Experimental Mineralogy In Situ on the Sea Floor

 Professor Jill Pasteris and her colleagues, research scientists Dr. John Freeman and Dr. Brigitte Wopenka, are collaborating with oceanographers at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California, and chemical engineers from Colorado School of Mines. From their research ships, the MBARI group under Dr. Peter Brewer deploys advanced remotely operated vehicles (ROV’s) that can take samples and make videos on the sea floor at depths to 4 km. MBARI is particularly interested in making accurate geochemical and mineralogical measurements in conjunction with its on-going experiments involving the placement of liquid CO2 on the seafloor. The reason for these experiments is to help evaluate the feasibility and desirability of sequestering so-called greenhouse gases deep in the ocean. All three collaborative groups have agreed that Raman spectroscopy, an analytical technique typically applied in a laboratory setting, would be an excellent way to investigate the solid, liquid, and gas products of reactions on the ocean floor. The challenge is to carry out such analyses in situ at several kilometers depth.

Pasteris’ group has helped MBARI to select and specify a Raman spectroscopic system that MBARI will purchase and encapsulate in pressure-resistant housings to enable its operation on the sea floor. The Washington University group will be involved in testing the "underseaworthy" Raman spectrometer and in interpreting the spectra that eventually will be taken. Such measurements have never been made on the sea floor. When Pasteris participated in a 4-day MBARI scientific cruise in March 2000, she got to experience firsthand (complete with seasickness, alas) what it will be like to take and interpret Raman measurements in real time from the sea floor.

Clathrate hydrates are considered to be one possible answer to the question of how humans could benignly re-package (sequester) the CO2 that we now dump in such huge abundance into the atmosphere due to the burning of carbon-based fossil fuels. Well recognized laboratory experiments on the pressure-temperature phase equilibria of CO2 and water indicate that below about 500 m in the ocean, CO2 and water should react to form a solid, ice-like compound called a clathrate hydrate. Clathrate hydrates are minerals that occur naturally on the sea floor, most commonly as methane (i.e., CH4, natural gas) clathrate hydrate. In these minerals, water molecules form a framework similar to that in ice, but many of the voids (sites) that are created are filled with gas molecules. In typical clathrate hydrates, there is about one gas (CO2 or CH4) molecule for every 6 water molecules.

The possibility of injecting CO2 into the ocean deeps brings up several questions. At what depth and temperature does liquid CO2 react to form a stable compound, such as a clathrate hydrate, that sinks to the sea floor? How long does that compound remain stable on the ocean floor, and what affects its longevity? What are the environmental impacts to both the local geology and biology of large-scale emplacement of CO2 deep in the ocean?

Dr. Brewer’s group at MBARI is the first to place liquid CO2 on the deep ocean floor. They successfully formed and videotaped the formation of synthetic CO2 clathrate hydrate at over 3600-m depth in Monterey Bay. Now they want to be able to take Raman spectra of the actual phase that forms in order to confirm its identity and to specify its crystal structure. The group at the Colorado School of Mines, under Dr. Dendy Sloan, plans to use Raman spectroscopy to measure the concentration of CO2 in the seawater at incremental distances from the clathrate. These data will provide a means of evaluating the long-term stability of the clathrate and its rate of conversion to dissolved CO2 in seawater. Pasteris’ group will focus its attention on mineralogical changes that may occur in sediments when CO2 is introduced. It is well known that the pH of the immediately surrounding seawater should plummet, which could cause dissolution of carbonate sediments or even acid attack on silicate minerals.

During the experiments on the cruise in March, 2000, Pasteris and her MBARI collaborators placed 5 different mineral and rock samples (mostly carbonates) at 3000-m depth in an artificial corral (see photograph) that then was filled partially with liquid CO2. The samples included polished blocks of fine-grained limestone, coarse-grained limestone, marble, and calcite-veined serpentinite, as well as a large cleavage rhomb from a single crystal of calcite. In April 2000, the MBARI group will return to retrieve the samples from the seafloor so that Pasteris’ group can determine if they underwent appreciable dissolution. In principle, the different grain sizes of the calcite will affect its dissolution rate, and the cleaved or polished surfaces of the samples will make it readily evident if and where any dissolution occurred.

 

 

FIGURE CAPTION: Photograph taken on the sea floor at 3015-m depth in Monterey Bay. Open-ended, PVC tube, about 54-cm inside diameter and 10 cm high, resting directly on clay-rich sea floor; 5 rock slabs are affixed to inner surface. PVC acts as a "corral" and reaction vessel for liquid CO2 injected via vertical tube connected to a tank on the nearby remotely operated vehicle. Each rock slab has vertical white paint stripe to act as barrier against reaction, thereby providing "control" material that will not undergo dissolution. Large, heart-shaped coalesced globule of liquid CO2 resting on clayey sediments in right half of corral. Additional CO2 has just been injected from the vertical tube, seen as colorless bubbles in left half of corral. Pasteris’ group will study dissolution effects on the rock samples after they have resided at depth for about 6 weeks. Photograph courtesy of Peter Brewer, Monterey Bay Aquarium Research Institute.