A Comparative Study Of Organic Geochemical Proxies In Holocene Aeolian And Lacustrine Sediments From The Shiyang River Drainage Basin

Aeolian and lake sediments are commonly used in past global changes. Organic geochemical proxies of sediment contain a wealth of palaeoenvironmental information, so it is an effective way to study past global changes. Understanding the influencing factors and mechanisms of these natural proxies is the premise of using exactly these indicators. A comparative study of organic geochemical indicators, from basin-wide sediments, is useful for understanding the complex source, transport and deposition processes of sediments, and it provides beneficial reference to track the sources, movement and deposition process of organic matter. It also can provide basis for distilling environmental information from sediments.In the study, we chose the HYH eolian deposit section in the upstream of the Shiyang River drainage basin and the QTH01lacustrine deposit section of the terminal Zhuye Lake as objects. We compared the contents, variable characteristics and mutual relationships among TOC, C/N and δ13Corg of the two sedimentary sections. This comparative study lays the foundation for research of deposition mechanisms of organic matter. There are160samples collected from the HYH section, in which17samples were used for AMS14C dating;292samples were collected from the QTH01section of Zhuye Lake, including19dating results in this section. The samples of the two sections were analyzed for TOC, C/N and δ13Corg results.According to the comprehensive and comparative data analysis, the result shows that, in the HYH section, the TOC percentage and C/N ratio of paleosol sedimentary layers are higher than the adjacent loess deposition layers. The δ13Corg values of adjacent loess layers are high in contrast to the adjacent paleosol sediments. During the Holocene, TOC percentage and C/N ratio in lacustrine sedimentary layers of QTH01section are relatively high, and the δ13Corg values are low. In the sand layers, TOC and C/N ratios are low while the δ13Corg values are high. The C/N ratios mostly reflect the input of organic matter in QTH01section sediments.Since～12500cal yr BP, the overall change features and variation ranges of TOC percentages for the HYH and the QTH01sections are similar. It indicates that deposition of organic matter from the same watershed have inner links and correspond basin-wide climate change in a similar way. In different sedimentary layers, mean values of TOC percentages, C/N ratios and δ13Corg and their relationships are well related to sedimentary facies. When TOC levels are high, C/N is relatively high too, but δ13Corg values are relatively low, and vice versa. The similarity reflects that the same watershed has similar and inherent patterns of carbon, nitrogen, organic carbon isotopic ratios and sediment transport processes. It also shows the sedimentary organic matter in this basin with similar deposition, decomposition and buried mechanisms.TOC percentages, C/N ratios and δ13Corg of the upstream river basin holocene sediments are higher than those of the lake sediments, reflecting that there are differences of primary productivity, vegetation types, vegetation coverages and the degree of influence of environmental changes between the upstream and downstream. Changing trends of TOC, C/N ratio and813Corg of the two sections are also different. It is connected to differences of organic matter input quantity and preservation ability. There are differences of primary productivity, the complexity of the depositional environment and semimetal diagenesis. Since～12500cal yr BP, there are no significant correlations among TOC percentage contents, C/N ratios and δ13Corg values in the HYH section. In the QTH01lacustrine sediments, the correlations between TOC and C/N ratios are positive, but δ13Corg values are negatively correlated with TOC and C/N ratios. This reflects the microscopic differences of organic geochemical proxies in different natural sections.Environmental changes in Shiyang River Basin during the Holocene is divided into4phases:～12500-～11000cal yr BP, the hydrothermal condition of drainage basin was poor, which is not conducive to vegetation growth;～11000-～7500cal yr BP, the environmental conditions were improved compared with the last phase;～7500-～3400cal yr BP, the Holocene climatic optimum appeared in the Shiyang River Basin;～3400-0cal yr BP, environments of the Shiyang River Basin tended to be drought.The study lacks of in-depth exploration for the change mechanisms of organic geochemical proxies. Modern sediment research is needed in order to improve this study. It is the future direction of this study.