Extreme events and environmental changes: Tracing sedimentary processes in Central Vietnam coastal lagoons

Abstract

Sediment cores, obtained in 2004Sediment cores, obtained in -2005Sediment cores, obtained in and in 2008 from seven coastal lagoons of Central Vietnam (Lang Co, Nuoc Man, Nuoc Ngot, Thi Nai, O Loan, Thuy Trieu and Dam Nai), were analysed to assess changes in 210 Pb, porosity, grain size, and depth distributions of selected major and trace elements (Al, As, Ni, and Zn). The aim was to trace sedimentary processes in areas periodically subject to extreme events. The occurrence of lateral sedimentary inhomogeneities has been hypothesised to explain the differences observed at depth for some tracer profiles, whereas topmost levels present evidence that accounts for the loss of a sediment layer (from 2.0 to 7.5 cm thick) between the two samplings. These losses might be attributed to major typhoons that impacted the coasts of Central Vietnam during the study period.

Exploring the possibility to detect recent temporal changes in highly disturbed sedimentary records through sampling repetitions and core comparisons of porosity and sand content

“…By comparing the intercept and the angular coefficients of the equations representing the linear semilogaritmic plot of 210 Pb versus depth, Giuliani et al (2011) concluded that a sediment layer several centimeters thick was removed from the Thi Nai lagoon site sometimes between 2005 and 2008, and displaced elsewhere. Similar conclusions were drawn by Romano et al (2013b), who used the test of parallelism through the analysis of covariance Table 1) (ANCOVA) in order to determine whether the regression slopes of 210 Pb profiles (m in the equation y=mx+ a, that resulted from the application of the ordinary least square regression algorithm) measured in 2005 and 2008 differed significantly. The hypothesis of covariance was verified, since calculated F at α 0.05 and p >0.05 was below F crit (Romano et al 2013b).…”

“…Similar conclusions were drawn by Romano et al (2013b), who used the test of parallelism through the analysis of covariance Table 1) (ANCOVA) in order to determine whether the regression slopes of 210 Pb profiles (m in the equation y=mx+ a, that resulted from the application of the ordinary least square regression algorithm) measured in 2005 and 2008 differed significantly. The hypothesis of covariance was verified, since calculated F at α 0.05 and p >0.05 was below F crit (Romano et al 2013b). The cause of sediment removal was identified with dredging activities linked to the construction of the NHEZ.…”

“…However, repeated sedimentary sequences of 210 Pb were able to highlight major changes occurred in the time interval between two following sampling campaigns (Giuliani et al 2011;Romano et al 2013b). By comparing the intercept and the angular coefficients of the equations representing the linear semilogaritmic plot of 210 Pb versus depth, Giuliani et al (2011) concluded that a sediment layer several centimeters thick was removed from the Thi Nai lagoon site sometimes between 2005 and 2008, and displaced elsewhere.…”

“…3a), whereas the sediment layer between surface and 34.5-cm depth in the 2008 TN-01 core must have been put into place by a mixed combination of natural sedimentation and active dumping of waste materials. The impact of natural events (such as major seasonal typhoons) cannot be ruled out either (Giuliani et al 2011;Romano et al 2013b). However, the closeness of the sampling site to the NHEZ provides much strength to the Banthropogenic hypothesis^.…”

“…The 2008 and 2010 core profiles present relative subsurface maxima at 8-and 14-cm depth, respectively. 2010 surficial PCB values have been already discussed in Romano et al (2013b).…”