Evidence

The light molybdenum isotope signatures (δ98Mo) of the Fe-Mn carbonates of the Sinqeni Iron Formation (Pongola Supergroup) suggest the existence of Mn oxides at 2.95 Ga and the accompanying preferential adsorption of light Mo isotopes (95Mo) onto Mn oxides

Light Mo isotope compositions (as low as -1‰) of Mn-rich sediments of the Senqini Iron Formation has been reported. Mo adsorption onto Mn oxides is the only known process that uptakes light Mo isotope with a large isotope fractionation. Although the Mn redox cycling has been proposed to be active in the Pongola Supergroup sediments, the light Mo isotope signatures are inherited, implying the occurrence of Mn oxide at 2.95 Ga. The positive correlation observed between the Fe/Mn ratio and the δ98Mo value of the iron formation sediments further supports the Mn oxidation and its interaction with Mo. The occurrence of Mn oxides provides evidence for the rise of oxygenic photosynthesis three billion years ago.

Authors

Planavsky, N.J., Asael, D., Hofmann, A., Reinhard, C.T., Lalonde, S.V., Kundsen, A., Wang, X., Ossa Ossa, F., Pecoits, E., Smith, A.J.B., Beukes, N.J., Bekker, A., Johnson, T.M., Konhauser, K.O., Lyons, T.W., Rouxel, O.J.

Abstract

The early Earth was characterized by the absence of oxygen in the ocean–atmosphere system, in contrast to the well-oxygenated conditions that prevail today. Atmospheric concentrations first rose to appreciable levels during the Great Oxidation Event, roughly 2.5–2.3 Gyr ago. The evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise, but it has proved difficult to constrain the timing of this evolutionary innovation1,2. The oxidation of manganese in the water column requires substantial free oxygen concentrations, and thus any indication that Mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing. Mn oxides are not commonly preserved in ancient rocks, but there is a large fractionation of molybdenum isotopes associated with the sorption of Mo onto the Mn oxides that would be retained. Here we report Mo isotopes from rocks of the Sinqeni Formation, Pongola Supergroup, South Africa. These rocks formed no less than 2.95 Gyr ago3 in a nearshore setting. The Mo isotopic signature is consistent with interaction with Mn oxides. We therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen.

Citation

Download BibTeX   |   Download Endnote

Evidence

The carbon (δ13C) and oxygen (δ18O) isotope of the Fe-Mn carbonates from the Sinqeni Formation, Pongola Supergroup indicate the precipitation of carbonates was not in equilibrium with seawater (in terms of C and O isotopes), implying the reductive Mn cycling in the sediments and subsequent authigenic carbonate formation via early diagenesis

The light δ13C (-22.3 to -13.5‰, VPDB) of Fe-Mn carbonates from the Sinqeni Formation, South Africa indicate the carbon source for the carbonate precipitation was primarily from organic carbon instead of the seawater dissolved inorganic carbon (DIC). The δ18O values of Fe-Mn carbonates (-21.1 to -8.6‰, VPDB) imply the carbonates precipitated in a pore-water environment instead of the water column of the ocean. The two pieces of evidence suggest reduction of Mn oxides coupled with degradation of organic carbon within the sediments account for the formation of authigenic Fe- Mn carbonates. This finding precludes the deposition of Mn carbonates directly from the water column, which confirms the deposition of Mn oxides onto the sediment-water interface at ~3 Ga. Free oxygen in the water column was necessary to prevent Mn oxides from reduction during settling.

Authors

Ossa Ossa, F., Hofmann, A., Wille, M., Spangenberg, J.E., Bekker, A., Poulton, S.W., Eickmann, B., Schoenberg, R.

Abstract

Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; ∼2.46–2.32 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O, Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. δ13C values between −22.3 and −13.5‰ VPDB, and δ18O values between −21.1 and −8.6‰ VPDB for Fe–Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe–Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. δ56Fe IRMM-014 values between −1.27 and 0.14‰ and δ98Mo NIST3134+0.25 values between −0.46 and 0.56‰ co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe–Mn carbonates, the primary light Fe and Mo isotopic signature of Fe–Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced O2, Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative Fe values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined δ56Fe and δ98Mo data indicate pervasive oxygenation of seawater with the O2 content in the photic zone likely reaching levels higher than the maximum value of 10 μM proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations.

Citation

Download BibTeX   |   Download Endnote

Evidence

The significant manganese enrichment in the Mesoarchean (2.95-2.98 Ga) sedimentary sequences of the Pongola Supergroup, South Africa, was caused by oxidation of Mn(II) via molecular oxygen

Systematic petrographic and geochemical investigation show that the MnO content of the primary Pongola Supergroup sedimentary depositions is as high as 8.5 wt.%. Sediments are characterized by manganoan siderite, ferroan rhodochrosite, and other Mn-Fe-rich mineral phases. The unusual enrichment of manganese in the sediments of the 2.95-2.98 Ga iron formations is interpreted as the oxidation of dissolved Mn by free oxygen in the ancient water column, which labels the emergence of oxygenic photosynthesis at ~3 Ga.

Authors

Ossa Ossa, F., Hofmann, A., Vidal, O., Kramers, J.D., Belyanin, G., Cavalazzi, B.

Abstract

An unusual sediment-hosted manganese deposit is described from the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. MnO contents up to 15 wt.% were observed in marine clastic and chemical sedimentary rocks. Mn enrichment is interpreted to have resulted from the hydrothermal alteration of manganiferous shale and BIF parent rocks, the primary MnO contents of which are as high as 8.5 wt.%. A detailed mineralogical and petrographic study shows that these parent rocks are characterized by manganoan siderite, ferroan rhodochrosite and other Mn–Fe-rich mineral phases, such as kutnohorite and Fe–Mn-chlorite. Their hypogene alteration gave rise to a diversification of mineral assemblages where ferroan tephroite, calcian rhodochrosite, rhodochrosite, pyrochroite, pyrophanite, cronstedtite, manganoan Fe-rich chlorite and manganoan phlogopite partially or totally replaced the previous mineral assemblage. Thermodynamic modeling performed on chlorite phases associated with the described mineral assemblages illustrates a decrease of average crystallization temperatures from ca. 310 °C during early metamorphic stages to ca. 250 °C during a hydrothermal stage. Mineral transformation processes were thus related to retrograde metamorphism and/or hydrothermal alteration post-dating metamorphism and gave rise to progressive Mn enrichment from unaltered parent to altered rocks. The timing of hypogene alteration was constrained by 40Ar/39Ar dating to between about 1500 and 1100 Ma ago, reflecting tectonic processes associated with the Namaqua-Natal orogeny along the southern Kaapvaal Craton margin. Manganiferous shale and BIF of the Mozaan Group may represent the oldest known examples of primary sedimentary Mn deposition, related to oxidation of dissolved Mn(II) by free oxygen in a shallow marine environment. Oxygenic photosynthesis would have acted as a first-order control during Mn precipitation. This hypothesis opens a new perspective for better constraining secular evolution of sediment-hosted mineral deposits linked to oxygen levels in the atmosphere-hydrosphere system during the Archean Eon.

Citation

Download BibTeX   |   Download Endnote

Evidence

Both field observations and modeling studies show that Mn oxide reduction in a modern ferruginous water column is fast enough to eliminate the possibility of Mn oxide settling on to the sediment-water interface

Lake Matano, Indonesia is a persistently stratified lake with a deep (>~120 m) anoxic (ferruginous) water column. Manganese primarily occurs as a dissolved Mn(II) phase below the chemocline due to Mn oxides reduction either by ferrous ions (Fe2+) or CH4. 1-D reactive transport modeling based on Mn oxidation/reduction kinetics also suggests that there is no way for Mn oxides to survive during their settling to the sediment-water interface. By analogy, the Mn oxides were unable to deposit on the sediment-water interface without free oxygen in the water column under the pervasive ferruginous condition of the Archean ocean. Therefore, the evidence of Mn oxides reduction within the sediments of Senqini Formation implies the existence of free oxygen in the water column at ~3 Ga

Authors

Jones, C., Crowe, S.A., Sturm, A., Leslie, K.L., MacLean, L.C.W., Katsev, S., Henny, C., Fowle, D.A., Canfield, D.E.

Abstract

This study explores Mn biogeochemistry in a stratified, ferruginous lake, a modern analogue to ferruginous oceans. Intense Mn cycling occurs in the chemocline where Mn is recycled at least 15 times before sedimentation. The product of biologically catalyzed Mn oxidation in Lake Matano is birnessite. Although there is evidence for abiotic Mn reduction with Fe(II), Mn reduction likely occurs through a variety of pathways. The flux of Fe(II) is insufficient to balance the reduction of Mn at 125 m depth in the water column, and Mn reduction could be a significant contributor to CH4 oxidation. By combining results from synchrotron-based X-ray fluorescence and X-ray spectroscopy, extractions of sinking particles, and reaction transport modeling, we find the kinetics of Mn reduction in the lake's reducing waters are sufficiently rapid to preclude the deposition of Mn oxides from the water column to the sediments underlying ferruginous water. This has strong implications for the interpretation of the sedimentary Mn record.

Citation

Download BibTeX   |   Download Endnote

Evidence

The light chromium isotope (δ53Cr) signatures of Nsuze paleosol indicate the redox mobilization of Cr from its igneous mineral hosts occurred ~3 Ga driven by atmospheric oxygenation

The Cr isotope compositions (δ53Cr) of the lower chloritic palaeosol from the Pongola Supergroup, South Africa are lighter than the igneous value of -0.123 ± 0.102‰, which is interpreted as the mobilization of oxidized Cr(VI) (CrO42-; enriched in heavier Cr isotopes) and corresponding retention of immobile Cr(III) in the paleosol with a light δ53Cr signature. The oxidation of Cr(III) from the igneous mineral hosts was achieved by Mn oxides reduction under the atmospheric O2 levels 3 × 10-4 times present atmospheric levels (PAL). The atmospheric oxygenation suggested by Cr isotope data is consistent with the presence of free oxygen in the water column and the Mn enrichment in the sediments of 2.95-2.98 Ga Pongola Supergroup via extensive Mn oxide oxidation.

Authors

Crowe, S.A., Døssing, L.N., Beukes, N.J., Bau, M., Kruger, S.J., Frei, R., Canfield, D.E.

Abstract

It is widely assumed that atmospheric oxygen concentrations remained persistently low (less than 10−5 times present levels) for about the first 2 billion years of Earth’s history1. The first long-term oxygenation of the atmosphere is thought to have taken place around 2.3 billion years ago, during the Great Oxidation Event2,3. Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6–2.7 billion years ago4,5,6. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billion-year-old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation from the Pongola Supergroup, South Africa. We find extensive mobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimum estimate for atmospheric oxygen concentrations at that time of 3 × 10−4 times present levels. Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300–400 million years earlier than previous indications for Earth surface oxygenation.

Citation

Download BibTeX   |   Download Endnote