史前海洋存在缺氧的“死亡区”-凯发k8官网

史前海洋存在缺氧的“死亡区”
文章来源:未知 文章作者:enread 发布时间:2011-01-06 03:45 字体: [ ] 
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the oceans became oxygen-rich as they are today about 600 million years ago, during earth's late ediacaran period. before that, most scientists believed until recently, the ancient oceans were 1 oxygen-poor for the preceding(在前的) four billion years. now biogeochemists at the university of california-riverside (ucr) have found evidence that the oceans went back to being "anoxic(缺氧的) ," or oxygen-poor, around 499 million years ago, soon after the first appearance of animals on the planet.

they remained anoxic for two to four million years.

the researchers suggest that such anoxic conditions may have been commonplace over a much broader 2(间隔) of time.

"this work is important at many levels, from the steady growth of 3 oxygen in the last 600 million years, to the potential impact of oxygen level 4(波动,起伏) on early evolution and 5(多样化,变化) of life," said enriqueta barrera, program director in the national science foundation (nsf)'s division of earth sciences, which funded the research.

the researchers argue that such fluctuations in the oceans' oxygen levels are the most likely explanation for what drove the explosive diversification of life forms and rapid 6 7 that marked the cambrian period some 540 to 488 million years ago.

they report in this week's issue of the journal nature that the transition from a generally oxygen-rich ocean during the cambrian to the 8 oxygenated ocean we have today was not a simple turn of the switch, as has been widely accepted until now.

"our research shows that the ocean fluctuated between oxygenation states 499 million years ago," said paper co-author timothy lyons, a ucr biogeochemist and co-author of the paper.

"such fluctuations played a major, perhaps 9, role in shaping the early evolution of animals on the planet by driving 10 and clearing the way for new organisms to take their place."

oxygen is necessary for animal survival, but not for the many bacteria that thrive in and even demand life without oxygen.

understanding how the environment changed over the course of earth's history can give scientists clues to how life evolved and flourished during the critical, very early stages of animal evolution.

"life and the environment in which it lives are intimately linked," said benjamin gill, the first author of the paper, a biogeochemist at ucr, and currently a postdoctoral researcher at harvard university.

when the ocean's oxygenation states changed rapidly in earth's history, some organisms were not able to cope.

oceanic oxygen affects cycles of other biologically important elements such as iron, phosphorus(磷) and nitrogen.

"disruption of these cycles is another way to drive biological crises," gill said. "a switch to an oxygen-poor state of the ocean can cause major extinction of species."

the researchers are now working to find an explanation for why the oceans became oxygen-poor about 499 million years ago.

"we have the 'effect,' but not the 'cause,'" said gill.

"the oxygen-poor state persisted likely until the enhanced burial of organic matter, originally 11 from oxygen-producing 12(光合作用) , resulted in the accumulation of more oxygen in the atmosphere and ocean.

"as a kind of negative feedback, the abundant burial of organic material facilitated by anoxia(缺氧症) may have bounced the ocean to a more oxygen-rich state."

understanding past events in earth's distant history can help refine our view of changes happening on the planet now, said gill.

"today, some sections of the world's oceans are becoming oxygen-poor--the chesapeake bay (surrounded by maryland and virginia) and the so-called 'dead zone' in the 13 of mexico are just two examples," he said.

"we know the earth went through similar 14 in the past. understanding the ancient causes and consequences can provide essential clues to what the future has in store for our oceans."

the team examined the carbon, 15 and molybdenum(钼) contents of rocks they collected from localities in the united states, sweden, and australia.

combined, these analyses allowed the scientists to infer the amount of oxygen present in the ocean at the time the 16(石灰岩) and 17 were deposited.

by looking at successive rock layers, they were able to compile the biogeochemical history of the ocean.



点击收听单词发音收听单词发音  

1 bkqzs3     
adv.比较...地,相对地
参考例句:
  • the rabbit is a relatively recent introduction in australia.兔子是相对较新引入澳大利亚的物种。
  • the operation was relatively painless.手术相对来说不痛。
2 85kxy     
n.间隔,间距;幕间休息,中场休息
参考例句:
  • the interval between the two trees measures 40 feet.这两棵树的间隔是40英尺。
  • there was a long interval before he anwsered the telephone.隔了好久他才回了电话。
3 6eayr     
adj.大气的,空气的;大气层的;大气所引起的
参考例句:
  • sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面温度与大气环流是密切相关的。
  • clouds return radiant energy to the surface primarily via the atmospheric window.云主要通过大气窗区向地表辐射能量。
4 5ffd9bfff797526ec241b97cfb872d61     
波动,涨落,起伏( fluctuation的名词复数 )
参考例句:
  • he showed the price fluctuations in a statistical table. 他用统计表显示价格的波动。
  • there were so many unpredictable fluctuations on the stock exchange. 股票市场瞬息万变。
5 8scxf     
n.变化,多样化;多种经营
参考例句:
  • the seminar was to discuss diversification of agriculture. 该研讨会讨论的是农业多种经营。 来自辞典例句
  • firm diversification is increasingly achieved by the means of takeover and merger. 通过接管和兼并,厂商经营范围日益多样化。 来自辞典例句
6 ctqz7m     
adj.进化的;演化的,演变的;[生]进化论的
参考例句:
  • life has its own evolutionary process.生命有其自身的进化过程。
  • these are fascinating questions to be resolved by the evolutionary studies of plants.这些十分吸引人的问题将在研究植物进化过程中得以解决。
7 nfkzmg     
n.人员流动率,人事变动率;营业额,成交量
参考例句:
  • the store greatly reduced the prices to make a quick turnover.这家商店实行大减价以迅速周转资金。
  • our turnover actually increased last year.去年我们的营业额竟然增加了。
8 gfuzd     
adv.完全地,全部地,彻底地;充分地
参考例句:
  • the doctor asked me to breathe in,then to breathe out fully.医生让我先吸气,然后全部呼出。
  • they soon became fully integrated into the local community.他们很快就完全融入了当地人的圈子。
9 usaxg     
adj.支配的,统治的;占优势的;显性的;n.主因,要素,主要的人(或物);显性基因
参考例句:
  • the british were formerly dominant in india.英国人从前统治印度。
  • she was a dominant figure in the french film industry.她在法国电影界是个举足轻重的人物。
10 spwzp     
n.熄灭,消亡,消灭,灭绝,绝种
参考例句:
  • the plant is now in danger of extinction.这种植物现在有绝种的危险。
  • the island's way of life is doomed to extinction.这个岛上的生活方式注定要消失。
11 6cddb7353e699051a384686b6b3ff1e2     
vi.起源;由来;衍生;导出v.得到( derive的过去式和过去分词 );(从…中)得到获得;源于;(从…中)提取
参考例句:
  • many english words are derived from latin and greek. 英语很多词源出于拉丁文和希腊文。 来自《简明英汉词典》
  • he derived his enthusiasm for literature from his father. 他对文学的爱好是受他父亲的影响。 来自《简明英汉词典》
12 a2czp     
n.光合作用
参考例句:
  • in apple trees photosynthesis occurs almost exclusively in the leaves.苹果树的光合作用几乎只发生在叶内。
  • chloroplasts are the structures in which photosynthesis happens.叶绿体就是光合作用发生的地方。
13 1e0xp     
n.海湾;深渊,鸿沟;分歧,隔阂
参考例句:
  • the gulf between the two leaders cannot be bridged.两位领导人之间的鸿沟难以跨越。
  • there is a gulf between the two cities.这两座城市间有个海湾。
14 f7c7eeee199dc0ef47fe322cc223be88     
n.[意]情节;剧本;事态;脚本
参考例句:
  • further, graphite cores may be safer than non-graphite cores under some accident scenarios. 再者,根据一些事故解说,石墨堆芯可比非石墨堆芯更安全一些。 来自英汉非文学 - 环境法 - 环境法
  • again, scenarios should make it clear which modes are acceptable to users in various contexts. 同样,我们可以运用场景剧本来搞清楚在不同情境下哪些模式可被用户接受。 来自about face 3交互设计精髓
15 ps4wc     
n.硫,硫磺(=sulphur)
参考例句:
  • sulfur emissions from steel mills become acid rain.炼钢厂排放出的硫形成了酸雨。
  • burning may produce sulfur oxides.燃烧可能会产生硫氧化物。
16 11a69209c16420403e5c4e59a1a1368f     
n.石灰岩( limestone的名词复数 )
参考例句:
  • the isotopic signatures of most ancient limestones indicated the same process. 大多数古代石灰岩的同位素特征说明了同样的过程。 来自辞典例句
  • there are four principal types of limestones. 石灰岩有四种主要类型。 来自辞典例句
17 470a61efb6b07051a4ac46629a4b3e81     
n.页岩( shale的名词复数 )
参考例句:
  • sandstones are, as a rule, interbedded with shales. 砂岩通常都与页岩互层。 来自辞典例句
  • oil-yielding organic matter is contained in the shales. 在这些页岩里有产油的有机物质。 来自辞典例句
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