Texas Coastal Hypoxia Linked to Brazos River Discharge as Revealed by Oxygen Isotopes     

Steven F. DiMarco  Josiah Strauss  Nelson May  Ruth L. Mullins-Perry  Ethan L. Grossman  David Shormann 《Aquatic Geochemistry》2012,18(2):159-181

Hypoxic conditions in the coastal waters off Texas (USA) were observed since the late 1970s, but little is known about the causes of stratification that contribute to hypoxia formation. Typically, this hypoxia is attributed to downcoast (southwestward) advection of waters from the Mississippi–Atchafalaya River system. Here, we present evidence for a hypoxic event on the inner shelf of Texas coincident with the presence of freshwater linked to high flow of the Brazos River in Texas. These conclusions are based on hydrographic observations and isotopic measurements of waters on the inner shelf near the Brazos River mouth. These data characterize the development, breakdown, and dispersal of a hypoxic event lasting from June through September 2007 off the Texas coast. Oxygen isotope compositions of shelf water indicate that (1) discharge from the Brazos River was the principal source of freshwater and water column stratification during the 2007 event, and (2) during low Brazos River discharge in 2008, freshwater on the Texas shelf was derived mainly from the Mississippi–Atchafalaya River System. Based on these findings, we conclude that the Mississippi–Atchafalaya River System is not the sole cause of hypoxia in the northern Gulf of Mexico; however, more data are needed to determine the relative influence of the Texas versus Mississippi rivers during normal and low flow conditions of Texas rivers.  相似文献   

Nonparametric Estimation to Reconstruct the Deformation History of an Active Fold in the Caspian Basin     

I. R. Sánchez-Borrego  J. I. Soto  M. Rueda  I. Santos Betancor 《Mathematical Geosciences》2016,48(8):985-1011

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Distribution and enrichment of trace metals and arsenic at the upper layer of sediments from Lerma River in La Piedad,Mexico: case history     

Benjamín Villalobos-Castañeda  Raúl Cortés-Martínez  Nuria Segovia  Otoniel Buenrostro-Delgado  Ofelia Morton-Bermea  Ruth Alfaro-Cuevas-Villanueva 《Environmental Earth Sciences》2016,75(23):1490

Lerma River is one of the largest rivers in Mexico. Over the past 20 years, unplanned population growth occurred along its course and the river has been used as the only outlet for industrial and domestic wastewater disposal. The aim of the present study was to determine trace metals such as Cr, Ni, Cu, Zn, Fe, Pb, and arsenic concentrations at the upper layer of sediments of the Lerma River meander in La Piedad, Michoacan, Mexico. Sediment samples were collected from eight different sites during the rainy and dry seasons. All samples were physically characterized, and concentration values of trace metals and As were determined. On the basis of protection criteria for freshwater sediments, concentrations of Fe, Zn, Cu, Ni, and Pb were found to exceed the lowest effect level; moreover, the concentrations were found to exceed the severe effect level at some sites, particularly for Cu. Statistical analyses showed significant differences between sampling seasons for Fe and As, and among sites for Ni, Cu, Zn, and Pb. In addition, the enrichment factor indicates the following order Zn > Cr > Cu > Ni > Pb > As, and the geoaccumulation index (I _geo) indicates contamination in the following order Zn > Cr > Cu > Ni > As > Pb. The Lerma River meander in La Piedad shows a reduction in pollution by trace metals and arsenic near the drain area and downstream of the meander. However, there are significantly higher concentrations of these elements in sediments of sites located in the middle part of the city.  相似文献   

Anchors for the cosmic distance scale: the Cepheid QZ Normae in the open cluster NGC 6067     

D. Majaess  L. Sturch  C. Moni Bidin  M. Soto  W. Gieren  R. Cohen  F. Mauro  D. Geisler  C. Bonatto  J. Borissova  D. Minniti  D. Turner  D. Lane  B. Madore  G. Carraro  L. Berdnikov 《Astrophysics and Space Science》2013,347(1):61-70

Cepheids are key to establishing the cosmic distance scale. Therefore it’s important to assess the viability of QZ Nor, V340 Nor, and GU Nor as calibrators for Leavitt’s law via their purported membership in the open cluster NGC 6067. The following suite of evidence confirms that QZ Nor and V340 Nor are members of NGC 6067, whereas GU Nor likely lies in the foreground: (i) existing radial velocities for QZ Nor and V340 Nor agree with that established for the cluster ( $-39.4\pm0.2(\sigma_{\bar {x}}) \pm1.2 (\sigma)~\mbox{km/s}$ ) to within 1 km/s, whereas GU Nor exhibits a markedly smaller value; (ii) a steep velocity-distance gradient characterizes the sight-line toward NGC 6067, thus implying that objects sharing common velocities are nearly equidistant; (iii) a radial profile constructed for NGC 6067 indicates that QZ Nor is within the cluster bounds, despite being 20′ from the cluster center; (iv) new BVJH photometry for NGC 6067 confirms the cluster lies d=1.75±0.10 kpc distant, a result that matches Wesenheit distances computed for QZ Nor/V340 Nor using the Benedict et al. (Astron. J. 133:1810, 2007, HST parallaxes) calibration. QZ Nor is a cluster Cepheid that should be employed as a calibrator for the cosmic distance scale.  相似文献   

Cosmological magnetic fields: their generation,evolution and observation     

Ruth?Durrer  Andrii?NeronovEmail author 《Astronomy and Astrophysics Review》2013,21(1):62

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gamma-ray astronomy and ultra-high-energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range.We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them.  相似文献   

Sediment characteristics and internal architecture of offshore sand ridges on a tideless continental shelf (western Mediterranean)     

Ruth Durán  Jorge Guillén  Marta Ribó  Gonzalo Simarro  Araceli Muñoz  Albert Palanques  Pere Puig 《地球表面变化过程与地形》2020,45(14):3592-3606

An integrated approach combining swath bathymetry, an extensive dataset of vibrocores and high-resolution seismic reflection data was used to assess the origin and evolution of offshore sand ridges on a tideless continental shelf (Gulf of Valencia, western Mediterranean). The sand ridges are located in the mid-outer shelf at 55–85 m water depth, obliquely oriented to the shoreline. They are 1.5 to 7 m high, with a wavelength between 600 and 1,100 m and a mean height-to-wavelength ratio of 0.004. The sand ridges are composed of well-sorted medium sand and are partially covered by a mud layer, evidencing a moribund stage. They overlie an erosion surface that locally crops out at the seafloor and is interpreted as the Holocene wave-ravinement surface. In the sediment cores, this surface corresponds to an erosional lag composed of coarse sand and gravel with pebbles. Small topographic irregularities on this surface are interpreted as shoreline-associated features that may act as the precursor for ridge development. Their preservation within the sand ridges could be related to the hardness of these features. Internally, the sand ridges display high-angle dipping reflections, indicating ridge migration towards the southeast in the direction of the present-day sediment transport direction. The presence of interbedded mud layers, associated with these reflections, indicates intermittent episodes of mud deposition when active. The internal architecture of some small ridges also provides new insights into their transition from an active to moribund state, as evidenced by a change in the geometry of the internal units from progradational to aggradational, finally being overlain by onlapping finer deposits over the flanks and in the troughs. The Gulf of Valencia sand ridge field constitutes a valuable potential sand resource of 22 million m³ of well-sorted medium and coarse sand with limited mud content, which must be preserved as a strategic sand reservoir. © 2020 John Wiley & Sons, Ltd.  相似文献   

Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling     

Roger D. Gonçalves  Elias H. Teramoto  Bruno Z. Engelbrecht  Miguel A. Alfaro Soto  Hung K. Chang  Martinus Th. van Genuchten 《Ground water》2020,58(3):432-440

This study presents an extension of the concept of “quasi-saturation” to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S_y), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S_y. Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table.  相似文献   

格陵兰冰心中的矿物粉尘记录     

Urs Ruth  张光威 《海洋地质前沿》2006,22(9):21-22

矿物粉尘在当今气候研究中发挥着重要的作用。矿物粉尘气溶胶具有的发射性质使其直接影响着大气圈的辐射平衡。矿物粉尘也以多种间接方式积极参与了气候活动,其中包括成为云层凝聚中心来决定云盖层的规模和特征以及可能控制着遥远的营养贫瘠的大洋与大陆地区的原始生物生产力。  相似文献   

A Preliminary Trophic Model of the Continental Shelf, South-western Gulf of Mexico     

S. Manickchand-Heileman  L.A. Soto  E. Escobar 《Estuarine, Coastal and Shelf Science》1998,46(6):885-899

A preliminary mass-balance trophic model was constructed to determine the flow of energy in a community of fish and invertebrates on the continental shelf of the south-western Gulf of Mexico. Input parameters were taken from the literature, except for the biomass of fish groups which was obtained from trawl surveys in the study area. The model consists of 12 fish groups, five invertebrate groups, phytoplankton and detritus. Results indicate an imbalance between primary production and consumption, with only about 10% of primary production being consumed in the water column. Most of the primary production is exported to detritus which forms the basis of the food-web, with a detritivory/herbivory ratio of 2·5. Benthic invertebrates play a significant role in transferring energy from detritus to higher trophic levels. Eight discrete trophic levels were found, with very reduced flow at levels higher than the fifth. The highest fractional trophic level is 4·3, and consists of sharks. Analysis of mixed trophic impacts showed that detritus and lower trophic levels have a significant positive impact on other groups in the system. Mean transfer efficiency is 9·2%. Some whole system properties are also given. Of net primary production, 6·7% is required to sustain current catch levels, suggesting that the resources in this area are fully exploited.  相似文献   

Understanding the dynamics and contamination of an urban aquifer system using groundwater age (14C, 3H,CFCs) and chemistry          下载免费PDF全文

Jürgen Mahlknecht  Arturo Hernández‐Antonio  Christopher J. Eastoe  Carol Tamez‐Meléndez  Rogelio Ledesma‐Ruiz  José A. Ramos‐Leal  Nancy Ornelas‐Soto 《水文研究》2017,31(13):2365-2380

The quality of the groundwater supplying drinking water to the Guadalajara metropolitan area has deteriorated due to both endogenic and exogenic processes. Previous studies of this complex neotectonic volcanic environment suggest that the sources of contamination here are underground fluids derived from an active volcanic center and surface wastewater derived from regional land‐use intensification. This study uses isotopic, gaseous, and chemical signatures to more comprehensively characterize this groundwater flow and its contamination paths. Groundwater is mainly recharged at the La Primavera Caldera to the west and is discharged into the Santiago River to the east. The exception to this trend is the Toluquilla area, where groundwater most likely represents rainfall originating from outside the basin limits. Evaporation affects groundwater in these areas, especially waters that have been affected by recycling below urban areas in the Atejamac area and by intensive agricultural activity in the Toluquilla area. Additionally, we present evidence that groundwater flow through alluvial sediments and tuffs in deeper wells mixes with a lower aquifer unit in basaltic‐andesitic rocks, which are in contact with hydrothermal fluids. Groundwater ages range from postbomb in the western and northwestern regions of the study area (i.e., the Atemajac aquifer unit) to Late Pleistocene in the southern and southeastern regions (i.e., the Toluquilla aquifer unit). Recently recharged water records little mixing and is located mostly in or near the La Primavera volcanic system. As groundwater undergoes gravitational flow towards discharge areas, it mixes with older water components. Chloride and sodium concentrations above natural background levels are primarily related to volcanic activity, nitrate is associated with human activities, and sulfate originates from both anthropogenic sources and water–rock interactions. Nitrate originating from land‐use activities (such as sewers, septic tanks, landfills, and agricultural fields) that is introduced into the deeper part of the groundwater system is expected to travel with the groundwater to the discharge areas because oxidizing conditions will prevent microbial reduction. See Supplementary Information.  相似文献