## 5 Discussion
### Paleosalinity Implications of Carbonate Rocks  
Based on the relationships between the salinity and \(\delta^{13}\)C and \(\delta^{18}\)O of carbonates [26; 27], [PERSON] and [PERSON] proposed an empirical equation to calibrate the paleosalinity reconstruction [28], as expressed by:  
\[Z=2.048\times(\delta^{13}\text{C}+50)+0.498\times(\delta^{18}\text{O}+50),\]  
where \(Z\) represents the relative paleosalinity. For consistency, all isotopic compositions are expressed on the PDB scale. When \(Z<120\), the depositional environment is freshwater, whereas in the opposite case, the depositional environment is seawater. The equation has been widely and successfully used in the paleosalinity reconstruction of the strata in the Tarim Basin [29]. The \(Z\) values of the carbonate samples in this study range from 120.33 to 131.67, all greater than 120 (Table 2), which suggests that the carbonate rocks of the Upper Quilitage Formation in the study area were formed in the carbonate platform of the marine environment.  
The lithologic characteristics of the carbonate rocks in the Upper Quilitage Formation suggest open or limited carbonate platform deposition. In the nearshore environment, a negative excursion of \(\delta^{13}\)C values (from about 3% in the lower part to about 0% in the middle part) may indicate injection of a large amount of freshwater or rainwater, reduction in evaporation rates, and decrease in biological productivity [30]. The decreasing trend of \(\delta^{13}\)C and \(Z\) values is shown in a few samples (the middle part of the middle part) in which \(Z\) values approach 120 (Figure 2). The oxygen isotope values become more positive from the middle part of the middle part to the top of the upper part. This could record an evaporation increase in the part.  
In addition, Sr/Ba and Th/U values are also important indicators of paleosalinity [31; 32; 33]. The chemical behaviors of Sr and Ba are different in the supergene processes of fresh and seawater. In freshwater, both elements exist in the form of aqueous carbonate; when fresh water is mixed with seawater, the reaction of Ba\({}^{2+}\) and seawater-derived SO\({}_{4}\)\({}^{2-}\) will form insoluble barite. Indeed, the tendency of Sr to be more enriched than Ba in seawater results in an increase in Sr/Ba ratios in carbonate materials as salinity increases [16; 34]. Generally, the Sr/Ba value of freshwater sediment is less than one, while that of seawater sediment is greater than one [16]. Generally, an Sr/Cu value between 1 and 10 indicates a wet climate,  
Figure 2: Scatter diagram shows the relationship between \(\delta^{13}\)C and \(\delta^{18}\)O of the carbonates of Upper Quilitage Formation in Kekedigankake section, Keping area. The blue dots represent each of the 40 samples analyzed and studied.  
while \(>\)10.0 indicates a dry climate [35]. Generally, V/Cr values less than 2 suggest an aerobic environment, ranging from 2 to 4.25 suggest an anaerobic environment, while greater than 4.5 imply an anoxic environment [36]. The Sr/Ba values of 27 samples ranged from 3.42 to 24.39, with an average value of 11, indicating that the water salinity in the study area was high enough during the formation of the Upper Quilitage Formation. It has been proposed by [PERSON] et al. that when the Th/U value is less than seven [37], the carbonate is deposited in the marine environment. The maximum Th/U value of the carbonate rocks in this study is 1.32, indicating marine deposition, which is consistent with the result of the Sr/Ba indicator.