## 5 Discussion
### Paleoproductivity and Paleowater Depth  
Productivity refers to the ability of organisms to produce organic matter within a unit of time and area. Phosphorus (P) is one of the essential elements for biological growth [30]. Organisms in the water body enrich P through metabolic processes and store it within their bodies. When these organisms die and are buried in sediments, P becomes an effective indicator for evaluating paleoproductivity. Nickel (Ni), Copper (Cu), and Zinc (Zn) exist in sediments in the form of organic-metal ligands and are released during the decomposition of organic matter in sulfidic environments. They are released during organic matter decomposition and can be preserved in sediments through reactions with pyrite in a sulfide environment. Therefore, these three heavy metals can not only indicate the reducing nature of the environment but can also be used to assess productivity [31]. Titanium (Ti) is difficult to transport, and its content mainly originates from the lake itself. To eliminate errors caused by evaluating with a single indicator, the ratios of P/Ti, Ni/Ti, Cu/Ti, and Zn/Ti (where X represents P, Ni, Cu, or Zn) are more reliable for evaluating paleoproductivity. The higher the X/Ti ratio, the higher the paleoproductivity in the region.  
Figure 11: Relationship between Ca/(Ca + Fe) and Sr/Ba.  
Statistical analysis and organization of the geochemical data from the dark mudstone of Ying IV in the Xuijaweizi area show that the P/Ti ratio ranges from 0.15 to 1.34, with an average value of 0.60; the Ni/Ti ratio ranges from 115.4 to 514.3, with an average value of 213.2; the Cu/Ti ratio ranges from 47.05 to 181.2, with an average value of 96.05; and the Zn/Ti ratio ranges from 435.98 to 2547.55, with an average value of 909.51 (Figure 12). Compared with the average values of these four ratios in Ying IV mudstone from the Shuangchengnan area of the same basin, the values from the Xuijaweizi area are relatively higher. Therefore, it is concluded that the paleoproductivity of the water body during the Ying IV period in the Xuijaweizi area was of moderate-to-high productivity.  
The aggregation and dispersion of elements are correlated with paleowater depth to some extent. Many transition elements and trace elements are good indicators of paleowater depth. Compared to the mass fraction of a single element, the ratios between major elements or trace elements are more indicative. The Fe/Mn ratio is selected to quantitatively distinguish between deep-water and shallow-water environments. A ratio less than 100 indicates a deep-lake environment, a ratio between 100 and 150 indicates a semi-deep lake environment, and a ratio greater than 150 indicates a shallow-water environment. The (Fe + Al)/(Ca + Mg) ratio is chosen as a qualitative indicator of paleowater depth. Fe and Al have higher contents in terrigenous clastics, while Ca and Mg are mainly present in carbonates. The contents of Ca and Mg have a negative relationship with those of Fe and Al, that is, the greater the water depth, the smaller the (Fe + Al)/(Ca + Mg) ratio [32]. In the study area, the Fe/Mn ratio of Ying IV ranges from 11.9 to 63.7, with an average value of 34.3, all indicating a deep-water environment. The (Fe + Al)/(Ca + Mg) ratio ranges from 1.18 to 9.95, with an average value of 5.4. Based on a comprehensive judgment, the paleowater body of Ying IV in the Xuijaweizi area is considered to be a deep-lake environment.  
Figure 12: Evolution of ancient productivity and paleowater depth in Ying IV of the Xuijaweizi Fault Depression.