Earth's surface underwent a dramatic transition ~2.3 billion years ago when atmospheric oxygen first accumulated during the Great Oxidation Event, but the detailed composition of the reducing early ...atmosphere is not well known. Here we develop mercury (Hg) stable isotopes as a proxy for paleoatmospheric chemistry and use Hg isotope data from 2.5 billion-year-old sedimentary rocks to examine changes in the Late Archean atmosphere immediately prior to the Great Oxidation Event. These sediments preserve evidence of strong photochemical transformations of mercury in the absence of molecular oxygen. In addition, these geochemical records combined with previously published multi-proxy data support a vital role for methane in Earth's early atmosphere.
SignificanceThe 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 Δ
S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed
S-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.
Emerging evidence suggests that atmospheric oxygen may have varied before rising irreversibly ∼2.4 billion years ago, during the Great Oxidation Event (GOE). Significantly, however, pre-GOE ...atmospheric aberrations toward more reducing conditions—featuring a methane-derived organic-haze—have recently been suggested, yet their occurrence, causes, and significance remain underexplored. To examine the role of haze formation in Earth’s history, we targeted an episode of inferred haze development. Our redox-controlled (Fespeciation) carbon- and sulfur-isotope record reveals sustained systematic stratigraphic covariance, precluding nonatmospheric explanations. Photochemical models corroborate this inference, showing Δ36S/Δ33S ratios are sensitive to the presence of haze. Exploiting existing age constraints, we estimate that organic haze developed rapidly, stabilizing within ∼0.3 ± 0.1 million years (Myr), and persisted for upward of ∼1.4 ± 0.4 Myr. Given these temporal constraints, and the elevated atmospheric CO₂ concentrations in the Archean, the sustained methane fluxes necessary for haze formation can only be reconciled with a biological source. Correlative δ13COrg and total organic carbon measurements support the interpretation that atmospheric haze was a transient response of the biosphere to increased nutrient availability, with methane fluxes controlled by the relative availability of organic carbon and sulfate. Elevated atmospheric methane concentrations during haze episodes would have expedited planetary hydrogen loss, with a single episode of haze development providing up to 2.6–18 × 1018 moles of O₂ equivalents to the Earth system. Our findings suggest the Neoarchean likely represented a unique state of the Earth system where haze development played a pivotal role in planetary oxidation, hastening the contingent biological innovations that followed.
The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, ...timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte-Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability.
The Great Oxidation Event (GOE) represents a crucial juncture in Earth history, signifying the rise in atmospheric oxygen from parts per million to percent levels at ∼2.45–2.32 billion-years-ago ...(Ga). Although planetary oxygenation undoubtedly led to the inception of the contemporary Earth system, the trigger(s) and mechanism(s) controlling this chemical reorganisation remain elusive. Quantitative estimates of the atmosphere's composition in the prelude to the GOE are central to understanding this oxygenation event. Previous analyses of 2.65–2.5 Ga sediments from the Griqualand Basin (South Africa) invoke a tantalising picture of an unusual Earth environment, alluding to an atmosphere periodically dominated by a layer of organic particles (“haze”) formed from methane photolysis. However, as yet this hypothesis has remained untested. Here we present four new coupled carbon and quadruple sulphur isotope records from distal, time equivalent (2.7–2.5 Ga), sedimentary successions from South Africa and Western Australia. These extended records reveal similar chemostratigraphic trends, supporting a dynamic terminal-Neoarchaean atmosphere, oscillating between a hazy state at elevated methane concentrations, and a haze-free anoxic background state. We suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks. These data question the canonical view of a simple, unidirectional planetary oxygenation and signify that the overture to the GOE was governed by complex feedbacks within the Earth–biosphere system.
•We present four ∼2.7–2.5 Ga coupled carbon and quadruple sulphur isotope records.•These data depict a biospheric overhaul of atmospheric chemistry at ∼2.7 Ga.•Post-2.7 Ga atmospheres were dynamic, switching between haze-free and hazy states.•We suggest that amplified biogenic CH4 fluxes drove these atmospheric aberrations.•Biology apparently regimented atmosphere chemistry in the prelude to the GOE.
The molybdenum (Mo) stable isotope system has been applied to a variety of geochemical and environmental problems. In the absence of a universally accepted zero-delta reference material, different ...groups report their data relative to their adopted in-house standards. Rigorous comparison of results generated in different laboratories using different analytical approaches is only possible if the in-house standards are of identical Mo isotope composition. To determine potential isotopic differences among various standards, the δ
98
Mo (
98
Mo/
95
Mo) values of ten Mo standard solutions were measured as part of this study. For six of these solutions, four laboratories carried out an intercalibration. In contrast to previous results, δ
98
Mo of various in-house standards were found to differ by up to 0.37‰. Renormalisation of our new and published Mo-isotope data available for seawater taken from various sites and the USGS rock reference material SDO-1 relative to NIST-SRM-3134, provides a much better agreement among reported δ
98
Mo values for these samples. Relative to NIST-SRM-3134, the δ
98
Mo of SDO-1 is 0.80 ± 0.14‰ (2s), while oxic, open-ocean seawater is characterised by an average δ
98
Mo of 2.09 ± 0.10‰ (2s). This intercalibration provides a solid platform for comparing and amending existing δ
98
Mo values. In addition, we recommend that future Mo isotope studies adopt NIST-SRM-3134 as a universal zero-delta reference material.
δ
98/95
Mo of different in-house standards are not identical. This intercalibration allows comparison to the proposed common delta-zero reference material NIST SRM 3134.
The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in ...the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.
Climate‐driven thawing of Arctic permafrost renders its vast carbon reserves susceptible to microbial degradation, serving as a potentially potent positive feedback hidden within the climate system. ...While seemingly intuitive, the relationship between thermally driven permafrost losses and organic carbon (OC) export remains largely unexplored in natural settings. Filling this knowledge gap, we present down‐core bulk and compound‐specific radiocarbon records of permafrost change from a sediment core taken within the Alaskan Colville River delta spanning the last c. 2,700 years. Fingerprinted by significantly older radiocarbon ages of bulk OC and long‐chain fatty acids, these data expose a thermally driven increase in permafrost OC export and/or deepening of mobilizable permafrost layers over the last c. 160 years after the Little Ice Age. Comparison of OC content and radiocarbon data between recent and Roman warming episodes likely implies that the rate of warming, alongside the prevailing boundary conditions, may dictate the ultimate fate of the Arctic's permafrost inventory. Our findings highlight the importance of leveraging geological records as archives of Arctic permafrost mobilization dynamics with temperature change.
Plain Language Summary
Temperature rise in the Arctic is likely causing enhanced thawing of perennially frozen soil (permafrost), leading to potential decomposition of organic matter and release of greenhouse gases. Models forecasting the potential release of permafrost organic carbon (OC) largely rely on historical records or experimental results over the past decades, leaving large uncertainties for long‐term predictions. In this study, a sediment core from the Alaskan Colville River delta was analyzed to provide a sub‐centennial long‐term record of changes of permafrost OC export. The radiocarbon results of bulk OC demonstrated a close association with temperature change, highlighting the increase of permafrost OC export and/or deepening of mobilizable permafrost layers for the past 160 years as a result of Arctic warming. The 2,700‐year record also implies that some factors like the rate of warming and the temperature before warming may need to be considered in climate models for better predictions.
Key Points
Arctic warming has likely caused an increase of permafrost organic carbon export and/or deepening of mobilizable permafrost layers over the last 160 years
Bank erosion is likely a key mechanism of mobilizing permafrost to the coast under warming conditions
The rate of warming and the prevailing boundary conditions may be important modulators of permafrost thawing
Sulfate-driven anaerobic oxidation of methane (SD-AOM) is ubiquitous in marine sedimentary environments, governing the global methane budget and the redox evolution of Earth's surface. Tracing SD-AOM ...and organoclastic sulfate reduction (OSR) from the pyrite archive is key to the reconstruction of SD-AOM activity and paleoenvironmental interpretation. However, discriminating the origins of pyrite – basically SD-AOM and OSR – is commonly challenging due to frequent overlap of δ34Spy values. Multiple sulfur isotopes of pyrite are expected to be an effective tool to distinguish between OSR and SD-AOM, yet variable uncertainties and unknowns remain. Here we investigated the δ34S and Δ33S values of pore-water sulfate and authigenic pyrite from a piston core taken on the continental slope of the South China Sea. A positive Δ33S - δ34S correlation of pore-water sulfate is observed in the upper OSR-dominated zone, resulting in Δ33S and δ34S values of sulfate diffusing into the sulfate-methane transition zone (SMTZ) >0.1‰ and >30‰ larger than the corresponding values of seawater sulfate. A negative Δ33S - δ34S trajectory of pore-water sulfate and pyrite is observed for the SMTZ, agreeing with low sulfur isotope fractionation characteristic for SD-AOM and a diagnostic large Δ33S - δ34S field of SD-AOM-derived pyrite. These findings elucidate that the multiple sulfur isotope systematics of pyrite in methane-bearing sediment depends on (1) mass transport effects of dissolved sulfate and sulfide, (2) the relative contribution of OSR and SD-AOM to the pore-water sulfide and pyrite pools, and (3) the sulfur isotope fractionation during microbial sulfate reduction. Our study highlights the importance of mass transport dynamics on the isotopic composition of pyrite, a factor that needs to be considered in any attempt to reconstruct the origin of early diagenetic pyrite and the paleoenvironmental setting with multiple sulfur isotopes.
•SD-AOM leads to negative Δ33S - δ34S trajectories of sedimentary pyrite.•SD-AOM causes negative Δ33S - δ34S correlation of sulfate at low methane flux.•SD-AOM-derived pyrite occupies a larger Δ33S - δ34S field than OSR-derived pyrite.•Δ33Spy - δ34Spy patterns vary with methane flux and sedimentary environment.
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