南海上层水体黑碳颗粒动力学的同位素示踪

Black carbon, representing one of the most important "missing carbon sinks", has been proved to play a crucial role in driving global warming following CO2 in the last years. Its emission and diffusion in the atmosphere have been extensively investigated. However, its behaviors in the ocean are very poorly understanood although the ocean is the most important potential sink, resulting in the impossibility of evaluating the contribution of black carbon to marine carbon cycle. This project will focus on the particle dynamics of particulate black carbon (i.e. PBC) in the upper layer of the South China Sea (SCS). Strategically, specific scientific questions are firstly proposed, followed by field investigation accompanying simulation experiments. Finally, the data will be synthetically discussed to reach our goals. The sources of PBC and its export from the euphotic layer, in both shelf and basin regions, have been selected as the primary scientific questions of highlighting its basic cycling in the SCS. Methodologically, multiple isotopes, including stable isotope (13C) and radioactive nuclides (i.e. 14C, 210Po, 210Pb, 234Th, 137Cs, and 230Th) will be used to reveal the following information: (1) the sources of PBC in the upper layer of the SCS, (2) the temporal-spatial patterns of PBC and its relation with hydrological parameters, (3) the turnover rates, export fluxes of PBC with respect to the upper layer and the dominating factors, (4) the quantitative contribution of PBC to particulate organic carbon standing crop and its influence on evaluating the biological pump efficiencies. These forthcoming results would provide the foundation of evaluating the role of black carbon in the carbon cycling of the South China Sea.

黑碳是全球碳循环中被忽略的碳失汇之一，近几年发现其在气候变暖中的作用仅次于CO2，故其排放和在大气中的扩散备受关注。但海水作为黑碳重要的归宿地，其中黑碳的信息极度匮缺，无法评估其对海洋碳循环的贡献。本项目立足已有研究基础，以黑碳的颗粒动力学为切入点，以颗粒态黑碳（PBC）的来源与输出为突破口，选择南海为研究区域，注重陆架区与海盆区的对比研究，通过核心科学问题聚焦-现场调查-受控模拟实验-综合分析，采用多种同位素（13C、14C、210Po、210Pb、234Th、137Cs、230Th）联合示踪的方法，研究南海上层水体PBC的同位素特征，揭示其主要来源、时空格局及其与水文动力条件关系，探明PBC循环与输出关键过程的速率与通量特征及主控因素，定量评估PBC对颗粒有机碳的贡献及其对评估生物泵效率的影响，综合研究黑碳对南海碳循环的贡献。项目可望在边缘海上层海水中黑碳的源汇研究方面取得突破。

课题研究了南海沿岸、陆架/坡和海盆区上层水体黑碳含量的空间特征和源-汇格局，并应用210Po和234Th等同位素示踪了颗粒态黑碳（PBC）的输出与再悬浮等颗粒动力学过程。高含量PBC通常出现在南海北部受径流影响的沿岸海域。PBC输出通量随离岸距离增加而指数减小，表明陆源输入的PBC在近岸海域快速沉降，对海盆区上层海水PBC贡献很小。由北部陆架区到陆坡区，PBC输出真光层的通量下降约30%，至海盆区降低约1个数量级。该空间特征有力地证实了南海北部陆架区是PBC的重要汇区。PBC输出通量对总颗粒有机碳（POC）输出通量的贡献在陆架、陆坡和海盆区分别为14%、8%和<5%，揭示了忽略PBC可造成南海生源POC输出通量不同程度的高估。课题首次获得了陆架区PBC的再悬浮通量，该通量占POC再悬浮通量的29%，明显高于PBC对POC输出通量的贡献，揭示了PBC比其他POC组分更难降解的特性。发现陆架区再悬浮的PBC可扩散进入海盆中层水体，导致中层水体PBC输出通量高于上层水体的现象，这一现象反映了跨陆架输运是调控南海PBC沉积后再分布的重要过程。湄公河等径流输入调控着南海西部混合层中溶解态黑碳（DBC）的空间格局。质量平衡计算表明径流和大气沉降是南海DBC的主要来源，其输入的DBC有超过1/3在真光层降解，其余DBC通过混合作用或颗粒物沉降进入中层水体，光降解和颗粒物沉降是黑碳迁出南海上层水体的两条主要途径。研究发现南海中层水中DBC组成与盐度之间存在显著相关性，为DBC作为示踪中深层水团混合的新示踪剂提供了有力的证据。课题共发表了11篇SCI学术论文，其中1篇论文被AGU的地球与空间科学新闻杂志报道，共7人次参加了学术会议交流，培养了1名博士和3名硕士，完成了课题计划任务。