Carbon on CUSTARD: Episode 2

Mr Gareth Lee & Dr Maribel García Ibáñez, University of East Anglia, UK


EPISODE 2: Instruments used for measuring dissolved carbon in the ocean

‘Morning Laurel, morning Lucy’. That’s how the day starts. The same greeting every day, every week and every year. That’s the way they like it. Consistency and repetition. A routine. The same routine… always. Step out of the routine and they start playing up, and boy can they play up when they want to! They have very specific needs too. Seawater, and lots of it. In fact, they drink it by the gallon, one every 7 hours. Sodium chloride solution too. Again, lots of it. About a gallon every 8 hours. Orthophosphoric acid. A dash every half hour or so. Ditto hydrochloric acid and magnesium perchlorate.

You may at this point be wondering who on earth Laurel and Lucy could be? Have we discovered stowaways from a far-away land? Have we encountered a strange kind of chemically fixated sea monster new to science?

VINDTA instrument on board RRS Discovery

Sadly, no. Laurel and Lucy are in fact our ‘Versatile INstruments for the Determination of Total Alkalinity’ or VINDTAs for short. It’s hard not to anthropomorphise such beastly instruments. They are temperamental to say the least and I am convinced that anyone who has used them will say the same. Treat them well and they will shine. Treat them badly and they will punish you. Even the manual states that to operate them you have to ‘like’ them.

‘Bubble, bubble, toil and trouble’ LEFT: is the DIC purge tube which strips CO2 gas from seawater samples. RIGHT: Coulometry cell measuring concentration of CO2 in our samples

In theory, two simple processes to determine two parameters allow us to understand the dissolved carbon system in the ocean: titration to determine total alkalinity (TA) and coulometry to determine total dissolved inorganic carbon (DIC). In reality it can be challenging. You can spend 23 ½ hours watching the instruments intensely and the moment you leave them they leak all over the floor. Another favourite is regularly stopping when you pop out to make tea. They seem to know you have left them and sit idle until you return. How they know is beyond belief.

During this research expedition, our day starts and ends at midday or mid-night (depending on our shift ). Running a single sample on Laurel or Lucy takes around 23 minutes each and we collect anywhere from 20 to 50 samples per day. so to keep us happy, and sane, we have a good supply of music, books and pod casts. The occasional visitor is a refreshing change too.

Sea water samples are collected from the ocean’s surface with the ‘Continuous Underway Seawater Sampling’ system that is built in to the RRS Discovery (left); and for sampling seawater at various depths of the water column we deploy a CTD rosette (right).

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