ABSTRACT As the wife and field assistant of geologist Walter Alvarez for the past 56 years, I have shared in adventures on five different continents. The quest to explore the history of our planet has given us insight and understanding of human history and culture as well. From the semi-arid Guajira desert of Colombia to the network of bike paths in Holland, to witnessing the September 1969 Revolution in Libya, from living in a medieval Italian hill town, visiting the Silk Road cities in Soviet Central Asia, participating in the plate tectonic revolution, helping found the Geological Observatory of Coldigioco, and continuing in the stimulating environment of the University of California, Berkeley, our lives have been rich in experiences. Linking it all together reminds us of the nearly infinite number of contingencies and decisions that shape each of our lives and contribute to our shared human history.

ABSTRACT The chapter consists of five sections. The first one provides an introduction to the collaboration between Walter Alvarez and the author in the interdisciplinary study of the Capitoline Hill in the early 1990s. The second section turns to how we first met in Rome in 1970 and our parallel pathways over the next two decades that saw each of us take the lead in developing a big new idea based on innovative interdisciplinary research. The third section gives the earth scientist essential background on the study of early Rome: such things as the seven kings of Rome, the original topography of the early city, and the syndrome of the eternal Rome. The fourth section develops an overview on the work that we conducted on the Capitoline Hill and the new results obtained. In the fifth section, we step back and consider the contribution of Walter Alvarez to our subsequent research and publications on early Rome and the emergence of the field of geoarchaeology in the study of ancient Rome.

ABSTRACT One of the most recent intellectual enterprises of the scientist honored in this book, Walter Alvarez, is the dive into big history. Alvarez’s research and worldview contributed directly to the foundations of this transdisciplinary field. In this paper, the relevance of big history to the emergent idea of planetary health is demonstrated. Since big history studies both natural and human-made systems over long periods of time, it is uniquely positioned to help address the three categories of challenges identified within planetary health: imagination, knowledge, and governance. Most extensively, this paper focuses on governance challenges. A case study on the Ethiopian health system illustrates that governance frameworks can be reimagined in such a way that incentives and efforts of actors within the system are aligned, producing better human health outcomes. For Homo sapiens to have a better chance of survival than dinosaurs did 66 million years ago, these lessons will need to be applied more broadly in the planetary health domain.

ABSTRACT The structural style at depth of the Umbria-Marche fold-and-thrust belt, which occupies the outer province of the Northern Apennines of peninsular Italy, has long been debated and interpreted in terms of thin-skinned or thick-skinned deformation models, respectively. Thin-skinned models predict that the Mesozoic–Tertiary sedimentary cover was detached along Upper Triassic evaporites and translated northeastward along stepped thrust faults above a relatively undeformed basement. On the other hand, thick-skinned models predict the direct involvement of conspicuous basement slices within thrust-related folds. A description of selected examples in the southeastern part of the Umbria-Marche belt reveals that some compressional structures are indeed thin-skinned, their style being controlled by rheological properties of a mechanically heterogeneous stratigraphy containing multiple décollements, whereas other structures are genuinely thick-skinned, their style being dominated by the reverse-reactivation of pre-orogenic normal faults deeply rooted within the basement. Therefore, the contrast of thin- versus thick-skinned structural styles, an issue that has generated a long-lasting debate, is only apparent, since both styles are documented to coexist and to have concurred in controlling the final compressional geometry of the fold-and-thrust belt.

ABSTRACT The Sibillini Mountains, which make up the southern part of the Umbria-Marche Apennines, were struck by a series of earthquakes in 2016, including five with magnitudes greater than 5. The largest event, M w 6.5, occurred on 30 October 2016. A M w 5.9 earthquake on 26 October ruptured several faults in the northern third of the Vettore–Bove fault system, and the M w 6.5 event produced surface ruptures along the entire 30-km length. Ground surveys conducted shortly after these earthquakes showed that many, but not all, of the surface ruptures corresponded to previously mapped faults. Also, some faults that had been mapped as Quaternary did not produce surface ruptures during the earthquakes. In this study, we present the results of detailed field mapping that was conducted prior to the 2016 earthquakes and provide evidence that all of the surface ruptures in the northern part of the Vettore–Bove fault system occurred along preexisting faults. Paleostress analysis shows that the reactivated faults had been active prior to 2016 in stress fields with similar orientations to the modern-day stress field. In addition, we show that one fault segment, which is the southern continuation of a major fault that slipped during the 2016 earthquakes, was not reactivated because it was unfavorably oriented.

ABSTRACT The formation of the “expansion breccia” observed in the Lower Cretaceous Maiolica limestone in the Umbria-Marches region of Italy is attributable to a fluid-assisted brecciation process that occurred during the late Miocene exhumation of the Northern Apennines. The hydrothermal fluids probably originated as brine solutions trapped in the Burano anhydrite while it was in a plastic state. The migration of the Burano from the plastic to the brittle domain during unroofing resulted in liberation and injection of over-pressured hydrothermal fluids into the overlying limestone, causing hydraulic fracturing. Mapping of breccia morphology along a 400-m transect showed structures produced by different flow regimes, with chaotic and mosaic breccia characterizing the core parts of the section and mineral-filled fractures and veins in the margins. Based on the clast size in the chaotic breccia, the estimated velocities for fluidizing the aggregates of clasts and sustaining the clasts in suspension are, respectively, 15 cm/s and 65 cm/s. Crack growth was probably the main mechanism for the fragmentation of the limestone. Explosion fracturing patterns were only sporadically observed in the breccia, indicating substantial heat loss of the over-pressured fluids during their ascent to the Earth’s surface.

ABSTRACT Numerous examples of transverse drainages in the Apennines inspired early, forward-thinking models to describe how rivers established and maintained their courses as mountains were being raised beneath them. We assemble the rate of baselevel fall (τ- U ) and associated channel χ-z data of ten transverse rivers draining the Apennine pro-wedge using a channel stream power linear inverse approach. We apply the results to evaluate competing models of transverse drainage development as well as the underlying dynamic and tectonic processes responsible for Apennine topography. The channel inversion approach employs the simplifying assumption of uniform uplift and erosion at the catchment scale, but accounts for variable rock erodibility as the first-order determinant of regional, mean channel steepness. Accordingly, local deviations in channel steepness are interpreted by the model as transient upstream-propagating waves of base-level fall originating at the catchment mouth. Modeled timing, rate, and unsteadiness of these base-level falls are broadly consistent with geomorphic, geologic, thermochronologic, and paleo-elevation isotopic data, indicating that the Apennines emerged impulsively at ∼2.5 Ma at rates ranging from ∼0.2–0.3 mm/yr for the central Apennines to rates of ∼0.7 mm/yr for the southern Apennines. Syn-deformation and foreland-propagating superposition dominate transverse drainage development for the northern and north-central Apennines, which are underlain by an intact Adriatic slab. In contrast, further south where a slab window separates the Adriatic slab from the base of the Apennine wedge, dynamic uplift prevails and the transverse drainages have developed in response to regional superposition and integration of catchments through spillover and headwater capture processes.

ABSTRACT The Cenozoic accretionary complex in the Calabrian Arc, southern Italy, contains hectometric- to kilometric-scale exposures of basalt, gabbro, and serpentinite that have been interpreted as dismembered fragments of Alpine Tethys ocean crust because of their incomplete nature with respect to the traditional view of a complete ophiolite sequence. We present new geologic mapping, geochemistry, and geochronology of one of these units at Timpa di Pietrasasso near the town of Terranova di Pollino in the Basilicata region that exposes Jurassic Tethyan pillow basalt and chert that are separated from gabbro and serpentinite by a fault. The gabbro in the footwall is Permian in age, indicated by U-Pb zircon ages of 284 ± 6 Ma, 293 ± 6 Ma, and 295 ± 4 Ma, linking it to gabbros that underplated continental crust after the Permo-Carboniferous Variscan Orogeny. The gabbro first underwent amphibolite-facies metamorphism, then developed a greenschist-facies mylonitic foliation near the fault surface that is crosscut by undeformed Jurassic-aged dikes of Tethyan origin, indicating that deformation is early Tethyan or pre-Tethyan in age. The underlying serpentinite is tectonically interleaved with blocks of Variscan lower crust, indicating that the missing upper plate of the extensional detachment complex was continental in origin. These features indicate that the Timpa di Pietrasasso unit preserves a low-angle detachment fault that developed in a hyperextended continental margin of the Alpine Tethys.

ABSTRACT Two contrasting field relationships may reflect different tectonic settings of subduction initiation preserved in orogenic belts. “Hot” subduction initiation assemblages include a large ophiolite unit (up to kms thick, extending tens to hundreds of km along strike) with supra subduction zone (SSZ) geochemical affinity that structurally overlies a thin (<500 m thick) sheet of high-pressure (HP), high-temperature (HT), primarily metamafic rocks called a metamorphic sole. The ophiolite generally lacks burial metamorphism and includes variably serpentinized peridotite at its base. The sole structurally overlies subduction complex rocks made up of oceanic materials (igneous part of oceanic crust and overlying pelagic sedimentary rocks, and clastic sedimentary rocks of trench fill affinity) and/or passive margin assemblages; some of the subduction complex may be metamorphosed under HP-low temperature (LT) conditions (such as blueschist facies). The field relationships suggest initiation of subduction within young (<15 My) and “hot” oceanic lithosphere and that the sole represents the first slice(s) of material transferred from the subducting to upper plate. Examples include the Neotethyan and northern Appalachian ophiolites and units beneath them, and the Coast Range ophiolite and subjacent Franciscan subduction complex of California. “Cold” subduction initiation assemblages lack SSZ ophiolite and island arc components and a metamorphic sole. Instead, the upper plate above the subduction complex is made up of continental lithosphere that last experienced significant heating during a passive-margin forming rift event. The protoliths of the rocks subducted were >70 My in age at the time of subduction initiation. The HP-LT subduction complex is composed of slices of continental crust and oceanic crust representing parts of a hyperextended continental margin. These field relationships suggest initiation of subduction along a continental margin within old (“cold”) hyperextended continental lithosphere. Examples include the Apennine subduction zone, exposed in Calabria, Italy, and the Alpine orogenic belt, both remnants of the Alpine Tethys.

ABSTRACT Two-dimensional depth-migrated seismic data were used to interpret and analyze extension and salt deposition in the ocean-continent transition (OCT) along 720 km of the southern Gulf of Mexico rifted margin. The OCT is characterized by alternating areas of salt-filled, fault-bounded outer troughs overlying a shallow Moho and salt perched at a level above the top of oceanic crust. Normal faults and the limit of oceanic crust are both offset by two sets of transfer faults and paleo–transform faults, respectively, that trend NNW-SSE and N-S. The patterns define five OCT segments that show propagation of both rifting and spreading to the NE, an abrupt jump in pole location, and rifting/spreading nuclei that link up laterally. Salt was deposited during outer trough formation to the SW but prior to it in the NE, where salt consequently flowed from proximal locations into the growing trough during decoupled thick-skinned extension. The salt was deposited at least 0.5–1.5 km below global sea level, with precipitation initially confined to the oldest troughs (in the west) and subsequently spreading to cover the entire basin in a deep brine over a period of at least 5 m.y. Possible siliciclastic strata interbedded with the salt were likely sourced from the south and southeast, and hypersaline conditions waned gradually during punctuated marine flooding over another 5–10 m.y. The Gulf of Mexico was thus a giant evaporite basin formed in a deep depression during late-synrift mantle exhumation in a magma-poor setting, analogous to the South Atlantic salt basins and possibly the Red Sea and southern Moroccan/Scotian margins.

ABSTRACT The reduced Jurassic sedimentary sequences deposited on a structural high in the Umbria-Marche Apennines, as well their relationships with adjacent expanded basinal sequences, have been reconstructed through detailed, interdisciplinary study of the Sasso di Pale and Monte Serrone areas near Foligno, Italy. The physiographic features of the basin originated in the Early Jurassic (latest early Pliensbachian), when extensional tectonic activity broke up a shallow water platform where the Calcare Massiccio had been deposited, and the area evolved from an edge-stepped structural high to a distally steepened ramp. The biostratigraphic framework of this paper is mainly based on calcareous nannofossils, which are a useful tool for dating condensed Jurassic successions. Although the sections studied have limited thickness and much lateral facies variation, the sedimentary evolution can be traced and interpreted within a wider Jurassic environmental perspective. In the upper Pliensbachian–lower Bajocian interval, local sea-level variations are compatible with the global sea-level curve. Furthermore, some of the characteristic events—such as the Pliensbachian–Toarcian crisis, the Early Toarcian Jenkyns Event, and the Middle Jurassic carbonate crisis—can be recognized. The present study shows how the reconstruction of local paleogeography can fit into a more general framework and how regional and global signals can be recognized even in a small structural high such as the one we have investigated.

ABSTRACT At present, the Global Stratotype Section and Point (GSSP) for the base of the Bartonian remains the only GSSP of the Paleogene System to be defined by the International Subcommission on Paleogene Stratigraphy (ISPS) and the International Commission on Stratigraphy (ICS). Here, we present the results of an integrated, high-resolution study of calcareous plankton and benthic foraminifera biostratigraphy and a detailed magneto-, chemo-, and cyclostratigraphic analyses carried out through the upper Lutetian to the upper Priabonian pelagic sediments of the Bottaccione Gorge section near Gubbio, central Italy, to check its stratigraphic completeness and constrain in time the optimal interval for defining and positioning the GSSP for the base of the Bartonian Stage. The high-resolution and solid integrated stratigraphic framework established at Bottaccione confirmed the completeness of the section, which meets the ICS recommendations for a potential designation as a GSSP for the base of the Bartonian Stage. Thus, the Bottaccione section was compared with the parastratotype section of the Bartonian in its type area, Alum Bay, UK. On this basis, two reliable criteria for defining and positioning the Bartonian GSSP at Bottaccione are provided: (1) the base of magnetic polarity chronozone C18r as the primary correlation criterion and (2) the base of the calcareous nannofossil Dictyococcites bisectus , which defines the CNE14/CNE15 zonal boundary as a secondary correlation criterion.

ABSTRACT As impact events are known to have had severe effects on the geological and biological evolution of the Earth, the need to detect potentially hazardous objects that might collide with the Earth, and to possibly protect our planet from asteroid impacts, has been recognized in recent years. Planetary defense covers human activities to address potential impacts of Near-Earth Objects on Earth. Once the immediate threat of such an impact is obvious, this fact, along with the intended countermeasures, will have to be communicated to the public. There is a parallel to the recent coronavirus (COVID-19) situation: an imminent threat and the required response are being communicated. Reactions between acceptance and cooperation all the way to denial, conspiracy theories, fake news, and active opposition can be observed. It is evident that these factors will have to be considered in the strategy for communicating the asteroid threat.

ABSTRACT The Bottaccione Gorge at Gubbio, in central Italy, has been an important source of information about Cretaceous and Paleogene Earth history. At the much younger end of the historical continuum, it is also important for understanding the early history of Gubbio itself, for which the only written, although somewhat ambiguous, evidence comes from the Tavole eugubine, the unique bronze tablets which are a kind of Rosetta Stone for the Umbrian language. The role of the Bottaccione Gorge is debated in the history of Gubbio. The road through the gorge, crossing the Monti di Gubbio, is an important element for explaining the location of the city. One of the first settlements (late Bronze Age) is recognized from archaeological evidence at the top of a morphological fault scarp on the slope of Monte Ingino. In the Iron Age, the city described in the Tavole eugubine developed, in which Okri (fortress), Tota (city), and three sacred gates are mentioned. The locations of Okri , Tota , and the gates are still under study. According to the most likely hypothesis, Tota would have developed in the plain, on the right bank of the Torrente Camignano, while the initial settlement would have been transformed into Okri , to which the sacred gates would belong. Another gate may have been placed at the entrance to the Bottaccione Gorge. When the Eugubini (the people of Gubbio) built the new, post-Roman Gubbio in the twelfth century, they still identified, as the most suitable place for a fortified city, the location above the scarp on the slope of Monte Ingino, and they built two new gates at its lateral ends. The city was likely equipped with a third gate that faced the Bottaccione Gorge. In the thirteenth century, the Bottaccione Aqueduct was built to bring water to the highest point of Gubbio. Thus, two waterways—one natural (Torrente Camignano) and the other artificial—still branch off from Bottaccione to reach Gubbio at two different points that determine the lowest and highest levels of the city.

ABSTRACT Travertine deposits preserved within ancient aqueduct channels record information about the hydrology, temperature, and chemistry of the flowing water from which they precipitated. However, travertine is also chemically reactive and susceptible to freshwater diagenesis, which can alter its original composition and impact reconstructions of aqueduct operation, maintenance, and climate. Hydraulic reconstructions, in combination with a suite of high-resolution optical, laser, electron, and X-ray microscopy analyses, have been used to determine the original crystalline structure and diagenetic alteration of travertine deposited in the Anio Novus aqueduct built in A.D. 38–52 at Roma Vecchia. Age-equivalent travertine deposits, precipitated directly on the mortar-covered floor at upstream and downstream sites along a 140-m-long continuous section of the Anio Novus channel, exhibit consistent crystalline textures and stratigraphic layering. This includes aggrading, prograding, and retrograding sets of travertine linguoid, sinuous, and hummocky crystal growth ripples, as well as sand lags with coated siliciclastic grains deposited on the lee slope of ripple crests. The original aqueduct travertine, which is similar to travertine formed in analogous natural environments, is composed of shrub-like, dendritically branching aggregates of 1–3-μm-diameter euhedral calcite crystals. Dark brown organic matter-rich laminae, formed by microbial biofilms and plant debris, create stratigraphic sequences of high-frequency, dark–light layering. This hydraulic and petrographic evidence suggests that large, radiaxial calcites diagenetically replaced the original aqueduct travertine shrubs, forming upward-branching replacement crystals that crosscut the biofilm laminae. While this diagenetic process destroyed the original crystalline fabric of the calcite shrubs, the entombed biofilm laminae were mimetically preserved. These integrated approaches create the type of depositional and diagenetic framework required for future chemostratigraphic analyses of travertine deposited in the Anio Novus and other ancient water conveyance and storage systems around the world, from which ancient human activity and climatic change can be more accurately reconstructed.

ABSTRACT Stratigraphic analysis of two sections of a fluvial strath terrace exposed on the left bank of the Esino River near the village of Trocchetti (province of Ancona, Marche region of central Italy), and the study of a large landslide located near the village of San Cristoforo, a few kilometers down valley from the Trocchetti fluvial terrace, provide evidence for two catastrophic environmental events, namely: (1) the aggradation on the riverbed of coarse, chaotic gravel due to a violent flashflood; and (2) the formation of a large ephemeral lake as the consequence of the landslide that barred the river channel at San Cristoforo. Archaeological and historical information about the lost Roman city of Tuficum , which was located just a kilometer upriver from the Trocchetti terrace, and ceramic artifacts found in the chaotic gravel unit, led us to the hypothesis that both the flashflood and the landslide were induced by the sudden, severe climate change of the Late Antique Little Ice Age (mid-sixth century to mid-seventh century CE).

ABSTRACT In this paper, we present old and new data about our integrated interdisciplinary stratigraphic study of sedimentary deposits preserved in the Grotta dei Baffoni Cave of the Frasassi hypogenic cave complex, including sedimentological, paleontological, archaeometric, and radiometric analyses. This research work allowed us to reconstruct the geologic, environmental, and human history of this part of the northeastern Apennines of Italy for the past 200,000 years, from the late Middle Pleistocene to the Present. Accumulation of alluvial sediment began in this cave ~200,000 years ago, when an entrance was breached by the Sentino River during its process of incision and deepening of the Frasassi Gorge coupled with regional tectonic uplift. Flooding of the cave went on until the entrance sill of the cave was lifted up to an elevation that could no longer be reached by the river, sometime in the mid–Late Pleistocene. After this, windblown dust (i.e., loess) and coarser carbonate clasts derived from the disintegration of the vaults due to cryogenic processes and/or seismically induced collapses of the limestone vaults, accumulated on this now-dry underground environment. The stratigraphy of an ~4-m-thick sedimentary deposit accumulated in the vast atrium room of the cave was measured, sampled, and documented in two excavation trenches in 1952 by archaeologist Anton Mario Radmilli. By collecting a dozen stratigraphically located osteological finds for 14 C dating, and revisiting artifacts collected by Radmilli, which are archived respectively in the Museum of Natural History of Verona and in the National Museum of Archaeology of Ancona, we assessed that the cave was frequented by wild animals, such as cave bear and ibex, starting in the mid–Late Pleistocene. Dating of charcoal particles from subsurface sediments in the inner part of the cave suggested that fires were lit in this cave by Epigravettian visitors during the Younger Dryas cold period. Scarce archaeological evidence nevertheless suggests that man began using this underground environment for worship practices probably in the early Neolithic. Human bones in the lower part of one of Radmilli’s excavations yielded early Eneolithic ages. No other human bones were found in overlying levels of this excavation, but the typology of animal bones and associated ceramic artifacts, corroborated by our 14 C dates, suggest that this cave was utilized as a worship or ritual place until the early Middle Bronze Age. After that, the cave was sporadically used as a shelter for herders until recent times.

ABSTRACT The Middle Paleozoic section of the Appalachian Plateau exhibits a mechanical stratigraphy defined by layers that emit seismic energy with unique signatures in response to a strain energy accumulated on time scales associated with local, regional, and plate-scale processes. The Earth is in a state of frictional equilibrium, which means that even small changes in effective stress cause brittle failure and the concomitant release of ambient seismic energy. Stress changes as low as 0.001 MPa, the level of stress changes during Earth tides or the transmission of a fluid pressure wave, can activate failure on critically oriented fractures. These phenomena lead to a release of ambient seismic energy, which can be mapped using seismic emission tomography (SET) methods to image fracture networks emitting coherent seismic waves. We used a buried array of 54 sondes to identify active fracture networks over a contiguous volume of 3.76 km 3 within Middle Paleozoic rocks hosting two Marcellus gas shale wells drilled under the Appalachian Plateau of Lycoming County, Pennsylvania, USA. We sampled ambient seismic emissions before and after two stimulations and found that the pattern was repeatable. The fracture patterns illuminated by ambient seismic emissions defined a mechanical stratigraphy populated by clouds of seismic activity separated by packages of beds emitting relatively less seismic energy. The unique attribute of the beds emitting less seismic energy is a lower least horizontal stress (S hmin ) relative to adjacent mechanical units in the section. These low stress beds include the bottom portion of both the Marcellus and Burket/Geneseo black shales. There are three thicker mechanical units carrying clouds of higher energy emissions. These three units include siltstones of the Brallier above the Burket/Geneseo package, silty shale beds of the Mahantango between the Marcellus and Burket/Geneseo packages, and Silurian-Devonian carbonates below the Marcellus package. In map view, emission patterns in the Brallier follow Alleghanian J2 joints. Patterns in the Mahantango are consistent with slip along columnar joint zones like those cutting upward in outcrops of shale on the Appalachian Plateau. In sum, SET reveals a mechanical stratigraphy based on the release of strain energy from three major units of the Middle Paleozoic section.

ABSTRACT The existence of an ~26–36 m.y. rhythm in interrelated global tectonism, sea-level oscillations, climate, and resulting sedimentation patterns during Phanerozoic time (the last 541 m.y.) has long been suspected. A similar underlying ~26.4–27.5 m.y. cycle was reported independently in episodes of extinctions of marine and non-marine species. Subsequent spectral analyses of individual geologic events of the last 260 m.y., including changes in seafloor spreading and subduction, times of hotspot initiation and intraplate volcanism, eruptions of Large Igneous Provinces (LIPs), tectonic events, sea-level fluctuations, oceanic anoxia, atmospheric carbon dioxide levels, and global climate have revealed evidence for the 26–36 m.y. cycle and the temporal association of events with an apparent overall periodicity of ~27.5 m.y. modulated by an ~8–9 m.y. cycle. The proposed episodes of geologic activity and environmental and biotic change may result from cyclical internal Earth processes that affect changes in mantle convection, plate motions, intraplate stresses, and/or periodic pulses of mantle-plume activity. Recently, the ~30 m.y. cycle has been linked to Earth’s long-term orbital changes within the Solar System, and it may also affect tectonism and climate. I also note considerable evidence for a similar ~30 m.y. cycle in the ages of terrestrial impact craters, which suggests possible astronomical connections. The shared geologic cycle time, formally ranging from ~26 to 36 m.y. (depending partly on varying data sets, geologic timescales, and statistical techniques utilized) is close to the estimated interval (~32 ± 3 m.y.) between our cyclical crossings of the crowded mid-plane region of the Milky Way Galaxy. Here I outline a proposed astrophysical pacing for the apparent pulses of both impact cratering and rhythmic geological episodes.

ABSTRACT Although the ~200 impact craters known on Earth represent only a small fraction of the craters originally formed, the available data suggest an excess of craters by one order of magnitude, in number, in the interval ca. 470–440 Ma during the Ordovician. Most of these “excess” craters may be related to the breakup of the L-chondrite parent body (LCPB) in the asteroid belt at 465.8 ± 0.3 Ma. This is the only obvious peak in the crater-age record that can currently be attributed to an asteroid breakup and shower event. Spatial crater densities in regions with high potential for crater preservation (e.g., Canada and Scandinavia) support a one order-of-magnitude increase in the flux of large (>0.1 km) impactors following the LCPB breakup. A similar pattern as seen in the cratering record is emerging in studies of the flux of micrometeoritic chrome spinel through the Phanerozoic, with so far only one major spike in the flux, and associated with the LCPB breakup. Similarly, the record of K-Ar and (U-Th)/He gas retention ages of recently fallen meteorites only locates one major breakup, the LCPB event, during the Phanerozoic. On the other hand, astronomical backtracking studies of the orbits of asteroid family members indicate ~70 major family-forming breakups within the past ~540 m.y., which apparently have not left any clear imprint in Earth’s geological record. The chrome-spinel grains recovered in our studies dominantly represent large micrometeorites (>300 µm) and as such are also representative of the flux of larger meteorites to Earth. An observed, nearly constant flux of ordinary chondritic chrome-spinel grains throughout the Phanerozoic, except after the LCPB event, indicates that the present situation—with a clear dominance of ordinary chondritic matter in the large (>500 µm) micrometeorite and macroscopic meteorite fractions—has prevailed at least for the last 500 m.y. This is also supported by generally high ratios in our samples of chrome-spinel grains from ordinary chondrites compared to other types of spinel-bearing meteorites. The chrome-spinel data together with the abundance of fossil meteorites (1–21 cm in diameter) on the Ordovician seafloor also sets an upper limit at one order of magnitude on the increase in flux of large (>0.1-km-diameter) L-chondritic projectiles to Earth following the LCPB. Such an increase would not stand out in the global cratering record if ordinary chondritic impactors had only represented a small fraction of all Phanerozoic impactors. We argue that the origin of impactors delivered to Earth during the past 500 m.y. has mirrored the flux of large micrometeorites and meteorites, with ordinary chondrites being an important or, most likely, the dominant (in numbers) component throughout.