Coupled surface and subsurface flow modeling of natural hillslopes in the Aburrá Valley (Medellín,Colombia)

Numerical results are presented of surface-subsurface water modeling of a natural hillslope located in the Aburrá Valley, in the city of Medellín (Antioquia, Colombia). The integrated finite-element hydrogeological simulator HydroGeoSphere is used to conduct transient variably saturated simulations. The objective is to analyze pore-water pressure and saturation variation at shallow depths, as well as volumes of water infiltrated in the porous medium. These aspects are important in the region of study, which is highly affected by soil movements, especially during the high-rain seasons that occur twice a year. The modeling exercise considers rainfall events that occurred between October and December 2014 and a hillslope that is currently monitored because of soil instability problems. Simulation results show that rainfall temporal variability, mesh resolution, coupling length, and the conceptual model chosen to represent the heterogeneous soil, have a noticeable influence on results, particularly for high rainfall intensities. Results also indicate that surface-subsurface coupled modeling is required to avoid unrealistic increase in hydraulic heads when high rainfall intensities cause top-down saturation of soil. This work is a first effort towards fostering hydrogeological modeling expertise that may support the development of monitoring systems and early landslide warning in a country where the rainy season is often the cause of hydrogeological tragedies associated with landslides, mud flow or debris flow.     

The role of the Río de la Plata bottom salinity front in accumulating debris

The Río de la Plata, one of the most important South American estuarine environments, is characterized by a bottom salinity front that generates an ecotone between the river and the estuary. Based on bottom trawls and costal sampling we describe the distribution, types, and amount of debris found in the bottom and shoreline across this front. Plastics and plastic bags were the main debris types in both areas. Concentrations of total debris upriver the front were always significantly higher than downriver the front showing that the front acts as a barrier accumulating debris. Moreover, a large part of debris end ups accumulated in the coastal area upriver the frontal position. This area is particularly sensitive because the coastline encompasses an UNESCO Man and the Biosphere Reserve and a Ramsar site, and due to the ecological significance of the front for many valuable species.     

The Geysers-Cobb Mountain Magma System, California (Part 2): timescales of pluton emplacement and implications for its thermal history

Over 400 ion microprobe U-Pb isotopic ages measured for zircons extracted from 24 geothermal wells that penetrate the Geysers Plutonic complex (GPC) allow us to conclude that the entire known extent of the GPC crystallized during the early Pleistocene. Nine samples of the microgranite porphyry that forms the shallow cupola (100-1,500 m below sea-level, mbsl) of the GPC yield the oldest model U-Pb age (1.75 ± 0.01 Ma after correction for initial U series disequilibrium; errors 1σ). Twelve samples from the main intrusive phase (orthopyroxene-biotite granite) present at depths >1,250 mbsl define a crystallization age of 1.27 ± 0.01 Ma. This coincides with the age determined for a structurally and compositionally distinct body of granodiorite (1.25 ± 0.01 Ma; N = 5 samples) that is intruded over a similar depth range. Two petrographically distinct varieties of orthopyroxene-biotite granite yield ages of 1.46 ± 0.03 (GPC21-6000) and 1.16 ± 0.02 Ma (CA5636 74F 21; three samples). U-Pb zircon ages for dikes intruded in metagraywacke country-rocks overlap with those obtained from the main body of the GPC and include the youngest material identified (dike sample NEGU2 ST1-7700: 1.11 ± 0.03 Ma). Overall, the U-Pb results demonstrate that the main body of the GPC (∼300 km³) was emplaced and crystallized within the upper crust over a short time interval (0.2 Ma) that overlaps with zircon crystallization ages of overlying silicic volcanic units.     

Measurement of surface water runoff from plots of two different sizes

Intensities and amounts of water infiltration and runoff on sloping land are governed by the rainfall pattern and soil hydraulic conductivity, as well as by the microtopography and soil surface conditions. These components are closely interrelated and occur simultaneously, and their particular contribution may change during a rainfall event, or their effects may vary at different field scales. The scale effect on the process of infiltration/runoff was studied under natural field and rainfall conditions for two plot sizes: small plots of 0·25 m² and large plots of 50 m². The measurements were carried out in the central region of Chile in a piedmont most recently used as natural pastureland. Three blocks, each having one large plot and five small plots, were established. Cumulative rainfall and runoff quantities were sampled every 5 min. Significant variations in runoff responses to rainfall rates were found for the two plot sizes. On average, large plots yielded only 40% of runoff quantities produced on small plots per unit area. This difference between plot sizes was observed even during periods of continuous runoff. Copyright © 2002 John Wiley & Sons, Ltd.     

Subsidence and strike-slip tectonism of the upper continental slope off Manzanillo, Mexico

The direction of convergence between the Rivera and North American plates becomes progressively more oblique (in a counter-clockwise sense as measured relative to the trench-normal direction) northwestward along the Jalisco subduction zone. By analogy to other subduction zones, the forces resulting from this distribution of convergence directions are expected to produce a NW moving, fore-arc sliver and a NW–SE stretching of the fore-arc area. Also, a series of roughly arc parallel strike-slip faults may form in the fore-arc area, both onshore and offshore, as is observed in the Aleutian arc.In the Jalisco subduction zone, the Jalisco block has been proposed to represent such a fore-arc sliver. However, this proposal has encountered one major problem. Namely, right-lateral strike-slip faulting within the fore-arc sliver, and between the fore-arc sliver and the North American plate, should be observed. However, evidence for the expected right-lateral strike-slip faulting is sparse. Some evidence for right-lateral strike-slip faulting along the Jalisco block–North American plate boundary (the Tepic–Zacoalco rift system) has been reported, although some disagreement exists. Right-lateral strike-slip faulting has also been reported within the interior of the Jalisco block and in the southern Colima rift, which forms the SE boundary of the Jalisco block.Threefold, multi-channel seismic reflection data were collected in the offshore area of the Jalisco subduction zone off Manzanillo in April 2002 during the FAMEX campaign of the N/O L'Atalante. These data provide additional evidence for recent strike-slip motion within the fore-arc region of the Jalisco subduction zone. This faulting offsets right-laterally a prominent horst block within the southern Colima rift, from which we conclude that the sense of motion along the faulting is dextral. These data also provide additional evidence for recent subsidence within the area offshore of Manzanillo, as has been proposed.     

Nitrate temporal and spatial patterns in 12 water-supply wells, Yucatan, Mexico

 The nitrate concentration in 12 water-supply wells were monitored for the period April 1992 to March 1993. Each water-supply well was sampled once a month. The nitrate concentrations in the 12 wells ranged from 7 to 156 mg/l. Two water-supply wells (Chacsinkin and Peto) showed concentrations that reached 3.5 times the maximum permissible limit for the Drinking Water Standard (45 mg/l). A third water-supply well (Akil) exceeds the norm for 7 out of 12 months. The use of nitrogen-rich fertilizers are responsible for high nitrate concentrations in groundwater in the southern part of Yucatan, Mexico where intensive agricultural practices exist. Received: 14 December 1999 · Accepted: 2 May 2000     

Re–Os isotope systematics for the Lince–Estefanía deposit: constraints on the timing and source of copper mineralization in a stratabound copper deposit, Coastal Cordillera of Northern Chile

The Lince–Estefanía stratabound copper deposit in the Michilla district is one of the most important deposits in the Coastal Cordillera of northern Chile and is one of the most representative of this type of deposit. Chalcocite and bornite characterize the main stage of hypogene copper sulfide mineralization. Rhenium and osmium isotopes are used here to constrain the age of hypogene mineralization and the source of osmium contained in these ore minerals. A Re–Os isochron yielded an age of 160±16 Ma (2σ), with an associated initial ¹⁸⁷Os/¹⁸⁸Os ratio of 1.06±0.09 (mean square of weighted deviates=1.8). This age is consistent with available geochronological data from volcanic rocks that host the mineralization and associated alteration phases. The high initial ¹⁸⁷Os/¹⁸⁸Os ratio indicates a lower crustal component for the source of Os and, by inference, the Cu sulfides that contain this Os. Late hematite occurs as an isolated phase or, more commonly, is associated with the chalcocite–bornite and supergene chalcocite–covellite associations. Analyses performed on pure hematite indicate a disturbance of the Re–Os system, and hence, this mineral phase is not useful as a Re–Os geochronometer.     

Form roughness and the absence of secondary flow in a large confluence–diffluence,Rio Paraná, Argentina

Confluence–diffluence units are key elements within many river networks, having a major impact upon the routing of flow and sediment, and hence upon channel change. Although much progress has been made in understanding river confluences, and increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry, most studies have been conducted in laboratory flumes or within small rivers with width:depth (aspect) ratios less than 50. This paper presents results of a field‐based study that details the bed morphology and 3D flow structure within a very large confluence–diffluence in the Río Paraná, Argentina, with a width:depth ratio of approximately 200. Flow within the confluence–diffluence is dominated largely by the bed roughness, in the form of sand dunes; coherent, channel‐scale, secondary flow cells, that have been identified as important aspects of the flow field within smaller channels, and assumed to be present within large rivers, are generally absent in this reach. This finding has profound implications for flow mixing rates, sediment transport rates and pathways, and thus the interpretation of confluence–diffluence morphology and sedimentology. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of fossil diatoms on the growth of two syngeneic murine leukemias

The effect of micronized frustules of fossil diatoms (DS) on tumor growth and survival of mice receiving an ip inoculations of either a spontaneous lymphoblastic leukemia (LB) or P-388 lymphocytic leukemia (P-388) was investigated. A multi-dose study was designed, mice receiving 1000, 100, 10 and 1 μg of a suspension of DS, either ip or sc on day 1, 4 and 9 after tumor inoculation. With LB the best results were observed when DS were inoculated by the ip route, no effect being observed with the sc route. The best therapeutic effect resulted with 1 μg DS: 33% of the mice were alive 24 days after tumor inoculation, when all the controls had died. With P-388, only a temporary effect with the 10 μg DS dose was observed. In no case was it possible to prolong survival. It is concluded that at low doses of DS therapeutic effect is greatest, while toxicity due to SiO₂ is greatly reduced.