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

位于合恩角和南极洲之间的德雷克海峡只是一个狭窄的海峡，但它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接在一起，因此阻碍了太平洋和大西洋之间的海水流动。大约3200万年前，一旦德雷克海峡开放，冰冷的深海海水就能够环绕南极洲周围的地球，形成南极绕极流，或称ACC。许多人认为，这是南极洲冰化的主要原因，并帮助世界从温室条件下降到冰屋条件；然而，也有人将冰川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有一大片未知年代的地形--中斯科舍海，那么模拟这一“门户”的开放并评估其在南极冰川中的作用将会更加简单。它显然是海洋的，因此能够向东输送深流，它的时代和历史是至关重要的。如果它是在德雷克海峡之前形成的，或者与德雷克海峡同时形成，那么ACC冰川和南极冰川可能都是在数百万年内彼此开始的。然而，如果它形成得更晚，南乔治亚等大陆碎片可能会阻止深水的流动，直到冰川开始很久之后，这支持了那些认为这两次事件没有联系的人。该项目是美英合作的，已经在纳撒尼尔·B·帕尔默号上分配了一个月的研究时间，价值100万美元。纳撒尼尔·B·帕尔默号是一艘美国科考船，其大小和破冰能力足以承受天气和冰层条件。这次探险定于2008年4月至5月，也就是国际极地年的中期。它的目的是在中斯科舍海进行一次详细的磁力调查，以确定可用来定义洋壳和确定洋壳年代的磁条异常。实事求是地说，勘测将从海底挖出岩石，然后在核反应堆中进行辐射，并用Ar-Ar方法测定年代。朱利安·皮尔斯(加的夫大学)将负责岩石的收集和年代测定；他的美国同事(德克萨斯大学奥斯汀分校的伊恩·达尔齐尔和拉里·劳弗)将负责该地区的磁力调查和构造板块构造史。也许值得注意的是，德雷克通道的开放也为地幔在地球深处的流动形成了一扇大门。太平洋目前正在收缩，因为它被板块消失的俯冲带包围，而大西洋正在扩张，因为它被形成板块的海脊所主导。因此，物质平衡需要下伏地幔从太平洋到大西洋的流动。大约25年前，最先在K-T边界发现陨石撞击的科学家沃尔特·阿尔瓦雷斯提出，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，地幔流动将像海水流动一样，通过德雷克海峡输送。在2001年与英国南极调查局同事共同撰写的一篇《自然》论文中，朱利安·皮尔斯利用铅和钕的同位素证明了太平洋地幔不同于南大西洋的地幔，因此可以通过疏浚的火山岩的同位素指纹来追踪通过德雷克海峡的太平洋地幔流动。这表明，太平洋和大西洋地幔之间的分界线现在至少部分位于神秘的中斯科舍海的某个地方。因此，通过分析中斯科舍海的疏浚岩石，将有可能准确地找出太平洋地幔迁移了多远，并研究这两类地幔之间的边界。这反过来将告诉我们地幔循环的速度和原因，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。

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'.