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Abstract: Salinity increase in the subtropical gyre system may have pre-conditioned the North Atlantic Ocean for a rapid return to stronger overturning circulation and high-latitude warming following meltwater events during the Last Glacial period. Here we investigate the Gulf Stream – subtropical gyre system properties over Dansgaard–Oeschger (DO) cycles 14 to 12, including Heinrich ice-rafting event 5. During the Holocene and Last Glacial Maximum a positive gradient in surface dwelling planktonic foraminifera δ18O (Globigerinoides ruber) can be observed between the Gulf Stream and subtropical gyre, due to decreasing temperature, increasing salinity, and a change from summer to year-round occurrence of G. ruber. We assess whether this gradient was a common feature during stadial-interstadial climate oscillations of Marine Isotope Stage 3, by comparing existing G. ruber δ18O from ODP Site 1060 (subtropical gyre location) and new data from ODP Site 1056 (Gulf Stream location) between 54 and 46 ka. Our results suggest that this gradient was largely absent during the period studied. During the major warm DO interstadials 14 and 12 we infer a more zonal and wider Gulf Stream, influencing both ODP Sites 1056 and 1060. A Gulf Stream presence during these major interstadials is also suggested by the large vertical δ18O gradient between shallow dwelling planktonic foraminifera species, especially G. ruber, and the deep dwelling species Globorotalia inflata at site 1056, which we associate with strong summer stratification and Gulf Stream presence. A major reduction in this vertical δ18O gradient from 51 ka until the end of Heinrich event 5 at 48.5 ka suggests site 1056 was situated within the subtropical gyre in this mainly cold period, from which we infer a migration of the Gulf Stream to a position nearer to the continental shelf, indicative of a narrower Gulf Stream with possibly reduced transport. [Copyright &y& Elsevier]

Abstract: The Western Boundary Undercurrent (WBUC), off eastern America, is an important component of the Atlantic Meridional Overturning circulation and is the principal route for southward transport of North Atlantic waters and southward return of Southern Source Water (SSW). Here a direct flow speed proxy (mean grain size of the sortable silt) is used to infer the vigour of flow of the palaeo-WBUC at Blake Outer Ridge, (ODP Site 1060, depth 3481 m) during Marine Isotope Stage (MIS) 3. The overall shape of the flow speed proxy record shows a complex pattern of variability, with generally more vigorous flow and larger-scale flow variations between 35 and 60 ka than in the younger part of MIS 3 and MIS 2 (<35 ka). Six events of reduced bottom flow vigour (Slow Events, SEs) occur. These appear uncorrelated with Heinrich events, but are instead synchronous with the warming phases of Antarctic Warm Events A-1 to A-4 (with one new one, A-1a and one poorly defined, ‘A-0’). This indicates that Antarctic climate exerts a stronger control on deep flow vigour in the North Atlantic during MIS 3 than Northern Hemisphere climate. The correspondence of SEs with Antarctic warming suggests a weaker WBUC flow due to reduced volume flux at SSW source or reduced SSW density. Because the variability of the lower limb of the WBUC was not connected to sharp North Atlantic changes in temperature, it is unlikely that the Dansgaard/Oeschger cycles were associated with a mode of MOC variation involving whole-ocean overturn, but more likely with perturbations of only the shallow Glacial Gulf Stream–Glacial Northern Source Intermediate Water cell. Nutrient proxies (benthic carbon isotopes and Cd/Ca of Uvigerina peregrina) at this site show similar trends to the GRIP δ 18O record. This correlation has previously been attributed mainly to hydrographic and flow changes but is here shown to be better explained by variations in surface ocean productivity and subsequent decomposition of 12C rich organic material on the sea floor. [Copyright &y& Elsevier]

Abstract: Pleistocene stable carbon isotope (δ 13C) records from surface and deep dwelling foraminifera in all major ocean basins show two distinct long-term carbon isotope fluctuations since 1.00 Ma. The first started around 1.00 Ma and was characterised by a 0.35‰ decrease in δ 13C values until 0.90 Ma, followed by an increase of 0.60‰ lasting until 0.50 Ma. The subsequent fluctuation started with a 0.40‰ decrease between 0.50 and 0.25 Ma, followed by an increase of 0.30‰ between 0.25 and 0.10 Ma. Here, we evaluate existing evidence and various hypotheses for these global Pleistocene δ 13C fluctuations and present an interpretation, where the fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon due to ventilation changes and/or changes in the production and export ratio. Our model indicates that to satisfy the long-term ‘stability’ of the Pleistocene lysocline, the ratio between the amounts of change in the organic and inorganic carbon burial fluxes would have to be close to a 1:1 ratio, as deviations from this ratio would lead to sizable variations in the depth of the lysocline. It is then apparent that the mid-Pleistocene climate transition, which, apart from the glacial cycles, represents the most fundamental change in the Pleistocene climate, was likely not associated with a fundamental change in atmospheric pCO2. While recognising that high frequency glacial/interglacial cycles are associated with relatively large (100 ppmv) changes in pCO2, our model scenario (with burial changes close to a 1:1 ratio) produces a maximum long-term variability of only 20ppmv over the fluctuation between 1.00 and 0.50 Ma. [Copyright &y& Elsevier]

Hemipelagic intervals in four giant Calypso piston cores from the Balearic Abyssal Plain (western Mediterranean Sea) were studied in order to determine fluctuations in the supply of terrigenous sediments to the basin during the last 130,000 years and possible climatic modulation of terrigenous sediment and particle flux. Carbonate records from hemipelagic intervals in these cores display distinct ‘Atlantic’ type cycles, where glacial periods show, on average, 20% lower calcium carbonate contents than interglacial period equivalents. The decrease in calcium carbonate seen during glacials may be the result of increased dilution by terrigenous (aeolian and fluvial) particles derived from aeolian and/or fluvial sources. Higher glacial aeolian particle fluxes to the western Mediterranean Sea may have been caused by changes in the prevailing atmospheric conditions, resulting in drier and colder climatic conditions in the source areas and more frequent and intense outbreaks of dust transport. The colder and drier climatic conditions in combination with a lowered sea level would allow rivers to dump their sediment load closer to the shelf edge, promoting enhanced fluvial input into the basin. Besides significant calcium carbonate variations that correlate with glacial/interglacial cycles, several short-term abrupt changes in the calcium carbonate content are recorded, which appear coincident with enhanced magnetic susceptibility values, especially the events within Marine Isotope Stage (MIS) 3. The timing of these short-term events may be linked with cooling events in the North Atlantic, particularly Heinrich Events and cold events within MIS 5. These short-term trends may possibly be explained by increased terrigenous particle transport, due to drier and colder climate in the Mediterranean borderlands. Our results further show that about 90% of the sedimentary sequence of the Balearic Abyssal Plain consists of turbidites. Generally, these appear not to have caused significant erosion, and an almost complete continuous time stratigraphy is present in the intercalated hemipelagic intervals. [Copyright &y& Elsevier]