High-resolution stratigraphy of the Mediterranean outflow contourite system in the Gulf of Cadiz during the late Pleistocene: The impact of Heinrich events E. Llave a, * , J. Scho ¨ nfeld b , F.J. Herna ´ndez-Molina c , T. Mulder d , L. Somoza a , V. Dı ´az del Rı ´o e , I. Sa ´nchez-Almazo f a Instituto Geolo ´gico y Minero de Espan ˜a, Servicio de Gelogı ´a Marina, C/Calera 1, 28760 Tres Cantos, Madrid, Spain b IFM-GEOMAR Leibniz-Institute of Marine Sciences, Wischhofstr. 1-3, D-24148 Kiel, Germany c Facultad de Ciencias del Mar, Univ. de Vigo, 36200 Vigo, Spain d De ´partement de Ge ´ologie et Oce ´anographie, UMR CNRS 5805 EPOC, Univ. Bordeaux 1, Avenue des faculte ´s, 33405 Talence Cedex, France e Instituto Espan ˜ol de Oceanografı ´a, C/Puerto Pesquero s/n, 29640 Fuengirola, Spain f Departamento de Estratigrafı ´a y Paleontologı ´a, Facultad de Ciencias, Univ. de Granada, C/Fuentenueva s/n, 18002 Granada, Spain Received 6 August 2004; received in revised form 11 November 2005; accepted 17 November 2005 Abstract A detailed, high-resolution stratigraphic analysis of the Mediterranean Outflow contourite system at the continental slope of the Gulf of Cadiz has been carried out through the correlation between a dense network of seismic reflection profiles (sparker, airgun, 3.75 kHz and parametric echosounder — TOPAS), Calypso giant piston and standard gravity cores. From such correlation we determine a stacking pattern constituted by four main seismic units (a–d) that are internally structured into ten subunits. Each subunit shows a single sequence formed by transparent seismic facies at the base to smooth, parallel reflectors of moderate to high amplitude facies at the top, being well correlated in the cores with a coarsening-upward sequence. The latest Pleistocene–Holocene deposits form glacial/interglacial depositional sequences related to cycles with a frequency range below the Milankovitch band that corresponds to millennial timescale climatic changes such as Dansgaard–Oeschger (1.5 ka) and Bond Cycles (10–15 ka). Oxygen isotope records of planktonic foraminifera and the occurrence of ice-rafted debris (IRD) in the most recent contourite subunits show clear evidence of the influence of the North Atlantic climatic conditions, especially the climatic Heinrich events (H) in the slope sedimentation of the Gulf of Cadiz and then in the circulation of the Mediterranean Outflow Water (MOW). The coarser contourite deposits are mostly associated with the Last Glacial Maximum, Younger Dryas and Heinrich events on the central area of the middle slope. During globally cooler conditions, the MOW was denser so that it was more active in deeper areas than today. On the other hand, during warm periods the MOW became less dense favoring an increased intensity of the MOW on the distal area of the upper slope. Therefore, spatial and vertical fluctuations of the MOW contourite system are strongly affected by global climate and oceanographic changes, being clearly influenced by iceberg discharges and probably also, by the resumption of thermohaline circulation in the North Atlantic Ocean during ice melting periods. D 2005 Elsevier B.V. All rights reserved. Keywords: Gulf of Cadiz; contourite deposits; Late Pleistocene; paleoclimate changes; Heinrich events; Mediterranean Outflow Water; paleoceanography 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.11.015 * Corresponding author. Tel.: +34 91 7287276; fax: +34 91 7287202. E-mail address: [email protected] (E. Llave). Marine Geology 227 (2006) 241 – 262 www.elsevier.com/locate/margeo
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www.elsevier.com/locate/margeo
Marine Geology 227
High-resolution stratigraphy of the Mediterranean outflow contourite
system in the Gulf of Cadiz during the late Pleistocene:
The impact of Heinrich events
E. Llave a,*, J. Schonfeld b, F.J. Hernandez-Molina c, T. Mulder d, L. Somoza a,
V. Dıaz del Rıo e, I. Sanchez-Almazo f
a Instituto Geologico y Minero de Espana, Servicio de Gelogıa Marina, C/Calera 1, 28760 Tres Cantos, Madrid, Spainb IFM-GEOMAR Leibniz-Institute of Marine Sciences, Wischhofstr. 1-3, D-24148 Kiel, Germany
c Facultad de Ciencias del Mar, Univ. de Vigo, 36200 Vigo, Spaind Departement de Geologie et Oceanographie, UMR CNRS 5805 EPOC, Univ. Bordeaux 1, Avenue des facultes, 33405 Talence Cedex, France
e Instituto Espanol de Oceanografıa, C/Puerto Pesquero s/n, 29640 Fuengirola, Spainf Departamento de Estratigrafıa y Paleontologıa, Facultad de Ciencias, Univ. de Granada, C/Fuentenueva s/n, 18002 Granada, Spain
Received 6 August 2004; received in revised form 11 November 2005; accepted 17 November 2005
Abstract
A detailed, high-resolution stratigraphic analysis of the Mediterranean Outflow contourite system at the continental slope of the
Gulf of Cadiz has been carried out through the correlation between a dense network of seismic reflection profiles (sparker, airgun,
3.75 kHz and parametric echosounder — TOPAS), Calypso giant piston and standard gravity cores. From such correlation we
determine a stacking pattern constituted by four main seismic units (a–d) that are internally structured into ten subunits. Each
subunit shows a single sequence formed by transparent seismic facies at the base to smooth, parallel reflectors of moderate to high
amplitude facies at the top, being well correlated in the cores with a coarsening-upward sequence. The latest Pleistocene–Holocene
deposits form glacial/interglacial depositional sequences related to cycles with a frequency range below the Milankovitch band that
corresponds to millennial timescale climatic changes such as Dansgaard–Oeschger (1.5 ka) and Bond Cycles (10–15 ka). Oxygen
isotope records of planktonic foraminifera and the occurrence of ice-rafted debris (IRD) in the most recent contourite subunits show
clear evidence of the influence of the North Atlantic climatic conditions, especially the climatic Heinrich events (H) in the slope
sedimentation of the Gulf of Cadiz and then in the circulation of the Mediterranean Outflow Water (MOW). The coarser contourite
deposits are mostly associated with the Last Glacial Maximum, Younger Dryas and Heinrich events on the central area of the
middle slope. During globally cooler conditions, the MOW was denser so that it was more active in deeper areas than today. On the
other hand, during warm periods the MOW became less dense favoring an increased intensity of the MOWon the distal area of the
upper slope. Therefore, spatial and vertical fluctuations of the MOW contourite system are strongly affected by global climate and
oceanographic changes, being clearly influenced by iceberg discharges and probably also, by the resumption of thermohaline
circulation in the North Atlantic Ocean during ice melting periods.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Gulf of Cadiz; contourite deposits; Late Pleistocene; paleoclimate changes; Heinrich events; Mediterranean Outflow Water;
paleoceanography
0025-3227/$ - s
doi:10.1016/j.m
* Correspondi
E-mail addr
(2006) 241–262
ee front matter D 2005 Elsevier B.V. All rights reserved.
argeo.2005.11.015
ng author. Tel.: +34 91 7287276; fax: +34 91 7287202.
Fig. 3. (A) Sparker seismic profile indicating the main seismic units within the Quaternary contourite sedimentary record from Llave et al. (2001). (B) 3.75 kHz seismic profile across the mounded
Faro–Albufeira drift indicating the main Late Pleistocene seismic units. (C) TOPAS seismic profile across the Faro–Cadiz sheeted drift indicating the main seismic units within the main Late
Pleistocene seismic units. In B and C, MD9923-36 and MD9923-41 core locations are showed but see Fig. 5 for further details.
E.Llave
etal./Marin
eGeology227(2006)241–262
247
Fig. 4. Spatial distribution and main depocenters of the Late Pleistocene–Holocene seismic unit. White dotted lines indicate the depocenters
orientation. Black lines show the channel locations, and black dotted lines the canyon locations. The thickness unit is milliseconds (twtt).
E. Llave et al. / Marine Geology 227 (2006) 241–262248
oldest to youngest) (Table 1 and Fig. 3B and C). Units
a, b, and d were further subdivided, each into minor
subunits (a1, a2, a3, b1, b2, b3, b4, c, d1 and d2). The
subunits are bounded by continuous reflectors of
strong-medium amplitude underlined by a change in
seismic facies. Within depositional sequences from a to
d there is a marked cyclic trend of seismic facies (Fig.
3) comprising a semi-transparent, low amplitude and
not well stratified pattern unit in the lower part that
passes up into an smooth, highly stratified parallel
reflectors of moderate-high amplitude in the upper
part. This facies evolve at the top in an erosive contin-
uous surface of high amplitude (Fig. 3).
The Late Pleistocene–Holocene seismic unit isopach
map shows a constant thickness displaying a flatter
geometry over all the middle continental slope (Fig.
4). A decrease in thickness is observed close to con-
touritic channels, in sectors affected by diapiric struc-
tures and Guadalquivir Bank uplift (Fig. 4). Several
depocenters are revealed by the spatial distribution
achieve in the isopach map (Fig. 4): (i) in the Faro–
Albufeira mounded drift the seismic unit reaches a
maximum thickness of 100 ms (twtt) with a ENE–
WSW direction; (ii) in the southwestern part of the
Faro–Cadiz sheeted drift sector, the maximum thick-
ness is of 80 ms (twtt) showing a NE–SE direction; (iii)
E. Llave et al. / Marine Geology 227 (2006) 241–262 249
E. Llave et al. / Marine Geology 227 (2006) 241–262250
lower sedimentation rates in the northern core
MD9923-36. The predominance of mud deposition
and higher sedimentation rates in core MD9923-41
points, on the other hand, to a generally lower near-
bottom current strength at this location. The sand con-
tent is much lower at the top of MD9923-41 than in the
uppermost samples of core MD9923-36 which also
indicates less winnowing and a lower current velocity
during the Late Holocene (Heilemann, 2000) on the
southern Faro–Cadiz sheeted drift as compared to the
Faro–Albufeira mounded drift area (Fig. 5).
Two IRD peaks occurred in core MD9923-41 that
corresponds to Heinrich events H1 and H4 (Fig. 5B). In
core MD99923-36, however, several peaks with mas-
sive abundances of lithic particles are recognized (Fig.
5A). The majority of lithic grains from these levels were
subrounded quartz. The grains were commonly glauco-
nite-impregnated and are therefore considered as rede-
posited shelf sand and not as IRD. Only those levels
were designated to Heinrich events where at least 5% of
the grains were identified as dolomite derived from the
Laurentide Ice Sheet (Baas et al., 1997; Schonfeld and
Zahn, 2000; Schonfeld et al., 2003).
The ANAS01-21 and ANAS01-22 gravity cores
were collected in the Faro–Cadiz sheeted and the de-
formed sheeted drifts, respectively. Both areas are char-
acterised by high sedimentation rates (Fig. 6). The
dominance of mud with scattered shell fragments
throughout is the main lithological feature of these
cores (Fig. 6). Thin sand layers are present at the
bottom and top of the cores only (Fig. 6). Two sandy
and muddy sand layers (d21 and d22) of about 15–75 cm
in thickness were identified within the most recent unit
d of the ANAS cores by a correlation with the upper-
most part of the MD cores (Fig. 6).
The correlation of seismic profiles with the cores
reveals the lithology of the different seismic units (Figs.
3, 5 and 6):
– Unit a: its upper part has been correlated with the
bottom of core MD9923-36. The seismic transparent
facies is hence related to sediments mainly com-
posed of mud, whereas seismic reflective facies are
related to coarser, silty sediments in the core.
– Unit b: the subunits b1, to b4 are characterised by
transparent facies at the base and more reflective
facies towards the top. They are related to predom-
inantly muddy deposits overlain by silty sediments
as observed in the cores.
– Unit c: is the most reflective unit in the Late Pleis-
tocene deposits. It is characterised by several high
amplitude reflectors intercalated with more transpar-
ent and thinner ones. These are related to silty/sandy
horizons with intercalated mud layers.
– Unit d: is in general very transparent and related to
extended muddy horizons in the top part of the
cores. Within this unit, there are more reflective
bodies on the top of d1 and d2 subunits. They are
related to silty/sandy beds, whereas the transparent
bodies are related to mud deposits.
4.3. Chronostratigraphy
The chronostratigraphic framework of IMAGES
cores MD9923-36 and ’41 revealed a Late Pleistocene
to recent age for the seismic units a, b, c and d (Fig. 5).
The chronostratigraphy of core MD992341 is based
on radiocarbon datings and the recognition of Heinrich
events. A close correlation of the oxygen isotope curve
with the parallel-core M39008-3 (Cacho et al., 2001)
inferred additional age control points for the upper part
of core MD9923-41 (Fig. 7).
The chronostratigraphic framework was further im-
proved in the lower part of this core by correlation of
the planktonic oxygen isotope record with the GISP2
ice core d18O record (Meese et al., 1997). Midpoints of
sudden cold–warm transitions of the planktonic oxygen
isotopes reflecting the beginning of Dansgaard/Oesch-
ger interstadials were tied to equivalent structures of the
GISP2 record (Dansgaard et al., 1993; Shackleton et al.,
2000) (Fig. 7).
The chronostratigraphy of core MD9923-36 is based
on the recognition of Heinrich events and a comparison
with stacked standard records (Imbrie et al., 1984;
Martinson et al., 1987). The age-model was refined
by graphic correlation of the planktonic oxygen isotope
curve with core MD9923-41 down to Heinrich event
H4, and with core MD952042 below this level (Cayre
et al., 1999; Shackleton et al., 2000) (Fig. 7). The
graphic correlation was performed with Analyse Series
Version 1.1 (Paillard et al., 1996).
The planktonic d18O records of cores MD9923-36
and MD9923-41 exhibit typical glacial to interglacial
variations (Fig. 5). According to the age models, our
records reach back in time to between approxi-
mately 24 ka (ANAS01-21) and 91.85 ka (MD9923-
36) (Tables 2 and 3). The planktonic d18O records
from cores or intervals with high sedimentation rates
show a millennial-scale variability during the last
glacial–interglacial transition and in the earlier part
of the records (Cacho et al., 2001), which can be
used for graphical correlation (Colmenero-Hidalgo et
al., 2004). The short-term variability seemingly mir-
rors Dansgaard/Oeschger cycles as observed in Green-
Fig. 6. ANAS01-21 and ANAS01-22 gravity cores and the TOPAS seismic profiles which crosses the deformed sheeted drift and Faro-Cadiz
sheeted drift respectively. For location of these ANAS01 gravity cores in the CDS of the Gulf of Cadiz see Fig. 1B.
E. Llave et al. / Marine Geology 227 (2006) 241–262 251
land ice cores (Dansgaard et al., 1993; Meese et al.,
1997). AMS 14C-datings ascertained the correlation of
the d18O curves from different cores. In particular the
comparison of oxygen isotope curves and AMS-dat-
ings from gravity core M93008-3 and giant piston
core MD9923-41 (Fig. 5B), which are 1.15 km
Fig. 7. Correlation of that data with the parallel-core M39008-3 (Cacho et al., 2001) in the upper part of the MD9923-41 core and with the GISP2
ice core d18O record in the lower part of the core. Thin gray lines are the correlations (given in the Tables 2 and 3). Thick gray line is H1. Crosses
are midpoints of cold/warm transitions which were used for correlating MD9923-41 with GISP2. Dots are 14C-datings and H as defined by IRD
scans.
E. Llave et al. / Marine Geology 227 (2006) 241–262252
apart, revealed variations in sediment thickness of
only a few tens of centimeters (Table 2). As such,
no significant stretching of the uppermost core meters
by the Calypso device is considered (Thouveny et al.,
2000; Skinner and McCave, 2003).
The age model of the cores is corroborated by the
recognition of Heinrich climatic events as depicted by
horizons with coarse terrigenous debris (Fig. 5). In
core MD9923-36, H1, H2, H4, and probably also H6
were identified. Core MD9923-41 shows only H1 and
H4 (Mulder et al., 2002). The concentration of ice-
rafted debris is with 2 to 12 grains N250 Am per gram
dry sediment about one order of magnitude lower than
off northern and southern Portugal (Baas et al., 1997;
Schonfeld and Zahn, 2000; Schonfeld et al., 2003).
The occurrence of IRD in core MD9923-36 (Fig. 5A)
is superimposed by probably shelf-derived sands as
described above. In both cores, Heinrich events and
cold climatic periods show higher sand contents, no-
tably the Younger Dryas, H4, and Marine Oxygen
Isotope Stage (MIS) 4. The sand content, i.e. grain
size and magnetic susceptibility curves show along the
MIS 3 interval in core MD9923-41 an inverse but
coherent cyclicity of D/O climatic oscillations as in-
ferred by the d18O curve (Fig. 5B).
Sand-rich, contouritic beds are marked by a high
reflectivity in the seismic records. This correlation
allows a chronological assessment of the contouritic
sedimentation. The oldest seismic unit a is developed
between the MIS 6 (135 ka) and the Heinrich event
H6 (59.4 ka). It has to be emphasized that the
boundary between both units is drawn with the top
of the sand-rich, highly reflective and stratified MIS 4
interval, where H6 is situated (Fig. 5). The seismic
unit b is positioned between H6 (59.4 ka) and H3
(30.5 ka). H3 is not depicted by IRD in this area, but
it coincides with a couple of time-equivalent sandy
contourites deposited during late D/O Interstadial 5.
The following seismic unit c is bracketed by H3
(30.5 ka) and H2 (23.8 ka), again with a sandy
contourite on top. The youngest seismic unit d is
developed between the H2 (23.8 ka) and present.
The subunits differentiated within each seismic unit
are limited by other contouritic beds developed dur-
Table 2
Age-model for core MD992341
Depth (cm) 14C Lab.
number
14C-age
(ka B.P.)
1sigma-
error (yr)
Correlation
with core
Depth therein
(cm)
14C-age§
(ka B.P.)
Calibrated
age (cal. ka)
Remarks
0 0.000 Core top
5 KIA14636 1.585 25 1.143
25 M39008-3 8 2.660 2.863
65 KIA14637 5.845 35 6.257
95 M39008-3 68 6.954 7.874
151 M39008-3 112 7.901 8.844
205 M39008-3 172 8.320 9.283
255 KIA14638 9.120 50 9.732
285 M39008-3 332 9.450 10.717
315 M39008-3 347 9.686 11.017
370 KIA14639 11.130 50 12.689
395 M39008-3 418 11.348 13.344
445 SU81-18 250 12.460 14.705
475 M39008-3 463 13.540 16.201 H1
485 KIA14640 14.210 80 16.465
565 KIA14641 15.010 110 Discarded1
580 KIA14642 15.720 100 18.206
675.9 GISP2 193,883 20.832 c/w transition
732.5 GISP2 198,598 23.405 Trans. to IS2
755 KIA14643 20.940 130 24.257
805 KIA14644 21.530 190 24.846
937.5 GISP2 205,577 27.832 Trans. to IS3
980.2 GISP2 207,552 29.011 Trans. to IS4
1005 KIA14645 26.290 240 29.776
1111.9 GISP2 212,681 32.296 Trans. to IS5
1176.6 GISP2 214,647 33.618 Trans. to IS6
1275.8 GISP2 217,689 35.273 Trans. to IS7
1285 KIA14646 32.040 560 Discarded2
1412.8 GISP2 223121 38.388 Trans. to IS8
1435 KIA14647 33.250 570 38.850 Diff. corr.3
1470 MD952039 1020 34.150 39.379 H4
1602.8 GISP2 227,549 41.151 Trans. to IS10
1678.8 GISP2 230,290 42.529 Trans. to IS11
1777.3 GISP2 235,822 45.371 Trans. to IS12
The chronostratigraphic framework was further improved by correlation of the oxygen isotope curve with the parallel-core M39008-3 (Cacho et al.,
2001) in the upper part of the core and with the GISP2 ice core d18 O record in the lower part of the core.1Radiocarbon age is too young possibly due to recrystallization of pteropod shell.2Radiocarbon age is too young, possibly due to contamination with tap water precipitates.3This dating is in the range of low geomagnetic intensities und high age offsets around IS8.
The data provided by Voelker et al. (2000) suggest a correction of 5600 yr.§Reservoir correction of 400 yr substracted.
E. Llave et al. / Marine Geology 227 (2006) 241–262 253
ing Heinrich events, the Younger Dryas and other
cold climatic periods.
A correlation of the lithological succession and
new AMS 14C-datings corroborate the Holocene and
Late Pleistocene age of the most recent contouritic
sand layers recovered with the ANAS gravity cores
(Fig. 6). ANAS01-21 was retrieved from the Faro–
Cadiz sheeted drift, and revealed that the last minor
depositional sequence d2 developed since the Younger
Dryas to present. Core ANAS01-22 was recovered
from an eroded edge of the Faro–Cadiz sheeted
drift, and the fine-grained deposits sampled corre-
spond to the lower part of the b2 depositional se-
quence (developed between H6 and H5 since MD
core datings). This shows that the muddy deposits
close to the bottom of the b2 unit have an age of
around 47 ka (Fig. 6).
5. Discussion
The determination of the stacking pattern and evo-
lution of the contourite depositional system in the Gulf
of Cadiz through dating and facies analysis contribute
answering questions of MOW variability in time and
Table 3
Age-model for core MD992336
Depth
(cm)
Correlation
with core
MD992341
(cm)
Correlation
with core
MD952042
(m)
Calibrated
age
(cal. ka)
Remarks
20 10 0.000
35 30 3.221
45 85 7.164
55 90 7.519
70 95 7.874
135 151 8.844
151 205 9.283
185 280 10.034
200 285 10.199
225 305 10.458
235 315 10.588
255 345 11.447
265 370 11.913
285 440 14.669
290 475 16.201 H1
330 555 17.746
385 590 18.455
400 610 18.881
410 620 19.094
420 635 19.413
440 650 19.732
470 670 20.157
495 690 20.583
525 730 21.227
545 751 21.644
560 778 22.214
590 795 22.888
601 805 23.026
625 825 23.303
655 845 23.766 Ca. H2
685 920 27.736
700 960 28.614
710 985 29.209
720 1010 29.655
730 1035 30.102
755 1095 32.123
765 1135 32.913
775 1155 33.455
795 1220 34.560
810 1270 35.287
825 1305 35.929
830 1325 36.700
840 1375 38.320
850 1435 38.663 Ca. H4
875 1485 39.469
970 1424 40.749
1025 1512 43.616
1055 1544 44.829
1080 1588 46.215
1160 1668 50.979
1220 1750 56.357
1315 1848 60.936
1355 1876 62.220
1403 1953 65.554 Event 4.2
1505 1982 70.549
Table 3 (continued)
Depth
(cm)
Correlation
with core
MD992341
(cm)
Correlation
with core
MD952042
(m)
Calibrated
age
(cal. ka)
Remarks
1600 2053 74.981
1660 2072 76.710
1685 2085 77.893
1715 2100 79.259
1780 2115 80.624
1801 2123 81.353
1830 2157 84.499
1840 2170 85.721
1875 2200 88.542
1900 2213 89.764
1945 2233 91.645
The chronostratigraphy is based on the correlation of the planktonic
oxygen isotope curve with core MD9923-41 down to Heinrich event
H4, and with core MD952042 below this level (Cayre et al., 1999;
Shackleton et al., 2000).
E. Llave et al. / Marine Geology 227 (2006) 241–262254
space related to climatic changes. These results
revealed high frequency cyclicity in the contourite
sedimentary record, with a frequency range below
the Milankovitch band that corresponds to cooling
Bond Cycles (10–15 ka), which ended in an Heinrich
climatic event (Bond et al., 1993). The coincidence
between climate and contourite cyclicity discloses a
straightforward interpretation of Late Pleistocene–Ho-
locene MOW dynamics in the context of northern
hemisphere climate variability.
5.1. Architectural stacking patterns on late quaternary
contourite deposits
A broad Late Quaternary contourite system of about
100–75 ms (twtt) thickness is develop in a mid-slope
setting at 500 to 1100 m water depth. It is characterised
by a sigmoid–oblique prograding, stacking pattern in
the northern margin (Faro–Albufeira mounded drift),
where is located the main depocenter, and by a broad
aggrading stacking pattern in its basinward prolonga-
tion (Faro–Cadiz and Bartolomeu Dias sheeted drift).
This contourite depositional system (CDS) is a conse-
quence of MOW interaction with an irregular sea bot-
tom, Coriolis forcing and the decreasing speed of
different undercurrent branches along the slope
(Melieres, 1974; Malod, 1982; Maldonado and Nelson,
1999; Nelson et al., 1999; Llave et al., 2001; Hernan-
dez-Molina et al., 2003).
The internal seismic signature of the CDS is char-
acterised by a sedimentary succession that comprises
continuous high amplitude reflections with parts that
display an acoustically parallel internal seismic signa-
Fig. 8. Late Pleistocene climatic curve as depicted by the Greenland ice core record (Dansgaard et al., 1993; Grootes et al., 1993; Meese et al., 1997)
in relation to sedimentation rate changes in core MD9923-36 and MD9923-41. Note that changes from cool to warm conditions are accompanied
with a decrease in sedimentation rate in the deeper core MD9923-36 but a slight increase in sedimentation rate, if at all, in the shallower core ’41.
Grey bars indicate the Heinrich climatic events.
E. Llave et al. / Marine Geology 227 (2006) 241–262 255
ture of low to medium amplitude reflections. The re-
petitive nature of the succession has made possible to
distinguish minor seismic units a, b, c and d, and to
further subdivide them into ten subunits. Cyclic
changes in the sedimentation rates during the Late
Pleistocene are also observed, where in general terms
high values are registered during cold climatic condi-
tions (Rogerson, 2002), as it is traced by the MD9923-
41 core (Fig. 8). On the other hand, higher sedimenta-
tion rates during ice-melting periods or cool–warm
transitions are described in those deposits located in
the northern area of the CDS of the Gulf of Cadiz, as it
is depicted by the MD9923-36 core data (Fig. 8).
The non-uniform distribution of age-control points
in the cores and possibility of hiatuses in the sediment
successions displayed by seismic sections do not justify
a serious application of frequency analyses at the pres-
ent stage. However, the time distance between seismic
unit and subunit boundaries and the repetitive changes
are related to the climatic variations during the Late
Fig. 9. Paleocirculation sketch during: (A) glacial conditions, when the MOW is denser, deeper and then the principal and southern branches (PB and SB) are the main fluxes; and (B) interglacial
conditions, period when the MOW is not so dense and flows in less deep waters, being the intermediate branch and the Mediterranean upper core (IB and MU) the principal nucleus.
E.Llave
etal./Marin
eGeology227(2006)241–262
256
E. Llave et al. / Marine Geology 227 (2006) 241–262 257
Pleistocene, suggesting an influence of 5th order pre-
cession cycles (19–23 ka). Precessional cycles have
already been constrained to drive the terrigenous supply
at the northern Iberian margin (Thomson et al., 2000)
and on the continental shelf and upper slope of the Gulf
of Cadiz (Hernandez-Molina et al., 2002). Then the
cyclic trend in the sedimentary facies is in common
mode with the cyclic climatic changes.
Anyway, the tight coupling of sandy contourites
with Heinrich climatic events and D/O stadials super-
imposes a millennial-scale variability. A common pat-
tern in this different variability is the transition from
warm or interglacial climatic conditions to a cold inter-
val, which is volumetrically much more important due
to mud deposition than the formation of thin sandy
contourites during cold climatic phases or cold–warm
transitions.
In this sense, it is proposed that the contourite
stacking pattern on the middle slope of the Gulf of
Cadiz is influenced by the repetition of cold and
warm climatic intervals (Fig. 8). This Late Pleistocene
climate variability exerted (Dansgaard et al., 1993;
Grootes et al., 1993; Meese et al., 1997) have direct
influenced on Gulf of Cadiz’s water masses circulation
pattern and then on the depositional systems.
5.2. Changes in the MOW contourite depositional sys-
tem as response to global climate changes: the impact
of Heinrich events
An enhanced Mediterranean circulation during the
Heinrich events and Glacial Maximum conditions in
comparison to warmer intervals has been proposed by
Cacho et al. (2000). A smaller and denser MOW that
would mix more vigorously with North Atlantic waters
is supposed to have prevailed during cool stages (Bar-
inger and Price, 1999). This view is controversial since
the MOW volume was certainly lower due to a reduced
cross-section of Gibraltar Strait during the glacial sea
level lowstands (Bryden and Stommel, 1984; Zahn,
1997; Matthiesen and Haines, 1998). This setting may
well have diminishing the water exchange between the
Mediterranean Sea and the Atlantic Ocean (Bethoux,
1984; Bryden and Stommel, 1984; Duplessy et al.,
1988). But, on the other hand, owing to this reduced
exchange, the lowered temperatures (Paterne et al.,
1986; Rohling et al., 1998), and a generally dryer
Mediterranean, the glacial MOW had a significantly
higher salinity and density (Zahn et al., 1987; Thunell
and Williams, 1989; Zahn, 1997; Schonfeld, 1997;
Cacho et al., 2000). Then, an intensive and deeper
MOW is originated (Thomson et al., 1999; Schonfeld
and Zahn, 2000; Rogerson, 2002). A deep and vigorous
MOW would result in a stronger interaction with the
seafloor at greater depths, being the Southern and Prin-
cipal Branches (SB and PB) the main MOW fluxes
(Fig. 9A), facilitate the transport and deposition of
coarser material, and finally would develop higher
sand contents in contourites, as can be observed in
the sedimentary record of the MD9923-41 southern
core. This scenario agrees well with our observations
as sandy contourites are developed in different places
under these different climatic conditions. Therefore, a
variable spatial influence of the MOW during each
climatic stage can be considered (Fig. 9A): the lower
Mediterranean branch enhanced during cool periods,
favoring the development of sandy contourites in the
central area of the middle slope (Fig. 9A).
During warm climatic periods and at high sea-level,
riverine sediments and terrigenous sands were trapped
on the shelf and predominantly fine suspension
reached the distal areas of the margin. The density
of the MOW was lower than during cool climatic
conditions (Zahn et al., 1987; Schonfeld, 1997;
Cacho et al., 2000), and then the interaction of the
MOW with the seafloor was more intensive at shal-
lower depths, result of the Intermediate Branch and
Mediterranean Upper Core (IB and MU) main fluxes
(Fig. 9B). At these warm conditions, sandy contourites
developed in shallower areas where in general the
upper Mediterranean branches were enhanced. This
conclusion again is in agreement with our observation
of higher, Late Holocene sand content in the top of the
northern core MD9923-36. It also offers an explana-
tion why many authors reported a period of winnow-
ing and contourite sand deposition associated with
interglacials instead of during glacials: they considered
sediment cores from depths near the top of today’s
upper MOW core, in the northern area of the Gulf of
Cadiz (Faro Drift), (Faugeres et al., 1984, 1985c,
1986; Gonthier et al., 1984; Stow et al., 1986),
where there is an intensification of the MOW during
warm climatic conditions.
6. Summary and conclusions
The strategy for reconstructing the Late Quaternary
Mediterranean Outflow Water (MOW) contourite sys-
tem variability and evolution has been based on seismic
and sedimentologic facies, sedimentary characteristics,
microfossils, benthic stable oxygen and carbon isotopes
and magnetic susceptibility studies. The combined ap-
proach, by the correlation of ultrahigh-resolution seis-
mic reflection profiles with giant piston and gravity
E. Llave et al. / Marine Geology 227 (2006) 241–262258
cores, facilitates an analysis of sedimentary facies with
a robust chronostratigraphic framework. The 20 m
Calypso piston cores recovered have pushed back the
record of change to around 45 ka, revealing very high
mean accumulation rates of 40 cm/ka. The composition
of these contourite facies is typically of mixed biogenic
and siliciclastic inputs. The core analyses has made
possible to reconstruct the general sedimentary condi-
tions and determine the main controlling factors for the
formation of these contourite deposits, demonstrating
the viability of a reliable, high-resolution chronostrati-
graphy in the CDS deposits, and hence the identifica-
tion of paleoceanographic changes.
Oxygen isotope records during the Late Pleistocene
to Holocene of planktonic foraminifera and the occur-
rence of ice-rafted debris in the most recent contourite
subunits show clear evidence of the influence of the
climate variability and Heinrich events in slope sedi-
mentation of the Gulf of Cadiz and then in the circula-
tion of the MOW. The presence of IRD in sandy
contourite beds in the Gulf of Cadiz middle slope is
further constrained. Up to four IRD layers related to
Heinrich events H1 through H6 have been detected in
20 m long giant piston cores. This study indicates a
close connection between the North Atlantic and the
hydrologic conditions in the Gulf of Cadiz.
During the Late Pleistocene to Holocene, major
contouritic sedimentation took place during cool cli-
matic conditions, where more sediment reached the
slope. Using contourite grain size as a proxy for
MOW strength, the data show MOW intensification
during cold phases having also played a stronger role
during these cold intervals in deeper waters because an
increase in density. This led to a higher sand content
in contourite deposits showing a reflective seismic
facies. Our observations indicate indeed an enhance-
ment of the lower MOW branch during climatic cool-
ing and a stronger upper MOW branch circulation
during warm intervals. In this context, the spatial
occurrence of sandy contourites was controlled by
variations in bathymetric position and current strength
of the MOW.
This new information complemented the compre-
hension of the paleoceanographic patterns on the
basis of the high-resolution seismic work. Thus we
conclude that the combined study of sedimentary
archives and seismic records is a key to understand
the cyclic nature of sedimentary, the past MOW
circulation pattern, and climatic changes, providing
implications for a better predictability of the environ-
mental consequences of present and future climatic
changes.
Acknowledgments
This work was supported by the following pro-
jects CICYT: PB94-1090-C03-03 (FADO), MAR-98-
02-0209 (TASYO) and Ren2002-04117-C03-01/02/03
(GADES). In addition, our results are related to the
Special Actions: REN2002-11669-E (MVSEIS Project)
and REN2002-11668 (MOUNDFOURCE Project), as
well as the IGCP-432 Project dContourites, Bottom
Currents and Palaeocirculations.T J. Schonfeld acknow-