Paleogene Climate and Deep-water Evolution in the Southwest Atlantic: Seismic Reflection and Coring Investigations in Support of IODP Proposal 862-Pre

今天的西南大西洋是一个关键区域：(i)太平洋和大西洋盆地之间的深水通过南极环极流（ACC）进行交流；（ii）填充海洋盆地最深处的南极边缘产生的深水的分散。南极海流是地球上最大的洋流系统，沿顺时针方向畅通无阻地绕南极大陆流动。穿过德雷克海峡（位于南极洲和南美洲之间的狭窄而深的海沟），ACC的平均体积运输是巨大的，估计每秒1亿到1.5亿立方米。因此，行政协调会是太平洋和大西洋之间水面和深水通讯的主要管道。没有这种联系，就不可能出现全球翻转环流，而在现代海洋中，全球翻转环流调节着全球热量、盐、营养物质和碳的运输，并对所有时间尺度上的全球气候产生重大影响。我们提出的项目是为了解决有关古近纪时间间隔（约6600万至2300万年前）的几个基本问题，这些问题几十年来一直困扰着海洋地质学家和古气候学家：ACC是什么时候开始的？它的建立对南极气候、翻转环流和全球纬向热输送有何影响？ACC的发展是否触发了3400万年前南极冰川的开始，或者它的作用完全次于大气中二氧化碳水平的下降？对这些问题的新见解只能通过在关键区域（如西南大西洋）进行新的地质研究来获得。以前在西南大西洋进行的古海洋学研究的一个主要问题是缺乏一个深度样带的地点，专门定位来追踪古近纪水团性质的变化。在这个区域，复杂的海底测深限制和控制着与南极大陆有关的深水和来自南极的底水的流动。该地区深水通道的演变及其与现代深水环流模式的逐步发展的关系尚不清楚。我们建议在西南大西洋亚南极福克兰高原东部地区进行UK-IODP现场调查调查（SSI），以进行地震反射调查和活塞取心作业。与阿尔弗雷德·韦格纳研究所（AWI）的巡航船一起，这些调查数据将为新的国际海洋发现计划（IODP）探险提供完整的建议，以钻探地点的深度样带。这些地点被选为地质水质量测量站，可以重建古近纪的深水环流模式。我们的主要目的是测试长期以来关于大气二氧化碳下降和ACC开始在驱动新生代降温和南极冰川作用中的相对作用的相互竞争的假设。UK-IODP SSI数据集，以及AWI领导的同伴巡航获得的数据，将有助于对深水沉积过程、漂移沉积和底流侵蚀的历史以及深水流的演变和西南大西洋的沉积特征有新的认识。UK-IODP SSI和AWI的综合结果将作为独立的、可发布的数据集，为大型海洋地质学家和古海洋学家提供兴趣，并为英国领导的IODP钻井建议及时提供坚实的基础，以便计划在南大西洋进行JOIDES Resolution作业。

The present-day Southwest Atlantic Ocean is a key region for: (i) the communication of deep waters between the Pacific and Atlantic ocean basins via the Antarctic Circumpolar Current (ACC); and (ii) the dispersal of deep waters produced on the Antarctic margin that fill the deepest parts of the ocean basins. The ACC is the largest oceanic current system on Earth and flows unimpeded clockwise around the Antarctic continent. Across the Drake Passage (a narrow, but deep trough between Antarctica and South America) the average volume transport of the ACC is enormous, estimated at 100 to 150 million cubic meters per second. The ACC is therefore the major conduit for surface and deep-water communication between the Pacific and Atlantic Oceans. Without this connection, global overturning circulation would not be possible, which, in the modern ocean, regulates the global transport of heat, salt, nutrients, and carbon and has a significant influence on global climate across all timescales.Our proposed project is developed to tackle several fundamental questions concerning the Paleogene time interval (~66 to 23 million years ago) that have frustrated marine geologists and palaeoclimatologists for decades: When was the ACC initiated? What was the impact of its establishment on Antarctic climate, on overturning circulation, and global latitudinal heat transport? Did the development of the ACC serve as the trigger for the initiation of Antarctic glaciation 34 million years ago or was its role completely secondary to that of declining atmospheric carbon dioxide levels? New insight into these problems can only be gained through new geological studies in key areas, such as the Southwest Atlantic Ocean.A major problem with previous palaeoceanographic studies in the Southwest Atlantic has been the lack of a depth-transect of sites, purposely situated to trace Paleogene changes in water mass properties. In this region, complex seafloor bathymetry restricts and controls the flow of both deep waters associated with the ACC and Antarctic-sourced bottom waters. The evolution of the deep passageways in this area, and their connection to the progressive development of the modern patterns of deep-water circulation, are unknown.We propose a UK-IODP Site Survey Investigation (SSI) to undertake seismic reflection survey and piston coring operations on the eastern Falkland Plateau region of the subantarctic southwest Atlantic Ocean. Together with a companion Alfred Wegener Institute (AWI) cruise, these survey data will allow development of a full proposal for a new International Ocean Discovery Program (IODP) expedition to drill a depth transect of sites. These sites have been chosen to serve as a geological water mass gauging station that will allow reconstruction of deep-water circulation patterns through the Paleogene. Our primary aim is to test long-standing competing hypotheses on the relative roles of declining atmospheric carbon dioxide and the initiation of the ACC in driving the onset of Cenozoic cooling and Antarctic glaciation.The UK-IODP SSI datasets, together with data derived from the companion cruise led by AWI, will contribute new understanding of deep-water sedimentary processes, the history of drift sedimentation and bottom-current erosion in conjunction with the evolution of deep-water flow, and sediment characteristics of the Southwest Atlantic Ocean. The combined results of the UK-IODP SSI and AWI cruises will function as stand-alone, publishable datasets of interest to a large community of marine geologists and palaeoceanographers, and provide a robust foundation for a UK-led IODP drilling proposal in time for planned JOIDES Resolution operations in South Atlantic.