The effect of rock composition on cyanobacterial weathering of crystalline basalt and rhyolite

The weathering of volcanic rocks contributes significantly to the global silicate weathering budget, effecting carbon dioxide drawdown and long‐term climate control. The rate of chemical weathering is influenced by the composition of the rock. Rock‐dwelling micro‐organisms are known to play a role in changing the rate of weathering reactions; however, the influence of rock composition on bio‐weathering is unknown. Cyanobacteria are known to be a ubiquitous surface taxon in volcanic rocks. In this study, we used a selection of fast and slow growing cyanobacterial species to compare microbial‐mediated weathering of bulk crystalline rocks of basaltic and rhyolitic composition, under batch conditions. Cyanobacterial growth caused an increase in the pH of the medium and an acceleration of rock dissolution compared to the abiotic controls. For example, Anabaena cylindrica increased the linear release rate () of Ca, Mg, Si and K from the basalt by more than fivefold (5.21–12.48) and increased the pH of the medium by 1.9 units. Although A. cylindrica enhanced rhyolite weathering, the increase in was less than threefold (2.04–2.97) and the pH increase was only 0.83 units. The values obtained with A. cylindrica were at least ninefold greater with the basalt than the rhyolite, whereas in the abiotic controls, the difference was less than fivefold. Factors accounting for the slower rate of rhyolite weathering and lower biomass achieved are likely to include the higher content of quartz, which has a low rate of weathering and lower concentrations of bio‐essential elements, such as, Ca, Fe and Mg, which are known to be important in controlling cyanobacterial growth. We show that at conditions where weathering is favoured, biota can enhance the difference between low and high Si‐rock weathering. Our data show that cyanobacteria can play a significant role in enhancing rock weathering and likely have done since they evolved on the early Earth.     

Could land‐based early photosynthesizing ecosystems have bioengineered the planet in mid‐Palaeozoic times?

The Ordovician and Silurian periods were times of major geological activity as regards palaeogeography, volcanism and climate change, the last of these evidenced by a series of cooling episodes and glaciations that climaxed in the Hirnantian (Late Ordovician). The presence of cryptospores in the Darriwilian (Middle Ordovician) marked the advent of higher plants on land. A critical survey of direct (mega‐ and microfossils) and some indirect evidence in succeeding rocks indicates the presence of algae, Bacteria, Cyanobacteria, Fungi, probable lichens, cryptophytes and basal tracheophytes. Similar associations of photosynthesizers and decomposers occur today in cryptogamic covers (CCs), for example biological crusts, except that bryophytes replace cryptophytes (basal embryophytes) and tracheophytes are absent. Thus, extant CCs, which make significant contributions today to global carbon and nitrogen fixation and prevention of erosion, provide an excellent analogue for the impacts of early land vegetation on both lithosphere and atmosphere. As a prerequisite to assessing impacts in Ordovician–Silurian times, with particular consideration of parameters used by climate modellers, the effects of a number of abiotic factors on the growth and survival of extant cryptogamic ground covers and their environmental impacts are reviewed. Factors include photosynthetically active radiation, ultraviolet radiation, temperature, water, oxygen, carbon dioxide, nitrogen, phosphorus, iron, surface roughness and albedo. A survey of the nature and extent of weathering facilitated by such vegetation concludes that it was limited based on depth of weathering when compared with that from rooted tracheophytes today, with minor effects on carbon dioxide drawdown. As global net productivity from Ordovician–Silurian CCs was very probably lower than today, and while the small fraction of intractable material in their organic carbon would have resulted in a more rapid turnover of terrestrial biomass, we conclude that there was decreased possibility of long‐term organic carbon burial. Hence, there would have been very limited increase in atmospheric oxygen and decrease in carbon dioxide resulting from carbon burial.     

Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach

Global weathering of calcium and magnesium silicate rocks provides the long-term sink for atmospheric carbon dioxide (CO(2)) on a timescale of millions of years by causing precipitation of calcium carbonates on the seafloor. Catchment-scale field studies consistently indicate that vegetation increases silicate rock weathering, but incorporating the effects of trees and fungal symbionts into geochemical carbon cycle models has relied upon simple empirical scaling functions. Here, we describe the development and application of a process-based approach to deriving quantitative estimates of weathering by plant roots, associated symbiotic mycorrhizal fungi and climate. Our approach accounts for the influence of terrestrial primary productivity via nutrient uptake on soil chemistry and mineral weathering, driven by simulations using a dynamic global vegetation model coupled to an ocean-atmosphere general circulation model of the Earth's climate. The strategy is successfully validated against observations of weathering in watersheds around the world, indicating that it may have some utility when extrapolated into the past. When applied to a suite of six global simulations from 215 to 50 Ma, we find significantly larger effects over the past 220 Myr relative to the present day. Vegetation and mycorrhizal fungi enhanced climate-driven weathering by a factor of up to 2. Overall, we demonstrate a more realistic process-based treatment of plant fungal-geosphere interactions at the global scale, which constitutes a first step towards developing 'next-generation' geochemical models.     

Geomicrobiology of a Weathering Crust from an Impact Crater and a Hypothesis for its Formation

Understanding the role of microbe-mineral interactions in rock weathering is vital to an understanding of nutrient availability to the biosphere and, in so far as weathering influences carbon dioxide drawdown, climate control. We studied a weathering crust on a resurge tsunami deposit (Loftarstone) from the ～ 455 Ma old Lockne impact crater, central Sweden with an integrated approach using XRD, electron microprobe analysis, SEM-EDS and GCMS analysis of organics. The lichens and fungal hyphae network preferentially weather the chlorite in the Loftarstone compared to feldspars and quartz. We demonstrate, using a fungal isolate (identified by ITS sequencing), that biologically induced dissolution of the calcite component produces cavities which increase the surface area of interaction between the biota and the rock substrate. The weathering crust exfoliates from the rock surface in sheets, which we attribute to the dissolution of the calcite matrix. We present a hypothesis for the crust development. As well as providing insights into weathering on substrates derived from a diversity of high-energy geological disturbances, such as impact events and tsunamis, the weathering crust provides a model system to understand weathering processes in other common lithologies with mixed mineralogies at small spatial scales, including many sedimentary rocks. This work reveals how each different clast plays a unique part in the weathering process, leading to a well-defined weathering sequence.     

Biogenic Enhancement of Weathering and the Stability of the Ecosphere

A dynamic model of the global carbon cycle is used to determine the influence of biotic amplification of weathering on the overall stability of the biosphere. It takes into account the most important processes for the long-term evolution of the Earth. The model is solved under the condition of slow changing luminosity, volcanic activity, and continental area. We find that for large enough amplification factors the system has two stable states, the abiotic and biotic solution. Furthermore, this leads to an extension of the life span of the biosphere by 0.7 Gyr compared to previous studies underestimating the effect of biogenic enhancement of weathering. It can be shown that the biosphere is resilient to random perturbation of the global carbon cycle for the next 0.5-1.0 Gyr.     

西南不同岩性混合小流域化学风化特征

岩石化学风化影响着全球碳循环和气候变化,化学风化速率的估算及控制因素一直是研究的热点。为探究不同岩性混合小流域内化学风化速率及影响因素,于2018年9月对印江河流域、石阡河流域及余庆河流域采集河水样品并分析水化学特征。结果表明:河水的总溶解性固体(TDS)平均值为244 mg·L^-1,高于世界河流平均值(100 mg·L^-1);TDS值的空间差异显示,岩性分布不同导致离子浓度的明显变化。流域中的优势阴阳离子分别为HCO₃^-和Ca²⁺,表明流域碳酸盐岩风化对河水水化学组成起主导作用;通过正演模型解析不同端元(大气、人为、硅酸盐岩和碳酸盐岩)对河流中总溶解阳离子贡献发现,支流中碳酸盐岩贡献变化明显(55.0%～93.9%),空间差异主要受岩性影响;印江河、石阡河和余庆河的硅酸盐岩风化速率分别为4.4、2.8和2.5 t·km^-2·a^-1,相应的CO₂消耗速率为45×10³、18×10^3...     

Rock geochemistry induces stress and starvation responses in the bacterial proteome

Interactions between microorganisms and rocks play an important role in Earth system processes. However, little is known about the molecular capabilities microorganisms require to live in rocky environments. Using a quantitative label‐free proteomics approach, we show that a model bacterium (Cupriavidus metallidurans CH34) can use volcanic rock to satisfy some elemental requirements, resulting in increased rates of cell division in both magnesium‐ and iron‐limited media. However, the rocks also introduced multiple new stresses via chemical changes associated with pH, elemental leaching and surface adsorption of nutrients that were reflected in the proteome. For example, the loss of bioavailable phosphorus was observed and resulted in the upregulation of diverse phosphate limitation proteins, which facilitate increase phosphate uptake and scavenging within the cell. Our results revealed that despite the provision of essential elements, rock chemistry drives complex metabolic reorganization within rock‐dwelling organisms, requiring tight regulation of cellular processes at the protein level. This study advances our ability to identify key microbial responses that enable life to persist in rock environments.     

Actinobacteria—An Ancient Phylum Active in Volcanic Rock Weathering

A molecular biological analysis of Icelandic volcanic rocks of different compositions and glassiness revealed the presence of Actinobacteria as an abundant phylum. In outcrops of basaltic glass they were the dominant bacterial phylum. A diversity of Actinobacteria were cultured from the rocks on rock-agar plates showing that they are capable of growing on rock-derived nutrient sources and that many of the taxa identified by molecular methods are viable, potentially active members of the community. Laboratory batch-culture experiments using a Streptomyces isolate showed that it was capable of enhancing the release of major elements from volcanic rocks, including weathered basaltic glass, crystalline basalt and komatiite, when provided with a carbon source. Actinobacteria of a variety of other sub-orders were also capable of enhancing volcanic rock weathering, measured as Si release. However, most strains did not significantly increase the weathering of the silica-rich rock, obsidian. These data show that Actinobacteria can contribute to volcanic rock weathering and, therefore, the carbonate-silicate cycle. Given their ancient lineage, it is likely they have played a role in rock weathering for over two billion years.     

Evolution of trees and mycorrhizal fungi intensifies silicate mineral weathering

Forested ecosystems diversified more than 350 Ma to become major engines of continental silicate weathering, regulating the Earth''s atmospheric carbon dioxide concentration by driving calcium export into ocean carbonates. Our field experiments with mature trees demonstrate intensification of this weathering engine as tree lineages diversified in concert with their symbiotic mycorrhizal fungi. Preferential hyphal colonization of the calcium silicate-bearing rock, basalt, progressively increased with advancement from arbuscular mycorrhizal (AM) to later, independently evolved ectomycorrhizal (EM) fungi, and from gymnosperm to angiosperm hosts with both fungal groups. This led to ‘trenching’ of silicate mineral surfaces by AM and EM fungi, with EM gymnosperms and angiosperms releasing calcium from basalt at twice the rate of AM gymnosperms. Our findings indicate mycorrhiza-driven weathering may have originated hundreds of millions of years earlier than previously recognized and subsequently intensified with the evolution of trees and mycorrhizas to affect the Earth''s long-term CO₂ and climate history.     

The carbon cycle and carbon dioxide over Phanerozoic time: the role of land plants

A model (GEOCARB) of the long-term, or multimillion year, carbon cycle has been constructed which includes quantitative treatment of (1) uptake of atmospheric CO₂ by the weathering of silicate and carbonate rocks on the continents, and the deposition of carbonate minerals and organic matter in oceanic sediments; and (2) the release of CO₂ to the atmosphere via the weathering of kerogen in sedimentary rocks and degassing resulting from the volcanic-metamorphic-diagenetic breakdown of carbonates and organic matter at depth. Sensitivity analysis indicates that an important factor affecting CO₂ was the rise of vascular plants in the Palaeozoic. A large Devonian drop in CO₂ was brought about primarily by the acceleration of weathering of silicate rock by the development of deeply rooted plants in well-drained upland soils. The quantitative effect of this accelerated weathering has been crudely estimated by present-day field studies where all factors affecting weathering, other than the presence or absence of vascular plants, have been held relatively constant. An important additional factor, bringing about a further CO₂ drop into the Carboniferous and Permian, was enhanced burial of organic matter in sediments, due probably to the production of microbially resistant plant remains (e.g. lignin). Phanerozoic palaeolevels of atmospheric CO₂ calculated from the GEOCARB model generally agree with independent estimates based on measurements of the carbon isotopic composition of palaeosols and the stomatal index for fossil plants. Correlation of CO₂ levels with estimates of palaeoclimate suggests that the atmospheric greenhouse effect has been a major factor in controlling global climate over the past 600 million years.     

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol,Northwest Territories,Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox‐sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle‐reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady‐state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³Cr values, and possibly lower than the igneous inventory (–0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂O₂, but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.     

Heavy Metal Concentration in the Urban Environment of Addis Ababa,Ethiopia

This work assesses the issue of whether the measured concentrations of heavy metals in soil, rocks, surface and ground waters in Addis Ababa can be related to anthropogenic contamination or natural weathering of rocks. Heavy metal analyses of rocks, soils, streams, springs and boreholes have been carried out to identify the presence of potentially harmful solutes. The maximum concentration of total chromium measured is 269 ppm in the northern, industry-free zone of Addis Ababa in the B ₂-horizon of soil profile (cambisol). The Ni/Cr ratio in the rocks is higher than soils, which could indicate the presence of high concentrations of Cr in soils is from weathering processes. A comparative study of different samples collected from various parts of the city indicates that the chemical composition of the hydrothermally affected volcanic rocks plays an important role in increasing heavy metal concentration in the study area. The fresh country rocks contain relatively low concentrations of heavy metals, as shown by background values. The statistical evaluation indicates that the hydrothermally altered rocks contain far higher mean heavy metal concentrations than the fresh acidic rocks (background values). Consequently, soils derived from altered rocks are enriched with respect to heavy metals. From this study it was possible to observe that the rock and soil outcrops of Addis Ababa are anomalously rich in heavy metals derived from hydrothermal activity. Therefore, heavy metal concentrations in the surrounding rocks and soils are related to geogenic sources whereas anthropogenic contribution as a cause of these concentrations is minor.     

Critically testing olivine‐hosted putative martian biosignatures in the Yamato 000593 meteorite—Geobiological implications

On rocky planets such as Earth and Mars the serpentinization of olivine in ultramafic crust produces hydrogen that can act as a potential energy source for life. Direct evidence of fluid–rock interaction on Mars comes from iddingsite alteration veins found in martian meteorites. In the Yamato 000593 meteorite, putative biosignatures have been reported from altered olivines in the form of microtextures and associated organic material that have been compared to tubular bioalteration textures found in terrestrial sub‐seafloor volcanic rocks. Here, we use a suite of correlative, high‐sensitivity, in situ chemical, and morphological analyses to characterize and re‐evaluate these microalteration textures in Yamato 000593, a clinopyroxenite from the shallow subsurface of Mars. We show that the altered olivine crystals have angular and micro‐brecciated margins and are also highly strained due to impact‐induced fracturing. The shape of the olivine microalteration textures is in no way comparable to microtunnels of inferred biological origin found in terrestrial volcanic glasses and dunites, and rather we argue that the Yamato 000593 microtextures are abiotic in origin. Vein filling iddingsite extends into the olivine microalteration textures and contains amorphous organic carbon occurring as bands and sub‐spherical concentrations <300 nm across. We propose that a martian impact event produced the micro‐brecciated olivine crystal margins that reacted with subsurface hydrothermal fluids to form iddingsite containing organic carbon derived from abiotic sources. These new data have implications for how we might seek potential biosignatures in ultramafic rocks and impact craters on both Mars and Earth.     

Expression of Glucose‐6‐Phosphate Dehydrogenase and 6‐Phosphogluconate Dehydrogenase in Oxidative Stress Induced by Long‐Term Iron Toxicity in Rat Liver

Reactive oxygen species (ROS) are highly reactive and oxygen‐containing molecules that are derived by metabolic activities or from environmental sources. Toxicity of heavy metals including iron has the ability to generate ROS in all living organisms. The pentose phosphate pathway enzymes, which are glucose 6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase, produce nicotinamide adenine dinucleotide phosphate (NADPH) that enables cells to counterbalance the oxidative stress via the action of the glutathione system. The results presented here have shown that toxic and nontoxic levels of iron have a strong effect on the expression of both genes. While toxic levels of iron exhibited significant changes in enzyme activity, nontoxic levels had no effect on enzymes in rat liver. Our results are the first evidence to elucidate how oxidative stress induced by long‐term iron toxicity affects both enzymes at the enzymatic and molecular level and also to determine any possible correlation between the enzymatic and molecular levels.     

Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust‐amended agricultural soil

Land‐based enhanced rock weathering (ERW) is a biogeochemical carbon dioxide removal (CDR) strategy aiming to accelerate natural geological processes of carbon sequestration through application of crushed silicate rocks, such as basalt, to croplands and forested landscapes. However, the efficacy of the approach when undertaken with basalt, and its potential co‐benefits for agriculture, require experimental and field evaluation. Here we report that amending a UK clay‐loam agricultural soil with a high loading (10 kg/m²) of relatively coarse‐grained crushed basalt significantly increased the yield (21 ± 9.4%, SE) of the important C₄ cereal Sorghum bicolor under controlled environmental conditions, without accumulation of potentially toxic trace elements in the seeds. Yield increases resulted from the basalt treatment after 120 days without P‐ and K‐fertilizer addition. Shoot silicon concentrations also increased significantly (26 ± 5.4%, SE), with potential benefits for crop resistance to biotic and abiotic stress. Elemental budgets indicate substantial release of base cations important for inorganic carbon removal and their accumulation mainly in the soil exchangeable pools. Geochemical reactive transport modelling, constrained by elemental budgets, indicated CO₂ sequestration rates of 2–4 t CO₂/ha, 1–5 years after a single application of basaltic rock dust, including via newly formed soil carbonate minerals whose long‐term fate requires assessment through field trials. This represents an approximately fourfold increase in carbon capture compared to control plant–soil systems without basalt. Our results build support for ERW deployment as a CDR technique compatible with spreading basalt powder on acidic loamy soils common across millions of hectares of western European and North American agriculture.     

Iron reduction in the metal-rich guts of wood-feeding termites

Termites play important roles in lignocellulose and humus turnover in diverse terrestrial ecosystems, and are significant sources of global atmospheric methane and carbon dioxide. All known termite species engage in obligate, complex nutritional symbioses with their gut microbes to carry out such processes. Several hundred microbial species, representing a broad phylogenetic and physiological diversity, are found within the well‐bounded, microliter‐in‐scale gut ecosystem of a given termite. However, most of these species have never been obtained in laboratory culture, and little can be said about their functional roles in the gut community or symbiosis. Herein, an unappreciated facet of the gut chemistry and microbiology of wood‐feeding termites is revealed: the redox metabolism of iron. Gut fluids from field‐collected termites contained millimolar amounts of ferrous iron and other heavy metals. When iron(III) hydroxides were amended to a filter paper diet of Zootermopsis nevadensis, a dampwood termite collected in the San Gabriel Mountains of Southern California, the specimens accumulated high levels of iron(II) in their guts. Additionally, iron was reduced at rapid initial rates in anoxic gut homogenates prepared from field‐collected Z. nevadensis specimens. A Clostridium sp. and a Desulfovibrio sp. were isolated from dilution‐to‐extinction enrichments of Z. nevadensis gut contents and were found to reduce iron(III), as did the termite gut spirochete Treponema primitia. The iron in the guts of wood‐feeding termites may influence the pathways of carbon‐ and electron‐flow, as well as microbial community composition in these tiny ecosystems of global importance.     

Neutrophilic Iron-Oxidizing Bacteria in the Ocean: Their Habitats,Diversity, and Roles in Mineral Deposition,Rock Alteration,and Biomass Production in the Deep-Sea

The importance of metals to life has long been appreciated. Iron (Fe) is the fourth most abundant element overall, and the second most abundant element that is redox-active in near-surface aqueous habitats, rendering it the most important environmental metal. While it has long been recognized that microorganisms participate in the global iron cycle, appreciation for the pivotal role that redox cycling of iron plays in energy conservation among diverse prokaryotes has grown substantially in the past decade. In addition, redox reactions involving Fe are linked to several other biogeochemical cycles (e.g., carbon), with significant ecological ramifications. The increasing appreciation for the role of microbes in redox transformations of Fe is reflected in a recent surge in biological and environmental studies of microorganisms that conserve energy for growth from redox cycling of Fe compounds, particularly in the deep ocean. Here we highlight some of the key habitats where microbial Fe-oxidation plays significant ecological and biogeochemical roles in the oceanic regime, and provide a synthesis of recent studies concerning this important physiological group. We also provide the first evidence that microbial Fe-oxidizing bacteria are a critical factor in the kinetics of mineral dissolution at the seafloor, by accelerating dissolution by 6–8 times over abiotic rates. We assert that these recent studies, which indicate that microbial Fe-oxidation is widespread in the deep-sea, combined with the apparent role that this group play in promoting rock and mineral weathering, indicate that a great deal more attention to these microorganisms is warranted in order to elucidate the full physiological and phylogenetic diversity and activity of the neutrophilic Fe-oxidizing bacteria in the oceans.     

Rock weathering creates oases of life in a High Arctic desert

During primary colonization of rock substrates by plants, mineral weathering is strongly accelerated under plant roots, but little is known on how it affects soil ecosystem development before plant establishment. Here we show that rock mineral weathering mediated by chemolithoautotrophic bacteria is associated to plant community formation in sites recently released by permanent glacier ice cover in the Midtre Lovénbreen glacier moraine (78°53′N), Svalbard. Increased soil fertility fosters growth of prokaryotes and plants at the boundary between sites of intense bacterial mediated chemolithotrophic iron‐sulfur oxidation and pH decrease, and the common moraine substrate where carbon and nitrogen are fixed by cyanobacteria. Microbial iron oxidizing activity determines acidity and corresponding fertility gradients, where water retention, cation exchange capacity and nutrient availability are increased. This fertilization is enabled by abundant mineral nutrients and reduced forms of iron and sulfur in pyrite minerals within a conglomerate type of moraine rock. Such an interaction between microorganisms and moraine minerals determines a peculiar, not yet described model for soil genesis and plant ecosystem formation with potential past and present analogues in other harsh environments with similar geochemical settings.     

Biogeochemical modelling of the rise in atmospheric oxygen

Understanding the evolution of atmospheric molecular oxygen levels is a fundamental unsolved problem in Earth's history. We develop a quantitative biogeochemical model that simulates the Palaeoproterozoic transition of the Earth's atmosphere from a weakly reducing state to an O₂‐rich state. The purpose is to gain an insight into factors that plausibly control the timing and rapidity of the oxic transition. The model uses a simplified atmospheric chemistry (parameterized from complex photochemical models) and evolving redox fluxes in the Earth system. We consider time‐dependent fluxes that include organic carbon burial and associated oxygen production, reducing gases from metamorphic and volcanic sources, oxidative weathering, and the escape of hydrogen to space. We find that the oxic transition occurs in a geologically short time when the O₂‐consuming flux of reducing gases falls below the flux of organic carbon burial that produces O₂. A short timescale for the oxic transition is enhanced by a positive feedback due to decreasing destruction of O₂ as stratospheric ozone forms, which is captured in our atmospheric chemistry parameterization. We show that one numerically self‐consistent solution for the rise of O₂ involves a decline in flux of reducing gases driven by irreversible secular oxidation of the crust caused by time‐integrated hydrogen escape to space in the preoxic atmosphere, and that this is compatible with constraints from the geological record. In this model, the timing of the oxic transition is strongly affected by buffers of reduced materials, particularly iron, in the continental crust. An alternative version of the model, where greater fluxes of reduced hydrothermal cations from the Archean seafloor consume O₂, produces a similar history of O₂ and CH₄. When climate and biosphere feedbacks are included in our model of the oxic transition, we find that multiple ‘Snowball Earth’ events are simulated under certain circumstances, as methane collapses and rises repeatedly before reaching a new steady‐state.     

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m^–2 yr^–1 for young soils to 0.7 g M^–2 yr^–1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m^–2 yr^–1 for young soils and 0.01 g m^–2 yr^–1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO₂ removal by weathering, however, as soils age, rates of CO₂ consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.