## 4 Discussion  
### Glacial-Interglacial Variability in \(\delta^{13}\)C\({}_{\text{org}}\)  
As an index of vegetation coverage, TOC was often used to reflect the variation of vegetation biomass from the study area in loess studies [39]. The \(\delta^{13}\)C\({}_{\text{org}}\) composition obtained from the loess-paleosol sequences derived from terrestrial higher plants has been used to reconstruct the variation in C\({}_{3}\)/C\({}_{4}\) relative abundance in past geological history periods [4; 5; 6]. The values of \(\delta^{13}\)C\({}_{\text{org}}\) composition and C\({}_{4}\) abundance indicated that the climate was colder and drier during the L1L2 (MIS 4) stage than during the L1S1 (MIS 3) stage [19; 20; 21]. The growth tendency of C\({}_{4}\) plants was more advantageous in the humid and warm climate and increased rainfall conditions [6; 11; 39; 40; 41]. Magnetic susceptibility was commonly used as the proxy index for precipitation [42; 43]. Previous studies have confirmed that there was a positive correlation between rainfall and frequency-dependent susceptibility (\(\chi_{\text{fd}}\)) through experiments on modern surface soil [44]. The \(\chi_{\text{fd}}\) for the CQS loess section was taken as an indicator of the precipitation change over the past 76-39 ka (Figure 3d). Previous studies have shown that there was a positive correlation between the Hm/(Hm + Gt) ratio and temperature for soils in the study area [45]. The \(\delta^{13}\)C\({}_{\text{org}}\) and C\({}_{4}\) abundance in the CQS profile revealed similarities with changes of \(\chi_{\text{fd}}\) and Hm/(Hm + Gt) (Figures 3b-d and 4d). The change of \(\delta^{13}\)C\({}_{\text{org}}\) composition recorded the regional differences of local plants and climate change. Previous studies have shown that the distribution ranges of the \(\delta^{13}\)C\({}_{\text{org}}\) of C\({}_{3}\) plants and C\({}_{4}\) plants were \(\sim\) 22% to \(-\)30% and \(\sim-\)9% to \(-\)19%, respectively [1; 2]. The average value of the \(\delta^{13}\)C\({}_{\text{org}}\) in the CQS loess section was \(-\)22%. This indicated that C\({}_{3}\) plants were one of the vegetations in the CQS area. [PERSON] et al. [12] studied the modern surface soil \(\delta^{13}\)C\({}_{\text{TOC}}\) data of eastern China, Australia and the Great Plains of North America around the North Pacific, and found that C\({}_{4}\) plants could grow when the mean annual temperature was higher than 12 \({}^{\circ}\)C. The current mean annual temperature of CQS in Shengshan Island is 15-17 \({}^{\circ}\)C, which is significantly higher than 12 \({}^{\circ}\)C. This showed that the vegetation of CQS contained C\({}_{4}\) vegetation. The average value of C\({}_{4}\) relative abundance in the CQS area was only 21.31%, and it indicated that C\({}_{4}\) plants were not the main vegetation in this area. Figure 4: Vertical distribution characteristics of (**a**) Fe\({}_{2}\)O\({}_{3}\) concentration, (**b**) CIA, (**c**) Hm/Gt ratio and (**d**) Hm/(Hm + Gt) ratio for the CQS section. To sum up, the \(\delta^{13}\)C\({}_{\text{org}}\) composition for the CQS loess section revealed that C\({}_{3}\)/C\({}_{4}\) mixed vegetation was the main vegetation in this study area, among which C\({}_{3}\) plants were the dominant vegetation during the LG. Comparison between \(\delta^{13}\)C\({}_{\text{org}}\) Record of the CQS Loess Section and Other Existing \(\delta^{13}\)C\({}_{\text{org}}\) Records from Central and Northern China  
Figure 5 presents the comparisons of the \(\delta^{13}\)C\({}_{\text{org}}\) records for the CQS loess section with other existing \(\delta^{13}\)C\({}_{\text{org}}\) records for the China Loess Plateau (CLP) [7, 37, 46] and Tianshan Mountains [16] and also displays the marine oxygen isotope stages record [47]. Except for the Yuanbao loess section west of the CLP and the Axike loess section north of the Tianshan Mountains, the curves of \(\delta^{13}\)C\({}_{\text{org}}\) composition for the CQS, Luochuan, Lantian and Xunyi loess sections showed a similar trend (Figure 5a-d).