## 3 Data Processing Methods  
### Calculation Methods of Geochemical Background Values and Geochemical Baseline Values  
Calculation Method 1 for Geochemical Background Values: Iterative Method [27; 28]. In geochemical data processing, traditional statistical parameter calculations are based on the assumption that the data follow a normal or log-normal distribution. However, in most cases, the data used for these calculations do not conform to such distributions. Therefore, \begin{table}
\begin{tabular}{c c} \hline \hline
**Analytical Methods** & **Target Elements** \\ \hline Inductively Coupled Plasma Mass Spectrometry (ICP-MS) & Ag, Bi, Cd, Ce, Co, Dy, Er, Eu, Gd, Ho, I, La, Lu, Mo, Nd, \\  & Ni, Pb, Pr, Rb, Sb, Sm, Tb, Th, Tm, Yb \\ \hline X-ray Fluorescence Spectroscopy (XRF) & Al\({}_{2}\)O\({}_{3}\), Ba, Br, CaO, Cl, Cr, TE\({}_{2}\)O\({}_{3}\), Ga, K\({}_{2}\)O, MgO, Mn, \\  & Na\({}_{2}\)O, Nb, P, S, SiO\({}_{2}\), Sr, Ti, Y, Zn, Zr \\ \hline Inductively Coupled Plasma Atomic Emission Spectrosco (ICP-AES) & Be, Cu, Li \\ \hline Atomic Fluorescence Spectroscopy (AFS) & As, Ge, Hg, Se \\ \hline Graphite Furnace Atomic Absorption Spectroscopy (AAS) & Au \\ \hline Alternating Current Arc-Emission Spectroscopy (AES) & B, Sn \\ \hline Gas Chromatography (GC) & TC, N \\ \hline Potentiometry Method (POT) & F, pH \\ \hline Volumetric Method (VOL) & Corg. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Analysis Methods and Elemental Composition (Data sources: Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences). Figure 1: Desert Sampling Location Map (Data and base map sources: Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences). by removing values that deviate more than three times the standard deviation from the mean, the data are adjusted to approximate a normal or log-normal distribution. At this point, the similarity between the calculated mean and the original data (before removal of values exceeding three times the standard deviation) should be assessed. When they are essentially consistent or very close, it indicates satisfactory calculation results. The iterative method follows these steps: (1) Calculate the mean (\(X_{1}\)) and standard deviation (\(Sd_{1}\)) of the original data set for each element across the entire area; check if the data conform to a normal distribution. If they do, then \(X_{1}\) is considered the background value. If not, proceed to step (2); (2) Remove a batch of values based on \(X_{1}\pm 3 Sd_{1}\) and obtain a new data set. Then, calculate the mean (\(X_{2}\)) and standard deviation (\(Sd_{2}\)) of this new data set and check if the data conform to a normal distribution. If they do, then \(X_{2}\) is considered the background value. If not, proceed to step (3); (3) Repeat step (2) until no more high-value points exist, then calculate the mean (\(X_{n}\)) and standard deviation (\(Sd_{n}\)) of the final dataset. \(X_{n}\) is taken as the background value \(C_{01}\), and \(n+1\) is the number of times the data conform to a normal distribution. The calculation process is carried out using Microsoft Visual Basic for Applications [15; 29; 30; 31; 32]. Calculation Method 2 for Geochemical Background Values: Frequency Distribution. Histogram Method [33; 34]. For those whose histogram is unimodal and close to normal distribution, the general processing method is to connect the left top Angle \(S\) of the maximum frequency column with the left top Angle \(Q\) of the latter group of frequency columns, and the right top Angle \(T\) of the maximum frequency column with the right top Angle \(P\) of the latter group of frequency columns. The projection of the two lines intersecting on the coordinate axis is the mass value \(M_{0}\), which can be used as the background value. Then, draw a horizontal line defined by 0.6 times the maximum frequency, intersecting both sides of the frequency density curve, and the length between the intersection point and the median value is the mean square error \(\sigma\).