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Climate clues glued to rocks.

Rock varnish is a black, paper-thin coating that forms slowly over tens of thousands of years as clays, trace elements, organic matter and whatever other particulates happen to fall out of the air are cemented to rock surfaces by iron and managanese oxides. "It's just astounding," says Ronald I. Dorn at the University of California at Los Angeles (UCLA). "There are entire mountain ranges in the Sinai Peninsula, as in the western United States, that are completely coated with varnish -- top to bottom."

Because varnishes are found worldwide -- especially in deserts, where they can remain chemically stable for 100,000 years and more -- Dorn and a number of other researchers think they could be useful in understanding the environmental conditions of the past. Dorn has published a few papers showing how rock varnishes record the alkalinity of past environments, as well as how varnishes can be used to date geologic and archaeological objects. Now, in the March 22 SCIENCE, he and Michael J. De Niro, also at UCLA, describe a method for estimating past aridity by measuring the ratio of carbon-13 to carbon-12 contained in varnish organic matter.

The researchers believe that the ratio in the varnish reflects that of plants that lived nearby. They also expect that different kinds of plants will have different ratios: So-called C.sub.3 vegetation--plants that produce acids containing three carbon atoms during the first step of photosynthesis -- would have a lower ratio than C.sub.4 plants that make acids with four carbons. And because C.sub.3 plants tend to live in cool, humid places while C.sub.4 and another type of plant prefer hot, arid locales, the ratio in the varnish reveals how arid or humid a region was.

At present, Dorn must scrape off a lot of varnish from rocks in order to get enough organic material for conventional isotope measurements. So, at best, the researchers' view of past climates is limited to general trends that persisted over a minimum of 100,000 years. In the future, however, Dorn hopes to improve the sensitivity of the technique by using an accelerator, which requires much less organic matter to measure the carbon isotope composition.
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Title Annotation:rock varnish
Publication:Science News
Date:Mar 30, 1985
Words:364
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