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

点击翻译按钮获取中文摘要

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.