利用Picarro测试石笋包裹体水氢、氧同位素的方法

Measurement of oxygen and hydrogen isotopic ratios of speleothem fluid inclusion water using Picarro

基于当前石笋包裹体水提取测试过程中面临的制约条件,本研究设计了用于提取石笋包裹体水的真空装置,将其与水同位素分析仪Picarro L2140-i相连接组成石笋包裹体水氢、氧同位素(δD、δ18O)测试系统.该系统能够快速地提取石笋包裹体中的水,并对其δD和δ18O的值进行测量.包裹体水真空提取装置使用了以高压气缸为驱动的破碎研磨器,具有良好的破碎效果;在120°C真空条件下破碎获得的包裹体水随后被高纯氮气带入到Picarro L2140-i中进行测量分析.为了验证包裹体水测试系统的可靠性和准确性,本研究分别将石笋包裹体水真空提取装置以及Picarro液态水自动进样器与Picarro L2140-i主机连接,对相同的水样品进行了对比测试.两种方法测得的δ18O和δD值在误差范围内接近.使用该系统对东亚季风区石笋样品进行的测试结果显示:石笋包裹体水的δ18O和δD值分布在全球大气降水线附近,表明测试过程中分馏作用较小;包裹体含水量对测试精度有一定影响,对于水汽浓度为2500~50000 ppm(1 ppm=1μL/L)的样品,δ18O、δD的标准差分别为0.5‰、2‰,与同类型系统相当.研究结果表明该系统能如实测量石笋包裹体水的δ18O和δD值,将为利用水-碳酸钙平衡分馏方程重建古温度提供重要的技术支撑.

A speleothem fluid inclusion extraction and isotope analysis system was established based on a vacuum extraction device and a Picarro L2140-i wavelength scanned cavity ring-down spectroscopy(WS-CRDS)water isotope analyzer.WS-CRDS allows to measure the stable oxygen and hydrogen isotope composition(δ18 O andδD)of small water samples simultaneously,which is faster,simpler,and requires less maintenance compared to currently used continuous-flow isotope ratio mass spectrometry(CF-IRMS).The precision of water isotopic analysis utilizing the WS-CRDS is often superior to that of CF-IRMS.Therefore,the use of Picarro for isotope analysis has rapidly gained popularity.The oxygen isotopic composition of speleothem calcite(δ18 Oc)is one of the main proxies of speleothem-based paleoclimate reconstructions,but the interpretation ofδ18 Ocremains complex as it can be influenced by many factors.Speleothem fluid inclusions contain the characteristics of cave drip water.Theδ18 O andδD values of fluid inclusion water provide direct information about paleoprecipitation,and can be further used to reconstruct paleotemperature when combined withδ18 Oc.The present paper contains a detailed description of the system as we have designed it,and provides an introduction to the system’s performance.The vacuum extraction device,connected to the Picarro analyzer,is designed to release the water trapped in speleothem fluid inclusions by pneumatically-operated crushing of discrete speleothem calcite chips.The whole device is kept at a temperature of 120°C to ensure the total and instantaneous vaporization of the water without isotopic fractionation.The vaporized water is carried by a flow of high-purity nitrogen to a 40 m L volume for homogenization,and is then introduced into the Picarro analyzer forδ18 O andδD analysis.A syringe injection unit was added to the vacuum extraction device to be able to measure standard water,withδ18 O andδD values similar to the speleothem samples,before and during measurements.To verify the precision and accuracy of the system,the same standard water samples were measured with an automated Picarro L2140-i set-up that includes an autosampler and a Picarro evaporation unit.Theδ18 O andδD values of standard waters as measured by both systems are comparable.Using the crushing device,speleothem samples from the East Asian Monsoon region yield pairedδ18 O/δD values within±0.5‰of the Global Meteoric Water Line(GMWL),indicating no isotopic fractionation during the measurements.The precision is a function of the water amount released and of the isotopic composition of the sample.The reproducibility of crushed speleothem samples,whose water vapor concentration is between 2500 and 50000 ppm,is 0.5‰forδ18 O and 2‰forδD(1 SD),similar to results obtained from other published measuring systems.