Late slowdown of the Atlantic Meridional Overturning Circulation during the Last Glacial Inception: New constraints from sedimentary ( 231 Pa/ 230 Th) Abel Guihou a,b, ⁎, Sylvain Pichat b , Sìlvia Nave c , Aline Govin a , Laurent Labeyrie a , Elisabeth Michel a , Claire Waelbroeck a a Laboratoire des Sciences du Climat et de l'Environnement (LSCE/IPSL, CEA-CNRS-UVSQ), Gif/Yvette, France b Laboratoire des Sciences de la Terre, Université de Lyon, Ecole Normale Supérieure de Lyon, Lyon, France c Laboratorio Nacional de Energia e Geologia, Departamento de Geologia Marinha, Estrada da Portela, Zambujal. Apartado 7586, 2721-866 Amadora, Portugal abstract article info Article history: Received 16 May 2009 Received in revised form 18 November 2009 Accepted 20 November 2009 Available online 16 December 2009 Editor: M.L. Delaney Keywords: AMOC Last Glacial Inception ( 231 Pa/ 230 Th) insolation ice-sheet growth Our study gives new constraints on the response of Atlantic Meridional Overturning Circulation (AMOC) export to various forcings during the Last Glacial Inception. The decay corrected excess sedimentary ( 231 Pa/ 230 Th) activity ratio (hereafter referred to as (Pa/Th)) has been measured over that period in two deep cores from the Western (SU90-11, 44°04′N, 40°01′W, 3645 m) and Eastern (MD01-2446, 39°03′N, 12°37′W, 3547 m) basins of the North Atlantic. Both records display significant changes despite the relatively short half-life of 231 Pa (∼ 32 kyr) compared to the period we investigate. The (Pa/Th) variability does not correlate to changes in local opal flux normalized to 230 Th. Moreover, the (Pa/Th) profiles display a high degree of coherency with indirect proxies of AMOC activity such as the benthic foraminifera δ 13 C and the mid-latitude summer Sea Surface Temperature in nearby reference cores. These additional pieces of evidence support our interpretation of the (Pa/Th) as reflecting AMOC export. The (Pa/Th) repeatedly underwent rapid changes during the Last Glacial Inception associated with the extension of ice rafted detritus in the North Atlantic, highlighting the control of ice-sheet dynamics through freshwater forcing on AMOC export. AMOC export remains large during periods of ice-sheet growth and its decreases lag the Northern Hemisphere summer insolation forcing. AMOC modulation appears driven by ice-sheet dynamics, itself driven by the seasonal insolation gradient between low and high Northern Hemisphere latitudes and the associated intensity of the meridional oceanic and atmospheric circulation. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ocean circulation plays an important role in regulating the climate system but the details of its response to external forcing remain vague (e.g. Ruddiman and McIntyre, 1981; Imbrie et al., 1992; Khodri et al., 2001). The Last Glacial Inception is a good candidate to investigate the feedbacks of the ocean circulation on the massive build-up of ice sheets. MIS 5 is characterized by a climate optimum (MIS 5.5) that lasted 9–10 kyr with a mean temperature about 2 °C warmer and a sea level high stand 4–6 m higher than present, consequence of a smaller Greenland ice sheet and/or West Antarctic Ice Sheet (e.g. Duplessy et al., 2007; Rohling et al., 2008). This climatic optimum is followed by progressive climate deterioration towards the glacial MIS 4. Yet, we do not know precisely how Atlantic Meridional Overturning Circulation (hereafter referred to as AMOC) responds to orbital forcing and to the massive build-up of ice-sheet over the northern continents. Previous studies have led to contradictory results. Some studies, focused on the early stage of the glacial inception (the transition from MIS 5.5 to MIS 5.4), proposed that AMOC responds early after the warm MIS 5.5 as a positive feedback to the decrease in Northern Hemisphere summer insolation and the associated cooling of the high northern latitudes (Adkins et al., 1997; Cortijo et al., 1999; Khodri et al., 2001). On the contrary, other studies proposed that AMOC does not respond directly to the decrease in Northern Hemisphere summer insolation but remains active well after the inception of the growth of high northern latitude ice sheets, thus supporting this growth by the subsequent heat and moisture transport from low to high latitudes (Ruddiman and McIntyre, 1979; McManus et al., 2002). However, most of these studies were based on proxies which are indirectly linked with AMOC export. For instance, benthic foraminifera δ 13 C is used as a tracer of water masses (Curry et al., 1988; Duplessy et al., 1988) but does not directly depend on their dynamics (LeGrand and Wunsch, 1995). This proxy is controlled by the biological cycling of 13 C depleted organic matter (Kroopnick, 1985), air–sea exchanges (Lynch-Stieglitz et al., 1995), and the transfer of carbon from the continents to the ocean (Shackleton, 1977). The decay corrected excess sedimentary ( 231 Pa/ 230 Th) ratio (hereafter referred to as (Pa/Th)) has been used in the past decade Earth and Planetary Science Letters 289 (2010) 520–529 ⁎ Corresponding author. Laboratoire des Sciences du Climat et de l'Environnement (LSCE/ IPSL, CEA-CNRS-UVSQ), Gif/Yvette, France. Tel.: +33169824357; fax: +33169823568. E-mail address: abel.guihou@lsce.ipsl.fr (A. Guihou). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.11.045 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl