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 Trace elements A group of elements that are present in the human body in very small amounts but are nonetheless important to good health. They include chromium, copper, cobalt, iodine, iron, selenium, and zinc. Trace elements are also called micronutrients. , 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 a·stound tr.v. a·stound·ed, a·stound·ing, a·stounds To astonish and bewilder. See Synonyms at surprise. [From Middle English astoned, past participle of astonen, ," says Ronald I. Dorn at the University of California The University of California has a combined student body of more than 191,000 students, over 1,340,000 living alumni, and a combined systemwide and campus endowment of just over $7.3 billion (8th largest in the United States). at Los Angeles Los Angeles (lôs ăn`jələs, lŏs, ăn`jəlēz'), city (1990 pop. 3,485,398), seat of Los Angeles co., S Calif.; inc. 1850. (UCLA UCLA University of California at Los Angeles UCLA University Center for Learning Assistance (Illinois State University) UCLA University of Carrollton, TX and Lower Addison, TX ). "There are entire mountain ranges in the Sinai Peninsula, as in the western United States Noun 1. western United States - the region of the United States lying to the west of the Mississippi River West Santa Fe Trail - a trail that extends from Missouri to New Mexico; an important route for settlers moving west in the 19th century , 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 al·ka·lin·i·ty n. The alkali concentration or alkaline quality of a substance that contains alkali. alkalinity 1. the quality of being alkaline. 2. 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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