Study On Sedimentary And Carbon Burial Characteristics And Expansion History Of Typical Mangroves In Guangxi

Mangroves are widely distributed in the upper part of intertidal zone in tropical and subtropical regions.They are characterized by high productivity and rapid deposition rate,and are of great significance for preventing coastal erosion and organic carbon burial.Mangrove expansion and its driving mechanism,mangrove deposition and carbon burial,and carbon reserve estimation are all hot topics at present.Typical coastal mangrove wetland in Guangxi were chosen as the research object,collected surface sediment samples and columnar.The particle size characteristics and sedimentary dynamic environment characteristics of were studied.While,the sedimentary facies division was carried out to reveal the evolution of the sedimentary environment.Based on the mangrove tree ring and historical remote sensing satellite image,the stand age and expansion history of mangrove were studied,and the main factors affecting the expansion of mangrove were discussed.In addition,the deposition rate of mangrove was estimated based on the sediment ²¹⁰Pb_ex specific activity,stand age and mangrove sedimentary layer thickness.At last,the spatial distribution and influencing factors of organic carbon content were analyzed,and the burial rate of organic carbon in mangrove was estimated also.The surface sediment of mangrove in Guangxi are mainly sandy silt in the inner belt and the front edge belongs to silty sand.The distribution of sediment types in Yuzhou Flat and Tieshan Bay is contrary to this.The mean particle size ranged from 49.5～192.3μm,with the smallest particle size in the inner of Dandu sea and the coarsest particle size is appeared in the interior side of Tieshan Bay.The sorting coefficient is 0.86～3.68,which belongs to poor sorting.The grain size characteristics of mangrove sediment were mainly determined by sedimentary dynamic environment,plant density and tidal gully development.The tidal-controlled mangroves are represented by Pearl Bay,where tidal gullies develop and the average particle size fluctuates from land to sea,and the particle size mainly depends on the distance from tidal gullies.The fluvial mangroves are represented by the Nanliu River Estuary,the tidal gully system is poorly developed,and the particle size mainly depends on the tidal intake at the sample site.The wave-controlled mangroves are represented by the Jinhai Bay,the tidal gully system not developed,and the main factor controlling the average particle size is the intensity of wave disturbance.The analysis of probability cumulative frequency curve shows that the hydrodynamic environment of mangrove front edge in Guangxi is weak in Pearl Bay,but strong in Yuzhou Flat and Jinhai Bay.The age of mature mangrove in Guangxi is mainly between 22 and 45 years,and the increase of rainfall is the direct cause of mangrove expansion in recent years.The short sedimentary history made the mangrove sediments shallower,with a thickness between 4 and 28cm,the maximum appeared in Nanliu River Estuary.Under the mangrove sediments is bare flat deposition or salt marsh deposition,which represents two evolution types of intertidal sedimentary environment,one is bare flat to mangrove,another is salt marsh to mangrove.This evolution actually reflects the expansion of mangrove to bare flat and salt marsh,respectively.Based on ²¹⁰Pb dating method,estimated the deposition rate of mangroves was 1.7～7.3 mm/a,and that was fastest in Nanliu River Estuary.The average mangrove deposition rate estimated by the thick-age method was 1.1～15 mm/a,which has high reliability and low experimental cost,and can be used as a comparison and supplement to the ²¹⁰Pb dating method.The sedimentation rate of mangrove is mainly controlled by sediment supply and sedimentary environment.Affected by storm surge,the deposition rate of open coast is higher.Estuarine mangroves have the fastest deposition rate because of their abundant sediment supply.The organic carbon content of mangrove surface sediments in Guangxi is 0.3～4.3%,with the highest value in the front of Dandu Sea and the lowest value in Yuzhou Flat.The organic carbon content of the surface sediments in mangrove is affected by the hydrodynamic,submergence time and deposition rate.The carbon burial rate was 27.71～452.37 g/（m²·a）,which was mainly controlled by the deposition rate.And the value was highest in the front area of Nanliu River Estuary and lowest in the inner area of Tieshan Bay.The results can be used for reference in the further study of mangrove deposition and expansion history in Guangxi,and can also provide support for the assessment of mangrove carbon burial potential.