A rchive Date
[ 09-08-2000 ]
Category
[ Science ]
sub-Categoy
[ Geology ]
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[Paper places creation of ozone layer
By MICHAEL SMITH- CNEWS Science
Wednesday, Aug. 09, 2000
There is a legend about a group of blind men who had heard of the elephant, but didn't know what it was. So they went to investigate. One felt the beast's side and thought the elephant was like a wall. One felt the animals's leg and thought it must be like a tree. And ... but you get the idea.
Which brings me to the ozone layer. The ozone layer - that blanket of protective poison that lies 80 kilometres or so above our heads - has been getting a lot of attention since scientists noticed a few years back that it was getting a shade ragged.
This is of concern, of course, because ozone protects us from much of the ultraviolet light that would otherwise be pouring down on us, causing lethal sunburns, random mutations and generally making life miserable.
But the ozone layer wasn't always there and it's an open question when it first wrapped the Earth in its protective folds. And, because you can't have an ozone layer without oxygen, knowing when it arose also tells you when there was abundant oxygen. And that, in turn, gives you clues as to when Life As We Know It could have first arisen.
Which is why scientists are greeting a paper in this week's Science with some interest. Atmospheric chemist Mark Thiemens of the University of California at San Diego, with colleagues, says he has geological evidence that the ozone layer was fully formed 2.2 billion years ago.
The conclusion rests on the changing ratios of four isotopes of sulphur, which may seem a long way from oxygen and ozone, but actually has a close link, Thiemens says.
Isotopes of an element are variations that have a different atomic weight but almost identical chemical properties. Sulphur-32, for instance, is the most common isotope of that element and its nucleus has 16 protons and 16 neutrons. Sulphur-33, a rarer isotope, has an extra neutron.
Most of the sulphur around is S-32, but there are traces of S-33, S-34 and S-36 in well-known proportions. But the energetic rays of ultraviolet light interfere with the natural proportions, enhancing S-33 and S-36 at the expense of the other two, Thiemens says.
So here's the argument: With no ozone, UV light pours through the atmosphere, reacting with sulphur oxides in the atmosphere and shifting the isotopic proportions. Those oxides, later deposited into rock, still show the UV effect. But the ozone layer, when it formed, would "turn off" the UV.
So very old rock should have extra shares of S-33 and S-36. But newer rock, formed after the ozone layer arose, should not. And if you choose your rocks carefully, you can see when the change occurs - in other words, when the ozone layer first occurred.
And when Thiemens and colleagues carried out the tests, they found that the ozone layer was fully formed by 2.2 billion years ago.
Neat, simple and conclusive, you might think. But science is rarely so cut and dried and the arguments are just beginning.
Sherlock Holmes used to opine that whenever you've eliminated the impossible, whatever remains must be true. But nature is tricky and eliminating the impossible is ... well ... almost impossible.
So geoscientist Hiroshi Ohmoto of Penn State University is a skeptic. Ohmoto says the Thiemens data are interesting, but the interpretation, although logical, is suspect. "I'm not sure that's the only explanation of the data," he said.
And until the impossible has been eliminated, "we really don't know what's causing the sulphur isotope variation."
Ohmoto, of course, has an axe to grind: He's convinced (and he has his own evidence) that the ozone layer formed much earlier than Thiemens thinks.
On the other hand, planetary geologist John Delano of the State University of New York at Albany welcomes the Thiemens work - not as a new result, but as something that confirms what he and other researchers think they know from other lines of evidence.
"It's a very interesting result, supportive of other independent data," he told me. "The case is building."
So who's right?
Well, like the blind men, scientists will keep investigating the elephant. Unlike the blind men, though, they'll compare and combine their impressions and - eventually - build up a pretty complete picture. It just won't happen in one blinding flash, in one paper, in one journal.
www.origins.rpi.edu/
psarc.geosc.psu.edu/index.htm ]
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