Sedimentary Records Of Environmental Evolutions Reflected By The Radionuclides And Organic Carbon Storage In The Coastal Wetlands Of The Northern Shandong Penisula

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 238 U and poor in 226 Ra. The concentration of 238 U was determined by the terrestrial input while the concentration of 226 Ra was determined by the tide intrusion. The physicochemical environment of the sediment in YER wetland was conducive to the enrichment of 238 U while the salty seawater would lead to a great loss of 226 Ra 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/226 Ra could reflect the relative degree of sediment accumulation and seawater erosion. Low accumulation rate and frequent tide intrusion tended to lead higher 238U/226 Ra values while wetland accretion tended to lead lower 238U/226 Ra 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 238 U, 232 Th and 226 Ra 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 238 U, 226 Ra and 238U/226 Ra, 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 sinkBecause 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.