Impact Ejecta Found at Paleocene-Eocene Boundary in New Jersey

Impact ejecta at the Paleocene-Eocene boundary

Authors:

Schaller et al

Abstract:

Extraterrestrial impacts have left a substantial imprint on the climate and evolutionary history of Earth. A rapid carbon cycle perturbation and global warming event about 56 million years ago at the Paleocene-Eocene (P-E) boundary (the Paleocene-Eocene Thermal Maximum) was accompanied by rapid expansions of mammals and terrestrial plants and extinctions of deep-sea benthic organisms. Here, we report the discovery of silicate glass spherules in a discrete stratigraphic layer from three marine P-E boundary sections on the Atlantic margin. Distinct characteristics identify the spherules as microtektites and microkrystites, indicating that an extraterrestrial impact occurred during the carbon isotope excursion at the P-E boundary.

Evidence of an Impact Produced Polymorph in NeoArchean Spherules

Shock-metamorphosed rutile grains containing the high-pressure polymorph TiO2-II in four Neoarchean spherule layers

Authors:

Smith et al

Abstract:

At least 17 spherule layers are presently known within stratigraphic units deposited between ca. 3.47 and 2.49 Ga. The spherule layers contain varying amounts of formerly molten, millimeter-sized and smaller spherules. The aggregate thickness of spherules in these layers commonly ranges from ∼1 cm to as much as a few decimeters. Several lines of evidence support the interpretation that the spherule layers represent distal impact ejecta layers. Previously, only one shock-metamorphosed grain (quartz) had been documented from the spherule layers. Therefore, a key diagnostic criterion for the impact origin of these layers has remained elusive for 30 years. We report the discovery, using micro-Raman spectroscopy, of shock-induced TiO2-II, a high-pressure polymorph of TiO2, in 34 grains from four Neoarchean spherule layers deposited between ca. 2.65 and 2.54 Ga. As all the TiO2-II-bearing grains contain rutile, we interpret them as shock-metamorphosed rutile grains. Shock-metamorphosed rutile grains, which may be more abundant in the upper parts of three of the layers, provide unambiguous physical evidence to further support an impact origin for these four layers. Our results demonstrate that TiO2-II can survive for >2.5 b.y. in supracrustal successions that have undergone low-grade metamorphism. Because TiO2-II transforms to rutile at a temperature ≥440 °C, TiO2-II in impact ejecta layers is a potential geothermometer. To our knowledge, this is the first report of a shock-induced, high-pressure polymorph formed by an Archean impact.

Evidence of an Impact From the PaleoArchean in Australia

A new ∼3.46 Ga asteroid impact ejecta unit at Marble Bar, Pilbara Craton, Western Australia: a petrological, microprobe and laser ablation ICPMS study

Authors:

Glikson et al

Abstract:

The Archean record contains seventeen asteroid impact ejecta units interpreted in terms of terrestrial vestiges of an extended late heavy bombardment (LHB) (, and ). Correlated impact ejecta units include the 3.47 Ga in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa, and Pilbara region of Western Australia, with multiple ejecta units in the 3.25–3.22 Ga and 2.63–2.48 Ga intervals. This paper reports the discovery and investigation of a new impact ejecta unit within the Marble Bar Chert Member (MBCM) of the felsic volcanic Duffer Formation, east Pilbara Craton, Western Australia. The age of the MBCM is constrained by a 3459±2 Ma U-Pb zircon date from the uppermost volcanic unit of the Duffer Formation and by a 3449±3 Ma U-Pb zircon date from the overlying felsic volcanic Panorama Formation, stratigraphically above the intervening un-dated Apex Basalt. The ejecta unit, observed in a drill core (ABDP 1) ∼4 km south-southwest of Marble Bar, consists of multiple lenses and bands of almost totally silicified impact spherules 1–2 mm in diameter. All internal primary textures of the spherules have been destroyed. Nonetheless, Fe-rich spherule rims, largely composed of secondary siderite, are well preserved. Chemical analyses of the rims reveal iron-magnesium carbonate displaying high Fe, Mg, Ni, Co and Zn. Whole-rock and in-situ analyses (X-ray fluorescence, Inductively Coupled Plasma Mass Spectrometry (ICPMS), electron-microprobe (EMP) and EMP-calibrated laser ICPMS) reveal that the rims contain high Ni abundances and high Ni/Cr ratios (less than 50). The spherules are separated by an arenite matrix and spherule lenses also occur within bedded chert. The spherules are particularly visible over some ∼14 m of true stratigraphic thickness in which chert breccia is interpreted to represent a tsunami-generated diamictite affected by hydrothermal fragmentation and veining. Despite the almost total silicification of the MBCM whole-rock nickel sulphide (NIS) results indicate high Ir (2 ppb) and a low Pd/Ir ratio (2.0), consistent with geochemical features of impact ejecta units. Dense concentrations of spherules at the 57-58 m level and the 77 m level of the core, separated by banded chert, raise the possibility of two distinct impact events. Stratigraphic and isotopic age data distinguish between the 3459–3449 Ma age of the MBCM ejecta unit and ∼3470.1 ± 1.9 Ma impact ejecta units in the Antarctic Creek Member, Mount Ada Basalt, about 40 km to the west of Marble Bar. In combination with a 3472 ± 2.3 Ma impact unit in the Barberton greenstone belt, these impact ejecta units record large Paleoarchean asteroid impacts, significant for understanding early earth bombardment rates and early crustal evolution.

Where did all the Impactors Come From During the PaleoArchean?

Spherule layers, crater scaling laws, and the population of ancient terrestrial impactors

Authors:

Johnson et al

Abstract:

Ancient layers of impact spherules provide a record of Earth's early bombardment history. Here, we compare different bombardment histories to the spherule layer record and show that 3.2–3.5 Ga the flux of large impactors (10–100 km in diameter) was likely 20–40 times higher than today. The E-belt model of early Solar System dynamics suggests that an increased impactor flux during the Archean is the result of the destabilization of an inward extension of the main asteroid belt (Bottke et al., 2012). Here, we find that the nominal flux predicted by the E-belt model is 7–19 times too low to explain the spherule layer record. Moreover, rather than making most lunar basins younger than 4.1 Gyr old, the nominal E-belt model, coupled with a corrected crater diameter scaling law, only produces two lunar basins larger than 300 km in diameter. We also show that the spherule layer record when coupled with the lunar cratering record and careful consideration of crater scaling laws can constrain the size distribution of ancient terrestrial impactors. The preferred population is main-belt-like up to ∼50 km in diameter transitioning to a steep distribution going to larger sizes.

Colombian Island has 2 cm Spherulites From the Chicxulub Impact that Wiped out the Dinosaurs

A team of scientists working on the Colombian island of Gorgonilla, in the Pacific, has found fragments of the meteorite that 66 million years ago caused the extinction of the dinosaurs, sources close to the investigation said Tuesday.

“(We have) evidence of the impact of the meteorite, (they are very tiny) particles that are the result when the meteorite hit,” geologist Herman Bermudez told Spanish news agency EFE.

Those remains, called spherulites, which are small, rounded bodies that commonly occur in vitreous igneous rocks, are part of the continental shelf material that is currently the Yucatan Peninsula and they were found with fragments of the meteorite.

The spherulites are 2 cm (about 0.8 inches) in diameter and look like marbles, being made of glass.

Under the microscope small crystals from the meteorite can be seen and they show its composition.

When the meteorite hit Earth, it blasted out a crater 200 km (125 mi.) in diameter known today as Chicxulub, which is on the Yucatan Peninsula.

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Possible Spherule Beds From Orosiria PaleoProterozoic Vredefort Impact

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Authors:

Huber et al

Abstract:

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was less than 2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic.

Darmouth Group Finds Evidence of Younger Dryas Impact

The impact occurred about 12,900 years ago, at the beginning of the Younger Dryas period, and marks an abrupt global change to a colder, dryer climate with far-reaching effects on both animals and humans. In North America, the big animals all vanished, including mastodons, camels, giant ground sloths and saber-toothed cats. Their human hunters, known to archaeologists as the Clovis people, set aside their heavy-duty spears and turned to a hunter-gatherer subsistence diet of roots, berries and smaller game.

It is not disputed that these powerful environmental changes occurred, but there has long been controversy over their cause. The classic view of the Younger Dryas cooling interlude has been that an ice dam in the North American ice sheet ruptured, releasing a massive quantity of freshwater into the Atlantic Ocean. The sudden influx is thought to have shut down the ocean currents that move tropical water northward, resulting in the cold, dry climate of the Younger Dryas.

But Sharma and his co-authors have discovered conclusive evidence linking an extraterrestrial impact with this environmental transformation. The report focuses on spherules, or droplets of solidified molten rock expelled by the impact of a comet or meteor. The spherules in question were recovered from Younger Dryas boundary layers at sites in Pennsylvania and New Jersey, the layers having been deposited at the beginning of the period. The geochemistry and mineralogy profiles of the spherules are identical to rock found in southern Quebec, where Sharma and his colleagues argue the impact took place.

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