Constraining the rise of oxygen with oxygen isotopes

Reconciling discrepant minor sulfur isotope records of the Great Oxidation Event

Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences. Given its origin in oxygen-free photochemistry, mass-independent sulfur isotope fractionation (S-MIF) is widely accepted as a geochemical fingerprint of an anoxic atmosphere. Nevertheless, S-MIF recycling through oxidative sulfide weathering-commonly termed the crustal memory effect (CME)-potentially decouples the multiple sulfur isotope (MSI) record from coeval atmospheric chemistry. Herein, however, after accounting for unrecognised temporal and spatial biases within the Archaean-early-Palaeoproterozoic MSI record, we demonstrate that the global expression of the CME is barely resolvable; thereby validating S-MIF as a tracer of contemporaneous atmospheric chemistry during Earth's incipient oxygenation. Next, utilising statistical approaches, supported by new MSI data, we show that the reconciliation of adjacent, yet seemingly discrepant, South African MSI records requires that the rare instances of post-2.3-billion-year-old S-MIF are stratigraphically restricted. Accepting others' primary photochemical interpretation, our approach demands that these implied atmospheric dynamics were ephemeral, operating on sub-hundred-thousand-year timescales. Importantly, these apparent atmospheric relapses were fundamentally different from older putative oxygenation episodes, implicating an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event.

Investigation of water-soluble organic constituents and their spatio-temporal heterogeneity over the Tibetan Plateau

Investigating the migration and transformation of carbonaceous and nitrogenous matter in the cryosphere areas is crucial for understanding global biogeochemical cycle and earth's climate system. However, water-soluble organic constituents and their transformation in multiple water bodies are barely investigated. Water-soluble organic carbon (WSOC) and organic nitrogen (WSON), and particulate black carbon (PBC) in multiple types of water bodies in eastern Tibetan Plateau (TP) cryosphere for the first time have been systematically investigated. Statistical results exhibited that from south to north and from east to west of this region, WSOC concentrations in alpine river runoff were gradually elevated. WSOC and nitrogenous matter in the alpine river runoff and precipitation in the glacier region presented distinct seasonal variations. WSON was the dominant component (63.4%) of water-soluble total nitrogen in precipitation over high-altitude southeastern TP cryosphere. Water-soluble carbonaceous matter dominated the carbon cycle in the TP cryosphere, but particulate carbonaceous matter in the alpine river runoff had a small fraction of the cryospheric carbon cycle. Analysis of optical properties illustrated that PBC had a much stronger light absorption ability (MAC-_PBC: 2.28 ± 0.37 m² g^-1) than WSOC in the alpine river runoff (0.41 ± 0.26 m² g^-1). Ionic composition was dominated by SO₄^2-, NO₃^-, and NH₄⁺ (average: 45.13 ± 3.75%) in the snow of glaciers, implying important contribution of (fossil fuel) combustion sources over this region. The results of this study have essential implications for understanding the carbon and nitrogen cycles in high altitude cryosphere regions of the world. Future work should be performed based on more robust in-situ observations and measurements from multiple environmental medium over the cryosphere areas, to ensure ecological protection and high-quality development of the high mountain Asia.

Bulk and grain-scale minor sulfur isotope data reveal complexities in the dynamics of Earth's oxygenation

The permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF–bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ³³S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed ³³S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth’s oxygenation.

The disappearance of mass-independent sulfur isotope fractionation (S-MIF) within the c. 2.3-billion-year-old (Ga) Rooihoogte Formation has been heralded as a chemostratigraphic marker of permanent atmospheric oxygenation. Reports of younger S-MIF, however, question this narrative, leaving significant uncertainties surrounding the timing, tempo, and trajectory of Earth’s oxygenation. Leveraging a new bulk quadruple S-isotope record, we return to the South African Transvaal Basin in search of support for supposed oscillations in atmospheric oxygen beyond 2.3 Ga. Here, as expected, within the Rooihoogte Formation, our data capture a collapse in Δ^3×S values and a shift from Archean-like Δ³⁶S/Δ³³S slopes to their mass-dependent counterparts. Importantly, the interrogation of a Δ³³S-exotic grain reveals extreme spatial variability, whereby atypically large Δ³³S values are separated from more typical Paleoproterozoic values by a subtle grain-housed siderophile-enriched band. This isotopic juxtaposition signals the coexistence of two sulfur pools that were able to escape diagenetic homogenization. These large Δ³³S values require an active photochemical sulfur source, fingerprinting atmospheric S-MIF production after its documented cessation elsewhere at ∼2.4 Ga. By contrast, the Δ³³S monotony observed in overlying Timeball Hill Formation, with muted Δ³³S values (<0.3‰) and predominantly mass-dependent Δ³⁶S/Δ³³S systematics, remains in stark contrast to recent reports of pronounced S-MIF within proximal formational equivalents. If reflective of atmospheric processes, these observed kilometer-scale discrepancies disclose heterogenous S-MIF delivery to the Transvaal Basin and/or poorly resolved fleeting returns to S-MIF production. Rigorous bulk and grain-scale analytical campaigns remain paramount to refine our understanding of Earth’s oxygenation and substantiate claims of post-2.3 Ga oscillations in atmospheric oxygen.

Microbial Diversity and Sulfur Cycling in an Early Earth Analogue: From Ancient Novelty to Modern Commonality

Life emerged and diversified in the absence of molecular oxygen. The prevailing anoxia and unique sulfur chemistry in the Paleo-, Meso-, and Neoarchean and early Proterozoic eras may have supported microbial communities that differ from those currently thriving on the earth’s surface. Zodletone spring in southwestern Oklahoma represents a unique habitat where spatial sampling could substitute for geological eras namely, from the anoxic, surficial light-exposed sediments simulating a preoxygenated earth to overlaid water column where air exposure simulates oxygen intrusion during the Neoproterozoic era. We document a remarkably diverse microbial community in the anoxic spring sediments, with 340/516 (65.89%) of genomes recovered in a metagenomic survey belonging to 200 bacterial and archaeal families that were either previously undescribed or that exhibit an extremely rare distribution on the current earth. Such diversity is underpinned by the widespread occurrence of sulfite, thiosulfate, tetrathionate, and sulfur reduction and the paucity of sulfate reduction machineries in these taxa. Hence, these processes greatly expand lineages mediating reductive sulfur-cycling processes in the tree of life. An analysis of the overlaying oxygenated water community demonstrated the development of a significantly less diverse community dominated by well-characterized lineages and a prevalence of oxidative sulfur-cycling processes. Such a transition from ancient novelty to modern commonality underscores the profound impact of the great oxygenation event on the earth’s surficial anoxic community. It also suggests that novel and rare lineages encountered in current anaerobic habitats could represent taxa that once thrived in an anoxic earth but have failed to adapt to earth’s progressive oxygenation.

A 200-million-year delay in permanent atmospheric oxygenation

The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth's habitability¹. Early atmospheric oxygenation occurred over a protracted period of extreme climatic instability marked by multiple global glaciations^2,3, with the initial rise of oxygen concentration to above 10^-5 of the present atmospheric level constrained to about 2.43 billion years ago^4,5. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago⁴, which represents the estimated timing of irreversible oxygenation of the atmosphere^6,7. Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron-sulfur-carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10^-5 of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated.

The Great Oxidation Event preceded a Paleoproterozoic "snowball Earth"

The causality between the Paleoproterozic Great Oxidation Event (GOE) and a global “snowball Earth” glaciation has remained unresolved due to an inability to determine their relative timing. We present quadruple sulfur isotope data from northwest Russia which constrain the GOE between 2,501 and 2,434 Ma. These are the tightest temporal and stratigraphic constraints ever presented for the GOE and show that the GOE predates Paleoproterozic glaciation in Russia and snowball Earth deposits in South Africa. Our results preclude hypotheses of Earth’s oxygenation in which global glaciation precedes or causes the evolution of oxygenic photosynthesis.

The inability to resolve the exact temporal relationship between two pivotal events in Earth history, the Paleoproterozoic Great Oxidation Event (GOE) and the first “snowball Earth” global glaciation, has precluded assessing causality between changing atmospheric composition and ancient climate change. Here we present temporally resolved quadruple sulfur isotope measurements (δ³⁴S, ∆³³S, and ∆³⁶S) from the Paleoproterozoic Seidorechka and Polisarka Sedimentary Formations on the Fennoscandian Shield, northwest Russia, that address this issue. Sulfides in the former preserve evidence of mass-independent fractionation of sulfur isotopes (S-MIF) falling within uncertainty of the Archean reference array with a ∆³⁶S/∆³³S slope of −1.8 and have small negative ∆³³S values, whereas in the latter mass-dependent fractionation of sulfur isotopes (S-MDF) is evident, with a ∆³⁶S/∆³³S slope of −8.8. These trends, combined with geochronological constraints, place the S-MIF/S-MDF transition, the key indicator of the GOE, between 2,501.5 ± 1.7 Ma and 2,434 ± 6.6 Ma. These are the tightest temporal and stratigraphic constraints yet for the S-MIF/S-MDF transition and show that its timing in Fennoscandia is consistent with the S-MIF/S-MDF transition in North America and South Africa. Further, the glacigenic part of the Polisarka Formation occurs 60 m above the sedimentary succession containing S-MDF signals. Hence, our findings confirm unambiguously that the S-MIF/S-MDF transition preceded the Paleoproterozoic snowball Earth. Resolution of this temporal relationship constrains cause-and-effect drivers of Earth’s oxygenation, specifically ruling out conceptual models in which global glaciation precedes or causes the evolution of oxygenic photosynthesis.