@article{b21fe330f4ef41c6921a4204f44d0ad6,
title = "Progressive ocean oxygenation at ~2.2 Ga inferred from geochemistry and molybdenum isotopes of the Nsuta Mn deposit, Ghana",
abstract = "Recent geochemical data suggest the occurrence of an O2 overshoot during the mid-Paleoproterozoic (~2.3–2.0 Ga). This O2 overshoot appears to be consistent with carbon isotope records that suggest high burial rates of organic carbon during that period, the so-called Lomagundi Event. However, little is known about the changes in the ocean redox conditions associated with the O2 overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (δ98/95Mo) isotopes, and total organic carbon contents of Mn-ore and phyllite samples from the Nsuta Mn deposit in the Birimian Supergroup of Ghana which were deposited during the O2 overshoot (at ~2.2 Ga). The Mn-ore samples contain early diagenetic rhodochrosite (Mn carbonate). The trace element compositions and Re-Os isotopes of the Mn-ore samples suggest that the rhodochrosite originated from primary manganese oxides (MnO2) deposited at ~2.2-Ga. The δ98/95Mo values of the least-altered Mn-ore samples range between −1.10‰ and −0.55‰ (relative to NIST3134), suggesting seawater δ98/95Mo values of 1.85 ± 0.18‰ (1SD) during the O2 overshoot. Such high seawater δ98/95Mo values can be best explained by enhanced removal of isotopically light Mo through adsorption onto Mn oxides. To form extensive Mn-oxide deposits, bottom seawater with O2 concentrations of > 10 μM would have expanded at ~2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic.",
keywords = "Geochemistry, Molybdenum isotope, Nsuta Mn deposit, Ocean paleoredox, Oxygen overshoot, Paleoproterozoic",
author = "Goto, {Kosuke T.} and Yasuhito Sekine and Takashi Ito and Katsuhiko Suzuki and Anbar, {Ariel D.} and Gordon, {Gwyneth W.} and Yumiko Harigane and Teruyuki Maruoka and Gen Shimoda and Teruhiko Kashiwabara and Yutaro Takaya and Tatsuo Nozaki and Hein, {James R.} and Tetteh, {George M.} and Nyame, {Frank K.} and Shoichi Kiyokawa",
note = "Funding Information: K. T. G. appreciates Mariah Mikesell for assistance with the sample preparation and J. R. H. thanks BHP Billiton for field support in Takoradi, Ghana and at the Nsuta mine. Eva St{\"u}eken, Alexandre Raphael Cabral, and an anonymous reviewer are thanked for their constructive feedback. This study was a part of “Scientific Research on Genesis of Marine Resources,” a project of Next-generation Technology for Ocean Resources Exploration of the Cross-ministerial Strategic Innovation Promotion Program (SIP). This study was also supported by JSPS KAKENHI (Grant No. JP15H02142 , 26257211 , and JP19K14832 ) and MEXT KAKENHI (Grant No. JP17H06456 ). Funding Information: K. T. G. appreciates Mariah Mikesell for assistance with the sample preparation and J. R. H. thanks BHP Billiton for field support in Takoradi, Ghana and at the Nsuta mine. Eva St{\"u}eken, Alexandre Raphael Cabral, and an anonymous reviewer are thanked for their constructive feedback. This study was a part of “Scientific Research on Genesis of Marine Resources,” a project of Next-generation Technology for Ocean Resources Exploration of the Cross-ministerial Strategic Innovation Promotion Program (SIP). This study was also supported by JSPS KAKENHI (Grant No. JP15H02142, 26257211, and JP19K14832) and MEXT KAKENHI (Grant No. JP17H06456). Publisher Copyright: {\textcopyright} 2021 Elsevier B.V.",
year = "2021",
month = apr,
day = "20",
doi = "10.1016/j.chemgeo.2021.120116",
language = "English",
volume = "567",
journal = "Chemical Geology",
issn = "0009-2541",
publisher = "Elsevier",
}