pH Changes around a CO2 Seep using pH Robust Probes in Sediment

Max Planck Institute for Marine Microbiology, Microsensor Group, Bremen, Germany
Involved scientists: Prof. Dirk deBeer

Aim of the experiment was to detect the impact of pH evolution around a CO2 seep, creating acidity. A sediment transect was measured with pH Robust Probes from PyroScience and pH electrodes for comparison. The sediments were obtained from a site in the North Sea where in the future CO2 may be pumped in an empty gas reservoir at 3 km depth. Aim was to find out how a possible leak could be detected, what the effects are and how wide the effects spread from the leak. In situ experiments were performed with artificial CO2 leaks, by pumping CO2 gas through a pipe to 3 m below the sediment surface. We were surprised to see how small the effects were and how local. Therefore, we repeated the experiment in the laboratory, with the same sediments in an aquarium with a gas entry in the bottom, connected to a CO2 bottle.

Transient of pH dynamics in the sediments along transects from the vent were measured using PyroScience optodes and electrodes. The sensors were positioned into sediment at different distances to the gas-vent (0cm, 2cm, 4cm and 8cm distance). The vent can be used to introduce N2 or CO2 gas into the sediment and therefore induce a pH change.  

First, the seep was flushed with N2, to locate the vent. This flushing induces a pH increase. After an hour, CO2 was flushed which induced an instant pH decrease in the vents, and much later at distance from the vents.

As seen in Fig. 2, It is visible that the closer the sensor is to the seed, the faster the pH value drops to pH 6.0. The sensor with largest distance to the vent shows stable pH values over the entire measurement time.

After the experiment of a week, the optode and electrode was checked for drift. The optodes showed a drift of about 0.13 unit up whereas the electrodes drifted at least 0.5 units. After stabilization of the pH a series of pH profiles were measured across the vent, from which contour plots were constructed (see Fig. 2). This showed a slightly branched acidification pattern, as the bubble paths meandered a bit. Most important, the pH effect is localized to the CO2 channels and does not spread into the surrounding area. This can be predicted with engineering rules, assuming the CO2 channel a cylinder in an infinite medium. Acidification should be limited to a distance of about 3 diameters of the venting channel. This is indeed the case.


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