| 黄土中钙结核的碳氧同位素研究(二)碳同位素及其古环境意义 |
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| 引用本文： | 韩家楙,姜文英,吕厚远,吴乃琴,郭正堂.黄土中钙结核的碳氧同位素研究(二)碳同位素及其古环境意义[J].第四纪研究,1995,15(4):367-377. |
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| 作者姓名： | 韩家楙 姜文英 吕厚远 吴乃琴 郭正堂 |
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| 作者单位： | 中国科学院地质研究所 |
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| 基金项目： | 中国科学院留学经费择优支持项目,国家自然科学基金 |
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| 摘 要： | 从洛川黄土剖面离石黄土各层古土壤中钙结核的碳同位素分析结果，探讨了钙结核碳同位素变化所反映的古土壤形成时的植被状况，还借助于“干燥度”，结合由钙结核氧同位素变化所获得的古温度资料，估算了各层古土壤形成时长时段的年平均降水量。所得结果表明，各层古土壤形成时温度和湿度状况有更复杂的变化。大部分古土壤形成时的年平均降水量与现代相差不多(增幅在５０ｍｍ左右)，但发育最好的Ｓ５形成时，年平均降水量超过现在约２００ｍｍ。古土壤的碳氧同位素能很好指示古环境，但研究工作还有待深入。
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| 关 键 词： | 古土壤 钙结核 碳同位素 干燥度 |
CARBON AND OXYGEN ISOTOPE COMPOSITIONS OF CARBONATE CONCRETIONS IN LOESS PART 2:CARBON ISOTOPE AND PALEO-ARIDITY |
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| Affiliation: | Institute of Geology, Chinese Academy of Sciences |
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| Abstract: | Carbonate is a common component in soils and Quaternary sediments. The stable isotopic compositions of secondary carbonates in Quaternary deposits, mainly the δ18O and δ13C values, were used as the important paleoenvironment data carriers,from which the information of the paleoclimatic and paleoecological conditions were extracted. Loess is one kind of the Quaternary sediments rich in carbonates. During the paleosol formation original carbonates became soluble bicarbonates in the local environment with high CO2 partial pressure. These soluble bicarbonates reprecipitated in water lost and/or the decreasing of CO2 content. Isotopic exchange took place during this dissolution and reprecipitation cycle.The carbonate concretions related to paleosols in Lishi Loess (Formation) of the Luochuan loess section were systematically collected. The measurement of carbon and oxygen isotopic compositions was carried out in the laboratory. Statistical analysis on the obtained δO and δC values was optimistic about the possibility of carrying out quantitative paleoclimatic and paleoecological reconstruction. In the first part of this paper the authors discussed the variation of δO values in different paleosols and an attempt was made to correlate the δO values to the paleotemperature when paleosols were formed. What is reported here is the variation of the carbon isotopic compositions of paleosol carbonates in Lishi Loess (Formation) of Luochuan and the paleoclimatic and paleoecological significance of the carbon isotopic composition.δ13C values range from -2.78 to -7.19‰, with the maximum value in less developed paleosol S2-1 and the minimum value in best developed paleosol S5-2. It changes in a opposite direction to that of δ18O. For δ18O, the minimum value is measured in less developed paleosol S2-1 and the maximum value in best developed paleosol S5-2. The variation magnitude between different paleosols (about 4.4‰) is bigger than that of δO(1.5‰).The carbon isotopic composition of pedogenic carbonate is controlled by the isotopic composition of soil CO2 in soil gases, which is usually well correlated with the proportion of C3 and C4 photosynthesis in the local ecosystem. Using the δC values of different paleosols the proportion of C3 and C4 biomass was estimated for each paleosol by geochemical calculation. The results imply that for most paleosols there should be almost same proportion of the C3 and C4 biomass and only for those well developed paleosols with higher C3 biomass up to 66% (S5-2) and for less developed paleosol with lower C, biomass (35%, S2-1) indicating the local ecosystem in Luochuan area is dominated by the grass vegetation during most of paleosol formation except for the well developed paleosol S5 which was developed under the vegetation with some trees and shrubs.C4 plants, which are adapted to high light and water stressed conditions, have δ13C values between about -9 and -19‰ quite different from that of C3 plants (between about -22 and -34‰). The C4 photosynthesis pathway is thought to have evolved in the Tertiary in response to lower atmospherict CO2 conditions. C4 dominate ecosystems are favored by considerable water stress during the growing season in the warm part of the year. It means that the increase of proportion of C4 biomass indicates the increase of the aridity of the environment. In this study, for simplification, the ratio of annual mean temperature (T) of annual mean precipitation (P) is used as the index of environment aridity. Based on the statistical analysis of the δC values of modern soils and the data of aridity delivered from the annual mean temperature and annual mean precipitation of the locality where the soil developed,the relation between the δC values of soil and the aridity can be obtained. An attempt is made using this relation to estimate the long term annual mean precipitation during time of each paleosol formation. The results are comparable to previous conclusions obtained from other methods. More work has to be done on the mechanism of concretion formation an |
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