Reconstructing millennial-scale variability in deep-water circulation of the North Atlantic during the early Pleistocene

摘要：对末次冰期和去冰期反复、大而突然的气候变化的观测引起了人们对未来气候突变可能性的关注。然而，关于过去这种变化的机制仍然存在相当大的不确定性，甚至在某种程度上，我们还没有完全了解它们的影响是如何在整个气候系统中传播的。格陵兰冰芯和北大西洋沉积物的记录表明，在末次冰期期间，北半球高纬度地区反复遭受非常大而突然的温度波动，通常被称为Dansgaard-Oeschger （D-O）振荡。一个单一的D-O振荡包括在短短几十年内突然变暖事件（格陵兰岛的年平均气温通常上升10 - 10摄氏度），随后是几个世纪的逐渐冷却，然后突然回到寒冷的状态。这些转变与深海环流强度的变化有关，这可能影响了向北大西洋和北欧海的海洋热输送。人们越来越认识到，要了解气候突变的过程，有必要记录气候系统在各种不同边界条件下的变率（例如日照、全球冰量、大气二氧化碳水平）。在这里，我们建议使用海洋钻探计划第164号腿982站点期间获得的北大西洋超极地海洋沉积物岩心，来检查冰岛苏格兰溢出水（北大西洋深水的前兆，负责启动全球热盐环流的水团）的深海流速千年尺度变化的性质，以响应在此之前立即变化的轨道和亚轨道气候制度。以及所谓的“中更新世气候转变”（MPT；约120 - 70万年前）。在这一转变期间，轨道强迫的主要影响从倾角强迫（41,000年）转变为离心率强迫（100,000年）和调制岁差（23,000年）。该项目中生成的古洋流数据将与现有的深海通气（基于底栖碳同位素）和表层海洋变异性（浮冰碎屑、稳定同位素）的高分辨率记录相结合，以建立这些重要气候成分之间关系的详细视图，并进一步了解它们的千年尺度变异性是如何通过中更新世边界条件的变化而演变的。这种古观测对于为气候模式提供输入和评估气候和冰盖行为的数值模拟具有重要意义。

Summary: The observation of repeated, large and abrupt climatic shifts during the last glacial and deglacial periods has raised current concern over the possibility of abrupt climate change in the future. Yet there is still considerable uncertainty concerning the mechanisms of such changes in the past, even to the extent that we do not fully understand how their effects are transmitted throughout the climate system. Records from Greenland ice cores and North Atlantic sediments suggest that high latitudes in the Northern Hemisphere were repeatedly subjected to very large and abrupt fluctuations in temperature during the last glaciation, commonly referred to as Dansgaard-Oeschger (D-O) oscillations. A single D-O oscillation comprised an abrupt warming event (typically >10 degrees C rise in mean annual temperature over Greenland) within just a few decades, followed by a few centuries of more gradual cooling before an abrupt return to cold conditions. These transitions have been linked to variations in the strength of the deep ocean circulation, which may have affected marine heat transports to the North Atlantic and Nordic Seas. It is increasingly recognised that to understand the processes of abrupt climate change it is necessary to document the variability of the climate system during a wide range of different boundary conditions (e.g. insolation, global ice volume, atmospheric CO2 levels). Here we propose to use marine sediment cores from the sup-polar North Atlantic obtained during Ocean Drilling Programme Leg 164, Site 982, to examine the nature of millennial-scale variability in deep ocean flow speeds of Iceland Scotland Overflow Water (a precursor of North Atlantic Deep Water, the watermass responsible for initiating the global thermohaline circulation) in response to the changing orbital and sub-orbital climate regimes immediately prior to, and across the so called 'mid-Pleistocene climate transition' (MPT; ~1.2-0.7 million years ago). During this transition the dominant influence of orbital forcing changed from obliquity forcing (41,000 years) to eccentricity (100,000 years) modulated precession (23,000 years) thereafter. The palaeocurrrent data generated within this project will be integrated with existing high-resolution records of deep ocean ventilation (based on benthic carbon isotopes) and surface ocean variability (ice rafted debris, stable isotopes) to establish a detailed view of the relationships among these important climate components and further our understanding of how their millennial-scale variability evolved through changing boundary conditions of the mid-Pleistocene. Such palaeo-observations are important for providing input to climate models and evaluating numerical simulations of climate and ice-sheet behaviours.