Influence of Intra- and Interspecific Interactions on Short-term Movement of the Amphipod Corophium volutator in Varying Environmental Conditions

Movement of animals on intertidal mudflats can have profound consequences for population and community dynamics. Such movement is often influenced by biotic interactions, but the abiotic environment can also be important, affecting the strength of species’ interactions and even changing their outcome. Here, we focused on two biotic interactions and examined their effect on movement of the burrow-dwelling amphipod Corophium volutator: intraspecific interactions (specifically, the effect of conspecific density) and interspecific interactions with a competitor–predator, the mud snail Ilyanassa obsoleta (=Nassarius obsoletus). We assessed the magnitude of the temporal variation and examined the influence of pertinent abiotic variables (lunar cycle, presence of tide pools). Despite substantial temporal variation in movement levels, density of conspecifics did not influence emigration or immigration rates of C. volutator. Mud snails negatively affected immigration of C. volutator, and this was consistent across all lunar phases. However, mud snails influenced mortality and emigration of C. volutator in a contrasting way relative to lunar phases; sometimes amphipods evaded snails and survived, but at other times did not emigrate and were killed. The presence of tide pools did not change the nature or strength of the snail–amphipod interaction. Our results further clarify the complex interactions between C. volutator and I. obsoleta and suggest that resources on the mudflat are sufficient to sustain high densities of amphipods in a density-independent manner. Further, they highlight the importance of considering multiple environmental variables when examining interspecific interactions.     

Palaeomagnetism of the loess/palaeosol sequence in Viatovo (NE Bulgaria) in the Danube basin

The results of a palaeomagnetic investigation of a 27 m thick loess/palaeosol sequence in Viatovo (NE Bulgaria) are presented in this paper. The sequence consists of topsoil S₀, seven loess horizons (L₁–L₇) and six interbedded palaeosols (S₁–S₆) overlying a red clay (terra rossa) complex. Magnetic viscosity experiments, IRM acquisition, AMS analysis and NRM stepwise alternating and thermal demagnetisation experiments of pilot samples were implemented for precise determination of the characteristic remanence and construction of a reliable magnetostratigraphical scheme. Analysis of IRM acquisition curves using the expectation – maximization algorithm of Heslop et al. [Heslop, D., Dekkers, M., Kruiver, P., van Oorschot, H., 2002. Analysis of isothermal remanent magnetization acquisition curves using the expectation – maximization algorithm. Geophys. J. Int., 148, 58–64] suggests that the best fitting is obtained by three coercivity components. Component 1 corresponds to SD maghemite/magnetite, while component 2 is probably related to the presence of oxidised detrital magnetites. The third component shows varying coercivities depending on the degree of pedogenic alteration of the samples and probably reflects the presence of detrital magnetite grains oxidised at different degree.

The relevance of the Viatovo section as a key representative sequence for the loess cover in the Danube basin is confirmed by the presence of geomagnetic polarity changes in the lower part of the sequence. The youngest one recorded in the seventh loess unit L₇ can be identified as corresponding to the Matuyama/Brunhes palaeomagnetic polarity transition. Two normal magnetozones were found in the red clay complex, probably corresponding to the Jaramillo and Olduvai subchronozones of the Matuyama chron.     

Growth forms,dispersal strategies and taxonomic spectrum in a semi-arid shrubland in SE Spain

We have identified the dispersal strategies of major growth forms in a semi-arid Mediterranean shrubland of Cabo de Gata Natural Park (SE Spain), testing (1) whether dispersal strategies and antitelechoric mechanisms vary among growth forms and families, and (2) whether dispersal traits that identify dispersal strategies can be used to predict different ways of plant survival in response to seasonal changes in their habitat. In the survey, the data of 17 dispersal traits were collected of 140 species and analysed using cluster analysis and nonlinear principal components analysis. For shrubs and forbs, low diaspore release height, spheroidal diaspores with a low mass and small size and a low to high diaspore number were associated with restricted spatial dispersal. In contrast, medium to high release height, relatively large and heavy diaspores varying in shape and a high diaspore number were associated with developed spatial dispersal (telechory). Telechory is higher in shrubs than in forbs and grasses, where atelechory/antitelechory predominate. Dispersal was synchronized mainly with the drought period (July and August) and the beginning of the rainy season (October–December). 68% of shrubs and 61.6% of forbs were anemochores, whereas zoochory (6% of shrubs) and ballistic species (8% of shrubs and 6.9% of forbs) were less common. Ombrohydrochory was well represented among forbs (27.3%) but rare among shrubs (4%). The main antitelechoric mechanisms were: bradyspory (30.7%), myxospermy (15.7%) and trypanocarpy (29.4%, only for grasses). Species with restricted spatial dispersal and antitelechoric mechanisms are well adapted to survival in semi-arid climatic conditions, and should be taken into consideration in conservation planning.     

Late Pleistocene–Holocene environmental and climatic history of a freshwater paramo ecosystem in the northern Andes

This study presents the first multi-proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD.     

A luminescence‐based chronology for the Harletz loess sequence,Bulgaria

The Harletz loess‐palaeosol sequence is located in northwestern Bulgaria and represents an important link between well‐studied loess sequences in eastern Romania and further sites to the west of the Carpathians (e.g. Serbia and Hungary). The aim of this study was to establish a chronostratigraphy of the deposits, using various methods of luminescence dating, together with basic stratigraphical field observations as well as magnetic properties. Luminescence dating was carried out using the quartz fine grain fraction and a SAR protocol, and the feldspar coarse grain fraction, applying the MET‐pIRIR protocol. Due to underestimation of the quartz fine grain fraction in the lower parts of the sequence, the resulting chronology is mainly based on the feldspar ages, which are derived from the stimulation temperature at 150 °C. A comparison with nearby sequences from Serbia, Hungary and Romania, and interpretations obtained through the stratigraphical and sedimentological signature of the sequence, supports the established chronology. Our data suggest that the prominent palaeosol (soil complex) in the upper quarter of the sequence was formed during MIS 5. It would follow that large parts of the Last Glacial loess overlying this palaeosol were probably eroded, and that the thick loess accumulation underlying this soil complex can be allocated to the penultimate glacial (MIS 6). A prominent MIS 6 tephra, which has been reported from other sequences in the area, is also present at Harletz.     

P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, northwest Iberian Massif, Spain): Implications for crustal subduction

Eclogite lenses in the Agualada Unit (western Ordenes Complex, Spain) contain the peak mineral assemblage garnet (prograde rim: Alm = 48 mol%, Prp = 30 mol%), omphacite (Jd max = 36 mol%), quartz, rutile and rare zoisite, which equilibrated at T = 700 °C and P > 12–14 kbar. Garnet shows discontinuous growth zoning, with a pyrope-poor intermediate zone (Alm = 51 mol%, Prp = 10 mol%) between a core zone where pyrope is slightly higher (Alm = 46 mol%, Prp = 16 mol%) and areas just inward from the rims where the maximum pyrope contents (Alm = 48 mol%, Prp = 30 mol%) are recorded. In atoll interiors, garnet contains inclusions of a first generation of omphacite (Jd max = 40 mol%). This omphacite is replaced in the matrix by a second generation (Jd max = 36 mol%) with higher Fe/Fe + Mg ratio. The compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the Agualada Unit, characterized by: (1) eclogite-facies metamorphism (T = 585 °C, P > 12–13 kbar), followed by (2) cooling during a slight decompression (T = 500 °C, P > 11–12 kbar), and (3) a final increase in P and T to form the garnet rim-matrix omphacite mineral assemblage. The Agualada Unit is part of a subduction complex which affected the Gondwana margin at the beginning of the Variscan cycle. The P-T evolution of the Agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge. The cooling event recorded by the Agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit, whereas the final progradation reflects the emplacement of the Agualada Unit directly under the mantle wedge. This evolution fits well with previously presented the retical models, both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges. P-T paths such as the one for the Agualada Unit eclogites, probably reflect a prolonged structural evolution. Although evidently rarely preserved, such paths are probably the rule rather than the exception during plate convergence.     

Magmatism in the Iberian Pyrite Belt: petrological constraints on a metallogenic model

Igneous formations associated with massive sulphide deposits in the Iberian Pyrite Belt (IPB) are essentially composed of basic lavas and dolerites, and dacitic to rhyolitic volcanites; intermediate lavas are subordinate. The basic rocks show variable geochemical characteristics: lavas and dolerites comparable to recent within-plate alkaline basalts seem restricted to the western and southern parts of the IPB, whereas basic rocks comparable to continental tholeiites or arc-related basalts occur across the whole belt. The felsic rocks are classified as calc-alkaline and belong to the “low-Al₂O₃ and high-Yb type”. At given SiO₂, Al₂O₃ and TiO₂ contents, they show variable Zr, Nb, and HREE contents. Heavy-rare-earth element fractionation decreases from the dacites to the rhyolites ([Gd/Yb]_N ∼ 1), whereas the negative Eu-anomaly becomes more pronounced. The characteristics of the rhyolites are typical of sulphide-fertile volcanic packages. Trace-element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase. Partial melting of an amphibolite protolith, which appears as the most probable model for the origin of this dacitic magma, requires a high T/P gradient in the crust. The occurrence of alkaline basalts and continental tholeiites is consistent with formation of the IPB in a tensional tectonic setting. However, the associated island-arc tholeiites suggest a location in a domain of plate convergence. Emplacement in a fore-arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints. A high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the IPB.

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Resumen (translated by E. Pascual) Las formaciones ígneas asociadas con los depósitos de sulfuros masivos de la Faja Pirítica Ibérica (IPB) se componen esencialmente de doleritas y lavas básicas y de rocas volcánicas dacíticas a riolíticas; las lavas intermedias son poco abundantes. Las rocas básicas muestran caracteres geoquímicos variables: lavas y doleritas comparables a basaltos alcalinos intraplaca recientes parecen hasta ahora restringidos a las partes W y S de la IPB, mientras que rocas básicas de caracteres comparables a las de toleítas continentales o basaltos relacionados con arcos aparecen en toda la zona. Las rocas ácidas se clasifican como calcoalcalinas del tipo “low-Al₂O₃, high-Yb”. Para un contenido dado en SiO₂, Al₂O₃ y TiO₂, muestran contenidos variables en Zr, Nb y REE. El contenido en tierras raras pesadas decrece de dacitas a riolitas ([Gd/Yb]_N ∼ 1), al tiempo que la anomalía de Eu se hace más pronunciada. La modelización de elementos trazas sugiere que las rocas evolucionaron a partir de un magma parental dacítico mediante cristalización fraccionada de hornblenda y plagioclasa. La fusión parcial de un protolito anfibolítico, que parece el modelo más plausible para el origen del magma dacítico, requiere un elevado gradiente T/P en la corteza. La existencia de basaltos alcalinos y de toleítas continentales es congruente con la formación de la IPB en un entorno tectónico distensivo. El emplazamiento en una cuenca “fore-arc”, en un segmento cortical de reciente acreción, se contempla como una hipótesis posible para explicar los caracteres geológicos y petrológicos. Un alto gradiente térmico, junto con la erupción en una cuenca submarina extensional, pueden haber sido los dos ingredientes clave en el desarrollo de los depósitos de sulfuros masivos en la IPB.

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Received: 3 March 1996 / Accepted: 7 April 1997     

Further determinations of upper-atmosphere rotational speed from analysis of satellite orbits

The average angular velocity of the upper atmosphere, which we take as Λ times the Earth's angular velocity, can be evaluated by analysing the changes in the orbital inclinations of satellites. In this paper the nine most suitable orbits now available are analysed and values of Λ are found for heights between 200 and 260 km. The results, which are more accurate than in our previous studies, confirm that Λ 1, i.e. that the atmosphere rotates faster than the Earth at these heights, and show that Λ increases with height, from 1.1 at 210 km to 1.4 at 260 km. This corresponds to mean west-to-east winds of 30 m/s at 210 km, increasing to 130 m/s at 260 km height. Results from one satellite indicate that the wind is probably strongest at times near sunset, with Λ = 1.5 ± 0.1 at 200 km height in August 1966. Comparisons are made with previous observational results and some of the suggested theoretical explanations are outlined.