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

德雷克海峡位于合恩角和南极洲之间，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。曾经，南美洲和南极洲是连在一起的，因此阻止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（Drake Passage）开通后，冰冷的深海海水环绕着南极洲的地球仪流动，形成了南极绕极流（ACC）。许多人认为，这是南极洲冰川作用的主要原因，并帮助世界从温室环境变成了冰室环境;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。如果不是德雷克海峡东端存在一个年龄未知的大地形：中央斯科舍海，模拟这个“门户”的开放并评估其在南极冰川作用中的作用将更加简单。它显然是海洋性的，因此能够向东输送深层洋流，它的年龄和历史是关键。如果它在德雷克海峡之前或同时形成，ACC和南极冰川可能都在几百万年内开始。然而，如果它形成得较晚，那么像南格鲁吉亚这样的大陆碎片可能会在冰川作用开始很久之后才阻挡住深水的流动，这就支持了那些认为这两个事件没有联系的人。这个项目是美国和英国的合作项目，已经在纳撒尼尔B上分配了一个月的研究时间，价值100万美元。帕尔默号是一艘美国研究船，其尺寸和破冰能力足以承受天气和冰况。这次考察定于2008年4月至5月进行，正值国际极地年中期。其目的是在中斯科舍海进行详细的磁力调查，以查明可用于确定洋壳和确定洋壳年代的磁条异常。为了使调查真实，将从海底挖掘岩石，然后在核反应堆中进行辐照，并通过Ar-Ar方法确定年代。朱利安皮尔斯（卡迪夫大学）将负责收集和测定岩石的年代;他的美国同事（来自奥斯汀得克萨斯大学的伊恩·达勒和拉里·劳弗）将负责磁力调查和构建该地区的板块构造史。也许值得注意的是，德雷克海峡的开放也形成了地球深处地幔流动的门户。太平洋目前正在缩小，因为它被板块消失的俯冲带所包围，而大西洋正在扩大，因为它被板块形成的山脊所主导。因此，物质平衡需要底层地幔从太平洋流向大西洋。大约25年前，首先在K-T边界发现陨石撞击的科学家Walter Alvarez提出，太平洋周围大陆的俯冲带和根部可能是太平洋地幔逃逸的障碍，地幔流动，就像海水流动一样，将通过德雷克海峡。在2001年的《自然》杂志上，朱利安·皮尔斯与英国南极调查局的同事一起，用铅和钕的同位素证明了太平洋地幔与南大西洋地幔的不同，因此太平洋地幔流可以通过疏浚火山岩的同位素指纹来追踪德雷克海峡。这表明，太平洋和大西洋地幔之间的边界现在至少有一部分位于神秘的中央斯科舍海之下。通过分析中斯科舍海的疏浚岩石，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。这反过来又会告诉我们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“不可接近的地球”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'.