Dating core sediment by applying the 210Pb method and verifying by residual of dioxin (during the Vietnam war) in Can Gio biosphere reserve

Application of methods using natural and artificial radionuclides such as 210Pb, 137Cs and 241Am in sediments to estimate chronology and accumulation rates of sediments, has been extensively developing for the recent past decades. In general, their results had have been worldwide agreed in many certain areas, but other areas have not yet. Can Gio mangrove forest (CGM) located in the downstream of Dong Nai—Sai Gon River, southwards of Ho Chi Minh City to the East Sea, where its sedimentation and erosion have complicatedly changed for decades. A number of 13 sediment samples of the core in the CGM was analyzed for 210Pb, 137Cs and 241Am. The results showed unsupported 210Pb very low concentrations and irregular decline with time, consequently, their sediment ages and accumulation rates could be extensively deviated. Otherwise, in the study area this method has not validate by any independent time marker or by method using artificial radionuclides (137Cs and 241Am) because of very low detection of their concentration in the sediment. Thus, the paper applied residue of dioxins concentration in sediment as a marker to demonstrate the accuracy of the 210Pb method. To carry out this work, the second core has been drilled close to the first core in distance of 0.5 m with ten collected sediment samples. The analyzed results showed that there was a peak of dioxins concentration. With the immobilize characteristic of dioxin, the time of herbicide spraying during the Vietnam war should be occurred at the depth of the highest dioxins concentration detected. This depth is at 95 cm. The spraying time was 1968–1972. The depth comparison between two cores for dioxin and 210Pb, respectively, presenting that there is a high age gap. In the same year, the 210Pb method showed at between 45.5 ÷ 49.5 cm in depth. As a marker, the residue of dioxin demonstrates that the 210Pb method should not be used to age the sediment of the Can Gio mangrove forest (CGM).