Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

The Oracle of Delta (pCO2)

Posted by sogasex on March 8, 2008

By Bob Castle, NOAA/AOML

When the ancient Greeks needed answers to important questions, they consulted the Oracle of Delphi. For this experiment, we need to find an area of the ocean that meets certain requirements. One of these is that the CO2 concentration (pCO2) in the surface water and the air must differ by at least 40 parts per million (ppm). This difference is what we refer to as delta pCO2. Today in the open ocean, the atmospheric CO2 stays relatively constant at around 380 ppm (although it increases from year to year), but the CO2 content in surface sea water can vary widely. At our present location, surface water pCO2 is running at 340 ppm or less, for a delta pCO2 of 380 – 340 = 40 ppm, thus satisfying this important requirement.

Since we cannot consult the Oracle of Delphi to find a suitable site, we rely on instruments to tell us about currents, wind speeds, and other parameters, including delta pCO2. One of these is the underway pCO2 system that our group installed when the Ron Brown was commissioned in July, 1997. It records 8 surface water and 3 atmospheric measurements every hour while the ship is steaming. Delta pCO2 for the last 24 hours can be readily determined by looking at the screen (see photo below) or the data can be downloaded to look at longer periods of time (see map below). Considering the importance of CO2 to this experiment, it has become our “Oracle of Delta”.

In the time since the system was first installed, we have collected hundreds of thousands of pCO2 measurements from all the world’s oceans except the Arctic. These data and similar data collected by other research groups help us understand the fate of the carbon dioxide released into the atmosphere from burning of fossil fuels. In addition, the system on the Brown has been used to test instruments like Mike DeGrandpre’s autonomous SAMI sensors (see “SAMI I am”).

The Greeks had only one Oracle of Delphi, but we are fortunate to have many “oracles”. They include instruments aboard ship, satellite imagery (see “Satellites”), and data provided by shore-based scientists. And unlike the Oracle of Delphi, who couched her answers in the form of riddles, our oracles provide answers that are much easier to understand.


Screen shot of underway pCO2 system. Blue dots are pCO2 concentrations in air (~ 383 ppm). Green dots show surface water pCO2 concentrations.


Map showing delta pCO2 at various positions as the Ronald H. Brown steamed around the projected study site. Red, orange, yellow and green dots represent deltas > 40 ppm, blue dots represent deltas < 40 ppm.


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