IODP Expedition 351, Philippine Sea: Neogene NW Pacific Deep Water Circulation

世界海洋的深层温盐环流，特别是与经向(从极地到极地区域)颠覆环流有关的环流，与过去地质历史中的长期和短期气候变化密切相关。随着环流路径、水流速度和海洋上升流的改变，与大气接触的二氧化碳(保存在海水中)的量也会改变，热量(也保存在海水中)在高纬度和低纬度之间的传输也会改变。气候既影响深海环流(例如，第四纪海冰的增长)，也受到海洋变化的影响(例如，南大洋深处的冰川二氧化碳储存；通过巴拿马地峡输送暖水)。尽管太平洋环流很重要，但作为世界上最广泛的海洋，太平洋环流的历史还没有受到新近纪(过去2600万年)的限制。现代太平洋环流在很大程度上是由低盐度的表层水向南流动构成的，受环极深水从南方(大于4公里水深)向北流动驱动。这个重要的环极深水由南极底层水和北大西洋深水的混合物组成，由太平洋以外的冷密下沉的高纬度水团形成。环极地深水充斥着太平洋深处，是较浅(约1-3公里水深)返回南太平洋中心水的来源。新近纪(约23 Ma至今)见证了全球气候的巨大变化，从中新世中期的温暖世界(1400万年前)到北半球冰川的“冰屋”(300万年前)。对温暖世界中太平洋经向翻转环流的强度的了解尚未得到解决，一些研究表明，在始新世(约5000万年前)，环流增强，而另一些研究则减少。一些研究试图重建中新世(2300万至500万年前)的太平洋深层环流，但主要由于可获得的材料，主要集中在中等深度的太平洋中央水。一些研究表明，随着巴拿马地峡在新近纪关闭，大西洋水域的影响正在减弱。其他人认为，中新世中期太平洋的环流速度较慢，但与今天的南方来源相似。如果是真的，我们预计北太平洋的底层水域在14 Ma之后会有一个更大的绕极深水“特征”。这一假设可以用来自菲律宾海(西北太平洋)的新的国际海洋发现方案岩心来验证，这些岩心监测整个新近纪的深水(水深4.5公里)，并具有良好的鱼牙记录，用来测量作为水团示踪剂的ND同位素。南大洋水的Nd值(-8)比太平洋中部水的负值(-3.5)更大，南大洋对环极地深水的影响体现在西北太平洋的现代Nd同位素剖面上。

Deep thermohaline circulation of the world's oceans, specifically in relation to meridional (from polar to polar regions) overturning circulation, is closely linked to long and short term climate changes in the geological past. As circulation pathways, flow speeds, and upwelling of the oceans changes, the amount of CO2 (which is held in ocean water) in contact with the atmosphere changes, and heat (also held in ocean water) transport between high and low latitudes changes. Climate both impacts deep ocean circulation (e.g. growth of sea ice in the Quaternary), and is impacted by oceanic changes (e.g. glacial CO2 storage in the deep Southern Ocean; transport of warm water through the Isthmus of Panama). Despite its importance, however, the history of deep (greater than about 4 km water depth) Pacific Ocean circulation, the most extensive of the world's oceans, has not yet been constrained for the Neogene (the last 26 million years). Modern Pacific circulation is composed largely of lower salinity surface water flowing broadly southwards, driven by Circumpolar Deep Water from the south (greater than about 4 km water depth) flowing northwards. This important Circumpolar Deep Water is composed of a mixture of Antarctic Bottom Water and North Atlantic Deep Water, formed by cold dense sinking high latitude water masses outside of the Pacific. Circumpolar Deep Water fills the deep Pacific, and is a source of the shallower (~1-3 km water depth) return southern flow Pacific Central Water. The Neogene (~23 Ma to Recent) has seen large changes in global climate, from a mid Miocene warm world (before 14 million years ago), to the 'ice-house' of Northern Hemisphere Glaciation (after 3 million years ago). Understanding the strength of the Pacific meridional overturning circulation in a warmer world is yet to be resolved, with some studies suggesting enhanced circulation and others reduced in the Eocene (around 50 million years ago). Several studies have attempted to reconstruct Pacific deep circulation over the Miocene (23 to 5 million yeas ago), but have focussed on intermediate depth Pacific Central Water largely due to available material. Some studies suggest there was a waning influence of Atlantic waters as the Isthmus of Panama closed through the Neogene. Others suggest the mid Miocene Pacific had a slower circulation, but with a similar southern source to today. If true, we would expect bottom waters in the North Pacific to have a greater Circumpolar Deep Water 'signature' after 14 Ma. This hypothesis can be tested with new International Ocean Discovery Program cores from the Philippine Sea (NW Pacific), that were monitoring deep waters (4.5 km water depth) throughout the Neogene, and have an excellent record of fish teeth with which to measure Nd isotopes as a water mass tracer proxy. Southern Ocean water Nd isotopes have a more negative value (-8) than Pacific Central Water (-3.5), and the Southern Ocean's influence on Circumpolar Deep Water is shown in modern Nd isotope profiles from the NW Pacific.