Influence of Organic Enrichment and Spisula subtruncata (da Costa, 1778) on Oxygen and Nutrient Fluxes in Fine Sand Sediments

[EN] The role of labile organic material and macrofaunal activity in benthic respiration and nutrient regeneration have been tested in sublittoral fine sand sediments from the Gulf of Valencia (northwestern Mediterranean Sea). Three experimental setups were made using benthic chambers. One experiment was performed in-situ through the annual cycle in a well-sorted fine sand community. The remaining experiments were carried out with mesocosms under laboratory conditions: one with different concentrations of organic enrichment (mussel meat and concentrated diatoms culture), and the other adding two different densities of the endofaunal bivalve Spisula subtruncata. Biochemical variables in surface sediment and changes in oxygen consumption and nutrient fluxes throughout incubation period were studied in each experiment. In the in situ incubations, dissolved oxygen (DO) fluxes showed a strong correlation with sedimentary biopolymeric fraction of organic carbon. Organic enrichment in the laboratory experiments was responsible for increased benthic respiration. However, sediment response (expressed as DO uptake and dissolved inorganic nitrogen—DIN—release) between oligotrophic and eutrophic conditions was more intense than between eutrophic and hypertrophic conditions. S. subtruncata abundances close to 400 and 850 ind m−2 also intensified benthic metabolism. DO uptake and DIN production in mesocosms with added fauna were between 60 and 75 % and 65–100 % higher than in the control treatment respectively. The results of these three experiments suggest that the macrobenthic community may increase the benthic respiration by roughly a factor of two in these bottoms, where S. subtruncata is one of the dominant species. Both organic enrichment and macrobenthic community in general, and S. subtruncata in particular, did not seem to have a relevant role in P and Si cycles in these sediments.This research was supported by the Conselleria d'Educacio (Generalitat Valenciana). We are very grateful for the valuable comments of anonymous reviewers on previous version of the manuscript.Sospedra, J.; Falco, S.; Morata, T.; Rodilla, M. (2016). Influence of Organic Enrichment and Spisula subtruncata (da Costa, 1778) on Oxygen and Nutrient Fluxes in Fine Sand Sediments. Estuaries and Coasts. doi:10.1007/s12237-016-0174-1SAller, R.C., and J.Y. Aller. 1998. The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments. Journal of Marine Research 56: 905–936.Aminot, A., and M. Chaussepied. 1983. Manuel des analyses chimiques en milieu marin. Brest: Centre National pour l’Exploitation des Oceans.Arnosti, C., and M. Holmer. 2003. 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Thermo-mechanical behavior of a granodiorite from the Liquiñe fractured geothermal system (39°S) in the Southern Volcanic Zone of the Andes

Fractures and faults in granitic rocks play an important role in geothermal systems because they permit the circulation of hot fluids. However, the thermo-hydro-mechanical behavior of granitic rocks has predominantly been studied at temperatures exceeding 300 °C but many geothermal systems experience temperatures much lower than this. The aim of this study was to evaluate how the depth, temperature, and amount and rate of mechanical loading associated conditions, that are realistic in low temperature geothermal system, influence the physical properties of geothermal reservoir hosting rock. We carried out both room temperature and low temperature thermo-mechanical tests on a granodiorite sample from the Liquiñe area, Chile, and performed post-experimental X-ray microtomography analysis to numerically estimate the permeability of the generated fractures. The results showed that both rock strength and rock stiffness decreased with increments of temperature treatment related to the development of thermal crack damage at temperatures > 150 °C and through the development of sub-critical cracking at constant temperatures between 50–75 °C. Slowest deformed samples also exhibited lower strengths, attributed to the development of sub-critical cracking. The cyclic triaxial loading test indicated that significant mechanical fracture damage was only initiated above 80% of the peak stress regardless of the number of repeated loading cycles at lower stresses. Low-temperature treatment appears to be a conditioning factor, but not the dominant factor in controlling the physical properties of reservoir hosting rocks. Our findings indicate that thermal crack damage is likely important for developing microfracture related permeability at depths between around 2–6 km where the temperature is sufficiently high to induce thermal cracking. At shallower depths, such was previously estimated the reservoir of Liquiñe, thermal crack damage is only generated adjacent to fractures that remain open and circulate the hot fluids but sub-critical cracking over time reduces the strength of rocks in lower temperature regimes. These processes combined to produce a geothermal reservoir in Liquiñe which likely first required the presence of a highly fractured fault zone

Variabilidad estacional de la comunidad de fitoplancton de las aguas superficiales de la zona costera de Gandía en el sur del Golfo de Valencia

[EN] Seasonal variability in the phytoplankton community of the coastal area of Gandia in the south of the Gulf of Valencia (Western Mediterranean Sea) was examined in relation to physical and chemical surface water variables (i.e. salinity, nutrients, dissolved oxygen and temperature). This small area presents most of the point and non-point nutrients inputs that affect coastal areas as wastewater discharges through submarine outfall, river discharges and groundwater discharges from a detritic aquifer. Furthermore, surface channels that drain the Safor wetland, which is used mainly for agricultural crops, outflow into the confined harbour. The main objective of the study was to observe the variations in phytoplankton groups as a response to environmental variables during different seasons and understand which species could be used as indicators of anthropogenic pressure. For this purpose, the taxonomic composition of the micro-phytoplankton communities at 32 fixed stations was determined in four sampling campaigns from summer 2010 to spring 2011. The results indicate that nutrient inputs mainly from the Serpis river and channels that drain the Safor Wetland determine the composition and abundance of the phytoplankton community, and that several key environmental factors such as water temperature, radiation, nutrients, and the molar ratios of nutrients influence seasonal phytoplankton assemblages. However, the discharge of effluent from a sewage treatment plant through the submarine outfall did not appear to have a significant impact on the phytoplankton community. The phytoplankton community comprised two main groups: diatoms and dinoflagellates and a total of 108 taxa were identified. The diatom population primarily flourished in autumn and winter whereas in spring, dinoflagellate bloom occurred with high radiation, very low DIP and high DIN:DIP and low DSi:DIN molar ratios. In this paper we discuss the possible rationale for these nutrient changes. Furthermore, potentially blooming species were detected in the Gandia harbour and in the mouth of the Serpis river at Venecia Beach.[ES] La variabilidad estacional de la comunidad de fitoplancton de las aguas superficiales de la zona costera de Gandía en el sur del Golfo de Valencia (Mediterráneo occidental) fue examinada, teniendo en cuenta su relación con diferentes variables físicas y químicas (p.e. salinidad, nutrientes, oxígeno disuelto, temperatura, etc.). En esta pequeña área se dan la mayoría de las entradas de nutrientes, tanto puntuales como difusas, que afectan las áreas costeras: descargas de aguas residuales mediante emisarios submarinos, aportes fluviales y descargas de aguas subterráneas procedentes de acuíferos costeros. Además, los canales superficiales que drenan el humedal de La Safor, cuyo uso fundamental es la agricultura, vierten en las aguas confinadas del puerto. El objetivo principal de este estudio fue observar las variaciones de los grupos de fitoplancton como respuesta a las variaciones ambientales en diferentes estaciones del año, y determinar que especies pueden ser utilizadas como indicadores de presión antrópica. Para ello, se determinó la composición taxonómica de la comunidad de micro-fitoplancton en 32 puntos de muestreo fijos en cuatro campañas de muestreo, desde verano de 2010 hasta primavera de 2011. Los resultados muestran que la entrada de nutrientes, principalmente a través del Río Serpis y los canales que drenan el humedal, determinan la composición y abundancia de la comunidad de fitoplancton. Además diversos factores ambientales clave como temperatura del agua, radiación, nutrientes, así como las relaciones entre nutrientes influyen en las diferentes asociaciones de fitoplancton observadas en cada estación. Por otro lado, la descarga del efluente procedente de la depuradora de aguas residuales no tuvo un impacto significativo sobre la comunidad de fitoplancton. La comunidad de fitoplancton estuvo formada principalmente por dos grupos: diatomeas y dinoflagelados. Se identificaron un total de 108 taxones. Las diatomeas proliferaron principalmente en otoño e invierno. Los dinoflagelados proliferaron en primavera, bajo condiciones de elevada radiación, baja concentración de DIP, razones DIN:DIP elevadas y razones DSi:DIN bajas. En este documento se discute la justificación de los cambios en la concentración de nutrientes. Además, se detectaron especies potencialmente nocivas en el Puerto de Gandía y en la desembocadura del río Serpis, en la playa de Venecia.Financial support for this research was provided by Ministry of Education, Culture and Sport, Government of Spain, through the Training Program for University Teachers (FPU). We would like to express our deepest thanks to Margarita Fernández and Vanessa Castan of IRTA (Research Institute of Technology, food and Agriculture).Gadea, I.; Rodilla, M.; Sospedra, J.; Falco, S.; Morata, T. (2013). Seasonal dynamics of the phytoplankton community in the Gandia coastal area, southern Gulf of Valencia. Thalassas. Revista de Ciencias del Mar. 29(1):35-58. http://hdl.handle.net/10251/46133S355829

Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

We present the detection and analysis of molecular hydrogen emission toward ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of twelve regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0--0 transitions of H$_2$ at 28.2 and 17.1${\,\rm \mu m}$ are detected in the IRS spectra for ten regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3$\sigma$ detection threshold is achieved for lines at 12.2 and 8.6${\,\rm \mu m}$. The excitation diagrams of the detected H$_2$ transitions are used to determine the warm H$_2$ gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range $86-137\,{\rm K}$. The bulk of the excited H$_2$ gas is found at these temperatures and contributes $\sim$5-17% to the total gas mass. We find a tight correlation of the H$_2$ surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photo-electric heating in photodissociation regions. We find the excitation of H$_2$ by this process is equally efficient in both atomic and molecular dominated regions. We also present the correlation of the warm H$_2$ physical conditions with dust properties. The warm H$_2$ mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H$_2$ excitation in photodissociation regions.Comment: Accepted for publication in MNRA

Systematic Molecular Differentiation in Starless Cores

(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap

Molecular Clouds as Ensembles of Transient Cores

We construct models of molecular clouds that are considered as ensembles of transient cores. Each core is assumed to develop in the background gas of the cloud, grow to high density and decay into the background. The chemistry in each core responds to the dynamical state of the gas and to the gas-dust interaction. Ices are deposited on the dust grains in the core's dense phase, and this material is returned to the gas as the core expands to low density. The cores of the ensemble number typically one thousand and are placed randomly in position within the cloud, and are assigned a random evolutionary phase. The models are used to generate molecular line contour maps of a typical dark cloud. These maps are found to represent extremely well the characteristic features of observed maps of the dark cloud L673, which has been observed at both low and high resolutions. The computed maps are found to exhibit the general morphology of the observed maps, and to generate similar sizes of emitting regions, molecular column densities, and the separations between peaks of emissions of various molecular species. The models give insight into the nature of molecular clouds and the dynamical processes occurring within them, and significantly constrain dynamical and chemical processes in the interstellar medium.Comment: 29 pages, 8 figures. Accepted for publication in Ap

Young starless cores embedded in the magnetically dominated Pipe Nebula

The Pipe Nebula is a massive, nearby dark molecular cloud with a low star-formation efficiency which makes it a good laboratory to study the very early stages of the star formation process. The Pipe Nebula is largely filamentary, and appears to be threaded by a uniform magnetic field at scales of few parsecs, perpendicular to its main axis. The field is only locally perturbed in a few regions, such as the only active cluster forming core B59. The aim of this study is to investigate primordial conditions in low-mass pre-stellar cores and how they relate to the local magnetic field in the cloud. We used the IRAM 30-m telescope to carry out a continuum and molecular survey at 3 and 1 mm of early- and late-time molecules toward four selected starless cores inside the Pipe Nebula. We found that the dust continuum emission maps trace better the densest regions than previous 2MASS extinction maps, while 2MASS extinction maps trace better the diffuse gas. The properties of the cores derived from dust emission show average radii of ~0.09 pc, densities of ~1.3x10^5 cm^-3, and core masses of ~2.5 M_sun. Our results confirm that the Pipe Nebula starless cores studied are in a very early evolutionary stage, and present a very young chemistry with different properties that allow us to propose an evolutionary sequence. All of the cores present early-time molecular emission, with CS detections toward all the sample. Two of them, Cores 40 and 109, present strong late-time molecular emission. There seems to be a correlation between the chemical evolutionary stage of the cores and the local magnetic properties that suggests that the evolution of the cores is ruled by a local competition between the magnetic energy and other mechanisms, such as turbulence.Comment: Accepted for publication in ApJ. 15 pages, 5 figures, 9 table