Grain size characteristics of core sedimentation from the Gaoqiao mangrove area in Zhanjiang, Guangdong Province of southern China

Mangrove coast is a special type of biological coast and it provides a variety of ecosystem services. To understand the evolution of the habitat of mangrove wetland, sediment cores along a hydrodynamic gradient in the inner and outer gulf of Yingluo Bay, the grain size characteristics (average grain size, sorting coefficient, skewness and kurtosis) of the sediments in the horizontal and vertical directions were analyzed using the moment method. The results showed that the sediments were mainly composed of silt and sand components, the surface samples of mangrove zone showed a trend of refinement with the maturation and deposition process of mangrove forests. The grain size distribution curve showed that the outer mangrove was more affected by hydrodynamic conditions than inner mangrove. The deposition processes of inner zone and outer zone were similar. The inner deposition was complicated by the influence of hydrodynamic conditions and biological processes. However, the sedimentary environment of the pure stand of Avicennia marina was relatively stable.Grain size parameters were affected by the hydrodynamic conditions and biogeography processes, and spatial heterogeneity was significant. In horizontal direction, the average particle size of the inner sample line of the bay turned to be small (texture coarsening) from inside to outside. The average particle size of the outer samples showed no obvious trends but become finer in intermediate zone distributed with Rhizophora stylosa trees. The sediment segregation was poor to worse, the frequency curve was negative biased to positive bias, and the peak presented from flat to sharp distribution. In vertical direction, the average particle size decreased from the surface to deeper layer, the separation became worse, the skewness increased, and the peak state increased too, reflecting the deposition process of the forest understory environment.The correlation analysis showed that the grain size parameters of the sediments were related to different volume levels, the average grain size and kurtosis parameters were positively correlated with the content of clay and grains, and negatively correlated with the sand content, the separation coefficient and skewness were negatively correlated with the fine particulate matter content. The average grain size was positively correlated with the kurtosis, but negatively correlated with the sorting coefficient and the skewness. The separation coefficient was negatively correlated with the kurtosis, and the skewness was negatively correlated with the kurtosis. A variety of roles were involved in the formation of sediment, and mangrove forests changed the hydrodynamic conditions of the coast, which affected the deposition process in turn. The results are significant to understand the biomechanical processes of mangrove wetlands on a century scale.