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Speed1 Lake Sediment Could Better Date Ancient Finds
Core samples found at the bottom of a Japanese lake could provide much more precise timelines for important archeological finds and climate-history questions.
Radiocarbon dating is the best-known and most widely used method to determine the age of organic material, such as bone or wood or plant matter. All such material contains radioactive carbon atoms, known as carbon-14, that decay at an understood and measurable rate.
Oxford University radiocarbon dating expert Christopher Ramsey and his colleagues were looking for organic material preserved for long periods of time in a still and airless environment, where radiocarbon levels would not have been affected by interactions with ocean water or groundwater.
They found what they were looking for in core samples taken from deep sediment at the bottom of Japan’s Lake Suigetsu. Ramsey, who reports on the find in Science, says the alternating layers of fossilized leaves and algae that settled to the lake-bottom offer a perfectly preserved record of more than 50,000 thousand years.
“What’s special about Lake Suigetsu is that we have an alternative dating technique that we can use there, which is essentially counting the annual layers, which are deposited in the sediment of Lake Suigetsu," Ramsey says. "And so putting that together with the radiocarbon dates, it gives us a sort of tool, if you like, for saying if we’ve got a radiocarbon measurement for a particular value, then we can now put that into an accurate, absolute date.”
Ramsey says radiocarbon as a dating tool must be anchored in time with some other technique to compare for accuracy. The only other physical record of atmospheric carbon comes from tree rings, which don’t go back nearly as far as the Japanese lake sediment. [284]
Speed2 “The step forward here is that for the first time we have a complete record that actually covers the whole last 50,000 years or so of radiocarbon in the atmosphere with known age samples," he says, "whereas before we only had that for the last 12,000 or 13,000 years.
The radiocarbon in the core-sample leaf fossils, like the carbon in tree rings, comes directly from the atmosphere. It is not subject to the chemical changes that can affect radiocarbon in ocean-floor sediment or in cave environments, where dripping groundwater and minerals form structures known as stalagmites and stalactites.
Ramsey says dating using the lake-sediment record is more accurate.
“People have tried to do this before, but they have been limited to using marine sediments and using stalagmites and stalactites in caves. And all of those have the problem that the carbon in the oceans and the stalagmites and stalactites isn’t exactly the same as that in the atmosphere. So it’s a bit more difficult to work out what’s going on with those.”
According to Ramsey, the new record will help refine the dating of organic material by centuries. It can also help clarify the chain of events that led to the advance and retreat of ice sheets during the last ice age.
“It will enable us to pinpoint much more precisely, exactly when changes take place in the environment and when we get changes in the archeological record, exactly how those relate to other changes that are taking place in the world which are recorded in things like the Greenland ice cores where we have a very good record of the climate."
Ramsey adds the improved radiocarbon reference will also provide more precise dates for when Neanderthals died out and modern humans spread across Europe.
[284]
Speed3 Citizens in Space
Citizens in Space, a project of the United States Rocket Academy, is dedicated to citizen science and citizen space exploration. Citizens in Space is a nonprofit project working with (not for) the companies developing new commercial spacecraft. Our goal is to enable ordinary people to fly in space as citizen astronauts (citizen space explorers) and to enable citizen scientists to fly experiments into space. For the first phase of our project, we have acquired an initial contract for 10 suborbital spaceflights with one of the new space transportation companies—XCOR Aerospace.
We will be making payload space on these flights available to citizen scientists. Professional researchers will be eligible, too, if they play by certain rules. We will fly these experiments free of charge, but any experiment submitted to us must be licensed as open-source hardware. We expect to fly up to 100 small experiments in our initial flight campaign. Our hope is that the experiment hardware developed through this project will be replicated widely by citizen scientists and flown many times on a wide variety of vehicles in the future. For information on the rules for submitting payloads, see the Call for Experiments.
Along with the general call for experiments, we are offering a $10,000 prize for one particularly interesting experiment in the High Altitude Astrobiology Challenge. We will also have a $5,000 reserve prize for the best experiment which does not win the High Altitude Astrobiology Challenge. [242]
Speed4 The Mysterious Case of the Missing Noble Gas
Xenon has almost vanished from Earth's atmosphere. Geoscientists think it might have disappeared in space
The evidence is in every breath of air, but answers are harder to come by. Xenon, the second heaviest of the chemically inert noble gases, has gone missing. Our atmosphere contains far less xenon, relative to the lighter noble gases, than meteorites similiar to the rocky material that formed the Earth.
The missing-xenon paradox is one of science’s great whodunits. Researchers have hypothesized that the element is lurking in glaciers, minerals or Earth’s core, among other places.
“Scientists always said the xenon is not really missing. It’s not in the atmosphere, but it’s hiding somewhere,” says inspector, sorry, professor Hans Keppler, a geophysicist at the University of Bayreuth in Germany. He and his colleague Svyatoslav Shcheka are the latest geoscientists to tackle the case, in a report published today in Nature.
Elementary, my dear Watson They went looking for answers in minerals. Magnesium silicate perovskite is the major component of Earth’s lower mantle — the layer of molten rock between the crust and the core, which accounts for half the planet’s mass. The sleuthing scientists wondered whether the missing xenon could be squirreled away in pockets in this mineral. “I was quite sure that it must be possible to stuff noble gases into perovskite,” says Keppler. “I suspected xenon may be in there.”
The researchers tried dissolving xenon and argon in perovskite at temperatures exceeding 1,600 ºC and pressures about 250 times those at sea level. Under these extreme conditions — similar to those in the lower mantle — the mineral sopped up argon yet found little room for xenon.
Those results may sound disappointing, but they gave Keppler and Shcheka an idea. What if xenon isn’t hiding at all?
More than 4 billion years ago, Earth was molten. Meteorites bombarded the planet, causing it to lose much of its primordial atmosphere. Keppler and Shcheka suggest that argon and the other noble gases hid in perovskite, but most of the xenon could not dissolve in the mineral, and disappeared into space. [343]
Speed5 “This is completely different from what everybody else is saying. They are saying the xenon is here but it's hiding somewhere. We are saying it is not here because very early in Earth’s history it had no place to hide,” says Keppler.
When Earth cooled, argon and other noble gases started seeping out of the perovskite and filling the atmosphere. Xenon, dissolved in the mineral at only trace levels, could in turn make up only trace amounts of the atmosphere.
As further support for their hypothesis, the scientists point out that the relative ratios of three noble gases — xenon, krypton and argon — in the atmosphere roughly correspond to their solubility in perovskite.
The theory may also explain why lighter isotopes of xenon are even more depleted from the atmosphere than heavier ones. “Nobody has ever been able to explain this,” says Keppler. He and Shcheka suggest that over billions of years, when xenon was seeping into space, lighter isotopes were most likely to escape. Case closed?
Ignoble gas Not so fast, says Chrystele Sanloup, a geoscientist at Pierre and Marie Curie University in Paris: “I don't think this discovery accounts for the missing xenon.” She notes that the theory does not totally explain all of the excess heavy xenon in the atmosphere, nor for additional xenon made from the radioactive decay of uranium and plutonium in rocks.
Besides, any explanation for Earth’s missing xenon should also apply to Mars, where the atmosphere also has a dearth of the noble gas. Keppler and Shcheka suggest that here, too, the ancient xenon escaped into space: the planet’s puny gravitational field prevented it from holding onto the gas. As a result, all xenon currently found on Mars is what little could dissolve in perovskite.
But Sanloup doubts that Mars has enough (if any) perovskite to explain the xenon in its atmosphere. Until the mystery of missing Martian xenon is solved, she says, the jury is still out on where Earth’s went. [328]
同一主题的另外一个网站上的一篇。有兴趣的同学可以比较一下,个人感觉贴出来的这篇更像GMAT的风格。 http://www.sciencenews.org/view/generic/id/345672/description/Depths_hold_clues_to_dearth_of_xenon_in_air
[Obstacle] The new world of DNA
A long-term effort to catalogue all the bits of the human genome that do something has released its results
http://www.economist.com/node/21562186?bclid=0&bctid=1839125904001
WHEN John Keats read George Chapman’s translation of Homer he felt, in his elevated, poetical way, like “some watcher of the skies/When a new planet swims into his ken”. So may many biologists feel when they get their hands on the first full release of analysis from a project called ENCODE—a release which includes some 30 research papers, six of them in the journal Nature, and a huge amount of well-curated data being made freely available online (yes, there’s even an app). This is biology on a scale that takes hundreds of people years of their lives, costing as much as all but the biggest telescopes used by today’s watchers of the skies. And it reveals a new world.
The revelation’s effect may be poetic in its grandeur. Its nature, though, is prosaic. It is a parts list: ENCODE stands for Encyclopedia of DNA Elements. The consortium that created it—442 members in 32 institutes around the world—has used the increasingly impressive tools available for sequencing genomes to mount a systematic analysis of 147 different types of human cell, attempting to say just what each part of the genome is doing in them. Their results confirm on a grand scale what has become clear over the decade since the Human Genome Project first produced a sequence of the three billion “letters” of which the genome is made: there is a great deal more to genomes than their genes.
When genes were first given a molecular basis, it was a fairly simple one. A gene was a piece of DNA that described a protein. When a cell had need of that protein it would cause a copy of the gene to be transcribed from DNA into RNA, a similar molecule capable of taking on more diverse forms. That RNA transcript would then be translated to make a protein. The bits of the genome which describe proteins this way have long been known to be only a fraction of the whole—a bit more than 1%—though it was accepted that some of the surrounding DNA was necessary to get the transcription machinery on and off the genes, thus turning them on or off as required. Human genes proved to be longer than might have been expected, with the RNA transcripts edited and rearranged before being translated into protein. Still, it seemed as if only a small fraction of the genome was actually doing anything, and that a lot of the rest was, or might as well be, “junk”.
Junk, schmunk Now ENCODE has shown that fully three-quarters of the genome is transcribed into RNA at some stage in at least one of the body’s different types of cell. Some transcripts are whittled down more or less immediately, but 62% of the genome can end up in the form of a transcript that looks stable. There is a sense in which these transcripts are the basic constituents of the genome—its atoms, if you like. The transcripts which are associated with genes describing proteins are just one type among many.
All this RNA has a wide variety of uses. It regulates what genes actually make protein and how much is made in all sorts of complicated ways; some transcripts are millions of times more common than others. Even ENCODE has not been able to catalogue all of this diversity, but it has made headway in clarifying what to look for.
Whereas 62% of the genome may be turned into finished transcripts in some cell or other, only about 22% of the DNA ends up in such transcripts in the typical cell. This is because of molecular switches that turn parts of the genome on and off depending on what the cell in question is up to. Such switches are as worthy of their place in the parts list as the locations of particular regions that code for proteins. They are, though, harder to find—and, it turns out, much more numerous.
That you need a profusion of such switches to get the right pattern of genes turned on and off in a given cell at a given time is obvious. But the scale of the regulatory system has taken even some of its cartographers by surprise. Ewan Birney of the European Bioinformatics Institute, who was the lead co-ordinator of ENCODE’s data-analysis team, says he was shocked when he realised that the genome’s 20,000-odd protein-coding genes are controlled by some 4m switches.
The ENCODE parts list makes available to biologists the places where RNA is transcribed; where proteins attach themselves to DNA to turn genes on or off; where the DNA is chemically altered from its normal state; where it is linked to the protein scaffolding that it is wrapped around in unusual ways; and more. This is fascinating to people interested in the question of how genomes switch from state to state—say, from the state of a stem cell, which can grow into almost anything, to that of a muscle cell, committed to an existence of contraction and expansion. It is also interesting for people who want to understand how cells go wrong.
One of the hot areas of research since the human genome was originally sequenced has been genome-wide association studies. These look at many possessors of an interesting trait, or sufferers from a disease, to see where they seem to have unusual DNA in common. Many of the places in the genome deemed relevant to disease in this way have turned out not to be actual genes. The ENCODE studies now show, though, that they often contain regulatory elements.
So, for example, a number of sites in the genome that appear relevant to Crohn’s disease—an inflammation of the digestive tract—are not associated with any known protein-making gene. But the parts list says those regions contain, or are close to, a particular kind of genetic switch turned on and off in various types of immune cell. This should help researchers focus on the specific immune-system problems that underlie the disease.
Another way in which ENCODE could have an impact on medicine is simply by showing doctors what cells of a specific type look like on a molecular level. There is a lot of hope, and hype, around the idea of “regenerative medicine” that would reprogram cells. Tim Hubbard, the director of informatics at Britain’s Sanger Institute, a factory-sized sequencing lab, says one thing ENCODE offers the world is a much better idea of what it looks like for a cell to be programmed to be a muscle cell, or a stem cell, or whatever. Thus it offers a way to check whether the genomes of artificially reprogrammed cells—which might, for example, be intended to serve as new nerve cells after a spinal injury—really are working like the genomes of the cells they seek to mimic. To be able to look at the pattern of a genome’s activity in such detail could open a door to worlds of new therapy as well as new knowledge. [1170]
剩余部分 No end in sight Keats saw the right response to such revelations as rapt, silent awe. For the ENCODErs there will be quick celebrations and a resumption of the effort. Impressive as it is, ENCODE is far from the last word. For one thing, its expertise and carefully calibrated techniques need to be spread far and wide—to be adopted and made useful by people doing clinical research. And there is more basic research to do. Only six of the 147 cell types looked at in ENCODE were studied in the amount of detail now possible. The others still await their close-up.
And then there are more questions. So far ENCODE has looked only at cells from one person for each of the cell types studied. That is a reasonable simplification; in terms of how the genome works, the difference between what’s turned on and off in a liver cell and a skin cell is far greater than the difference between how one person’s skin cells work and those of their neighbour, however genetically different the neighbour. But it will be helpful to get a sense of differences between your liver cells and your neighbour’s—especially if you are ill and he is healthy.
Most beguiling to biologists, though, is the difference between one of your liver cells and another. Spectacularly sensitive as they are, the techniques used by ENCODE and other cutting-edge research still need to take material from many cells in order to put together a picture. But this will blur subtleties—and even hide mechanisms if some cells work one way and some another. Hence a new interest in finding ways to look at what is going on in single cells, not least because that will be the way that models of how the switching systems work can most easily and thoroughly be tested. That, according to Dr Hubbard, is the thrilling frontier for labs like those that worked on ENCODE. In a decade that frontier could go as far beyond ENCODE as ENCODE has surpassed the original genome-sequencing efforts. [339] |
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