Progressive ocean oxygenation at ~2.2 Ga inferred from geochemistry and molybdenum isotopes of the Nsuta Mn deposit, Ghana

Kosuke T. Goto, Yasuhito Sekine, Takashi Ito, Katsuhiko Suzuki, Ariel D. Anbar, Gwyneth W. Gordon, Yumiko Harigane, Teruyuki Maruoka, Gen Shimoda, Teruhiko Kashiwabara, Yutaro Takaya, Tatsuo Nozaki, James R. Hein, George M. Tetteh, Frank K. Nyame, Shoichi Kiyokawa

Research output: Contribution to journalArticlepeer-review

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.

Original languageEnglish (US)
Article number120116
JournalChemical Geology
Volume567
DOIs
StatePublished - Apr 20 2021

Keywords

  • Geochemistry
  • Molybdenum isotope
  • Nsuta Mn deposit
  • Ocean paleoredox
  • Oxygen overshoot
  • Paleoproterozoic

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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