Evidence for an oxygenation event 1.4 billion years ago, during the Middle Proterozoic 'Boring Billion'.

The Middle Proterozoic Era, between 1.8 billion years ago and 800 million years ago, is often known as the 'Boring Billion', due to an apparent lack of significant change in the Earth's biology, climate, and ocean chemistry. Oxygen levels were much lower than today, although there is considerable debate as to exactly how low, with some experts arguing for levels between 1% and 10% of modern levels, while others believe the figure should be between 0.1% and 1%. In the past decade a number of studies have come to the conclusion that oxygen levels fluctuated during the Middle Proterozoic, with a series of transient oxygenation events occurring over the era, with studies of redox-sensitive trace metal enrichments and biomarkers preserved in the middle Xiamaling Formation of North China suggesting that one of these occurred about 1.4 billion years ago.

Photosynthesis by living organisms is the major source of oxygen in the Earth's atmosphere, however an increase in the rate of photosynthesis is not in itself sufficient to change the oxygen level in the Atmosphere; a number of substances present on the Earth's surface reacting with free oxygen, which has a buffering effect on the atmosphere. 

Oxygen can only begin to build up in the atmosphere once substances such as organic carbon and pyrite are buried in sufficient quantities to neutralise this buffering effect. Plausible causes of such burial are events such as upwellings of poorly oxygenated waters from the deep sea, or increased weathering on land particularly of phosphate rich flood basalts from Large Igneous Provinces, leading to nutrient-rich runoffs entering the sea. Either of these scenarios would have provoked a spike in biological activity, with phytoplankton absorbing the carbon from carbon dioxide in the atmosphere while releasing the oxygen, then sinking and being buried beneath sediments when they died. Another possible scenario is an increase in sulphur, in the form of sulphate from the erosion of calcium sulphate (gypsum), or in the form of sulphur dioxide another product of Large Igneous Province volcanism, entering the (iron-rich) Proterozoic ocean, and forming pyrite minerals which then sunk and were buried, removing iron from the ocean and allowing oxygen to build up. 

The emplacement of the black shales of the Xiamaling Formation was roughly coeval with an episode of Large Igneous Province volcanism about 1.4 billion years ago, which suggests a connection between the volcanism and the apparently oxygen enriched environment in which these deposits were laid down, although direct evidence for this has yet been found.

Large Igneous Province formation during the Phanerozoic has been linked to changes in atmospheric composition and ocean chemistry, global climate change, and significant changes in the evolution of the biosphere. It is reasonable to assume that Large Igneous Province formation during the Proterozoic would have had similarly environmental impacts, altering the redox balance of the oceans, and leading to the burial of much organic material within sediments and raising the proportion of oxygen in the atmosphere. Volcanic emissions linked to Large Igneous Province emplacement could be expected to increase the amount of both carbon dioxide and sulphur dioxide in the atmosphere, leading to an increase in photosynthesis and pyrite burial. Subsequent weathering of volcanic rocks on land would lead to increased levels of nutrients entering the oceans, further increasing biological productivity, and leading to the deposition of carbon-rich black shale deposits.

In a paper published in the journal Geophysical Research Letters on 19 January 2024, Lei Xu of the China University of Geosciences (Beijing), Maxwell Lechte of the Department of Earth and Planetary Sciences at McGill University, Xiaoying Shi, also of the China University of Geosciences (Beijing), Wang Zheng of the School of Earth System Science at Tianjin University, Limin Zhou of the National Research Center of Geoanalysis, Kangjun Huang of the State Key Laboratory for Continental Dynamics and Early Life Institute at Northwest University, Xiqiang Zhou of the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, and Dongjie Tang, again of the China University of Geosciences (Beijing), present the results of a geochemical study of the Xiamaling Formation within the Yanliao Basin of North China, which examined proxies for palaeo-productivity such as phosphorus content, total organic carbon, and trace element ratios, as well as data on molybdenum isotope ratios and sulphur isotope ratios with pyrite, with the aim of determining the relationship between large-scale magmatism and the oxygenation of surface waters in the Middle Proterozoic.

Geological setting of the study area. (a) Simplified paleogeographic map of the approximately1.4 billion-year-old strata, showing the spatial and temporal distributions of approximately 1.4–1.3 billion-year-old large igneous provinces and continental rift zone. (b) Extent of the Yanliao Basin during the period from about 1.42–1.32 billion years ago.  (c) Simplified paleogeographic map of Yanliao Basin showing localities of the Xiamaling Formation sections. (d)–(f) Simplified maps showing geology of the Jixian area, Zhaojiashan and Jizhentun area, and the Renjiazhuang area. Xu et al. (2024).

The Xiamaling Formation was laid down in an open marine setting within the extensional Yanliao Basin, on the North China Craton, the last sedimentary series laid down prior to the breakup of the ancient supercontinent of Nunu. Palaeomagnetic evidence suggests that at this time the craton was located between 10°N and 30°N. 

It was not possible to find a single location where the entire Xiamaling Formation was exposed, so Xu et at. investigated four separate exposures, each of which exposed a part of the sequence, at the Tielingzi, Zhaojiashan, Jizhentun, and Renjiazhuang villages across the Jixian-Huailai-Xiahuayuan-Chengde region. The Xiamaling Formation disconformably overlies the Tieling Formation (i.e. it lays over the Tieling Formation, but it was not laid down directly in sequence, there was a time interval, and possibly erosion and deformation, between the end of Tieling Formation deposition and the onset of Xiamaling Formation deposition) and is in turn disconformably overlain by the Changlongshan Formation. The Xiamaling Formation is made up primarily of dark shale and siltstone, and can be divided into four subunits, names Members I-IV, with Member I being the lowest. The four members record a large transgressive-regressive cycle (increase and decrease in global sealevels, which would cause water to transgress onto land and then regress from it in a coastal environment), the Member II recording the peak of the transgression. The four members are quite distinctive, and can easily be distinguished at the different locations, making corelation between the sites reasonably easy. The two most useful sites for the study proved to be those at Zhaojiashan and Jizhentun, which were easy to compare to one-another due to their proximity (only about 10 km apart). The boundary between the Tieling and the Xiamaling formations was exposed at Jixian, which is about 180 km from Zhaojiashan and Jizhentun, but still easy to correlate with the other two exposures.

Stratigraphic correlation of the Xiamaling Formation in North China, with key ages, volcanic ash beds and lithographic sequences. Xu et ai. (2024).

Member I of the Xiamaling Formation disconformably overlies the Tieling Formation, apparently as part of the transgressive process (i.e. the area was above water and subject to erosion between the deposition of the two formations. Members I and II contain well-preserved horizontal lamination but lack obvious wave-agitated structures or crossing-bedding, which is consistent with deposition in a sub-tidal environment below the fair-weather wave base. Member III and the lower part of Member IV have no recognizable sedimentary structures indicative of storm-wave influence, which implies that they were deposited below the storm wave base, deeper than about 100 m. 

The age of the Xiamaling Formation has been well constrained, with a bentonite (clay derived from volcanic ash) layer at the base of Member one having yielded a zircon which has been uranium-lead dated to 1.418 billion years before the present (± 14 million years). Zircon minerals are formed by the crystallisation of cooling igneous melts. When they form they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements. However, it is likely that the unconformity at the base of the Xiamaling Formation means that the lowermost part of Member I is of slightly different ages in different places. Zircons from bentonite and tuff layers within Member III have yielded dates of 1.3922 and 1.3844 billion years before the present, and a date of 1.323 billion years before the present has been obtained from Member IV. Thus, the Xiamaling Formation can be safely considered to be between 1.42 and 1.32 billion years old.

Xu et al. collected 300 new samples of shale and siltstone for their study, with 22 samples coming from Member I at Tielingzi, 73 samples from Member II at Zhaojiashan, and 152 samples from Members III and IV at Jizhentun. Furthermore, 53 black shale samples were obtained from Member III at Renjiazhuang. These samples were then cleaned, dried, ground into powder, and then analysed for trace elements, total sulphur, total organic carbon, and mercury concentration.

Macroscopic depositional features of the Xiamaling Formation. (A) Grey silty shale in the Member I (Jixian section). (B) Green fine-grained sandstone in the basal Member II (Zhaojiashan section). (C) Alternating red and green shale beds in the middle Member II (Zhaojiashan section). (D) Alternating green and black shale beds in the upper Member II (Zhaojiashan section). (E) Alternating siliceous rocks and black shale beds in the basal Member III (Jizhentun section). (F) Black shale with volcanic ash layers in the middle Member III (Jizhentun section). (G) Green shale with limestone concretions in the upper Member IV (Jizhentun section). (H) Black shale with volcanic ash layers in the middle Member III (Renjiazhuang section). Xu et al. (2024).

There is a clear corelation between mercury levels and total organic carbon in the shales of the Xiamaling Formation, but these seem independent of total sulphur, aluminium, and iron levels. This suggests that the formation can be split into three stages. Stage I, comprising Member I and the lower and middle parts of Member II, has low mercury levels, and a low mercury/total organic carbon ratio. Stage II, comprising the upper part of Member II and all of Member III, shows a  sharp spike in the mercury/total organic carbon ratio. In Stage III, which comprises Member IV, this returns to the 'normal' level seen in Stage I. 

Palaeo-productivity, as evidenced by phosphorus and total organic carbon levels and enrichment in copper, zinc, and nickel, show a similar trend, with a peak in Stage II, slightly after the mercury/total organic carbon ratio peak. The total organic carbon peak is more closely aligned with the total organic carbon/reactive phosphorus ratio than it is with the mercury/total organic carbon ratio. Stage II shows peaks in total organic carbon (which rises 26% by weight) phosphorus (increases 0.26% by weight), the phosphorus/aluminium ratio, the copper/aluminium ratio, the zinc/aluminium ratio, and the nickel/aluminium ratio. Proxies for palaeo-productivity also rise in deposits of equialent age across North China and northern Australia during the same time interval. 

The possibility of an oxygenation event 1.4 billion years ago has recently faced some robust challenges. There is no obvious chromium fractionation in the shales of the Xiamaling Formation, something which would be expected if atmospheric oxygen levels were higher than about 1% of modern levels. Nor does Member III show any enrichment in vanadium, something which might be expected if oxygen levels reached 4% of today's levels. An alternative hypothesis has been suggested, in which the observed changes in chemistry reflect the basin receiving runoff from a different terrestrial catchment area, rather than any wider change in oceanic or atmospheric chemistry. It is also possible that the in the low oxygen conditions of the Middle Proterozoic, the shallow seas and atmosphere could have had a weak buffering effect on each-other, making it dangerous to make assumptions about atmospheric chemistry from marine sediments. 

Xu et al. argue that the evidence supports a rise in oxygen levels in both seawater and atmosphere about 1.4 billion years ago, and that the black shales deposited at this time in the North China, North Australian, and possibly other cratons, are evidence for this. A number of geochemical lines of evidence support this. For example, a higher proportion of heavy molybdenum isotopes is found in shales of the lower part of Member III of the Xiamaling Formation than in other Middle Proterozoic strata, closer to that seen in modern seawater, which suggests an ocean with oxygen levels approaching modern levels. Studies of uranium isotopes sediments of the Velkerri Formation of northern Australia have suggested that about /75% of the seafloor was oxygenated about 1.4 billion years ago. Furthermore, the proportion of the isotope sulphur³⁴ in pyrite drops in the higher part of Member III of the Xiamaling Formation, which is thought to be indicative of elevated levels of sulphate in the ocean, which in turn is likely to be a result of higher erosion on land, due to an oxygenated atmosphere.

On this basis Xu et al. argue that even if global oxygen levels did not rise significantly 1.4 billion years ago, there is significant evidence for at least a localised oxygenation event in the Yanliao Basin. Analysis of iron minerals and other oxygen sensitive elements in the sediments of Member IV of the Xiamaling Formation all suggest that the seafloor was oxygenated at the time of deposition, which is unlikely without wider marine oxygenation. Limestones from the upper part of Member IV have higher iodine to calcium and magnesium ratios, which is considered to be another indicator of an oxygenated water column. 

Large Igneous Province emplacement is known to have occurred between about 1.4 and 1.3 billion years ago, with a small peak in volcanic activity at about 1.42 billion years ago, followed by a rapid onset of Large Igneous Province volcanism at 1.40 billion years ago, peaking at about 1.38 billion years ago. This has been recorded on most continents, and is assumed to have had a significant impact on the Proterozoic environment. The widespread black shale emplacement at about 1.4 billion years ago is generally thought to be connected to this Large Igneous Province emplacement event, although establishing a direct causative relationship has proved difficult, largely because of the poor age constraint of most Middle Proterozoic sediments. Recent studies have used high mercury levels found in many of these black shales to suggest a link to volcanic events. However, mercury can be scavenged into sediments by organic matter, sulphides, clay minerals, and iron-magnesium oxyhydroxides. Xu et al.'s results suggest that there is a correlation between mercury and organic carbon in the Xiamaling Formation, but no correlation between mercury and sulphides, iron, or aluminium, which supports the hypothesis of a link between mercury and organic carbon deposition in this sequence.

In Stage II of the Xiamaling Formation there is a spike in mercury/organic carbon deposition, which could have been caused either by enhanced scavenging of mercury from seawater by organic material, or increased amounts of mercury entering the water column. Fluctuating redox conditions can also lead to an increase in mercury deposition, but iron and trace metal analysis suggests that redox conditions were relatively stable during Stage II. A euxinic (low oxygen, high sulphur) environment, which is thought to have been present, would lead to higher mercury deposition into sediments, where the sulphur would be reduced by the actions of bacteria, leading to an increased amount of both mercury and organic material within the sediments. However, there is a drop in the amount of sulphur Stage III, where there is no reason to believe the waters would have been less euxinic than during Stage II. This makes a higher mercury inputs into the ocean during Stage II the most likely explanation.

A variety of sources can lead to raised mercury levels in the oceans. In the modern world this is often linked to the combustion of wood and/or coal, but neither of these is likely in the Middle Proterozoic. More plausible scenarios include hydrothermal activity, remineralization of organic carbon from deep seawater, enhanced continental weathering, and volcanic aerosols. The sediments of the Xiamaling Formation record a positive europium anomaly, which is unlikely to be associated with enhanced hydrothermal activity. There is no sign of any organic matter remineralization during Stage II, which makes it unlikely this was the source of the mercury. The known extensive Large Igneous Province volcanism from approximately the time when these deposits were being laid down gives a very plausible origin for the mercury present, and the mercury/total organic carbon ratios, as well as being high, are similar to those found in Phanerozoic sediments which are known to be associated Large Igneous Province emplacement, further supporting Xu et al.'s hypothesis. Large Igneous Province emplacement would have been sporadic over the interval 1.40-1.35 billion years ago, making it unlikely to have ceased during Xiamaling Stage III. However, Stage II does appear to represent the most intense. The available mercury/total organic carbon and zircon data suggests that volcanism began about 1.41 billion years ago, with the onset of the oxygenation event at about 1.40 billion years ago.

The availability of phosphorous has been a limiting factor for life throughout Earth's history, and is a major control on ocean productivity, the burial of organic matter, and the production of oxygen. A sudden influx of nutrients including phosphorus, into Middle Proterozoic basins such as the Yanliao in North China and the McArthur Basin in northern Australia would have provided a stimulus for biological production, increasing the rates of both organic matter burial and oxygen production. An upwelling of nutrient-rich deep seawaters has been suggested as a source of the increased production seen in the Xiamaling Formation, but Xu et al. argue that the weathering of volcanic rocks following a major interval of Large Igneous Province emplacement is a more plausible cause. Such Large Igneous Provinces are composed largely of mafic rocks, which have a much higher phosphorus content than either ultramafic or felsic rocks. A link between the 1.4 billion year ago Large Igneous Province emplacement and the subsequent spike in oxygen has previously been suggested, but evidence to support this has been absent.

Correlation of geochemical data from the Xiamaling Formation and the Velkerri Formation based on age, volcanic ash and stratigraphic sequence. Xu et al. (2024).

Xu et al. demonstrate that mercury levels first increase 135 m from the base of the Xiamaling Formation during Stage II. This is slightly before the increase in nutrient elements (phosphorus, copper, nickel, zinc) and elevated organic matter and pyrite burials, which starts 150 m above the base. This provides a clear chronological link between the onset of volcanism and the subsequent oxygenation event. The higher phosphorus levels seen in Stage II than in either Stage I or Stage III suggest that weathering of the new igneous rocks provided significant source of this element, raising the phosphorus levels in the oceans leading to a boom in biological activity within the oceans. This manifests in a sharp increase in the amount of organic carbon being buried, with a maximum being reached 173 m above the base of the Xiamaling Formation.

Xu et al. propose that weathering of phosphorus from the Large Igneous Provinces provoked a major increase in ocean productivity about 1.4 billion years ago, leading to an increased rate of organic carbon burial and a rise in atmospheric oxygen. This rise in atmospheric oxygen would have further increased the rate of erosion on land, leading to high levels of sulphur entering the oceans, leading to the development of euxinic conditions, increasing the rate of pyrite burial, and further oxygen production. The high levels of organic carbon and available phosphorus suggest that phosphorus was being efficiently recycled within the oceans of the time, further raising oxygen production.

Xu et al. are careful to point out that this is not the only known instance of Large Igneous Province weathering prompting an oxygenation event; both the Great Oxidation Event and Neoproterozoic Oxidation Event, the two most significant oxygenation events in Earth's history, were closely associated with episodes of Large Igneous Province emplacement. 

The high mercury and total organic carbon deposition rates in the middle Xiamaling Formation strongly suggest that the deposition of this formation was influenced by the 1.40-1.35 billion years ago Large Igneous Province emplacement episode. The increased levels of mercury and organic carbon were linked to high inputs of nutrients, particularly phosphorus, into the ocean, fuelling higher biological production, increased pyrite burial, and an oxygenation event at about 1.4 billion years ago. 

See also...

Undestanding the relationship between carbon dioxide and the formation and environmental impact of Large Igneous Provinces.

Large Igneous Provinces are defined as the geologically rapid emplacement (over 100 000-1 000 000 years) of hundreds of thousands to millions of cubic kilometres of lava at the surface and the associated intrusive bodies. They are dominated by thick successions of lavas known as flood basalts. These vast igneous provinces have formed several times throughout Earth’s history, on almost all of the major continents and also in the oceans. Large igneous provinces are often found far from plate boundaries. Detailed studies of individual Large Igneous Provinces have shown that they are formed of igneous rocks with diverse compositions, ranging from tholeiitic basalts, to occasional rhyolites, to strongly alkaline magmas such as lamproites and carbonatites. The generation and emplacement of Large Igneous Province magmas is linked to rapid, large-scale outgassing of volatile molecules and elements, including sulphur, water, halogens, and carbon dioxide. This surface outgassing is facilitated by extensive subterranean magmatic plumbing systems that form important pathways for the transfer of mantle and crustal carbon to the atmosphere. Among magmatic gases, carbon dioxide (CO₂) is particularly vital to the life cycle of Large Igneous Province magmatism and its climatic consequences. The centrality of CO₂ in the environmental perturbations that coincide with some Large Igneous Provinces, such as the Deccan Traps (India), Siberian Traps (Russia), Karoo–Ferrar (southern Africa and Antarctica, respectively), Ontong Java Plateau (Pacific Ocean), Columbia River Basalt Group (northwestern USA), and the Central Atlantic Magmatic Province (northwest Africa, southwest Europe, northeast and southeast North America), renders Large Igneous Province-driven climate stress an important palaeoclimate analog for the present-day climate. However, the origins, budget, isotopic composition, and fate of Large Igneous Province carbon remain pressing and challenging questions due to the evanescence of CO₂ in carbon-saturated mafic magmatic systems.

The hardened lava flows of the Deccan Traps, in western India, may have played a role in the demise of the Dinosaurs. Gerta Keller/Science.

In a paper published in the journal Elements on 2 October 2019, Benjamin Black of the Department of Earth and Atmospheric Sciences at the City College of New York, and Sally Gibson of the Department of Earth Sciences at the University of Cambridge, discuss the relationship between the outgassing of carbon dioxide and the emplacement of Large Igneous Provinces.

Water, CO₂, sulphur, and halogens are among the most abundant constituents in volcanic gases. While some fraction of these gases is released during volcanic eruptions, the remainder may be discharged diffusely through crust, unaccompanied by volcanic eruptions. This cryptic degassing can form an important part of the overall gas release budget, especially for CO₂. Furthermore, if the establishment of lithospheric plumbing systems during the initial development of a Large Igneous Province precedes the onset of flood basalt volcanism at the surface, or if intrusive magmas solidify after the last eruptions, cryptic degassing could either precede or postdate active volcanism.

Because no Large Igneous Provinces are forming at the present day, studies of recent, analogous, volcanic activity provide an important source of information about the release of Large Igneous Province carbon. Eruptions of tholeiitic flood basalt lavas (subalkaline basalts, basalts that contain less sodium than some other basalts) may bear similarities to present-day fissure eruptions in Hawai‘i (Kï'lauea Volcano) and Iceland (Laki and Holuhraun Volcanoes). The smaller volume alkaline magmas associated with some Large Igneous Province may have had similar origins to those in the East African Rift.

Lava from a fissure on the  Kï'lauea Volcano, Hawai'i,  flows through a well-established channel to the ocean south of Kapoho. USGS.

At sites of present-day volcanism, both eruptive and cryptic CO₂ release can be constrained through ground-based and airborne measurements. More direct constraints on carbon in Large Igneous Province magmas can potentially be provided by petrologic and geochemical methods, but such data are scarce and require cautious interpretation. Blebs of melt trapped inside growing crystals (commonly known as 'melt inclusions') can record magmatic water, CO₂, sulphur, and halogen concentrations at the time of entrapment and prior to eruption. However, any CO₂ that has already exsolved (come out of solution and escaped) at the time of melt entrapment cannot be reliably reconstructed. Alternatively, trace element ratios such as CO₂/Niobium and CO₂/Barium from un-degassed samples can be used to infer original CO₂ content for partly degassed, primitive magmas. This is because Niobium and Barium exhibit a similar incompatible behaviour to CO₂ during mantle melting but do not degas.

Although all Large Igneous Province consist of a wide variety of magma types, estimates of volatile contents have mainly been derived for tholeiitic flood basalts. Most previous estimates of the CO₂ content of flood basalts have relied on analogous, well-studied basaltic systems, such as Kï'lauea and Laki, to arrive at concentrations of 0.5–0.9 % CO₂ (by weight) in flood basalt magmas. Given that in continental settings, flood basalt magmas may require several percent weight of dissolved water and/or exsolved CO₂ to be sufficiently buoyant to erupt through low-density felsic continental crust (crust enhanced in silicon, oxygen, aluminium, sodium, and potassium), these CO₂ concentrations may be underestimates.

Lave fields deposited during the 1783-84 Laki Eruption in Iceland. Ulrich Latzenhofer/Fotopedia.

Alternative estimates of the CO₂ concentrations in flood basalt magmas have been derived from olivine-hosted melt inclusions. These are rare and typically found in primitive flood basalts, so they may not necessarily be representative of the main phase of more fractionated tholeiitic magmatism. Moreover, analyses of CO₂ in olivine-hosted melt inclusions in tholeiitic magmas reflect only the dissolved amount, which forms an indeterminate fraction of the total CO2 released per km3 of magma. Thus far, measurements of CO₂ in olivine-hosted melt inclusions from flood basalts have overlooked the CO₂ in shrinkage vapour bubbles (which may dominate the total CO₂ content) and so are minimum estimates. If Large Igneous Province magmas reach CO₂ saturation at high pressures, CO₂ concentrations in melt inclusions represent lower limits on initial concentrations. Consequently, it is not surprising that CO₂  measurements for melt inclusions in the Siberian Traps are lower than the estimates for flood basalts based on measurements from Hawai‘i and Iceland.

The extent of the End Permian Siberian Traps Volcanism. Jo Weber/Wikimedia Commons.

The use of volatile/nonvolatile trace element ratios (such as CO₂/niobium and CO₂/barium) to estimate original CO₂ concentrations must be applied with care to flood basalt magmas. This is because processes such as recharge, assimilation, and fractional crystallisation in crustal magma chambers can significantly modify concentrations of strongly incompatible trace elements. Primitive high-Magnesium oxide lavas, known as picrites, may sidestep this issue and thereby provide a window into initial CO₂ concentrations. Using this approach, barium and niobium concentrations in picrites from the Siberian Traps and the North Atlantic Igneous Province suggest original melt CO₂ concentrations of between 0.1 and 2 CO₂ by weight. This large range for flood basalts, along with the current lack of data for more alkaline magmas, emphasises the need for further direct constraints on carbon in Large Igneous Provinces.

Large igneous provinces occupy broad areal extents, up to 1 000 000 km² and are widely believed to have resulted from the impingement and lateral spreading of upwelling high-temperature mantle plume heads with diameters of up to 2000 km at the base of the lithosphere. According to their site of emplacement, large igneous provinces may be categorised as oceanic or continental. Large Igneous Provinces emplaced on the continents may draw carbon from three main reservoirs: the convecting mantle, the subcontinental lithospheric mantle, and sedimentary rocks and fluids in the crust. The main source of carbon for oceanic large igneous provinces is the convecting mantle.

 Summary diagram of Large Igneous Province (LIP) carbon fluxes ( expressed as mega tonnes of carbon per year: Mt C y ̄¹), isotope ratios (rxpressed as δ13C, in units of per mil, ‰), and various types of geological reservoirs, all placed in context. The carbonatite ledge represents a major inflection in the CO²-bearing peridotite solidus. Black & Gibson (2019).

Carbon dioxide behaves highly incompatibly during mantle melting, meaning that it partitions almost entirely into the melt phase. Consequently, the initial CO² concentrations of magmas are determined by the carbon concentration of their mantle source and by the degree of partial melting. The high helium³/helium⁴ ratios in some large igneous province magmas suggest their parental melts are formed from deep-sourced, primordial material brought up in mantle plumes. In addition to primordial carbon, mantle plumes are also likely to contain carbon that has been recycled, due to subduction and subsequent entrainment of oceanic crust by the plume. However, the fate of carbon during plate tectonic recycling is not well known. Furthermore, while some Large Igneous Province melts have incompatible trace element and strontium, neodymium, lead and hafnium isotopic ratios similar to oceanic basalts, and they appear to be derived solely from a mantle plume source, the geochemistry of many large igneous province melts testifies to additional contributions from the overlying lithosphere.

Variations in the depth and degree of melting in upwelling mantle plumes associated with flood basalts are well-established, for example, from incompatible trace element ratios that reflect the presence or absence of garnet in the residue during melting. Numerical models indicate that the extent of partial melting that occurs during upwelling of mantle plumes is primarily controlled by the temperature of the convecting mantle and the thickness of the overlying lithosphere. If the lithosphere becomes thinner through the course of Large Igneous Province magmatism, due to synemplacement (fracturing) extension or erosion, the amount of melting will be lowest at the earliest stages of plume impact and the carbon concentration in these melts will be high. Because the lithosphere is of nonuniform thickness, the amount of melting in the plume will also vary spatially at any given time, and regions of pre-existing thinning or weakness will focus plume upwelling and melting.

Thinning or removal of the subcontinental lithospheric mantle may also potentially mobilize carbon. While this large and ancient Earth reservoir has been proposed as a major repository for volatiles, in part due to the infiltration of small-fraction, volatile rich, convecting, mantle-sourced melts over long periods of geological time, its carbon budget and isotopic composition are poorly known. The most concentrated accumulation of carbon is likely to be either at a depth of approximately 75 km, where experimental studies have shown that there is a depression in the CO₂-bearing peridotite solidus, or in regions where redox freezing traps carbon as graphite or diamonds, which can later be oxidized by percolating carbonatitic melts.

Mantle plume–lithosphere interactions through the life cycle of continental Large Igneous Provinces (LIPs) can influence the melting regime of the convecting mantle and the subcontinental mantle lithosphere. (1) During the initial impingement of a mantle plume (red) beneath thick continental lithosphere (yellow and brown), low-degree partial melts from metasomatised lithospheric mantle predominate. (2) Over the course of Large Igneous Province magmatism, the lithosphere undergoes significant thinning through extension and/or foundering. Formation of tholeiitic basalts through adiabatic decompression melting in the mantle plume imposes a maximum lithospheric thickness during main-phase flood basalt magmatism of about 70 km. Black & Gibson (2019).

A final, but potentially important, source of carbon to Large Igneous Province magmas comes from crustal fluids and sedimentary rocks (such as evaporites, coals, carbonates, and hydrocarbons). The flux of carbon from these crustal sources depends on the country rock lithologies, the input and distribution of heat, and the fraction of gases that reach the atmosphere.

Large igneous provinces are commonly emplaced over several million years, but the main pulse of flood basalt volcanism occurs on timescales of a million years or less. The flux of CO₂ is, therefore, likely to vary during Large Igneous Province emplacement. and through the course of individual Large Igneous Province eruptions. Both the evolving CO₂ flux and the ratio of carbon isotopes depend on magma emplacement rates, melting conditions, carbon sources, and flushing of CO₂ through the magmatic system as a fluid phase that is not bound to magma transfer. All of these factors are likely to shift through the life cycle of a Large Igneous Province.

The tempo of magma emplacement can be constrained through studies of geochronology, physical vulcanology, palaeomagnetism, radiogenic isotope systems, or proxies such as mercury deposition. These lines of evidence retain significant uncertainties, but they do generally support the existence of short-timescale variations in volcanic activity superposed on gradually shifting long-term mean volcanic fluxes.

Geochronologic studies show that low-degree, incompatible trace element–rich, lithospheric melts often pre- and postdate the main phase outpourings of flood basalts. If lithospheric removal occurs during Large Igneous Province emplacement, this could also trigger a pulse of devolatilisation from both the foundering lithospheric material and the residual subcontinental lithosphere under a steeper geothermalmal gradient.

Thermomechanical transitions may modulate the depths of magma storage and, therefore, the country rock lithologies that are to be subjected to heating and devolatilisation; crustal metamorphism also requires heating of large volumes of rock, and outgassing from the cold upper crust may, therefore, lag behind the onset of volcanism.

On the timescales of individual eruptions, carbon outgassing can be decoupled from volcanic flux, for example when CO₂ partitions into a fluid phase. Ground-based measurements of the Holuhraun (Iceland) fissure eruption of 2014–2015 revealed that CO₂/SO₂ ratios in the volcanic plume were higher by a factor of 10 during the earliest days of the eruption.

Flipping the causal relationship, evolving CO₂ concentrations during fractionation of magmas in the deep crust have also been hypothesised to exert control over the eruptibility of flood basalt magmas. Carbon dioxide may, therefore, play a role in shaping the tempo of volcanic activity and outgassing.

The depths at which Large Igneous Province magmas become saturated in an exsolved CO₂-rich phase, and the mobility and fate of the exsolved fluid, are critical to understanding their overall carbon outgassing history. The proportion of a magmatic volatile substance, one that is initially dissolved in the melt, that reaches the atmosphere can be thought of as the outgassing efficiency. For CO₂, it is commonly assumed that the outgassing efficiency is close to 100% for extrusive flood basalt magmas, due to the very low solubility of CO₂ in basaltic melt at one atmosphere pressure. In conjunction with an assumed CO₂ content of 0.5% weight in a primitive basaltic melt, this efficiency implies an approximate CO₂ yield of 14 megatonnes per  cubic kilometre of erupted magma. Importantly, if the CO₂ outgassing efficiency deviates significantly from 100%, carbon isotope fractionation due to partial degassing may shift the net isotope ratio of the carbon that is released.

Estimates of CO₂ outgassing that are based solely on emplacement rates of flood basalts do not account for the potential flux of CO₂ from associated intrusive magmas. Outgassing from CO₂ saturated magma bodies in the permeable upper crust may take place through gradual, passive degassing in conjunction with emissions during eruptions. In the less permeable lower crust and lithospheric mantle, dike formation and magma ascent may provide one of the only avenues for CO₂-rich exsolved fluids to reach the surface. In this case, CO₂ initially exsolved at depth could 'flush' shallower magmas, increasing CO₂ release beyond what would be expected from the volume of erupted flood basalts. The hypothesis of large-scale CO₂ flushing in the complex magmatic plumbing systems associated with flood basalts receives some support from studies of Icelandic fissure eruptions. Comparison between trace element concentrations and melt inclusion CO₂ content from the 1783–1784 Laki (Iceland) fissure eruption suggests that about 60% of the initial CO₂ cargo was degassed in the lower-to-middle crust.

 A fissure eruption in Hawai'i. Wikimedia Commons.

The importance of deep intrusive degassing depends on the relative volumes of intrusive and extrusive magmas. Based on petrology and seismic imaging of high-velocity layers near the Moho (the Mohorovičić Discontinuity,  the boundary between the Earth's crust and the mantle), previous researchers have inferred that the ultramafic cumulates that may underlie Large Igneous Provinces are comparable in volume to the erupted lavas. A range in intrusive/extrusive ratio of 0.5 to 4 implies that 30%–80% of Large Igneous Province magmas do not erupt. The efficiency with which these deep intrusive magmas degas and transfer CO₂ to the atmosphere is uncertain. It has been estimated than a 40–60% degassing rate of intrusive magmas occurred  in the Laki plumbing system, but this rate could be higher if crystallisation in deep magma bodies drove further CO₂ exsolution. Assuming homogeneous CO₂ content, 50% intrusive degassing, and efficient transfer of CO₂ to the atmosphere via flushing through the magmatic system and eruption, the magmas that do erupt could carry 1.25 to 3 times their native CO₂ content assuming an intrusive/extrusive ration of 0.5 to 4. This 'excess carbon' load is, in some ways, analogous to the 'excess sulfur' released from some arc volcanic eruptions that tap a sulphur-rich exsolved phase.

The Mohorovičić Discontinuity, or Moho (red). Geology.com

The fluxes of CO₂ during Large Igneous Province magmatism, and, consequently, the viability of CO₂ as a significant driver of climate change, have been the subject of debate. Perhaps the best evidence for major perturbation of the global carbon cycle by Large Igneous Provinces comes from carbon isotope records and independent palaeoclimate and  atmospheric CO₂ content proxies. In brief, these lines of evidence indicate four major aspects of carbon cycle disruption: (1) strong warming coinciding with some Large Igneous Provinces (e.g., the Siberian Traps and the North Atlantic Igneous Province), with more ambiguous evidence in other cases; (2) spikes in atmospheric CO₂ content; (3) negative carbon isotope excursions (drop in the proportion of carbon¹² in sediments); (4) ocean acidification. Taken together with geochronology aligning these environmental changes to Large Igneous Province magmatism, the most straightforward interpretation is that they were primarily caused by carbon release related to Large Igneous Province emplacement, possibly supplemented with light carbon from clathrates (chemical substances consisting of a lattice of water molecules that traps or contains other molecules) or from the metamorphism of sedimentary organic material. Accurately determining the CO₂ budget of diverse Large Igneous Province magmas is critical to testing hypotheses regarding the causal links between magmatism, warming, and carbon cycle perturbation.

On the timescales of silicate weathering and water–rock reactions, Large Igneous Provinces may play an important role as carbon sinks, because CO₂ reacts with calcium and magnesium in basalts to form carbonate minerals. By analogy to mid-ocean ridge settings, where the balance between net outgassing and net sequestration of carbon is uncertain, the capacity for  Large Igneous Provincecarbon sequestration on longer timescales may rival the magnitude of potential outgassing. This balance, and the timescales of atmospheric CO₂ draw down, may depend on the extent of subaerial versus submarine volcanism in a given Large Igneous Province and the potential for water–rock reactions in hydrothermal systems.

Large igneous provinces are dominated volumetrically by vast outpourings of flood basalt lavas and their intrusive equivalents. The frequent association of alkaline and carbonatite magmatism with flood basalts in Large Igneous Provinces attests to the petrologic importance of carbon during the generation of Large Igneous Province magmas, though when and how plumbing systems associated with diverse magma types interact with each other is an unresolved question. Previous estimates of the CO₂ content of flood basalts range from 0.5% to 0.9% by weight. Revised estimates of the CO₂ content of Hawaiian (Kï'lauea) and Icelandic (Laki) basalts, along with incompatible trace elements from flood basalt picrites, suggest that primitive flood basalt magmas may commonly comprise about 1% CO₂ by weight, or possibly more. However, such primitive magmas comprise a small fraction of the erupted lavas. More detailed numerical models and geochemical measurements are required to understand the carbon concentrations in flood basalts and other magma types in Large Igneous Provincess. For example, the carbon delivery potential of voluminous tholeiite lavas in flood basalt successions is important but poorly constrained. While their parental magmas result from high degrees of partial melting and are, therefore, less likely to be intrinsically CO₂-rich they may receive a boost from CO₂-rich fluids released from deep intrusive magmas.

A fissure formed during the 1783-4 Laki eruption in Iceland. Alan Robock/Eos. 

Based on the shifting contributions of deep convecting mantle, lithospheric mantle, and crustal sources, together with the variable flux from deep intrusive magmas, the CO₂ flux from Large Igneous Province magmas probably evolves through the entire magmatic cycle. As a consequence, Large Igneous Province carbon emissions are unlikely to scale directly with volumetric eruption rates, which offer, at best, a partial picture of the tempo of carbon outgassing. Nevertheless, gross estimates based on erupted volume provide a starting place for situating Large Igneous Provinces in the context of the global carbon cycle. For a total Large Igneous Province magma volume of 1 000 000–10 000 000 km³ emplaced over 100 000–1 000 000 years, and carrying  about 1% weight in CO₂, the mean annual flux would be 30–3000 mega tonnes of CO₂ per/year, with the potential for orders of magnitude deviation from this mean rate over the course of the magmatic cycle. The Laki fissure eruption released an estimated 300–900 mega tonnes of CO₂ over the course of 8 months. For comparison, the present-day global subaerially released flux of CO₂ from all volcanoes has been estimated at 300–600 mega tonnes of CO₂ per/year. During geologically brief intervals of intense outgassing, Large Igneous Provinces are likely to dominate the global flux of deep carbon to the atmosphere.

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Evidence for a connection between large igneous province eruption and black shale deposition in the Mesoproterozoic.

The Mesoproterozoic Era is an arbitrarily defined period of geological history, lasting from 1600 to 1000 million years ago. Unlike more recent geological time periods the Mesoproterozoic is defined arbitrarily, without reference to geologic events, as the absence of a widespread fossil record combined with the fact that the landmasses that occurred at this time have long since broken up and reformed in different combinations, makes in hard to use the kind of events that are used to define the boundaries of more recent periods, such as mass extinctions or large igneous province emplacements. The Mesoproterozoic is divided into three Periods, again defined arbitrarily upon dates rather then geological events, with the Calymmian lasting from about 1600 to about 1400 million years ago, the Ectasian lasting from 1400 to 1200 years ago, and the Stenian, lasting from 1200 to 1000 years ago.

In a paper published in the journal Geology on 25 September 2018, Shuan-Hong Zhang of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciences, Richard Ernst of the Department of Earth Sciences at Carleton University and the Faculty of Geology and Geography at Tomsk State University, Jun-Ling Pei and Yue Zhao, also of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciences, Mei-Fu Zhou of the Department of Earth Sciences at The University of Hong Kong, and Guo-Hui Hu, again of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciencesn, describe the correlation of a series of igneous eruptions and black shale depositions from the Mesoproterozoic, which they suggest could be used to define the boundaries between geological periods in the way that happens with more modern geological sequences.

Zhang et al. observe that a number of Mesoproterozoic large igneous provinces have been dated to approximately 1380 million years ago, including the Mashak Large Igneous Province in on the eastern margin of the Baltica Craton (now in the southern Urals), the Hart River–Salmon River Arch Large Igneous Province on the western margin of the Laurentia Craton (the western United States), the Midsommersø–Zig-Zag Dal Large Igneous Province in northern Greenland, the Chieress Large Igneous Province in the Anabar shield of the northern Siberia Craton, the Kunene-Kibaran Large Igneous Province in the Congo Craton of Central Africa, the Pilanesberg Large Igneous Province in the Kalahari Craton of Southern Africa. Smaller igneous features in other cratons including, West Africa, Amazonia, and East Antarctica, have been dated to the same time.

Furthermore, at least two black shale deposits, the Xiamaling Formation in northern China and the Velkerri Formation in North Australia, have been dated to the same time, with several other similar deposits, while not yet directly dated, appear to be off similar age based upon their stratigraphic positions (i.e. they are found in between rocks older than 1380 million years and rocks younger than 1380 million years) including the Dzhelindukon and Vedreshev formations in Siberia, the Bijaigarh Shale and black shales from the Srisailam Formation in India, and the Serra do Garrote Formation of Brazil.

Representative stratigraphic columns of black shales in Xiamaling Formation in North China Craton (A–C) and Velkerri Formation in North Australian Craton (D–G). Zhang et al. (2018).

Black shales are typically associated with major oceanic anoxia events, such as the the one associated with the End Permian Extinction. The presence of widespread Mesoproterozoic black shales has previously been associated with generally low oxygen levels during this era, but this is no longer considered to be the case, as oxygen levels in the atmosphere are thought to have begun to rise quite quickly after the end of the deposition of banded ironstone formations about 1800 million years ago (these formations are thought to have formed as oxygen released by early Algae or photosynthetic Bactria reacted with iron in the water of the oceans, causing it to settle out as rust, with oxygen starting to build up in the atmosphere once this iron had been used up), and multicellular fossils interpreted as Eukaryotic Algae (Seaweeds) are known from about 1560 million years ago onwards.

Based upon this, Zhang et al. see evidence for a widespread igneous event about 1380 million years ago, which may have been associated with the breakup of the ancient supercontinent of Nuna, and which led to a global anoxic ocean event. Furthermore, they argue that since this event is recorded in strata around the world, that the date of 1380 million years ago should be adopted as the boundary between the Calymmian and Ectasian periods, rather than the arbitrary date of 1400 million years ago.

Distribution of ca. 1380 Ma large igneous provinces (LIPs) and black shales in paleogeographic reconstruction map of Nuna supercontinent. Zhang et al. (2018).

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Understanding the conection between the Siberian Traps volcanism and the End Permian Extinction.

The formation of Large Igneous Provinces have been associated with three of the five largest extinction events in the Earth's fossil record, including the most severe of these, the End Permian Extinction, in which it has been estimated 96% of all species then present on Earth were wiped out, and which is generally linked to a release of greenhouse gasses (methane and carbon dioxide) associated with the emplacement of the Siberian Traps Large Igneous Province. However these Large Igneous Provinces typically take several million years to form, while extinction events are thought to happen over only a few thousand years, which implies that the connection between the two events is not simple.

In a paper published in the journal Nature Communications on 31 July 2017, Seth Burgess of the Volcano Science Center of the U.S. Geological Survey, James Muirhead of the Department of Earth Sciences at Syracuse University, and Samuel Bowring of the Earth, Atmospheric, and Planetary Sciences Department at the Massachusetts Institute of Technology, publish a detailed examination of the timelines of both the Siberian Traps vocanic episode and the End Permian Extinction, and make deductions about the conections between these two events.

Burgess et al. used a timeline for the emplacement of the Sibetian Traps Large Igneous Province previously developed by Seth Burgess and Samuel Bowring, and published in a paper in the journal Science Avances in 2015. This timeline divided the formation of the Siberian Traps into three phases, Stage 1, Stage 2 and Stage 3. Stage 1 lasted from 252.24 to 251.907 million years ago, and saw an extensive period of volcanic extrusion, as magma from a plume deep within the Earth's interior burst through the lithosphere and erupted at the surface in a series of vast pyroclastic eruptions. Stage 2 lasted from 251.907 to 251.583 million years ago, and saw a cessation of surface eruptions, which were replaced by the emplacement of extensive volcanic sills (sheets of volcanic rock injected in between older layers of sedimentary rocks) across the Tunguska Basin of Siberia. In Stage 3, which lasted from 251.583 to 250.2 million years ago, this sill emplacement stopped and was replaced by a second phase of eruptive surface volcanism.

Comparing these events to the timescale of the Permian Extinction, Burgess et al. found that Stage 1 of the Siberian Traps vocanism had relatively little impact on either the Permian biosphere or its climate (as determined by carbon isotope ratios in marine sediments), with only limited signs of climatic instability around the poles. However the onset of Stage 2 was associated with both a sudden very sharp change in carbom isotope levels, thought to be indicative of the release of large amounts of greenhouse gasses into the atmosphere and a subsequent rise in ocean temperature of about 10℃, and also the sudden and more-or-less complete collapse of the Earth's biosphere. The remainder of Stage 2 is associated with a smaller, more steady increase in greenhouse gasses, but limited further impact on the biosphere, while Stage 3 was associated with a gradual lowering of greenhouse gas levels.

This implies that the greenhouse gasses associated with the End Permian Extinction were not derived from the magma plume which caused the Siberian Traps volcanism, but rather were released from the sediments of the Tunguska Basin by metamorphic reactions during the emplacement of the sills during Stage 2 of the episode. This was particularly rapid at the onset of this phase, when magma was first passing through the sediments, causing the emission of stored carbon; further emplacement of igneous material had limited effect as the majority of this carbon had already been released.

Time series of Siberian Traps LIP emplacement. (a) Pre-emplacement basin. (b) Emplacement of a volcanic load during Stage 1. The feeder system is unresolved, and most likely situated below lavas in (f). (c) Beginning of Stage 2, with lateral sill complex growth, widespread heating, and greenhouse gas generation. (d) Continued sill emplacement during Stage 2. (e) Renewed extrusive magmatism during Stage 3. Burgess et al. (2017).

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