DEPOSITIONAL RECORDS OF ENVIRONMENTAL EVOLUTION DURING THE RECENT 150 YEARS IN THE INTERTIDAL ZONE OF LONGMEN ISLAND, GUANGXI PROVINCE

A sediment Core LM01 was collected in May, 2007 from the intertidal zone of the Longmen Island, Guangxi province. Grain size, biogenic elements (TOC, TN and TP)and heavy metals (Hg, Cu, Pb, Zn, Cd, Cr, As)were analyzed for the core sediment in order to study the environmental evolution in the recent 150 years. Vertical grain size distribution shows a finer trend upwards from 40 cm in depth, indicating the decrease in sedimentary dynamics in the recent 60 years. Enrichment factors indicated that natural inputs prevailed up to the late 1970s, and since then the excess fluxes of heavy metals have showed a consistently increasing trend. The molar C/N ratio revealed that terrigenous organic matters decrease toward the surface, due to the decreases in mangrove forest by human activities recently. Integrated from all indicators, the environmental evolution of the Longmen Island in the recent 150 years can be divided into two stages:(1)before the 1970s, it was characterized by the relatively low heavy metal pollution and scarce eutrophication; (2)after the 1970s, the concentrations of heavy metals, total phosphorus and oil-related organic matters increase significantly, indicating the anthropogenic impact.