加拿大华人论坛 加拿大生活信息Articles mimic to ielts readings

在加拿大

Lost City Pumps Life-essential Chemicals At Rates Unseen At Typical Deep Ocean Hydrothermal Vents ScienceDaily (Feb. 5, 2008) ― Hydrocarbons -- molecules critical to life -- are being generated by the simple interaction of seawater with the rocks under the Lost City hydrothermal vent field in the mid-Atlantic Ocean.Being able to produce building blocks of life makes Lost City-like vents even stronger contenders as places where life might have originated on Earth, according to Giora Proskurowski and Deborah Kelley, two authors of a paper in the Feb. 1 Science. Researchers have ruled out carbon from the biosphere as a component of the hydrocarbons in Lost City vent fluids.Hydrocarbons, molecules with various combinations of hydrogen and carbon atoms, are key to cellular life. For instance, cell walls can be built from simple hydrocarbon chains and amino acids are short hydrocarbon chains hooked up with nitrogen, oxygen or sulfur atoms."The generation of hydrocarbons was the very first step, otherwise Earth would have remained lifeless," says lead author Proskurowski, who conducted the research while earning his doctorate from the University of Washington and during post-doctoral work at Woods Hole Oceanographic Institution.Some researchers believe the first building blocks of life made their way from outer space while others hypothesize that the right ingredients were generated by geological process on Earth, perhaps at hydrothermal vent systems where seawater seeps into the seabed and picks up heat and minerals until the water is so hot it vents back into the ocean.The Lost City hydrothermal vents, discovered by Kelley and others during a National Science Foundation expedition in 2000, are formed in a very different way than the black smoker vents scientists have known about since the 1970s. Black smokers are so named because it can appear as if smoke is billowing from them. In fact the smoke is actually dark iron- and sulfur-rich minerals precipitating when scalding vent waters -- as hot as 760 F --meet the icy cold depths. The spires and mounds that form are darkly mottled mixes of sulfide minerals.In contrast, structures at the Lost City hydrothermal vent field are nearly pure carbonate, the same material as limestone in caves, and they range in color from white to cream to gray. The structures drape the cliffs at Lost City and range from the size of tiny toadstools to the 18-story column, named Poseidon, that dwarfs most known black smoker vents by at least 100 feet. The field was named Lost City in part because it is on top of a submerged mountain named Atlantis and was discovered by chance during an expedition on board the research vessel Atlantis.Water venting at Lost City is generally 200 F. The fluids do not get as hot as the black smokers because the water is not heated by magma but rather by heat released during serpentinization, a chemical reaction between seawater and mantle rock.That's also the reason for all the hydrocarbons.Naturally occurring carbon dioxide is locked in mantle rock. At Lost City, the reaction between the rock and seawater produces 10 to 100 times more hydrogen and the hydrocarbon methane than a typical black smoker system found along mid-ocean ridges, the Science co-authors found.The Lost City system forms hydrocarbons in higher concentrations and with more complexity than do typical black smoker systems on mid-ocean ridges, says Kelley, a University of Washington professor of oceanography who was the principal investigator for a 2005 National Oceanic and Atmospheric Administration's expedition that gathered the samples analyzed for the Science paper.The hydrocarbons being produced at Lost City are not formed from atmospheric carbon dioxide dissolved in seawater because none of the carbon carries the radioisotopic signature that would be present if they had been exposed to sunlight, Proskurowski says.Analysis of rock from Lost City shows that the hydrocarbons are not coming from the living biosphere. Rock in contact with seawater has a very consistent ratio of carbon dioxide to helium. But the rock at Lost City had a strikingly different ratio. It turns out that the depleted amount of carbon dioxide in the rocks roughly equals the amount of hydrocarbons being produced in the fluids, he says."The detection of these organic building blocks from a non-biological source is possible evidence in our quest to understand the origin of life on this planet and other solar bodies," Proskurowski says.Lost City is exceptional, Kelley says, because chemical reactions in the seafloor produce acetate, formate, hydrogen and alkaline fluids. All these substances may have been key to the emergence of life, according to work published recently by Michael Russell and A.J. Hall of Glasgow and William Martin of Germany. In addition, acetate and formate found in Lost City fluids may have been an important energy source for the ancestors of methanogens, microorganisms that live off the methane at places like Lost City. It's perhaps one more bit of evidence about where life may have originated, Kelley says.The Lost City hydrothermal vent field is about 2,300 miles east of Florida, on the Mid-Atlantic Ridge, at a depth of 2,600 feet. Microorganisms there thrive in alkaline vent fluids, some nearly as caustic as liquid drain cleaner. This contrasts to the previously studied black-smoker vents where organisms have adjusted to acidic water. Lost City microbes live off methane and hydrogen instead of the carbon dioxide that is the key energy source for life at black-smoker vents.Although nobody has found another field like Lost City, Kelley says she's sure others exist because there are so many other places where mantle rock has been thrust up through the seafloor, exposing it to seawater and serpentinization. It is likely that even more mantle rock was present in the oceans of early Earth, Kelley says.Journal reference: Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field. Giora Proskurowski, Marvin D. Lilley, Jeffery S. Seewald, Gretchen L. Früh-Green, Eric J. Olson, John E. Lupton, Sean P. Sylva, and Deborah S. Kelley. Science. 1 February 2008. 319: 604-607 [DOI: 10.1126/science.1151194] (in Reports)Other co-authors of the paper, "Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field," are Marvin Lilley and Erick Olson from the University of Washington, Jeffrey Seewald and Sean Sylva from Woods Hole Oceanograhic Institution, Gretchen Früh-Green from the Swiss Federal Institute of Technology and John Lupton with NOAA's Pacific Marine Environmental Laboratory.Adapted from materials provided by University of Washington

评论
2008/08/02 - 2010/06/30 Vancouver2010/07/01 - 2012/05/31 Toronto2012/06/01---------------- Montreal回复: An article quite similar to ielts readingsTipping Elements In Earth's Climate SystemScienceDaily (Feb. 7, 2008) — A number of key components of the earth's climate system could pass their 'tipping point' this century, according to new research.The term ‘tipping point’ is used to describe a critical threshold at which a small change in human activity can have large, long-term consequences for the Earth’s climate system."Society may be lulled into a false sense of security by smooth projections of global change," the researchers around Timothy Lenton from the British University of East Anglia in Norwich and Hans Joachim Schellnhuber from the Potsdam Institute for Climate Impact Research report. Global change may appear to be a slow and gradual process on human scales. However, in some regions anthropogenic forcing on the climate system could kick start abrupt and potentially irreversible changes. For these sub-systems of the Earth system the researchers introduce the term "tipping element".In this new research, lead author Prof Tim Lenton of the University of East Anglia (UEA) and colleagues at the Postdam Institute of Climate Impact Research (PIK), Carnegie Mellon University, Newcastle University and Oxford University have drawn up a shortlist of nine tipping elements relevant to current policy-making and calculated where their tipping points could lie. All of them could be tipped within the next 100 years. The nine tipping elements and a prediction of the time it would take them to undergo a major transition are:Melting of Arctic sea-ice (approx 10 years) Decay of the Greenland ice sheet (more than 300 years) Collapse of the West Antarctic ice sheet (more than 300 years) Collapse of the Atlantic thermohaline circulation (approx 100 years) Increase in the El Nino Southern Oscillation (approx 100 years) Collapse of the Indian summer monsoon (approx 1 year) Greening of the Sahara/Sahel and disruption of the West African monsoon (approx 10 years) Dieback of the Amazon rainforest (approx 50 years) Dieback of the Boreal Forest (approx 50 years) Arctic sea-ice and the Greenland Ice Sheet are regarded as the most sensitive tipping elements with the smallest uncertainty. Scientists expect ice cover to dwindle due to global warming. The West Antarctic Ice Sheet is probably less sensitive as a tipping element, but projections of its future behavior have large uncertainty. This also applies to the Amazon rainforest and Boreal forests, the El Niño phenomenon, and the West African monsoon. "These tipping elements are candidates for surprising society by exhibiting a nearby tipping point," the authors state in the article that is published in PNAS Online Early Edition. The archetypal example of a tipping element, the Atlantic thermohaline circulation, could undergo a large abrupt transition with up to ten percent probability within this century, according to the UN climate report from 2007.Given the scale of potentially dramatic impacts from tipping elements the researchers anticipate stronger mitigation. Concepts for adaptation that go beyond current incremental approaches are also necessary. In addition, "a rigorous study of potential tipping elements in human socio-economic systems would also be welcome," the researchers write. Some models suggest there are tipping points to be passed for the transition to a low carbon society.Highly sensitive tipping elements, smallest uncertaintyGreenland Ice SheetWarming over the ice sheet accelerates ice loss from outlet glaciers and lowers ice altitude at the periphery, which further increases surface temperature and ablation. The exact tipping point for disintegration of the ice sheet is unknown, since current models cannot capture the observed dynamic deglaciation processes accurately. But in a worst case scenario local warming of more than three degrees Celsius could cause the ice sheet to disappear within 300 years. This would result in a rise of sea level of up to seven meters.Arctic sea-iceAs sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation than white sea-ice so that the warming is amplified. This causes more rapid melting in summer and decreases ice formation in winter. Over the last 16 years ice cover during summer declined markedly. The critical threshold global mean warming may be between 0.5 to 2 degrees Celsius, but could already have been passed. One model shows a nonlinear transition to a potential new stable state with no arctic sea-ice during summer within a few decades.Intermediately sensitive tipping elements, large uncertaintyWest Antarctic Ice SheetRecent gravity measurements suggest that the ice sheet is losing mass. Since most of the ice sheet is grounded below sea level the intrusion of ocean water could destabilize it. The tipping point could be reached with a local warming of five to eight degrees Celsius in summer. A worst case scenario shows the ice sheet could collapse within 300 years, possibly raising sea level by as much as five meters.Boreal forestThe northern forests exhibit a complex interplay between tree physiology, permafrost and fire. A global mean warming of three to five degrees Celsius could lead to large-scale dieback of the boreal forests within 50 years. Under climate change the trees would be exposed to increasing water stress and peak summer heat and would be more vulnerable to diseases. Temperate tree species will remain excluded due to frost damage in still very cold winters.Amazon rainforestGlobal warming and deforestation will probably reduce rainfall in the region by up to 30 percent. Lengthening of the dry season, and increases in summer temperatures would make it difficult for the forest to re-establish. Models project dieback of the Amazon rainforest to occur under three to four degrees Celsius global warming within fifty years. Even land-use change alone could potentially bring forest cover to a critical threshold.El Niño Southern Oscillation (ENSO)The variability of this ocean-atmosphere mode is controlled by the layering of water of different temperatures in the Pacific Ocean and the temperature gradient across the equator. During the globally three degrees Celsius warmer early Pliocene ENSO may have been suppressed in favor of persistent El Niño or La Niña conditions. In response to a warmer stabilized climate, the most realistic models simulate increased El Niño amplitude with no clear change in frequency.Sahara/Sahel- and West African monsoonThe amount of rainfall is closely related to vegetation climate feedback and sea surface temperatures of the Atlantic Ocean. Greenhouse gas forcing is expected to increase Sahel rainfall. But a global mean warming of three to five degrees Celsius could cause a collapse of the West African monsoon. This could lead either to drying of the Sahel or to wetting due to increased inflow from the West. A third scenario shows a possible doubling of anomalously dry years by the end of the century.Indian summer monsoonThe monsoon circulation is driven by a land-to-ocean pressure gradient. Greenhouse warming tends to strengthen the monsoon since warmer air can carry more water. Air pollution and land-use that increases the reflection of sunlight tend to weaken it. The Indian summer monsoon could become erratic and in the worst case start to chaotically change between an active and a weak phase within a few years.Lowly sensitive tipping elements, intermediate uncertaintyAtlantic thermohaline circulationThe circulation of sea currents in the Atlantic Ocean is driven by seawater that flows to the North Atlantic, cools and sinks at high latitudes. If the inflow of freshwater increases, e.g. from rivers or melting glaciers, or the seawater is warmed, its density would decrease. A global mean warming of three to five degrees Celsius could push the element past the tipping point so that deep water formation stops. Under these conditions the North Atlantic current would be disrupted, sea level in the North Atlantic region would rise and the tropical rain belt would be shifted.The paper also demonstrates how, in principle, early warning systems could be established using real-time monitoring and modelling to detect the proximity of certain tipping points.Journal article: Lenton, T. M., Held, H., Kriegler, E., Hall, J. W., Lucht, W., Rahmstorf, S. and Schellnhuber, H. J. (2008). Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences, Online Early Edition. February 4, 2008.Adapted from materials provided by Potsdam Institute for Climate Impact Research, via EurekAlert!, a service of AAAS

  ·新西兰新闻 警方用DNA调查北岛乡村尸骸 当地56年前有儿童离奇失踪
·新西兰新闻 北岛消防员处置车祸现场 发现死者为自己的妻子