Numerous previous studies of the DWBC have made use of the grain size of the detrital silt fraction of cores from the crest of the BOR in order to study paleocurrent intensity [Johnson et al., 1988; Haskell et al., 1991; Haskell and Johnson, 1993; Bianchi et al., 2001; Yokokawa and Franz, 2002]. Here we use the grain size proxy, which provides an estimate of relative changes in near-bottom flow intensity of the depositing paleocurrent [McCave et al., 1995; McCave and Hall, 2006], with near-bottom flow speeds decreasing with distance from the fast flowing core of the DWBC [Stahr and Sanford, 1999].
 The proxy can be used where the source sediment is characterized by a broad range of grain sizes and the distance to the core site is sufficient for a sorted signal to develop [McCave et al., 1995; Bianchi et al., 2001; McCave and Hall, 2006]. These conditions are met on the BOR as its supply of terrigenous sediment is principally transported by the DWBC [Heezen et al., 1966] from the American continental margin. Bianchi et al.  confirmed that at Sites 1060 and 1062 on the BOR there was no significant sediment input from ice rafted debris (IRD) or turbidity current influence during the MIS 5 interval. Likewise, visual inspection and core logs from Sites 1059 and 1057, which are both located on the ridge crest and so less likely to be affected by turbidites and debris flows, show no evidence of any major down-slope deposition events.
 It has been shown that the DWBC changed position and migrated vertically in the water column during past climatic cycles [e.g., Ledbetter and Balsam, 1985; Johnson et al., 1988; Haskell et al., 1991; Bianchi et al., 2001]. As pointed out by Bianchi et al. , at any given point in time, no single value of the along the BOR is representative of the overall relative flow velocity of the DWBC. Therefore any sedimentological paleocurrent data from the BOR must be viewed in terms of both changing vigor at one or more given localities and position/depth of the DWBC. The records in this investigation (Figure 3) are interpreted using the same principles outlined by Bianchi et al. , making the assumption that production rates of LNADW are a major control on the vertical movement of the DWBC. Therefore, as each of the core sites is located at water depths above the present-day fast flowing primary DWBC core, a synchronous increase in at both depths is indicative of a shoaling of the DWBC core and a reduction in LNADW production.
5.1. Paleocurrent Variability During the Holocene
 Recent studies have suggested that the modern hydrographic regime in the western North Atlantic may have only been established ∼7 kyr B.P. when the formation of LSW started [Hillaire-Marcel et al., 2001; Cottet-Puinel et al., 2004]. The Holocene records of cores 39GGC and 43GGC younger than ∼7 kyr should therefore be most akin to the present-day hydrographic setting at the BOR and these data offer us the chance to “ground truth” the configuration of proxy data, employed for the MIS 5 interval, to the modern setting.
5.2. Paleocurrent Variability During MIS 5 and 4
 The boundaries of the MIS 5 substages [Shackleton, 1969] discussed in this study are defined according to those set out by Heusser and Oppo  and are represented by areas in yellow in Figure 3. Comparing the for Sites 1057 and 1059 across the entire MIS 5 interval and into MIS 4 reveals the changing relationship of the relative flow speeds recorded at each site. Site 1057 shows a slight increasing trend in the throughout the record, a trend that is not present at Site 1059, although the increases in unison at both sites during the latter part of cold substages 5d, 5b and during early stage 4 in conjunction with previously identified marine cold events (e.g., C19, C20, C21 and C23 [McManus et al., 1994; Oppo et al., 2001; Heusser and Oppo, 2003]). The δ13C variations at Site 1057 also show a general increase to heavier values throughout the record, intersected by lighter excursions, indicating declining or reduced deep ocean ventilation during the larger cold events (C19–C24). Site 1059 records more pronounced excursions during these cold episodes, but the δ13C values during the warm substages (5c and 5a) remain similar to MIS 5e values.
 During MIS 5e and early 5d (Figure 3e), the shallower Site 1057 records lower grain size values (mean = 15.4 μm) than Site 1059 (mean = 17.1 μm) and these are the lowest values throughout the record and are also lower than those observed in core 43GGC during the Holocene. The offset between the two sites is reasonably constant (1.7 μm ± 0.75 μm) and this observation strongly corroborates one of the key assumptions in this work that sedimentation in the sortable silt range on the crest of the BOR is predominantly controlled by deepwater currents rather than simple down slope sediment movement where coarser grain sizes would always be found at shallower depths [Haskell et al., 1991; Haskell and Johnson, 1993]. Throughout the MIS 5e interval the benthic δ13C (Figure 3e) at both sites are similar at around 0.7‰ and imply the presence of a well mixed water mass between the sites. To further investigate the extent of this water mass, benthic δ13C data were incorporated from two deeper sites (Figure 4), ODP Site 1060 on the BOR (3480 m water depth [Bianchi et al., 2001]) and core GPC-9 on the BahOR (4758 m water depth [Keigwin et al., 1994]). The δ13C data from ODP Site 1060 [Bianchi et al., 2001, Figure 4c] and GPC-9 [Keigwin et al., 1994, Figure 4] also suggest the presence of a similar water mass at greater depths.
Figure 4. Extended depth range benthic isotope (δ18O and δ13C) and CaCO3 (wt %) data. (top to bottom) (a) Benthic δ18O, (b) benthic δ13C, and (c) CaCO3 versus age (kyr B.P.). Records are shown in red (Site 1057, 2584 m water depth), blue (Site 1059, 2985 m water depth), black (Site 1060, 3480 m water depth), and green (GPC-9, 4758 m water depth). Data for Site 1060 are from Bianchi et al.  and are based on Cibicidoides wuellerstorfi, Cibicidoides spp., and Uvigerina spp. The δ18O results for C. wuellerstorfi and Cibicidoides spp. have been corrected by +0.64‰ to account for species-dependent isotopic fractionation, and a correction of +0.7‰ has been applied to the δ13C data for Uvigerina spp. (see Bianchi et al.  for further details). The GPC-9 data from Keigwin et al.  only show the Cibicidoides spp. record in this study. Warm interglacial/substages are shown in yellow. Vertical dashed lines refer to previously identified cold events (labeled) [Oppo et al., 2001; Heusser and Oppo, 2003].
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 The benthic δ13C records during MIS 5e are indicative of a water column that is dominated by northern source waters (NSW). However, the benthic δ13C signature is slightly lighter than that experienced by core 39GGC in the Holocene (Figure 3f) and the δ13C of ∼1‰ [Kroopnick, 1985; Curry and Oppo, 2005] associated with contemporary LNADW. This suggests poorer ventilation during MIS 5e than the Holocene, with δ13C values similar to those expected for the present-day BW of Stahr and Sanford [1999, section 1.2] where the influence of AABW is believed to slightly decrease the benthic δ13C signature [Keigwin et al., 1994]. Today, BW on the BOR is restricted to water depths below 3400 m and only contributes about 15% to the overall transport [Stahr and Sanford, 1999]. However, it is suggested that during peak MIS 5e a NSW mass with a similar benthic δ13C value to present-day BW extended throughout the water column, at least up to ∼2600 m water depth, at the BOR.
 The lower recorded at Site 1057 is suggestive of a weakened LSW production, which would have presumably caused a shoaling or possibly even a loss of the upper dynamic limb of the DWBC, which is LSW-sourced at these depths today. In the absence of any significant influence from the shallower DWBC core Site 1057 would be distant and isolated from the deeper fast flowing core of the DWBC. The formation and properties of the North Atlantic intermediate and deep water over the last interglaciation are contentious. Hillaire-Marcel et al.  suggest that LSW formation was absent during the last interglacial and advocate the presence of a single water mass originating from the Nordic Seas overlain by a thin buoyant surface layer. While Rasmussen et al.  conclude that LSW, with a fairly similar composition as today, was generated throughout the peak of MIS 5e. They also suggest the presence of a benthic foraminiferal “Atlantic assemblage” that does not appear to be linked to overflow water from the Nordic Seas. The compiled benthic δ13C record presented here strongly support the presence of a uniform water mass, dominated by LNADW below ∼2500 m on the BOR, during peak MIS 5e, with evidence from the Site 1057 flow speed record of a weakened LSW. At 2584 m water depth Site 1057 is located at the base of LSW influence on the BOR and therefore highly sensitive to relative changes in LSW and LNADW production and DWBC flow. However, we need to be cautious as, clearly, on the basis of our data alone we cannot rule out continued vigorous production of SLSW ventilating shallower depths.
 The benthic δ13C records (Figure 4b) suggest an apparent reorganization in the hydrography and flow of the DWBC starting close to the MIS 5e–5d boundary at ∼117–110 kyr B.P. At this time a significant lowering of δ13C values was experienced at the two deeper sites (Sites 1060 and GPC-9) indicative of decreasing ventilation, while conditions at the two shallower sites (1057 and 1059) experience a smaller shift toward heavier values. In terms of the records this transition is much less obvious but is marked by the onset of gradually increasing at Site 1059 and 1057 (Figure 3e). These data support a shoaling of the water column structure in response to a decrease in LNADW production at that time [Adkins et al., 1997; Hall et al., 1998]. Such a transition is consistent with the indication of a reduction in the depth of the deep core of the DWBC suggested in the Site 1060 record at ∼118–117 kyr [Bianchi et al. 2001]. Bianchi et al.  suggest that it was only after ∼113 kyr that the core of the DWBC shoaled above Site 1060. This study further suggests that the reduced LNADW between ∼118–113 kyr is part of a longer transition. At ∼111 kyr B.P., a significant shift in the relationship between the and benthic δ13C is observed at Sites 1057 and 1059 (Figures 3e and 3f): a increase at Site 1057 is responsible for the flow speed records at both sites converging and subsequently behaving more coherently through the remainder of MIS 5 and into MIS 4. We suggest that such behavior is indicative of the reestablishment of a stronger and possibly deeper secondary core of the DWBC after ∼111 kyr. The strengthening and deepening of the shallower core would increase the flow speeds recorded at Site 1057 while possibly depressing the depth of the deeper flowing DWBC core. Coincident with this shift, the benthic δ13C records diverge and Site 1057 benthic δ13C values increase to typically >1‰ while the long-term values at Site 1059 remain unchanged. Immediately following this divergence the benthic δ13C at GPC-9 reach minimum values (Figure 4b). The increased benthic δ13C values at Site 1057 imply the development of a clear hydrographic boundary in the ∼500 m of water column that separate the two sites. The shallower, more nutrient depleted water mass has benthic δ13C values slightly higher than those experienced in the Holocene at 39GGC, implying that this water mass is of a northern origin and may represent the initiation of, or a similar water mass to Glacial North Atlantic Intermediate Water (GNAIW) formation. This is supported by Chapman and Shackleton [1998, 1999] and Chapman et al.  who suggest that the gradual increase in δ13C values in core SU90-03 (40°N, 32°W, 2475 m water depth) in the North Atlantic during MIS 5, could signify a long-term change in the depth and/or production rate of NADW. This, in conjunction with the reestablishment of the shallower core of the DWBC, could also explain the long-term increase in the evident at Site 1057 throughout the record (Figure 3e). However, it should be noted that Oppo and Lehman  documented a similar rising trend in benthic δ13C within MIS 5 in subpolar North Atlantic core V29-202 (2658 m water depth), and argued that it was driven by the changing composition of tropical surface feed waters, driven by either biological or thermodynamic processes. However, our grain size records demonstrates that a change in DWBC geometry occurred over MIS 5 and suggests that observed benthic δ13C values cannot be explained solely by changes in the preformed composition of the surface waters, but must involve a physical change in deep ocean circulation.
 Curry and Oppo  demonstrate that the sharp boundary at ∼2 km in the subpolar North Atlantic between northern and southern source water masses during the LGM is eroded as GNAIW flows southward toward the BOR study sites and mixes with Southern Ocean waters. At comparable latitudes to the BOR the whole depth range from 2–4 km appear to be in a mixing zone. Although the precise geometry described by Curry and Oppo  during the LGM may differ slightly from that experienced within MIS 5 our results suggest that once the δ13C gradient was established it persisted throughout the subsequent glaciation and that at least 1059 lay in a water mass gradient most of the time. This is supported by data from GPC-9 [Keigwin et al., 1994, Figure 6c], which show a similar benthic δ13C pattern to the shallower two sites but with a more pronounced influence of AABW.
 The latter parts of cold substage 5b and, to a lesser extent, 5d are characterized by a transient interval of increased associated with significant positive excursions in the planktonic δ18O records at each site indicative of sea surface cooling (cold events: C24, C23 and C21; see section 5.3; Figures 3c and 3e). A lack of benthic foraminifera, possibly due to the increased influence of corrosive southern-sourced AABW, particularly during the MIS 5b event, precludes a clear observation of the hydrographic changes associated with these intervals but do hint at reduced water column ventilation. An increase in dissolution during these intervals is consistent with the reduced CaCO3 observed (Figure 4c). The maxima in DWBC flow speeds during these intervals are indicative of a rapid shoaling of the DWBC to a depth at which the fast flowing core is close to Sites 1059 and 1057 and most likely increased influence of AABW.
 MIS 4 is only partly represented in our records. However, it is during this interval that the largest changes in DWBC flow speed and hydrography are recorded. Two δ13C excursions centered at ∼75 and ∼69 kyr, consistent with surface cooling events C20 and C19, are associated with a substantial increase in the flow speed at Site 1057 and a smaller increase at the deeper Site 1059 (Figures 3c and 3e). This suggests a significant decrease in NADW/GNAIW production leading to a rapid shoaling of the deeper DWBC core more proximal to and possibly above Site 1057. These flow speed changes are accompanied by similarly abrupt excursions in the benthic δ13C records at both sites, and have also been previously documented on the BahOR [Keigwin et al., 1994]. In the case of the younger C19 event benthic δ13C values fall to below −0.5‰ at both Site 1057 and 1059, characteristic of a water mass similar to unmodified AABW during the LGM [Oppo and Fairbanks, 1987; Curry et al., 1988]. During the interstadial intervals surrounding these cold events the benthic δ13C and values return to similar levels as those observed during MIS 5a. The correlation of these events in the deep ocean to other proxies such as the δ18O NGRIP record which provides a temperature proxy record for the northern North Atlantic (Figure 3) highlights the potential global significance of these oscillations.
5.3. Surface–Deep Ocean Links
 Oppo et al.  have previously made a detailed study comparing the planktonic δ18O record at Site 1059 to its benthic δ13C record, finding virtually synchronous oscillations between the two proxies suggestive of a persistent surface-deepwater linkage from early in MIS 5e. Oppo et al.  argue that SST variability provides the simplest explanation for the suborbital oscillations (4–10 kyr pacing) in planktonic δ18O apparent during MIS 5, as they are similar in magnitude and timing to the planktonic δ18O variations in MIS 3, which have been previously attributed to SST [Keigwin and Boyle, 1999; Sachs and Lehman, 1999]. In this study it is clear that planktonic δ18O [Oppo et al., 2001] evidence for surface cooling, particularly during cold events C24–C19, is not only associated with weak NADW (low benthic δ13C), but also corresponds to a shoaling DWBC as represented by increasing values (Figure 3).
 No specific SST proxy measurements are available for MIS 5 from the BOR, so in Figure 5 we compare the Site 1057 and 1059 proxy records with the high-resolution SST record derived from measurements of the unsaturated alkenone ratio in core MD95-2036 recovered from the nearby Bermuda Rise (33°41.444′N, 57°34.548′W, 4462 m water depth [Lehman et al., 2002]). The correlation between the SST at MD95-2036 and the planktonic δ18O at Sites 1057 and 1059 (Figure 5a) confirms the presence of a series of synchronous abrupt cooling events at each site. Intriguingly, comparison of the alkenone-derived SST record and the planktonic δ18O record of Site 1059 also reveals a lack of shorter-timescale structures in the alkenone record compared to the planktonic δ18O record. This finer structure in the Site 1059 planktonic δ18O data could be a function of changes in sea surface salinity (SSS), which is not recorded in the purely temperature-related alkenone record. Alternatively, the finer-timescale structure may be lost in the alkenone record because of mixing of alkenones of different ages in any one sample [Ohkouchi et al., 2002], or it could result from changes in seasonality, preservation or depth habitat between the two proxies [e.g., Popp et al., 2006].
Figure 5. Comparison of the alkenone-derived SST from Bermuda Rise core MD95-2036 [Lehman et al. 2002] and Sites 1059 and 1057 proxy records. (a) MD95-2036 alkenone-derived SST along with planktonic δ18O for Sites 1057 and 1059. (b) SST normalized using a simple second-order polynomial regression along with results for Sites 1057 and 1059. (c) Normalized SST along with the benthic δ13C data for Sites 1057 and 1059. Vertical dashed lines refer to previously identified cold events (labeled) [Oppo et al., 2001; Heusser and Oppo, 2003].
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 Lehman et al.  showed a strong correlation between their alkenone record and the benthic δ13C data of GPC-9 on the BahOR [Keigwin et al., 1994]. They observed a clear correspondence between low SST and low δ13C values, which is also recorded in the record of Hall et al.  in MD95-2036, as evidence of reduced NADW formation. This has resulted in speculation that local suppression of deepwater formation is one likely mechanism for the amplification of the direct cooling effects of iceberg melting and ice sheet discharge [Lehman et al., 2002]. The relationship described by Lehman et al.  together with the findings of Oppo et al.  are further substantiated by the benthic δ13C record of Site 1059 that shows a strong relationship between colder SSTs and low benthic δ13C (Figure 5c), suggesting reduced LNADW formation during cold intervals with a greater influence of AABW. During MIS 5 and the 5/4 transition, this relationship is less pronounced at the shallower Site 1057 probably in part because of the lower resolution and the proximity of GNAIW that maintains high δ13C values. Comparison of the MD95-2036 alkenone SST record, the Site 1059 planktonic δ18O record and Site 1059 (Figure 5b) reveals a striking and persistent relationship throughout MIS 5 and the 5/4 transition. This suggests a very tight linkage between SST in the subtropical western North Atlantic and DWBC activity which extends north to the Bermuda Rise. The link between these shallow and deep locations may lie in the Nordic Seas. During the last interglacial surface temperature variations in the Nordic Seas coincided with deepwater changes [Fronval and Jansen, 1996; Fronval et al., 1998], with the majority of fluctuations being related to reductions in the AMOC. Furthermore, Oppo et al.  suggest that during deglacial and glacial periods ocean-ice interactions and deepwater variability may amplify suborbital variability. They suggest that during the penultimate deglaciation NADW production varied between the Nordic Seas and open North Atlantic positions, similar to the situation thought to occur during the LGM [Boyle and Keigwin, 1987], and this occurred in parallel with SST oscillations. It is suggested that this situation was not confined to the penultimate deglaciation but played an important role throughout MIS 5 and into MIS 4, with warm water penetrating into the Nordic Seas resulting in melting of ice and a weakening of NADW formation [Oppo et al., 2001]. The effects of these intervals of weaker and shallower NADW formation are not confined to the marine environment. They can also be linked to cold events in the terrestrial pollen record of the southeastern United States [see Heusser and Oppo, 2003] and the larger oscillations (C19–C24) have cold counter parts in the Greenland Ice core records (Figure 3). Although the cause of such suborbital variability remains unknown, the observed variations in deepwater circulation and the associated northward transport of heat are likely to have played an important role communicating the climatic variation throughout the wider circum-North Atlantic region [Heusser and Oppo, 2003].