Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion

Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion.Editor’s summary The geologic boundary between the Cretaceous and Paleogene eras 66 million years ago is marked by worldwide deposits from an impact at modern Chicxulub, Mexico. The impact coincides with a mass extinction that wiped out non-avian dinosaurs and many other species. Fischer-Gödde et al . measured ruthenium isotopes in the impact deposits and compared them with multiple classes of meteorites, which represent potential impactor compositions. They found that the Chicxulub impactor was a carbonaceous asteroid that formed in the outer Solar System. Additional measurements of five other impacts showed that those were due to silicate asteroids that formed in the inner Solar System. —Keith T. SmithAn impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion.Editor’s summary The geologic boundary between the Cretaceous and Paleogene eras 66 million years ago is marked by worldwide deposits from an impact at modern Chicxulub, Mexico. The impact coincides with a mass extinction that wiped out non-avian dinosaurs and many other species. Fischer-Gödde et al . measured ruthenium isotopes in the impact deposits and compared them with multiple classes of meteorites, which represent potential impactor compositions. They found that the Chicxulub impactor was a carbonaceous asteroid that formed in the outer Solar System. Additional measurements of five other impacts showed that those were due to silicate asteroids that formed in the inner Solar System. —Keith T. Smit

Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth's final stages of accretion

Petrology and geochemistry of granulite xenoliths from Udachnaya and Komsomolskaya kimberlite pipes, Siberia

Lower crustal xenoliths from the Udachnaya and Komsomolskaya kimberlite pipes in Siberia are mainly meta-igneous mafic garnet granulites, with subordinate feldspar-rich garnet granulites. Pressure and temperature estimates are interpreted as the conditions in the lower crust at the time of the last granulite-facies metamorphic event (800–890°C) followed by cooling to 610–720°C with a pressure decrease from 1·2 to 0·8 GPa. Most of the xenoliths show minor alteration. Leaching experiments demonstrate that their isotopic, major and trace element compositions have been affected by interaction not only with the host kimberlite but also with a fluid mobilized from local sedimentary country rocks. To obtain unambiguous compositional data we have calculated the composition of selected samples using modal analyses, electron microprobe and laser ablation inductively coupled plasma mass spectrometry data for constituent minerals. The reconstructed protoliths of most of the xenoliths were Fe-tholeiites of intraplate affinity, similar to some Archean basalts, whereas the others show characteristics of subduction-related magmas. However, the mafic granulites are strongly depleted in Rb, Th and U, which were removed by a small-degree partial melt. A protolith age of c. 3 Ga is supported by a disturbed Sm–Nd isochron, Nd and Hf model ages, and published U–Pb ages of zircon cores

Early Cambrian Food Chain: New Evidence from Fossil Aggregates in the Maotianshan Shale Biota, SW China

International audienceThree categories of fossil aggregates are recognized in the Lower Cambrian Maotianshan Shale biota from SW China: (1) elliptical aggregates with randomly distributed exoskeletal remains of typically small- to medium-size bivalved arthropods (e.g., ostracode-like bradoriids, phyllocarid- like waptiids), hyoliths, and trilobites are interpreted as coprolites, possibly produced by anomalocaridids (e.g., waptiid-rich coprolites) and/or unknown epibenthic predators; (2) elongate, ribbon-like aggregates composed of oriented small hyolith shells, interpreted as the feces of infaunal carnivorous worms such as priapulids; and (3) concentric aggregates, typically with a central nucleus (e.g., remains of medusoid eldoniids or bivalved arthropod carapaces) and peripheral exoskeletal fragments, possibly generated by bottom currents whirling around carcasses. These new coprolite data add to morphofunctional information obtained from fossil organisms and indicate that predation occurred at different levels of the water column with: (1) endobenthic predators (diverse priapulid fauna) feeding near the sediment-water interface; (2) epibenthic predators/scavengers (almost exclusively arthropods); (3) predators living in the lower levels of the water column (e.g., anomalocaridids); and (4) mid-water predators exploiting upper levels in the water column (e.g., eldoniids, ctenophores, chaetognaths). Communities living at or close to the water-sediment interface (epibenthic sensu stricto, meiobenthic, and demersal animals) were exposed to a multidirectional predatorial pressure from infaunal, epifaunal, and mid-water predators. Although predation was diverse, nothing indicates that the food chain extended beyond the level of primary carnivores. Animals already had acquired complex behaviors such as hunting (e.g., anomalocaridids, priapulids) and predator avoidance in which sensory systems were involved. The example of theMaotianshan Shale indicates that the burst of anatomical innovations (new body plans) that characterizes the early Cambrian alsowas accompanied by the rapid development of new feeding strategies and by an unprecedented expansion of ecological interactions (prey-predator relationships)