11:00 〜 11:15
[BCG05-08] Limited Mo isotopic fractionation in ~2.35-Gyr-old clastic sedimentary rocks in the Huronian Supergroup
キーワード：大酸化イベント、モリブデン同位体、ヒューロニアン累層群
Atmospheric oxygen rose to appreciable levels during the early Paleoproterozoic, which is often referred to as the Great Oxidation Event (GOE). The disappearance of mass-independent fractionation of sulfur isotopes (MIF-S) in sedimentary rocks suggests the initial rise of atmospheric O2 levels to above 10-5 times the present atmospheric level (PAL) occurred by ~2.43 Ga (e.g., Warke et al., 2020 PNAS 117, 13314–13320). However, recent investigations reveal the preservation of MIF-S in younger sedimentary rocks deposited between 2.35 and 2.25 Ga. (Philippot et al., 2018 Nat. Commun. 9, 2245; Poulton et al., 2021 Nature 592, 232–236). These MIF-S records may reflect fluctuations in atmospheric O2 levels around10-5 PAL between 2.43 to 2.25 Ga (Poulton et al., 2021). Alternatively, the MIF-S signals may capture oxidative weathering of Archean sulfides possessing MIF-S signals (Philippot et al., 2018).
To further understand the atmospheric redox conditions during the early Paleoproterozoic, we analyzed Mo isotopic compositions (δ98/95Mo) of ~2.35-Gyr-old clastic sedimentary rocks from the Espanola and Serpent formations in the Huronian Supergroup. Small MIF-S signals and low Mo concentrations (~1–2 ppm) were previously reported from the studied interval (Yamada et al., 2008 JpGU; Nakamura et al., 2014 JpGU), although the MIF-S signals may reflect the mobilization of detrital pyrite by late fluids (Cui et al., 2018 Astrobiology 18, 519–538). The analyzed samples have an average δ98/95Mo of +0.16 ± 0.33 ‰, which is similar to the average Archean upper continental crust δ98/95Mo (+0.03 ± 0.18‰) estimated from Archean and early Paleoproterozoic glacial diamictites (Greaney et al., 2020 EPSL 534, 116083). The unfractionated δ98/95Mo requires limited oxidative weathering of Mo as well as little authigenic Mo enrichment during deposition. Our results, therefore, suggest low atmospheric O2 conditions at ~2.35 Ga and support the oscillations in atmospheric O2 levels after its initial rise at ~2.43 Ga.
To further understand the atmospheric redox conditions during the early Paleoproterozoic, we analyzed Mo isotopic compositions (δ98/95Mo) of ~2.35-Gyr-old clastic sedimentary rocks from the Espanola and Serpent formations in the Huronian Supergroup. Small MIF-S signals and low Mo concentrations (~1–2 ppm) were previously reported from the studied interval (Yamada et al., 2008 JpGU; Nakamura et al., 2014 JpGU), although the MIF-S signals may reflect the mobilization of detrital pyrite by late fluids (Cui et al., 2018 Astrobiology 18, 519–538). The analyzed samples have an average δ98/95Mo of +0.16 ± 0.33 ‰, which is similar to the average Archean upper continental crust δ98/95Mo (+0.03 ± 0.18‰) estimated from Archean and early Paleoproterozoic glacial diamictites (Greaney et al., 2020 EPSL 534, 116083). The unfractionated δ98/95Mo requires limited oxidative weathering of Mo as well as little authigenic Mo enrichment during deposition. Our results, therefore, suggest low atmospheric O2 conditions at ~2.35 Ga and support the oscillations in atmospheric O2 levels after its initial rise at ~2.43 Ga.