青藏高原湖泊萝卜螺碳酸盐二元同位素温度计研究及其在特征时段的应用

Carbonate clumped isotope (△47) is a new developed tool for quantitative paleotemperure reconstruction. There are vast of lakes distributed on the Tibetan Plateau. But the paleoclimate reconstruction using △47 is still limited. The gastropod Radix widely distributed living in the freshwater and oligohaline to mesohaline lakes. The shell of Radix is a good material for △47 measurement. This project will recover the particular period change using Radix shell △47 mainly with the help of modern process study. First, we select about 10 lakes for observing the water temperature and lake level. We also collect the Radix for measuring △47 and oxygen and carbon isotopes simultaneously from the lakes. After comparing the △47 value and temperature of different time span, we will contribute some useful data for calibrating the △47-temperature, and the calibration line used for Tibetan Radix shell of may be selected. Then we will know what time span of lake water temperature of △47 reflects. With the help of model to air-lake temperature,using the lake temperature-air temperature and observation data, we will esteblish the ralitionship between lake temperature and air temperature. Comparing the caculated lake and mearsured water oxygen isotope, we will evaluate the validity of oxygen caculation derived from shell carbonate. We will also compare the carbon and oxygen istopes and element ratio in Radix shell of different lakes for finding their potential envionmental significance. A envionmental change sequence will also be retrieved using a lake core from Xuru Co. With comparing of environmental change sequence in three lakes respectively, we will use those proxies studied above to recover the envionmental change of particular period when gastropod layer appeared.

碳酸盐二元同位素（△47）是古气候学研究中新发展起来的古温度定量指标。青藏高原湖泊广布，利用湖泊沉积物△47进行古温度重建的研究还比较少见。腹足类萝卜螺是青藏高原淡水系统最常见种，其螺壳是进行△47分析的良好材料。本项目拟在监测湖水温度的前提下，通过采集具有一定温度梯度的湖泊中的现代萝卜螺，分析螺壳的△47，对比螺壳△47值与监测的湖水温度的关系，确定螺壳△47的水温定量指示意义；利用模型计算和湖泊气象资料与湖面温度对比等方法，建立起水温与气温的关系；对比计算出的现代湖水氧同位素与现代湖水氧同位素实测值，评价螺壳反推湖水氧同位素的有效性；对比螺壳常规同位素以及元素比值的空间差异，分析他们的环境意义。利用湖泊岩芯建立许如错全新世中期以来的环境变化序列。与各湖泊已有的和本项目重建的湖芯环境序列相比较，将以上现代过程的结果应用得到纳木错、沉错和许如错湖相剖面螺壳层出现时段的环境恢复上来。

在青藏高原湖泊盆地中经常会发现一些具有萝卜螺壳层的古湖相剖面出露。研究这些古湖相剖面对于理解湖泊自身及其环境演化过程具有重要意义。本项目研究的内容分为两部分，一是通过现代过程调查，分析现生萝卜螺壳体参数与水体环境之间的关系；二是在陆上古湖相剖面测年和螺壳参数的辅助下，利用许如错和沉错的湖泊岩芯，重建全新世某一时段湖面波动和环境变化的历史，并分析驱动因素。结果表明，所选湖泊年均温或夏季温度空间差异较大，为验证水温和螺壳同位素的关系打下坚实基础。许如错湖泊岩芯XRF元素和磁化率指标重建了中晚全新世5500 cal a BP以来的湖面波动和径流变化历史，发现5500~4000 cal a BP印度季风最为强盛，湖面最高，4000~2000 cal a BP季风减弱，湖面下降，径流减弱；2000 cal a BP以来，湖面虽有波动，但保持低水位。最后一次高湖面应该出现在2000 cal a BP前后。利用沉错湖泊岩芯指标重建的YD以来的湖泊水位和环境变化历史可分为4个阶段，YD阶段，13.2~11.5 cal ka BP，水位最低，冷干程度最强，印度季风最弱；早全新世最适宜期，11.5~8 cal ka BP，水位最大，最暖湿，印度季风最强；中全新世冷干期，8~6 cal ka BP，此阶段与纳木错相吻合，是区域尺度对季风变化的响应；6 cal ka BP以来，较高水位时期，出现几次冷干事件，陆上高湖相沉积物年龄在这个范围内，开始时间多为冷干事件之后。最后一次高湖面出现在大约900 cal BP左右。总而言之，沉错、许如错和纳木错的湖面波动主要受印度季风控制，最后一次高湖面出现时期，沉错是在大约900 cal BP，其他湖泊出现在2000 cal BP左右。