Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Chalk Talk

Posted by sogasex on March 5, 2008

By Dave Drapeau, Bigelow Laboratory for Ocean Science

If you’ve been paying attention then you already know that one of the main objectives of this cruise is to look at concentrations of CO2 and its exchange between the ocean and the atmosphere here in the Southern Ocean. From a biological viewpoint, carbon which enters the surface ocean from the atmosphere as CO2 can have several different fates:

1) CO2 may be ‘fixed’ into the cells of microscopic plants (phytoplankton) that live in the surface waters through the process of photosynthesis (read more about this in Bob’s blog). These can be eaten (grazed) by larger organisms and the carbon passed up through the food chain (maybe that tuna salad you had for lunch?) however at each step along the way some of that carbon is ‘breathed’ back into the ocean or atmosphere as CO2.

2) Under certain conditions the phytoplankton can ‘bloom’ and produce a lot more biomass than the local grazers can eat. Eventually some of these cells sink and become part of the oceanic sediment. This is one mechanism for the oceans to remove carbon from the surface ocean and hence the atmosphere. These types of sediments may eventually form oil which, when removed and burned, releases this stored carbon back into the atmosphere.

3) There is special group of phytoplankton however that use CO2 in a unique way. In addition to fixing carbon into their cellular material as all plants do they also cover themselves in plates (liths) made of calcite (calcium carbonate), more commonly known as chalk. The most common of these coccolithophores (see picture below) look lot like tiny soccer balls (or if you’ve just spent a week in Chile, futbols).

There are several theories about the purpose of these plates but regardless of ‘why’ they do it this group of phytoplankton can have a dramatic impact on the cycling and fate of carbon in the ocean and atmosphere. Because it is heavy and also not very good to eat (unless you have indigestion) most of the this chalk eventually finds it’s way to the bottom thus removing a lot of carbon from the ocean/atmosphere system. The White Cliffs of Dover were created over millions of years from the skeletons of these tiny organisms. You can see in the image below that the seas around the UK are still very rich in coccolithiphores.

Very little work has been done looking at these species in the Southern Ocean, however satellite imagery suggests that they could be very important in this area (see picture below). As Pete mentioned a few days ago (see March 2nd blog) we need to keep in mind the limitations of remotely sensed (satellite) data and, whenever possible, go and see for ourselves. That’s why I’m here! Is it calcite we’re seeing in this image? Is the satellite being fooled by something in the atmosphere or lots of bubbles in the water from all the wind and waves down here?

Stay tuned.

coco.jpg

Emiliania huxleyi, a common coccolithophore, covered with calcite (chalk) plates.

map.jpg

Satellite image of calcite derived from an experimental algorithm and MODIS AQUA data. Notice high concentrations throughout the Southern Ocean.

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