| 36Cl断代法 |
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| 引用本文： | 黄麒.36Cl断代法[J].第四纪研究,1997,17(3):240-247. |
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| 作者姓名： | 黄麒 |
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| 作者单位： | 中国科学院盐湖研究所!西安，710043 |
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| 基金项目： | 国家自然科学基金，美国国家自然科学基金 |
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| 摘 要： | 本文讨论了36Cl的成因、36Cl断代法的原理及其在第四纪年代学中的应用范围和测试方法，重点论述了大陆蒸发盐的36Cl断代法。作者等曾应用该法测得了柴达木盆地1．4MaB．P．以来的石盐年龄。该法也已在冰碛物、火山岩等第四纪地质体的测年中获得了成功。有理由可以相信36Cl断代法将在第四纪年代学研究中得到更广泛的应用。
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| 关 键 词： | 36Cl断代法 大陆蒸发盐 冰碛物 |
| 收稿时间： | 1997-02-09 |
| 修稿时间： | 1997-05-21 |
METHOD FOR CHLORINE-36 DATING |
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| Huang Qi.METHOD FOR CHLORINE-36 DATING[J].Quaternary Sciences,1997,17(3):240-247. |
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| Authors: | Huang Qi |
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| Institution: | Institute of Salt Lakes, Chinese Academy of Sciences, Xi'an 710043 |
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| Abstract: | Chlorine-36 production, principles and experimental method of 36Cl dating are briefly described.1 Chlorine-36 productionChlorine-36 is produced in atmosphere, rocks or water near the land surface（epigene）, and within rocks and water below the earth's surface （hypogene）.2 Experimental MethodFor 36Cl measurement by accelerator mass spectrometry （AMS）, the samples are prepared by dissolving away about half of amount of the halite and rocks in distilled water for removing any exchanged or recently precipitated component. The samples are then dissolved in distilled water or deionized water, and the chlorine precipitated as AgCl. The supernatant is discarded. AgCl is purified by repeated dissolution in concentrated ammonia and Ba（NO3）2 solution. The solution reprecipitated by addition of HNO3. Centrifugalized and dried at 80℃. AgCl is placed in small tightly sealed bottles and protected from light, and 36Cl measurement by AMS.3 Chlorine-36 DatingChlorine-36 dating of continental evaporites.Water entering closed basin lakes, such as those in the Chaidamu Basin, carries 36Cl and stable chlorine from meteoric, epigene and hypogene sources. Due to the high solubility of chloride, the chlorine isotopes will accumulate in the lake water until halite saturation is achieved. The 36Cl will then be locked into the halite crystals,and will decay with time. The age of the halite may be computed by comparing the initial 36Cl / Cl ratio （at the time of deposition） with measured one in the halite by using the basic equation:where λ36 is the decay constant (2.303 × 10-6/ a)Successful dating halite by 36Cl depends on two conditions: （1） constant of known input 36Cl / Cl ratio, （2） negligible cosmic-ray production after deposition, and no significant halite translocation. These conditions have been tested on the evaporite deposits in the Searles Lake （U.S.A.） and Salt Lakes in Chaidamu Basin （China）, and have been found valid there. They also support the hypothesis that 36Cl production has been fairly constantly over the last million years, and a conclusion that has important implications for 36Cl dating of halite.Chlorine-36 dating of volcanic rocks.Because of the reaction of cosmic rays with elements in minerals. Chlorine-36 accumulates in rocks exposed at the earth's surface. This suggests that the ratio of 36Cl to stable chlorine can be used as a geochronometer. Reasonable agreement has been obtained between measurements of 36Cl in volcanic rocks known age and calculations of chlorine-36 production based on rock composition and cosmic-ray flux.The buildup of 36Cl could be a useful tool for measuring the surface exposure time of young volcanic rocks17].Chlorine-36 dating for glacial deposits.Deposits from mountain glaciers provide an important record of Quaternary climatic fluctUations, but it is difficult to date directly. A chronology has been obtained for glacial deposits at Bloody Canyon, California, by measurement of 36Cl produced by cosmic rays in boulders exposed on moraine crests. Although the timing of the glaciations correlates well with peaks of global ice volume inferred from the marine oxygen isotope record, the relative magnitudes differ markedly. The lengths of the moraines dating show that the early glacial episodes were more extensive than those during the later Wisconsin, and indicate that the transition from interglacial to full glacial conditions was rapid16].This confirms the constancy of cosmic-ray flux and 36Cl accumulates in rocks exposed at the earth's surface over the last million years, and implies that 36Cl dating can be used for accurately dating of halite sediments, young volcanic rocks, and glacial deposits |
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| Keywords: | chlorine-36 dating continental evaporites glacial deposits |
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