| 摘要: |
| 洞穴石笋因其精准的定年和蕴含的多种代用指标，已成为古气候和古环境变化研究的重要载体之一。在石笋研究的多种代用指标中，碳酸盐碳同位素由于受到多重因素的影响，应用其重建过去环境变化历史一直局限在少数几个研究地点。洞穴上覆土壤有机质是洞穴石笋中有机质的重要来源，除此之外，洞穴原位产生的微生物及基岩中的有机质也有贡献。由于主要继承了地表植被和土壤有机质的同位素组成，石笋有机质碳同位素组成的时间序列能够很好地指示地表植被的演变及土壤过程的变化。本文对石笋有机质碳同位素组成的研究现状进行了综述，介绍了最新建立的石笋有机质碳同位素组成分析方法，并指出应用这一指标研究环境变化需要解决的关键问题和未来面临的挑战。 |
| 关键词: 石笋 有机质 碳同位素组成 环境变化 |
| DOI：10.7515/JEE181007 |
| CSTR：32259.14.JEE181007 |
| 分类号: |
| 基金项目:国家重点研发计划（2017YFA0603401）；黄土与第四纪地质国家重点实验室重大研究项目；黄土与第四纪地质国家重点实验室开放基金(SKLLQG1723) |
| 英文基金项目:National Key Research and Development Program of China (2017YFA0603401); Key Project of State Key Laboratory of Loess and Quaternary Geology; Open Fund of State Key Laboratory of Loess and Quaternary Geology (SKLLQG1723) |
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| Progress and challenge of organic carbon isotope composition research in stalagmite |
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XUE Gang, CAI Yanjun, CHENG Peng, MA Le, CHENG Xing
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710054, China
4. Xi’an Accelerator Mass Spectrometry Center, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope Speleothem has been one of the most important geological archives for reconstructing the paleoclimate and paleoenvironment because of multiple advantages, such as, absolute ages, high-resolution proxies. Many speleothem oxygen isotope composition sequences have been reconstructed 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 understand the changes of climate and environment due to the multiple affecting factors like the types and density of vegetation, soil process and vadose zone process or other environment factors, thus it only be applied to reconstruct past climate and environment change in few and specific study sites. Inversely, organic carbon isotope composition of stalagmite has significant potential and advantages that straight reflects the information of organic matter from overlying karst system. Although the organic carbon research still exists some problems on the source and content of organic matter, analytical technique and interpretations of organic proxies, yet there were two organic carbon isotope composition sequences have been published in recent years and provided practical and useful analytical technique. In this study, we reviewed and introduced the source of stalagmite organic matter, the advantages of organic matter proxies, and the development of analytical technique according to the published research, and we also proposed the key problems and future challenges on organic matter proxies research in speleothem. Materials and methods In this study, our stalagmite sample was collected from Xiaosumang Cave, Qinghai. We ground a portion of our study sample being the homogenized powder. The sub-samples were firstly and completely digested by phosphoric acid solution to remove the inorganic carbon. Once the digestion was complete, the residual solution and oxidant (50 mL super clean water +2.0 g K2S2O8 + 100 μL H3PO4) were processed with vacuum. After that, the organic carbon was oxidized to CO2 by oxidant in high temperature (95℃). At last, the CO2 was purified in off-line vacuum system and subsequently measured by Isotope Ratio Mass Spectrometer. Results The stalagmite organic matter has specific advantages to reconstruct the paleoclimate and paleoenvironment, the study of its isotope composition is attracting attentions increasingly. There were two studies have displayed corresponding organic carbon isotope composition sequences and analytical techniques (Blyth et al, 2013b; Li et al, 2014), they proposed that the organic carbon isotope composition possibly affected by soil microbe, temperature and precipitation. Even so, there are some problems still need to be solved or developed: (1) clarify the source of organic matter, it possibly derived from epikarst soil layer, cave microorganism or vadose zone, and (2) the low amount of organic matter in stalagmite, the scientists had summarized that the organic carbon only is 0.01%—0.3% of total carbon (Blyth et al, 2016). (3) A comprehensive and ideal analytical technique need to be developed. In this study, we developed a new method which is modified from a useful analytical technique (Lang et al, 2012) to study the organic carbon isotope composition, the results show that the δ13Corg varies between −22.1‰ and −22.6‰, the standard deviation is 0.2‰. Discussion The published researches had shown two Holocene organic carbon isotope sequences which ascribe the variations of organic carbon isotope composition to microbial activities due to the respiration and degradation, changes in temperature and precipitation. To better understand the organic carbon isotope composition, it is crucial to clarify the source of organic matter in speleothem. By using multiple method, such as luminescence lamina character, monitor experiment and IR analyses, the organic matter is thought to be mainly derived from epikarst soil zone, however, by analyzing the biomarker, some studies proposed that the organic matter possibly comes from in-cave. Thus, clarifying the source of organic matter is still need to be further studied. Conclusions This paper reviewed the progress of research on stalagmite organic carbon isotope composition: discussing the possible source of organic matter in speleothem, summarizing the affecting factors and implications of organic carbon isotope composition. We also analyzed some important problems in this research area and provided some possible solutions for low abundance organic matter in speleothem and the source of organic matter. Furthermore, this paper also reviewed analytical techniques from different studies, and pointed out the challenges in the field of organic matter of speleothem. Most importantly, we provided a practical method and measured the organic carbon isotope composition of our study sample. Recommendations and perspectives The organic carbon isotope research in speleothem is an important work, to avoid or solve some problems, (1) we need to further create or improve analytical technique to reduce the sample size and obtain higher resolution organic carbon isotope proxy. (2) To distinguish the source of organic matter, we speculate that liquid chromatography-mass spectrometry (LC-MS) technique possibly can be used. (3) To better understand the implications of organic carbon isotope composition, we need to further enhance the monitoring work in karst system. |
| Key words: stalagmite organic matter carbon isotope composition environment change |
