## 4 Discussion
### Glacial-Interglacial Variability in \(\delta^{13}\)C\({}_{\text{org}}\)
The \(\chi_{\text{fd}}\) was mainly affected by iron oxide minerals, which were usually generated in the warm, humid with strong pedogenesis and weathering environment. The precipitation in the CQS study area could increase with the strengthening of the East Asian summer monsoon during the L1S1 (MIS 3) stage. Some researchers considered that the \(\delta^{18}\)O values of Chinese stalgmites could indicate the summer monsoon precipitation and monsoon intensity [58; 59; 60]. The more negative \(\delta^{18}\)O values for stalgmites indicated stronger Asian monsoons and more rainfall [53]. The \(\delta^{18}\)O record of the stalgmites was basically consistent with the \(\delta^{13}\)C\({}_{\text{org}}\) composition of the CQS loess section (Figure 6a,d). Thus, precipitation was one of the main driving factors of variations of \(\delta^{13}\)C\({}_{\text{org}}\) composition in the CQS loess section. The effect of temperature on the \(\delta^{13}\)C\({}_{\text{org}}\) composition was also very complex. High temperature could advance the activity of plant enzymes, enhance the photosynthetic efficiency of plants, and thus promote the growth of plants [1]. C\({}_{4}\) plants generally had a relatively competitive growth advantage in high-temperature environments [3]. The curve changes of the Hm/(Hm + Gt) ratio, the \(\delta^{13}\)C\({}_{\text{org}}\) composition and C\({}_{4}\) abundance from the CQS loess profile were similar, and it indicated that temperature had a significant impact on the change of relative abundance of C\({}_{4}\) plants on the East China Sea Island (Figure 6a-c). Moreover, the positive similarity among C\({}_{4}\) abundance of the CQS loess section, the ocean temperature records of the Antarctica Dome C ice cores and the GRIP marine oxygen isotope records, indicated that the change of C\({}_{4}\) abundance was closely related to temperature (Figure 6b,f,g). [PERSON] et al. [61] considered that a low temperature was beneficial to the growth of C\({}_{3}\) plants by studying the \(\delta^{13}\)C composition of n-alkanes of CLP. [PERSON] et al. [62] studied the differences in the soil organic carbon isotope records of three typical regions in the middle latitudes of the northern hemisphere during the LG and considered that the high temperature in the growing season had an obvious effect on the growth of C\({}_{4}\) plants, and at the same time, the temperature range had a certain threshold. To sum up, the temperature was also one of the important factors to control the \(\delta^{13}\)C\({}_{\text{org}}\) composition and C\({}_{4}\) abundance of the CQS loess deposition. Carbon isotope composition was the result of C\({}_{3}\) and C\({}_{4}\) photosynthesis of atmospheric CO\({}_{2}\) by plants [7]. Previous studies indicated that a higher atmospheric CO\({}_{2}\) concentration was more beneficial to the growth of C\({}_{3}\) plants, while a lower atmospheric CO\({}_{2}\) concentration was more beneficial to the growth of C\({}_{4}\) plants [6; 7; 63]. [PERSON] and [PERSON] [64] quantitatively reconstructed the atmospheric CO\({}_{2}\) concentration by using the \(\delta^{13}\)C composition of tree rings; the \(\delta^{13}\)C composition of tree rings decreased by 2.0% \(\pm\) 0.1% for every 100 ppm increase in atmospheric CO\({}_{2}\) concentration. It indicated that the \(\delta^{13}\)C composition decreased with an increase in atmospheric CO\({}_{2}\) concentration. Figure 6a,h shows that there was no negative correlation between atmospheric CO\({}_{2}\) concentrations and the \(\delta^{13}\)C\({}_{\text{org}}\) record of the CQS loess profile. In general, the change of atmospheric CO\({}_{2}\) concentration was unlikely to drive the change of \(\delta^{13}\)C\({}_{\text{org}}\) composition in the CQS loess deposition during the LG.