2010.8 阅读背景资料/原文详细版

myice

2.1.4. 海豚声音的背景资料：
Every dolphin has its own unique 'signature whistle'. Listening to these whistles is one way to identify specific dolphins and track their whereabouts.
Echolocation - the location of objects by their echoes - is a highly specialized faculty that enables dolphins to explore their environment and search out their prey in a watery world where sight is often of little use. As sound travels four and a half times faster in water than in air, the dolphin's brain must be extremely well adapted in order to make a rapid analysis of the complicated information provided by the echoes.
Although the ability to echolocate has only been proven experimentally for a few odontocete species, the anatomical evidence - the presence of the melon, nasal sacs and specialized skull structures - suggests that all dolphins have this ability.
The dolphin is able to generate sound in the form of clicks, within its nasal sacs, situated behind the melon. The frequency of this click is higher than that of the sounds used for communication and differs between species. The melon acts as a lens which focuses the sound into a narrow beam that is projected in front of the animal.
When the sound strikes an object, some of the energy of the soundwave is reflected back towards the dolphin. It would appear that the panbone in the dolphin's lower jaw receives the echo, and the fatty tissue behind it transmits the sound to the middle ear and thence to the brain. It has recently been suggested that the teeth of the dolphin, and the mandibular nerve that runs through the jawbone may transmit additional information to the dolphin's brain.
As soon as an echo is received, the dolphin generates another click. The time lapse between click and echo enables the dolphin to evaluate the distance between it and the object; the varying strength of the signal as it is received on the two sides of the dolphin's head enable it to evaluate direction. By continuously emitting clicks and receiving echoes in this way, the dolphin can track objects and home in on them.
The echolocation system of the dolphin is extremely sensitive and complex. Using only its acoustic senses, a bottlenose dolphin can discriminate between practically identical objects which differ by ten per cent or less in volume or surface area. It can do this in a noisy environment, can whistle and echolocate at the same time, and echolocate on near and distant targets simultaneously - feats which leave human sonar experts gasping.

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2.2.2. 关于地球降温的参考文章：
The geologic record tells a story in which continents removed the greenhouse gas carbon dioxide from an early atmosphere that may have been as hot as 70 degrees Celsius (158 F). At this time the Earth was mostly ocean. It was too hot to have any polar ice caps. Lowe hypothesizes that rain combined with atmospheric carbon dioxide to make carbonic acid, which weathered jutting mountains of newly formed continental crust. Carbonic acid dissociated to form hydrogen ions(离子), which found their way into the structures of weathering minerals, and bicarbonate, which was carried down rivers and streams to be deposited as limestone(石灰石) and other minerals in ocean sediments.
...... both caused further cooling, perhaps a temperature drop of 40 to 50 degrees Celsius. and the Earth's first glaciation may have occurred 2.9 billion years ago
New continents formed and weathered, again taking carbon dioxide out of the atmosphere: About 3 billion years ago, maybe 10 or 15 percent of the Earth's present area in continental crust had formed. By 2.5 billion years ago, an enormous amount of new continental crust had formed -- about 50 to 60 percent of the present area of continental crust. During this second cycle, weathering of the larger amount of rock caused even greater atmospheric cooling, spurring a profound glaciation about 2.3 to 2.4 billion years ago.
"So eventually the carbon dioxide level climbs again," Lowe says. "It may never return to its full glorious 70 degrees Centigrade level, but it probably climbed to make the Earth warm again."
Continents played key role in collapse and regeneration of Earth's early greenhouse, geologists say
If a time machine could take us back 4.6 billion years to the Earth's birth, we'd see our sun shining 20 to 25 percent less brightly than today. Without an earthly greenhouse to trap the sun's energy and warm the atmosphere, our world would be a spinning ball of ice. Life may never have evolved.
But life did evolve, so greenhouse gases must have been around to warm the Earth. Evidence from the geologic record indicates an abundance of the greenhouse gas carbon dioxide. Methane probably was present as well, but that greenhouse gas doesn't leave enough of a geologic footprint to detect with certainty. Molecular oxygen wasn't around, indicate rocks from the era, which contain iron carbonate instead of iron oxide. Stone fingerprints of flowing streams, liquid oceans and minerals formed from evaporation confirm that 3 billion years ago, Earth was warm enough for liquid water.
Now, the geologic record revealed in some of Earth's oldestrocks is telling a surprising tale of collapse of that greenhouse -- and its subsequent regeneration. But even more surprising, say the Stanford scientists who report these findings in the May 25 issue of the journal Geology, is the critical role that rocks played in the evolution of the early atmosphere.
"This is really the first time we've tried to put together a picture of how the early atmosphere, early climate and early continental evolution went hand in hand," said Donald R. Lowe, a professor of geological and environmental science who wrote the paper with Michael M. Tice, a graduate student investigating early life. NASA's Exobiology Program funded their work. "In the geologic past, climate and atmosphere were really profoundly influenced by development of continents."
The record in the rocks
To piece together geologic clues about what the early atmosphere was like and how it evolved, Lowe, a field geologist, has spent virtually every summer since 1977 in South Africa or Western Australia collecting rocks that are, literally, older than the hills. Some of the Earth's oldest rocks, they are about 3.2 to 3.5 billion years old.
"The further back you go, generally, the harder it is to find a faithful record, rocks that haven't been twisted and squeezed and metamorphosed and otherwise altered," Lowe says. "We're looking back just about as far as the sedimentary record goes."
After measuring and mapping rocks, Lowe brings samples back to Stanford to cut into sections so thin that their features can be revealed under a microscope. Collaborators participate in geochemical and isotopic analyses and computer modeling that further reveal the rocks' histories.

The geologic record tells a story in which continents removed the greenhouse gas carbon dioxide from an early atmosphere that may have been as hot as 70 degrees Celsius (158 F). At this time the Earth was mostly ocean. It was too hot to have any polar ice caps. Lowe hypothesizes that rain combined with atmospheric carbon dioxide to make carbonic acid, which weathered jutting mountains of newly formed continental crust. Carbonic acid dissociated to form hydrogen ions, which found their way into the structures of weathering minerals, and bicarbonate, which was carried down rivers and streams to be deposited as limestone and other minerals in ocean sediments.
Over time, great slabs of oceanic crust were pulled down, or subducted, into the Earth's mantle. The carbon that was locked into this crust was essentially lost, tied up for the 60 million years or so that it took the minerals to get recycled back to the surface or outgassed through volcanoes.
The hot early atmosphere probably contained methane too, Lowe says. As carbon dioxide levels fell due to weathering, at some point, levels of carbon dioxide and methane became about equal, he conjectures. This caused the methane to aerosolize into fine particles, creating a haze akin to that which today is present in the atmosphere of Saturn's moon Titan. This "Titan Effect" occurred on Earth 2.7 to 2.8 billion years ago.
The Titan Effect removed methane from the atmosphere and the haze filtered out light; both caused further cooling, perhaps a temperature drop of 40 to 50 degrees Celsius. Eventually, about 3 billion years ago, the greenhouse just collapsed, Lowe and Tice theorize, and the Earth's first glaciation may have occurred 2.9 billion years ago.
The rise after the fall
Here the rocks reveal an odd twist in the story -- eventual regeneration of the greenhouse. Recall that 3 billion years ago, Earth was essentially Waterworld. There weren't any plants or animals to affect the atmosphere. Even algae hadn't evolved yet. Primitive photosynthetic microbes were around and may have played a role in the generation of methane and minor usage of carbon dioxide.
As long as rapid continental weathering continued, carbonate was deposited on the oceanic crust and subducted into what Lowe calls "a big storage facility ... that kept most of the carbon dioxide out of the atmosphere."
But as carbon dioxide was removed from the atmosphere and incorporated into rock, weathering slowed down ú there was less carbonic acid to erode mountains and the mountains were becoming lower. But volcanoes were still spewing into the atmosphere large amounts of carbon from recycled oceanic crust.
"So eventually the carbon dioxide level climbs again," Lowe says. "It may never return to its full glorious 70 degrees Centigrade level, but it probably climbed to make the Earth warm again."
This summer, Lowe and Tice will collect samples that will allow them to determine the temperature of this time interval, about 2.6 to 2.7 billion years ago, to get a better idea of how hot Earth got.
New continents formed and weathered, again taking carbondioxide out of the atmosphere. About 3 billion years ago, maybe 10 or 15 percent of the Earth's present area in continental crust had formed. By 2.5 billion years ago, an enormous amount of new continental crust had formed -- about 50 to 60 percent of the present area of continental crust. During this second cycle, weathering of the larger amount of rock caused even greater atmospheric cooling, spurring a profound glaciation about 2.3 to 2.4 billion years ago.
Over the past few million years we have been oscillating back and forth between glacial and interglacial epochs, Lowe says. We are in an interglacial period right now. It's a transition ú and scientists are still trying to understand the magnitude of global climate change caused by humans in recent history compared to that caused by natural processes over the ages.
"We're disturbing the system at rates that greatly exceed those that have characterized climatic changes in the past," Lowe said. "Nonetheless, virtually all of the experiments, virtually all of the variations and all of the climate changes that we're trying to understand today have happened before. Nature's done most of these experiments already. If we can analyze ancient climates, atmospheric compositions and the interplay among the crust, atmosphere, life and climate in the geologic past, we can take some first steps at understanding what is happening today and likely to happen tomorrow."

myice

2.1.12. 蝙蝠和蛾的英文文献

1
Bats are active at nights. Most of them hunt for insects by sending high pitched sounds (Sound is made by vibrating things. The faster things vibrate, the higher the pitch. The rate at which vibrations are produced is, pitch).

Bats produce sounds by their mouth or nose. When the sound waves hit an object an echo comes back. The bat’s ears have a complex set of folds that help determine the position of the object. Based on the intensity of echo, a bat can know how big an object is. A smaller object will reflect less sound waves and the echo will be small. If the object is an insect, the bat can know in which direction the insect is moving. A lower pitch echo will mean that the insect is moving away and a higher pitch will mean the opposite. This mechanism is known as echolocation. Bats navigate themselves and hunt insects by echo locating objects and prey. A wide variety of insects are eaten by bats, except Tiger moths.

Tiger moths belong to the family Arctidae. They are world wide in distributio0n. Most of them are night fliers. They derive their name from their bold contrasting coloration of gold and black, resembling the stripes of a tiger. The wings are thin and elegant, having fine scales and a span of ¾ to 3 inches. The larvae of Tiger moths, the woolly bears feed on a variety of plants and accumulate toxins in their skin. Adult moths acquire these toxins. They are therefore bad tasting insects.

The Tiger moth has survived bat predation over millions of years. The moth has a special organ called Tymbal organ on its meta thorax. This organ has thin membranes which are vibrated to produce ultrasonic sounds (high pitch sounds similar to bats that humans cannot hear).The moth has also a Tympanal organ on the thorax which functions as a hearing organ.

With this apparatus, the Tiger moth is capable of hearing bat’s sounds. It evades the bat, by a series of evasive maneuvers of loops, spirals and dives. It produces high frequency click sounds or squeaks. These sound waves perceived by the bat as multiple echoes, leave the bat confused and unable to locate or target the moth.

It was earlier believed that the Tiger moth jams the sonar system of bats by its ultrasounds. But experiments have revealed that the Tiger moth is more intent on making its presence felt by its sounds and warn the predator of its toxins and bad taste. It is more a chemical message to bats to seek their dinner somewhere else.

The aerial battles of Tiger moths and bats continue to baffle scientists.

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2
Moths Mimic Sounds to Survive; Science Daily (May 30, 2007)
(This research is to be published in the May 29 issue of the Proceedings of the National Academy of Sciences.)

The research was conducted by Jesse Barber, a doctoral student in biology at Wake Forest. William E. Conner, professor of biology at Wake Forest, co-authored the study.

This is the first study to definitively show how an animal species uses acoustic mimicry as a defensive strategy.

The research was conducted by Jesse Barber, a doctoral student in biology at Wake Forest. William E. Conner, professor of biology at Wake Forest, co-authored the study.

In response to the sonar that bats use to locate prey, the tiger moths make ultrasonic clicks of their own. They broadcast the clicks from a paired set of structures called “tymbals.” Many species of tiger moth use the tymbals to make specific sounds that warn the bat of their bad taste. Other species make sounds that closely mimic those high-frequency sounds.

“We found that the bats do not eat the good-tasting moths that make the similar sounds,” said Barber, who has worked on this research for four years.

In the study, other types of moths that were similar in size to the sound-emitting moths, but did not make sounds, were gobbled up by the bats.
The researcher trained free-flying bats to hunt moths in view of two high-speed infrared video cameras to record predator-prey interactions that occur in fractions of a second. He also recorded the sounds emitted from each moth, as well as the sounds made by the bats.

All the bats quickly learned to avoid the noxious moths first offered to them, associating the warning sounds with bad taste. They then avoided a second sound-producing species even though it was not chemically protected. This is similar to the way birds avoid butterflies that look like the bad-tasting Monarch.

The two species of bats used were big brown bats and red bats. Barber raised the bats in the lab so behavior learned in the wild would not influence the results of the experiment.

Barber said anecdotal observations have suggested that animals such as snakes, owls and bees use acoustic mimicry. This study takes the next step and provides the definitive experimental evidence for how mimicking sounds helps an animal survive.

myice

6篇极像原文的文章 感谢gitarrelieber

1.1.3. 《灰色经济》的英语原版出处：
The Economist (@ 18 June 2004)

If so, then depending on your local laws you may have been participating in what economists call the "informal" or "grey" economy. In essence, the grey economy consists of legal activities whose participants fail to pay tax or comply with regulations. The informal (or "underground" or "parallel" economy) is often taken to mean something broader, including illegal activities such as prostitution and drug dealing as well, although there is no agreed strict definition.

The grey economy is often thought of as something found at the margins of poor countries, such as a hawker stand in Thailand or a roadside vendor in Ghana. But that is misleading. Although it represents a greater share of total output in poor countries, it exists in rich and poor places alike. Recent research suggests that the grey economy is growing. Moreover, a new study suggests that it may be slowing the overall economic growth of developing countries.

By its very nature, the informal economy's size in any country is hard to observe. In a paper published a couple of years ago ("Size and Measurement of the Informal Economy in 110Countries Around the World," World Bank Working Paper, July 2002), Friedrich Schneider, of the Johannes Kepler University of Linz, exhaustively examined the ways of estimating it. There are two basic approaches. The first is direct: you could ask people whether they dodge taxes, or look at the results of spot tax-audits. However, people are unlikely to confess to breaking the law, and tax inspectors do not usually check on a random sample of the population. So the second method, indirect detective-work, is better. For example, you might compare data on cash transactions or electricity consumption with official output figures. If the use of cash or electricity is growing much faster than the measured economy, this might indicate that the informal share of total activity is rising.

Using such techniques, Mr. Schneider estimated that the informal economy in developing countries in 2000 was equivalent to 41% of their official GDP. In Zimbabwe, the figure was 60%. In Brazil and Turkey, around half of non-farm workers are in the informal sector. In OECD countries the share of the informal economy was lower, but far from negligible, at 18%.

There is little mystery about why the informal economy exists. There are a lot of advantages to operating in the shadows. For a start, there are no income taxes to pay. Avoiding social-security charges, which often drive a chunky wedge between take-home pay and employers' wage bills, can both cut labour costs and thicken wage packets. People can also save a fair bit by ignoring safety, environmental and health rules, not to mention intellectual property rights.

Indeed, in cross-country comparisons, the more expensive and more complicated are taxes and regulations, the bigger is the informal economy as a share of GDP. That explains why, among rich countries, Spain, Greece, Italy and Belgium have some of the largest grey economies and why America, Canada and Switzerland have much smaller ones. In recent years, the growth in the grey market in some poor countries may owe a lot to the International Monetary Fund's austerity programs, which increase taxes and thus encourage many entrepreneurs to opt out.

A booming grey economy sounds like good news, if only because many of the officially jobless are in fact earning a living. So if the poorest are winning, who loses? The entire economy does, according to a new study by Diana Farrell of the McKinsey Global Institute. The price for having a large grey economy can be much lower productivity. Grey firms tend to be small and want to stay that way lest they come to the attention of the authorities. However, their small scale limits their ability to make the most of new technology and business practices.

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2.1.1. 《微眼动》的英语原版出处：
Windows on the Mind (Scientific American Magazine @ August 2007)

And yet only recently have researchers come to appreciate the profound importance of such “fixational” eye movements. For five decades, a debate has raged about whether the largest of these involuntary movements, the so-called microsaccades, serve any purpose at all. Some scientists have opined that microsaccades might even impair eyesight by blurring it. But recent work has made the strongest case yet that the seminuscule ocular meanderings separate vision from blindness when a person looks out at a stationary world.

Indeed, animal nervous systems have evolved to detect changes in the environment, because spotting differences promotes survival. Motion in the visual field may indicate that a predator is approaching or that prey is escaping. Such changes prompt visual neurons to respond with electrochemical impulses. Unchanging objects do not generally pose a threat, so animal brains – and visual systems – did not evolve to notice them. Frogs are an extreme case. A fly sitting still on the wall is invisible to a frog, as are all static objects. But once the fly is aloft, the frog will immediately detect it and capture it with its tongue.

Frogs cannot see unmoving objects because, as Helmholtz hypothesized, an unchanging stimulus leads to neural adaptation, in which visual neurons adjust their output such that they gradually stop responding. Neural adaptation saves energy but also limits sensory perception. Human visual system does much better than a frog’s at detecting unmoving objects, because human eyes create their own motion. Fixational eye movements shift the entire visual scene across the retina, prodding visual neurons into action and counteracting neural adaptation. They thus prevent stationary objects from fading away.

The results of these experiments, published in 2000 and 2002, showed that microsaccades increased the rate of neural impulses generated by both LGN and visual cortex neurons by ushering stationary stimuli, such as the bar of light, in and out of a neuron’s receptive field, the region of visual space that activates it. This finding bolstered the case that microsaccades have an important role in preventing visual fading and maintaining a visible image. And assuming such a role for microsaccades, our neuronal studies of microsaccades also began to crack the visual system’s code for visibility. In our monkey studies we found that microsaccades were more closely associated with rapid bursts of spikes than single spikes from brain neurons, suggesting that bursts of spikes are a signal in the brain that something is visible.

In our experiments, we asked volunteers to perform a version of Troxler’s fading task. Our subjects were to fixate on a small spot while pressing or releasing a button to indicate whether they could see a static peripheral target. The target would vanish and then reappear as each subject naturally fixated more – and then less – at specific times during the course of the experiment. During the task, we measured each person’s fixational eye movements with a high-precision video system.

As we had predicted, the subjects’ microsaccades became sparser, smaller and slower just before the target vanished, indicating that a lack of microsaccades– leads to adaptation and fading. Also consistent with our hypothesis, microsaccades became more numerous, larger and faster right before the peripheral target reappeared. These results, published in 2006, demonstrated for the first time that microsaccades engender visibility when subjects try to fix their gaze on an image and that bigger and faster microsaccades work best for this purpose. And because the eyes are fixating – resting between the larger, voluntary saccades – in the vast majority of the time, microsaccades are critical for most visual perception.

myice

2.3.1 《金星氢元素逃逸》 英语原版出处：
Global Climate Change on Venus (New Light on the Solar System; Special Editions)
Author: Mark A. Bullock and David H. Grinspoon

THE STUNNING DIFFERENCES between the climates of Earth and Venus today are intimately linked to the history of water on these two worlds. Liquid water is the intermediary in reactions of carbon dioxide and surface rocks that can form minerals. In addition, water mixed into the underlying mantle is probably responsible for the low-viscosity layer, orasthenosphere, on which Earth’s lithospheric plates slide. The formation of carbonate minerals and their subsequent descent on tectonic plates prevent carbon dioxide from building up. Models of planet formation predict that the two worlds should have been endowed with roughly equal amounts of water, delivered by the impact of icy bodies from the outer solar system. But, when the Pioneer Venus mission went into orbit in 1978, it measured the ratio of deuterium to ordinary hydrogen within the water of Venus’s clouds. The ratio was an astonishing 150times the terrestrial value. The most likely explanation is that Venus once had far more water and lost it. When water vapor drifted into the upper atmosphere, solar ultraviolet radiation decomposed it into oxygen and either hydrogen or deuterium. Because hydrogen, being lighter, escapes to space more easily, the relative amount of deuterium increased. Why did this process occur on Venus but not on Earth? In 1969 Andrew P. Ingersoll of the California Institute of Technology showed that if the solar energy available to a planet were strong enough, any water at the surface would rapidly evaporate. The added water vapor would further heat the atmosphere and set up what he called the runaway greenhouse effect. The process would transport the bulk of the planet’s water into the upper atmosphere, where it would ultimately be decomposed and lost. Later James F. Kasting of Pennsylvania State University and his co-workers developed a more detailed model of this effect. They estimated that the critical solar flux required to initiate a runaway greenhouse was about 40 percent larger than the present flux on Earth. This value corresponds roughly to the solar flux expected at the orbit of Venus shortly after it was formed, when the sun was 30 percent fainter. An Earth ocean’s worth of water could have fled Venus in the first 30 million years of its existence. A shortcoming of this model is that if Venus had a thick carbon dioxide atmosphere early on, as it does now, it would have retained much of its water. The amount of water that is lost depends on how much of it can rise high enough to be decomposed—which is less for a planet with a thick atmosphere. Furthermore, any clouds that developed during the process would have reflected sunlight back into space and shut off the runaway greenhouse. So Kasting’s group also considered a solar flux slightly below the critical value. In this scenario, Venus had hot oceans and a humidstratosphere. The seas kept levels of carbon dioxide low by dissolving the gas and promoting carbonate formation. With lubrication from water in theasthenosphere, plate tectonics might have operated. In short, Venus possessed climate-stabilizing mechanisms similar to those on Earth today. But the atmosphere’s lower density could not prevent water from diffusing to high altitudes. Over 600 million years, an ocean’s worth of water vanished. Any plate tectonics shut down, leaving volcanism and heat conduction as the interior’s ways to cool. Thereafter carbon dioxide accumulated in the air.

This picture, termed the moist greenhouse, illustrates the intricate interaction of solar, climate and geologic change. Atmospheric and surface processes can preserve the status quo, or they can conspire in their own destruction. If the theory is right, Venus once had oceans—perhaps even life, although it may be impossible to know.

myice

2.1.5. 《八哥学说话》英语原版出处：
Social influences on vocal development
Author: Charles T. Snowdon, Martine Hausberger

The vocal talent of starlings has been known since antiquity, when Pliny considered their ability to mimic human speech noteworthy. Ornithologists know that this species possesses a rich repertoire of call and songs, composed of whistles, clicks, snarls, and screeches. In addition, starlings are well known for their ability to mimic the sounds of other animals or even mechanical noises. Descriptions of starling song in the past reflect the difficulty of describing all the variety of sounds included. Witherby mentioned a “lively rambling melody of throaty warbling, chiring, clicking and gurgling notes interspersed with musical whistles and pervaded by a peculiar creaking quality.”
This complexity explains why detailed studies of starling song have delayed long after the arrival of the sound spectrograph. As mentioned by West & King, “the problem with starlings is that they vocalized too much, too often and in too great numbers, sometimes in choruses numbering in the thousands. Even the seemingly elementary step of creating an accurate catalogue of the vocal repertoire of wild starlings is an intimidating task because of the variety of their sounds.”

Chaiken have compared the sons of young males raised in different social conditions: either with a wild-caught adult song tutor, individually housed but tape-tutored by a tape-recording or raised in total isolation. All birds had been taken from the nest at an early age (8-10 days) and were hand raised. Untutored birds produced mostly an abnormal song, where even the basic organization of song was missing. In contrast, both tape- and live-tutored birds developed songs with a normal basic organization, but with some syntactical abnormalities for the tape-tutored birds. Tape-tutored birds had repertoires half as large as those of live-tutored birds. Large differences occurred between both groups of birds in their …

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2.2.1. 《火山熔岩》
The Origin of the Land under the Sea (Scientific American Magazine @ February 2009)
Author: Peter B. Kelemen

Knowledge of the intense heat and pressure in the mantle led researchers to hypothesize in the late 1960s that ocean crust originates as tiny amounts of liquid rock known as melt—almost as though the solid rocks were “sweating.” Even a minuscule release of pressure (because of material rising from its original position) causes melt to form in microscopic pores deep within the mantle rock. Explaining how the rock sweat gets to the surface was more difficult. Melt is less dense than the mantle rocks in which it forms, so it will constantly try to migrate upward, toward regions of lower pressure. But what laboratory experiments revealed about the chemical composition of melt did not seem to match up with the composition of rock samples collected from the mid-ocean ridges, where eruptedmelt hardens. Using specialized equipment to heat and squeeze crystals from mantle rocks in the laboratory, investigators learned that the chemical composition of melt in the mantle varies depending on the depth at which it forms; the composition is controlled by an exchange of atoms between the melt and the minerals that makeup the solid rock it passes through. The experiments revealed that as melt rises, it dissolves one kind of mineral, orthopyroxene, and precipitates, or leaves behind, another mineral, olivine. Researchers could thus infer that the higher in the mantle melt formed, the moreorthopyroxene it would dissolve, and the more olivine it would leave behind.(melt上升时， 溶解Ort产生Oli, 所以melthigher， 溶解的Ort越多，产生的/留在身后的Oli也越多) Comparing these experimental findings with lava samples from the mid-ocean ridges revealed that almost all of them have the composition of melts that formed at depths greater than 45kilometers. This conclusion spurred a lively debate about how meltis able to rise through tens of kilometers of overlying rock while preserving the composition appropriate for a greater depth. If melt rose slowly in smallpores in the rock, as researchers suspected, it would be logical to assume that all melts would reflect the composition of the fashallowest part of the mantle,at 10 kilometers or less. Yet the composition of most mid-ocean ridge lavas amples suggests their source melt migrated through the uppermost 45 kilometers of the mantle without dissolving any orthopyroxene from the surrounding rock. But how? （疑大概为狗狗第一段的背景内容）

In the early 1970s scientists proposed an answer: the melt must make the last leg of its upward journey along enormous cracks. Open cracks would allow the melt to rise so rapidly that it would not have time to interact with the surrounding rock, nor would melt in the core of the crack ever touch the sides. Although open cracks are not a natural feature of the upper mantle— the pressure is simply too great—some investigators suggested that the buoyant force of migrating melt might sometimes be enough to fracture the solid rock above, like an icebreaker ship forcing its way through polar pack ice. Adolphe Nicolas of the University of Montpellier in France and his colleagues discovered tantalizing evidence for such cracks while examining unusual rock formations called ophiolites. Typically, when oceanic crust gets old and cold, it becomes so dense that it sinks back into the mantle along deep trenches known as subduction zones, such as those that encircle the Pacific Ocean. Ophiolites, on the other hand, are thick sections of old seafloor and adjacent, underlying mantle that are thrust up onto continents when two of the planet’s tectonic plates collide. A famous example, located in the Sultanate of Oman, was exposed during the ongoing collision of the Arabian and Eurasian plates. In this and other ophiolites, Nicolas’s team found unusual, light-colored veins called dikes, which they interpreted as cracks in which melt had crystallized before reaching the seafloor. The problem with this interpretation was that the dikes are filled with rock that crystallized from a melt that formed in the uppermost reaches of the mantle, not below 45 kilometers, where most mid-ocean ridge lavas originate. In addition, the icebreaker scenario may not work well for the melting region under mid-ocean ridges: below about 10 kilometers, the hot mantle tends to flow like caramel left too long in the sun, rather than cracking easily.

To explain the ongoing mystery, I began working on an alternative hypothesis for lava transport in the melting region. In my dissertation in the late 1980s, I developed a chemical theory proposing that as rising melt dissolves orthopyroxene crystals, it precipitates a smaller amount of olivine, so that the net result is a greater volume of melt. Our calculations revealed how this dissolution process gradually enlarges the open spaces at the edges of solid crystals, creating larger pores and carving a more favorable pathway through which melt can flow. As the pores grow, they connect to form elongate channels. In turn, similar feedbacks drive the coalescence of several small tributaries to form larger channels. Indeed, our numerical models suggested that more than90 percent of the melt is concentrated into less than 10 percent of the available area. That means millions of microscopic threads of flowing melt may eventually feed into only a few dozen, high porosity channels 100 meters or more wide. Even in the widest channels, many crystals of the original mantle rock remain intact, congesting the channels and inhibiting movement of the fluid. That is why melt flows slowly, at only a few centimeters a year. Over time, however, so much melt passes through the channels that all the soluble orthopyroxene crystals dissolve away, leaving only crystals of olivine and other minerals that the melt is unable to dissolve. As a result, the composition of the melt within such channels can no longer adjust to decreasing pressure and instead records the depth at which it last “saw” an orthopyroxene crystal. One of the most important implications of this process, called focused porous flow, is that only the melt at the edges of channels dissolves orthopyroxene from the surrounding rock; melt within the inner part of the conduit can rise unadulterated.

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2.1.4. 《研究地质时间》
Scientific American @ Jan 1990
The Yugoslav astronomer Milutin Milankovitch refined and formalized the hypothesis in the 1920’s and 1930’s.The astronomical pacemaker he advocated has three components, two that change the intensity of the seasons and a third that affects the interaction between the two driving factors. The first is the tilt of the earth’s spin axis. Currently about 23.5 degrees from the vertical, it fluctuates from 21.5 degrees to 24.5 degrees and back every 41,000 years. The greater the tilt is, the more intense seasons in both hemispheres become: summers get hotter and winter colder.

The second, weaker factor controlling seasonality is the shape of the earth’s orbit. Over a period of 100,000 years, the orbit stretches into a more eccentric ellipse and then grows more nearly circular again. As the orbital eccentricity increases, the difference in the earth’s distance from the sun at the orbit’s nearest and farthest points grows, intensifying the seasons in one hemisphere and moderating them in the other. (At present the earth reaches its farthest point during the Southern Hemisphere winter; as a result, southern winters are a little colder – than their northern counterparts.)

A third astronomical fluctuation governs the interplay between the tilt and eccentricity effects. It is the precession, or wobble, of the earth’s spin axis, which traces out a complete circle on the background of stars about every 23,000 years. The precession determines whether summer in a given hemisphere falls at near or a far point in the orbit– in other words, whether tilt seasonality is enhanced or weakened by distance sesonablity. When these two controllers of seasonality reinforce each other in one hemisphere, they oppose each other in the opposite hemisphere.

Milankovitch calculated that these three factors work together to vary the amount of sunshine reaching the high northern latitudes in summer over a range of some 20 percent – enough, he argued, to allow the great ice sheets that advanced across the northern continents to grow during intervals of cool summers and mild winters. For many years, however, the lack of an independent record of ice-age timing made the hypothesis untestable.

In the early 1950’s Cesare Emiliani produced the first complete record of the waxings and waning of past glaciations. It came from a seemingly odd place, the sea floor. Single-cell marine organisms called foraminifera house themselves in shells made of calcium carbonate. When the foraminifera die, sink to the bottom and contribute to these a-floor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen (oxygen 18) to ordinary oxygen (oxygen 16) in the carbonate preserves the ratio of the two oxygen atoms in the water molecules.

It is now understood that the ratio of oxygen isotopes in seawater closely tracks the proportion of the world’s water that is locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation a tiny bit more readily than molecules containing the lighter isotope. Hence, as water evaporated from warm oceans moves away from the source, its oxygen 18 preferentially returns to the oceans in precipitation. What ultimately falls as snow on ice sheets and mountain glaciers is relatively depleted of oxygen 18. As the oxygen 18-poorice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen 18 becomes in seawater – and hence in the sediments.

Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the cycles Milankovitch had predicted. A chronology for the combined record showed in 1976 that the record contains the very same periodicities as the orbital process.

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Others have found that during the last ice age the earth’s mountain glaciers also expanded. The evidence – from the heaps of debris plowed up by the glaciers, knows as moraines – is as clear in the tropics and the southern temperate latitudes. On all the mountains studied so far, regardless of geographic setting or precipitation rate, the snow line descended by about one kilometer, corresponding to a drop in temperature of about five degrees Celsius.

Where organic material was trapped in the moraines, radio carbon dating shows that the glaciers advanced and retreated on the same schedule. They fluctuated near their maximum extent between about 19,500 and 14,000 years ago, about the same time as the glaciations of northern ice sheets began to shrink, the mountain glaciers underwent a dramatic retreat that sharply reduced their size by about 12,500 years ago.

How could changes in summer sunshine at the latitude of Iceland have caused glaciers to grow and retreat in New Zealand and the southern Andes? If orbital cycles do indeed drive glacial cycles by acting directly on northern ice sheets, the response to seasonality changes in the high northern latitudes must be strong enough to override the effects of the very different changes in the Southern Hemisphere. One possibility is that the northern ice sheets themselves translate Northern Hemisphere seasonality into climatic change around the world.