铀同位素反演古海洋环境的研究进展

罗劬侃; 曹建华; 钟亮; 白冰; 王奇岗; 廖红为; 宗克清; 覃汉莲

doi:10.11932/karst20240412

铀同位素反演古海洋环境的研究进展

doi: 10.11932/karst20240412

罗劬侃^{1, 2, 3,},
曹建华^{2, 3},
钟亮^{2, 3},
白冰^{2, 3},
王奇岗^{2, 3, 4},
廖红为^{2, 3, 5},
宗克清¹,
覃汉莲^{2, 3,}

1.
中国地质大学地球科学学院, 湖北武汉 430078
2.
中国地质科学院岩溶地质研究所/自然资源部、广西岩溶动力学重点实验室/联合国教科文组织国际岩溶研究中心, 广西桂林 541004
3.
广西平果喀斯特生态系统国家野外科学观测研究站, 广西平果 531406
4.
中国地质大学环境学院, 湖北武汉 430078
5.
中国地质大学地质探测与评估教育部重点实验室, 湖北武汉 430078

基金项目: 广西自然科学基金项目“238U/235U对泥盆纪-石炭纪古海洋环境转型的指示意义——以桂林南边村全球辅助层型剖面为例”（2022GXNSFBA035593）

详细信息

作者简介:
罗劬侃（1982－），女，副研究员，硕士，主要从事古环境重建方面的研究。E-mail：luoqukan11@163.com

中图分类号: P736
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- 文章访问数: 27
- HTML浏览量: 6
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- 被引次数: 0
出版历程
- 收稿日期: 2023-06-25
- 录用日期: 2024-04-15
- 修回日期: 2024-02-17
- 刊出日期: 2024-10-31

Research advance for uranium isotope as a quantitative proxy for paleo-oceans anoxic or oxic environment

LUO Qukan^{1, 2, 3
,},
CAO Jianhua^{2, 3},
ZHONG Liang^{2, 3},
BAI Bing^{2, 3},
WANG Qigang^{2, 3, 4},
LIAO Hongwei^{2, 3, 5},
ZONG Keqing¹,
QIN Hanlian^{2, 3
,}

1.
School of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430078, China
2.
Institute of Karst Geology, CAGS/Key Laboratory of Karst Dynamics, MNR & GZAR/International Research Center on Karst under the Auspices of UNESCO, Guilin, Guangxi 541004, China
3.
Pingguo Guangxi, Karst Ecosystem, National Observation and Research Station, Pingguo, Guangxi 531406, China
4.
School of Environmental Studies, China University of Geosciences, Wuhan, Hubei 430078, China
5.
Key Laboratory of Geological Survey and Evaluation of Ministry of Education, China University of Geosciences, Wuhan, Hubei 430078, China

摘要

摘要: 铀同位素因可定量反演全球尺度古海洋缺氧洋底分布面积占比(%)而被广泛应用在埃迪卡拉纪末期以来的重要大洋缺氧或生物事件中。通过对国内外相关文献进行综述，系统总结了利用铀同位素开展定量反演的原理、方法与成果，初步构建了铀同位素定量反演的还原性海洋洋底面积占比（%）与大气氧气浓度、大洋缺氧或生物事件的耦合关系，发现：（1）铀同位素反演结果与各缺氧或生物事件吻合度较高，表明铀同位素确实为有效的全球尺度深时尺度定量反演指标；（2）还原性海洋洋底扩张与大气氧气浓度变化之间普遍存在滞后性，推测与海平面、海洋生产力、海洋内部环流变化及底层水氧化还原反应的滞后性相关。指出铀同位素反演受样品后期成岩、风化蚀变作用的影响，可能存在解译误差；铀同位素单指标解译结果存在精准度偏低的缺点，需采用多指标综合反演的方法提升反演精度。
- 铀同位素 /
- 定量反演 /
- 铀同位素模型 /
- 大洋缺氧 /
- 生物事件
Abstract: Compared with other elements that are sensitive to redox environments (U, Mo, V, S, Fe, Cu, Zn, Ni), uranium isotopes have the advantage to quantitatively reflect long-term and global-scale paleoredox of oceans, due to its uniform composition in the oceans globally, and quantitative relationship with the proportion of the anoxic seafloor areas in ancient oceans. Therefore, uranium isotopes are widely used in the events such as the Late Ediacaran, Early Cambrian, Ordovician/Silurian, Late Devonian, Devonian/Carboniferous, Permian/Triassic, Triassic/Jurassic, and OAE 2 (Oceanic Anoxic Event 2). Through a review of previous papers, the author has systematically summarized the principles, methods, and results of using uranium isotopes for quantitative analysis of paleoredox. It has found that the core of current uranium isotopes quantitative method is the mass balance box model of seawater uranium (mass balance box model) and its isotopes and the related variants. By simplifying the uranium mass balance box model based on the proportion and fractionation coefficient of different types of uranium sinks, the redox environment and duration of ancient oceans can be quantitatively reflected. However, there may be ambiguity in the interpretation regarding whether the marine environment contains sulfur or not. In addition, by reviewing previous papers, the author has set up a preliminary coupling relationship among the proportion of anoxic seafloor area (%) reflected from uranium isotopes, atmospheric oxygen concentration, and oceanic anoxic or biological events. It has found that: (1) the uranium isotope analysis results were highly consistent with the occurrence time and extent of various anoxic or biological events, especially being coincident with the events that occurred in the Late Ediacaran, Late Ordovician, Late Devonian Frasnian-Famennian, Devonian/Carboniferous, Permian/Triassic, and Triassic/Jurassic periods, indicating that uranium isotopes are indeed an effective global scale and deep-time scale proxy for quantitative analysis on paleoredox of oceans. (2) The trend of anoxic seafloor expansion is not completely consistent with the trend of the atmospheric oxygen concentration changes. Usually, there is a lag. Only the trend of atmospheric oxygen concentration changes in the Triassic/Jurassic period is similar to the trend of oceanic redox environment changes. Analysis suggests that uranium isotope analysis results indicate the proportion of anoxic seafloor area, that is, the proportion of anoxic bottom seawater area, which is actually the change of redox environment of oceanic bottom water. At first, the change in atmospheric oxygen concentration affects shallow seawater, which first responds to the changes in the atmospheric redox environment, and then gradually affects the biological productivity of the seawater, gradually transmitting the environmental changes caused by redox evolution to the bottom water, and thus leading to a lag reaction of anoxic seafloor's expansion or shrink; or due to climatic changes, sea level rise and fall can cause changes of ocean internal circulation (such as thermohaline ocean circulation), leading to changes in productivity and causing anoxic seafloor area expansion or shrink, which may also result in hysteresis effects. There should be a kind of coupling relationship between the atmospheric redox evolution and ocean redox evolution, it deserves to be paid more attention for further study in future. In addition, through a review and analysis of previous research results, the author believes that there are several problems with the quantitative analysis of global scale paleoredox environments of oceans by using uranium isotopes, such as low accuracy, ambiguity, and the representativeness of samples. It is suggested to improve this method from the following two aspects further: (1) although uranium isotopes have been widely used in many important biological events or oceanic anoxic events since the Ediacaran period, the representativeness of sampling sites for some research results still needs to be improved. Although uranium isotopes are assumed to have global homogeneity theoretically, this assertion has not fully considered the influence of post-diagenesis, weathering and alteration in each sampling site on the uranium isotope composition that reflects the original seawater features. It is necessary to calibrate and verify the analysis results of different samples from different sampling sites of the same period to improve the quantitative analysis accuracy further. (2) The single proxy analysis results of uranium isotopes provides a wide range of anoxic seafloor area proportion, showing insufficient accuracy. It is necessary to carry out comprehensive multi-proxy isotopic quantitative analysis, by adding the results of isotopes like Mo, Tl, and others, which is believed could effectively improve the analysis accuracy for the same event. Finally, the author believes that if the accuracy of uranium isotopes method for quantitative analysis could be improved by comprehensive analysis with combination of other isotopes analysis, it is necessary to find out the coupling relationship between atmospheric redox evolution and marine redox evolution, especially the time effects of marine redox evolution to the atmospheric evolution. Hence, it is also possible to find out the response mechanism of oceanic redox conditions to the atmospheric redox evolution. The coupling relationship and the response mechanism may help the related researchers to predict the impact range and extent of oceanic redox evolution after a certain periods of lagging to the atmospheric redox evolution that is caused by natural and human factors. The results may provide great support for addressing global climate change.
- uranium isotope /
- quantitative reflection /
- uranium isotope model /
- oceanic anoxic events /
- biological events

HTML

图 1 海洋中铀同位素平衡模型（海洋中主要的溶解铀来源于陆地河流，海洋铀会包括还原性/缺氧沉积物、局限性滞海洋盆沉积、海相碳酸盐、铁锰结核及蚀变大洋洋壳）（据[59]）

Figure 1. U isotope budget model in the oceans (The main sources of dissolved uranium in the ocean are terrestrial rivers, and marine uranium can include reduced/anoxic sediments, localized stagnant ocean basin deposits, marine carbonates, ferromangan-manganese nodules, and altered macrocrust) (According to [59])

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图 2 利用Lau铀汇箱式模型（A图：δ²³⁸U和B图：[U]值）计算出晚二叠纪-中晚三叠纪古海洋还原性面积占比f_anox（据文献[37]）

注：虚线为平均值，延长线上、下端为最大和最小值

Figure 2. Proportion of anoxic seafloor area (f_anox, %) of Late Permian–Late-Middle Triassic by using Uranium Box Model (Figure A: δ²³⁸U and Figure B: [U] value) (According to literature[37])

Note: The dotted line is the average value, and the upper and lower ends of the extension line are the maximum and minimum values.

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图 3 中元古代海洋三类铀汇质量守恒模型测算 (据文献[49]）

橙线代表各种可能的模型迭代下的最小洋底面积，蓝色代表最大洋底面积，红色块状部分为中元古代海水δ²³⁸U_SW（–0.43%至–0.73‰），图中虚线交汇处，代表含S的还原性静海洋底面积占比最大为7%。

Figure 3. Estimation of three-sink mass balance modeling on Mid-Proterozoic seawater (According to literature [49])

The orange line represents the minimum seafloor area under various possible model iterations; the blue line represents the maximum seafloor area, and the red block represents the δ²³⁸U_SW (–0.43% to –0.73‰) of the Mesoproterozoic seawater. The intersection of dotted lines in the figure represents the maximum 7% of the seafloor area of the anoxic quiet sea containing S.

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图 4 显生宙主要大洋缺氧事件（或生物大灭绝/生命大爆发事件）发生时期大气氧气浓度与还原性洋底面积占比变化耦合关系图（据[68]修改）

注：深蓝色实线为GEOCARBSULFOR生物地球化学模型模拟出的大气氧气含量变化范围^[77]；深蓝色虚线为COPSE生物地球化学模型模拟出的大气氧气含量变化范围^[78]；灰色不规则区域代表通过化石木炭重建的大气氧气含量变化曲线^[79]。灰色线条代表通过地球化学指标、寒武纪生物群和大火燃烧记录重建的大气氧气浓度阈值^[80−84]。浅蓝色方框代表对应年代还原性海洋洋底面积占比变化范围。

Figure 4. Coupling relationship between atmospheric oxygenation concentration, anoxic seafloor area (f_anox, %) during the oceanic anoxic events (or mass extinction/life explosion) (Modified based on [68])

Note: The deep blue solid line represents the range of atmospheric oxygen content simulated by the GEOCARBSULFOR biogeochemical model ^[77]; the deep blue dashed line represents the range of atmospheric oxygen content simulated by the COPSE biogeochemical model^[78]; the gray irregular region represents the curve of atmospheric oxygen content change reconstructed from fossil charcoal^[79]. The gray lines represent the threshold values of atmospheric oxygen concentration reconstructed from geochemical indicators, Cambrian biota, and fire burning records^[80−84]. The light blue box represents the range of the proportion of anoxic seafloor area for the corresponding period.

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