Global Gradients Research Articles

Spatial Scale and Temporal Component of Selection in Side‐Blotched Lizards

Spatial variation in selection has long been recognized as promoting population divergence and in maintaining genetic polymorphisms, but selection at a fine spatial scale is seldom measured directly. We analyzed spatial and temporal variation in selective regimes on egg size using long-term population data of the side-blotched lizard (Uta stansburiana). Juvenile survival rates varied between years at a small spatial scale that was reflected as a strong interaction between the local neighborhood level and year. Spatially and temporally variable selection acted jointly on egg mass, which presumably would facilitate the maintenance of high additive genetic variance for this trait. Local selection gradients calculated at the neighborhood level were significantly correlated with the annual global selection gradients calculated at the metapopulation level. However, there was substantial variance in these local selective regimes, which suggests that strong local selection could go undetected if the analysis was limited to the global level. We also investigated the degree of spatial synchronization among outcrop in local selection gradients. The degree of synchrony was higher among later-clutch hatchlings than among first-clutch hatchlings, and we suggest that more intense density- and frequency-dependent selection on egg size later in the season is responsible for this effect.

Is there a dichotomy in the Dark Matter as well as in the Baryonic Matter properties of ellipticals?

We have found a correlation between the M / L global gradients and the structural parameters of the luminous components of a sample of 19 early-type galaxies. Such a correlation supports the hypothesis that there is a connection between the dark matter content and the evolution of the baryonic component in such systems.

Voltage–temperature characteristics in model coils using a Nb3Sn cable-in-conduit conductor for ITER

Abstract Two types of model coils (MC) were recently tested in order to assess possible designs of the international thermonuclear experimental reactor (ITER) conductor and Joints for the Toroidal field (TF) coils. One of these MC is wound using a stainless-steel-jacketed conductor, the Toroidal field model coil (TFMC), and the other a Ti-jacketed conductor, the Toroidal field coil insert (TFCI). This paper is dedicated to the electrical behaviour of these conductors through the V–T characteristics. In a first part we present the analysis of experimental results with a global thermo-electromagnetic model in which the current distribution is supposed uniform among the strands. Emphasis is put on all elements making the operating conditions of strands in a coil different from those of a single strand test (global strain, magnetic field and temperature gradients). The second part is dedicated to current distribution non-uniformity effects, either considered at the strand scale (TFMC) or at the subcable scale (TFCI). For this purpose, CEA developed a code (ENSIC) based on an electrical model network simulating conductor lengths and the associated joints. This model is applied to both coil tests. The last part includes a discussion on the validity and precision of the results provided by both models, compared to the experimental measurements.

INDO-PACIFIC BIODIVERSITY OF CORAL REEFS: DEVIATIONS FROM A MID-DOMAIN MODEL

Understanding the nature and causes of global gradients in species richness is a perennial ecological problem, and recent work has highlighted the need to assess these gradients relative to an appropriate statistical expectation. This paper examines latitudinal and longitudinal gradients in species richnesses of corals and reef fishes in the Indo-Pacific domain and compares them with gradients predicted by a mid-domain model in which geographic domains are located at random between the latitudinal and longitudinal boundaries of this region. We test for significant differences between observed and predicted species-richness patterns, and we identify regions that are enriched or depauperate in species, relative to expectation. In addition, we move beyond previous mid-domain analyses by directly comparing observed spatial distributions of geographic ranges with those predicted by a mid-domain model. This comparison indicates precisely how species-richness anomalies are produced by nonrandomness in the distribution of species ranges. For both corals and fishes, large and statistically significant differences exist between observed latitudinal and longitudinal species-richness gradients and those predicted by mid-domain models. Longitudinally, species richness is markedly higher than predicted along the African coast and, to a lesser extent, within the Indo-Australian Archipelago (IAA), and it is markedly lower than expected in the eastern Pacific. Latitudinally, species richness becomes increasingly higher than predicted as one moves from the equator to the tropical margins; then it becomes sharply lower than predicted beyond the tropics. Unexpectedly, differences between observed and predicted spatial distributions of range endpoints and midpoints reveal a pattern of nonrandomness that is highly congruent with the hypothesis that gyres in the Indian and Pacific Oceans, with the IAA forming a porous boundary between them, have a major influence on Indo-Pacific species-richness patterns. Our analyses indicate that the perspective offered by a focus on explaining nonrandomness in the location of geographic ranges (rather than explaining why species numbers vary in space) is likely to dramatically alter our assessments of alternative explanations for global species-richness gradients.

Response of planktic foraminiferal size to late Quaternary climate change

Geographical size distribution within entire Holocene foraminiferal assemblages is related to global environmental gradients such as temperature, primary productivity, and environmental variability. This study demonstrates that these correlations are also recognizable in late Quaternary assemblages from three locations in the South Atlantic on temporal and latitudinal scales. The size response to temporal paleoenvironmental changes during glacial‐interglacial cycles mimics the geographic Holocene size variability. The amplitude of size variability is directly related to the amplitude of the climatic fluctuations as shown by the stable size‐temperature relationship over time. The documented changes in the assemblage size are caused by species replacement and intraspecific size variability. The relative importance of these processes depends on the environmental setting. Species have been shown to reach their maximum size and abundance under certain optimum conditions and decrease in size if environmental conditions differ from these optima. We confirm that late Quaternary species sizes were largest at paleotemperatures identical to Holocene ones.

Modelling Dictyostelium discoideum Morphogenesis: the Culmination

The culmination of the morphogenesis of the cellular slime mould Dictyostelium discoideum involves complex cell movements which transform a mound of cells into a globule of spores on a slender stalk. We show that cyclic AMP signalling and differential adhesion, combined with cell differentiation and slime production, are sufficient to produce the morphogenetic cell movements which lead to culmination.We have simulated the process of culmination using a hybrid cellular automata/partial differential equation model. With our model we have been able to reproduce the main features that occur during culmination, namely the straight downward elongation of the stalk, its anchoring to the substratum and the formation of the long thin stalk topped by the spore head.We conclude that the cyclic AMP signalling system is responsible for the elongation and anchoring of the stalk, but in a roundabout way: pressure waves that are induced by the chemotaxis towards cyclic AMP squeeze the stalk through the cell mass. This mechanism forces the stalk to elongate precisely in the direction opposite to that of the chemotactically moving cells. The process turns out to be ‘guided’ by inactive ‘pathfinder’ cells, which form the tip of the stalk. We show that the entire development is enacted by means of the aforementioned building blocks. This means that no global gradients or different modes of chemotaxis are needed to complete the culmination.MPEG movies of the simulations are available on-line: http://www-binf.bio.uu.nl/stan/bmb.

Palaeobiogeography and the Ordovician and Mesozoic-Cenozoic biotic radiations

Abstract Curves of taxonomic diversity through geological time consistently show major evolutionary radiations during the Ordovician Period and from the mid-Mesozoic to the present day. Both intervals were characterized by marked biotic provincialism, reflecting episodes of major continental break-up and global tectonism, and their later histories featured steep global climatic gradients. The Ordovician radiation can be recognized at a wide spectrum of taxonomic levels from species to class, and the biogeographical patterns associated with the radiation of individual clades reflect a complex combination of plate distribution, tectonic activity, sedimentary environment, sea-level rise and, ultimately, glaciation. The true scale of the mid-Mesozoic-Cenozoic biotic radiation is currently a topic of intense debate but there is no doubt that it affected plants and animals in both the marine and terrestrial realms. The role of land bridges and ocean gateways in controlling the formation of biodiversity patterns has been a persistent theme in Mesozoic-Cenozoic biogeography, and a complex set of Neogene tectonic events probably aided the development of both latitudinal and longitudinal provinces during the Cenozoic. The present volume highlights some of the successes across a spectrum of approaches to unravelling the Ordovician and Mesozoic-Cenozoic radiations within the context of palaeobiogeography.

Spiral Galaxies with [ITAL]HST[/ITAL]/NICMOS. II. Isophotal Fits and Nuclear Cusp Slopes

We present surface brightness profiles for 56 of the 78 spiral galaxies observed in the HST/NICMOS2 F160W snapshot survey introduced in Paper I of this series, as well as surface brightness profiles for 23 objects out of the 41 that were also observed in the F110W filter. We fit these surface brightness profiles with the Nuker law of Lauer et al. and use the smooth analytical descriptions of the data to compute the average nuclear stellar cusp slopes γ in the 01–05 radial range. Our main result is the startling similarity between the nuclear stellar cusp slopes γ in the near-infrared compared with those derived in the visual passband. This similarity has several implications: (1) Despite the significant local color variations that are found in the nuclear regions of spirals and that are documented in Paper I, there are typically little or no optical-NIR global color gradients, and thus no global stellar population variations, inside ~50–100 pc from the nucleus in nearby spirals. (2) The large observed range of the strength of the nuclear stellar cusps seen in the HST optical study of spiral galaxies reflects a physical difference between galaxies and is not an artifact caused by nuclear dust and/or recent star formation. (3) The dichotomy between R1/4 bulges, with steep nuclear stellar cusps γ ~ 1, and exponential bulges, with shallow nuclear stellar cusps γ < 0.3, is also not an artifact of the effects of dust or recent star formation. (4) The presence of a surrounding massive disk appears to have no effect on the rise of the stellar density distribution within the innermost hundred parsecs of the R1/4 spheroids. These results imply a breakdown within the family of exponential bulges of the nuclear versus global relationships that have been found for the R1/4 spheroids. Such a breakdown is likely to have significant implications concerning the formation of exponential bulges and their connection with the R1/4 spheroids.

A critique of Phanerozoic climatic models involving changes in the CO 2 content of the atmosphere

A critique of Phanerozoic climatic models involving changes in the CO 2 content of the atmosphere

Homogenization of the Stellar Population along Late-Type Spiral Galaxies

We present a study of the broadband UBV color profiles for 257 Sbc barred and nonbarred galaxies, using photoelectric aperture photometry data from the literature. Using robust statistical methods, we have estimated the color gradients of the galaxies, as well as the total and bulge mean colors. A comparative photometric study using CCD images was done. In our sample, the color gradients are negative (reddish inward) in approximately 59% of the objects, are almost null in 27%, and are positive in 14%, considering only the face-on galaxies, which represent approximately 51% of the sample. The results do not change, essentially, when we include the edge-on galaxies. As a consequence of this study we have also found that barred galaxies are overrepresented among the objects having null or positive gradients, indicating that bars act as a mechanism of homogenization of the stellar population. This effect is more evident in the U-B color index, although it can also be detected in the B-V color. A correlation between the total and bulge colors was found that is a consequence of an underlying correlation between the colors of bulges and disks found by other authors. Moreover, the mean total color is the same irrespective of the gradient regime, while bulges are bluer in galaxies with null or positive gradients, which indicates an increase of the star formation rate in the central regions of these objects. We have also made a quantitative evaluation of the amount of extinction in the center of these galaxies. This was done using the Wide Field Planetary Camera 2 (WFPC2) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Space Telescope (HST) archival data, as well as CCD B, V, and I images. We show that although the extinction in the V-band can reach values up to 2 mag in the central region, it is unlikely that dust plays a fundamental role in global color gradients. We found no correlation between color and O/H abundance gradients. This result could suggest that the color gradients are more sensitive to the age rather than to the metallicity of the stellar population. However, the absence of this correlation may be caused by dust extinction. We discuss this result by considering a picture in which bars are a relatively fast, recurrent phenomenon. These results are not compatible with a pure classical monolithic scenario for bulge and disk formation. On the contrary, they favor a scenario in which both these components are evolving in a correlated process in which stellar bars play a crucial role.

Stable carbon isotope distribution of particulate organic matter in the ocean: a model study

The stable carbon isotopic composition of particulate organic matter in the ocean, δ13CPOC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ13CPOC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ13CPOC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ13CPOC are much greater than in δ13CDIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ13CPOC. Understanding the factors that control isotope variability is a prerequisite when applying δ13CPOC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ13CPOC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO2 in seawater and the organic material produced by algae, εP, is a function of the ambient CO2 concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, εP, depends on the CO2 concentration in seawater and on the (instantaneous) growth rates, μi, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ13CPOC distribution. The interhemispheric asymmetry of δ13CPOC can mostly be attributed to the interhemispheric asymmetry of CO2 concentration in the water. However, the strong seasonal variations of δ13CPOC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.

Effects of Stenting on Blood Flow in a Coronary Artery Network Model

The effect of stenting on blood flow is investigated using a model of the coronary artery network. The parameters in a generic non‐linear pressure–radius relationship are varied in the stented region to model the increase in stiffness of the vessel due to the presence of the stent. A computationally efficient form of the Navier–Stokes equation is solved using a Lax–Wendroff finite difference method. Pressure, vessel radius and flow velocity are computed along the vessel segments. Results show negative pressure gradients at the ends of the stent and increased velocity through the middle of the stented region. Changes in local flow patterns and vessel wall stresses due to the presence of the stent have been shown to be important in restenosis of vessels. Local and global pressure gradients affect local flow patterns and vessel wall stresses, and therefore may be an important factor associated with restenosis. The model presented in this study can be easily extended to solve flows for stented vessels in a full, anatomically realistic coronary network. The framework to allow for the effects of the deformation of the myocardium on the coronary network is also in place.

Distinguishing climatic and tectonic forcing of turbidite sedimentation, and the bearing on turbidite bed scaling: Palaeocene-Eocene of northern Spain

Mineral composition and carbon isotope signals of pelagic limestones and hemipelagic marlstones reveal the driving processes of interbedded turbidites. The Palaeocene to lower Eocene lower bathyal Zumaia series in northern Spain between Zumaia and Getaria (W of San Sebastian) offers this opportunity. This deep-marine series records the transition from a carbonate system (Palaeocene) to a siliciclastic system (early Eocene), mainly controlled by the initiation of Pyrenean uplift in the latest Palaeocene. During the tectonically quiet Palaeocene, varying carbonate turbidite deposition was controlled by climate-dependent production of (foramol association) carbonate debris on the carbonate shelves bordering the basin. The late Palaeocene period was characterized by a cycle of increasing and then decreasing turbidite sedimentation. Increased carbonate production was driven by comparably cool/dry climates in the late Palaeocene Atlantic region, and global marked temperature gradients which enhanced wind-driven Ekman-upwelling of nutrients. In contrast, during warm/perennially humid periods (early Palaeocene and the Palaeocene-Eocene transition) the dynamics of the carbonate turbidite deposition was sluggish, due to decreased carbonate production. In the carbonate system, frequency-size statistics of bed thicknesses show that turbidite generation occurred non-scale invariant. From the earliest Eocene, increasing siliciclastic input from the rising and west advancing Pyrenean chain is observed. After a transitional period, the highly dynamic lower Eocene siliciclastic system evolved on a prograding deep-sea fan under a perennially humid and seasonally wet climate. Several periods of increased tectonic activity and sediment input are recognized by clay mineral assemblages in interturbidite beds. Turbidite bed frequency-size statistics show a power-law distribution, implying that during such periods (particularly NP 12), the depositional system temporarily reached a self-organized critical state. The evolution of the system to critical state appears to be mainly driven by siliciclastic sediment input.

Durability Scaling of Cracking in HPC Structures Subject to Hygromechanical Gradients

The cracking of concrete is one of the main causes of limited life spans of concrete structures. Besides external actions, cracking can be induced by phenomena involving hygromechanical coupling. Such phenomena involve two types of scale effects—one related to local or global diffusion gradients, the other to size effects on cracking in concrete structures. This paper presents results of a research program on the durability of nuclear cooling towers, with minimum reinforcement, constructed with normal and high-strength concrete, when subjected to hygric and thermal gradients. To this end, a combined experimental-numerical approach is applied involving 1:2 scale structural tests and finite-element durability analysis using a probabilistic crack approach. The model is presented and then validated with experimental data. Finally, from 1:1 scale numerical analysis of the cracking due to drying, it is shown that the structural durability performance, in terms of crack opening, is governed by the evaporable water in concrete; it scales both the magnitude and the time of drying, and thus the crack opening and its long-term propagation. At higher moisture content, this durability performance is only slightly influenced by a high strength concrete matrix or by an increased steel reinforcement.

Similar glacial and Holocene deep water circulation inferred from southeast Pacific benthic foraminiferal carbon isotope composition

We present Holocene and last glacial maximum (LGM) oxygen and carbon isotope measurements on Planulina wuellerstorfi in six southeast Pacific cores. Sedimentation rates are low in this part of the ocean, and measurements were made on individual foraminiferal shells in order to identify the Holocene and glacial individuals on the basis of their extreme δ18O. The new δ13C data were combined with previous P. wuellerstorfi data for interpretation of global thermohaline circulation. Data from the Southern Ocean were examined closely for regional coherency and a few anomalous δ13C values suspected of having productivity overprint were removed. The resulting global δ13C distributions and gradients indicate that the deep water circulation was similar during the Holocene and LGM. This interpretation brings δ13C data to a better agreement with Cd/Ca data and marks a sharp contrast with a widely held view based on δ13C measurements that the glacial Southern Ocean was the terminus of the thermohaline circulation. The proposed presence of glacial North Atlantic Deep Water does not necessarily contradict the postulated presence of Glacial North Atlantic Intermediate Water.

Abundance Distributions in Barred Galaxies

AbstractI review the evidence for global abundance gradients in barred galaxies to be flatter when compared with those of normal galaxies. The finding that the abundance gradient becomes flatter as the strength of the bar increases, illustrates the effectiveness of radial flows induced by bars at transporting large amount of gas across galaxy disks.

Order and disorder in auditory cortical maps

Recent physiological experiments suggest that several basic receptive field properties of neurons show non-uniform spatial distributions in the primary auditory cortex of cats and primates. The spatial distribution patterns of some of these receptive field parameters are suggestive of a parallel coding scheme for processing sound information onto several superimposed cortical ‘maps’. The representations of these parameters in the auditory cortex are compatible with general features of self-organizing mapping algorithms, as the spatial representations exhibit global parameter gradients with overlaid functional patchiness. Recent studies have also revealed that within a ‘representational’ map, the degree of local coherence varies over a wide range and that the map can contain a substantial degree of disorder in its parametric representation of sounds. Although the causes and consequences of this representational disorder are not known, it may reflect yet unresolved organizational principles in the auditory cortex.

A new role for the plasmaphere in “quiet” ionospheric electrodynamics

Global scale longitudinal gradients of pressure in the plasmasphere may be formed naturally by ionospheric processes, or caused by electrostatic fields of ionospheric dynamo origin. It is shown that plasmaspheric gradients of pressure, orthogonal both to the magnetic field ( B) and to grad B, generate geophysically significant field-aligned currents. Considering the ionosphere and plasmasphere as a coupled electrodynamic system, these currents alter non-negligibly the self-consistent ionospheric electric field and current. Criteria are established for this coupling mechanism (a kind of plasmaspheric impedance) to be significant. This has implications for the relationships of ionospheric electric fields and currents, F-region drifts, and magnetic variations, due to upper atmosphere tides and winds.

Radar wind profiler observations of solar semidiurnal atmospheric tides

Semidiurnal solar tides in the mid‐latitude troposphere are investigated using harmonic analysis of 404 MHz radar profiler wind data obtained from a wide longitude zone in the U.S. The tides are apparent above a 1000‐m‐deep surface layer and increase in amplitude with height, attaining speeds of 0.5–0.7 m/s at 5–7 km. Observed wind characteristics agree well with tidal characteristics obtained with a dynamical model driven by observed global semidiurnal horizontal pressure gradients.

The influence of bars on the chemical composition of spiral galaxies

view Abstract Citations (212) References (79) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Influence of Bars on the Chemical Composition of Spiral Galaxies Martin, Pierre ; Roy, Jean-Rene Abstract Imaging spectrophotometry in the nebular lines Hα, Hβ, [O III] λ5007 and [N II] λ6584, using narrow-band interference filters, has been performed of 82, 55, and 79 H II regions in the barred spiral galaxies NGC 925, NGC 1073, and NGC 4303, respectively. The O/H abundance gradients were derived from the abundance indicators [O III],/Hβ and [N II]/[O III], calibrated by Edmunds & Pagel (1984). The global O/H gradients in NGC 925 [{DELTA} log (O/H)/{DELTA}R = -0.033 dex kpc^-1^] and NGC 1073 [{DELTA} log (O/H)/{DELTA}R = -0.048 dex kpc^-1^] are flatter than the gradients observed in normal galaxies of the same morphological type. The abundance gradient in NGC 4303 is identical to that found in normal spiral galaxies. A comparison is made between the O/H gradients of normal galaxies and of a sample of galaxies showing a barred morphology. It is concluded that the global abundance gradients of spiral galaxies with a barred structure are in general shallower than gradients of normal galaxies. The slopes of O/H gradients are analyzed as a function of two properties of bars: the relative length of the bar with respect to the size of the disk, and the bar ellipticity, defined by the axis ratio of the bar. It is found that gradients are flatter when the length or the ellipticity of the bar increases. This result is consistent with recent models of radial flows. These observations indicate that large-scale mixing of the interstellar gas occurs across the disks of barred spiral galaxies and affects the radial distribution of elements. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/173917 Bibcode: 1994ApJ...424..599M Keywords: Abundance; Barred Galaxies; Galactic Structure; H Alpha Line; H Beta Line; H Ii Regions; Hydrogen; Nitrogen; Oxygen; Spiral Galaxies; Astronomical Models; Charge Coupled Devices; Image Analysis; Imagery; Interstellar Gas; Kinematics; Optical Fibers; Spectrophotometry; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: STRUCTURE; GALAXIES: SPIRAL; ISM: H II REGIONS full text sources ADS | data products SIMBAD (24) NED (24) CDS (1)