Abstract: Background, aim, and scope Speleothem has been one of the most important geological archives for reconstructing the paleoclimate and paleoenviroment because of multiple advantages, such as, absolute age, high resolution. Many speoleothem oxygen isotope composition sequences has been constructed by researchers around the world, they provided abundant scientific data for exploring local and global climate and environment change. However, the carbon isotope composition of carbonate has uncertainties to explain the climate and environment, it only was applied to reconstruct past climate and environment change in few and specific positions. Because there are many factors can affects the value of carbon isotope including the types and density vegetation, soil process and vadose zone process or other environment factors. Inversely, Organic carbon isotope composition of stalagmite has significant potential and advantages that straight reflects the information of organic matter. Although this research still exists some problems including the source and content of organic matter, analytical method, explanations, interestingly, there were two organic carbon isotope composition sequences has been published in recent years and also provided practical and useful analysis methods. In this paper, we reviewed and introduced the source of stalagmite organic matter, the advantages of organic matter and analytical method, we also proposed that the key problems which needs to be solved and future challenges, if we use the organic carbon isotope composition of stalagmite to study the environment change. Materials and methods In this paper, our stalagmite sample was collected in Xiaosumang Cave, Qinghai. We ground this stalagmite sample being the homogenized powder. The sub-samples were firstly and fully digested by phosphoric acid solution to remove the inorganic carbon. Once the digestion was finished, the residual solution reacted with oxidant (50ml super clean water+2.0g K2S2O8+100μL H3PO4). After that, The organic matter was oxidized to CO2 by oxidant in high temperature (95℃). At last, the CO2 was purified in off-line vacuum system and measured by Isotope Ratio Mass Spectrometer. Results The stalagmite organic matter has specific advantages to reconstruct the paleoclimate and paleoenvironment, and the organic carbon isotope composition attracts attentions increasingly, there were two publications had displayed corresponding organic carbon isotope sequence and analytical method, the researchers also proposed that the organic carbon isotope composition possibly affected by soil microbe, temperature and precipitation. Even so, there are some problems still need to solve. (1) The source of organic matter, it possibly derived from epikarst soil layer, cave microorganism or vadose zone, and (2) the amount of organic matter is low in stalagmite, the scientist had summarized that the organic carbon only is 0.01-0.3% of total carbon (Blyth et al., 2016). (3) A comprehensive and ideal analysis method. Furthermore, we also developed a new method to study the organic carbon isotope composition, the experimental result show that the δ13Corg from -22.1 to -22.6‰, the standard deviation is 0.2‰. Discussion The published analytical method had shown two Holocene organic carbon isotope sequences, which explained that microbe effects can affect the organic carbon isotope composition because of respiration or degradation. In addition to this, the temperature and precipitation also has effects on organic carbon isotope composition. One hand, if climate condition is dry, it will extend the residence time of organic matter in soil increasing the microbial degradation on organic matter. In another hand, if climate condition is warm, the microbial activities will be strong resulting in enhanced degradation. Moreover, to better explain the organic carbon isotope composition, make the source of organic matter clear is crucial. By using multiple method, such as luminescence lamina character, monitor experiment, IR analyses, and researchers proposed that the organic matter mainly derived from epikarst soil zone, however, by analyzing the biomarker, some researchers proposed that organic possibly comes from in-cave. Thus, make sure the source of organic matter is very vital. Conclusion This paper reviewed the progress on stalagmite organic carbon isotope composition: talking about few possibilities of source of organic matter, summarizing the effecting factors of organic carbon isotope composition. We also discussed some important problems in this research and provided some possible solving methods about low abundance organic matter, the source of organic matter, the explanation of organic carbon isotope composition. Furthermore, this paper also summarized analysis methods of different researchers on different stalagmites, and pointed out the challenges in the future and the direction of possible breakthroughs. Most importantly, we measured the organic carbon isotope composition and provided a new method for scientists. Recommendations and perspectives The organic carbon isotope research is an important work, to avoid or solve some problems, (1) we need to further design or improve analysis method reducing the sample size or obtaining higher resolution organic carbon isotope proxy. (2) To distinguish the source of organic matter, we speculate that liquid chromatography - mass spectrometry (LC-MS) technique or other methods possibly can be work.(3) To better understand the meaning of organic carbon isotope composition, we need to enhance studying about the whole karst system (from epikarst to cave).
Keywords: stalagmite; organic matter; carbon isotope composition; environment change