湖泊现生自生碳酸盐二元同位素Δ47与温度定量关系的现代过程研究

Palaeoclimatologists have been paying closely attention to temperature reconstruction as temperature is one of important parameters for assessing the climate change in the future. Based on the theory that the carbonate clumped isotope (Δ47) is dependent of temperature, 50 lakes distributed in Qinghai-Tibetan Plateau and Inner Mongolia-Xinjiang Plateau, which have great gradients of temperature, salinity, elevation, and latitude, will be selected to study the clumped isotope thermometry in lacustrine authigenic carbonates from modern lakes. Monitoring, sampling and analysis will be conducted for 2 years. How temperature during the period of formation of a single lacustrine carbonate relates to climate (water and air temperature) will be determined. Some potential temperature-independent factors, such as chemical composition of the lake water, carbonate type and its content, which could influence clumped isotopes of lacustrine carbonate, will be discussed. Then we will establish temperature-Δ47 relationship for total carbonates (calcite, aragonite, dolomite, and ostracoda shells) and only for calcite, aragonite, dolomite, and ostracoda shells, respectively, and discuss the relationship between carbonate Δ47 and temperature changes caused by latitude and elevation variations. Finally we will assess the precision, deviation, uncertainty, applicability of these relationships for paleotemperature reconstruction. This study can provide scientific basis for temperature reconstructions during the geological time over the China or even over the whole world by using our established clumped isotope thermometry in lacustrine authigenic carbonates.

温度作为评估未来气候变化的重要参数之一，是古气候学家关注的重点。本项目将基于碳酸盐二元同位素Δ47只受温度影响的理论，在我国青藏高原和蒙新高原选择具有一定海拔、纬度、温度和盐度等自然梯度的湖泊50个左右，开展湖泊现生自生碳酸盐二元同位素Δ47与温度关系的现代过程研究。通过2年的监测、采样和分析测试：确定湖泊自生碳酸盐主要形成季节的水温同气温的关系；研究在天然湖泊这一复杂的自然系统中，湖泊自生碳酸盐Δ47是否受湖水水化学条件、碳酸盐种类及含量等其它要素的影响 ；建立湖泊自生碳酸盐（方解石、文石、白云石、介形类壳体）Δ47同温度（水温、气温）的函数关系，并探讨因海拔高度和纬度效应引起的温度变化与湖泊自生碳酸盐Δ47的关系 ，评估它们在古温度重建中的可靠性和适用性（误差、精度、适用范围、不确定性等）。为利用我国乃至全球的湖泊自生碳酸盐Δ47 进行地质历史时期古温度的重建提供重要的科学依据。

湖泊是气候变化研究的重要载体，湖泊水体和湖泊沉积物碳酸盐的同位素组成可以用于区域水汽循环及气候环境变化的研究。本项目采集了中国西部33个代表性湖泊的表层湖水及表层沉积物的自生碳酸盐样品，系统地探讨了湖泊海拔、水温、水化学组成等对自生碳酸盐氧同位素和团簇同位素分馏的影响。获得的主要结果如下：（1）中国西部33个湖泊夏季湖水氢氧同位素存在良好的线性关系且呈现明显的空间分异特征，这种空间分异特征与降水氢氧同位素的空间分异特征基本一致。表明湖水氢氧同位素组成受控于大气降水所带来的氢氧同位素的初始组分，与降水水汽来源及局地蒸发过程密切相关。（2）自然条件下湖水pH值、氧同位素交换反应速率的变化可能造成湖泊自生碳酸盐沉淀过程中的氧同位素动力学非平衡分馏，从而使得氧同位素分馏系数与湖水温度之间的负相关性并不显著。但无论同位素分馏是否达到平衡，碳酸盐氧同位素组成总是取决于湖水的氧同位素组成。湖泊自生碳酸盐氧同位素组成的变化可以反映降水和局地蒸发过程的变化。（3）湖泊自生碳酸盐团簇同位素Δ47与湖泊夏季平均水温存在较好的线性负相关关系，而和海报高度存在明显的线性正相关关系，由此建立的Δ47与温度和海拔经验公式可应用于古温度和古海拔重建。此外，湖岸双壳类和腹足类的生理适应机能导致其壳体碳酸盐团簇同位素对温度变化敏感度较低，其Δ47并不能准确记录湖泊夏季平均水温。项目获得的结果，为今后利用湖泊自生碳酸盐氧同位素重建湿度变化、利用自生碳酸盐团簇同位素重建古湖水温度、古海拔高度、青藏高原隆升等方面提供了科学依据。