Miguel Island Research Articles

Copulatory process in Oxychilus (Drouetia) atlanticus (Morelet and Drouët, 1857) (Pulmonata: Zonitidae)

Summary Oxychilurr (Drouetia) atlanticus (Pulmonata: Zonitidae) is an endemic hermaphroditic species from São Miguel island (Azores). The copulatory process is a mutually simultaneous event and takes about 24 h from the beginning of penetration until the disconnection of the partners. Copulation is characterized by the progressive and complete evertion of the penis and penial caecum. The latter, everting completely inside the bursa copulatrix, is fundamental to the release of the anterior portion of the spermatophore, which is the first to be transferred. The anterior portion of the spermatophore is hook-shaped, aiding its anchorage to the bursa, and the posterior portion ends in a hood with an aperture through which the spermatozoa are released.

Analysis of gravimetric anomalies in Furnas caldera (São Miguel, Azores)

Furnas Volcano (São Miguel Island, Azores) is a steep-sided caldera complex resulting from several collapses. In this paper, we analyse and interpret the gravity survey carried out by the Instituto de Astronomı́a y Geodesia (Spain) and the gravity data published by the Instituto Nacional de Meteorologia e Geofisica (Portugal). The resulting Bouguer anomaly reveals the presence of a regional trend, several local features and a component of noise. In separating the regional trend we use a robust polynomial fitting. Further, a covariance analysis and a least squares prediction are applied to filter the noise and to model the local anomaly. A first signal map represents the main local anomaly, while the secondary signal map shows superficial anomalies. This distribution reveals a local main minimum close to the 1630 A.D. and Gaspar craters. Several possible alignments are shown that lead to a model, mainly with SW–NE and SE–NW orientations, of the negative anomaly, with the pattern of the low density zones showing a relationship with the volcanic features. Applying a stabilized linear method of gravimetric inversion, we obtain a density contrast distribution of the sources of the filtered local Bouguer anomaly, considering as unknowns the anomalous densities of a fixed discrete distribution of rectangular prisms which represents the subsoil volume. A mixed minimization criteria for the weighted residuals and the weighted density contrast allow us to obtain a solution as a discrete three-dimensional density contrast model. The obtained anomalous densities model shows a clear relation with the structure of the caldera complex and tectonic trends, with the principal body of negative contrast density being associated with the subsurface structure of the caldera.

Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/ 3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from São Miguel and Terceira. The eastern part of São Miguel island displays radiogenic helium ( 4He/ 3He > 140,000, R/ R a<5.1) and lead (20.00, 15.75 and 40.33 for 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb). The 207Pb/ 204Pb and 208Pb/ 204Pb ratios for São Miguel are unusually radiogenic for oceanic basalts. Terceira basalts contain relatively unradiogenic/primitive 4He/ 3He ratios, with a minimum value of 64,000 ( R/ R a=11.3), and relatively high lead isotopic ratios ( 206Pb/ 204Pb = 20.02, 207Pb/ 204Pb = 15.64 and 208Pb/ 204Pb = 39.35). We propose that the Terceira source has a composition produced by a mixing between recycled oceanic crust (high 206Pb/ 204Pb) and entrained lower mantle (high 3He) material. The São Miguel island isotopic signature may be due to sampling of local (km-size) heterogeneity located at relatively shallow depth. The preferred origin of this heterogeneity is the Jurassic delamination of subcontinental lithosphere, which occurred during rifting and opening of the North Atlantic. The primitive helium ratios were also observed on the Mid Atlantic ridge at 38.5°N, reflecting plume–ridge interaction, whereas radiogenic ratios (>100,000) were observed at latitude higher than 40°N and may reflect the influence of the São Miguel component at the ridge.

Mineralogy, micromorphology and chemical analysis of andosols on the Island of São Miguel (Azores)

A study concerning soil genesis was conducted on surface and buried volcanic soils of São Miguel island (Azores). Of eight soil profiles, three were selected for detailed study of pedogenesis. A series of techniques which include chemical, micromorphological, XRD and DXRD analysis, micropicking on impregnated soil blocks, TEM-EDS and IR spectroscopy analyses were used. The soils showed an incipient to moderate degree of weathering, as typically occurs in many young volcanic districts. Four different stages of soil development were clearly identified. The first stage of soil development was characterised by dominance of primary minerals and some allophane and ferrihydrite formation, with an Fe o/Fe d ratio of about 0.86 (LOM soil). The second stage of soil development was indicated by halloysite and ferrihydrite formation in the lower horizons along with an Fe o/Fe d ratio of about 0.62 (SAN surface soil). The third stage of soil development was characterized by halloysite and ferrihydrite formation in the lower horizon but also by clay illuviation and stronger pumice weathering, and an Fe o/Fe d ratio of about 0.51 (SAN buried soil). The fourth stage of soil development concerned the LIX soils (buried soils older than 5000 years BP) which had a much higher content in hydrated halloysite and considerable amounts of goethite and hematite. These soils exhibit the most expressed weathering features and an Fe o/Fe d ratio of about 0.25. The presence of young surface soils and well-developed buried soils can be explained by (i) the longer pedogenesis of buried soils or, possibly, by (ii) the presence of a climate more seasonal than that of the present day.

40Ar/ 39Ar ages and paleomagnetism of São Miguel lavas, Azores

We present new 40Ar/ 39Ar ages and paleomagnetic data for São Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. 40Ar/ 39Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K–Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across São Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The 40Ar/ 39Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0–0.1 Ma) and up to 0.78 Myr (0–0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78–0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction.

Biometrics, weights, breeding and moulting seasons of passerines in an Azores cloud forest

Information on biometrics, weights, breeding and moulting seasons is presented for several passerines, namely Chaffinch, Fringilla coelebs moreletti, Azores Bullfinch, Pyrrhula murina, Goldcrest, Regulus regulus azoricus and Blackcap, Sylvia atricapilla atlantis, captured from mid to high altitudes in S. Miguel island, Azores. The Chaffinch is the only species presenting sexual dimorphism in bill measurements, the Azores Bullfinch and the Goldcrest are sexually dimorphic in wing‐length, while the Blackcap showed no sexual dimorphism. All species bred between June and August and started moulting in August. Predominantly granivorous birds seemed to start breeding slightly later than predominantly insectivorous birds.

A three‐dimensional gravity inversion applied to São Miguel Island (Azores)

Gravimetric studies are becoming more and more widely acknowledged as a useful tool for studying and modeling the distributions of subsurface masses that are associated with volcanic activity. In this paper, new gravimetric data for the volcanic island of São Miguel (Azores) were analyzed and interpreted by a stabilized linear inversion methodology. An inversion model of higher resolution was calculated for the Caldera of Furnas, which has a larger density of data. In order to filter out the noncorrelatable anomalies, least squares prediction was used, resulting in a correlated gravimetric signal model with an accuracy of the order of 0.9 mGal. The gravimetric inversion technique is based on the adjustment of a three‐dimensional (3‐D) model of cubes of unknown density that represents the island's subsurface. The problem of non‐uniqueness is solved by minimization with appropriate covariance matrices of the data (resulting from the least squares prediction) and of the unknowns. We also propose a criterion for choosing a balance between the data fit (which in this case corresponds to residues with rms of the order of 0.6 mGal) and the smoothness of the solution. The global model of the island includes a low‐density zone in a WNW‐ESE direction and a depth of the order of 20 km, associated with the Terceira rift spreading center. The minimums located at a depth of 4 km may be associated with shallow magmatic chambers beneath the main volcanoes of the island. The main high‐density area is related to the Nordeste basaltic shield. With regard to the Caldera Furnas, in addition to the minimum that can be associated with a magmatic chamber, there are other shallow minimums that correspond to eruptive processes.

Magnetic study of the Furnas caldera (Azores)

A local ground magnetic study of the Furnas caldera (S. Miguel Island, Azores) has provided new insight into the magnetic structure of this volcano. Analysis of the data comprised removal of the IGRF, reduction to the pole, pseudogravity integration and upward continuation. Also, a spectral method was applied to estimate the depth to the magnetic sources, as well as a 2.5D forward modelling technique. Magnetic properties obtained at the laboratory for some representative sample rocks were considered in the modelling process. The most relevant features are the existence of an important negative anomaly inside the caldera and of an intense positive anomaly to the south of the coast. The former points out a decrease in the magnetization of the caldera filling materials with respect to the surrounding rocks, which could be explained as the result of post-eruption processes such as hydrothermal alteration. This is expected as Furnas has an active hydrothermal system probably related with a magmatic reservoir at high temperature. The positive anomaly suggests the existence of a strongly-magnetized body beneath the south coast.

Slow inflation of the Furnas Volcano, São Miguel, Azores, suggested from initial leveling and global positioning system measurements

The Furnas volcano, São Miguel island, Azores is an active stratovolcano, with a 6‐km‐diameter caldera. In order to mitigate volcanic risk in the area, crustal deformation research based on optical leveling tilt measurements and lake leveling was initiated at the volcano in 1991. Progressive tilt as high as 4 µrad/yr has been observed, but the measurements do not all fit a simple model. The Global Positioning System (GPS) was used to measure a network of fourteen stations at the volcano in 1993, and again in 1994. Horizontal displacements in the one year period between the surveys are less than 1 cm, but significant with 95% confidence. Stations are being displaced away from the caldera and strain analysis indicates areal dilation of 0.84±0.40 µstrain/yr of the volcanic edifice. The areal dilation can be explained by slight inflation of the volcano. The location of the center of inflation is rather uncertain, but based on both the GPS and the leveling data the preferred location is in the northern part of the caldera. A central uplift rate of &gt;7 mm/year is suggested.

Stygiolobus azoricus gen. nov., sp. nov. Represents a Novel Genus of Anaerobic, Extremely Thermoacidophilic Archaebacteria of the Order Sulfolobales

On the basis of three isolates (strains FC6T [T = type strain], FC4, and RG1) of extremely thermophilic chemolithoautotrophic archaebacteria obtained from solfataric fields on ao Miguel Island, Azores, the new genus Stygiolobus is described. These isolates grow obligately chemolithotrophically by reduction of S0 with H2 (H2-S0 lithotrophy) and are the first strictly anaerobic members of the order Sulfolobales. With a DNA G+C content of 38 mol%, the Stygiolobus isolates resemble Sulfolobus spp., which, however, are faculatively organotrophic and aerobic S0 oxidizers. The new isolates are also distinct from Acidianus spp., which resemble Stygiolobus by growing by H2-S0 lithotrophy. However, Acidianus spp. can also grow aerobically by S0 oxidation and have G+C contents of 31 mol%. At this time, only one species of the genus Stygiolobus is known, Stygiolobus azoricus sp. nov.; the type strain of S. azoricus is strain FC6 (= DSM 6296).

Radiocarbon Dates for Lava Flows and Pyroclastic Deposits on São Miguel, Azores

We report 63 new radiocarbon analyses of samples from São Miguel, the largest (ca 62 × 13km) and most populous (ca 150,000 inhabitants) island in the Azores archipelago (Fig 1). The samples are mainly carbonized tree roots and other plant material collected from beneath 20 mafic lava flows and spatter deposits and from within and beneath 42 trachytic pyroclastic flow, pyroclastic surge, mudflow, pumice-fall, and lacustrine deposits and lava flows. One calcite date is reported. The samples were collected during geologic mapping of the entire island (Moore, in press A; sample locations are shown on this map). Nine 1:25,000-scale topographic maps, published in 1983 by the Portuguese Army Cartographic Service, cover the island; samples and locations described below refer to these named sheets.

Seismic hazard analysis in zones of time and space interdependence: An application to São Miguel Island, Azores

The Azores Islands are located in the mid-Atlantic region near the triple junction where the Euro-Asiatic-African-American plates join together. Seismic activity in the area is very high, as can be observed either from historical events since the fifteenth century, from present day microseismicity, and from direct and indirect measurements of recent tectonic deformation. Volcanic activity is also present throughout the region.

Phenocrystic olivines from the eastern Azores

SummaryPhenocrystic and xenocrystic olivines and one olivine nodule sample, all from volcanics in the eastern Azores, range in composition from Fo87 to Fo69. Most are phenocrysts from the Nordeste alkali basaltic complex, eastern São Miguel Island, in which olivine-bearing rocks range from ankaramite to latite. CaO, MnO, and NiO trends are related to Mg-Fe contents, both for the concentrates as a whole, and also within analysed zoned crystals. It is shown that anomalous plots of Δ2ϑ(220LiF—062olivine) are related to pronounced Mg-Fe zonal gradients and, in samples from some flows, to mixtures of different generations of olivine. Regressions of Δ2ϑ vs. Fo (mol %) and FeO (wt %) for the least-zoned olivines are in close accord with those for Hawaiian olivines reported by Murata and others (1965).MnO percentages increase linearly with FeO; this and the Hawaiian trend are indistinguishable. Limited data for the most magnesian olivines indicate that NiO increases with FeO up to ∼ 15 wt % of FeO; the trend then decreases with further iron enrichment. This trend is apparent from other olivine data, and its relationship to the onset of pyroxene crystallization is discussed. CaO in olivine phenocrysts is inversely related to normative anorthite percentages in host rocks; the trend appears to be controlled largely by co-precipitation and fractionation of calcic plagioclase.