Geochemical and mineralogical evidence of tectonic and sedimentary factors controlling the origin of ferromanganese crusts associated to stratigraphic discontinuities (Betic Cordilleras, SE of Spain)

Ferromanganese crusts were found in carbonates of tectonostratigraphic units located in the northern and southern areas of the eastern External Subbetic of the Betic Cordilleras (SE Spain). The crusts are associated with four stratigraphic discontinuities of the Jurassic pelagic swells sequences: D1 (Late Carixian-Early Domerian), D2 (Middle Toarcian-Early Bajocian), D3 (Middle Bathonian-Middle Oxfordian), D4 (Early Tithonian-Late Albian). Two main textural types of crusts are distinguished. Type I crusts are thin and characterized by the presence of goethite, quartz, albite and phyllosilicates. Moreover, they show Si, Al, Mg, Na, Ti and K contents close to the European Shale Composite contents and Fe/Mn ratios (>350) higher than type II crusts. Type II crusts occur as thicker banded laminae and/or macrooncoids. They consist mainly of goethite and Mn-oxyhydroxides, which are enriched in REE, Co, Ni and Cu and show a strong Ce positive anomaly. After stratigraphical, mineralogical and geochemical data, the crust formation would be produced by the exposition of bottom sediments during long periods to a thin layer of oxidizing sea and porewater enriched in metallic elements. The textural and compositional variations between crusts can be explained by taking into account the bathymetric conditions. In shallower swells, the precipitation of a thick layer of banded type II crusts and in deeper areas, thin type I crusts were formed. Organic influence was only important in crusts from D3 of the northern area where textural evidence indicates the existence of seasonal periodically alternation between organism accretion and fine sedimentation. These were preceded and followed by phases in which the inorganic precipitation of oxides prevailed together with the fine sedimentation.     

The use of principle component analyses in characterising trace and major elemental distribution in a 55 kyr peat deposit in tropical Australia: Implications to paleoclimate

The Lynch’s Crater peat deposit in NE-Australia is a sensitive environmental archive located in the tropical Southern Hemisphere. This unique deposit illustrates that local and regional changes had a profound effect on the local Australian ecosystem over the past 55 kyr. To obtain a proxy of past climate changes, trace and major element geochemistry analyses were applied to a 13 m peat core from the crater. Principle component analysis (PCA) was used to identify the main factors that control elemental distribution in the peat and to add interpretative strength to the geochemical behavior of selected major and trace elements. For example, Sc, Al, Cu, and Pb were found to be related to increased erosion of the basin soils, and from this, several periods of significant flux from atmospheric input and/or terrigenous run-off were identified. Geochemically mobile elements during rock weathering and pedogenesis, such as Mg, Ca, and Sr helped to identify the peat ombrotrophic-minerotrophic boundary at ∼1.5 m depth and offered important information about fluxes of these nutrients to the mire and their dynamics within the deposit. Arsenic and V comparisons between the peat record (high concentrations in some peat sections) and in local basin rocks (very low concentrations), suggested the presence of a long range, atmospheric dust source early in the formation of the mire. The Lynch’s Crater peat record presents a continuous record of environmental change in tropical Australia and contributes new understanding to geochemical processes in peatlands.     

Stratigraphic framework of gas accumulations in the Ría de Pontevedra (NW Spain)

Knowledge of marine geological environments in which shallow gas is accumulating is becoming increasingly important in global studies of climate change because a measurable proportion of the total methane source comes from continental margins. Previous studies have revealed that coastal environments represent important geological environments where microbial methane is being generated, is accumulating, and is being released. In the Ría de Pontevedra, at least 4.5 km² of seafloor in the innermost part of the ría is underlain by sediments containing natural gas. Seismic interpretation contributes new findings for the definition of periods and geological environments in which the gas could have been generated, and is accumulating and released in the Ría de Pontevedra. Groundtruthing the seismic data (facies, environments) makes it possible to identify favourable geological environments for gas generation in the sedimentary infill of the Ría de Pontevedra. Sequence stratigraphy based on high-resolution seismic profiles and post-Last Glacial Maximum sea-level records makes it possible to establish the stratigraphic architecture of the ría and to define the periods in which gas could have been generated. The results of this study show that the sedimentary infill is composed of a fifth-order sequence developed since the Last Glacial Maximum. Within this sequence, gas appears to have accumulated in the Holocene deposits associated with the latest transgressive and highstand system tracts. Seismic analysis shows that gas could have been generated in different geological environments in the Ría de Pontevedra. If coastal environments at times of lower sea level were similar to those of the present, organic-rich mud deposits (deposited mainly in lakes, estuaries and floodplains) could have survived transgression and remained buried as potential gas sources in the inner part of the ría.     

The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite

Schwertmannite is a ubiquitous mineral formed from acid rock drainage (ARD), and plays a major role in controlling the water chemistry of many acid streams. The formation of schwertmannite was investigated in the acid discharge of the Monte Romero abandoned mine (Iberian Pyrite Belt, SW, Spain). Schwertmannite precipitated from supersaturated solutions mainly owing to the oxidation of Fe(II) to Fe(III) and transformed with time into goethite and jarosite. In a few hours, schwertmannite precipitation removed more than half of the arsenic load from solution, whereas the concentration of divalent trace metals (Zn, Cu, Pb, Cd, Ni, and Co) remained almost unchanged. In the laboratory, natural schwertmannite was kept in contact with its coexisting acid water in a flask with a solid-liquid mass ratio of 1:5 for 353 days. During this time, the pH of the solution dropped from 3.07 to 1.74 and the concentrations of sulfate and Fe increased. During the first 164 days, schwertmannite transformed into goethite plus H₃O-jarosite but, subsequently, goethite was the only mineral to form. Some of the trace elements, such as Al, Cu, Pb, and As were depleted in solution during the first stage as schwertmannite transformed into goethite plus H₃O-jarosite. On the contrary, the transformation of schwertmannite to goethite (with no jarosite) during the second stage released Al, Cu, and As to the solution. Despite the variation in their concentrations in solution, approximately 80% of the total Al and Cu inventories and more than 99% As and Pb remained in the solid phase throughout the entire aging process.     

Tracing the flow of organic matter from primary producers to filter feeders in Ria Formosa lagoon, Southern Portugal

The flow of organic matter along the main navigation channel of Ria Formosa, Portugal, was assessed using determinations of suspended particulate matter (SPM), particulate organic matter (POM), and chlorophyll a (chla) concentrations in conjunction with stable isotope values of primary producers, particulate matter, and two filter feeders. SPM in the lagoon is dominated by inorganic particles comprising 80% of total weight with organic matter averaging about 20%. The algal component of the POM averaged about 5% with the remainder comprised of detritus. The δ¹³C values of primary producers ranged from ?9.1‰ in the intertidal seagrassZostera noltii to ?30.7‰ in the red seaweedBostrychia scorpioides revealing underlying differences in the mechanisms of carbon uptake. The δ¹³C value ofB. scorpioides, which develops entangled on the salt marsh speciesSpartina maritima, suggests that its main source of inorganic carbon is atmospheric CO₂. The δ¹³C values of the high marsh macrophyteSarcocornia perennis significantly increased with distance from the ocean while δ¹³C values ofZ. noltii decreased, probably because higher decomposition of organic matter at inner stations lowers the δ¹³CO₂ value in the water. The δ¹⁵N values of Ulvales, seagrasses, and marsh plants significantly increased from outer stations to inner stations. This increase may be due either to recycling of nitrogen (N) within the marsh (with loss of light N₂ or NH₄) or to inputs of isotopically heavy N from sewage. The δ¹⁵N values of particulate matter showed an opposite trend, which indicates higher microbial degradation of organic matter at the inner lagoon. The data demonstrate that the seston in the lagoon is a mixture of detritus from lagoon primary producers with a minor contribution of microalgae. The filter feeders are most likely assimilating a mixture of phytoplankton and microphytobenthos. Digestion of lagoon seston is selective. The δ¹⁵N values of both muscle and digestive gland of filter feeders showed the opposite gradient of particulate matter indicating that the depleted δ¹⁵N of SPM at inner stations was not assimilated or even ingested. Stable isotopes values did not differ between the filter-feeders—the musselMytilus galloprovincialis collected on buoys and the clamTapes decussatus collected in the sediment—suggesting a considerable mixture of benthic-pelagic organic matter throughout the water column. Assessment of the changes in isotopic decomposition of detritus as it decays is required to refine our understanding of organic matter transfers in detrital food webs.     

Analysis of the spatial variation of heavy metals in the Guadiana Estuary sediments (SW Iberian Peninsula) based on GIS-mapping techniques

This work reveals the usefulness of the GIS (Geographic Information Systems) mapping techniques to show the distribution of pollutants along an estuarine environment, as the final stage of a thorough study. In the case of study, the environmental quality of the sediments in the Guadiana river estuary was determined by means of a complete geochemical characterization consisting on the calculation of enrichment factors for the most important metals and metalloids (compared with the local background of non-contaminated sediments). The obtained results were depicted in “enrichment distribution maps” which evidenced a distribution of the elements in two groups: Group-I, elements with natural origin (Al, Fe, Mn, Co, and Cr) distributed homogeneously along the basin, and Group-II, elements associated to anthropic origin (As, Cd, Cu, Pb, Ni and Zn) with clear punctual sources besides a high concentration all over the estuary. The enrichment factors for the elements of Group-II are indicative of the existence of a noticeable diffuse historical mining pollution associated with the acid mine drainage generated in the internal zones of the basin, which could overlap minor pollution inputs from other human activities. The obtained results lead us to a reconsideration of the traditionally thought “unpolluted” environment when it was compared to nearby estuaries.     

Variability of mangrove ecosystems along the Brazilian coast

Brazilian mangroves extend from 4°30′N to 28°30′S, varying greatly in growth form, species distribution patterns, and stand structure, in spite of a limited floristic diversity. We divided the Brazilian coastline into eight units, within which physiographic and climatic conditions are relatively uniform, and described mangrove occurrence, species distribution and structural attributes characteristic of each segment. In general, greates mangrove coverage and greatest forest stature are found in areas with a large surplus of rainfall over potential evapotranspiration and macrotidal regimes. An exception was the segment containing the mouth of the Amazon river, where freshwater systems dominate over brackish or marine associations. We believe that the variability in species associations and the dominance of each in a given environment is predominantly determined by the characteristics of the landforms that can be colonized by each species in a given region. The type, size, and frequency of occurrence of available landforms is a function of the particular mix of fluvial, tidal, and wave energies found in a region. Different species colonize these sites depending on their adaptations and edaphic preferences. Climate affects mangrove colonization and growth. We suggest that Brazilian mangroves play a minor role in modifying the geomorphic setting; the spatial arrangement of the various forest types is a response to the underlying topography and edaphic conditions, and to the constraints imposed by climatic and hydrologic factors. The spatial arrangement of species does not necessarily show successional processes, but may be the result of direct and differential colonization on appropriate substrates.     

Phytoplankton production and seasonal biomass variation of seagrass,Ruppia maritima L., in a tropical Mexican lagoon with an ephemeral inlet

Plankton metabolism andRuppia maritima biomass were measured seasonally during 1982–83 in El Verde Lagoon, a small coastal lagoon with an ephemeral inlet on the Pacific Coast of Mexico. Total net aquatic primary production was 521 g C m^?2 y^?1. The water column was slightly heterotrophic, with an annual P/R ratio of 0.89. Our analysis indicates that tropical and subtropical coastal lagoons with restricted or seasonal inlets have generally higher net aquatic primary productivity levels than lagoons with permanently open inlets. We hypothesize that this is due to retention of nutrients and plankton stocks during the dry season. The seasonal pattern of water column metabolism was related to rainfall and riverflow, with higher values generally occurring during the wet season. Net production and respiration were about three times lower during the 1982 dry season as compared to the 1983 dry season which received considerable rains due to abnormal climatic conditions. The biomass ofR. maritima ranged from zero to 620 g dry wt m^?2. Growth occurred only during the dry season and there were two distinct biomass peaks representing two separate crops. The second crop was heavily epiphytized with nitrogen-fixing algae. There was an apparent succession in dominance of water column productivity over the year, withRuppia dominating during the dry season and phytoplankton more important during the wet season.     

Analysis of swarms of high-frequency seismic events at Nevado del Ruiz volcano, Colombia (January 1986–August 1987): Development of a procedure

The determination of signal properties like frequency, amplitude, “signature” of the sequence of arrivals of P- and S-phases and the subsequent computation of hypocenter parameters, are the steps of a procedure in the analysis of swarms of high-frequency seismic events from Nevado del Ruiz volcano, which could be useful in other volcanic areas. The use of this procedure on Ruiz data led to the determination of various seismic source volumes, which together with hypocenters of non-swarmed events and the identification of geological features such as faults which cross the area, alignment of domes and volcanic edifices, and the record of low-frequency seismic events not centered in the area of the active vent, suggest the existence of a magma chamber of a rather complex geometry. This hypothesis would suggest that magma, emplaced in zones of weakness (faults) that affect the area, may act as the source of seismic activity. This demands a revision of the methodology used in the deformation measurements, and in the analysis of released energy and event occurrence too. The relations between hypocenters (sources), location and relative energy of the swarms, as well as the general volcanic activity (tremor, ash and gas emission, deformation) could turn out to be a possible means to assist in the identification of premonitory activity and thus be relevant to the process of monitoring and forecasting.