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.
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.
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.
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.
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