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C0 2 CHEMISTRY DEEP IN THE OCEAN Video camera records reactions of C02 with seawater to form a solid hydrate
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n a series of deceptively easy-looking experiments, a team of ocean chem ists and engineers from Monterey Bay Aquarium Research Institute (MBARI), Moss Landing, Calif., and Stanford University have captured on videotape what happens to carbon diox ide when it is released in the deep ocean. Their footage may help evaluate whether that is a good place to seques ter some of the C0 2 produced by burn ing fossil fuels that is now building up in the atmosphere. Peter G. Brewer, senior scientist at MBARI; Franklin M. Orr Jr., professor of petroleum engineering at Stanford; and their colleagues performed a smallscale field test of what would happen to C0 2 injected into the ocean, either in gaseous or liquid form, at various depths [Science, 2 8 4 , 943 (1999)]. Al though there are considerable data from theoretical models and laboratory experiments on the impact of discharg ing C0 2 directly into the deep ocean, there are little actual field data, Orr ex plains. These experiments are very diffi cult to do, he points out; they require a remotely operated vehicle that can per-
form them and record the data. 'We had to draw on the capability built up at MBARI over the past decade to be able to do this experiment," he says. "This is one of the first examples where somebody has gone out into the field to make observations, even on a small scale, of what happens to C0 2 at various depths" in the ocean, says E. Eric Adams, senior research engineer in the department of civil and environ mental engineering at Massachusetts Institute of Technology. Adams is part of an international team that plans to conduct its own field experiments in the next few years on the effects of dispers ing C0 2 into deep seawater. "It's a beautiful piece of work," says Wallace S. Broecker, professor of geo chemistry at Columbia University, who also studies the potential impact of dis charging large amounts of C0 2 into the deep oceans. Broecker thinks the ex periments are valuable because they confirm many of the predictions made earlier based on limited laboratory data. The experiments find that in many ways C0 2 in the ocean behaves as mod els have predicted. In particular, they verify that because liquid C0 2 is more compressible than water, at depths be * > | 7* low about 3,000 meters it becomes denser than wa '£f« · ysr~- en ter and sinks. Higher in y - --< * © the water column, it's lighter than water and ris es. The researchers also found that at both 350 meters and 3,650 meters, C0 2 reacts with seawater -Ato form a solid hydrate. . -v. Some models had pre ι * » · * - ) * » »tt«*t«* dicted that if C0 2 were in jected into the ocean at Liquid carbon dioxide spills from a 4-L beaker depths greater than 3,000 on the ocean floor, beneath 3,650 meters of meters, it would settle into seawater. Some of the C 0 2 has reacted with wa depressions in the ocean ter to form a nearly transparent solid hydrate, floor to form "ponds" with barely visible at the bottom of the beaker, caus ing the remaining C 0 2 to overflow. an icelike layer of hydrate on top. So the researchers CD
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MAY 10,1999 C&EN
tried it: They put a glass beaker on the ocean floor, 3,650 meters below the sur face, filled it about half full with liquid C0 2 , and set up a video camera to record what happened. 'What didn't happen was what every body imagined, which was that a layer of hydrate would form over the top," Orr says. "Instead, the liquid C0 2 in the bea ker seemed to be expanding. As we watched over a period of about an hour, it expanded and expanded until eventu ally the C0 2 spilled over the edge of the beaker." It formed discrete puddles that rolled downhill on the ocean floor, even tually out of view of the camera. It's now clear, Orr explains, that hy drate particles were forming and sink ing, rather than floating, in the liquid C0 2 . Because the hydrate contains about six water molecules for each mol ecule of carbon dioxide, it filled more space than the original C0 2 did, push ing the C0 2 out of the beaker. "Surely in the next 30 or 40 years, we are going to go to disposing of C0 2 as one of the partial solutions to the C0 2 problem," Broecker says, "and there are only two places that you can put it. One is in the ocean, the other is in saline aquifers on the continents. I think the publication of this paper will get people to think about the ocean approach as something that should be seriously considered." That would be a very useful outcome, he says. "Unless we spend the next 10 years looking into these possibilities and finding out all of the costs and environ mental consequences, we'll just have to do it later and have wasted a lot of time." Rebecca Rawls
EPA set to tighten vehicle emission, gasoline standards Calling it "one of the most important steps we can take to clean the air we breathe and protect the health of all Americans," President Bill Clinton an nounced during his weekly radio address on May 1 the Environmental Protection Agency's long-awaited proposal tighten ing emission standards for motor vehi cles and sulfur-control requirements for gasoline. EPA is proposing for the first time that both automobiles and light-duty trucks—including pickups, minivans,
