5.1. Glacial to Interglacial Changes in the NE Atlantic
The production of NADW and its export to the Southern Ocean have been identified as a major control in the meridional heat and salt transfer between the Northern and Southern Hemispheres. The long-term behavior of the sources of NADW has been investigated in detail through (geochemical) analysis of deep-sea cores at many sites [e.g., Boyle and Keigwin, 1982; Raymo et al., 1997]. Such data have shown that the production of Lower (L)NADW was suppressed during glacials, which severely reduced the meridional heat transport and thus amplified the Pleistocene glaciations [Raymo et al., 1990]. Physical evidence for changes in deep ocean circulation on these timescales has also been sought. Regardless of the merits (as discussed in section 3.2) of using a flow speed parameter calculated by difference from a time series of assumed input flux, the reconstructions of MM'95 and McCave et al. [1995b], based on the Biogeochemical Ocean Flux Study (BOFS) suite of cores, between 50–60°N and 15–25°W, and ranging in water depth between 1100–4045 m, remain the spatially most comprehensive attempt at examining differences between last glacial and Holocene paleocurrents in the North Atlantic. These data were the first to provide physical confirmation that during the last glacial maximum, the flow of LNADW was slower than in the Holocene and that this was followed by alternating flow increases (early deglacial, Bølling-Allerød, early Holocene) and decreases (Heinrich-1, Younger Dryas) (Figure 14). Comparison of Figure 14 with the recently published Pa/Th flow records of McManus et al.  and Gherardi et al.  shows them to be remarkably similar.
Figure 18. Sedimentological parameters for core MD95-2040 on the Iberian margin plotted against age [Hall and McCave, 2000]. (a) SPECMAP stack of benthic δ18O values [Martinsson et al., 1987]. (b) Terrigenous silt/clay ratio (wt% 2–63 μm/wt% <2 μm). (c) ; the line under the data in stage 2 indicates the part of the record believed to be unreliable as a current indicator because of possible downslope contamination. The vertical dashed lines indicate the boundaries between Marine Isotope Stages (MIS) achieved by correlating the benthic isotopic record for the core to SPECMAP. Note the clear relationship of slower flow in cold/cool periods from stages 6 to 3 with a lag of a few thousand years relative to isotopic shifts.
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5.2. Gardar Drift and Bermuda Rise at the Penultimate Glacial Termination
Records from two cores from different North Atlantic basins, but both under the influence of NADW flow, one close to its origin and the other down-stream in its evolution are illustrated here (Figures 19b and 19c). The first is from (NEAP 18K, 52°46′N, 30°20.7′W, 3275 m water depth) at the southern extremity of Gardar Drift. This was deposited from Iceland Scotland Overflow Water (ISOW) in the Iceland Basin [Bianchi and McCave, 2000] but was covered by Southern Source Water (SSW) at the LGM [Bertram et al., 1995]. Core MD95-2036 was taken from eastern Bermuda Rise (33°41.4′N, 57°34.5′W, 4462 m water depth), where the sediments are also deposited under current influence [McCave et al., 1982; Laine et al., 1994]. This core is also sited under LNADW at present, with SSW (originally Antarctic Bottom Water) at greater depths over the adjacent Sohm Abyssal Plain (5500 m depth). Because bottom currents exert primary control on sediment deposition and there is negligible interference by direct fall-out from icebergs at either site during the interglacial, they are optimal locations to record bottom water flow changes.
Figure 19. Near bottom current flow speeds for (b) NEAP 18K (Gardar Drift): (c) MD95-2036 (Bermuda Rise) from Hall et al.  together with similar data from (d) ODP Site 1060 and (e) ODP Site 1062 (both Blake-Bahama Outer Ridge) from Bianchi et al. . Also shown for comparison is (a) the δ18O record from NGRIP [North Greenland Ice Core Project Members, 2004]. The gray bands numbered I to IV distinguish events of deceleration followed by acceleration which, on the basis of independent age models for the two sites in Figures 19b and 19c, match very closely in time, demonstrating coordinated flow changes across the North Atlantic [Hall et al., 1998].
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The two cores show several synchronous events (labeled I to IV) in the record interpreted as deceleration followed by acceleration of the deep current (Figures 19b and 19c). Events II to IV are closely correlated with cooling episodes recorded in the δ18O of the NGRIP Greenland ice core (Figure 19a). Core NEAP 18K shows an abrupt and large decrease in the (from about 16 to 11 μm (Figure 19b, event I), indicative of slowing flow speeds, from ∼130 ka to 129 ka, just before full stage 5e. A similar flow speed decrease is seen on Bermuda Rise about 1000 years earlier, which is also clearly represented in the hydrographic Cd/Ca record of that site [Adkins et al., 1997]. These termination events [cf. Hall et al., 1998, Figure 3] are important in view of recent concepts that infer a redistribution of heat and salt within the ocean as a means to stimulate or slow down the vigor of the MOC.
Inferred flow speeds at both sites increase rapidly into full interglacial conditions (event I), suggesting strengthening NADW flow. A notable feature in both records is the sharp decrease in the flow speed seen at about 120 ka reaching a minimum value at ∼119–118 ka (event II). During that event between 120 and 118 ka, the abundance of the warm species N. pachyderma (d.) in NEAP 18K decreases from 35% to <5% and planktonic δ18O values increase by ∼0.65‰, which, with negligible ice volume effect, suggest a cooling of the northward-flowing surface water temperature of 2 to 3°C. These data indicate that the flow slowed down in an event that marked, if not the end [Adkins et al., 1997], then the beginning of the end of the last interglacial, and which was not clearly marked at this time in any chemical proxy. The flow speed evidence here was the key. However, subsequent flow speed and hydrographic reconstruction of the DWBC on the Blake Ridge and Bahama Outer Ridge (ODP Site 1060, 3480 m water depth and Site 1062, 4760 m water depth; Figures 19d and 19e) by Bianchi et al.  suggest that the LNADW production changes inferred at the ∼118 ka event were more complex than suggested above and involved an additional vertical migration of the flow axis of the DWBC. This latter observation is important as it highlights the influence of spatial variability of proxy reconstruction within a DWBC system that, under changing source water production, may vary both its velocity and vertical position in the water column. Such effects can only be recognized by the use of multiple sites forming a depth transect across the current of interest.
This approach was adopted by Yokokawa and Franz , who integrated SS measurements and magnetic properties for the interval MIS 10.2–8.3 (350 to 250 ka) using ODP Sites 1055–1062. These BBOR sites provide an intermediate and deep water transect across the DWBC from 1800 m to 4760 m water depths. The data show a similar signal to the DWBC reconstruction of Bianchi et al.  with the fast flowing core located >3000 m water depth during interglacial periods and a shoaling to around 2200 m water depth during glacial periods. Through the transitions between these periods, the fast flowing core of the DWBC decreased in depth and its intensity increased. On its own the SS proxy provides no indication of flow direction. Yokokawa and Franz  employed the azimuth of the maximum axis of the magnetic anisotropy ellipsoid (Kmax), as an additional current flow direction indicator. For the BOR overall dominant flow directions inferred for the warm intervals were broadly parallel to the bathymetric contour lines. Interestingly, at the deepest site (Site 1062, 4762 m water depth) the directional data suggest the possibility of changing flow direction between warm and cold periods, but whether this relates to changing mode of particle alignment with speed or an inflow along the western side of the basin is unknown.
5.3. Holocene Iceland-Scotland Overflow Water Variability
The global significance of millennial-scale climate shifts during the last glacial cycle is widely accepted. Several high-resolution paleoclimatic investigations of ice cores [e.g., O'Brien et al., 1995] and marine sediment cores [e.g., Bond et al., 1997] suggest a continuation, although much less distinct, of this variability throughout the Holocene. Bianchi and McCave  inferred changes in the current strength of Iceland Scotland Overflow Water (ISOW) from core NEAP 15K (51°07.09′N, 21°52.09′W, 2848 m water depth) recovered from the Gardar drift in the South Iceland Basin. They found evidence for a quasiperiodic ∼1500-year variability throughout the Holocene which they also linked, through the northward heat transport in the surface ocean, to climate events in Northern Europe (Figure 20), notably by showing enhanced ISOW flow speeds during the Medieval Warm Period and a subsequent decrease during the Little Ice Age. In addition, a similar Holocene ISOW flow speed record from core NEAP 4K [Hall et al., 2004] located some 1100 km north of NEAP 15K on Björn Drift (61°29.91′N, 24°10.33′W, 1627 m water depth) reports evidence for significant (>95% confidence level) concentration of variance at 1000 and 400 year periods, although the millennial scale cyclicity was poorly defined with additional weaker (>90%) 1400-year and 700-year cycles. Such data are in agreement with evidence of a more prominent 900–1000 year and 400–550 year periodicities in Holocene climate proxies [Stuiver and Braziunas, 1989; Chapman and Shackleton, 2000; Schulz and Paul, 2002; Risebrobakken et al., 2003]. However, we should keep in mind that the identification of spectral peaks in such time series are not without controversy, and a reanalysis of Bianchi and McCave's  data has suggested that the record is better described as containing a spectral continuum without significant spectral lines, possibly related to aliasing of an annual cycle [Wunsch, 2000]. Additionally, it is intriguing that in neither NEAP 15K nor -4K do times of reduced ISOW flow speeds coincide with the IRD events described by Bond et al. .
Figure 20. North Atlantic Holocene paleoenvironmental proxy records from Bianchi and McCave . Data are shown on a calendar years B.P. (and AD/BC) basis. (a) δ18O data from central Greenland GISP2 ice core with Gaussian interpolation using a 300-year window. Solid and dashed arrows between 0 and 7.5 kyr B.P. represent periods of general warming or cooling which match relative decreases or increases in the intensity of ISOW flow, respectively. (b) record for NEAP-15K with Gaussian interpolation using a 300-year window. (c) Planktonic foraminiferal δ18O data from the Sargasso Sea [Keigwin, 1996], which mainly reflects changes in sea surface temperature.
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Recently, Ellison et al.  report results from a multiproxy study of core MD99-2251 recovered from southern Gardar Drift (57°26.87′N, 27°54.47′W; 2620 m water depth). This core is near NEAP 15K and has a mean sediment accumulation rate of ∼110 cm kyr−1 over the interval of 9,200–7,200 years ago. The data show that the “8.2 ka event” was marked by two distinct cooling events at 8,490 and 8,290 years ago. An associated decrease in was interpreted as evidence for a significant reduction in ISOW flow speeds. These data provide the strongest evidence yet that surface salinities followed by deep ocean flow changes were forced by meltwater outbursts from a multistep final drainage of the proglacial lakes Aggasiz and Ojibway. Clear identification of these rapid changes is not possible in Bianchi and McCave's  results (even with sedimentation rate of 40 cm kyr−1) and needs the very high sedimentation rates found here to get a signal uncompromised by bioturbation. Of course, ISOW is only one component of the precursor water masses that ultimately comprise North Atlantic Deep Water. Future work should be aimed a providing an integrated reconstruction of each these precursors (i.e., Denmark Strait Overflow Water and Labrador Seawater, as well as ISOW) in order to better identify the dominant modes of Holocene (and older) deep water variability and surface ocean climate in the North Atlantic.
5.4. Flow Into Southern Hemisphere Ocean Basins
Recent work by Gröger et al. [2003a] at ODP Sites 927 located on the Ceara Rise in the western equatorial Atlantic (5°27.7′N, 44°28.8′W, 3315 m water depth) found clear evidence for weakened LNADW flow and decreased ventilation during the glacial periods in the time interval 800 ka to 300 ka (MIS 20-8) (Figure 21c). Such a signal contrasts in antiphase with the Southern Hemisphere record of Hall et al.  from ODP Site 1123 (Figure 21b). This site is located on the northeast flank of the Chatham Rise, east of New Zealand (41°47.2′S 171°29.9′W, 3290 m water depth) beneath the southwest Pacific DWBC. The Site 1123 data in Figures 21a and 21b clearly show faster flow in glacial periods and slower during interglacials over the past 1.2 Myr. Significant spectral peaks were identified at each of the orbital frequencies. These were coherent with benthic records of both oxygen and carbon isotopes at 98% (for 100 and 41 ka) and 90% confidence (for 23 ka). Hall et al.  suggest that the pattern of increased glacial DWBC flow speeds was related to greater glacial production of AABW/Circumpolar Deep Water (CDW), a feature supported by the diatom tracer data of Stickley et al.  and evidence for a more vigorous glacial flow of CDW seen in the grain size data from cores recovered on a sediment drift south of Shag Rocks passage, in the Drake Passage outflow region [Pudsey and Howe, 1998; Howe and Pudsey, 1999]. These results resolved the paradox that, although the DWBC has a very large flux, presently it appears to be slow-moving around the New Zealand Margin (shown by current meters, nepheloid layers, bottom photographs), yet there are extensive scours around volcanic pinnacles on the seabed [McCave and Carter, 1997] due to the faster flow in the past. In addition, further downstream, these enhanced flows may have produced the circulation changes that drove the glacial increases in sediment focusing recorded in the central equatorial Pacific over the past 300 kyr [Marcantonio et al., 2001]. A clear relationship was also documented between the benthic carbon isotope gradient between Sites 1123 and 849 (Δδ13C(1123-849)) and with periods of reduced ventilation inferred from the isotopes (high Δδ13C(1123-849)) associated with reduced DWBC flow speeds. Cross-spectral analysis of these records show they are >90% coherent with zero phase lags at the eccentricity and obliquity periods. Such a signal requires a substantial input of nutrient-depleted LNADW to the Southern Ocean during interglacials in addition to the reduced flux of LNADW in glacials, as suggested in the Site 927 data. Waters feeding the Pacific DWBC include LNADW which is modulated by the Antarctic Circumpolar Current (ACC) in the South Atlantic, where these waters are mixed with cold deep waters (Antarctic Bottom waters, AABW) from source regions in the Weddell and Ross Seas and Adelie Coast, to form Circumpolar Deep Water (CDW).
Cross-spectra between the Site 927 and benthic δ18O show that minimum flow speeds occur around 7.6 ka after maximum ice volume at the eccentricity period [Gröger et al., 2003b]. In contrast, a similar comparison at Site 1123, over the past 1.2 Myr, found a zero phase lag (of opposite sign) between and benthic δ18O at the 100 kyr period, and a small lag of 2.1 kyr ± 1.4 kyr at the obliquity period. This suggests a much slower coupling between ice-volume induced changes in LNADW production and the velocity of LNADW in the equatorial Atlantic, than is seen in the southern Hemisphere between AABW production and the flow speed of the DWBC entering the Pacific Ocean. This observation is consistent with the possible strong dependence of AABW production on the winds over the Southern Ocean [Rahmstorf and England, 1997].
Although not specifically using it must be mentioned that the first attempts to relate grain size to relative paleocurrent speeds were in the South Atlantic [Ledbetter and Johnson, 1976; Ellwood and Ledbetter, 1977; Ledbetter, 1984]. One of the early results of this work was the conclusion that the deep flow of AABW through Vema Channel leaving Argentine Basin was stronger at glacial maxima, similar to the result of Hall et al.  for the SW Pacific (but see also section 5.5). Also in the South Atlantic, Kuhn and Diekmann  used the abundance of the SS proxy (SS%) to infer flow strength over the past 590 kyr, at 4624 m water depth on the north side of Agulhas Ridge in the S Cape Basin (ODP Site 1089; 40°56.18′S, 09°53.64′E) under a westward deep geostrophic flow [Tucholke and Embley, 1984]. The SS% was used in preference to the in this study because of a suggested modification of terrigenous silt grain-size distribution caused by the removal of biogenic opal required for reliable measurement. Near bottom waters at Site 1089 are believed to be related to the outflow of the deepest layers of the Weddell Sea. SS% showed well defined glacial-interglacial alternations with more vigorous flow during interglacial periods, suggestive of higher production rates of dense bottom water and invigoration of regional contour currents. Kuhn and Diekmann  suggested this variability may be related to the presence of floating ice shelves augmenting deep water formation during high sea level stands. Comparison with the SW Pacific DWBC flow speed record [Hall et al., 2001], which is influenced by a substantially shallower component of CDW than that recorded at Site 1089, may suggest a strong depth related variability in both production and current activity in the Southern Ocean water masses on glacial-interglacial timescales.
Over the past few years oceanographic interest in the Indian Ocean has increased. This is partly because it lies under the path of the so-called “Indonesian Throughflow,” which feeds into the S. Equatorial Current, which in turn feeds the Agulhas Current that leaks warm salty water into the South Atlantic. At present only a single paleocurrent study has been published from the Indian Ocean. McCave et al.  examined bottom flow through the Madagascar–Mascarene Basin into Amirante Passage over the last glacial-interglacial cycle. These sediments under the DWBC are plastered up against the Madagascar Ridge south of Madagascar, and lie at the foot of Farquhar Ridge north of the island in the entrance to Amirante Passage [cf. McCave et al., 2005, Figures 1 and 3]. Low sedimentation rates hampered high resolution reconstruction at shallow depths, but core WIND 28K in the entrance to Amirante Passage (10°09.23′S, 51°46.15′E, 4157 m water depth, with a sedimentation rate of 4 cm kyr−1) offers an initial indication of glacial-to-interglacial behavior in the major deep inflow path to the Indian Ocean (Figure 22). In general the results, perhaps surprisingly, suggest bottom flows varied only slightly between glacial and interglacials, with faster flow in the warm periods of the last interglacial and minima in cold periods. The dominant feature of the WIND 28K records are four pulses of substantially faster flow that correspond to positive benthic δ18O shifts of 0.5–1‰. McCave et al.  emphasize the correspondence of these pulses to global cooling phases and major falls in sea level, suggesting sharp increases of bottom-water density, and a transient local geostrophic effect. In contrast to the deep inflow to the SW Pacific (shown in Figure 21) peak flow speeds in the Amirante Passage appear limited to the periods of inferred density variation, not to the subsequent periods of uniform higher density (i.e., glacial maxima). McCave et al.  conclude that during the inferred periods of density change and high flow speeds, the deep near-bed waters of the Indian Ocean inflow were strongly stratified. This process culminated in the basins filling up with uniformly higher density water, thereby removing the density contrast between the lower inflow and water above, resulting in slower flow speeds in the glacial maxima.
It is of interest and possible significance that Ledbetter's [1986a] reworking of the Vema Channel data indicated that the flow speed maxima also occurred at interglacial to glacial transitions, specifically MIS 7 to 6 and 5a to 4. Like the Indian Ocean Amirante Passage data, this too is from flow through a choke-point. More work is clearly needed in each of the southern hemisphere gateways to characterize the glacial-interglacial, let alone suborbital, variation in deep- and shallow-water masses and input strength to the major ocean basins, particularly the depth-related variability in the flow of CDW from the ACC.
5.5. Pre-Quaternary Studies of Flow Variability
Several notable attempts to conduct paleocurrent reconstruction using the SS proxy beyond the Quaternary have been made. Extending their ODP Site 927 Pleistocene time series, Gröger et al. [2003b] report evidence for a stepwise reduction of glacial LNADW flow speeds from the late Pliocene toward the Pleistocene. Of note, they identify a three phase shift in LNADW current strength during the transition from mid-Pliocene warmth to the large-scale Northern Hemisphere Glaciation (NHG) at ∼3.2 Ma. Between ∼3.5 and 3.2 Ma, LNADW was found to be highly variable with large amplitude fluctuations gradually diminishing and giving way to remarkably stable flow during the initial phase of NHG between ∼3.2 and 2.75 Ma. Synchronous with the first occurrence of large-scale continental ice and the marked decrease in convection within the GIN seas, LNADW flow speeds substantially decreases and the high amplitude variability returns. Gröger et al. [2003b] and colleagues suggest that because of the high degree of sensitivity of NADW production to changes in surface water salinity, the high-amplitude fluctuations of LNADW circulation prior to ∼3.2 Ma are linked to changes in the Atlantic salinity budget, while after 2.75 Ma they are primarily controlled by ice sheet forcing. The cause of inferred stability of LNADW during the Northern Hemisphere cooling between ∼3.2 and 2.75 Ma remains unclear.
Orbital control on the dynamics of the deep Pacific inflow has also been confirmed during the period of global ice accumulation associated with the expansion of the East Antarctic Ice Sheet (EAIS) in the middle Miocene from ∼15.5 to 12.5 Ma [Hall et al., 2003] (Figure 23). Cyclic variation in and lithological records from Site 1123 were found to be dominated by the 41 kyr orbital obliquity cycle. This result is perhaps not surprising but clearly indicates a strong coupling between the variability in the speed of the DWBC and high-latitude climate forcing that may have persisted within the MOC for at least the past 15 Myr. Long-term changes in flow speed during the interval also suggest an intensification of the DWBC under an inferred increase in Southern Component Water production. This occurred at the same time as decreasing Tethyan outflow and major EAIS growth between ∼15.5 and 13.5 Ma. These results provide the only direct physical evidence that a major component of the MOC was associated with the middle Miocene growth of the EAIS.
Further evidence that MOC changes were critical to the development of the Neogene icehouse climate are suggested in the prominent shift in ocean circulation recorded at ODP Site 1170 (047°09′S 146°03′E, 2704 m water depth) and ODP Site 1171 (048°30′S 149°07′E, 2148 m water depth) located on the South Tasman Rise at ∼24 Ma, just prior (by 50–60 kyr) to the Mi-1 event marking the Oligocene-Miocene boundary (OMB). The flow speed record from Site 1170 record is shown along with the corresponding benthic δ18O record in Figure 24. The clear increase in at 24 Ma is taken by Pfuhl and McCave  as independent evidence for a strengthening of the west-to-east flow through the Tasman Gateway, which was well known to have opened at ∼33 Ma, the Eocene-Oligocene Boundary [Stickley et al., 2004]. This flow speed increase is accompanied by a clear increase in benthic δ18O at the site which, together with additional isotopic evidence from other Southern Ocean sites, is plausibly attributed to the inception of a full, i.e., modern-type, ACC circulation pattern at the OMB. A requirement for the complete establishment of full circumpolar circulation is the tectonic opening of the Drake Passage south of America to deep throughflow. However, the timing of the Drake Passage opening is poorly constrained and there has been considerable debate in the literature over two alternative time ranges based mainly on paleomagnetic tectonic reconstructions [e.g., Barker and Burrell, 1977; Lawver and Gahagan, 1998; Livermore et al., 2004]. An early Drake Passage opening at ∼31–28.5 Ma and a later or younger age range between ∼22 and 17 Ma. The combination of and isotopic data presented by Pfuhl and McCave  strongly support a deep DP-opening near the OMB at 23 Ma instead of a much earlier date close to the Eocene-Oligocene boundary.