In this study, two sediment cores (*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 d 13 C to trace the origins of SOC. The cores were analyzed by high-resolution sections for d 13 C, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of d 13 C, TOC, and C/N ratios ranging from -28.19 to -22.5%, 2.14-30.94 mg/g, and 10.29-18.32, respectively. The d 13 C and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and d 13 C values in mangrove sediments. However, the shifts of d 13 C 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 d 13 C \ -25%, and C/N \ 12 and d 13 C [ -25% indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying d 13 C, 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 d 13 C and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in d 13 C from the subtidal, through to tidal flat, and to the intertidal mangrove. d 13 C and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.
