Japan Geoscience Union Meeting 2015

Presentation information


Symbol B (Biogeosciences) » B-PT Paleontology

[B-PT23] Decoding the history of Earth: From Hadean to Modern

Mon. May 25, 2015 2:15 PM - 4:00 PM 104 (1F)

Convener:*Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Katsuhiko Suzuki(Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology), Chair:Kentaro Nakamura(Department of Systems Innovation, School of Engineering, University of Tokyo)

2:30 PM - 2:45 PM

[BPT23-15] In-situ iron isotope analysis of pyrite and organic carbon/nitrogen isotope ratios from the Middle Proterozoic sediments

*Kazumi YOSHIYA1, Yusuke SAWAKI1, Manabu NISHIZAWA2, Tsuyoshi KOMIYA3, Takafumi HIRATA4, Shigenori MARUYAMA1 (1.Tokyo Institute of Technology, 2.JAMSTEC, 3.The University of Tokyo, 4.Kyoto University)

Oxygenation of Earth's surface is deeply linked to evolution of life. Independent evidence suggests that the Earth's atmospheric oxidation state is increased in two steps: (1) from 2,400 to 2,300 million years ago, and (2) around 600 million years ago (Holland, 2002; Holland, 2006). In contrast, the ocean was mostly reducing during the Archean, whereas the Phanerozoic was as oxygenated as it is now. Compared with Archean and Phanerozoic time, the redox status of middle Proterozoic (1.8-1 billion years ago) ocean remains little known. Canfield considered that the middle Proterozoic deep ocean was globally sulfidic condition (Canfield, 1998). On the other hand, Planavsky and others considered that deep-ocean was globally iron-rich anoxic condition, and sulfidic conditions are restricted to biologically productive ocean margin and restricted marginal basin (Planavsky et al., 2011).
Here we show iron isotope analysis of individual pyrite grains and whole rock carbon/nitrogen isotope analyses of middle Proterozoic sediments, mainly mudstones and black shales, from four drillcore samples (Mount Young 2, McArthur River 2, Urapunga 4 and 5) in McArthur Basin, Northern Australia.
Pyrites from the Wollogorang Formation of the Tawallah Group show the wide variation of δ56Fe values from -2 to +2 ‰. It suggests that the occurrence of partial oxidation, so their depositional environment of the Wollogorang Formation was ferruginous condition. δ15NTN values of the black shale in the Wollogorang and Barney Creek formations are from +4 to +7 ‰, relatively high values. The high δ15NTN values suggest the occurrence of partial denitrification in the water-column. δ15NTN values of black shale in the Wollogorang and Barney Creek Formations suggested that middle proterozoic sulfidic condition did not persist for long periods as previous studies insisted.