Nanometre-size diamonds in the Cretaceous/Tertiary boundary clay of Alberta

The Transformation Mechanism of Graphite to Hexagonal Diamond under Shock Conditions

The formation of a hexagonal diamond represents one of the most intriguing questions in materials science. Under shock conditions, the graphite basal plane tends to slide and pucker to form diamond. However, how the shock strength determines the phase selectivity remains unclear. In this work, using a DFT-trained carbon global neural network model, we studied the shock-induced graphite transition. The poor sliding caused by scarce sliding time under high-strength shock leads to metastable hexagonal diamond with an orientation relationship of (001)G//(100)HD+[010]G//[010]HD, while under low-strength shock due to long sliding distance cubic diamond forms with the orientation (001)G//(111)CD+[100]G//[110]CD, unveiling the strength-dependent graphite transition mechanism. We for the first time provide computational evidence of the strength-dependent graphite transition from first-principles, clarifying the long-term unresolved shock-induced hexagonal diamond formation mechanism and the structural source of the strength-dependent trend, which facilitates the hexagonal diamond synthesis via controlled experiment.

A Tunguska sized airburst destroyed Tall el-Hammam a Middle Bronze Age city in the Jordan Valley near the Dead Sea

We present evidence that in ~ 1650 BCE (~ 3600 years ago), a cosmic airburst destroyed Tall el-Hammam, a Middle-Bronze-Age city in the southern Jordan Valley northeast of the Dead Sea. The proposed airburst was larger than the 1908 explosion over Tunguska, Russia, where a ~ 50-m-wide bolide detonated with ~ 1000× more energy than the Hiroshima atomic bomb. A city-wide ~ 1.5-m-thick carbon-and-ash-rich destruction layer contains peak concentrations of shocked quartz (~ 5-10 GPa); melted pottery and mudbricks; diamond-like carbon; soot; Fe- and Si-rich spherules; CaCO₃ spherules from melted plaster; and melted platinum, iridium, nickel, gold, silver, zircon, chromite, and quartz. Heating experiments indicate temperatures exceeded 2000 °C. Amid city-side devastation, the airburst demolished 12+ m of the 4-to-5-story palace complex and the massive 4-m-thick mudbrick rampart, while causing extreme disarticulation and skeletal fragmentation in nearby humans. An airburst-related influx of salt (~ 4 wt.%) produced hypersalinity, inhibited agriculture, and caused a ~ 300-600-year-long abandonment of ~ 120 regional settlements within a > 25-km radius. Tall el-Hammam may be the second oldest city/town destroyed by a cosmic airburst/impact, after Abu Hureyra, Syria, and possibly the earliest site with an oral tradition that was written down (Genesis). Tunguska-scale airbursts can devastate entire cities/regions and thus, pose a severe modern-day hazard.

Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds

The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.

Nanosecond formation of diamond and lonsdaleite by shock compression of graphite

The shock-induced transition from graphite to diamond has been of great scientific and technological interest since the discovery of microscopic diamonds in remnants of explosively driven graphite. Furthermore, shock synthesis of diamond and lonsdaleite, a speculative hexagonal carbon polymorph with unique hardness, is expected to happen during violent meteor impacts. Here, we show unprecedented in situ X-ray diffraction measurements of diamond formation on nanosecond timescales by shock compression of pyrolytic as well as polycrystalline graphite to pressures from 19 GPa up to 228 GPa. While we observe the transition to diamond starting at 50 GPa for both pyrolytic and polycrystalline graphite, we also record the direct formation of lonsdaleite above 170 GPa for pyrolytic samples only. Our experiment provides new insights into the processes of the shock-induced transition from graphite to diamond and uniquely resolves the dynamics that explain the main natural occurrence of the lonsdaleite crystal structure being close to meteor impact sites.

Quantifying the distribution of nanodiamonds in pre-Younger Dryas to recent age deposits along Bull Creek, Oklahoma panhandle, USA

High levels of nanodiamonds (nds) have been used to support the transformative hypothesis that an extraterrestrial (ET) event (comet explosion) triggered Younger Dryas changes in temperature, flora and fauna assemblages, and human adaptations [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104(41):16016-16021]. We evaluate this hypothesis by establishing the distribution of nds within the Bull Creek drainage of the Beaver River basin in the Oklahoma panhandle. The earlier report of an abundance spike of nds in the Bull Creek I Younger Dryas boundary soil is confirmed, although no pure cubic diamonds were identified. The lack of hexagonal nds suggests Bull Creek I is not near any impact site. Potential hexagonal nds at Bull Creek were found to be more consistent with graphene/graphane. An additional nd spike is found in deposits of late Holocene through the modern age, indicating nds are not unique to the Younger Dryas boundary. Nd distributions do not correlate with depositional environment, pedogenesis, climate perturbations, periods of surface stability, or cultural activity.

Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments

The long-standing controversy regarding the late Pleistocene megafaunal extinctions in North America has been invigorated by a hypothesis implicating a cosmic impact at the Allerød-Younger Dryas boundary or YDB (approximately 12,900 +/- 100 cal BP or 10,900 +/- 100 (14)C years). Abrupt ecosystem disruption caused by this event may have triggered the megafaunal extinctions, along with reductions in other animal populations, including humans. The hypothesis remains controversial due to absence of shocked minerals, tektites, and impact craters. Here, we report the presence of shock-synthesized hexagonal nanodiamonds (lonsdaleite) in YDB sediments dating to approximately 12,950 +/- 50 cal BP at Arlington Canyon, Santa Rosa Island, California. Lonsdaleite is known on Earth only in meteorites and impact craters, and its presence strongly supports a cosmic impact event, further strengthened by its co-occurrence with other nanometer-sized diamond polymorphs (n-diamonds and cubics). These shock-synthesized diamonds are also associated with proxies indicating major biomass burning (charcoal, carbon spherules, and soot). This biomass burning at the Younger Dryas (YD) onset is regional in extent, based on evidence from adjacent Santa Barbara Basin and coeval with broader continent-wide biomass burning. Biomass burning also coincides with abrupt sediment mass wasting and ecological disruption and the last known occurrence of pygmy mammoths (Mammuthus exilis) on the Channel Islands, correlating with broader animal extinctions throughout North America. The only previously known co-occurrence of nanodiamonds, soot, and extinction is the Cretaceous-Tertiary (K/T) impact layer. These data are consistent with abrupt ecosystem change and megafaunal extinction possibly triggered by a cosmic impact over North America at approximately 12,900 +/- 100 cal BP.

Early precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47-2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia

Pioneering studies of Precambrian impact fallout units and associated tsunami deposits in the Hamersley Basin, Pilbara Craton, Western Australia, by B.M. Simonson and S.W. Hassler, document a range of tsunami deposits associated with impact fallout units whose impact connection is identified by associated microtektites and microkrystites (condensation spherules). The impact connection of these particles is demonstrated by iridium anomalies, unique platinum group elements patterns, and Ni-rich mineral phases. Densely packed tsunami-transported fragments and boulders overlie microkrystite units of the >2629 +/- 5 Ma top Jeerinah Impact Layer (JIL). Tsunami events closely follow spherule settling associated with the 2561 +/- 8 Ma Spherule Marker Bed SMB-1 and SMB-2 impact events, Bee Gorge Member, Wittenoom Formation. The two impact cycles are separated by a stratigraphically consistent silicified black siltstone, representing a "Quiet Interval." The SMB turbidites display turbulence eddies, climbing ripples, conglomerate pockets, slumps, and waterlogged sediment deformation features. Consequences of tsunami in the probably contemporaneous Carawine Dolomite (Pb-Pb carbonate ages of approximately 2.56-2.54 Ga), eastern Hamersley Basin, include sub-autochthonous below-wave base excavation and megabrecciation of sea floor substrata, resulting in a unique 10-30-m-thick spherule-bearing megabreccia marker mapped over a nearly 100-km north-south strike distance in the east Hamersley Basin. The field relations suggest a pretsunami settling of the bulk of the spherules. Tsunami wave effects include: (1). dispersal of the spherule-rich soft upper sea floor sediments as a subaqueous mud cloud and (2). excavation of consolidated substrata below the soft sediment zone. Excavation and megabrecciation included injection of liquefied spherule-bearing microbreccia into dilated fractures in the disrupted underlying carbonates. Near-perfect preservation of the spherules within the basal microbreccia veins suggests tsunami-induced hydraulic pressures locally exceeded lithostatic pressure. Late-stage settling of spherule-bearing mud clouds in the wake of the tsunami is represented by an abundance of spherules in the uppermost microbreccia zones of the megabreccia pile. From the deep below-wave base facies of the Carawine Dolomite, tsunami wave amplitudes may have exceeded 200 m depth. The approximately 2.47-2.50 Ga DGS4 (S4 Macroband, Dales Gorge Member, Brockman Iron Formation) fallout units include exotic chert and carbonate boulders transported by tsunami following settling of a 10-20-cm-thick microkrystite and microtektite-rich unit. Seismic perturbations preceding deposition of the JIL and SMB fallout units are marked by rip-up clasts. The geochemistry of microkrystites and microtektites suggests impact fallout originated from impacts in simatic/oceanic crustal regions, although tsunami waves may have originated from seismically reactivated faults and plate margins located at distance from the impact craters.

Comparing the evidence relevant to impact and flood basalt at times of major mass extinctions

The five major mass extinctions identified in 1982 by Raup and Sepkoski have expanded to six, with the suggestion that the Permian-Triassic extinction was a double event. Is there a general explanation for great mass extinctions, or can they result from different triggers, or even from internal system instabilities? The two most-discussed candidates for a general extinction mechanism are impacts and flood-basalt eruptions. A compilation of evidence for impact at the times of mass extinctions shows that this cause is abundantly confirmed in the case of the Cretaceous-Tertiary extinction and the late Eocene, which is a time of minor and gradual extinction, but little or no evidence connects other major extinctions to impact. On the other hand, there is a remarkable time correlation between flood basalts and four major extinctions, but no other evidence that flood basalts cause mass extinctions. The evidence for an impact-extinction linkage is strikingly different from that for a connection between flood basalts and extinctions. Flood basalts cover larger areas than craters and their associated thick ejecta blankets, which are thus less likely to be found. Impacts distribute proxies globally at instantaneous time horizons, whereas flood-basalt events are extended in time, and no remote proxies have been recognized. Many global killing mechanisms have been proposed in the case of impacts, but few have been suggested for flood basalts. It is possible that flood basalts are triggered by impact, but it is not obvious how impacts could result from anything other than chance. The hypothesis that impacts are the general cause of mass extinctions has not received supporting evidence, but has not been falsified. The hypothesis that flood basalts are the general cause of mass extinctions is supported by evidence from timing, but is not susceptible to falsification. Other candidates for general extinction causes, especially sea-level changes and system instabilities, would require separate treatment. The question is still very much open.

A diamond trilogy: superplumes, supercontinents, and supernovae

Diamond is a remarkable mineral and has been long recognized for its unusual physical and chemical properties: robust and widespread in industry, yet regally adorned. This diversity is even greater than formally appreciated because diamond is recognized as an extraordinary recorder of astrophysical and geodynamic events that extend from the far reaches of space to Earth's deep interior. Many diamonds are natural antiques that formed in presolar supernovae by carbon vapor deposition, in asteroidal impacts and meteorite craters by shock metamorphism, and in Earth's mantle 1 to 2 billion years after planetary accretion from fluids and melts. The carbon in diamond is primordial, but there are unexplained isotopic fractionations and uncertainties in heterogeneity.

Elemental carbon as interstellar dust

C 60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in cireumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature—low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented.

Multiple factors in the origin of the Cretaceous/Tertiary boundary: the role of environmental stress and Deccan Trap volcanism

A review of the scenarios for the Cretaceous/ Tertiary (K/T) boundary event is presented and a coherent hypothesis for the origin of the event is formulated. Many scientists now accept that the event was caused by a meteorite impact at Chicxulub in the Yucatan Peninsula, Mexico. Our investigations show that the oceans were already stressed by the end of the Late Cretaceous as a result of the long-term drop in atmospheric CO2, the long-term drop in sea level and the frequent development of oceanic anoxia. Extinction of some marine species was already occurring several million years prior to the K/T boundary. The biota were therefore susceptible to change. The eruption of the Deccan Traps, which began at 66.2 Ma, coincides with the K/T boundary events. It erupted huge quantities of H2SO4, HCl, CO2, dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature. The result was a major reduction in oceanic productivity and the creation of an almost dead ocean. The volcanism lasted almost 0.7 m.y. Extinction of biological species was graded and appeared to correlate with the main eruptive events. Elements such as Ir were incorporated into the volcanic ash, possibly on soot particles. This horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the K/T boundary. An oxidation front penetrated this horizon leading to the redistribution of elements. The eruption of the Deccan Traps is the largest volcanic event since the Permian-Triassic event at 245 Ma. It followed a period of 36 m.y. in which the earth's magnetic field failed to reverse. Instabilities in the mantle are thought to be responsible for this eruption and therefore for the K/T event. We therefore believe that the K/T event can be explained in terms of the effects of the Deccan volcanism on an already stressed biosphere. The meteorite impact at Chicxulub took place after the onset of Deccan volcanism. It probably played a regional, rather than global, role in the K/T extinction.