A multidisciplinary study

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/95
Comunicações Geológicas (2011) 98, 93-98
ISSN: 0873-948X; e-ISSN: 1647-581X
A multidisciplinary study of an organic-rich mudstone in the
Middle Holocene on the Northern coast of Portugal
Estudo multidisciplinar de um argilito rico de matéria orgânica do
Holocénico Médio na costa Norte de Portugal
H. Ribeiro1*, A. Bernal2, D. Flores1,3, J. Pissarra2,4, I. Abreu1,2, J. V. Romani5, F. Noronha1,3
Artigo original
Original article
Recebido em 06/06/2011 / Aceite em 21/07/2011
Disponível online em Outubro de 2011 / Publicado em Dezembro de 2011
© 2011 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: A multidisciplinary study was carried out in an organic-rich
mudstone level from Castelo do Neiva (Viana do Castelo) beach usually
covered by gravels and sands in the intertidal zone. Palynomorphs (pollen
and foraminifera) and wood macrofossil remains were analyzed and a
petrographic study of dispersed organic matter was performed. The
formation was dated, by 14C, from 5880 ± 60 BP, corresponding to
middle Holocene. The palynomorphs analysis revealed the absent of
foraminifera. Pollen results showed a clear dominance of Quercus
associated with Corylus and Alnus, as well as the presence of hydrophytes
species, such as Myriophyllum and Typha angustifolia, and the Lycophyta
Isoetes. The macroremains analysis was coherent with pollen results,
showing the presence of Quercus robur L. wood. The organic matter of
this level is at an early evolutionary stage compatible with a peat stage,
which deposition was taken in a forest swamp. The results showed the
presence of a fresh water environment occupying the area of the
nowadays beach, suggesting different climatic conditions and sea level
relatively to present times. At that time sea level was lower and climate
conditions would correspond to a period of colder winters and hotter
summers than present.
Keywords: Castelo do Neiva, organic-rich deposit, plant macroremains,
pollen, petrography.
Resumo: Foi estudada uma camada rica em matéria orgânica localizada
na zona intertidal, habitualmente coberta por cascalhos e areia, na praia
de Castelo do Neiva (Viana do Castelo). Efectuaram-se análises de
macrorestos de madeira e palinomorfos (pólenes e foraminíferos) e foi
feito um estudo petrográfico da matéria orgânica dispersa. A unidade deu
datações 14C de 5880 ± 60 BP, correspondendo ao Holocénico médio. A
análise de palinomorfos revelou a inexistência de foraminíferos. A nível
polínico verificou-se predomínio de Quercus associado a Corylus e Alnus
a presença de espécies hidrófitas, como Myriophyllum e Typha
angustifolia, e de Isoetes. Os macrrorestos identificados correspondem a
madeira de Quercus robur L.. A nível petrográfico verificou-se que a
matéria orgânica está numa fase evolutiva inicial compatível com uma
turfa. Os resultados mostraram a existência de um ambiente de água doce
onde hoje se localiza a praia, sugerindo diferentes condições climáticas e
um nível do mar mais baixo do que na actualidade.
Palavras-chave: Castelo do Neiva, matéria orgânica, macrorrestos de
plantas, pólenes, petrografia.
1
Centro de Geologia da UP, Rua do Campo Alegre, 687, 4169-007 Porto,
telephone:+351220402775.
2
Departamento de Biologia da Faculdade de Ciências da UP, Rua do Campo Alegre,
s/n, 4169-007 Porto.
3
Departamento de Geociências, Ambiente e Ordenamento do Território da
Faculdade de Ciências da UP.
4
BIOFIG – Centro de Biodiversidade e Genómica Integrativa & Funcional.
5
Instituto Universitário de Xeoloxia Isidro Parga Pondal, Universidade da Coruña.
Spain.
*Corresponding author /Autor correspondente: [email protected]
1. Introduction
Over the years intensive research has been performed using a
multiproxy approach in order to understand how past climate
changes affected sea level dynamics, ecosystems and to
reconstruct existing palaeoenvironments. A multidisciplinary
approach using, among others, geological records, botanical
remains or pollen distribution patterns allows to have more
precise indicators to reconstruct environment changes and mean
high tide level in the course of the Holocene (Freund et al.,
2004).
The Holocene is an epoch of particular interest since
climatic changes and human actions played a role in landscape
modifications (Messerli et al., 2000; Roberts et al., 2011). During
this time period, coastal zones were particularly vulnerable to sea
level fluctuations which influenced the available land for plants
(García-Amorena et al., 2007). Therefore cold climatic episodes
caused loss of diversity in northern Europe but allowed a refuge
for biota survival in central and southern parts of this continent
(Cheddadi et al., 1997; Carrión et al., 2010). Due to its vast
coast, Portugal occidental margin is an adequate local to the
study of sea-level fluctuations and impacts during the Holocene.
However, due to human development and land use intensification
it is not always easy to find undisturbed Holocene environments.
In the northern coast of Portugal, several Holocene remains
of peat bog sediments, ancient lagoon bottoms (Tijucas), and
soils of ancient forest (podsoils) have been studied aiming to
characterize the past vegetation and climatic conditions and to
relate them with sea-level fluctuations (Granja & de Groot, 1996;
Granja, 1999; Gómez-Orellana et al., 2001; García-Amorena et
al., 2007; Granja et al., 2010).
The aim of this work was study one organic-rich mudstone
found in the coastal region of Castelo do Neiva (Viana do
Castelo), where nowadays the beach is present. Through
geochronological, paleobotanical and petrographic studies it was
characterized the existing past vegetation, contributing to the
knowledge of mid-Holocene vegetation dynamics in Portuguese
coastal areas.
2. Study area
The studied organic-rich mudstone (Fig. 1) from Castelo do
Neiva (Viana do Castelo) beach (N 41º 37,472; W 8º 49,046 and
N 41º 37,353; W 8º 48,936) at north of the Neiva River was
observed in a low tide and a marine over-washing occurred, as it
94
is located on a level of the beach usually covered by pebbles and
sands. The beach is classified, by Soares de Carvalho et al.
(2006), as shingle beach, with beach-cusps consisting of quartzite
and black schist pebbles, fringed by rock outcrops in the
foreshore and a foredune. The bedrock consists on a black shale
of Ordovician age, outcropping as cliffs during low tide
presenting a N135ºE sub vertical foliation, sub parallel to the
shoreline. At East outcrops a Variscan two mica granite.
H. Ribeiro et al. / Comunicações Geológicas (2011) 98, 93–98
For pollen content studies the sample was submitted to
chemical treatment preparation for defloculation, to remove
humic substances and acetolysis with the aim to obtain a residue
rich in pollen. These procedures were done according to the
methodology proposed in Faegri & Iverson (1989). The residue
obtained was stored in phenolated glycerine. For pollen
identification and counting, slides were mounted and observed at
a transmitted light microscope (×400) along full lengthwise
traverses. Pollen counts for each type identified were then
converted into percentages of the total pollen counts. Pollen
grains were classified by morphological characteristics and
identified, where possible, by comparison with bibliographic
material (Reille, 1992; 1995; 1998).
3.3. Macroremain analysis
For histologic observation, collected macroremains were
numbered, resin-embedded, sectioned, according to well-defined
orthogonal planes (cross radial and tangencial) and observed
under a reflected light microscope. Samples were identified
according to the recommendations of the IAWA Committee
(1989) and using the identification key by Schweingruber (1990).
3.4. Petrographic analysis
Fig. 1. A and B - Outcrop of the organic-rich mudstone. C - Sample of the material
dated 5880 ± 60 14Cyr BP. The formation was observed in marine overwash in the
Castelo do Neiva beach (Viana do Castelo).
Fig. 1. A e B - Afloramento do argilito rico de matéria orgânica. C - Amostra do
material datado de 5880 ± 60 14Cyr BP. O afloramento foi observado na zona
intertidal na praia de Castelo do Neiva (Viana do Castelo).
3. Material and methods
3.1. Sample collection and dating
Sampling was conducted manually with a spatula after removing
the live biological material like algae and washing with the local
seawater to remove some sand. After, still on the sampling place,
the sample was washed with freshwater to remove any salt and
marine water, dried and enclosed in a plastic bag.
The geochronological data was obtained by 14C, performed
in the Tandem Laboratory at the Uppsala University (Sweden).
A continuous vertical sampling profile was not performed
because the organic-rich mudstone was exposed for only three
days during the summer time holidays.
3.2. Palynomorph analysis
The studied organic-rich unit was studied for foraminifera and
pollen. For both analyses, three sub-samples with a volume about
5 cm3 were separated from the interior of the collected sample, to
ensure that it was not exposed to air. Samples were dried at 60 °C
and weighed to compare equal amounts.
For foraminifera analysis, samples were sieved at 0.063 mm
and 1mm diameter and washed with distilled water until it was
clean according the methodology recommended by Boltovskoy
(1965). The sediment remaining in the sieve was stained with
methylene blue solution to facilitate the distinction of the
foraminifera shells. For foraminifera identification and counting,
was used a binocular microscope.
The organic-rich material was studied using organic petrography
techniques. The sample was ground, and homogenized following
ISO standard procedures. Thereafter, a general petrographic
characterization was carried out. The petrographic composition
and random huminite reflectance were performed following
standard procedures as well as ICCP recommendations (ISO
7404-2; ISO 7404-3; ISO 7404-5; ICCP, 1963, 1971).
The petrographic composition, established by maceral
analysis, was carried out by the point count method (500
individual determinations), using both reflected and fluorescent
blue lights, and the nomenclature adapted follows the
nomenclature of the ICCP System 1994 for the huminite and
inertinite maceral groups (Sýkorová et al., 2005, ICCP, 2001;
respectively), and Taylor et al. (1998) for the liptinite group.
Reflectance measurements were taken on the maceral ulminite B
using a Leitz Orthoplan microscope equipped with a DiscusFossil system under standard conditions.
In order to complement the petrographic composition, a
proximate analysis for the determination of moisture, ash and
volatile matter contents of the sample was carried out, in
accordance with ISO standards (ISO 589, 2008; ISO 1171, 2010;
ISO 562, 2010).
A complementary study by DRX (fraction <2 µm) was
performed, for identification of mineral content, at the LNEGIGM, using a PA118RMIN apparatus.
4. Results and discussion
4.1. Dating
The studied sample from Castelo do Neiva corresponds to a fine
dark material, containing organic matter, pollen, wood remains and
diatoms. This material (Fig. 1) was dated of 5880 ± 60 yr BP by
14
C, corresponding to Middle-Holocene. This time corresponds to a
period after the last glacial-interglacial transition and to the
beginning of the human impact (Roberts et al., 2011). At this time
sea level was about 4 m lower (Santos et al., 1993; Santos & Vidal
Romani, 1993; López Cancelo, 2004) and climate conditions
would correspond to a period of colder winters and hotter summers
than present, as well as atmospheric CO2 concentrations similar to
Multidisciplinary Study of a Mudstone
95
those levels observed prior to the industrial revolution (Cheddadi
et al., 1997).
An organic-rich mudstone also occurs on some units from
Aguçadoura Formation (Soares de Carvalho et al., 2006)
described as “a sedimentary sequence that includes a unit with
fine plastic and dark (wet) or ligh grey (dry) deposits, containing
peat and wood remains of trunks, branches and roots” and
occurring on different sites to south of River Neiva (Soares de
Carvalho et al., 2006). The geochronology of the Aguçadoura
Formation, by 14C dating, performed on different levels of peat,
indicated the age limits for this deposit between 360 ± 40 and
4270 ± 50 yr BP (Granja & Soares de Carvalho, 2003; Soares de
Carvalho & Granja, 2003). The age of the studied unit from
Castelo do Neiva has a similar age as the lower limit reported for
the Aguçadoura unit (Granja & Soares de Carvalho, 2003; Soares
de Carvalho & Granja, 2003), possibly indicating that the
Aguçadoura unit can be extended to the north of the Neiva river.
1997; Santos Fidalgo et al., 1997; Roberts et al., 2011), mainly
due to higher summer temperatures. Therefore, based on the
pollen record in Castelo do Neiva, it can be inferred the existence
of a high soil humidity environment by the presence of Alnus and
Salix, colder conditions than present ones during winter but
higher summer temperatures by the presence of Quercus, and
finally, indicated by the abundance of hydrophytes taxons the
existence of a permanent water environment.
Araújo (1991) reported, for the Portuguese coast a sea-level
highstand between 6800-5000 BP. The pollen record obtained in
Castelo do Neiva, suggests that this highstand level was lower
than the present sea-level, being consistent with the findings of
Costa Casais et al. (1996) which suggested that the Mougás
deposit level, dated of 5530 ± 60 BP, remained far from the
present hightide position.
4.2. Palynomorphs analysis
Tabela 1. Espectro polínico e respectivas percentagens observado nas amostras da
praia de Castelo do Neiva (Viana do Castelo).
%
Pollen type
%
Cupressaceae
Pinus spp.
Quercus deciduous
Corylus spp.
Alnus spp.
Salix spp.
Ulmus spp.
Olea europaea
Total
Erica spp.
Pistacia spp.
Sambucus type
Lonicera spp.
Hedera spp.
Total
Nymphaea
Myriophylum spp.
Typha angustifolia
Lemna spp.
Total
0.8
0.6
27.9
6.4
4.7
0.8
0.1
0.1
41.3
1.1
0.1
0.1
0.3
0.2
1.9
6.7
4.6
4.3
0.2
15.7
Poaceae
Urtica
Arum type
Cyperaceae
Chenopodiaceae
Lytrum spp.
Plantago spp.
Rumex spp.
Brassicaceae
Umbeliferae
Lamiaceae
Asteraceae Liguliflorae
Artemisia spp.
Fabaceae
Liliaceae
Total
Monolete
Trilete
Isoetes spp.
Total
16.4
9.5
3.0
2.1
1.8
0.9
0.9
0.7
0.4
0.4
0.2
0.1
0.1
0.1
0.1
36.7
1.1
0.3
1.2
2.7
Herbs
Pollen type
Pteridophyta
Trees
Shrubs
Hydrophites
Foraminifera results showed that the studied organic-unit was
sterile on these fossils, indicating the continental character of this
unit that never had a direct connection to the sea.
Pollen results showed a dominance of Quercus forest
associated with Corylus and Alnus, as well as the Lycophyta
Isoetes and hydrophytes, such as Nymphaea, Myriophyllum and
Typha angustifolia (Table 1).
During the Early-Holocene a progressive expansion of
Quercus and other mesothermophilous trees was reported in NW
Iberia, being observed a maximum extent of most European
forests approximately 6000 years ago (Carrión et al., 2010). This
timing matches the age of the Castelo do Neiva unit, where the
presence of a dominant arboreal vegetation can indicate the final
stages of a refuge for deciduous woods in the Iberian Peninsula
as already reported in other studies (Santos Fidalgo et al., 1997;
García-Amorena et al., 2007; Muñoz Sobrino et al., 2007;
Carrión et al., 2010).
In a study of the pollen content of the Mougás deposit
(Pontevedra district in Galicia) with similar isotopic age (5530 ±
60 BP at 83-110cm from the top) (Costa Casais et al., 1996), it
was reported the predominance of arboreal species (GómezOrellana et al., 1998), with similarities to that observed in
Castelo do Neiva. Also, at 75 km south from Castelo do Neiva, in
Cortegaça, it was reported a contemporary lagunar level (Granja
& de Groot, 1996) with arboreal prevalence of Pinus, the
presence of Quercus, Salix, Alnus and Corylus, as well as the
dominance of Ericaceae and Poaceae at the shrub and at the
herbaceous levels respectively (Gómez-Orellana et al., 2001).
However in Cortegaça taxa such Myrioplylum and Nymphaea
were absent, indicating different settlement conditions between
the two areas. In fact, material such as wood remains of trunks,
branches and roots and diatoms found at the Aguçadoura
Formation (south of Neiva River) (Soares de Carvalho & Granja,
2003) can reveal the presence of a system of coastal lagoons,
swamps and more extensive river estuaries. In this study the
pollen types found probably reveal the existence of swamp
conditions, possibly belonging to a unit older than the previous
ones, eventually representing a lower level of Aguçadoura
Formation.
The similarity found within these apart areas concerning the
dominance of arboreal vegetation around 6.000 BP, confirms a
general colonization of Pinus followed by Quercus along the
Northwest coast of the Iberian Peninsula.
Considering the climatic conditions, the period around 6000
BP is considered to represent a Holocene climatic Optimum, with
mean temperature values higher than today (Cheddadi et al.,
Table 1. Pollen spectra and respective percentages from Castelo do Neiva beach
(Viana do Castelo).
4.3. Macroremain analysis
From five collected samples, four corresponded to periderm
structures (rhytidome with some cells of phloem) (Fig. 2A and
B), which do not allow identification to the species level. The
other sample showed to be wood of Quercus robur L. (Fig. 2C to
G). This analysis was coherent with pollen results, being an
important additional information, since macroremains for species
identification are considered to give the most reliable information
on the type of local vegetation communities (Birks & Birks,
2000; García-Amorena et al., 2007).
Species description: Quercus robur L.
The wood sample, with evident growth rings and a
noticeable change in diameter of pores between early-wood and
late-wood, showed a ring-porous pattern with one file (rarely
two) of large pores in the early wood and small pores in the latewood distributed in diagonal pattern or radial files. Large vessels
in early-wood are occluded by tyloses and have a diameter about
105 micrometers. Vasicentric tracheids and aggregates of axial
diffuse parenchyma cells are visibles in radial section.
Homocellular rays are uniseriate and multiseriate (10 cells wide).
96
4.4. Petrographic analysis
Results from maceral analyses, reflectance measurement and
proximate analysis are shown in Table 2. Representative
microphotographs of the various maceral associations can be
seen on Fig. 3.
The reflectance of this sample is 0.21% indicating that the
organic matter of this organic-rich mudstone is at an early
evolutionary stage.
The organic fraction of this organic-rich mudstone is
attributed mainly to macerals of the huminite group (84%), with
16% of macerals from the liptinite group.
The sample comprises mainly humic tissues (telohuminite,
52%) represented by ungelified tissues (textinite). A high
percentage of these tissues exhibited fluorescence indicating the
presence of cellulose and the low grade of evolution of the
organic matter. The cellular lumens of these tissues were
normally found to be empty and occasionally filled with mineral
matter. Humic detritus (detrohuminite, 16%) as well as gels
(gelohuminite, 16%) were identified.
The liptinite content of this sample is 16% represented by
sporinite, with spore and pollen showing different shapes and
ornamentations, cutinite and liptodetrinite (Fig. 3). Resinite in a
H. Ribeiro et al. / Comunicações Geológicas (2011) 98, 93–98
globular form was also identified but not counted during the
maceral analysis. A few oxidized tissues were observed but no
inertinite was counted in this sample.
The mineral matter content of the organic-rich mudstone
from Castelo do Neiva is very high (70%) (Table 2), mainly
represented by clay minerals, muscovite and quartz, confirmed
by XRD. No pyrite was observed, which is indicative of the
absence of sulphur and the presence of fresh-water conditions
during the deposition of the organic matter.
The ash yield (66.6%) is similar to the mineral matter
content determined in the maceral analysis and the percentage of
the volatile matter is in agreement with the rank of the organic
matter.
Calder´s petrographic indices (Calder et al., 1991) slightly
modified by Hacquebard (1993): vegetation index (VI = 1.08; VI
= (Humotelinite + Fusinite + Semifusinite + Resinite +
Suberinite)/(Humocollinite + Humodetrinite + Inertodetrinite +
Sporinite + Cutinite + Alginite + Liptodetrinite + Fluorinite)),
and groundwater influence index (GWI = 1.90; GWI =
(Humocollinite + Humodetrinite + mineral
matter) /
(Humotelinite)); indicate that the organic matter was preserved
under wet conditions with a water level that always remained
above the topographic surface of the basin.
Fig. 2. Rhytidome and wood of Quercus robur L.. Phloem fibers and sclereids in transverse section (A) and phloem rays in radial section (B) (arrows). Transverse sections
showing early wood with larger pores than in the late wood (C) and pores in a radial pattern at the end of the late wood (arrows) (D). Tangential sections showing tyloses
(arrows) within a vessel member (E) uniseriate rays (ru) and a portion of the multiseriate ray (rm) (F). Homocellular ray in radial section (G). Bar = 100 µm.
Fig. 2. Ritidoma e madeira de Quercus robur L.. Secção transversal mostrando fibras do floema e esclereídeos (A) e secção radial mostrando raios floémicos (B) (setas).
Superfície transversal mostrando poros de maior diâmetro no lenho inicial (Primavera) do que no lenho final (Outono) (C) com um padrão radial dos poros do lenho final
(Outono) (setas) (D). Secção longitudinal tangencial mostrando um elemento de vaso com tilose (setas) (E), raios unisseriados (ru) e parte de um raio multisseriado (rm) (F).
Secção Longitudinal radial mostrando um raio homocelular (G). Barras de escala = 100 µm.
Multidisciplinary Study of a Mudstone
97
Furthermore, relatively higher values for the groundwater
influence index (GWI >1) confirmed this fact and justified the
absence of inertinite in this sample. The values found for these
petrographic indices suggest the deposition of the organic matter
in reotrophic conditions close to an inundated forest swamp.
Table 2. Maceral composition (vol. %, mineral-matter-free), mineral matter content
(vol.%), ulminite B random reflectances (Rr, %), standard deviation (s); and,
proximate data analysis (moisture, as received; ash, dry basis; volatile matter, dry
ash free basis).
Tabela 2. Composição maceral (% em vol., sem matéria mineral), teor em matéria
mineral (% vol.), refletância da ulminite B (Rr, %), desvio padrão (s); dados da
analise imediata (humidade, como recebido; cinzas, seca ao ar; matérias voláteis,
seco sem cinzas).
Telohuminite
52
Sporinite
6
Detrohuminite
16
Cutinite
7
Gelohuminite
16
Liptodetrinite
3
Total Huminite
84
Total Liptinite
16
Mineral matter
70
M, ar%
8.1
Rr, %
0.21
Ash, d%
66.6

0.04
VM, daf%
65.2
Fig. 3. Different aspects of the organic-rich sample. Textinite with wide open cell
walls occasionally filled with mineral matter (A and B); Sporinite, composed by
pollen and spores (C and D); Cutinite (E); Resinite in a globular shape (F). Bar = 50
µm.
Fig. 3. Diferentes aspectos petrográficos da amostra rica em matéria orgânica.
Textinite com vacuolos celulares ocasionalmente preenchidos por matéria mineral
(A e B); esporinite constituída por pólen e esporos (C e D); cutinite (E); resinite com
forma globular (F). Barra de escala = 50 µm.
5. Conclusions
The results of this multidisciplinary research point out that the
organic matter of the Castelo do Neiva mudstone is at an early
evolutionary stage, compatible with a peat stage, corresponding
to Middle-Holocene. Its deposition was taken in a rheotrophic
hydrological regime, where the water level always remained
above the topographic surface of the basin, in a forest swamp.
Climatically it was formed during a period considered to
represent a Holocene climatic Optimum, with annual mean
temperature higher than today. The presence of an environment
with woody and hydrophytes plants occupying the area of the
nowadays beach as well as the absence of pyrite suggests the
existence of a wetland environment without marine influence.
Acknowledgments
The first author benefits from a scholarship (SFRH/BPD/43604/
2008) financed by QREN-POPH and FCT. The authors would
like to thank Professors João Pais and Manuela Marques for
suggestions that improved the manuscript.
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