EXTINCTION OF THE DINOSAURS

　　Paleontologistshave argued for a long time that the demise of the dinosaurs was caused byclimatic alterations associated with slow changes in the positions ofcontinents and seas resulting from plate tectonics. Off and on throughout theCretaceous (the last period of the Mesozoic era, during which dinosaursflourished), large shallow seas covered extensive areas of the continents. Datafrom diverse sources, including geochemical evidence preserved in seafloorsediments, indicate that the Late Cretaceous climate was milder than today’s.The days were not too hot, nor the nights too cold. The summers were not toowarm, nor the winters too frigid. The shallow seas on the continents probablybuffered the temperature of the nearby air, keeping it relatively constant.

　　At the end ofthe Cretaceous, the geological record shows that these seaways retreated fromthe continents back into the major ocean basins. No one knows why. Over aperiod of about 100,000 years, while the seas pulled back, climates around theworld became dramatically more extreme: warmer days, cooler nights; hottersummers, colder winters. Perhaps dinosaurs could not tolerate these extremetemperature changes and became extinct.

　　If true,though, why did cold-blooded animals such as snakes, lizards, turtles, andcrocodiles survive the freezing winters and torrid summers? These animals areat the mercy of the climate to maintain a livable body temperature. It’s hardto understand why they would not be affected, whereas dinosaurs were left toocrippled to cope, especially if, as some scientists believe, dinosaurs werewarm-blooded. Critics also point out that the shallow seaways had retreatedfrom and advanced on the continents numerous times during the Mesozoic, so whydid the dinosaurs survive the climatic changes associated with the earlierfluctuations but not with this one? Although initially appealing, thehypothesis of a simple climatic change related to sea levels is insufficient toexplain all the data.

　　Dissatisfactionwith conventional explanations for dinosaur extinctions led to a surprisingobservation that, in turn, has suggested a new hypothesis. Many plants andanimals disappear abruptly from the fossil record as one moves from layers ofrock documenting the end of the Cretaceous up into rocks representing thebeginning of the Cenozoic (the era after the Mesozoic). Between the last layer ofCretaceous rock and the first layer of Cenozoic rock, there is often a thinlayer of clay. Scientists felt that they could get an idea of how long theextinctions took by determining how long it took to deposit this one centimeterof clay and they thought they could determine the time it took to deposit theclay by determining the amount of the element iridium (lr) it contained.

　　It has notbeen common at Earth’s since the very beginning of the planet’s history.Because it usually exists in a metallic state, it was preferentiallyincorporated in Earth’s core as the planet cooled and consolidated. lr is foundin high concentrations in some meteorites, in which the solar system’s originalchemical composition is preserved. Even today, microscopic meteoritescontinually bombard Earth, falling on both land and sea. By measuring how manyof these meteorites fall to Earth over a given period of time, scientists canestimate how long it might have taken to deposit the observed amount of lr inthe boundary clay. These calculations suggest that a period of about onemillion years would have been required. However, other reliable evidencesuggests that the deposition of the boundary clay could not have taken onemillion years. So the unusually high concentration of lr seems to require aspecial explanation.

　　In view ofthese facts, scientists hypothesized that a single large asteroid, about 10 to15 kilometers across, collided with Earth, and the resulting fallout createdthe boundary clay. Their calculations show that the impact kicked up a dustcloud that cut off sunlight for several months, inhibiting photosynthesis inplants; decreased surface temperatures on continents to below freezing; causedextreme episodes of acid rain; and significantly raised long-term globaltemperatures through the greenhouse effect. This disruption of food chain andclimate would have eradicated the dinosaurs and other organisms in less thanfifty years.

恐龍的滅絕

　　很長時間以來，古生物學家們認為恐龍的滅亡是與因地質構造而引起的海洋和大陸位置變遷相關的氣候變化所致。在整個白堊紀(中生代的最后的一段時間，這時恐龍正值繁盛)，廣闊的淺海覆蓋了大量的陸地。各方面的數據，包括海床沉積中的地理化學證據，都表明白堊紀后期的氣候比現在的氣候要溫和得多。白天不是很熱，夜間也不是很寒冷。夏天不是太炎熱，而冬天也不是太寒冷。大陸上的淺海可能使其附近的空氣少受影響，以保持相對穩定的溫度。

　　在白堊紀后期，地質資料表明這些淺海都從大陸退回到主要的海洋盆地內了，沒有人明白為什么。大約在100000年內，海洋收縮了，世界的氣候也隨之變得更極端：白天更熱，夜間更冷，夏天更熱，冬天更冷。恐龍或許就是因為無法忍受這種嚴峻的氣溫變化因而滅絕。

　　如果真是這樣，那么為什么冷血動物，比如蛇類、蜥蜴、烏龜和鱷魚卻能夠幸免于寒冬和酷夏呢?這些動物都是依賴于氣溫以使其身體保持適合生存的溫度。很難理解它們為什么毫不受影響，然而恐龍卻如此的無能以至于無法適應，尤其是有些科學家認為恐龍是熱血動物。批評者們也指出淺海在中生代曾有過無數次的進入大陸而又退回盆地的過程，所以為什么恐龍在前面的海洋起伏中能幸免于難，而在這一次中卻不能呢?盡管最初人們這樣認為，但是簡單的與海平面高度有關的氣候變化假設是不足以解釋所有數據的。

　　對傳統的關于恐龍滅絕解釋的不滿使得人們反過來驚奇的發現了新的理論。當人們對比白堊紀后期的巖層資料和新生代(中生代后面的一個時期)早期的資料時發現很多植物和動物都突然的消失了。在白堊紀最后的一層巖石和新生代的第一層巖石之間，常有一層很薄的粘土。科學家們感覺到他們可以通過確定這層一厘米厚的粘土層中元素銥的含量來推測其的沉積時間，進而推測大滅絕所用的時間。

　　自從地球的形成一來，銥元素在地球的便面上就不常見。因為它通常是以金屬狀態存在，并隨著地球的冷卻和固結而優先的合并到地核中了。在一些隕石中，依可能會高度富集，而這里常保存著太陽系內原始的化學組成。直到今天，小型的隕石也在連續不斷的撞擊地球，并掉落在陸地和海洋中。通過確定在一段給定時間內掉落在地球上的這種隕石的數量，科學家們就可以確定沉積隔層粘土的形成時間。這種計算表明形成這種沉積可能需要一百萬年。然而其他可靠的證據則表明沉積這層粘土可能不需要一百萬年。所以這種不正常的銥的富集可能需要一種特殊的解釋。

　　考慮到這些事實，科學家們就假設有一個較大的小行星，直徑差不多有10到15公里，曾與地球相撞，所以碰撞揚起的灰塵等就形成了這層粘土層。他們的計算表明撞擊揚起的灰塵遮擋了陽光達幾個月之久，阻止了植物的光合作用，將陸地上的氣溫降到了零點之下，導致酸雨，通過溫室效應造成了長期的、嚴重的全球升溫。這種對食物鏈和氣候的極大擾亂將可使恐龍和其他生物在不到50年的時間內絕跡。