## 5 Discussion  
### Paleoclimate  
The term paleoclimate denotes climatic regimes in Earth's geological past. Sedimentary elemental enrichment patterns are controlled by depositional environmental dynamics, particularly climatic conditions [22]. Major element oxide compositions in sedimentary sequences serve as classical proxies for paleoclimate reconstructions. For example, under arid climatic conditions, ancient lakes may become more alkaline due to evaporation, leading to authigenic precipitation of MgO and CaO in palustrine facies. Thermodynamic partitioning favors MgO precipitation over CaO under elevated temperatures, thus, driving a systematic increase in Mg/Ca ratios with progressive aridity. Low values of Mg/Ca indicate warm climates, while high values indicate arid climates [23]. Sr enrichment is preferentially associated with evaporative regimes, though its concentration may be modulated by aqueous carbonate ion activity. According to the relationship diagram between Sr/Cu and CaO (Figure 6), the relationship between the two is poor, with a relationship coefficient R\({}^{2}\) of only 0.0859. After excluding the influence of carbonate rocks, Sr/Cu can be used as a good proxy indicator for paleoclimate reconstruction. Generally, an Sr/Cu ratio greater than 10 indicates arid climate conditions, while a ratio between 1 and 10 indicates warm and humid climates [24; 25]. The Sr/Cu ratios of Ying IV mudstone in the study area range from 4.25 to 11.92, with an average value of 7.48, indicating an overall transitional climate environment from semi-warm and humid to semi-arid. The Si/Al ratio in sedimentary rocks can also reflect the climatic characteristics of the deposition period. If the climate is humid, intense chemical weathering can cause the transport and migration of  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Depth/m** & **V** & **Cr** & **Co** & **Ni** & **Cu** & **Zn** & **Rb** & **Sr** & **Ba** & **Tl** & **Zr** \\ \hline
3864 & 42.739 & 80.314 & 6.687 & 44.996 & 18.311 & 238.942 & 80.798 & 191.760 & 6476.34 & 0.411 & 307.347 \\
3874 & 56.822 &  
where CaO* refers only to Ca in silicates, not in phosphates or carbonates. A CIA value between 50 and 70 indicates a low degree of weathering, corresponding to a cold and dry paleoclimate during sedimentation. A CIA value between 70 and 85 indicates a moderate degree of weathering, corresponding to a warm and humid paleoclimate. A CIA value between 85 and 100 indicates a high degree of weathering, corresponding to a hot and arid paleoclimate. In the formula, all oxides are expressed in moles. CaO* refers specifically to CaO in silicate minerals, avoiding interference from CaO in carbonate and phosphate minerals. During diagenesis, potassium (K) may become abnormally enriched through metasomatic processes. The A-CN-K ternary diagram (Al2O3 - (CaO* - Na2O) - K2O) is commonly used to determine whether potassium correction is necessary. As shown in Figure 8, the CIA values of the samples correlate well with the ideal weathering trend line, indicating that the samples have not been affected by potassium metasomatism. The CIA values of Ying IV mudstone in the study area range from 65.38 to 80.23, with an average value of 76.55, indicating moderate weathering. This suggests that the overall climate in the study area was warm and humid. Additionally, the (Al2O3 + K2O + Na2O) - SiO2 cross-plot also indicates that the paleoclimate of Ying IV in the Xuijaweizi area was generally warm and humid (Figure 9).