The Experts below are selected from a list of 11412 Experts worldwide ranked by ideXlab platform
Christian Koeberl - One of the best experts on this subject based on the ideXlab platform.
-
geochemistry of 2 63 2 49 ga impact Spherule layers and implications for stratigraphic correlations and impact processes
Precambrian Research, 2009Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Alex Shukolyukov, W U Reimold, Gunther W LugmairAbstract:Thin layers rich in Spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long-distance stratigraphic correlation of Precambrian successions. Seven formations in Western Australia and South Africa clustered around the Archean-Proterozoic boundary each contain a single Spherule layer, all of which were deposited between ca. 2.49 and 2.63 Ga. Analyses of 25 samples from 6 of the 7 Spherule layers and 23 samples from closely associated strata free of Spherules revealed an extraterrestrial component (ETC) in all six layers, based on PGE ratios and/or Cr isotopic composition. The amount of ETC varies from marginally detectable to clear and obvious; it generally amounts to ca. a few percent. Both PGE ratios and Cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition. In contrast, all Spherule layers older than ca. 3.0 Ga that have been analyzed were produced by carbonaceous chondritic impactors. Normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in PGE ratios favors ballistic emplacement of melt droplets rather than Spherule formation via vapor condensation. The geochemical data also provide a means to test proposed intra- and intercontinental stratigraphic correlations. Cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2.63 Ga. In contrast, clear geochemical differences between the middle layers, both deposited ca. 2.54 Ga, suggest they were not produced by the same impact event. The youngest, banded iron formation-hosted layers on both continents have also been correlated, but no geochemical data are available on the African layer to test this at present.
-
iridium anomalies and shocked quartz in a late archean Spherule layer from the pilbara craton new evidence for a major asteroid impact at 2 63 ga
Geology, 2004Co-Authors: Birger Rasmussen, Christian KoeberlAbstract:A thin (1‐5 mm) Spherule layer in ca. 2.63 Ga shale from the Jeerinah Formation (Pilbara craton, northwestern Australia) has been identified at two new localities. The layers have Ir concentrations as high as 15.5 ppb, significantly higher than the surrounding carbonaceous shale (,1.5 ppb). The sand-sized Spherules display quench and devitrification textures and are interpreted as former silicate melt droplets that were replaced by K-feldspar, carbonate, and sulfide during diagenesis. In one Spherule-layer sample, an angular quartz grain (;100 mm in size) with planar deformation features was found, which represents the oldest known shocked grain in distal ejecta by .2000 m.y. The survival of shocked quartz in ca. 2.63 Ga rocks, which have undergone multiple metamorphic events, suggests that their absence in other impact ejecta layers may not only be a question of preservation. The presence of shocked quartz and anomalously high Ir contents in a layer containing melt Spherules provides compelling evidence for an extraterrestrial impact with a target area that was at least partly silicic, favoring a continental impact site. Estimates based on geochemical data suggest that the Spherule layer comprises as much as 2‐3 wt% of a chondritic meteorite component. If proposed correlations with the Carawine (eastern Pilbara craton) and Monteville (South Africa) layers are correct, then the combined ejecta blanket represents fallout from a single major impact with an areal distribution of .32,000 km 2 , among the largest yet documented in the Precambrian rock record.
-
Geochemistry of Cenozoic microtektites and clinopyroxene-bearing Spherules
Geochimica et Cosmochimica Acta, 2004Co-Authors: B.p. Glass, Heinz Huber, Christian KoeberlAbstract:Abstract We have determined the major and trace element compositions of 176 individual microtektites/Spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) Spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx Spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/Spherules from a given layer can be distinguished from microtektites/Spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/Spherule came from based only on trace element compositions. The cpx Spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx Spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx Spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx Spherules have values that lie off the mixing curves in a direction that suggests Ni loss, probably as a result of solution of a Ni-rich phase (olivine?). The Ni-rich Australasian microtektites also have Ni values that lie close to mixing curves between the average upper continental crust and chondrites. However, both the cpx Spherules and HNi Australasian microtektites appear to have Ir (and to a lesser extent Au) contents that are much too low to have Ni/Ir ratios similar to chondritic values. We have no explanation for the low-Ir and -Au contents except to speculate that they may be the result of a complex fractionation process. The Ivory Coast and North American microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of, or nature of, a meteoritic component in them. Trace element compositions are consistent with derivation of the Cenozoic microtektite/Spherule layers from upper continental crust. The normal Australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content. The source rock for the high-Mg Australasian microtektites is not known, but the HMg microtektites do not appear to be normal Australasian microtektites that were simply contaminated by meteorites or ultramafic rocks. The average Ivory Coast microtektite composition can be matched with a mixture of target rocks at the Bosumtwi crater. The average composition of the North American microtektites suggests an arkosic source rock, but with graywacke and quartz-rich end members. However, we could not match the composition of the North American microtektites with lithologies in impact breccias recovered from the Chesapeake Bay impact structure that is believed to be the source crater. Likewise, we could not match the composition of the cpx Spherules with mixtures of basement rocks and overlying sedimentary deposits (for which compositional data are available) at the Popigai impact crater that may be the source crater for the cpx Spherules. This may be because the cpx Spherules were derived, in large part, from clastic surface rocks (sandstones and shales) for which no compositional data are available.
-
geochemical evidence for an impact origin for a late archean Spherule layer transvaal supergroup south africa
Geology, 2000Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Wolf Uwe ReimoldAbstract:A Late Archean layer rich in sand-sized Spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville Spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the Spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous Spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the Spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville Spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville Spherule layer and Early Archean Spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian Spherule layer is unique and needs to be characterized individually, as is true for the impact Spherule layers of the Phanerozoic.
-
Ocean Drilling Project Hole 689B Spherules and upper Eocene microtektite and clinopyroxene-bearing Spherule strewn fields
Meteoritics & Planetary Science, 1999Co-Authors: B.p. Glass, Christian KoeberlAbstract:— Montanari et al. (1993) reported a positive Ir anomaly in the upper Eocene sediments from Ocean Drilling Program Hole 689B on the Maud Rise, Southern Ocean. Vonhof (1998) described microtektites and clinopyroxene-bearing (cpx) Spherules associated with the Ir anomaly in Hole 689B and suggested that they belong to the North American and equatorial Pacific cpx strewn fields, respectively. We searched a suite of 27 samples taken through the Spherule layer from Hole 689B, and we recovered 386 microtektites and 667 cpx Spherules. We studied the petrography of the microtektites and cpx Spherules and determined the major element compositions of 31 microtektites and 14 cpx Spherules using energy dispersive x-ray analysis. We also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis. We found that the peak abundance of cpx Spherules is ∼2 cm below the peak abundance of the microtektites (∼128.7 m below sea floor), which suggests that the cpx Spherule layer may be slightly older (∼3–5 ka). The microtektites are mostly spherical and are generally transparent and colorless. They are similar to the North American microtektites in composition, the biggest differences being their generally lower Na2O and generally higher Zr, Ba, and Ir (up to 0.3 ppb) contents. We agree with Vonhof (1998) that the Hole 689B microtektites probably belong to the North American tektite strewn field. We calculate that the number of microtektites (>125 μm)/cm2 at Hole 689B is 52. This number is close to the concentration predicted by extrapolation of the trend of concentration vs. distance from the Chesapeake Bay structure, based on data from other North American microtektite-bearing sites. Thus, the North American strewn field may be at least four times larger than previously mapped. The Hole 689B cpx Spherules range from translucent yellow to opaque black, but most are opaque tan to dark brown. They are generally spherical in shape and all are < 125 μm in diameter. Some contain Ni-rich spinels in addition to clinopyroxene microlites. The cpx Spherules are petrographically and compositionally similar to cpx Spherules previously found in the northwestern Atlantic Ocean, Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. The abundance and widespread geographic occurrence of these Spherules suggest that the strewn field may be global in geographic extent. Assuming a global extent, we estimate that there may be at least 25 billion metric tons of cpx Spherules in the strewn field. Based on age, size, and geographic location, we speculate that the 100 km diameter Popigai crater in northern Siberia may be the source of the cpx Spherule layer.
Bruce M Simonson - One of the best experts on this subject based on the ideXlab platform.
-
shock metamorphosed rutile grains containing the high pressure polymorph tio2 ii in four neoarchean Spherule layers
Geology, 2016Co-Authors: Frank C Smith, Bruce M Simonson, Alexandra E Krulldavatzes, B. P. Glass, Joseph P Smith, Karl S BookshAbstract: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 TiO 2 -II, a high-pressure polymorph of TiO 2 , in 34 grains from four Neoarchean Spherule layers deposited between ca. 2.65 and 2.54 Ga. As all the TiO 2 -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 TiO 2 -II can survive for >2.5 b.y. in supracrustal successions that have undergone low-grade metamorphism. Because TiO 2 -II transforms to rutile at a temperature ≥440 °C, TiO 2 -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.
-
first detection of extraterrestrial material in ca 2 49 ga impact Spherule layer in kuruman iron formation south africa
Geology, 2015Co-Authors: Bruce M Simonson, Steven Goderis, Nicolas J BeukesAbstract:Thin layers rich in formerly molten Spherules interpreted as distal ejecta from large impacts by extraterrestrial bodies have been found in 8 stratigraphic units deposited between ca. 2.63 Ga and 2.49 Ga and attributed to a minimum of 4 separate impacts. Here we report geochemical evidence of extraterrestrial material in the only one of these Spherule layers where it has not been previously reported, the Kuruman Spherule layer (KSL) in the Kuruman Iron Formation, a banded iron formation (BIF) in the Griqualand West Basin (South Africa). We identified the KSL in 3 drill cores separated by as much as ∼350 km and analyzed 2 core samples that have a mean Ir concentration of ∼12.9 ppb and nearly chondritic interelement ratios of platinum group elements Ir, Ru, Pt, and Rh. This suggests that the samples contain ∼1%–3% by mass extraterrestrial material even though the Spherules are highly diluted by ambient sediment. Our geochemical data strongly support the correlation of the KSL with the Dales Gorge Spherule layer (DGSL) in a penecontemporaneous BIF in the Hamersley Basin (Western Australia). The KSL and DGSL are close matches in terms of major and various trace element contents and the DGSL has a comparable Ir content of ∼11.5 ppb. Therefore it is very likely the KSL and DGSL are distal ejecta from a ca. 2.49 Ga impact by a single extraterrestrial object >10 km across. The lack of any significant changes in the stratigraphic succession in either basin also implies that large impacts alone are not sufficient to cause long-term changes in Earth’s surface environments.
-
paraburdoo Spherule layer hamersley basin western australia distal ejecta from a fourth large impact near the archean proterozoic boundary
Geology, 2011Co-Authors: Scott W Hassler, Bruce M Simonson, D Y Sumner, Louis BodinAbstract:We report the discovery of the Paraburdoo Spherule layer, which consists entirely of replaced impact-produced melt Spherules. It is distal ejecta from a large impact close to the Archean-Proterozoic boundary in the Hamersley Basin of Western Australia. The Spherule layer occurs in the Paraburdoo Member of the Wittenoom Formation and was deposited ca. 2.57 Ga (date via U-Pb age interpolation) in a deep shelf environment. The layer consists of microkrystites rich in plagioclase and ferromagnesian crystals, replaced by K-feldspar and a phlogopite-like sheet silicate, respectively. The skeletal textures indicate rapid cooling of a melt with mafic to ultramafic composition and suggest oceanic target rocks. The Paraburdoo Spherule layer is 2 cm thick, normally graded, and was probably deposited by suspension settling, as there is no evidence of reworking; it is strikingly similar to the Reivilo Spherule layer in the Griqualand West Basin (South Africa), appears to provide a third impact-related high-resolution stratigraphic correlation between these two basins, and points to a high frequency of large impacts around the Archean-Proterozoic boundary.
-
geochemistry of 2 63 2 49 ga impact Spherule layers and implications for stratigraphic correlations and impact processes
Precambrian Research, 2009Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Alex Shukolyukov, W U Reimold, Gunther W LugmairAbstract:Thin layers rich in Spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long-distance stratigraphic correlation of Precambrian successions. Seven formations in Western Australia and South Africa clustered around the Archean-Proterozoic boundary each contain a single Spherule layer, all of which were deposited between ca. 2.49 and 2.63 Ga. Analyses of 25 samples from 6 of the 7 Spherule layers and 23 samples from closely associated strata free of Spherules revealed an extraterrestrial component (ETC) in all six layers, based on PGE ratios and/or Cr isotopic composition. The amount of ETC varies from marginally detectable to clear and obvious; it generally amounts to ca. a few percent. Both PGE ratios and Cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition. In contrast, all Spherule layers older than ca. 3.0 Ga that have been analyzed were produced by carbonaceous chondritic impactors. Normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in PGE ratios favors ballistic emplacement of melt droplets rather than Spherule formation via vapor condensation. The geochemical data also provide a means to test proposed intra- and intercontinental stratigraphic correlations. Cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2.63 Ga. In contrast, clear geochemical differences between the middle layers, both deposited ca. 2.54 Ga, suggest they were not produced by the same impact event. The youngest, banded iron formation-hosted layers on both continents have also been correlated, but no geochemical data are available on the African layer to test this at present.
-
correlating multiple neoarchean paleoproterozoic impact Spherule layers between south africa and western australia
Precambrian Research, 2009Co-Authors: Bruce M Simonson, D Y Sumner, Nicolas J Beukes, Sarah Johnson, Jens GutzmerAbstract:Abstract Well-studied successions in the Griqualand West Basin (South Africa) and the Hamersley Basin (Western Australia) that both straddle the Archean–Proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities. Each succession hosts a minimum of three impact Spherule layers, implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years. The occurrences of the youngest Spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact. Close similarities in stratigraphic setting, age, and petrographic characteristics of the Spherules, as well as the restriction of distinctive irregular melt particles to only these layers, suggest the oldest layers in the two basins were likewise formed by a single impact. In contrast, the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the Spherules, suggesting they are products of different impact events. These results suggest that using impact Spherule layers to establish a global network of high-resolution stratigraphic markers for early Precambrian successions may be an achievable goal, but careful comparisons between potentially correlative layers will be necessary to achieve it.
Paul Ahlquist - One of the best experts on this subject based on the ideXlab platform.
-
Cowpea chlorotic mottle bromovirus replication proteins support template-selective RNA replication in Saccharomyces cerevisiae
PLOS ONE, 2018Co-Authors: Bryan S. Sibert, Xiaofeng Wang, Amanda K. Navine, Janice G Pennington, Paul AhlquistAbstract:Positive-strand RNA viruses generally assemble RNA replication complexes on rearranged host membranes. Alphaviruses, other members of the alpha-like virus superfamily, and many other positive-strand RNA viruses invaginate host membrane into vesicular RNA replication compartments, known as Spherules, whose interior is connected to the cytoplasm. Brome mosaic virus (BMV) and its close relative, cowpea chlorotic mottle virus (CCMV), form Spherules along the endoplasmic reticulum. BMV Spherule formation and RNA replication can be fully reconstituted in S. cerevisiae, enabling many studies identifying host factors and viral interactions essential for these processes. To better define and understand the conserved, core pathways of bromovirus RNA replication, we tested the ability of CCMV to similarly support Spherule formation and RNA replication in yeast. Paralleling BMV, we found that CCMV RNA replication protein 1a was the only viral factor necessary to induce Spherule membrane rearrangements and to recruit the viral 2a polymerase (2apol) to the endoplasmic reticulum. CCMV 1a and 2apol also replicated CCMV and BMV genomic RNA2, demonstrating core functionality of CCMV 1a and 2apol in yeast. However, while BMV and CCMV 1a/2apol strongly replicate each others' genomic RNA3 in plants, neither supported detectable CCMV RNA3 replication in yeast. Moreover, in contrast to plant cells, in yeast CCMV 1a/2apol supported only limited replication of BMV RNA3 (
-
Host ESCRT proteins are required for bromovirus RNA replication compartment assembly and function.
PLOS Pathogens, 2015Co-Authors: Arturo Diaz, Xiaofeng Wang, Jiantao Zhang, Abigail Ollwerther, Paul AhlquistAbstract:Positive-strand RNA viruses genome replication invariably is associated with vesicles or other rearranged cellular membranes. Brome mosaic virus (BMV) RNA replication occurs on perinuclear endoplasmic reticulum (ER) membranes in ~70 nm vesicular invaginations (Spherules). BMV RNA replication vesicles show multiple parallels with membrane-enveloped, budding retrovirus virions, whose envelopment and release depend on the host ESCRT (endosomal sorting complexes required for transport) membrane-remodeling machinery. We now find that deleting components of the ESCRT pathway results in at least two distinct BMV phenotypes. One group of genes regulate RNA replication and the frequency of viral replication complex formation, but had no effect on Spherule size, while a second group of genes regulate RNA replication in a way or ways independent of Spherule formation. In particular, deleting SNF7 inhibits BMV RNA replication > 25-fold and abolishes detectable BMV Spherule formation, even though the BMV RNA replication proteins accumulate and localize normally on perinuclear ER membranes. Moreover, BMV ESCRT recruitment and Spherule assembly depend on different sets of protein-protein interactions from those used by multivesicular body vesicles, HIV-1 virion budding, or tomato bushy stunt virus (TBSV) Spherule formation. These and other data demonstrate that BMV requires cellular ESCRT components for proper formation and function of its vesicular RNA replication compartments. The results highlight growing but diverse interactions of ESCRT factors with many viruses and viral processes, and potential value of the ESCRT pathway as a target for broad-spectrum antiviral resistance.
-
Nodavirus-Induced Membrane Rearrangement in Replication Complex Assembly Requires Replicase Protein A, RNA Templates, and Polymerase Activity
Journal of Virology, 2010Co-Authors: Benjamin G. Kopek, Erik W. Settles, Paul D. Friesen, Paul AhlquistAbstract:Eukaryotic positive-strand RNA [(+)RNA] virus genome replication universally occurs on rearranged host intracellular membranes (1, 37, 49). Membrane rearrangements used by different viruses include, but are not limited to, membranous webs of vesicles (24, 56), double-membrane vesicles (41), and double-membrane layers (52). Among the most common virus-induced membrane rearrangements are 50- to 80-nm membrane invaginations or Spherules which are associated with RNA replication by alphaviruses, bromoviruses, nodaviruses, flaviviruses, tymoviruses, tombusviruses, and other viruses (23, 35, 44, 48, 51, 62). Such replication-associated membrane rearrangements are often induced by one or a few viral nonstructural proteins. The membranous web formed by hepatitis C virus (HCV) is induced by HCV protein NS4B (19). Double-membrane vesicles formed by the equine arterivirus are induced by the viral nsp2 and nsp3 proteins (55). Endoplasmic reticulum (ER) Spherules formed by brome mosaic virus (BMV) are induced by BMV RNA replication protein 1a (51). To better understand the mechanisms of (+)RNA virus replication complex formation, including membrane rearrangement, we examined Flock House virus (FHV) Spherule formation. FHV belongs to the family Nodaviridae and the genus Alphanodavirus, whose members naturally infect insects (9). FHV encapsidates a bipartite, single-stranded, positive-sense RNA genome whose RNAs are capped but not polyadenylated (Fig. (Fig.1)1) (9). RNA1 (3.1 kb) encodes multifunctional viral replicase protein A (112 kDa), the only FHV protein required for RNA replication (8). During replication, RNA1 also produces subgenomic RNA3 (387 nucleotides [nt]), which encodes the RNA interference (RNAi) suppressor protein B2 (12 kDa) (22, 26, 31). Genomic RNA2 (1.4 kb) encodes the capsid precursor protein α (43 kDa) (21). FIG. 1. Schematic of FHV genome components and the corresponding expression cassettes used in Drosophila expression plasmids. Each FHV component is expressed by a baculovirus IE1 promoter in a plasmid that also contains the baculovirus transactivating hr5 enhancer. ... FHV infection induces the formation of ∼50-nm-diameter membranous vesicles, or Spherules, between the mitochondrial outer and inner membranes (35). Three-dimensional electron tomographic imaging shows all such Spherules to be invaginations of the outer mitochondrial membrane, with interiors connected to the cytoplasm through ∼10-nm-diameter open necks (28). We previously showed that protein A and FHV RNA synthesis localize to the interiors of these Spherules, which thus represent the FHV RNA replication complex (28). In addition to providing RNA-dependent RNA polymerase and likely capping functions for RNA synthesis (8, 9, 21, 27), protein A has major roles in replication complex assembly. Protein A localizes to mitochondrial outer membranes through an N-terminal mitochondrial targeting and transmembrane sequence (34) and, in a step separable from RNA synthesis, recruits FHV RNA templates to mitochondria (59, 60). Protein A also self-interacts in vivo through multiple domains in ways important for RNA replication (16). Consistent with this, immunogold localization of protein A, biochemical results, and other data show that transmembrane, self-interacting protein A is present at ∼100 molecules per Spherule, implying that protein A forms a dense shell-like structure lining the interior Spherule membranes (28). In this study, we examined the requirements for forming the FHV Spherule RNA replication compartments. Although protein B2 can interact with protein A (5), we found that B2 and FHV capsid proteins were dispensable for Spherule formation. Protein A was required for Spherule formation but, unlike the case for many (+)RNA viruses for which one or more nonstructural proteins suffice for replication-associated membrane rearrangements, was insufficient. The results show that Spherule formation requires a replication-competent RNA template, occurs downstream of RNA template recruitment, and depends on protein A polymerase activity. Thus, in contrast to many other (+)RNA viruses, the membrane rearrangements associated with FHV RNA replication are tightly linked to viral RNA synthesis. These findings have mechanistic implications for RNA replication, RNA silencing, and virus control.
Wolf Uwe Reimold - One of the best experts on this subject based on the ideXlab platform.
-
geochemical evidence for an impact origin for a late archean Spherule layer transvaal supergroup south africa
Geology, 2000Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Wolf Uwe ReimoldAbstract:A Late Archean layer rich in sand-sized Spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville Spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the Spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous Spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the Spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville Spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville Spherule layer and Early Archean Spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian Spherule layer is unique and needs to be characterized individually, as is true for the impact Spherule layers of the Phanerozoic.
-
early archaean Spherule beds in the barberton mountain land south africa no evidence for impact origin
Precambrian Research, 1995Co-Authors: Christian Koeberl, Wolf Uwe ReimoldAbstract:Abstract Detailed petrographical and geochemical studies are reported for several well-documented occurrences of Spherule-rich layers in the Barberton Mountain Land, South Africa, which were previously inferred by Lowe and co-workers to be the result of early Archaean meteorite impacts. In contrast, we made the following observations. The textures of most Spherules are not necessarily the result of impact, but could instead be the product of radial or intersertal growth of crystals during secondary mineral formation. There is no difference in the content of siderophile elements between Spherule layers and layers devoid of Spherules. The siderophile element abundances are high where sulphide minerals (e.g., pyrite, gersdorffite, and chalcopyrite) and/or chromite are present, independent of the presence or absence of Spherules. High contents of, e.g., Ir (up to 2700 ppb), Ni (0.96 wt%), and Cr (1.6 wt%) were found in various samples. Abundances of these elements in chondritic meteorites are approximately 600 ppb, 1.4 wt%, and 0.35 wt%, respectively, resulting in respective meteoritic components in these samples of 450%, 70%, and 460%. We do not accept these high concentrations as primary meteoritic signatures. Impact melt rocks have typically « 1% of a meteoritic component. Furthermore, Spherules of any kind are very rare in known impact deposits (including the K-T boundary). If present at all, they are not usually associated with any significant Ir or PGE anomaly. The high abundances of the siderophile elements in some Barberton samples and their enrichment in secondary minerals indicate that these elements have been remobilized and reconcentrated. The PGE interelement ratios have changed during remobilization as well. Thus, the PGE abundance patterns and ratios are not primary and cannot be used as an argument in favour of an impact origin either. Nickel-rich Cr-spinels, which are found in some of the Barberton Spherule samples, have low Fe 3+ Fe (total) ratios. These ratios are incompatible with an extraterrestrial or impact origin, because such impact-derived spinels (including those found at the K-T boundary) are highly oxidized and also have different chemical compositions. There is also no indication of any evidence of shock metamorphism associated with the Barberton Spherule layers, which is the commonly accepted definitive criterion for recognition of an impact origin. This is unusual, because such evidence is preserved even in heavily altered samples from deeply eroded Archaean impact structures. We conclude that, while these Spherule layers are clearly unusual and deserve further attention, there is no convincing evidence for an origin by impact. We suggest that they were formed by volcanic processes, followed by extensive hydrothermal alteration.
B.p. Glass - One of the best experts on this subject based on the ideXlab platform.
-
Sr and Nd analyses of upper Eocene Spherules and their implications for target rocks
Meteoritics & Planetary Science, 2006Co-Authors: Shaobin Liu, D. A. Papanastassiou, H. H. Ngo, B.p. GlassAbstract:Upper Eocene impact ejecta has been discovered all over the world. The number of upper Eocene impact layers and the geographic distribution of each layer, based on major chemical composition and biostratigraphic data, are not agreed upon. We have performed four Sr-Nd isotopic analyses of clinopyroxene-bearing Spherules (cpx Spherules) and three Sr-Nd analyses of microtektites from five Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites in the South Atlantic and Indian Oceans. Our data support the hypothesis that there is only one cpx Spherule layer in upper Eocene sediments. We also find that the microtektites associated with the cpx Spherule layer in the South Atlantic and Indian Oceans are not part of the North American tektite strewn field, but belong to the same event that produced the cpx Spherules. The microtektites, together with cpx Spherules, are more heterogeneous than microtektites/tektites from other strewn fields. No direct link has been established between the microtektites from this study and possible target rock at the Popigai crater.
-
Spherule LAYERS—RECORDS OF ANCIENT IMPACTS
Annual Review of Earth and Planetary Sciences, 2004Co-Authors: Bruce M Simonson, B.p. GlassAbstract:▪ Abstract A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact Spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact Spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact Spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, Spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A reco...
-
Geochemistry of Cenozoic microtektites and clinopyroxene-bearing Spherules
Geochimica et Cosmochimica Acta, 2004Co-Authors: B.p. Glass, Heinz Huber, Christian KoeberlAbstract:Abstract We have determined the major and trace element compositions of 176 individual microtektites/Spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) Spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx Spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/Spherules from a given layer can be distinguished from microtektites/Spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/Spherule came from based only on trace element compositions. The cpx Spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx Spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx Spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx Spherules have values that lie off the mixing curves in a direction that suggests Ni loss, probably as a result of solution of a Ni-rich phase (olivine?). The Ni-rich Australasian microtektites also have Ni values that lie close to mixing curves between the average upper continental crust and chondrites. However, both the cpx Spherules and HNi Australasian microtektites appear to have Ir (and to a lesser extent Au) contents that are much too low to have Ni/Ir ratios similar to chondritic values. We have no explanation for the low-Ir and -Au contents except to speculate that they may be the result of a complex fractionation process. The Ivory Coast and North American microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of, or nature of, a meteoritic component in them. Trace element compositions are consistent with derivation of the Cenozoic microtektite/Spherule layers from upper continental crust. The normal Australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content. The source rock for the high-Mg Australasian microtektites is not known, but the HMg microtektites do not appear to be normal Australasian microtektites that were simply contaminated by meteorites or ultramafic rocks. The average Ivory Coast microtektite composition can be matched with a mixture of target rocks at the Bosumtwi crater. The average composition of the North American microtektites suggests an arkosic source rock, but with graywacke and quartz-rich end members. However, we could not match the composition of the North American microtektites with lithologies in impact breccias recovered from the Chesapeake Bay impact structure that is believed to be the source crater. Likewise, we could not match the composition of the cpx Spherules with mixtures of basement rocks and overlying sedimentary deposits (for which compositional data are available) at the Popigai impact crater that may be the source crater for the cpx Spherules. This may be because the cpx Spherules were derived, in large part, from clastic surface rocks (sandstones and shales) for which no compositional data are available.
-
Upper Eocene Impact Ejecta/Spherule Layers in Marine Sediments
Geochemistry, 2002Co-Authors: B.p. GlassAbstract:Abstract Microtektites with compositions and ages similar to those of the North American tektites have been found in deep-sea deposits in the Gulf of Mexico, Caribbean Sea, northwestern Atlantic Ocean and on Barbados. Unmelted impact ejecta (including shocked quartz and feldspar with multiple sets of planar deformation features, coesite, stishovite, and reidite, a high-pressure polymorph of zircon) have been found associated with this microtektite layer. These microtektites appear to belong to the North American tektite strewn field based on their geographic location, age (∼35 Ma), and composition. Clinopyroxene-bearing Spherules (cpx Spherules) are closely associated with the North American microtektitestherefore, occurrences of cpx Spherules in the Indian Ocean and equatorial Pacific were originally interpreted to indicate a major extension of the North American strewn field. However, at a few sites it is clear that the cpx Spherules are older than the North American microtektites and it is now accepted that they belong to a different impact event. The major crystalline phase in the cpx Spherules is clinopyroxene, as their name implies, but they also contain Cr- and Ni-rich spinels. The cpx Spherules generally have lower SiO 2 and Al 2 O 3 and higher FeO, MgO, and CaO contents compared with the North American microtektites. The cpx Spherules also have high Ni, Cr, Co, and Ir contents compared to the North American microtektites and to the average upper continental crust. The cpx Spherule layer is associated with a positive Ir anomaly and with the extinction of several taxa of Radiolaria. Flattened pancake Spherules composed of clay have been found in upper Eocene deposits at Massignano, Italy, associated with a positive Ir anomaly. The pancake Spherules are believed to be diagenetically altered cpx Spherules. Shocked quartz with multiple sets of planar deformation features have been found associated with the Spherule layer at Massignano. The number of upper Eocene microtektite/Spherule layers is debated, but most authors agree that there are probably only two: the North American microtektite layer and the cpx Spherule layer. Geographic variations in abundance, age, composition, and Sr and Nd isotopic data are consistent with the North American microtektites (and tektites) being derived from the 90 km-diameter Chesapeake Bay structure. Based on the nature of the shocked quartz found associated with the Spherule layer at Massignano and the age and Sr and Nd isotopic composition of the cpx Spherules, it has been suggested that the 100 km-diameter Popigai impact structure in northern Siberia may the source crater for the cpx strewn field.
-
upper eocene impact ejecta Spherule layers in marine sediments
Chemie Der Erde-geochemistry, 2002Co-Authors: B.p. GlassAbstract:Abstract Microtektites with compositions and ages similar to those of the North American tektites have been found in deep-sea deposits in the Gulf of Mexico, Caribbean Sea, northwestern Atlantic Ocean and on Barbados. Unmelted impact ejecta (including shocked quartz and feldspar with multiple sets of planar deformation features, coesite, stishovite, and reidite, a high-pressure polymorph of zircon) have been found associated with this microtektite layer. These microtektites appear to belong to the North American tektite strewn field based on their geographic location, age (∼35 Ma), and composition. Clinopyroxene-bearing Spherules (cpx Spherules) are closely associated with the North American microtektitestherefore, occurrences of cpx Spherules in the Indian Ocean and equatorial Pacific were originally interpreted to indicate a major extension of the North American strewn field. However, at a few sites it is clear that the cpx Spherules are older than the North American microtektites and it is now accepted that they belong to a different impact event. The major crystalline phase in the cpx Spherules is clinopyroxene, as their name implies, but they also contain Cr- and Ni-rich spinels. The cpx Spherules generally have lower SiO 2 and Al 2 O 3 and higher FeO, MgO, and CaO contents compared with the North American microtektites. The cpx Spherules also have high Ni, Cr, Co, and Ir contents compared to the North American microtektites and to the average upper continental crust. The cpx Spherule layer is associated with a positive Ir anomaly and with the extinction of several taxa of Radiolaria. Flattened pancake Spherules composed of clay have been found in upper Eocene deposits at Massignano, Italy, associated with a positive Ir anomaly. The pancake Spherules are believed to be diagenetically altered cpx Spherules. Shocked quartz with multiple sets of planar deformation features have been found associated with the Spherule layer at Massignano. The number of upper Eocene microtektite/Spherule layers is debated, but most authors agree that there are probably only two: the North American microtektite layer and the cpx Spherule layer. Geographic variations in abundance, age, composition, and Sr and Nd isotopic data are consistent with the North American microtektites (and tektites) being derived from the 90 km-diameter Chesapeake Bay structure. Based on the nature of the shocked quartz found associated with the Spherule layer at Massignano and the age and Sr and Nd isotopic composition of the cpx Spherules, it has been suggested that the 100 km-diameter Popigai impact structure in northern Siberia may the source crater for the cpx strewn field.
