Evolution of Pliocene paleoceanography in the subpolar Atlantic during the onset of Northern Hemisphere Glaciation (NHG) NOW MODIFIED TO: Mid-Pliocene evolution of Mediterranean Outflow Water (MOW) and its salt discharge to the Northeast Atlantic during t

在拉布拉多海，深层对流单体目前形成了北大西洋上层深水，并与北欧海的深水源一起控制着北大西洋温盐环流（THC）的强度和向北方高纬度地区的相关热量输送。这种对流是高度敏感的淡水干扰所引起的融水从格陵兰岛和（冰川）Laurentide冰盖，使这片海的关键领域，研究过去的气候变化。例如，更新世北大西洋气候的大幅度千年尺度变化与副极地西北大西洋的冰-海相互作用和拉布拉多海水形成的变化密切相关。为了更好地理解拉布拉多海古海洋学在上新世晚期北方半球冰川作用（NHG）和中美洲航道同期关闭期间的作用，需要测试三个假设，促进热量、盐和水分向北方高纬度地区的输送：（一）欧亚大陆北方上新世晚期降水的增加导致北冰洋经东格陵兰向北大西洋的淡水输出增加电流（EGC）和削弱北大西洋THC，从而加速NHG;二）在拉布拉多海的深水形成占主导地位，在不太严重的晚上新世冰川;三）千年尺度的气候变率仅限于扩大冰盖推进西北大西洋货架的时间。我们建议重建EGC和LSW在IODP站点U1307在NHG（3.3-2.6 Ma）的开始，通过有孔虫Mg/Ca和稳定同位素记录与亚千年的时间分辨率的演化。对1307号站点保存完好、连续的上新世晚期沉积物剖面进行精确的年龄控制，也将有助于实现第303航次船上小组的总体目标，即为北大西洋气候变化建立一个年代地层框架。

In the Labrador Sea deep-reaching convection cells presently form Upper North Atlantic Deep Water and govern - together with deepwater sources in the Nordic Seas - the strength of North Atlantic thermohaline circulation (THC) and the related heat transport to northern high latitudes. This convection is highly sensitive to freshwater disturbances induced by meltwater from the Greenland and (glacial) Laurentide ice-sheets and makes this sea a key area to study past climate change. For example, large-amplitude, millennialscale variability in North Atlantic climate in the Pleistocene was intimately linked to ice-sea interaction in the subpolar northwest Atlantic and to changes in Labrador Sea Water formation. Three hypotheses need to be tested to arrive at a better understanding of the role of Labrador Sea paleoceanography during the Late Pliocene onset of Northern Hemisphere Glaciation (NHG) and coeval closure of the Central American Seaway, promoting heat, salt, and moisture transport to northern high latitudes: i) Increased Late Pliocene precipitation in northern Eurasia has led to enhanced freshwater export from the Arctic Ocean to the North Atlantic via the East Greenland Current (EGC) and weakened North Atlantic THC, thus accelerated NHG; ii) Deepwater formation in the Labrador Sea was dominant during less severe Late Pliocene glacials; iii) Millennial-scale climate variability was confined to times of enlarged ice-sheets advancing over northwest Atlantic shelves. We propose to reconstruct the evolution of EGC and LSW at IODP Site U1307 over the onset of NHG (3.3-2.6 Ma) by means of foraminiferal Mg/Ca and stable-isotope records with submillennial time resolution. Precise age control for the well preserved, continuous Late Pliocene sediment section at Site 1307 will also contribute to a Leg-303 Shipboard Party s general objective, that is to generate a chronostratigraphic framework for North Atlantic climate variability.