An Interview with Denis Pierrot
Interview by: Emma Pontes
I’m pleased to introduce Physical Chemist and Co-Chief Scientist of GOMECC-3, Denis Pierrot. On a normal day, he can be found in the computer lab overseeing CTD operations, but today, he’s kind enough to escort me to the Hydro Lab where the pCO2 Underway System lives. pCO2 is slightly different than regular CO2 concentration; it is the partial pressure of CO2 in a liquid or gas. On the Ron Brown, we have a pCO2 Underway System which Denis proudly tells me is state of the art, and the result of collaboration between several esteemed scientists around the world.
In Image 1, you can see the conglomerate of wires, equilibrators, and pumps that make up the system. A water line is connected to the system which pumps surface water from outside our ship in a constant flow into the system. The equilibrator (red circle) sprays the incoming water in a thin sheet which allows the air inside to become equilibrated with the water. This means that the pCO2 of the water will equal the pCO2 of the air inside the equilibrator. This equilibrated air is then sent to a gas analyzer (yellow circle) that measures the pCO2. In this manner, the pCO2 of the water is measured indirectly (by measuring equilibrated air pCO2) and graphed neatly on the laptop attached.
Additionally, the pCO2 Underway System has a gas line (green circle) that draws in air from the bow of the ship, ensuring the cleanest air possible with no contamination from ship emissions. This line is connected to the gas analyzer and the incoming air is measured for its pCO2 directly. The goal of this system is to measure the pCO2 of the water AND the atmosphere. The interaction and difference between ocean and air pCO2 is the basis of Denis’ work.
Image 2 shows a graph of the output from the pCO2 Underway System. The blue and brown dots represent pCO2 of the water, while the purple and green crosses represent pCO2 of the atmosphere (purple line). The goal here is to determine where and when the ocean acts as a source and sink of CO2. The ocean is a source of CO2 when the water pCO2 is higher than that of the atmosphere (above the purple line). Conversely, the ocean is a sink of CO2 when atmospheric pCO2 is higher than that of the water (below the purple line). Since gases always move from high to low concentration in an endless quest for equilibrium, Denis and his colleagues can tell if the ocean is releasing (source) or absorbing (sink) CO2 and at what rate. The rate of this CO2 exchange between ocean and atmosphere is called flux. Denis hopes to create an up to date flux map of the Gulf of Mexico complete with spatial and temporal attributes. Flux is constantly changing both seasonally and temporally, and is dependent on wind, ocean current, temperature, and other atmospheric factors.
One quarter of anthropogenic CO2 is dissolved into the ocean. This has important and unfavorable implications for calcifying organisms such as certain plankton, corals, sea urchins, and any other creature that relies on carbonate to build its shell or skeleton. CO2 is an acidic gas that lowers the pH of water which it is dissolved in. A lower pH means more acidic water, which also means less available carbonate for calcifying critters to utilize in their shell building. A more acidic ocean is one with fewer ecologically important corals (and other carbonate-reliant species) and fewer commercially important seafood items like shellfish. Denis is very passionate about his work, which has ecological and commercial implications under ocean acidification conditions.