Article

Speciation of rare earth elements in surface sediments of Lagoon of Aveiro (N Portugal)

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Abstract

The goal of this work is to determine the distribution of the rare earth elements in the surface sediments in the Ria de Aveiro (N Portugal) coastal lagoon. Concentrations of REE (La, Ce, Nd, Sm) and related elements (Th, Y and Sc) were evaluated by total digestion of sediments in the fine fraction of 33 grab-samples of surface sediments. The same method was followed for Al, Ca, S and Zr, related to terrigenous sources, but also, in the cases of Ca and S, to biogenic and quimiogenic processes. Concentrations of REE and related elements were additionally assessed by a three-step sequential chemical extraction (E1-3) procedure coupled with ICP–MS determination. The geochemical results were processed by statistical analysis combining textural and mineralogical data (XRD techniques). The results show that mobile La, Ce, Nd, Sm and Y are mostly adsorbed by amorphous Mn hydroxide (E3), secondly associate with organic matter (E2) and only in a very small extent are adsorbed on clay mineral structural sheets or associated with carbonates (E1). Thorium and Sc are almost exclusively linked with E2. Ytrium shows a very high percentage of availability, whilst Ce, La, Sc and Th are by far less mobile. Results of cluster analysis reveals that sediments are enriched in: i) quartz, feldspars and anhydrite in the most hydrodynamic areas with good connection with the ocean, where tidal currents are stronger and water is salty and well-oxygenated; ii) mud, TOC, phyllosilicates, siderite, pyrite and S in low hydrodynamic areas. In these areas, pyrite and siderite can result from reducing bacterial activity in subsurface sediments. Early diagenetic changes in the sediments would also influence the elements adsorption to clay minerals and their co-precipitation with carbonates (E1). The highest values of REE, in E1, occur in distal locations in relation to the mouths of rivers and streams. The retention of the elements in this phase may occur mainly in the lagoon, depending on the availability of carbonates and clay minerals and their assemblage’s composition. Concentrations of REE and related elements adsorbed by organic matter (E2) and by amorphous Mn hydroxide (E3) tend to rise near the rivers mouths and may be related to geochemical processes that take place upstream in the course of the rivers before the entry of sediments into the lagoon. Elemental concentrations retained in the structure of mineral phases (R), also tend to reach high values near the rivers mouths but in low hydrodynamic areas. There is some fractionation between Th and the light REE which is more evident in the Murtosa channel that is fed mainly by the Antuã river.

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Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H2S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.
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Textural, mineralogical, geochemical and microfaunal data (benthic foraminifera) were studied along the OMEX core KSGX 40 recovered in the Galicia Mud Deposit, of the NW Iberian outer continental shelf, off the Ría de Vigo (North of Spain). This core included the records of the last ca. 4.8 ka cal BP and consists, from the base to the top, of a sedimentary sequence exhibiting gradual upward decrease in grain size. Sediments of this core are mainly siliciclastic, largely composed of quartz, K-feldspars, plagioclases, and phyllosilicates (mica/illite, kaolinite and chlorite) showing a great continental influence in this zone. Two periods of deposition of finer sediments are registered between ∼ 2.2–1.2 ka cal BP and ∼ 0.5–0 ka cal BP.Since the last ∼ 2.2 ka BP, but mainly during both muddy intervals, the Galicia Mud Deposit was nourished with a lower and finer supply of detrital minerals compensated by higher amounts of organic matter, as it is suggested by a Benthic Foraminifera High Productivity (BFHP) proxy. Processes involved in organic matter degradation by aerobic organisms led to depressed levels of oxygen in the sediments, as shown by a Benthic Foraminiferal Oxygen Index (BFOI). Peaks of redox-sensitive elements, like Mn, Fe, Cu, Ni, Cr, Co, Zn, Ni and Pb as well as the presence of diagenetic minerals, such as pyrite, suggest the development of anoxic conditions beneath the sedimentary surface and early diagenetic changes due to high organic matter flux, in both muddy intervals.Two different hydrodynamic regimes were inferred through the analysis of the different proxies (textural, mineralogical, geochemical and benthic foraminifera): (1) A strong hydrodynamic regime between ∼ 4.8 and 2.2 ka cal BP characterized by the prevalence of winter storms, which gave rise to a deep mixed layer on the shelf. (2) Weak hydrodynamic regime between ∼ 2.2–1.2 ka cal BP and ∼ 0.5–0 ka cal BP with a high predominance of upwelling and an increase in oceanic stratification.
Article
A numerical model was used to predict toxic microalgal transport and dispersion in Ria de Aveiro in Portugal. A previously developed Lagrangean particle tracking model coupled to a calibrated two-dimensional hydrodynamic model of Ria de Aveiro was used. Microalgae were regarded as passive particles and the methodology used allowed the determination of their trajectories, as induced by the tidal currents predicted by the hydrodynamic model. The model assumes Ria de Aveiro as vertically homogeneous and does not take into account the vertical distribution patterns of microalgae. Simulations were carried out during extreme spring and neap tides, with microalgal released at the mouth of the lagoon at the local flood. The maximum and minimum areas affected during the occurrence of toxic microalgal blooms were estimated to evaluate the suitability of the distribution of the sampling stations included in the local monitoring program. It was found that the tidal currents greatly determine the microalgal horizontal distribution and dispersal in the lagoon. The results confirmed that the locations of water and bivalve shellfish sampling stations, postulated by INIAP/IPIMAR, in the context of the local harmful algal bloom (HAB) program, were appropriate, although some possible refinements were identified.
Caracterização hidráulica e aluvionar da Ria de Aveiro Utilização de modelos hidráulicos no estudo de problemas da Ria
  • C M Vicente
Vicente, C.M., 1985. Caracterização hidráulica e aluvionar da Ria de Aveiro. Utilização de modelos hidráulicos no estudo de problemas da Ria. In: Jornadas da Ria de Aveiro, vol. III – Ordenamento da Ria de Aveiro. Câmara Municipal de Aveiro, Portugal.