*6.2. Physicochemical Properties Index of Sediment Lixivium: A New Index Reflecting the Changes of Coastal Environment*

Different sedimentary characteristics can be formed because of the differences in the dynamic conditions and the biological species living in the environment, as well as climate changes and sources of sediment. Therefore, sedimentary characteristics can also be used to invert the sedimentary environment. The distribution of diatom species in the sediments is controlled by environmental variables in the area, such as water temperature, salinity, depth, size of the water body, water depth, pH, and nutrients [44]. Diatoms are extremely important indicators for reconstructing the environment of ancient coastal areas [45–52]. The geochemical elements Sr and Ba are both soluble in water and migrate with water. The concentration of sulfate ion in water increases because of evaporation or seawater intrusion into the water environment. Then the Ba ions in the water body form barium sulphate and precipitate first. The solubility of strontium salts (sulphate and carbonate) would be slightly higher than that of barium salts, and the strontium salts would be precipitated after barium sulphate precipitation [53]. The change in the Sr and Sr/Ba ratio could indicate changes in salinity [39–43].

Sediment lithology, diatom species, geochemical elements and ratios, and physicochemical properties of sediment lixivium in the GX and CYK sections of the Subei Basin corresponded well. Their changing trends indicated that both profiles could be divided into three stages of sedimentary environments (Figures 8 and 9), implying that these physicochemical property indexes could indicate the evolution of the coastal sedimentary environment. Furthermore, Figures 8 and 9 show that the physicochemical property indexes agree well with the geochemical elements and ratios, as well as the diatom species generally. In particular, the stratigraphic boundary indicator at the beginning of transgression is highly consistent (the position of the dotted line in the lower part of Figures 8 and 9). However, some slight differences are detected in the details of the record. The first difference is that in the process of transgression, some minor fluctuations recorded in the diatom species, and geochemical elements and ratios are not reflected in the physicochemical property indexes of sediment lixivium. For example, at a depth of 370–300 cm in the GX profile, the diatom species, geochemical elements, and ratios fluctuated significantly; however, the physicochemical property indexes of sediment lixivium were not recorded. This could be ascribed to the rapid deposition rate, loose sediments, and easy infiltration of the upper salty water when transgression occurred, which obscured the small fluctuation records. The second difference is that at the end of transgression, the boundary line indicated by the physicochemical property indexes of sediment lixivium was slightly lower than was the boundary line indicated by the diatom species, geochemical elements, and the ratios. This could be ascribed to the fact that after transgression, the research section was located in a freshwater lake sedimentary environment and, because of the long-term immersion in fresh water, the soluble solids in the adjacent strata were dissolved and diluted.

## **7. Conclusions**

Based on a study of the experimental process and change mechanism of TDS, EC, and SAL of sediment lixivium in the sea–land interaction zone of the Subei Basin, as well as comparison with geochemical elements and ratios and diatom species, the following conclusions can be drawn:

(1) Through preliminary tests, a reasonable method of preparing sediment lixivium and the appropriate time for determining the physicochemical properties of sediment lixivium were determined. Ultrapure water had to be used in the process to ensure that the solvent did not contain soluble solids and to reduce the introduction of external errors. The reasonable time for testing the physicochemical property indexes of the sediment lixivium was determined as 120 h (fully stirred daily) or 168 h (no stirring) after the production of sediment lixivium. The physicochemical properties measured at this time were stable, which could reflect the difference in the water environment in the sediment deposition to the greatest extent. It should be noted that the test time for the physicochemical properties of sediment lixivium on the same profile should be tested simultaneously.


**Author Contributions:** Conceptualization, Q.S. and Y.C.; methodology, Y.C. and S.Z.; software, Q.S. and Y.C.; investigation, Q.S. and Y.C.; data curation, S.Z. and Y.C.; writing—original draft preparation, Q.S.; writing—review and editing, Q.S.; project administration, Q.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Grant No. 41671195 and 41801005); Marine science and technology innovation project of Jiangsu province (No. JSZRHYKJ202002).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon request.

**Acknowledgments:** The authors would like to thank editors and reviewers for their valuable comments for the improvement of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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