Instantaneous and inverse coupling between the South and North Atlantic during the last glacial period

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The last glacial period was punctuated by repeated, large, millennial-scale (hundreds to thousands of years) shifts in northern hemisphere climate known as Dansgaard-Oeschger (D-O) oscillations. These events were characterised by the extremely rapid alternation between cold and warmer conditions with temperature increases over Greenland and NW Europe sometimes exceeding 10 degrees C within a few decades. The discovery of D-O oscillations has provided a major stimulus for climate research and has fuelled debate over the possible nature of 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 were propagated around the Earth. In contrast with the abrupt nature of changes occurring in the Northern Hemisphere, those observed in Antarctica and the Southern Hemisphere in general display a more gradual response, which was approximately 'out-of-phase' (i.e. opposite) with the north. This relationship has given rise to the notion of a 'seesaw' which acts to redistribute heat between the Northern and Southern Hemispheres. The seesaw represents changes in the nature of circulation within the Atlantic Ocean; strong circulation leads to warming in the North Atlantic (as in the modern climate) while a weakened circulation leads to cooling across the North Atlantic and a warming in the South Atlantic and Antarctica. More specifically, the seesaw converts the abrupt northern signal into a reversed but equally abrupt signal in the South Atlantic. This South Atlantic signal is then converted into the more gradual Antarctic signal by the large heat capacity of the Southern Ocean. To date, no evidence for the 'South Atlantic' signal has been observed. We have unpublished pilot data from a South Atlantic marine sediment core which show, for the first time, very rapid changes during the late glacial and deglacial periods that were instantaneous (within dating uncertainty) and of opposite sign to those observed in the North Atlantic. We suggest that these shifts represent the 'missing-link' for explaining the transmission of abrupt signals between the high latitudes of both hemispheres. This proposal aims to extend these records back to ~100 kyr ago. This will enable us to investigate the propagation of abrupt climate shifts between the high latitudes of both hemispheres during the main phase of millennial-scale climate variability (the last glacial period) without the added complexity of orbital-scale (several thousands of years) variability which characterised the last deglacial period. We will also investigate the onset of millennial-scale climate variability during the transition from interglacial to full glacial conditions at around 75 kyr ago. The study will represent the first test for the direct transmission of abrupt, inter-hemispheric climate shifts throughout the last glacial cycle. We will use a multi-proxy approach to reconstruct hydrographic changes in the surface South Atlantic during the period ~100 to ~27 kyr ago. Planktonic foraminiferal (surface dwelling zooplankton) faunal assemblages will be employed to study rapid shifts in the frontal systems of the Antarctic Circumpolar Current (ACC) which represents the largest flow of water in the modern ocean. We will also use geochemical analyses (Mg/Ca ratios and oxygen isotopes) of single species of planktonic foraminifera to investigate changes in the background temperature of the region, which we predict will reveal more gradual changes. We will use species which live at different water depths within the upper water column (thermocline) to detect changes in the physical properties of the thermocline during abrupt climate change events. Finally we will employ benthic (bottom-dwelling) foraminiferal assemblage counts to investigate changes in surface ocean productivity, which may be related to changes in the strength of the ACC.