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Ancient manganese (Mn) deposits are primarily characterized by the presence of Mn(II) carbonates that likely formed by the diagenetic reduction of precursor Mn(IV) oxides. As such, Precambrian sedimentary Mn deposits have been used as a line of evidence for the evolution of oxygen in Earth's surface environments. However, recent studies have shown that these Mn(II)-carbonates have the ability to directly accumulate within anoxic water columns, where free oxygen does not play a role in their formation. This alternative pathway casts uncertainty on the robustness of using ancient Mn deposits to constrain the redox fabric of the past marine water columns. Here, we investigate the Wafangzi Mn and Fe ore deposit from the 1.45 billion-year-old Tieling Formation, North China. The deposit contains Mn(II,III) mineral phases (hausmannite, braunite) as inclusions, or unreacted residues, trapped within Mn(II) carbonate (Ca-rhodochrosite). Some nodules and oolites of Mn(II) and Fe(II)-carbonate phases are also present and display a banded structure with concentric rings. Mn(III) oxide (manganite) is present in a paragenetic assemblage along with hematite and replacement textures with braunite. The negative carbon isotope composition (δ13C, –7‰ to –4‰) from Mn(II) carbonate samples in the Wafangzi Mn deposit which are distinct from that of contemporaneous seawater (~0‰), along with petrographic and speciation analyses, collectively suggest that the Mn(II,III)- and Fe(II)-bearing mineral phases formed through the diagenetic reduction of primary Mn(IV)/Fe(III) minerals coupled to the oxidation of organic matter. Therefore, the Wafangzi Mn deposit suggests the presence of sufficiently oxygenated marine waters, overlying anoxic ferruginous deeper waters with a transitional manganous water layer that could have driven the redox cycling of Mn, Fe, and C. Given the contemporaneous economic Mn deposits in the 1.45 Ga Ullawarra Formation in Western Australia, our findings imply the existence of a transient, and perhaps widespread, pulsed oxygenation event in the mid-Proterozoic oceans. [ABSTRACT FROM AUTHOR]

Pure marine chemical sediments are archives of geochemical proxies for the composition of seawater and may provide information about the ancient hydrosphere–atmosphere system. The early stage of the Emeishan large igneous province (ELIP) was characterized by the subaqueous eruption of mafic igneous rocks around the J. altudaensis zone of the Capitanian Stage that has been proposed to have contributed to the Guadalupian mass extinction. However, detailed mechanisms and the impact of the eruption on the Guadalupian marine environment have yet to be assessed. Here, to examine the Guadalupian marine environment, we studied major and trace element concentrations, particularly rare earth element and yttrium data, along with high-precision Sr-Nd isotope ratios, of three types of Mn ores (i.e., clastic, massive, and oolitic) and siliceous limestones from the Zunyi Mn deposit in South China formed following the early-stage eruption of the ELIP. Our results indicate that the clastic Mn ores contain notable detrital mafic aluminosilicates. In contrast, the massive and oolitic Mn ores and siliceous limestones preserved the pristine geochemical signatures of the Middle–Late Permian seawater characterized by distinctly low (87Sr/86Sr)i and high εNd(t) values. These data indicate a strong impact of the early-stage submarine eruptions of the ELIP on the marine environment in South China and worldwide, likely through intensive seawater–rock interaction. [ABSTRACT FROM AUTHOR]

Unlike modern Mn(IV)-dominated manganese nodules, ancient manganese deposits primarily contain Mn(II) (manganous) mineral phases. The general assumption is that these manganous minerals formed post-depositionally from a Mn(IV) precursor phase during diagenesis and metamorphism. However, recent studies have shown that aqueous Mn(II) may combine with either CO 3 2– (aq) or H 4 SiO 4(aq) to nucleate and precipitate as primary manganous phases within anoxic waters. These findings thus make it difficult to assess what comprised the primary mineral assemblage of ancient Mn deposits, which in turn, has implications for understanding the redox fabric of the past marine water column. Here, we investigate the paragenesis and oxidation state of various Mn minerals (oxides, carbonates, and silicates) from the late Devonian Xialei and middle to late Permian Zunyi manganese deposits in South China. Through characterization of the micro-scale distribution, composition, and speciation of different Mn-bearing mineral phases, we provide evidence to support an oxidative mechanism during Mn cycling at the time of deposition. We document the presence of a residual oxide phase (hausmannite; Mn 3 O 4) that represents a primary to early formed Mn(III)-bearing oxide preserved in Mn(II)-carbonate nodules. Additionally, we observe spherulitic textures comprised of mixed-valence Mn(II,III,IV)-silicates enclosed in diagenetic Mn(II)-carbonate laminations. Our results allow for the construction of a paragenetic model for these deposits beginning with the reduction of a primary Mn(IV) phase and the subsequent precipitation of intermediate Mn(III)-oxide or -silicate mineral phases. Following continued exposure to reducing conditions during early diagenesis, Mn(III)-oxides were then replaced by Mn(II)-carbonates. Similarly, Mn(III)-silicates were transformed into various Mn(II)-silicates during late-stage diagenesis and metamorphism. This model suggests that the accumulation of various divalent Mn mineral phases in ancient sediments may, therefore, be indicative of deposition under a water column with a sufficient oxidizing potential to induce Mn(II) oxidation. [ABSTRACT FROM AUTHOR]

Microbial activity has been proposed to play an important role in the genesis of manganese (Mn) carbonate deposition in natural sediments, with microbial reduction of Mn(IV) oxides as a crucial factor that enhances the saturation state of Mn carbonates in pore waters. Recently, the nucleation of Mn carbonates on cellular templates has been suggested to represent a further microbial control. However, microstructural evidence for this model remains tentative and the Mn carbonate nucleation pathway is still unconstrained. Here we report petrographic and in-situ chemistry that supports the direct role(s) of microbes in the formation of Mn carbonates in the giant Malkansu Mn metallogenic belt at West Kunlun, Northwest China. The Mn carbonate ores exhibit negative δ13C values (−10.2 to −12.7‰) that indicate the microbial reduction of Mn oxide. Microscopic data show that the Mn carbonates exhibit spheroidal rod and dumbbell-like shaped morphologies with a well-defined core-shell structure. These carbonates are intimately associated with net-like organic matter. An incubation experiment was also carried out showing that biogenic Mn carbonate minerals share a strikingly similar crystal form with the natural Mn ore samples. Together, these results provide further evidence for a dual role of microbes in Mn‑carbonate ore formation: Mn(II) and carbonate produced by the microbial reduction of Mn(IV) oxides, and the nucleation of Mn carbonate minerals on cellular surfaces and/or organic matrices. Potentially, these Mn carbonates may have self-assembled to form rod- and dumbbell-like shaped morphologies, analogous to other carbonate systems. The unique core-shell structure observed herein also indicates that the formation of Mn‑carbonates mediated by microbes follows a heterogeneous nucleation pathway. [ABSTRACT FROM AUTHOR]