Evidence for current-controlled morphology along the western slope of Hatton Bank (Rockall Plateau, NE Atlantic Ocean)

A multibeam bathymetric and high- (airgun and sparker) to very high-resolution (Topas) seismic study of the western slope of Hatton Bank (NE Atlantic), located between 600 m and 2,000 m water depth, has revealed a highly variable range of current-controlled morphological features. Two major seabed areas can be distinguished: (1) a non-depositional area corresponding to the top of the bank and (2) a depositional area in which the Hatton Drift has developed. Both areas are characterised by distinct morphologies associated either with rock outcrops and rocky ridges or with smooth surfaces, slides and bedforms controlled mainly by bottom currents interacting with the topography of the bank. The water depth separating the morphological areas probably coincides with the boundary of the Labrador Sea Water and the Lower Deep Water. Morphological features identified in the study area include contourite channels (moats, furrows and scours), fields of sediment waves, edges of contourite deposits, ponded deposits, scarps, gullies, ridges, depressions, slides and slide scars. These morphological features do not necessarily reflect present-day conditions but may have been associated with past current events, consistent with earlier interpretations.     

Larval fish abundance and physical forcing in the Gulf of Alaska, 1981–2003

The present study investigates ecological patterns and relationships to environmental variables among a time-series of larval fish species abundance from late spring surveys (1981–2003) in the northwest Gulf of Alaska (GOA). Links between interannual variation in species abundance and the physical environment were explored using generalized additive modeling (GAM). Trends in larval abundance and connections with physical variables displayed patterns that indicate unique and complex responses among species to environmental forcing during the larval period. In particular, the observed patterns suggest that ontogenetic-specific responses, representing sub-intervals of early life, are important. In addition, a notable degree of synchrony in larval abundance trends, and similarity in links with physical variables, were observed among species with common early life history patterns. The deepwater spawners, northern lampfish, arrowtooth flounder, and Pacific halibut, were most abundant in the study area during the 1990s, in association with enhanced wind-driven onshore and alongshore transport. Years of high abundance for Pacific cod, walleye pollock, and northern rock sole were associated with cooler winters and enhanced alongshore winds during spring. High larval abundance for spring–summer spawning rockfish species and southern rock sole seemed to be favored by warmer spring temperatures later in the time-series. This apparent exposure–response coupling seems to be connected to both local-scale and basin-scale environmental signals, to varying degrees depending on specific early life history characteristics. Understanding such ecological connections contributes to the evaluation of vulnerability and resilience among GOA species’ early life history patterns to fluctuating climate and oceanographic conditions. This investigation also provides crucial information for the identification of “environmental indicators” that may have a broad-spectrum effect on multiple species early life history stages, as well as those that may be more species-specific in exerting control on early life history survival. Of particular interest was the emergence of the EP–NP (East Pacific–North Pacific) teleconnection index as the top-ranked variable in the GAM models exploring the connections between late spring larval abundance and the physical environment. The EP–NP index represents an important and often primary mode of spring–summer atmospheric variability in the northeast Pacific, with a strong expression in the GOA, and its connection with species in this study implies that it may be a climate mode of significant ecological importance.     

Landscape and climatic changes during the end of the Late Prehistory in the Amblés Valley (Ávila, central Spain), from 1200 to 400 cal BC

Correlations between ecological and cultural changes occurred during a short period between the end of Subboreal and the beginning of Subatlantic in the Amblés Valley (Ávila, central Spain) are analyzed, taking into account palaeopalynological and archaeological data. Plant dynamics from pollen analyses, both from archaeological sites and peat bogs, have been interpreted in relation to human settlements and the transformation of economic practices. These provided a comprehensive hypothesis on human/climate interactions at the beginning of the 1st millennium cal BC. There was an ecological crisis in the region that lasted for a century (ca. 850–760 cal BC). This was especially sharp due to both the geographical constraints of the area and its previous agrarian history. This event implied a sudden and abrupt climatic change from xeric and warm conditions (Subboreal) to more humid and cooler ones (Subatlantic). Environmental stress derived from climatic crisis could be an important factor in the explanation of the historical process, whose main consequences were both the origin of the Iron Age peasant villages and the end of the ‘dehesa’ type landscape.     

Three monthly coral Sr/Ca records from the Chagos Archipelago covering the period of 1950–1995 A.D.: reproducibility and implications for quantitative reconstructions of sea surface temperature variations

In order to assess the fidelity of coral Sr/Ca for quantitative reconstructions of sea surface temperature variations, we have generated three monthly Sr/Ca time series from Porites corals from the lagoon of Peros Banhos (71°E, 5°S, Chagos Archipelago). We find that all three coral Sr/Ca time series are well correlated with instrumental records of sea surface temperature (SST) and air temperature. However, the intrinsic variance of the single-core Sr/Ca time series differs from core to core, limiting their use for quantitative estimates of past temperature variations. Averaging the single-core data improves the correlation with instrumental temperature (r > 0.7) and allows accurate estimates of interannual temperature variations (~0.35°C or better). All Sr/Ca time series indicate a shift towards warmer temperatures in the mid-1970s, which coincides with the most recent regime shift in the Pacific Ocean. However, the magnitude of the warming inferred from coral Sr/Ca differs from core to core and ranges from 0.26 to 0.75°C. The composite Sr/Ca record from Peros Banhos clearly captures the major climatic signals in the Indo-Pacific Ocean, i.e. the El Niño–southern oscillation and the Pacific decadal oscillation. Moreover, composite Sr/Ca is highly correlated with tropical mean temperatures (r = 0.7), suggesting that coral Sr/Ca time series from the tropical Indian Ocean will contribute to multi-proxy reconstructions of tropical mean temperatures.     

Paleomagnetic and rock-magnetic survey of eocene dike swarms from the Tecalitlan area (Western Mexico): Tectonic implications

For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacán block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacán block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α₉₅ = 8.9°. The corresponding mean paleomagnetic pole position is P_lat = 75.7°, P_long = 166.6°, K = 31, A₉₅ = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacán Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.     

Recent observations of the WR central star of the planetary nebula LMC-N66

The recent evolution of the central star of the planetary nebula LMC-N66 is presented. Before 1987, it showed a weak continuum with aT _eff120 000 K andL _bol25 000L and in a few years it developed strong WR features (P Cygni line profiles in N v at 124.0 nm and C IV at 155.0 nm, wide Heii emission, etc.) typical of a WN 4.5. Additionally the stellar continuum increased by a large factor and the absolute visual magnitude of the star changed from + 1.24 in 1987 to–2.57 in January, 1995. The WR features and enhanced continuum, evidencing a powerful mass-loss event remained with small variations for more than 5 years. Recent ultraviolet and optical data shows that the mass-loss seems to have diminished abruptly in the last three months.     

Reconstructing seawater δO from paired coral δO and Sr/Ca ratios: Methods, error analysis and problems, with examples from Tahiti (French Polynesia) and Timor (Indonesia)

We compare several statistical routines that may be used to calculate δ¹⁸O_sw and SSS from paired coral Sr/Ca and δ¹⁸O measurements. Typically, the δ¹⁸O_coral-SST relationship is estimated by linear regression of coral δ¹⁸O vs. SST. If this method is applied, evidence should be given that at a particular site SST and SSS do not co-vary. In the tropical oceans, SST and δ¹⁸O_sw (SSS) often co-vary, and this will bias the estimate of the regression slope of δ¹⁸O_coral-SST. Using a stochastic model, we show that covariance leads to a bias in the coefficients of the univariate regression equations. As the slope of the δ¹⁸O_coral-SST relationship has known, we propose to insert this value for γ₁ in the regression models. This requires that the constants of the regression equations are removed. To omit the constants, we propose to center the regression equations (i.e., to remove the mean values from the variables). The statistical error propagation is calculated to assess our ability to resolve past variations in δ¹⁸O_sw (SSS). At Tahiti, we find that the combined analytical uncertainties of coral δ¹⁸O and Sr/Ca equal the amplitude of the seasonal cycle of δ¹⁸O_sw (SSS). Therefore, we cannot resolve the seasonal cycle of SSS at Tahiti. At Timor, the error of reconstructed δ¹⁸O_sw (SSS) is lower than the magnitude of seasonal variations of δ¹⁸O_sw (SSS), and the seasonal cycle of δ¹⁸O_sw (SSS) can be resolved.     

Aspects of Mechanical Behavior and Modeling of a Tropical Unsaturated Soil

The research studies the applicability of two elastoplastic models for the collapse prediction of the lateritic soil profile from Southeastern Brazil. These tropical soils have peculiar geotechnical behavior, due to their mineralogical composition and porous structure coming from intense process of formation. Two elastoplastic models were analyzed: the Barcelona Basic Model (BBM) and another one based on BBM, however developed for tropical soils. Oedometric tests with suction control were performed at three distinct depths of the soil profile. The BBM was not suitable for the upper layer of the soil profile, because BBM considers the compressible behavior of the soil in function of the reduction of the elastoplastic compressibility index with the increase of the matric suction. The model developed for tropical soils showed better suited to the compressible behavior of the soil profile, resulting in good prediction of the collapse potential, mainly by accepting increasing values of the elastoplastic compressibility index of the soil profile with the matric suction rise.     

An astrophysical oscillator strength for the S ii 94.7-nm resonance line and S abundances in DLAs

By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle spectrograph, we determine an 'astrophysical' f value for the S  ii   λ 94.7-nm line: f =0.00498_−0.00138^+0.00172 , error at 1 σ level. This is almost a factor of 30 smaller than the guessed value found in the Kurucz data base (  f =0.1472) , which was until now the only one available for this transition. We use our 'astrophysical' f to investigate the S abundance in two damped Ly α absorption systems (DLAs) observed with the UV–Visual Echelle Spectrograph (UVES) at the European Southern Observatory's 8.2-m Kueyen telescope. In the case of the absorber at z _abs=3.02486 towards QSO 0347-3819, we find a sulphur column density which is consistent, within errors, with that determined by Centurión et al. by means of the λ 125.9-nm line, thus providing an external check on the accuracy of our f value. For the damped absorber at z _abs=4.4680 towards BR J0307-4945, we determine a high value of the S abundance, which, however, is probably the result of blending with Ly α forest lines.