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Ross Mitchell等-NC:雪球地球时期生命存续之谜新解
2021-07-07 | 作者: | 【 】【打印】【关闭

  雪球地球是地球经历过的最极端的气候事件。当今冰盖只出在位于高度的大比如格陵兰岛和南极洲,与之相反,雪球地球名思,意味着整个地球都被厚冰包裹住了。    

  加州理工学院的Joseph Kirschvink和哈佛大学的Paul Hoffman及其团队最初提出雪球地球假说时,他的极端气候事件引了很大的争Gabrielle Walker撰写的一本引人入书详细90年代末到2000年代初那些地学家之的精彩辩论     

  在雪球地球得广泛可的今天,人很容易忘早期的分歧——但有的科学家从不忘。一项刚发表在Nature Communications上的新发现终于拔除了迄今存在于雪球地球假里的两根“肉中刺”。     

  “雪球地球假的最大挑之一就是,生命似乎幸存了下来。”本文作者之一,南安普大学的地学家Thomas Gernon     

  “所以要么雪球地球从来没有生,要么生命以某种方式逃了极端冰川作用来的灾。”Gernon道。     

  另外一个雪球地球的挑来自于岩石记录,在那个期沉的地似乎有太多的相在雪球地球期,冰封的海洋会完全和大气绝联系。没有了正常的海洋和空气之的交流,多在正常气候下会生的沉在雪球地球期应该    

  被称作雪球地球假的‘沉’。 中科院地与地球物理研究所的Ross Mitchell研究员,他也是本文的第一作者。     

  些岩层记录里的旋回看上去非常像是冰盖前和后退的气候旋回。”Mitchell道。雪球地球预测整个海洋都会被埋藏在冰下,而岩层记录里的化性被认为与之相悖。 

  学家决定检验岩里旋回的真正成因。他们选定的目岩石是位于南澳大利弗林德斯国家公园里的“条建造”。     

  建造是一种含有薄的独特沉岩。在期的地球上,条建造很常,但自那以后它基本上底消失,直到重7亿年前的雪球地球期。     

  来自于海底的液系。”Gernon道。通常来,大气会快速氧化,所以条建造无法累。但是在雪球地球期,海洋与大气被隔断,富集从而形成条建造。    

  本文的地学家们发现建造的薄积记录了冰川前和后退的据。 尤其是地旋回的周期和地球道形状的化(又称奇循)呈关性。     

  地球着太阳转动道形状(也即离心率)呈周期性化,地球自转轴斜(斜度)和晃(差)也有似的周期性化。些天文旋回改着地球表面能接收到的太阳射的量,话说,它控制着气候化。    

  然在雪球地球期,地球的气候系得完全不同,但地球幸福地此一无所知,只是一如以往地行着化。”Mitchell道。     

  并未参与本研究,治梅森大学的Linda Hinnov教授说这项发现称得上“非凡”,并表示很期待能够检验新研究提出的一,即全球性的冰川可能地球的旋有减速作用。

  的祖先所经历过的冰期旋回在雪球地球期也一在运行着。     

  些持不停的气候旋回致的冰川前和退在雪球地球期和在今天无二致。Gernon     

  雪球地球期的冰盖是动态的而非静发现解决了两个前文提到的期存在的谜题。沉旋回和生命的存两者都得益于地球道不舍昼夜的变动    

  研究成果发表于国际顶级专业期刊Nature CommunicationsMitchell R N, Gernon T M, Cox G M, et al. Orbital forcing of ice sheets during snowball Earth[J]. Nature Communications, 2021, 12: 4187.)(原文链接 

 

A new way for life to survive Snowball Earth    

  Snowball Earth is the most extreme climate event Earth has ever seen. As opposed to ice sheets being restricted to continents only at high latitudes like Greenland and Antarctica today, the aptly named Snowball Earth would have engulfed the entire planet in thick ice.    

  When it was first proposed by Joseph Kirschvink of the California Institute of Technology and Paul Hoffman and his team at Harvard University, the extreme climatic event was an extremely contentious hypothesis. A page-turner written by Gabrielle Walker offers an inside look into the great debates among geologists in the late 1990s and early 2000s.     

  Now that the Snowball Earth hypothesis is generally accepted, it is easy to forget those early disagreements—but some scientists never forgot. A new discovery just published in Nature Communications finally reconciles what had remained, until now, two thorns in the side of the Snowball Earth hypothesis.     

  “One of the most fundamental challenges to Snowball was that life seems to have survived,” says Thomas Gernon, geologist at the University of Southampton and co-author of the study.     

  “So either Snowball didn’t happen, or life somehow avoided a bottleneck during the severe glaciation,” explains Gernon.     

  The other early challenge to the Snowball hypothesis came from the rocks themselves. There appeared to be too much variation in the layers of sediment deposited at that time.     

  During the Snowball, the frozen ocean would have been entirely cut off from the atmosphere. Without the normal exchange between the sea and air, many variations in climate that normally occur simply wouldn’t have.     

  “This was called the ‘sedimentary challenge’ to the Snowball hypothesis,” says Ross Mitchell, professor of the Institute of Geology and Geophysics Chinese Academy of Sciences in Beijing, China and the lead author. 

  “The highly variable rock layers appeared to show cycles that looked a lot like climate cycles associated with the advance and retreat of ice sheets,” explains Mitchell. Such variability was thought to be at odds with a static Snowball Earth entombing the whole ocean in ice.     

  The geologists decided to test the precise origin of the cycles in the sedimentary rocks. The rocks they targeted were “banded iron formation”, or BIF, in the Flinders Ranges National Park of South Australia.    

  BIF are unique sedimentary rocks with very thin layers of very iron-rich sediment. Although BIF had once been abundant on ancient Earth, they had essentially gone extinct until their reappearance during Snowball Earth about 700 million years ago. 

  “The iron comes from hydrothermal vents on the seafloor,” explains Gernon. Normally, the atmosphere oxidizes any iron immediately, so BIF typically doesn’t accumulate. But during the Snowball, with the ocean cut off from the air, iron was able to accumulate enough to form BIF.     

  The geologists discovered that the thin layers of BIF recorded evidence of the advance and retreat of ice sheets. In particular, the tempo of the cycles related to Earth’s orbital variations, known as Milankovitch cycles.    

  Earth’s orbit around the sun changes its shape, or eccentricity, and the tilt and wobble of Earth’s spin axis, known as obliquity and precession, also undergo cyclic changes. These astronomical cycles change the amount of incoming solar radiation that reaches Earth’s surface and, in doing so, they control climate. 

  “Even though Earth’s climate system behaved very differently during Snowball, Earth’s orbital variations would have been blissfully unaware and just continued to do their thing,” explains Mitchell.     

  Linda Hinnov, Professor at George Mason University who was not involved in the study, describes the discovery as “remarkable” and looks forward to testing one of the implications of the study that the global glaciation might have slowed Earth’s rate of rotation.    

  The ice age cycles that our ancestors endured would have also been operational during the Snowball.     

  “These ongoing orbital climate cycles caused the Snowball ice sheets to advance and retreat just like they do today,” says Gernon.     

  Now knowing that Snowball ice sheets were dynamic, not static, solves the two long-standing riddles. Both the sedimentary cycles and the survival of life were aided and abetted by Earth’s ongoing orbital variations.     

(Top) Sedimentary rocks of South Australia including the Holowilena Ironstone studied here that is sandwiched between glacial deposits. (Bottom) Field photo of the Holowilena Ironstone showing fine sedimentary layering ideal for this study.  

  The article can be found online: https://www.nature.com/articles/s41467-021-24439-4

 
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