Seawater knows how to hang around.
Our human ancestors came from the oceans, and even today, about 70 percent of the human body is mildly salty water.
Many rocks contain seawater trapped in their crystals hundreds of millions of years ago.
That fossil water is telling scientists about the Earth's history.
A little more than a year ago, a Pennsylvania researcher reported finding a 250-million-year-old bacterium. He found the germ trapped, he said, in a tiny pocket of brine. The brine was nested inside a crystal.
Some scientists didn't believe it. The germ, they contended, had slipped into the brine pocket way more recently than any 250 million years ago.
Even so, plenty of scientists are poking around inside crystals formed millions of years ago, prospecting for trapped fluids and gases.
The waters trapped inside crystals have a fancy name: They're called fluid inclusions. Robert Goldstein, a Kansas University distinguished professor of geology, has written a whole book about them.
His article in the November issue of Science magazine described what fluid inclusions can tell us about the environment of the ancient Earth.
Some fluid inclusions are found in salt crystals and other minerals that were deposited as seas dried up. Others are bottled up in glacial ice and have information about prehistoric atmospheres.
The size of inclusions makes them a difficult study. They may be only a few micrometers wide only one-tenth the width of a human hair.
Another challenge, says Goldstein, is mining the inclusions without tainting their watery contents. Still another is making sure that nothing within the inclusion space has affected the fluid's makeup since it was sealed off from the world.
This problem arose with one of the early reported inclusions. In the late 1980s, a scientist found gas bubbles trapped in amber and thought he'd found remnants of the ancient atmosphere.
Actually, he'd found remnants of the amber's breakdown.
Even with all these problems, fluid inclusions have started to yield secrets about Earth's history. Inclusions in rocks a half a billion years old are telling us, for example, that the oceans have changed.
The mineral aragonite routinely crystallizes out of today's seawater. In ancient seas, aragonite dissolved.
That tells us, Goldstein says, that the ancient atmosphere was much richer in carbon dioxide than today's and that ocean chemistry has changed.
In fact, fluid inclusions tell us there have been large carbon dioxide swings in the last half billion years.
What really blows me away, though, is the idea that fluid stays locked up in crystals for hundreds of millions of years.
And get this: Some of these tiny pockets of fluid contain a bubble. The bubble is maybe one-fiftieth the width of a human hair. Subtle temperature variations around the crystal can keep that bubble dancing for a hundred million years, Goldstein says.
It also boggles me to think that humans, like the crystals of a great many rocks we pick up, are full of the sea. Our blood is about 1 percent salt, sea water about 3.5 percent salt.
Rocks and people: Trace us back far enough, and you'll see that we both come from a cradle of brine.
Roger Martin is a research writer and editor for the Kansas University Center for Research and editor of Explore, KU's research magazine Web site, which can be found at www.research.ukans.edu. Martin's e-mail address is rmartin@kucr.ukans.edu.
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