PALAE0
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
Late Quaternary sedimentation on the Portuguese continental
margin: climate-related processes and products
J.H. Baas a,,, j. M i e n e r t a, F. A b r a n t e s b, M.A. Prins c
a GEOMAR Research CenterJbr Marine Geosciences, Wischhofstr. 1-3, Building 4, 24148 Kiel, Germany
b Instituto Geol6gico e Mineiro, Departemento de Geologia Marinha, Rua Academia das Ci6ncias, 19 20,
1200 Lisbon, Portugal
c Comparative Sedimentology Division, Institute o f Earth Sciences, PO Box 80.021, 3058 TA Utrecht, The Netherlands
Received 27 June 1996; revision 28 October 1996; accepted 28 October 1996
Abstract
The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment
gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted
periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity
currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice sheets
(Heinrich events, lithofacies D) and large amounts of pteropods (lithofacies F). Settling of biogenic particulate
material (lithofacies E) prevailed during the Holocene, when sea level and sea surface temperatures were high and
terrigenous shelf-input was low.
Downslope transport was dominant on the northern part of the Portuguese margin, culminating in frequent
turbidity current transport between 35 and 70 ka. This may be due to a humid climate and a high fluvial input.
Pteropod muds are confined to cores south of 41°N. Prominent peaks in pteropod concentration were radiocarbon
dated at 17.8 and 24.6 ka. Layers rich in ice-rafted debris (IRD) were found along the entire margin. The base of
these layers have been dated at 13.6-15.9 14C ka, 21.0-22.0 14C ka, 33.8 14C ka and _+64.5 ka, which correspond
well with the ages of Heinrich events 1, 2, 4 and 6 in the central North Atlantic. Heinrich events 0 (10.5 ka), 3 (27
ka) and 5 (50 ka) rarely influenced sedimentation on the Portuguese slope. A mineralogical study of the IRD within
Heinrich layers suggests that most icebergs were derived from the Laurentide Ice Sheet in the Hudson Strait and
Hudson Bay area through the Labrador Current and the Canary Current and flowed in a southward direction along
the margin. I R D from European ice sheets may have been mixed in during Heinrich event 6. On their way along the
margin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The
southernmost latitude studied (37°N) may be close to the southeastern extension of iceberg transport during Heinrich
events. © Elsevier Science B.V. All rights reserved.
Keywords: Portugal; Continental margin; Palaeoclimate; Palaeoceanography; Heinrich event; Bottom current
* Corresponding author. Present address: Universit6 de Rouen, Laboratoire de Geologie, 76821 Mont-Saint-Aignan, France. Fax:
+ 23514-7022. E-mail: [email protected].
0031-0182/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved
PH S0031-0182 (96)00135-6
2
,I.H. Baasetal. /Pa[aeogeograp/1,1'. Pcihu'oclhnutolo%y, Palaeoecolo%y 130/1997) 1 23
1. Introduction
The late Quaternary sedimentary record on the
continental margin off Portugal documents a wide
range of biogenic and lithogenic sediment types
(Duplaix et al., 1965; Monteiro and Moita, 1971;
Kudrass, 1973, 1993: Siedler and Seibold, 1974;
Thiede, 1977; Monteiro et al., 1980; Gonthier
et al., 1984; Faugeres et al., 1985; Stow et al.,
1986; Vanney and Mougenot, 1990; Abrantes.
1991; Mienert, 1993: Lebreiro et al., 1996;
Sch6nfeld, 1996; Zahn et al., 1996). Until now, no
systematic attempt has been made to classify these
sediments into lithofacies and to interpret them in
terms of depositional process, source area and
relation to climate and sea level change. The
present-day hydrography and morphology provide
important information on the processes that have
been, and in part still are, responsible for sediment
dispersal on the slope. Sediment gravity flows
transport shelf sediments into bathyal and abyssal
depths, dominantly through E-W-trending submarine canyons (Vanney and Mougenot, 1990) and,
to a lesser extent, across intercanyon areas. The
warm and saline water masses of Mediterranean
Outflow Water ( M O W ) flow, after passing the
Strait of Gibraltar, along the southern and western
coast of Portugal in 600 1500m water depth,
forming mud ripples, and, on a larger scale, contourite drifts (e.g., Zenk, 1971; Faugbres et al.,
1984; Gonthier et al., 1984; Stow et al., 1986; Zahn
et al., 1987: Kudrass, 1993; Sarnthein et al., 1994).
Active winnowing takes place in the upper part of
MOW, where mean current velocities are highest
(about 12 cm/s) and light transmissivities are low
(Zenk and Armi, 1990). The resuspended fraction
is transported downslope to form fine-grained
deposits near the lower boundary of the MOW
( Kudrass, 1993: Sch6nfeld, 1996). Other potential
sediment transport agents are turbulent eddies
associated with MOW. These so-called "meddies'
are 40-50 km in diameter, and have azimutal
velocities of 20 30 cm/s and transversal velocities
of 4-5 cm/s (Prater and Sanford, 1992; Zenk et al.,
1992). Below ± 2 0 0 0 m , North Atlantic Deep
Water ( N A D W ) masses flow in a southward direction along the slope. Present-day current velocities
in the N A D W are much lower than in the MOW
(Meincke et al., 1975). The area is further characterised by coastal upwelling during the summer
months when strong northeastern trade winds
induce offshore displacement of surface waters
followed by upwelling of deeper, nutrient-rich
waters (Fifiza, 1983). Abrantes (1991) showed
evidence from diatom assemblages that upwelling
was increased during the last glaciation. It was
shown recently that terrigenous sediment was
introduced into the area by iceberg-transporting
ocean surface currents during the Late Pleistocene
(Mienert, 1993; Sch6nfeld et al., 1993: Lebreiro
et al., 1996; Baas and Mienert, 1996; Zahn et al.,
1996).
Here, high-resolution sedimentological and
paleoceanographical records from sediment cores
in a N S transect along the Portuguese margin are
presented. The main objective is the classification
of sediment types into a sedimentary facies scheme,
which makes up the framework for the reconstruction of responses of sedimentary processes to
changes in late Quaternary climate. Special emphasis is given to terrigenous sediment transport by
contour currents, sediment gravity flows, and icebergs into intercanyon areas during the last
70,000 yr.
2. Material and methods
2.1. Location
In 1993, the R.V. Poseidon cruise 200-10 investigated the Portuguese continental margin as part
of the research programme "European North
Atlantic Margin ( E N A M ) : Sediment Pathways,
Processes and Fluxes" (Mienert, 1993; Mienert
et al., 1993). A large number of gravity cores and
box cores was recovered along six latitudinal transects between 37.3 and 4 1 . 8 N and in water depths
between 245 and 2551 m. Nine gravity cores,
between 1.18 and 5.75 m in length, were selected
for the present study. Their locations, depths, and
recovered lengths are summarised in Fig. I. Except
for core PO200-10-24-2, which is located close to
the Aveiro submarine canyon, all cores were recovered well within intercanyon environments.
J.H. Baas et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
4-2
37"19.49'N
9"31 .IO'W
1276
O.
3
6-2
8-2
18-1
21-1
24-2
28-2
32-1
37"49.30"N
9"302(YW
1086m
37°38.46'N
9"55 91"W
2200m
3~'37,70'N
9"55.50~/
1967m
40°32.90'N
9"40,95~/
2381m
40~34.12'N
9"28,9TW
150Om
41 "29.3ffN
9"43 26~/
2155m
41 "38.05'N
9"28.94~V
1844m
b,gl
Z
Z, Ch
Z,Ch
Ch
g
pt
100
b, tam
g¢,pt
Ch,Z
b
b
Ch
Ch
b
Ch
Ch
b
Ch
:-_:-_:_-_-_l
:
Ch
tam
:
pt
gl
pt, Ch
tam
tam
l
tam
tam
b, FU
FU
•
.
:
,-;o
l
;
;._, .'.
,.=".%- •
•
°
l
:
l
:
l
l
!
!
:
=
:
:
FU, curt
FU, curt, gl
llUl(~aciesA 1
41111-
.,= .'.
.. °.
- k
IT[]
I~ofacms A2
C] Ih'hofacl~sA3
~
Kthofaciel B1
=t,Ch
~(~actas B2
li~ofacies C
lithofacies O
lithofacies E
ii
:
i
c.
l
i
~ ' ~
lithQfacmsF
b
Ch
tam
FU
gl
1:4
Z
biotud~lted
Chono~te~
traces
laminated
fining upward aequenc~
glauconke
plefopods
Zoophyc~traces
Ch
12 " W
1 0WV
8"W
Fig. 1. Lithofacies distribution in cores f r o m the Portuguese margin. N u m b e r s above logs denote core name, latitude, longitude and
water depth. All coare names have prefix PO200-10.
4
J.H. Baas et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
2.2. Analytical methods
Most cores were run through a Multi Sensor
Core Logger (MSCL), measuring bulk volume
magnetic susceptibility, G R A P E density and compressional (P)-wave velocity at 2 cm intervals using
the methods described by Chi (1995). After macroscopic description, X-ray photographs were made
from sediment slabs, using an anode voltage of
35 kV and an exposure time of 11 15 s. Detailed
core descriptions revealed pelagic and hemipelagic
sediments with variable admixture of silty material
(mainly obvious from colour changes and grey
level variations on X-ray photographs) and distinct
sand- and pebble-bearing horizons.
The cores were sampled at 10cm intervals.
Additional sampling was done across coarsegrained layers at a spacing of 1 5 cm. The samples
were washed on 0.063 mm mesh. The coarse fraction was further split in the following subfractions:
0.063-0.125, 0.125-0.250, 0.250 0.500, 0.500
1.000 and > 1.000 mm, using an ATM Sonic Sifter.
The weight of each subfraction was converted to
the percentage of dry bulk sample weight. In some
cores, silt and clay concentrations were determined
using the standard pipette method. Detailed grain
size analysis with a Malvern Particle Sizer was
done on selected <0.063 mm fractions from cores
PO200-10-28-2 and PO200-10-6-2. Measurements
comprise bulk samples and samples decarbonated
with 3% hydrochloric acid in order to distinguish
between biogenic and lithogenic depositional
mechanisms.
The number of terrigenous grains per gram bulk
dry sediment weight was determined from
0.250 0.500 mm splits. Mineralogical analysis was
done on detrital grains > 0 . 2 5 0 m m from layers
particularly rich in terrigenous material. Generally,
more than 200 grains per sample were analysed
under a microscope. The percentages of quartz,
feldspar (K-feldspar and plagioclase) and detrital
carbonate (dolomite and limestone) were determined. Two additional classes were distinguished:
volcanogenic minerals and a 'rest' class with heavy
minerals, rock fragments and assessoria. Where
present, the number of red quartz and red hematite-stained quartz was counted separately.
Census counts of the planktonic foraminifer
Neogloboquadrina pachyderma were used for examining paleoclimatic conditions. The amount of
pteropods shells and pteropod fragments was
expressed as the weight percentage of pteropods
in the 0.250 0.500 mm subfraction per gram bulk
dry sediment. On the northern Portuguese margin
the number of pteropods is negligible. A LECO
CS-125 infrared analyser was used to determine
total organic carbon ( T O C ) and calcium carbonate content, following standard methodology (cf
Baumann et al., 1993).
Oxygen and carbon isotope ratios were measured on _+20 specimens of the planktonic foraminifer Globigerina bulloides and up to 10
specimens of the benthic foraminifer Cibicidoides
wuellerstorfi at the Leibnitz Laboratory of the
University of Kiel (Dr. H. Erlenkeuser). Given
values are calibrated to the PDB scale. The total
reproducibility amounts to 0.09%o for 61sO and
0.04%,, for 813C. Accelerator mass spectrometer
(AMS) radiocarbon age determinations were done
at the University of Arhus on Globigerina bulloides
(Dr. J. Heinemeier). A reservoir correction of 400
years was applied to all ~4C ages (Bard et al., 1989 ).
3. Lithofacies
The sediments found in the cores from the
Portuguese margin dominantly consist of hemipelagic muds with variable admixture of terrigenous
silt, sand and pebbles, and biogenic components,
mainly foraminifera and pteropods. The degree of
bioturbation is moderate to high, according to the
classification scheme of Reineck (1967). Based on
the analytical data summarised in Figs. 2 and 3,
the following lithofacies were defined (Fig. 4):
3.1. LithoJacies A__[braminfferal terrigenous mud
3.1.1. Description
Lithofacies A consists of greyish olive, foraminiferal terrigenous mud with low, but variable bulk
sand content. This hemipelagic mud may be homogeneous or contain faint current-induced sedimentary structures. Occasionally, Chondrites and
Zoophycos trace fossils were observed. Chondrites
tends to concentrate directly below sandy terrige-
Z H. Baas et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
PO200-10-4-2
PO200-10-6-2
PO200-10-8-2
5
P0200-10-15-1
magnetic susceptibility (~cgs)
10
20
30
I
i
I
0
0
10
20
30
30
40
50
60
0
,
0
100
100
5O
200
2O0
IO0
300
150
4oo
2OO
50O
25O
~'
400
H1
51111
~'H
600
.
,
-
20
,
.
40
.
600
•
.
0
60
.
,
20
.
,
40
-
60
0
,
.
20
.
.
6O
.
40
10
20
30
0
5
300
0
30
60
90
120
number of detdtal grains per gram bulk sediment
PO200-10-21-1
0
10
i
0
50
100 ,
A
H~,~
20
i
PO200-10-24-2
PO200-10-28-2
magnetic susceptibility (•cgs)
30
p
H1a
H1b~
0
0
10
'
20
"
~o
/~
30
J
150
~
100
,
90
i
120
20
|
40
i
60
f
~ H 1 ?
1
200"~HH~
H4a
/
80
i
60
0
0
100
"0
,
30
30
i
50
loo
~>
300
20
*
,o
60
25O
10
=
~o
E
£
I:.
0
0
PO200-10-32-1
120
0
30
150
'
~
~
2OO
~
2~,
)
~
Hla
25O
3o0 )H6 ~ . ~
>
J
Hlb
,
,
,
60
90
120
350
H6
,
0
30
60
90
120
30
60
90
120
number of detrital grains per gram bulk sediment
Fig. 2. Magnetic susceptibility and amount of detrital grains in cores from the Portuguese margin. H = H e i n r i c h layer.
6
J.H. Baas et al. ," Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
%
PO200-10-4 -2
0
0.1
'
0
100 .
oE 200
v
J= 300
400
PO200-10-6 -2
0,2
0
' 0t
100
H1
b
pteropods
0.25
~
PO200-10-21-1
0.5
0
' 0
~
50
H0
100
300 ~
150
300
400 } H2
200
l
600
600 ~
300
% TOC
PO200 -10 -28 -2
0
20 30
i
i
i
5(3
100
2
i;
50
100
50
H0
Hla
100 1
""---~
Hlb
.
PO200-10-6-2
40 50
i
0
8~'C (Oloo) C . w u e l l e r s t o r f i
PO200-10 -28 -2
10
50
6013
% CaCO3
02 04 06 08
C
i
i
i
I)O200-10-21-1
25
o
A 200
250
~
0
......... dex
sin
PO200-10-6 -2
0.2
100
500 ~
2OO
0.1
J
200
500
10o
% N. pachyderma
H1
H2
130200-10-8-2
-05 05
15 25
0
0 . . . . . /. .
0
/
100
/
50
t
H1
20O
? lOO
05
PO200.10-21-1
1
-05
50,
0
05
1
HO
Hla
100- H l b ~
H:a 150
I
150
200
250
4OO
250
350
H2
2OO
5OO
250
60O
300
H1HIb~
200.250.150.
:300.
~"O (Oloo) GI. bulloides
PO200-10-6-2
3
2
1
PO200-104-2
0
0
3
2
.....
1
PO200-10-21-1
0
-~'0
3.5
2.5 15
. . . . . .
3.5
0 i
100
200
H1
151)
400
500
600
3
2
1
. . . . . .
50
100
300
1=O200-10-32.2
PO200-10-28-2
0.5
/0
300
1
2.5
i
15
,
H I ~
i
0,5
0
50
IO0
150
200
2oo
250
250
300
30O
350
35O
~-
Fig. 3. Analytical data from selected cores. (a) % pteropods. (b) % N. paehyderma. (c) % TOC and % CaCO 3. (d) 8~:~C(%~,)measured
on C. wuellerstolfi. (e) ~5~aO(%,) measured on Gl. bu/loides. H = Heinrich layer: TOC= total organic carbon.
JmH. Baas et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
lithofacies
% sand
further characteristics
°ram'n"e a'te r' e ° 'sm"d
increased
clastic input
[: A3-fdrb~mih#eralS'and}/te[i'iggnc)}]s.rYitidl"".: ".: '".: "".t
~B 1-~J°ra~--~m
i~nifer~-ra
sr~iltyr~te
I ri~g
r er~nu~s
° m~i--~
depositional process
"background"
sedimentation
"background"
sedimentation
's 'u8 IIIIIIII II
I
7
NNi~
]
%CaCO3 low, %TOC high,
magnelicsusceptibilityhigh,
GRAPE/p-wave velocity high,
sharplowerboundarysurface
silty bottom flow
Boumasequences
turbidity current
detrital carbonate,6 '~C/ 8~60low,
magneticsusceptibility/GRAPE/pwave velocity high, Nq.
pachyderma(sin) high, Chondrites
Heinrich event
sandy bottom flow
highdiversityofforaminifera,
%CaCO3 high, Nq. pachydetma
(sin)low
...................
~
. . . . .
-'~_ ~t
highconcentrationsof pteropodshells
Fig. 4. Summary of lithofacies and depositional processes.
nous mud beds with scattered terrigenous clasts
(lithofacies D, see below). Lithofacies A bears
pteropod shells adjacent to pteropod muds (lithofacies F) (Figs. 1 and 3a). Three subfacies were
distinguished:
3.1.1.1. Lithqfacies A 1--Joraminiferal terrigenous
mud.
Lithofacies A1 is characterised by bulk
sand fractions smaller than 4%. Sand-sized terrigenous clasts are rare to absent. Rare sedimentary
structures include faint horizontal lamination and
fine-coarse intercalations. Foraminiferal terrigenous mud is the dominant lithofacies in cores from
the southern and part of the central Portuguese
margin (Fig. 1).
3.1.1.2. Lithofacies A2 siltyJbramin(fbral terrigenous mud.
Lithofacies A2 is the silty equivalent
of lithofacies A1. The percentage of grains
>0.063 mm is between 4 and 10%. Terrigenous
sand content is low. Lithofacies A2 mainly occurs
in cores from the northern area and part of the
central Portuguese margin (Fig. 1 ).
3.1.1.3. Lithofacies A3--sandy foraminiferal terrigenous mud.
Lithofacies A3 is the sandy equivalent of lithofacies A1. In core PO200-10-32-1
(Fig. 1) it contains abundant bioturbation, small,
but significant amounts of detrital grains in the
0.250-0.500 mm subfraction, and alternations of
fine- and coarse-grained intervals on a centimetre
scale. The bulk sand content is larger than 10%.
3.1.2. Interpretation
Lithofacies A represents the glacial background
sedimentation in the study area. Current influence
and coarse terrigenous input increase from south
to north along the margin. The terrigenous fraction
probably originates from eolian transport and shelf
escape of geostrophic flows. At least part of the
terrigenous sand may have been mixed in from
adjacent lithofacies D beds by bioturbation.
Lithofacies A3 represents periods of relatively
high current activity and/or detrital sediment
input. The centimetre-scale grain size fluctuations
are interpreted as relatively short-term changes in
current strength, resulting in varying degrees of
8
J. tt. Baas et al. /Palaeogeogral)hy, PahteoclhTtatology, Palaeoecology 130 (1997) 1 23
current winnowing and/or terrigenous sediment
input. Lithofacies A3 corresponds to the upper
slope contourite facies of Sch6nfeld et al. (1993),
which was interpreted as the product of current
winnowing by MOW. Further evidence for this
origin is given by the higher abundance of this
lithotype in adjacent Poseidon cores from 900 m
water depth (not shown here), i.e., close to the
present-day core of MOW.
3.2. Litho['acies B
beds
jbraminl"Ji, ral terri,genous mud
3.2.1. Description
Lithofacies B comprises beds of greyish olive to
dark olive foraminiferal, terrigenous mud. The
internal organisation of the mud beds suggests
that they were formed during distinct events. This
distinguishes them from lithofacies A. Characteristic features of lithofacies B beds are a
homogeneous distribution of coarse grains, sharp
and erosive to gradual lower boundary surfaces,
internal erosional surfaces with an undulating
character, current laminations (mainly cross lamination and horizontal lamination) and fining
upward sequences. The beds are also characterised
by increased G R A P E density and P-wave velocity,
and some by increased magnetic susceptibility.
3.2.1.1. Lithofacies B1 Joramin(/eral silo' terrigenous mud beds.
Bulk sand content in lithofacies
BI is between 4 and 10%. It further contains up
to 6 detrital grains per gram bulk sediment in the
medium sand fraction and relatively large percentages of total organic carbon (Figs. 2 and 3c).
Lithofacies BI was found in the lower half of core
PO200-10-28-2 from the northern and in core
PO200-10-15-1 from the central part of the
Portuguese margin (Fig. 1) and is associated with
the foraminiferal sandy terrigenous mud beds ( lithofacies B2), described below.
3.2.1.2. Litho['acies B2@'oramin([eral sandy terrigenous mud beds.
Lithofacies B2 is the sandy
equivalent of lithofacies B1. It consists of fining
upward sequences from sandy mud (> 10% grains
>0.063 ram) to mottled, partly silty, hemipelagic
mud. Terrigenous material is largely confined to
the subtraction <0.250mm. Up to 12 detrital
grains per gram bulk sediment were counted in
the medium sand fraction (Fig. 2). Foraminiferal
sandy terrigenous mud beds are confined to core
PO200-10-15-1 and the basal parts of the cores
from the northern Portuguese margin (Fig. 1 ). At
least one lithofacies B2 bed (252-280 cm in core
PO200-10-28-2) contains numerous glauconite
grains in the 0.250 0.500 mm subfraction. In the
same core, CaCO3 content is relatively low and
total organic carbon content is high (Fig. 3c).
The mineralogical composition of a lithofacies
B2 bed from core 28-2 is given in Fig. 5. The sand
fraction >0.250 mm contains about 50% quartz,
20% of volcanogenic grains, and only small
amounts of feldspar, detrital carbonate and
red(-stained) quartz. The rest group with rock
fragments and heavy minerals is relatively large.
3.2.2. Interpretation
Lithofacies B is inferred to represent bottom
flow deposits, based on the homogeneous distribution of detrital grains, the presence of current
lamination, and the overall silty to sandy nature
of the layers. In this paper, bottom flows are
defined as all near bottom currents which are able
to reshape the sediment surface, including contour
currents and turbidity currents. A further distinction of bottom flow type will be made below.
Current influence was largest in lithofacies B2.
Based on differences in sand content, lithofacies
B1 and B2 are classified as silty and sandy bottom
flow deposits, respectively. Glauconite grains were
probably derived from the adjacent shelf, where
this type of grain is numerous (Alveirinho Dias
and Nittrouer, 1984).
3.3. Litho/itcies C
turbidite deposits
3.3.1. Description
A turbidite deposit was found at the base of
core PO200-10-28-2 (Fig. 1). It has a grey colour
and shows a typical fining upward Bouma-type
sequence from homogeneous sandy mud through
lenticular bedded and flaser bedded sandy/silty
mud to unstructured mud. The bulk sand content
at the base of the bed is high (about 19%). The
maximum number of detrital grains in the medium
J.H. Baas et aL/ Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
Depth in core
P0200-10
Lithofacies
n
Detrital
carbonate
Totalquarlz
I
2H;~om
~(H01
~
6-2;178cm
DC.~I
32~
I
I
15-1;270cm
DIH1]
214
21-1;64cm
D[H1]
39~
24-2;11Ocm
D[HI]
33~
28-2; 88cm
D{HI)
207
6-2; 43~m
D {H2}
192
28-2; 128cm
D(H2}
209
28-2; 165cm
D {H4}
341
32-1; 133cm
D{H4]
~37
I
I
I
[]
I
I
•
I
I
28-2;295,298cm
D(H6]
228 ~
I
I
I
I
I
I
/
I
I
28-2; 273,270cm
B2
226
I
28-2;328,330cm
C
236 ~
I
0
10
20
30
40
50
60
70
0
Volcanogenic
clasts
Feldspar
I
•
•
I
I
I
I
•
•
I
I
•
I
•
•
I
I
•
I
I
I,
20
30 0
Red(-stained)
quartz
Rest
•
I
10
9
10
I
30
0
10
I
I
I
~I,
20
I
,
20
30
n,
0
10
20
30
0
10
20
30%
Fig. 5. Mineralogy of selected samples from Heinrich layers, bottom flow deposits and turbidites, n = n u m b e r of grains; H =
Heinrich layer.
sand fraction is about 10. The presence of large
amounts of well-rounded glauconite grains contribute to the sandy character of lithofacies C.
The mineralogical composition of the sand fraction >0.250 mm is similar to that of the sandy
bottom flow deposit (lithofacies B2) described
above. About 50% is quartz. Red(-stained) quartz
is rare. The bed contains relatively large numbers
of volcanogenic clasts, rock fragments, and heavy
minerals (_+20%), and low numbers of detrital
carbonate and feldspar (Fig. 5).
3.3.2. Interpretation
Lithofacies C was formed by deposition from a
waning turbidity current, thus generating a classical Bouma sequence. The presence of well-rounded
glauconite grains probably indicates a shelf origin
of the turbidity current.
3.4. Lithofacies D--sandy terrigenous mud beds
with scattered terrigenous clasts
3.4.1. Description
Lithofacies D is well defined on X-ray photographs (Fig. 6). It consists of distinct sandy terrige-
nous mud beds that contain scattered terrigenous
clasts ranging in size from fine sand to pebbles.
Current lamination is absent. Further characteristics are high percentages of grains >0.063 mm,
large numbers of detrital grains in the medium
sand fraction (up to 130 grains per gram bulk
sediment), and sharp lower boundary surfaces
(Fig. 6). Lithofacies D beds have increased numbers of left-coiling Nq. pachyderma (Fig. 3b), low
concentrations of foraminifera, depleted 6180
values, and negative 613C anomalies (Fig. 3d-e).
Most beds are accompanied by increased magnetic
susceptibility (Fig. 2), GRAPE density and P-wave
velocity. Chondrites traces are often present below
lithofacies D.
Lithofacies D was found in all cores from the
Portuguese margin. Several beds are composed of
two peaks in detrital grain content separated by
an interval of decreased detrital grain content, e.g.,
in core PO200-10-24-2 (79 118 cm depth; Fig. 2)
and core PO200-10-28-2 (158-180cm depth;
Fig. 2).
The mineralogical composition of lithofacies D
beds is remarkably constant along the margin.
Some examples are shown in Fig. 5. Most beds
.L tl. Baas et aL / Pulaeogeography, Pahwoclimatology. Palaeoecolog.v 130 (1997) 1 23
11)
P0200-10-21-1
28.5 cm
53.5 c m
.....i " '
',..~;~G
.-.:::ii. "
"
"
O.
.::.:.:
"
-7-
.'
72 c m
53.5 cm
Fig. 6. X-ray photographs of core PO200-10-21-1, showing
Heinrich layers 1 (HI) and 0 (HO). The numbers above and
below the drawings indicate depth in core. Note that the right
drawing fits at the base of the left drawing. Stippled areas indicate increased a m o u n t s of sandy detrital material in mud.
Pebbles are shown in black. White, circular spots are trace fossils. Subhorizontal and inclined lines denote changes in colour,
structure, and/or texture. Note the sharp base of Heinrich layer
1 at 70 cm.
contains around 60% quartz, up to _+ 10% volcanogenic grains, feldspar and grains in the rest group
(mainly rock fragments and heavy minerals), and,
most typically, between 15.9 and 24.5% detrital
carbonate. Exceptions are the lithofacies D beds
at 280-308 cm in core PO200-10-28-2, and at the
base of core PO200-10-32-1, which contain 11.8
and 5.5% detrital carbonate, respectively. The surfaces of large detrital carbonate grains show linear
pits and scratches. Red(-stained) quartz is rare in
this lithofacies.
3.4.2. hTterpretation
The detritus in the detrital carbonate-bearing
beds of lithofacies D is interpreted as ice-rafted
debris (IRD), based on the scattered distribution
of clasts on X-ray photographs, the poor sorting
of the sediment with grain sizes ranging from silt
to gravel, and the linear scratches on detrital
carbonate grains. It is proposed that these layers
were formed during Heinrich events (Heinrich,
1988; Bond et al., 1992, 1993; Broecker et al.,
1992), distinct periods of instability of the
Laurentide Ice Sheet leading to massive release of
debris-carrying icebergs into the North Atlantic
during the Late Pleistocene. It was showil that the
high fluxes of IRD during Heinrich events are
associated with ( 1 ) low concentrations of foraminifera,
(2)
dominance
of
the
left-coiling
Neogloboquadrina pachyderma species in the
remaining foraminiferal fauna (Heinrich, 1988:
Broecker et al., 1992), indicating southward penetration of arctic water, (3) depletion in planktonic
8180 (e.g., Heinrich, 1988; Bond et al., 1992, 1993;
Broecker et al., 1992: Andrews et al., 1993 ), denoting salinity drops due to extensive meltwater discharge, (4) decreased benthic ~3C ratios (e.g.,
Keigwin and Lehman, 1994; Sarnthein et al., 1994:
Maslin et al., 1995; Zahn et al., 1996), evidencing
a reduction or even a termination in deep water
convection, (5) high concentrations of detrital
limestone and dolomite (Andrews and Tedesco,
1992: Bond et al., 1992; Bond and Lotti, 1995),
(6) a clay mineralogy which differs from ambient
sediment (illite and chlorite instead of smectite)
(Jantschik and Huon, 1992), (7) low porosity/high
bulk density (Bond et al., 1992), (8) sharp bases,
indicating rapid deposition of IRD (Bond et al.,
1992), (9) increased magnetic susceptibility in
the open ocean (Grousset et al., 1993; Mienert
and Chi, 1995; Lebreiro et al., 1996), and (10)
increased grey level reflectance (Grousset et al.,
1993). These criteria apply well to lithofacies D
beds. They contain increased concentrations of
Nq. pachyderma (sin), are depleted in 61sO and
813C, contain about 20% detrital dolomite in
the >0.250 mm fi'action, and most beds have
increased
GRAPE
density
and
magnetic
susceptibility.
The presence of twinned peaks in IRD content
in the Heinrich layers from the Portuguese margin
may support ice sheet model calculations by Alley
and MacAyeal (1994), as discussed in a separate
paper (Baas and Mienert, 1996).
Chondriles traces are made by endichnial deposit
feeders. The burrowing depth of Chondrites is
proportional to the degree of bottom water oxy-
J.H. Baas et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
genation (Bromley and Ekdale, 1984). The deepest
traces occur in well-oxygenated sediment and are
usually tiered with other oxygen-related species
like Zoophycos, Thalassinoides and Planolites. In
anaerobic environments Chondrites migrates to the
sediment surface and is often the single species
present. The concentrations of Chondrites traces
immediately below Heinrich layers may therefore
be a proxy for bottom water stagnation during
Heinrich events, as proposed by Sarnthein et al.
(1994) and also indicated by benthic foraminiferal
faunal changes (Baas et al., 1996).
3.5. Lithofacies E foram ooze/mud
3.5.1. Description
Lithofacies E consists of greyish olive to dark
olive foram oozes and muds with high carbonate
contents (up to 50%), high numbers of foraminifera, and very low concentrations of coarse terrigenous material. Lithofacies E is clearly different
from other lithofacies by the abundance of wellpreserved, high-diversity foraminiferal assemblages. The larger species are visible as distinct
spheres on X-ray photographs. Weight percentages
of bulk grains >0.063 mm are intermediate to
high with an average of 6% and peak values of
20%. With few exceptions, the number of leftcoiling, arctic Nq. pachyderma species and pteropods are low.
Foram oozes/muds were found along the entire
Portuguese margin. They are confined to the top
sections of sediment cores (Fig. 1). The thickest
intervals are present in cores from the southern
margin. In these intervals, the carbonate content
and the percentages of grains > 0.063 mm suddenly
increase at the base of the intervals, and thereafter
show a gradual decrease towards the top. Similar
trends are present for the percentages of grains
>0.125 mm, but not for the percentages of grains
> 0.250 mm, denoting the upper size range of most
foraminiferal tests. Lithofacies E contains abundant Zoophycos-type trace fossils in core
PO200-10-15-1 from the central part of the
margin. 6180 values are low with an upward
decreasing trend at the base of the lithofacies
(Fig. 3).
11
3.5.2. Interpretation
The presence of large numbers of foraminifera,
their high diversity, high carbonate contents, low
arctic Nq. pachyderma (sin) percentages, low and
upward decreasing oxygen isotope values, and low
detrital sand content suggest an interglacial origin
for lithofacies E. AMS 14C dating gives a Holocene
age to lithofacies E.
3.6. Lithofacies F--pteropod mud
3.6. l. Description
Lithofacies F consists of greyish olive muds rich
in pteropod shells and shell fragments. In other
lithofacies pteropods are present only in small
quantities, i.e., less than 0.015% per gram bulk
sediment in the 0.250-0.500 mm subfraction. The
pteropod muds contain at least one order of magnitude more specimens and pteropods comprise up
to 84% of the medium sand fraction. On X-ray
photographs, large numbers of pteropods occur in
distinct layers, which are often less than a few
millimetres thick. Lithofacies F is further characterised by increased bulk sand content and reduced
magnetic susceptibility.
Lithofacies F is confined to distinct depths well
below lithofacies E in cores from the southern and
central Portuguese margin (Fig. 1 ). Two pteropod
mud intervals were found at 268-310cm and
502-520 cm depth in core PO200-10-6-2 (Fig. 3a).
The upper interval contains numerous simple,
straight species (Creseis type). The pteropod assemblage in the lower interval is dominated by winded
species (Limacina type).
3.6.2. Interpretation
Pteropod shells consist of easily dissolvable aragonite. The large numbers of pteropods in lithofacies F may indicate a decrease in the rate of
dissolution due to a deepening of the aragonite
compensation depth and/or increased productivity
of pteropod shells (cf. Melkert et al., 1992).
4. Stratigraphy and core correlation
The age control and stratigraphic correlation
between cores is primarily based on AMS 14C
12
J. tl. Bars et al. /Pa&eogeogmphy, Palaeoclimatology, Pahwoecology 130 (1997) 1 23
dating and oxygen isotope records (Table 1).
Oxygen isotope event 3.1 (25.42 ka; Martinson
et al., 1987) was found near the base of core
PO200-10-6-2 (cf. Zahn et al., 1996). A characteristic negative 81sO anomaly, dated at 17.46 ka in
core PO200-10-32-2, was used as stratigraphic fix
point for the northern and central part of the
margin. The Younger Dryas ( 10.5 ka) is characterised by a distinct oxygen isotope anomaly. Oxygen
isotope event 1.1 (8.8 ka; Bard et al., 1989), the
Holocene climatic optimum, was positioned at
minimum oxygen isotope ratio at the end of deglaciation. Additional chronostratigraphic control for
core PO200-10 8-2 was obtained by correlating the
planktonic 81sO record with a continuously AMS
14C-dated oxygen isotope curve from nearby core
SU81-18 ( 3 7 4 6 ' N , 10 I I ' W ; Bard et al., 1989). In
the cores where isotope ratios and radiocarbon
dating are lacking, physical properties measurements and Bond et al. ( 1993)'s Heinrich layer ages
were used to establish a preliminary age model.
The age control points were used to convert the
data from the depth domain to the time domain
for the last 40 ka (Fig. 7). Linear interpolation
between age control points in cores where age
control is based on isotope ratios and radiocarbon
dating, gives the following age ranges for the base
of Heinrich layers (lithofacies D; Table 1):
13.6 15.9, 21.0 22.0 and 33.8 ka. These ages correspond well with established ages for Heinrich
layers 1 (14.6 ka), 2 (21.4 ka), 4 (35.5 ka) in the
central North Atlantic (Bond et al., 1992, 1993)
and Heinrich layer 4 (33.5 ka) on the southern
Portuguese margin (Zahn et al., 1996). The anomalous age estimates of 11.2 ka for Heinrich layer
1 and 30.4 ka for Heinrich layer 4 in core
PO200-10-32-1 are probably caused by the poor
quality of the oxygen isotope record and the lack
of age control points. Moreover, oxygen isotope
ratios were measured on samples from parallel
core PO200-10-32-2. This may have led to further
age offsets.
The Younger Dryas normally shows up as
a change in grey-level on X-ray photographs,
indicating increased silt content.
In core
PO200-10-21-1 a distinct lithofacies D bed with
scattered I R D of characteristic mineralogy and
low 613C values (Figs. 2, 3 and 5) was found at
_+ 10 ka. This age may correspond to Heinrich
event 0 further northeast (Bond et al., 1993;
Andrews et al., 1995).
The upper pteropod-rich interval ( lithofacies F )
in core PO200-10-6-2 (Fig. l) has an interpolated
radiocarbon age of 16,6 18.0 ka. The peak pteropod concentration was dated at 17.8 ka. This
interval is inferred to be time-equivalent with
lithofacies F horizons in PO200-10-4-2 and
PO200-10-21-1 (Fig. 1). Similar pteropod-rich
layers were found at 18 ka in cores off Northwest
Africa between 15 and 2 7 N (Sarnthein et al.,
1982). This period of enhanced deposition of
pteropod shells may thus have been synchronous
over the East Atlantic margin between 15 and
40.5 N. The lower lithofacies F interval in core
PO200-10-6-2 was formed at 24.2-24.8 ka with
peak values at 24.6 ka (Fig. 7a).
Below Heinrich layer 4, core PO200-10-28-2
contains several lithofacies B and C beds. The ages
of these bottom flow and turbidite deposits were
approximated by correlating the oxygen isotope
and magnetic susceptibility records with core
D11957P from Tore Seamount (39 03'N, 12 36'W:
Fig. 7b; Lebreiro et al., 1996). Linear interpolation
of sedimentation rates suggests that silty bottom
flow deposits were formed at 38.0, 41.6, 48.4 and
52.8 ka. A sandy bottom flow deposit was generated at 58.6 ka. Increased amounts of volcanic
fragments were deposited at 275 cm (Fig. 5). The
interpolated age of 57.6 ka correlates with Ash
Zone II, which has been dated at 57.5 ka in the
central North Atlantic (Ruddiman and McIntyre,
1984). The age at the base of the lithofacies D bed
at 280 305 cm is 64.5 ka, which corresponds well
with previous age estimates for Heinrich event 6
(e.g., Bond et al., 1993; Mienert and Chi, 1995;
Lebreiro et al., 1996). The turbidite at the base of
core PO200-10-28-2 was deposited about 70 ka
ago. The absence of Heinrich event 5 in the detrital
sand record supports the findings of Lebreiro et al.
(1996). Heinrich layer 3 is well established by
increased I R D content in the Tore Seamount core.
In contrast, only few detrital sand grains were
found in core PO2001-0-28-2 from the northern
Portuguese margin (Fig. 2), Yet, a silty/sandy bed
with underlying C77ondrites traces is present at
128 142 cm in core PO200-10-28-2. Its lower con-
J.H. Baas et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
13
Table 1
S u m m a r y of chronostratigraphic age control points and inter- and extrapolated ages of Heinrich layers
Chronostratigraphic control points ( A M S
14C)
Core
PO200-10
Depth
(cm)
14C-bull.
(res.0.4 ka)
Error ~4C
(ka)
6-2
6-2
6-2
6-2
21-1
21-1
21-1
28-2
28-2
28-2
28-2
28-2
28-2
32-2
32-2
(32-2
3
75
155
201.5
44
54
70
0
20
50
160
170
182
1.5
60
170
0.86
4.52
13.27
14.5
9.33
10.06
13.61
1.82
6.67
10.55
32.5
33.5
33.85
7.88
17.46
>41 )
0.075
0.17
0.15
0.33
0.075
0.16
0.09
0.15
0.085
0.1
0.41
0.46
0.52
0.13
0.24
Chronostratigraphic control points (oxygen isotope ratios)
Core
PO200-10
Core depth
(cm)
Age
(ka)
Oxygen
isotope event
Reference
6-2
6-2
6-2
8-2
8-2
8-2
8-2
8-2
8-2
8-2
21-1
21-1
28-2
28-2
100
140
540
110
140
160
190
220
230
280
29
129
28
118
8.8
10.5
25.42
8.8
10.7
12.4
13.4
14
14.2
15.9
8.8
17.46
8.8
17.46
1.1
Y. Dryas
3.1
1.1
Bard et al. (1989)
Bard et al. (1989)
Martinson et al. ( 1987)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
Bard et al. (1989)
dated anomaly, PO200-10-32-2
Bard et al. (1989)
dated anomaly, PO200-10-32-2
1.1
1.1
Inter- and extrapolated ages of Heinrich layers
Core
PO200-10
Base H1
depth
Base H1
age
Base H2
depth
Base H2
age
6-2
8-2
190
189
(258) a
70
(105P
94
22
14.2
14.5
(15.2) a
13.6
(15.9) a
15.0
'11.2 'b
435
22.0
128
21.0
21-1
28-2
32-1/2
aSecondary peak in I R D content.
bRough estimates, see text for discussion.
Base H4
depth
Base H4
age
180
139
33.8
'30.4 'b
14
J.H. Baas et ~tl. ' Palaeogeogr¢q~l )', Palc;eoelimato/ogy, Palaeoeeology 130 (1997) 1 23
a
PO200-10-6-2
PO200-10-8-2
...... % p t e r o p o d s
--
2
0
3
I
p
1
I
t
I
PO200-10-21-1
PO200-10-32-112
n u m b e r of detrital grains per gram bulk s e d i m e n t x 0.01
3
i
0
I
2
i
1
I
i
0
I
3
I-- i
2
I
t
1
I
a " O ( ° l o o ) GI. bulloides
0
3
0
[
2
t
+
1
i
0
I
p
~0
<
<
5
-
Pt
S
./~
20
5
f 5
10
15
HI?
10
15
15
o
"o
2O
2O
2O
H2
25
±25
25
25
3O
30
30
30
PO200-10-28-2
--
n u m b e r of detrital grains per
g r a m bulk s e d i m e n t x 0.01
D11957P:
Tore Seamount
8"O(°1oo) GI. bulloides
3
2
1
0
_~:
3
F---~ ~
2
I
10
m a g n e t i c susceptibility (~cgs)
o
i
10
H2~
20
20
H4
~
40
60
q
~
40
£
8
50
Ash II
lO
2O
i
i
3O
i--~
10
H1
H2
2O i
H4
&
40
0
0 ~ -
~H2
3?
H3
H4
30
i
20
H2
10
2O
I~111
10
H1
,,~
D11957P:
Tore Seamount
PO200-10-28-2
H4
4O
5O
60
60
6O
70
70
70
7O
80
~80
80
80
Fig. 7. (a) 8JSO ratio, IRD content, and pteropod content against radiocarbon age for cores PO200-10-6-2. PO200-1(l-8-2,
PO200-10-21-1 and PO200-l(l-32-1/2, based on the age control points given ill Table 1. Heinrich layers (HO H4) and pteropod-rich
muds (Pt) are shown. (b) 81sO ratio and IRD content against radiocarbon age for core PO200-10-28-2 and correlation of magnetic
susceptibility and oxygen isotope ratio with core D11957P from the Tore Seamount. Heinrich layers HO to 116 are indicted.
J. ILL Baas et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
tact has an interpolated age o f 26.1 ka, which is
within 1 ka o f the established age o f Heinrich
event 3 (27 ka; Bond et al., 1993). Further study
is required to verify if this bed is associated with
Heinrich event 3.
Fig. 8 shows that mean sedimentation rates for
the studied part o f the late Q u a t e r n a r y are generally higher on the southern margin than on the
northern margin, that is o f the order o f 20 c m / k a
in the south and less than 10 c m / k a in the north.
The sedimentation rates increase sharply a r o u n d
Heinrich layers. For example, sedimentation rates
rose to 34 c m / k a during deposition o f the lower
part o f Heinrich layer 4 in core PO200-10-28-2,
15
which is m o r e than six times the average sedimentation rate.
5. D e p o s i t i o n a l p r o c e s s e s
5.1. BottomJlows
Laser Particle Sizer analysis was done on
selected samples ( f r a c t i o n < 0 . 0 6 3 ram) from the
northern core PO200-10-28-2 and the southern
core PO200-10-6-2 to examine whether the b o t t o m
flow deposits (lithofacies B) were formed by erosional or depositional processes (Fig. 9a). It is
radiocarbon age (ka)
0
10
20
30
40
I
f
I
100
200
x
300
u
._E
.=
~.
•'o
400
--o---e-~
--x---o---o-A
--e--
P 0200-10-4-2
p 0200-10-6-2
P0200-10-8-2
P0200-10-15-1
P0200-10-21-1
P0200-10-24-2
P0200-10-28-2
P0200-10-32-2
500
600 ±
Fig. 8. Depth against radiocarbon age for the studied cores off Portugal. Line between 60 and 170 cm for core PO200-10-32-2denotes
minimum sedimentation rate, as the oldest radiocarbon age is >41 ka.
Z 14: Baas et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
16
a
5 cm - Itho~des E (HC4Ocene)
135 cm - lllhof~d i s A10~ DIgit s)
1so ~ - lilhofl¢l$ D (H1)
10
PO2~-I 0-~2
~
1
10
1o0
1
10
Grain size (micron)
r
.
.
.
.
.
.
.
.
.
.
.
.
.
::.
10
100
Gra~l size (mioro~)
18 ~ - IR~ofldes E (HOlocene)
350 cm * I~o(=¢~es AI
310 cm - llth ofacWs F (ptecopod mud)
.
100
Groin size (micron)
108 PO200-1q~5-2
PO200-10-21-2
F'\
1
10
100
1
10
Grain ~ze (micron)
100
10
Grain ~ e (mizen)
100
Gr=in size ( m i c ~ )
Ie5 cm - lithofacils O (H4)
48 cm - lltho f t c ~ A2 (Y. DWIS)
lO
=
e
'/i
4
i,
o
1
10
IO0
10
G r i n Size (micron)
330 con - lill~ofac~es C (turbKIte)
208 cm - Mhoftctes B 1 (~lty boUonl 1tow d=po~d()
10
PO200-10-2¢2
PO200-1
6
~
~o
Grain l l z l ( m i e n )
100
too
258 cm - lithob ties B2 (ssndy bo~ orn ft(~/aepo~t)
!
PO200-10-211-2~c/~ , ~ ~
' . \~..I
7"
4
0
10
Gm~n ~ze (micron)
..............
IO0
Grain ~ze ( m ~ )
. . . . . . . .
1
10
G~In ~ze (nd~m)
100
10
100
Grlk~ ~ze (mtrz~)
0
lO
lOO
Grain size (micron)
Fig. 9. (a) Grain size distributions for selected from cores PO200-10-6-2 and PO200-10-28-2, measured with a Malvern Laser Particle
Sizer. H = Heinrich layer. (b) Grain size distributions in current-winnowed sediments from the Nova Scotian Rise, showing increase
in sorting and modal grain size with increasing current velocity (after McCave et al., 1995).
J.H. Baas et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1-23
assumed that erosion is mainly associated with
sediment winnowing by contour currents flowing
along the Portuguese margin, while deposition is
primarily linked with gravitational currents flowing
from the shelf and upper slope towards the deeper
basin. PO200-10-28-2 was recovered at 2155m
water depth near the top of the present-day
NADW. PO200-10-6-2 was recovered in 1086 m
water depth within the lower part of MOW with
typical ~13C ratios of about 1.5%o (Fig. 3d).
Samples from both bottom flow deposits and other
lithofacies were analysed for comparison (Fig. 9a).
In general, the differences between grain size
distributions for various lithotypes are small. Most
bulk and decarbonated samples show a modal
grain size between 4 and 9 ~tm and a gradual
decrease in frequency towards larger sizes. Yet, the
modal grain size is smaller in the southern core
(Fig. 9a). Holocene samples usually have a pronounced mode in the 4-5 Jam range, and relatively
low weight percentages of grains in the 20-63 lam
fraction. In the northern core the 4-5 ~tm mode is
confined to the bulk sediment (Fig. 9a). The
Younger Dryas sample from core PO200-10-28-2
has an anomalously high modal grain size of
14 ~tm, whereas in core PO200-10-6-2 the Younger
Dryas contains relatively large amounts of grains
in the > 10 ~tm fraction. Lithofacies B1 and B2
beds tend to have slightly larger modal grains sizes
than other lithofacies, or at least a broad high
frequency range in the fine silt fraction. The base
of the turbidite bed in core PO200-10-28-2 shows
a similar pattern (Fig. 9a).
In order to interpret these observations in terms
of erosion and deposition, the grain size distribution curves were compared with similar curves for
current-influenced sediments from the Nova
Scotian Continental Rise. It was found that the
modal grain size and its frequency increase with
increasing current velocity, and thus with increasing winnowing of the sediment surface (Fig. 9b;
McCave, 1985; McCave et al., 1995). The grain
size distribution curves for the Portuguese margin
are all similar to the low-velocity curve on the
Nova Scotian Rise. This implies that current winnowing was insignificant in the studied cores. We
infer that the bottom flow deposits were formed
by depositional downslope-directed currents, as
MOW and N A D W were probably not strong
17
enough to resuspend large volumes of material
from the sediment surface. Winnowing activity of
MOW is confined to shallower water, as implied
by high volumes of sand in upper slope sediments
(Kudrass, 1993; Mienert, 1993). An exception
occurs at the Younger Dryas event at the northern
site. The relatively high modal grain size may
signify minor current sorting during this cold
period. Further evidence for the depositional
nature of the bottom flow deposits is given by the
similarity with the grain size distribution curves of
the Holocene foram muds/oozes, hemipelagic
muds and Heinrich layers, which all involve simple
settling from suspension. The slightly increased
modal grain size for the bottom flow deposits may
indicate minor winnowing, but an alternative
explanation ensues from the Laser Particle Sizer
data of the turbidite deposit at the base of core
PO200-10-28-2. It is unlikely that current winnowing, if present at all, was able to penetrate to the
base of the turbidite, which was at least 25 cm
below the sediment surface. The cause for the
increase in modal grain size, found in both the
bottom flow deposits and the turbidite deposit,
should therefore be sought in the depositional
process rather than in surface erosion. We hypothesise that lithofacies B I and B2 were formed
below low-concentration turbidity currents without a clear division of sedimentary structures into
Bouma-type sequences. The slight increase in
modal grain size may be the signature of grain size
segregation within these turbidity currents.
The Holocene high frequency mode at 4-5 ~tm
and accompanying low coarse silt concentrations
probably result from a dominance of detrital clay
and fine silt and small biogenic constituents (coccoliths?), and a lack of fragments of foraminifera
of coarse silt size in this relatively 'fresh' sediment.
Foraminiferal shell fragments normally make up
the bulk of this fraction at deeper stratigraphic
levels (cf. McCave et al., 1995). The presence of
small biogenic constituents is confirmed in core
PO200-28-2, where the 4-5 ~tm peak disappeared
after carbonate dissolution.
5.2. Heinrich events
Heinrich layers were deposited along the entire
Portuguese continental margin. Melting icebergs
18
J.H. Baas el aL ,; Palaeogeography, Palaeoclimatology, PalaeoecohzKv 130 ( 1997j 1 23
released their sediment load during Heinrich events
l, 2, 4 and 6, and possibly during Heinrich event
3 and the Younger Dryas (Heinrich event 0).
During Heinrich events 1 and 2, the input of IRD
on the northern and central part of the margin
was about three times larger than on the southern
margin. The decrease in IRD content mainly
occurred between 3 9 . 6 N (core PO200-10-15-1)
and 37.8:N (core PO200-10-6-2), signifying an
increased melting rate between these latitudes. The
low concentrations of IRD in the southernmost
cores (e.g., core PO200-10-4-2) further suggest that
3 7 N is close to the maximum southward extension
of icebergs in the eastern North Atlantic. The
observed distribution of IRD denotes an east- to
southward transport of icebergs along the margin.
At present, the Eastern Boundary (Canary)
Current flows southward along the Iberian
Margin. Kipp (1976) constructed distribution
maps of foraminiferal assemblages from core top
samples, showing a tongue of polar taxa extending
southward along the Iberian margin parallel to the
Eastern Boundary Current (his fig. 22). Several
studies demonstrate that a similar current has been
present during the late Quaternary (e.g., Thiede,
1977; Wang et al., 1995). Moreover, thickness
distributions of Heinrich layers 1 and 2 in the
North Atlantic suggest enhanced eastward transport of icebergs between 40 and 50'N (Dowdeswell
et al., 1995), which may have been continuous
towards the Iberian margin (cf. Ruddiman, 1977;
Robinson et al., 1995). A glacial equivalent of the
present-day Canary Current is therefore regarded
to be responsible for the incursion of polar Nq.
pachyderma (sin) species and the supply of IRD
to the Portuguese margin during Heinrich events.
It thus formed a southeastward branch diverging
from the major northeastward transport path
(Bond et al., 1992).
Several data demonstrate that Heinrich events
had a weaker signature at the Portuguese margin
than in the central North Atlantic. Most Heinrich
layers described by Bond et al. (1992) contain
close to 100% IRD per total number of entities.
The IRD content in the Portuguese cores is usually
much less than 85%. Moreover, the peak height in
bulk magnetic susceptibility is smaller than at
higher latitudes, also indicating lower IRD content
(cf. Grousset et al., 1993; Mienert and Chi, 1995).
Broecker et al. (1992) found that more than 80%
of the total number of planktonic foraminifera in
Heinrich layers consists of left-coiling Nq. pachyderma as opposed to 20 80% off Portugal (e.g.,
Fig. 3b). Estimated IRD fluxes on the northern
Portuguese margin are somewhat less than in the
central North Atlantic (Bond et al., 1992; Van
Krefeld et al., 1996). This is probably due to the
branching of iceberg transport paths described
above and increased melting of ice along the
southeastern branch. The weakening of deep water
convection during Heinrich event I was perceived
as far south as 37.5'N, signifying the dramatic
decreases in benthic $13C ratios in cores PO200-106-2 and PO200-10-8-2, and the dominance of low
oxygen-related Chondrites trace fossils and benthic
foraminifera in the same cores (Baas et al., 1996).
The uniform mineralogical composition of
Heinrich layers 1, 2 and 4 provides an important
tool tbr correlating cores across the Portuguese
margin and for distinguishing Heinrich events from
local input of shelf-derived sand. Local, mediumgrained sand generally contains less quartz and a
wider range of rock mineral types, like volcanic
clasts, rock fragments and heavy minerals.
However, the major indicator is detrital carbonate,
which is abundant in Heinrich layers, but occurs
in trace quantities only in bottom flow deposits.
Detrital carbonate (dolomite and calcite) is typical
for Heinrich layers in the central North Atlantic
(Andrews and Tedesco, 1992; Bond et al., 1992).
Its origin has been traced back to carbonatic rocks
underlying the kaurentide Ice Sheet in the Hudson
Strait and Hudson Bay area (Bond et al., 1992;
Andrews and Tedesco, 1992; Andrews et al., 1994).
Concentrations of 15.9 24.5% detrital carbonate
suggest that the Laurentide Ice Sheet was the main
source for the IRD on the Portuguese margin.
Bond and Lotti (1995) defined additional sources
for IRD in central North Atlantic Heinrich layers.
Basaltic glass has an Icelandic origin, and hematite-stained grains dominantly originate from the
Gull" of St. Lawrence. An alternative source for
red-stained clasts are Continental Red Beds. which
are frequent on, for example, the British Isles.
With few exceptions, the concentrations of volcanogenic and
hematite-stained
particles in
J.H. Baas et al. /Palaeogeography, Palaeoclhnatology, Palaeoecology 130 (1997) 1 23
Portuguese Heinrich layers are low (<6%). This
excludes Iceland, the British Isles and the Gulf of
St. Lawrence as major sources for IRD. The
dominantly northeastern Canadian origin confirms
the results of Sr and Nd isotope studies in a nearby
core from Noroit Seamount (46°182q, 15"~04'W)
for Heinrich layers 1 and 4 (Revel et al., 1996).
The same authors found that the I R D in Heinrich
layers 2 and 6 was dominantly supplied by icebergs
from the British and Fennoscandian ice masses.
An enhanced influence of European sources may
explain the decreased concentrations of detrital
carbonate in Heinrich layer 6 on the northern
part of the Portuguese margin. Although a
Fennoscandian source for the rock minerals from
the present study area cannot be excluded, as no
key minerals are known from this area, a dominant
European source for the IRD in Heinrich layer 2
is improbable, because of the close similarity in
mineralogical composition with Heinrich layers 1
and 4. We speculate that this discrepancy is caused
by a geographical position of the Noroit Seamount
outside of the main transport path of Canadian
icebergs during Heinrich event 2. Interesting to
note in this respect is that detrital carbonate is
rare in a core from the Gulf of Biscay further to
the east (47.7~N, 8.1°W; unpublished data by the
first author), which may corroborate with the
above explanation. Icebergs may not have reached
the Portuguese margin during Heinrich event 5,
and presumably few icebergs were able to cross
the North Atlantic during Heinrich event 3 and
the Younger Dryas (Heinrich event 0).
The picture emerging is that of a Portuguese
continental margin periodically intruded by a preHolocene Canary Current carrying large amounts
of icebergs originating from a collapsing
Laurentide Ice Sheet. On their way along the
Portuguese margin, the icebergs rapidly melted,
leading to a greatly reduced flux of IRD off
southern Portugal. Fig. 10 shows how this fits in
with the general surface circulation patterns in the
North Atlantic, as given by Robinson et al. (1995).
During most Heinrich events, icebergs cross the
Atlantic in the Labrador Current, and then take
the Canary Current towards the Portuguese
margin. The alternative path is the Northeast
Atlantic Current towards northern Europe. IRD
19
of European and East Greenland origin is mainly
transported to southern latitudes in a cyclonic gyre
between 45 and 65°N, comprising the Northeast
Atlantic Current, the westward extension of the
Norwegian Sea Current, and the Central North
Atlantic Gyre (Fig. 10). Apparently, this circulation cell did not supply significant amounts of
IRD to Portugal during Heinrich events 1, 2 and
4. It may have done so, however, during Heinrich
event 6. Finally, the Subtropical Gyre at 40°N
may have taken up Canadian icebergs via the
Nova Scotian Current, a southern branch of the
Labrador Current. The icebergs incorporated in
this trajectory should contain increased concentrations of hematite-stained grains (Bond and
Lotti, 1995). The scarcity of this type of grain in
the studied sediments suggests that the Portuguese
margin was beyond the main transport path of
these icebergs.
6. Geological history
The late Quaternary depositional history of the
Portuguese margin has been reconstructed with
the sedimentological and paleoceanographical data
given above. Pelagic sedimentation prevails in the
Holocene. In pre-Holocene times, settling of biogenic particulate matter was supplemented with
sea level- and climate-controlled input of terrigenous material from local and distant sources by
low-concentration turbidity currents, contour currents and icebergs.
Data records for the period between 70 and 25
ka are available only for the northern Portuguese
margin. There, terrigenous input is high between
70 and 35 ka. Low-concentration turbidity currents
transported sand and silt from the shelf and upper
slope towards the deeper basin. Main events
occurred at +70, 58.6, 52.8, 48.4, 41.6 and 38.0
ka. The enhancement of shelf-to-basin transport
probably followed increased erosion of shelf sediment at low sea level, as global climate was cold
and water was stored in continental ice sheets.
This situation was interrupted by two periods of
melting of ice masses at 64.5 ka (Heinrich event
6) and 50 ka (Heinrich event 5), of which only
the former had a major influence on sedimentation
20
J.H. Baas et al. / Pulaeogeography, Palaeoclmtatology, Palaeoecolog 3 130 (1997) 1 23
70 °
60 °
50<'
40 °
30 °
20"
10 °
0°
Fig. 10. Ocean surface circulation patterns and transport paths of icebergs during Heinrich events 1, 2 and 4 (modified after Robinson
et al., 1995). The main pathway of icebergs from the Laurentide Ice Sheet towards Europe, including the Portuguese margin, is
shown by the thick line, which is primarily based on a compilation of data of Kipp (1976), Ruddiman (1977), Bond et al. (1992),
Grousset et al. (1993), Dowdeswell et al. (1995), Gwiazda et al. (1996), Lebreiro et al. (1996) and Revel et al. (1996). Note the
branch directed towards the south starting at about 20°W. Thin lines denote additional trajectories of iceberg transport. CC= Canary
Current; CNAG=Central North Atlantic Gyre; I C = Irminger Current; L C = Labrador Current; NEAC=Northeast Atlantic Current:
NSC=Norwegian Sea Current; N S C C = N o v a Scotian Current: SG=Subtropical Gyre.
on the Portuguese margin. Large numbers of melting icebergs passed the Iberian margin in a glacial
Canary Current, while forming Heinrich layer 6.
The Canary Current was supplied with icebergs
from the Laurentide Ice Sheet through the
Labrador Current and possibly from European ice
sheets through the Central North Atlantic Gyre.
Volcanic ash was transported by eolian flows from
Iceland to northern Portugal at 57.5 ka, forming
ash zone II. A new period of ice sheet instability,
starting at 33.8 ka, led to the formation of Heinrich
layer 4 on the Portuguese margin. This cold period
lasted for about 1.5 ka, and involved low bottom
water oxygenation. The icebergs mainly originated
from the Laurentide Ice Sheet. After this event,
normal glacial conditions were re-established until
Heinrich event 3 commenced at about 27 ka.
According to Zahn et al. (1996), only the meltwa-
ter signal, but no icebergs, reached the southern
margin. The present data suggest that some icebergs reached the Portuguese margin.
Heinrich events 1 and 2 have similar signatures
as Heinrich event 4. Polar water temperatures
prevailed, deep water convection was weakened,
and icebergs were derived mainly from the
Laurentide Ice Sheet through the Labrador and
Canary currents. An increase in sea surface temperature caused intensified melting of icebergs south
of 39.6°N. The quiescent periods between these
Heinrich events were characterised by increased
preservation of pteropod shells, culminating in
pteropod-rich intervals at 24.2 24.8 and 16.6 18.0
ka. These intervals are confined to the central and
southern part of the Portuguese margin. The presence of large numbers of easily dissolvable pteropods probably indicates a decrease in the rate of
J.H. Baas et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 130 (1997) 1 23
dissolution due to a deepening of the aragonite
compensation depth and/or increased productivity
of pteropods. No apparent relation with climatic
change was found.
The transport of terrigenous material from the
Portuguese shelf towards the deep Atlantic basin
was enhanced during sea level drops in the last
glacial period. Terrigenous input was highest in
the northern part of the study area. This may have
a climatic cause. The present-day Iberian continent
is divided into two major climatic zones (Monteiro
et al., 1980). The continental shelf north of Nazar6
Canyon (40°N) has a humid hinterland with a
relatively high relief, high rainfall, and a high river
discharge. The climate south of Nazar6 Canyon is
arid with low fluvial input. We hypothesise that a
similar climatic zonation was responsible for
enhanced shelf sedimentation and consequently
enhanced shelf-to-basin sediment transport during
the glacial period on the nortern margin.
No evidence for current winnowing in water
depths of 1086 m on the southern and 2155 m on
the northern margin was found. Current velocities
of NADW probably were not high enough to
erode the sediment bed. Current winnowing by
MOW is confined to upper slope sediments.
Downslope transport of terrigenous material
decreased and biogenic production increased
following the sea level rise and sea surface temperature increase during deglaciation. The temporary
return to glacial conditions around 10.5 ka was
recorded on the Portuguese margin by increased
input of terrigenous silt and sand. The Younger
Dryas is a meltwater event equivalent to Heinrich
events in the central North Atlantic (Broecker,
1994), but only few icebergs reached the
Portuguese margin. Minor sediment winnowing
occurred on the northern part of the Portuguese
margin during the Younger Dryas. Holocene sediments have a typical interglacial signature with a
dominance
of biogenic
over
terrigenous
sedimentation.
Acknowledgements
The authors wish to thank the captain, crew
and scientific staff of R.V. Poseidon for collecting
21
the sediment cores, Jan Heinemeier for AMS 14C
analysis, Helmut Erlenkeuser for stable isotope
analysis, Wilma Rehder for X-ray photography,
and all students at GEOMAR for laboratory support. The suggestions by John Andrews and an
anonymous reviewer to improve the final version
of the manuscript were much appreciated. This
work is part of the research programme "ENAM
(European North Atlantic Margin) Sediment
Pathways, Processes and Fluxes", financed by the
European Union under MAST II.
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