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Article / Book Information
題目(和文)

SO2光化学反応における硫黄同位体非質量依存分別の実験的研究とその太古代大気への適用

Title(English)

Experimental study on sulfur mass-independent fractionation during
SO2 photochemistry and its application to Archean atmosphere

著者(和文)

遠藤美朗

Author(English)

Yoshiaki Endo

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11053号,授与年月日:2019年3月26日,
学位の種別:課程博士,
審査員:上野 雄一郎,中本 泰史,横山 哲也,綱川 秀夫,奥住 聡

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第11053号,
Conferred date:2019/3/26,
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

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（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

学生氏名：
Student’s Name

遠藤 美朗

コース

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

指導教員（主）：Academic Supervisor(main)（理学上野雄一郎）

指導教員（副）：Academic Supervisor(sub)要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）Earth's early atmosphere is essential to understand origin of life, evolution of life, andclimate of that time. Before 2.4 Ga, there are believed to be no or very low level of molecularoxygen in the ancient atmosphere. However, there is little consensus about reducing gas
species and compositions of the atmosphere at that time. Sulfur mass-independent
fractionation (S-MIF) recorded in Archean sedimentary rocks is expected to constrain thechemical and physical states of the ancient atmosphere. Specific photochemical reactions
were shown to cause large S-MIF by previous studies (e.g. Farquhar et al., 2001). However,
the most basic characteristics of Archean S-MIF, which is Δ36S/Δ33S slope, have yet to bereproduced by experiments. In order to clue the ancient atmospheric chemistry, it is essential
to understand the mechanism of S-MIF including 36S. I have conducted a series ofexperiments to examine S-MIF during SO2 photochemistry.
In Chapter 2, because I noted that fractionation factors in photolysis can be calculated fromisotopologue absorption cross sections, I measured photoabsorption cross sections of SO2
isotopologues (32SO2, 33SO2, 34SO2, and 36SO2) from 190 to 220 nm. This is the first report of36SO2 absorption cross sections. Observed cross sections clearly depended on sample

pressures. Owing to careful analysis, I found that this pressure dependence stems fromexperimental artifact, and the dependence could be corrected. As a result, high accuracy andhigh precision (less than 1% of error) of cross sections were achieved. Fractionation factors ofSO2 photolysis were estimated to be smaller than a previous study (Danielache et al., 2008).
The magnitude of fractionation factors was found to be similar to or below that of Archean
S-MIF.
In Chapter 3, I conducted SO2 photochemical experiments under reducing CO atmosphereand examined sulfur isotope fractionations. In previous studies, because of a limitation ofsample amount for quadruple sulfur isotope analysis, isotope fractionations in SO2 photolysis
could not be examined under optically thin SO2 condition which should be realistic
atmospheric condition. I developed a new analytical method of small sample (10 nmol S) forsulfur isotope analysis, and the experiment under optically thin SO2 condition was achieved.
As a result, the observed S-MIF by photochemical experiments reproduced the characteristicof Archean Δ36S/Δ33S slope about −1, for the first time. Moreover, it was found that theS-MIF occurs in self-shielding in SO2 photolysis and photoexcited SO2 chemistry. Sulfur
mass-independent fractionation in final products reflects mixing of these two mechanisms. Inatmosphere, self-shielding should reflect partial pressure of SO2 and S-MIF in excited SO2chemistry should reflect partial pressure of CO or CH4. Accordingly, to reproduce theArchean Δ36S/Δ33S slope, the atmosphere was estimated to contained several kilopascals ofCO or CH4 in the Archean atmosphere.
In Chapter 4, I focused on SO2 absorption line width dependence of self-shielding. In orderto test total pressure dependence of S-MIF, I conducted SO2 photochemical experimentschanging both partial pressures of SO2 and total pressures. Total pressure dependence ofS-MIF in SO2 photolysis was clearly shown experimentally. Analysis based on the SO2
absorption line width found that the total pressure dependence comes from pressure
broadening of SO2 absorption lines. At high total pressure such as above 1 bar, the Δ36S/Δ33S
slope was far from that of the Archean. Based on Chapter 3's model, to reproduce large S-MIFof late Archean, it was found that the Archean atmospheric pressure should be below 1 bar, orthe SO2 was photolyzed in the upper atmosphere.
Finally, In Chapter 5, the possible Archean atmospheric states estimated from quadruple
sulfur isotopes were discussed. Based on the results from Chapter 2 to Chapter 4 and using
numerical photochemical model, the Δ33S and Δ36S values were modeled under various SO2column densities, partial pressures of CO, and total pressures. When the results werecompared to the geological large S-MIF of late Archean, and to reproduce the S-MIF, it wasfound to be important that S-MIF in excited SO2 was conserved in final products. Reducing
gas species were discussed using their reaction rate constants with excited SO2. Consequently,
it was found that hydrocarbons including CH4 cannot explain the late Archean S-MIF and it
was concluded that CO is possibly reasonable as reducing gas of excited SO2.
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 of 2000 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).