Abstract
Abstract. Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9–14 (300–540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north – south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.
Highlights
The Brunhes polarity chron encompasses the last 780 ka and its middle section is often considered as a warm period during the last 1000 ka, when warm surface waters penetrated polewards and sea levels were generally higher than at Present (Droxler et al, 2003)
While the interglacial δ18O levels are similar at the two sites and for Marine Isotope Stages (MIS) 11c in agreement with those recorded for core MD01-2443 (Fig. 6f; de Abreu et al, 2005), G. inflata δ13C values at site MD01-2446 are generally higher (Fig. 8d) indicating that more nutrients were available in the offshore waters either because of lower nutrient consumption or because the waters offshore had already higher preformed nutrient concentrations
Planktic foraminiferal δ18O values and alkenone derived Sea surface temperature (SST) indicate that surface water temperature and salinity conditions during the interglacials MIS 11c and MIS 9e were not much different and very stable along the North Atlantic Current (NAC) and Portugal Current (PC)
Summary
Based on temperature related proxy records from the oceans (Hodell et al, 2000; Lea et al, 2003; McManus et al, 2003; de Abreu et al, 2005; Helmke et al, 2008) and from Antarctica (Petit et al, 1999; Jouzel et al, 2007) it was an unusually long lasting interglacial and northern heat piracy, i.e. the enhanced advection of warm waters from the South into the North Atlantic, was at its maximum (Berger and Wefer, 2003). The definition of an interglacial as the period of ice volume minimum/sea-level highstand (Shackleton, 1969), shortens this interval to 409 to 396 ka (based on the LR04 chronology; Lisiecki and Raymo, 2005). During the transition to MIS 10 the Gulf Stream waters at ODP Site 1056 experienced higher temperature variability linked to cooling episodes in the surface and thermocline waters (Chaisson et al, 2002; Billups et al, 2004), episodes that are contemporary with those recorded at ODP Site 980
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