J. Nannoplankton Res. 27 (2), 2005, pp.159-172 © 2005 International Nannoplankton Association
ISSN 1210-8049 Printed by Cambridge University Press, UK

159

Calcareous nannoplankton as a tracer of the marine
influence on the NW coast of Portugal over the last
14 000 years
C. Guerreiro
Centre of Geology, University of Lisbon, Building C6, 6.4.67, Campo Grande, 1749-016 Lisboa, Portugal; cataguerreiro@gmail.com

M. Cachão
Centre of Geology, University of Lisbon, Building C6, 6.4.67, Campo Grande, 1749-016 Lisboa, Portugal & Department of Geology, Faculty of
Sciences, University of Lisbon, Building C6, 6.4.55, Campo Grande, 1749-016 Lisboa, Portugal

T. Drago
INIAP, IPIMAR, CRIPSUL, 8700-305 Olhão, Portugal
Manuscript received 22nd March, 2005; revised manuscript accepted 19th August, 2005

Abstract A calcareous nannoplankton taphonomic and palaeoecological study was performed on cores from the
Minho and Douro Estuaries (NW Portugal) and from the NW Portuguese middle continental shelf. Because certain
nannoplankton species are taphonomically resistant to the highly energetic and aggressive conditions between theestuaries and the shelf, their sedimentary record acts as an oceanic tracer in such coastal domains. For the Late
Quaternary, this allows interpretation of eustatic variation from the induced sequence of palaeoenvironmental
changes. Palaeogeographically, the time-series pattern of nannofossil abundances in these coastal sediments is interpreted as follows: (1) persistently high amounts of nannofossils (2x107 nannofossils/g) are related to marine environmental conditions, produced by post-glacial flooding of pre-existing valleys; (2) the gradual increase in intermittency of nannofossil abundances through time indicates a regressive trend, first due to recovery of the estuarine conditions, followed by a total absence of nannofossils, indicating evolution towards complete emersion either by fluvial (Core CPF1) or barrier-beach sediments (Cores M1, M2, D1A and D1B). During the Late Holocene, the nannoplankton assemblages were similar to those found in Present Day neritic conditions. The main taxa comprise
Gephyrocapsa spp., Helicosphaera carteri, Emiliania huxleyi and Coccolithus pelagicus sensu lato. An unexpected
and extremely high percentage of H. carteri in the Douro Estuary’s mouth was likely related to local confinement,
which probably provided adequate and particular nutrient concentrations that prompted opportunistic behaviour inthis species.
Keywords Calcareous nannoplankton, taphonomy, Quaternary eustatic variations, NW Iberia, Minho Estuary,
Douro Estuary, coastal palaeogeography

1. Introduction
The littoral zone is a region of great instability, in whichmarine and terrestrial domains interplay, both characterised as being particularly sensitive to a variety of climatic, oceanographic and geological processes. Theirecosystems are highly productive, constantly changing
their interface, and controlled by global (e.g. glacio-eustacy and climate), as well as local, factors (such as sedimentary discharge and antropism) (Drago et al., 2002).
Estuaries represent the main channels for supply ofnutrients and terrigenous material to the ocean. Most
Present Day Portuguese estuaries have lost their typical‘v’-shaped profiles and acquired flat-bottomed valleys, asa result of sedimentary infilling induced by Holocene
base-level elevation (Andrade, 1998). Consequently, theytend to exhibit extensive sedimentation in distal alluvial
plains, forming estuarine flats and marshes, and frequently have undergone intense infill of the valley’s interior
sections. This infill represents high-resolution sedimentary sequences that allow documentation of marine/fluvial

interactions during the Upper Quaternary (Freitas, 2003).
Chemically, estuaries are characterised by a high spatial
and temporal variability of environmental conditions, particularly as far as salinity is concerned (Ortega et al.,1994). The transformation of certain elements from asolution state into a particulate phase, and their subsequent stock-piling in the sediments, turns estuaries into
sources of nutrients and a number of metals that are commonly transported by rivers (Ortega et al., 1994).
River discharge into the ocean frequently produces
temperature, salinity and turbidity plumes along the coastline, due to the different physical parameters that distinguish fresh from marine water (Oliveira et al., 1994).
According to Baumann et al. (2002), nutrients transported by upwelling promote high biological productivity
along coasts, such as the Portuguese west coast, mainly inthe proximity of capes. In such an upwelling coastal
region as that of the study area, phytoplankton assemblages may be controlled by certain trophic axes, related
to turbulence degree and nutrient availability (Margalef,160

Guerreiro, Cachão, Drago

1997). Therefore, regions located in proximity to continental margins, which are characteristically turbid, areassociated with situations where spatial confinement of
nutrient-rich conditions may occur (Cachão & Moita,
2000). On the other hand, understanding of the physical,
geological and biological processes that occur above themiddle continental shelf is also important to allow tracking and quantification of the nutrient flux that is being
introduced into the shelf-break zone (Oliveira et al.,2002), as well as the possible influence of the nannoplankton, as recorded in both the water-column and thesedimentary record.
Considered as an oligotrophic group, some calcareous
nannoplankton species seem to have developed delicate
and highly elaborate morphologies, possibly to favour
buoyancy (e.g. Discosphaera tubifera, Umbellosphaeraspp., Rhabdosphaera spp.; see Siesser & Winter, 1994;
Young, 1994). This may have allowed them to dwell in
stratified, nutrient-poor oceanic water-masses and, consequently, to be globally represented in oceanic oozes.
Other taxa evolved large coccoliths/spheres (e.g.
Coccolithus pelagicus: Cachão & Moita, 2000) or developed as r-type opportunists (e.g. Emiliania huxleyi:
Brand, 1994), possibly allowing them to compete fornutrients in more turbulent water-conditions, such as
coastal environments. Their sedimentary record, however,is modified by the hydrodynamics of bottom conditions(Roth & Berger, 1975) that can cause resuspension anddispersion of the nannofossils, and that may prevent themfrom being deposited with coarser sediments on the innershelf. Diluted by high sedimentation rates, their deposition occurs both on the middle and outer sectors of the
shelf, and on paralic low-energy inlets of the estuaries andlagoon coastal systems (Ferreira & Cachão, 2005; Ribeiro
et al., in prep.).
Previous studies have already shown the usefulness ofnannofossils in multi-proxy studies, tracking marine sedimentation within paralic systems, such as estuaries andlagoons, and contributing valuable information topalaeoenvironmental and palaeogeographic reconstructions (Freitas et al., 1999; Drago et al., 2003). Using datafrom the NW Portuguese coast and shelf, the present
study aims to add additional support to the use of calcareous nannoplankton to this type of study.
2. Environmental setting of the studyarea
2.1 General oceanography of thePortuguese middle continental shelf
The Portuguese middle continental shelf presents a generally gentle and regular morphology, with four deep incisions: the submarine canyons of Nazaré, Lisboa, Setúbal
and São Vicente. The Oporto Canyon is also one of theimportant morphological characteristics of the Portuguese
shelf (Dias, 1987; Figure 1).
The Atlantic coast of Iberia represents the northern
limit of the upwelling zone associated with the North

Figure 1: Location map showing NW Portugal submarine canyons andcontinental shelf

Atlantic Anticyclone Gyre (Woster et al., 1976). This is ahydrographically complex region, characterised by the
confluence of different water-masses (Fiúza, 1984; Rios
et al., 1992; Fiúza et al., 1998) and currents (Frouin et al.,
1990; Haynes & Barton, 1990; Dias et al., 1996; Fiúza etal., 1998). Consequently, Portugal’s location on the eastern margin of the North Atlantic Ocean somewhat determines the climatic and oceanographic characteristics ofthe adjacent oceanic region (Dias, 1987; Oliveira et al.,2002), while meteorological conditions are controlled by
the Azores High Pressure System.
Summer over the Iberian middle continental shelf ischaracterised by an equator-ward superficial current,
which is controlled by northerly to north-westerly windsthat are favourable to upwelling (Fiúza, 1983; Vitorino,
1989; Silva, 1992). Immediately below this southwardflowing current, a polar-ward current was detected. The
flux pattern made by both currents is called the Iberian
Mid-Shelf Water-Cross Summer Circulation. In the winter, southerly to south-westerly strong winds arefavourable to the establishment of a downwelling regime
over the shelf, induced by a polar current that persists at
all depths (Vitorino, 1989). These patterns are also controlled by the coastal morphology, the upper shelfbathymetry and by local winds (Fiúza, 1983).
2.2 The Minho Estuary
The Minho River (Figure 2) reaches the ocean through asoftly contoured estuary, oriented approximately
NNE–SSW. Its width is <500m for the lower 25km, gradually increasing in the upper 15km, to reach its maximum
width (about 2km) at the confluence with the Coura River,Calcareous nannoplankton as a tracer...Portugal
Figure 2: Location mapof distal sectors of
Minho, Coura andDouro Rivers, NW
Portugal. Studied cores:
M1,
M2(MinhoEstuary); CPF1 (Coura
Estuary); D1A, D1B,
D2 (Douro Estuary);and FF1GM92 (NW
Portuguese
middleshelf)

161

conditions may reach the river’s mouth.
The main marsh comprises the area where the tributary issues from the Coura River’s mouth, associated witha few small islands and a small marshy area (Figure 2).
The Coura River is vertically homogeneous during spring
high-tide, losing the marine signature 4km upstream of
the confluence with the Minho Estuary. According toMoreno et al. (2003), this extreme salinity gradient,which characterises the lower estuary during each tidecycle, may be causing a strong ecological pressure on theenvironment, which controls microfaunal (benthic
foraminiferal) distributions.
2.3 The Douro Estuary
The Douro River (Figure 2) is one of the most importantof Iberia’s rivers, being the third longest (927km) and thefirst in hydrographic basin area. It has very irregularflows, which, in its lower section, may reach 17 000m3/s,
during winter, decreasing to <100m3/s during summer. It
has the highest rate of sedimentary discharge, representing the NW Iberian margin’s main source of fine sediment. Near the river’s mouth, >80% of the total terrigenous input is transported in suspension (Dias, 1987).
The Douro Estuary is narrow and funneled, partly
barred by a sand-barrier (Cabedelo). The estuary has aminimum width of 135m near Dom Luis Bridge (about
6km upstream of the river’s mouth) and a maximum
width of 1300m near Arrabida’s Bridge. In the presence oflarge water-volumes (>10 000m3/s), the sand-barrier isdestroyed by drainage; this is gradually rebuilt, during
decreased flow regimes.

at Caminha. The River Minho’s mouth is narrow, controlled by underwater outcrops, and exhibits a submersed
bar with two shallow (~1m) channels, one trending south,
the other north. These outcrop ridges reach the water’ssurface in certain locations near the northern channel(Fatela et al., 2003).
The Minho Estuary is affected by a meso- to microtidal system, showing amplitudes of approximately 4m.
This value may be significantly amplified during stormsurge episodes. As a result of the tidal regime and theestuary’s flatness, tides reach up to 40km upstream, causing intense silt-up, which is reflected in immersion of 7080% of the estuary’s bottom during spring low-tides(Alves, 1996).
High sea-water penetration into the estuary is shownby temperature/salinity (T/S) profiles performed during
spring high-tides (summer 2002: Moreno et al., 2003).
These measurements reveal weak estuarine/strong marine
characteristics (salinity = 35–30‰: Moreno et al., 2003).
The estuary water remains vertically homogeneous until
4km upstream, after which it becomes partially mixed forthe next 10km. At low tide, however, almost null salinity

3. Material and methodology
A total of 362 samples were analysed for this study: (1) 23
surface-sediment samples from the Minho Estuary; (2)201 samples selected from Cores CPF1 (11.5m thick), M1(28.7m thick) and M2 (21m thick) from the Minho
Estuary; (3) 108 samples from D1A (0–7.9m depth), D1B(10.1–19.7m depth) and D2 (20-40m depth), all recoveredfrom the Douro Estuary; and (4) 35 samples selected fromCore FF1MG92 (3m thick), recovered from thePortuguese middle shelf, off Oporto, at 100m depth(Figure 2). Sample interval varied from 10 to 20cm,
depending on lithology and the texture of the sediments.
Since conditions favourable to the settling of calcareous
nannoplankton are characterised by low hydrodynamics,
silty-clay-richer intervals were sampled in more detail.
As a first stage, a coccolith abundance index (CAI)
was determined on randomly selected 30mm traverses ofthe smear-slide, based on the methodology described inCachão & Moita (2000). Since the CAI tends to optimise
the occurrence of nannoplankton in the sedimentary
record, this rippled smear-slide-based technique was used
for preliminary recognition and a semiquantitative estimate of abundance.
Absolute abundances were subsequently determinedon all samples that were found to contain nannofossils,

162

Guerreiro, Cachão, Drago

using the previous method. Slides were prepared according to the random settling procedure (Flores & Sierro,1997), adapted for neritic (continental shelf) and paralic(estuarine and lagoonal) facies (Ferreira & Cachão, 2003)
by modifying sediment weights and pipetted volumes.
Quantification of the calcareous nannofossil content (nannofossil/g) of each sample was determined according to
the equation:
V
Pa
1
N=n x
x
x
(1)
Vp
Oa
W

at Station 3 (14x106 nannofossils/g). A distinct predominance of Gephyrocapsa ericsonii (5x104 nannofossils/g),
G. oceanica and Emiliania huxleyi (both with 6x104 nannofossils/g) was observed.
4.1.2 Core samples
M1 and M2 recovered the entire Holocene sedimentary
sequence. Their lithologies, predominantly fine to veryfine sands, display similar variation patterns with depth,
reaching the granitic substrate at 28.7m and 21m, respectively (Figure 4). CPF1 consists of 11.6m of homoge-

where N = nannofossils/g, n =counted number of nannofossils, V= volume of the preparation bottle(10 000ml), Vp = volume pipetted
into the Petri dish with coverslip,
Pa = Petri dish area, Oa =observed area, and W = sedimentweight.
Slides were observed with an
optical polarising microscope(Olympus BX-40) at 1250x magnification. Core samples and slides
are stored in the Nannofossil andCoastalSedimentology
Laboratories (NanoLab and
ProCoast) of the Department of
Geology, Faculty of Sciences,
University of Lisbon.
4. Results
4.1 Minho Estuary
Figure 4: Absolute abundances of calcareous nannoplankton (nannofossils/g) and silty-clay fraction
4.1.1 Surface-sedimentpercentages, Cores M1 and M2, Minho Estuary. Model stages: II – first estuarine stage; III – floodsamples
ed ria stage; IV – second estuarine stage; V – Present Day coastal drift-driven sedimentation stage.
Only eight of the 23 surface-sediRadiocarbon ages in yrs BP
ment samples collected from the
Minho Estuary contained calcareous nannoplankton neous silty-clayey, organic-rich sediments (Figure 5).
(Figure 3). Highest abundance and diversity was observed
Highest abundances were found in CPF1, with values
reaching 5.2x106 nannofossils/g. Core
M2 displayed maximum abundancesof 4.2x105 nannofossils/g, whilst Core
M1 had abundance peaks slightly<3x105 nannofossils/g. Calcareous
nannoplankton diversity is slightly
higher in CPF1 (Figure 6) than in M1
and M2 (Figures 7 and 8, respectively), although within the same magnitude.
Maximum values of species abundance are (in decreasing order):Gephyrocapsa spp., with the predominance of G. oceanica (1.3x106 nannofossils/g), followed by G. muellerae(6.4x105 nannofossils/g) and G. ericnannofossils/g),
sonii(4.7x105
Figure 3: Absolute calcareous nannoplankton abundances (nannofossils/g) in surface sediment
Helicosphaera
carteri(8.2x105 nansamples, Minho EstuaryCalcareous nannoplankton as a tracer...Portugal

163

nitude occurred, with assemblages majorly dominated by
H. carteri (Table 1).
In Core D1A, only the lowest sample (-5.17m) was
productive, containing modest numbers of nannofossils.
H. carteri is present, but replaced in dominance by G.
oceanica, G. ericsonii and E. huxleyi (Table 1).

Figure 5: Absolute abundances of calcareous nannoplankton (nannofossils/g) and silty-clay fraction percentages, Core CPF1, Coura Estuary.
Model stages: III – flooded ria stage; IV – second estuarine stage; V –Present Day fluvial stage. Radiocarbon ages in yrs BP

nofossils/g), Emiliania huxleyi (6.2x105 nannofossils/g),
Calcidiscus leptoporus s.l. (8.8x104 nannofossils/g),
nannofossils/g),
Syracosphaera
spp.
(1.8x104
Umbilicosphaera sibogae s.l. (1.8x104 nannofossils/g),
Coccolithus pelagicus s.l. (1.4x104 nannofossils/g) andBraarudosphaera bigelowii (5.4x103 nannofossils/g).
Ascidian spicules were detected sporadically (1.2x104
spicules/g). Despite the fact that Syracosphaera spp., U.
sibogae and ascidian spicules present higher maximum
values than C. pelagicus, this latter species is generally
more consistently present in the studied samples.
Both Cores CPF1 and M2 contain G. oceanica as themost abundant species, followed by H. carteri. In Core
M1, this situation is reversed, with H. carteri showing
slightly higher abundances.
4.2 Douro Estuary
Cores D1A, D1B and D2 recovered complementary sections of the sedimentary record of the distal Douro
Estuary: D2 the lowermost part, D1B the intermediate
portion and D1A the uppermost part (Figure 9).
Core D2 is practically barren of nannofossils, with the
exception of the uppermost portion. Only the top of thecore registered irregular abundances and modestly diverse
assemblages, with a few tens of coccoliths per short traverse (<200 CAI). G. muellerae and H. carteri dominate
these assemblages (Table 1).
Core D1B has more consistently present and abundantnannofossils. Above the initial portion, devoid of nannofossils, the middle part has modest and irregularly present
assemblages (with magnitudes similar to those found inD2). Between -12m and -10m, a pronounced and persistent increase in nannofossils of two to three orders of mag-

4.3 NW Portuguese middle continental
shelf
Recovered from the Portuguese middle continental shelf,Core FF1GM92 represents a typical neritic marine environment and thus its calcareous nannofossil abundanceand diversity record is used here as a proxy for an environment that maintained proximity to the continental discharge from the Douro river-mouth (Figure 10). In the
majority of the samples, G. muellerae is the dominant
species, reaching absolute abundances >1x108 nannofossils/g, followed by E. huxleyi (5.2x105 to 2.9x107 nannofossils/g), G. oceanica (1.4x105 to 2.3x107
nannofossils/g), G. ericsonii (9.0x104 to 3.0x107 nannofossils/g), and H. carteri (max. 1.0x107 nannofossils/g)(Figure 11). Core FF1GM92 contained higher numbers ofnannoplankton than those from the estuaries, except for
samples -2.61m, -0.66m and -0.31m, which contained<300 CAI. The calcareous nannoplankton assemblages
are similar to those found in the estuarine cores, but withmuch higher absolute abundances. For example, C. leptoporus (max. 3.4x106 nannofossils/g), C. pelagicus (max.
8.7x106 nannofossils/g) and Syracosphaera spp. (max.
6.3x106 nannofossils/g) are significantly higher in abundance in shelf sediments than in the coastal estuarine sediments.
5. Discussion
Gephyrocapsa muellerae and Emiliania huxleyi are significantly abundant in samples from the western Iberia
middle shelf. High abundances have been previously
noted in temperate water-masses (e.g. Jordan & Kleijne,
1994) and are used as indicators of the Temperate
Biogeographic Zone (Zone 2 of McIntyre & Bé, 1967;
Winter et al., 1994). Considering that the Portuguese continental shelf is located between the Temperate andSubtropical Biogeographic Zones, the high abundances ofthese two species is predictable in Portuguese waters(Cachão & Moita, 2000). The presence of species such as
Calcidiscus leptoporus is probably related to waters coming from south of the Azores Front (Cachão & Moita,
2000). Despite obvious differences between nannofossil
abundances in the estuaries and on the shelf, it is possible
to interpret these variations for the last 14kyr in terms ofdilution by variable sedimentary rates, caused by changes
in fluvial discharge.
5.1 Taphonomy versus ecology of thecalcareous nannoplankton in coastal
domains
Although subjected to the influence of oceanic advective

164

Guerreiro, Cachão, Drago

Figure 6: Absolute abundances (nannofossils/g) of the most importantnannoplankton species found in Core
CPF1, Coura Estuary

Figure 7: Absolute abundances (nannofossils/g) of
the most important nannoplankton species found inCore M1, Minho Estuary

Figure 8: Absolute abundances (nannofossils/g) of
the most important nannoplankton species found inCore M2, Minho EstuaryCalcareous nannoplankton as a tracer...Portugal

165

blages are less diverse in comparison to open ocean assemblages(Roth & Berger, 1975). However,taxa such as Coccolithus pelagicus s.l., Helicosphaera carteri,
e
s
a
a
ii
u
i
on
nic
ller
gic
eri
ley
s
a
t
e
a
c
x
e
r
l
Gephyrocapsa oceanica and E.
i
u
oc
ca
er
hu
pe
m
Core
Height (m)
E.
G.
C.
G.
H.
G.
huxleyi may compete in coastal
environmental conditions, andtheir productivity signals are high
-0.57
0
0
0
0
0
0
enough to be easily distinguishedD1A -3.68
0
0
0
0
0
0
-3.83
0
0
0
0
0
0
in paralic sediments, even when
-4.12
0
0
0
0
0
0
terrigenous sedimentary rates
-4.57
0
0
0
0
0
0
may be high.
-5.17
27
6
53
9
56
54
-10.55
30
2844
156
42
276
171
Nannoliths are dimensionally-10.71
27
4743
135
45
180
324
and
hydrodynamically equivalent(84.4%)-11.27
42
420
201
39
330
351
to silt particles. Thus, their depo-11.51
54
1323
63
81
567
432
sition necessarily implies the-11.69
45
621
135
45
216
369
-11.83
45
522
81
27
198
171
maintenance of low to null
-12.18000000hydrodynamic bottom condi-12.35
0
0
0
0
0
0
tions. Such conditions are rarely-12.6622
60
00-12.74000000
present in high-energy coastal
0
0
0
0
0
0
D1B -12.88
domains. However, the complex-13.3800
0
0
00-13.474
7
10
02
7interplay of continental (terrige-13.58
3
2
7
3
4
18
nous discharge, variations in-13.83418
82
1
6-14.08000
0
00availability of organic material,-14.26000
0
00more or less important local-14.41000
0
00changes in coastal morphology,-14.810
5
4
53
13-15.18102102
6etc.) and marine (storm-surge-15.53
4
5
32
7
4
15
episodes; dissolution processes-16.0700
0
0
00-16.1800
211
13
related to transport from the-16.58000
0
00
plankton production area to the-16.85000
0
00-17.50000000
coast; possible seasonal varia-18.15
0
0
0
0
0
0
tions that may have affected pro-18.60
0
0
0
0
0
0
ductivity, etc.) factors implies-24.870
112
91
1-25.0612
4
15
1
2
that only the general total nanno-25.30
5
47
4
46
4
5
fossil curve is robust enough to-25.58000000-25.8200
0
0
0
0
allow interpretation of global-26.00
2
14
0
49
7
3
eustacy in the NW sector of the-26.1400
0
0
00-26.36
0
22
1
36
9
11
Portuguese littoral zone.
-26.71
4
27
7
40
7
4
Along the coast, the settling of-27.0800
1
4
0
0
nannofossils in marshes and other-27.48000
0
00-28.95
0
0
0
0
0
0
D2
favourable depositional environ-29.25
0
0
0
0
0
0
ments has taphonomic and eco-29.70
0
0
0
0
0
0
-30.04000000logical meaning. Taphonomical-30.28
0
0
0
0
0
0
ly, this may reflect a more or less-30.53000000-30.65000000extended biostratonomical advec-30.90
0
0
0
0
0
0
tion by tides, storm-induced-31.50000000
oceanic downwelling currents,-31.78000000-31.86000000
and/or coastward wave move-32.00
0
0
0
0
0
0
ments, whilst diagenetically, they
-32.1100
0
0
00-32.3000
0
0
00
are exhibiting a significant resist-32.44
0
0
0
0
0
0
ance to chemical dissolution and-32.53000
0
00-32.5700
0
0
00
breakage processes, caused byorganic matter oxidation in seditransport mechanisms, nannoplankton species densities
ments
typicalofcoastalenvironments. Their abundancesand relative abundances in pelagic environments show
also
reflect
dilutionbyterrigenous fluvial discharges,
good correlation between the water-column and the sediboth
insideandoutsidetheestuaries.
ment (Kinkel et al., 2000). Coastal nannoplankton assemTable 1: Coccolith abundance index (CAI) of C. pelagicus, H. carteri, G. oceanica, G. muellerae,
G. ericsonii and E. huxleyi in Cores D1A, D1B and D2, Douro Estuary. H. carteri percentage of total
assemblage added for sample –10.71m

166

Guerreiro, Cachão, Drago

availability), in most cases only
a taphonomical analysis is
extractable. This is the case forthe assemblages found in thesurface-sediment samples of the
Minho Estuary and from thecores that were recovered off the
Minho and Douro Estuaries,
with the exception of Core D1B(Douro Estuary), which has
revealed a sufficiently highabundance of H. carteri to beinferred as a palaeoecologicalrecord.
There is no evidence for theoccurrence of significant dissolution in the studied coastal
cores, since both more and lessresistant species are present inalmost all samples (with the
exception of a few surface sediment samples from the Minho
Figure 9: Coccolith Abundance Index (CAI) and silty-clay fraction percentages in Cores D1A, D1B
and D2, Douro Estuary. Model stages: I – initial fluvial stage; II – first estuarine stage; III – flooded
Estuary). The observed abunria stage; V – Present Day coastal drift-driven stage. Radiocarbon ages in yrs BP
dances of E. huxleyi and G. ericsonii are a good indication of no
significant diagenetic elimination, since these nannofossils, due to their small size and low resistance to dissolution, are usually the first to be affected by chemical corrosion in the water-column, at the sediment/surface interface, or within the sediment (Schmidt, 1978).
Only eight surface-sediment samples had calcareous
nannoplankton, all from the terminal part of the Minho
Estuary, close to the ocean. Samples from Porto da
Capitania (Station 3) had the higher abundances, mainly
because this was a more silty facies, potentially morefavourable to the deposition of nannofossils, in contrast to
the remaining samples that varied from fine to coarse
sands.
It was predictable to find G. oceanica and H. carteri
in surface-sediment samples below such high-energy
depositional conditions, because of their robustness.
However, high abundances of E. huxleyi and G. ericsonii
were also registered, indicating that, although coccolithsof these species may be affected by dissolution, they seem
Figure 10: Absolute abundances of calcareous nannoplankton (nannoto endure these coastal hydrodynamic conditions.
fossils/g) and silty-clay percentage of Core FF1GM92, Portuguese midAlthough G. oceanica, E. huxleyi and G. ericsonii
dle continental shelf. Radiocarbon ages in yrs BP
generally predominate, C. leptoporus was the dominant
species at Station PR6 (Figure 3). This species is practiEcologically, assemblage composition is particularly
cally absent from all remaining samples. Since C. leptoinfluenced by species that proliferated in the water-colporus is directly related to oceanic water-masses off theumn close to shore, just prior to post mortem transport and
Iberian shelf, approaching the coast mainly during winterdeposition. These taxa are in competition for nutrients
downwelling conditions (Oliveira et al., 2000), its domiwith other neritic phytoplankton groups, such as diatoms
nance inside the estuary may be related to a storm event.
and dinoflagellates.
Despite the fact that various calcareous nannoplankton
5.2 The Holocene palaeoenvironmental
species may indicate certain (palaeo)environmental/ecoevolution model
logical influences (e.g. temperature, turbulence, nutrient
Over the shelf, nannofossil assemblages from Core

Calcareous nannoplankton as a tracer...Portugal

167

M2, respectively, the nannofossil record began tooscillate during a welldefined sector, 8 to 10m
thick (Stage IV; Figure 4).
This is interpreted asreflecting estuarine conditions, which, in this distal
section of the palaeo-valley, are mainly controlled
by coastal drift-driven sedimentation. This indicatesthat the coastal sedimentary
system started to prograde,
probably due to the
Holocene deceleration insea-level rise (Dabrio et al.,
2000; Lario et al., 2002),
marking a regressive trendof the coastline. In the final
2-3m (Core M1), nannofosFigure 11: Absolute abundances of C. pelagicus, H. carteri, G. muellerae, G. oceanica, G. ericsonii, E. huxleyi, C. leptoporus and Syracosphaera spp. (nannofossils/g) of Core FF1GM92, Portuguese middle continensils are absent. This is intertal shelf
preted as reflecting thecomplete recovery of terFF1GM92 are not significantly different from those thatrigenous
sedimentationbycoastaldrift at this location incharacterise the Present Day water-masses of thetheestuary(StageV;
Figure4).
Portuguese middle continental shelf (Cachão et al., 2000),
Core CPF1 (Sapal do Coura; Figure 5) showed an
and one may assume that similar oceanographic characoverallnannoplankton abundance pattern similar to thoseteristics prevailed during the time-interval (475yr) regisobservedin Cores M1 and M2. However, because Core
tered by this core. As already mentioned, although coastal
CPF1
did
not reach the substrate, it does not record thedepositional conditions may have varied considerably, theentiresedimentary
sequence, particularly the pre-ria
overall similarities in nannofossil variation patterns in all
stage.
Nannoplankton
are significant and persistent fromcores of the Minho (CPF1, M1 and M2) and Douro
the
base
of
the
core,
compatible
with marine-influenced
Estuaries (D1A, D1B and D2) can be interpreted as a prisedimentation
withinafloodedsectionof the palaeo-valmarily depositional signal, caused by changes in marineley,reflecting
the
ria
stage
at
the
beginningof its sedidriven sedimentation and bottom hydrodynamism,
mentary
record(StageIII;
Figure5).
enabling determination of their variations in space andAt around -10.2m, a transition from persistently hightime for this sector of the northern Portuguese coast.
nannofossil abundances to oscillating abundances wasnoted. This is interpreted as evolution towards an estuar5.2.1 Minho Estuary
Nannofossils are present from almost the base of Cores ine stage, which seems to have occurred at a slightly highM1 and M2 . The presence of nannofossils at the earlier er depth than the beginning of the coastline progradation
stage of the sedimentary record in these cores indicates in M1 and M2 (Stage IV; Figure 5). Core CPF1’s persistsedimentation is marine, being initiated soon after the ent nannofossil record began between 8.9 and 8.2ka,coastline passed over these core locations. The different about 3.35 and 4kyr before M1 and M2, respectively. This
depths at which Cores M1 and M2 reached the substrate indicates that (1) marine sedimentation ended much earli(Figure 4) reflect their relative positions inside the Minho er in the inner sector of the Minho Estuary, which is plaupalaeo-valley. Due to these altimetric differences, marine sible, or (2) the age models for CPF1 and/or M1+M2 aresedimentation started later in M2 (-19m MSL; Stage III in not accurate enough.
In all these cases, the same general pattern of nannoFigure 4), a time when marine sedimentation was alreadyfossilabundance was obtained. A first stage (not regisdeveloping in M1, located in a deeper section of theteredatCPF1), in which the nannofossil record is sparse
palaeo-valley (Stage III in Figure 4). A short stage of
andirregular,
attributed to the beginning of marine-influweak and intermittent nannofossil record in the base of
enced
sedimentationin that region, is interpreted as a
M2 is interpreted as a palaeo-estuary sedimentation stage
more-or-lessopenestuary.
In the second stage, nannofosat this more external part of the palaeo-valley (Stage II in
sils
are
well
represented
andhave a persistent record,
Figure 4). Upwards, nannofossils soon become persistentmovingtowardsamarine-influenced
environment, whichin both cores.
isinterpretedasaflooded
ria.
The
third
stage is characUpwards, at -15m and -13m depth in Cores M1 and168

Guerreiro, Cachão, Drago

terised by the return to an irregular profile of nannofossil
abundance and diversity. This is interpreted as reflecting
sediment progradation and silt-up over the coastal area
due to a deceleration of sea-level rise, between 8ka (for
CPF1) and 5 to 4ka (for M1 and M2). This time-interval
is in agreement with the first symptoms of sand-barrierconfinement in coastal lagoons on the SW Portuguese
coast, registered at 5.4ka (Cearreta et al., 2003; Freitas etal., 2003). This third stage is interpreted as a return to an
estuarine sedimentary environment. A fourth stage, better
documented at CPF1 than at M1 (not analysed in M2), ischaracterised by a sequence of barren samples, which isinterpreted as reflecting fluvial sedimentation (CPF1) orcoastal drift-driven sedimentation (M1, M2, D1A and
D1B).
Samples from Core CPF1 contain higher absoluteabundances of all species in samples from its palaeo-estuary stage (Stage IV; Figure 5) than from the Present Day
Minho Estuary, except for G. ericsonii. This means thatthe palaeo-estuary probably was broader than the Present
Day estuary, with lower sand content, and therefore providing better conditions for nannoplankton deposition.
Abundances in Core CPF1 are one order of magnitude
higher than those registered at M1 and M2. These higher
values are related to the marshy conditions of the CouraRiver, from which Core CPF1 was recovered. This regionis characterised by low hydrodynamics, with a propensity
for silt and clay deposition. On the contrary, Cores M1
and M2 were recovered off the Minho river-mouth, in ahigher-energy environment, interpreted as the palaeo-valley of the Minho River. Although closer to the ocean,Cores M1 and M2 contain less abundant and diverse
nannoplankton assemblages, indicating that near-bottomhydrodynamic conditions are the primary parameter controling settling and the fossil record in coastal environments. This corresponds to favourable conditions for nannofossil deposition in an inner estuary, compared to thecoastal region.
5.2.2 Douro Estuary
Cores D1A, D1B and D2 combined exhibit CAI abundances and general distribution patterns similar to thoseobserved in the cores from the Minho Estuary.
Abundances within the Douro Estuary (a few tens of nannofossils per short traverse) suggest an environment inwhich marine sedimentation was beginning to dominate
the region (Stage II; Figure 9), but which was still showing significant ecological restrictions, compatible with alagoon system or confined estuary of some sort. An
increase in abundance of two to three orders of magnitude
occurred between -12m and -10m (Core D1B), suggestingthat the previously restricted marine conditions developed
into open-marine sedimentation (Stage III; Figure 9). Thisorder of nannofossil abundance shows similar magnitudes
to typical Present Day inner shelf marine environments,
with the exception that, in this case, assemblages aremainly dominated by H. carteri (>80%). According to theCore D1B sedimentary record, environmental conditions
must have changed drastically soon after this high abundance of H. carteri in the Douro Estuary, probably as aresult of the development of a sand-bar (Naughton et al.,
2002; Figure 9).
5.3 The opportunistic behaviour of
Helicosphaera carteri in coastal
environments
The high abundance of H. carteri reported above (Figure12) is interpreted as opportunistic behaviour in a coastal
environment since: 1) it completely dominates the assem-

10µm
Figure 12: Example of the abundance of helicoliths (seen in crossednicols), together with terrigenous minerals, in Sample -10.71m (Core
D1B, Douro Estuary; for location see Figure 2)

blages, reaching 84.4% and abundances of 2.4x108 coccoliths/g, relative to the silty-clay fraction, or 3.2x107 coccoliths/g, relative to the bulk sample (Core D1B, Sample-10.71m); 2) there is no known current mechanism able toeither separate a single nannoplankton species from offshore, or to resuspend a single species from nearby
marine sediments, for deposition elsewhere. Additionally,
there is no possibility of reworking from upstream, since
the geological units carved by the Douro River watershed
are Palaeozoic metamorphic or magmatic lithologies(Cachão & Moita, 2000); 3) samples from surrounding
offshore areas show relatively lower amounts of helicoliths. In fact, samples from the oceanic domain of seamounts close to the Madeira Archipelago (32˚14’N,
16˚51’W) disclosed abundances of helicoliths of around7x107 coccoliths/g. Samples from the Galicia Sea-mountCore N3KF21 (30-32cm; Figure 13A) disclosed 4.3x107
coccoliths/g, while top-core samples from MD95 2040
(Figure 13A) disclosed 1.2x108 coccoliths/g (Cachão etal., 2002); 4) previous study of the Portuguese continen-Calcareous nannoplankton as a tracer...Portugal

169

Ria (at Galici; Figure 2),
north of the Minho Estuary,show a brief interval (fromabout 1693 to 578yrs BP),in which increases in H.
carteri relative abundance
have
been
recorded,although never reaching>20% of the total assemblage (Alvarez, 2000). In
addition, the two moments
in which H. carteri increases in abundance inside theVigo’s ria and the Douro
Estuary do not coincide intime, which indicates these
are local, rather thanregional, events.
From the evidence
given above, it is suggested
that the preferential predominance of H. carteri in
the Douro Estuary cannot
be attributed to particularly
Figure 13: Statistical data of H. carteri percentages (assemblages of 300 coccoliths >3µm length), surfacefavourable
offshore oceanic
sediment samples, Minho, Douro, Tagus and Guadiana Estuaries. Location map of Cores N3KF21 (Galicia
Sea-mount), MD95 2040 (~WSW of Douro Estuary), and from a sea-mount in the Madeira Archipelago.
or neritic marine condiData for Guadiana Estuary from Ferreira et al. (2005); data for Minho, Douro and Tagus Estuaries fromtions, but rather to local
Orlando (unpublished report)
particular and specificpalaeoenvironmental contal shelf has revealed that H. carteri is more abundant inditions
insidetheestuary
thatallowed the proliferation of
surface sediments from northern sectors (~30 to 40%)H.
carteriinsuchatransitional
domain. At the present
than in the south (<20%; Cachão, 1993). H. carteri,
stageofknowledge,acertaindegreeof confinement andalthough fluctuating, is also revealed to be one of the twonutrient
enrichment
issuggestedashavingprovided suitmost abundant nannoplankton species represented in theableconditions
thatallowedtheopportunistic
behavior ofcores recovered off the Minho Estuary and FF1GM92
H.
carteriinthese
coastalareas.
(Figure 11), but never reaching abundances of >50%.
Complementary data from the immediately surrounding
areas of the estuaries of the Minho, Douro, Tagus and 6. Conclusions
Guadiana (Figure 13B) indicate a tendency for higher val- Although calcareous nannoplankton are mostly confinedues of helicoliths in the northern estuaries compared to the to oceanic environments, the results obtained from thesouthern ones. On the other hand, sediments from the NW Portuguese littoral zone indicate that certain taxa cannorthern inner shelf have shown that H. carteri does not be used as proxies for interpreting marginal palaeoenvihave a distinct record in sediments affected by river out- ronments. Here, specifically, we have used them to interpret fluctuations between the relative predominance oflets, unlike C. pelagicus (Cachão & Moita, 2000).
These findings raise new questions concerning the oceanic versus terrestrial sedimentation that have affectedpalaeoecological behaviour of this species. H. carteri is the Portuguese coast as a result of eustatic sea-level risecommonly present, both as an extant and fossil compo- that followed the last deglaciation.
Core FF1GM92, recovered from the Portuguese midnent, in Quaternary coastal and oceanic thanato- anddlecontinental shelf, contains typical shelf calcareous
taphocoenoses. In the oceanic domain, its relative abunnannofossil
assemblages. The presence of both temperate
dances rarely exceed 20%, with maxima in the(Gephyrocapsa
muellerae, Emiliania huxleyi) and submesostrophic Eastern Equatorial Tropical Divergence belt
tropical(Gephyrocapsa
oceanica, Calcidiscus lepto(Ziveri et al., 2004). Regions influenced by river disporus)
speciesisinaccordancewith the location of thecharge (e.g. the estuarine Ebro River: Cros, 2002), with
shelf
in
the
marginal
eastern
Atlantic, between the
low salinity and nutrient-richness (Giraudeau, 1992), or
TemperateandSubtropical
Biogeographic
Zones.
that show moderate to high coccolithophore productivity
With
respecttothesolution-proneE.
huxleyi andrates (Ziveri et al., 1995, 2004), namely, the upwelling
Gephyrocapsa
ericsonii,
theirpresenceinourmaterial,
area off NW Africa (Ziveri et al., 2001), all have highandindeedtheirsimilarabundances
to
the
more
robustabundances of H. carteri. Cores recovered from the Vigo

170

Guerreiro, Cachão, Drago

Gephyrocapsa oceanica, G. muellerae andHelicosphaera carteri, allows us to inferthat the nannofloras reflect depositional
conditions rather than post-depositional
diagenesis. This has allowed us to use theassemblages to interpret variations in themarine influence through time on our sampled locations.
The unexpectedly high numbers of H.
carteri in the Douro Estuary are probably
related to a degree of local environmental
restriction, which may have concentrated
nutrients, thus allowing what we interpret
to be opportunistic behaviour.
Marine-driven sedimentation within the
Present Day estuarine environments of NW
Portugal has changed through time, as indiFigure 14: Schematic diagram representing the proposed Holocene palaeoenvironmencated by variations in counts of the nantal evolution model for the studied region
nofloral assemblages, which were similar
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The authors wish to express thanks to Pedro Fernandes, Sílvia
Serina and Selma Gabriel (IPIMAR) for yielding sedimentological data. To Áurea Narciso and Conceição Freitas (Dept.
Geology, Fac. Sciences, Univ. Lisbon) for their contributions
and generosity during the discussion of the data. The text further
benefited from extensive review by Luïsa Cros and Jackie Lees.
This work was funded by the project CANAL (FCT
POCTI/32724/PAL/2000).

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Taxonomic appendix

Family COCCOLITHACEAE Poche, 1913 emend. Young &Bown, 1997
Genus Coccolithus Schwartz, 1894
Coccolithus pelagicus sensu lato includes:
Coccolithus pelagicus (Wallich, 1877) Schiller, 1930
Coccolithus pelagicus subsp. braarudii (Gaarder, 1962) Geisen
et al., 2002
Coccolithus pelagicus subsp. azorinus (Gaarder, 1962) Parente& Cachão in Parente et al., 2004
Order Isochrysidales Pascher, 1910 emend. Edvardsen &Eikrem in Edvardsen et al., 2000
Family NOELAERHABDACEAE Jerkovic, 1970 emend.
Young & Bown, 1997
Genus Emiliania Hay & Mohler in Hay et al., 1967
Emiliania huxleyi (Lohmann, 1902) Hay & Mohler, 1967
Genus Gephyrocapsa Kamptner, 1943
Gephyrocapsa ericsonii McIntyre & Bé, 1967
Gephyrocapsa muellerae Bréhéret, 1978
Gephyrocapsa oceanica Kamptner, 1943
Order Syracosphaerales Hay, 1977 emend. Young et al., 2003
Family SYRACOSPHAERACEAE(Lohmann,
Lemmermann, 1903
Genus Syracosphaera Lohmann, 1902
Syracosphaera pulchra senso lato includes:
Syracosphaera histrica Kamptner, 1941
Syracosphaera pulchra Lohmann, 1902

1902)

Order Zygodiscales Young & Bown, 1997
Family HELICOSPHAERACEAE Black, 1971 emend. Jafar &Martini, 1975
Genus Helicosphaera Kamptner, 1954
Helicosphaera carteri var. carteri (Wallich, 1877) Kamptner,
1954

For taxonomic references, see Jordan et al. (2004) and Monniot& Buge (1971).

Nannolith-bearing taxa

Kingdom Chromista Cavalier-Smith, 1986
Division Haptophyta Hibberd, 1972
Class Prymnesiophyceae Hibbert, 1976 emend. Cavalier-Smithet al., 1996

Family BRAARUDOSPHAERACEAE Deflandre, 1947
Genus Braarudosphaera Deflandre, 1947
Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre,
1947

Order Coccolithales Schwartz, 1932 emend. Edvardsen etal., 2000

Kingdom Animalia
Phylum Chordata
Subphylum Urochordata
Class Ascidiaceae

Family CALCIDISCACEAE Young & Bown, 1997
Genus Calcidiscus Kamptner, 1950
Calcidiscus leptoporus sensu lato includes:
Calcidiscus leptoporus (Murray & Blackman, 1898) Loeblich &Tappan, 1978
Calcidiscus quadriperforatus (Kamptner, 1937) Quinn &Geisen in Sáez et al., 2003
Genus Umbilicosphaera Lohmann, 1902
Umbilicosphaera sibogae senso lato includes:
Umbilicosphaera sibogae (Weber-van Bosse, 1901) Gaarder,
1970
Umbilicosphaera foliosa (Kamptner, 1963) Geisen in Sáez etal., 2003

Order Aplousobranchiata Berril, 1950
Family DIDEMNIDAE Giard, 1872
Fossil ascidian spicules
Genus Micrascidites Deflandre & Deflandre-Rigaud, 1956
Micrascidites vulgaris Deflandre & Deflandre-Rigaud, 1956