Particle size and magnetic properties of Holocene estuarine

Particle size and magnetic properties of Holocene estuarine
deposits from the Doñana National Park(SW Iberia):
evidence of gradual and abrupt coastal sedimentation
and L.
wirh 7 figures and 1 tabIe
Summary. High resolurion parricle size and environmental magnetic analyses were carried
out on two cores from marsh sediments in the Doñana National Park (SW Spain), in an investigation of gradual and abrupt coastal change. Complementary palaeontological analyses were
also undertaken. Whilst the balance between sea leve! change and fluvial sediment input is seen
to have a strong influence on the gradual evolution of the Guadalquivir estuary, via the deve!opment and degradation of spit barrier systems, two episodes of abrupt environmental change
are recorded by the input of coarser sediment to the back-barrier. These episodes would appear
to be associated with high energy conditions as a result of storm or tsunami events. The multidisciplinary methodology presented here enables differentiation between these two episodes,
the earlier of which (ca. 2400 yrBP) is probably due to a combination of enhanced erosion
potential of the barrier and tsunami activity, and the later of which is the result of climatic
instability during historical times (16th-18th centuries).
Zusammenfassung. Partikelgrofle und magnetische Eigenschaften '!Jan Estuarinablagerungen aus dem Halaún im Natianalpark van Dañana (SW Iberia): Beweis für graduelle und
abrupte Küstensedimentatian. - 1m Rahmen einer Untersuchung über graduelle und abrupte
Küstenveranderungen wurden Hochauflosungsanalysen bezüglich der Partike!gro/;e und
magnetische Umgebungsanalysen in zwei Kernen aus Sumpfsedimenten im Nationalpark von
Doñana (SW Spanien) vorgenommen. Zusatzliche palaontologische Analysen wurden ebenfalls durchgeführt. Wahrend das Gleichgewicht zwischen den Veranderungen auf dem Meeresspiegel und der Zufuhr von Flu/;sedimenten, über die Entwicklung und den Abbau von Barrieresystemen wie Sandbanke, einen gro/;en Einflu/; auf die graduelle Evolution des Guadalquivir-Flu/;bettes zu haben scheint, werden zwei Episoden abrupter Umweltveranderung
durch die Zuführung von rauherem Sediment in die hintere Barriere verzeichnet. Diese Episoden scheinen mit Bedingungen hoher Energie als Ergebnis eines Sturmes oder einer Tsunami zusammenzuhangen. Die multidisziplinare Methodologie, die hier vorgestellt wird,
ermoglicht eine Differenzierung zwischen diesen beiden Episoden, von denen die erste (ca.
2400 yrBP) wahrscheinlich auf eine Kombination von erhohtem Erosionspotential der Barriere und der Tsunami-Aktivitat zurückzuführen ist, und die zweite als Ergebnis klimatischer
lnstabilitat wahrend bestimmter historischer Abschnitte anzusehen ist (16.-18.Jahrhundert).
Résumé. Taille et propietétés magnétiques de les dépóts de l'estuaire holoeene du Pare National de Doñana (SW Iberia): évidenee de sédimentation littoral brusque ou graduel. - Dans le
Pare National de Doñana (SW d'Espagne), les carottes de deux sondages effectués dans la
marais ont fait l'objet d'une détermination de taille et d'une analyse magnétique environnementale, et ce, dans le but de rechecher sur les échanges brusques ou graduels du littoral. Ce
travail a été complété par une analyse paléontologique. Puisque le bilan entre las oscillations
du niveau de la mer et la sédimentation fluvial a une influence considérable sur l'évolution de
j'estuaire de Gualquivir, via le développement ou la dégradationde la barriere littoral, on a
identifié deux épisodes d'échange environnemental avec l'enregistrement de sediments grossiers derriere la barriere. Ces épisodes sont liés ade fortes conditions hydrodynamiques, résultat de tempetes ou de tsunamis. La méthodologie multidisciplinaire présentée dan s ce rapport
a permis de différencier entre ces deux épisodes; le premier (2,400 yrBP) est probablement dti
a la combinaison de l'érosion potentielle et l'activité du tsunami, et le deuxieme est le résultat
d'instabilité climatique récent (16 éme au ls éme siecle).
Introduction: the Guadalquivir marshland and barrier system
The Doñana N ational Park is situated in the mouth of the Guadalquivir river (SW
Spain) and is characterised by marshland and an extensive spit barrier and dune complexo The Guadalquivir marshlands are the result of the progressive infilling of the Guadalquivir estuary during the Holocene due to the development and progradation of a
spit barrier system at its mouth, as observed in most of the estuaries of this area (fig. 1).
This sector of the coast is a wave-dominated, mesotidal environment with southwesterly prevailing winds and a west-to-east littoral drift that beco mes reoriented to
the south-east near the Strait of Gibraltar due to the orientatÍon of the coast.
The study of the Holocene evolution of the Guadalquivir estuary and other
adjacent estuaries (fig. 1) have been described by several authors (ZAZO et al. 1994,
LARIO et al. 1995, GOY et al. 1996, LARIO 1996, LARIO et al. 1996, RODRIGUEZ RAMIREZ 1996, RODRIGUEZ RAMIREZ et al. 1996, DABRIO et al. 1999), fram which it has
been possible to reconstruct the general pattern and sequence of Late Pleistocene and
Holocene deposition.
Around ca. 18000 yrBP sea level was between -125 and -120 m relative to the
present level (HERNÁNDEZ MOLINA et al. 1994). This was followed by a rapid rise
which started approximately ca. 16000-14000 yrBP, when sea level was displaced by
100 m or so to about -25 to -20 m by 9600 yrBP. This datum is recorded at the base
of the organic and peat deposits in cores from the Guadalete and Odiel estuaries
(DABRIO et al. 1995, GOY et al. 1996, LARIO 1996, ZAZO et al. 1996a, DABRIO et al.
1999). From 9600 yrBP until at least ca. 8000 yrBP, a deceleration in the rate of sea
level rise occurred, as recorded by the organic and peat deposits and shelly lags found
in various cores (DABRIO et al. 1995, GOY et al. 1996). A change in the environmental magnetic properties and grain size parameters in these cores has been related to
changes in the dynamics of the depositional environment (LARIO 1996). The deceleration in sea-Ievel rise to at least ca. 10000 yrBP has been related to the Younger Dryas
event (LARIO 1997) and to the sea level stÍllstand that has been registered during this
event in the North Atlantic (MAYEWSKI 1994, STANLEY 1995).
After 8000 yrBP, a renewed acceleration in the rate of sea level rise rate took
place. In the estuaries of the AtlantÍc coast, maximum marine conditions were
reached between 7600 and 7100 yrBP (GOY et al. 1996, DABRIO et al. 1999), and may
Gulf of Cadiz
DOÑAN'" H. . '.
La Atunara
60 Km
Gibraltar strait
Fig. 1.
Location of the study area and core sites.
spit barrier
morphosedimentary units
"' ~I
• Sanlucar de Barrameda
have continued until ca. 6800-6500 yrBP (LARJO 1996). At ca. 6500-6000 yrBP a further deceleration in the rate of sea level rise is recorded, or perhaps even a fall, as
reflected in the input of river sediments to the coastal zone, which is identified from
the enviranmental magnetic properties and grain size analyses carried out in sediments fram Guadalete marshlands (Bay of Cadiz), as well as fram a freshwater input
as revealed by the isotopic signatures determined fram analyses carried out on microand macrofauna from the same sediments (DABRIO et al. 1999).
As has been observed in the interpretation of sorne sea level curves from around
the world, once the transgressive maximum is reached, a relaxation of the glacioeustatic component is praduced and redistribution of oceanic water becomes the
dominant factor influencing the nature of sea level change (MORNER 1994, 1996). This
is responsible for the development of the morphosedimentary units present in this
littoral area and, equally, for the evolution of coastal morphology. In this area, a series
of negative and positive oscillations in sea level occurred and, together with the sedimentary dynamics at this time, favoured the development of spit barrier systems
(ZAZO et al. 1994, LARJO et al. 1995). ZAZO et al. (1994) proposed that the pro gradation of the barriers developed in two main phases separated by a large erosional gap
related to clima tic instability. The first of these phases comprises morphosedimentary
units H¡ and H 2 , and the second is made up of units H3 and H 4 (fig. 1). The morphosedimentary units consist of spit bar units that contain sets of beach ridges separated
by large swales (i. e. gaps) or erosional surfaces that truncate beach ridges and swales.
Radiocarbon data from this and later papers date the large erosional gap between the
first and second progradation phases to between 2600 and 2350 yrBP and at same
time a change between agradation sedimentary conditions to progradation conditions
have been also observed (GOY et al. 1996, ZAZO et al. 1996b). After this event, the second progradation phase started and the build up of large systems of spit barriers in
the coastline reduced the connection of the estuary with the open se~. Simultaneously,
the back-barrier evolved progressively through tidallagoons to its present-day stage
dominated by tidal flats and marshes (GOY et al. 1996). An erosional gap has also been
identified in this second phase between morph6sedimentary units H3 and H 4, as
defined by ZAZO et al. (1994), that has been dated to between 800 and 500 yrBP (GOY
et al. 1996, LARJO 1996).
In addition to gradual change, the region is also susceptible to abrupt events. The
Gulf of Cadiz is an are a of high seismicity associated with the Azores-Gibraltar plate
boundary that forms the westernmost part of the lithospheric boundary between the
Eurasian and African plates. This boundary defines a zone of earthquake activity,
aligned roughly east-west, that extends from the mid-Atlantic ridge near the Azores
toward the Strait of Gibraltar (LOPEZ ARROYO & UDIAS 1972, UDIAS et al. 1976,
BUFORN et al. 1988). Seismic events in this area are also responsible for the generation
of tsunamis, which are recorded in both historical and modern instrumental data
(GALBIS 1932 1940, MEZCUA & MARTÍNEZ 1983, CAMPOS 1992). Several papers
describe coastal changes generated by the tsunami associated with the 1755 AD Lisbon earthquake (MARTÍNEZ SOLARES et al. 1979, ANDRADE 1992, DAWSON et al. 1995,
DABRJO et al. 1997), but sedimentary and/or morphological records of other paleotsunamis have not yet been found in this region.
This paper focuses on the importance of low-frequency, high-magnitude events
(storms and tsunamis) in the Holocene evolution of the Doñana coast. To this end, a
multi-disciplinary methodology has been applied to the sedimentary record in the
back-barrier in an attempt to distinguish between gradual and abrupt coastal change
and, if possible, between the sedimentary record of storm surges and tsunami events.
M ethodology
Two cores were obtained by means of an Eijkelkamp percussion drill and a Russian
corer. Core LL (Lucio del Lobo) reached a depth of 7.30 m and core LP (Lucio del
Pescador) extended to a depth of 7.60 m (fig.1). Core LL was sampled every 20 cm
and core LP every 10 cm. High resolution particle size analysis was completed on
these samples using a Coulter LS 130 laser sizer. The software provided by Coulter
was used to calculate statistical parameters such as mean, mode and median using the
arithmetic method (Coulter Corporation 1992). Measurements in SI units were converted into phi (<p) using the formula <p = -log2D (grain diameter in mm). Skewness,
kurtosis and standard deviation (or sorting) were calculated using the methodology
of FOLK & WARD (1957), once the grain sizes at the relevant percentiles given by the
Coulter sizer were converted to phi. These parameters were then grouped into classes
(McMANUS 1988) as summarised in table 1. Although no universal model exists to
distinguish past depositional environments in sediments of this nature (McMANUS
1988), it is generally accepted that mean grain size and sorting are hydraulically controlled (GRIFFITHS 1967) and can be positively correlated with energy of the environment and the degree of sediment processing (LONG et al. 1996), providing that there
is no additional input of sediment or change in sediment provenance.
Table 1
Groups of particle size parameters following the criteria of McMANUS (1988).
Use of the parameters
Measures of common
size and average size
Measures of spread
about the average
sorting or graphic
standard deviation
Well-sorted sediments present low values
Well-sorted sediments present values 1
Measures of preferenti al spread to one side
of the average
Reveals the symmetry of a granulometric
distribution. Values between + 0.1 to - 0.1
show a symmetrical distribution.
Negative and positive values are related
to the amount of coarse or fine sediment
Measures of concentration or pcakedness of
the distribution of the
grains to the central size
Related both to the dispersion and the
normality of the distribution. The use in
geological interpretations is not clear
(BoGGS 1987)
Grain size analyses
...... ~ ...
[lll coarse-very coarse
\, \, medium/
~ fiI
shell/shell fragments
1 roots/roots remains
5 6 7
MEAN (phi)
8 6.5
0.6 0.8
MODE (phi)
8 ·0.4
1.2 1
40 60 80
The magnetic properties measured in each sediment sample were magnetic susceptibility, anhysteretic remanent magnetisation (ARM), saturation isothermal remanent magnetisation (SIRM) and reverse field remanence measurements at -20 mT,
-30 mT, -40 mT, -50 mT, -100 mT and -300 mT. Low- and high-frequency (0.47
and 4.7 kHz) magnetic susceptibility were measured with a Bartington MS2 susceptibility meter single-sample sensor. Fields of ARM were grown using a Molspin AE
demagnetiser. The AF peak field was 100 mT with a steady DC biasing field 0.04 mÍ
SIRM was generated in a Trilec pulse magnetiser with a field of 1000 mT. The reverse
fields were generated in a Molspin pulse magnetiser. At each stage sample remanences were measured using a Minispin low speed fluxgate magneto meter. All these
measurements were recalculated in relation to the weight of the sample and the following parameters and quotients were obtained: Magnetic 5u5ceptibility (X), corresponding to the low frequency (LP) magnetic susceptibility; XHP (high frequency
magnetic susceptibility), XPD and XPD% (frequency dependent magnetic susceptibility expressed as the difference between XHP and XLP as a mass-specific value and a
percentage of XLP: 100(XLF-XHP/XLF); SIR M; XARM (normalised anhysteretic remanent
magnetisation); XARM/SIRM; SIRM/X; SIR.1\1/XARM; XARM/X; backfield IRMs (reverse
field isothermal remanent magnetisations) expressed as a percentage of SIRM
(IRM_ zomT, IRM_ 30mT, IRM_ 4ornT, IRM- sornT, IRM-100rnT, IRM-30ornT); and HIRM
(hard isothermal remanent magnetisation, being the difference between SIRM and
IRM_ 30orn y). The use and interpretation of these parameters have been largely
described in previous literature (THOMPSON & OLDFIELD 1986, MAHER 1988, OLDFIELD 1990, HUTCHINSON 1993, PECK et aL 1994, STONER et al. 1996). The full range
of magnetic data are presented in LARIO (1996), but here we focus on the parameters related to magnetic grain size, the concentration of magnetic mineral s and mineralogy as used by Thompson and OLDFIELD (1986), Yu & OLDFIELD (1989) and
PECK et al. (1994).
Lithological data and particle size analyses
Lucio del Pescador
Based on the lithological section and grain-size analyses from core LP, the following
levels have been differentiated (LARIO 1996) (fig. 2):
level m. (top of the core) Compact brown silty-clays.
levell. Thin (mm) packages of carbonate sands and muds with remains of shell
fragments. Presence of evaporites and iron oxides. A small change in grain size is represented in the mean and median, as well as poor sorting in the mean/median ratio,
skewness and sorting.
leve! k. Homogeneous brown silty-clays. Homogeneity is recorded in all grain
size parameters.
Fig.2. Grain size properties of core LP. Arrows show high-energy episodes. Cf) Black cireles
show samples for dating. Radiocarbon AMS dates in roots and shells at the same leve! are
2490 ± 60 yrBP (UtC-4026) and 2490 ± 105 yrBP (reservoir effect corrected, UtC-4031) respective!y (modified from LARIO 1996).
leve! j. Violet clays with silt lenses. Intense root bioturbation and evidence of
pseudogley and hydromorphism.
leve! i. Grey clays with millimetre-thick lenses of very fine sands and silts.
Bivalve shell fragments are also presento
leve! h. Grey silty-clays characterised by scattered reduction spots at the base
with fine paralle! lamination. Intensive bioturbation in association With roots is
present towards the topo
leve! g. Compact grey silty-clays. The homogeneity of this leve! is represented
in all the grain size parameters.
leve! f. Clays with sorne irregular silt lenses and fine horizontallamination. A
leve! with greater abundance of clays is represented by a decrease in the grain size
(mean, median and mode). A sharp change in skewness, kurtosis and sorting at the
bottom of this leve! is also observed.
leve! e. Clays with silt lenses and fine horizontallamination. Well defined leve!
in all the grain size parameters; homogeneous and well sorted.
leve! d. Alternations of silts and silty-clays with horizontallamination. At the
top there is a change to dark silts that include sand lenses. This alternation is well represented in the mean, median and mode grain size parameters.
leve! c. Silty level. Better sorting is illustrated by the mean of the mean/median
ratio and sorting.
leve! b. Alternations of silts and homogeneous clays. At the bottom are some
laminations of silt and fine sands.
leve! a. (bottom of the core). Clays with laminations of bivalve shell fragments
and foraminifera. It is also possible to differentiate a leve! with reduction spots, and
a sub-leve! (al) characterised by a small change in grain size skewness, kurtosis and
sorting parameters.
3.2 Lucio del Lobo
Regrettably it was not possible to obtain a lithological section from this core, and the
samples available for grain size analyses have not enabled a detailed record to be
established. However, the characteristics of the measured grain size parameters show
a similarity with the data from core LP (fig. 3). Near the base of the core (7.00 m
depth), a leve! that records an input of coarse sediments, most of them shell fragments, is observed. Between approximate!y 5 and 4 m depth there is evidence of episodes of dynamic instability, as revealed by an increase in coarse material and by
bimodal grain size distributions. Similarly, there is an input of poorly-sorted coarse
sediments at approximately 1.40 depth. This may well be related to storm events, but
because coarser sediment influxes may also be due to changes in facies (i. e. channel
migration), coupled with the fact that graduallithological boundaries with the lower
and upper units are recorded, the interpretation of a storm event is somewhat equivocal. Indeed, channel migration or reorganisation of fluvial network would also be
induced by high energy episodes.
Grain size analyses
~< \
MEAN (phi)
MEDIAN (phi)
3 4 5 6 7 8
MODE (phi)
Grain size properties of core LL (modified from LARro 1996).
0.2 0.7 0.8 0.9 1.0 1.5
2.5 O
coarse-very coarse
40 80 80 100
soils and
Environmental Magneti8m
o 20 40
~~c:'.-~ lransil from
ope~ esruary lo
. reslneted estuary
O 10 20
and slorms
IRM reverse
50 100
O 20
40 60 80 100
~ ........ 1.....
I\I( . . .
, b-l.dr,ICb-rl.CT_,r,b-,,,,b-r-l,::rr::b+:,::r,,c±r-~''''.,:ln-cl¡::;-r::,b-"d,Ct,::r.,c:::ln
.. _:h!-::ol;
1c' 1i-=(-:,71)_-=r:::h,-=.I.~
, ~Tt-::ln:lr-::rt_;I=<Ic-"I-:±JT~'=+r;-=,
' ~rl-::~Til(=ir,=<lI--j¡-,'}=+¡~-:::rr18+±rf-~=,"-"=.~rr.W=.,'Tl_,
8 ¡----
Fig, 4,
L!' ,
Environmental magnetie properties of eore LP, with high energy events being highlighted as in fig, 2 (modified from IID..ARIO 1996),
Magnetic analyses
Lucio del Pescador
The same levels as differentiated on the basis of the lithology and grain size characteristics can be recognised in sorne of the magnetic properties (fig. 4). Parameters
re!ated to magnetic grain size (X, XFD, SIRM, XARM, XARM/SIRM and XARM/X) and mineralogy (SIRM, SIRM/X, HIRM, Soft and IRM reverse fie!ds) reveal the most pr6minent changes. At approximate!y 7.10 m depth there is an increase in magnetic properties related to an increase in the concentration of magnetic minerals, the majority
of which would appear to be fine single domain grains, reflecting a change in the
dynamics of the depositional environment and/or the provenance of sedimento However, the magnetic properties can also be influenced by diagenesis and authigenesis,
as illustrated by the enhanced SIRM/X ratio and divergence of the reverse field IRM
values in this leve! and, hence, the presence of authigenic greigite (OLDFIELD 1990).
In our case this would appear to be associated with the high energy episode, as indicated by the particle size data, that occurred ca. 2400 yrBP, although it is more like!y
to be linked with the evidence of reductive diagenesis in leve! a. A subtle change in
the magnetic properties (SIRM/X, HIRM and IRM reverse), being more prominent
in the parameters re!ated to magnetic mineralogy (higher coercivity magnetic minerals such as haematite and goethite), is observed at a depth of ca. 3.5 m. This coincides
with the grain size and palaeontological data that record a change at this depth which
is linked to the transition between a marine-influenced estuary and closed estuarine
conditions, although a causallinkage is difficult to ascertain as the magnetic properties are very similar aboye and be!ow this leve!. The enhanced contribution from
higher coercivity magnetic minerals (HIRM) between a depth of 1.80 m and the top
of the core may be re!ated to soil deve!opment and hydromorphism processes. This
sequence is interrupted by a period of enhanced magnetic concentration, characterised by a high proportion of very fine-grained superparamagnetic grains (XFD), indicating a marked change in sediment provenance which, coupled ~ith the particle size
data, may represent flooding of the upper soil horizon, probably due to storm and/or
tsunami events.
4.2 Lucio del Lobo
A marked increase in the values of most of the magnetic parameters, characterised
by an increase in the concentration of magnetic minerals and a significant contribution from fine magnetic grain sizes, is observed at a depth of 6.5 m which is associated, as in core LP, with high energy sedimentary events (fig.5). Re!ative uniformity of the magnetic properties (even though there are sorne variations) is observed
upward to a depth of about 3.50 m. This is indicative of a change in sediment provenance and, coupled with the particle size data, in the energy of the deposition environment, probably due to the loss of a coastal sediment supply and to the predominance of river sediment input, as well as to the loss of coarse coastal sediments as
opposed to fine-grained material. Like core LP, the increase in the contribution from
high coercivity magnetic minerals and the reduction in the magnetic concentration
(primarily fine single domain grains) between depths of 1.50 and 0.50 m can also be
re!ated to soil deve!opment and hydromorphism, although the high energy event
----------------------Envlronmanlal Magnatlsm
soils and
(x10·'Am'kg') (x10';"\g') (~1d"mA')
O 125 250
O 100 200 O
O 10 20
X" "X
transit from
opan estuary to
restricled estuary
and storms
200 O
50 100
IRM reverse
O 20 40
60 60 100
Fíg, S,
Envíronmental magnetíe properties of eore LL (modified from LARIO 1996),
recorded in core LP is not observed here (probably because this core was sampled
every 20 cm).
Environmental interpretation and the relationship between the barrier
and estuary infilling
The plot of mean grain size against sorting (fig. 6a) reveals that most of the sediments
have been deposited in a transitional environment between partially open estuary and
closed basin conditions with a clear contribution of river input and sporadic high
energy events (TANNER 1991, LARIO et al. 1996, LARIO 1996). This confirms that the
estuary was becoming enclosed during this period, and that it is possible to associate
this period of sedimentation with the second progradation phase observed in the spit
barrier systems dated to between 2350 yrBP and present day. This agrees with previous studies which suggest that from ca. 2400 yrBP the progradation of the barrier prevailed, with a progressive restriction in the communication with the open sea beco ming more marked after ca. 800 yrBP (GOY et al. 1995, LARro et al. 1996, ZAZO et al.
A significant change in the depositional environment appears at a depth of
about 6.5-7 m, being characterised by relatively coarser sediment input and a change
in the magnetic properties. This may be indicative of a possible fluvial contribution
coupled with high-energy episodes that re-mobilised sediments and favoured the
progradation of the barrier. Indeed, more sediment wiU have been included in the
sediment budget from both fluvial input and erosion of the littoral units (sand cliffs,
spit barriers) from the AIgarve (South Portugal) and Gulf of Cadiz. Hence, the inner
estuary became infilled via sediment entrapment. The relative uniformity of the
measured parameters indicate continuous and stable sedimentation until a depth of
approximately 4-3.50 m, although minor variations enable the differeLltiation of
smaller levels and short episodes of higher energy, as reflected in the grain size analyses by coarse material input. Between 3.5 and 2.70 m (dependent on which of the
cores is considered), a significant change in fauna diversity and abundance have been
also recorded (GOY et al. 1995). In both cores, remains of benthic and planktonic
foraminifera, radiolarians, spicules of sponges, charophytes, ostracods, fragments
and spicules of echinoderms, briozoans, bivalves and gastropods have been found, as
well as frequent remains of plants. The presence of milliolids between depths of 3
and 5.50 m indicate a greater marine influence, since this group does not tend to be
developed where there are restricted marine conditions. Planktonic foraminifera are
relatively abundant in sorne samples, even though it is very probable that sorne of
these are reworked, as is indicated by the poor state of preservation. A similar conclusion can be made from the scarce radiolarians that appear. Remains of echinoderms appear only between 3 and 5.50 m, with a similar distribution to the milliolids, and appear to be related to an enhanced marine influence in the estuary. The sporadie presence of charophytes towards the top of the core LP indicates proximal conditions in the study area. From these data, it can be interpreted that the greatest
marine influence occurred between 3 and 5.50 m depth. Below these levels the
marine influence is less evident, even though the conditions were not so restricted as
those registered aboye 3m, where estuarine conditions prevailed with only a very
minor influence from the open sea. The reduction of autochthonous coccoliths in the
Closed basin
/ /
H.E.- High Energy
L.E.- Low Energy
and storm
mean «(jl)
Fig.6 a) Diagram of mean grain size versus sorting of sediments from eores LL and LP
(modified from LARIO 1996). The sedimentary environments are modified from the classifieation of TANNER (1991). b) This should be also interpreted in terms of ehanges in the hydrodynamie eonditions and energy of the depositional environment.
upper part of the eore supports this progressive restriction in 'the open sea influence
(LARIO 1996).
The changes in grain size and magnetic properties that appear at 4-3 m depth
which are associated with a reduction in the marine influence, may be correlated with
geomorphological changes observed at ca. 800 yrBP in the spit barrier systems by
LARIO et al. (1995) and GOY et al. (1996) that record a rapid development of these
systems and, consequently, restriction in the back barrier environment. This is also
related to a sudden marine fauna impoverishment and drop in diversity (GOY et al.
The relative uniformity in grain size parameters to a depth of approximately
1.50-1.00 m suggests a stability in the sedimentological conditions which correspond
to a marsh system influenced by fluvial conditions characterised by distal flood -plain,
distal crevasse-splay and levee sediments.
Moreover, a marked change in sorne magnetic parameters (X, XFD, XARM and
HIRM) observed between 1.50 and 0.50 m depth is indicative of a period of environmental stability associated with soil development and hydromorphism. In core LP,
which is more influenced by channel dynamics, this level also contains an episode that
represents an inwash of coarse material and dissection of the upper soil horizon. This
may be associated with the high rate of sedimentation and barrier progradation
related to flood and storm episodes which occurred from ca. 500 yrBP, and particularly during 16th, 17th and 18th centuries (LARIO et al. 1996, ZAZO et al. 1996a). Final
infilling of the estuary took place under comparatively stable conditions, but equally
influenced by fluvial processes.
This interpretation can be correlated with the evidence obtained from the studies of the Doñana spit barrier system (ZAZO et al. 1994, LARIO et al. 1995). Of the
three morphosedimentary units observed in the Atlantic are a (H b H3 and H 4), the
two most recent units are recorded in Doñana, corresponding to the second progradation phase, and all three units are present in La Algaida. As noted previously, a large
erosional gap can be identified over the entire region. This is dated to between 2600
and 2350 yrBP and is probably a consequence of a moderate sea level rise, perhaps
linked with climatic instability and storm episodes (ZAZO et al. 1994, LARIO 1996).
These events would be the cause of the degradation of unit H 2 • At 4.00-3.50 m depth
in the cores, it would seem be that at least unit H3 carne to an end and, consequently,
the communication with the open sea was severely restricted. The almost total infilling of the estuary promoted the formation of hydromorphic soils during the last millennium. At the same time, the barrier continued to prograde, forming unit H 4 and
acting as a littoral sediment trap for the resulting spit barrier. In the areas most influenced by fluvial conditions (core LP, figs.2 and 4), a period of climatic instability,
dated to 400-500 yrBP, can also be identified as an episode that interrupted the phase
of hydromorphic soil development via the input of coarser sediment of different
Episades af coarser sediment input
As the bottom of core LP has been dated to ca. 2500 yrBP (GOY et al. 1996; fig. 2),
the sedimentary sequence registered in the cores encompasses the second progradation phase recorded in this littoral area. From the interpretation of particle size analyses, sorne episodes of coarser sediment input to the inner estuary during this' infilling
phase have been recognised (LARIO 1996, LARIO et al. 1996). These episodes are characterised by an increase in the mean grain size (fig. 2 and 3). The mean/median ratio,
usually with values close to unity (indicating well sorted sediments), reveals a marked
decrease in sorting at this time. Skewness, sorting and, to a lesser extent, kurtosis also
reveal a marked change in environment at these levels. These granulometric characteristics are typically bimodal distributions, as can be confirmed once the particle size
distribution diagrams of the samples are investigated (fig. 7). These particle size signatures are clearly identified in core LP, but they can only be inferred to be so for
core LL because it lacks the necessary resolution.
Episode 1 (located at ca. 7 m depth, figs. 2 and 3) presents the enhanced input of
coarser sediment, marked by an increase of silt and fine sand, as well by the presence
of marine shell fragments that were separated with a 740 !lm sieve before use of the
Coulter sizer. Episode 2 (located at 0.70 m depth in core LP and probably at ca.
1.20 m in core LL, figs. 2 and 3) also displays these granulometric characteristics, but
they are less marked and shell fragments are not presento As noted previously, these
characteristics are indicative of an increase in the hydraulic energy of the depositional
environment. A bivariate plot of mean grain size versus sorting, interpreted only in
terms of change of the hydrodynamic conditions (fig. 6b), reveals that these coarse
grained units were deposited under different hydrodynamic and energy conditions
than the remainder of the coreo Changes in magnetic properties are also registered in
Sample LP-67 (7.1 m depth)
Episode 1 of
coarse material input
E 2
Grain size (,....m)
Sample LP-5 (0.7 m depth)
coarse material input
Grain size (,....m)
Fig.7. Diagram of particle size analyses distribution of episodes identified in fig. 2. The bimodal distributions reflect the input of coarse sediment (arrows) associated with the high
energy of these events.
the same levels (figs. 4 and 5). The magnetie properties of the sediments are eontrolled
by the eomposition, eoneentration and grain size of magnetie minerals and refleet
subtle lithologieal variations whieh ean be interpreted in terms of ehanges in the
dynamies of the depositional environment (SroNER et al. 1994, SroNER et al. 1995).
Also, these ehanges may refleet variations in sediment provenanee and/or transport
proeesses, as well as diagenetie overprinting of the de tri tal magnetie signature (SroNER et al. 1996). Moreover, an inerease in suseeptibility and remanent intensity often
eorrespond to obvious lithologieal ehanges (SUrTILL et al. 1982). Episodes 1 and 2
show a marked ehange in most magnetie properties, whieh are interpreted in terms
of ehanges in sediment souree and depositional energy. After the high energy Epi-
sode 1, the previous magnetic characteristics become re-established and reveal it as a
sporadic episode not likely to be related to changes in sea level or sediment budget.
In core LL, a marked change in magnetic mineralogy is observed at 6.2 m depth, but
this magnetic signature seems to be related to the presence of greigite (OLDFIELD .
1990). The magnetic properties related to Episode 2 are more complex, but thi~epi­
so de may be linked to changes in the upper sedimentary units (between 1.60 and
0.40 m depth) that seem to be related to the development of hydromorphism features
in the soils and to flood episodes that occurred in historical times (principally during
16th, 17th and 18th centuries) during periods of climatic instability, as is supported
by historical data (BORJA 1992).
From palaeontological analyses it would appear that Episode 1 of coarse sediment influx (at ca. 7.10 m in core LP) is related to the input of shell fragments and an
increase in echinoderm fragments. After this episode, communication with the sea
was re-established, as revealed by the presence of brackish species of foraminifera and
diatoms (LARIO 1996). At ca. 4-3 m depth, every trace of marine conditions disappears and the total infilling of the estuary under marsh conditions would appear to
have taken place.
Episode 1, centred around 2400 yrBP (ca. 2600-2500 CalBP), is clearly of
marine origin and is probably associated with the high energy eros ion episode that
has been recorded in all the spit barrier systems from this littoral zone as a large erosional gap between different morphosedimentary units (ZAZO et al. 1994, LARIO et
al. 1995, GOY et al. 1996, LA..RIO 1996). This episode is accompanied not only by input
of silt and coarse sediments (that exhibit bimodal and multimodal particle size distributions), as has been observed by DAWSON et al. (1995) in the Algarve (5 Portugal),
but also by the presence of shell fragments. Although the presence of multimodal particle size distributions or the input of coarse sediment provides a contradictory
record in the study of tsunami events (5HI et al. 1995, DAWSONl::t al. 1996), DAWSON
et al. (1995) found sedimentary units which they interpreted as tsunami deposits to
be extremely rich in shell fragments.
RODRIGUEZ RAMIREZ et al. (1996) described a phase of extensive erosion in the
Doñana spit barrier between the H 2 and H3 morphosedimentary units described by
ZAZO et al. (1994). This resulted in erosion of the Doñana H 2 and the isolation of the
La Algaida H 2 morphosedimentary units. Moreover, LARIO (1996), in the nearby spit
barrier system of Punta Umbria (fig. 1), found that a similar episode linked with a
large erosional gap dated to between 2600 and 2350 yrBP (2700-2400 CalBP) was
responsible of the segmentation of the spit and the initiation of a new drainage system
(Punta Umbria ria) that allowed the installation of the Roman fisheries. Due the
breakdown of the spit barrier, marine conditions were re-established in the estuary,
with an increase in fauna diversity and abundance (GOY et al. 1996). There is sorne
evidence to suggest that large-scale tsunamis are often associated with profound reorganisation of coastal barrier systems (BOURROUILH-LE JAN & TALANDIER 1985,
DAWSON 1994). ANDRADE (1992) found that tsunami inundation in the Algarve
region, associated with the Lisbon earthquake of 1755 AD, appears to have been
accompanied by widespread submergence, barrier breaching and the deposition of a
large washover fan, as well as the complete reorganisation of the back-barrier drainage system. Also associate with the 1755 Lisbon earthquake DABRIO et al. (1998)
found the same features affecting the Valdelagrana spit barrier (Gulf of Cádiz, fig. 1).
Even though all the morphosedimentary units of the spit barrier systems are
separated by an erosional gap, as has been observed in several geomorphologic studies (ZAZO et al. 1994, LARIO et al. 1995, LARIO 1996), there is no record of this erosional hiatus in cores from the back-barrier. However, the erosional gap dated
between 2600 and 2350 yrBP appears to be formed by the conjunction 6f various situations that provoked this evento Taking the evidence for a signifi~ant reorganisation
of coastal barrier system into account, there are indications of the occurrence, perhaps co-incident with a period of storm activity, of a very rapid high energy episode
such a tsunami evento This tsunami may not have been recorded as a consequence of
its magnitude, but rather as a result of the specific erosional conditions at this time.
This is supported by erosional conditions, linked with climatic instability (storm episo des) and slow sea level rise, that have been recorded over the entire south Iberian
littoral, not only in the Atlantic area that has been cited previously, but also in the
Mediterranean are a (GOY et al. 1986, SOMOZA et al. 1991, ZAZO et al. 1994, ZAZO et
al. 1996b). Historical data reveal that an earthquake in the Gulf of Cadiz took place
near the time of this episode, at 600 BC (GALBIS 1932, CAMPOS 1992), but the first
one noted in the historical record of tsunamis took place at 218 BC (GALBIS 1932)
and has been described by the same author as:
" .. .la isla de Cádiz y toda la Marina frontera de Andalucía padeció grandes terremotos o temblores [... ] la mar anegó muchos lugares que primero fueron descubiertos lanzando fuera de si multitud de pescados, unos comunes, y otros nunca vistos."
(" ... The Island of Cadiz and all the Andalucian marine boundary suffered large
earthquakes and shakes [... ] the sea flooded a lot of sites that before it were uncovered throwing up a multitude of fishes, sorne of them common and well known and
others never seen. ")
High resolution particle size analyses and environmental magnetic analyses have
proved valuable tools in the identification of both gradual and abrupt changes in the
deposition of estuarine sediments, particularly in terms of hydrodynamic conditions
and depositional energy. Classification of these sediments by means of a combination
of grain size data enable us to place the sequence obtained from the Doñana cores at
a transition between a partially closed estuary to a restricted and infilled es tu ary,
which corresponds with the regional sequence observed in other estuaries in the
region during the Late Holocene (ca. 2500 yrBP to the present). In these sequences,
rapid episodes of coarser sediment input have been recognised, which may be related
to high energy events. Changes in the magnetic properties also reveal simultaneous
changes in sediment provenance and processing.
High energy storm and tsunami episodes willleave similar sedimentary signatures in coastal deposits, but the combination of particle size analyses, environmental magnetic analyses, and geomorphological interpretation enable differemiation
between the two high energy episodes observed. In the Atlantic Iberian littoral, a
record of tsunami activity at ca. 2400 yrBP has been recorded as a rapid erosional episode. However, this eros ion potential has been amplified by the simultaneous occurrence of coastal eros ion linked with clima tic instability and a slight rise in sea level
rather than as a consequence of tsunami magnitude.
The authors should like to acknowledge financial support from Spanish DGICYT
Projects PB95-109 and PB95-946 and DGES Projects PB98-514 and PB98-2§5 and
Fundación Areces Project "Cambios climáticos y variaciones del nivel del mar ... "
This research is also a contribution to IGCP Project 43.7 and the INQUA Shorelines
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Addresses of the authors: J. LARIO, e ZAZO & L. LUQUE, Dep. Geología, Museo Nacional
de Ciencias Naturales, CSreJose Gutierrez Abascal, 2. E-28006, Madrid, Spain. (lario@ PLATER, Dep. Geography, University ofLiverpool. P. O. Box 147, L69 3BX Liverpool, UK. - J. L. GOY & F. J. SIERRO, Dep. Geologia, Universidad de Salamanca. E-37008 Sa1amanca' Spain. - eJ. DABRIO, Dep. Estratigrafía, Facultad de Geología, Universidad Complutense, E-28040 Madrid, Spain. - F. BOR}A, Area Geografía Fisica, Universidad de Huel"a, Campus del Carmen, E-21007 Huelva, Spain.
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