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.
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.
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 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.
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.
See also...
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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