Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka

Earth and Planetary Science Letters 202 (2002) 465^480 www.elsevier.com/locate/epsl

Climatic and oceanographic changes in the Northeast Atlantic re£ected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka Eva Moreno a;1 , Nicolas Thouveny a; , Doriane Delanghe a , I. Nick McCave b , Nick J. Shackleton c a

Centre Europe¤en de Recherche et d’Enseignement en Ge¤osciences de l’Environnement (CEREGE), Europo“le de l’Arbois, BP 80, 13545 Aix en Provence Cedex 04, France b Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK c Godwin Laboratory, Department of Earth Sciences, University of Cambridge, Pembroke Street, Cambridge CB2 3SA, UK Received 18 December 2001; received in revised form 4 June 2002; accepted 24 June 2002

Abstract Rock magnetic parameters measured along two giant piston cores MD95-2040 (40‡34PN, 9‡51PW) and MD95-2042 (37‡47PN, 10‡09PW) collected off the Portuguese Margin, related to other proxy-climatic data, have been used to reconstruct magnetic mineralogical changes of, in relation to environmental and climatic conditions over the North Atlantic, Western Europe and Northwest Africa during the last three climatic cycles (since isotope stage 10). Thin discrete layers containing coarse grains of titano-magnetite are associated with events of iceberg discharge during Heinrich events 1^6 [Heinrich, Quat. Res. 29 (1988) 142] that have equivalent events in isotope stages 5^8. Concentrations of fine-grained (Ti-) magnetite and hematite/goethite, varying in phase opposition, are directly linked with alternations of cold and warm climatic periods. Spectral analyses of the rock magnetic signals reveal Milankovitch periods at 100 and 41 ka, confirming the relationship with long-term climatic changes at high latitudes. The nature (Ti-magnetite) and size range of the finest ferrimagnetic fraction as well as its variation, suggest a control by deep currents carrying a colloidal/clayey fraction from remote sources (Iceland, Faeroes, mid-Atlantic Ridge). Variation of hematite/goethite contents is linked with transport by rivers and winds from the neighbouring continent. A tight correlation with the D^O cycles in Greenland ice records confirms that North Atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka. C 2002 Elsevier Science B.V. All rights reserved. Keywords: Portugal; continental margin; sedimentation; magnetic properties; climate change; ocean circulation; Pleistocene

1. Introduction * Corresponding author. Tel.: +33-442-971558; Fax: +33-442-971590. E-mail address: [email protected] (N. Thouveny). 1

Present address: Istituto Nazionale di Geo¢sica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy.

The Portuguese Margin is a key geographic location for the reconstruction of the Pleistocene climatic and oceanographic history of the North Atlantic region. It is characterised by high sedimentation rates allowing high resolution studies.

0012-821X / 02 / $ ^ see front matter C 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 7 8 7 - 2

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Fig. 1. (a) Position of the referenced cores in the North Atlantic: Me69-19 [1], T88-9P [37], SU90-24 and PS2644-5 [47]. (b) Bathymetry of the studied area and position of the cores MD95-2040 and MD95-2042 as well as the reference cores used for the age model: MD95-2039 and SU81-18.

The sedimentary sequences in this area were known to contain evidences of repeated iceberg discharges associated with Heinrich layers [2^6] that appear to be correlative with cold temperature phases in Greenland at least over the 10^40 ka interval [7]. Modern deep sea waters along the Portuguese Margin are mostly in£uenced by North Atlantic Deep Water (NADW) [8], whose formation has been demonstrated to be drastically reduced during glacial periods [9^14]. The convection regime during glacial periods produced intermediate water analogous to present day Labrador Sea Water with similar depth of penetration and was probably active further south. The formation of Lower NADW (LNADW ; Iceland^Scotland Over£ow Water coming from the Nordic seas) in the eastern North Atlantic was strongly reduced. This regime was associated with a strong northern cooling and with the installation of widespread cold, dry and windy climatic conditions over the North Atlantic and Europe [15]. These changes led to a replacement of LNADW by Southern Source Water (SSW) below a depth of 2000 m [16]. Changes in the thermohaline cir-

culation were also associated with Heinrich events [16^20] and D^O cycles [21,22]. The Portuguese Margin is also a key site for the record of oscillations of the Polar Front between its present location (60^70‡N across the Norwegian Sea) and its location at 40‡N during the glacial maximum [23].

2. Geographical setting This study focuses on two giant piston cores: MD95-2042 and MD95-2040 (Fig. 1), collected in 1995 o¡ the Portuguese Margin by the French RV Marion Dufresne with a Calypso piston corer of the French Institute for Polar Research and Technology (IFRTP) [24]. C-14 ages obtained on other cores SU81-18 (37‡46PN, 10‡11PW) [25] and MD95-2039 (40‡34PN, 10‡20PW) [26] have been used for time-scale construction (Fig. 1). Core MD95-2042 was collected at a depth of 3146 m, 140 km o¡shore from the Tagus and Sado river mouths. Core MD95-2040 was collected at a depth of 2665 m east of Oporto Seamount, about 100 km from the coastline ; MD95-

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2039 was collected at 3381 m depth, west of the seamount.

b

b b

3. Magnetic parameters b

Magnetic parameters have been measured on a set of 8 cm3 cubic specimens continuously subsampled along the cores, and normalised by the wet mass. Low frequency (0.92 kHz) magnetic susceptibility (M) was measured with a Geofyzika Kappabridge KLY2 susceptibility meter. The anhysteretic remanent magnetisation (ARM), and isothermal remanent magnetisation (IRM) were measured with a cryogenic magnetometer 2G 760R. Saturation IRM (SIRM) and back IRM were produced in a steady ¢eld of 2.8 T and back ¢eld of 0.3 T with a pulse magnetiser. ARM was imparted in a 0.1 mT bias ¢eld combined with a linearly decaying alternating ¢eld of 80 mT and expressed as anhysteretic susceptibility (Marm ) (i.e. divided by the strength of the steady ¢eld). The respective signi¢cances of magnetic parameters as mineralogical indices were introduced by several authors [27,28] :

b

467

SIRM and M are proxies for coarse (Ti-) magnetite. ARM is a proxy of ¢ne-grained (Ti-) magnetite. The Marm /M ratio is inversely related to the grain size (Ti-) magnetite [29,30]. Hard IRM (HIRM) = (IRM30:3 T +SIRM)/2 is an index of the hematite or goethite content [31]. The SIRM/M ratio was used to detect the ferrimagnetic sulphide content [32].

4. Chronostratigraphy The construction of the time scale uses two different age models. The ¢rst one is based on series of radiocarbon ages and N18 O markers correlated with the SPECMAP record [33,34]. High precision correlation between susceptibility pro¢les (Fig. 2) enabled the relocation of chronological data available from neighbouring cores MD952039 and SU81-18 in the studied cores. Intercore correlation and depth-to-time transformation were achieved through linear interpolation procedures between consecutive markers using Analys-

Fig. 2. Correlation between susceptibility records of cores SU81-18, MD95-2042, MD95-2040 and MD95-2039.

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Table 1 Age models for cores MD95-2039 and MD95-2042 Depth MD95-2042 (cm)

Depth MD95-2039 (cm)

Reference

204.82

^

14

236.83 266.49 283.86 298.15 345.97 361.91 377.81 392.08 425.16 456.31 471.74 502.29 530.91 547.3 576.78 419.97 480.58 480.58 839.29 839.29 968.19 1160.13 ^ ^

^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 168.5 208.5 208.5 542.5 542.5 648 818.5 ^ ^

MD95-2042 [5] (MD95-2039)

MD95-2039 [25] N18 Oben

1760 1980 2100 2160 2260 2400 2540 2620 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

1415 1550 1660 1710 1760 1850 1900 1970 2030 2230 2300 2330 2365 2402 2510 2540 2600 2620 2660 2740 2790

(1822.31) (1976.95) (2123.28) (2196.55) (2278.65) (2432.05) (2517.25) (2631.76)

14

C

C

MIS position 4 [34] 5 [34] 5.1 [34] 5.2 [34] 5.3 [34] 5.4 [34] 5.5 [34] 6 [34] 6.2 [34] 6.3 [34] 6.4 [34] 6.41 [34] 6.42 [34] 6.5 [34] 6.6 [34] 7 [34] 7.1 [34] 7.2 [34] 7.3 [34] 7.4 [34] 7.5 [34]

Age model 1 (yr BP)

Depth MD95-2042 (cm)

Depth MD95-2039 (cm)

Reference

Age model 2 (yr BP) 18

5 971

0

Plankton N O MD95-2042 N18 O GISP2 corr. [36]

7 660 8 489 9 756 10 486 11 934 12 424 12 828 13 744 14 353 14 596 14 887 15 952 16 399 17 049 17 169 14 304 15 396 15 892 23 278 23 956 26 382 32 271

305 434 1014 1054 1178 1218 1271 1302 1338 1442 1486 1558 1618 1684 1755 1876 1967 1990 2033 2140 2492 2580 2594 2626

82 266 577 746 818 874 918 946 988 1068 1128 1188 1251 1300 1362 1459 1530 1570 1606 1709 1846 1933 1952 1969

Age model 1 (yr BP)

^

MD95-2039 [25]

58 690 73 910 79 250 90 950 99 380 110 790 123 820 129 840 135 100 142 280 152 580 161 340 165 350 175 050 183 300 189 610 193 070 200 570 215 540 224 890 240 190

^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

0

11 630 14 590 27 910 29 090 32 370 33 720 35 330 36 330 38 500 41 220 42 630 45 360 47 070 52 230 56 670 62 220 66 160 72 890 73 160 82 900 116 000 128 000 129 100 132 000 MIS position

Age model 2 (yr BP)

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2230 2300 2330 2365 2402 2510 2540 2600 2620 2660 2740 2790 2810 2900 2980 3110 3240 3320 3340 3380 3470

6.3 [34] 6.4 [34] 6.41 [34] 6.42 [34] 6.5 [34] 6.6 [34] 7 [34] 7.1 [34] 7.2 [34] 7.3 [34] 7.4 7.5 [34] 8 [34] 8.2 [33] 8.3 [33] 8.4 [34] 8.5 [34] 8.6 [33] 9 [33] 9.1 [33] 9.2 [33]

142 280 152 580 161 340 165 350 175 050 183 300 189 610 193 070 200 570 215 540 224 890 240 190 244 180 249 000 257 000 265 670 288 540 299 000 303 000 310 000 320 000

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Table 1 (Continued). MD95-2042 [5] (MD95-2039)

MD95-2039 [25] N18 Oben

MIS position

Age model 1 (yr BP)

^

MD95-2039 [25]

MIS position

Age model 2

^ ^ ^ ^ ^ ^ ^ ^ ^ ^

2810 2900 2980 3110 3240 3320 3340 3380 3470 3520

8 [34] 8.2 [33] 8.3 [33] 8.4 [34] 8.5 [34] 8.6 [33] 9 [33] 9.1 [33] 9.2 [33] 9.3 [33]

244 180 249 000 257 000 265 670 288 540 299 000 303 000 310 000 320 000 331 000

^

3520

9.3 [33]

331 000

(yr BP)

Age model 1 is based on calibrated 14 C ages obtained in cores SU81-18 [24] and MD95-2039 [25] and on the MIS limits. In core MD95-2042 we have noted the depth position of the MIS estimated from core MD95-2039 planktonic N18 O record transferred to core MD95-2042 [5] and we give the benthic N18 O record in core MD95-2039 [25]. Due to the gap between the two records, the benthic MIS limits from core MD95-2039 were used as a chronological template for both cores MD95-2039 and MD95-2042. The second age model was built on the GISP2 time scale based on the N18 O^N18 O correlation between core MD95-2042 and GISP2 [36]. The depth^age relationship of the bottom part of the MD95-2039, beyond the GISP2 time scale, is the same as for the ¢rst age model.

eries software [35]. Table 1 presents AMS 14 C ages available from cores SU81-18 [25] and MD95-2039 [26] (radiocarbon ages of core SU81-18 enter only in the MD95-2042 age model because their relocation in the top of cores MD95-2039 and -2040 is insu⁄ciently accurate). Isotopic stratigraphies are based on the planktonic N18 O record [5,36] from core MD95-2042 down to marine isotope stage (MIS) 6, and on the benthic N18 O record from core MD95-2039 [26] down to the MIS 9/10 boundary, because

oxygen isotope records are still unavailable for core MD95-2040). Phase lags observed between the planktonic and benthic N18 O records of the two cores (Table 2) have been con¢rmed by the recent comparison of planktonic and benthic N18 O records along one single core (MD95-2042). According to Shackleton et al. [36], the benthic N18 O record presents smooth £uctuations resembling the Vostok ice isotope record whereas the planktonic N18 O record presents large amplitude £uctuations resembling the Greenland ice records; these

Table 2 Estimated ages of the susceptibility peaks from core MD95-2040 between stages 6 and 8 compared to the ages of the IRD peaks from core TP88-9P [39]

M peak S10 S11

S12 S13 S14 S15

S16

Sub-peak

a b c a b

a b c

Age (ka) 136 158 162 165 181 175 190 200 221 224 227 253

IRD peak core T88-9P (48‡23PN, 25‡05PW) [39]

Age (ka)

h10

164^167

h11

182^183

h12 h13

189 201

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MIS 6 6 6 6 6 6 6/7 7 7 7 7 8

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Fig. 3. Depth^time relationship of cores MD95-2042 (left) and MD95-2039 (right) using two di¡erent age models. The ¢rst age model is based on all the 14 C available from cores SU81-18 and MD95-2039 and isotopic stage limits (see Table 1) from core MD95-2039. The second age model is based on the GISP2 time scale and MIS limits. The data have been transferred between cores using the correlation between susceptibility records.

di¡erences have been assigned to the di¡erent responses of the surface and deep ocean to climate. In order to account for this phase shift and to produce equivalent age models for the three cores (MD95-2039, MD95-2040 and MD95-2042), we used the benthic N18 O record of MD95-2039 as the common chronostratigraphic reference for age model No. 1. On the other hand, the high resolution of the planktonic N18 O record of core MD95-2042 and its similarity to the Greenland ice records has been used to generate a second, alternative, age model based on the GISP2 time scale [36] that has been preferably used for core intervals covering the last climatic cycle (core MD95-2042, 16 upper m of MD95-2040 and 20 upper m of MD95-2039). Table 1 presents the reference points used for the age model based on the GISP2 time scale for MD95-2042 and their location in MD95-2039 [36]. The age/depth relationship is presented in Fig. 3. Di¡erences between the two models are small but because the GISP2 model is often used as chronostratigraphic reference, it was preferably used for the ¢nal time scale of the last 90 ka.

5. Results The combination of rock magnetic parameters (Figs. 4 and 5) allowed identi¢cation of the principal magnetic phases: coarse ice rafted magnetite, ultra-¢ne magnetite and hematite/goethite and their interpretation in terms of proxy-climatic data. 5.1. Abrupt short signatures and Heinrich events Multiple M and IRM peaks indicate large and abrupt increases of the concentration of coarse ferromagnetic grains. At the same depths, grain size-dependent parameters (Marm /M and ARM30 / ARM) indicate a coarsening of the mean magnetic grain size that was con¢rmed by scanning electron and optical microscopy analyses of sieving and magnetic extracts. Large grains of Timagnetite are associated with coarse quartz and feldspar grains ( s 60 Wm). Calibrated radiocarbon ages of peaks S1^S4 coincide with the average ages of Heinrich layers H1^H4 [7]. Peaks S5 and S6 reasonably correspond with Heinrich layers H5 and H6; peak S7

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Fig. 4. Bulk (black) and carbonate corrected (grey) magnetic records from core MD95-2042 compared to planktonic N18 O record obtained on Globigerina bulloides [36]. Susceptibility and IRM records indicate variations in coarse magnetite; ARM and Marm /M indicate variations in ¢ne magnetite and magnetic grain size, respectively; HIRM indicates the hematite/goethite content.

Fig. 5. Bulk (black) and carbonate corrected (grey) magnetic records from core MD95-2040 compared to the benthic N18 O record obtained on Cibicidoides wuellerstor¢ [25]. Magnetic parameters have been interpreted in the same way as in core MD95-2042 (see Fig. 3).

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occurs at the transition MIS 4/5a; peaks S8 a,b and S9 a,b occur in sustages 5b and 5d. Susceptibility peaks have also been identi¢ed before the last glacial, their ages have been indicated in Table 2. During the complex interglacial 5, three peaks have been identi¢ed. Peak S7 occurs at the transition MIS 4/5a, while peaks S8a,b and S9a,b occur in sub-stages 5b and 5d. These events can be related to the IRD events H7, H8 and H9 identi¢ed by Heinrich [1] in core Me69-19 (47‡19PN, 19‡41PW). S7 and S8 are older than H7 and H8 (Table 2) but this lag is probably due to the fact that our age model is based on the GISP2 time scale. In core MD95-2040, older susceptibility peaks have been labelled peaks S10^S12 (stage 6), S13 (transition 6/7), S14 and S15 (stages 7b and d), and S16 (stage 8) (Table 2). Their magnetic signatures M, SIRM and Marm /M have similar shapes but smaller amplitudes than typical Heinrich events. Microscope analyses of the coarse grain size fraction ( s 63 Wm) con¢rmed the presence of IRD grains in some of these layers, further evidencing IRD events in this time interval as ¢rst identi¢ed by [37]. The duration of the susceptibility events between 130 and 160 ka BP (Fig. 6) is similar to that of Heinrich events [38,39] : 1.5^2 ka, with a notable exception for S14 and S15, which have longer durations. Complex structures of peaks S11^S14 can be compared with peaks S1 and S2; double or even triple peaks suggest the occurrence of successive arrivals of iceberg rafts possibly from di¡erent origins. More than 50 IRD peaks had been identi¢ed in the sub-polar North Atlantic for the last 500 ka, associated with low SST events at the time of ice cap growth, including cold sub-stages in MIS 5, 7 and 9 [40]. In core T88-9P (48‡23PN, 25‡05PW) from the central North Atlantic, six IRD discharges occurring during MIS 6 and 7 have been labelled h8^h13 [37]; our susceptibility peaks S11, S12, S13 and S14 are, respectively, synchronous with h10, h11, h12 and h13 (Table 2). The occurrence of iceberg discharges during interstadial or interglacial complexes (MIS 5 and 7) suggests that Heinrich-type events occurred even

Fig. 6. Susceptibility record from core MD95-2040 between 120 and 260 ka. The labels indicate the susceptibility peaks S10^S16. Grey band indicates the mean duration of a Heinrich event during the last glacial period [37].

when ice caps were in a growing phase after their period of minimum volume. 5.2. High frequency variability and Dansgaard^Oeschger cycles Greenland atmospheric temperature records of the last glacial epoch point out short period oscillations (1000^3000 yr), called Dansgaard^ Oeschger (D^O) cycles, composed of alternating stadials and interstadials [41^44]. Similar oscillations were identi¢ed in marine sedimentary records from the North Atlantic [45^47] and Alboran Sea [48] and in West European lacustrine sediments [49,50]. As we have pointed out in the previous section, the strong similarity between the MD95-2042 planktonic N18 O record and the GRIP and GISP2 N18 O records indicate that the deep and surface waters along the Portuguese Margin also reacted to these millennial variations [36]. In Fig. 7 we show the M, HIRM, Marm /M and N18 O records from core MD95-2042 compared to the N18 O records from GISP2 for the interval 10^65 ka. Linear correlation coe⁄cients (R) between the GISP2 N18 O record and the planktonic N18 O, M, HIRM and Marm /M from core MD95-2042 are, respectively, 30.77, 30.27, 30.33 and 0.65. Magnetic mineral concentration and size, espe-

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5.3. Low frequency signals and glacial^interglacial alternation

Fig. 7. Core MD95-2042: comparisons between N18 O from GISP2 Greenland ice (a) with planktonic N18 O (b), Marm /M (magnetite grain size) (c), HIRM (hematite and goethite contents, on a reverse scale) (d) and M (e). Numbers label interstadial periods [41]. Note the strong similarity of HIRM and Marm /M with isotope records even at ultra-¢ne scale.

cially the parameter Marm /M related with grain size, are in correlation with D^O cycles and Heinrich events (H1^H4). Dominant M peaks correspond to Heinrich events, but minor peaks are correlative with second-order stadial phases documented in the Greenland temperature records. HIRM and Marm /M records £uctuate in phase with D^O cycles : high HIRM values, pointing out enhancements in high coercivity minerals (hematite or goethite) occur during cold stadials, and high Marm /M values pointing out enhancements in ¢ne-grained (Ti-) magnetite occur during temperate events of interstadials. The origin of the magnetic fractions carrying these signals is discussed on the basis of complementary arguments presented in the following subsection.

Long-term trends of magnetic concentration, grain size and mineralogy also correspond with the wide N18 O glacial^interglacial variations (Figs. 3 and 4). The correction for CaCO3 contents in cores MD95-2042 [51] and MD95-2040 [52] ampli¢es the contrasts of Marm and HIRM signals. Magnetic mineralogy £uctuates between two end-members : the ¢rst one, associated with interglacials or interstadials, is enhanced with ¢ne (single-domain to pseudosingle-domain) magnetite while the second, associated with glacials or stadials, is enhanced with high coercivity minerals. In order to check the climatic signi¢cance of these magnetic signatures, we performed spectral analyses using two methods (Maximum Entropy (ME) and Blackman^Tukey (BT) [35]) on susceptibility, Marm /M and HIRM records from core MD95-2040. Data were evenly spaced each 0.4 ka; the records were detrended and normalised to a zero mean. The construction of the time scale, built partially by correlation of N18 O with the SPECMAP stack, itself tuned to insolation parameters, obviously introduces Milankovitch periods in the N18 O record, however, we emphasise that our test aims to verify the climatic in£uence on magnetic records that are not a priori supposed to follow the same patterns. Di¡erent power spectra are revealed for the susceptibility (M), Marm /M and HIRM records (Fig. 8). The susceptibility record, tracer of iceberg discharges, is not strongly marked by Milankovitch periodicities: the eccentricity (100 ka) period is absent and only minor peaks appear near the obliquity (41 ka) and precession (19^23 ka) periods. In contrast, the Marm /M and HIRM records present major periods at 100 and 41 ka and weaker periods near 19^23 ka. These results con¢rm the strong in£uence of glaciation^deglaciation rythms on sedimentary inputs. We will now attempt to identify in£uences on the sedimentation in order to interpret our records in terms of climatic variability during the time interval concerned.

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Fig. 8. Spectral analysis of M, Marm /M and HIRM records from core MD95-2040 for the 0^340 ka window. They have been expressed as the logarithm of the spectral power density versus frequency (cycles/ka). They were obtained using the BT and ME methods, using Analyseries software [35]. For the BT method, we have indicated the bandwidth and the 80% con¢dence interval. Grey bands represent the main orbital periodicities of eccentricity (100 ka), obliquity (41 ka) and precession (23 ka).

6. Interpretation and discussion 6.1. Origin of the ¢ne magnetite content Glacial/interglacial oscillations in ¢ne magnetite and hematite/goethite content are likely to be related to the bulk sediment composition. In Fig. 9 we have compared the HIRM and Marm /M records from core MD95-2040 with other granulometric data [53] from the same core: sortable silt mean

size SS [13], clay content and silt/clay ratio. A relationship appears between the SS [53] and the Marm /M indicator of magnetic grain size (except for unreliable data of stage 2 [53]): coarse (respectively ¢ne) SS and low (respectively high) clay contents are associated with smaller (coarser) magnetite grain size. Clay content, silt/clay ratio and sortable silt mean size were interpreted as indicators of bottom £ow speed [13]: in core MD95-2040 during the last 150 ka these parameters document changes of the bottom current from glacial to interglacial conditions: warm (respectively cold) periods correspond to faster (respectively slower) bottom currents in this area [53]. Comparison with magnetic proxies reveals that small (respectively large) sizes of magnetite coincide with fast (respectively slow) currents. This apparent paradox can be explained by the fact that the size of the ¢ne magnetite carrying most of the ARM (i.e. smaller than 1 Wm) is well below the minimum size on which hydrodynamic sorting is active. We therefore suggest that the grain size of the magnetite population is controlled by the sediment sources or pathways, rather than by £ow speed. Nowadays, the bottom waters of the Iberian Margin are determined by the Mediterranean Out£ow Water (MOW) between 600 and 1500 m, and North (East) Atlantic Deep Water (NEADW) from 1500 m down to about 2900 m; below, a southern source silicate signature identi¢es the Lower Deep Water [13]. During glacials, advection of nutrient-rich SSW occurred deeper than V2000 m below the glacial NADW [12,54,55]. On the other hand, the advection of the MOW was probably reduced during the last glacial due to a lowered sea level [3]. Changes in the intensity or in the origin of the deep circulation during glacials could be responsible for changes of sediment components delivered to the Iberian Margin, resulting in quantitative and qualitative changes of the magnetic fraction. The hypothesis of a change in bottom water mass is consistent with benthic N13 C data: in core MD95-2042 benthic N13 C changes are synchronous with planktonic N18 O changes and also

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475

Fig. 9. Comparison between Marm /M and HIRM records and sedimentological data from core MD95-2040 [52] for the last 150 ka. The sedimentological data used were the terrigenous clay (wt% of total sediment 6 2 Wm); the terrigenous silt/clay ratio (wt% 2^ 63 Wm/wt% 6 2 Wm) and the sortable silt mean size (Wm). The horizontal bar in stage 2 indicates the portion of the record where Hall and McCave [52] consider the sortable silt record to be unreliable as a current indicator, being probably a¡ected by downslope processes.

with the Greenland ice N18 O changes, showing that the ventilation of the deep ocean by NADW was suppressed during cold events in the North Atlantic. Reduced ventilation of deep water during Heinrich events is also indicated by the distribution of trace fossils and benthic foraminiferal assemblages in two sediment cores from the Portuguese Margin at 2160 and 1100 m water depths [4]. The link between £uctuations of LNADW and ¢ne magnetite has previously been suggested [46,47] : magnetic parameters in cores located between East Greenland (67‡N) and the Bermuda Rise (33‡N) exhibit short-term variations in phase with D^O cycles. Comparison with our data shows a correlation of the Marm /M record of core MD95-2042 with the ARM30 records from subpolar cores SU90-24 (62‡40PN; 37‡22PW) and PS2644-5 (67‡42PN, 21‡45PW) (Fig. 10) [47]. In the latter case, the ARM records re£ect £uctuations of the magnetite deposition by NADW waters. On the Portuguese Margin, these £uctua-

tions can be related to the concentration of ¢negrained magnetite. The principal source of ¢ne magnetite in the eastern North Atlantic being the Tertiary Thulean basaltic province (Iceland^ Faeroes^Northwest Scotland), it was deduced that deep currents may have exerted a major control on magnetic concentration and grain sizes by winnowing. The results of these studies suggest that during MIS 3, the NEADW circulation was less active during stadials than during interstadials [46,47]. Following this argument, it is reasonable to conclude that the exportation of ¢ne-size lithic material to sites located downstream was under the in£uence of the strength of the NADW. Although the Portuguese Margin is not on the direct NADW pathway, the very ¢ne size of the magnetite ( 6 1 Wm), associated with low settling velocity and slow aggregation onto larger fastsinking particles [56], might be appropriate for spreading LNADW to bring ¢ne magnetite from Nordic basaltic sources to the Portuguese Margin where it is scavenged by the abundant sinking

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Fig. 10. Comparison between the Marm /M record of core MD95-2042 with the ARM30 records from sub-polar cores SU90-24 and PS 2644 [49]. The age scale of cores SU90-24 and PS 2644 is given [60]. All records re£ect £uctuations in the magnetic concentration between Dansgaard^Oeschger interstadials. Numbers reported in the ¢gure correspond to interstadials.

particles resulting from upwelling productivity [57]. Fluctuations in the LNADW production lead to £uctuations in deep currents along the Iberian Margin as observed in [53]. Further analyses of isotopic content (Sr, Nd, Pb) may help to trace the origins of the ¢nest magnetic fraction and check the proposed hypothesis. 6.2. Origin of the hematite/goethite content During cold stages 2, 4, 5b, 5d and 6, the abundance of both the clay fraction and the hematite/ goethite content suggests that the hematite/goethite may have the same source and pathway as the clay fraction. A probable source of clays is the Portuguese coastal area, where low sea level and

aridity during cold periods favour the eolian erosion and sediment transport from shore and shelf areas, resulting in faster accumulation rates [2]. Sediment £ux studies using (230 Thxs )0 con¢rm that the clay £ux is larger during glacial periods [26]. It was also suggested [2] that clay supplies partly come from downslope in turbidity currents in canyons. Hematite/goethite, as a product of continental rock alteration and soil formation, is transported to the ocean by rivers and winds. For example, near the Azores [27], o¡shore Senegal and on the Sierra Leone Rise [31], glacial intervals contain higher hematite/goethite contents originating from arid regions of North Africa. Moreover, the concentration variation of terrestrial dusts in Greenland ice demonstrated the increase of atmo-

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spheric turbulence over high latitudes during glacial stages [58]. Eolian dusts can originate from the nearest continent (Iberian Peninsula) and from remote areas of Sahara and Sahel. The coast of Portugal is characterised by weak westerly winds in winter and relatively strong northerly and northwesterly winds in summer [59]. The dust transport from Sahara is favoured by strong low pressure over Southwest Europe [60], but wind patterns suggest that the Portuguese coast would contribute much more than North Africa. The hypothesis of an eolian contribution is supported by the pollen analyses of core SU81-18: two coeval peaks in wind-transported terrestrial elements (pollen and spores), mainly from dry, steppe-type formations, at 15 and 10.3 ka, re£ect the dominance of anticyclonic circulation during the cold phases of the last deglaciation [61]. Although pollen grains found in cores MD952042 and MD95-2039 were mostly attributed to the catchment area of river Douro and Tagus [62,63], the eolian transport of pollen grains must have been signi¢cant or even dominant during cold and dry phases.

477

al from remote areas; comparison with East and South Greenland data suggest a link with the lower NADW regime. The concentration of hematite/goethite ^ high during glacials and low during interglacials ^ appears to react ^ like the clay fraction ^ to sea level, aridity and wind strength over the continent. Further geochemical studies will be necessary in order to identify the sources of particles and test these hypotheses.

Acknowledgements The cores were collected during the IMAGES 101 cruise (chief scientists : L.Labeyrie, F. Bassinot, Y. Lancelot and J.L. Turon). We are greatful to Y. Balut, chief of the operation at Institut FrancUais pour la Recherche et la Technologie Polaires (IFRTP), as well as to the technical and navigation crews of the RV Marion Dufresne. We thank E. Bard and D. Pailler for providing CaCO3 data used to express magnetic parameters on a carbonate-free basis. We thank Prof. S.K. Banerjee and F. Grousset and an anonymous reviewer for constructive comments that helped to improve this manuscript.[AC]

7. Conclusions Rock magnetic properties measured along two giant sedimentary cores from the Portuguese Margin allowed identi¢cation of long and short-term environmental/climatic £uctuations during the last three climatic cycles. Sixteen events of iceberg rafting (Heinrich-like) reached the Portuguese Margin since I.S. 8 glacial maximum; however, a major ampli¢cation started at stage 5b, with a maximum for the Heinrich event 4. Concentrations of ¢ne-grained magnetite and hematite/goethite oscillate in phase opposition at the rhythms of sea surface and atmospheric temperatures. Short and long-term variability of the sedimentation in this area thus appear to be primarily forced by high latitude climatic oscillations. The concentration of ¢ne-grained magnetite ^ high during interglacials, low during glacials ^ is interpreted as an index of the ability of deep current to transport very ¢ne-grained detrital materi-

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