The application of δ13C and C/N ratios as indicators of organic carbon sources and paleoenvironmental change of the mangrove ecosystem from Ba Lat Estuary, Red River, Vietnam

In this study, two sediment cores (similar to 70 cm) were collected from separate mangrove forests straddling the Ba Lat Estuary, Red River of northern Vietnam, to examine the origins of sedimentary organic carbon (SOC) and reconstruct the paleoenvironment. In addition, mangrove leaves and particulate organic matter were collected and measured for delta C-13 to trace the origins of SOC. The cores were analyzed by high-resolution sections for delta C-13, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of delta C-13, TOC, and C/N ratios ranging from -28.19 to -22.5aEuro degrees, 2.14-30.94 mg/g, and 10.29-18.32, respectively. The delta C-13 and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and delta C-13 values in mangrove sediments. However, the shifts of delta C-13 and C/N ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources, with values of C/N > 12 and delta C-13 < -25aEuro degrees, and C/N < 12 and delta C-13 > -25aEuro degrees indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying delta C-13, C/N ratios, and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments (subtidal, tidal flat, and intertidal mangrove). General trends in delta C-13 and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in delta C-13 from the subtidal, through to tidal flat, and to the intertidal mangrove. delta C-13 and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.