Sedimentary Environment Evolvement of Xiaohai Lagoon in the Hainan Island

The paper analyzes granularity of column sediment XHK04-01 and XHK04-04, and their 210Pb and determine total organic carbon、nitrogen、sulfur content of column sediment XHK04-01 and XHK04-04, and finally using difference of C / N, C / S in sediments researches the evolvement of sediment environment. The results indicate: middle sediment and southern sediment of Xiaohai is mostly silver sand, illuminating that middle and southern hydrodynamic conditions of Xiaohai become weak; effects of seawater on sedimentary environment gradually decrease and the overlying water of sediment gradually desalt, but southern water is more desalted less salt than middle water. Southern sediment rates was 0.623 cm/a; total organic carbon and total nitrogen was steady before 60s in 20th century, and terrene organic carbon was about 80 per cent of total organic carbon; total organic carbon and total nitrogen rase rapid after 60s in 20th century, especially since 1973, and terrene organic carbon was about 45 per cent of total organic carbon. Autochthonic organic carbon rose. Middle Sediment rates was 0.216 cm/a, and total organic carbon and total nitrogen was steady before 50s in 20th century, terrene organic carbon was about 70 per cent of total organic carbon; total organic carbon and total nitrogen rose rapid after 50s in 20th century, and terrene organic carbon was about 47 percent of total organic carbon; autochthonic organic carbon rose. Southern and middle sedimentary environment of Xiaohai was steady from 1750s to 1950s, Southern and middle sedimentary environment of Xiaohai changed a lot in the past fifty years. The reasons are related to the narrow door of Xiaohai lagoon, building the dyke and the changing fluvial course.