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Evolution of the Drake Passage/Scotia Sea Deep Ocean Current and Mantle Gateway

德雷克海峡/斯科舍海深洋流和地幔门户的演化

基本信息

批准号：
NE/F004974/1
负责人：
Julian Pearce
金额：
$ 27.87万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2008
资助国家：
英国
起止时间：
2008 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FF004974%2F1
关键词：
Evolution Drake Passage Scotia Sea

项目摘要

Drake Passage, which lies between Cape Horn and Antarctica, is only a narrow strait but it has had a huge effect on the evolution of the Earth's climate. At one time, South America and Antarctica were joined, so blocking the flow of seawater between the Pacific and Atlantic Oceans. Once Drake Passage opened, some 32 million years ago, cold, deep ocean water was able to circle the globe around Antarctica, forming the Antarctic Circumpolar Current, or ACC. This, many believe, was the main cause of the glaciation of Antarctica and helped plunge the world from Greenhouse to Icehouse conditions; however others attribute the glaciation to changes in greenhouse gas concentrations and the Earth's orbital cycles. Modelling the opening of this 'gateway' and assessing its role in the Antarctic glaciation would be more straightforward were it not for the presence of a large terrain of unknown age at the eastern end of Drake Passage: the Central Scotia Sea. Apparently oceanic and therefore able to transmit the deep current eastwards, its age and history are critical. If it formed before, or at the same time as, Drake Passage, the ACC and Antarctic glaciation could both have initiated within a few million years of one another. If, however, it formed later, continental fragments such as South Georgia could have blocked the flow of deep water until long after glaciation had started, so supporting those who argue that the two events were not connected. This project, a US-UK collaboration, has already been allocated a month of research time worth 1$M on the Nathaniel B. Palmer, a US research vessel with the size and ice-breaking capability needed to withstand the weather and ice conditions. The expedition is scheduled for April-May 2008, in the middle of the International Polar Year. It aims to carry out a detailed magnetic survey in the Central Scotia Sea to identify the magnetic stripe anomalies that can be used to define and date oceanic crust. To ground-truth the survey, rocks will be dredged from the sea floor, then irradiated in a nuclear reactor and dated by the Ar-Ar method. Julian Pearce (Cardiff University) will be responsible for the collection and dating of the rocks; his American colleagues (Ian Dalziel and Larry Lawver from the University of Texas in Austin) will be responsible for the magnetic survey and constructing a plate tectonic history of the region. Remarkably perhaps, the opening of Drake Passage also formed a gateway for flow of mantle in the deep Earth. The Pacific Ocean is presently shrinking because it is surrounded by subduction zones where plates are lost, while the Atlantic ocean is expanding because it is dominated by ridges where plates are created. Mass balance therefore requires Pacific-to-Atlantic flow of the underlying mantle. Some 25 years ago, Walter Alvarez, the scientist who first identified the meteorite impact at the K-T boundary, proposed that the subduction zones and roots of continents around the Pacific would have acted as barriers to escape of Pacific mantle and that mantle flow, like seawater flow, would be funnelled through Drake Passage. In a 2001 Nature Paper with colleagues from the British Antarctic Survey, Julian Pearce demonstrated using isotopes of Pb and Nd that Pacific mantle was different from mantle of the South Atlantic and so Pacific mantle flow could be traced through Drake Passage by isotopic fingerprinting of dredged volcanic rocks. This showed that the boundary between the Pacific and Atlantic mantle now lies, at least in part, somewhere beneath the enigmatic Central Scotia Sea. By analysing the dredged rocks from the Central Scotia Sea, it will thus be possible to find out exactly how far the Pacific mantle has migrated and study the boundary between the two types of mantle. That in turn will inform us about the rates and causes of mantle circulation and the ways in which different domains of mantle interact, so increasing our knowledge of the 'inaccessible Earth'.

德雷克海峡位于合恩角和南极洲之间，虽然是一条狭窄的海峡，但却对地球气候的演变产生了巨大的影响。南美洲和南极洲一度连在一起，阻碍了太平洋和大西洋之间的海水流动。大约 3200 万年前，德雷克海峡开通后，寒冷的深层海水就能够围绕南极洲环绕地球，形成南极绕极流 (ACC)。许多人认为，这是南极洲冰川作用的主要原因，并帮助世界从温室条件转变为冰室条件。然而，其他人将冰川作用归因于温室气体浓度和地球轨道周期的变化。如果不是因为德雷克海峡东端——斯科舍海中部——存在一大片年龄未知的大地形，对这个“门户”的开放进行建模并评估其在南极冰川作用中的作用将会更加直接。显然是海洋性的，因此能够将深海流向东传送，其年龄和历史至关重要。如果它是在德雷克海峡之前或同时形成的，那么ACC和南极冰川作用可能都在几百万年内开始。然而，如果它形成得更晚，那么像南乔治亚岛这样的大陆碎片可能会阻塞深水的流动，直到冰川作用开始很久之后，因此支持了那些认为这两个事件没有联系的人。该项目是美英合作的项目，已在 Nathaniel B. Palmer 号上分配了一个月的研究时间，价值 100 万美元，这艘美国研究船的尺寸和破冰能力足以承受天气和冰况。这次探险计划于 2008 年 4 月至 5 月，即国际极地年中期进行。它的目的是在中部斯科舍海进行详细的磁力调查，以确定可用于定义洋壳和测年的磁条异常。为了确定调查的真实情况，将从海底挖出岩石，然后在核反应堆中进行辐照，并通过 Ar-Ar 方法测定年代。朱利安·皮尔斯（卡迪夫大学）将负责岩石的收集和测年；他的美国同事（奥斯汀德克萨斯大学的伊恩·达尔齐尔和拉里·劳弗）将负责磁力勘测并构建该地区的板块构造历史。值得注意的是，德雷克海峡的开口也形成了地幔在地球深处流动的门户。太平洋目前正在萎缩，因为它被板块消失的俯冲带包围，而大西洋正在扩张，因为它主要由板块形成的山脊所控制。因此，质量平衡需要底层地幔从太平洋流向大西洋。大约 25 年前，首次发现 K-T 边界陨石撞击的科学家沃尔特·阿尔瓦雷斯 (Walter Alvarez) 提出，太平洋周围的俯冲带和大陆根部将成为太平洋地幔逃逸的障碍，地幔流将像海水流一样通过德雷克海峡流入。 2001 年，朱利安·皮尔斯 (Julian Pearce) 与英国南极调查局的同事在《自然》杂志上发表的一篇论文中，利用 Pb 和 Nd 同位素证明，太平洋地幔与南大西洋的地幔不同，因此可以通过疏浚火山岩的同位素指纹来追踪穿过德雷克海峡的太平洋地幔流。这表明太平洋和大西洋地幔之间的边界现在至少部分位于神秘的中部斯科舍海下方的某个地方。通过分析从中部斯科舍海挖出的岩石，将有可能准确地查明太平洋地幔迁移了多远，并研究两种地幔之间的边界。这反过来又将使我们了解地幔循环的速率和原因，以及地幔不同区域相互作用的方式，从而增加我们对“难以接近的地球”的了解。