論文 / 著書情報
Article / Book Information
題目(和文)

13C-13C二重置換同位体分子の生物地球化学とその炭化水素起源への応用

Title(English)

Biogeochemistry of 13C-13C clumped isotopologue and its implications
on hydrocarbon origin

著者(和文)

田口宏大

Author(English)

Koudai Taguchi

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12649号,授与年月日:2024年3月26日,
学位の種別:課程博士,
審査員:上野 雄一郎,GILBERT ALEXIS ROMAI,横山 哲也,尾﨑 和海,癸生川 陽子

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12649号,
Conferred date:2024/3/26,
Degree Type:Course doctor,
Examiner:,,,,学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

系・コース：
Department of, Graduate major in

地球惑星科学
地球惑星科学

学生氏名：
Student’s Name系
コース

田口 宏大

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

審査員主査：
Chief Examiner（理）

上野 雄一郎要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）Clumped isotope geochemistry is a new field of study using multiple isotopic substitutions within a moleculeand potentially provides information for origins of organic molecules on Earth and beyond. However, due totechnical difficulty, its measurement in organic molecules is still limited to a few simple molecules.
Here, we have developed a new fluorination method for measuring 13C-13C clumped isotopologue
abundances (∆13C13C) in ethane, ethene, and ethanol (Chapter 2). Reproducibility of the whole protocol,
including chemical modification steps and measurement of isotopologues, is better than ±0.09‰. We reported
the 13C-13C clumped isotopologues of ethanol produced by the fermentation of glucose from C3, C4, and CAM
plants. All these bio-ethanol show a very narrow range of ∆13C13C values irrespective to their δ13C variation,suggesting that the glucose tends to “scramble” C-atoms through the Calvin-Benson-Bassham (CBB) cycle,
eventually leading to a constant 13C-13C distribution in sugars.
The developed method was standardized and calibrated against a stochastic reference scale using isotopicallyenriched 13C-13C-labeled ethanol (Chapter 3). We found that the chemical reaction steps in the fluorination
method could change the 13C distribution in ethene, leading to a scale compression for observed ∆ values. Thus,
we provided the empirical transfer function to standardize the ∆13C13C values. After standardization of thefluorination method, we constructed a stochastic reference frame assuming that the clumped isotopologues
distribution in biogenic glucose assuming isotopic equilibrium (+0.2‰ against stochastic distribution) at 25°C
through carbon scrambling during the CBB cycle. The equilibrium ∆13C13C value in glucose was calculatedusing density functional theory. Consequently, the reference scale for ∆13C13C analysis was established.
To understand actual varioation of 13C-13C clumping in nature, we have measured various ethane (C2H6)
samples (Chapter 4). Most natural ethane gas from various geological settings show similar ∆13C13C values
compared to the bio-ethanol. On the other hand, abiotically-synthesized ethane from CH4 in laboratory shows
distinctively low 13C-13C abundances compared to the stochastic distribution (anti-clumping) as well as thethermogenic ethane. We modeled the abiotic ethane formation reaction using a collision frequency of
isotopologues and found that the collisional frequency model predicts the anti-clumping in abiotic ethane.
Remarkably, the model prediction matches with a few putative abiotic ethane from natural geological settings,
such as Kidd Creek Mine. Therefore, ∆13C13C analysis can provide additional evidence for abiological originof some natural gas on Earth.
To understand the change in ∆13C13C during thermal clacking, we conducted hydrothermal experiments forabout two years using lignin and docosane (C22H46) as starting materials (Chapter 5). The results demonstrated
that thermogenic ethane has a relatively high 13C-13C abundance compared to stochastic distribution, whichmatches thermogenic ethane from various geological settings. Also, we modeled the ethane formation from thethermal cracking of precursor organics and demonstrated that 13C distribution in thermogenic ethane is inheritedfrom the C-C bonding in the precursor molecule (i.e., bio-glucose). In addition, if thermal maturity increases
further, the 13C-13C distribution in ethane may shift toword equilibrium at the temperature of formation.
To understand the tolerance of the 13C-13C clumped isotope signature against ethane decomposition, we
conducted theoretical calculations for the 13C-13C clumped isotopic fractionation through an ethane dissociation(C2H6 → CH3 + CH3) reaction using quantum chemical calculations and transition state theory (Chapter 6). We
found that the 13C-13C clumped isotopic fractionation during ethane dissociation reactions does not show
significant variation at plausible geological temperatures in the subsurface on Earth, even though isotope
fractionation occurs in the thermal cracking. The observed ∆13C13C variations in natural gas ethane may reflect
mixing between reservoirs of different maturity levels.
Finally, the established systematics of clumped ethane isotopologues was applied for distinguishing theorigins of hydrocarbons from serpentinite mud volcanos at the Mariana forearc region (Chapter 7). The Marianahydrocarbons are potentially abiotic in orign, though thermogenic origin has not been discarded because
subducting slabs contain organic-rich sediment (i.e., potential source of thermogenic ethane). Using the 13C-13C
clumped isotope signatures, we demonstrated that hydrocarbons are mainly of thermogenic origin. In summary,the approach distinguishes biogenic, abiotic, and thermogenic organic compounds and can provide an important
step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments.
備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800

Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository

Website (T2R2).