Palynological Assemblage And Environment Evolution Of The Coastal Zone Within The Last Hundred Years In Guangxi

In this paper, grain size analysis, palynological analysis and 210Pb measurement are carried out on surface sediments from 14 sections and core sediments from 8 cores in the Guangxi coastal zone. According to the results from surface sections, we discuss the relationship between palynological records and sedimentary environment, and further to the mechanism on palynological distribution. Then, the extracted proxies are used to interpret the palynological record, grain size and Pb from core sediments, reconstruct the evolution of sedimentary environment in the coastal zone of Guangxi within the last hundred years on the 210Pb chronological frame, and indicate the potential factors on the evolution.Analysis of Grain size and palynology is carried out on 79 surface sediments from 14 sections in the coastal zone of Beilun estuary, Zhenzhu bay, Qinzhou bay and Yingluo bay. The results show that the palynological distribution is highly correlated with the sedimentary condition in coastal zone. High palynological concentration occurs in the mangrove forest and inner dyke, while low palynological concentration is in the bare tidal flat which is in a much stronger hydrodynamic system. This reflected palynological distribution is controlled by hydrodynamic force, especially the hydrodynamic sorted action similar to sediment materials. In General, fern spore content is low in the coastal zone, and tend to increase seaward, which is associated with its enrichment by floating transport and hydrodynamic sorting action. From mountainous area near the coastal zone, Pinus pollen obviously prevails over other terrestrial pollen, ascribing to the relative enrichment by wind and fluvial transports in a long distance and a floating way in sea. Pollen of species dominated in tropical rainforest and seasonal rain forest occur in very low contents. Poaceae pollen seems to be associated with the distance from dyke, indicated by its higher contents near dykes. Pollen content of Casuarinaceae dominant in planted shelterbelt trends to decrease seaward with an increase in distance from the dyke. Mangrove pollen are clearly characterized by in-situ deposition in mangrove tidal zone, where terrestrial palynological grains are mainly transported by water and deposit in very low content, whereas mangrove pollen of dominant species in communities prevail over others in very high content. Besides compositions of communities, palynological distribution in mangroves is also affected by characteristics of plant itself (e.g. pollination way), hydrodynamic conditions and elevation, etc. Content of mangrove pollen in non-mangrove tidal areas is generally less than 10%, which could be a boundary value indicating an allochthonous record. According to the relationship between palynological records and sediment units obtained from surface sections,6 proxies including palynological concentration, disaccate pollen, Casuarinaceae pollen, mangrove pollen, fern spore, and mean grain size of sediment, are selected as indicators on the sedimentary environment. Threshold values of indicators are established for the coastal various sediment zones in order to reconstruct the paleo-sedimentary environment recorded in cores.210Pb specific activities are measured on samples from 8 sediment cores to determine the chronological information. Core F11 is not calculated sedimentation rate due to strong disturbance on samples. The average sedimentation rates in cores F14, LM01, Q24, Q32, Q37, C11 and 018 are 0.67 cm/a,0.61 cm/a,0.25 cm/a,1.68 cm/a,0.63 cm/a,0.70 cm/a and 0.44 cm/a, respectively, and the ages at the bottom of each cores are about 1888 AD,1846 AD, 1727 AD,1956 AD,1864 AD,1916 AD and 1816 AD, accordingly.Results from palynological analysis and grain size analysis on 8 sediment cores reflect the evolution sedimentary environment within last several hundred years, under the 210Pb chronological framework, on the basis of threshold values of indicators and the mechanism on palynological distribution obtained from surface sections.In general, the coastal line tends to landward migrate within the last hundred years However, regional differences on coastline migration can be recognized on decade to annual time scales. Coastline of Zhenzhu bay had slightly migrated landward since 20th century. Qinzhou bay shows the trend of westward expansion and eastward reduction. In the western coast area, a sharp decline of Rhizophora and Aegiceras pollen content in western coastal area reflects a degradation of mangrove in the early 20th century. The western coastline obviously migrates landwards, and high tidal zone evolutes into low tidal zone. The area of tidal flat reduces due to the restricted space by human activities such as building dykes in 1960s and 1970s. The east coastline has obviously migrated seaward, corresponding to the rapid deposition with more materials from Qinjiang River since 1980s. Terrestrial influence is intensified, shown by an increase in arboreal pollen and Poaceae pollen. This perhaps is associated with human activities, such as reclamation and diking, which accelerate the evolution from low tide zone to high tidal zone. The coastline of Baihai area has migrated landward since the middle of 20th century, resulting in an evolution from low tidal zone to the high tidal zone. Yingluo bay is characterized by an evolution to low tidal zone with a declination of mangrove beach after 1970's. In addition, an increase in Poaceae pollen (>40μm) content reflects the intensified human cultivated activity for the exploit in the coastal zone after about 20th century.