南海深部生物地球化学-物理耦合过程对海---气界面CO2通量的调控

The contemporary coastal ocean, characterized by abundant nutrients and high.primary productivity, is generally seen as a significant CO2 sink at the global.scale. However, mechanistic understanding of the coastal ocean carbon cycle .remains limited,leading to the unanswered question of why some coastal systems .are sources while others are sinks to atmospheric CO2. Recently, we proposed .a new hypothesis:in addition to the processes identical to those in the open .ocean such as thermodynamic and biological pump controls, both land input .and exchange with the open ocean are significant determinants of the CO2 fluxes .in the coastal ocean. .There are at least two distinct settings: River-dominated Ocean Margins (RiOMar) .and Ocean-dominated Margins (OceMar). In a simplified scheme, RiOMar, as .recognized previously, is featured by concurrent inputs of autotrophic (nutrients) .and heterotrophic (organic matter) loadings, while OceMar is characterized by .concurrent off-site inputs, typically from depth, of nutrients and dissolved .inorganic carbon (DIC). This proposal is to test if the hypothesis stands using.the basin area of thelargest marginal sea of the Pacific, the South China Sea as.a case. We are to answer the following critical questions: (1) what are the.fluxes of the non-local sourced DIC and nutrients transported into the deep .South China Sea basin through the Luzon overflow? (2) how do the major.biogeochemical and physical processes modulate the transport of DIC and.nutrients from the depth to the upper South China Sea? (3) how the upper.metablic processes control the air-sea CO2 fluxes in the upper South China Sea..We contend that OceMar, in contrast to the previously recognized .River-dominated Ocean Margins, is a significant concept to improve our .mechanistic understanding of the coastal ocean carbon cycle. .Moreover, the approach this proposed research is going to adopt shall be.applicable to many other systems with similar physical settings, thereby.to enhance the impact of the South China Sea Deep program.

边缘海碳循环是地球系统碳循环的重要环节，但至今我们依然不能回答为什么有些陆架边缘海是大气CO2的汇，有些是源，这主要缘于我们对控制边缘海碳循环的主要过程和关键机理尚有诸多不明之处。基于申请团队扎实的研究基础，项目拟验证假设为：南海盆地的CO2源汇格局与深部过程密切相关，南海存在源自相邻大洋深部的外源溶解无机碳，当外源无机碳被输送至其上层时，如果不被同时上涌的营养盐所支持的有机碳生产完全消耗，其过剩部分即会逸出至大气。故项目拟解决如下关键科学问题：(1) 输入南海深部的外源无机碳、营养盐的通量有多大？(2)南海深部及至南海上层的过程中调控无机碳、营养盐输运的关键生物地球化学和物理过程是什么？(3) 南海上层新陈代谢过程如何控制海-气界面CO2通量？项目直面南海的碳循环过程与机理研究，揭示深部过程与上层海-气界面交换的关联，这是解决现代南海碳循环问题的核心，对认识南海碳循环演变也具重要意义。

本项目以南海为研究区域，对历史资料进行集成分析，并开展四个航次的现场观测及数值模式的开发和研究工作，深入南海碳循环的过程和机理研究，提出真光层双层结构假设及其对源汇格局的调控，并将研究视野拓展至全球尺度，揭示南海碳循环机理的全球意义。课题显著推进了南海深部计划的三大核心内容之南海生物地球化学研究，并为南海作为全球边缘海研究示范做出了贡献。. 本项目集成分析了2000年以来南海36个航次的海表CO2分压数据，确认南海海-气CO2通量的季节分布特征，进一步提高碳通量估算的准确性，南海向大气释放CO2的年通量为13.1  1012 g C。. 外源的溶解无机碳（DIC）和营养盐影响南海的源汇格局，本项目估算DIC和无机氮（DIN）通过吕宋海峡输入南海的净通量分别为4422 Tg C yr-1和50.9 Tg N yr-1，对该通量空间及季节变化的深入探讨，对厘清南海CO2海气通量的调控因子具有重要意义。. 本项目基于所构建的中国海多尺度数值模拟系统（CMOMS）揭示了南海独特的三层环流结构，并对其形成的动力学机理提出新解说。通过高分辨率物理-生物地球化学耦合模型对南海碳酸盐系统、营养盐及相应过程进行模拟。模型很好地反映南海DIC垂直输运的空间变化，其变异尺度主要为中尺度和亚中尺度，说明南海内部水体辐聚/辐散和中尺度涡旋是影响南海营养盐和无机碳垂直通量分布的主要过程。. 本项目对大洋主控型边缘海（OceMar）碳循环的概念框架开展了进一步的论证和深入解析研究，在全球其他边缘海验证了该解析方法的通用性，初步揭示了南海碳循环研究的全球意义，显著提升了对边缘海碳循环过程和机理的认识。.基于所得营养盐通量的垂向结构，本项目提出将真光层视为两层结构的新假设，即营养盐耗尽层（NDL）和富营养盐层（NRL），发现进入两层的DIC和营养盐有效通量比值与Redfiled比值不同，这可能是造成南海为大气CO2弱源的原因之一。. 本项目实施以来在Journal of Geophysical Research、Limnology and Oceanography等国际期刊上发表9篇相关论文；1篇论文已接收；参与组织专辑一期，先后9次在国际重要学术会议被邀请作报告。已培养博士5名、硕士4名。