中国科学院海洋研究所机构知识库

氮（N）是生命活动不可或缺的元素，氮气（N₂）是现代地球大气的主要成分，岩石中的氮则是地球内部物质循环的重要示踪元素。大气和地幔是地球最重要的氮储库，但是氮在大气和地幔之间的交换非常缓慢，原因是大气N₂性质稳定，难以被生物直接利用，也难以直接被固定在岩石中参与物质循环。大气N₂必须通过固氮过程被转化为其他形式的氮（如NH₃或NH₄⁺等），才能进入地球其他圈层。现代地球的固氮由生物活动主导，而太古代之前的早期地球缺乏生物活动，固氮只能由非生物过程主导。然而，已知的非生物固氮方式不能满足早期地球上生命起源的氮需求，无法主导早期大气中N₂分压的重大改变，也难以对大气−地幔氮同位素组成不平衡的现象作出解释。蛇纹石化作为一种广泛发生在早期地球表层的水岩反应，其潜在的非生物固氮作用并未受到前人重视。本论文旨在利用高温高压水热实验证明蛇纹石化可以将大气N₂还原为NH₃，并探讨蛇纹石化固氮对于早期地球氮循环的影响，以期推动解决氮循环中的重大科学问题。

蛇纹石化（serpentinization）普遍发生在含铁矿物/岩石和水之间，其最重要的特征是产氢（H₂），原理是矿物中的Fe²⁺被氧化为Fe³⁺，导致H₂O中的氢被还原为H₂。而工业合成氨反应则表明N₂可以和H₂在高温高压和催化剂的作用下迅速反应产生NH₃。蛇纹石化合成氨反应具有理论可行性，但尚无实验证据证明。本研究选用早期地球表层最常见的岩石——橄榄岩、H₂O和N₂作为初始物，使用自研的钛合金高温高压水热装置，进行“橄榄岩−H₂O−N₂”体系的水热实验。为确保上述实验能够顺利开展，本人及所在的团队自主设计了新型高温高压设备，将传统的软密封不锈钢水热釜升级为硬密封钛合金水热釜，有效避免釜体本身与水反应产生H₂，进而干扰实验结果。本研究利用包括X射线晶体衍射、热重分析和拉曼光谱在内的多种分析手段对固体反应产物进行了详尽分析并建立了相应的标准曲线，准确地测定了产物的蛇纹石化程度。通过对反应产物的高精度色谱分析，在气体和液体产物中分别检测到了H₂和NH₄⁺，证明了N₂在蛇纹石化中被转化为了NH₃，进而对控制蛇纹石化产物的矿物种类和影响蛇纹石化产H₂和产NH₃速率的影响因素进行了定性和定量分析。在初始物中额外加入CO₂，发现CO₂可以提高NH₃的产率。

在上述高温高压实验的基础上，本论文进一步探讨了蛇纹石化固氮过程对于早期地球氮循环的影响，主要包括生命起源、大气演化和深部氮循环三个方面。

（1）生命起源：1953年，Miller将由甲烷（CH₄）、NH₃、H₂和H₂O组成的还原性大气进行放电实验以模拟早期地球表层闪电过程，成功合成了丰富的氨基酸。由于Miller-Urey反应首次实现了自然条件下无机物向生命必需有机物的转变，因此被认为是最重要的前生命反应之一。然而，地质证据表明地球早期大气的主要成分是CO₂和N₂，并非Miller所设想的还原性大气。在CO₂和N₂气氛中，Miller-Urey反应合成氨基酸的效率大幅下降，生命起源受阻。解决这一难题的关键是，为前生命反应寻找充足的氮源。蛇纹石化在早期地球表层广泛发生，是早期地球表层最重要的固氮过程。蛇纹石化的固氮效率高出其他非生物固氮途径至少3个数量级，可以为前生命反应提供充足的NH₃，促进生命起源。基于蛇纹石化实验，本研究进一步对早期地球表层氨基酸的合成和火星表面芳香族化合物的成因提出了新的模型。

（2）大气演化：N₂是大气最主要的组成部分，大气中N₂分压直接影响地球表层环境的宜居性。地质证据表明，大气中N₂分压变化大致经历了三个阶段：冥古代时期（pN₂>2.6 bar）、太古代时期（pN₂≈0.5 bar）和显生宙时期（pN₂=0.8 bar）。但是，传统固氮过程难以解释早期大气N₂分压大幅改变的原因。本研究基于蛇纹石化实验，对早期地球大气成分演变进行了模拟计算，证明蛇纹石化是导致冥古代大气N₂丢失的主要原因。此外，本研究合理推测地球表层蛇纹石化强度的减弱是元古代大气N₂分压回升的主要原因，并为此设计了水热实验，开展了模拟研究。

（3）深部氮循环：大气（δ¹⁵N=0‰）和地幔（δ¹⁵N=−5 ± 2‰）在氮同位素上存在显著的不平衡。由于自太古代以来的沉积物和蚀变洋壳普遍具有显著的富集重氮同位素的特征（δ¹⁵N=−5‰至+50‰），现有的板片俯冲再循环模式并不能解释地幔整体亏损重氮同位素的原因。传统观点认为原始地幔更加亏损重氮同位素，主要由还原的顽火辉石球粒陨石（δ¹⁵N=−45‰至−15‰）增生而来，但这一观点与其他元素和同位素证据相悖，且无法解释太古代地幔和现代地幔氮同位素组成基本不变的事实。本研究通过实验产物的氮同位素分析指出，蛇纹石化的产物——蛇纹石具有较高的N含量和亏损重氮同位素的特征。蛇纹石可能正是长期寻找的“¹⁵N亏损端元”，对于深部氮循环具有重要指示意义。

综上，本研究以蛇纹石化固氮的水热实验为核心，证明了蛇纹石化固氮反应，探讨了该反应对早期地球氮循环的影响，主要包括为前生命反应提供氨源、改造早期大气和改变地幔氮同位素特征等。蛇纹石化固氮是早期地球表层的重要过程，将在未来的地球氮循环研究中发挥更大的作用。

Nitrogen (N) is an indispensable element for life, and dinitrogen (N₂) is the main component of the modern atmosphere. Also, N in rocks is an important tracer of the material cycle insides Earth. The atmosphere and mantle are two of Earth’s most important N budgets. However, the N exchange between the two budgets is too slow because atmospheric N₂ is inert. The atmospheric N₂ is useless for the creatures and cannot be fixed in rocks directly to participate in the deep N cycle. Nitrogen must be fixed as other forms (such as NH₃ and NH₄⁺), which is N fixation, to be available to other Earth’s spheres. Biological activities dominate nitrogen fixation on the modern Earth. In contrast, early Earth (Hadean and Archean) lacked creatures, and only abiotic processes could achieve N fixation. However, the known abiotic N fixation pathways cannot meet the N needs in the origin of life on early Earth, nor can they explain the significant changes of atmospheric N₂ partial pressure in Archean and N isotopic disequilibrium between the atmosphere and mantle. Serpentinization is a common water-rock reaction that widely occurred on the surface of early Earth, and previous researchers have neglected the potential of abiotic N fixation in serpentinization. This paper aims to demonstrate that serpentinization can reduce atmospheric N₂ to NH₃ by using hydrothermal experiments. Also, this paper explores the impact of N fixation in serpentinization on the N cycle of the early Earth and helps better understand big science questions.

      Serpentinization commonly occurs between Fe-containing minerals/rocks and water. The key to serpentinization is hydrogen production (H₂). Ferrous iron (Fe²⁺) in the mineral is oxidized to ferric iron (Fe³⁺), and the hydrogen in H₂O is reduced to H₂. Also, N₂ can quickly react with H₂ to produce NH₃ under high P-T conditions and the catalysts, called industrial NH₃ synthesis. Thus, NH₃ synthesis in serpentinization is theoretically available; however, no experimental evidence has proved this assumption. This study chose peridotite, which is the most common rock on the surface of early Earth, H₂O and N₂ as initial materials for hydrothermal experiments and used a new kind of hydrothermal vessel for the experiments of “peridotite−H₂O−N₂” system. To make the experiments successful, our team independently advanced hydrothermal instruments. We upgraded the traditional soft-seal stainless steel vessel to a novel hard-seal Ti-alloy vessel and thus eliminated the interference from H₂ production in the reaction of the instrument itself and water. With high-resolution spectrum analysis of the products, H₂ and NH₄⁺ are detected in the gases and fluids, respectively, and thus demonstrate the NH₃ synthesis in serpentinization. Besides, we added CO₂ as an additional material and found that CO₂ could increase NH₃ production. Furthermore, the solid products were analyzed in detail using many methods, including X-ray crystal diffraction, thermogravimetric analysis, and Raman spectrum, and the corresponding standard curves were found to calculate the serpentinization extent of the products. Finally, this study analyzed the factors qualitatively and quantitatively that influence the mineral species of products, serpentinization rates, H₂ production rates, and NH₃ production rates.

Based on the above hydrothermal experiments, this paper further explores the impact of the N fixation in serpentinization on the N cycle of early Earth, including the origin of life, atmospheric evolution, and deep N cycle.

(1) Origin of life: In 1953, Miller conducted discharge experiments on a reducing atmosphere composed of methane (CH₄), NH₃, H₂, and H₂O to simulate surficial lightning on early Earth and succeed in synthesizing abundant amino acids. The Miller-Urey reaction first changed inorganics into organics necessary for life, which is considered one of the most critical prebiotic reactions. However, geological evidence after the 1970s showed that the main components of the Earth’s early atmosphere were CO₂ and N₂ and did not support a reducing atmosphere on early Earth like Miller had assumed. In neutral atmospheres filled with CO₂ and N₂, synthesizing amino acids in the Miller-Urey reaction's efficiency drops significantly, and life's origin is hindered. Finding sufficient N sources for prebiotic reactions is one key to solving this problem. Serpentinization occurred extensively on the early surface and was the most crucial N fixation process on the early surface. The efficiency of N fixation in serpentinization is at least three orders of magnitude higher than other abiotic N fixation pathways, and serpentinization can provide sufficient NH₃ for prebiotic reactions, thereby promoting the origin of life. Based on serpentinization experiments, this study proposes a new model for synthesizing amino acids on the surface of early Earth and the formation of aromatic compounds on the Martian surface.

(2) Atmospheric evolution: N₂ is the main component of the atmosphere, and its partial pressure directly affects the habitability of the Earth’s surface environment. Geological evidence shows that the change of N₂ partial pressure in the atmosphere has gone through three main stages: the Hadean period (pN₂>2.6 bar), the Archean period (pN₂≈0.5 bar), and the Phanerozoic period (pN₂=0.8 bar). However, traditional N fixation cannot explain the dramatic changes in atmospheric pN₂. Based on the serpentinization experiment, this study simulated the evolution of the early Earth’s atmospheric composition and demonstrated that serpentinization was the main reason for the loss of atmospheric N₂ in the Hadean. In addition, this study reasonably predicts that the weakening of surface serpentinization intensity is the main reason for the rising atmospheric N₂ partial pressure in the Proterozoic and designs a hydrothermal experiment to carry out the simulation.

(3) Deep N cycle: There is a significant disequilibrium in the N isotopic composition of the atmosphere (δ¹⁵N=0‰) and mantle (δ¹⁵N=−5 ± 2‰). The sediments and altered oceanic crust since Archean have significantly enriched heavy N isotopic signatures (δ¹⁵N=−5‰ to +50‰), the typical model of slab subduction and recycling cannot explain why the whole mantle is depleted in heavy N isotope. The traditional view is that the primitive mantle was much more depleted in heavy N isotopes and mainly accreted from reduced enstatite chondrites (δ¹⁵N=−45‰ to −15‰). However, this view opposes other elemental and isotopic evidence and cannot explain the truth that the Archean mantle has the same isotopic composition as modern. This study pointed out that the serpentinization product has a high N content and depleted δ¹⁵N characteristic based on the N isotopic analysis of experimental products. Serpentine may be the isotopic δ¹⁵N-depleted end member, which has been missed and has important significance for the deep N cycle.

In summary, this study focuses on the hydrothermal experiment of N fixation in serpentinization and explores its impact on the N cycle of early Earth, which mainly includes providing NH₃ sources for prebiotic reactions, altering the early atmospheric pN₂ and changing the mantle’s N isotopic composition. Nitrogen fixation in serpentinization was a vital process on the early surface and will play a more significant role in the research of the Earth’s N cycle.

图目录

图 1‑1  地球主要储库中氮储量和氮同位素特征... 1

图 1‑2  经典生物地球化学氮循环... 2

图 1‑3  冥古代和始太古代的大事件时间轴... 4

图 1‑4  蛇纹石化产氢及其副产物示意图... 6

图 1‑5  Miller-Urey实验所使用的仪器... 9

图 1‑6  前生命反应发展历史... 10

图 1‑7  地质历史时期大气pN₂变化... 12

图 1‑8  深部氮循环示意图... 13

图 1‑9  常见矿物离子的离子半径比较... 14

图 1‑10  蛇纹石化中氮分配与分馏的可能机制... 15

图 1‑11  博士论文基本架构和思维导图... 16

图 2‑1  蛇纹石化水热装置设计图... 21

图 2‑2  传统不锈钢水热釜和新型钛合金水热釜对比... 23

图 2‑3  钛合金高温高压水热装置（南方科技大学，交叉学院）... 24

图 2‑4  钛合金高温高压水热装置（中科院海洋所，深海中心）... 24

图 3‑1  蛇纹石化合成氨反应示意图... 31

图 3‑2  蛇纹石化实验产物的XRD特征峰（a）和TGA结果（b）... 33

图 3‑3  天然蛇纹岩的FTIR谱图... 33

图 3‑4  实验初始物和产物的拉曼光谱（a，b），产物的穆斯堡尔谱以及铁分布（c）和电子显微镜照片（d，e）... 34

图 3‑5  含水镁石的天然蛇纹岩（a）和蛇纹石−橄榄岩混合样品（b）的TGA结果... 35

图 3‑6  蛇纹石-橄榄石和蛇纹石-橄榄岩混合样品的TGA标准曲线... 36

图 3‑7  橄榄石（a）、蛇纹石（b）和蛇纹石−橄榄岩混合样品（c）的XRD结果... 36

图 3‑8  蛇纹石-橄榄岩混合样品的XRD标准曲线... 37

图 3‑9  橄榄石（a）、实验产物（b）和蛇纹石−橄榄岩混合样品面扫描（c）的拉曼光谱... 38

图 3‑10  蛇纹石-橄榄石（a）和蛇纹石-橄榄岩（b）混合样品的拉曼光谱标准曲线... 38

图 3‑11  实验产物的蛇纹石化程度（a）和其中各矿物的质量百分数（b）。... 39

图 3‑12  MgO-SiO₂-H₂O体系的热力学三角图... 40

图 3‑13  各影响因素对蛇纹石化速率（Log J）的影响，包括（A）温度（B）压力（C）溶液盐度（D）水岩比（E）岩石颗粒粒径（F）反应时长... 41

图 3‑14  气相色谱和离子色谱结果及其相应的标准曲线... 42

图 3‑15  蛇纹石化中的产氢（a）与产氨（b）... 43

图 4‑1  Miller-Urey反应示意图... 47

图 4‑2  前生命反应的理论框架... 48

图 4‑3  海底热液系统示意图... 49

图 4‑4  冥古代地球表层氨基酸合成示意图... 51

图 4‑5  已登录火星表面的探测器（a）和水岩反应证据（b）... 53

图 4‑6  火星表面地形和地下生态系统模型... 55

图 5‑1  地质历史时期大气pN₂变化... 58

图 5‑2  早期大气中CO₂和N₂分压的估算... 60

图 5‑3  冥古代地球大气可能的演化模型... 62

图 5‑4  太古代至今的全球火山岩Ni含量变化（a，b）、科马提岩喷发比例（c）以及大气pN₂变化（d）（改自Liu et al. (2021)）... 64

图 6‑1  地球上氮的常见存在形式（A）、氮在大气、海洋和地球内部的含量（B、C、D）以及地球各个储库的氮储量（E）... 67

图 6‑2  蚀变洋壳与本实验蛇纹石化产物中的氮含量与同位素特征... 68

图 6‑3  深部氮循环示意图（a）和氮通量简略图（b）... 69

表目录

表 1‑1  博士论文工作量一览表... 19

表 2‑1  初始橄榄岩的主量元素成分... 26

表 2‑2  蛇纹石化实验初始物状态和实验条件... 27

表 3‑1  蛇纹石化产物中各矿物的比例... 39

表 3‑2  蛇纹石化中H₂与NH₃的产量... 45

表 5‑1  类地行星早期大气pN₂估算... 61

表 5‑2  类地行星早期大气pCO₂估算... 61