中国东部降水三氧同位素的时空变化及其对石笋古气候研究的指示意义

The climatic significance of speleothem δ18O variability has been controversial mainly due to the complex hydroclimatic controls on the processes affecting the origin transport and precipitation of water molecules. Modern observations also indicate that precipitation amount and water vapor source both affect the variability of precipitation δ18O in a monsoonal setting. How to distinguish the processes those are responsible for the variability of precipitation δ18O have been subject of investigation by climate scientists for decades. Isotope pair (δ18O and δD) has long been used for this purpose. In this project, we intend to use a new secondary index (17Oex) of water molecule to circumvent this problem. Studies show that the 17Oex value of the precipitation reflects the evaporation of water which is affected by the relative humidity in moisture source region but do not depend on the temperature of the evaporation. Recently, this project applicant has made a breakthrough in measuring 17Oex value of CO2 gas samples with very high precesion (5 per meg). This simple technique is extended to measure the 17Oex value of water samples by measuring the water equilibrated CO2. Based on this technique, the current project will first analyze the 17Oex value of precipitation in three locations of East China including Beijing, Nanjing and Guilin. Their variation characteristics at seasonal to inter annual timescales and the controlling factors will be studied. The results, seasonal to inter-annual variations in water vapor source of precipitation over East Asian monsoon region will be used to address the issues related to the moisture source and their recycle history that in turn potentially shed light to test many intriguing hypothesis related to controls on hydroclymatic process in paleoclimate at different timescales.

石笋δ18O指标的气候意义一直存在争议，这主要是由于水汽从源区到形成降水的过程中影响因素较多造成的。现代观测记录表明季风区降水量和水汽源区的变化对降水δ18O的影响较大，但如何区分二者的影响权重一直困扰着气候学家。本项目将发掘一种新的研究手段—三氧同位素，通过测定降水17Oex值来尝试解决这一问题。研究表明降水17Oex值与水汽的冷凝和蒸发过程无关，它基本上只受水汽源区相对湿度变化的影响，可以指示降水水汽源变化。近来，本项目申请人成功突破了CO2样品中17Oex的测试技术，并正在发展水体17Oex测试方法。本项目将首次测试北京、南京、桂林三个地区的降水17Oex，明确中国东部降水17Oex的季节-年际变化特征，研究东亚季风区降水水汽源的变化规律及其影响因素，为将来利用石笋17Oex指标研究不同时间尺度上的降水水汽源变化历史及其与气候变化的联系提供依据。

本项目建立了通过质谱和光谱两种技术高精度测定水中三氧同位素（17O-excess）的方法并将其应用于样品测试，得到的17O-excess数据表明该方法的测试精度小于10 per meg (1σ SD)，能够分辨出降水17O-excess的微小变化。对于USGS45、USGS47、GISP三种国际标准的测定结果与前人研究在误差范围内一致，证明了测试方法的稳定性和可靠性。基于此方法首次测试了南京、桂林、北京三个典型亚洲季风区不同季节的降水17O-excess值，为研究东亚季风区的降水水汽源区的变化及其影响因素提供了重要证据。数据表明：东亚季风区降水δ18O以及d-excess存在明显的季节性变化，而17O-excess的变化在不同的年份表现不一致。水汽源的位置在不同的季节的变化与降水过程中的二次蒸发以及不同源区的水汽混合可能是影响中国东部地区降水三氧同位素的主要因素。本研究将推动国内降水三氧同位素指标的理论应用并为后续的大气降水-洞穴滴水-现代碳酸盐沉积的三氧同位素指标的联合测试研究打下基础。