring other parameters such as dissolved oxygen and nutrients, and we have become fast friends (something about sampling at 3 am while listening to some great music creates an instant bond). We have been fortunate to have some incredible experiences including outrunning tropical storm Franklin in the previous week. In addition to this, we just witnessed a meteor shower on August 12th that lasted the majority of the night!

We have a mere few days left of our 35 day cruise, and it has gone by far too quickly. It has been interesting to view the profiles of DIC along the varying transect lines and observe trends and changes in different areas and depths. This last week is going to be one of the busiest, since the transect lines and CTD stations are so close together. I am excited to continue to learn about inorganic carbon profiles in the Gulf, and see how the remainder of our results turn out. Here’s to the last week at sea, and the adventures that it will bring!