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Distribution of living benthic foraminifera on the northern Gulf of Cadiz continental shelf

Publication . Mendes, Isabel; Dias, J. A.; Schonfeld, J.; Ferreira, Óscar

Living (stained) benthic foraminiferal faunas were analyzed in 51 surficial sediment samples taken from the northern Gulf of Cadiz continental shelf between the mouths of the Guadiana and Guadalquivir rivers. The distribution and abundance of 26 species with relative abundance .5% were related to water depth, sediment type, river discharge, water temperature, salinity, turbidity, and primary productivity. Hierarchical classification using R- and Q-mode cluster analyses, and individual distributions, allowed these taxa to be categorized into four general groups. Group 1 is represented by Bolivina ordinaria and Hopkinsina atlantica. These species are linked to river discharge, and are associated with muddy sediments. They are considered the most opportunistic species in the assemblage, and are associated with the limits of productivity. Group 2, contains Ammonia beccarii, Eggerelloides scaber, Elphidium gerthi, Quinqueloculina laevigata, and Q. stelligera, and is characteristic of shallow water depths. Its species are associated with different sediment types and their distribution is influenced by river discharge. Group 3 is dominated by Bulimina aculeata, B. elongata, Elphidium excavatum, E. cuvillieri, Epistominella vitrea, and Rectuvigerina phlegeri. These species display the highest abundances between 30–100 m water depths. They prevail in muds associated with weak hydrodynamics, low oxygenation, and high organic matter. Group 4 includes species that are more abundant in deep stations and is divided into two subgroups. Subgroup 4A includes Bolivina catanensis, B. italica, B. striatula, Cassidulina laevigata, Stainforthia sp., Nouria polymorphinoides, and Nouria sp., with higher abundances in one or two samples. Subgroup 4B, with Brizalina dilatata, B. spathulata, Bulimina marginata, Nonionella iridea, N. stella, and N. turgida, has patchy distribution. Higher abundances of Nonionella species off the Guadiana River are related to low temperature and salinity.

2012Journal article

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Benthic foraminifera as palaeo-environmental indicators in the Northern Gulf of Cadiz

Publication . Mendes, Isabel Maria de Paiva Pinto; Dias, João Manuel Alveirinho; Schönfeld, Joachim

This thesis is an investigation of benthic foraminifera as palaeo-environmental indicators on the Northern Gulf of Cadiz continental shelf, northeast Atlantic Ocean. The main objective was to determine the environmental indicator value of living benthic foraminifera and their application to Holocene palaeoenvironmental reconstruction. Relationships between living benthic foraminiferal fauna and both physical and chemical parameters were analysed in surficial samples collected from the continental shelf between the Guadiana and Guadalquivir Rivers. Living benthic foraminiferal faunas were characterised in detail and associations of the most abundant species classified into four groups. These groups allowed the characterisation of the modern environment and associated physical and chemical parameters. Sedimentological and benthic foraminiferal analyses, combined with a chronological framework, were performed on three Holocene sequences from contrasting sedimentary environments. A comparison between the most abundant species in living faunas and those occurring in Holocene shelf sediments was also made. An integrated approach using all the available information sheds light on paleoenvironmental changes, which were distilled into five evolutionary scenarios. Each scenario is a representation of the environmental changes that characterised the three studied environments at different times during the Holocene. They indicate the influence of various factors, including sea-level changes, river discharge and substrate properties, that are mirrored by different benthic foraminiferal assemblages. The thesis demonstrates that the modern ecological preferences of common benthic foraminiferal species have direct applicability to interpreting the palaeoenvironmental record.

2010Doctoral thesis

Open access

Inner shelf paleoenvironmental evolution as a function of land–ocean interactions in the vicinity of the Guadiana River, SW Iberia

Publication . Mendes, Isabel; Rosa, F.; Dias, J. A.; Schönfeld, J.; Ferreira, Óscar; Pinheiro, J. P.

This study investigates the land-ocean interactions along the northern margin of the Gulf of Cadiz in the vicinity of the Guadiana River. Benthic foraminifera and sedimentological characteristics were analysed in a sedimentary sequence spanning ca. 5000 years (core 8, 22 m water depth) retrieved from the inner shelf prodeltaic wedge. The analyses were integrated in a temporal framework based on accelerated mass spectrometry radiocarbon dating. Paleoenvironmental changes and sediment transfer mechanisms from the continent to the shelf were investigated and related to climatic oscillations and anthropogenic impact in the region. The results allowed the identification of two main periods of deposition. The first period, from ca. 5150 cal. BP (core base) to 1500-1200 cal. BP, is characterised by a mix of fine and coarse sediments, relatively constant percentages of terrigenous and bioclasts, and benthic foraminifera species characteristic of coastal environments. These features indicate an environment strongly influenced by discharge from the Guadiana River. The second period, from ca. 1500-1200 cal. BP to ca. 200 cal. BP (core top), is characterised by silt-clay fraction dominated sediments, an increase in terrigenous sediment towards the top, and benthic foraminifera species characteristic of environments with low levels of energy. An increase in the level of human occupation associated with changes in climate led to widespread erosion and soil loss to the continental shelf during this period, with possible silting up of the Guadiana eastern distributary, allowing the transport of high amounts of sediment to the shelf by the western distributary that led to the enhancement of fine sedimentation and the formation of the prodeltaic wedge in the area of the studied core.

2010Journal article

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Living, dead and fossil benthic foraminifera on a river dominated shelf (northern Gulf of Cadiz) and their use for paleoenvironmental reconstruction

Publication . Mendes, Isabel; Dias, J. A.; Schönfeld, J.; Ferreira, Óscar; Rosa, F.; Lobo, F. J.

Same-area comparison between patterns of Recent (living) and Holocene benthic foraminiferal assemblage composition is essential to validate their utility as proxies for paleoenvironmental reconstructions. Such reconstructions have scarcely been attempted in shelf environments. In this study, we compared living (stained), dead and Holocene benthic foraminiferal assemblages from the Gulf of Cadiz continental shelf off the Guadiana River. On average, 99% of the living benthic foraminiferal species were preserved in the dead assemblage and 95% in the fossil record. Several common species were assessed as indicators for certain environmental factors, including river discharge, supply of terrestrial organic matter, heavy metal concentrations, oxygen levels, substrate properties, hydrodynamic energy levels, sea-level rise and human impact. The paleoenvironmental reconstruction depicted five stages of Holocene environmental evolution of the Guadiana shelf over the last 11500 Cal yr BP. The characteristics of the stages, and the transitions between them, were in particular driven by changes in sea-level, influx of terrestrial organic matter and limits of productivity.

2013Journal article

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Natural and human-induced Holocene paleoenvironmental changes on the Guadiana shelf (northern Gulf of Cadiz)

Publication . Mendes, Isabel; Dias, J. A.; Schönfeld, J.; Ferreira, Óscar; Rosa, F.; Gonzalez, R.; Lobo, F. J.

Three contrasting sedimentary environments on the continental shelf off the Guadiana River (northern Gulf of Cadiz) were integrated in a chronological framework and analysed in terms of sedimentology and benthic foraminiferal assemblages to understand the Holocene paleoenvironmental evolution. The analysed environments differ in terms of their depositional regimes and benthic foraminiferal assemblages. However, a dominant fluvial origin of the sand fraction was observed in all three environments. Holocene sedimentary processes were mainly controlled by natural (sea level changes and climate variations) and human-induced processes (e.g. deforestation, agriculture) along four evolutionary stages. The three older stages were mainly influenced by natural processes, such as sea level variations and fluvial inputs, whereas the most recent stage reflects a combination of climatic- and human-induced processes. A deepening of sedimentary environments related to a period of rapid sea level rise, strongly influenced by river discharges occurred from c. 11,500 to c. 10,000 cal. yr BP. A reduction in sediment export to the shelf, as a result of the continuous and rapid sea level rise and enhanced estuary infilling reflects the second stage, from c. 10,000 to c. 5000 cal. yr BP. The beginning of the third stage, from c. 5000 to c. 1500–1000 cal. yr BP, is marked by a sea-level slowdown and the relatively stable climate and environmental conditions. The fourth stage, from c. 1500–1000 cal. yr BP to Recent times, reflects the intensification of human-induced processes and climatic variability in the Guadiana River basin. This stage also reflects modern depositional conditions, with the formation of a proximal prodeltaic wedge and a distal muddy body.

2012Journal article

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