Grain-Size Composition And Distribution Characteristics Of Sediments In The Chanthaburi Coast,Gulf Of Thailand And Its Environmental Significance

The coastal sediment records the complex sedimentary dynamic characteristics of the coastal area and the evolution of the marine environment.The increasingly intensified human activities and resource exploitation have greatly changed the environments of the estuarine and coastal area.Gulf of Thailand,the largest gulf of the South China Sea,is located on the Sunda shelf which lies in the west of the Pacific Ocean and its affiliated seas.And the Chanthaburi and Welu rivers are located in the eastern part of Gulf of Thailand.In this thesis,grain-size analysis and AMS¹⁴C dating analysis were used to study the sedimentary characteristics,sedimentary hydrodynamic conditions,grain-size transport trend,sedimentary environment and sediment source of the Chanthaburi coast.Combined with physical geography,geological background,hydrological and meteorological features of the study area,the contribution of sea material from Chanthaburi River and Welu River to the coastal sedimentary environment was illustrated.The thesis not only provides a theoretical basis for the development and construction of the Chanthaburi coast area.It also provides theoretical support for the study of the land-sea interaction process in the middle and small estuaries.At the same time,it is of great significance to promote the"One Belt And One Road"strategy between China and Thailand as well as China-Thailand marine cooperation.The results shows that there are seven types of surface sediments on this coast,with grain sizes???generally consisting of sand and silt.The results of grade-standard deviation analysis indicate that sediment grain size of component a?44.19?m-105?m?is a sensitive indicator of environmental change.The spots with high content of sensitive grain size a was mainly distributed in the coastal areas of the Chanthaburi and Welu estuaries and the northwestern part of the study area,indicating that the hydrodynamic changes in these three areas were more significant.The sediment presents relatively complex transport trends which are mainly characterized by northwestward and northeastward transport from sea to land.The sediments at the mouths of the Chanthaburi Estuary and the Welu River oscillate under the influence of tidal currents.Based on the results of grade-standard deviation analysis and grain-size trend analysis,the study area was divided into three provinces,representing different sedimentary environments and material sources.Province I is distributed in the nearshore of the Chanthaburi and Welu estuaries.Surface sediments in this area are mainly terrigenous sediments,mixed with small amounts of sediment from the surrounding sea areas.Province II is distributed in the northwestern part of the study area.The surface sediments in this area are relatively coarse sediments,remaining in the denuded state.Province III is distributed in the central part of the study area and most of the coastal areas.The fine-grained sediment in the area is brought by tidal currents and other processes.Compared with temperate tidal-controlled estuaries in eastern China,the two tropical estuaries in the study area have smaller suspended sediment contents,runoff amounts,and tidal ranges.And sediment grains are coarser,indicating that the hydrodynamic conditions are stronger.The surface sediments of the study area mainly show transport trends from sea to land,which has large difference from other estuaries.The main reasons for the similarities and differences in the transport trends of sediments in these estuaries are the differences in hydrodynamic conditions and the specifics of regional topography.The lithology of the WLE08,WLE10 and WLE12 cores is clayey silt in the Chanthaburi Coast.The upper and lower layers are relatively uniform,with poor sorting and positive skewness.They were formed since the Mid-Late Holocene.The grade-standard deviation method was used to obtain the sensitive grain-size component of the core samples.Each core sample corresponds to two sensitive grain-size?coarse and fine?components and the typical depth of these two components is analyzed,and the result show that the sediments are almost tidal sedimentary.Since the mid-late Holocene,controlled by tidal currents,the sedimentary hydrodynamic environment of the Chanthaburi coast is relatively stable.In the early Holocene transgression event,the study area reached the modern sea level around8.25 cal.ka BP.The deposition rate in the study area was faster in the Middle Holocene,then slowed down,and increased in the late Holocene.Since the mid-late Holocene,the intensity of the Asian Summer Monsoon in the study area has decreased.The study area went through a relatively strong summer monsoon fluctuation period between 8.3 and 6.5 cal.ka BP,and experienced a relatively weak stable period between 6.5 and 3 cal.ka BP.Since 1.5k cal.ka BP,the South Asian Summer Monsoon has strengthened.During the 8.2 cal.ka BP period,the sensitive S_C2/S_C1 value decreased sharply,indicating that there was a large weakening period of the South Asian summer monsoon.This is consistent with the famous“8.2 cal.ka BP global climate abrupt event”,which is similar to the South Asian Summer Monsoon recorded by the stalagmite?¹⁸O of Oman,and also has many responses with the East Asian Summer Monsoon recorded by the stalagmite?¹⁸O of Dongge Cave,reflecting the regional and even global connection of climate change.