In this study, total organic carbon (TOC), total nitrogen (TN), and stable carbon isotopes (δ¹³C) were measured in surface intertidal saltmarsh and bare tidal flat sediments along the Rudong coast. The distribution and sources of organic carbon were examined under different depositional environments based on C/N ratios and a two-terminal mixing model. The results showed that the average TOC content of the vegetated saltmarsh sediments, bare tidal flat areas near vegetation (BF1), and bare tidal flat areas far from vegetation (BF2) were 4.05, 2.72, and 1.22 mg/g, respectively. The mean δ¹³C value within the vegetated saltmarsh area was −22.37‰, and the C/N ratio was 9.3; the corresponding values in the BF1 area were −23.27‰ and 7.95, respectively; and in the BF2 area, the corresponding values were −21.91‰ and 5.36, respectively. These C/N ratios reflect an increasing marine contribution with distance from the vegetated zone. Combined with the two-terminal mixing model, the organic carbon in the vegetated saltmarsh sediments was dominated by terrestrial sources, while the bare tidal flat sediments were more influenced by marine sources, and the bare tidal flat sediments nearer to the vegetated zone (BF1) were influenced by a combination of vegetation, marine sources, and other terrestrial factors.

In current studies, the physicochemical properties of water, such as total dissolved solids, salinity, and electrical conductivity, are used mainly to investigate changes in the properties of surface water and groundwater. In our experimental study, we aimed to introduce the physicochemical properties of water bodies into the field of paleoenvironmental changes. We employed the physicochemical property indexes of sediment lixiviums in two research sections of the sea–land interaction zone in the eastern margin of the Subei Basin (China). Preliminary tests determined that the optimal solvent for preparing the sediment lixiviums is ultrapure water; the use of this water can prevent errors caused by soluble solids in the solvent. Using a container with a lid to prepare the sediment lixiviums could reduce errors caused by evaporation. Furthermore, we determined the appropriate process and duration for testing the physicochemical properties of sediment lixiviums. The optimal time for testing the physicochemical properties was 120 h (mixture fully stirred daily) or 168 h (no stirring). The weight of the sediment, volume of the solvent, and test time should be consistent in the same research section. Comparing the physicochemical property indexes of sediment lixiviums with geochemical elements and diatom indicators, we found that these indexes show obvious indications of transgression, and have an obvious advantage in indicating transgression.