Sulphur cycling in a Neoarchaean microbial mat

Abstract

Multiple sulphur (S) isotope ratios are powerful proxies to understand the complexity of S biogeochemical cycling through Deep Time. The disappearance of a sulphur mass-independent fractionation (S-MIF) signal in rocks <~2.4 Ga has been used to date a dramatic rise in atmospheric oxygen levels. However, intricacies of the S-cycle before the Great Oxidation Event remain poorly understood. For example, the isotope composition of coeval atmospherically derived sulphur species is still debated. Furthermore, variation in Archaean pyrite δ³⁴ S values has been widely attributed to microbial sulphate reduction (MSR). While petrographic evidence for Archaean early-diagenetic pyrite formation is common, textural evidence for the presence and distribution of MSR remains enigmatic. We combined detailed petrographic and in situ, high-resolution multiple S-isotope studies (δ³⁴ S and Δ³³ S) using secondary ion mass spectrometry (SIMS) to document the S-isotope signatures of exceptionally well-preserved, pyritised microbialites in shales from the ~2.65-Ga Lokammona Formation, Ghaap Group, South Africa. The presence of MSR in this Neoarchaean microbial mat is supported by typical biogenic textures including wavy crinkled laminae, and early-diagenetic pyrite containing <26‰ μm-scale variations in δ³⁴ S and Δ³³ S = -0.21 ± 0.65‰ (±1σ). These large variations in δ³⁴ S values suggest Rayleigh distillation of a limited sulphate pool during high rates of MSR. Furthermore, we identified a second, morphologically distinct pyrite phase that precipitated after lithification, with δ³⁴ S = 8.36 ± 1.16‰ and Δ³³ S = 5.54 ± 1.53‰ (±1σ). We propose that the S-MIF signature of this secondary pyrite does not reflect contemporaneous atmospheric processes at the time of deposition; instead, it formed by the influx of later-stage sulphur-bearing fluids containing an inherited atmospheric S-MIF signal and/or from magnetic isotope effects during thermochemical sulphate reduction. These insights highlight the complementary nature of petrography and SIMS studies to resolve multigenerational pyrite formation pathways in the geological record.

Figure 1

Simplified geological map of Transvaal and Griqualand West sediments, the approximate location of BH1‐SACHA (star) and the inferred fault trace of the Kheis sole thrust fault. Insert: the red rectangle shows the location of the main map relative to other Transvaal sediments and the Kaapvaal craton (beige). Adapted from Altermann and Wotherspoon (1995)

Figure 2

BH1‐SACHA bulk sulphur isotope data plotted against core depth (m) with the corresponding sedimentary log. Brecciated zones largely reflect post‐depositional tectonic deformation. The pyritised microbialites analysed in this study were sampled at a depth of 3,184 m (grey band). Lokam., represents Lokammona. Data from Izon et al. (2015) and stratigraphic log adapted from Altermann & Siegfried, 1997

Figure 3

(a) Reflected light scanner image of sample‐3184 from the Lokammona Formation showing the areas of SIMS analyses (white dots). Blue dot = Figure 6 analysis area; green dot = Figure 7 analysis area. Divisions on the scale bar represent 1 mm. (b) A trace of the image in (a) showing laminae compositions, type examples of sedimentary structures discussed in the text and the location of the normal microfault. Modal percentages of laminae compositions are shown in brackets (pyrite/matrix)

Figure 4

BSE images of different sedimentary structures in sample‐3184. (a) Wavy crinkled internal lamination composed of type 1 pyrite and disseminated pyrite. (b) Wavy crinkled internal lamination composed of type 1 pyrite with type 2 pyrite overgrowths. (c) Pyrite concretion. Note that the concretion both cross‐cuts and causes deformation of the lamination. (d) The normal microfault that is infilled by quartz and clay minerals. Note the microfault and vein cross‐cut type 1 pyrite; type 2 pyrite cross‐cuts the microfault and vein. Therefore, the relative order of formation is as follows: pyrite 1 precipitated first, brittle deformation caused microfault formation, the microfault was infilled, and finally type 2 pyrite precipitated. py, py 1, py 2 and qtz represent pyrite, type 1 pyrite, type 2 pyrite and quartz, respectively

Figure 5

Plot of multiple S‐isotope data (δ³⁴S and Δ³³S) measured via SIMS. SIMS error bars are 1SE for each measurement (n = 15 cycles). Green triangles represent a mixed signal, where the area of analysis sampled both type 1 and type 2 pyrite. The orange line is the Archaean reference array as described by Ono et al. (2003)

Figure 6

Combined petrography with SIMS S‐isotope data shows the typical textural, δ³⁴S and Δ³³S characteristics of type 1 pyrite. (a) Reflected light photomicrograph of the analytical grid location. (b) BSE image of the analysis area overlain by a SIMS δ³⁴S image constructed by spline interpolation of the analytical grid (n = 31). Note that the analysis area is composed of a ~500‐μm aggregate of anhedral to subhedral, 1‐ to 20‐μm pyrite crystals. (c) Plot to show SIMS δ³⁴S against Δ³³S data (‰). Note the variation in δ³⁴S data is significant; the variation in Δ³³S data is not significant. Error bars represent 1SE for each measurement (n = 15 cycles) (d) BSE image of the analysis area overlain by a SIMS Δ³³S image constructed by spline interpolation of the analytical grid (n = 31)

Figure 7

The contrasting textural and S‐isotope signatures of type 1 and type 2 pyrite. (a) Reflected light photomicrograph of the area of analysis. (b) BSE image of the analysis area overlain by a SIMS δ³⁴S image constructed by spline interpolation of the analytical grid (n = 15). Note the ~300‐μm cubic type 2 pyrite grain, overgrowing type 1 pyrite aggregates. (c) Plot to show SIMS δ³⁴S against Δ³³S data (‰). Note the significant difference in Δ³³S for type 1 and 2 pyrite. Error bars represent 1SE for each measurement (n = 15 cycles). Blue circles = type 1 pyrite; green circles = type 2 pyrite. (d) BSE image of the analysis area overlain by a SIMS Δ³³S image constructed by spline interpolation of the analytical grid (n = 15)

Figure 8

Graphs showing the calculated δ³⁴S values of the hydrogen sulphide instantaneous product relative to the proportion of sulphate consumed (f), the initial δ³⁴S_sulphate composition and the MSR fractionation factor (α_{source‐product}). The grey rectangles correspond to the range of δ³⁴S_sulphide compositions measured in type 1 pyrite in sample‐3184. (a) α = 6‰, (b) α = 12‰

Figure 9

Hypothesised formation of post‐lithification pyrite with an anomalous Δ³³S signal. Sulphur can be sourced via three pathways: (A) seawater, (B) dissolution of sulphate‐ or sulphide‐bearing minerals or (C) the disproportionation of SO ₂ in magmatic‐hydrothermal or magmatic steam environments