In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

Abstract

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/ Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28-to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions. | 307MARIN-CARBONNE Et Al.

The Dziani Dzaha Lake: A long‐awaited modern analogue for superheavy pyrites

“…Significant spatial and stratigraphic variability in sulfates and sulfides δ 34 S values are increasingly documented (e.g., Fike et al, 2015; Liu et al, 2019; Pasquier et al, 2017, 2021; Richardson, Newville, et al, 2019; Ries et al, 2009; Thomazo et al, 2019), highlighting the potential role of local processes and hampering the calculation of global pyrite burial rates by simple isotopic mass balance. The isotope signatures of sedimentary sulfate or pyrite are also increasingly suspected of being impacted by secondary processes (e.g., Aller et al, 2010; Farquhar et al, 2013; Marin‐Carbonne et al, 2020; Meyer et al, 2017; Richardson, Keating, et al, 2019). In other words, individual stratigraphic successions may not capture the global sulfur cycle but rather regionally controlled sulfur cycles in partially restricted basins or diagenetic processes (Fike et al, 2015; Liu et al, 2019; Paiste et al, 2020; Pasquier et al, 2021).…”

High‐spatial‐resolution measurements of iron isotopes in pyrites by secondary ion mass spectrometry using the new Hyperion‐II radio‐frequency plasma source

“…The reproducibility obtained on the SpainCR standard (July 2018) is close to that measured in Balmat pyrite with a value of ±0.28‰ (2SD, n = 61). Published data obtained using the 16 O − Duoplasmatron source 68 on three days of analysis show a reproducibility of ±0.44‰ (2SD, n = 17) on the same grain of Balmat, which highlights the better stability of the Hyperion‐II source than of the Duoplasmatron.…”

“…Dots are δ 56 Fe values measured with the Hyperion‐II radio‐frequency plasma source in February 2018 (blue dots) and April 2018 (white dots) sessions. Gray diamonds are δ 56 Fe data from the Duoplasmatron source 68 …”

Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation

“…The interpretation of these pyrite Fe isotopic compositions is not straightforward because they could be controlled by several things: (i) the size of oxidizing iron sinks that removed isotopically heavy Fe 3+ -oxyhydroxides, leaving an isotopically light dissolved Fe 2+ pool from which pyrite formed (9, 10); (ii) microbial dissimilatory Fe 3+ reduction (DIR) that preferentially releases an isotopically light Fe 2+ pool (12,13); and/or (iii) a kinetic isotope effect (KIE) accompanying partial pyrite precipitation, which produces isotopically light pyrite (14,15). The relative importance of these processes remains debated (9)(10)(11)(12)(13)(14)(15)(16)(17)(18), and this uncertainty has hindered quantitative interpretation of the ancient iron cycle. Consequently, Fe isotope records have not yet constrained the degree to which Fe removal on highly productive continental margins was a net sink or source for early O 2 (8).…”

“…They most likely inherited their Fe isotopic compositions from pyrite precipitated in porewater near the sediment-seawater interface, but in some cases dissolution-reprecipitation has eradicated their primary textural features and caused recrystallization into massive forms. In situ work on Archean pyrites suggests that these secondary texture-altering processes do not eradicate primary sedimentary Fe isotopic signatures (18). A major source of iron to porewaters would have been downward diffusion of overlying Fe 2+ -rich seawater into the sediments (9).…”