Variation Of Sediment Environment Quality Under The Influence Of Human Activities And Its Impact On The Mangrove Development Over The Past 100 Years In The Coastal Zone Of Guangxi Province

In the paper, 210Pb data, grain-size characteristics, major and trace elements were carried out on six sediment cores, including the pollen andδ¹³Corg,δ¹⁵N, TOC, TN analysis of two sediment cores (O18 and Q37). According to the results, mangrove development over the past 100 years was reconstructed in the coastal zone of Guangxi province, and its relationship with the response to human activities.Based on the CIC (constant initial concentration) model, the sedimentation rates of six cores were calculated by using the excess 210Pb activities. And the average sedimentation rates of cores Q37, Q24, LM01, C11, O18 and F14 were 0.63 cm/a, 0.25 cm/a, 0.61 cm/a, 0.70 cm/a, 0.44 cm/a and 0.67 cm/a, respectively. The ages at the bottom of each core are approximately 1864 AD, 1727 AD, 1846 AD, 1916 AD, 1816 AD and 1888 AD. Furthermore, the grain-size characteristics of six cores, including Mz,δ, Sk and KG, were conducted and found that the silt is the main components of the sediments, followed by sand and clay fractions.In order to compensate the mineralogical and grain-size variations of heavy metals, a common approach is to normalize the geochemical data using one element as a grain-size proxy, e.g., Al, Fe, Sc, Cs, Li, Rb, and Mn. For this study, we have chosen Al as the normalizing element for the coastal sediments of Guangxi. For these reasons, regional background levels have been established by linear regression from the data of the deep sediment layers.According to the indicators, environmental evolution of the Guangxi coastal sediments over the past 100 years can be divided into three stages: (1) before the end of 1940s characterized by the relatively low excess metal fluxes, indicating natural inputs; (2) between the late 1940s and the early 1980s, with the local development of the aquaculture, the excess Cu elements were inputed by using the traditional disinfectants (e.g., copper-contained); (3) after the early 1980s, excess metal fluxes were increased with the rapid development of industry and agriculture along the coastal zone, especially in recent 10 years.According to the relationship between palynological records and vegetation types obtained from surface sediments, it found that mangrove pollen is clearly characterized by in-situ deposition in mangrove intertidal zone, whereas mangrove pollen of dominant species in communities prevails over others in very high content. Besides, palynological distribution in mangrove communities is also affected by characteristics of pollination way and hydrodynamic conditions. The content of mangrove pollen in non-mangrove tidal areas is generally less than 10%, which could be considered an allochthonous input. And the palynological distribution is also related with the vegetation characteristics, e.g., the content of mangrove pollen (Avicennia marina) in Dongwei has a significant positive relationship with its coverage, height and diameter. Therefore, the pollen assemblages and its contents can be used as a good indicator to trace the source of maternal plants and its development.Based on the analysis of pollen spectra in the core O18 and Q37, it found that the content of mangrove pollen in the bottom of the cores was relatively high (slightly more than 10%), indicating an in-situ deposition. And the disparity of fern spores was generally low, indicating that the hydrodynamic condition was relatively weak, which may be related to the flourish on the overlying mangrove beach. In the surface sediments, the content of mangrove pollen was evidently reduced (less than 10%), an indicated an allochthonous input. Furthmore, the increase of Pinus pollen should be related with the planting shelter forest along the coast, and the increase of Poaceae pollen reflects the intensity of human activities for cultivation near to the coastal areas. Meanwhile, the degradation of mangrove is corresponded to the increase of in-situ hydrodynamic condition, which leads to the increased disparity of fern spores. Based on the analysis ofδ¹³Corg,δ¹⁵N, TOC and TN in the core O18 and Q37, three sources (Planktonic algae, terrestrial inputs and mangrove) of organic material in sediments were separated by usingδ¹³Corg-C/N mixed model. And the burial flux of organic carbon and the contribution of mangroves source over the past 100 years were reconstructed. It was found that the low values ofδ¹³Corg were almost corresponding to the high values of C/N (no linear relationship withδ¹⁵N), and showed significant negative correlation between them. This is more consistent with the conclusions obtained by the Predecessors in the surface sediments. It indicates thatδ¹³Corg and C/N still have an important role in reconstructing the contribution of end-member organic matter, despite of the inevitable early diagenesis and degradation. The sources of organic matter in core O18 is mainly of mangrove and planktonic algae, however, the contribution of terrestrial inputs is relatively low. Furthermore, there is a significantly decreasing trend in mangrove organic matter from the 12 cm up to 4 cm (1980 AD 1997 AD). And the organic source of core Q37 between 90 cm and 50 cm (1864 AD 1927 AD) is mainly dominated by mangroves, and then mainly dominated by terrigenous inputs. The mainly reason which is caused sharply decrease of mangrove organic matter is due to in-situ human activities, e.g., artificial ponds.In order to study the relationship between mangrove development and human activities, the first thing is to reconstruct the evolution of mangrove development, which needs to be combined with pollen, stable carbon isotope compositions and grain-size characteristics. They have showed a consistent trend between pollen and mangrove organic matter (δ¹³Corg and C/N), which can be used as an indicator for reconstructing the evolution of mangrove development recently. Compared with the time span of ¹00 years, the data of social statistics in Guangxi was shorter, which can not meet the needs of academic research. Accordingly, the results indicated that sediment cores can be alternative approach for the records of human activities (excess metals fluxes, Pinus pollen and Poaceae pollen) in developing countries, where usually lack of the environmental monitoring data. Finally, it reveals the temporal evolution of human activities (industrial and agricultural), and its impact on the mangrove development.