Evidence for the oxidation of Earth's crust from the evolution of manganese minerals

The study investigates how Earth's shallow crust became oxidized over geologic time and seeks a quantitative proxy for spatially averaged crustal redox state. Oxygenic photosynthesis and the Great Oxidation Event (GOE) profoundly altered Earth's surface environments, yet the relationships and timing between atmospheric oxygenation and crustal oxidation remain uncertain. Prior reconstructions of atmospheric O2 exist, but direct metrics for crustal redox evolution are scarce. The authors propose that manganese (Mn), a redox-sensitive and abundant crustal element that occurs in multiple oxidation states across numerous minerals, can serve as a proxy to track crustal oxidation through time. By leveraging large mineralogical databases, they aim to quantify temporal trends in Mn mineral oxidation states and evaluate how these trends relate to atmospheric oxygen histories, including possible time lags reflecting the rate at which the shallow crust equilibrates with the atmosphere.

Multiple proxies have been used to reconstruct Earth's atmospheric oxygen history, including carbon and sulfur isotopes, trace elements in marine sediments, and organic biomarkers. The GOE (~2.5 Ga) marks sustained atmospheric O2 accumulation, potentially preceded by transient 'whiffs' of oxygen. After a long period of relative stasis, oxygen levels rose notably near 600 Ma and fluctuated into the Phanerozoic. Oxidative weathering greatly increased mineral diversity, with more than half of known mineral species arising from oxidation processes. Previous mineral-based redox proxies, such as rhenium in molybdenite, indicate crustal oxidation changes lagging atmospheric O2 by hundreds of millions of years, suggesting minerals can record subsurface redox evolution. However, a comprehensive, quantitative assessment of redox changes recorded by a widely distributed, multi-valent element like Mn across geologic time had not been undertaken.

Data sources: The authors compiled occurrences of Mn-bearing minerals from mindat.org (as of 20 November 2015) and ages from the Mineral Evolution Database (MED, rruff.info/ima). The dataset includes 22,064 Mn mineral–locality pairs, of which 2,666 have associated geologic ages. Definition and classification: An Mn mineral is any IMA-approved species with Mn as an essential element. Mn occurring as a minor constituent in other minerals (typically Mn2+ substituting for Fe2+) and geologically young ocean-floor Mn nodules do not affect temporal trends considered. All 560 known Mn mineral species were assigned to lists corresponding to predominant Mn oxidation states +2, +3, and +4. Oxidation states were determined by charge balance, structural literature for sulfides, and published spectroscopic/structural refinements. A small number of minerals containing multiple Mn valences (e.g., hausmannite, Mn2+Mn3+2O4) appear on multiple lists. Temporal binning and metrics: For the 2,666 dated pairs, occurrences were binned into 50 Myr intervals. For each bin, the frequency of occurrences per Mn oxidation state was tabulated. Mineral fractions for each oxidation state were computed as occurrences of that state divided by total Mn occurrences in the bin. The average Mn oxidation state per bin was calculated as 2x + 3y + 4z, where x, y, z are the fractions of +2, +3, and +4 states, respectively. Bins with five or fewer occurrences were excluded from fraction and average calculations. Uncertainty estimation: Fits of raw occurrence counts to discrete distributions (Poisson, binomial, negative binomial, geometric, hypergeometric) were evaluated via BIC in R; the negative binomial best fit (size ≈ 0.622, μ ≈ 11.569, p ≈ 0.051). Given small p, standard error for counts was approximated as √N; errors on fractions were √N/T (T = total bin counts). Errors on average oxidation states were derived from propagation of variance among occurrences. Comparison with atmospheric oxygen: Phanerozoic average Mn oxidation states were compared to atmospheric O2 reconstructions (e.g., Bergman et al. 2004), fitting both to y = A sin(ωt) + B cos(φt) via iterative least-squares, yielding oscillation periods and phase shifts. The time lag was computed as [φ(O2)/ω(O2)] − [φ(Mn)/ω(Mn)]. Oxygen reconstructions were shifted forward by the derived lag to assess correlation of minima and maxima with Mn oxidation state trends.

- Mn mineralization frequency peaks align with supercontinent assembly intervals, with distinctive behavior around Rodinia (peaks before and after but few during). - Prior to ~600 Ma, trends are less certain due to fewer preserved deposits; however, during Kenorland (3.0–2.5 Ga), the average Mn oxidation state increased from +2 to >+2.5, potentially reflecting Mn-oxidizing metabolisms predating cyanobacterial photosystem II and/or localized 'whiffs' of O2 with preservation biases. - Post-GOE, there is a long-term rise in the proportion of Mn3+ and Mn4+ mineral occurrences relative to Mn2+, especially evident over the last 600 Ma. The average Mn oxidation state increases from 600 Ma to present, with multi-point minima and maxima exceeding uncertainties. - Atmospheric O2 reconstructions exhibit local maxima (~550–600, 250–300, 50–100 Ma) and minima (~400–450, 150–200 Ma, and near present) that closely mirror Mn oxidation state patterns when a time lag is considered. - Sinusoidal fits yield oscillation periods of ~35.7 Myr (O2) and ~35.0 Myr (Mn), and a phase-derived time lag of ~66 ± 1 Myr, indicating changes in Mn oxidation state follow atmospheric O2 by about 66 Myr on average. - Shifting O2 reconstructions forward by ~66 Myr produces strong correspondence between Mn oxidation state extrema and atmospheric O2 trends, implying direct atmospheric control on shallow crustal oxidation on geologic timescales. - The ~66 Myr lag contrasts with longer lags inferred from deeper-forming molybdenite rhenium data, suggesting depth-dependent equilibration rates and offering a strategy to resolve redox evolution with depth using different mineral groups.

The results substantiate Mn mineral oxidation states as a quantitative proxy for the redox evolution of Earth's shallow crust. The strong, lagged correlation with atmospheric oxygen reconstructions implies that atmospheric oxygenation drives crustal oxidation but requires tens of millions of years for equilibration, reflecting processes such as oxidative weathering, brittle fracturing, fluid circulation, and tectonic mixing. Peaks in Mn mineralization coincident with supercontinent cycles emphasize tectonic regulation of mineral formation and preservation. The early Kenorland-era rise in Mn oxidation suggests pre-GOE oxidative processes or localized oxygen availability, though preservation biases may partly influence early records. The shorter lag for shallow, Mn-bearing minerals relative to deeper molybdenite indicates a depth gradient in oxidation timescales, pointing to a layered progression of crustal redox equilibration. These findings bridge atmospheric histories with subsurface redox dynamics and demonstrate the power of large mineral databases to interrogate Earth system co-evolution.

This study introduces a mineralogical proxy for shallow crustal redox state based on the distribution and oxidation states of Mn minerals through time. The authors demonstrate a coherent, lagged relationship between atmospheric oxygen fluctuations and the average oxidation state of Mn in the crust over the last billion years, with an average lag of ~66 Myr. The work quantifies aspects of crustal oxidation timing and highlights the role of tectonics and surface processes in mediating equilibration with the atmosphere. Future research should extend this approach to other redox-sensitive metals (e.g., first-row transition elements) and mineral groups to reconstruct redox evolution at varying crustal depths and across reservoirs (crust, ocean), thereby refining constraints on oxygen fugacity through deep time.

- Pre-600 Ma trends carry greater uncertainty due to sparse and potentially biased preservation of mineral deposits, which may favor more oxidized Mn minerals. - Bins with ≤5 occurrences were excluded, reducing temporal resolution in the deep past. - The proxy predominantly samples minerals forming in the upper ~1 km of the crust; deeper crustal redox states are less directly constrained by Mn minerals. - Individual mineral-locality records may have errors, though large sample sizes mitigate random errors. - Only one atmospheric O2 reconstruction (with available data) was used for quantitative sinusoidal fitting; other reconstructions were compared qualitatively after time-shifting. - Classification of oxidation states in some minerals required literature-based interpretation, introducing potential systematic uncertainties.