lecture 4

　　Permian extinction(二叠纪大灭绝)

　　二叠纪大灭绝发生在大约2.5亿年前二叠纪末期，是地球历史上规模最大的一次灭绝。海洋生物中大约一半的科级单元生物都灭绝了，而种级单元的灭绝达到了90%以上。占领海洋近3亿年，曾经无比繁盛的海洋软体动物几乎灭绝殆尽。在陆地上也发生了巨大变化，大量植物、昆虫以及四足动物灭绝了，让位于发展到现代的新生物门类，生态系统彻底更新。

　　人们普遍认为，这次灭绝的原因似乎来自地球本身。在这一时期，大陆合并形成泛大陆，海岸线和浅海减少，全球变暖，海洋可能变得缺氧。这一灭绝宣告了古生代的结束和中生代的开始。

　　五次地球性集体大灭绝

　　现在公认的全球性集群大灭绝有5次，即奥陶纪、晚泥盆纪、二叠纪、三叠纪及白垩纪大灭绝。据研究，在地球上多细胞生物的六亿年进化历史中，曾经有四十亿种动植物在地球上生活过。而现在生存的生物仅仅只有几百万种，绝大多数在地球上生活过的生物都灭绝了。

　　第一次物种大灭绝，又称奥陶纪大灭绝。

　　奥陶纪(Ordovician Period，Ordovician)，地质年代名称，是古生代的第二纪，开始于距今5亿年，延续了6500万年。奥陶纪亦分早、中、晚三个世。奥陶纪是地史上海侵最广泛的时期之一。在板块内部的地台区，海水广布，表现为滨海浅海相碳酸盐岩的普遍发育，在板块边缘的活动地槽区，为较深水环境，形成厚度很大的浅海、深海碎屑沉积和火山喷发沉积。

　　奥陶纪末期曾发生过一次规模较大的冰期，其分布范围包括非洲，特别是北非、南美的阿根廷、玻利维亚以及欧洲的西班牙和法国南部等地。

　　第二次生物大灭绝，又称泥盆纪大灭绝。

　　泥盆纪(Devonian period)，地质年代名称，古生代第四纪，约开始于4.05亿年前，结束于3.5亿年前，持续约5000万年。泥盆纪分为早、中、晚3个世，地层相应的分为下、中、上3个统。泥盆纪古地理面貌较早古生代有了巨大的改变。表现为陆地面积扩大，陆相地层的发育，生物界的面貌也发生了巨大的变革。陆生植物、鱼形动物空前发展，两栖动物开始出现，无脊椎动物的成分也显著改变。

　　第三次生物大灭绝，又称二叠纪大灭绝。

　　二叠纪(Permian period)是古生代的最后一个纪，也是重要的成煤期。二叠纪分为早二叠世， 中二叠世和晚二叠世。二叠纪开始于距今约2.95亿年，延至2.5亿年，共经历了4500万年。二叠纪的地壳运动比较活跃，古板块间的相对运动加剧，世界范围内的许多地槽封闭并陆续地形成褶皱山系，古板块间逐渐拼接形成联合古大陆(泛大陆)。陆地面积的进一步扩大，海洋范围缩小，自然地理环境的变化，促进了生物界的重要演化，预示着生物发展史上一个新时期的到来。

　　距今2.5亿年前的二叠纪末期，发生了有史以来最严重的大灭绝事件，估计地球上有96%的物种灭绝，其中90%的海洋生物和70%的陆地脊椎动物灭绝。三叶虫、海蝎以及重要珊瑚类群全部消失。陆栖的单弓类群动物和许多爬行类群也灭绝了。这次大灭绝使得占领海洋近3亿年的主要生物从此衰败并消失，让位于新生物种类，生态系统也获得了一次最彻底的更新，为恐龙类等爬行类动物的进化铺平了道路。 科学界普遍认为，这一大灭绝是地球历史从古生代向中生代转折的里程碑。其他各次大灭绝所引起的海洋生物种类的下降幅度都不及其1/6，没有使生物演化进程产生如此重大的转折。

　　科学家认为，在二叠纪曾经发生海平面下降和大陆漂移，造成了最严重的物种大灭绝。所有的大陆聚集成了一个联合的古陆，富饶的海岸线急剧减少，大陆架也缩小了，生态系统受到了严重的破坏，很多物种的灭绝是因为失去了生存空间。更严重的是，当浅层的大陆架暴露出来后，原先埋藏在海底的有机质被氧化，这个过程消耗了氧气，释放出二氧化碳。大气中氧的含量减少，对生活在陆地上的动物非常不利。随着气温升高，海平面上升，又使许多陆地生物遭到灭顶之灾， 海洋里也成了缺氧地带。地层中大量沉积的富含有机质的页岩是这场灾难的证明。

　　这次大灭绝是由气候突变、沙漠范围扩大、火山爆发等一系列原因造成。

　　第四次生物大灭绝，又称三叠纪大灭绝。

　　三叠纪(Triassic period)是中生代的第一纪，爬行动物和裸子植物崛起，位于二叠纪(Permian)和侏罗纪(Jurassic)之间。

　　始于距今2.5亿年至2.03亿年，延续了约5000万年。海西运动以后，许多地槽转化为山系，陆地面积扩大，地台区产生了一些内陆盆地。新的古地理条件导致沉积相及生物界的变化。从三叠纪起，陆相沉积在世界各地，尤其在中国及亚洲其它地区都有大量分布。古气候方面，三叠纪初期继承了二叠纪末期干旱的特点;到中、晚期之后，气候向湿热过渡，由此出现了红色岩层含煤沉积、旱生性植物向湿热性植物发展的现象。植物地理区也同时发生了分异。

　　距今1.95亿年前的三叠纪末期，估计有76%的物种，其中主要是海洋生物灭绝，此次灾难并无特别明显的标志，只发现海平面下降之后又上升，出现大面积缺氧的海水。

　　第五次生物大灭绝，又称白垩纪大灭绝或恐龙大灭绝。

　　白垩纪(Cretaceous Period，Cretaceous)是中生代最后的一纪，始于距今1.37亿年，结束于距今6500万年，其间经历了7000万年。无论是无机界还是有机界在白垩纪都经历了重要变革。位于侏罗纪之下、新生界之上。白垩纪是中生代地球表面受淹没程度最大的时期，在此期间北半球广泛沉积了白垩层，1822年比利时学者J.奥马利达鲁瓦将其命名为白垩系。白垩层是一种极细而纯的粉状灰岩，是生物成因的海洋沉积，主要由一种叫做颗石藻的钙质超微化石和浮游有孔虫化石构成。

　　在五次大灭绝中，这次大灭绝事件最为著名，因长达14000万年之久的恐龙时代在此终结， 海洋中的菊石类也一同消失。其最大贡献在于消灭了地球上处于霸主地位的恐龙及其同类，并为哺乳动物及人类的最后登场提供了契机。这次灾难来自于地外空间和火山喷发，在白垩纪末期发生的一次或多次陨星雨造成了全球生态系统的崩溃。撞击使大量的气体和灰尘进入大气层，以至于阳光不能穿透，全球温度急剧下降，黑云遮蔽地球长达数年(零点几至几个百万年)之久，植物不能从阳光中获得能量，海洋中的藻类和成片的森林逐渐死亡，食物链的基础环节被破坏，大批的动物因饥饿而死，其中就是恐龙。

　　小行星撞击说

　　支持小行星撞击说的科学家们推断，这次撞击相当于人类历史上发生过最强烈地震的100万倍，爆炸的能量相当于地球上核武器总量爆炸的1万倍，导致了2.1万立方公里的物质进入大气中。由于大气中大量高密度的尘埃，太阳光不能照射到地球上，导致地球表面温度迅速降低。没有阳光，植物逐渐枯萎死亡;没有植物，植食性的恐龙饥饿而死;没有植食性动物，肉食性的恐龙失去食物来源，在绝望和相互残杀中缓慢消亡。几乎所有的大型陆生动物都未能幸免于难。小型的陆生动物，像一些哺乳动物依靠残余的食物勉强为生，终于熬过了最艰难的时日，等到了古近纪陆生脊椎动物的再次大繁荣。

　　撞击假说的支持者发现了许多有力的证据，来证明他们的观点。最有力的证据来自在K/T(白垩纪和古近纪)地质界线上发现的铱异常和冲击石英。科学家推测，这种高含量的铱元素就是撞击地球的小行星带来的，冲击石英在撞击过程中形成，但同时撞击所形成的撞击坑却未被找到，多数的陨石坑被认为其大小与推测不相符合。

　　美国人查特吉约提出了一种类似的假说。他认为在白垩纪末期撞击地球的凶手不是一颗小行星或者陨石，而是彗星雨。大量的彗星雨撞击到地球上，形成一个环绕地球一周的撞击带，其中有2块巨大的彗星体成为了恐龙大灭绝的“主犯”：一块形成了墨西哥湾附近的巨大的陨石坑，另外一块撞击到印度大陆上，形成的陨石坑比墨西哥湾附近的陨石坑还大。

　　盘点5次生物大灭绝

　　在地球的发展史上，生命从无到有，再到多样化，经历了长达数亿年的时间。为了更直观地理解地球的演化历史，法国科学家里夫形象地把46亿年的时间压缩成了一天：这一天的前1/4 的时间，地球上是一片死寂;时针指向凌晨6时，最低级的藻类开始在海洋中出现，它们持续的时间最长;一直到了20时，软体动物才开始在海洋与湖沼中活动;23时30分，恐龙出现，但只“露脸”了仅仅10分钟便匆匆离去;在这一天的最后20分钟里，哺乳动物出现，并迅速分化;23时50分，灵长类的祖先登场，在最后的2分钟里，它们的大脑扩大了3倍，成为人类。

　　地球史好似一场演出，将各类生物比作舞台上的演员，它们依次登台，演绎了一场精彩而隆重的晚会。但是，有人登场，就会有人退场。

　　Extinction events

　　The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: (1) End Ordovician (Ordovician-Silurian extinction), (2) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction), (4) End Triassic (Triassic-Jurassic extinction), and (5) End Cretaceous (Cretaceous-Tertiary extinction). (See geologic time scale for an overview of these time periods.)

　　These and a pair of other extinction events acting as "book ends" for the Big Five are highlighted below：

　　1 End Ordovician extinction (about 444 million years ago). Two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased causing the first die-off, and then, when sea levels rose rapidly between five hundred thousand to a million years later, a second great die-off occurred. On hypothesis is that a gamma ray burst may have triggered this extinction (Jha 2005).

　　2 Late Devonian extinction (about 360 million years ago). Near the Devonian-Carboniferous transition a prolonged series of extinctions led to the elimination of about 70 percent of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.

　　3 End Permian extinction (about 251 million years ago). At the Permian-Triassic transition (the Permian-Triassic extinction event) about 95 percent of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53 percent of marine families, 84 percent of marine genera, and an estimated 70 percent of land species (including plants, insects, and vertebrate animals.)

　　4 End Triassic extinction (about 200 million years ago). At the time of the Triassic-Jurassic transition, about 20 percent of all marine families as well as most non-dinosaurian archosaurs, most therapsids (except the order from which mammals descended), and the last of the large amphibians were eliminated.

　　5 End Cretaceous extinction (about 65 million years ago). At the time of the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50 percent of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet impact event, although there is not a consensus on this theory.

　　6 End Cambrian extinction (about 488 million years ago). A series of mass extinctions at the Cambrian-Ordovician transition eliminated many brachiopods and conodonts (a group of small eel-like vertebrates characterized by multiple pairs of bony toothplates) and severely reduced the number of trilobite species.

　　7 Holocene extinction (Present day). A 1998 survey by the American Museum of Natural History found that 70 percent of biologists view the present era as part of a mass extinction event, the Holocene extinction event. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.

　　Causes for mass extinction

　　Some of the hypotheses for the causes of mass extinction events are:.

　　1 Impact events. The impact of a sufficiently large asteroid or comet could create large tsunamis, global forest fires, and reduction of incoming sunlight due to large amounts of dust and smoke in the atmosphere. Taken together, it is not surprising that these and other related effects from an impact event might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinction (about 65 mya) is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Ordovician extinction (about 444 mya), End Permian extinction (about 251 mya), End Jurassic extinction (about 145 mya), and End Eocene extinction (about 40 mya).

　　2 Climate change. Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing that the recent cycles of ice ages are believed to have had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician (about 444 mya), End Permian (about 251 mya), and Late Devonian (about 360 mya).

　　3 Volcanism. The formation of large igneous provinces through the outflow of up to millions of cubic kilometers of lava in a short duration is likely to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous extinction (about 65 mya), End Permian extinction (about 251 mya), End Triassic extinction (about 200 mya), and End Jurassic extinction (about 145 mya).

　　4 Gamma ray burst. A nearby gamma ray burst (less than 6,000 light years distance) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living there. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as a possible explanation for the End Ordovician extinction (about 444 mya). However, a recent study by leading gamma ray burst researchers says that gamma ray bursts are not possible in metal rich galaxies like our own (Stanek et al. 2006).

　　5 Plate tectonics. The opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian extinction (about 251 mya).

　　Other hypotheses, such as the spread of a new disease or simple competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

　　The Big Five extinction events(五次大灭绝英文背景知识阅读)

　　Ordovician-Silurian extinction

　　The Ordovician-Silurian extinction (about 444 mya), which may have comprised several closely spaced events, was the second largest of the five major extinction events in Earth history in terms of percentage of genera that went extinct. (The only larger one was the Permian-Triassic extinction (about 251 mya).

　　The End Ordovician extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician period and the following Silurian period. During this extinction event, there were several marked changes in the isotopic ratios of the biologically responsive elements carbon and oxygen. These changes in the isotopic ratios may indicate distinct events or particular phases within one event. At that time, all complex multicellular organisms lived in the sea, and of them, about 100 marine families covering about 49 percent of genera (a more reliable estimate than species) of fauna became extinct (Rohde 2005). The bi-valve brachiopods and the tiny, colonial bryozoans were decimated, along with many of the families of trilobites, conodonts, and graptolites (small, marine colonial animals).

　　The most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic eon, which ended the long, stable greenhouse conditions typical of the Ordovician period. The event was preceded by a fall in atmospheric CO2, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. Evidence of these has been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the oceans, and the interglacials free it, causing sea levels repeatedly to drop and rise. During the glaciation, the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992).

　　The shifting in and out of glaciation stages incurred a shift in the location of bottom water formation—from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

　　The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.

　　Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

　　The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

　　Late Devonian extinction

　　The Late Devonian extinction was one of the five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes (the jawless fishes) suddenly disappeared. A second strong pulse closed the Devonian period.

　　Although it is clear that there was a massive loss of biodiversity toward the end of the Devonian, the extent of time during which these events took place is still unclear, with estimates as brief as 500 thousand years or as extended as 15 million years, the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions.

　　Anoxic conditions in the seabed of late Devonian ocean basins produced some oil shale. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the coral-like stromatoporoids, and the rugose and tabulate corals. The reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian corals) did not recover until the Mesozoic era.

　　The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: A recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not known well enough to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms (armored fishes). Freshwater species, including our tetrapod (four-legged vertebrates) ancestors, were less affected.

　　Reasons for the late Devonian extinctions are still speculative. Bolide (asteroids, meteorites) impacts could be dramatic triggers of mass extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extraterrestrial impact has been identified in this case.

　　The "greening" of the continents occurred during Devonian time: By the end of the Devonian, complex branch and root systems supported trees 30 m (98 ft) tall, and the deposits of organic matter that would become Earth's earliest coal deposits accumulated. But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The covering of the planet's continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere, and since CO2 is a greenhouse gas, reduced levels may have helped produce a chillier climate. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. Evidence, such as glacial deposits in northern Brazil (located near the Devonian South Pole), suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region. Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very end of the Devonian, it is more likely to be a result, rather than a cause of the drop in global temperatures.

　　McGhee (1996) has detected some trends that lead to his conclusion that survivors generally represent more primitive or ancestral morphologies. In other words, the conservative generalists are more likely to survive an ecological crisis than species that have evolved as specialists.

　　Permian-Triassic extinction

　　The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago, defining the boundary between the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.

　　For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10 percent of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100 percent of the available fossil record (Eshet et al. 1995). However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have only been found in shallow marine deposits (Wignall 1996). Alternatively, others argue that fungal hypha (long, branching filament) are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record (Erwin 1993).

　　At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 million years ago (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

　　Explanatory theories

　　Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.

　　Plate tectonics. At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.

　　Impact event. When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exists in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event. If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater. Others have claimed evidence of a possible impact site off the coast of present-day Australia.

　　Supernova. A supernova occurring within ten parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultraviolet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovas suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe, believed to be caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within approximately 6,000 light years would produce the same effect.

　　Volcanism. The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years. The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated whether volcanic activity, over such a long time, could alter the climate enough to kill off 95 percent of life on Earth. There is evidence for this theory though. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has been located.

　　Atmospheric hydrogen sulfide buildup. In 2005, geoscientist Dr. Lee R. Kump published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started a warming of the atmosphere. The warming itself did not seem to be large enough to cause such a massive extinction event. However, it could have interfered with the ocean flow. Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates. The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide. Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era show deformities that could have been caused by ultraviolet radiation.

　　Methane hydrate gasification. In 2002, a documentary, The Day the Earth Nearly Died, summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike—rotting vegetation—seemed insufficient. Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by the upwelling of frozen methane hydrate from the seabed. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.

　　A combination. The Permian extinction is unequaled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.

　　Triassic-Jurassic extinction

　　The Triassic-Jurassic extinction event occurred 200 million years ago and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. Twenty percent of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event opened an ecological niche allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangea started to break apart.

　　Several explanations for this event have been suggested, but all have unanswered challenges.

　　▪ Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.

　　▪ Asteroid impact. As yet, no impact crater can be dated to coincide with the Triassic-Jurassic boundary.

　　▪ Massive volcanic eruptions. Such eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide, which would cause either intense global warming (from the former) or cooling (from the latter). However, the isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO2 composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic-Jurassic boundary suggesting that there was a rise in atmospheric CO2 and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO2 outgassing and catastrophic dissociation of gas hydrates. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian era.

　　Cretaceous-Tertiary extinction

　　Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.

　　The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred about 65.5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of the Tertiary period.

　　The duration of this extinction event, like many others, is unknown. Many forms of life perished, encompassing approximately 50 percent of all plant and animal families, including the non-avian dinosaurs. Barnosky et al. (2011) and dos Reis et al. (2014) place the species lost at 76 percent. Many possible causes of the mass extinctions have been proposed. The most widely accepted current theory is that an object from space produced an impact event on Earth.

　　The extinction event is also known as the K-T extinction event and its geological signature is the KT boundary. ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C." "T" is the abbreviation for Tertiary a long-standing geological name for the period following the Cretaceous that has, in some scientific circles, been supplanted by the alternate name “Paleogene.")

　　A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation, and Scollard Formation, including six or seven families of theropods (the "lizard-hipped" dinosaurs that were also carniverous) and a similar number of Ornithischian ("bird-hipped") dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes (primitive birds) and Hesperornithiformes (toothed and perhaps flightless diving birds). The last of the pterosaurs (flying reptiles that occurred in a great range of sizes) also vanished. Mammals suffered as well, with marsupials and multituberculates (rodent-like, tree-dwelling mammals) experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. As much as 57 percent of the plant species in North America may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99 percent annihilated, yet still manage to survive.

　　Darkness from an impact-generated dust cloud (Alvarez et al. 1980), one of the main theories for the extinction, would have resulted in reduction of photosynthesis both on land and in the oceans. On land, preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains. Mammals of the Late Cretaceous are not considered to have been herbivores. Many mammals fed on insects, larvae, worms, snails and so forth, which in turn fed on dead plant matter. During the crisis when green plants would have disappeared, mammals could have survived because they lived in "detritus-based" food chains. In stream communities, few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

　　Theories

　　Impact Theory (Alvarez hypothesis). In 1980, a team of researchers, led by Nobel Prize-winning physicist Luis Alvarez, discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 million years ago, contain a concentration of iridium hundreds of times greater than normal. They suggested that the dinosaurs had been killed off by an impact event from a ten-kilometer-wide asteroid. The theory is supported by the relative abundance of iridium in many asteroids and the similarity between the isotopic composition of iridium in asteroids and K-T layers, which differs from that of terrestrial iridium. Iridium is very rare on the Earth's surface, but is found more commonly in the Earth's interior and in extraterrestrial objects such as asteroids and comets. Furthermore, chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments strongly supports the impact theory and suggests that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.

　　The blast resulting from such an impact would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, may have created a hurricane of unimaginable fury, and certainly would have thrown massive amounts of dust and vapor into the upper atmosphere and even into space. A global firestorm may have resulted as the incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30–35 percent) during the late Cretaceous. This high O2 level would have supported intense combustion. The level of atmospheric O2 plummeted in the early Tertiary (Paleogene) period.

　　In addition, the worldwide cloud would have blocked sunlight for months, decreasing photosynthesis and thus depleting food resources. This period of reduced sunlight, a "long winter," may also have been a factor in the extinctions. Gradually skies would have cleared, but greenhouse gases from the impact would be assumed to cause an increase in temperature for many years.

　　Radar topography reveals the 180 kilometer (112 mile) wide ring of the Chicxulub crater (image courtesy NASA/JPL-Caltech)

　　Although further studies of the K-T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for more than a decade. The discovery of the Chicxulub Crater in the Yucatan, as well as various types of debris in North America and Haiti, has lent credibility to this theory. Most paleontologists now agree that an asteroid did hit the Earth 65 million years ago, but many dispute whether the impact was the sole cause of the extinctions. The age of the Chicxulub crater has been revised to approximately 300,000 years before the K-T boundary. This dating is based on evidence collected in northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs some 10 meters below the K-T boundary. This finding supports the theory that one or many impacts were contributory, but not causal, to the K-T boundary mass extinction.

　　Deccan traps. Several paleontologists remained skeptical about the impact theory, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, but instead occurred gradually over about ten million years, a time frame more consistent with longer-term events such as massive volcanism. Several scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. Luis Alvarez, who died in 1988, replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely, or at least partly, due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), and these may have contributed to the extinctions.

　　Multiple impact event. Several other craters also appear to have been formed at the K-T boundary. This suggests the possibility of near-simultaneous multiple impacts from perhaps a fragmented asteroidal object, similar to the Shoemaker-Levy 9 cometary impact with Jupiter.

　　Supernova hypothesis. Another proposed cause for the K-T extinction event was cosmic radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The fallout from a supernova explosion should contain the plutonium isotope Pu-244, the longest-lived plutonium isotope (half-life 81 million years) that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially countering this hypothesis.

　　Overview of explanation. Although there is now general agreement that there was at least one huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, it is difficult to directly connect this to mass extinction, and in fact there is no clear linkage between an impact and any other incident of mass extinction, although research on other events also implicates impacts.

　　One interesting note about the K-T event is that most of the larger animals that survived were to some degree aquatic, implying that aquatic habitats may have remained more hospitable than land habitats.

　　The impact and volcanic theories can be labeled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the [[[dinosaur]]s were in decline for a period of about 10 million years before the asteroid hit. (A study by Fastovsky & Sheehan (1995) counters that there is no evidence for a slow, 10-million-year decline of dinosaurs.) Slower mechanisms are needed to explain slow extinctions. Climatic change, a change in Earth's magnetic field, and disease have all been suggested as possible slow-extinction theories. As mentioned above, extensive volcanism such as the Deccan Traps could have been a long-term event lasting millions of years, still a brief period in geological time.

　　Holocene extinction event or the "Sixth Extinction”

　　The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species during the modern Holocene epoch. The extinctions vary from mammoths to dodos, to species in the rainforest dying every year. Because some believe the rate of this extinction event is comparable to the "Big Five" mass extinctions, it is also known as the Sixth Extinction, although the actual numbers of extinct species are not yet similar to the major mass extinctions of the geologic past.

　　The Holocene epoch extends from the present day to back about 11,500 years ago. An interglacial period, the Holocene starts late in the retreat of the Pleistocene glaciers. Human civilization dates entirely to the Holocene.

　　In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene/Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction event.

　　The observed rate of extinction has risen dramatically in the last 50 years. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses.

　　The Pleistocene or Ice Age extinction

　　The Ice Age extinction event is characterized by the extinction of many large mammals weighing more than 40 kg (88 lb). In North America, around 33 of 45 genera of large mammals went extinct, in South America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in sub-Saharan Africa only 2 of 44. Only in South America and Australia did the extinction occur at family levels or higher. The two main hypotheses concerning this extinction are: (1) the animals died off due to climate change (the retreat of the polar ice cap), and (2) the animals were exterminated as a result of human activity: The "prehistoric overkill hypothesis" (Martin 1967).

　　The prehistoric overkill hypothesis is not universally applicable and is imperfectly confirmed. For instance, there are ambiguities around the timing of sudden extinctions of marsupial Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval. In North America, the culture that has been connected with the wave of extinctions is the paleo-Indian culture associated with the Clovis people, who were thought to throw spears to kill large animals. The chief opposition to the prehistoric overkill hypothesis has been that populations of humans, such as the Clovis culture, were too small to be ecologically significant.

　　An alternative to the theory of human responsibility is Tollmann's bolide theory, a more controversial hypothesis, which claims that the Holocene was initiated by an extinction event caused by bolide (asteroid or meteorite) impacts.

　　Among the major megafauna exterminated about 9,000 to 15,000 years ago were the woolly mammoth, the woolly rhinoceros, the Irish elk, the cave lion, the cave bear, and saber-toothed cats.

　　Recent extinctions

　　In more recent years, within the past 2,000 years, a large number of species have become extinct in ways more clearly linked to human dispersal or activity. Around 1500 C.E., several species became extinct in New Zealand after Polynesian settlers arrived, including ten species of Moa (giant flightless ratite birds). It is currently estimated that among the bird species of the Pacific, some 2,000 species have gone extinct since the arrival of humans (Steadman 1995). In Madagascar, starting with the arrival of humans about 2,000 years ago, nearly all of the island's megafauna became extinct, including the Aepyornism, or elephant bird (a giant flightless ratite bird); 17 of 50 species of lemur; and a giant tortoise. Starting about 500 years ago, a number of species became extinct upon human settlement of the Indian Ocean islands, including several species of giant tortoise on the Seychelles and the Macscarene islands. Notable examples of modern extinctions of mammal fauna include the Thylacine or Tasmanian tiger (Thylacinus cynocephalus); the Quagga (a zebra relative); the Dodo, the giant flightless pigeon of Mauritius; the Great Auk of islands in the north Atlantic; and the Passenger Pigeon of North America, which became extinct in 1914.

　　Human impacts

　　According to a report by the Center for Biodiversity and Conservation (1999), there is a general pattern that has emerged related to human activity in the past 50,000 years. After the emergence of modern humans, few known extinctions occur in those areas of longest human occupancy (Africa and Eurasia), and those that occur are spread out. But the migration of human beings into other areas is linked to the loss of many large vertebrate species.

　　For example, about 50,000 years ago, Indonesia lost about 50 percent of its large mammals when human beings migrated there, and the movement of human beings into Australia 60,000 to 40,000 years ago resulted in large mammals and other vertebrates disappearing. In North and South America, there was a loss of some 135 mammal species, including 70 percent of North America's large mammals, between 12,500 and 10,000 years ago, when humans migrated from Asia. The settlement of Madagascar (2,000 years ago), the West Indies (7,000 years ago), islands of the Mediterranean Sea (10,000 years ago), Hawaii (1,600 to 1,400 years ago), and New Zealand (1,200 to 800 years ago) all coincided with extinction episodes. Notably, all terrestrial vertebrates outside of Africa and Asia that weighed more than 1,000 kilograms have become extinct.

　　Among the human activities currently considered as impacting extinctions are overhunting (either directly, or indirectly by decimation of prey populations), introduction of infectious diseases (perhaps carried by associated animals such as rats or birds), increased interspecific competition, habitat destruction, and the introduction of exotic species. The destruction of large mammals could have had even wider impacts on the ecosystems of which they were part.

　　Many biologists believe that we are at this moment at the beginning of an accelerated anthropogenic mass extinction. Eldredge has stated "It is…well established that the earth is currently undergoing yet another mass extinction event…and is clear that the major agent for this current event is Homo sapiens” (Eldredge 1999). E.O. Wilson of Harvard, in The Future of Life (2002), estimates that at current rates of human destruction of the biosphere, one-half of all species will be extinct in 100 years.

　　Those who are skeptical about the current mass extinction argue that even if the current rate of extinction is comparable or higher than the rate during a great mass extinction event, as long as the current rate does not last more than a few thousand years, the overall effect will be small. There is still hope, argue some, that humanity can eventually slow the rate of extinction through proper ecological management. Current socio-political trends, others argue, indicate that this idea is overly optimistic. Many hopes are set on sustainable development.

　　Wilkes Land Crater (威尔克斯地陨石坑)

　　威尔克斯地陨石坑位于南极洲东部的威尔克斯地(Wilkes Land)，位在冰层下方1.6公里处。威尔克斯地陨石坑先后被科学家研究过两次，分别命名为：威尔克斯地异常(Wilkes Land anomaly)、威尔克斯地质量瘤(Wilkes Land mass concentration)。

　　Wilkes Land Anomaly (威尔克斯地异常)

　　在1962年，R.A. Schmidt根据地质物理学研究，首次提出在南极洲威尔克斯地的冰原下方，有个大型撞击陨石坑[1]。R.A. Schmidt更提出，该陨石坑是澳亚散落区(Australasian strewnfield)的玻璃陨石来源。在1976年，J.G. Weihaupt也支持这个撞击陨石坑理论[2]。威尔克斯地异常的冰层下方，被认为有个直径243公里的凹陷地形，最深处达848米，并有重力异常现象。在1979年，C.R. Bentley根据空气回波声学调查结果，认为该地并没有上述的陨石坑。

　　Wilkes Land mass concentration(威尔克斯地质量瘤)

　　在2006年3月，俄亥俄州立大学的拉尔夫·冯·弗雷泽(Ralph von Frese)与Laramie Potts利用NASA的重力量测及气候监控人造卫星，宣称发现一个质量瘤，直经约300公里，名为威尔克斯地质量瘤。而一个更大型的地形结构，环绕者威尔克斯地质量瘤。弗雷泽等人认为该地冰原下方有个大型陨石坑，直径达500公里。造成该撞击坑的撞击物体直径可能达到 55.60 公里。

　　由于该地形位于冰原之下，无法直接采集样本研究。该质量瘤也可能是由热柱或大规模火山爆发形成。如果该地形是由陨石撞击产生，该陨石的直径应是造成希克苏鲁伯陨石坑的陨石的四、五倍宽，希克苏鲁伯撞击事件被认为是白垩纪第三纪灭绝事件的主因。

　　由于地表上的质量瘤，会随者时间消失，冯·弗雷泽等人认为威尔克斯地质量瘤的年代，距今5亿年以内。而澳洲在1亿年前与，自冈瓦那大陆分开。冯·弗雷泽等人推测该次撞击事件，使这个地区的地壳厚度变薄，进而造成大陆的分裂。撞击事件的发生时间与规模，使它们可能与二叠纪-三叠纪灭绝事件有关联。二叠纪-三叠纪灭绝事件发生于2.5亿年前，是自多细胞生物出现以来，地质年代中规模最大的灭绝事件。板块重构二叠纪-三叠纪灭绝事件时的大陆分布发现该撞击坑的对蹠点就是西伯利亚暗色岩，弗雷泽等人在2009年发表的论文仍处争议的理论指出，形成威尔克斯地陨石坑的撞击在撞击点的对蹠点引发火山爆发。但在这之前已经有其他二叠纪-三叠纪之间的撞击事件候选地点。澳洲西北外海的贝德奥高地，也被认为是造成的二叠纪末灭绝事件的撞击陨石坑。但是，撞击事件是否与这次灭绝事件有关联，仍处在争论中。

　　Bedout High in Australia (贝德奥高地)

　　贝德奥高地(Bedout High)又译比多岛隆起，位在澳洲西北海岸外约250公里，界于甘宁盆地(Canning basin)与罗巴克盆地(Roebuck basin)之间。这个隆起的海底地形，由海底盆地环绕，直径约30公里。在70年代与80年代，当地曾先后有两次石油探勘工作

　　撞击陨石坑

　　John Gorter另根据Lagrange-1探勘工作挖出的火成岩，估计贝德奥高地的形成年代接近二叠纪末期。,加州大学圣塔巴巴拉分校的地质化学家卢安·贝克(Luann Becker)等人提出进一步的理论，宣称探勘工作挖出的岩石是角砾岩，而只有撞击事件才能形成其中的熔融结晶;贝克等人并估计这些岩石的年代为2亿5010万年前(误差值为±450万年)。新测定的年代与二叠纪-三叠纪灭绝事件的年代相当接近，贝克等人推论贝德奥高地的撞击事件造成该次灭绝事件。铬同位素也证实该岩石带有外太空物质。

　　反对意见

　　并非所有的撞击地质学家，都同意贝德奥高地是撞击事件造成的陨石坑，包含以下反对意见：

　　1. 被认为是撞击事件熔融形成的角砾岩岩石，也具有火成玄武岩的特征。被误认的结晶构造，可能是与海水接触、以及被上层沉积物积压所造成的变形。

　　2. 如此大型的陨石坑，应会有分布广泛的喷出物质。但在澳洲的二叠纪/三叠纪交界地层，没有发现撞击产生的喷出物质。

　　3. 该撞击理论的证据，在方法上有瑕疵。例如贝德奥高地并没有类似希克苏鲁伯陨石坑的环状重力图。

　　4. 根据更详细的地质物理学重新检验，贝德奥高地可能是板块运动造成的隆起地形，而非大型撞击造成的陨石坑。

　　Yucatan Crater(墨西哥尤卡坦陨石坑)

　　在墨西哥尤卡坦半岛有一个大得惊人的陨石坑，直径大约有180公里，深900米!它就是闻名于世的“奇科苏卢布”陨石坑，它埋在数百米的沉积岩下面，即使你走在上面，也不一定察觉到这是一个陨石坑。

　　这个陨石坑是以墨西哥的小镇“奇科苏卢布”命名的，小镇就在陨石坑附近。墨西哥国家石油公司(PEMEX)在50年代进行石油勘探时首次发现了这个大坑。PEMEX公司的地质学家发现那里的地心引力和磁力不规则，然后对墨西哥湾进行了更深的钻探。PEMEX公司没有发现石油，但还是在60年代和70年代进行了多次钻探，而且钻得更深。对岩芯进行检查后，地质学家意识到，他们发现了一种非同寻常的结构!

　　后来经过11年的研究，研究人员最后把尤卡坦半岛的地心引力不规则现象归因于一次碰撞事件。人们开始相信太空中一个巨大的天体曾经撞向了尤卡坦半岛，在那里留下了一个巨大的陨石坑。成千上万公里的地球外壳蒸发、熔化或者被弹射出去，形成了一个大火球，横扫整个世界。那次大碰撞相当于1亿兆吨TNT炸药爆炸，“奇科苏卢布”陨石坑因此跻身世界七大谜团。

　　人们推测，在6500万年前，一颗直径大约10公里的陨石从天而降，飞快地撞击地球，引起了巨大的海啸和全球大火，大地被淹没，森林被烧毁，烟尘遮天蔽日终年不散，植物因无法进行光合作用而枯死，动物则因得不到食物而大量灭绝。恐龙也被认为是在这场灾难中永远地告别了地球。

　　作者介绍

　　王佳Sam，教授课程TOEFL Junior听力、TOEFL听力、SSAT词汇、ISEE词汇，毕业于北京大学，语言学专业硕士研究生，专攻TOEFL/TOEFL Junior听力，及SSAT/ISEE词汇等科目教学，资深规划师，提分权威教师。具备过硬的专业知识、丰富的教学经验，能够因材施教，严格要求的同时注重方法，授课风格简洁明快，能够引发学生思考，激励进步，促进学生养成主动学习的好习惯。众多高分学员被哥伦比亚大学、南加大、纽约大学、杜克大学等著名录取。现任北京新东方北美美国高中项目教学主管，曾任北美VIP项目听力教研组长、VIP项目专员，2015财年北京学校“优秀教师”。

　　多位SSAT词汇学生考取总分2300+词汇750+高分，被Choate Rosemary Hall、St. Paul、Deerfield Academy 等排名前十的美国私立高中录取。

(编辑：Joe)