| 湖泊硅藻氧同位素:一种前景广阔的陆相古气候指标 |
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| 引用本文： | 汉景泰,陈贺海,李东,郭正府,储国强,刘嘉麒.湖泊硅藻氧同位素:一种前景广阔的陆相古气候指标[J].第四纪研究,2006,26(4):634-642. |
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| 作者姓名： | 汉景泰 陈贺海 李东 郭正府 储国强 刘嘉麒 |
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| 作者单位： | 中国科学院地质与地球物理研究所,北京,100029 |
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| 基金项目： | 中国科学院知识创新工程项目;国家自然科学基金 |
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| 摘 要： | 硅藻氧同位素已日益成为陆相古气候重建的一种重要手段。文章在简述这一领域已取得的基本认识基础上,着重介绍了近年来的重要进展。主要进展包括: 1)在硅藻的分离纯化方面,规范了重液分离法和新的重力差异流体分离法;2)完善了分步氟化法,创生了高温碳还原法的氧同位素制取技术;3)通过实验培养和天然湖泊监测实验,证实了硅藻氧同位素与温度的分馏平衡关系;4)湖泊硅藻氧同位素可以反映古温度、气候干旱事件和大气降水来源变化。同时,对目前硅藻氧同位素在湖泊沉积古气候研究中存在的主要问题作了讨论和展望。
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| 关 键 词： | 硅藻氧同位素 湖泊 古气候 |
| 文章编号： | 1001-7410（2006）04-634-09 |
| 收稿时间： | 2006-03-03 |
| 修稿时间： | 2006-04-01 |
LACUSTRINE DIATOM OXYGEN ISOTOPES: A PALEOCLIMATIC PROXY WITH GREAT POTENTIAL OF BROAD APPLICATION IN TERRESTRIAL RECORDS |
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| Han Jingtai,Chen Hehai,Li Dong,Guo Zhengfu,Chu Guoqiang,Liu Jiaqi.LACUSTRINE DIATOM OXYGEN ISOTOPES: A PALEOCLIMATIC PROXY WITH GREAT POTENTIAL OF BROAD APPLICATION IN TERRESTRIAL RECORDS[J].Quaternary Sciences,2006,26(4):634-642. |
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| Authors: | Han Jingtai Chen Hehai Li Dong Guo Zhengfu Chu Guoqiang Liu Jiaqi |
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| Institution: | Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029 |
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| Abstract: | Diatom oxygen isotopes applied for paleoclimatic reconstruction started from 1970s first for marine sediments and later extended to lacustrine sediment. As the technologies and methodologies involved are rather complicated, it developed slowly until recent years during which substantial progress had been made and great potential for broad applications to terrestrial sedimentary sequences was demonstrated. This paper is aimed to provide a brief and comprehensive review for the major achievements with discussions of the major problems remained. Diatom frustules are composed of amorphous silica through absorbing dissolved silica during growth. It contains an outer layer with Si-OH bond and the inner tetrahedrally bonded portion structured with Si-O bonds. The Si-OH bond is easily broken and the oxygen is exchangeable with surroundings under natural low temperature conditions. The inner Si-O bond, however, is much stronger and requires much more energy to be broken, keeping the oxygen stable in most circumstances. Dissolution of diatom frustules can cause a significant change of diatom flora in sediments from its original living flora, or completely destroyed, especially under alkaline environment. Some diatom oxygen isotope studies suggest little discrimination between species in term of the isotopic composition. Isolation and purification of the diatom frustules constitute one of the crucial parts in diatom oxygen isotope analysis, as most of diatoms is a few to tens of μm in size and is commonly mixed with clay and silt minerals. A modified traditional separation method (heavy liquid separation) involves four major steps, removal of organic matter and carbonates, size differentiation through sieving, differential settling and removal of mineral particles using pipette, and heavy liquid separation and cleaning of chemical contaminants with distilled water. A newly applied method with a specifically designed device SPLITT (gravitational split-flow lateral-transport thin) isolates the diatom frustules by adjusting the water (or other liquid) flow speed and sorts out the preferred size and density fractions. Since pure diatom samples are rarely achieved, purity estimation is recommended for calibration of the measured isotope values.Oxygen isotope extraction techniques involve a series of processes to remove the exchangeable component of the hydrous layer. Currently, stepwise fluorination (SWF) method is routinely employed. For removal of the exchangeable component, stoichiometrically deficient reagent (ClF3 or BrF5) is applied to the sample in the oxygen extraction line under low temperature and vacuum conditions. Subsequently, excessive reagent is used to the sample at high temperature for complete release of the oxygen. For isotope measurement, the yield oxygen is converted to CO2 by reacting with graphite in the same fluorination line. Some modifications to the SWF method include CIE (controlled isotope exchange) that is occasionally used in the case of the sample amount being insufficient for SWF method, and a method that removes the oxygen in the hydrous layer by dehydration at high temperature. A very recently developed method called iHTR (inductive high temperature carbon reduction), which dehydrates the sample up to above 1000℃ with a number of steps under vacuum, then disassociates the oxygen from Si-O bond by raising temperature to 1550~1750℃, and converts the oxygen to CO, instead of CO2, for isotope measurement. Although this method demonstrates a high accuracy and needs the sample amount one order less in magnitude than other methods, it has not been clarified whether isotope exchange occurred through atomic diffusion during dehydration at high temperature.A number of studies have constructed mathematical relationships between diatom isotope fractionation and temperature based on laboratory cultivation, though only a few species, natural lake diatom flora monitoring and diatom frustules from sediments. In spite of some debate, they show that the diatom oxygen isotopes are thermodynamically equilibrated with the water. However, the temperature fractionation effect suggested by different studies remains discrepant. Previous study shows a coefficient of 0.5 ‰ /℃, while 0.2 ‰ /℃ is demonstrated by most of later studies. No applicable diatom oxygen isotope thermometer has been established.The environmental factors that affect diatom oxygen isotope composition include temperature, oxygen isotope composition of the water in which the diatoms grow. These are more complicated in lake environment by the changes in precipitation, seasonal precipitation distribution pattern and variations of the pattern, precipitation-evaporation balance status. The dominant factor(s) must be identified for a sedimentary environment prior to the use of diatom oxygen isotopes as a paleoclimatic proxy. Up to date, a few studies use δ18 Odiatom as a paleotemperature indicator, while others refer the vertical variations of δ18 Odiatom in a sedimentary sequence to be an indicator of paleowater changes, for example, changes in amount of precipitation, precipitation-evaporation balance status (including dry event), water resources, or air mass source.Some fundamental problems need to be clarified urgently. These include 1) the effect of biogenic silica maturity on the oxygen isotope composition, 2) the temperature fractionation effect of different species, 3) the ecological studies of the diatom flora in a given lake, especially the seasonality and growing depth in the lake for the dominant and other notable members of the diatom flora, and 4) refining the mathematical expression of the relationship between δ18 Odiatom and temperature through more strict investigations. |
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| Keywords: | lacustrine diatom oxygen isotope paleoclimate |
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