Environmental changes since 39 ka reflected by diatom in core sediments from Dongzhaigang Harbor, Hainan Island

    A 39,000-year record of sedimentary environmental changes, based on high-resolution grain size and diatom records from core ZK13-22, in the eastern shore of the Dongzhaigang harbor, Hainan Island, make it possible to study the relationship between environmental changes in the study area and the sea level changes in the South China Sea.

    The results show that during the period from 39.4 to 15.3 ka B.P., the grain size of the core ZK13-22 sediments was relatively coarse, and no diatoms were observed in the corresponding layer, suggesting that the study area was mainly in a terrestrial environment. Between 15.3 to 10.3 ka B.P., the grain size decreased during post-glacial period, the plankton species (Cyclotella striata and Paralica sulcata), which are marine species living in estuarine areas, was above 70% on average. The content of the benthic species Nitzschia cocconeiformis reached as high as 17%, indicating a rise in sea level in the South China Sea, marine waters intruded onto the Dongzhaigang harbor and reached the core site, and during this transgressive interval, the study area changed into an intertidal environment. From 10.3 to 7.6 ka B.P., the sediment particle size reached its lowest value throughout the borehole, while the species diversity and abundance of diatoms peaked, dominated by eurythermal intertidal and coastal planktonic species, the core site generally showed a enhanced marine influence and reduced freshwater input, shallow marine environment developed in situ. Between 8.0 to 7.6 ka B.P., the content of Rhizosolenia bergonii peaked, suggesting that the sea water temperature and salinity were relatively high during this period, possibly related to the intensified warm currents in the region. Since 7.6 ka B.P., the grain size increased significantly, diatoms only appeared at 4.4 ka B.P.. During this period, the relative abundances of Cyclotella striata and Paralica sulcata in the sediments climbed to 29% and 26% respectively. This change indicates enhanced hydrodynamic conditions, increased riverine influence, and sea level fluctuating decreases. Correspondingly, the the core site gradually shifted to an estuarine-intertidal environment. During the period from 4.4 to 3 ka B.P., the sediment grain size increased sharply, the study area transitioned to a terrestrial depositional environment.