The terrestrial record of Late Heavy Bombardment

New Astronomy Reviews 81 (2018) 39–61

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The terrestrial record of Late Heavy Bombardment a,⁎

Donald R. Lowe , Gary R. Byerly a b

T

b

Department of Geological Sciences, Stanford University, Stanford, CA 94305-2115, United States Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Archean Spherules Meteorite impacts Barberton Greenstone Belt Crustal evolution

Until recently, the known impact record of the early Solar System lay exclusively on the surfaces of the Moon, Mars, and other bodies where it has not been erased by later weathering, erosion, impact gardening, and/or tectonism. Study of the cratered surfaces of these bodies led to the concept of the Late Heavy Bombardment (LHB), an interval from about 4.1 to 3.8 billion years ago (Ga) during which the surfaces of the planets and moons in the inner Solar System were subject to unusually high rates of bombardment followed by a decline to present low impact rates by about 3.5 Ga. Over the past 30 years, however, it has become apparent that there is a terrestrial record of large impacts from at least 3.47 to 3.22 Ga and from 2.63 to 2.49 Ga. The present paper explores the earlier of these impact records, providing details about the nature of the 8 known ejecta layers that constitute the evidence for large terrestrial impacts during the earlier of these intervals, the inferred size of the impactors, and the potential effects of these impacts on crustal development and life. The existence of this record implies that LHB did not end abruptly at 3.8–3.7 Ga but rather that high impact rates, either continuous or as impact clusters, persisted until at least the close of the Archean at 2.5 Ga. It implies that the shift from external, impact-related controls on the long-term development of the surface system on the Earth to more internal, geodynamic controls may have occurred much later in geologic history than has been supposed previously.

1. Introduction One of the many outcomes of the NASA Apollo program and the 6 manned lunar landings between 1969 and 1972 was the direct sampling and return of lunar rocks and the establishment of a more exact, quantitative lunar chronology. The results include recognition that the heavily cratered lunar highlands formed about 4.0 billion years before the present (= 4.0 Ga) or slightly earlier and the more lightly cratered lunar maria representing vast, low-relief basaltic plains with much fewer, mainly smaller craters, formed 3.9–3.1 Ga but mostly between 3.7–3.9 Ga. Abundant radiometric evidence for widespread impact melt formation 4.1–3.8 Ga and the subsequent abrupt termination of largescale cratering at about 3.8 Ga led to the concept of a Late Heavy Bombardment (LHB) or lunar cataclysm (Tera et al., 1974) during which the Earth, Moon, and presumably other planetary surfaces were bombarded by large bolides at rates substantially higher than those that prevailed before about 4.1 Ga (Fig. 1(A)). Based on this lunar chronology and studies of lunar cratering, the LHB was thought to have extended from about 4.1 or 4.0 Ga until 3.8 Ga (Fig. 1(A)). After 3.8 Ga, the impact rate was interpreted to have dropped abruptly until, somewhere between 3.6 and 3.0 Ga, depending on the cratering rate curve one prefers, it reached a value that differed little from the very low

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impact rate of today (Neukum et al., 2001; Valley et al., 2002; Koeberl, 2006). A number of investigators have questioned the concept of a Late Heavy Bombardment, suggesting instead a more continuous decline in the impact flux (Fig. 1(B)) (e.g. Neukum et al., 2001; Boehnke and Harrison, 2016). The paucity of terrestrial rocks older than about 3.8–3.9 Ga is consistent with this concept of profound crustal modification and disruption by large impacts during LHB, and the apparent absence of terrestrial craters and impact features in preserved early Archean terrestrial rocks, which range from about 3.55 to 3.0 Ga, seemed for many years to be consistent with the inferred low impact rates at this time. However, it should be noted that Green (1972) and others compared the thick mafic volcanic sequences in early Archean terrestrial greenstone belts to the basaltic lunar maria and hypothesized that they may reflect large terrestrial impacts that accompanied the large mariaforming lunar impacts. These and other possible effects of large impacts on early terrestrial tectonics will be considered in Section 4.3. Evidence for ancient terrestrial impacts comes in two main forms: craters and ejecta layers. Craters, both terrestrial and lunar, have been regarded by some as possible meteor impact structures since they were first studied but the wide acceptance of the role of impacts in generating ancient terrestrial structures is a more recent event. Many more-

Corresponding author. E-mail address: [email protected] (D.R. Lowe).

https://doi.org/10.1016/j.newar.2018.03.002 Received 17 October 2017; Received in revised form 28 February 2018; Accepted 20 March 2018 Available online 26 March 2018 1387-6473/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Inferred terrestrial cratering rates. (A) Cratering rate calculated by Neukum et al. (2001) from lunar data shows about 15–18X present rate at 3.5 Ga. This model does not consider a Late Heavy Bombardment. (B) Cratering rate of Valley et al. (2002) shows a distinct LHB and an even lower impact rate at 3.5 Ga than suggested by Neukum et al. (2001), perhaps only 1.2X present low levels.

four or five times the mass of the impactor, that is ejected from the crater, often into or above the upper atmosphere, spread around the Earth as a rock vapor cloud, and condensed to form liquid silicate droplets that chill and fall to the earth as solid, spherical to subspherical bodies termed spherules. Spherules are commonly <0.1 to about 1 or 2 mm in diameter. They include glassy particles termed microtektites and partially or fully crystalline grains termed microkrystites (Glass and Burns, 1988). Between 2.5 and 10 crater diameters from the impact site, ejecta may include both spherules and small unmelted or partially melted ballistic particles. (3) Impacts also generate a great deal of fine dust that is ejected into the atmosphere and subsequently carried by air currents as non-ballistic particles to locations potentially around the globe. These are analogous to volcanic plumes that can cloud the atmosphere with dust for months. It was dust clouds that are thought to have contributed to global cooling and the severe climatic changes that drove collapse of the Late Cretaceous ecosystems and extinction of the dinosaurs about 65 million years ago. General discussions of ejecta layers are provided by Smit (1999), Simonson and Glass (2004), Glass and Simonson (2012), and Glass and Simonson (2013).

or-less circular zones of surface deformation and rock brecciation were commonly termed cryptovolcanic or cryptoexplosion structures before about 1965 because there was often little or no evidence for an associated igneous event and because of the general reluctance to accept a major role for impacts in later Earth history. Work by Robert Dietz on the Vredefort structure, South Africa (Dietz, 1961), and the Sudbury structure, Ontario, Canada (Dietz, 1964) pioneered the modern advocacy of a major role for impacts in Earth history. An ejecta layer represents materials thrown out from an impact site at the time of and as a result of the impact (Fig. 2). Ejecta takes three forms: (1) Discrete particles, either liquid or solid, that are thrown from the impact site, follow more-or-less ballistic trajectories, and fall either as coarse ejecta close to the impact site or at more distant sites as finer, distal ejecta (Fig. 2). Simonson and Glass (2004) and Glass and Simonson (2012) suggest that ejecta deposited >2.5 crater diameters from the crater should be called distal ejecta and closer deposits should be termed proximal ejecta. Proximal ejecta includes blocks and boulders of target rock, mineral grains, melted and partially melted materials, accretionary grains called accretionary lapilli, and some spherical melt droplets. (2) More distal ejecta deposited >10 crater diameters from the crater consists primarily of glassy particles. These include rare larger (> 1 cm) particles called tektites representing melted masses of the target rock and abundant smaller grains, most <1 mm in diameter, representing ballistic liquid silicate droplets. Many ejecta layers, especially >10 crater diameters from the impact site, include spherules representing either melted or vaporized target and bolide material. Large impacts generate large amounts of rock vapor, up to

In the present report, we provide a review of the distribution, ages, and characteristics of the Archean ejecta layers and the Barberton Greenstone Belt layers in particular, discuss the possible influence of large impacts on early Archean crustal development, and consider the implications of this preserved terrestrial impact history on the nature of Late Heavy Bombardment and the late Solar System impact history. All of the Archean ejecta layers reported to date are distal layers composed largely of spherules and fine ash and dust. Coarser ejecta layers have not yet been recognized in Archean sequences. Fig. 2. Ejecta blankets. (A) Moltke, a small lunar crater 6.6 km in diameter, with a proximal ejecta blanket showing large blocks very near the crater and dunes in finer ejecta within about 2 crater diameters of the impact site. (B) Kuiper, a 60-km diameter crater on Mercury, shows a proximal ejecta blanket to a few crater diameters from the impact site surrounded by a rayed distal blanket. Photos from NASA.

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Fig. 3. Main events, rock sequences, and time scales for the early Earth. Blue–purple show the named time intervals for early events. The terms Early and Late Archean are here used to designate intervals of Archean time before and after 3.0 Ga, respectively. The main episodes of impacting are shown in red, with those younger than 3.5 Ga representing impact layers preserved in the geologic record. The development of the Earth's crust as preserved today is shown in green for the main periods of pre-continental volcanic and sedimentary greenstone belt activity, orange for the main periods of formation of new blocks of continental crust in the areas of older greenstone deposition, and yellow for the main ages of the sedimentary sequences deposited on the continental blocks after they had formed and been eroded down to form stable platforms (Lowe, 1992). No continental blocks are known older than about 3.1 Ga. Craton cover sediments include shallow-water quartz-rich sediments eroded mainly from the underlying blocks of continental crust and carbonate sediments containing the earliest records of the flourishing shallow-water microbial communities that lived on these shallow platform surfaces. All of the Archean terrestrial impact layers discovered to date are preserved in the Early Archean greenstone belts or in the Late Archean craton cover sequences on the Early Archean cratons. Abbreviations: LHB, Late Heavy Bombardment; EAC, main interval of Early Archean craton formation; LAC, main interval of Late Archean craton formation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2. Archean ejecta layers

et al., 2003; Lowe et al., 2014) and possibly 2 others from the Pilbara Block (Glikson et al., 2004; Glikson et al., 2016). This study will focus on the 8 ejecta layers in the Barberton Greenstone Belt.

The oldest putative impact craters include the ∼3.023 Ga Maniitsoq structure in West Greenland (Garde et al., 2012) and the ∼2.400 Ga Suavjarvi structure in Russia (Mashchak and Naumov, 1996). The oldest widely accepted impact crater is the 2.023 Ga Vredefort structure in South Africa. According to Glass and Simonson (2012) 28 distal impact layers were known as of 2012. Glikson et al. (2016) suggests that 17 asteroid ejecta units of Archean are known, although some may be correlative. We count at least 14 well-defined Archean impact beds, and, depending on the validity of multiple beds reported from single outcrops or core (e.g., Schulz et al., 2017), perhaps as many as 19. Perhaps the best known ejecta layer is that associated with the Cretaceous–Tertiary (K–T) impact 65 million years ago (Alvarez et al., 1980). At distal localities around the globe, far from the impact site in Yucatan, Mexico, this layer includes a thin, 3–5 mm-thick layer of spherules formed by condensation of a rock vapor cloud. Archean (pre2.5 Ga) and near Archean ejecta layers occur in two clusters. Fig. 3 shows these impact clusters within the overall context of events and rock sequences characterizing the preserved terrestrial early rocks record. One group of 4 ejecta layers was generated by large impacts 2.63 to 2.49 Ga and is known from sedimentary sequences in the Hamersley Basin, Western Australia, and the Griqualand West Basin, South Africa. These layers were first reported by Simonson (1992) and the spherule layers, spherules, and their correlations and implications have been extensively documented and explored by Hassler et al. (2000), Hassler and Simonson (2001), Hassler et al. (2011), and many others. The second, older major cluster of Archean ejecta layers occurs in rocks 3.472–3.225 Ga in the Barberton Greenstone Belt (BGB), South Africa, and in greenstone belts of the Eastern Pilbara Craton, Western Australia. These greenstone belts are deformed but often relatively unsheared terranes made up of volcanic and sedimentary rocks that were not deposited on or in close association with older blocks of continental crust. The evolution of these belts ultimately culminated in the formation of new blocks of continental crust, the Kaapvaal Craton in South Africa and the Pilbara Craton in Western Australia, at about 3.1–3.0 Ga (Fig. 3). The spherule beds in these ancient greenstone sequences are the oldest known terrestrial ejecta deposits. Documented ejecta layers in these greenstone belts include 8 known beds in the Barberton Greenstone Belt, South Africa (Lowe and Byerly, 1986; Lowe

3. The BGB spherule beds 3.1. Geologic setting The numbering and locations of the 8 known impact layers in the BGB are shown in Figs. 4, 5 and Table 1. The BGB is made up of interlayered volcanic and sedimentary rocks, the Swaziland Supergroup, that is 12–15 km thick and ranges in age from at least 3.552 Ga to 3.220 Ga (Fig. 5). The oldest exposed rocks are volcanic rocks of the Onverwacht Group. These predominantly mafic and ultramafic (Fe and Mg-rich, low SiO2) lavas dominated BGB development for nearly 300 million years, from >3.552 Ga to 3.260 Ga. They were low-viscosity melts that erupted to form broad, low-relief, mainly submarine volcanic plains. At least twice during the accumulation of this sequence, mafic and ultramafic volcanism was interrupted by periods of more silicic volcanism that formed high-standing, steep-sided volcanic cones, the tops of which probably extended above sea level and served as sources of erosional debris and as shallow-water platforms that hosted the growth of extensive shallow-water biological communities and widespread chemical sedimentation. Throughout this long volcanic evolution, thin sedimentary layers were deposited locally and regionally during periods of volcanic quiescence. These layers are composed largely of marine chemical sediments, biological materials representing the remains and products of early marine microbial communities, and pyroclastic sediments formed by the accumulation of ash and dust from distant volcanic eruptions. These sediments were lithified early, in large part by silicification through interaction with silica-charged marine waters, to form finely crystalline silica-rich rocks called chert. Absent is debris derived from the weathering and erosion of older, uplifted rocks pointing to the absence of mountains and crustal deformation. This was a prolonged period of volcanism and sedimentation on what was probably a relatively stable, submarine volcanic surface. Following Onverwacht volcanism, the area of the Barberton Greenstone Belt was affected by deformation and uplift. The Fig Tree Group (Fig. 5), up to about 1000 m thick, includes abundant ash and dust representing felsic volcanism but is composed mainly of 41

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Fig. 4. (A) Index map showing the location of the Barberton Greenstone Belt, “B” within South Africa. (B) Geology of the central part of the Barberton Greenstone Belt showing the type localities of the 8 known impact layers, the known outcrop extent of S1 (circles), and the studied outcrop of S7 (squares). To date, spherules are known from S7 only at the type locality just north of the Komati River. Beds S4, S5, and S6 are known from only the type localities (Table 1). Modified from Lowe et al. (2003).

3.2. History of the BGB impact layers

conglomerate, sandstone, and mudstone derived by the weathering and erosion of older parts of the greenstone belt itself. This was a period of mainly local uplift and deformation and the deposition of sediments in adjacent or nearby basins. The Moodies Group at the top of the sequence includes up to 3000 m of sandstone, conglomerate, and a minor component of shale. These rocks record uplift and deep erosion through the greenstone belt rocks and into the underlying intrusions, most of which represent deep-seated silicic magmas that were emplaced at the same time as silicic lavas were erupted on the surface to form the felsic volcanic units of the Onverwacht and Fig Tree Groups. Moodies time saw regional uplift, the formation of substantial mountain ranges, and the deposition of thick, coarse sediments eroded from these mountains in surrounding basins. This rock sequence provides an important window on events and processes that took place on the Earth over a period exceeding 350 million years in length during the formative stages of early Earth history.

The distribution of known impact layers in the BGB is shown in Fig. 5 and the specific locality information in Fig. 4 and Table 1. The BGB impact layers were discovered when Lowe and Byerly (1986) reported the presence of layers of quenched liquid silicate droplets in Archean greenstone belt sequences 3.552–3.220 Ga in South Africa and Western Australia. They documented the presence of layers containing spherical, originally glassy as well as crystalline particles, and suggested that they formed through the condensation of globe-encircling, impact-produced clouds of rock vapor, a process discussed by Stevenson (1985). De Wit (1986) and Buick (1987) argued that the spherules represent ocelli, variolites, or other particles formed within and subsequently eroded from mafic volcanic rocks. Lowe et al. (1989) identified four spherule beds in the BGB, termed S1 (stratigraphically lowest) through S4 (highest), and demonstrated that S3 and S4 are sites of major iridium enrichment. Kyte et al. (1992) analyzed platinum 42

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Fig. 5. Stratigraphic column of rocks in the Barberton Greenstone Belt, South Africa. The ages of key units and the stratigraphic positions of the 8 known spherule beds are shown.

Table 1 Information on the type localities of the BGB spherule beds. Spherule Bed

Locality number (Lowe)

Locality number (Byerly)

Latitude and longitude

S1 S2 S3 S4a S5a S6a S7 S8

SAF-478 SAF-272 SAF-179, 349, 380 SAF-179, 349, 380 SAF-384 SAF-233 SAF-316b SAF-383

SA117, 449 SA312 SA306 SA504 SA560 SA22 SA963-1 SA130

26° 56.411′ S., 30° 52.931′ E. 25° 53.811′ S., 31° 00.017′ E 25°° 54.907′ S., 31° 01.135′ E. 25° 54.907′ S., 31° 01.135′ E. 25° 53.286′ S., 30° 52.445′ E. 25° 54.542 S., 31° 02.717′ E. 26° 01.270′ S., 30° 59.774′ E. 25° 54.148′ S., 31° 02.876′ E.

a b

Only known locality. Only confirmed locality with spherules. 43

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Fig. 6. Outcrop of spherule bed S2. The hammer, which is 40 cm long, lies just below the spherule bed. Stratigraphic top is to the right. S2 appears as a light colored sandy layer with angular clasts of dark chert ripped up from underlying rocks (1). Above the spherule bed is a layer of massive dark silicified sediment (2) representing the fallout layer of material that settled out of the water column after passage of the impact-generated tsunami. Together, these layers form the event bed at this locality. Surrounding sediments both below (3) and above (4) these beds are banded chert.

ocean waters (Lowe and Byerly, 2015). More recently Schulz et al. (2017) have studied the International Continental Scientific Drilling Program (ICDP) core BARB5 from the BGB and recognized what appear to be several separate spherule layers in a 20–25 cm-thick interval in what Lowe and Byerly (1999) regarded as the S3 spherule bed. The core shows abundant soft-sediment deformation and evidence of sliding and folding within this interval and only a single bed is present in surface exposures a few hundred meters away. We consider that the apparent multiple spherule beds in the core probably reflect early post-depositional downslope flowage, viscous mixing, and repetition of what was originally a single impact layer.

group elements (PGE) in S4, reported Ir levels as high as 360 ppb nearly chondritic relative abundances of Ir, Os, and Pt, and attributed fractionation of Pd and Au to later hydrothermal alteration. Byerly and Lowe (1994) analyzed the mineral spinel, a type of chromium aluminum oxide, within S3 spherules. These spinels show dendritic and skeletal octahedral forms, have very high Ni and ferric iron contents, and are composed dominantly of the chalcophile elements Fe, Ni, Cr, and V. They are distinctly different from those in most terrestrial ultramafic volcanic rocks (komatiites) and other magmatic rocks and were interpreted as having formed through condensation of rock vapor clouds following large impacts. As discussed by Lowe et al. (2003), a number of investigators initially questioned the impact interpretation of the BGB spherule beds (Koeberl et al., 1993; Koeberl and Reimold, 1995; French, 1998; Ryder et al., 2000; and Reimold et al., 2000). Koeberl et al. (1993), Koeberl and Reimold (1995) and Reimold et al. (2000) noted that samples of spherule beds collected in the vicinity of gold mines in the northern BGB show non-chondritic ratios among siderophile elements and noted a correlation between platinum group element (PGE) and chalcophile element abundances. They suggested that elevated PGE levels in the spherule beds are the result of post-depositional mineralization and hydrothermal alteration and that the spherule beds formed through unspecified volcanic processes. Shukolyukov et al. (2000) and Kyte et al. (2003) demonstrated that spherule beds S2, S3, and S4 have non-terrestrial Cr isotopic ratios. These results confirmed the presence of a significant extraterrestrial component in the BGB spherule beds and affirmed their impact origin, which had been proposed initially based largely on geological arguments. These units are an important terrestrial record of large meteorite impacts on the early Earth and a key window on the possible role of such impacts on the early evolution of the Earth's early life, crust, ocean, and atmosphere. Since these early studies, additional research has identified a total of 8 spherule beds in the BGB sequence (Lowe et al., 2014), S2 has been shown to have been responsible for widespread crustal fracturing and disruption (Lowe, 2013), and it has been inferred that the impacts associated with S5 and S8 were so large that they resulted in superheating of the atmosphere and evaporation of up to several hundred meters of

3.3. General features of the BGB impact layers The 12–15 km-thick BGB volcanic and sedimentary sequence presently includes 8 known spherule beds, <1 to about 3 m thick, that collectively represent most of the known record of Early Archean terrestrial impacts (Fig. 5). Based on detrital zircons, these beds span a period from about 3.472 Ga to 3.225 Ga, about 250 million years, and stratigraphic positions from H4c in the upper Hooggenoeg Formation of the Onverwacht Group to roughly the middle of the Fig Tree Group. Rocks older than the lowest known spherule bed, S1, are mostly volcanic flow rocks with very little interbedded sedimentary material or have been metamorphosed and sheared and their potential for preserving ejecta layers seriously compromised. Rocks above the highest spherule bed, S5, are all detrital sediments and any impact-generated debris falling into those environments would have been quickly eroded and mixed with terrestrial sediments. We would suggest that the discovered impact layers in the BGB may represent only a small proportion of the large impacts that occurred during this interval of time. The BGB spherule beds were numbered S1 through S8 (Fig. 5), more-or-less in the order discovered. All appear as layers of coarse sand- to gravel-sized detritus (Fig. 6) and are characterized by the presence of spherules. They occur at recognizable stratigraphic positions that, because of the folding and faulting in the BGB, are often repeated a number of times across the belt. Beds S1, S6, S7, and S8 occur within the Onverwacht Group and, with the exception of S7, are within relatively thin sedimentary layers, mostly <10 m thick, 44

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have been largely converted to serpentine minerals, involving recrystallization and hydration (Byerly, 1999). The sedimentary rocks and many of the tops of the volcanic flow sequences interacted extensively with sea water following their deposition and there is an ongoing debate about the extent to which their alteration reflects this early rock-sea water interaction or whether it reflects interactions with later, hot hydrothermal fluids moving upward through the igneous and sedimentary pile. In most sedimentary units and spherule beds, the more mobile elements, such as Na, Mg, Fe, and Ca have been depleted and the rocks are composed largely of SiO2 (silica), Al2O3, and K2O. Both SiO2 and K2O were introduced during alteration (Hanor and Duchac, 1987, 1990; Hessler and Lowe, 2006; Krull-Davatzes et al., 2012). In many rocks, alteration also involved the precipitation of carbonate minerals, including dolomite, CaMg(CO3)2, iron-rich dolomite, and/or calcite (CaCO3). The resulting alteration assemblage of minerals includes mostly microcrystalline quartz (chert), microcrystalline phyllosilicates and micas such as sericite and chlorite, some oxides, and carbonates. The only original minerals remaining tend to be virtually insoluble components such as coarse grains of quartz, iron oxides, zircons, and chromites. The immobile components, including major element oxides Al2O3 and TiO2; trace elements Cr, Ni, Sc, Th, Zr; the rare earth elements; and the platinum group elements have had their absolute abundances changed but ratios to one another are often unchanged. These ratios have served as much of the basis for estimating original compositions of rocks within the BGB.

deposited during breaks in the predominantly mafic to ultramafic volcanic activity. Spherule bed S7 occurs within a thick sedimentary sequence deposited on an inactive volcanic edifice that was being eroded and gradually subsiding below sea level (Tice and Lowe, 2006). Four layers, S2, S3, S4, and S5 occur within the Fig Tree Group, mostly toward the base. S2, deposited at about 3.260 Ga, widely marks the base of the Fig Tree group in the central and southern part of the BGB, where it rests directly on silicified sediments or volcanic rocks at the top of the Onverwacht Group. In the northern BGB, S3, about 3.243 Ga, marks the base of the Fig Tree Group. It is thought that these terranes represent two separate crustal blocks of differing age and history. Four of the impact layers are known only from single outcrops or very restricted areas of outcrop: S4, S5, S6, and S8 (Fig. 4). Away from these areas, the stratigraphic positions of the beds are not well exposed (S5), are within thick sections of current-deposited clastic detritus so that infalling spherules were probably mixed and diluted with other debris (S4), or currents and waves removed most spherules and any other evidence of an impact event at that horizon (S6 and S8). The most widespread and well exposed spherule layers are S1, S2, and S3, largely because of structural repetition and because the spherules fell into a wide range of preserved sedimentary environments, including many representing relatively quiet, often deep-water settings where there was little potential for post-depositional erosion (S2 and S3). In one structural belt, S1 is present for nearly 20 km continuously along strike (Fig. 4). S1 has also been identified in the Archean Warrawoona Group in the Pilbara Craton of Western Australia (Byerly et al., 2002). The S7 layer is widely marked by a layer of conglomerate and may coincide with a widespread unconformity. It is known to contain spherules only locally at the type locality (Fig. 4, Table 1). It has not been examined around the central part of the Onverwacht Anticline but is presumably present as a more-or-less continuous layer from the western to the eastern edge of the study area. In the field, spherule beds are layers of sandstone that range from a few centimeters to perhaps 5 m thick, but most are less than 1 m thick (Fig. 6). Under closer scrutiny, even with the naked eye but especially with a hand lens, the sand-sized particles can be seen to include varieties that are highly spherical, mostly from <0.01 cm to about 0.2 cm in diameter. These are spherules and, under the microscope, many show features indicating that they formed as liquid silicate droplets (Lowe and Byerly, 1986; Lowe et al., 1989). All of the spherule beds are also absent in some areas where their stratigraphic position is exposed, probably mostly through post-impact erosion. The subtle records of smaller impacts may take careful geochemical and isotopic studies to reveal. Much of the record has probably been lost due to erosion, dilution through mixing with other sediments, and post-depositional alteration.

3.5. Main components and physical features of the BGB impact layers Figs. 7–10 show stratigraphic sections of rocks associated with the impact event beds S1, S2, S6, and S8. Additional stratigraphic sections of S2, S3, S4 are available in Lowe et al. (2003), sections of S8 are provided by Lowe et al. (2014), and of S5 in Lowe and Byerly (2015). S7 is still too poorly known to provide a detailed measured section. Table 1 provides coordinates of the type localities. It is important to note that the actual spherule-bearing units are only one component in an association of bed types that constitute an impact layer or event bed: a bed formed as a direct consequence of a single large impact. The complex and compound event beds vary from about 1 to 5 m thick. In the discussion below, we summarize the main layer types that comprise the eight BGB impact event beds: most are shown in Figs. 7–10. 3.5.1. Spherule beds Spherules are key components of all BGB ejecta layers discovered to date and important clues, especially in the field, to identification of the impact origin of sedimentary layers in which they occur (Simonson, 2003). They are spherical to ovoid particles from about 0.01 to 4 mm in diameter. Most are 0.1–1 mm across (Figs. 11 and 12). Some were glassy particles (microtektites), others show crystalline rims and probable glassy interiors, and others show pseudomorphs of internal quench crystallites throughout (microkrystites). Spherules vary in internal structuring, the degree of crystallinity, surface texture, and internal impurities. Additional details about the spherules and photographs of the spherule types in individual spherule beds is given by Lowe et al. (2003, 2014) and Krull-Davatzes et al. (2006). There are two main types of spherule beds. (1) In some localities, spherule beds are composed of ∼10–35 cm of nearly pure spherules (Fig. 13(A)). We have interpreted these layers to have been deposited mainly by the fall of spherules out of the atmosphere with little or no reworking in surface environments by waves or currents. These beds are commonly graded from coarse at the base to fine at the top and usually contain only spherules and little other material that could represent ejecta particles. (2) The deposition of most impact layers was accompanied and followed by strong waves and/or currents, even in quiet, deep-water submarine settings. The second type of spherule bed contains spherules mixed with locally derived detritus ripped-up from the sea floor (Fig. 13(B)) and many show current-produced structures. We

3.4. Alteration of greenstone belt rocks and spherule beds Rocks in the BGB are over 3 billion years old. They have suffered through a number of periods of deformation and heating shortly after and during the long period following their deposition. As a consequence, all rocks of the Barberton belt have been extensively altered. All show the effects of thermal alteration at temperatures of at least 320 °C (Xie et al., 1997; Tice et al., 2004; Krull-Davatzes et al., 2012). However, in most parts of the central and southern greenstone belt away from faults and the marginal contacts with surrounding intrusive igneous rocks, greenstone-belt rock are largely unsheared. Original compositions have been severely altered but original structures and textures down to a few microns across are often well preserved. The far northern and northeastern parts of the BGB have been pervasively sheared and there most original small-scale structures and textures have been obliterated. The styles and effects of alteration in the BGB vary and depend on the original compositions and textures of the rocks, the settings of alteration, and the source(s) of the altering fluids. Thick ultramafic flows 45

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cross-stratification representing current-produced dunes developed on the sea floor (Fig. 7(A) and Lowe et al., 2003). The underlying layers are thinly and evenly banded chert showing no evidence for strong current activity or deposition under current-active conditions (Fig. 7(A)). Where the spherule beds contain ripped-up chunks of sedimentary and volcanic materials mixed with spherules and the immediately overlying and underlying beds were deposited under quiet, low-energy conditions, it unambiguously indicates that the high-energy events coincided with and were probably directly related to the impacts and fall of impact-produced debris. Krull-Davatzes et al. (2006) has shown that compositional grading in spherule bed S3 where fall-deposited reflects the complex evolution of the impact rock-vapor plume. In other cases, the spherules fell into environments characterized by frequent current or wave activity and not only do the spherule beds show evidence of mixing but underlying and overlying units show current- and/or wave-produced structures and textures as well. 3.5.2. Late stage fallout beds Some spherule beds are overlain by massive layers of mainly finegrained detritus, 50 cm to about 2 m thick, lacking spherules (Figs. 6, 7B, and 8). These layers commonly have a black or greenish in color, depending on the source of the detritus. Where the tsunamis ripped up soft carbonaceous sea-floor sediments, these capping detrital layers are commonly dark gray to black, graded, and composed largely of fine mixed detrital sediments and carbonaceous matter. Toward their bases, they may show evidence of weak current activity in the form of ripples, probably reflecting the waning stages of tsunami-related current and wave activity. Above, they are composed largely of massive to laminated, fine particulate detritus and carbonaceous matter cemented by very fine microquartz (chert). Where the spherule beds are interbedded with mafic or ultramafic volcanic rocks, the late stage fallout beds are commonly greenish because of the presence of Cr-bearing micas and they commonly contain detritus eroded from the underlying volcanic units (Fig. 13(B)), including Cr-bearing spinels. These late-stage fallout layers represent fine sediments stirred up by the tsunamis, which promoted ocean mixing and erosion of the sea floor. Some may also represent atmospheric dust that settled back to the surface following the impact. This fine suspended debris settled slowly back to the sea floor following the impacts, passage of the tsunamis, and coarse-sediment and spherule deposition. Spherule beds S2 and S3 are widely overlain by late stage fallout layers (Figs. 7(B) and 8) and fallout layers are also associated with S8 and S6 (Figs. 9 and 10). 3.5.3. Stromatolitic laminated crusts and layers containing laminated crust debris Lowe and Byerly (2015) concluded that some of the BGB impact events were sufficiently large to have deposited enough heat into the atmosphere to cause boiling of the surface layers of the ocean. Sleep et al. (1989) proposed this process for early boiling and sterilization of the oceans and the effects of smaller but still large impactors have been discussed by Segura et al. (2013). Boiling of the surface layers of the ocean would be accompanied by drops in sea level and precipitation of the dissolved components in sea water as evaporative crusts and deposits. This process would be particularly effective in shallow water, where evaporation of thin surface films in areas of wave activity and splash would force direct precipitation of crusts on surface sediments and rocks. Lowe and Byerly (2015) provided examples of such crusts deposited as a consequence of partial ocean evaporation following the S5 and S8 impacts. These deposits are preserved mainly as laminated silica crusts developed across erosional surfaces on underlying rocks, including ultramafic volcanic rocks, spherule beds, and older, non-impact-related sedimentary units (Fig. 14(A)). They contain abundant dispersed carbon, tourmaline, and fine clays. The carbon is interpreted to have been derived from hyperthermophylic microbial communities (Byerly et al., 1986) that encrusted the sea floor during boiling of the surface ocean layers, tourmaline as an insoluble

Fig. 7. Stratigraphic sections of impact event beds S1 and S2. (A) S1 is a current-structured bed deposited within a sequence of fine-grained, finely structured sediments showing few signs of current activity. The current activity recorded by S1 is thought to reflect passage of a tsunami generated by the impact and related sea-floor movements. (B) S2 at the locality shown in Fig. 6 is a prominent spherule layer containing clasts of ripped-up carbonaceous mud and smaller pieces of chert eroded from underlying sea-floor sediments. The current activity is thought to reflect the passage of an impact-generated tsunami. S2 is overlain by a layer of black chert representing a fallout layer composed of fine sediment that settled out of the water column after passage of the tsunami. Sedimentary rocks both below and above the impact layers are fine banded carbonaceous cherts deposited mainly under quiet-water conditions.

have interpreted these mixed beds, which are common to all spherule beds except S4, as the deposits of tsunamis generated by the impacts themselves or by crustal movements resulting from the impacts. In some cases, the tsunamis eroded materials from the sea floor and mixed it with fall deposited spherules that were accumulating simultaneously as the waves passed (Fig. 13(B)). The coincidence of the tsunamis with the impact events is especially evident where overall sedimentation before and after spherule-bed deposition occurred under quiet, deepwater conditions. Bed S1 in all localities is a unit of current deposited spherules mixed with ripped-up detritus and in some localities it shows 46

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Fig. 8. Stratigraphic section of the S2 event bed at the SAF-483 locality. It includes not only the spherule layer, which has been worked by waves and/or currents associated with the impact-generated tsunami and the spherules mixed with locally derived ripped-up material, but a capping tsunami-deposited sand layer with thin barite seams originating from fluids venting from nearby crustal fractures. The greenish fallout layer consists mostly of fine ash and dust and is capped by a thick barite layer.

precipitate out of the boiling sea water, and clays as fine sea-floor sediments stirred up by waves and currents and incorporated into the crusts during evaporation. While spherule beds S7 and S8 locally show in situ laminated silica crusts, much more widespread is debris eroded from such crusts (Fig. 14(B)). Successive silica laminations in such crusts were originally separated by thin microbial layers and/or layers of other, more soluble salts. Over short times, the microbial layers degraded and more soluble evaporates dissolved, allowing break-up of the crusts and dispersal of the debris. Evaporative crusts would have developed in the uppermost intertidal to shallow subtidal zones where evaporation would have driven direct precipitation of silica and other dissolved salts. This was also a zone where the deposits would have been subject to wave and current activity associated with breaking waves along the coast and tidal currents and to exposure and erosion as sea level fell. The erosion of these laminated crusts produced abundant small, curved silica chips that were transported into surrounding environments and became common components in the sedimentary system. Laterally from where spherule bed S8 is present, the impact layer is characterized by up to 2 m of laminated stromatolite-crust chips mixed with sparse spherules (Fig. 14(B)). Similar layers have been described by Djokic et al. (2017) from 3.48 Ga rocks in the Pilbara Block, Western Australia. These were

interpreted as silica deposits around thermal springs. When the examples from the BGB were deposited, the entire ocean was a thermal pool and the layers can be found throughout the outcropping extent of the layers. The most widespread in situ stromatolitic laminated crusts discovered in the BGB to date are associated with spherule bed S8 (Figs. 10 and 14), where they crop out for more than 10 km along strike. The S5 spherule bed is capped by a layer of stromatolite-crust chips up to 15 cm thick. Because the depositional setting for S5 where it has been studied is deep-water, these laminated crust chips may have been produced locally during the transitory drop in sea level or transported bu current from shallow-water sites of production. Fragments of laminated silica crusts provide an important clue in the search for other layers formed by large impacts. 3.5.4. Chert dikes Spherule bed S2 widely marks the base of the Fig Tree Group, directly overlying 20–60 m of Onverwacht sedimentary rocks composed largely of silicified biogenic sediments that in turn overlie thick sequences of ultramafic volcanic rocks. The upper 50–150 m of the Onverwacht Group are widely cut by narrow subvertical bodies of silicified carbonaceous and volcaniclastic sediments termed chert dikes 47

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Fig. 9. Stratigraphic column of beds associated with spherule bed S6. The event layers include S6 and an overlying fallout layer of tuff-like sediment. These occur within a section of well-layered banded chert. The Green Sandstone Bed (Byerly et al., 2017) is a current-deposited sandstone unit that is probably unrelated to spherule bed S6.

3.6. Mineralogical and geochemical features of impact beds

that formed coincident with the S2 impact (Fig. 15). Evidence presented by Lowe (2013) indicates that these chert dikes formed by the shattering of the upper 100+meters of near-surface sedimentary and volcanic rocks by the S2 impact and the downward flowage of loose, surficial sediments into the resulting open fractures. Loose spherules resulting from the impact were transported as much as 60 m into the fractures during the downward flow of soft sedimentary materials, but sediments from higher levels of the sedimentary sequence are absent in the dikes. While similar small chert dikes occur beneath many other sedimentary layers in the Onverwacht Group, this is the only regionally fractured stratigraphic interval identified to date. It speaks to the disruptive effects of large impacts on the planetary crust, even at considerable distances from the impact sites.

3.6.1. Shocked and high-pressure, high-temperature minerals As emphasized by Glass and Simonson (2013) and as reported widely from the Cretaceous–Tertiary (K–T) boundary layer, damaged mineral grains resulting from the intense shock of large impacts are widespread in many ejecta layers. The most common and widespread shocked mineral is quartz, which shows multiple sets of planar deformation features within individual quartz grains (Fig. 16). Shocked quartz is directly associated with many impact craters and ejecta layers but the high pressures required to deform the quartz do not occur in volcanoes. Shocked zircons have also been reported from impact deposits. Two other minerals, coesite and stishovite, are polymorphs of SiO2 formed at high temperatures and pressures associated with impacts and commonly associated with shocked quartz. Shocked minerals, such as quartz and zircon, and shock metamorphic features have not been observed in the BGB spherule beds. However, the BGB impacts appear to have involved impacts on Archean oceanic crust, probably composed largely of komatiite and basalt (Byerly and Lowe, 1994; Krull-Davatzes et al., 2014), which would have yielded no quartz or zircon. All ferromagnesian minerals in the spherule beds have been completely altered. No primary minerals remain that could be expected to show shock features.

3.5.5. Barite The fractures associated with spherule beds S2 and S3 served as conduits for the later movement of fluids through the upper part of the volcanic and sedimentary sequence. Ascending fluids spilled out onto the surrounding land surface and/or sea floor and locally deposited layers of barite (BaSO4). Barite occurs within the lower part of the Fig Tree Group in the S2 and S3 intervals throughout much of the central and northern BGB. We have not found barite or chert-dike complexes associated with the other known BGB impact layers and it seems likely that other processes of crustal fracturing, such as later tectonic events, could have produced barite layers known from other BGB stratigraphic units (Reimer, 1980).

3.6.2. Iridium and platinum group element (PGE) anomalies Unusual abundances of the platinum-group elements, especially iridium, have played a key role in the identification of Phanerozoic 48

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Fig. 10. Stratigraphic section of rocks associated with spherule bed S8. In this section, S8 is contained within a large slide block bounded below by a unit of breccia marking the base of the slide. The spherule bed was disrupted during sliding and mixed with soft black carbonaceous sediment that may have been a fallout layer and fragments of laminated silica crust. The block is capped by an intact layer of laminated silica crust deposited after sliding but during continued boiling and evaporation of the oceans.

within the condensing vapor clouds and subsequently in the wave- and current-active surface environment.

impact layers, including the K–T boundary layer (Alvarez et al., 1980), as well as the impact layers in the BGB. Lowe et al. (1989, 2003) reported higher-than-background Ir levels in the BGB spherule beds, including some beds that yield samples with Ir levels above those in chondritic meteorites. Kyte et al. (1992) demonstrated that similar elevated levels of other platinum-group elements are present as well. Byerly and Lowe (1994) studied Cr-rich spinels in the BGB spherule beds, especially S3, which is distinguished by high Ir contents in many sections. Kyte et al. (2003) reported Ir levels of background BGB rocks, away from the spherule beds, of <0.6 to about 2.6 ppb and associated spherule beds S3 and S2 with from 104 to 725 ppb and from 2 to 6 ppb, respectively. Byerly and Lowe (1994) identified spinels (chromites) in the spherule beds as an unusual Ni-rich variety like those in meteorites and concluded that the Ir and other platinum-group elements were carried in the spinels. Mohr-Westheide et al. (2015) likewise confirmed spinel as the main platinum group element carrier phase in spherule bed S3. The unusual abundances of the Ir in the BGB rocks does not reflect hydrothermal concentration of Ir but is rather attributable to hydraulic fractionation of the spinels and spinel-bearing spherules

3.6.3. Cr isotope anomalies Cr has been used to characterize the spherule beds, although it is also a ubiquitous component of the ultramafic volcanic rocks in the BGB. Because the Cr is also carried in spinel in the volcanic rocks and spinel is a mineral that is moderately resistant to weathering and alteration processes, it is widely released into surface environments as a detrital material. Some sands in the Fig Tree Group are virtually heavy mineral lags composed largely of spinel concentrated by hydraulic processes during transport and deposition. Like iridium, the absolute abundance of Cr is not directly related to impact processes but the Cr/Ir ratio in komatiites tends to be distinctly higher by a factor of about 10 in komatiites than in BGB impact layers (Fig. 17) and may help to distinguish impact from volcanic units. Cr isotopes have also proven to be very useful in distinguishing impact layers in the BGB from normal Cr-rich layers of volcanic origin. Shukulykov et al. (2000) and Kyte et al. (2003) analyzed Cr isotopes in 49

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Fig. 11. Thin section photomicrographs of spherule beds. (A) The lowest part of spherule bed S2 at locality SAF-206 (Lowe et al., 2003). Near its base, S2 is a bimodal mixture of large, clear spherules (1), now composed entirely of quartz, and small, darker spherules now composed of phyllosilicate minerals, such as micas (2). These may represent alumina-poor and more alumina-, iron-, and magnesium-rich liquid droplets, respectively. The quartz forming the clear spherules was introduced during alteration. A similar bimodal spherule assemblage is widely developed in many spherule beds. (B) A meter above (A), the spherules in S2 are quite different from those at the base. They include a population of small clear quartz spherules (1) and larger more compositionally diverse spherules containing phyllosilicates and opaque grains. Some are composed largely of Cr-rich mica (2), others show evidence of spherules merging (3) and edge-centered inward-growing quench crystallites (4). Others had vacuoules (5). (C) Spherule bed S1 in Western Australia. The bed includes small, clear spherules (1) as well as darker spherules showing edge-centered, inward-growing quench crystallites (2) and crystalline textures throughout (3). Two spherules show welding, yielding a dumbbell-shaped particle (4). In all cases shown, these beds are composed of particles that solidified from liquid droplets that condensed out of heterogeneous rock vapor clouds generated by large asteroid impacts. All photographs from rock thin sections taken in plane polarized light.

the BGB impact beds S1, S2, S3, and S4 (Fig. 18). All meteorite classes studied so far have different ratios of 53Cr to other Cr isotopes than terrestrial rocks. This reflects an early Mn/Cr fractionation and possibly heterogeneous distribution of the now-extinct parent radionuclide 53Mn (half-life, t1/2 = 3.7 m.y.) in the early Solar System. Carbonaceous chondrites also have an excess of 54Cr due to a pre-solar component (Shukolyukov and Lugmair, 2000, 2001). Thus, precise measurements of Cr isotope abundances can distinguish terrestrial from extraterrestrial materials and the carbonaceous chondrites from other meteorite groups (Fig. 18) and were critical in the general acceptance of the BGB spherule beds as impact layers. Their results indicate that the asteroids that formed the S2, S3, and S4 ejecta layers in the BGB were carbonaceous chondrites. S1 yielded too little Cr for isotopic analysis.

3.6.4. Other geochemical signatures of impact deposits A number of other geochemical features have been used to infer that some beds contain impact-related extra-terrestrial components. Cr-spinels in some BGB spherules contain unusually high levels of Ni, similar to those in meteorites, suggesting an impact origin (Byerly and Lowe, 1994; Krull-Davatzes et al., 2010; Mohr-Westheide et al., 2015). Platinum group element-rich nuggets and mineral phases have been used as evidence for extraterrestrial components in some layers (MohrWestheide et al., 2015). Re–Os isotope ratios and ratios of elements within the siderophile element group have been used to reinforce an impact origin for S3 in drill core (Schulz et al., 2017). No doubt additional criteria supporting an impact origin for these layers will be identified as they are better studied. 4. Discussion 4.1. Evidence for an impact origin of the BGB spherule beds As summarized by Lowe et al. (2003) a wide range of geological and geochemical evidence collectively indicates that the BGB spherules represent particles formed by large meteorite impacts on the early Earth (Glass and Simonson, 2013): (1) All impact layers identified in the BGB are characterized by spherules, although they are very sparsely distributed in S7. A variety of natural spherical to sub-spherical particles that occur in greenstone belts might be confused with spherules produced by impacts. Accretionary lapilli (Fig. 19) are spherical grains from <0.1 to about 2 cm in diameter and hundreds of layers of accretionary lapilli are present in the BGB volcanic sequence (Lowe, 1999; Stiegler et al., 2008). Accretionary lapilli can also form in the dust clouds produced by impacts. However, the BGB accretionary lapilli lack evidence for an impact origin and their textural and compositional properties indicate that they have been constructed within volcanic clouds and eruption columns from volcanic ash and dust grains. Fountains of mafic lava produce abundant liquid silicate droplets that chill and are spread as glassy particles around the vents. However, lava fountain droplets are seldom dispersed more than a few hundred meters from the vent and have a uniform composition like that of the fountaining lava. The complex original compositions of the spherules in the individual BGB impact beds, their common wide extent, and the common lack of evidence of closely associated mafic volcanism all suggest that the spherules have not formed by lava fountaining. Carbonate ooliths are sand-sized spherical particles of calcium 50

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Fig. 12. Thin section photomicrographs of spherule types. (A) Spherule bed S5 is composed of two contrasting spherule types, large, now nearly pure silica spherules (1) and small, mica-rich spherules (2), many of which are crushed in this thin section. The mica-rich material commonly coats the surfaces of the larger spherules. This bimodal association of spherules is present in many of the spherule beds. The clear spherules, now composed largely of quartz due to later replacement, probably consisted originally of a non-aluminous glass. (B) Spherule bed S6 also includes larger, more siliceous spherules (1) and smaller micaceous spherules (2) but also shows a variety of more mixed types, such as the layered spherule (3) and the dark spherule that contains a chunk of more quartzose material (4). (C) Spherules from the middle of the S2 spherule bed, locality SAF-206. Spherules from this bed can also been seen in Fig. 11(A) and (B). Some of the darker, mica and oxide-rich spherules show internal cavities and vacuoles and others show well-developed crystalline quench textures (1). (D) Clear, now-siliceous spherules with very pitted and rugose surfaces, perhaps formed by abrasion or corrosion during flight. All photographs taken in plane polarized light.

deposition widely eroded and reworked the spherules and stripped off the soft, rapidly deposited impact layers in many areas. (3) With the exception of S5, which is interbedded with sandstones composed of silicic volcaniclastic debris, these units were not accompanied by local volcanism, are not localized around possible volcanic vents, and do not display regular proximal-to-distal facies, which might be expected if they originated as ballistic volcanic particles such as lava-fountain droplets. (4) The spherule beds are unique beds in the sections where they occur. Similar particles are absent outside of the beds except in rare instances where spherules or pieces of spherule beds have been eroded and deposited as part of younger sedimentary units. In several areas, where spherules fell into environments characterized by soft, muddy sediments, they foundered into the muds, the spherule layers stretched and deformed plastically, and/or have been injected downward or upward through post-depositional soft-sediment mobilization (Lowe et al., 1989, Fig. 3). (5) Edge-centered inward radiating and lath-shaped and prismatic crystal pseudomorphs, dendritic spinels, pseudomorphs after barred olivine, and welded particles indicate that the spherules formed as liquid silicate droplets.

carbonate that have formed in many modern and ancient currentactive shallow-water settings where carbonate sediments are deposited. However, no significant sedimentary carbonate units are known in the BGB or in any sedimentary sequence until about 3.1 Ga. The most likely origin for these spherules is as melt ejecta or as condensates from large, impact-produced rock vapor clouds. (2) The event beds for S1, S2, S3, S7, and S8, where not removed by erosion or by post-depositional shearing, are of regional extent. The spherules were deposited as blankets that mantled surfaces in every depositional environment into which they fell. In quietwater settings, they accumulated as graded fall deposits with little or no subsequent current working. In shallower water, spherules were widely reworked by tsunamis or by ambient waves and/or currents. Over wide areas they were removed by erosion and in some areas they were greatly thickened by reworking. These were regional and possibly global layers, not the products of local processes or events. S4, S5, and S6 have been identified only in local outcrops. The corresponding horizons in other sections are either poorly exposed or, where exposed, the spherule beds and associated impact-related units are not present, in most cases we feel due to erosion. The tsunamis that followed spherule 51

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Fig. 13. Spherule beds. (A) Spherule bed S3 where composed of nearly pure spherules. (B) Spherule bed S2 where spherules are mixed with sand- to gravel-sized chunks of sediment ripped up from the sea floor. The bulk of the visible grains are gray to clear pieces of ripped-up siliceous sea-floor sediments (1) that were still soft when eroded.

preclude an origin by simple erosion of volcanic or any other rocks, as suggested by some (de Wit, 1986; Buick, 1987). (8) The presence of compositionally diverse spherules in most beds (Figs. 11 and 12) indicates that they did not form from single homogeneous magmas, like spherules formed as lava fountain droplets. (9) Beds S3 and S4 are sites of major Ir anomalies; S1, S2, S6, and S8 host smaller but still significant Ir anomalies (Lowe et al., 1989; Kyte et al., 1992; Koeberl et al., 1993; Reimold et al., 2000). Ir has not been measured in S7 and no anomaly had been seen in S5. (10) The more immobile platinum group elements, Pt, Os, and Ir,

(6) The spherules are unlike any volcanic particles of ultramafic, basaltic, or felsic composition, varieties of which occur throughout the BGB. Most spherules exhibit internal textures, structures, and alteration compositions unlike those in ocelli, variolites, accretionary lapilli, and other volcanic structures and particles within the BGB. (7) The spherules were formed as smooth spheres and oval-shaped grains, unlike particles produced by weathering and erosion, and locally comprise nearly 100% of the particles in spherule beds where not reworked and mixed with rip-up detritus. Their structuring, extreme sphericity where undeformed, and abundance 52

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Fig. 14. Laminated silica crust. (A) Laminated silica crust formed by ocean evaporation that shows inclined stromatolite-like domes and columns. These crusts are thought to form by ocean evaporation following large impacts that raise surface temperatures to above boiling. They probably hosted microbial mats during growth. (B) Laminated silica crust chips formed by the break-up of silica crust, probably in wave- or current-active zones that were exposed as sea level dropped due to ocean evaporation. Both photos are from crusts associated with spherule bed S8.

Fig. 15. Chert dikes formed by crustal fracturing associated with the large S2 impact. (A) These chert dikes are jagged bodies composed of dark gray to black carbonaceous sediments that were injected downward from the sea floor into open fractures generated by the impact. The originally soft sediments have been later cemented by silica to form the hard dikes. The chert dikes here include some containing abundant angular blocks quarried off the dike walls (1) and dikes composed of uniform, fine-grained black chert (2). Bedding is horizontal. (B) A dike formed as an open fracture that was later filled by the precipitation of quartz and other forms of silica out of fluids that filled the cavities. Bedding is horizontal. The geologic pick is 40 cm long.

expected in impact-related materials (Byerly and Lowe, 1994; Krull-Davatzes et al., 2010). (12) The Cr-isotopic signatures in samples from beds S2, S3, and S4 (Fig. 17) indicate the presence of extraterrestrial Cr components and suggest carbonaceous chondrite projectiles (Shukolyukov

display roughly chondritic ratios in S3 and S4, consistent with derivation from chondritic impactors (Kyte et al., 1992; Schulz et al., 2017). (11) Spinels in S3 display compositions unlike those of spinels in komatiites within the BGB and consistent with compositions 53

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Fig. 16. A grain of shocked quartz showing multiple planar deformation features formed by the extreme pressures and shearing associated with impact events. The grain is 0.13 mm across. Eocene Chesapeake Bay impact crater. Public domain photomicrograph by Glen A. Izett, U.S. Geological Survey.

there irregular particles similar in composition and size to the spherules that may represent ballistic debris. It seems likely that all of the BGB spherule layers are very distal ejecta deposits formed by the condensation of rock vapor clouds ejected above the atmosphere following large impacts at very distant impact sites (Lowe et al., 1989; Kyte et al., 1992; Byerly and Lowe, 1994; Lowe et al., 2003). Arguments of HYPERLINK \l "bib49" Koeberl et al. (1993), Koeberl and Reimold (1995), Reimold et al. (2000), and others against an impact origin of the BGB spherule beds have included (1) the high PGE and siderophile element abundances in the spherule beds, in some cases exceeding chondritic levels, (2) the non-chondritic ratios of platinum group elements and lack of correlation of siderophile elements (e.g., Ni and Ir) (3) the correlation between siderophile element abundances and the contents of chalcophile elements, such as S, As, Sb, and Se; (4) the presence of local PGE enrichment in country rocks surrounding the spherule beds; (5) the compositions of Ni-rich spinels in BGB spherule samples, which they suggest are different from those found in known impact ejecta and meteorites; and (6) the absence of

et al., 2000; Kyte et al., 2003). (13) Ultramafic volcanic rocks in the BGB have a Cr to Ir ratio that is very similar to that of the Earth's primitive mantle but distinctly different from that in the spherule beds (Fig. 17) (Lowe et al., 1989). The extreme Cr and Ir levels in S3 and S4 have a Cr to Ir ratio that are similar to that in carbonaceous chondrites. At lower Cr and Ir values, the trends for S3 and S4 are more dispersed, in part reflecting analytical uncertainties at sub-ppb levels but also probably reflecting dilution of one or both elements by non-impact sources. Even at the highly dispersed Cr/Ir seen at Ir levels below 10 ppb, the discrimination between komatiites and spherule beds is complete, with both S1 and S2 beds showing compositions consistent with an impact origin. The BGB spherule beds lack splash particles, such as stretched, teardrop-shaped, and flattened button-shaped grains, which are common to tektites and ballistic liquid droplets. No large exotic ballistic clasts have been identified in the BGB spherule beds, and only in S1 are

Fig. 17. Comparison of ultramafic volcanic rocks (komatiites) and impact layers in the BGB in terms of Cr vs Ir. Komatiites contain high Cr levels but only moderate to low Ir abundances. Impact layers tend to contain less Cr and higher Ir. The main carriers of both Cr and Ir are spinels. These results provide one means of distinguishing impact layers from komatiites or layers composed of debris eroded from komatiites.

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Fig. 19. Accretionary lapilli form by the sticking together of either fine volcanic ash and dust grains within an atmospheric eruption cloud or in an impactgenerated cloud of debris. These accretionary lapilli are of volcanic origin. They show concentric structuring and many have central nuclei composed of siltsized grains. Here, the largest lapilli, which are about 1 cm across, are composed entirely of very fine ash and dust. Under the microscope, the particles making up these lapilli can be seen to be of volcanic origin.

Wit et al., 1982; Duchac and Hanor, 1987; Hanor and Duchac, 1990; and many others). Rocks along the northern, frontal part of the BGB have been widely subject to sulfide and Au mineralization, which could be expected to change ratios of more mobile (e.g., Ni, Au, Fe, S, Cu) to immobile (e.g., Ir) elements, and penetrative shearing and faulting, which might be expected to blur the spatial distribution of even immobile components, such as Ir. Observations that samples collected from such areas no longer preserve primary element ratios and distributions, especially at a small scale, should not be surprising. The unusually high, often super-chondritic and spatially heterogeneous Ir abundances in the BGB spherule beds have been problematic to some in terms of an impact origin. Shukolyukov et al. (2000), however, noted that Ir abundances as high and higher as those observed in the BGB spherule beds have been noted in the K–T boundary layer and meteorites. Lowe et al. (2003) and Kyte et al. (2003) suggested that both low and high Ir abundances could reflect a number of vapor plume and post-fall sedimentary processes, including dilution by low-Ir detritus and hydraulic concentration of the spinel carrier minerals in waveand current-active surface environments. More recent papers (e.g., Mohr-Westheide et al., 2015; Schulz et al., 2017) have re-emphasized these possibilities.

Fig. 18. Cr isotopic analysis of the BGB spherule beds and other relevant rocks. The x-axis is the normalized e53Cr value. e53Cr is derived from the 53Cr/52Cr ratio and normalized to an extra-solar component using 54Cr. By definition Earth is set as e = 0. From top down: (1) samples from BGB impact layers S2, S3, and S4 (open circles) systematically and with statistical significance plot to values of e = −0.3 to −0.4, while sedimentary (upper 3 black symbols) and igneous rocks from the BGB plot at the terrestrial value of e = zero; (2) Carbonaceous chondrites (light gray diamonds) also plot at values of e = −0.3 to −0.4; 3) A variety of terrestrial and lunar rocks and standards (lower black circles) plot e = 0; and 4) Ordinary chondrites and martian meteorites (open diamonds) plot at values of e = + 0.2. This plot implies that the BGB spherule deposits must reflect impacts of large carbonaceous chondrite bodies with the Earth and that most of the Cr in the impact beds is derived from the bolides. From Shukolyukov et al. (2000) and Kyte et al. (2003).

shocked minerals and metamorphism, which occur "even in 2-Ga-old impact structures, such as Vredefort and Sudbury". The principal problem with the first four these observations is that they derived mainly from samples collected in and around gold mines and mineralized zones in the northern part of the BGB and failed to evaluate long-known processes of alteration and metasomatism that have affected rocks throughout the BGB (Smith and Erlank, 1982; de

4.2. Sizes of the BGB impactors Estimating the size of the asteroids that produced the BGB impact 55

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section, the impact ratio between the Earth and the Moon is about 20:1. That is, for every asteroid of a given size that strikes the Moon, roughly 20 similarly-sized impactors should strike the Earth. For the 8 large impactors recorded in the Barberton Belt, if they were the only large impactors to strike the Earth during the 3.72–3.225 Ga interval, there would be less than 50% probability that a similar sized asteroid would have struck the Moon. This is consistent with the absence of very large lunar craters younger than about 3.7 or 3.6 Ga. However, it seems likely that these large impacts would have been part of a broad spectrum of continuing impacts that should have included many smaller, but still large objects. On the Moon, the four largest craters to form after Orientale, which has an age of perhaps 3.8–3.9 Ga, are Iridum, Humboldt, Tsiolkovsky, and Hausen, which have diameters of about 250, 207, 180 km, and 167 km, respectively, quite small compared to the sizes of the craters implied by the estimated sizes of the Early Archean terrestrial impactors. These are not much larger than craters such as Vredefort and most of these probably formed on the early side of the range between 3.2 and 3.7 Ga. Hence, evidence for massive impacts on the Moon at times that roughly overlap those of the inferred large terrestrial impacts and spherule bed-formation is sparse. It could be that the 8 BGB impact layers discovered to date represent a substantial proportion of the very large terrestrial impacts that occurred over this interval and that no comparably large impacts occurred on the Moon after the formation of Orientale. The imprecision inherent in the current estimates of the sizes of the BGB impactors, however, would also allow that only some of the Archean impactors recorded in the geologic record were indeed big, and these likely made the thickest Archean spherule beds. Others could have been more consistent with the sizes of bodies that made the largest post-Orientale craters on the Moon. Thus, our interpretation of the Archean impact spherule beds is statistically consistent with what we see on the Moon in terms of postOrientale craters, but most terrestrial impacts of the Archean era likely made Vredefort- rather than Orientale- or Imbrium-sized craters, which have yet to be clearly identified or distinguished in the BGB.

layers is problematic in the absence of craters and more proximal ejecta deposits. Making such estimates depends on there being direct or indirect relationships between asteroid size and properties of the distal ejecta deposits. Most obviously, the total amount of ejecta produced could be related to the impactor size. This approach has been taken by a number of authors, although the very non-uniform distribution of distal ejecta layers, such as lunar ejecta rays, raises questions about estimating global characteristics of the layers from their properties in individual outcrops. Sleep et al. (1989) suggested that the gross thickness of S3, approximately 35 cm of pure spherules locally, was consistent with an energy release for a bolide 30 km diameter, assuming the 35 cm was representative of the worldwide layer thickness. Kyte et al. (1992) calculated an Ir abundance for S4 consistent with a bolide greater than 20 km in diameter, and Byerly and Lowe (1994) use the Ir abundances in S2 and S3 to suggest bolides of 30 km in diameter for these impacts. Shukolyukov et al. (2000) found the extraterrestrial chromium in S4 required a chondritic source about one third greater than that previously estimated from Ir. Lowe et al. (1989) used the maximum diameter of spherules condensed from impact rock vapor clouds as suggested by O'Keefe and Ahrens (1982), approximately 3 mm in S3, to infer a bolide size in the range of 20–50 km. Melosh and Vickery (1991) use several spherulesize models to suggest that these Archean impact bolides could have had diameters in excess of 100 km. Byerly and Lowe (1994) also did a detailed size analysis for S3 and conclude that using the average size for the smallest population of spherules, those with Ni-spinel, which have an average diameter of 0.85 mm, yields a bolide size of 24 km. More recently, Johnson and Melosh (2012a) have modeled the general process of spherule formation in rock vapor plumes and related spherule size to impactor size, impactor velocity, and where in the vapor plume spherule formation takes place. Johnson and Melosh (2012b) used a combination of the thickness of the spherule layers and spherule sizes to estimate sizes for the S1, S2, S3, and S4 impactors of 29–53, 37–58, 41–70, and 33–53 km, respectively. Spherule layer S5 is about 22–25 cm thick but the spherules are mixed with volcanic ash and show evidence of working by currents. S6 appears to be a fall deposited layer, 15–20 cm thick, composed of pure spherules grading upward into ashy fallout material. S7 is too poorly known to estimate bed thickness. S8, known mainly from a single locality (Fig. 10) where it occurs in a slide block (Lowe and Byerly, 2015). It is disturbed but probably included at least 10–15 cm of pure spherules. The spherules in these beds show a size range similar to those of S1–S4. These measurements suggest that a similar size range of impactors was involved in the formation of all of the BGB impact layers. It must be emphasized that the lateral discontinuity of the spherule beds due to post-depositional erosion and the wide reworking by currents during and after deposition render these only general estimates of impactor size. However, the consistency of the results and the thicknesses of the layers involved, which are all thicker than virtually any known terrestrial impact layers formed during the last 2.5 billion years, collectively suggest that these were very large bodies, mainly or entirely asteroids, that were bombarding the Earth between 3.472 and 3.225 Ga. In summary, a variety of parameters has been used to estimate the size of the BGB Archean impactors, including bed thickness, spherule size, Ir content, and extraterrestrial Cr abundance. All yield comparable estimates of 20–50 km diameter for the bolides. Glikson (2001) scales these diameters by a factor of 20 to suggest that the craters produced were in the range of 400–1000 km in diameter. This range compares well with the sizes of lunar impact basins, which formed 300–500 million years earlier. The smaller lunar maria, Bailly, Schrodinger, and Medeleev are all about 300 km in diameter, while the largest basins, Orientale, Imbrium and Crisium, are about 1000 in diameter. The formation of several craters on Earth, each 400–1000 km diameter, would have had profound effects on both geological and biological processes. One final note on impactor sizes: because of the Earth's larger cross-

4.3. Tectonic effects of large impacts The earliest history of the Earth involved accretion through impacts, including late disruption of the young Earth by a large impact to form the Moon at about 4.5 Ga. A key question is whether the early Earth that we see in the terrestrial geologic record starting at about 3.8 Ga but especially following the inferred initiation of BGB greenstone belt activity at about 3.65 Ga (Drabon et al., 2017) was still being influenced by large impacts or whether crustal development and surface geodynamics had shifted to internal controls and impacts played only a minor role in overall crustal development. A key question remaining to be answered is when did this fundamental shift in the control of surface geodynamics and evolution of the Earth's crust and surface system take place? Many authors have suggested that impacts can have severe effects on the crust hundreds or thousands of kilometers from the impact sites. More uncertain, however, are the possible effects of large impacts on the overall terrestrial geodynamic system. From reconfiguring plates and plate boundaries to inducing major magmatic episodes (e.g., Grieve et al., 2006; Glikson 2001, 2008), impacts have been suggested as having potentially profound influences on the geodynamic system and surface environments. However, it is still not clear what the prevailing geodynamic system was in the Early Archean, before about 3.0 Ga. While many suggest a broad plate-tectonic geodynamic regime throughout Earth history (e.g., Turner et al., 2014; Krull-Davatzes et al., 2014), there is an increasingly widely held view that some other tectonic regime may have prevailed before about 3.0 Ga (e.g., O'Neill and Debaille, 2014). Lowe et al. (1989, 2003) noted that two of the BGB impact layers, S2 and S3, regionally mark the contact between the Onverwacht and Fig Tree Groups. This boundary represents the transition from a period 56

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crust as late as the end of the Archean at 2.5 Ga.

of perhaps 300 myr or longer of predominantly quiescent, mafic and ultramafic volcanism, during which there was little or no tectonic uplift and deformation, to an interval of active tectonism involving crustal shortening, uplift, mountain building, and deep erosion into the greenstone belt rocks and deeper-level intrusive roots. Deformation ultimately culminated in the intrusion at about 3.1 to 3.0 Ga of large volumes of granitic magma, stabilization, and formation of the Kaapvaal Craton, one of the earliest known continental blocks. In the central, southwestern, and southern parts of the BGB, this contact is marked by spherule bed S2, approximately 3.260 Ga. This impact was widely associated with extensive fracturing of underlying Onverwacht rocks (Lowe, 2013; Sleep and Lowe, 2014). In the northern part of the belt, which appears to represent a structurally separate and younger block, the transition is marked by S3 formed at ∼3.240 Ga. The diachronous character of this contact suggests that in different areas, the transition from a volcanic anorogenic regime to an interval of deformation and uplift occurred at different times, although probably all within a 20–25-million year interval following 3260 Ma. In both cases, however, this transition was directly associated with a large asteroid impact. These coincidences suggest that a series of large impacts or an impact cluster may have been responsible for disrupting the previously existing crust and geodynamic regime, perhaps characterized by a relatively stable, thick, mafic crust (e.g. Drabon et al., 2017), and the eventual global transition into a regime that may have been much more like a modern plate tectonic system. Glikson (2001, 2008) concluded that an impact cluster at this time played a major role in terrestrial crustal evolution and triggered major oceanic volcanic activity that resulted in a large-scale flux of iron into the oceans and the subsequent deposition of major banded iron formation. The specific details of this transition and the precise nature of geodynamic systems both before and after this transition remain unclear. It is now widely recognized that large-asteroid impacts related to the Late Heavy Bombardment from about 4.0 to 3.8 could have severely disrupted and fragmented any pre-existing crust (Marchi et al., 2014) and perhaps postponed the development of a stable, long-lived continental crust until the impact flux waned. Fragments of an old “primordial” crust may be reflected in the Hadean zircons 4.0–4.4 Ga from the Jack Hills, Western Australia (e.g., Kemp et al., 2010). A number of investigators have suggested that an early crust was disrupted by impacts during the LHB to be followed by a period of voluminous magmatism to form volcanic provinces resembling lunar maria (Marchi et al., 2014). Hansen (2015) suggested that large impacts might have disrupted an early, thin lithosphere, forming a widespread igneous province that evolved to form greenstone belts with a melt residue that developed a complementary lithospheric mantle. Both Green (1972) and Frey (1977, 1980) had previously suggested that large impacts produced maria-like basins on the early Earth that may have evolved into modern ocean basins. These may what we are seeing in the pre3.0 Ga greenstone belts like the Barberton belt. Evidence from these early greenstone belts suggests that this period of greenstone activity, which probably ranged from >3.650 to about 3.200 Ga, was followed by a period of crustal instability and disruption triggered again by large asteroid impacts, the 3.260–3.220 Ga impact cluster. After perhaps 100 to 200 million years of instability, the first more modern, thick, buoyant, high-standing, stable cratons were formed, represented today by the Kaapvaal and Pilbara cratons in South Africa and Western Australia, respectively. A Late Archean, post-3.0 Ga generation of greenstone belts (Fig. 3) is represented by volcanic-sedimentary sequences embedded within the basement on every continent. This period of greenstone evolution ended at about 2.6 Ga (Lowe, 1992) and was followed by yet another major episode of crustal disruption and the formation of large areas of new continental crust 2.6–2.5 Ga. It may be more than just coincidence that this episode of Late Archean greenstone-belt disruption and continental crust formation coincided fairly closely with the cluster of large impacts at 2.63–2.49 Ga. Perhaps large impacts were playing a key role in the evolution of the continental

4.4. Biological effects of large Archean impacts Large impacts have the potential to influence biological communities primarily through their effects on the surface environment, including the temperature and composition of the ocean and atmosphere (Toon et al., 1997). Large impacts can result in boiling of the oceans or ocean surface layers by superheating the atmosphere. Sleep et al. (1989) suggest that in the distant Hadean, impacts of extremely large asteroids, >∼440 km in diameter, would have delivered enough heat to the Earth to have boiled and evaporated the oceans and sterilized the planet's surface. Any products of preceding intervals of biological evolution would have been obliterated, the clock reset, and the subsequent history of life and its evolution would have started anew. Smaller impacts, however, could still have had severe effects on extant life. A depth of water corresponding to the photic zone would have been evaporated by a 190-km diameter object. Such large impacts would have superheated the world's atmosphere and generated near-surface ocean temperatures at or above boiling for hundreds to thousands of years (Sleep et al., 1989). More recent calculations (Segura et al., 2013) have shown that thinner layers of ocean water would have been evaporated by smaller impacts, and that even bolides 50–100 km across could have evaporated tens of meters of ocean water. Following impacts like those recorded in the BGB, the upper layer of the ocean would have effectively been a global thermal pool. Such impacts would have obliterated all but the most extreme hyperthermophilic organisms in surface and near-surface waters. Smaller impacts can generate and disperse large amounts of dust in the atmosphere. This debris could spread globally, block sunlight, and cause cooling of the Earth's surface. Such effects have been cited as the primary or contributing cause of the global mass extinctions, including that of the dinosaurs and large marine cephalopods called ammonites at the end of the Cretaceous about 65 million years ago (Alvarez et al., 1980). We take the presence of abundant organic carbon throughout all known Archean and Hadean sedimentary sequences to be an indication that life was present and influencing the surface world as far back as there is a geologic record. However, this life would have been profoundly influenced by the large, Early Archean impacts discussed here. Any global surface temperature changes consequent on these multiple large impacts would have had a profound filtering effect on early life. After the last sterilizing impact, the progenitors of all later life would have been the thermophiles that would have survived the last largest impact and their descendants. 4.5. Implications of the BGB spherule beds for the nature of the Late Heavy Bombardment A number of recent papers have focused on evaluating Archean and late Hadean impact rate(s) and impact-rate continuity based in part on the spherule layers described here (Bottke et al., 2012; Johnson and Melosh, 2012b; Lowe et al., 2014; Bottke and Norman, 2017; and others). Lowe et al. (2003) summarized the difficulties in interpreting the terrestrial Archean impact record, then limited to only four known Early Archean BGB spherule layers and one from Western Australia and a number of Late Archean impact layers, in terms of the Archean terrestrial impact flux. This problem has been revisited by Glikson (2008) and was reviewed by Johnson and Melosh (2012b). A more recent view of the Late Heavy Bombardment is presented by Bottke and Norman (2017). While the authors have explored much of the BGB over the past 40 years, we have visited only a small proportion of the actual outcrops. With half of the spherule beds discovered to date represented by only individual outcrops, the potential for at least an equal number of as yet undiscovered major spherule beds remains high. Issues of selective and 57

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50 km bolides, this would represent a rate about 320 times the current rate. The numbers of beds representing large-body impacts 3.26–3.22 Ga and 2.65–2.5 Ga suggest that actual impact rates during these relatively short intervals were considerably higher than these estimates. It is possible that impact clusters during the Archean were concentrated in a few narrow windows, one ∼3.26–3.22 Ga and another at 2.65–2.5 Ga, both of which just happened to coincide with the deposition of a series of major sedimentary units that would favor preservation of fall-deposited impact-generated materials. The S2, S3, S4, and S5 group of impact beds formed from ∼3.260 to ∼3.225 Ga reflects an impact recurrence interval of about 1 per 8.75 million years. These results suggest that whether there were impact spikes or not during the Early Archean, impact rates were substantially above present levels from at least 3.260 Ma to 3.225 Ga (Fig. 20). The impact cluster at 2.65–2.5 Ga suggests that either high impact rates or impact-rate spikes persisted to the close of the Archean. It should be noted, however, that these two impact clusters occur in thick, well-preserved rock sequences in which environments that would favor their preservation were widely developed. It could be argued (Lowe et al., 2003) that the intervening interval from about 3.225 to 2.65 Ga is less well represented in the geologic record and then mainly by depositional settings in which spherule beds and impact layers might be overwhelmed by influxes of detrital materials or destroyed with current and wave active shallow marine environments. If such were the case, the apparent clusters might actually be part of a more continuous impact history that persisted to or beyond the close of the Archean at 2.5 Ga. The older part of the BGB and the Pilbara Craton include several spherule beds and Glikson et al. (2004) have suggested another impact cluster 3.48–3.34 Ga. This cluster is less well constrained but may exist. There is evidence for at least two impacts in the BGB between about 3.340 and 3.308 Ga (S8 and S6, respectively), two impacts and perhaps more between about 3.472 (S1) and 3.416 (S7) Ga. The 3.472 Ga age for S1 is based on detrital zircons and similar ages are encountered for felsic igneous units 2000 m lower in the section so the actual age of S1 may be considerably younger than this, with a minimum age of about

incomplete preservation, the probability that only a small proportion of the time represented by the BGB is actually represented by rocks, and the as-yet undiscovered record of what must have been a large number of smaller impacts complicate interpretations of the preserved terrestrial Archean record of the late LHB at this time. In addition to the cluster of at least four impact layers in the Fig Tree Group between 3.260 and 3.225 Ga, Hassler and Simonson (2001) and Hassler et al. (2011) have identified layers representing at least four large impacts 2.64–2.5 Ga in Western Australia and South Africa. These Late Archean impact layers occur within sedimentary units within major basin-fill sequences, including iron formation and similar sedimentary rocks deposited under mainly quiet and probably deep-water conditions, not unlike those that characterized deposition of parts of the Fig Tree Group. This depositional setting, where sedimentation is likely to be slow but more-or-less continuous and where major erosion events are rare, offers the ideal opportunity to preserve infalling debris from large and small impacts. Moreover, such deposits are very non-uniformly distributed over geologic time. There are a number of such units in Early Archean greenstone belts deposited between 3.6 and 3.2 Ga, a few in the interval 3.0–2.7 Ga, and a large number in Late Archean greenstone belts 2.65–2.5 Ga. It may be that the present distribution of known impact-generated beds within Archean rocks reflects, at least in part, the temporal distribution of depositional conditions favorable to their accumulation and preservation rather than an actual variability in the impact rate. Ryder (2003) suggested that the terrestrial impact rate declined from ∼100 times to about twice the present-day rate between ∼3.8 and 3.0 Ga, averaging ∼15 times current levels over this interval. The lunar cratering rate of Neukum et al. (2001) similarly shows an impact rate about 15 times current levels at 3.5 Ga, about 3 times current levels at 3.2 Ga, and the same as current levels after about 2.8 Ga. The analysis of Chapman (2004) suggests that 20 km bolides have an average present-day impact frequency of about 1 impact every 1 billion years and that 50 km objects have a frequency of about 1 every 10 billion years (Fig. 20). The 8 BGB spherule beds between 3.472 and 3.225 Ga represent, at a minimum, one >20 km impact every 31 million years. For 20 km bolides, this would be about 32 times present rates and for

Fig. 20. Relationship between impactor diameter and interval between impacts based on current impact rates. The inferred diameters of the asteroids represented by the BGB impact layers, 20–50 km, are shown in gray. Modified from Harris (2008) and Chapman (2004). 58

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Fig. 21. Models for the early impact history of the Earth. (a) Black curve. The continuous exponential decline model of Neukum et al. (2001) lacking a LHB. (b) Yellow curve. An impact rate evolution curve like that proposed by Valley et al. (2002), which shows a spike for the Late Heavy Bombardment followed by a gradual decline to present low rates at about 3.5–3.0 Ga. (c) Blue. A model in which an exponential decline like that of Valley et al. (2002) is punctuated by impact spikes at LHB, 3.5–3.2 Ga, and 2.65–2.40 Ga. (d) Red. A continuous exponential decline model but with elevated rates extending to at least the close of the Archean at 2.5 Ga. The terrestrial impact record favors model d or a curve between c and d.

the Eastern Pilbara Block, Western Australia, provide records of the terrestrial processes, events, sedimentary systems, crustal development, life, and the nature of the ancient ocean and atmosphere between about 3.55 and 3.22 billion years ago. Perhaps one of our abiding conclusions is that the early Earth was a very non-uniformatiarianistic world. While water ran downhill even in the Early Archean, the composition of the crust, ocean, and atmosphere were very different than they are today. Life was limited to prokaryotic microbes and a host of chemical sediments that are virtually unknown today were deposited out of the early oceans. The early surface temperature may have been as high as 70–85 °C (Knauth and Lowe, 2003) or so low that glaciers covered much of the landscape. In our studies over the past 45 years we have attempted to develop a better understanding of the conditions under which these rocks formed and their implications for the evolution of the early crust. One of the major discoveries has been that the Archean Earth even then was still subject to frequent, very large asteroid impacts. Our studies indicate that between about 3.47 and 3.22 billion years ago, the Earth was struck by at least 8 and probably many more large asteroids between 20 and 50 km in diameter and some perhaps much larger. These objects were part of a continuing rain of extraterrestrial objects striking the Earth that commenced at the start of the Late Heavy Bombardment sometime before 4.0 Ga, gradually declined over the next billion and a half years and ended sometime after 2.5 Ga. None of these late-bombardment objects struck in the area where rocks of the Barberton Greenstone Belt were being deposited: if one had, existing BGB rocks would not be preserved today. It is likely that these impacts were a contributing factor in the scarcity of rocks older than 3.0 Ga today. But a record of large impacts at distant locations on the Earth is preserved in the 8 known spherule layers in the BGB and similar beds in the Pilbara Craton. We suspect that many smaller impacts either left no

3.445 Ga, the age of felsic volcanic rocks in H6 a few hundred meters higher in the section. These ages suggest that large impacts were recurring events between about 3.472 and 3.300 Ga. Older rocks are either volcanic units or have been metamorphosed and are less likely to contain preserved spherule beds. Overall, while impact clusters may exist, there is evidence for a persistent and perhaps rather continuous bombardment of the Earth by large asteroids from about 3.472 to 3.225 Ga at rates substantially above those prevailing in younger geologic time. This conclusion is consistent with conclusions reached by Bottke and Norman (2017) based largely on lunar and martian impact records. These observations require that the history of the Archean on Earth must be reinterpreted in terms of the role that frequent large impacts and/or impact clusters may have had on crustal, biological, and environmental evolution. Long term crustal development in the interval before 3.0 Ga may, in fact, have been controlled as much by impact processes as by indigenous terrestrial geodynamics. The overall record suggests to us that the more-or-less continuous decline in impact frequency from 3.8 and perhaps even 4.2 Ga to 3.2 Ga suggested by Neukum et al. (2001) may be correct but that their relatively low estimates of the prevailing impact rate during that interval must be increased by a factor of from 10 to over 100 (Fig. 21). Sedimentary sequences like that in the Barberton Greenstone Belt provide a rich record not only of terrestrial events and processes but also of the late Late Heavy Bombardment and its effects on the Earth and its life. This record has been only partially illuminated to date and will never be fully utilized without extensive additional field-based studies.

5. Conclusions Sedimentary rocks in Barberton Greenstone Belt, South Africa, and 59

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record or had their records lost within the background rain of chemical, pyroclastic, and detrital sediments. Many impacts probably occurred during episodes of volcanic activity and their deposits have been lost by incorporation into lava flows. In the end, however, we would argue that there is a much more extensive record of the impact history of the early Earth in terrestrial rocks than has been discovered to date. These rocks should continue to be explored and studied to better document these events and their effects on the early crustal, environmental, and biological evolution of the Earth.

and Planetary Science Conference, (A89-10851 01-91). Cambridge and New York/ Houston, TX. Cambridge University Press/Lunar and Planetary Institute, pp. 455–458 1988. Glass, B.P., Simonson, B.M., 2012. Distal impact ejects layers: spherules and more. Elements 8, 43–48. Glass, BP., Simonson, B.M., 2013. Distal Impact Ejects Layers: A Record of Large Impacts in Sedimentary Deposits. Springer, Heidelberg, pp. 716. Glikson, A.Y., 2001. The astronomical connection of terrestrial evolution: crustal effects of post–3.8 Ga mega-impact clusters and evidence for major 3.2 ± 0.1 Ga bombardment of the Earth–Moon system. J. Geodyn. 32, 205–229. Glikson, A.Y., 2008. Field evidence of eros-scale asteroids and impact-forcing of Precambrian geodynamic episodes, Kaapvaal (South Africa) and Pilbara (Western Australia) Cratons. Earth Planet. Sci. Lett. 267, 558–570. Glikson, A.Y., Allen, C., Vickers, J., 2004. Multiple 3.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. 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Acknowledgments We are grateful to Stanford University and Louisiana State University, which provided funds to support this research; the National Science Foundation, Grant Numbers EAR8406420 and EAR8904830 to DRL, which funded key parts of the early stages of this research; the NASA Exobiology Program grants NCC-2-721, NAG5-98421, and NNG04GM43G to DRL; the UCLA Astrobiology program of the NASA Astrobiology Institute; the Mpumalanga Parks Board, especially Mr. Johan Eksteen, and Sappi Forest Products, which allowed us access to private lands; and the many mining geologists and interested Barberton residents who have helped the authors in this study throughout the years. We would also like to thank Bill Bottke and Bruce Simonson for very helpful reviews and suggestions concerning the manuscript. References Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science 208 (4448), 1095–1108. Boehnke, P., Harrison, T.M., 2016. Illusory late heavy bombardment. Proc. Natl. Acad. Sci. 113, 10802–10806. http://dx.doi.org/10.1073/pnas.1611535113. Bottke, W.F., Norman, M.D., 2017. The late heavy bombardment. Ann. Rev. Earth Planet. Sci. 45, 619–647. org/10.1146/annurev-earth-063016-020131. Bottke, W.F., Vokrouhlický, D., Minton, D., et al., 2012. An Archaean heavy bombardment from a destabilized extension of the asteroid belt. Nature 485, 78–81. http://dx. doi.org/10.1038/nature10967. Buick, R., 1987. Comment on Early Archean silicate spherules of probable impact origin, South Africa and Western Australia. Geology 15, 180–181. Byerly, G.R., 1999. Komatiites of the Mendon Formation: late-stage ultramafic volcanism in the Barberton Greenstone Belt. In: Lowe, D.R., Byerly, G.R. (Eds.), Geologic Evolution of the Barberton Greenstone Belt, South Africa. Geological Society of America, pp. 189–211 Special Paper 329. Byerly, B.L., Lowe, D.R., Byerly, G.R., 2017. Exotic heavy mineral assemblage from a large Archean impact. Lunar Planet. Sci. XLVIII, 1582. Byerly, G.R., Lowe, D.R., 1994. Spinel from Archean impact spherules. Geochim. Cosmochim. Acta 58, 3469–3486. Byerly, G.R., Lowe, D.R., and Walsh, M.M., 1986, Stromatolites from the 3,300–3,500Myr Swaziland Supergroup, Barberton Mountain Land, South Africa. Nature, vol. 319, pp. 489–491. Byerly, G.R., Lowe, D.R., Wooden, J.L., Xie, X., 2002. An Archean impact layer from the Pilbara and Kaapvaal Cration. Science 297, 1325–1327. Chapman, C.R., 2004. The hazard of near-Earth asteroid impacts on Earth. Earth Planet. Sci. Lett. 222, 1–15. Dietz, R.S., 1961. Vredefort ring structure: meteorite impact scar? J. Geol. 69, 499–516. https://doi.org/10.1086/626768. Dietz, R.S., 1964. Sudbury structure as an astrobleme. J. Geol. 72, 412–434. https://doi. org/10.1086/626999. Djokic, T., Van Kranendonk, M.J., Campbell, K.A., Walter, M.R., Ward, C.R., 2017. Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits. Nat. Commun. 8, 15263. http://dx.doi.org/10.1038/ncomms15263. Drabon, N., Lowe, D.R., Byerly, G.R., Harrington, J.A., 2017. Detrital zircon geochronology of sedimentary rocks of the 3.6–3.2 Ga Barberton Greenstone Belt: no evidence for older continental crust. Geology 45, 803–806. Duchac, K.C., Hanor, J.S., 1987. Origin and timing of the metasomatic silicification of an early Archean komatiite sequence, Barberton Mountain Land, South Africa. Precambrian Res. 37, 125–146. French, B.M., 1998. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures 954. Lunar and Planetary Institute Contribution, Houston, TX, pp. 120. Frey, H., 1977. Origin of the Earth's ocean basins. Icarus 32, 235–250. Frey, H., 1980. Crustal evolution of the early Earth: the role of major impacts. Precambrian Res. 10, 195–216. Garde, A.A., McDonald, I., Dyck, B., Keulen, N., 2012. Searching for giant, ancient impact structures on Earth: the Mesoarchaean Maniitsoq structure, West Greenland. Earth Planet. Sci. Lett. 197, 337–338. http://dx.doi.org/10.1016/j.epsl.2012.04.026. Glass, B.P., Burns, C.A., 1988. Microkrystites: a new term for impact-produced glassy spherules containing primary crystallites. In: Proceedings of the Eighteenth Lunar

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