过去500ka以来南海沉积物不同形态氮含量与同位素变化及其控制因子

Nitrogen is a productivity-limiting factor in most oligotrophic seas, alteration of nitrogen inventory therefore regulates the marine productivity on a millennial timescale and modulate atmospheric CO2 through biological pumps. The South China Sea is a typical oligotrophic marginal sea located at the end of the pathway of North Pacific intermediate water. Due to the complete utilization of nitrate, the total nitrogen isotope of sediments (δ15NTN) can reflect the changes of subsurface nitrate isotope and then served as the indicator of the variation of oceanic nitrogen pool. Total nitrogen (TN) in sediments contains both organic nitrogen (ON) and inorganic nitrogen (IN), and large amounts of IN will mask the isotope signal of TN, which was previously ignored. Studies in the modern South China Sea show that both the planktonic foraminifer shell-bound nitrogen isotope (δ15NFB) and the organic nitrogen isotopes of the sinking particles (δ15NON) from euphotic zone are exhibiting the similar isotopic signatures with subsurface nitrate. However, recent studies have shown that the δ15NFB in the South China Sea are not consistent with the records of sedimentary δ15NTN, and the δ15NTN values are systematically lower than that of the δ15NFB, and their differences are co-varied with sea level change and terrestrial minerals variation. The study on sinking particles by sediment trap have found that lateral transport was common in the South China Sea and that lateral materials exhibit similar characters with terrigenous mineral particles. Therefore, the project proposed the hypothesis that the lateral transport of a large amount of clay-fixed nitrogen with depleted nitrogen isotopes resulted in a low value of δ15NTN and masked the changes of δ15NTN during glacial-interglacial periods. In order to peel off the effects of clay fixed inorganic nitrogen, the project first attempts to use the bacterial method to directly measure the δ15NON of sediments to unlock the climatic and environmental information in organic and inorganic nitrogen isotopes.

海洋氮储库变化在千年尺度上透过生产力调控大气CO2。位于北太平洋中层水末端的南海是寡营养盐海区，由于硝态氮被完全利用其沉积物总氮同位素反映海洋氮储库变化。沉积物总氮（TN）包含有机氮（ON）与无机氮（IN），过去IN常被忽略，TN被当作ON应用，但具有较低δ15N值的IN会降低δ15NTN的值。现代水柱研究显示浮游有孔虫壳体同位素（δ15NFB）与真光层输出颗粒氮同位素(δ15NON)均记录了光区底部硝酸盐的同位素信号。而沉积记录显示南海δ15NFB 系统性高于δ15NTN其差值与海水面及陆源物质同步变化。陆源颗粒侧向传输在南海普遍存在，因此本项目提出假设：来自侧向搬运的黏土矿物夹带大量IN导致δ15NTN总体性偏低，掩盖了δ15NTN冰期-间冰期的变化。为剥离IN的影响，本项目首次尝试细菌法直测沉积物有机氮同位素，评估有机氮及无机同位素的气候及环境意义，提升氮同位素在南海应用的理论基础。

沉积物总氮含量（TN）及其同位素（δ15NTN）是反演海洋氮储库变化的重要工具。沉积物中总氮的赋存形态主要有有机氮（ON）和无机氮（IN），其中ON主要由生源碎屑组成，其含量与同位素记录了上覆水体氮循环信号，而固定于2:1型粘土矿物晶格层中的固定态铵则是IN的主要存在形式。由于在开阔大洋沉积物中IN含量较低而常被忽略，因此大部分研究用TN代替ON用于古海洋氮循环研究。然而近年来许多研究表明在沉积物中IN占比较高，且δ15NIN相对δ15NON较轻，因而IN及其同位素对TN的稀释作用不能忽略。因此，本研究首次采用反硝化细菌法测量沉积物有机氮含量及其氮同位素，获得了南海MD972142钻孔不同形态氮含量及其氮同位素结果。结果显示IN平均占比为51%，最高可达68%。尽管TN%与ON%、δ15NTN和δ15NON间的变化趋势相似，并未受到高占比的IN的影响而发生显著偏移，但δ15NON的指标敏感度更高，表现为变化幅度较δ15NTN更明显。.前人发现南海浮游有孔虫的FB-δ15N记录了上层海洋硝酸盐氮同位素信号且与代表真光区输出生产力的δ15Nsink值近似。由于壳体保护作用，沉积物的FB-δ15N受早期成岩作用影响较小，而沉积物δ15NON则额外记录了早期成岩作用改造的信号，通过对比二者差异可以评估不同时期水体及沉积物氧化还原变化。δ15NON与FB-δ15N的对比结果显示二者变化趋势相反，间冰期δ15NON较FB-δ15N正偏，冰期相反。二者的差值Δδ15N(ON-FB) 与表征底层水氧化还原环境的Mn含量变化同步，说明二者间的差异与早期成岩改造有关。在间冰期，底层水为氧化环境，沉积物中的有机质经历氧化条件下的早期成岩改造，氮同位素增大，导致Δδ15N(ON-FB) 为0-3‰ 的正值；冰期，底层水环境偏还原，还原环境下的早期成岩改造致使有机质氮同位素减小，δ15NON偏移次表层硝酸盐δ15N信号，Δδ15N(ON-FB) 呈现 -3-0‰ 的负值。.δ15NIN与日照正相关，且记录了其来源地的气候信息。然而，δ15NIN亦具有区域差异，其中MD972142钻孔的δ15NIN较St.3钻孔的δ15NIN偏重，表明其IN源区的差异，这可能与其源区的土壤溶液δ15NNH4+以及IN与土壤溶液中NH4+的分馏系数有关，具体机制还需未来进一步的研究确定。