*3.4. Diatom Analysis Method*

We selected 26 and 25 samples for diatom identification from the GX and CYK sections, respectively. Before identification, the sediments were pre-treated with hydrochloric acid and hydrogen peroxide. Subsequently, the samples were made into slices for observation under a microscope [38].

#### **4. Test Process of Physicochemical Properties of Sediment Lixivium**

Choice of solvent: Water is the most commonly used solvent; moreover, distilled, deionized, purified, or ultrapure water is used often as a solvent in experiments. The physicochemical properties of the different types of water obviously differ. As regards distilled water, because of process differences, the lower the number of distillations, the greater the TDS of the water; the TDS will be close to zero after multiple distillations. Deionized water uses an anion/cation exchange resin to remove the anions/cations in the water; the TDS of the water can be reduced to less than 20 mg·L −1 . Pure water contains a certain amount of dissolved solids, and the TDS is usually between 0 and 50 mg·L −1 . Ultrapure water removes the conductive medium in the water almost completely and removes or reduces the non-dissociated colloidal substances and organic substances in the water to an extremely low level; the TDS test result is zero. Accordingly, aiming to minimize the influence of the solvent on the test results, we selected ultrapure water as solvent to prepare the sediment lixivium.

Preparation of sediment samples: Sediment shows differences in water content, particle size, cementation degree, and the like, because of the influence of the sedimentary environment, sediment source, compaction, and other factors. We intended to eliminate the influence of the precipitation rate of the attached substances on the sediment in the solvent, which is caused by the difference in the water content and the degree of cementation of the sediment. To achieve this, we uniformly dried the samples at a low and constant temperature. Subsequently, we ground them into powder and passed them through 200 mesh sieves. The experimental samples prepared in this manner could precipitate the attached soluble substances more quickly after being dissolved in the solvent.

Choice of container: Open containers are not suitable for storage during the experiment, as the sediment powder has to remain in the container for an extended time after dissolving. The solvent will evaporate and reduce, leading to a large error in the measured results. To minimize the error, a container with a lid should be chosen for the production of the sediment lixivium and the container should be covered to prevent or reduce evaporation.

Preparation of sediment lixivium and determination of the test time: For comparison, the weight of the sample taken from the same section and the volume of added ultrapure water should be the same. We weighed 5 g of dried and ground sample with an electronic balance, placed it in a 150 mL container with a cover, added ultrapure water (100 mL), and stirred the mixture using a glass rod. Subsequently, we measured the physicochemical properties. After covering the container tightly and letting it stand for 24 h, we measured the physicochemical properties again, and then used a glass rod to fully stir the mixture again. This process was repeated six times. We discovered that the physicochemical

properties tended to be stable (Figure 4). To determine whether stirring the sediment lixivium every day is conducive to accelerating the dissolution of substances, we set up a reference group for comparative experiments. The samples in the reference group were stirred fully during the preparation but were not stirred again afterward. These samples were also measured every 24 h. After seven tests, the results tended to be stable (Figure 2). Based on the test results, we concluded that if the sediment lixivium were stirred fully every day, the best test time should be 120 h. However, if the sediment lixivium were fully stirred only when it was prepared and then left unstirred, the best test time should be 168 h. The test instrument was a multiparameter water quality meter (model SX751; Shanghai Sanxin Instrument Factory, Shanghai, China). During the test, the probe of the instrument was immersed completely in the upper clarified solution of sediment lixiviums, and each index was measured three times, after which the average value was calculated.

**Figure 4.** Change curve of the physicochemical property indexes of sediment lixivium with and without stirring with time (different color symbol lines represent the test result of different sediment lixivium, with the top same color symbol line being the test results under stirring, and the bottom the test results without stirring. (**A**): Change curve of TDS of sediment lixivium with time; (**B**): Change curve of EC of sediment lixivium with time; (**C**): Change curve of SAL of sediment lixivium with time).

#### **5. Results**

#### *5.1. Variation Characteristics in Physicochemical Properties of Sediment Lixivium*

The three physicochemical property indexes TDS, EC, and SAL were measured in sediment lixiviums of the GX and CYK sections, with all showing obvious changes (Figure 5, Table 1). The changes in the three indices indicated that both the GX and CYK profiles could be divided into three stages. In stage I, the three indices TDS, EC, and SAL were all in the low-value stage and showed a gradual increasing trend. In stage II, the three indices all showed high values, and there were certain fluctuations in the CYK section. In stage III, the three indexes all showed low values. In addition, Figure 5 shows extremely good consistency among the TDS, EC, and SAL, with the calculated correlations between TDS and EC, and TDS and SAL all being close to 1 (Figure 6).

## *5.2. Variation Characteristics of Geochemical Elements Sr and Sr/Ba Ratio*

The Sr/Ba ratios are also often used to indicate the changes in salinity and distinguish continental and marine sedimentary environments [39–43]. Sr and Sr/Ba ratios were selected for geochemical analysis (Figure 7, Table 2). The changes in Sr and Sr/Ba indicated that both the GX and CYK profiles could be divided into three stages. In stage I, the Sr and Sr/Ba ratios were low with small fluctuations. In stage II, the Sr and Sr/Ba ratios were high, with obvious fluctuations, and in stage III, the Sr and Sr/Ba ratios became low again with small fluctuations.

In addition, in stage II of the GX profile, it could be divided into two sub stages according to the changes of Sr and Sr/Ba. In stage II-1, Sr and Sr/Ba increased rapidly and then decreased, and the overall values were low (average values: 119.98 mg·kg <sup>−</sup><sup>1</sup> and 0.26, respectively); in stage II -2, Sr and Sr/Ba were overall high values (average values: 141.17 mg·kg <sup>−</sup><sup>1</sup> and 0.32, respectively) with small fluctuations. **(mg∙L−1 (μs∙cm−1 (g∙kg−1 (mg∙L−1 (μs∙cm−1 (g∙kg−1**

**Figure 5.** Variation characteristics of physicochemical property indexes of sediment lixivium in GX (**a**) and CYK (**b**) profiles.

**Table 1.** Changes of physicochemical properties of sediment lixiviums of GX and CYK profiles.


**Figure 6.** Correlation between TDS and EC, and TDS and SAL in sediment lixivium study section (blue dotted line is the trend line).

**Figure 7.** Variation characteristics of Sr and Sr/Ba ratio of sediments in GX (**a**) and CYK (**b**) profiles. **Figure 7.** Variation characteristics of Sr and Sr/Ba ratio of sediments in GX (**a**) and CYK (**b**) profiles.


**(mg∙kg−1**

**Table 2.** Changes of Sr and Sr/Ba of sediment of GX and CYK profiles.

**(mg∙kg−1**

#### *5.3. Variation Characteristics of Freshwater–Saltwater Diatom Proportion –*

The diatom fossils of 26 and 25 samples were identified from GX and CYK profiles, respectively, and a total of 34 species of diatoms were identified. Among these were eight freshwater and 26 brackish water diatom species. The changes in the combination of diatom species indicated that the GX and CYK profiles could be divided into multiple diatom distribution zones from bottom to top (Figures 8 and 9). The distribution zones reflected a freshwater sedimentary environment, tidal flat–shallow sea sedimentary environment, and freshwater sedimentary environment, respectively. –

**Figure 8.** Comparison of lithology, TDS, Sr/Ba, and diatoms of GX section and sedimentary.

**Figure 9.** Comparison of lithology, TDS, Sr/Ba, and diatoms of CYK section and sedimentary environment.

## **6. Discussion**

## *6.1. Reasons for Changes in Physicochemical Properties of Sediment Lixivium in Coastal Areas*

The physicochemical properties of water reflect the amount of TDS, size of EC, and level of SAL in the water. TDS in the water was significantly correlated positively with EC and SAL (Figure 6). The physicochemical properties were related mainly to the amounts of soluble substances attached (or adsorbed) by the sediment itself. When sediments are deposited, the concentration of TDS in the water will differ when the sedimentation environment differs (i.e., fresh water and salt water). These soluble substances can change the total amount of TDS attached to the sediments through adsorption (precipitation), precipitation (dissolution), ion exchange, and other methods. When the amount of TDS in the water is high, the amount of TDS attached to the sediments is also large, and vice versa. When the dried and ground sediment powder was dissolved in a sufficient amount of ultrapure water, the soluble solids attached to the sediment were released because of the extremely low TDS concentration in the ultrapure water, thereby increasing the amount of

TDS in the solution. The larger the amount of TDS in the water of the original sedimentary environment, the larger would be the amount of TDS released into the ultrapure water, and vice versa. Therefore, the water environment of sediment deposition can be reconstructed according to the dissolved TDS (EC, SAL) in the ultrapure water.
