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A The cumulative age-distribution of bolides capable of creating basin-sized impact structures on Earth number as y-axis. After Bottke et al. 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. After Johnson and Melosh The Chicxulub Mexico impact is shown in red. C A summary of key events and preserved pieces of evidence relevant to planetary evolution. The K—Pg [formerly the K—T] extinction event shown as a red vertical bar. Data from Krissansen-Totton et al.

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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 e. The delivery of extraterrestrial matter to Earth happens with two dominant size classes of objects: the tiniest, and the largest.

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At the other end of the spectrum, cosmic dust, having particle diameters in the micrometre range, constitutes the second significant source of extraterrestrial matter Peucker-Ehrenbrink et al. 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 C reservoir, this simple logic is complicated by impact physics.

Comparing the delivery of extraterrestrial elements of a refractory nature i. 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 Alexander et al.

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. 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 e.

With currently available data, modelling cannot unequivocally rule out either scenario. This line is interpreted to date the timing of volatile element loss and homogenization. The age conspicuously coincides with the more widespread preservation of terrestrial rocks, i.

If future lunar data confirm the existence and timing of the LHB, 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 LHB 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 e.

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 e.

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 Fig.

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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 Cr e. A particular advantage of studying spherule beds is that they are preserved within a stratigraphic context Simonson and Glass This provides additional sedimentological information and geochemical evidence of potential environmental disruption.

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. Johnson and Melosh concluded that most of the spherule bed—forming bolides were 20—50 km in diameter and would have excavated transient craters of up to 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 2, Ma 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. 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.

Notwithstanding this limitation, it is evident from the existing global compilation Fig.

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. 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 km away in the Fe-rich sedimentary strata of the Lake Superior region of North America e.

The layer is a breccia containing lithic fragments some shocked , devitrified glasses of various kinds, as well as accretionary lapilli Fig. Of critical importance is that the breccia layer occurs within a Palaeoproterozoic sedimentary context Fig. The bolide is believed to have hit a foreland basin covered by relatively shallow water e.

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  • These contained very little carbon. However, Petrus et al. If similar in composition to comet Halley, which has 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.

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    After Cannon et al. The rock abounds with millimetre-sized accretionary lapilli that formed in the impact plume. Photo width is 5 cm. 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. Cannon et al. No re-occurrence of the dominant deposition of iron formation after the impact has yet been observed. 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.

    A Potential mass of CO 2 released as a function of bolide diameter for three scenarios. After Artemieva et al. After Alexander et al. After Delsemme The largest impact events preserved on Earth e. This transition stunts the growth of the final crater depth. The transient crater depth is likely an over-estimate due to uncertainties about excavation, deformation, and displacement styles.

    Note that although the horizontal scale is the bolide diameter as in Figure 3A, the depicted ranges differ. Data from Collins et al. 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 — km across but thanks to its remnant now being folded, there is an unparalleled opportunity to study transects from the shocked basement into the differentiated melt sheet and across the basin fill e.

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    Of particular interest is the 1, m thick unit that overlies the crystallized melt sheet. It consists of breccias and tuffs that collectively are far too thick to represent the fallback from the impact. Instead, the first m of chaotic breccias most likely formed through a fuel-coolant interaction, when seawater flooded onto the superheated melt sheet e. The remaining stratigraphy is characterized by sustained deposition of subaqueous volcanic products bombs, lapilli and ash that are more mafic than the average target rocks.