|关键词||放射性核素 有机碳 环境演变 沉积物 滨海湿地|
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。
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=axb）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 (Raeq) and the external hazard index (Hex) 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=axb). 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-1 and 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.
|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.|