Measuring carbon at sea

by Eithne Tynan

My name is Eithne Tynan and I am a chemical oceanographer at the University of Southampton. I am involved in the Ragnarocc project and I’m in charge of the CO2 measurements on this cruise. This is a big task and one that involves a five person team, six different instruments, boxes and boxes of consumables and a large supply of chocolates and haribos! As was explained on an earlier post the measurements on this transect will be combined with those on the subtropics cruise next year to calculate the carbon budget for the North Atlantic. So how do we go about transforming a sample of salty seawater into a number that tells us how much CO2 there is in the ocean? Well, easy, we take last year’s measurements and just adjust them up a little bit. No, just kidding!

I’ll try to keep it simple. Sampling is the first task and a critical part of our job: we are effectively measuring the amount of dissolved CO2 gas in the water, which means we want to minimize any contact with the atmosphere. This is why there is an established protocol around the CTD where samples for gases are drawn before any others. So once we have our sample in the bottle, we ‘kill’ it with poison to stop any biological activity which could change the CO2 concentration. At this stage with the bottle properly sealed, it can be stored in a cool place for several months before analysis. However, by the end of the cruise we will have collected around 2300 samples, so we have brought our instruments to sea to analyse the samples as we go. One great thing about been at sea is that the lab is running for 24hours, seven days a week, and in the seven weeks this cruise lasts we will have analysed what would take one person about 5 months in the lab.

So this brings me to our beloved VINDTA, the main instrument we use for our measurements. We have two of these on board and as Penny said today “it looks like a mad scientist’s machine”. One thing for sure, they definitely make you mad at times….that’s when you reach for the emergency stash of chocolate! We have two of these VINDTAs on board and Becky, one of our team members, renamed them Golden Girl and Princess Precious, you can guess which one has been giving us more trouble on this trip! Basically, on the VINDTA the seawater sample gets stripped of all the CO2 dissolved in it by adding some acid. This released CO2 gas gets pumped into a chemical solution that changes color according to the amount of CO2 injected into it. Using mathematical equations the color change is converted to dissolved inorganic carbon (DIC), et voila, that’s the total amount of CO2 that is in that particular sample of the ocean. Chemistry is definitely cool!

We have just reached the coast of Scotland and we don’t have long to go now before we finish science, so perfect timing for a big thank you to the awesome carbon team on this cruise: Becky Garley, Claudia Fry, Jen Clarke and Alex Griffiths. Thanks for the smooth sailing!

Images: The carbon team, by Eithne Tynan

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