David Thistle

Current Research Interests

Environmental effects of CO₂ sequestration in the deep sea

Background

Since the beginning of the industrial revolution, the concentration of CO₂ in the atmosphere has increased from 275 ppm to 370 ppm and is increasing by ~1.5 ppm yr-¹. Because CO₂ is a greenhouse gas, the increasing concentration will exacerbate global warming. The ocean has a large capacity to absorb CO₂ and is being considered as a reservoir in which CO₂ could be sequestered. One strategy involves collecting CO₂ before it enters the atmosphere and injecting it into the deep ocean, where it is predicted to stay for hundreds of years. Together with Drs. James Barry and Peter Brewer from Monterey Bay Aquarium Research Institute and Drs. Kevin Carman and John Fleeger from Louisiana State University, my graduate student Linda Sedlacek and I are studying the ecological consequences for deep-sea animals of storing CO₂ in the ocean.

First Experiment

Our goal in this experiment was to begin to study the dose-response relationship between sequestered CO₂ and the deep-sea infauna. At 3250 m depth off Monterey, CA, USA (CO₂ Canyon, Figure 1), we transported CO₂ to the seabed with the remotely operated vehicle Tiburon (Figure 2) of Monterey Bay Aquarium Research Institute and pumped approximately 20 l of liquid CO₂ into each of three open-topped containers on the seabed (Figure 3).

The liquid CO₂ was denser than seawater at this depth and formed a pool in each container. Subsequently, the CO₂ dissolved into the near-bottom water, and CO₂-rich seawater flowed across the surrounding seabed. When we returned after 30 days, we collected sediment cores (7-cm inner diameter) from an area ~2 m away from a container (a region expected to have been exposed to CO₂-rich seawater) and from an area ~75 m away, which should not have been influenced by the CO₂ (Figure 4).

Carbon-dioxide concentration is difficult to measure directly in seawater. In previous work, pH has been used instead because changes in CO₂ concentration change the pH. We used microelectrodes to measure a vertical pH profile in the bottom 2 mm of water and the top 8 mm of sediment in each core and found that the pore-water pH was consistently more acidic (~0.75 pH unit) in the cores taken near the containers than in those taken farther away. The increase in acidity was far greater than natural variability (which is 0.1 pH unit).

Although relatively little is known about the effects of reduced pH or elevated CO₂ concentrations on deep-sea animals, they are adapted to stable pH and CO₂ conditions and are thought to have much less buffering capacity than do shallow-water species; therefore, even slight changes in pH may have important influences on metabolic activities.

To test for effects on the infaunal invertebrates, we chose harpacticoid copepods (Figure 5) as a representative taxon. These miniature crustaceans (body length < 1 mm) are ubiquitous and abundant in sediment, and one can assess their condition by inspection of the internal organs through their transparent body walls. We examined each adult harpacticoid and scored its condition. The proportion of the animals that were scored as alive when collected was significantly lower in the samples taken near the containers for each of three sediment layers; these samples were from the cores in which the pore water was more acidic.

Our results suggest that the CO₂-rich seawater we created lowered the pore-water pH, killing harpacticoids (and presumably other infaunal invertebrates). pH is predicted to be lowered in mixing zones that will extend 100s of kilometers around proposed CO₂ disposal sites, so further assessment of the dose-response relationship is needed to allow policy makers to evaluate the environmental consequences of sequestration as part of the solution to the CO₂ problem.

Experiment 2

In fall 2003, we repeated our experiment at a site (CO₂ West, Figure 1) at 3600 m. We deployed substantially more (140 liters) CO₂ in order to explore different portions of the dose-response relationship. Linda Sedlacek is investigating the consequences for the infauna.