Description: © The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Climate of the Past 6 (2010): 531-552, doi:10.5194/cp-6-531-2010.

Description: 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.

Description: The Fundacao para a Ciencia e a Tecnologia (FCT) through the PORTO (PDCT/MAR/58282/2004) and SEDPORT projects (PDCTM/ 40017/2003), and postdoctoral (SFRH/BPD/21691/2005) and PhD (SFRH/BP/13749/2003) fellowships funded A. V. and T. R. Additional funding to T. R. and J. G. was provided by the Consolider-Ingenio 2100 Project CE-CSD2007-0067.

Description: We establish the relationship between shell oxygen(6le0) and carbon (S13C) isotopic composition and size and thickness for the planktonic foraminifers Orbulina universa and Neogloboquadrina dutertrei (size only). The degree to which the stable isotopic composition of individual shells correlates with shell size limits their applicability to paleoenvironmental reconstructions and must therefore be examined carefully. We selected shelis from four intervals from a western and an eastern equatorial Atlantic core corresponding to the glacialhnterglacial extremes of the last 150,000 years (isotope stages 1, 2, 5e, and 6). We estimate the shell thickness of 0, universa using a mathematical relationship between the geometry and physical properties of a calcite sphere. Thickness measurements on selected shells of this species confirm that the calculated thickness, once it has been corrected for shell porosity, is equivalent to the measured thickness. There are no consistent trends between 0. universa 6l80 or 613C values and shell size or thickness for the size range 450-900 pm. For N. dutertrei, there are no systematic fluctuations between shell 6l80 and size between 450-700 pm. N. dutertrei shells larger than 500 pm show no 613C : size relationship. We are confident that the stable isotopic variability contained in 0. universa (450-900 pm) and N. dutertrei (〉500 pm) assemblages analyzed does not fluctuate with differences in shell morphology. The stable isotope data from these size ranges can thus be confidently used for paleoenvironmental reconstructions.

Description: The ~100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope ( 18 O, 13 C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in 13 C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric p CO 2 . The amplitude of glacial-interglacial changes in 13 C increased across the MPT, but this was not mirrored by changes in nutrient content. We interpret this in terms of air-sea CO 2 exchange effects, which changed the 13 C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO 2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric p CO 2 . Conversely, following the establishment of the ~100 k.y. glacial cycles, 13 C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO 2 into the North Atlantic following the MPT.