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Reconstructing regional to hemispheric-scale climate variability requires the application of spatially representative and climatically sensitive proxy archives. Large spatial networks of dendrochronologies have facilitated the reconstruction of atmospheric variability and inferred variability in the Atlantic Ocean system. However, the marine environment has hitherto lacked the direct application of the spatial network approach because of the small number of individual absolutely-dated marine archives. In this study we present the first analyses of a network of absolutely-dated annually-resolved growth increment width chronologies from the marine bivalves Glycymeris glycymeris and Arctica islandica. The network contains eight chronologies spanning > 500 km along the western British continental shelf from the southern Irish Sea to North West Scotland. Correlation analysis of the individual chronologies and a suite of climate indices, including the Atlantic Multidecadal Oscillation (AMO), Central England surface air temperature (CET), northeast Atlantic sea surface temperatures (SST's) and the winter North Atlantic Oscillation (wNAO), demonstrates that, despite the large geographical distances been sites and the heterogeneous nature of the marine environment, the increment width variability in these series contains an element of coherence likely driven by a common response to changing environmental forcing. A nested Principal component analysis (PCA) was used to construct five composite series which explain between 31% and 74% of the variance across the individual chronologies. Linear regression analyses indicate that the composite series explain up to 41% of the variance in Northeast Atlantic SSTs over the calibration period (1975–2000). Calibration verification (reduction of error [RE] and coefficient of efficiency [CE]) statistics indicate that the composite series contains significant skill at reconstructing multi-decadal northeast Atlantic SST variability over the past two centuries (1805–2010). These data suggest that composite series derived from sclerochronology networks can facilitate the robust reconstruction of marine climate over past centuries to millennia providing invaluable baseline records of natural oceanographic variability.

There is much uncertainty surrounding the mechanisms that forced the abrupt climate fluctuations found in many palaeoclimate records during Marine Isotope Stage (MIS)-3. One of the processes thought to be involved in these events is the Atlantic Meridional Overturning Circulation (MOC), which exhibited large changes in its dominant mode throughout the last glacial period. Giant piston core MD95-2006 from the northeast Atlantic Ocean records a suite of palaeoceanographic proxies related to the activity of both surface and deep water masses through a period of MIS-3 when abrupt climate fluctuations were extremely pronounced. A two-stage progression of surface water warming during interstadial warm events is proposed, with initial warming related to the northward advection of a thin warm surface layer within the North Atlantic Current, which only extended into deeper surface layers as the interstadial progressed. Benthic foraminifera isotope data also show millennialscale oscillations but of a different structure to the abrupt surface water changes. These changes are argued to partly be related to the influence of low-salinity deepwater brines. The influence of deepwater brines over the site of MD95-2006 reached a maximum at times of rapid warming of surface waters. This observation supports the suggestion that brine formation may have helped to destabilize the accumulation of warm, saline surface waters at low latitudes, helping to force the MOC into a warm mode of operation. The contribution of deepwater brines relative to other mechanisms proposed to alter the state of the MOC needs to be examined further in future studies. Copyright 2008 by the American Geophysical Union.

The import of relatively salty water masses from the Indian Ocean to the Atlantic is considered to be important for the operational mode of the Atlantic Meridional Overturning Circulation (AMOC). However, the occurrence and the origin of changes in this import behavior on millennial and glacial/interglacial timescales remains equivocal. Here we reconstruct multiproxy paleosalinity changes in the Agulhas Current since the Last Glacial Maximum and compare the salinity pattern with records from the Indian-Atlantic Ocean Gateway (I-AOG) and model simulations using a fully coupled atmosphere-ocean general circulation model. The reconstructed paleosalinity pattern in the Agulhas Current displays coherent variability with changes recorded in the wider I-AOG region over the last glacial termination. We infer that salinities simultaneously increased in both areas consistent with a quasi interhemispheric salt-seesaw response, analogous to the thermal bipolar seesaw in response to a reduced cross-hemispheric heat and salt exchange during times of weakened AMOC. Interestingly, these hydrographic shifts can also be recognized in the wider Southern Hemisphere, which indicates that salinity anomalies are not purely restricted to the Agulhas Current System itself. More saline upstream Agulhas waters were propagated to the I-AOG during Heinrich Stadial 1 (HS1). However, the salt flux into the South Atlantic might have been reduced due to a decreased volume transport through the I-AOG during the AMOC slowdown associated with HS1. Hence, our combined data-model interpretation suggests that intervals with higher salinity in the Agulhas Current source region are not necessarily an indicator for an increased salt import via the I-AOG into the South Atlantic.

Six Ocean Drilling Program (ODP) sites, in the Northwest Atlantic have been used to investigate kinematic and chemical changes in the “Western Boundary Undercurrent” (WBUC) during the development of full glacial conditions across the Marine Isotope Stage 5a/4 boundary (~70,000 years ago). Sortable silt mean grain size measurements are employed to examine changes in near bottom flow speeds, together with carbon isotopes measured in benthic foraminifera and % planktic foraminiferal fragmentation as proxies for changes in water-mass chemistry. A depth transect of cores, spanning 1.8–4.6 km depth, allows changes in both the strength and depth of the WBUC to be constrained across millennial scale events. measurements reveal that the flow speed structure of the WBUC during warm intervals (“interstadials”) was comparable to modern (Holocene) conditions. However, significant differences are observed during cold intervals, with higher relative flow speeds inferred for the shallow component of the WBUC (~2 km depth) during all cold “stadial” intervals (including Heinrich Stadial 6), and a substantial weakening of the deep component (~3–4 km) during full glacial conditions. Our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation (involving Glacial North Atlantic Intermediate Water) that was quantitatively distinct from preceding cold stadial events. Furthermore, our chemical proxy data show that the physical response of the WBUC during the last glacial inception was probably coupled to basin-wide changes in the water-mass composition of the deep Northwest Atlantic.

In order to monitor the evolution of the British-Irish Ice Sheet (BIIS) and its influence in surface ocean structure during marine isotopic stages (MIS) 2 and 3, we have analyzed the sediments recovered in core MD04-2829CQ (Rosemary Bank, north Rockall Trough, northeast Atlantic) dated between similar to 41 and similar to 18 ka B.P. Ice-rafted debris flux and composition, Ar-40/Ar-39 ages of individual hornblende grains, multispecies planktonic stable isotope records, planktonic foraminifera assemblage data and faunal-based sea surface temperatures (SSTs) demonstrate a close interaction between BIIS dynamics and surface ocean structure and water properties in this region. The core location lies beneath the North Atlantic Current (NAC) and is ideal for monitoring the shifts in the position of its associated oceanic fronts, as recorded by faunal changes. These data reveal a succession of BIIS-sourced iceberg calving events related to low SST, usually synchronous with dramatic changes in the composition of the planktonic foraminifera assemblage and with variations in the stable isotope records of the taxa Neogloboquadrina pachyderma (sinistral coiling) and Globigerina bulloides. The pacing of the calving events, from typically Dansgaard-Oeschger millennial timescales during late MIS 3 to multicentennial cyclicity from similar to 28 ka B.P., represents the build-up of the BIIS and its growing instability toward Heinrich Event (HE) 2 and the Last Glacial Maximum. Our data confirm the strong coupling between BIIS instabilities and the temperature and salinity of surface waters in the adjacent northeast Atlantic and demonstrate the BIIS's ability to modify the NAC on its flow toward the Nordic Seas. In contrast, subsurface water masses were less affected except during the Greenland stadials that contain HEs, when most intense water column reorganizations occurred simultaneously with the deposition of cream-colored carbonate sourced from the Laurentide Ice Sheet.

Three depth transects containing a total of 33 sediment cores were investigated along the Blake Outer Ridge in the western subtropical North Atlantic. Sortable silt mean () grain size and stable isotope records were used to assess the position and relative intensity of the Western Boundary Undercurrent (WBUC) during the Holocene, the Last Glacial Maximum (LGM), and the Younger Dryas (YD) intervals. The Holocene reconstruction is consistent with modern physical and chemical hydrographic measurements in the area, suggesting a deep position for the fast flowing core of the WBUC (3000–4000 m, deepening to ∼4500 m water depth on the ridge flanks) and a water column dominated by North Atlantic Deep Water (NADW). The LGM and YD reconstructions show that a comparable hydrographic regime was present during both these intervals, suggesting a similar mode of circulation that was appreciably different from the Holocene reconstruction. The WBUC's zone of maximum flow speed during these intervals is suggested to have shifted above 2500 m water depth, consistent with nutrient depleted Glacial North Atlantic Intermediate Water formation with an increasing influence of Southern Source Water (SSW) beneath. Below 4000 m water depth, results hint at increased SSW flow vigor during both the LGM and YD with higher flow speeds than during the Holocene. This study provides a framework for aiding the interpretation of time series records of paleocurrent flow speed changes in the region of the WBUC.

We present high-resolution records of upper ocean temperatures derived from Mg/Ca ratios of surface-dwelling Globigerina bulloides and subsurface-dwelling Neogloboquadrina pachyderma sinistral and the relative abundance of N. pachyderma sinistral for the period 55–8 ka BP from NE Atlantic sediment core MD01-2461. Millennial-scale temporal variability and longer-term trends in these records enable us to develop a detailed picture of past ocean conditions such as a weakening of thermocline intensity from marine isotope stage 3 (MIS 3) to the last glacial maximum (LGM). The correspondence of all temperature proxies and convergence of paired oxygen isotope (δ 18O) records from both planktonic species implies a breakdown in the thermocline and year-round mixing of the upper water column through the LGM, perhaps related to decreasing insolation and additional cooling in association with the expansion of the circum–North Atlantic ice sheets. Millennial-scale divergence in surface and subsurface temperatures and δ 18O across the last glacial correspond to meltwater release and the development of a strong halocline associated with both Heinrich (H) events and instabilities of the NW European ice sheet. During such episodes, G. bulloides Mg/Ca appears to record ambient, even warming summer sea surface temperatures across H events while the other proxies record maximum cooling.

Sortable silt mean grain sizes together with oxygen and carbon isotopic data produced on the benthic foraminiferal species Fontbotia wuellerstorfi are used to construct high-resolution records of near-bottom flow vigour and deep water ventilation at a core site MD02-2589 located at 2660 m water depth on the southern Agulhas Plateau. The results suggest that during glacial periods ( marine oxygen isotope stages 2 and 6, MIS 2 and MIS 6, respectively), there was a persistent contribution of a well-ventilated water mass within the Atlantic to Indian oceanic gateway with a delta(13)C signature similar to present-day Northern Component Water (NCW), e. g., North Atlantic Deep Water (NADW). The records of chemical ventilation and near-bottom flow vigor reflect changes in the advection of northern source waters and meridional variability in the location of the Antarctic Circumpolar Current and its associated fronts. We suggest that during Termination II (TII), changes in chemical ventilation are largely decoupled from near-bottom physical flow speeds. A mid-TII climate optimum is associated with a low-flow speed plateau concurrent with a period of increased ventilation shown in the benthic delta(13)C of other Southern Ocean records but not in our benthic delta(13)C of MD02-2589. The climate optimum is followed by a period of southern cooling around 128 ka coincident with a stronger influence of NCW to interglacial levels at around 124 ka. All proxy records show a near synchronous and rapid shift during the transition from MIS 5a-4 (73 ka). This large event is attributed to a rapid decrease in NADW influence and replacement over the Agulhas Plateau by southern source waters.

[1] We have compiled results obtained from four high sedimentation rate hemipelagic sequences from the Celtic sector of the NW European margin ( NE Atlantic) to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles. We focus on periods characteristic of deglacial transitions. We adopt a multiproxy sedimentological, geochemical, and micropaleontological approach, applying a sampling resolution down to ten microns for specific intervals. The investigation demonstrates the relationships between the Bay of Biscay hydrography and the glacial/deglacial history of both the proximal British-Irish Ice Sheet (BIIS) and the western European continent. We identify recurrent phases of laminae deposition concurrent with major BIIS deglacial episodes in all the studied cores. Evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale. We discuss their impact at a global scale considering the present and past key location of the Bay of Biscay versus the Atlantic Meridional Overturning Circulation (AMOC).

Records from Ocean Drilling Program Sites 1057 and 1059 (2584 m and 2985 m water depth, respectively) have been used to reconstruct the behavior of the Deep Western Boundary Current (DWBC) on the Blake Outer Ridge (BOR) from 130 to 60 kyr B.P. (marine isotope stage (MIS) 5 and the 5/4 transition). Site 1057 lies within Labrador Sea Water (LSW) but close to the present-day boundary with Lower North Atlantic Deep Water (LNADW), while Site 1059 lies within LNADW. High-resolution sortable silt mean (inline equation) grain size and benthic δ 13C records were obtained, and changes in the DWBC intensity and spatial variability were inferred. Comparisons are made with similar proxy records generated for the Holocene from equivalent depth cores on the BOR. During MIS 5e, inline equation evidence at Site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the LSW-sourced shallower limb of the DWBC that occupies these depths today. In contrast, the paleocurrent record from the deeper site suggests that the fast flowing deep core of the DWBC was located close to its modern depth below 3500 m. During this interval the benthic δ 13C suggests little chemical stratification of the water column and the presence of a near-uniform LNADW-dominated water mass. After ∼111 kyr B.P. the inline equation record at Site 1057 increases to reach values similar to Site 1059 for the rest of MIS 5. The strengthening of flow speeds at the shallow site may correspond to the initiation of Glacial North Atlantic Intermediate Water formation also suggested by a divergence in the benthic δ 13C records with Site 1057 values increasing to ∼1.2‰. Coupled suborbital oscillations in DWBC flow variability and paleohydrography persisted throughout MIS 5. Comparison of these data with planktonic δ 18O records from the sites and alkenone-derived sea surface temperature (SST) estimates from the nearby Bermuda Rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical North Atlantic SST and DWBC flow.

Variations in the near-bottom flow speed of Iceland-Scotland Overflow Water (ISOW) are documented in a 230-year-long deep-sea sediment record from the eastern flank of Reykjanes Ridge in the subpolar North Atlantic at (sub)decadal time scales. For recent decades, the ISOW palaeocurrent reconstructions show similarities with observational hydrographic data. Furthermore, recent ISOW flow changes fall mostly within the range of its variability of the past 230 years. The record also reveals a hitherto unrecognized coupling of deep flow speeds in the subpolar North Atlantic with the North Atlantic Oscillation (NAO) index, with more (less) vigorous ISOW flow during negative (positive) phases of the NAO. Our results suggest that the changes in ISOW vigor are largely controlled by the transport and characteristics of Labrador Sea Water rather than variations in the overflow itself, with implications for the meridional overturning of the Atlantic Ocean and climate.

High-resolution (125–500 year temporal resolution) sortable silt mean grain size data of subtropical northwest Atlantic Ocean Drilling Program (ODP) Leg 172, Site 1061, monitor fluctuations of the Deep Western Boundary Current (DWBC) throughout marine oxygen isotope stages (MIS) 12–10 (330–460 kyr) in great detail. Comparison with foraminiferal stable isotope data from the same site and ODP Site 1063 (Bermuda Rise, data of Poli et al. (2000)) shows that the DWBC shoals during glacial times in relation to a reduced North Atlantic Deep Water production and increased advection of southern ocean waters. Superimposed fluctuations in the flow speed/position of the DWBC at a semiprecession scale are persistent throughout MIS 10–12 and independent of the glacial state. Additional millennial-scale variations in DWBC flow speed parallel surface water cooling events and ice-rafted debris episodes especially at the transition of MIS 11/10, indicating the sensitivity of deep current flow variations to changes in the surface water conditions. These millennial-scale variations are amplified when benthic foraminiferal δ 18O values exceed ∼3.8‰ and may be related to the transition of an ice volume threshold where climate variability is possibly amplified by feedback mechanisms associated with ice sheet growth.

The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice-rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

Today the deep western boundary current (DWBC) east of New Zealand is the most important route for deep water entering the Pacific Ocean. Large-scale changes in deep water circulation patterns are thought to have been associated with the development of the East Antarctic Ice Sheet (EAIS) close to the main source of bottom water for the DWBC. Here we reconstruct the changing speed of the southwest Pacific DWBC during the middle Miocene from ∼15.5–12.5 Ma, a period of significant global ice accumulation associated with EAIS growth. Sortable silt mean grain sizes from Ocean Drilling Program Site 1123 reveal variability in the speed of the Pacific inflow on the timescale of the 41 kyr orbital obliquity cycle. Similar orbital period flow changes have recently been demonstrated for the Pleistocene epoch. Collectively, these observations suggest that a strong coupling between changes in the speed of the deep Pacific inflow and high-latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 Myr. Furthermore, long-term changes in flow speed 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 evidence that a major component of the deep thermohaline circulation was associated with the middle Miocene growth of the EAIS and support the view that this time interval represents an important step in the development of the Neogene icehouse climate.

Tuber magnatum Picco is the most expensive of the truffles and a great deal of research has been carried out in an attempt to solve the mysteries of its ecology and biology. However, considerable work remains to be done particularly on those secrets of its life cycle that remain a mystery. It is known that T. magnatum is heterothallic, but it has yet to be determined how fertilization occurs between the two strains of different mating types. It is also known that the white truffle is an ectomycorrhizal fungus, and its mycorrhizas can be produced in greenhouses, but then they seem to disappear in the field. The role of other soil microorganisms, fungi and bacteria, on its soil mycelial development and fructification is intriguing but is far from being completely understood. All these uncertainties have made the cultivation of T. magnatum extremely difficult and only recently have we had the scientific proofs that it is possible. Even so, many questions remain unanswered and the management practices of T. magnatum plantations are still to be better defined to also enable the taming of this truffle.