Microtektites and mass extinctions: evidence for a late devonian asteroid impact

Glass spherules, similar to microtektites, have been found near the Frasnian-Famennian boundary (F/F) (Upper Devonian) at Senzeilles, Belgium, contemporaneous with one of the largest marine mass extinctions of the Phanerozoic. These spherules exhibit a wide range of compositions and display teardrop, dumbbell, and compound morphologies analogous to microtektites. In addition, they lack crystallites, have few or no vesicles, and have a low content of volatile material. These characteristics are supportive of an impact origin. The Siljan Ring (Sweden) and Charlevoix structure (Quebec,Canada) are candidate craters of this age. The presence of microtektites near the F/F boundary supports the hypothesis that an impact caused the Upper Devonian worldwide benthic mass extinctions.     

Evidence for a major meteorite impact on the Earth 34 million years ago: implication for eocene extinctions

A deep-sea core from the Caribbean contains a layer of sediment highly enriched in meteoritic iridium. This layer underlies a layer of North American microtektites dated at 34.4 million years ago and coincides with the extinction of five major species of Radiolaria. It is suggested that a massive, chemically undifferentiated meteorite collided with the earth, producing the tektites and leading to extinctions 34 million years ago.     

Cretaceous extinctions: evidence for wildfires and search for meteoritic material

Clay samples from three Cretaceous-Tertiary boundary sites contain 0.36 to 0.58 percent graphitic carbon, mainly as fluffy aggregates of 0.1 to 0.5 micrometers-apparently a worldwide layer of soot. It may have been produced by wildfires triggered by a giant meteorite. This carbon, corresponding to a global abundance of 0.021 +/- 0.006 gram per square centimeter, could have greatly enhanced the darkening and cooling of the earth by rock dust, which has been suggested as a cause of the extinctions. The surprisingly large amount of soot (10 percent of the present biomass of the earth) implies either that much of the earth's vegetation burned down or that substantial amounts of fossil fuels were ignited also. The particle-size distribution of the soot is similar to that assumed for the smoke cloud of "nuclear winter," but the global distribution is more uniform and the amounts are much greater, suggesting that soot production by large wildfires is about 10 times more efficient that has been assumed for a nuclear winter. Thus cooling would be more pervasive and lasting. No trace of meteoritic noble gases and no meteoritic spinel were found in these carbon fractions. Accordingly, limits can be set on the mass fraction of the meteorite that escaped degassing (相似文献   

Early eocene vertebrates from baja california: evidence for intracontinental age correlations

Newly discovered fossils support a Wasatchian (early Eocene) age for the Punta Prieta vertebrate fauna of Baja California and reveal the utility of land mammal ages on a continental scale. Dispersal scenarios for late Paleocene and early Eocene vertebrates usually invoke heterochrony for similar, but geographically separated, faunas or taxa. Such heterochrony is not supported by independent geochronologies or adequate geographic samples.     

Hind limbs of eocene basilosaurus: evidence of feet in whales

New specimens of middle Eocene Basilosaurus isis from Egypt include the first functional pelvic limb and foot bones known in Cetacea. These are important in corroborating the intermediate evolutionary position of archaeocetes between generalized Paleocene land mammals that used hind limbs in locomotion and Oligocene-to- Recent whales that lack functional pelvic limbs. The foot is paraxonic, consistent with derivation from mesonychid Condylarthra. Hind limbs of Basilosaurus are interpreted as copulatory guides.     

Inositol isomers: occurrence in marine sediments

A combined gas chromatographic-mass spectrometric technique was used to identify and quantitate the occurrence of myo-, chiro-, and scyllo-inositol in marine sediments. The most abundant isomer was myo-inositol. These inositols were found in all the organic-rich sediment samples examined, and the amount of inositol decreased steadily with the age of the sample. A small fraction of the inositols occurred as hexaphosphate esters.     

Origin of whales in epicontinental remnant seas: new evidence from the early eocene of pakistan

Pakicetus inachus from the early Eocene of Pakistan is the oldest and most primitive cetacean known. The dentition of Pakicetus resembles that of carnivorous mesonychid land mammals as well as middle Eocene cetaceans. The otic region of the cranium lacks characteristic specializations of whales necessary for efficient directional hearing under water. Pakicetus occurs with a land-mammal fauna in fluvial sediments bordering epicontinental Eocene remnants of the eastern Tethys seaway. Discovery of Pakicetus strengthens earlier inferences that whales originated from terrestrial carnivorous mammals and suggests that whales made a gradual transition from land to sea in the early Eocene, spending progressively more time feeding on planktivorous fishes in shallow, highly productive seas and embayments associated with tectonic closure of eastern Tethys.     

Isoleucine epimerization for dating marine sediments: importance of analyzing monospecific foraminiferal samples

The rate of isoleucine epimerization in fossil planktonic foraminifera is strongly species-dependent. Alloisoleucine/isoleucine ratios of two species of the same age can vary by more than a factor of 2. This finding, in combination with the known temporal and spatial variability of foraminiferal assemblages, demonstrates the critical importance of basing geochronological studies of marine sediments on monospecific samples. One rapidly epimerizing species generates a calibration curve of potentially high precision for dating sediments between the ages of about 50,000 to 400,000 years.     

Seismic evidence for water deep in Earth's upper mantle

Water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone. Observations of converted seismic waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a depth of 410 kilometers, most likely explained by as much as 700 parts per million of water by weight.     

Microbial autotrophy: a primary source of organic carbon in marine sediments

The chemoautotrophic fixation of carbon dioxide by bacteria is responsible for an appreciable component of the organic carbon in a sulfide-rich marine mud. A peak of carbon dioxide fixation (at 40 centimeters subbottom) coincides with peaks in the organic carbon content, the ratio of carbon to nitrogen, and bacterial cell counts. Stimulation of fixation by thiosulfate and inhibition by anaerobic conditions implicate the chemoautotrophic sulfur bacteria as primary producers in this environment.     

Carbon dioxide exchange between air and seawater: no evidence for rate catalysis

It has been suggested that enzymatic catalysis plays a major role in regulating the mass transport of carbon dioxide from the atmosphere into the oceans. Evidence for this mechanism was not found in a series of gas exchange experiments in which the gas transfer rate coefficients for samples obtained from various natural seawaters, with and without the addition of carbonic anhydrase, were compared with those from artificial seawater. Wind-induced turbulence appears to be the major factor controlling the ocean's response to anthropogenic increases in atmospheric carbon dioxide.     

Isotopic evidence for variations in the marine calcium cycle over the Cenozoic

Significant variations in the isotopic composition of marine calcium have occurred over the last 80 million years. These variations reflect deviations in the balance between inputs of calcium to the ocean from weathering and outputs due to carbonate sedimentation, processes that are important in controlling the concentration of carbon dioxide in the atmosphere and, hence, global climate. The calcium isotopic ratio of paleo-seawater is an indicator of past changes in atmospheric carbon dioxide when coupled with determinations of paleo-pH.     

Reexamination of glucose-6-phosphatase activity in the brain in vivo: no evidence for a futile cycle

Glucose-6-phosphatase activity in the rat brain in vivo was estimated by measuring the differential loss of tritium and carbon-14 from the glucose pool labeled by a mixture of [2-3H]glucose and [U-14C]glucose. The results provide no evidence of significant dephosphorylation of glucose-6-phosphate and do not support the hypothesis of a futile cycle involving glucose-6-phosphatase activity in the brain.     

Seismic images of crust and upper mantle beneath Tibet: evidence for Eurasian plate subduction

Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300 degrees C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.     

北京城市绿地群落结构对降温增湿功能的影响

\t\t\t\t\t目的\t\t\t\t\t城市绿地具有改善环境、保护环境和美化环境的重要作用;它在提升人居生活环境、打造宜居城市方面具有不可替代的地位。为更好地了解中国新兴卫星城市的绿地状况,以南京仙林大学城为例,对绿地群落的物种组成和多样性进行了调查分析,以期为此类绿地建设提供科学参考。\t\t\t\t\t\t\t\t\t\t\t\t\t方法\t\t\t\t\t在南京仙林大学城的绿地随机设置30个20 m×20 m的样方进行分层调查;利用重要值、多度、多样性指数等对所调查的结果进行分析。\t\t\t\t\t\t\t\t\t\t\t\t\t结果\t\t\t\t\t(1)仙林大学城公共绿地共有木本植物27科36属41种,以蔷薇科、木犀科和木兰科植物居多;整体上,外来物种的运用比例偏高,乔木层尤为明显。(2)重要值结果表明香樟、东京樱花、银杏及金边冬青卫矛、石楠、海桐分别为乔木层和灌木层的优势物种;同时这些物种的多度明显高于其他物种。(3)乔木径阶结构表明：该区域以中小径级树木为主,仍有较大的发展空间。(4)群落垂直结构不高,乔木层多数个体集中在3～6 m;灌木层高度集中在0.4～1 m。(5)与其他城市绿地多样性指数比较,该区绿地多样性明显偏低。\t\t\t\t\t\t\t\t\t\t\t\t\t结论\t\t\t\t\t改善群落结构特征、提升物种多样性、加强景观特色营造是今后仙林大学城公共绿地建设应着重考虑的方面。\t\t\t\t     

Antarctic glacial history from analyses of ice-rafted deposits in marine sediments: new model and initial tests

Contrasts between the latitudinal distributions of ice-rafted debris deposited in deep-sea sediments during Pleistocene glacial and interglacial periods are predicted by a new model. The model requires the existence of a restricted zone where rates of deposition of ice-rafted debris are essentially independent of glacial-interglacial cycles. Initial tests and published results show that the concept is valid in the Southern Ocean and that it provides a new means of diagnosing major migrations of climatic zones.     

South american geochronology: radiometric time scale for middle to late tertiary mammal-bearing horizons in patagonia

Radiometric (potassium-argon) age determinations for basalts and tuffs associated with middle to late Tertiary mammal-bearing horizons in Patagonia, southern Argentina, permit refinement of boundaries and hiatuses between beds of Deseadan (early Oligocene) through Friasian (middle to late Miocene) age. At two localities beds of Deseadan age are overlain by basalts, which gave dates of 33.6 and 35.4 million years ago; 34.0 million years ago is tentatively accepted as a terminal date for known Deseadan. At several localities beds of Colhuehuapian age are underlain by basalts, which gave dates ranging from 28.8 to 24.3 million years ago; 25.0 million years is tentatively taken as a basal age for known Colhuehuapian. The paleontological hiatus between known Deseadan and known Colhuehuapian is thus in the order of 9.0 million years. Two tuffs from the Santa Cruz Formation (Santacrucian) gave ages of 21.7 and 18.5 million years. Plagioclase and biotite concentrates of an ignimbrite from the Collón Curá Formation (Friasian) gave ages ranging from 15.4 to 14.0 million years.