Surface pollen distribution patterns in Beibu Gulf and corresponding sediment dynamics environment

The Beibu Gulf is a semi-enclosed gulf in the northwest of the South China Sea. We palynologically analyzed 306 surface sediment samples from the eastern Beibu Gulf to improve bioclimatic interpretation of fossil pollen records there. Surface pollen assemblages could be classified into five pollen regions based on the distributions of total, arboreal, herbaceous and fern pollen concentrations. Four high-concentration and three low-concentration subregions could be distinguished within these regions. The distribution patterns of surface pollen assemblages were consistent with those of grain sizes of surface sediments. Sediments from regions with high pollen concentrations were very fine and fine silts (>7.0 Φ, <0.008 mm), whereas those with low pollen concentrations were fine sand (<3.5 Φ, >0.088 mm). Sedimentary heterogeneity of surface pollen assemblages was closely related to pollen source, transportation and sedimentation controlled by ocean currents, tides and waves, and oceanic bottom topography. Fern spores exhibited higher percentages along the east margins of the region, while arboreal types like Pinus increased in concentration towards the center. Herbaceous pollen appeared in high percentages around seacoasts near their source areas. Dacrydium and mangrove pollen were distributed near their source regions at low percentages. We discuss the sediment dynamic environments in the eastern Beibu Gulf based on surface pollen distributions. The estuary region is an important access to the sea basin and a depositional site for terrestrial pollen grains. Coastal regions can accumulate pollen due to the back-and-forth movements of tides there. Although ocean currents on a gulf scale tend to spread pollen grains, the interaction of multiple currents could lead to pollen accumulation and deposition. Low surface pollen concentrations in the northeast shallow-water regions of the eastern Beibu Gulf could be attributed to repeated washing and sediment floatation caused by severe wave activities during the summer. Strong tides in Qiongzhou Strait frequently wash the seabed and disadvantage pollen deposition, leading to low surface pollen concentrations there.