This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

/pdf/traces-of-catastrophe-a-handbook-of-shock-metamorphic-28xgtwpf9g.pdf

Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

Mass extinctions manifest in Earth’s geologic record were turning points in biotic evolution. We present 40 Ar/ 39 Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.

/pdf/time-scales-of-critical-events-around-the-cretaceous-1piu0w7mcf.pdf

Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary

The Chicxulub crater ejecta stratigraphy is reviewed, in the context of the stratigraphy of underlying and overlying rock sequences. The ejecta sequence is regionally grouped in (a) thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater, and exposures near the border of Yucatan and Belize; (b) Gulf of Mexico region, <2500 m from the crater, with up to 9 m thick, complex, tsunami-wave influenced, tektite-bearing sequences in shallow marine (<500 m deep) environments and tektite bearing, decimeter thick gravity-flow deposits in deep water sites; (c) an intermediate region between 2500 and 4000 km from the crater where centimeter thick, tektite-bearing layers occur, and (d ) a global distal region with a millimeter thin ejecta layer. The distal ejecta layer is characterized by sub-millimeter sized microkrystites, often rich in Ni-rich spinels and (altered) clinopyroxene. Wherever present, the ejecta layers mark exactly the sudden mass-mortality horizon of the K/T boundary. What exactly caused the mass mortality is still uncertain, but it appears the main event leading to the K/T mass extinctions.

The global stratigraphy of the cretaceous-tertiary boundary impact ejecta

We review the major impact-associated mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 104 Mt (i.e., impact frequencies less than one in 6 × 104 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 104 or 105 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 104–105 Mt (intervals up to 3 × 105 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 105 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 105 to 106 Mt (intervals up to 2 × 106 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild-fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 106 and 107 Mt (intervals up to 1.5 × 107 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km², and the resulting smoke would reduce light levels even further. At energies above 107 Mt, blast and earthquake damage reach the regional scale (106 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world's ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 109 Mt (>108 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96RG03038

Environmental Perturbations Caused by the Impacts of Asteroids and Comets

The 1783-1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated approximately 250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (approximately 60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was approximately 7.0 and 15.0 Mt, respectively. Furthermore, approximately 75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months, or even 1-2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposhere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods.

/pdf/sulfur-chlorine-and-fluorine-degassing-and-atmospheric-4pml25p6js.pdf

Sulfur, chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

THE 50-cm-thick Cretaceous/Tertiary boundary layer at Beloc in Haiti contains spherules of silicic black glass, 1–8 mm in diameter, which have been attributed to impact fusion of continental crust1. Calcic yellow glass (up to 30 wt% CaO) forms a coating on and streaks within some black glass spherules, and has been attributed to melting of carbonate-rich sediment1, or organic-rich or pyritic limestone2. Here we present new trace-element and stable and radiogenic isotope data which show that the silicic glass is derived from continental crust of andesitic composition, whereas the high-Ca glass formed by melting of evaporite-rich sediment. This is confirmed by melting experiments with evaporite and andesite at 1,200–1,400 °C, which approximately reproduce the high-Ca glass. The temperature-dependent variation of sulphur content in synthetic high-Ca glasses indicates a formation temperature of 1,300 °C for the Haiti glasses. The geology of the impact site inferred from the geochemistry of the Haiti glasses matches the lithologies found in the 180-km Chicxulub structure, which occurs in Cretaceous evaporite deposits in Mexico. The high sulphur content of the calcic glasses suggests that the impact may have generated significant emissions of sulphur dioxide to the atmosphere, causing short-term global cooling.

Geochemical constraints on source region of Cretaceous/Tertiary impact glasses

The stratigraphic distribution of Ni-rich spinels in cretaceous-tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

We have measured an anomalous iridium concentration in the clays of the Cretaceous/Tertiary boundary in the Bidart section, thus confirming the planetary character of this anomaly. The iridium anomaly is synchronous with a very sharp minimum of the 13C/12C ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary. The integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the Pacific Ocean. In spite of a careful search, we have found no evidence of any exotic material.

Ph. Bonté

Papers

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

Geochemical constraints on source region of Cretaceous/Tertiary impact glasses

The stratigraphic distribution of Ni-rich spinels in cretaceous-tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

An iridium rich layer at the Cretaceous/Tertiary boundary in the Bidart Section (southern France)