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| Titel |
Toward estimation of origin of methane at ancient seeps — Carbon isotopes of seep carbonates, lipid biomarkers, and adsorbed gas |
| VerfasserIn |
Yusuke Miyajima, Yumiko Watanabe, Akira Ijiri, Akiko Goto, Robert Jenkins, Takashi Hasegawa, Saburo Sakai, Ryo Matsumoto |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143713
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| Publikation (Nr.) |
 EGU/EGU2017-7462.pdf |
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| Zusammenfassung |
| Methane is generated mainly by microbial or thermal degradation of organic matter, and the
origin of methane can be estimated based on its stable carbon isotopic signature. Seafloor
seepages of methane-charged fluids have been a major source of methane to the ocean, and
knowing the origin of methane at the methane seeps can provide valuable insights into the
subsurface fluid circulation and biogeochemical processes. Methane seeps in the geological
past are archived as authigenic methane-derived carbonate rocks, which precipitate via an
alkalinity increase facilitated by microbially mediated anaerobic oxidation of methane. Here
we attempted to estimate origins of methane at ancient seeps, based on several
proxies preserved within the seep carbonates. We examined methane-seep carbonate
rocks in the Japan Sea region, collected from lower Miocene to middle Pleistocene
sediments at 11 sites on land, and also carbonate nodules collected from the seafloor off
Joetsu, where thermogenic methane is seeping. Carbon isotopic compositions of the
carbonates and lipid biomarkers of methane-oxidizing archaea within them were
analyzed. In order to directly know original isotopic signatures of methane, we also
attempted to extract adsorbed methane through acid dissolution of the powdered
carbonates.
Early-diagenetic carbonate phases show various δ13C values between −64.7 and −4.7‰ vs.
VPDB, suggesting either biogenic or thermogenic, or both origins of methane. A lipid
biomarker pentamethylicosane (PMI) extracted from the ancient carbonates has δ13C values
mostly lower than −100‰ , whereas that from the modern methane-derived carbonate
nodule has a higher value (−80‰ ). The δ13C values of the seeping methane (−36‰ ) and
PMI in the modern Joetsu seep carbonate shows an offset of −44‰ . If this carbon isotope
offset was similar at the ancient seeps, the δ13C values of PMI indicate that methane at
ancient seeps in the Japan Sea region was biogenic in origin, with δ13C values lower than
−50‰ .
Acid dissolution of the Miocene to Pliocene carbonates released methane with δ13C values
mostly around or higher than −50‰ , which conflicts with the estimation based on
biomarkers. Moreover, the Pleistocene and modern samples released only trace amounts of
methane. It is thus highly possible that the extracted methane was mostly adsorbed on
the carbonates within zones of thermogenic generation of hydrocarbons during
burial.
In conclusion, we can roughly estimate origins of methane at ancient seeps based on δ13C
values of carbonates and biomarkers. However, in order to directly analyze methane
contained in ancient seepage fluids, exploration of gas or fluid inclusions trapped within
carbonate crystals is necessary. |
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