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

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'.