基于放射性核素的山东半岛北部滨海湿地沉积环境演变与有机碳储库的讯息解析

	基于放射性核素的山东半岛北部滨海湿地沉积环境演变与有机碳储库的讯息解析
	王启栋
学位类型	博士
导师	宋金明
	2016-05-20
学位授予单位	中国科学院大学
学位授予地点	北京
学位专业	海洋化学
关键词	放射性核素有机碳环境演变沉积物滨海湿地
其他摘要	滨海湿地不仅是人类经济活动最密集的区域，更是重要的生态缓冲区，在降低近海污染、保护海岸稳定以及应对全球气候变化等方面发挥了重要作用。然而随着气候问题的日益突出和经济社会的快速发展，滨海湿地生态环境的变化不断加剧。为科学地保护和利用滨海湿地，揭示滨海湿地环境演化的历史规律和驱动机制以及预测滨海湿地对全球变化的响应已成为当下的前沿和热点问题。本学位论文基于放射性核素变化解析了山东半岛北部典型滨海湿地的环境演变讯息，评估了人类活动影响下滨海湿地沉积有机碳储库的特征和变化趋势，获得了一系列新的结果和认识： 1. 黄河口湿地沉积物中的放射性核素可明确指示沉积环境的变化，黄河输水输沙与核素含量呈正相关，当黄河改道及调水调沙等工程使湿地加积增长时，核素含量显著增加。近几十年来黄河口湿地的沉积过程因受黄河频繁改道以及径流量变化的影响而变得十分复杂，沉积物岩芯放射性核素、生源要素和微/痕量元素的垂直变化记录了黄河河道变迁、输水输沙变化以及河口湿地的加积与侵蚀交替等过程，黄河口湿地沉积环境极不稳定。黄河口湿地沉积物中放射性核素的含量与分布受有机碳含量及粘土含量的影响。与黄河泥沙相比，黄河口湿地沉积物呈现富²³⁸U贫²²⁶Ra的特征。²³⁸U的含量主要取决于陆源输入，而黄河口湿地沉积物的理化环境有助于²³⁸U的富集；²²⁶Ra的含量则取决于潮水的入侵，盐度较高的海水会导致沉积物中²²⁶Ra的流失。放射性核素的含量变化与沉积环境的改变密切相关，当黄河输水输沙不足导致湿地受到侵蚀时，核素的含量逐渐降低，而当黄河改道及调水调沙等工程使湿地加积增长时，核素的含量则显著提升。此外，²³⁸U/²²⁶Ra能够反映滨海湿地加积与受到侵蚀的相对程度，湿地以侵蚀为主，²³⁸U/²²⁶Ra升高，湿地以加积为主，²³⁸U/²²⁶Ra降低。黄河口新生湿地的沉积速率不断变化，1976年黄河改道之初沉积速率较高，改道之前以及发生长时间断流时沉积速率较低，而近70年的平均沉积速率约为1.0 cm·a^-1。沉积物的理化参数及生源要素的含量随深度的变化较大，反映了不断变化的沉积环境，如粒度组成的波动与黄河水沙输送量的变化有关；45 cm处含水率的突然升高表明沉积物受地下水的影响；而上层沉积物中C/N的升高说明陆源物质输入的增多。大部分金属的含量高于黄河河道沉积物而低于渤海沉积物，因此其在30-50 cm深度出现的低值区与1976年黄河改道初期大量黄河泥沙的快速堆积有关。 2. 昌邑柽柳林湿地表层沉积物中的放射性核素由于物质来源的不同和人类活动的影响而具有明显的区域差异，沉积物岩芯中放射性核素的剧烈垂直变化则记录了重大历史沉积事件以及人类活动对滨海湿地沉积环境的影响。根据核素的垂直分布，昌邑湿地的历史沉积过程可被划分为四个阶段：缓慢沉积阶段，受黄河河道迁徙影响下的沉积环境剧烈变化阶段，黄河归流后的稳定阶段，以及在人类活动影响下的退化、增长交替阶段。昌邑湿地沉积物中²³⁸U、²³²Th、²²⁶Ra等放射性核素的含量与黄河口湿地沉积物十分接近，说明昌邑湿地与黄河口湿地沉积物母质相同，均为近现代黄河的冲淤物。不同区域表层沉积物中的放射性水平存在显著差异，等效镭活度（Ra_eq）及放射性危害指数（H_ex）等参数均显示，靠近河流的区域放射性水平最高，表明河流物质输入是沉积物中放射性核素的重要来源；而靠近堤坝、沟渠、道路等人工设施的区域，沉积物放射性水平也明显偏高，表明建造施工等人类活动给沉积物带来外源物质的输入。在百年尺度内，昌邑湿地沉积环境较稳定，沉积速率仅为0.23 cm·a^-1。沉积物岩芯中生源要素的垂直分布主要与表层植被有关，而沉积物的理化性质则主要取决于水文条件。常量元素和重金属的垂直变化较小，而稀土元素和放射性核素则具有较高的变异性，体现了在更长时间尺度上物质来源的差异。放射性核素的含量与理化参数及生源要素的关系不密切，说明其受外部环境条件的影响较小。根据²³⁸U和²²⁶Ra及其比值的垂直分布，昌邑湿地历史沉积过程可被分为四个阶段：缓慢沉积阶段，受黄河河道迁徙影响下的沉积环境剧烈变化阶段，黄河归流后的稳定阶段，以及在人类活动影响下的退化、增长交替阶段。在不同的阶段，放射性核素及其比值的变化均能够反映重大历史沉积事件、物质来源的改变以及人类活动对滨海湿地沉积环境的干扰，具有重要的环境指示意义。 3. 黄河口新生湿地沉积有机碳的储量与植被覆盖度有关，植被密集区域沉积有碳储量较植被稀疏区域有显著提升，随着植物群落的演替黄河口新生湿地沉积物碳汇强度有显著增加的趋势；昌邑湿地沉积有机碳库的储量较低，主要原因是在水文环境的影响下沉积速率低、有机质矿化分解速率高，植被来源的有机质难以在深层沉积物中埋藏，而在人类活动影响下，昌邑湿地的水文环境条件仍在不断恶化，导致沉积有机碳库的储量呈下降的趋势，需要采取有效的治理措施，以恢复昌邑湿地沉积物的碳汇功能。黄河口新生湿地大部分区域植被稀疏，1 m以内沉积有机碳的储量区域差别不明显，而覆盖有密集碱蓬的区域沉积有机碳储量则明显较高，整个区域平均有机碳储量约为3.32 kg·m^-2，有机碳总储量约为1.99×10⁸ kg。昌邑湿地6月份和11月份表层沉积物有机碳的平均含量分别为0.47%和0.61%。表层有机碳的分布与植被的分布一致，并且受到沉积物粒度的影响；在沉积物岩芯中，有机碳的垂直分布则受到植被类型、水文条件、沉积物粒度以及重大气候灾害的影响。在有植被分布的区域，沉积物有机碳含量随深度的变化关系可以用幂函数（y=ax^b）来表示，而植被对沉积物有机碳的贡献仅限于0-20 cm深度。受防潮大坝的影响，昌邑湿地大部分区域由潮间带变为潮上带，沉积物含水率降低，尽管表层有植被覆盖，深层沉积物的有机碳含量甚至低于频繁淹水的光滩。受水文条件限制，昌邑湿地沉积速率较低，有机碳矿化速率高，并且有机碳向深层沉积物的迁移较弱，有机碳的平均埋藏通量仅为17.5 g·m^-2·a^-1。而1 m深度以内沉积有机碳的储量仅为1.795 kg·m^-2，远低于其他湿地甚至陆地生态系统。整个昌邑湿地沉积有机碳储库的大小估约为6.373×10⁷ kg。对比不同区域累积有机碳储量的垂直分布可以看出，潮间带区域变为潮上带后深层沉积物有机碳的储量明显减小。防潮大坝的修建，地下水的开采以及河流径流量的减小等因素导致昌邑湿地水文环境不断恶化。从长远角度讲，采取有效的治理措施，改善昌邑湿地沉积物水环境，有利于恢复滨海湿地生态功能的恢复，并使其变成有巨大潜力的碳汇。 ; The coastal wetlands are important ecological buffer areas where the human economic activities are very intensive, and they play a vital role in reducing the coastal pollution, protecting the shoreline and responding to global changes. However, with the global climate issues increasingly prominent and the economic society rapidly developing, the ecological environment changes in the coastal wetland are intensifying. In order to scientifically protect and develop the coastal wetlands, revealing the historical rules and driving mechanisms of environmental evolution of coastal wetlands and predicting the responses of coastal wetlands to global climate changes have become the research frontier and hot issue of the moment. This dissertation is devoted to illustrate the environmental evolutions of the typical coastal wetlands in the northern part of the Shandong peninsula and to evaluate the characteristics and changing tendency of the sedimentary organic carbon reservoir under the influence of human activities. A series of new results and understandings have been obtained as follows: 1. The radionuclides in the sediment of the Yellow River Estuary (YRE) wetland could clearly indicate the sedimentary environment changes. The concentration of radionuclides was positively related with the water and sediment discharge of the Yellow River. When the coastal wetland accreted and grew due to the channel migration of the lower Yellow River and the implementation of the water and sediment regulation project, the content of radionuclides increased correspondingly. Influenced by the frequent channel shift and runoff variation of the Yellow River, the sedimentary processes of the YRE wetland in recent decades were extremely complicated and unstable. However, processes like migrations of channel, variations of water and sediment discharge, as well as accretion and erosion alternation of the coastal wetland could all be recorded by the vertical distribution of radionuclides, biogenic and minor/trace elements. The concentration of radionuclides in the sediment of the YRE wetland was affected by the organic carbon content and clay content. Compared to the sediment in the Yellow River, the sediment of the YRE wetland was rich in ²³⁸U and poor in ²²⁶Ra. The concentration of ²³⁸U was determined by the terrestrial input while the concentration of ²²⁶Ra was determined by the tide intrusion. The physicochemical environment of the sediment in YER wetland was conducive to the enrichment of ²³⁸U while the salty seawater would lead to a great loss of ²²⁶Ra from the sediment. The concentration of radionuclide was closely related to the changes of sedimentary environment, and it gradually reduced when insufficient water and sediment discharge causing erosion of the wetland and significantly increased when projects like channel migration and water-sediment regulation promoting the wetland accretion growth. What’s more, the vertical distribution of ²³⁸U/²²⁶Ra could reflect the relative degree of sediment accumulation and seawater erosion. Low accumulation rate and frequent tide intrusion tended to lead higher ²³⁸U/²²⁶Ra values while wetland accretion tended to lead lower ²³⁸U/²²⁶Ra values. The sedimentation rate of the YRE wetland was high in the early years after the channel migration in 1976, and it was low before 1976 as well as when long time drying up of the Yellow River occurred. The average sedimentation rate in the recent 70 years was about 1.0 cm·a^-1. The physicochemical parameters of the sediment and the content of biogenic elements drastically varied with the depth, reflecting the constantly changing sedimentary environment. For instance, the variation of grain size composition was related to the variation of water and sediment discharge, and the sudden increase of water content at about 45 cm indicated the impact of underground water, and the higher C/N in the upper layer suggested the increased terrestrial material input. The contents of most metal elements in the YRE wetland sediment were higher than the Yellow River sediment and lower than the Bohai Sea sediment. Therefore, the relatively lower content of metal element at the layer 30-50 cm was due to the rapid accumulation of Yellow River sediment in the early years after channel migration in 1976. 2. The concentration of radionuclides in the surface sediment of Changyi wetland presented obvious regional difference because of the different material sources and the impact of human activities. The drastic vertical variation of radionuclides in the sediment profiles recorded significant historical deposition events and the impact of human activities on the coastal wetlands. The vertical distribution of biogenic elements, however, was mainly affected by external environment conditions such as vegetation and hydrology. According to the vertical distribution of radionuclides, the historical deposition process of Changyi wetland could be divided into four stages: slow deposition stage, drastic change stage affected by the Yellow River, stable stage and alternating stage affected by human activities. The concentrations of ²³⁸U, ²³²Th and ²²⁶Ra in the sediment of Changyi wetland were close to that in the sediment of YRE wetland, indicating that the sediment in these two wetlands all developed from the alluvial deposits of Yellow River. The radioactive levels of the surface sediment in different regions were significantly different. The radium equivalent activity (Ra_eq) and the external hazard index (H_ex) all showed that areas close to the river tended to had higher radioactive level, suggesting that material carried by the river was one of the important sources of radionuclides. Besides, areas close to dam, ditches and roads also had higher radioactive level, indicating that human activities also brought about external radionuclides input to the wetland. The sedimentary environment of the Changyi wetland in recent 100 years was stable and the sedimentation rate was about 0.23 cm·a^-1. The distribution of biogenic elements in sediment cores was mainly affected by the vegetation in the surface while the distribution of sediment properties was determined by the hydrological conditions. The vertical variations of major elements and heavy metals were small, but the rare earth elements and radionuclides presented high variability which reflected the changes of material sources on a larger time scale. The concentration of radionuclides had no relationship with the sediment properties, implying that it was less influenced by the external environment conditions. Base on the vertical distribution of ²³⁸U, ²²⁶Ra and ²³⁸U/²²⁶Ra, the historical deposition process of Changyi wetland could be divided into four stages. Overall, the radionuclides could serve as important environmental indicators since in different stages they could reflect significant historical deposition events, changes in material sources and the impact of human activities on the coastal wetlands. 3. In the YRE wetland, areas with higher vegetation coverage tended to have higher sedimentary organic carbon storage. Therefore, the sediment of new born YRE wetland would become a significant carbon sink with the succession of plant communities. However, because of the low sedimentation rate and high decomposition rate of organic matter caused by poor hydrological environment, organic matters derived from vegetation could hardly be buried in the deep sediment, resulting in a very low organic carbon storage in the sediment of Changyi wetland. Under the influence of human activities, the hydrology environment condition of Changyi wetland was deteriorating and the sediment organic carbon storage was declining. Thus, effective measures were proposed to be taken urgently to improve the hydrological condition and to recover the wetland function as a carbon sink Because of the low vegetation coverage, the organic carbon storages in the upper 1 m sediment had no significant difference among most areas of the new born YRE wetland. But in areas covered with dense Suaeda salsa, the sedimentary organic carbon storage was significantly higher. In the whole area, the average organic carbon storage in the 1 m upper sediment was about 3.32 kg·m^-2 and the total organic carbon storage was about 1.99×10⁸ kg. The SOC content of the surface soil in June and in November were 0.47% and 0.61%, respectively. The distribution of the surface SOC content was in accordance with the distribution of the wetland vegetation and was affected by the grain size composition. However, it was only in the upper 20 cm soil layer that the contribution of vegetation to the SOC was significant. Vertical distribution of SOC content had close relations with vegetation type, hydrological condition, soil particle size as well as big climate events. The relationship between the SOC content and the depth in the vegetation covered areas could be described by power function (y=ax^b). Because of the low sedimentation rate, weak downward migration and high decomposition rate of organic matter, the organic carbon burial flux was only 17.5 g·m^-2·a^-1and the organic carbon storage in the 1 m upper sediment was only 1.795 kg·m^-2, which was much lower than other wetland systems or even terrestrial ecosystems. The total organic carbon storage in Changyi wetland was 6.373×10⁷ kg. Under intensive human activities, the Changyi wetland was drying and the organic carbon storage was reducing. For the health of the ecosystem and long-term benefits of mankind, strategies were proposed to be taken urgently to restore the wetland and turn the Changyi coastal wetland to a considerable carbon sink in the future.
学科领域	海洋化学
语种	中文
文献类型	学位论文
条目标识符	http://ir.qdio.ac.cn/handle/337002/112553
专题	海洋生态与环境科学重点实验室
作者单位	中国科学院海洋研究所
第一作者单位	中国科学院海洋研究所
推荐引用方式 GB/T 7714	王启栋. 基于放射性核素的山东半岛北部滨海湿地沉积环境演变与有机碳储库的讯息解析[D]. 北京. 中国科学院大学,2016.

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