运用叶蜡脂肪酸C-14研究粤西近岸沉积物的来源与运移过程

Muddy sediments in the Pearl River estuary and the inner shelf along west Guangdong Province extend 400 km from the river delta to the Leizhou Peninsula, with sediment depth decreasing from ~20 m to several meters. Currently, there are two viewpoints on the transport of fluvial materials: one is the proximal sedimentation and then delivery gradually by resuspension; the other is the direct deposition of particles from water column sourced from the river mouth. Also, there are two viewpoints on the source of silty sediments that are coarser than the riverine mud: one is from the small rivers in the adjacent land; the other is the reworked relic shelf sediments. In this project, we propose to solve the above unknowns by analysis of C-14 of the terriginous leaf wax fatty acids. We will examine if there is a decreasing trend in fatty acid C-14 along the axis of mud deposit, which can be applied as evidence for the existence or not of the sedimentation-resuspension process. We will also investigate the fatty acid C-14 in the silty sediments. If the silty sediments were from reworking of the relic shelf sediments, fatty acid C-14 in them would be greatly depleted. This study will give us a better understanding of the source and transport of inner shelf sediments, which is valuable for the application and interpretation of sedimentary records in climatic and environmental change studies.

珠江河口及粤西内陆架泥质沉积由河口区向西南的雷州半岛方向呈舌状展布400余km，厚度由约20 m逐渐减薄至数m。目前关于这些由河流输入的泥质物的沉积过程主要有先储后运和随沿岸流搬运直接沉降两种观点，对沉积物中较粗粒度层的物质来源也有沿岸小型河流输入与陆架沉积物改造搬运的不同观点。针对上述问题，本申请计划以陆地高等植物来源的叶蜡脂肪酸的C-14为主要研究手段，分析河口和内陆架泥质区颗粒物和泥质物中叶蜡脂肪酸C-14的空间分布情况，查验由河口到泥质区远端是否存在显著的C-14减少趋势，由此判断沉积物是否经历了复杂的沉积-再搬运过程；比较不同粒级沉积物中叶蜡脂肪酸C-14的差异，查验粗粒级沉积物中是否显著贫C-14，由此判断沉积柱中的粗粒级层位是否来自于陆架“老”物质的改造和搬运。本项研究有助于加深对内陆架沉积物搬运过程和物质来源的科学认识，为运用和解释沉积记录中的气候环境意义奠定基础。

陆架边缘海沉积物中陆源有机碳进入海洋系统的迁移、转化和最终归宿是全球碳循环研究的重要内容。本项目采集了珠江口和南海北部陆架区的表层沉积物和柱状样沉积物柱样，开展了总有机碳含量（TOC）、总含氮量（TN）、总有机碳和单体脂肪酸的稳定和放射性碳同位素（δ13C和F14C）、粒度和一些关键生物标志物的分析，取得了如下一些重要结果和创新性的认识。1）揭示了南海北部陆架区陆源有机碳的空间分布特点，以及水动力过程对陆源有机碳“老化”的影响。发现珠江口、粤西泥质区的沉积物以陆源有机碳的贡献为主，贡献量分别为70±3%和54±10%。根据数据特点划分了两个不同的沉积区域：在区域一（包括珠江口和内陆架）中，表层沉积物中陆源有机碳的含量沿陆架运输方向逐渐减少，反映了沉积-再悬浮过程控制的陆源有机碳运输；在区域二（外陆架-陆坡），较强的底移搬运导致粗颗粒物和老有机碳聚集。2）基于C28+30脂肪酸和总有机碳δ13C、F14C和C/N比的四端元混合模型显示，估算了南海北部沉积陆源有机碳中化石有机碳和土壤有机碳的相对贡献，发现在粤西近岸泥质区，化石有机碳的埋藏效率显著高于土壤有机碳的埋藏效率，说明了在陆源有机碳输运和沉积过程中化石有机碳能够继续保持惰性，而大部分土壤有机碳却被再矿化了，提出边缘海土壤有机碳的再矿化可能是重要的大气二氧化碳“源”。3）通过钻孔沉积物剖面的脂肪酸同位素数据分析，揭示了人类活动对珠江流域的生态变化及其对南海北部沉积有机质的影响。结果显示，由于人类活动持续加强，珠江流域分别在1950年代和1970年代经历了两次明显的C4植物增加，而且内陆架泥质区沉积物中海洋和化石有机碳的堆积速率自1950年代以后一直在增加，表明人类活动不但增加了土壤和化石有机碳的输出入海，还增加了边缘海的初级生产力，也加速了土壤有机碳的氧化降解（激发效应）。