揽瓜阁俱乐部第四期 Day6 2021.01.23
【自然科学 - 生物】 What really killed the dinosaurs?Lingering doubts about the cause of a mass extinction are put to rest (The Economist - 575 字 短精读）
One thing about the prehistoric past which almost everybody thinks they know is that the dinosaurs (those, at least, that did not belong to the group of animals now known as birds) were wiped out more or less instantaneously by a collision between Earth and a very large space rock. The crater from that collision was discovered decades ago in southern Mexico. The effects of the giant waves created by the impact can be seen in places like Hell’s Creek, near Bowman, North Dakota, where marine creatures were swept far inland. And modelling suggests the planet would have been a pretty uncomfortable place for quite some time afterwards, with ejecta suspended in the atmosphere blotting out the sun, and acid rain changing the chemical composition of the oceans.
And yet... a small group of holdouts paint a different picture. Yes, they say, Earth was indeed hit by an asteroid or comet some 66m years ago at the end of the Cretaceous period. But that was either a coincidence or the straw that broke the planet’s ecological back. For the rocks also show that a series of huge volcanic eruptions was happening at the time in what is now India. Toxic and climate-changing gases from these eruptions, they suggest, were the underlying cause of the mass extinction that did for the dinosaurs—a point of view backed up by the fact that two earlier mass extinctions, those at the ends of the Permian and Triassic periods, coincided with similar eruptions while showing no sign of an asteroid strike. Conversely, several other large bolides are known to have arrived at various times in the past without accompanying extinctions.
A paper just published in Science, by Pincelli Hull of Yale University and her colleagues, casts further light on the matter. It shows that though the Deccan Traps eruptions, as this period of volcanism is known, went on over the course of around 1m years, they did most of their atmospheric damage about 200,000 years before the dinosaurs disappeared. So it was indeed, the team conclude, the impact that caused the extinction, not the volcanoes.
Dr Hull and her associates gathered data from hundreds of sources. These included analyses of fossil leaves, soil, mollusc shells, foraminifera (a type of shell-growing amoeba) and general marine sediments. Such sources record palaeotemperatures in several different ways. One is through the ratio in shells between two isotopes of oxygen. This ratio varies with ambient temperature in a predictable manner. A second is the ratio of magnesium to calcium in foraminifera shells. A third, called tex86, relies on the composition of the membranes of a group of single-celled organisms called Archaea. And so on.
By combining all of these data the researchers show pretty conclusively that the temperature rose by about 2°C between 400,000 and 200,000 years before the mass extinction. It then fell gradually back over the subsequent 200,000 years to its previous level. They interpret this temperature spike as a consequence of global warming brought about by carbon dioxide released in the Deccan Traps eruptions. This would also have coincided with the rise and decline of other nasties, such as sulphur dioxide, which the Deccan volcanism pumped into the atmosphere. By the time of the mass extinction, in other words, it looks as though things atmospheric had returned to normal. It therefore seems unlikely that the Deccan Traps were responsible for the extinction. Rather, it was the asteroid wot done it.
Source: The Economist
【自然科学-天文】 The search for dark matter in the recent majestic universe (WSY - 498字 短精读）
WHAT KIND OF particle could dark matter be made of? Astronomical observation and theory provide some general clues. It cannot be protons, neutrons, or anything that was once made of protons or neutrons, such as massive stars that became black holes. According to calculations of particle synthesis during the big bang, such particles are simply too few in number to make up the dark matter. Those calculations have been corroborated by measurements of primordial hydrogen, helium and lithium in the universe.
Nor can more than a small fraction of the dark matter be neutrinos, a lightweight breed of particle that zips through space and is unattached to any atom. Neutrinos were once a prominent possibility for dark matter, and their role remains a matter of discussion, but experiments have found that they are probably too lightweight [see “Detecting Massive Neutrinos,” by Edward Kearns, Takaaki Kajita and Yoji Totsuka; , August 1999]. Moreover, they are “hot”—that is, in the early universe they were moving at a velocity comparable to the velocity of light. Hot particles were too fleet-footed to settle into observed cosmic structures.
The best fit to the astronomical observations involves “cold” dark matter, a term that refers to some undiscovered particle that, when it formed, moved sluggishly. Although cold dark matter has its own problems in explaining cosmic structures [see “The Life Cycle of Galaxies,” by Guinevere Kauffmann and Frank van den Bosch; , June 2002], most cosmologists consider these problems minor compared with the difficulties faced by alternative hypotheses. The current Standard Model of elementary particles contains no examples of particles that could serve as cold dark matter, but extensions of the Standard Model—developed for reasons quite separate from the needs of astronomy—offer many plausible candidates.
By far the most studied extension of this kind is supersymmetry, so I will concentrate on this theory. Supersymmetry is an attractive explanation for dark matter because it postulates a whole new family of particles—one “superpartner” for every known elementary particle. These new particles are all heavier (hence more sluggish) than known particles. Several are natural candidates for cold dark matter. The one that gets the most attention is the neutralino, which is an amalgam of the superpartners of the photon (which transmits the electromagnetic force), the boson (which transmits the so-called weak nuclear force) and perhaps other particle types. The name is somewhat unfortunate: “neutralino” sounds much like “neutrino,” and the two particles indeed share various properties, but they are otherwise quite distinct.
Although the neutralino is heavy by normal standards, it is generally thought to be the lightest supersymmetric particle. If so, it has to be stable: if a superparticle is unstable, it must decay into two lighter superparticles, and the neutralino is already the lightest. As the name implies, the neutralino has zero charge, so it is unaffected by electromagnetic forces (such as those involving light). The hypothesized mass, stability and neutrality of the neutralino satisfy all the requirements of cold dark matter.
Source: WSY
【自然科学-植物】 Death by Lightning Is Common for Tropical Trees （科学美国人- 2分23秒 精听)
先做听力再核对原文哦~
The chance that a human being like you will be struck by lightning is minuscule. But what if you’re a tall tree in the tropics?
“Lightning happens in milliseconds. We can’t predict where it’s going to be, and we generally can’t find it after it’s happened, so what a hard thing to study.”
Evan Gora, an ecologist at the University of Louisville. Now, for the first time, Gora and his colleagues were able to quantify the effects of lightning strikes in tropical forests around the world—thanks to satellite data and a network of ground sensors.
“We saw that forests that have more lightning strikes hitting per hectare per year have fewer large trees per hectare, presumably because they’re killed by lightning. More biomass turns over every year, so basically, the lightning seems to be affecting the forests and causing trees to die. And then they have less total biomass.”
In a ground survey in Panama, the researchers found that a single lightning strike typically damages more than 20 trees. And within a year, five or six of them die. The scientists combined this figure with their satellite data from around the world to estimate how many trees in tropical forests die each year due to lightning.
“We think around 830 million trees are struck by lightning, and about a quarter of those, around 200 million, are killed. So that’s a lot. And as I mentioned before, we know that it’s not just a random tree in the forest: typically, it’s the largest trees.”
The study is in the journal Global Change Biology. [Evan M. Gora et al., Pantropical geography of lightning-caused disturbance and its implications for tropical forests.
Gora says the findings reveal that that lightning is one of the most important natural factors killing trees in the tropics. How climate change will affect lightning patterns is not well-understood. But some models predict more frequent lightning as the planet continues to warm—which could mean even more carbon-storing trees would disappear.
“Tropical forests basically function as our planet’s, you know, main terrestrial air conditioner. So if we’re decreasing the carbon stored in them, that’s going to have a big effect on their ability to continue, you know, conditioning our planet and taking up all that carbon that we’re producing.”
Gora’s work is just the beginning of this research. He hopes that future technological advances and fieldwork will help scientists better understand lightning’s role in ecosystems across the planet. The findings should be illuminating.
Source: Scientific American
【笔记格式要求】 同学们任选 2 片文章精读/精听并进行笔记打卡
精读笔记格式要求： 1.总结文章中心大意 2.总结分论点或每段段落大意 3.摘抄印象深刻或者觉得优美的句子 4.总结文章中的生词 5.记录阅读时间、总结时间、总时间
精听笔记格式要求： 1.逐句听写整篇文章 2.对照原文修改听写稿，标记出错原因 3.总结文章中心大意 4.总结精听过程中的生词 5.记录听写时间、总结时间、总时间
这里也给大家三点学习小建议哦~ 精读：如遇到读不懂的复杂句，建议找出句子主干，分析句子成分，也可以尝试翻译句子来帮助理解~ 精听：建议每句不要反复纠结听，如果听 5 遍都没听出来，那就跳过，等完成后再回听总结原因，时间宝贵，不要过于执着哦~
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发表于 2021-1-22 22:25:50
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