甲烷团簇同位素研究进展及其在表层地球系统碳循环研究中的应用

doi:10.13745/j.esf.sf.2022.2.74

摘要/Abstract

摘要：

团簇同位素是当今地球化学研究的前沿领域。有关团簇同位素技术原理、指示意义、模型校正等方面研究已取得初步成果,而在表层地球系统研究领域以及地理、环境和生态等领域的综合应用研究工作亟待加强。在表层地球系统中,碳循环过程涉及广泛且影响深远,而甲烷(CH₄)作为碳循环中关键环节的重要物质组成,揭示其来源及地球化学过程具有重大意义。甲烷团簇同位素技术的引入将为开展相关研究提供有效工具,该技术在揭示时间-空间不同尺度碳循环研究等领域具有广阔的应用前景。本文综述了团簇同位素的理论基础、测试分析现状及国内外主要研究成果。重点讨论了CH₄团簇同位素研究现状、理论模型和同位素不平衡的影响因素,并论述了团簇同位素在天然气、大气和水体CH₄成因和转化的应用典型案例。最后提出对开展不同实验室间测试结果对比、测定自然丰度下环境样品、探索准平衡过程的展望,以期对未来应用甲烷及其他物质,如二氧化碳的团簇同位素开展表层地球系统碳循环研究起到一定的借鉴作用。

关键词: 团簇同位素, 甲烷, 二氧化碳, 表层地球系统, 碳循环

Abstract:

Clumped isotopes are a frontier field of geochemical research today. Basic researches on the technical principles of clumped isotope techniques, indicative value of clumped isotopes, clumped isotope calibration, etc. have yielded preliminary results; whereas applications of clumped isotopes are scarce in the study of Earth surface systems as well as in geography, environmental sciences and ecology. In Earth surface systems, carbon cycling has far-reaching implications, and for methane (CH₄), an important part of the natural carbon cycle, revealing its source and geochemical process is of great importance. The introduction of methane clumped isotope method can provide an effective tool as likely it can be used to reveal carbon cycling at different scales in time and space. This article summarizes the theoretical basis of clumped isotopes and the current test analysis results, with focuses on the research status and theoretical models of methane clumped isotopes as well as the factors influencing isotopic disequilibrium, and discusses typical applications of clumped isotopes in studying methanogenesis and CH₄ transformation in natural gas, atmosphere and water. Finally, test result comparisons between different laboratories, sample collection under natural isotopic abundances, and discussion on quasi-equilibrium process are put forward to provide a reference for future application of methane clumped isotopes and others, such as carbon dioxide clumped isotopes, in studying carbon cycling in Earth surface systems.

Key words: clumped isotopes, methane, carbon dioxide, Earth surface systems, carbon cycle

中图分类号:

P593
X142

王欣楚, 刘丛强, 李思亮, 徐胜, 丁虎, 庞智勇, 帅燕华. 甲烷团簇同位素研究进展及其在表层地球系统碳循环研究中的应用[J]. 地学前缘, 2023, 30(2): 463-478.

WANG Xinchu, LIU Congqiang, LI Siliang, XU Sheng, DING Hu, PANG Zhiyong, SHUAI Yanhua. Methane clumped isotopes: Research progress and application in carbon cycling in Earth surface systems[J]. Earth Science Frontiers, 2023, 30(2): 463-478.

图/表 8

表1 CH4和CO2气体随机分布状态下同位素分子丰度

Table 1 Abundances and masses of CH4 and CO2 isotopologues in random states

气体	质量/amu	准确质量/amu	同位素分子	相对丰度	前提	来源文献
CH₄	16	16.031	¹²CH₄	98.80%	假设甲烷δ¹³C=0‰, δD=0‰	[40]
	17	17.035	¹³CH₄	1.11×10^-2
		17.038	¹²CH₃D	6.16×10^-4
	18	18.041	¹³CH₃D	6.92×10^-6
		18.044	¹²CH₂D₂	1.44×10^-7
	19	19.047	¹³CH₂D₂	1.62×10^-9
		19.050	¹²CHD₃	1.49×10^-11
	20	20.053	¹³CHD₃	4.68×10^-13
		20.056	¹²CD₄	5.82×10^-16
	21	21.060	¹³CD₄	6.54×10^-18
气体	质量/amu	准确质量/amu	同位素分子	相对丰度	前提	来源文献
CO₂	44	44.001	¹²C¹⁶O₂	98.4%	假设¹⁷O/¹⁶O和¹⁸O/¹⁶O 比值等于VSMOW标准, ¹³C/¹²C比值等于PDB标准	[35]
	45	44.993	¹³C¹⁶O₂	1.11×10^-2
		45.005	¹²C¹⁷O¹⁶O	7.48×10^-6
	46	46.005	¹²C¹⁸O¹⁶O	4.00×10^-3
		45.997	¹³C¹⁷O¹⁶O	8.4×10^-6
		46.009	¹²C¹⁷O₂	1.42×10^-7
	47	46.997	¹³C¹⁸O¹⁶O	44.4×10^-6
		47.009	¹²C¹⁷O¹⁸O	1.50×10^-6
		47.001	¹³C¹⁷O₂	1.60×10^-9
	48	48.009	¹²C¹⁸O₂	3.96×10^-6
		48.001	¹³C¹⁷O¹⁸O	1.68×10^-9
	49	49.001	¹³C¹⁸O₂	4.45×10^-9

图1 H2同位素交换反应能量差异及Δ变化(据文献[30,35]修改) a—反应平衡常数Keq和T的关系;b—D2分布和D分布的关系。

Fig.1 Hydrogen exchange reaction energy difference and Δi changes. (a) Keq-T plot showing temperature dependence of equilibrium constants for the homogeneous isotope exchange reactions. (b) Stochastic reference frame for the stable isotopologues of molecular hydrogen. Modified after [30,35].

图2 Δ18代表温度与环境/形成温度的比较(据文献[47]修改)

Fig.2 (a) Relationship between Δ18 and temperature and (b) prediction of methane source based on temperature correlation. Modified after [47].

表2 不同实验室开展甲烷团簇同位素测试的实验精度(±1 SE)对比(据文献[23,40,51,55-56,82-83])

Table 2 Comparison of experimental Δ13CH3D and Δ12CH2D2 values obtained by different laboratories for methane clumped isotopes. Adapted from [23,40,51,55-56,82-83].

作者	机构	时间	仪器	Δ¹³CH₃D精度 (± 1 SE)	Δ¹²CH₂D₂精度 (± 1 SE)
Stolper等^[40]	加州理工学院	2014	253 Ultra原型机	±0.23‰
Wang等^[23]	麻省理工学院	2015	可调谐激光光谱仪	±0.42‰
Shuai等^[51]	加州理工学院	2018	253 Ultra原型机	±0.23‰
Eldridge等^[82]	伯克利大学	2019	253 Ultra	±0.33‰	±1.35‰
Giunta等^[55]	加州大学洛杉矶分校	2019	Nu Panorama	±0.30‰	±1.00‰
Zhang等^[56]	东京工业大学	2021	253 Ultra	±0.31‰	±1.24‰
庞智勇^[83]	天津大学	2021	253 Ultra	±0.27‰	±1.35‰

图3 自然样品中甲烷δD、δ13C和Δ18的三维散点图(据文献[23,47,51,55,84]) 平衡分馏(右线)和动力学同位素分馏(左线)的总体趋势在图中均标出。

Fig.3 Three-dimensional scatter plots of methane δD, δ13C and Δ18 in natural samples. Adapted from [23,47,51,55,84].

图4 甲烷样品中Δ13CH3D和Δ12CH2D2的关系(据文献[55-56,58,89⇓⇓⇓⇓⇓⇓-96])

Fig.4 Relationship between Δ13CH3D and Δ12CH2D2 in methane samples. Adapted from [55-56,58,89⇓⇓⇓⇓⇓⇓-96].

图5 组合效应示意图(据文献[88-89])

Fig.5 Combination effect during methane formation. Adapted from [88-89].

图6 不平衡模型(DM)对空气中Δ13CH3D和Δ12CH2D2源估计(据文献[97]) 图中每个圆的面积与它所代表的通量成比例地缩放。

Fig.6 Atmospheric methane source estimation according to disequilibrium model (DM). Adapted from [97].

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