Progressive Ocean Oxygenation At Similar To 2.2 Ga Inferred From Geochemistry And Molybdenum Isotopes Of The Nsuta Mn Deposit, Ghana

Recent geochemical data suggest the occurrence of an O-2 overshoot during the mid-Paleoproterozoic (similar to 2.3-2.0 Ga). This O-2 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 O-2 overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (delta Mo-98/95) 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 O-2 overshoot (at similar to 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 similar to 2.2-Ga. The delta Mo-98/95 values of the least-altered Mn-ore samples range between 1.10% and 0.55% (relative to NIST3134), suggesting seawater delta Mo-98/95 values of 1.85 +/- 0.18% (1SD) during the O-2 overshoot. Such high seawater delta Mo-98/95 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 O-2 concentrations of > 10 mu M would have expanded at similar to 2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic.