南海北部湾沉积物有机质成岩矿化过程及其对孔隙水溶解无机碳（DIC)的贡献

Continental margin sediments are an important biogeochemical reactor, where the sedimentary organic carbon is transferred into dissolved inorganic carbon (DIC) by the active diagenetic processes. This DIC generation in these sediments is released into the overlying water column and acted as an important benthic source for seawater. In this project, the sediments of Beibu Gulf (South China Sea) are used as a research object. Based on the combining analysis of sediments and pore water geochemical and isotopic composition, our objectives are to: 1) understand the carbon burial efficiency, organic carbon loss and their controlling factors during the diagenesis and remineralization of terrigenous and marine organic carbon in different sedimentary environments; 2) clarify the main diagenetic mineralization pathways, mechanisms and their coupling relations occur within these sediments, focusing on sulfate reduction, iron manganese iron reduction, methane production and oxidation; 3) distinguish the DIC transformation pathways and fluxes among the sedimentary organic matter decomposition, carbonate dissolution and precipitation, methane production and oxidation (AOM) processes in these sediments; 4) reveal the response relationships between the DIC cycles and the sedimentary environments and pore water geochemical characters. On the basis of these researches, DIC cycle model driven by the sedimentary diagenetic processes will be constructed, and thus we can quantitatively assess the relative contributions of these biogeochemical processes to pore water DIC. This research is helpful for better understanding of the roles of the continental margin sediments in the balance of the global carbon cycle, and providing an important scientific basis for knowing the effects of diagenetic processes on eco-environment in global scale.

陆架边缘海沉积物是重要的生物地球化学反应器，活跃的有机质成岩作用将有机碳转化为溶解无机碳（DIC）并释放进入上层水柱，充当海水DIC重要的“源”。 本项目以南海北部湾沉积物为研究对象，通过分析沉积物及其孔隙水地球化学与同位素组成并结合培养实验，认识不同沉积环境下的有机质保存效率、成岩过程中不同来源有机碳丢失及其制约因素；厘清沉积物内部发生的主要成岩矿化途径，重点探讨硫酸盐还原、铁锰异化还原、甲烷产生和氧化等过程发生机制及其之间的耦合关系；查明这些过程之间的DIC迁移转化及其通量，揭示它们与沉积环境和孔隙水地球化学组成之间的响应关系。在此基础上，建立基于沉积物有机质成岩矿化作用驱动的DIC循环模式，定量评估这些生物地球化学过程对孔隙水DIC的相对贡献，为深入理解陆架边缘海沉积物有机质成岩矿化过程在全球碳循环收支平衡中的作用及其生态环境效应提供重要的科学依据。

纵观已有的海洋碳循环研究，学术界过多关注海水中碳的输入与转化过程，评估海水吸收CO2的能力以及海洋对气候变化的调节力和控制力。相比而言，有关海底沉积物中碳循环过程及其对上层海水的影响与贡献的研究明显不足。本项目采集到南海北部湾不同沉积环境下的重力柱样品，通过分析沉积物（TOC、δ13C-TOC、CaCO3等）及孔隙水地球化学组成（DIC、δ13C-DIC、Ca2+、Mg2+、Sr2+、Fe2+、Mn2+等），揭示了不同沉积环境下的沉积物地球化学特征差异及其对有机质保存与矿化的影响，重点查明了不同沉积环境下孔隙水硫酸盐剖面分布差异性及其控制因素，探讨沉积物有机质氧化分解、甲烷产生和氧化、碳酸盐沉淀等生物地球化学过程之间的耦合关系，借助数值模拟手段，定量估算出南海北部湾典型陆架边缘海沉积物有机质成岩矿化过程对孔隙水DIC的贡献及自生碳酸盐埋藏的影响。相关研究成果将为深入理解陆架边缘海沉积物有机质成岩矿化过程在全球碳循环收支平衡中的作用及其生态环境效应提供科学依据。