Collaborative Research: Glacial History of Snowball Earth

这是哈佛大学和汉密尔顿学院的首席研究人员共同提出的建议。他们将研究最近两个“雪球地球”时代的冰川历史，这两个时代早于地球上首次出现复杂的动物生命。(雪球地球是指整个海洋被向赤道流动的冰川覆盖的状态。这是由于冰-反照率反馈导致气候系统基本不稳定的表现。)雪球事件延长了数百万年，因为大气中的二氧化碳必须达到目前水平的数百倍，才能发生冰川消融。这是因为硅酸盐风化作用(消耗二氧化碳)减弱了，但火山放气作用没有减弱。以前用来测试雪球概念的冰川沉积物(硅质岩)实际上可能是在冰川消融期间形成的，在非常不同的条件下，造成了理论和观测之间的错误二分法。细粒的悬浮物位于斜晶岩之间或之下，以前被认为是间冰期或间冰期的，但它们可能更好地代表了雪球地球本身期间的海洋沉积。当雪球地球冰川消融时，溶解在海水中的亚铁应该以氧化铁的形式沉淀下来。如果前者代表雪球地球，后者代表雪球地球，则观察到富铁氧化物矿床跟随悬浮沉积，先于斜方辉石，符合滚雪球假说。悬浮沉积物局部与碳酸盐沉积物和其他开阔水域的迹象有关。模型表明，来自较高纬度的海洋冰川的入侵将使热带海洋即使在表面温度达到融点(由于温室压力)后仍被厚厚的冰层覆盖。因此，受海洋冰川入侵保护的热带大陆架和内陆海上将出现绿洲。在雪球绿洲中沉淀的碳酸盐具有独特的同位素模式，这是由巨大的大气碳库决定的。这些新概念可以消除对基于冰川记录的雪球地球概念的最有力的批评。将在东格陵兰峡湾地区、北奥斯特兰德群岛(斯瓦尔巴群岛)西北部和加拿大科迪莱拉北部的麦肯齐山脉进行地层测绘和测井。在这些地区，雪球地球的完整冰川历史非常发达和暴露。样品集将被收集和管理，用于合作的地球化学研究。该提案汇集了各自领域最有经验的两名研究人员，进行了一项以实地为基础的密切合作研究，以使用创造性和原创性的概念来测试一个影响深远的理论(雪球地球)。一些概念(例如，雪球)源自最近在另一个领域(地球物理流体动力学)的理论建模。哈佛团队先前在雪球地球问题上的工作的一个优势是同事、地球化学海洋学家丹尼尔·P·施拉格的强烈兴趣和参与。两名首席调查员都在文科院校任职，主要负责本科生教学。两者都将他们的研究元素融入到他们所教授的课程中，都将学生作为现场和实验室助理。他们定期为普通公众举办讲座，并经常受到媒体科学作家和电视制片人的咨询，内容涉及南极地球科学和雪球地球的各个方面。他们还为科学新闻课程做出了贡献。《科学美国人关于雪球地球》上的一篇文章是由一位首席研究员共同撰写的，许多小学教师在课堂上使用了这篇文章。

This is a collaborative proposal by Principal Investigators at the Harvard University and Hamilton College. They will study the glacial history of the two most recent "Snowball Earth" epochs, which preceded the first appearance of complex animal life on the planet. (Snowball Earth refers to a state in which the entire ocean is covered by Equatorward-flowing glaciers. It is the manifestation of a fundamental instability in the climate system due to ice-albedo feedback.)Snowball episodes expanded millions of years because atmospheric pCO2 must reach hundreds of times present levels for deglaciation to occur. This is achieved because silicate weathering (which consumes CO2) is attenuated, but volcanic outgassing is not. Glacial deposits(diamictite) previously used to test the snowball concept may actually have formed duringdeglaciation, under very different conditions, creating a false dichotomy between theory andobservations. Fine-grained suspension deposits between or beneath diamictites were previouslyregarded as interglacial or interstadial, but they may better represent marine sedimentationduring snowball Earth itself. When a snowball Earth deglaciates, ferrous iron dissolved inseawater should precipitate as iron oxide. Iron-oxide-rich deposits are observed to followsuspension deposits and precede diamictite, consistent with the snowball hypothesis if the former represent the snowball Earth and the latter its demise. The suspension deposits are locallyassociated with carbonate sediments and other indications of open water. Models imply thatinvasion by marine glaciers from higher latitudes will keep the tropical ocean covered by thickice even after the surface temperature reaches the melting point (due to greenhouse forcing).Consequently, 'oases' will open up on tropical shelves and inland seas that are protected frommarine glacial invasion. Carbonates precipitated in snowball oases have characteristic isotopicpatterns, dictated by the large atmospheric carbon reservoir. These new concepts could eliminatethe most potent criticisms of the snowball Earth concept based on the glacial record. Stratigraphic mapping and logging will be done in the fjord region of East Greenland, northwestern Nordaustlandet (Svalbard), and the Mackenzie Mountains of the northern Canadian Cordillera. In these areas, the full glacial history of snowball Earth is well developed and exposed. Sample sets will be collected and curated for collaborative geochemical studies.Intellectual merit. The proposal brings together two of the most experienced researchers intheir respective fields in an intensely collaborative field-based study to test a far-reaching theory(snowball Earth) using creative and original concepts. Some of the concepts (e.g., snowballoases) derive from recent theoretical modeling in yet another field (geophysical fluid dynamics).A strength of the Harvard team's prior work on the snowball Earth problem was the stronginterest and involvement of a colleague, the geochemical oceanographer Daniel P. Schrag.Broader impact. Both Principal Investigators hold appointments at liberal arts institutions, where the primary responsibility is to undergraduate instruction. Both incorporate elements of their research in courses they teach, and both include students as field and laboratory assistants. They regularly give lectures intended for the general public, and are frequently consulted by media science writers and television producers covering aspects of Antarctic geoscience and snowball Earth. They have also contributed to courses in science journalism. An article in Scientific American on Snowball Earth co-written by one of the Principal Investigators is used by many elementary school teachers in their classes.