ABSTRACT Australasian tektites represent the largest group of tektites on Earth, and their strewn field covers up to one sixth of Earth’s surface. After several decades of fruitless quest for a parent crater for Australasian tektites, mostly in the main part of the strewn field in Indochina, the crater remains undiscovered. We elaborate upon a recently suggested original hypothesis for the impact in the Alashan Desert in Northwest China. Evidence from geochemical and isotopic compositions of potential source materials, gravity data, and geographic, paleoenvironmental, and ballistic considerations support a possible impact site in the Badain Jaran part of the Alashan Desert. In further support of an impact location in China, glassy microspherules recovered from Chinese loess may be the right age to relate to the Australasian tektite event, perhaps as part of the impacting body. The most serious shortcomings of the commonly accepted Indochina impact location include signs of little chemical weathering of source materials of Australasian tektites, unlike highly weathered sedimentary targets in Indochina, and questionable assumptions about transport of distal ejecta.

ABSTRACT Layered deposits on Mars imaged by the three rovers are generally inferred to have been deposited by liquid water (or wind or volcanism), consistent with interpretations based on orbital imaging. This interpretation implies early Mars was warm and wet, despite long-standing problems with modeling this case. As an alternative hypothesis, rapid sediment deposition during Late Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (at least in Gale Crater) by chemically neutral groundwater can explain all the observed sediment features, such as ubiquitous low-angle cross-bedding, primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, high friability with low bulk densities, planar scoured unconformities, and rounded cobbles from rock tumbling. In other words, the ground-observed mineralogy, geochemistry, and sedimentology do not require and even are inconsistent with deposition from liquid water. Unlike the Moon, early Mars is believed to have had an atmosphere and water, perhaps mostly frozen. If so, impacts should have formed turbulent ground-hugging impactoclastic density currents capable of traveling hundreds of kilometers, and even globally. As terrestrial analogs, smaller-scale density currents are widespread around explosive volcanoes and nuclear test sites, whereas terrestrial impact analogs are lacking. Steam condensation on particles causes accretionary lapilli to form, grow to a maximum size, and fall out on layered deposits, and similar spherules have been observed by two of three rovers. Explaining these spherules as normal sedimentary concretions at Meridiani Planum required ignoring some of the observations. Ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could therefore actually have resulted from impact cratering, the dominant geologic process in the early solar system.

ABSTRACT Large bolide impacts in the Phanerozoic produced global change identifiable in the postimpact sediments. Aside from a few isolated examples, however, evidence of postimpact change associated with Precambrian impacts is sparse. This study used the Neoarchean Paraburdoo spherule layer as a case study to search for impact-induced change in the sediments above the spherule layer. We found possible minor sedimentary changes that may have been due to either a disturbance by bottom currents or changing diagenetic conditions. Contrary to the trends found with several post–Great Oxidation Event large bolide impacts, we found no evidence of shifts in tectonic regime, sediment weathering and deposition, or paleoenvironment induced by the Paraburdoo spherule layer impact, for which the impactor is estimated to have been approximately three times larger than the Cretaceous–Paleogene bolide. This lack of a clear signal of climatic shift may be due to one or more mechanisms. Either the Paraburdoo spherule layer’s deposition in several-hundred-meter-deep water within the Hamersley Basin of Western Australia was too deep to accumulate and record observable changes, or the Neoarchean’s high-CO 2 atmospheric composition acted as a threshold below which the introduction of more impact-produced gases would not have produced the expected climatic and weathering changes. We also report minor traces of elevated iron and arsenic concentrations in the sediments immediately above the Paraburdoo spherule layer, consistent with trends observed above other distal impact deposits, as well as distinctive layers of hematite nodules bracketing the spherule layer. These geochemical changes may record ocean overturn of the Neoarchean stratified water column, which brought slightly oxygenated waters to depth, consistent with the observation of tsunami deposits in shallower impact deposits and/or heating of the global oceans by tens to hundreds of degrees Celsius in the wake of the Paraburdoo spherule layer impact. Either or both of these mechanisms in addition to impact-induced shallow-water ocean evaporation may also have caused a massive die-off of microbes, which also would have produced a postimpact increase in iron and arsenic concentrations.

ABSTRACT Early Archean spherule layers, widely accepted to represent distal ejecta deposits from large-scale impact events onto the early Earth, have been described from several stratigraphic levels of the Barberton greenstone belt in South Africa. Recently, exploration drilling at the Fairview Gold Mine (25°43′53″S, 31°5′59″E) in the northern domain of the belt resulted in the discovery of a new set of spherule layer intersections. The Fairview spherule layers in drill cores BH5901, BH5907, BH5911, and BH5949 were intersected just a few meters apart, at about the same stratigraphic position within the transition from the Onverwacht Group to the Fig Tree Group. The Fairview spherule layers have petrographic and chemical similarities to at least three other well-known Barberton spherule layers (S2–S4), and multiple spherule layer bed intersections in drill cores BARB5 and CT3, all from about the same stratigraphic position. They are not uniform in composition, in particular with respect to abundances of highly siderophile elements. The highest concentrations of moderately (Cr, Co, Ni) and highly siderophile (Ir) elements are within the range of concentrations for chondrites and, thus, reinforce the impact hypothesis for the generation of the Fairview spherule layers. Iridium peak concentrations and Cr/Ir interelement ratios for spherule layer samples from drill cores BH5907, BH5911, and BH5949 suggest admixtures of 50%–60% chondritic material, whereas for the BH5901 spherule layer, only an admixture of 1% chondritic material is indicated. We discuss whether these four Fairview spherule layers represent the same impact event, and whether they can be correlated to any of the S2–S4, CT3, and BARB5 intersections.

ABSTRACT The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3 He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 μm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels.

ABSTRACT Two late Eocene impact spherule layers are known: the North America microtektite layer (from the Chesapeake Bay crater) and the slightly older clinopyroxene (cpx) spherule layer (from Popigai crater). Positive Ir anomalies occur at 5.61 m and 6.19 m above the base of a late Eocene section at Massignano, Italy. The age difference between the two anomalies is ~65 ± 20 k.y. The older Ir anomaly at 5.61 m appears to be associated with the cpx spherule layer. Although no impact spherules or shocked-mineral grains have been found associated with the upper Ir anomaly at 6.19 m, it has been proposed that it may be from the Chesapeake Bay impact. Comparison with other distal ejecta layers suggests that microtektites, but not shocked-mineral grains, from the Chesapeake Bay crater could have been thrown as far as Massignano. However, their absence neither supports nor disproves the hypothesis that the Ir anomaly at 6.19 m is from the Chesapeake Bay impact. On the other hand, the North American microtektite layer is not associated with an Ir anomaly. Furthermore, the average age difference between the cpx spherule layer and the North American microtektite layer appears to be ~18 ± 11 k.y., which is nearly one quarter the age difference between the two Ir anomalies at Massignano. This indicates that the Ir anomaly at 6.19 m is too young to be from the Chesapeake Bay impact, and thus is most likely not from the Chesapeake Bay impact.