Strong Chemical Gradients Research Articles

Ecosystem dynamics at a productivity gradient: A study of the lower trophic dynamics around the northern atolls in the Hawaiian Archipelago

Application of a lower trophic ecosystem model coupled with an ocean circulation model allows us to derive a time series of estimated plankton biomass for waters around Kure Atoll, Pearl & Hermes Atoll and Midway Islands, over the period 1964–2006. This time series shows considerable interannual and decadal variation in productivity, and changes in recent years that are coherent with observed changes at higher trophic levels. Further the model results show that the change in productivity can be viewed from the perspective that the atolls, at the northern edge of the subtropical gyre, are situated at a strong physical, chemical and biological gradient. The gradient moves north and south seasonally and interannually resulting in changes in lower trophic level productivity around the atolls fixed in position. Variation in biome boundaries or gradients may be a key factor in ecosystem dynamics in many ecosystems located near such boundaries.

Habitat Associations in Gastropod Species at East Pacific Rise Hydrothermal Vents (9°50′N)

At deep-sea hydrothermal vents on the East Pacific Rise (9 degrees 50'N), distinct megafaunal assemblages are positioned along strong thermal and chemical gradients. We investigated the distribution of gastropod species to determine whether they associate with specific megafaunal zones and to determine the thermal boundaries of their habitats. Gastropods colonized a series of basalt blocks that were placed into three different zones characterized by vestimentiferan tubeworms, bivalves, and suspension-feeders, respectively. Additional gastropods were collected on selected blocks from higher temperature vestimentiferan habitat and from grab samples of alvinellid polychaetes. On the blocks, gastropod species clustered into a "Cool" group (Clypeosectus delectus, Eulepetopsis vitrea, Gorgoleptis spiralis, and Lepetodrilus ovalis) whose species tended to be most abundant in the suspension-feeder zone, and a "Warm" group (Lepetodrilus cristatus, L. elevatus, L. pustulosus, and Cyathermia naticoides) whose species all were significantly more abundant in the vestimentiferan zone than elsewhere. The temperature ranges of Cool species were generally lower than the ranges of Warm ones, although both groups were present at 3 to 6 degrees C; also present was Bathymargarites symplector, which clustered with neither group. Three additional species, Rhynchopelta concentrica, Neomphalus fretterae, and Nodopelta rigneae, co-occurred with Warm-group species on selected blocks from hotter habitats. Although a few species were found only in alvinellid collections, most species were not exclusive to a specific megafaunal zone. We propose that species in the Cool and Warm groups occupy specific microhabitats that are present in more than one zone.

On the occurrence and ecological features of deep chlorophyll maxima (DCM) in Spanish stratified lakes

Deep chlorophyll maxima (DCM) are absolute maxima of Chlorophyll-a concentration among the vertical profile that can be found in deep layers of stratified lakes. In this manuscript I review the principal mechanisms that have been argued to explain the formation of DCM, which include, among others, in situ growth of metalimnetic phototrophs, differential impact of grazing between the different lake strata, and passive sedimentation to the layers where water density and cell density are equalized. The occurrence of DCM in Spanish lakes, as well as the main ecology characteristics of the oxygenic phototrophs that form DCM in these lakes is also reported. Cyanobacteria, either filamentous or unicellular, and cryptophytes, are the main components of most DCM found in the reported Spanish lakes, although diatoms, chrysophytes, dinoflagellates, and chlorophytes also contribute to these chlorophyll maxima. These organisms cope with strong physical and chemical gradients, among which those of water density, light and inorganic nutrient availability, and sulphide concentrations appear to be the most determinant factors influencing planktonic community structure.

Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: Evidence for a syn-glacial sulfidic deep ocean

We have analyzed δ34S (sulfate and pyrite), δ18Ocarbonate and δ13Ccarbonate, and major and trace elemental concentrations, including extractable Fe and Mn phases, in four sections of the Maieberg Formation, the cap-carbonate sequence to the Marinoan glaciation in northern Namibia. δ34Ssulfate profiles and other geochemical characteristics in the basal, transgressive cap dolostone (Keilberg Member) are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations. In the overlying interval that consists of rhythmites deposited during the transgressive high-stand, large geochemical disparities occur between sections that we interpret to have been deposited in open-ocean versus restricted settings. In the former, a large negative shift in δ34Ssulfate of ∼20‰ above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in δ18O and δ13C, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, δ34Ssulfate increases abruptly, δ18O and δ13C are invariant, and a spike in Mn and Fe concentrations is present, but subtle. These contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold, euxinic deep water and a cap of warm, oxic, brackish water that flooded the continental shelf during the post-glacial transgression. The geochemical differences between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the waters along the Otavi platform.

Ontogenetic vertical distribution and diel migration of the copepod Eucalanus inermis in the oxygen minimum zone off northern Chile (20–21° S)

The vertical and ontogenetic distribution, and diel vertical migration (DVM), of Eucalanus inermis in relation to the strong vertical gradient in oxygen concentration associated with an intense oxygen minimum zone (OMZ) were studied at a coastal area off northern Chile (20-21° S). A close relationship between the abundance of the whole copepod population and law oxygen waters was found, with most developmental stages remaining near the base of the oxycline (30-80 m) and within the upper zone of the OMZ (30-200 m). All stages performed DVM but not at all the stations, mainly between the 30-60 and 60-200 m strata; a small fraction (<20%) appeared in the surface layer (0-30 m) mostly at night. This strategy of movement would result in a better utilization of food resources since the strong physical and chemical gradients at the base of the oxycline and upper OMZ boundary might serve as a site of particle accumulation. A secondary fluorescence peak was, in fact, found at all the stations, coinciding with minimal dissolved oxygen (DO, <1 mL O 2 L -1 ) at the base of the oxycline or in the upper OMZ boundary. The relevance of the biogeochemical flux involved in this diel migration pattern was assessed by calculating the potential active input of carbon and nitrogen from the upper layers into deeper the OMZ.

Large‐Scale CO and [Ci] Emission in the ρ Ophiuchi Molecular Cloud

We present a comprehensive study of the ρ Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 1600 arcmin2 maps (2.3 pc2) with 0.1 pc resolution in submillimeter CO (4 → 3) and [C I] (3P1 → 3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) and pointed observations in the CO (7 → 6) and [C I] ( 3P2 → 3P1) lines. Within the large-scale maps, smaller spectral line maps of 3000 AU resolution over ~90 arcmin2 (0.2 pc2) of the cloud in CO, CS, HCO+, and their rare isotopomers are made at the Heinrich Hertz Telescope (HHT) in Arizona. Comparison of CO, HCO+, and [C I] maps with far-infrared observations of atomic and ionic species from the Infrared Space Observatory (ISO) far-infrared and submillimeter continuum emission and near-infrared H2 emission allows clearer determination of the physical and chemical structure of the ρ Oph PDR, since each species probes a different physical region of the cloud structure. Although a homogeneous plane-parallel PDR model can reproduce many of the observations described here, the excitation conditions needed to produce the observed HCO+ and [O I] emission imply inhomogeneous structure. Strong chemical gradients are observed in HCO+ and CS; the former is ascribed to a local enhancement in the H2 ionization rate, and the latter is principally due to shocks. Under the assumption of a simple two-component gas model for the cloud, we find that [C II] and [C I] emission predominantly arises from the lower density envelopes (103-104 cm-3) that surround denser cloud condensations, or clumps. The distribution of [C I] is very similar to that of C18O and is generally consistent with illumination from the far side of the cloud. A notable exception is found at the western edge of the cloud, where UV photons create a PDR viewed edge-on. The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD 147889 and a total gas column density that follows that of C18O, decreasing toward the edges of the cloud. In contrast to the conclusions of other studies, we find that no nonequilibrium chemistry is needed to enhance the [C I] abundance. Each spectral line is traced to a particular physical component of the cloud and PDR. Although CO rotational line emission originates from both dense condensations and diffuse envelopes, the millimeter-wave transitions mostly find their origins in envelope material, whereas the high-J submillimeter lines stem more from the dense clumps. Submillimeter HCO+ and infrared [O I] and [C II] emission indicate clump surface temperatures of 50-200 K, an ultraviolet radiation field with IUV ≈ 10-90, densities of 105-106 cm-3, and interior temperatures of ≤20 K. This study highlights the value of large-scale infrared and submillimeter mapping for the interpretation of molecular cloud physical and chemical structure, and important future observations are highlighted.

CCD Studies of δ Scuti Stars in Open Clusters

AbstractAsteroseismology on δ Scuti stars has until now produced very few convincing results – if we aim at doing strong tests of details of stellar modelling. The main reason for the lack of success is probably that these stars often rotate, which split nonradial oscillation frequencies into many more frequencies. These many frequencies and the fact that the more evolved δ Scuti stars contain a strong chemical composition gradient at the edge of the convective core, produce a very complicated eigenfrequency spectrum. In contrast to this, we expect, in principle, seismological studies of δ Scuti stars to be a very simple task: One has to compare theoretical oscillations in model stars with the observed oscillations. However, in order to produce convincing asteroseismological results, we need to do three things: (1) Detect as many eigenfrequencies as possible at high precision, (2) identify the eigenmodes and (3) improve the theoretical models. By observing δ Scuti stars in open clusters using CCDs, we have a possibility to improve on (1) and (2) as well as providing an opening for an improvement in the theoretical models by doing accurate calibrations of the basic cluster properties. In this paper I shall describe some of the results from CCD studies of δ Scuti stars in open clusters and identify some future prospects for this technique.

Adsorption Parameters for Radioactive Liquid-Waste Migration

Proton titration experiments have been conducted at the Idaho National Engineering and Environmental Laboratory on synthetic goethite and soil in an effort to develop adsorption parameters that will help predict migration of radioactive liquid waste. This is the initial step in a reactive transport project to understand contaminant migration in a system characterized by strong chemical gradients. For this stage, two levels of pretreatment were applied to the soil to remove carbonate minerals and soluble salts to focus on the remaining mineral fraction. Without some sort of treatment or conditioning, native soil has a large buffer capacity that interferes with proton titration experiments. In this report, results are presented from the initial stages of the project.

Microbiology in nuclear waste disposal: interfaces and reaction fronts.

It is now generally acknowledged that microbial populations will be present within nuclear waste repositories and that the consequences of such activity on repository performance must be assessed. Various modelling approaches--based either on mass balance/thermodynamics or on kinetics--have been developed to provide scoping estimates of the possible development of these populations. Past work has focused on particular areas of the repository which can be considered relatively homogeneous and hence can be represented by some kind of 'box' or 'mixing tank'. In reality, however, waste repositories include a range of engineering materials (steel, concrete, etc.) which are emplaced at depth in a rock formation. Strong chemical gradients--of the type which may be exploited by lithoautotrophic microbial populations--are likely to be found at the contacts between different materials and at the interface between the engineered structures and the host rock. Over the long timescales considered, solute transport processes will cause the locations of strong chemical gradients to move, forming reaction fronts. The high-pH plume resulting from the leaching of cement/concrete in some repository types is a particularly important example of such a reaction front. Redox fronts, which may occur in different areas of all kinds of repositories, also play an important role and would be locations where microbial activity is likely to be significant. In this paper, the key microbial processes expected at (or around) interfaces and fronts will be discussed, with particular emphasis on the development of quantitative models. The applicability of the models used wil be tested by considering similar fronts which can be found in natural systems.

Extended synchrotron X-ray reflectivity study of a Sm-based layer buried into CdTe(001)

We present an extended X-ray reflectivity study of a Sm-based epitaxial layer buried into CdTe(001) in order to investigate its structure. The measured reflectivity is modelled over a wide range of momentum transfer and the adjustment of the calculated intensity gives quantitative information about the electron density and interplanar distance profiles along the surface normal. Complementary experiments of photoemission spectroscopy allow the interpretation of the in-plane electron density in terms of chemical composition. High resolution electron microscopy shows that the very strong chemical gradients deduced from the model result from an original topology of the layer.

Consequences of riverine flooding for seston and theperiphyton of floating meadows in an Amazon floodplain lake

Algal biomass and the chemical composition of seston and floating meadow periphyton were measured in an Amazon floodplain lake during a period of channelized river inflow and a period of overbank flow. During both periods, sedimentation of riverine particulates and loss of dissolved nutrients created strong spatial chemical gradients from the river onto the floodplain which were stable for months at a time. The molar : P of dissolved inorganic nutrients supplied by river water was very low (NO3− + NH4+ : SRP &lt; 5) and decreased further as water entered the floodplain. Initial loss of sediment in surface waters increased the organic content of seston and periphyton and decreased the C : N of seston within 2 km of the river. More gradual loss of finer P‐rich particles caused 2−4‐fold increases in N : P and C : P ratios inside the floodplain. During channelized river inflow, epiphytic algal biomass peaked where inorganic turbidity was high (&gt;50 mg liter−1) and phytoplankton biomass was low. Later in the year, overbank flow elevated dissolved nutrient levels inside the lake and disrupted thermal stratification. This transition from lentic to lotic conditions coincided with the seasonal high for epiphytic algal biomass and the seasonal low for phytoplankton biomass. Overall, epiphytic algal biomass was 4–10 times higher than phytoplankton biomass on a per unit area basis. In situ bag experiments revealed that inorganic turbidity regulates the ability of attached algae to utilize nutrients supplied by river water. Epiphytic algal biomass decreased after contact with undiluted river water. Epiphytic algal biomass increased 5‐fold in river water diluted by 90%.

Mineral and whole-rock compositions of seawater-dominated hydrothermal alteration at the Arctic volcanogenic massive sulfide prospect, Alaska

The Arctic volcanogenic massive sulfide prospect, located in the Ambler mineral district of northwestern Alaska, includes three types of hydrothermally altered rocks overlying, underlying, and interlayered with semimassive sulfide mineralization. Hydrothermal alteration of wall rocks and deposition of sulfide and gangue minerals were contemporaneous with Late Devonian or Early Mississippian basalt-rhyolite volcanism. Whole-rock analyses of altered rocks surrounding the prospect indicate that strong chemical gradients exist in major and minor oxides and volatiles. These were not homogenized or overprinted by Late Jurassic (?) to Middle Cretaceous greenschist to glaucophane-schist facies metamorphism. Very low alkali contents and moderate to very high contents of Ba, F, and Mg are the major chemical characteristics of the pervasively altered rocks. The strong Mg metasomatism, high Ba contents, and limited extent of altered rocks suggest a rapid influx of relatively cold seawater into a hot hydrothermal vent system. Alteration developed asymmetrically around a linear fissure, suggesting fracture control of ore fluids rather than a point source. Micro-probe analyses of phyllosilicates from the Arctic area indicate two discrete mineral populations. Magnesian chlorite, barian white mica, and barian fiuorphlogopite in hydrothermally altered rocks have compositions distinct from similar minerals (chlorite, phengite, biotite) developed during high-pressure metamorphism in metapelitic and metavolcanic lithologies. These differences in mineral chemistry are the result of differences in protolith composition caused by hydrothermal alteration-metasomatism. Without mineral composition data, the contacts between some alteration assemblages and relatively unaltered metavolcanic and metasedimentary rocks with similar mineralogy cannot be determined.

Satellite image analysis techniques applied to oceanography

Infrared and visible spectral sensors on spacecraft can provide imagery of ocean conditions equivalent to a global network of in situ sensors. Oceanographers can now view all the world’s oceans during one day’s travel of a polar-orbiting satellite. The surface areas of complex ocean regions that have strong thermal and chemical gradients, such as the Gulf Stream in the Atlantic and the Kuroshio in the Pacific, are displayed in minute detail in the imagery provided by satellite infrared and visible sensors. Because of the vast quantities of data involved in satellite infrared and visible imagery, computer techniques must be used to derive oceanographic information. A variety of basic computer image analysis techniques are required to do so. These basic techniques include selective enhancement, geographic registration, absolute ocean radiation and multiple-image composition. Through the use of these techniques, satellite and conventional oceanographic data can be integrated to form synergistically a powerful analytical tool for modern ocean research.