Geochemical, textural, mineralogical and micropaleontological data used for climatic reconstruction during the Holocene in the Galician sector of the Iberian continental margin

Reconstrucción climática durante el Holoceno en el sector gallego del margen continental ibérico: Datos geoquímicos, texturales, mineralógicos y micropaleontológicos

V Martins¹, F. Rocha¹, C. Gomes¹*, V Gomes¹, J. Jouanneau², O. Weber² and J. Dias³

¹ Departamento de Geociências Universidade de Aveiro Campus de Santiago 3810-193 Aveiro, Portugal. * E-mail: cgomes@geo.ua.pt

³ Universidade de Algarve, UCTRA Campus de Gambelas 8000 Faro, Portugal.

Recibido en junio de 2003;
aceptado en abril de 2004.

Abstract

In the present study, Al, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb and Cd concentrations were determined in the sediments of the KSGX 40 core collected from the Galicia muddy patch (NW Iberian continental shelf). The results were integrated together with textural, mineralogical and micropaleontological (foraminifera) data in order to determine the possible climatic influence on the type of sedimentation that took place in the muddy patch. The geochemical data of the KSGX 40 core suggest that sediment composition and texture were significantly influenced, over the last 5.2 kyr, by the Holocene sea transgression and by climatic oscillations consisting of both relatively temperate/cold periods and relatively warm/wet periods.

Key words: geochemical data, marine sediments, climatic oscillations, Holocene, Galicia.

En el presente estudio se han determinado las concentraciones de Al, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb y Cd en un sondeo de sedimento recogido en el depósito lodoso de Galicia (NW de la plataforma continental ibérica). Estos resultados se han comparado con datos texturales mineralógicos y micropaleontológicos (foraminíferos) para estudiar la influencia climática en la sedimentación de este depósito lodoso. Los datos geoquímicos del sondeo KSGX 40 sugieren que la composición de los sedimentos y su tamaño de grano han estado influenciados durante los últimos 5.2 ka por la transgresión holocénica del nivel del mar y por oscilaciones climáticas que consisten tanto de periodos relativamente templados/fríos como de periodos cálidos/ lluviosos.

Palabras clave: datos geoquímicos, oscilaciones climáticas, Holoceno, sedimentos, depósito lodoso de Galicia.

Introduction

In this work we present and analyze data from the Ocean Margin Exchange (OMEX) core KSGX 40, collected in the Galicia muddy patch (NW Iberian continental shelf) (fig. 1).

The OMEX project aims to gain a better understanding of the physical, chemical, biological and sedimentological processes that occur at the ocean margin of the European continental shelf in order to quantify energy and matter fluxes across this boundary.

The main features of the NW Iberian continental shelf that define the circulation, transport and deposition of fine sediments were discussed by Jouanneau et al. (1998) and Dias et al. (2002a, b). Araujo et al. (2002) studied the geochemistry of the sediments from both the Galicia and Douro muddy deposits and Oliveira et al. (2002) studied the distribution of clay minerals in the same sediments. The Iberian continental margin is affected by coastal upwelling (Frouin et al., 1990), responsible for the high oceanic productivity in the region.

In the present work, the distribution of Al, Ca, Fe, Mg, Cu, Pb, Zn, Cr, Co, Ni and Cd concentrations in sediments from the KSGX 40 core is discussed and related to textural, mineralogical and micropaleontological (benthic and planktonic foraminifera) data with the aims of checking for a possible climatic change record during sedimentation.

Materials and methods

The OMEX KSGX 40 core (164 cm long) was collected from the muddy patch (42°14'98" N, 09°01'01" W) located at the Galician continental shelf off Ría de Vigo (fig. 1), at a sea depth of 115 m.

The grain size analysis was carried out using a Malvern 3600E laser diffraction particle sizer, which provided the grain size distribution in the 0.05-878 µm size range. Calcium carbonate content was determined by a gasometric method.

To reduce the effects of sediment variable grain size, only the fine fractions (<63 µm) were analyzed. These fractions were obtained by wet sieving and dried at 60°C. Chemical element concentrations in the sediment samples studied were determined following the method proposed by Lecomte and Sondag (1980): 1 g of sediment was digested with a mixture of 3 mL of HCl (37%), 2 mL of HNO₃ (65%) and 1 mL of HF (40%). After drying at 120°C, the residue was mixed with 10 mL of HNO₃. The resulting solution was centrifuged, filtered and mixed with demineralized water. Determinations of Al, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb and Zn were carried out by flame atomic absorption spectrometry on a GBC 600 spectrophotometer.

Mineralogical studies were conducted on the <2 µm sediment size fraction using X-ray diffraction techniques. The clay fraction (<2 µm) was obtained by conventional sedimentation techniques, according to Stokes' Law. Qualitative and semi-quantitative mineralogical analyses of the clay fractions followed the criteria recommended by Schultz (1964), Barahona (1974), Thorez (1976), Mellinger (1979) and Pevear and Mumpton (1989).

Biostratigraphic and palaeoecological studies based on benthic foraminifera and the assessment of the relative contents of terrigenous and biogenic components were carried out using a binocular on the >63 sediment fraction of samples collected every centimeter along the core. We determined the evolution of the total abundance of endofaunal species characteristic of the inner and middle shelf (less than 50 m depth), which are markers of both high organic matter contents and low oxygen contents, as well as that of suspension feeders associated with temperate/cold waters. Also, species diversity was assessed using the Shannon-Wiener index (according to Shannon, 1948):

For ¹⁴C dating using AMS (accelerator mass spectrometry), 10-20 mg of foraminifera shells were separated from the sediment fractions >125 µm corresponding to the sediment layers of 39-40 cm, 69-70 cm and 134-135 cm, in the laboratory of Beta Analytic Inc., Miami, Florida (USA).

The data obtained were subjected to multivariate statistical analysis (principal components analysis and Pearson correlations) using the Statistica (v. 5.1) software.

Results

Carbon-14 dating provided ages for three sediment layers (table 1). The intercalated ages were estimated on the basis of these values. Core KSGX 40 contains sediments that were deposited during the last 5200 years BP.

The KSGX 40 core consists, from the base to the top, of a sedimentary sequence exhibiting gradual upward decrease of grain size (fig. 2). In the upper section of the core, approximately from the depth of 100 cm upwards, the fine fraction of sediment <63 (silt + clay) is abundant (60-90%), whereas the sand fraction (>63 µm) becomes more abundant in the lower section of the core (fig. 2).

The relative abundance of non-cohesive sediment particles presents values > 65% in the 33-55 cm sections and below 76 cm (fig. 2). The CaCO3 content ranges from 2% to 18% of the dry sediment weight and is higher in the lower section of the core (fig. 2). Foraminifera and mollusc shells are important sediment biogenic components, significantly contributing to the total sediment CaCO3.

The values of the Kubler index of illite crystallinity (Kubler, 1964; Segonzac, 1969) and the kaolinite/illite ratio are higher in two particular sections of the core, between 164 and 120 cm, and between 85 and 45 cm, and show a tendency to increase between 20 and 0 cm (fig. 3).

In the 164-115 cm section, higher contents of kaolinite and smectite occur. In the 115-95 cm section, the abundance of these clay minerals decreases, but illite and chlorite contents increase. In the 90-50 cm section, smectite content increases and kaolinite content shows the highest values despite the reduction of kaolinite content starting at 85 cm (fig. 3). In the sediments of the upper 50 cm of the core, chlorite persists in higher contents whereas smectite is absent or rare. Between 18 and 30 cm, the kaolinite/illite ratio decreases notably (fig. 3).

Pyrite is always present along the core, both in the sand fractions as framboidal sedimentary deposits and in pyritized shells of benthic foraminifera.

Benthic foraminifera assemblages along core KSGX 40 consist of autochthonous individuals (not showing transport marks) and of transported shells (broken or abraded).

The total number of foraminifera (benthic and planktonic) shells per gram of sediment is, as a rule, higher below the depth of 80 cm down to the basis of the core (<6500 shells per gram of sediment), this section also being characterized by a higher number of autochthonous benthic foraminifera and transported shells (fig. 4).

Hence, the number of foraminifera shells increases in the bottom section of core KSGX 40, where the sediment grain size increases, as does the number of species transported from both the inner and middle shelf and the diversity of benthic foraminifera species, expressed by the Shannon-Wiener index (fig. 4).

The Shannon-Wiener index values (or diversity of benthic foraminifera) decrease in the upper 70 cm of the core, i.e., in the last 2380 ± 100 years cal BP (fig. 4).

Throughout the core, the thanatocoenosis of benthic foraminifera is mainly composed of infaunal species (fig. 5), such as Bolivina dilatata, B. ordinaria, B. pseudoplicata, B. skagerrakensis, Brizalina spathulata, Bulimina exilis, Buliminella tenuata, Chilostomella oolina, C. ovoidea, Fursenkoina loeblichi, F. pauciloculata, Globobulimina spp., Nonionella bradyi, N. iridea, N. stella, N. turgida, Sphaeroidina bulloides, Stainforthia complanata, S. feylingi, S. fusiformis and Uvigerina peregrine (Corliss and Emerson, 1990; Corliss, 1991; Buzas et al., 1993; Rathburn and Corliss, 1994; Alve and Murray, 1995; Bernhard and Sen Gupta, 1999; Ernst, 2002).

The relative abundance of infaunal specimens increases from the base to the top of the core, but particularly from the 100-cm level up to the top of the core (fig. 5).

The total relative abundance of species like Ammonia beccarii, Buliminella tenuata, Chilostomella oolina, C. ovoidea, Eggerelloides scaber, Fursenkoina loeblichi, F. pauciloculata, Globobulimina spp., Nonionella stella, N. turgida, Stainforthia complanata, S. feylingi and S. fusiformis, with higher tolerance to significant oxygen depletion in pore water, shows either dysoxic or microxic environments due to high availability of organic matter in the sediment (Mullineaux and Lohmann, 1981; van der Zwaan and Jorissen, 1991; Sen Gupta and Machain-Castillo, 1993; Moodley et al., 1997, 1998; Bernhard and Sen Gupta, 1999; van der Zwaan et al., 1999; Rijk et al., 1999; Ernst, 2002). This group of benthic for-aminifera shows higher values in the core section of 90-50 cm (fig. 5), where the relative abundance of sediment fine fraction is higher (fig. 2). In the lower section of the core, the percentage of passive suspension feeders such as Cibicides refulgens, C. ungerianus, Discorbis mira, D. williamsoni, Discorbis spp., Dyocibicides bisserialis, Gavelinopsis praegeri, Hanzawaia nitidula, Lobatula lobatula, Paumotua terebra and Planorbulina mediterranensis, is higher (Murray, 1991; Vergnaud-Grazzini et al., 1989) (fig. 5).

Also present in the assemblages of core KSGX 40 is another group of benthic foraminifera, such as Amphicoryna scalaris, Bigenerina nodosaria, Bolivina albatrossi, B. difformis, B. dilatata, B. striatula, B. seminuda, B. robusta, B. skagerrakensis, Brizalina pacifica, B. spathulata, Bulimina exilis, B. aculeate, B. alazanensis, B. marginata, Cassidulina laevigata var. carinata, C. teretis, Cassidulinoides bradyi, Chilostomella oolina, Eggerella bradyi, Globocassidulina subglobosa, Hoeglundina elegans, Hyalinea balthica, Melonis barleeanum, M. pompilioides, Neolenticulina peregrina, Nonionella bradii, Sphaeroidina bulloides, Trifarina angulosa, Uvigerina peregrina and Valvulineria bradyana; however it is generally better represented in temperate/cold waters of outer neritic and bathyal environments (Pujos, 1976; Blanc-Vernet et al., 1984; Cearreta, 1986, 1989, 1994; Mathieu, 1986; Murray, 1991; Alve and Murray, 1994; Banner et al., 1994; Levy et al., 1995; Martins and Carapito, 1999; Mendes et al., in press). The total abundance of this group of benthic foraminifera is lower in the 90-50 cm core section and higher in the 115-95 cm and 30-18 cm core sections (fig. 5).

The chemical data determined are shown in figures 6, 7 and 8. All samples have Cd concentrations below the detection limit (0.05 mg kg^-1).

The analysis of the logs shown in figures 6, 7 and 8 allowed the definition of two main zones in the core in relation to the distribution of the chemical element concentration values: zone 1, above the 80 cm level, is characterized by higher (than below this level) concentrations of Fe, Mn and Co, and two concentration maxima for Fe, Mn, Zn, Co, Cu, Cr and Pb identified approximately at 80-50 cm and 20-0 cm; zone 2, below the 80 cm level, is characterized by higher Ca, Mg and Al concentrations that decrease at 80-50 cm and 20-0 cm, where the elements referred to in zone 1 show higher concentrations.

The values of Ca concentration decrease, as a rule, from the base to the top of the core, whereas Fe, Mn and Co concentrations show an opposite trend.

Spearman's correlations between chemical element concentrations and textural, mineralogical and micropaleonto-logical parameters subjected to principal components analysis (fig. 9), allowed the definition of the following groups of parameters:

• Group 1, associated with the sediment sand fraction (>63 µm), comprises the following parameters: Ca and CaCO3 contents, foraminifera density and total number of suspension feeders. These parameters show significant positive correlations between them, whereas significant negative correlations were found with the parameters of group 2.

• Group 2, associated with sediment fine fractions (<63 and <15 µm), comprises the following parameters: Fe, Mn, Cu, Zn, Pb, Co and Cr concentrations. These concentrations exhibit significant positive correlations with the endofauna of benthic foraminifera and with the total number of species related to high organic matter and low oxygen contents.

• Group 3, associated with the clay minerals chlorite and illite and with benthic foraminifera characteristic of cold waters, comprises the following parameters: Al, Ni and Mg concentrations, which show significant positive correlations between them.

• Group 4, associated with the clay minerals kaolinite and smectite, consists of parameters considered markers of relatively warm and wet climates that prevail in the source areas of clay mineral genesis.

Principal components analysis (fig. 9) allows the distinction of two main factors important for the definition of the groups identified. Altogether, factors 1 and 2 explain 71% of the variance of the data used; these factors are shown in table 2. Factor 1 represents the distribution of the parameters of groups 1 and 2, whereas factor 2 is related to the parameters of groups 3 and 4.

Factor 1 seems to be associated with sediment texture. On the one hand, heavy metal concentrations in the sediments increase as the fine fraction and organic matter contents increase and oxygen content decreases. On the other hand, coarse sediments deposited under more energetic hydrodynamic conditions, expressed by higher concentrations of suspension feeding foraminifera, seem to favour the preservation of CaCO3, expressed by higher Ca, CaCO3 and foraminifera shell contents in the sediments.

The concentrations of Ni, Mg and Al are included in group 3, which is related to periods of cold climate. It is not easy to explain this association; however, it is known that plankton uses these chemical elements and that oceanic productivity is, as a rule, higher during cold episodes. Therefore, it is possible that the increase of the concentration of these elements could be related to the accumulation of biodetritus as a consequence of high oceanic productivity.

Discussion

The gradual grain size decrease found in core KSGX 40 could most probably be due to the sea transgression occurred during the Holocene. According to Zazo et al. (1996), after the Holocene transgressive maximum, reached at 6900 years BP, a small drop in the sea level occurred, followed by a small rise in its level between 2400 years and the present.

Some climatic changes that occurred during the Holocene appear to have conditioned the marine sedimentation regime. Based on the analytical data available it was possible to establish three zones in core KSGX 40.

Zone 1

Zone 1, corresponding to the 164-90 cm section, is characterized by: (a) coarse grain size sediment; (b) higher Ca, CaCO3 and foraminifera (number of benthic and planktonic foraminifera per gram) contents; (c) higher Mg, Ni and Al concentrations; (d) lower Fe, Mn, Cu, Zn, Co, Cr, and Pb concentrations; (e) higher number of foraminifera species characteristic of the inner and middle continental shelf and higher number of suspension feeders; and (f) illite + kaolinite + smectite + chlorite assemblages indicating a general temperate and wet climate with intercalated oscillations prevailing during sediment deposition. The occurrence of a relatively warm and wet period, corresponding to the core section from the base to the 120-cm level, allowed the input of higher amounts of sediments characterized by coarser grain size into the oceanic system. Together with the terrigenous components, foramin-ifera were also transported from shallower zones of the shelf. The accumulation of coarse sediments on the outer shelf allowed the easier removal of sediment interstitial water, a fact that favoured the preservation of CaCO3 expressed by the relatively high Ca content found in sediments and by the fair preservation of foraminifera shells. This rainy period was identified in Europe by Bohncke and Vanderberghe (1991), Zolitschka and Negendank (1993), and Magri (1997).

In the sediment section between 120 and 90 cm, the high content of illite + chlorite indicates the occurrence of relatively cold climatic conditions that prevailed in the continent at the time of the genesis of these clay minerals, 3800-3000 years cal BP. From the base to the top of this section, a decrease in smectite content associated with an increase in chlorite content was recorded in this subsection. Also, a decrease, in this same subsection, of the kaolite/illite ratio, allowed the identification of an episode of cold and dry climate, at least partially responsible for a decrease in sand supply to the deposit studied. This climatic cooling could be related to the thermal oscillation, the "Lobben Stage", around 3600 years BP (Ehlers, 1996) that caused the expansion of the glaciers of the Austrian and Swiss Alps.

Zone 2

The chemical elements Fe, Mn, Cu, Pb, Zn, Cr, Co and Ni are usually adsorbed onto sediments, particularly onto clay minerals, due to both their high specific surface area and global negative electric charge. This fact supports the significant positive correlation found between sediment fine silt + clay (<15 µm) content and the concentration of these elements.

The main source of Al is lithogenic. This element participates largely in the structures of both primary and secondary aluminosilicates existing in weathered products, which are introduced into the oceanic system transported mainly by the rivers. However, Al contents show a weak positive correlation with the sediment fine fraction content and a significant negative correlation with the chemical elements of group 1. The content of Al is reduced in this zone of the core, evidencing a decrease in the transport of fine clayey terrigenous sediments that could be compensated by higher deposition of organic matter, mostly transported laterally. Benthic foraminifera assemblages support this assumption, since a higher number of individuals associated with high organic matter content and infaunal species were found in this core section. A high consumption of oxygen by benthic fauna led to an oxygen depletion in sediments.

Biological, physical, chemical and diagenetic processes can change the sediment properties (Zuo et al., 1991; Nolting and Helder, 1991). Reducing conditions at the sediment-water interface create the conditions needed for diagenetic enrichment (i.e. occuring after sediment deposition) of metals that undergo a redox change and become less soluble, or of metals that form insoluble sulphides. In this zone, Fe and Mn redox behaviour could play a central role in the cycling of other metals that can be adsorbed onto Fe and Mn precipitates.

Zone 3

Zone 3 corresponds to the 50-0 cm section of core KSGX 40 and to a climate similar to the actual climate: Sub-Atlantic (cold and wet). This zone comprises two sub-zones. Sub-zone 3.1

Sub-zone 3.1 corresponds to core section 50-20 cm and is characterized by: (a) a slight increase in the sand fraction and a slight decrease in the <15 µm fraction; (b) decrease of the kaolinite/illite ratio; (c) increase of chlorite content; (d) increase of Al, Ca, Mg and Ni concentrations; and (e) reduction of Fe, Mn, Cu, Pb, Zn, Cr and Co concentrations. The increased Al content expresses the higher input of fine clayey terrigenous sediments relative to organic matter, justifying both the better preservation of CaCO₃ and the higher Ca content values in the sediments. Less deposition of fine particles may have determined the lower contents of Fe, Mn, Cu, Pb, Zn, Cr and Co.

The higher illite and chlorite contents, as well as the higher content of benthic foraminifera associated with cold waters, present in the 30-18 cm core section, could be related to the climatic cooling episodeóLittle Ice Age, a neo-historical glacial fluctuation (between XVI and XIX centuries, according to Grove, 2001)óthat seems to have caused lower sea water temperatures. On the other hand, the 46-35 cm core section could be related to the warmer episodeóLittle Climatic Optimumó as described by Brown (1998). During these two climatic episodes, dry periods alternating with huge river floods would have occurred, expressed by the slight increase of the sand fraction content and Al concentration in the sediments.

Sub-zone 3.2

Sub-zone 3.2, corresponding to the 20-0 cm core section, is characterized by: (a) a notable increase of the sediment fine fraction (<15 µm); (b) increased Fe, Mn, Cu, Pb, Zn, Cr and Co concentrations; (c) the increase of the kaolinite/illite ratio; (d) decrease of chlorite content; and (e) decrease of Al, Ca, Mg and Ni concentrations. This section, corresponding to the subrecent period, shows similarities with the 90-50 cm section, with a higher accumulation of finer sediments and organic matter because of a tendency in the sea level to rise and lower hydrodynamics in the area where core KSGX 40 was collected.

Conclusions

The presence of pyrite in core KSGX 40 indicates the occurrence of periods of anoxic conditions just below the sediment-water interface and/or inside foraminifera shells. Pyrite is a diagenetic mineral common in anoxic environments. It is formed by reaction of the sulphate ion with the reduced form of iron (Fe²⁺). This process is mediated by organic matter reaching the sea floor and then being oxidized in a characteristic sequence of reactions (Kaplan et al., 1963).

The origin of Mn, Cu, Co, Fe, Zn, Cr and Pb in the sediments is mainly related to detrital inputs from continental soils and weathered rocks.

Taking into account the mineralogical and geochemical data determined, the grain size distribution and the total content of benthic foraminifera species, it was possible to establish the zonography of the KSGX 40 core. Such zonography indicates the occurrence of some climatic oscillations during the Holocene that conditioned the sedimentary record of the Galician continental shelf muddy patch.

Climatic oscillations that occurred during the Holocene and oceanic conditions probably related to small sea level oscillations seem to have conditioned the inflow of these sediment types (texture and composition) into the ocean, as well as its redistribution and deposition.

The trace metal content of the sediments in this zone is a consequence not only of natural weathering but also of local influences, reflecting changes in hydrodynamic conditions and deposition of finer particles and organic carbon in sediments and influencing diagenetic changes.

Acknowledgements

This research was supported by the Industrial Minerals and Clays Research Center of the Foundation for Science and Technology (FCT) and by the University of Aveiro. This project was partly supported by the European Union within the Marine Science and Technology programme (contract MAS3-CT97-0076).

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