Estimating the phosphorus content of the Ediacaran seas.

The availability of phosphorus is considered to be a major limiting factor on biological productivity, and its availability in the oceans over geological timescales is thought to have been a significant control on the evolution of life. Most phosphorus found in the modern oceans derives from the weathering of continental rocks, and needs to be constantly replenished as it is taken up by living organisms in the photic zone, sinks, and is buried, as well as to a lesser extent being absorbed by iron (oxyhydr)oxide minerals. 

Buried organisms typically break down releasing their phosphorus as they are degraded by microbes, and iron (oxyhydr)oxide minerals often dissolve in reducing subsurface environments, also releasing their phosphorus. This can be reabsorbed by other mineral phases at the sediment surface, including apatite and vivianite, as well as more iron (oxyhydr)oxide minerals, or may be recycled back into the water column, promoting further biological productivity. Such recycling of phosphorus into the water column is thought to be promoted by euxinic conditions (low oxygen, high sulphur) but inhibited by ferruginous conditions (low oxygen, high iron), due to the high capacity for iron mineral formation, although conditions in the sediment are probably more important than in the water column; sulphide generation in shallow sediments is likely to lead to the release of phosphorus.

Calculating the amount of bioavailable phosphorus in ancient oceans is notoriously difficult. One method that has produced results has been to compare the ration of phosphorus to iron in iron-rich sediments, which can give some idea of the proportion of phosphorus in the water column. However, this is complicated by a number of factors, such as the proportion of dissolved silica in the water, which is known to have an influence on the uptake of phosphorus by iron (oxyhydr)oxide minerals. Siliceous Phytoplankton appeared in the oceans in the Cambrian, and are presumed to have lowered the amount of silica in the water column. Thus the Precambrian oceans should have had higher silica contents that those of the Phanerozoic. However, the rock record suggests that the silica content of the Neoproterozoic oceans was probably lower than was the case for Palaeoproterozoic and Mesoproterozoic oceans. Despite these uncertainties, it is generally accepted that the phosphorus content of the world's oceans increased significantly during the global glaciations of the Cryogenian Period.

Iron formations are not ubiquitous in the geological record, and there are long periods of time for which no such deposits are known, making it difficult to reconstruct the phosphorus content of the oceans using this method. Notably, there are few useful iron formations available for the Ediacaran Period, leaving researchers unclear about phosphorus concentrations in the post-Cryogenian oceans in which multicellular Animals first began to diversify. The oceans of the Ediacaran are thought to have undergone some severe redox fluctuations, with oxygen reaching the deep ocean in the places where the distinctive Ediacaran fauna first appeared. Phosphorus can also be measured in siliclastic rocks, which have a nearly unbroken record dating back to the Palaeoproterozoic. However, while some studies of the phosphorus content of these rocks suggests that the amount of phosphorus being incorporated into shales increased between about 800 million years ago and 635 million years ago, as more studies have been carried out, they have produced a picture in which the average amount of phosphorus in shales varies little over the Neoproterozoic and early Palaeozoic.

Depite this, calculations have suggested that the phosphorus content of the oceans increased significantly during the Ediacaran, due to rising sulphate concentrations in sediments, and the release of phosphorus by sulphate-reducing Bacteria. It has been argued that for much of the Proterozoic primary production, and therefore oxygen production, was supressed by limited recycling and consequently high rates of burial of phosphorus, leading to the oxygen-poor oceans seen over much of this time. In the Neoproterozoic, rising sulphate levels are thought to have helped phosphorus recycling, leading to more fertile seas and a rise in marine oxygen levels, although direct evidence for this model has yet to be found.

In a paper published in the journal Communications Earth & Environment on 19 January 2024, Xiuqing Yang of the School of Earth Science and Resources at Chang’an University, and the School of Earth and Environment at the University of Leeds, Jingwen Mao, also of the School of Earth Science and Resources at Chang’an University, and of the Key Laboratory for Exploration Theory & Technology of Critical Mineral Resources at the China University of Geosciences,  Fred Bowyer of the School of GeoSciences at the University of Edinburgh, Changzhi Wu, Rongxi Li, Chao Zhao, and Guowei Yang, again of the School of Earth Science and Resources at Chang’an University, and Simon Poulton, also of the School of Earth and Environment at the University of Leeds, document a newly discovered Ediacaran iron formation within the North Qilian Orogenic Belt of northwest China.

Present distribution of Neoproterozoic iron formations. Yang et al. (2024).

The iron formations of the North Qilian Orogenic Belt have only been very lightly metamorphosed, and comprise largely haematite and jasper, giving it good potential for the study of phosphorus cycling. Yang et al. conducted high-resolution petrographic, mineralogical and geochemical studies on the North Qilian iron formations, and compared these to other datasets from around the world, in order to create a scheme of phosphorus bioavailability across the crucial Ediacaran interval in the evolution of Earth's life.

The Kawa, Jiapigou and Xiaoliugou iron formations of the lower Zhulongguan Group of the Qilian Orogenic Belt have been dated to about 600 million years ago. They comprise mostly haematite and jasper, with smaller amounts of clay minerals,  magnetite, carbonate minerals and apatite. Well-defined banding is rare, but where present comprises separate bands of haematite-rich and jasper-rich laminae, between 0.5 mm and 5 mm in width. All samples were taken from open pit mines, with care being taken to avoid collecting where there were signs of weathering or late-stage hydrothermal alteration.

(a) Schematic tectonic map of China. (b) Simplified geological map of the Qilian Orogen Belt. (c) Geological map of the western segment of the North Qilian area, China. Yang et al. (2024).

In places where banding could be found, phosphorus rich grains were present in both the haematite and jasper layers. Notably, the haematite layers were dusty or microplaty, suggesting that they have retained their original mineralogy. Some courser haematite grains are present, probably as a result of late-stage diagenesis or low-grade metamorphism. Phosphorus is primarily concentrated within apatite grains, which also have a high calcium content. Phosphorus and calcium levels also correlate in analysis of bulk samples. Energy dispersive spectroscopy analysis of apatite grains suggests that these are predominantly carbonate fluorapatite, mostly less than 5 μm in diameter, though some, rare, larger grains may reach 50 μm. Bulk samples were found to have high phosphorus/iron ratios, but low organic 

Photomicrographs of Ediacaran iron formations and carbonate fluorapatite. (a) Thin section of iron formations with a typical banded structure, where the red laminae are jasper and the grey laminae are hematite (sample JPG-30, Banded Iron Formation). (b) False-colour scanning electron microscope mineral map of (a), where the red-violet colour shows carbonate fluorapatite, which is mainly distributed in jasper-rich laminae. (c) Scanning electron microscope images of fine-grained haematite particles (sample KW-6, Iron Formation). (d) Disseminated carbonate fluorapatite grains with a subhedral shape (sample KW-33, Iron Formation). (e) Euhedral carbonate fluorapatite grains (sample JPG-30, Banded Iron Formation). (f) Rare coarse-grained carbonate fluorapatite (sample JPG-28, Banded Iron Formation). (g) Haematite inclusions in a carbonate fluorapatite particle (sample JPG-26, Banded Iron Formation). (h) Energy dispersive spectroscopy spectrum from an apatite particle shown in (e), using a gold-plated thin section. Peaks for carbon, oxygen, fluorine, phosphorus and calcium confirm the mineral is carbonate fluorapatite. Abbreviations: Hem, Haematite; Qtz, Quartz. Yang et al. (2024).

Howthe phosphorus cycle works under ferruginous conditions is poorly understood, as is how this relates to iron formation deposition. Ferruginous oceans were prevalent for much of the Precambrian, and phosphorus levels typically low. It has generally been assumed that these phenomena are connected, with the low phosphorus levels being due to an iron trap, in which phosphorus atoms are bound into iron minerals and taken out of ocean circulation.

If phosphorus was in fact largely being bound into carbonate fluorapatite minerals in Archaean-Mesoproterozoic iron formations and ironstones, then it is possible that much of this phosphorus was being remineralized from biological sources. However, this is difficult to reconcile with the low organic carbon content of these Precambrian iron deposits. An alternative is possibility would be that the early oceans in fact had mush higher phosphorus contents that has previously been supposed, and that the binding of phosphorus into carbonate fluorapatite is simply a consequence of iron silicate precipitation under these conditions.

Stratigraphic column of Ediacaran iron formations in the North Qilian area, China. Yang et al. (2024).

Yang et al.'s study demonstrates that carbonate fluorapatite was the main sink for phosphorus in the Ediacaran iron formations of North Qilian, China, but does not provide any information on the process by which the phosphorus was bound in this way. Nevertheless, Yang et al. do feel able to make some inferences from the data. The lack of an association between phosphorus and aluminium suggests that the phosphorus was not being deposited as detrital particles (i.e. bound to clay minerals, which have high aluminium contents), which in turn suggests that this phosphorus was not derived directly from a terrestrial source. This does not preclude the phosphorus having been originally derived from terrestrial weathering, simply that any such phosphorus must have been dissolved in the ocean, where it could be scavenged by iron minerals, rather than being deposited in a particulate form with aluminium minerals.

The remineralization of phosphorus from organic matter to carbonate fluorapatite cannot be ruled out, however this would have been likely to lead to the formation of the precipitation of iron minerals such as magnetite and siderite as the organic carbon was oxidised. The dominance of haematite in the North Qilian iron formations, combined with the rareness of magnetite, makes this scenario improbable. Thus, the high phosphorus content of the North Qilian deposits compared to earlier iron formations is unlikely to be related to the remineralization of organic material. 

Field and microphotographs of iron formations from North Qilian. (a) Iron formations with haematite-rich laminae and jasper-rich laminae. (b) Iron formations with a jasper lens. (c) and (d) Microphotographs of iron formations; (c) was taken under reflected light, and (d) is a scanning electron microscope image. Abbreviations: Hem, Haematite; Qtz, Quartz. Yang et al. (2024).

This does not, however, imply that the main reason for the high phosphorus levels seen in the North Qilian iron formations was drawdown by iron minerals. The dominance of haematite in these formations implies that iron was being precipitated from the water column as a form of hydrated ferric oxide, probably when ferruginous waters were oxygenated during upwellings. Phosphorus could potentially have been absorbed during this process, but would have been released within the sediment as the ferrihydrite remineralized into more stable haematite. In modern hydrothermal deposits a correlation can be seen between iron and phosphorus because phosphorus adsorbs onto iron (oxyhydr)oxides, but there is no evidence for this happening in the North Qilian deposits. This suggests that the concentration of phosphorus in porewaters was above the saturation point for carbonate fluorapatites, due to phosphorus being released by the remineralization of ferrihydrates into haematite, which would have led to carbonate fluorapatite deposition. This would also help to explain the presence of haematite inclusions within carbonate fluorapatite grains.

Yang et al. also note that where banding is present, carbonate fluorapatite grains are found in both haematite and jasper laminae, but are more common within jasper. This is also consistent with the release of phosphorus during the remineralization of ferrihydrates. Studies of Mesoproterozoic iron deposits suggest that phosphorus was precipitated into carbonate fluorapatite, despite the water being supersaturated for the iron phosphate mineral vivianite, because iron ions were being removed from the water by the formation of iron silicates. In the North Qilian deposits, these iron ions were probably only ever present within pore water, again favouring the deposition of carbonate fluorapatite.

Images of banded iron formation samples JPG-30 (a), (c), (e), (g), (i), (k) and JPG-28 (b), (d), (f), (h), (j), (l) from the Jiapigou section. (a), (b) Thin section photographs; (c), (d) Scanning electron microscope images. (e)–(j) False-colour scanning electron microscope mineral map of haematite, quartz and carbonate fluorapatite. (k), (l) Energy dispersive spectroscopy elemental map of phosphorus. Yang et al. (2024).

Despite the difficulties associated with the determination of ocean phosphorus concentrations from ancient iron formations, Yang et al. believe that they can detect a significant change in phosphorus levels in the Ediacaran compared to earlier deposits. Experiments have determined that no more than about 10% of phosphorus present in iron formations when they form is likely to be subsequently lost due to post-depositional processes; far lower than the determined difference between the Ediacaran and earlier deposits. Studies of the rock record have determined four phases with their own distinctive iron/phosphorus ratios (with some gaps). The oldest of these covers the Archaean and Palaeoprotorezoic, the next the Cryogenian, then the Cambrian to the Jurassic, and finally the Cretaceous to Quaternary. The phosphorus/iron ratio observed in the North Qilian Formation are consistent with those from Ediacaran iron formations in Iran, and much higher than those observed in Archaean to Palaeoprotorezoic, Tonian, or Cryogenian deposits, and indeed much higher than is observed in Cambrian to Jurassic iron formations, falling closest to the levels seen in Cretaceous to Quaternary strata.

This large jump in phosphorus levels in Ediacaran iron deposits seems highly suggestive of elevated phosphorus levels in the Ediacaran oceans, to the extent that phosphorus was probably being adsorbed onto iron (oxyhydr)oxide minerals in preference to ions such as calcium or magnesium. The North Qilian iron formations are interlayered with dolostones and sandstones, which implies that they were being laid down in a shallow marine setting, making it unlikely that the phosphorus levels recorded were something restricted to deep ocean basins. Although the levels of dissolved calcium and magnesium in the Ediacaran seas is poorly understood, the high levels of phosphorus recorded in Ediacaran iron formations compared to Palaeozoic and Mesozoic examples implies that dissolved phosphorus levels were high in the Ediacaran, despite the higher levels of dissolved silica likely to have been present.

If this is correct, then the increase in the proportion of phosphorus in the Ediacaran seas was one of the largest seen in Earth's history. This is consistent with the average proportion of phosphorus in Ediacaran shales (which are about 0.34 % phosphorus by weight) compared to Tonian (0.09 % phosphorus by weight) or Cryogenian (0.13 % phosphorus by weight) shales. Tonian iron formations also show very low levels of phosphorus, supporting the idea that Tonian seas had very low phosphorus levels. This, while the precise reasons for and timing of phosphorus increases in Neoproterozoic oceans is unclear, multiple lines of evidence suggest that this increase was not a phenomenon restricted to the Cryogenian glacial phases, but rather something which intensified during the following Ediacaran Period.

Schematic representation of the evolution of marine redox state through the Neoproterozoic to Cambrian based on compiled palaeoredox proxy records. Yang et al. (2024).

High levels of terrestrial erosion have been proposed as a mechanism for the increase in phosphorus in the Cryogenian, though why this would continue significantly into the Ediacaran is unclear. An alternative is that recycling of phosphorus back into the water column may have played a significant role in keeping phosphorus levels high. Yang et al.'s findings suggest phosphates were mobilized during the remineralization of iron (oxyhydr)oxide minerals to haematite close to the sediment-water interface suggests a degree of phosphorus recycling was likely, but iron formations were relatively rare in the Ediacaran, making it unlikely that this system was having a major impact on the global environment.

Both the Ediacaran and Cryogenian oceans are thought to have been redox-stratified. Ferruginous conditions were probably widespread, but increased sulphate levels in the oceans may have been more important for phosphorus recycling, as sulphate-reducing Bacteria would have increased the rate at which organic phosphorus was remineralized. This would have been a marked difference between Ediacaran and earlier Neoproterozoic oceans, where ferruginous conditions were prevalent, but sulphate levels low.

Most phosphorus entering shallow-marine waters today does so in upwelling zones, where currents bring water upwards from the deep ocean. This source brings about 60 times more phosphorus into shallow  marine waters than all of the world's river systems combined. If this was also the case in the Ediacaran seas, then upwelling currents would have been the major source of the phosphorus which fuelled primary production in these seas, and therefore the Ediacaran rise in oxygen content, as well as the nutrients which fed the diversifying Eucaryotes of the Period; the phosphate-rich iron formations of North Qilian have been dated to approximately 600 million years ago, slightly younger that the Lantian assemblage of South China, which at 602 million years old is the ealiest known example of the macroscopic Ediacaran Fauna.

See also...

Understanding the influence of large bolide impacts on the Earth's carbon cycle and climate.

Many scientists suggest that the Earth has recently transitioned into a new period, the provisionally termed 'Anthropocene', this is defined to reflect the planet-wide effects of human activity. The Anthropocene could be described as a gigantic combustion experiment in which reduced, energy rich forms of carbon (e.g., coal, oil, gas, wood) are oxidised to CO₂, with additional significant atmospheric emissions from industrial and land-use activity. The cumulative atmospheric CO₂ release since 1750 AD is about 2000 gigatons. For comparison, the bolide strike that formed the Chicxulub structure in Mexico about 66 million years ago released between 425 and 1400 gigatons of CO₂. Thus, some large bolide impacts are comparable to the Anthropocene effect in terms of the rapid disruption of the carbon cycle and the potential for exceeding the currently unknown critical degree of perturbation. However, during the most intense bombardment period on the early Earth, the surface was poor in the carbon and sulphur rich sediments that exert the greatest control over climate perturbation. Time, therefore, provides a natural narrative for a review of environmental consequences.

In a paper published in the journal Elements on 1 October 2019, Balz Kamber of the School of Earth, Environmental and Biological Sciences at the Queensland University of Technology, and Joseph Petrus of the School of Earth Sciences at the University of Melbourne, and the Harquail School of Earth Sciences at Laurentian University, examine the relationship between bolide impacts, atmospheric carbon fluctuations, and the climate, as preserved in the Earth's rock record.

Large bolide impact events have become rare. At most, there is one strike of one roughly 10 km object every 100–200 million years. But the rich history of former bombardment is evident on the surfaces of inner solar system bodies, as well as from the few preserved impact features on Earth itself. Impact basins more than 1000 km across exist on our planetary neighbours and they are pockmarked with thousands of smaller impact features. Counting crater numbers and measuring the sizes of craters on images of the Moon’s surface qualitatively shows that most of the very large basins formed early in the history of the solar system. Dates for lunar samples constrain the bulk of the bombardment to have occurred within the first roughly 700 million years since planet formation. Importantly, however, significant events capable of producing basins hundreds of kilometres in diameter also happened in more recent history. It is instructive to study how these could have disrupted the complex interplay between biology and geology on Earth; that is, how they have affected the global biogeochemical carbon cycle. 

The Earth’s geological history is subdivided into eons, eras and periods. For the older eras, these subdivisions were defined with the appearance or disappearance of dominant rock types, whereas most boundaries of the younger eras and periods coincide with the rapid disappearance of organisms (mass extinctions).

Timeline of significant geologic events relative to geological age, subdivided into the four, colour-coded, geological eons (Hadean, Archaean, Proterozoic, Phanerozoic). The eons are subdivided into the geologic eras; the boundaries of the Proterozoic periods are shown but not named. (A) The cumulative age-distribution of bolides capable of creating basin sized impact structures on Earth (number as y-axis). (B) The ages of the six largest known terrestrial impact events (as solid lines during the Proterozoic and Phanerozoic) and the most prominent spherule layers (as broken lines, all during the Archaean). The Chicxulub (Mexico) impact is shown in red. (C) A summary of key events and preserved pieces of evidence relevant to planetary evolution. The End Cretaceous extinction event is shown as a red vertical bar. (D) Global data compilation of the carbon isotope compositions (δ13C as y-axis) of carbonate (blue symbols) and organic carbon (dark grey symbols). Kamber & Petrus (2019).

The Hadean Eon (4567 Ma to about 3850 Ma) is the oldest eon of Earth’s history, and it witnessed by far the largest number of impacts. Unfortunately, the Hadean geological record is very sparse and significant uncertainty exists about Earth’s evolution during that time. Nevertheless, three key questions are of great scientific importance because of their enduring legacy for the remainder of Earth history. They all relate to the Hadean impact history: (1) Did the Hadean bombardment deliver volatile elements, including water and carbon, to the early Earth? (2) What happened to the Earth’s vanished primordial crust? (3) Was life on Earth already established during the Hadean? 

The delivery of extraterrestrial matter to Earth happens with two dominant size classes of objects: the tiniest, and the largest. The fully formed Earth was struck by a few hundred bolides capable of causing very large (over 100 km) impact basins, and these objects contained at least one-third of the delivered extraterrestrial matter. At the other end of the spectrum, cosmic dust, having particle diameters in the micrometre range, constitutes the second significant source of extraterrestrial matter. Of the two types of impactors, comets and asteroids, comets are predominantly composed of volatile species (oxygen, carbon, hydrogen, nitrogen), whereas asteroids include undifferentiated chondritic bodies that also contain about 1.1% by weight hydrogen and about 1.8% by weight carbon, with organic molecules. Although intuitively, this suggests that the delivery of such matter to Earth during the Hadean might have contributed to the build-up of the hydrosphere and to the surficial carbon reservoir, this simple logic is complicated by impact physics.

Comparing the delivery of extraterrestrial elements of a refractory nature (i.e., with a high boiling temperature) to those of a volatile character (low boiling temperature) shows that most of the volatile cargo would be lost from cosmic dust upon its atmospheric entry, except for particles of less than 35 μm, which experience limited frictional heating. Impacts of larger bolides at velocities in excess of about 15 kilometres per second cause at least partial vapourisation of the target and intense heating to more than 10 000 K of the vapourised material, and this leads to the formation of a silicate vapour plume. The behaviour of the various chemical elements in these plumes, particularly atmospheric escape versus condensation and fallback to the Earth, is currently not fully understood. The isotopic systematics of light elements and noble gases suggest that late addition to the Earth from comets is unlikely to have been volumetrically important for water, nitrogen and carbon (the proportions of isotopes of each element in a body depend upon where it formed in the primordial disk from which the Solar System formed, with lighter isotopes mote proportionally abundant further from the Sun). Current evidence favours an origin of the terrestrial volatiles by early capture during planetary accretion rather than by late addition during very large impact events.

Regardless of the origin of volatiles, the rate at which the lunar surface was bombarded (and, by analogy, the Earth) can be reconstructed by combining crater density statistics with the known ages of rocks from the Moon’s surface. The largest uncertainty in this flux estimate arises from the paucity of samples returned from the older, more heavily cratered dark side of the Moon and the few direct dates for large lunar impact basins. There are two end-member models for the bombardment flux: one that envisages a spike in very large impacts between 3850 million years ago and 4200 million years ago (the Late Heavy Bombardment scenario) versus one that favours an exponentially decaying flux (the Accretion Tail Scenario'). With currently available data, modelling cannot unequivocally rule out either scenario. One of the strongest pieces of evidence in favour of the Late Heavy Bombardment remains the uranium/lead age line of lunar highland samples (uranium, decays into, amongst other things, lead at a known rate; it is possible to calculate the age of a mineral which would not have lead in in when it formed from the ratio between these elements) that was originally used to advance the concept of a late bombardment. This line is interpreted to date the timing of volatile element loss and homogenisation. The age conspicuously coincides with the more widespread preservation of terrestrial rocks, i.e., the Archaean–Hadean boundary. If future lunar data confirm the existence and timing of the Late Heavy Bombardment, one of the most significant environmental consequences of very large bolide impacts on Earth could have been the destruction of the protocrust. On Mars and Mercury, the ancient protocrusts persisted, despite bombardment, but the Late Heavy Bombardment on Earth may have been effective at crust destruction if the crust–mantle system had reached a vulnerable state, due, for example, to build-up of internal heat.

An artists impression of impacts on the Moon during the Late Heavy Bombardment. Australian National University.

With no supracrustal rocks of Hadean age preserved, the question of putative Hadean life and its effects on the carbon cycle cannot be studied directly. By contrast, the Archaean sedimentary record does contain samples with remains of organic (reduced) carbon, as well as carbonate, and there is clear evidence that the Archaean Earth was struck by very large bolides. No unequivocal Archaean impact basins have been found to date. Instead, the evidence for impacts comes from so-called spherule layers within sedimentary sequences. These tell-tale sediment layers are millimetre-to metre thick, laterally continuous, and contain spherules of various compositions, some with evidence for quench cooling, high pressure minerals, or shock features. The first important inference drawn from their distribution in time is that the Earth continued to be bombarded with large bolides well beyond 3850 million years ago and that the Archaean witnessed more large impacts than the later eons. Because many spherule beds are enriched in iron-loving (siderophile) elements, it has also been possible to incontrovertibly prove that some layers have had a contribution to their formation from a vapourized asteroid, for example via the isotope composition of Chromium.

A particular advantage of studying spherule beds is that they are preserved within a stratigraphic context. This provides additional sedimentological information and geochemical evidence of potential environmental disruption. Most of the well-preserved Archaean spherule beds from the Kaapvaal Craton of southern Africa and the Pilbara Craton of western Australia  show evidence for sedimentary redistribution caused by currents and/or waves. The consistent occurrence of spherules within reworked eroded local detritus rather than the pure deposits of constant thickness expected from fallout, strongly suggests that reworking was a consequence of the impact itself via tsunamis, impact-induced turbidity currents, or bottom return flows.

Previous studies have theorised that most of the spherule bed–forming bolides were 20–50 km in diameter and would have excavated transient craters of up to 100 km deep and final basins reaching several hundred kilometres in diameter. To date, no such basin has actually been discovered. One interesting area of future research is the question of shock-metamorphism of the lithospheric mantle during excavation and collapse of transient cavities well below the crust–mantle boundary. In terms of environmental and carbon isotope consequences, the impact that caused the 2629 million year old spherule layers in Western Australia and South Africa is particularly instructive because it is found within carbonate (mostly dolomite), which is conducive to chemical and isotopic analysis. The corresponding impact basin would have been about 100–150 km in diameter and, thus, represent a significant event. The carbon isotope values for reduced carbon across the spherule bed do not show an incontrovertible trend, but nonetheless indicate a significant general shift towards lighter carbon isotopes. By contrast, the shallow marine carbonate isotope values remain near constant across the spherule bed, but these data were obtained at a more limited spatial resolution that may not have captured the disruption of the global biogeochemical carbon cycle. More detailed isotopic studies across spherule beds are needed to explore to what extent the balance between the buried sedimentary reduced carbon and the dissolved oceanic carbon was disrupted by these impact events.

Carbonate is a less dominant sediment type in the Archaean supracrustal rock record than in the Proterozoic and the Phanerozoic. Therefore, it is impossible to produce a continuous global carbonate carbon isotope record that would cover all the 15 known Archaean spherule layers to test how representative the 2629 Ma event was. Notwithstanding this limitation, it is evident from the existing global compilation that the presently documented fluctuations in Archaean carbon isotope ratios were much less pronounced than in the Palaeoproterozoic and Neoproterozoic. Regardless of the potential of very large bolide impacts to temporarily disrupt the ancient carbon cycle, the apparent stability of the cycle itself, as well as the similarity of the predominant Archaean carbon isotope ratios with modern carbonate carbon, is astonishing. The oldest carbonates occur in the Isua Greenstone Belt of Southwest Greenland. They are between 3710 and 3810 million years old and, although not universally accepted as sedimentary in origin, some appear to have yielded carbon isotope ratios close to the modern-day value, and they have co-existing very light carbon preserved in putative biogenic graphite Due to the pervasive metamorphic overprint of the Isua rocks, some doubt remains as to whether the recorded carbon isotope values truly reflect the sedimentary system. Regardless, many more paired reduced carbon and carbon isotope values have been reported for younger Archaean sedimentary rocks, leaving little doubt as to the stability of the early terrestrial carbon cycle.

Photomicrographs of typical spherules collected from the 2130–1848 million year old spherule layer in northeast Midternæs, Greenland. (a) Spherule filled with finely crystalline radial-fibrous chalcedony and sericite in plane polarised light. Note marginal replacement by invasive dolomite and concentration of carbonaceous matter (black) along boundary between chalcedony and dolomite. (b) Same field of view as (a) between crossed polarisers. Arrows indicate planar interfaces between adjacent radial-fibrous aggregates. (c) Spherule filled with combination of coarser, equigranular quartz crystals (white) and fibrous sericite (grey) in plane polarised light. Sericite is unusually coarse and locally organized into radiating aggregates. (d) Part of spherule similar to (c) between crossed polarisers showing radiating sericite aggregates. Scale bars are 300 µm in (a), (b) and (c) and 100 µm in (d). Chadwick et al. (2001).

Of the six largest preserved terrestrial impact structures, three are Proterozoic in age: the 2023 million year old Vredefort impact structure in South Africa, the 1849 million year old Sudbury Basin in Canada, and the 580–590 million year old Acraman crater in South Australia. Due to deep erosion of the Vredefort structure and the lack of a confirmed corresponding impactite layer, it is impossible to reconstruct the environmental consequences of Earth’s largest preserved bolide impact. By contrast, both the Sudbury and Acraman events preserve remnant impact structures, as well as corresponding impactite layers in the sedimentary record. These two impact events are, therefore, more conducive to studying putative global environmental consequences.

The impact layer corresponding to the Sudbury Basin is found up to 700 km away in the iron-rich sedimentary strata of the Lake Superior region of North America. The layer is a breccia containing lithic fragments (some shocked), devitrified glasses of various kinds, as well as accretionary lapilli; this layer differs from the Archaean spherule beds. Of critical importance is that the breccia layer occurs within a Palaeoproterozoic sedimentary context. The bolide is believed to have hit a foreland basin covered by relatively shallow water, and the main excavated rocks were quartz-rich sandstones of the over 2200 million year old Huronian Supergroup and Archaean basement. These contained very little carbon. However, it has been argued that the bolide was likely a 15 km diameter comet (with a density of 0.6 g per cm³). If this body was similar in composition to comet Halley's Comet, which is estimated to be 18.4% carbon by mass, then the Sudbury object would have contained 195 gigatons of carbon and, if fully vapourised, would have released about 700 gigatons of CO₂, or about one-third of the CO₂ perturbation of the current Anthropocene experiment.

In the lead-up to the impact, the continental foreland basin of the Lake Superior region was ferruginous, with thick banded-iron formations being deposited. The Sudbury impact layer nearly always caps the iron formations and other ferruginous sediment, and subsequent deposition continues with different mud-sized detritus. No re-occurrence of the dominant deposition of iron formation after the impact has yet been observed. There is, thus, strong regional evidence that the Sudbury impact event caused a sharp change in basin water conditions over 700 km away. The disappearance of Palaeoproterozoic banded iron formation at about 1850 million years ago is a global phenomenon, and there is strong evidence for tsunami deposits within some of the impact layers at variable water depths, which has led to the proposal that that the Sudbury impact could have pervasively changed the regional, and probably the global, oceanic stratification, bringing to an end the long-lasting dominantly ferruginous state of the early Palaeoproterozoic deep oceans. The physical reasons for the inferred change remain to be established, however. It is currently unknown how an event such as the Sudbury impact could have disrupted the global oceanic iron supply and started the fickle oceanic states of the remaining Proterozoic. 

Stratigraphic relationship shown in six sedimentary logs between the Palaeoproterozoic Sudbury (Canada) impact layer (in blue) and the type of sediments that preceded and followed this event. Note the lack of deposition of banded iron formation after the impact event. The six logs relate to the following: Mesabi Iron Range (Minnesota, USA); Gunflint Iron Range (Minnesota, USA, and northwest Ontario, Canada); Gogebic Iron Range (Michigan and Wisconsin, USA); Iron River–Crystal Falls District (Michigan, USA); Marquette Iron Range (Michigan, USA); Baraga Basin and Dead River Basin (both in Michigan, USA). (Inset) Example of a lapilli-stone, one of the rock types that makes up the Sudbury impact layer. The rock abounds with millimetre-sized accretionary lapilli that formed in the impact plume. Photo width is 5 cm. Kamber & Petrus (2019).

The Sudbury Basin itself preserves the best-exposed and most accessible stratigraphy through a very large impact basin on Earth. It may originally have measured 170–200 km across but thanks to its remnant now being folded, there is an unparallelled opportunity to study transects from the shocked basement into the differentiated melt sheet and across the basin fill without the need for drilling. Of particular interest is the 1300 m thick unit that overlies the crystallised melt sheet. It consists of breccias and tuffs that collectively are far too thick to represent the fallback from the impact. Instead, the first 300 m of chaotic breccias most likely formed through a fuel-coolant interaction, when seawater flooded onto the superheated melt sheet The remaining stratigraphy is characterised by sustained deposition of subaqueous volcanic products (bombs, lapilli and ash) that are more mafic than the average target rocks. The observation of on-going igneous activity within a subaqueous impact basin has led to speculation that it could represent deeply sourced magmatism.

Numerical impact modelling has demonstrated that the depth of the transient cavity (created within less than a few seconds) nearly linearly increases with increasing bolide diameter, whereas the final depth of even a 500 km diameter basin is less than 3 km. The divergence in depth between transient cavity and final basin necessitates an ever-increasing material flow during the rebound and collapse of the original cavity. It has been proposed that the vertical component of this material flow could give rise to secondary decompression melting. In areas of unusually high continental heat flow and on weak plates (such as ocean basins), one environmental consequence of very large impact events could, therefore, be sustained, deeply sourced magmatism and the associated release of volatiles.

Regardless of this possibility, a final noteworthy aspect of the Sudbury crater fill is the progressive enrichment of the breccias and tuffs in reduced carbon. Studying the chemistry of the fine-grained ash-sized matrix of the crater fill, has led to the conclusion that the crater basin was likely cut off from the open ocean and so developed a distinctive water chemistry within it. The sustained magmatic activity within the basin supported base-metal deposition similar to volcanogenic massive sulfide ores, which otherwise occur at oceanic spreading sites. Apart from the destructive forces of very large impacts, one very different environmental consequence of subaqueous events could, thus, be the formation of enclosed 'ponds' (similar in shape to atolls), which contained chemical 'factories' (hydrothermal systems) producing organic molecules as potential building blocks for life. Whereas life had long been established by 1849 Ma, similar Hadean or early Archaean subaqueous impact basins should be considered as possible birth places of life and the kick-start of the terrestrial carbon cycle.

The structure of the Sudbury Basin. Natural Resources Canada/Wikimedia Commons.

The approximately 590 million year old Acraman impact occurred during a period of intense fluctuations in the carbon cycle in the late Neoproterozoic Era. The possibly 85–90 km diameter impact structure is now deeply eroded, but the corresponding impact layer can be traced for over 500 km. Palaeomagnetic data suggest a low latitude impact, which could potentially increase any resultant environmental effects. But in terms of the on-going Neoproterozoic fluctuations in carbon isotopes, the Acraman event seems to have been relatively minor, with only a small excursion towards a more negative reduced carbon isotope value; however, the detailed isotope stratigraphy is currently missing. The Acraman impactite layer coincides with a marked change in fossil plankton (Acritarch) successions and may have been more significant in terms of radiation than the preceding worldwide Marinoan Glacial Event of the Cryogenian Period. The main target lithology of the Acraman impact were acidic volcanic rocks poor in carbon. There may have been limited disruption of the global carbon cycle, although detailed carbon isotope stratigraphy is unavailable.

The remaining three largest terrestrial impact structures are Phanerozoic in age. The possible causal relationship between a large bolide impact and a Phanerozoic extinction event has been widely discussed, but there are two very clear observations. One is that there have been more significant extinction events during the Phanerozoic than there are very large impact structures to account for them; the second is that there were large impact events, such as the the 215 million year old Manicouagan Crater in Quebec, Canada, with no correlative mass extinction.

Against this backdrop, the exceptional coincidence between the End Cretaceous extinction event and the roughly180 km diameter, 66 million year old, Chicxulub impact structure in Yucatan stands out. It is still being debated whether the environmental effects of the bolide strike on their own were responsible for extinction or whether the Earth was struck at a time when its biology had already been pushed close to a tipping point by volcanic degassing and dropping sea-level. Regardless, it is widely agreed that the Chicxulub impact caused planet-wide climate disruption, as supported by the geological context of the impact site. In the late Cretaceous, the Yucatan Peninsula was a partially emerged platform composed of calcium carbonate and evaporite deposited on older sediments, themselves sitting on Precambrian basement. The bolide excavated through this 'fertile' stratigraphy at a site partly on land and partly submerged.

From a carbon cycle perspective, the presence of thick carbonate beds at the target site is of greatest relevance. The potential quantity of CO₂ devolatilised to high atmospheric altitude from bolides is dwarfed by that modelled to be ejected from a thick carbonate platform. On a 500–1000 year timescale, the effects of releasing 425–1400 gigatons of CO₂ into the atmosphere is climate warming, but in the case of Chicxulub, where limestone, sulfate and seawater were the target, the short term effect was dramatic SO₂-driven cooling with global annual mean surface air temperatures dropping by more than 20 °C, recovering only after 30 years. The bolide strike may also have caused massive wildfires and/or stratospheric emission of smoke from combustion of hydrocarbons within the target marine platform (oil and gas are produced to the north and west of the impact site). The nature of recovered molecules from incompletely combusted hydrocarbons preserved in the impact layer supports the idea that the bulk of the soot was released from reduced carbon contained within the impacted target rocks, amplifying SO₂-driven cooling along the equator and causing droughts. All this is consistent with the extinction patterns.

The End Cretaceous event, thus, emphasises a further aspect of impacts on the terrestrial surface, which is lithologically and geochemically highly diversified and evolved. Less than one-sixth of the current planetary surface has a suitable make-up to cause strong stratospheric cooling if hit by a large bolide; significant direct disruption of the global carbon cycle seems only likely from impacts onto thick carbonate targets.

The size and location of the Chicxulub Crater. Passant Rabie/Inverse.

Bolide impacts have affected the Earth’s carbon cycle in a multitude of ways. The widely held view that there are direct effects to the carbon cycle through environmental devastation and mass extinction, such as has been popularised with the End Cretaceous boundary event, is probably the exception rather than the rule. Most of the consequences of large impacts have been indirect. On the Hadean Earth, intense bombardment may have contributed to the destabilisation of the original crust, thereby possibly promoting plate motion that has become an integral part of carbon cycling through plate destruction. Subaqueous early impact basins may also have been self-contained production sites
of organic molecules that could have been potential cradles for life.

Throughout the Archaean, the Earth continued to be occasionally bombarded by large bolides, as inferred from thick beds of spherules that must have splashed down from giant melt and vapour plumes. The existing Archaean sedimentary carbon isotope record does not appear to show fluctuations of the magnitude seen in later times: however, the record is of limited temporal resolution. In general, partitioning of carbon between the reduced and oxidised pools has remained surprisingly constant. The much more pronounced carbon isotope excursions of the Palaeoproterozoic and Neoproterozoic do not coincide with known impact events. Instead, the two very large events, at 1849 and 590 million years ago, are traceable in the sedimentary record and are associated with the end of the deposition of banded iron formations and the radiation of Acritarch plankton, respectively. If future work demonstrates these to be causal relationships, they would illustrate the indirect influence of large impacts on the carbon cycle through the reorganisation of the ocean’s redox state and the disruption of biological evolution.

Most Phanerozoic mass extinctions are not coincident with very large impact events. The Chicxulub event, occurring at the End Cretaceous boundary, caused a moderate carbon isotope excursion and greatly disrupted the budget of climate-active gases in the atmosphere. This, in turn, led to a short-term abrupt cooling and a medium-term strong warming The lesson drawn for the Anthropocene is that the release of several thousand gigatons of CO₂ into the atmosphere may not leave a marked carbon isotope signal in the geological record. Instead, the Anthropocene is more likely to leave its legacy as a mass extinction from greenhouse-induced climate change on a biosphere already at a tipping point caused by habitat loss.

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Evaluating the possibility that the iron oxides in Banded Ironstone Formations formed post-depositionally.

Around two-and-a-half billion years ago, bands of iron oxides began to be deposited in ocean basins around the world. These Banded Ironstone Formations are considered to be evidence of oxygen-producing photosynthetic organisms colonising the oceans, leading to oxygen reacting with iron dissolved in the seawater to produce iron-oxide precipitates, which fell to the ocean floor. Once all of the dissolved iron had been reacted, oxygen began to build up in the atmosphere, resulting in the Great Oxidation Event of about 2.45 billion years ago,  marking the boundary between the Archean and Proterozoic Eons. However, in 2014 a team of scientists led by Birgir Rasmussen of Curtin University proposed an alternative explanation for the formation of Banded Ironstone Formations, arguing that the presence of the mineral greenalite in the Transvaal Supergroup of South Africa the and Hamersley Group of Australia implies that the iron was not oxidised when it was deposited, and that the ironstone was instead formed post-depositionally by the peculation of oxygen-rich waters through iron rich deposits.

In a paper published in the journal Nature Geoscience on 27 May 2019, Leslie Robbins of the Department of Earth & Atmospheric Sciences at the University of Alberta and the Department of Geology and Geophysics at Yale University, Sean Funk, also of the Department of Earth & Atmospheric Sciences at the University of Alberta, Shannon Flynn of the School of Natural and Environmental Sciences at Newcastle University, Tyler Warchola, also of the Department of Earth & Atmospheric Sciences at the University of Alberta, Zhiquan Li, again of the Department of Earth & Atmospheric Sciences at the University of Alberta, and of the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences, and the Key Laboratory of Mineral Resources at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Stefan Lalonde of the Laboratoire Géosciences Océan of the European Institute for Marine Studies, Benjamin Rostron, again of the Department of Earth & Atmospheric Sciences at the University of Alberta, Albertus Smith, Nicolas Beukes, and Michiel de Kock of the Paleoproterozoic Mineralization Research Group and National Research Foundation Centre of Excellence for Integrated Mineral and Energy Resource Analysis, at the University of Johannesburg, and Larry Heaman, Daniel Alessi, and Kurt Konhauser, again of the Department of Earth & Atmospheric Sciences at the University of Alberta, present the results of a computer simulation of the formation of Banded Ironstone Formations in the Hamersley Group of Australia, which seeks to assess whether such a model is plausible.

The Hamersley Group has an aerial exposure of over 100 000 square kilometres, is over 2.5 km thick, and comprises the Marra Mamba, Brockman, Weeli Wolli and Boolgeeda iron formations, together laid down between about 2.6 and 2.45 billion years ago. Robbins et al. constructed a model in which the target percentage of iron oxidised, permeability of the rock, hydraulic gradient and dissolved oxygen concentration could all be varied, producing a total of 3 802 356 permutations of the model.

Conceptual model for the postdepositional oxidation of greenalite by oxidative groundwater. (a)–(c) Deposition of a greenalite precursor phase (a) followed by postdepositional oxidation pathways that include early diagenetic fluids (b) or infiltration of a meteoric oxidizing fluid and flow parallel to iron-rich layers (c). (d) Banding preserved in the Joffre Member of the Brockman Iron Formation, Western Australia. Any postdepositional oxidation must be capable of explaining the preservation of the distinct banding characteristic of Banded Ironstone Formations. Robbins et al. (2019).

Robbins found that of the 3 802 356 permutations of the model, only 8.87% were capable of oxidising all of the iron in the Hamersley Group in the 2.45 billion years available since the deposits were laid down, and only ~1.06% were capable of doing this in the 250 million years available before the rocks of the Turee Creek Group covered the Hamersley Group to such an extent that compaction of the Hamersley deposits would have made the flow of oxygenated groundwater more-or-less impossible. Furthermore, all of these 'succesful' permutations of the model require high levels of permeability within the sediments (unlikely within a 2.5 km thick deposit in a deep marine environment) combined with a high oxygen content in the permeating water (again unlikely in deep marine environments today, and much less so in the Palaeonproterozoic, when only the upper part of the water column is thought to have been weakly oxygenated.

Robbins et al. also see a number of other problems with the post-depositional oxygenation model. These include the presence of millimetre scale bands in the deposits, which would almost certainly have been disrupted by peculating waters and oxygenation (iron oxides take up more space than unoxygenated iron compounds, and conversion of one to the other tends to result in the loss of fine structures, most notably in the flaking associated with rusting iron). Robbins et al. also note that Rare Earth Elements are present in the Hamersley Banded Ironstones in roughly the proportions expected in sediments deposited in marine environments, a ratio that would normally be disrupted by highly oxygenated waters flowing through the rock.

Robbins et al. conclude that the post-depositional oxygenation of the iron in the Hamersley Group is highly implausible, if not actually impossible. They further note that any such event would have to have not only affected the rocks of ironstones of the Hamersley Group, but also those of the Transvaal Supergroup, with an aerial extent of 110 000 square kilometres, and a thickness of 950 m, which is thought to have been laid down in a connected, adjacent basin to the Hamersley Group, as well as those of the Kursk Supergroup in Russia, the Krivoy Rog Supergroup in Ukraine, the Cauê Formation in Brazil, the Benchmark Iron Formation in the US and the Ruker Series in Antarctica.

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