青海湖水-气界面CO2交换的地球化学研究

Constructing “boundless carbon cycle”, designating all lateral and vertical carbon fluxes to and from the systems of inland waters, is the most fundamental frontier of the global carbon cycling study. Much progress has been made about the fluxes of CO2 emission from rivers, streams, lakes, reservoirs, ponds and other systems of inland waters around the world. However, data from salt lakes remains very sparse. As a result, the role of salt lakes as carbon source or sink in global carbon cycling, the strength of carbon source/sink, keeps to be disputed extensively and intensely. And factors and processes regulating CO2 production and emission in salt lakes remains unknown. In this proposal, we will conduct an observation of CO2 exchange on the water-atmosphere interface at Lake Qinghai, a huge water body with a salinity of about 15.5‰, using a static chamber technique. Combining with measurements of CO2 partial pressure in the surface water, the flux of CO2 exchange on the water-atmosphere interface in Lake Qinghai and surrounding small lakes could be calculated with a better precision. The concentrations and isotopic compositions of particulate organic C (POC), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) of lake samples that are collected at the different depth in the water columns of the lake will be determined. Processes of biogeochemical cycling of organic carbon and inorganic carbon will be discussed on the basis of temporal and spatial variations of concentration and isotopes of POC, DOC, DIC and PIC, with a focus on the coupling among organic carbon cycling, inorganic carbon cycling, and pH variation. The mechanism of how biogeochemical cycling of organic carbon and inorganic carbon regulate the CO2 concentrations and exchange fluxes in the salt lake will be revealed. A model based on the balance of carbon mass will be constructed, in which CO2 exchange on the water-atmosphere interface will be coupled with organic and inorganic carbon cycling. A comparison of CO2 concentrations and exchange fluxes between Lake Qinghai and surrounding small lakes whose salinity varies in a large range will be carried out, by which change of CO2 exchange flux on the water- atmosphere interface of salt lakes with evolution in the past and trend in the future of salt lakes will be discussed. Finally, the role of salt lakes to the local and global carbon cycling will be assessed on the basis of the result that will be obtained in this program.

针对内陆水体CO2释放的boundless碳循环模型是当前全球碳循环研究的前沿，其中盐湖是重要的组成部分，也是最薄弱的环节，目前仅有少量观测数据且争议很大。本次研究选取我国最大的微咸水湖青海湖及其周围的小湖泊作为研究对象，利用野外现场观测水-气界面CO2交换，结合走航式表层水体CO2浓度测定，充分揭示盐湖CO2时空分布的高变化性特征，准确估算水气界面CO2交换通量。通过对湖水颗粒有机碳、溶解有机碳、溶解无机碳和颗粒无机碳的浓度和同位素系统分析，揭示青海湖有机碳、无机碳的生物地球化学过程，认识微咸水湖OC、IC循环的相互作用对界面CO2交换的调控机制，建立以水-气界面CO2交换为核心的湖泊碳质量平衡模型。对比分析青海湖主湖与周围不同盐度小湖泊的CO2变化规律，认识盐湖演化以及未来全球变化背景下盐湖CO2交换的变化趋势。研究成果将为认识盐湖在区域及全球碳循环中的作用奠定坚实的基础。

盐湖CO2释放是全球碳循环的重要组成部分。由于观测数据少，盐湖的CO2又是碳循环研究的薄弱环节，也是争议最大的部分。项目选取我国最大的咸水湖青海湖作为研究对象，结合周边子湖以及其他不同盐度的盐水湖，观测水-气界面CO2交换通量，分析盐湖CO2产生机制和释放规律，揭示青海湖及其流域碳来源和转移过程以及CO2的产生机制和释放量，认识盐湖在区域及全球碳循环中的作用。. 项目获得了一批盐湖的CO2和CH4释放通量数据。青海湖主湖CO2释放通量很小，空间上和昼夜变化平均为10.9±3.3（10.0~12.4）和6.3±5.7（0.3~10.4）mmol/m2/d。子湖及其他盐湖分别为11.0±28.0和13.7±23.0mmol/m2/d。入湖河流布哈河从源头到入湖口CO2释放通量逐渐增加，平均为30.9± 20.8 mmol/m2/d，是主湖的3倍。青海湖主湖CH4的释放通量很低，平均为37.6±59.3umol/m2/d，子湖较高，平均为1108.3 ±1278.0umol/m2/d, 其他盐湖的CH4的释放通量变化较大，中值为3.8umol/m2/d，与青海湖主湖低的CH4释放一致。CO2和CH4通量具有同步变化，表明风速通过气体交换系数是调节释放通量的主要控制因素。. 主要入湖河流布哈河源头区以碳酸盐和蒸发盐风化为主，向下游硅酸盐风化的比例增加，CO2分压和CO2释放通量增加，有机质降解对CO2的贡献也在增加。青海湖湖区碳酸盐系统的深度剖面和昼夜变化表明无机碳酸盐沉淀和有机质代谢都对CO2有影响，相对贡献在时间和空间上存在快速变化。湖水无机碳CO32-含量高于HCO3-，CO32-与HCO3-的反向变化缓冲了pH波动，使得pH变化范围很小，对CO2的调节作用很小，而CO2与HCO3-之间存在显著的相关性，凸显了盐湖中碳酸盐缓冲体系对CO2的调节作用是青海湖CO2释放通量小的主要原因。