Gardar Drift Research Articles

Diatom phytoplankton response to Holocene climate change in the Subpolar North Atlantic

A detailed investigation of marine core MD99-2251 from the Gardar Drift has been undertaken to examine the extent of Holocene climate variability reflected by changes in diatom floral abundances in the subpolar North Atlantic. The results from this study provide both an overview of climate variability for the entire Holocene and a decadal-scale study focussed around the 8.2 kyr event, where ice rafted debris also has been quantified. The changing composition of diatom assemblages indicates a highly unstable early Holocene from 11.5 to 9 kyr with switches in the dominance of cool Subarctic floras and warmer North Atlantic Current floras. The presence of high productivity events in the diatom floras during this interval suggests that the core location was, at times, in close proximity of the Subarctic Front. An expansion in the importance of cold Arctic/Greenland Current floras occurred from 9 to 7 kyr, followed by a switch to a well developed warm North Atlantic Current flora between 7 and 5 kyr and then more stable conditions during the Late Holocene. Changes in sea surface hydrography, especially the relative strength of the warm North Atlantic Current, are considered to have had the greatest influence on the composition of diatom floral assemblages. The 8.2 kyr event is not recognised as a discrete climate perturbation in the diatom assemblage data, but this event occurred within a broad cooling from 9 to 7 kyr, where the presence of sea-ice and cold water species indicate an increased incursion of the Arctic and Subarctic water masses between 8.8 and 7.8 kyr. The 8.2 kyr event also was not marked by any increase in the delivery of ice rafted debris over the Gardar Drift.

North Atlantic Deep Water and climate variability during the Younger Dryas cold period

The Younger Dryas, the last large millennial-scale climate oscillation (12.9–11.6 ka), has been widely attributed to a massive meltwater discharge event that disrupted ocean circulation and plunged the circum–North Atlantic back into a near-glacial state. Low-resolution deep-water reconstructions indicate lower North Atlantic Deep Water (NADW) production during the Younger Dryas, though the Δ 14 C record requires some deep-water production. Herein, we reconstruct deep-water mass variations using a southern Gardar Drift sediment core with an expanded Younger Dryas section. We show that southern-sourced water invaded the deep North Atlantic to start the Younger Dryas, but was replaced by NADW within 500 yr. Southern-sourced waters briefly reappeared at the end of the Younger Dryas. These deep-water reorganizations to start and end the Younger Dryas suggest that increased meltwater fluxes were limited temporally and focused on regions where deep-water convection occurred during the deglaciation.

Threshold behavior of millennial scale variability in deep water hydrography inferred from a 1.1 Ma long record of sediment provenance at the southern Gardar Drift

[1] Integrated Ocean Drilling Program (IODP) Site U1314 of the North Atlantic is a critical sedimentary archive record of subpolar deep water from the southern Gardar Drift for which we derived an age model of orbital resolution for the last 1.8 Ma. This chronology combined with high-resolution (cm scale) X-ray fluorescence core scanning measurements of major elements allows tracking changes in terrigenous provenance during the last 1.1 Ma. Low Potassium to Titanium (K/Ti) ratios reflect enhanced transport of basalt-derived titanomagnetites during warm climate intervals, while high K/Ti ratios indicate a dominance of acidic sediment sources typical for glacial and stadial events. Changes in K/Ti and magnetic concentration at Site 1314 are coeval with fluctuations in smectite content and grain size data from nearby piston cores, suggesting that the provenance changes are mainly controlled by variable flow of the Iceland-Scotland Overflow Water, an important branch of North Atlantic Deep Water. Furthermore, K/Ti variations on orbital time scales show a striking similarity to the deep sea δ13C record from ODP Site 607. Pervasive features of the K/Ti time series during and after the Mid-Pleistocene Transition are suborbital changes similar to Dansgaard/Oeschger and Bond oscillations that appear to be strongly amplified during ice growth phases when global benthic δ18O was within the range of ∼4.1–4.6 per mil. The strong increase in variability of sediment provenance and subsequently deep hydrography at benthic δ18O values below ∼4.1 suggests that the extent of glaciations and, therefore, sea level corresponding to this value constitutes an important physical threshold that was persistent at least for the last 1.1 Ma.

Phase relationships of North Atlantic ice-rafted debris and surface-deep climate proxies during the last glacial period

We report stable isotope, core scanning XRF, and ice-rafted detritus (IRD) data in glacial-aged sediments from piston Core KN166-14-JPC-13 (hereafter referred to as JPC-13) retrieved at 53.1°N 33.5°W on the southern Gardar Drift, North Atlantic. A chronology was established by correlating millennial-scale features in the benthic δ 18O record to Portuguese Margin Core MD95-2042. Once the alignment of benthic δ 18O of JPC-13 to MD95-2042 is fixed, the relative timing of proxy variables is used to determine the phasing of changes in IRD and surface-deep hydrography. Each peak in North Atlantic IRD coincided with a decrease in benthic δ 18O that, in turn, has been linked to warming in Antarctica (Events A1–A7). The IRD pulses are followed shortly thereafter by abrupt decreases in planktonic δ 18O, indicating warming and increased heat transport to the subpolar North Atlantic associated with Greenland interstadials (GIS) 8, 12,14, and 16–17. Grain-size and elemental ( K/Ti) variations indicate that Iceland–Scotland Overflow Water (ISOW) was stronger during these long, warm interstadial periods of Marine Isotope Stage (MIS) 3. The results are consistent with interhemispheric phase relationships inferred from Iberian Margin sediment cores and with methane synchronization of Greenland and Antarctic ice cores.

Intra-annual variability of extremely rapid sedimentation onto Gardar Drift in the northern North Atlantic

North Atlantic sediment drifts are valuable archives for paleoceanographic reconstructions spanning various timescales. However, the short-term dynamics of such systems are poorly known, and this impinges on our ability to quantitatively reconstruct past change. Here we describe a high-resolution 319-day time-series of hydrodynamics and near-bottom (4 m) particulate matter flux variability at a 2600 m deep site with an extremely high sediment accumulation rate on the southern Gardar Drift in the North Atlantic. We compare our findings with the actual deposits at the site. The total annual particle flux amounted to ∼360 g m −2 yr −1, varied from ∼0.15 to >5.0 g m −2 day −1 and displayed strong seasonal compositional changes, with the highest proportion of fresh biogenic matter arriving after the spring bloom in June and July. Flux variability also depended on the changing input of lithogenic matter that had been (re)suspended for a longer time (decades). Active focussing of material from both sources is required to account for the composition and the magnitude of the total flux, which exceed observations elsewhere by an order of magnitude. The enhanced focussing or increased delivery appeared to be positively related to current velocity. The intercepted annual particle flux accounted for only 60% of the sediment accumulation rate of 600±20 g m −2 yr −1 (0.20±0.07 cm yr −1), indicating higher intra- and inter-annual variability of both the biogenic and lithogenic fluxes and/or advection of additional sediment closer to the seafloor (i.e. <4 m). This temporal variability in the composition and amount of material deposited highlights intra-annual changes in the flux of lithogenic material, but also underscores the importance of (reworked) sediment focussing and seasonality of the biogenic flux. All should be taken into account in the interpretation of the paleorecord from such depositional settings.

Surface and deep-water hydrography on Gardar Drift (Iceland Basin) during the last interglacial period

Changes in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric δ 13C gradient during the penultimate glaciation was similar to the last glaciation with high- δ 13C Glacial North Atlantic Intermediate water above ∼ 2000 m, and low- δ 13C water derived from the Southern Ocean below. During Termination II, low- δ 13C water was present throughout the water column with minimum values at intermediate depths (∼ 1500–2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic δ 18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low- δ 13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ∼ 124 ka. These changes may have affected Atlantic Meridional Overturning Circulation.

Rock magnetic properties of the Gardar Drift sedimentary sequence, Site IODP U1314, North Atlantic: Implications for bottom current change through the mid-Pleistocene

The sediment obtained at Integrated Ocean Drilling Program (IODP) Site U1314 on the Gardar Drift in the eastern flank of the Reykjanes Ridge was analyzed using rock magnetic techniques to study the depositional environment through the mid-Pleistocene climate transition. The study area is under the influence of Iceland–Scotland Overflow Water (ISOW). Consequently, the sedimentary record represents directly ISOW behavior over time. A change in the depositional environment was documented as magnetic property changes that are the result of changes in concentration and grain size of magnetic minerals, both of which decrease upward from around 0.9 Ma. Because these shifts are accompanied by a change in magnetic fabrics, which is interpreted as a change in bottom current directions from E–W to NNE–SSW, it is considered that the sedimentation pattern inferred from magnetic records at the site was modified by a change in the bottom current through the middle Pleistocene.

Paleomagnetic directions of the Gauss-Matuyama polarity transition recorded in drift sediments (IODP Site U1314) in the North Atlantic

Abstract The geomagnetic field direction during the Gauss-Matuyama (G-M) polarity transition was investigated from a high-accumulation-rate (≥10 cm/kyr) sediment core drilled in the Gardar drift in the North Atlantic at Site U1314 during Expedition 306 of the Integrated Ocean Drilling Program (IODP). A well-defined characteristic remanent magnetization was generally obtained by alternating field demagnetization. The consistency of the results with records from Icelandic lavas confirms that the North Atlantic drift sediments contain a high-fidelity record of the geomagnetic field change. During the G-M transition, the virtual geomagnetic pole (VGP) latitude shows north-south-north-south rebounding, with the three VGP paths falling within different longitudinal bands. Two of the three paths are close to or within the preferred bands in which transitional VGPs are suggested to be longitudinally confined. Three additional loops occur that approach mid-to-low latitudes from the North or South pole regions. In addition, the VGPs show rapid movement (directional jumps) between VGP clusters.

Influence of sediment drift accumulation on the passage of gravity-driven sediment flows in the Iceland Basin, NE Atlantic

The Maury Channel is a deep-sea sediment transport system located in the Iceland Basin and extends from the Icelandic plateau southwards towards the Charlie-Gibbs Fracture Zone (CGFZ). This study has utilised multibeam bathymetry and multi-channel seismic reflection survey data along 480 km of its 1200 km pathway. In the northern reach of the channel it is predominantly broad (>20 km) and shallow (∼10 m). Further to the south the channel narrows (5–10 km) and locally deepens to 150 m prior to finally discharging onto the Eriador Plain to the north of the CGFZ. DSDP Site 115 in the Iceland Basin can provide insight into the evolution of the system as it sampled a suite of volcaniclastic turbidites of unequivocal Icelandic provenance. This sequence produces distinct amplitude anomalies on seismic reflection profiles allowing it to be mapped over an area of at least 26,000 km 2. The southern edge of the high velocity unit is delimited by onlap onto the flanks of the Miocene (and younger) Gardar Drift. The drift appears to have initially acted as a barrier to southerly flows and promoted ponding of flows in the Maury Fan. Continued sediment supply from Iceland eventually filled the Maury Fan leading to the overspilling of the Gardar Drift dam. A result was the initiation of the Maury Channel. To the south of the drift, where the seabed is steep, flows are confined to the channel, whereas to the north of the drift, where the gradient is less, unconfined flow pathways dominate. The Maury Channel system highlights the interaction between turbidity currents and bottom currents on abyssal plains. The growth of sediment drifts not only mould the seafloor through their bathymetric development but also, through the building of seafloor topography, influence the passage and behaviour of gravity-driven sediment-laden flows along the seafloor.

Late Miocene to Pleistocene paleoceanographic records from the Feni and Gardar Drifts: Pliocene reduction in abyssal flow

Magnetic fabric analyses from two North Atlantic drift deposits provide proxies for determining relative variations in the strength of abyssal flow over the last 10 my. The data show a cessation of current-controlled sedimentation at the shallower Feni Drift (2417 m) at the time of onset of Northern Hemisphere glaciation (2.6 Ma). Drift formation ended nearly 2 my earlier (4.2 Ma) at the deeper Gardar Drift (3220 m), implying stepwise reduction in deep-water flow. Relatively light δ 18O values at the deeper Gardar Drift indicate a warmer, thus also more salty, water mass site prior to 6 Ma. We interpret this as representing Mediterranean Sea water, which flowed north at depths greater than that of the Feni Drift Site. The supply of Mediterranean Water to the North Atlantic was shut off as the Gibraltar Straits closed, causing the Messinian salinity crisis, and never returned to that position in the water column after the Mediterranean opened again.

High-resolution magnetostratigraphy of late quaternary sediments from Lake Baikal, Siberia: timing of intracontinental paleoclimatic responses

Sediment cores retrieved from 6 locations in Lake Baikal were subjected to a paleomagnetic study in order to establish detailed age models based on correlations of relative paleointensity records. Additional data were provided by calibrated accelerator mass spectrometry (AMS) 14C dating, as well as by documentation of geomagnetic excursions like Laschamp at ∼42 ka and Iceland Basin at ∼185 ka. Few intervals were affected by diagenetic features like selective reductive dissolution of magnetite and greigite mineralization (Demory et al., 2005-this issue), and those that were left out of paleointensity records. These records were tuned to the well-dated paleomagnetic record from ODP Site 984 Channell [Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) site 984 (Bjorn Drift) since 500 ka: comparisons with ODP site 983 (Gardar drift). J. Geophys. Res., B: [Solid Earth], 104 (10):22, 937-22, 951]. The complex shape of the resulting depth/age curves highlights the need for a high-resolution age model. We focused on the climatic boundary between marine isotopic stage (MIS) 7 and 6 where the Iceland Basin paleomagnetic excursion is clearly documented in the North Atlantic Channell et al.[Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and d 18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka. Earth Planet. Sci. Lett., 153 (1–2), 103-118] and in Lake Baikal Oda et al. [Oda, H., Nakamura, K., Ikehara, K., Nakano, T., Nishimura, M., Khlystov, O., 2002. Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet. Sci. Lett., 202 (1), 117-132] ; present study). During this period, we provide evidence for a return to cold conditions in the Lake Baikal region simultaneous to the sea surface cooling, but earlier than the global ice volume change observed in North Atlantic planktonic and benthic δ 18O records, respectively. The classical strategy of age model reconstruction, based on direct correlation of the climatic record from Lake Baikal sediments with the marine δ 18O reference curves is shown here to be unreliable. Moreover, this strategy does not consider (i) the nonlinearity of the age model in Lake Baikal sediments and (ii) the time lags between the global ice volume change and sea surface cooling observed in δ 18O marine records. Finally, the “Baikal 200” compilation of the paleointensity records established in this study provides a 200-ka-long synthetic paleomagnetic record for Central Eurasia.

Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12 000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction. The observed overall correlation between bottom flow speed, as given by the ‘sortable silt’ mean size index, and the bulk coccolith concentration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of sediment redistribution by bottom current. The down-core variations in coccolith assemblage structure (species %) indicate that changes in properties of surface waters south of Iceland occurred as two distinct steps at ca. 11.2 and 6 ka. The onset of North Atlantic Drift water influence over the Gardar Drift after 11.2 ka was associated with excess export flux of coccolith carbonate in the vicinity of an active frontal system. The period from 10 to 6 ka saw the progressive warming of the study area, which culminated between 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated changes in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established after a last cooling step between 3.5 and 2.8 ka. These long-term reorganisations of the surface hydrology are interpreted as the response of the North Atlantic to the combined force of the solar insolation and the waning Laurentide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi. These successive decreases in the export fluxes of E. huxleyi exhibit a distinct millennial pacing, in phase with previously recorded Holocene advection of cool, ice-bearing waters from the Greenland–Iceland seas to the British islands. From a clear analogy with the most recent extreme variation in global ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be found in long-term modulations of the atmospheric processes associated with the North Atlantic Oscillation (NAO).

Hydrography and sedimentation under the deep western boundary current on Björn and Gardar Drifts, Iceland Basin

Abstract The interaction of Northern Component Waters overflowing the Iceland–Scotland Ridge with sediments further south in the Iceland Basin has been responsible for highly variable sedimentation patterns since at least the late Miocene (∼5 million years ago), and has formed both Bjorn and Gardar Drifts. Presently this process is maintained by a flux of approximately 3 Sv (measured along the south Icelandic slope) of Iceland–Scotland Overflow Water (ISOW) with a mean regional flow direction to the SW, which affects sediment transport on Bjorn Drift below ∼1350 m depth and over most of the Gardar Drift. Coriolis deflection of ISOW flow causes it to move as a Deep Western Boundary Current (DWBC) in the Iceland Basin, which redistributes sediments under the influence of seabed topography. Here, hydrographic and sedimentological data are used to illustrate the effects and extent of the vigorous DWBC activity. The study presents a baseline against which inferred changes in the past can be assessed. North of ∼58°N bottom nepheloid layers with heavy suspended sediment loads are often encountered, particularly under the denser and faster-flowing core of ISOW on the eastern slope of Gardar Drift. In this area the presence of migrating, regular sedimentary waves is seen on 3.5 kHz profiles. This is consistent with both the near-bottom flow velocity probably in excess of ∼10 cm s −1 estimated by the geostrophic method and the presence of well-sorted foraminiferal sand on the seabed. South of ∼58°N dilute bottom nepheloid layers, associated with more tranquil conditions near the seabed (inferred from photographic and sedimentological evidence), show that currents on Gardar Drift are weak. Although most of the south Iceland Basin is mantled by both regular and irregular, non-migratory mudwaves, the absence of migrating structures (with the exception of current intensification associated with discrete basement highs) indicates that more sluggish currents characterise this region. Throughout the region highly variable mean Holocene sediment accumulation rates are inferred (2–40 cm ka −1 ) as a result of the DWBC interacting with the topography.

Geomagnetic palaeointensities and astrochronological ages for the Matuyama–Brunhes boundary and the boundaries of the Jaramillo Subchron: palaeomagnetic and oxygen isotope records from ODP Site 983

We have measured relative geomagnetic palaeointensity proxies, palaeomagnetic directions, and δ 18 O for the 700–1100 ka interval from ODP Site 983 (Gardar Drift, North Atlantic), where mean sedimentation rates are ca .13 cm kyr −1 . The age model was generated by matching the benthic δ 18 O data to the Ice Volume Model and confirmed by tuning the precessional components of both signals. For the Matuyama–Brunhes boundary (MBB) and the boundaries of the Jaramillo Subchronozone, the duration of the polarity reversal process, defined by virtual geomagnetic polar latitudes of less than 45°, is ca .5 kyr. Whereas the generally accepted astrochronological estimates for the boundaries of the Jaramillo Subchronozone lie within the polarity transitions as recorded at Site 983, the astrochronological age for the Matuyama–Brunhes polarity transition (780 ka) is ca .5 kyr older than the onset of this transition at Site 983 (775 ka). The polarity reversals lie within palaeointensity lows, with abrupt recovery of palaeointensity post reversal. There is no progressive (‘sawtooth’) decrease in palaeointensity within the Jaramillo Subchronozone or between the top of the Jaramillo and the MBB, but rather, within polarity chrons, several short intervals of low palaeointensity which sometimes coincide with high–amplitude secular variation. Orbital (100 and 41 kyr) periods are present in the palaeointensity record. As they are not obviously attributable to climate/lithology in these records, they may be a feature of the geomagnetic field itself.

Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: Comparisons with ODP Site 983 (Gardar Drift)

Normalized remanence (relative geomagnetic paleointensity) records for the last 500 kyr can be matched between two Iceland Basin Ocean Drilling Program (ODP) sites (984 and 983) and correlated with other high‐resolution records. Directional secular variation is, however, not easily correlated between sites due to inadequate recording, at these mean sedimentation rates (12–15 cm/kyr), of the characteristic high frequency variability of the directional record. Both sites record the Iceland Basin event at ∼186–189 ka in which the characteristic magnetization component rotates through 180° and back, coincident with a paleointensity low that lasted about 3 kyr. Other Brunhes Chron geomagnetic excursions appear to be manifest as intervals of higher‐amplitude secular variation during lows in paleointensity. There is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records; however, this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters.

North Atlantic Intermediate Waters in the Late Pliocene to Early Pleistocene

We generated benthic isotope records from Ocean Drilling Program (ODP) site 981 on the Feni drift (2173 m water depth) and from ODP site 983 on the Gardar drift (1983 m water depth) to examine the interaction between North Atlantic Deep Water (NADW) and Glacial North Atlantic Intermediate Water (GNAIW) formation from 2.0 to 1.4 Ma. We find NADW at both sites during interglacial periods, and a mix of NADW and Southern Ocean water at the Feini drift during most glacial periods. Prior to 1.7 Ma we find no evidence ofr GNAIW at the Gardar drift site. Instead, glacial Gardar drift δ13C values are as low or lower than values for all other sites in the North Atlantic and reflect continued glacial overflow from the Nordic seas. After 1.7 Ma Gardar drift δ13C values increase and suggest that there was GNAIW at the Gardar drift site during some glacial intervals. Overall, we find that NADW and GNAIW production changed around 1.7 Ma in concert with changes in sea surface temperature and salinity and in the Earth's obliquity cycle.

Relative geomagnetic paleointensity and δ18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka

At ODP Site 983, relative geomagnetic paleointensity and planktic and benthic δ 18O records have been acquired for the last 350 kyr. The mean sedimentation rate in this interval is 11.3 cm/kyr. Magnetic properties and hysteresis ratios indicate that pseudo-single domain magnetite is the remanence carrier. Volume susceptibility ( k), anhysteretic (ARM) and isothermal (IRM) remanence values vary by a factor of 3–4, well within the criteria usually cited for paleointensity studies. Natural remanent magnetization (NRM) is normalized by ARM and IRM to acquire the paleointensity proxy. Arithmetic means of NRM/ARM and NRM/IRM, calculated for five demagnetization steps in the 25–45 mT range, constitute the relative paleointensity estimates. Some paleointensity lows (particularly those at ∼40, ∼120 and ∼188 ka) are associated with directional excursions of the field, especially the event at ∼188 ka (referred to here as the Iceland Basin Event) that constitutes a short-lived polarity reversal. For the last 200 kyr, the records can be correlated with other high-resolution paleointensity records such as those from the Labrador Sea, Mediterranean/Somali Basin and Sulu Sea, implying that the millennial scale features are globally synchronous. A labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages.

Grain-size and SrNd isotopes as tracer of paleo-bottom current strength, Northeast Atlantic Ocean

A crucial question in modern paleoceanographic research is how vigourous was the bottom current in the glacial North Atlantic Ocean. Distribution of sediment grain size can provide such information, if the characteristics of the source functions are known. The interest of this paper is to use the SrNd isotopic composition of lithic particles as a tracer of origin, in order to determine possible changes in the source function, over the last 185 kyr. Grain-size distribution data are derived from a sediment core in the Icelandic basin (Gardar Drift). The distribution pattern reveals a well sorted silt size (modal size 20 μm) in the glacial deposits. Sr and Nd isotopic composition measured on separate size fractions indicates a more constant local origin (Iceland) for the silt fraction than for the fine fraction. These data suggest that the increase in the silt fraction during glacial periods is related primarily to an increase in the bottom current strength, rather than to changes in sediment supply (i.e. ice-rafting, turbidity current). The increase in vigour of the bottom current, recorded during the isotopic stages 6, 4 and the last post glacial periods, could have been initiated by a southward shift of the zone of deep water formation in the northern part of the Icelandic basin.

Glacial‐interglacial paleoenvironments of the eastern Atlantic Ocean: The Biogeochemical Ocean Flux Study (BOFS) Paleoceanography Program

The papers in this special section discuss many aspects of paleoenvironmental analysis performed using various proxies on cores collected in the northeastern and tropical eastern Atlantic during two cruises in the framework of the NERC Biogeochemical Ocean Flux Study Program (BOFS). Coring of the northern BOFS leg was carried out on Leg 3 of the 1989 BOFS program (RRS Discovery cruise 184 [McCave, 1989]). Forty‐one cores were taken on a North‐South transect at about 20°W, from the flanks of the Mid‐Atlantic Ridge (47°N) to Gardar Drift (59°N) at water depths between 1150 and 4050 m. Of these, 15 cores were taken using a 3 m‐ long, large‐diameter kastenlot corer designed by Zangger and Mc Cave [199O]. Cruise and core data are given by Manighetti et al. [this issue]. The southern leg coring was carried out on Leg C of the 1990 BOFS program (RRS Charles Darwin cruise 53 [Shimmield, 199O]). Forty‐six cores, including 14 using the kastenlot corer, were taken along a transect at about 2O°W, between 18 and 42°N, at depths between 3000 and 4900 m and focusing on an area off northwest Africa .

Cenozoic sediment accumulation on drifts in the northern North Atlantic

The northern North Atlantic sediment drifts have a much greater areal extent than has previously been indicated. The northern North Atlantic extends from the East Greenland to European continental shelves and from the Charlie‐Gibbs Fracture Zone to the Greenland‐Scotland Ridge. Within this region are seven major sediment drifts containing some of the best clues for pre‐Quaternary bottom water circulation in the North Atlantic. Feni Drift is the oldest in the region, originating near the Eocene‐Oligocene boundary. It was followed by accumulation of Bjorn, Gardar, Hatton, and Snorri Drifts from the late Early to Middle Miocene. Eirik Drift may have started to accumulate in the Late Miocene, and Gloria Drift in the Early Pliocene. Through analysis of the Cenozoic sediment mass/age distribution and the change in area of seafloor for the study area through time, it can be concluded that there is a deficit of Oligocene to Miocene sediment in the region. This could be explained by sediment cycling; the erosion of older sediment to produce young sediment. The mass of Pliocene to Quaternary sediment is much larger than that of the older sediment and may have been derived primarily through the erosion of Eocene and Oligocene sediment by deepwater currents. The sediment drift portion of the total regional sediment mass, increased significantly three times during the Cenozoic. Each of these drift growth phases, lasted between 3 and 4 m.y. The first two growth phases, from the Late Eocene to Early Oligocene and from the Early to Middle Miocene, were accompanied by northward shifts of depocenters to the newly forming sediment drifts. The third and largest growth phase occurred between 7 and 3 Ma, and may have been preceded by the initiation of accumulation on Eirik Drift, 8–7 Ma. This most recent growth phase was accompanied by high apparent accumulation rates over southern Gardar Drift as it expanded to the east. Erosion rates over Reykjanes Ridge may have been higher during the mid‐Pliocene than in the Quaternary. There are two main sites where dense water could have formed and flowed south through the Rockall Trough to start the accumulation of Feni Drift. If the atmosphere were cool enough, dense water could have formed in the Norwegian Sea or on the Faeroe Shelf. Dense water may also have formed on the Rockall Plateau due to salinity increase through evaporation in an arid climate. As an alternative to the hypothesis of Arctic Ocean water overflowing the Iceland‐Faeroe Ridge in the early‐middle Miocene, it is suggested that dense water formed along shallow segments of the Iceland‐Faeroe Ridge and then flowed into the South Iceland Basin to begin accumulation of Bjorn, Gardar and Snorri Drifts. The initiation of sediment drift formation in the South Iceland Basin may have been caused by a decreased rate of deep water production combined with a higher rate of detrital input due to a Middle Miocene uplift event along the Iceland‐Faeroe Ridge. A second uplift event along the Greenland‐Scotland Ridge and a decreased production of Northern Component Water in the early Pliocene may be responsible for increased sediment accumulation in Bjorn, Gardar and Eirik Drifts.