Variations And Paleoclimatic Significance Of Loess Organic Carbon Isotopes In Longxi Loess Plateau And Xinjiang Ili Basin

Scholars from both domestic and abroad have studied soil organic carbon isotopic composition (513Corg) of more than10loess profiles in the eastern Chinese Loess Plateau (CLP. refers to the part of the CLP east to the Liupan Mountains in this paper) since the Last Glacial (LG), basically found out the variation characteristics that the loess δ13Corg values of the loess accumulated during the LG is more negative than those of the paleosol developed during Holocene in this region, indicating that the relative abundance of C4plants was increased from the LG to the Holocene in the terrestrial ecosystems. Relative to the eastern CLP, while only few studies of loess813Corg have been conducted in the Longxi Loess Plateau (the western part of the CLP that west to the Liupan Mountains), and the variations of the relative abundance of C3/C4plants since the LG in the western CLP is not so consistent as the results of the eastern part of the CLP. To investigate variation characteristics and corresponding paleoclimatic significance of the loess δ13Corg in the Longxi Loess Plateau since the LG, we present the high resolution δ13Corg records from the Zhang Jiachuan loess profile located in the southeastern part of the Longxi Loess Plateau.Zhang Jiachuan loess profile is a typical loess/paleosol profiles accumulated since the LG, we collect165loess samples from this profile with the sampling density of2cm per sample. The813Corg values of these loess samples are ranged from-21.5%o to-23.4‰with an average value of-22.6‰, and the δ13Corg values were gradually more positive since the LG, indicating that the relative abundance of C4plants was increased from the LG to the Holocene in the study site which is similar to the results from the eastern CLP. Furthermore, we compared the813Corg results of Yanshi profile located in the eastern CLP、Jingyuan profile located in the northwest part of Longxi Loess Plateau with our results of Zhang Jiachuan profile since the LG. The comparison indicates that the variations of the relative abundance of C3/C4plants in terrestrial ecosystems in the Longxi Loess Plateau since the LG is more complex than that in the eastern CLP, which suggests that there still has some internal differences in C3/C4variations between different parts in the Longxi Loess Plateau. The relative abundance of the C4plants increased from the LG to the Holocene in the eastern and southern part of the Longxi Loess Plateau with much more small magnitude than that in the eastern CLP. However, in the northwestern part of the Longxi Loess Plateau, it seems that this area was predominated by C3plants and the signal from C4could be ignored in terrestrial ecosystems since the LG.The loess δ13Corg values from the Western Europe are extremely negative since the last interglacial (with most data are more negative than-24‰), and even more negative in paleosols formed during the interglacial periods, which is opposite to the results of the eastern CLP. The authors suggested that the Western Europe was predominated by C3plants since the last interglacial. As above mentioned, the northwestern Longxi Loess Plateau was also predominated by C3plants since the LG, with the negligible signal from C4plants (represented by the results of the Jingyuan profile). The arid Central Asia falls into the similar latitudinal band as the Western Europe and CLP, also with lots of loess distributed in this region (for example, the Xinjiang loess). However, until now, little is known about the variations and paleoclimatic significance of the loess δ13Corg in this region.Total organic carbon (TOC) concentrations and δ13Corg of the loess samples from Axike (AXK) loess/paleosol profile located in eastern Ili basin, eastern Central Asia, have been measured. Three stages with peak negative δ13Corg data occurs in the three paleosol layers observed in the field in lower part of the AXK profile, broadly corresponding to the stages with higher TOC concentrations, as well as MSfd values reported recently by other researchers. Therefore, loess δ13Corg data from AXK profile show a "negative correlation model" with the paleosol layers with strong pedogenesis intensity, which is opposite with the results from the eastern CLP, while consistent with previous reported data from the Western Europe and northwestern CLP. Which means the loess δ13Corg data from AXK profile recorded the responses of δ13C of local predominate C3plants to paleoclimatic variations. Numerous studys indicate that δ13C of modern C3plants mainly responded to the variations of local precipitation, and there is a negative correlation between δ13C of modern C3plants and local mean annual precipitation. Therefore, loess δ13Corg data from Xinjiang loess area may be a potential indicator for paleoprecipitation. Further investigation about the quantitative correlation between the δ13C of modern C3plants or/and surface soils in arid Central Asia and the corresponding precipitations needs to promote. After that, paleoprecipitation reconstruction based on the loess δ13Corg in this area is a feasible way for the quantitative paleoclimate reconstruction.