[B-PT05] Decoding the history of Earth: From Hadean to Modern

[BPT05-P11] Organic Nitrogen/Carbon isotope ratios from the Middle Proterozoic sedimentary rocks, McArthur Basin, Northern Australia

*Kazumi Yoshiya¹, Yusuke Sawaki², Manabu Nishizawa³, Tsuyoshi Komiya⁴, Shigenori Maruyama¹ (1.Earth-life Science Institute, Tokyo Institute of Technology, 2.Earth & Planetary Sciences, Tokyo Institute of Technology, 3.JAMSTEC, 4.The University of Tokyo)

Keywords:Nitrogen isotope ratio, Middle Proterozoic, McArthur Basin

Oxygenation of Earth's surface is expected to be deeply linked to evolution of life. Many of independent evidence suggest that the Earth’s atmospheric oxidation state is increased in two steps: from 2,400 to 2,300 million years ago, and around 600 million years ago (Holland, 2006). On the other hand, ocean was mostly dominated by reducing conditions during the Archean, whereas the ocean-atmosphere system in the Phanerozoic was as oxygenated as it is now. It has been generally assumed that the middle Proterozoic ocean was globally oxic at the surface and sulfidic (euxinic) at depth. Nitrogen limitation caused by trace metal scarcity has been proposed as an explanation for why eukaryotic diversification is delayed (Canfield, 1998; Anbar & Knoll, 2002).
Here we show nitrogen and carbon isotope compositions of middle Proterozoic sediments, mainly carbonate rocks, mudstones and black shales prepared from six drillcore samples (Mount Young 2, McArthur River 2, Urapunga 4, Urapunga 5, Jamison-1 and 14MCDDH002) in McArthur Basin, Northern Australia.
δ¹⁵N_TN values of the black shale in the Wollogorang and Barney Creek formations are relatively high, ranging from +4 to +7 ‰. The high δ¹⁵N_TN values likely reflect the predominance of partial denitrification in the water-column. δ¹⁵N_TN values gradually decrease from +7 to +1 ‰ stratigraphically upward, and the average δ¹⁵N_TN value is 3.5 ‰.
An increasing nitrate reservoir may have been responsible for the decreasing δ¹⁵N_TN value, which implying an ocean oxygenation in the middle Proterozoic.
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