during the past kyr B.P.

Transcription

1 PALEOCEANOGRAPHY, VOL. 14, NO. 5, PAGES , OCTOBER 1999 Northeast Atlantic sea surface circulation during the past kyr B.P. Susanne Lassen, 1 Eystein Jansen,2,3 Karen Luise Knudsen,4 Antoon Kuijpers, 1 Margrethe Kristensen,4 and Karen Christensen4 Abstract. A study was made of three cores from the Faeroe-Shetland gateway, based on planktonic foraminifera, oxygen isotopes, accelerator mass spectrometry 4C dates, magnetic susceptibility, and counts of ice rafted debris (IRD). The data, covering the period ka, show that during the Last Glacial Maximum the Arctic Front occupied a position close to the Faeroes, allowing a persisting inflow of Atlantic surface water into the Faeroe-Shetland Channel. The oceanographic environment during deposition of two IRD layers is influenced by Atlantic surface water masses during the lower IRD layer, with transport of icebergs from N-NW. Polar surface water conditions prevailed only during deposition of the upper IRD layer. There is no indication of surface meltwater influence the region during the deglaciation, but there is a persistent influence of Atlantic surface water masses in the region. Thus we conclude that during almosthe entire period (30-10 ka) the Faeroe-Shetland Channel was a gateway for transport of Atlantic surface water toward the Norwegian Sea. 1. Introduction The interaction between climate and ocean circulation in the North Atlantic and the Nordic Seas is of major Henrich et al., 1989, 1997; Henrich, 1998; Veum et al., importance for the understanding of the Quaternary 1992; Berger and dansen, 1995; Hebbeln et al., 1994; climatic variations. Today, the North Atlantic Drift Hebbeln and Wefer, 1997; Rosell-Meld et al., 1998; transports warm, saline Atlantic surface water from the Rasmussen et al., 1996a, b, 1997; Dokken and Hald, 1996] Atlantic Sea into the Norwegian Sea. This flow of warm who all suggested open ocean circulation in the Nordic waters from the tropics to the high latitudes is responsible Seas (at least seasonally). The presence of Atlantic surface for the comparably mild climate in NW Europe. Entering water in the Nordic Seas during glacial time has been the Nordic Seas, this surface water cools and sinks to recorded as far north as Fram Strait [Hebbeln et al., 1994; become the major source of deep water in the world Hebbeln and Wefer, 1997] and Denmark Strait oceans [Aagaard et al., 1985; Aagaard and Carmark, et al., 1995]. The Iceland- Faeroe Ridge has been 1994]. The locations of the areas of convection in the suggested to have been a pathway [Henrich et al., 1997; Greenland Sea are controlled by the major oceanographic Rasmussen et al., 1996a, b, 1997]. This pattern applies for boundaries in the surface waters and seasonal variations in the distribution of sea ice in the Nordic Seas. Currently, open ocean convection is located in realm of the Arctic water masses [Aagaard and Carmark, 1994]. In reconstructing past ocean circulation the location and fluctuations of the oceanic frontal zone (Arctic Front) of these water masses is thus an important parameter. Previous reconstructions of the surface ocean circulation Paper number 1999PA /99/1999PA Mapping, and Prediction (CLIMAP), 1981 ]. This was later challenged by several authors [e.g., Olausson, 1981; [Sarnthein the interstadials and the Last Glacial Maximum (LGM), whereas the stadials, including Heinrich events, were thought to have been characterized by very limited or no inflow of Atlantic surface water to the Nordic Seas and the Polar and Arctic Fronts situated south of the Faeroe Islands [Rasmussen et al., 1996a, b, 1997]. The purpose of our study is to reconstruct the location and fluctuations of the oceanographic fronts in the have suggested perennial sea ice cover in the Nordic Seas southern sector of the Faeroe-Shetland gateway during the during the last glacial period with a clockwise cold period ka B.P., i.e., a period of continental ice circulation cell [Climate: Long-Range Investigation, growth including the LGM and deglaciation and also repeated Heinrich events. More specifically, our aim has 1 Geological Survey of Denmark and Greenland, Copenhagen, Denmark. been to study whether the Faeroe-Shetland Channel was a 2Department of Geology, University of Bergen, Bergen, Norway. pathway for the inflow of Atlantic surface water to the 3Also at Nansen Environmental and Remote Sensing Center, Bergen, Nordic Seas during the LGM. Here we present an Norway. 4Department of Earth Sciences, University of Aarhus, Arhus, Denmark. investigation of planktonic foraminiferal fauna assemblages, stable oxygen isotopes, and ice-rafte debris Copyright 1999 by the American Geophysical Union. (IRD) from high-resolution cores from the NE Atla_n_t,_'c south of the Faeroe Islands, depicting the paleoceanographic fluctuations in this hydrographically crucial area. 616

2 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION 617 Iceland Figure 1. Regional bathymetric map showing the channels and banks south and east of the Faeroe Islands and the surrounding sea areas with the location of the three cores of this study: ENAM32 (60ø20'53"N, 09ø46'37"W; core length 4.10 m), 90OH1006 (60ø58'15"N, 07ø30'16"W; core length 2.69 m) and 88HM1007 (60ø44'69"N, 04ø30'89"W: core length 2.50 m). The position of core ENAM93-21 (62ø44'00"N, 03ø59'92"W; core length 11 m) north of the Faeroe Islands is also marked [Rasmussen et al., 1996a, b, 1997]. Arrows indicate the modern surface currents in the area. 2. Material Three cores have been studied from the NE Atlantic margin south(east) of the Faeroe Islands (Figure 1). The gravity core 88HM1007 (1125 m water depth) from the Faeroe-Shetland Channel was collected with R/V Haakon Mosby in 1988, and the gravity core 90OH1006 (874 m water depth) in the Faeroe Bank Channel was collected with R/V Olavur Halgi in Both were obtained by the Faeroese Museum of Geology. Core ENAM32 is a piston core from 1011 m water depth south of Bill Bailey Bank taken by the Geological Survey of Denmark and Greenland with R/V Pelagia in The results from analyses of these cores will be correlated with those from core ENAM93-21 (1020 m water depth) north of the Faeroe Islands as representative of cores from the area of the northern entrance of the Faeroe-Shetland Channel [Rasmussen et al., 1996a, b, 1997] (Figure 1). 3. Chronology For all ages we refer to reservoir-corrected (400 years) accelerator mass spectrometry (AMS) radiocarbon dates that were obtained from samples of mono species planktonic foraminifera (Neogloboquadrina pachyderma sinistral or dextral) (Table 1). The results in Figures 2 and 3 are plotted against age, assuming a constant sedimentation rate between each dated level. The sedimentation rates vary between 6 and 72 cm kyr - during the LGM and in the deglaciation intervals. In general, these sedimentation rates are high compared to other cores from the Atlantic area [Broecker et al., 1992; Rasmussen et al., 1996a]. The cores presented here complement each other as they have high-resolution intervals in different stratigraphic intervals of the LGM and the deglaciation period. Core ENAM32 covers the longestime period of the three cores, i.e., the past 30 14C kyr. The other two cores cover the LGM and the deglaciation period. Core 90OH1006 shows good resolution of the LGM, whereas the transition from LGM through the deglaciation period is reflected in better detail in core 88HM1007. Only the latter core contains a continuous deglacial period with the Bolling-Allerod (B- A) and Younger Dryas (YD) chrons. A common feature of all three cores is the lack or scarcity of Holocene sediments. Because of conflicting AMS dates from the upper part of core 90OH1006, indication of a condensed interval in the upper part of core ENAM32 (based on the oxygen isotopes), these parts of the cores will not be further included in the discussion. The reason for this is probably reworking and/or winnowing due to increased (Holocene) bottom current strength.

3 618 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION Table 1. Accelerator Mass Spectrometry 14C Dates Obtained on Planktonic Foraminifera From Selected Levels in Each of the Three Cores Core ENAM 32 90OH HM1007 Depth, m Sample Type 14C Age B.P. Rcor Laboratory Sedimentation Rate, B.P. Number 0.3 NPS 12, , AAR NPS 15, , AAR NPS 16, , AAR NPS 21, , AAR NPS 23, , AAR NPS 26, , AAR NPS 31, , AAR-2458 cm kyr NPS 15, , AAR NPS 16, , AAR NPS 18, , AAR NPS 19, , AAR NPS 10, , AAR NPS 14, , AAR NPS 15, , AAR NPS 15, , AAR NPS 15, , AAR NPS 18, , AAR The reservoir age is assumed to equal 400 years, and the Rcor column comprises the reservoir-corrected age. The dates are measured on Neogloboquadrina pachyderma sinistral (NPS). The laboratory numbers and calculated sedimentation rates for core sections are shown Lithology, IRD, and Magnetic Susceptibility The three cores studied display fairly uniform lithologies, mainly silty clays. The uppermost centimeters in all three cores have higher contents of (fine) sand. The glacial intervals are composed of dark grey to yellow grey silty clays, but in core ENAM32, there are also layers containing some silt and fine sand. IRD contents were counted for two of the cores: ENAM32 and 90OH1006, using the fraction >0.125 mm. In each sample, 300 grains were counted, and the total number of grains per 100 g dry sediment is presented in Figure 2. The IRD content of the cores is variable, but the composition is rather uniform (not shown) and consists mainly of quartz. Grains of other minerals and rocks such as detrital carbonate, volcanic, and plutonic rock fragments have also been identified. In core ENAM32, two levels with a major input of IRD are apparent. On the basis of the age of the layer boundaries these IRD layers have age ranges of (lower IRD layer (L-IRD)) and ka (upper IRDlayer (U-IRD)), respectively (Table 1 and Figure 2). The two IRD layers in core ENAM32 correspond to IRD layers recognized in the North Atlantic and the Norwegian Sea. Thus the age of the lower IRD event corresponds with the youngest dates of the H3 event in the North Atlantic [Andrews et at., 1998; Elliot et at., 1998] and NS6 [Baumann et al., 1995] and HP2 [Dokken and Hard, 1996] in the Norwegian Sea, while the upper IRD layer can be correlated to H2 in the North Atlantic [Bond et at., 1992; Andrews et at., 1998; Elliot et at., 1998] and NS4 [Baumann et at., 1995] and HP 1 [Dokken and Hard, 1996] in the Norwegian Sea. In addition, the LGM interval in the two cores is characterized by a high IRD content with especially large regular peaks in core 90OH1006. The magnetic susceptibility was measured at intervals of 5 cm on the whole core of ENAM32 [Kuij)9ers et at., 1998a, b] and on the split core of 90OH1006 (Figure 2). The results of magnetic susceptibility reveal high values for the LGM period, gradually decreasing through the deglaciation. This pattern is comparable to other cores from south of the Faeroe Islands [Kuij)9ers et at., 1998a, b]. A low magnetic susceptibility signal during IRD layers in core ENAM32 as well as in cores from the North Atlantic [Grousset at., 1993; Robinson et at., 1995] has also been observed in other cores from the Faeroe area [Kuij)gers et at., 1998a, b] as well as in cores from around south Greenland [Andrews et at., 1998]. 5. Quantitative Micropalaeontology and Stable Isotopes In accordance with the differences in resolution the cores were subsampled at different intervals, ranging from 1 to 10 cm. The planktonic foraminifera were picked from

4 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION 619 Magnetic susceptibility (CGSx 10 '6) No. of IRD/100g sediment No. of planktonic foraminifera/100g Z Figure 2. Magnetic susceptibility [Kuijpers et al., 1998a, b] and the abundance of ice-rafted debris (IRD) and planktonic foraminifera (per 100 g sediment) in cores ENAM32 and 90OH1006 where LGM is the Last Glacial Maximum, L-IRD is the lower IRD-event and U-IRD is the upper IRD-event. The dashed line marks the hiatus in core ENAM32. the mm fraction in cores 90OH1006 and ENAM32 and from the 0.1 mm fraction in core 88HM1007. In the latter core the foraminifera in the HM1007 were carried out at the Stable Isotope Laboratory, University of Bergen, whereas the measurements on core ENAM32 were carried out at the 1.0 mm fractions were concentrated using heavy liquid Kiel Stable Isotope Laboratory, University of Kiel. Both flotation CC14 (p=1.59 g cm _:3). Approximately 300 laboratories use a Finnigan MAT 251 mass spectrometer. planktonic foraminifera per sample were picked and Details on preparation, measurements and intercalibration identified. Faunal percentages and the number of of the two laboratories are reported by $arnthein et al. planktonic foraminifera per 100 g dried sediment were [ 1995]. Results are given with respecto Peedee belemnite calculated and presented in Figures 2 and 3. The (PDB), after calibration to the National Institute of planktonic foraminifera faunas contain up to eight species Standard and Technology (NIST) NBS 19 standard. but are largely dominated by a single species, the polar N. pachyderma sinistral. The foraminifera specimenshow no 6. Paleoceanographic Interpretation signs - of dissolution: Because of the high content... of and n:... :. _ subarctic specimens and the studied fraction ( and not >0.150 mm), paleotemperatures have not been estimated Heinrich Events (30-19 ka) Stable oxygen isotopes were measured on -20 specimens of N. pachyderma sinistral from the mm fraction. The measurements on core 90OH1006 and The three cores studied are located north of the North Atlantic IRD belt (40ø-55øN) [Ruddiman, 1977; Robinson et al., 1995]. This may explain why the IRD events were

5 ß 620 LASSEN ET AL.' NORTHEAST ATLANTIC SEA SURFACE CIRCULATION b SO %NPS %T.Q. %G.B B'A Degla " Figure 3. Oxygen isotopes (5'80, measured on Neogloboquadrina pachyderma sinistral) and percentage distribution of N. pachyderma sinistral, Turborotalia quinqueloba, and Globigerina hulloides versus age ( nc ka B.P.) in cores ENAM32, 90OH1006, 88HM1007, and ENAM93-21 [Rasmussen et al., 1996a, b, 1997]. Only very few specimens of G. hulloides were present in core ENAM93-21, and these are therefore not included (T. Rasmussen, personal communication, 1997). The lower IRD event (L-IRD) corresponds to Heinrich event 3 (H3); The upper IRD event (U-IRD) corresponds to Heinrich event 2 (H2). LGM is the Last Glacial Maximum; deglac. is deglaciation; B-A is Bolling-Allerod; and YD is Younger Dryas. The dashed line indicates the lower boundary of reworked surficial sediment in core ENAM32. Stars indicate the levels of AMS '4C datings (See Table 1).

6 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION 621 not immediately recognized during initial sampling but became apparent after IRD and planktonic foraminifera analyses and measurement of isotopes [e.g., Bond et al., 1992; Grousset et al., 1993]. According to Ruddiman [1977] the southern limit of the IRD belt marks the maximum distribution of polar surface water masses. Melting of icebergs is limited to the north, and the flux of IRD is lower than in the south [Grousset et al., 1993]. In ENAM32 the two IRD events are generally defined by a pronounced peak in IRD, a high percentage of N. pachyderma sinistral, a light oxygen isotopic signal, a low abundance of planktonic foraminifera, and a low magnetic susceptibility (Figures 2 and 3). A low abundance of foraminifera is a typical feature of Heinrich events in the main parts of the Atlantic and is probably caused by a combination of lithic dilution and low salinity linked to iceberg melting [e.g., Heinrich, 1988; Broecker et al., 1992; Grousset et al., 1993; Rasmussen et al., 1996b; Cortijo et al., 1997]. During the lower IRD layer the planktonic fauna are characterized by dominance of N. pachyderma sinistral and a relatively high content of Globigerina bulloides. Today, N. pachyderma sinistral dominates in polar and arctic water masses with seasonal sea ice cover [Phleger, 1965; Johannessen et al., 1994], whereas the distribution of G. bulloides is linked to the Atlantic surface water masses [Hemleben et al., 1989; Johannessen et al., 1994]. This fauna composition thus suggests cold surface water conditions affected by (seasonally) sea ice cover with some influence from Atlantic surface water. In contrast to other Heinrich layers including H2, the source area of the Atlantic H3 layer was originally thought to have been different from the Laurentide ice sheet [Bond et al., 1992; Grousset et al., 1993]. However, Bond and Lotti [ 1995] discovere detrital carbonate in H3 suggesting a Laurentide source area for this event as well instead of a Laurentide ice sheet must have been limited, as previously found in the Irminger Basin [Elliot et al., 1998]. The cores from the area north of the Faeroe Islands do not contain any significant IRD layer at the level that corresponds to H3. Moreover, Rasmussen et al. [ 1996a] recorded only a negligible amount of subarctic planktonic foraminifera. We suggest that this difference observed north and south of the Faeroe Islands is due to the influence of Atlantic surface water, implying enhanced melting in the southern area. However, this difference is not reflected in the oxygen isotopes, which reveal values around 3.2%o in both European source as argued by Grousset et al. [1993]. In layer (H2), whereas it was open during deposition of the general, however, H3 differs from H4, H2, and H1 in its lower IRD layer. In addition, the contribution from the low content of lithic grains regardless of proximity to or Laurentide ice sheet to the area may have been smaller position relative to the IRD belt [Bond and Lotti, 1995: during deposition of the upper IRD layer (H2) than during Elliot et al., 1998]; H3 might have been influenced by the lower IRD layer (H3). The planktonic oxygen isotope icebergs from both source areas. The presence of the values are -0.5%o higher north of the Faeroe Islands Icelandic ash grains from three different volcanic areas (S. [Rasmussen et al., 1996b] than south of the islands during Johnsen and K. Gronvold, personal communication, 1995) the H2-event. This also suggests less iceberg melting or in the lower IRD layer (H3) in our core ENAM32 indicates lower temperatures in the northern area. The temperature that some of the icebergs were transported to the area from minimum during the upper IRD event corresponds well the N-NW. This supports investigations of reworked with the peak in the relative abundance of the polar form nannofossils in the North Atlantic H3 and H2 by Rahman N. pachyderma sinistral. [1995], who suggested that the north European icebergs The abundance of planktonic foraminifera increases rd3ffed in an an i-dockwise loop thro the Norwegian- abruptlyin the period following the trd tayer. This can Greenland Sea and into the North Atlantic during these be ascribed to decreased iceberg discharge and thus events. Whether the IRD layers in core ENAM32 have a decreased IRD and freshwater input and/or increased pure Fennoscandic/Greenland source or some Laurentide production of planktonic foraminifera. The abundance input is still uncertain, but the very low content of increase coincides with a marked decrease in the relative carbonate grains indicates that the potential input from the abundance of N. pachyderma sinistral, pronounced peaks areas. The IRD signal of the upper IRD layer (H2) in core ENAM32 is less pronounced than that of the lower IRD layer (H3), and N. pachyderma sinistral reaches maximum values in this interval (>90%). Compared to the lower IRD layer the Atlantic influence is now markedly reduced as indicated by the low abundance of G. bulloides. Thus polar surface conditions probably prevailed south (this study) as well as north of the Faeroe Islands [Rasmussen et al., 1997]. According to the Greenland Ice Core Project (GRIP) ice core the period around 21,500 B.P. experienced the absolute temperature minimum, with temperatures more than 20øC lower than today's [Johnsen et al., 1995]. During this period the area of permanent and seasonal ice cover is expected to have increased. This could impede the iceberg's movement to the Faeroe area and explain the restricted signal in IRD and oxygen isotopes, corresponding to H2, in the entire Faeroe area [Rasmussen et al., 1996b; this study]. The anticlockwise circulation of the icebergs in the Nordic Seas [Rahman, 1995; Dokken and Hald, 1996; Hebbeln and Wefer, 1997] would thus have been displacementoward the east. The restricted IRD signal in our cores from south of the Faeroe suggests that the flow from the north across the Iceland-Faeroe ridge was blocked during deposition of the upper IRD

7 622 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION in Turborotalia quinqueloba, and a less significant increase in the relative abundance of G. hulloides. These ß. suggest an increased influence Of Atlantic surface water [Hemleben et al., 1989; Johannessen et al., 1994] Last Glacial Maximum (19-15 ka) During the LGM, there are distinct regional differences in the planktonic fauna across the Faeroe area. In general, the faunas from the area south of the Faeroes (this study) very much differ from those in the area north of the Faeroes [Rasmussen et al., 1996a, b, 1997]. Additionally, there are variations in the southern area from west during the LGM (Figure 3), which corresponds to the isotope isoline of the Arctic Front today [dohannesen et al., 1994; $arnthein et al., 1995]. The proximity of the Arctic front is also suggested for the area north of the Faeroe Islands [Rasmussen et al., 1996a]. The relative abundance of G. hulloides in all our cores suggests that inflow of surface water masses from the North Atlantic via the Faeroe-Shetland Channel into the Norwegian Sea continued during the LGM. However, this inflow must have been limited as it did not affect the core locations NE of the Faeroe Islands where a negligible amount of G. hulloides in ENAM93-21 and a low abundance of subarctic foraminifera in cores NAB1-04 and (ENAM32 and 90OH 1006) toward east (88HM 1007). The planktonic assemblage in the cores located more to the west comprises more T. quinqueloba (mainly 8-16% but up to 25%) and less N. pachyderma sinistral than core 88HM1007 from the Faeroe-Shetland Channel. Taking the NAB 1-10 were observed [Rasmussen et al., 1996a, b]). The high percentage of N. pachyderma sinistral in the Faeroe-Shetland Channel core (88HM1007) during the LGM is interpreted as a result of the presence of a mixed water mass related to the proximity of the Arctic Front sedimentation rates into account, planktonic productivity' rather than an indication of low temperatures and the in the areas more to the west is also calculated to have presence of a polar water mass [Johannessen et al., 1994]. been somewhat higher than in the Faeroe-Shetland The paleoceanographic interpretations of these results Channel. A comparison of these faunal records with the contradict previous reconstructions by $arnthein et al. present distribution and abundance of T. quinqueloba [1995]. They suggested that the transport of surface water around the modem Arctic Front [Johannessen et al., 1994] into the (eastern) Norwegian Sea totally stopped during the indicates a high influence of the Arctic Front in the Faeroe LGM because of the presence of a weak clockwise area throughout the LGM period. The planktonic /5280 meltwater gyre west of Ireland, which should have blocked values (corrected for the ice volume effect) are -3%0 the Atlantic water inflow and thus strengthened the paleo- Iceland Figure 4. Surface water conditions in the Faeroe region during the LGM with the Arctic Front locked over the Faeroe platform and the Faeroe Bank area. The inflow of Atlantic surface water (shown by dark shading) to the Norwegian Sea through the Faeroe Shetland Channel, limited to the NE-SW by the Arctic water mass (hatched), based on planktonic foraminiferesults shown in Figure 3 is shown.

8 LASSEN ET AL.: NORTHEAST ATLANTIC SEA SURFACE CIRCULATION 623 Irminger current. However, inflow of Atlantic waters to high latitudes (78øN) is confirmed by studies [e.g., Hebbeln et al., 1994]. Hebbeln et al. [1994] propose that the inflow was concentrated on the eastern side of the Nordic Seas. This would suggest an anticlockwise surface water circulation in the Nordic Seas [e.g., Kellogg, 1980; Hebbeln and Wefer, 1997; Henrich, 1997], which is supported by our results (Figure 4) Deglaciation (15-10 ka) The first indication of deglaciation in the northeastern Atlantic is a strong reduction in planktonic 5 80 following the LGM [Duplessy et al., 1981]. This meltwater signal is recorded all over the North Atlantic and in the Nordic Seas [e.g., Duplessy et al., 1981; Jansen and Veum, 1990; Kof and Jansen, 1994; $arnthein et al., 1995], but the deglaciation signal seems to be time transgressive through the area. The earliest signal is recorded in the Icelandic Sea at around 16 ka 4C B.P. [Kof and Jansen, 1994]. In core 88HM1007 (Figure 3)the first deglaciation signal is dated at the period between 15.5 and 15.0 ka 4C B.P., corresponding to the datings of the first deglaciation signals in the Norwegian Sea [Sarnthein et al., 1995] and west of Ireland in the North Atlantic [Jansen and Veum, 1990]. The deglacial record of core 88HM1007 is comparable to other records of North Atlantic cores [Weinelt et al., 1991; Ko9 and Jansen, 1994; Kroon et al., 1997] by displaying a gradual decrease in the planktonic 5 80 values after 15 ka [Ko9 and Jansen, 1994; $arnthein et al., 1995]. This is in contrast to the distinct isotopically light planktonic O peak found in the cores from the Norwegian Sea, indicating that the meltwater discharge from the Barents ice sheet did not affect the North Atlantic core sites [Weinelt et al., 1991]. Whether the southward drift of icebergs and meltwater as suggested by, for example, Weinelt et al. [1991] and $arnthein and Altenbach [1995], reached the Faeroe region is questionable. The planktonic fauna in core 88HM1007 does not change drastically from the LGM into the deglaciation period, as would be expected if the surface current direction changed to a southern direction. Instead, a minor increase in the relative abundance of G. bulloides would rather indicate a slight increase in the influence of Atlantic surface water. This may be an indication of the beginning of the formation of an ice-free corridor far north along the Norwegian coast, which occurred around 13.4 ka 4C B.P. [Ko9 et al., 1993]. It shows that meltwater cannot have had an important influence Norwegian Sea and Faeroe-Shetland Channel. to Berger and Jansen [ 1995] the circulation system the peak deglaciation can be regarded as estuarine. The outflow of meltwater at the western side of the Nordic Seas may have forced an inflow of Atlantic surface the coast of Norway because of the coriolis effect. This would fit with our planktonic data, but variations in the faunal distribution pattern suggest that this system was not operating continuously at all times. The planktonic faunas are still dominated by N. pachyderma sinistral during the B-A, but with lower percentages than during the early part of the deglaciation. T. quinqueloba shows a small peak together with a minor decrease in G. bulloides. The abundance of the planktonic foraminifera core 88HM1007 increases through the time interval. This faunal evidence suggests that the increase in the inflow of Atlantic waters during the deglaciation continues through the B-A, which is in accordance with the general idea of an ice-free corridor along the coast of Norway [e.g., Ko Karpuz and Jansen, 1992; Kof et al., 1993; $arnthein et al., 1995]. Ko9 et al. [ 1993] suggested that the Arctic Front was located close to the Faeroe area during the time interval ka 4C B.P. However, T. quinqueloba in core 88HM1007 and ENAM93-21 [Rasmussen et al., 1996b] never comprises more than 10% (Figure 3), reaching highest values in the latter core. This indicates that the entire Faeroe area was influenced by Atlantic surface water during the B-A and that the Arctic Front probably had a more north(west)erly position than that proposed by Kof et al. [1993] and Ko and Jansen [1994]. It should be mentioned, however, that the B-A interval spans several climatic fluctuations [Ko9 Karpuz and Jansen, 1992] and that the present cores may not contain the entire record of these climatic variations. The YD interval reflects fluctuations in composition of the planktonic fauna, suggesting a unstable climatic period. There is a change from dominance of N. pachyderma sinistral to dominance of the right coiled form of N. pachyderma together with an increase in the percentage of G. bulloides. This indicates that the inflow of Atlantic surface water masses continued and gradually increased through the YD. The increase in subpolar species may reflect a general temperature rise [Hemleben et al., 1989] during the YD. 7. Conclusion The study of three cores from the region south(east) of the Faeroe Islands has shown some intriguing features in the puzzle of fitting together the late Pleistocene paleoceanographic patterns of the North Atlantic and the Nordic Seas. Core ENAM32 contains two IRD layers (named the lower and the upper IRD layers) that can be correlated with H3 and H2 [Bond et al., 1993; Andrews et in the southeastern al., I998;Elliotetal., i998] ntheno Afianticand%ith According NS6/HP2 and NS4/HP1 [Baumann et al., 1995; Dokken during and Hald, 1996] in the Norwegian Sea, respectively. The ash composition of the lower IRD layer suggests a transport direction of icebergs from N-NW, confirming water along previousuggestions that at that time the circulation in the

9 624 LASSEN ET AL.' NORTHEAST ATLANTIC SEA SURFACE CIRCULATION marginal areasuch as the Faeroe-Shetland Channel and the Denmark Strait [Sarnthein et al., 1995]. The deglaciation pattern depicted by the b 80 in core Nordic Seas consisted of an anticlockwise loop [Rahman, 1995]. This IRD layer may have sources in both the 88HM1007 from the Faeroe-Shetland Channel is similar to Fennoscandian and the Laurentide ice sheets, though very few carbonate grains where identified. At the same time the southern Faeroese area was characterized by open ocean circulation with Atlantic surface water masses. A well-defined oceanic front must have been present along the Iceland-Scotland Ridge and separating these waters those recorded in other North Atlantic cores [Koq and Jansen, 1994] and appears to be unaffected by the flow of meltwater along the coast of Norway. The planktonic faunal composition does not change significantly from the LGM to the deglaciation, evidencing a continued influx of Atlantic surface water through the Faeroe-Shetland from polar water masses area north of the Faeroe Islands Channel during the deglaciation period. This could have [Rasmussen et al., 1996a]. been the initial stage of the ice-free corridor along the During deposition of the upper IRD layer, polar surface southern part of the Norwegian coast dated to 13.4 ka 4C water conditions dominated in the entire Faeroe Region, and the large extension of the permanent and seasonal ice cover may explain the virtualack of an IRD-signal. The B.P. [Kof and Jansen, 1994]. The present study also suggests that the circulation was highly variable through the deglaciation period. In summery, we thus can conclude passage between Iceland and the Faeroe Islands might that Atlantic surface water inflow through the Faeroehave been largely closed for iceberg drift because of the sea ice cover. The surface water conditions during the LGM are Shetland Channel prevailed most of the time from 30 to 10 ka, thus providing significant moisture source for the (rapid) build up of the LGM Scandinavian ice sheets. characterized by large N(E)-S(W) gradients in various hydrographic proxies. The planktonic faunand planktonic Acknowledgment. Cores 88HM1007 and 90OH1006 were collected as 5 80 values indicate the proximity of the Arctic Front to part of the Biofar project, and core ENAM32 was collected as part of the the Faeroe platform. However, the cores from the area ENAM-I project by the Geological Survey of Denmark and Greenland (MAST II program). Additional funding was provided by the ENAM-II south(east) of the Faeroes Islands record some inflow of project through the Geological Survey of Denmark and Greenland (Ph.D. Atlantic surface water during the LGM, which must have grant to S.L.). Part of the stable oxygen measurements were performed by been restricted, not having affected the area north of the H. Erlenkeuser of the Kiel laboratory (ENAM32). We would like to Faeroe Islands [Rasmussen et al., 1996a, b]. During the LGM the inflow of surface water from the Atlantic to the express our gratitude to J. Heinemeier of the AMS laboratory at the University of Aarhus, for measuring the 4C-dates and to T. Rasmussen for the use of data from core ENAM Special thanks are due to S. Nordic Seas seem thus to have been restricted to the Heier-Nielsen for providing valuable discussions and comments and to G. Scott, who improved the language. A. Jennings and an anonymous reviewer are greatly thanked for their constructive criticism. 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