Measuring pH changes in seawater

We are on day 11 of our research cruise to the North Sea to investigate water column chemistry, and biogeochemistry and ecology of the seafloor at a site where we will experimentally release carbon dioxide below the seafloor next year. That sounds like an odd scientific objective, experimentally releasing carbon dioxide, but the justification is a good one. Former oil and natural gas bearing formations hundreds or thousands of meters below the seafloor are among the sources of carbon that now accumulates in our atmosphere as carbon dioxide. Carbon dioxide emissions can be reduced with improved technology, improved efficiency, and increasing uses of renewable energy, but additional progress could also be made by taking an active role in capturing carbon dioxide at point sources, such as industrial power plants, and sequestering it in geologic formations that are the least likely to leak. Candidate formations include former oil and natural gas bearing reservoirs deep beneath the seafloor. If natural gas could be held naturally there for millions of years, it is likely a good location for sequestering carbon dioxide. An additional advantage of these formations is that the infrastructure that was developed to extract oil and natural gas from them could be used to inject carbon dioxide back into them.

The benthic lander developed by the Max Planck Institute for Marine Microbiology (MPI) on the back deck of the GEOMAR Research Vessel Poseidon

Our objectives in releasing carbon dioxide from the seafloor next year is to investigate the potential damage to local ecosystems that could be caused by a leak, and to develop technology to quantify carbon dioxide emission from the seafloor should a leak occur. To accomplish this, engineers at the Max Planck Institute for Marine Microbiology have adapted ion sensitive field effect transistors to be used for rapidly detecting pH fluctuations in seawater. We are using these sensors in a novel application. Marine sediments naturally take up oxygen and produce carbon dioxide as they respire organic matter, like we do. As carbon dioxide is produced in seawater, pH is reduced. We can use the change in pH to calculate the carbon dioxide production.

pH, dissolved oxygen, and water velocity (all lower left) are measured at 16 Hz to capture their turbulent fluctuations in water

To do this, we have adapted a technique from atmospheric research based on turbulence. The pH sensors are positioned 25 cm above the seafloor and measure at 16 Hz as turbulence transports eddies of low pH water upwards from the sediment surface. Eddies are monitored at the tip of the sensor with an acoustic velocity instrument. Combined, pH and vertical velocity can be used to calculate the vertical transport of hydrogen ions. In measuring from second-to-second, minute-to-minute, and hour-to-hour, we can use this technology to examine changes in the metabolism of benthic ecosystems over time. Improvements in this technology would allow us to watch changes in the metabolism of marine ecosystems from season-to-season, and year-to-year, helping us to do a better job of protecting them.

Deployment of the MPI benthic lander.

An array of fast sensors for turbulence measurements by the MPI lander at the seafloor (120 m depth).

Greetings from all on the ship

Dirk Koopmans

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