Reexamination of 2.5-Ga "whiff" of oxygen interval points to anoxic ocean before GOE

Abstract

Transient appearances of oxygen have been inferred before the Great Oxygenation Event (GOE) [∼2.3 billion years (Ga) ago] based on redox-sensitive elements such as Mo and S—most prominently from the ∼2.5-Ga Mount McRae Shale in Western Australia. We present new spatially resolved data including synchrotron-based x-ray spectroscopy and secondary ion mass spectrometry to characterize the petrogenesis of the Mount McRae Shale. Sediments were primarily composed of organic matter and volcanic ash (a potential source of Mo), with U-Pb ages revealing extremely low sedimentation rates. Catagenesis created bedding-parallel microfractures, which subsequently acted as fluid pathways for metasomatic alteration and recent oxidative weathering. Our collective observations suggest that the bulk chemical datasets pointing toward a “whiff” of oxygen developed during postdepositional events. Nonzero Δ³³S in trace-metal–poor, early diagenetic pyrite and the unusually enriched organic carbon at low sedimentation rates instead suggest that environmental oxygen levels were negligible ∼150 million years before the GOE.

Fig. 1.. Multiscale evidence for postdepositional sulfide mineralization.

(A) Stratigraphic column of ABDP-9 focused on Mount McRae Shale from (3). Interval where evidence for whiff of oxygen is highlighted by black arrow; this section notably coincides with fissile cleavage (see Fig. 4), clinochlore veins, and extensive pyrite mineralization. Monazite and xenotime, often intergrown with clinochlore veins, dated post-GOE between ∼1650 and 2200 Ma. BIF, banded iron formation; Fm., Formation. (B) Examples of early pyrite nodules deforming surrounding laminae with secondary pyrite halos (133.91 m) and pyrite in planar laminations closely associated with bedding-parallel clinochlore veins (132.24, 131.01, and 142.52 to 142.60 m). (C) Pyrite euhedra and clotted euhedra in sedimentary matrix concentrated against clinochlore seams (135.02 m); seams and clotted pyrite euhedra cross-cut by vertical clinochlore vein (135.44 m); compressional faulting of pyrite-rich seam with additional pyrite euhedra following fault (138.54 m); pyrite seams intergrown with clinochlore-filled overpressure veins with euhedral pyrite overgrowths (147.16 m); early diagenetic pyrite nodule that includes pyritized volcanic clasts (153.54 m). (D) Pyrite nodule with brighter (As-rich) overgrowth with an accessory metal sulfide (white) (135.02 m); euhedral pyrite crystals with As-rich pyrite overgrowth and cement (138.54 m); two examples of sulfide-mineralized volcaniclastic particles and glass (147.16 m); euhedral pyrite crystals cross-cutting shale particles (153.54 m).

Fig. 2.. Petrographic relationships of metals.

(A) Photographic scan of pyrite nodule at 138.09 m highlighting multiple generations of sulfide formation—early nodular formation with later rim and cross-cross pattern of veins. The circle containing irregular chips is an impregnated pyrite standard. The red rectangles show regions focused on in (B) and Fig. 3. (B) Synchrotron XRF map of the same thin section, with metals color-coded. Arsenic is found in the features representing postdepositional alteration events (the veins and the rim). White arrows point to a few of the more discrete copper-bearing phases. XRF maps were not standardized, so each element’s abundance is relative to highs/lows of the target region. (C) Correlation between molybdenum and arsenic using SIMS measurements in pyrite of the same sample. A diversity of molybdenum abundances was found with a strong correlation between high-molybdenum and arsenic-rich sites, which petrographically are shown to be late. The distinct trendlines suggest multiple generations or evolution of fluids.

Fig. 3.. SIMS measurements of S isotopes and metals.

(A) XRF map on the edge of the pyrite nodule at 138.09 m (Fig. 2) with overlay of reflected light images showing where SIMS measurements were collected. (B) Δ³³S and Mo abundance are plotted visually on the thin section of 138.09 m. Clear trends are noticeable with higher Δ³³S along veins that also have higher molybdenum (and arsenic) concentrations. Analysis of Δ³³S fine-grained pyrite outside the nodule also contains higher values and abundant arsenic. (C) Combined S isotope measurements from four samples where each measurement was labeled as early, mixed, or late based on petrography (see figs. S13 to S16). Early pyrite textures have slightly negative Δ³³S and near-zero to slightly negative δ³⁴S, whereas late textures show an increase in Δ³³ and disparate changes in δ³⁴S. Similar trends are noted between samples suggesting that patterns are not related to stratigraphy, but instead due to multiple generations of fluids moving through these samples, recrystallizing pyrite and affecting metal and S isotopic compositions. VCDT, Vienna Canyon Diablo Troilite.

Fig. 4.. Sulfur mineralogy and fissile cleavage within the Mount McRae Shale.

(A to C) Core scan of 147.21 m containing pyrite nodules (labeled white box), which show evidence for oxidation through dissolution/porous texture and mineral transformations in reflected light and backscatter electron (BSE) images. (D and E) BSE images overlain by elemental color maps via EDS reveals Ca, S precipitates in bedding-parallel fractures lined by precipitated silicate cements. Ablation due to electron beam (visible pits) suggests that these sulfur phases are hydrated. (F and G) Core scans displaying fissile nature of the rocks, with labeled locations of BSE images in yellow and XRF maps at multiple S energies in white. (H to K) XRF maps of total sulfur concentrations at 2490 eV (left column) display sulfur phases distributed throughout the sample. Endmember fitting of XRF maps at multiple S energies (2469.5, 2471, 2472.5, 2476, 2482.5, and 2490 eV) identifies these sulfur minerals (fig. S19), and a mineralogical map can be made (right column). Pyrite and sphalerite are present in distinct “hot spots” as disseminated crystals. However, sulfate occurs in widespread bedding-(sub)parallel fractures as discontinuous stringers and along fracture surfaces, consistent with BSE-EDS observations.

Fig. 5.. Schematic diagram of petrogenesis of Mount McRae Shale based on our observations and geochemical data.

(A) Deposition of organic carbon and air-fall volcaniclastic debris. (B) Early stages of diagenesis including pyrite nodule growth and pyrite permineralization of sediments. (C) Compaction during burial and thermal maturation of organic-rich sediments in catagenesis. (D) Hydrocarbon formation leads to episodic fluid overpressure, microfracturing, and hydrocarbon expulsion along bedding-parallel microfractures eventually developing into fissile cleavage. (E) Migration of cratonic fluids along microfractures accompanied by at least three episodes of metasomatic mineralization, including pyrite rich in both As and Mo. (F) Introduction of meteoric fluids and oxidative weathering resulting in pyrite oxidation and calcium sulfate precipitation along fissile cleavage.