Work Package 3

Impact of a CO2 leak on marine geochemical cycles

Sediment cores collected by divers during the CO2 release experiment.

Collection of gas bubbles by divers.

Collection of water samples from the Soel Mara.

Overall aim

To define and quantify the processes that determine the fate of CO2 seeping through seafloor sediments, and potentially in to the ocean environment and the atmosphere. This will be done via investigations of the biochemical transformation of CO2, including its potential to remobilise other chemical species.

CO2 is injected into a geological storage site as a gas or liquid. Some of the gaseous CO2 could, over time, be dissolved into the pore fluids in the geological rock reservoir. This means that any CO2 that escapes into the marine environment could occur in either a gas or liquid state.

CO2 release experiment

We have collected chemical data for sediments (Figure 1) and their pore fluids (fluids within the sediments), water, and the atmosphere, from the seafloor controlled CO2 release. Some parameters were monitored continuously throughout the release, but most of the data were collected at specific time points i.e before installation and before, during and after injection.

These data are now being evaluated, to assess the effects of a CO2 leak on:

  1. Seawater chemistry
  2. Flow of oxygen, dissolved inorganic carbon, alkalinity, key nutrients and other relevant chemistry across the sediment-seawater boundary.
  3. Sediment - pore fluid reactions, including the potential for release of metals that could be of harm to organisms.
  4. Flow of CO2 from seawater into the atmosphere.

The tasks within this work package are:

Task 3.1: Impact of a CO2 release on the sediment environment. (Scottish Association for Marine Sciences, National Oceanography Centre Southampton)
The sediment environment has been characterised by measuring and mapping the distribution of selected chemical parameters (such as pH, dissolved organic carbon and trace metals) within the sediment (Figure 2). This will enable us to determine the impact of a CO2 leak on their distributions. We have also measured the distribution of temperature and porosity (open spaces within the sediment), which are key inputs for the models described in work package 2. This work has been done on diver-collected sediment cores (Figure 3).

Task 3.2: Impact of a CO2 release on ocean chemistry. (National Oceanography Centre Southampton)
We have determined the distribution of inorganic carbon species, as well as oxygen and nutrients in the water before, during and after CO2 release (Figure 4). Where possible, these were monitored directly using instruments installed on the sea bed over the course of the release experiment. Our Japanese collaborators also deployed an autonomated submarine, equipped with a pH probe, to map out the changes in the acidity of seawater caused by CO2 release. An important part of this work was to assess the proportion of CO2 present as bubbles, versus the proportion that dissolved in seawater. The bubbles were sampled directly (by divers, Figure 3) and they were also monitored visually.

Task 3.3: Potential for release of CO2 into the atmosphere. (National Oceanography Centre Southampton, Plymouth Marine Laboratory)
In order to evaluate whether the CO2 released at the seafloor escaped in to the atmosphere, we measured the concentration of CO2 at the sea-surface and in the atmosphere. These data will be combined with local measurements of wind speed to allow air-sea flow rates to be calculated.