## 4 Discussion
### Differentiation Effect and Paleo-Environmental Evolution  
#### 4.2.1 Differentiation Effect  
The differentiation of LREE and HREE in both SC and TG profiles showed a LREE loss and HREE aggregation from topsoil to RSI, as shown in Table 2. The two reasons for the REE differentiation are: firstly, the leaching solution from upper layer was adsorbed and filtered by the clay layer, during which the LREE was absorbed; secondly, the stability of the complexes combined with the high concentration of CO\({}_{3}\)' can form relatively stable complex with REEs, and this stability increased with increasing elements ordinal number. Therefore, the HREE removed with weathering fluid was deposited at the RSI, resulting in enrichment of HREE [51].  
More LREEs were lost much more in the SC profile than in the TG profile. The SC location has a stable tropical monsoon climate that is high in temperature and precipitation due to the influence of the southeast and southwest monsoons from the Pacific and Indian Oceans. Although the differentiation of REEs was small, the content of REE contents in weathering crusts were different (Table 2).  
2.2 Differentiation Between \(\delta\)Ce and \(\delta\)Eu and Significance of Paleo-Environment  
Besides LREE and HREE, the geochemical parameters \(\delta\)Ce and \(\delta\)Eu showed obvious differentiation (Figure 3). Generally, Ce consists of Ce\({}^{3+}\) and Ce\({}^{4+}\) under oxidizing conditions. The latter is prone to hydrolysis and precipitates after adsorption by Fe and Mn oxides, then separates from the other REEs. This effect leads to a deficit of Ce. A \(\delta\)Ce value of 0.78 is generally adopted as the reference value to denote a redox environment [52]. Eu has two valence states: Eu\({}^{2+}\) and Eu\({}^{3+}\). The former can be oxidized to become the latter under alkaline oxidizing conditions. The reverse process can occur under strongly acidic and reducing conditions. Positive and negative anomalies occur when \(\delta\)Eu \(>\) 1 and \(\delta\)Eu \(<\) 1, respectively [53].  
The weathering profile of SC profile basically showed the stable positive abnormal process of Ce, indicating that the external REDOX environment remained stable during the weathering process since the profile was formed. The differentiation process of the TG profile was complicated. First, significant positive anomalies exist in the top of the profile, which may be related to the strong oxidation of surface tillage soil under tilling. Some studies suggested that abundant organic matter (humic acid) on the surface may also contribute to the formation of positive Ce anomalies on the surface, as the complexation of humic acid with Ce is significantly stronger than that of other REEs [54]. According to the fluctuation process of Ce in the TG profile and its negative anomaly, the weathered TG limestone profile experienced several relatively reductive climatic environments.  
Ce in the SC and TG profiles showed a positive anomaly at the upper layer, but a negative anomaly in the lower layer. In the oxidation environment, Ce was oxidized into Ce\({}^{4+}\), and then hydrolyzed and formed undissolved hydroxide. The weak acid condition accelerated the process because H\({}^{+}\) has a catalytic effect on Ce\({}^{4+}\) aggregation [55]. Therefore, the Ce in upper profile has a positive anomaly and less Ce leached downward. The RSI has an extremely negative anomaly interface for Ce.  
[PERSON] et al. [56] found that the layer of the RSI was the location with a reductive environment that experienced long-term groundwater erosion, which is why the Ce has a positive anomaly in the upper section and a negative Ce anomaly in the lower section. The fluctuation of Ce in the TG weathering profile indicates the abrupt transition of the REDOX environment in the TG profile.