²¹⁰ Pb_Exc,¹³⁷Cs Reference Profiles And Modern Sedimentation In The South Tianjin Coast

The study area was in the south Tianjin Coast. Sediment samples were collected from S4 and S5 sites in the study area,210Pb and 137Cs activity, grain size, and foraminifera were measured, and hydrologic data, to establish the study area 210Pbexc, 137Cs activity-depth regional reference profiles, approaching to the history of atmospheric sedimentation. Based on it, sediment samples were collected from the planning area to reclamation of Nangang industrial zone in the study area,210Pb and 137Cs activity were measured to find out modern sedimentation rates in this area, and then to provide basic data for evaluating the environmental geology of coastal reclamation project and other coastal engineering.The results show that two layers, in which the 210Pbexc activity is obviously lower in 3-5cm and 10-13cm, correspond to main peak and minor peak of 137Cs. The results of 210Pb dating (CIC model and CRS model) show that the main peak of 137Cs is 1963, corresponding to the global 137Cs fallout well; however, the minor peak is not 1986, corresponding to the accident of the Chernobyl reactor. Meanwhile, total 210Pbexc inventory in sediment from 1963 to 2013 is about 2.735Bq/cm2.The hydrological data shows that there were two flood events in 1963 and 1996 in the area, during the floods, the sits of S4 and S5 were inundated, and the sediments transported by the floods deposited here. The old sediments transported by floods contained low activity of 210Pbexc compared the surface sediments, and contained 137Cs, therefore, two easily recognizable sedimentary layers formed in the 210Pbexc and 137Cs activity-depth profiles. Moreover, during the floods, because of the terrigenous sediments without foraminifera deposited in this region, led to low foraminiferal abundance, meanwhile, high energy events led to high brokenness percentage of the foraminifera, coarse-grained and poorly sorted sediments.Therefore, in 137Cs regional reference profiles of the study area, approaching to the history of atmospheric sedimentation, main peak is 1963, and it can be used as an important reference year, and minor peak is not 1986. In 210Pbexc regional reference profiles, total 210Pbexc inventory in sediment from 1963 to 2013 is about 2.735Bq/cm2, and can be used to estimate deposition/erosion on the whole. Based on it, the modern sedimentation rates data of Nangang industrial zone is analyzed. The results show that sedimentation rate appear a decreasing trend from south to north on the intertidal areas; sedimentation rate appear an obvious decreasing trend from intertidal areas to 2m water depth of shallow sea; Sedimentation rate has not obvious change from 2 to 5m water depth of shallow sea. The data can be used to provide basic data for evaluating the environmental geology of reclamation project and other coast engineering.