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)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)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 isotopologueabundances (∆13C13C) in ethane, ethene, and ethanol (Chapter 2). Reproducibility of the whole protocol,including chemical modification steps and measurement of isotopologues, is better than ±0.14‰. 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 an 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 are at 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 the actual variations 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 as
the bio-ethanol. On the other hand, abiotically-synthesized ethane from CH4 in the laboratory shows
distinctively low 13C-13C abundances compared to the stochastic distribution (anti-clumping). We modeled theabiotic ethane formation reaction using a collision frequency of isotopologues and found that the collisionalfrequency model predicts the anti-clumping in abiotic ethane. Remarkably, the model prediction matches witha few putative abiotic ethane from natural geological settings, such as Kidd Creek Mine. Therefore, ∆ 13C13C
analysis can provide additional evidence for abiological origin of some natural gas on Earth.
To understand the change in ∆13C13C during thermal cracking, 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 towards equilibrium at the temperature of formation.
To understand the robustness 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 during 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 origin, although thermogenic origin has not been discarded becausesubducting slabs contain organic-rich sediments (i.e., potential source of thermogenic ethane). Using the 13C13
C clumped isotope signatures, we demonstrated that hydrocarbons are mainly of thermogenic origin.
In summary, the approach distinguishes biogenic, abiotic, and thermogenic organic compounds and canprovide an important step forward for discriminating the origin of organic molecules on Earth and in extraterrestrial environments.