Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Chalk Talk Part Deux, The Revenge

Posted by sogasex on March 25, 2008

By Dave Drapeau, Bigelow Laboratory for Ocean Sciences

Since a lot of you have probably been wondering since my last entry (see Chalk Talk) “How does he figure out how much calcium carbonate is in the water?” I figured I’d talk a little about the instruments we use for this. Most of our sampling uses the ships ‘Uncontaminated Seawater line’ which is has an outlet in most of the labs aboard. This is constantly pumping seawater from about 10 feet below the surface (we call it surface water because it’s very difficult to sample closer to the top without occasionally sucking in air, especially in an area where 10’ waves are the ‘little’ ones). So instead of putting our instruments in the water, several of us (Burke, Steve, Roberta, Matt, Bob, and myself) are bringing the water to our instruments, using this constant stream of surface water to map various properties as the ship steams around both inside and to and from the ‘patch’.

Our system (see picture below) starts with some basic oceanographic measurements like temperature, salinity and chlorophyll fluorescence. Then the water passes by an instrument which measures amount of optical backscatter from particles in the water. The reason we can estimate the amount of calcite in the surface ocean from satellites is because these particles reflect a lot of light. After the instrument gets a good idea of the amount of total backscatter in a sample a computer turns on a pump which adds a weak acid into the seawater stream. Just like a TUMS after dinner will get rid of your indigestion, any calcite in the water will dissolve as we drop the pH and the amount of backscattered light will decrease. We look at the difference in the backscattering at normal and low pH to determine what we call the ‘acid labile backscattering’. The computers which collect all the data are also connected to the ships navigation system which allows every data point to have a time and position to go with it.

We combine this continuous flow-thru measurement with water samples we filter and analyze for particulate calcium back at the lab. One assumption (generally true) we make is that all of the particulate calcium we see in our analysis is bound to carbon as calcite (CaCO3, aka calcium carbonate, aka chalk). This represents the amount of Particulate Inorganic Carbon (PIC) in the water as opposed to the carbon tied up in the cellular material of plants and animals which is labeled as POC, or organic carbon. This can be an important distinction as I mentioned in the previous blog because of the different fates of these two types of ‘fixed’ carbon in the ocean.

So is there a lot of PIC in the surface waters of the Southern Ocean???

Remember that satellite imagery showed what appeared to be a large band of high calcite surface water in the circumpolar area (see first blog), but that there were several possible explanations including whitecaps and bubbles from the constant high winds down here. Well, the unofficial answer is yes. Preliminary results show substantial amounts of PIC in surface waters both along the transect from Punta Arenas to the study site and in the study sites themselves (see picture below). The data need to be looked at more closely and correlated with chemical analyses so don’t tell anyone, yet, but the implication of these findings suggest that the Southern Ocean could represent one of the largest pools of marine PIC on earth making it even more important to study the fluxes of carbon between the ocean and atmosphere and how they might be changing.

I’d like to send a ‘Shout Out’ to Mrs. Jones’ 3rd grade class at Boothbay Region Elementary School. You guys have kept me busy with all the great questions and it’s been fun telling you about the cruise. And finally, Happy 7th Birthday to Henry Drapeau! I miss you buddy and wish I could be there.


Where the ‘Magic’ happens. Underway temperature, salinity, chlorophyll fluorescence, and total and acid labile optical backscattering.


The percentage of acid labile backscattering over the area of the first ‘patch’. Values greater than 0.1 (10%) are generally considered significant.


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