南海深部过程的海水14C示踪研究

Recent research on deep processes in the South China Sea has expanded into the geophysical survey, stratified mooring and seabed observations with the help of state-of-the-art technology. This evidence in conjunction with numerical modeling of physical oceanography revealed new insights into the structure of deep ocean circulation, the dynamics of oceanic sediment transport and deposition in the South China Sea. In contrast, isotope geochemical techniques that can provide unambiguous information on the sources and age of Dissolved Inorganic Carbon (DIC) in water masses have hardly been applied to studies of the South China Sea. In this project, we propose to apply accelerator mass spectrometry and other isotopic ratio mass spectrometry to measure DIC 14C and 13C, and oxygen and hydrogen stable isotope ratios on seawater from different water depths. Combined with previously published physical oceanographic, sedimentological and element geochemical observations and records, these new data will serve as a basis to unravel long- and short-term changes in the estuarine circulation scheme of the South China Sea. In particular, we hope to provide a better constraint of changes in the residence time of deep waters and the quality of the Pacific inflow. We shall also generate 14C and 13C measurements of foraminifera collected from seawater and from selected sediment cores to improve our understanding on past variations in paleo-reservoir age as the result of changes in climate and past ventilation processes. Finally, we shall measure 14C of different types of organic carbon in seawater and sediments for a better knowledge of their sources as well as providing preliminary basic data for investigating the role of deep processes of South China Sea in global carbon cycle.

近年南海深部过程研究应用最新技术，拓展了地球物理、锚系分层和海底观测等探索，结合物理海洋学数值模拟，正酝酿着对南海底层海流结构、物质搬运与沉积的海洋动力机制等深海过程的新认识。相比之下，能够提供水团来源和年龄信息的同位素地球化学分析手段，在南海深部过程研究中应用仍然较少。本项目拟利用加速器质谱和其它质谱技术测量南海不同深度海水溶解无机碳中的14C和稳定同位素13C，以及海水的氢、氧稳定同位素，结合物理海洋、沉积观测和元素地球化学分析的已有成果，为全面认识南海表层和深部环流的结构及其在不同季节可能的变化，及获得更加精准的南海海水滞留时间提供新的依据；测定南海现代海水中和海底表层沉积物中有孔虫的14C，结合若干钻孔样品有孔虫14C年龄特征，揭示南海表层水储库年龄与深部水团对流年龄随时间的变化；测定南海海水和表层沉积物中不同形态有机碳的14C，为研究南海深部过程在全球碳循环中的作用提供基础数据。

以往的南海深层环流研究主要依靠数值模式、诊断模型或者锚系浮标、卫星跟踪漂流浮标、深海示踪物等物理海洋学方法。本项目从地球化学的角度，测量南海大部分海域、吕宋海峡及西菲律宾海不同深度海水的多种同位素，目的是为更深入认识南海表层和深部环流的结构及其变化特征提供基础数据。本项目建立了一种用于高精度AMS-14C分析的海水溶解无机碳快速制样方法，建立了基于波长扫描光腔衰荡光谱技术的海水氢氧同位素测量实验室和海水溶解无机碳和全碱度测量实验室，已完成了105站位共计1155个海水样品的DI14C分析，以及154站位共计2139个样品的氢氧同位素的测定。基于这些同位素地球化学新数据，系统刻画了南海表层至深部各水团的放射性碳和氢氧稳定同位素特征，证明了吕宋海峡南北部上中层水团交换存在很强的空间差异，以及南海中层水与西太平洋之间活跃的水平交换过程；证明了吕宋海峡深层强烈的越密度面混合过程，并揭示了这一过程对进入南海的深层海水理化性质重要的调控作用；验证了南海深层环流的气旋型逆时针结构；给出了南海海盆内深层水团的精细14C年龄，确定了南海深层水的滞留时间为125+/-25年；发现了南海内部深层水上涌、涡旋对于局地水团的影响、南海北部陆坡中深层活跃的垂直混合等现象的同位素证据，对深化南海翻转流及内部强混合机制的认识具有参考价值。

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