Millennial-scale oceanic climate variability off the Western Iberian margin during the last two glacial periods

Marine Geology 196 (2003) 1^20 www.elsevier.com/locate/margeo

Millennial-scale oceanic climate variability o¡ the Western Iberian margin during the last two glacial periods Lu¤cia de Abreu a;b; , Nicholas J. Shackleton a , Joachim Scho«nfeld c , Michael Hall a , Mark Chapman d a

Department of Earth Sciences, Godwin Laboratory, University of Cambridge, Pembroke Street, New Museums Site, Cambridge CB2 3SA, UK b Departamento de Geologia Marinha, Instituto Geolo¤gico e Mineiro de Portugal, Estrada da Portela, Zambujal, 2720 Alfragide, Portugal c GEOMAR Research Center for Marine Geosciences, Wischofstr. 1-3, 24148 Kiel, Germany d School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 7 June 2002; accepted 29 January 2003

Abstract High-resolution palaeoclimate records recovered from the Iberian margin in core MD95-2040 exhibit large fluctuations in oceanographic conditions over the last 190 ka. Large-scale cooling of the surface ocean is indicated by the presence of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral), and in some instances the occurrence of ice-rafted debris (IRD). Ice-rafting episodes were prevalent in both of the last two glacials with greater intensity in Stages 2 through 4, than in Stage 6. The six youngest Heinrich events are well defined during the last glacial but detrital carbonate is absent from Heinrich layers HL6, HL5 and HL3. Dansgaard^Oeschger stadialequivalent sub-millennial IRD deposition events have been detected, in particular during Stage 3, allowing a good match with the cooling displayed in the Greenland ice core (GISP2). Sea-surface temperature off Portugal in Stage 6 was in general warmer than during the last glacial, pointing towards a weaker southward influence of polar water masses. Ice rafting occurred mainly in mid-MIS (Marine Isotope Stage) 6 (between 173 and 153 kyr) as a group of poorly differentiated, short-duration quasi-continuous events, mainly marked by the high abundance of sinistral N. pachyderma. Differences exist in IRD composition relative to the last glacial, with a reduced Canadian-derived detrital carbonate component, combined with an important contribution of volcanic particles. The lower magnitude and higher frequency of these events suggest that the higher temperatures would have induced iceberg waning closer to the source areas. < 2003 Elsevier Science B.V. All rights reserved. Keywords: Iberian margin; millennial-scale climate variability; sea-surface temperature; Stage 6; ice rafting; planktonic foraminifera

1. Introduction * Corresponding author. Tel.: +44-1223-334870; Fax: +44-1223-334871. E-mail address: [email protected] (L. de Abreu).

Over the last decade several ocean circulation reconstructions focussing on the last glacial period

0025-3227 / 03 / $ ^ see front matter < 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0025-3227(03)00046-X

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have suggested that millennial- and even centennial-scale, ice-sheet surges into the North Atlantic could alter the ocean’s heat conveyor and cause fundamental reorganisation of the ocean-climate system (Broecker, 1994; Cortijo et al., 1995). Episodes of ice-rafted debris (IRD) deposition indicate the transport pathway of the icebergs (e.g. Heinrich, 1988; Cortijo et al., 1997; Oppo et al., 1998; McManus et al., 1999) by the anticyclonic polar gyre from the eastern margin of the Laurentide (Bond et al., 1992; Broecker et al., 1992), Fenno-Scandinavian, Icelandic (Grousset et al., 1993, 2000; Bond and Lotti, 1995; Sa¤nchez Gon‹i et al., 1999; Snoeckx et al., 1999) and, more regionally, British-Irish ice sheets (Scourse et al., 2000; Richter et al., 2001). These episodes, referred to as ‘Heinrich events’ (Broecker et al., 1992), have the duration of a few hundred to a thousand years and recurrence intervals of approximately 7^10 ka (e.g. Bond et al., 1992; Bond and Lotti, 1995). Heinrich-type events punctuated not only glacial periods, but also more detailed records show prominent increases in iceberg calving at 1.5^3-ka intervals (Bond et al., 1993; Bond and Lotti, 1995; van Kreveld et al., 2000) that have been correlated with Dansgaard^ Oeschger (D^O) oscillations observed in the Greenland ice-core records (Dansgaard et al., 1993; Grootes et al., 1993). Palaeoceanographic studies of the Portuguese margin have revealed that iceberg rafting of sediments into the North Atlantic occurred south of the main ice-rafting belt (Ruddiman, 1977) beyond the area of maximum iceberg drift (e.g. Kudrass and Thiede, 1970; Lebreiro et al., 1996, 1997; Baas et al., 1997; Zahn et al., 1997; Abrantes et al., 1998; Zahn, 1998; Boelaert, 1998; de Abreu, 2000). However, the low resolution of most of these records does not enable the history of the surface water temperature variability in the subtropical North Atlantic at sub-Milankovitch scales to be fully described. In addition, very few records describe glacial climate variability beyond the last 65 ka and in particular Stage 6, the penultimate glacial period. Recent studies covering the penultimate glacial have been mainly focussed on sediments from the northern North Atlantic region (Oppo and Leh-

man, 1995; van Kreveld et al., 1996; Bout-Roumanzeilles et al., 1997; Bond et al., 1999; McManus et al., 1999). And the majority of these palaeoceanographic records are low resolution compared to those available for the most recent glacial. In the present work, we examine the e¡ects of large-scale rapid climate changes during the last two glacial periods, based on an exceptional highresolution record of palaeoceanographic changes for the last 190 ka collected o¡ the Iberian margin. This is a sensitive sector corresponding to the boundary of iceberg transport, and thus, the extreme extension of these events, and their relative importance can be detected. This study, based on IMAGES core MD952040, aims to assess and compare the di¡erences in magnitude and frequency of climatic changes during the last two glacial periods and detect pervasive, millennial-scale IRD deposition at the Western Iberian margin.

2. Area of study and oceanographic setting Detailed micropalaeontological, sedimentological and geochemical data from sediment samples were obtained from the topmost 22 m of deep-sea core MD95-2040 (40‡34,91PN, 9‡51,67PW; 35.24 m long) (Fig. 1). This core was collected with the Calypso giant piston corer during the IMAGES MD101 Scienti¢c Cruise on board the RV Marion Dufresne (Bassinot and Labeyrie, 1996). MD952040 was recovered at 2465-m water depth, well above the local carbonate compensation depth of ca 4 500 m (Crowley, 1983). The coring site is located 124.5 km west of the Portuguese coast, adjacent to the southeastern £ank of the Oporto Seamount, where high deposition rates allow the recovery of long, highly detailed sedimentary records. Sedimentation patterns are primarily in£uenced by seasonal high productivity and fall-out from intermediate nepheloid layers (Hall and McCave, 2000). In this area of the eastern part of the subtropical gyre, modern surface water characteristics are in£uenced by the descending branch of the North Atlantic Drift (Portugal Current) and by a sea-

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Fig. 1. Bathymetric map of the Iberian margin showing the location of core MD95-2040.

sonal upwelling regime. Intermediate waters derive from the Mediterranean out£ow and deepwater masses are controlled by the thermohaline equilibrium between North Atlantic Deep Water (NADW) and Antarctic Bottom Water. At the present day, the cored site is under the in£uence of NADW.

3. Methodology 3.1. Stable isotope measurements Stable isotope measurements were generally made on 30 specimens of the planktonic foraminifera species Globigerina bulloides picked from the 250^355-Wm-size fraction. Before analysis, the samples were treated with a 3% solution of hydrogen peroxide to remove traces of organic

contaminants, crushed and cleaned using an ultrasonic bath, rinsed in acetone and oven dried at 50‡C. Depending on the sample size, they were analysed using di¡erent mass spectrometers and preparation systems. The larger samples were reacted with 100% orthophosphoric acid at 90‡C using a VG Isotech Isocarb common acid bath system, and the carbon dioxide produced during this reaction was then analysed by a VG Isotech SIRA Series II mass spectrometer. For the smaller samples (less than 100 Wg) analyses were performed using a Prism mass spectrometer with a Micromass Multicarb sample preparation system. The results were calibrated to VPDB (Vienna PeeDee Belemnite) using the repeated analysis of an internal carbonate standard (Carrara Marble). The analytical precision was better than 0.08x (1c) for oxygen and 0.06x (1c) for carbon (in-

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Table 1 Chronostratigraphic model for core MD95-2040 Event

Depth

(analysis)

(cm)

Core top Base Holocene GIF 98 318a TI GIF 98 319a KIA13457b KIA13458b KIA13459b KIA13632b KIA13633b KIA13460b KIA13634b GIF 98 320a ^ ^ ^ ^ GIF 98 321a HL2 GIF 98 322a ^ GIF 98 323a ^ ^ Top HL3 GIF 98 324a Base HL3 Top D^O 5 Base D^O 6 Top D^O 7 Base D^O 7 KIA13635b ^ Top HL4 GIF 98 325a HL4 Base HL4 Base D^O 10 Base D^O 11 ^ Base D^O 12 Base HL5 D^O 14 Base D^O 14 D^O 16 Base D^O 16 D^O 17 HL6 ^ ^ Top D^O 19 Top D^O 20

2.5 131.5 152.5 181.5 221.5 221.5 239.5 257.5 266.5 275.5 278.5 281.5 290.5 293.5 305.5 341.5 377.5 488.5 510.5 512.5 530.5 572.5 575.5 608.5 647.5 722.5 728.5 751.5 766.5 778.5 815.5 821.5 833.5 854.5 866.5 881.5 898.5 928.5 950.5 965.5 995.5 1028.5 1064.5 1079.5 1097.5 1118.5 1127.5 1142.5 1196.5 1220.5 1253.5 1262.5

Species

Conventional AMS 14 C age (3400 yr)

^ ^ G. ^ N. N. N. N. N. N. N. N. N. ^ ^ ^ ^ N. ^ N. ^ N. ^ ^ ^ N. ^ ^ ^ ^ ^ N. ^ ^ N. ^ ^ ^ ^ ^ ^

^ ^ 10 600 ^ 13 610 13 130 13 370 14 020 14 070 14 070 14 050 14 200 14 510 ^ ^ ^ ^ 20 410 ^ 20 650 ^ 22 200 ^ ^ ^ 25 430 ^ ^ ^ ^ ^ 30 060 ^ ^ 32 490 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

^ ^ ^ ^ ^ ^ ^ ^ ^ ^

bulloides pachyderma pachyderma pachyderma pachyderma pachyderma pachyderma pachyderma pachyderma pachyderma

pachyderma pachyderma pachyderma

pachyderma

pachyderma

pachyderma

Error

Assigned age

(T)

Calibrated radiocarbon agesc (cal yr)

(cal yr)

^ ^ 100 ^ 130 80 70 70 70 70 90 80 130 ^ ^ ^ ^ 220 ^ 140 ^ 190 ^ ^ ^ 250 ^ ^ ^ ^ ^ 380 ^ ^ 490 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

^ ^ 12 463E ^ (16 232)E 15 656E 15 953E (16 709)E (16 766)E (16 766)E 16 743E 16 913E 17 273E ^ ^ ^ ^ 23 727* ^ 23 720* ^ 25 516* ^ ^ ^ (28 917)* ^ ^ ^ ^ ^ (34 064)8 ^ ^ 38 4908 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

0 11 547 12 463 14 643 ^ 15 656 15 953 ^ ^ ^ 16 743 16 913 17 273 17 429 17 741 19 240 20 300 23 727 (23 777) 23 720 24 363 25 516 25 700 26 180 29 000 ^ 30 220 31 520 33 720 34 480 35 420 ^ 36 880 38 360 38 490 39 379 40 160 41 540 42 600 43 420 45 500 46 920 51 009 52 420 55 460 56 940 58 320 59 440 62 300 64 000 67 250 69 800

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Source referenced ^ b b b

^ ^ ^ ^ ^ ^ ^ ^ ^ b b b b

^ b

^ b

^ b b b

^ b b b b b

^ b b

^ b b b b b b b b b b b b b b b b b

L. de Abreu et al. / Marine Geology 196 (2003) 1^20

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Table 1 (Continued). Event

Depth

(analysis)

(cm)

Base D^O 20 5b Base 5c ^ Peak 5e ^ ^ ^ Top Stage 7

1280.5 1343.5 1436.5 1499.5 1541.5 1556.5 1607.5 1934.5 2120.5

Species

^ ^ ^ ^ ^ ^ ^ ^ ^

Conventional AMS 14 C age (3400 yr) ^ ^ ^ ^ ^ ^ ^ ^ ^

Error (T) ^ ^ ^ ^ ^ ^ ^ ^ ^

Calibrated radiocarbon agesc (cal yr) ^ ^ ^ ^ ^ ^ ^ ^ ^

Assigned age

Source referenced

(cal yr) 73 650 92 000 111 000 115 248 126 364 133 423 140 788 161 624 184 000

b

R R R R R R R R

a Radiocarbon dates provided by the Laboratoire des Sciences du Climat et l’Environnement, and by the AMS Tandetron facility of Gif-sur-Yvette, France (J.-C. Duplessy). b Radiocarbon dates provided by P. Grootes (Leibniz Labor, Kiel, Germany). c Calibrated radiocarbon ages in brackets were not used in this model. Calibration with: E Calib 4.3 (Stuiver and Braziunas, 1993); * Laj et al. (1996); 8 Voelker et al. (2000). d R Martinson et al. (1987); b Stuiver and Grootes (2000); Meese et al. (1997).

ternal standard). The standard deviation of the analyses of replicate samples is 0.14x for N18 O and 0.08x for N13 C. 3.2. Foraminiferal assemblages and sea-surface temperature (SST) reconstruction The identi¢cation of the planktonic foraminiferal species, in the sediment fraction larger than 150 Wm, is based on Kennett and Srinivasan (1983). Twenty-¢ve di¡erent species of planktonic foraminifera were identi¢ed in core samples, all belonging to the living fauna in that area (Duprat, 1983). Species relative abundance data were used to identify ¢ve distinctive assemblages based on species distributions in surface water masses and surface sediments (e.g. Be¤, 1977; Ottens, 1991). The ¢ve assemblages are: polar, subpolar, temperate/cold subtropical, warm subtropical and tropical. The relationship between planktonic foraminiferal assemblages and SST data was estimated using a modern analogue technique ^ SIMMAX 28 ^ as described in P£aumann et al. (1996). The current EPOCH core top data set was used as reference for SIMMAX 28, containing 947 core tops covering the eastern North Atlantic and the South Atlantic between 88‡N and 43‡S, 35‡E and 60‡W, with a number of these located in upwelling areas. The analogue parameters include water temperature data for the four caloric seasons, four

depth ranges and the combined means of these depth ranges. Palaeoclimate estimates are derived by comparison of the downcore planktonic foraminiferal assemblage with the reference core top assemblage data, using the climatic character of only the most similar modern samples, providing the ‘modern analogue’ of the fossil sample. 3.3. Terrigenous particles The relative abundance and mineralogical composition of the terrigenous grains in the planktonic foraminiferal sample splits were determined. Two main groups of non-biogenic elements were identi¢ed: (i) particles of a diversi¢ed mineralogical composition (quartz, detrital carbonate, feldspar and volcanic grains) attributed to ice-rafting sources, and (ii) a background signal believed to be shelfderived particles and minerals formed in situ in the bathyal and abyssal areas (glauconite and pyrite). To further quantify patterns of IRD deposition the magnetic susceptibility was measured on dried samples of the 63^150-Wm sediment fraction using a Bartington Instruments MS2B sensor (Dearing, 1994).

4. Chronostratigraphy The age model for core MD95-2040 is based on

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a combination of oxygen isotope stratigraphy, 14 C dating and synchronisation of the SST record of MD95-2040 and the GISP2 N18 O data (Table 1). The Globigerina bulloides N18 O record reveals a long, high-quality, continuous record that documents the existence of high-amplitude and highfrequency variability over the last two glacials. Isotopic stages are recognised and ages assigned in accordance with SPECMAP (Imbrie et al., 1984; Martinson et al., 1987). The age model for the last climatic cycle was further re¢ned based on the following: (1) 14 C accelerator mass spectrometry (AMS) measurements provided by the radiocarbon laboratory of the Laboratoire des Sciences du Climat et l’Environnement, and by the AMS Tandetron facility of Gif-sur-Yvette, France (J.-C. Duplessy) and P. Grootes (Leibniz Labor, Kiel, Germany). These were calibrated with Calib 4.3 (Stuiver and Braziunas, 1993) up to 18 000 14 C years, and after Laj et al. (1996) and Voelker et al. (2000) for older ages. (2) GISP2-derived dates (Stuiver and Grootes, 2000; Meese et al., 1997). We correlated the very rapid warming events associated with D^O oscillations in the GISP2 record and the warming in sea-surface conditions in core MD95-2040 palaeotemperature record. This phase locking assumes a strong climatic coupling between Iberian sea-surface conditions and atmospheric temperatures over Greenland (Fig. 5). Mean linear sedimentation rates for this core were estimated, assuming a constant accumulation rate between dated levels. The sampling at 3-cm intervals gives an average time resolution of 180 years for Stage 6 and 180^200 years between Stages 4 and 2. The resultant age model for the last deglaciation and the Holocene is consistent with the planktonic N18 O record from the nearby, well-dated, core SU81-18 (Bard et al., 1989).

5. Results 5.1. Isotopic measurements A complex sequence of climatic oscillations, not identi¢ed in lower-resolution reference curves such as SPECMAP (Martinson et al., 1987) could

be observed during the last two glacials (Fig. 2). Such variability seems to result from SST and salinity changes o¡ the Iberian margin, as a result of melting icebergs, and in the case of core MD95-2040 possibly also in£uenced by river runo¡ (Roucoux et al., 2001). The stable isotopic composition along core MD95-2040 shows signi¢cant di¡erences between glacial Stages 6 and 4 through 2, with the most positive N18 O values occurring in Stages 6 (3.5x) and 2 (2.9x). In Stage 6, high-frequency oscillations between 0.5 and 1.5x are the most striking feature, superimposed on longer-term changes of approximately 2x. The heavy N18 O isotopic composition of Globigerina bulloides during the penultimate glacial in conjunction with the generalised higher SST values (Section 5.3), indicates a possible enrichment of seawater in heavy isotopes. This can be related to the entrapment of N16 O in the well-developed Stage 6 ice sheets. Glacial Stage 4 is clearly marked but presents a lighter oxygen signature (with maximal di¡erences between 0.63 and 0.25x from Stages 6 and 2 respectively). In oxygen isotope Stage 3 the planktonic N18 O record shows high-frequency oscillations, similar to those recorded in Stage 6, the most signi¢cant of which are between 1 and 1.5x. There is a trend towards heavier N18 O values as the glacial conditions became more pronounced and the global ice volume increased (e.g. Shackleton, 1987). Within this 30-kyr interval there are periods of lighter isotopic composition that are heavier than the full interglacial values (Fig. 2). The transition from Stage 3 to 2 is marked by a rapid enrichment of 0.3^0.5x as observed by Prell et al. (1986), Lebreiro et al. (1997) and Moreno et al. (1998). 5.2. Microfaunal study We observed 10 major periods of dominance of the monospeci¢c polar assemblage, composed of the sinistral variety of Neogloboquadrina pachyderma, during each of the last two glacial periods (Fig. 3). Peak abundances during the last glacial (Stages 4 to 2) are appreciably higher (by over 40%) than during the penultimate glacial (Stage 6) and throughout most of Termination II

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LGM

7

Stage 6LGM-equivalent Martinson et al. [1987]

Fig. 2. Comparison between Stage 6 and the last glacial based on micropalaeontological, geochemical and sedimentological criteria (percentage of IRD relative to the total number of sample elements) in core MD95-2040.

(46.4%). In Stage 6 sinistral N. pachyderma reached at most 50% of the planktonic foraminiferal population. When the polar species is not dominant, the subpolar (Globigerina bulloides, T. quinqueloba and dextral coiling Neogloboquadrina pachyderma) and temperate/cold subtropical (Globigerina falconensis, Globorotalia in£ata, Globorotalia scitula, Globorotalia hirsut, Globorotalia truncatulinoides, Globigerinita glutinata, Globigerinella aequilateralis and Orbulina universa) assemblages dominate.

This indicates the presence of warmer waters o¡ the Iberian margin. The lower numbers of temperate/cold subtropical relative to the 65^80% of subpolar species during Stage 6 suggests less favourable conditions than during the warm intervals of the last glacial period. The warm subtropical assemblage reaches high values during the early part of Stage 3 and within Stage 2, at the glacial maximum. Similar trends can be identi¢ed during the penultimate glacial, but are less distinct. A comparable shift towards

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Fig. 3. Glacial and interglacial distribution pattern of the planktonic foraminiferal assemblages in core MD95-2040 and estimated summer SST.

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Fig. 4. Glacial Stage 6. Comparison of IRD, Neogloboquadrina pachyderma (s) relative abundance and SST in core MD95-2040.

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warmer assemblages at the last glacial maximum has been observed along the northern margin of this subtropical gyre (Chapman and Maslin, 1999). 5.3. Reconstructing SSTs from planktonic foraminifera Summer SSTs from MD95-2040 show marked variability during the last glacial periods (Fig. 3). Although the nutrient levels and their interaction with SST should be considered when attempting to make palaeoenvironmental reconstructions, it seems probable that the temperature of surface waters is the main factor determining the distribution of planktonic foraminiferal species at this site. The role of temperature in controlling the foraminiferal assemblage composition is further supported by the agreement of our summer SST records with the N18 O (Figs. 4 and 5), the distribution of IRD associated with iceberg melting events (discussed in Section 5.4), and also the similarity of this summer SST curve to the alkenone-based SST reconstruction, in particular for Stage 6 (Pailler and Bard, 2002). During the penultimate glacial, on average, temperature changes were less severe than for the following glacial Stages (4 to 2). Summer SSTs have a mean value of 14‡C, reaching maximal values of 19‡C. The lowest temperatures (8‡C) were estimated during multiple cooling events, between 172 ka and 152 ka. During ice volume maxima, conditions were colder in Stage 6 than in Stage 2 (Fig. 4). The episodes of abrupt cooling, within Stage 6, not always detected in lower-resolution North Atlantic cores, may be direct analogues to the cooling events identi¢ed during the last glacial period (Bond et al., 1993). On the other hand, during the last glacial period cooling events seem to have been more extreme, being the lowest summer SST values estimated for glacial Stage 4 (4.8‡C) and Heinrich layers HL4 (5.5‡C) and HL1 (5.3‡C). SST minima

11

within Stages 4, 3 and 2 in this sector of the Iberian margin were between 5‡C and 12‡C colder than the mean Holocene temperature of 19‡C, and in good agreement with former temperature estimations for glacial/interglacial periods in the Northeast Atlantic (Lebreiro et al., 1997; Vidal et al., 1997; Chapman and Shackleton, 1999; Cayre et al., 1999). At, or just slightly prior to, the last glacial maximum (LGM) SSTs were approximately as warm as today, with di¡erences of less than 1^2‡C. Only at the end of the glaciation, HL1, temperatures of surface waters were signi¢cantly reduced. Abrupt and intensive SST changes have been identi¢ed in core MD95-2040 in between the most important cooling episodes corresponding to Heinrich events. These periods of rapid deterioration of sea-surface conditions (with SST £uctuations between 10‡C and 16‡C) exhibit, with very few exceptions, the same variability and frequency of the D^O events observed in the N18 O record of Greenland ice cores over the last 70 ka (Fig. 5). Thus a parallelism can be established allowing the assignment of D^O events as far back as D^O 19. The range of SST change within stadial/interstadial transition is generally less intensive than for Heinrich events, between 3‡C and 9‡C, reaching more extreme di¡erences between HL5 and HL4 (9‡C). However, much less variation is present between HL2 and HL1. 5.4. Ice-rafting events Distinct horizons with coarse terrigenous debris, which mark the southward extension of the well-known North Atlantic Heinrich events were identi¢ed in core MD95-2040. To de¢ne the limits of the main iceberg discharge and better characterise periods in£uenced by cold meltwater, the presence of terrigenous particles was used, together with the abundance of sinistral Neogloboquadrina pachyderma and the N18 O record. The IRD layers o¡ Portugal consist of a great variety of mineral types that vary between layers

Fig. 5. Last glacial period. Comparison of IRD, Neogloboquadrina pachyderma (s) relative abundance and SSTs in core MD952040. Numeric scale at the top corresponds to GISP2-equivalent interstadials (Dansgaard et al., 1993).

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Fig. 6. Downcore distribution of the main terrigenous content in MD95-2040. IRD and shelf-derived sediments such as glauconite abundance are expressed in relation to the total entities present in the sediment fraction larger than 150 Wm.

(Fig. 6). Quartz is the dominant mineral species throughout, with frequent detrital carbonate, a few feldspar grains, rare volcanic particles (black volcanic glass), ma¢c minerals and scarce basalt lithoclasts also present. Other non-ice-rafted particles, either authigenic and/or derived from the nearby continental shelf, such as glauconite and pyrite show up in core MD95-2040 in signi¢cant

amounts both within and outside Heinrich layers. The presence of glauconite can be used as an indicator of shelf or upper slope instability as glauconite-mica shelf sands are typical for this area, but this interpretation is not straightforward in our case as this authigenic mineral can have different origins. It can derive from the shelf or result from the erosion of Miocene outcrops that

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are present in the vicinity of core MD95-2040. Nevertheless, the presence of some mica in most of these levels suggests that the presence of glauconite during IRD events may be the result of iceberg ploughing and resuspension of shelf-edge sands. 5.4.1. Stage 6 Ice rafting o¡ the Iberian margin occurred mainly in mid-MIS 6 (between 173 and 153 kyr) as a group of poorly di¡erentiated, short-duration quasi-continuous events, mainly marked by the high abundance of sinistral Neogloboquadrina pachyderma. The main IRD deposition pulses fall within the present age model (Table 1) at 172, 167.5, 162, 161, 157, 155.5, 154.6, 150.6 and 147.6 ka. Several other peaks of sinistral N. pachyderma (over 20%) were detected, but these were not associated with any IRD input. Most IRD deposition events during the penultimate glacial are characterised by the presence of sharp-edged hyaline quartz grains (4^10% of the total entities present in the sample), rare feldspar (less than 0.1%) and volcanic particles (0.4^0.6%), but no detrital carbonate was identi¢ed in the sediment fraction larger than 150 Wm of core MD95-2040 through this interval (Fig. 6). Detrital carbonate, possibly of Canadian origin is recognised, however, further North (van Kreveld et al., 1996). Also Richter et al. (2001) identi¢ed detrital carbonate at the Feni Drift, which re£ects deposition in a region with low IRD supply. 5.4.2. Last glacial A clearer imprint of ice-rafting events in the form of IRD deposition exists during the last glacial period than during Stage 6 (Figs. 4 and 5) suggesting a more e¡ective transport of icebergs down to 40‡N. In the cold intervals of Stages 4, 3 and 2 six main terrigenous-rich levels were identi¢ed. In this chronology HL6 is dated between 61.9 and 58.3 cal ka, HL5 between 46.9 and 45.2 cal ka, HL4 between 40.2 and 38.3 cal ka, HL3 at 30.2^ 29.0 cal ka, HL2 between 24.3 and 23.1 cal ka and HL1 is dated between 17.6 and 14.9 cal ka. The sediments corresponding to Heinrich layers also contain evidence of extremely low-temperature surface water and the planktonic species Neo-

13

globoquadrina pachyderma (left-coiled) dominates the foraminiferal population reaching up to 90% (HL1). In terms of the relative abundance of IRD deposited and associated sea-surface palaeoenvironmental changes, the ice-rafting pulses of the last glacial di¡er signi¢cantly from each other and from those detected in Stage 6. The abundance of IRD, mainly corresponding to quartz grains, reaches its highest values in HL6 (27.5%), HL4 (30.7%) and HL2 (37.6%), where inclusively the double peak of HL2 referred to by Grousset et al. (2000) is represented in the percent record of sinistral Neogloboquadrina pachyderma. While in HL5 (16%), HL3 (3.5%) and HL1 (12.4%) the percentage of quartz is considerably lower (Fig. 6). Mineralogical di¡erences exist between the sediments deposited in distinct events, in particular the presence/absence of detrital carbonate and volcanic particles. Detrital carbonate was only noted in HL4, HL2 and HL1, while HL3 is particularly rich in volcanic particles (1.7%). 5.4.3. Ice-rafted detritus distribution at sub-Heinrich periodicities During Stage 6 it is di⁄cult to identify wellde¢ned events as in the last glacial, but they are still detected in the sediment fraction larger than 150 Wm. In Stages 3 through 2 the IRD in the fraction larger than 150 Wm disappears completely or is drastically reduced between main Heinrich layers, but some deposition of IRD continues to be present. This is associated with less intensive, short-duration, cooling episodes, with SST decreases of generally 1^3‡C, but reaching maximal di¡erences of 6^9‡C (Section 5.3), coeval with D^O stadials identi¢ed in the GISP2 ice core. The magnetic susceptibility record of the 63^ 150-Wm sediment fraction con¢rmed the presence of ¢ner IRD material, in agreement with Moreno et al. (1998, 2002) and Thouveny et al. (2000). Moreover, selected IRD counts of the 90^150Wm fraction suggest the ¢ner material has the same lithological composition as the coarser particles from the main Heinrich layers, including quartz, rare volcanic particles and some ma¢c minerals.

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This is the ¢rst time that such a highly resolved record of IRD deposition has been detected on the Iberian margin, and it shows that even relatively small-scale ice-sheet instability e¡ects can be traced in the eastern North Atlantic as far as 40‡N.

6. Discussion 6.1. Magnitude of SST changes during glacial periods The lowest SST estimates, 12^13‡C below modern values, occurred during glacial Stages 6 and 4 through 2 (Figs. 4 and 5). The present study reveals that none of the ice volume maxima (6, 4 and 2) corresponds to the most extreme sea-surface cooling. In fact, o¡ the Iberian margin the LGM temperatures were almost as high as today, and thus di¡ering from previous regional SST reconstructions by about +5‡C (e.g. CLIMAP, 1981; Ruddiman and McIntyre, 1981). These small glacial/interglacial di¡erences in temperature are supported by alkenone data from the same core (Pailler and Bard, 2002). However, there are signi¢cant glacial/interglacial di¡erences in the relative abundances of the warm subtropical elements of the fauna (Fig. 3). While CLIMAP reconstructions point to a signi¢cant temperature decrease during the LGM (SSTs reaching less than 10‡C), Cayre et al. (1999), de Vernal et al. (2000) and Sarnthein et al. (2000) argued that glacial temperatures in some areas of the mid-latitude North Atlantic di¡ered little from modern values. At the latitude of the Iberian margin the most signi¢cant SST drops coincided with the arrival of meltwater and IRD levels corresponding to the North Atlantic Heinrich events. Our SST record exhibits higher-frequency variability during glacials than during interglacials, which, apart from a few events, seem to have been relatively ‘stable’ periods. Within Stage 6 summer SSTs were never below 8‡C, while during the last glacial SSTs were lowered to less than 4‡C during Heinrich events. The high sampling resolution of this core reveals a strong similarity in terms of the frequency of SST variation between

some of the main cold intervals within the penultimate glacial and Stage 3. The limited contribution of Neogloboquadrina pachyderma (s) to the Stage 6 assemblage, relative to Stages 4, 3 and 2, suggests a reduced southward in£uence of polar water masses at that time (Thiede and MolinaCruz, 1978; Lebreiro et al., 1996, 1997; Cayre et al., 1999; Calvo et al., 2001) and consequently less extreme cooling. These results agree with the ¢ndings of Calvo et al. (2001), which suggested that a steeper N^S thermal gradient was established in mid-latitude North Atlantic during Stage 2, relative to Stage 6. Our records indicate that the polar front did not reach as far south as during the last glacial, thus making the penultimate glacial, in general, a warmer period in this region. The warmer ocean waters may have led to rapid melting, and resulted in comparatively fewer icebergs reaching the eastern North Atlantic, thus delivering less detritus. However, the faunal sequence of Termination II and the beginning of Substage 5e show an interesting aspect; there is a high concentration of sinistral Neogloboquadrina pachyderma and yet no associated IRD. 6.2. Ice-rafting events in Stage 6 and the last glacial period The lower amounts of ice-rafted particles that reached the Iberian margin during the penultimate glacial correspond to angular quartz grains and volcanic particles. The magnetic susceptibility record of the 63^150-Wm sediment fraction corroborates these observations (Fig. 4). This shows that although the magnetic signature of most icerafting events is weak for Stage 6, the longer-term signature, which characterises cold stages, is possibly enhanced by the existence of ¢ne magnetite grains, smaller than 63 Wm (e.g. Moreno et al., 1998, 2002; Thouveny et al., 2000; de Abreu, 2000). This pattern seems to re£ect the same irregularity and di¡erence in mineralogical composition as observed among the more pronounced, younger HL6-1. These results agree well with the Feni Drift record of IRD input, indicating that the iceberg discharge events were far less pronounced during

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Stage 6 than during the last glaciation (McManus et al., 1999; Richter et al., 2001). The absence of detrital carbonate during Stage 6 at this site suggests signi¢cantly less in£uence of Canadian sources, possibly related to the greater stability of an expanded Laurentide ice sheet. It also poses new questions about the importance of the European ice sheets during this glacial period. SST decreases of 5^8‡C associated with, and in some instances slightly leading, the input of IRD were identi¢ed during Stage 6, but a sequence of multiple, smaller-amplitude coolings was also detected, to which we could not associate any IRD signal in MD95-2040 (Figs. 4 and 5). This suggests this particular core was collected outside the area of migration of icebergs during the less intensive discharge phases within the penultimate glacial period. During the penultimate glacial the Earth was a¡ected by a cold extreme for a much longer time than during the last glacial. This fact may have favoured the development of more extensive Northern Hemisphere ice sheets (Crowley, 1994; van Andel and Tzedakis, 1996), which covered distinct geological provinces, increasing the albedo (re£ectivity) and leading to a greater cooling of marginal areas, and the consequent southward displacement of atmospheric high-pressure cells (Leroux, 1996; Bertrand et al., 2002). The extension, topography and importance of ice-covered areas during the penultimate glacial is still controversial ; mid-latitude continental terminal moraines show the presence of more extensive ice sheets relative to the last glacial (e.g. Tornqvist et al., 2000). For instance, one of the most extensive ice sheets covered northern Germany and Scandinavia during the corresponding Saale Glacial (e.g. Tornqvist et al., 2000; Dahlgren et al., 2002), in particular during the early, Drenthe substage (Nilsson, 1983; van der Wateren, 1994; Ehlers, 1996). Although this was true for eastern Europe and Germany, the Drenthe ice front oscillation was not extensive in the British Isles. In the high-altitude tropics, contrary to what has been inferred elsewhere, the apparent absence of Stage 6 moraines indicates that here the penultimate glaciation was less important than the LGM (e.g. Zreda and Shanahan, 1999). The cli-

15

mate may have been controlled to a great extent by regional or zonal factors, such as the insolation distribution (Shanahan and Zreda, 2000). During the last 65 ka, di¡erences in the character and amount of IRD deposited during the six Heinrich layers suggest the establishment of diverse palaeoenvironmental conditions that are characterised by the presence of lithic elements from di¡erent geographic origins. The existence of detrital carbonate, hematite-stained quartz grains, feldspar and black basaltic glass in most of the main IRD deposits (HL4, HL2 and HL1) suggests that icebergs derived from di¡erent source areas contributed to ice-rafting episodes (Bond and Lotti, 1995; Snoeckx et al., 1999; Scourse et al., 2000). Major di¡erences were observed for HL6, HL5 and HL3, where no detrital carbonate was identi¢ed. For the last event, the IRD provenance has been characterised through isotopic tracers as not dominantly derived from the Laurentide ice sheet (Grousset et al., 1993; Snoeckx et al., 1999). The distribution of volcanic particles in this core corresponds closely to other IRD assemblages, suggesting that they may have been delivered to the site by means of ice transport and correspond to the basaltic glass described in cores from the Denmark Strait (Bond and Lotti, 1995). Unlike other Heinrich layers, HL5 does not contain any volcanic elements, which, together with low total IRD abundances, points to a generally less intensive ice-rafting episode that is not normally well represented o¡ the Iberian margin. Conversely, HL3 has not been identi¢ed in some other North Atlantic areas and o¡ southern Portugal (Cortijo et al., 1995; Zahn et al., 1997; Chapman and Shackleton, 1999; Scho«nfeld and Zahn, 2000) but is well represented in core MD95-2040. This Heinrich layer in particular is unusually rich in volcanic particles, thus strengthening the idea that it is mainly composed of IRD from an Icelandic source (Kirby and Andrews, 1999; Snoeckx et al., 1999; Grousset et al., 2000). The magnitude of the SST reductions in MD952040 is very similar for all the Heinrich events and close to those described for mid-latitudes in the North Atlantic Ocean (Chapman and Shackleton, 1999), with minima around 3^6‡C. The SST esti-

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mates suggest that during events HL6, HL4 and HL1 (coinciding with part of Termination I) conditions were quite similar, but the magnitude of ice rafting seems to di¡er signi¢cantly, with HL4 being the most intense (Fig. 5). 6.3. Presence of sub-millennial-scale IRD events coeval with D^O stadials The high-frequency variability of the analysed records displays for the last glacial period not only intervals of abrupt major cooling associated with the Heinrich events, but also multiple, shortterm, slightly less intense SST decreases between the most important events that have partially similar micropalaeontological, and in some cases mineralogical characteristics. The actual SST values coeval with D^O stadials in the GISP2 N18 O record can be di¡erentiated in MD95-2040. They are characterised by signi¢cant SST drops (generally 1^3‡C, but reaching extreme changes of 9‡C) accompanied by the presence of sinistral Neogloboquadrina pachyderma (around 5^10%). IRD corresponds mainly to angular quartz grains, but a few rounded elements were also observed. No detrital carbonate was visually distinguished under the binocular microscope, which suggests that the central part of the Laurentide ice sheet did not contribute signi¢cantly to these events at the Iberian margin. These new results add evidence that IRD continued to be deposited between main Heinrich events within a time period of a few hundreds of years (McManus et al., 1999; Andrews, 2000). Until very recently these short-term events, coeval with some cold D^O stadials, had only been recognised in northern latitudes of the Atlantic Ocean (Bond et al., 1993, 1997; Bond and Lotti, 1995; Fronval et al., 1995; van Kreveld et al., 2000), although SST decreases in phase with D^O events had already been detected in a core from the Alboran Sea (Cacho et al., 1999, 2001). Core MD952040 seems to con¢rm that up to 40‡N a quasicontinuous ice-rafting history is recorded. Our results agree with suggestions that the processes behind these sub-Milankovitch climatic changes cannot be accounted for by just orbital

variations, but must be seen in a broader context. The high-frequency variability may derive from interactions between orbital forcing and the high sensitivity of continental ice caps to small variations, either in sea-level (e.g. Imbrie et al., 1993; MacAyeal, 1993) or ice-deposit thickness and con¢guration and its direct relation to changes in the thermohaline circulation. Such changes signi¢cantly a¡ect the stability of ice sheets, which in turn produce an important impact on the frequency and magnitude of iceberg discharges into the North Atlantic. The system must work in delicate, dynamic equilibrium between external, possibly orbital forcing and its ampli¢cation by internal thresholds; when it reaches a state of non-equilibrium, even small events may have signi¢cant repercussions.

7. Conclusions The high sediment accumulation rate of IMAGES core MD95-2040 allows the full amplitude of climatic variability o¡ the Iberian margin to be captured, and for the ¢rst time enables an exhaustive description of the penultimate glacial deposits at 40‡N. Extreme SSTs were generally less low during the penultimate glacial than during Stages 4 through 2, suggesting a reduced southward in£uence of polar water masses o¡ Portugal. None of the glacial maxima corresponds to the lowest SSTs, but in fact they reached nearly interglacial values, and in particular during the LGM, the SST was almost as warm as at present. The drift and melting of icebergs o¡ the Iberian margin seems to have been the main cause of abrupt SST drops during the last glacials, and a similar pattern of SST oscillations during Stages 6 and 3 can be identi¢ed. Signi¢cant di¡erences exist in terms of the magnitude and temporal distribution of iceberg discharge. The presence of IRD during both glacials and in some cold intervals within interglacials was noted. Ice rafting seems to have been a fairly continuous process, during both glacials, with greater intensity over the last glacial period. During Stage 6, ice sheets had a bigger extension than during the last glacial, thus one can

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assume that iceberg calving areas were probably not the same. The slightly lower sea-level that characterised the penultimate glacial may have contributed to the higher stability of ice sheets. The substantially lower amount of ice-rafted particles identi¢ed in several North Atlantic sites relative to the last glacial suggests the ice tongues from the Laurentide ice sheet did not reach the shelf edge during this period (Hiscott et al., 2001). Ice-rafting events were not as abrupt and catastrophic in Stage 6 as during Stages 4, 3 and 2. These episodes are not as easily di¡erentiated, but constitute a group of short-duration quasi-continuous events, better marked by the presence of sinistral Neogloboquadrina pachyderma than IRD. The lower magnitude and higher frequency of these events in the penultimate glacial at 40‡N suggests that the generally higher temperatures would have induced iceberg disintegration closer to the source areas. Quartz is always associated with ice-rafted deposits, but no detrital carbonate was detected in MD95-2040 during Stage 6 in the sediment fraction larger than 150 Wm. Volcanic particles, both reworked lithoclasts and volcanic glass, are also present in some levels, indicating a contribution of material derived from northern Britain and Icelandic ice sheets. In the last glacial, however, six major ice-rafting events are well constrained and it was possible to identify important di¡erences between them. The presence of quartz grains, some of which are hematite-stained, detrital carbonate, feldspar and black volcanic glass in HL4, HL2 and HL1 suggest that icebergs derived from di¡erent source areas contributed to ice-rafting episodes. No detrital carbonate was observed in HL6, HL5 and HL3, this last event being particularly rich in volcanic elements. This fact strengthens the assumption that HL3 is mainly composed of IRD from an Icelandic source. D^O stadial-equivalent IRD deposition events were described for the ¢rst time o¡ the Iberian margin at 40‡N. Due to the very high sedimentation rates which characterise the cored area, submillennial events were detected, allowing a good match with the short, abrupt coolings displayed in the Greenland ice core.

17

Suborbital-scale climate variations seem to be a feature of the climate system during the last glacial periods of the Pleistocene. Centennial periodicity during the penultimate glacial is similar to that observed in Stage 3 (V700 yr) and might be a function of ice-sheet instability associated with oceanic circulation changes. Although the group of mechanisms behind this variation is still not understood, possibly solar radiation changes may exert some control (Bond et al., 2001). However, it seems reasonable to surmise that a diverse combination of forcing factors and reaction times of the di¡erent components of the complex ocean^atmosphere^ice-sheet system would have produced a partially similar record in the deepsea sediments.

Acknowledgements The ¢rst author in particular is grateful to Tjeerd van Andel, Fa¤tima Abrantes, Susana Lebreiro, Antje Voelker, Isabel Cacho, Helen Pfuhl and David Cox for their valuable comments and suggestions; also to the reviewers Elsa Cortijo and James Scourse whose advice and constructive comments helped to improve the manuscript ; to Phil Gibbard for the enlightenment on the extension of the Saalian deposits in di¡erent parts of the European continent; to Simon Crowhurst for editing and advice; and to James Rolfe for his help in the preparation of many samples for stable isotopic analysis. Funding for L.A. was provided by the Portuguese FundacYa‹o para a Cie“ncia e a Tecnologia fellowship: PRAXIS XXI/BD/ 5881/95. Part of this research was supported by EC Grant ENV4-CT97-0643 and NERC Grant GR3/12889. We are grateful to the French MENRT, TAAF, CNRS/INSU and especially to IFRTP for the IMAGES coring operations aboard the Marion Dufresne. References Abrantes, F., Baas, J., Klitgaard, D., Loncaric, N., Rasmussen, T., Gaspar, L., Ha£idason, H., 1998. Hydrographic changes along the European Margin between 20 and 8 kyrs: Sediment £uxes. Mar. Geol. 152, 7^24.

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