中国南海IODP349航次深部沉积物孔隙水的生物地球化学的研究

Pore-water analysis is a central approach in marine biogeochemistry. During the recently accomplished IODP Expedition 349 in the South China Sea, high resolution pore-water samples were collected in subseafloor sediments of the East Sub-basin (Site U1431) to a depth of 550 mbsf (meters below seafloor), of the northern continent-ocean boundary (Hole U1432C) to a depth of 110 mbsf, and of Southwest Sub-basin (Site U1433) to a depth of 588 mbsf. Sites U1431, U1433 and Hole U1432C were drilled at water depths of 3289 to 4390 m. The preliminary results from onboard studies show that moderately high concentrations of methane occur at Site U1433 and Hole U1432C, but with background values of methane at Site U1431; and pore-water profiles of three sites are quite different from each other. Systematical geochemical analyses will be carried out, especially on DIC, DOC and acetate concentrations, and δ13C values of DIC in pore-waters, and δ13C values of TOC and TIC in sediments, to investigate the biogeochemical processes and their link with carbon cycling in deep subseafloor sediments in the South China Sea, and to determine the sources and methanogenesis of methane in U1432C and U1433, and to understand the impacts of the tectonics and sedimentary processes of the South China Sea on the biogeochemistry and carbon cycling in deep subseafloor sediments. This study will also provide a better understanding on the carbon cycling in deep subseafloor sediments in the global open oceans.

海底沉积物孔隙水是海洋生物地球化学研究的核心内容。刚刚完成的中国南海IDOP349航次在水深达3829-4390米的深海海盆，成功采集了东部次海盆站位U1431的550米以浅的、陆/洋壳边界钻孔U1432C的110米以浅的和西南次海盆站位U1433的 588米以浅的高分辨率的孔隙水样品。“决心号”上的初步研究显示， U1432C和U1433含有较高浓度的甲烷，而U1431的基本无甲烷，站位间孔隙水的地球化学差异较大。因此本项目拟对3个站位的孔隙水和相对应的沉积物展开系统的地球化学分析测定，尤其是孔隙水的DIC、DOC和乙酸浓度，以及DIC的δ13C，以及沉积物TIC的δ13C。揭示南海深部沉积物的生物地球化学过程和演化，及其与深部沉积系统碳循环的关联，探讨高浓度甲烷的可能来源和产生机制，探索它们与南海构造活动、沉积演化的关联。研究成果对深入认识全球远洋海底深部碳循环具有重要意义。

南海是西太平洋一个边缘海，位于欧亚、菲律宾海和印度-澳大利亚板块之间。本项目对IODP 349航次在东部和西南次海盆的残留洋脊的附近钻取的U1431站位和U1433站位的~500 m以浅沉积物孔隙水化学组成以及DOC、乙酸和DIC的δ13C，孔隙水对应层位沉积物的总无机碳和总有机碳的含量及其δ13C，开展了系统的研究。U1433的硫酸盐、碱度和δ13C-DIC均指示该站位存在硫酸盐甲烷过渡带（SMTz）于~30 m处。在SMTz以深，甲烷显著增加并在~100 m达到最大，同时δ13C-DIC在此处达到最大~0 ‰ PDB，表明甲烷正在此处产生。也即U1433站位沉积物由浅至深依次正在发生着氧化锰、氧化铁和硫酸盐还原、甲烷的厌氧氧化以及可能的甲烷生成，这与典型的深海沉积物的氧化还原序列一致；而U1431站位硫酸盐一直没有耗尽，最低保持在2-3 mM，这与基本检测不到甲烷相符。.U1431站位孔隙水的DOC浓度为0.7±0.29 mM，比U1433的2.1±0.9 mM 低了近一个数量级。U1431和U1433站位孔隙水DOC的δ13C分别为-22.8±2.1和-21.0±1.3 ‰PDB，均随着深度逐渐变重。U1431站位的沉积物TOC含量为0.28±0.11wt%，为典型深海沉积有机质含量。而U1433站位沉积物TOC含量较高为0.43±0.10 wt%。U1431和U1433站位δ13C-TOC分别为-23.1±0.8和-22.8±0.6 ‰PDB，因沉积物有机质选择性降解导致随深度逐渐变轻。两个站位δ13C-DOC都比其相应层位δ13C-TOC稍重~1-2 ‰，揭示了DOC主要来自有机质的无氧降解。.U1433站位基本上整个层位都能检测到乙酸，乙酸含量较高33±34 μM，其δ13C值为-45 ~ -18 ‰PDB，表明U1433站位高浓度的乙酸很可能来自甲烷的厌氧氧化。而U1431站位几乎没有检测到乙酸。.这个两个站位孔隙水的深度曲线的差异，主要是由于两个次海盆沉积物的沉积速率以及海底的地热梯度的不造成的。同时南海深海海盆深部沉积物的DOC和δ13C-DOC为南海乃至全球海洋的碳循环提供重要数据。