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，用于：(1)通过南极绕极流将太平洋和大西洋盆地之间的深水联系起来；(2)南极边缘产生的填满大洋盆地最深部分的深水扩散。ACC是地球上最大的洋流系统，顺时针方向畅通无阻地绕着南极大陆流动。穿过德雷克海峡(南极洲和南美洲之间的一个狭窄但很深的海槽)，ACC的平均运输量是巨大的，估计为每秒1亿至1.5亿立方米。因此，行政协调会是太平洋和大西洋之间进行表层和深水交流的主要渠道。如果没有这种联系，全球颠覆环流是不可能的，在现代海洋中，它调节着全球热量、盐、营养物质和碳的运输，并在所有时间尺度上对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是什么时候启动的？它的建立对南极气候、颠覆环流和全球纬度热量输送有何影响？ACC的发展是3400万年前南极冰川形成的导火索，还是它的作用完全次于大气二氧化碳水平的下降？只有对西南大西洋等关键区域进行新的地质学研究，才能对这些问题有新的认识。西南大西洋以往古海洋学研究的一个主要问题是缺乏专门用于追踪古近纪水团性质变化的地点的深度横断面。在该区域，复杂的海底水深测量限制和控制了与行政协调会有关的深水和源自南极的底层水的流动。该地区深水通道的演化及其与现代深水循环模式的逐步发展的联系尚不清楚。我们建议英国IODP现场调查调查(SSI)在亚南极西南大西洋的东福克兰高原地区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)的配套邮轮一起，这些调查数据将使我们能够制定一项新的国际海洋发现计划(IODP)探险的完整提案，以钻探遗址的深度横断面。这些地点被选为地质水质测量站，将允许重建通过古近纪的深水循环模式。我们的主要目的是检验关于大气二氧化碳下降和ACC启动在推动新生代降温和南极冰川开始中的相对作用的长期相互竞争的假说。UK-IODP SSI数据集，以及由AWI领导的伴随邮轮的数据，将有助于对深水沉积过程、漂流沉积和底流侵蚀的历史以及深水流动的演变，以及西南大西洋的沉积物特征有新的理解。英国IODP SSI和AWI巡航的综合结果将作为一个独立的、可发布的海洋地质学家和古海洋学家社区感兴趣的数据集发挥作用，并为英国领导的IODP钻探提议提供坚实的基础，以便及时在南大西洋计划的JOIDES分辨率作业。

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 sub-Antarctic 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.