Comparative dating of a Bison-bearing late-Pleistocene deposit,Térapa,Sonora, Mexico

A recently discovered Bison-bearing fossil locality at Térapa, Sonora, Mexico, had previously been dated to 440 ± 130 ka using whole rock ⁴⁰Ar/³⁹Ar on a basalt flow that impounds the deposit. This age is considerably older than the accepted age of about 240–160 ka for the migration of Bison into greater North America. The Térapa deposit also contains a mixture of fossils from extralimital or extinct tropical animals and temperate animals. Constraining the age of the deposit is critical to interpret the paleontologic and paleoclimatologic implications of this unique Sonoran fossil locality. Three additional geochronological methods have been applied to this deposit (infrared stimulated luminescence (IRSL), amino acid racemization (AAR), and radiocarbon) and the data from the original ⁴⁰Ar/³⁹Ar age were revisited. The IRSL data suggest that the impounding basalt flow and the sediments that abut it were emplaced 43 ka ago and that the oldest sediments were deposited shortly after. Two radiocarbon ages suggest the fossiliferous sediments were emplaced by 42 ka. Effective diagenetic temperatures inferred from the AAR results, combined with AAR data from a similar-age deposit in southern Arizona, are in accordance with the 40–43 ka age estimates. For the AAR results to corroborate the ⁴⁰Ar/³⁹Ar age, the effective diagenetic temperature for the area would need to be approximately 3 °C, which is unrealistically low for northern Mexico. The new geochronological results suggest the Térapa deposit and fossils are 40–43 ka old. The anomalously old ⁴⁰Ar/³⁹Ar age for the impounding basalt is probably the result of low ⁴⁰Ar* concentrations and inherited ⁴⁰Ar.     

Land use/cover, landforms and fragmentation patterns in a tropical dry forest in the southern Pacific region of Mexico

In spite of widely documented studies of deforestation rates and land use/cover changes in tropical dry forests in Mexico, relatively little is known about fragmentation patterns in such forests. This study defines the spatial distribution of landforms and land use/cover types the lower Papagayo River basin and examines their influence on fragmentation patterns and biological diversity in a tropical dry forest in that southern Pacific region. The land use/cover map was constructed from aerial photographs, Landsat TM imagery (2000) and fieldwork. Landform units were defined based on altitude, slope, lithology and morphology. Landscape fragmentation parameters were obtained using FRAGSTATS (version 3.3) considering the numbers of patches, mean, minimum and maximum patch size, edge density, total edge and connectivity. Results show tropical dry forest to be remnant vegetation (~11 per cent), characterized by isolation and low connectivity. Land use/cover types have different effects on fragmentation patterns. Agriculture and cattle raising produce similar numbers of patches, but with a different mean size; and human settlements have a scattered distribution pattern. The abandonment of rural agricultural livelihoods has favoured the expansion of secondary tropical dry forest characterized by continuity and high connectivity, which suggests a high regeneration potential from land abandonment. It can be concluded that tropical dry forest fragmentation and recovery at regional scales depend on such landscape attributes as lithology, slope, geomorphology and management.     

Venus wind map at cloud top level with the MTR/THEMIS visible spectrometer, I: Instrumental performance and first results

Solar light gets scattered at cloud top level in Venus’ atmosphere, in the visible range, which corresponds to the altitude of 67 km. We present Doppler velocity measurements performed with the high resolution spectrometer MTR of the Solar telescope THEMIS (Teide Observatory, Canary Island) on the sodium D2 solar line . Observations lasted only 49 min because of cloudy weather. However, we could assess the instrumental velocity sensitivity, per pixel of 1 arcsec, and give a value of the amplitude of zonal wind at equator at .     

Anatomical variation of five plant species along an elevation gradient in Mexico City basin within the Trans-Mexican Volcanic Belt,Mexico

Change in environmental conditions with altitudinal gradients induces morpho-anatomical variations in plants that have been poorly documented in intertropical regions. Five species with three life forms, cryptophyte (Alchemilla procumbens, Geranium seemannii), hemicryptophyte (Acaena elongata, Lupinus montanus), and phanerophyte (Symphoricarpos microphyllus), distributed along an altitudinal gradient in the Sierra Nevada of central Mexico, were studied. The aims were to identify and evaluate their morpho-anatomical modifications under the hypothesis that the sizes of individuals and of their wood and leaf cell types decrease as elevation increases. Three individuals per species per site were collected at seven locations along the altitudinal gradient (2949-3952 m). Their morpho-anatomical characters were analyzed through multiple regression analyses. Elevation was the variable that best explained anatomical changes in the leaf and wood of the five species. Canopy density and potassium content in the soil also contributed to explain the variation in anatomical variables along the gradient. As elevation increased a bimodal pattern was observed in various anatomical characters as in the leaf width of A. elongata, A. procumbens and G. seemannii and in the vessel diameter of A. procumbens, G. seemannii, and L. montanus. Other features as the vessel diameter of A. elongata, the fiber length of S. microphyllus, and the ray width of A. elongata increased as the elevation increased. Anatomical traits have a tendency to decrease in size but just toward the end of the gradient, which is probably related to changes in canopy density. The plant response to the altitudinal gradient is more focused on anatomical adaptations than morphological variation; it is also species dependent.     

Beitrag zur Tektonik Nordchiles

Zusammenfassung Großräumige geologische Kartierungen in den drei Nordprovinzen Chiles ergaben, daß die Kordilleren von weitgespannten Schuppenstrukturen beherrscht werden. Diese bestehen aus Grundgebirgs- und mesozoisch-känozoischen Deckgebirgsschollen, die durch N-S streichende und 40–60° nach Osten einfallende Überschiebungen bedingt sind. Die Überschiebungen verlaufen weitgehend parallel zu einer berechneten Fläche, auf der sich die Erdbebenhypozentren befinden.An den Rändern des Längstalgrabens finden sich örtlich neben normalen Störungen auch Überschiebungen. Kompressionstektonik herrscht gegenüber Dehnungstektonik vor.Die stratigraphische Position der Salzkordillere bei San Pedro de Atacama und eine Erklärung für die Entstehung der vulkanischen Eisenlagerstätte El Laco werden kurz skizziert.

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Regional mapping in northern Chile revealed large scale imbricate structures. The thrust sheets, derived either from the basement or from the postpaleozoic cover, have been moved along N striking, 40–60° E dipping thrust planes, which parallel the mathematically established plane comprising the foci of the earthquakes.At the rims of longitudinal valleys, in addition to normal faults overthrusts occur locally. Compressional tectonic is dominant with respect to tensional tectonic.The stratigraphic position of the Salt Ranges (Cordillera de la Sal) near San Pedro de Atacama and the genesis of the volcanic iron deposits of El Laco are briefly discussed.

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Resumen Estudios geológicos regionales en las tres provincias más septentrionales de Chile demuestran que en las cordilleras predomina una amplia estructura de escamas. Estas están constituidas por bloques de rocas antiquas del basamento paleozoico y de formaciones cobertizas mesozoico-cenozoicas que están controladas por fallas inversas y sobreescurrimientos de rumbo N-S y manteo de 40°–60° al este. Estos sobreescurrimientos son sensiblemente paralelos a la superficie en que se situan la mayor parte de los hipocentros sismicos.En los bordes del valle longitudinal o Pampa se encuentran frecuentemente fallas inversas junto a otras de tipo normal, indicando un predominio de procesos de compresión, sobre los de dilatación.Brevemente se discuta la posición estratigráfica de la Cordillera de la Sal, en el Salar de Atacama, y una explicación para la génesis del yacimiento volcánico de hierro del Laco.

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Résumé Des travaux géologiques récents ont démontré que le trait structural le plus important dans le nord du Chili est une alternance de blocs anciens et de formations de couverture méso-cénozoïques. Cette structure imbriquée est contrôlée par des failles inverses, parallèles au plan où la plus grande partie des tremblements de terre ont leur origine.Des failles inverses de faible pendage et même des nappes de charriage ne sont pas rares dans les formations sédimentaires jurassiques. La tectonique de compression prédomine sur la tectonique de tension.La position stratigraphique de la Cordillère de Sel près de San Pedro de Atacama et une explication pour la formation du gisement de fer volcanique de El Laco sont brièvement discutées.

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, , , . - , , N-S 40–60° . , . — . . San Pedro de Atacama El Laco.

     

A suggested explanation of some cases of reversed magnetism in rocks

Summary Some of the cases of reverse magnetic polarity of rocks might be explained by the influence of pre-existent normal magnetized surrounding rocks.     

Oceanic transport and storage of carbon emissions

Using a global carbon cycle model (GLOCO) that considers seven terrestrial biomes, surface and deep ocean layers based on the HILDA model and a single mixed atmosphere, we analyzed the response of atmospheric CO₂ concentration and oceanic DIC and DOC depth profiles to additions of carbon to the atmosphere and ocean. The rate of transport of carbon to the deepest oceanic layers is rather insensitive to the atmosphereic-ocean surface gas exchange coefficient over a wide range, hence discrepancies between researchers on the precise global average value of this coefficient do not significantly affect predictions of atmospheric response to anthropogenic inputs. Upwelling velocity, on the other hand, amplifies oceanic response by increasing primary production in the upper ocean layers, resulting in a larger flux into DOC and sediments and increased carbon storage; experiments to reduce the uncertainty in this parameter would be valuable.The location of the carbon addition, whether it is released in the atmosphere or in the middle of the oceanic thermocline, has a significant impact on the maximum atmospheric CO₂ concentration (pCO₂) subsequently reached, suggesting that oceanic burial of a significant fraction of carbon emissions (e.g. via clathrate hydrides) may be an important management option for limiting pCO₂ buildup. Our analysis indicates that the effectiveness of ocean burial decreases asymptotically below about 1000 m depth. With a constant emissions scenario (at 1990 levels), pCO₂ at year 2100 is reduced from 501 ppmv considering all emissions go to the atmosphere, to 422 ppmv with ocean burial at a depth of 1000 m of 50% of the fossil fuel emissions. An alternative scenario looks at stabilizing pCO₂ at 450 ppmv; with no ocean burial of fossil fuel emissions, the rate of emissions has to be cut drastically after the year 2010, whereas oceanic burial of 2 GtC/yr allows for a smoother transition to alternative energy sources.     

Iron,manganese and trace metal concentrations in seaweeds from the central west coast of the Gulf of California

Concentrations of Co, Cu, Fe, Mn, Ni, Pb and Zn in four macroalgae species (Ulva lactuca, Chondracanthus squarrulosus, Sargassum sinicola and Gracilariopsis lemaneiformis) were obtained for the first time from the central part of the west coast of the Gulf of California. Generally, no differences in metal concentrations were found among the different seaweed species, although spatial differences were apparent. Iron, Mn and Cu exhibited higher concentrations at the stations located in front of Angel de la Guarda Island, probably because of high vertical mixing processes present in the zone. The results were compared with dissolved metal concentrations reported for the Gulf of California (Cd, Mn and Fe) and the North Pacific Ocean. The resulting linear regression of the results vs. North Pacific Ocean concentrations indicated that the levels of Cu, Ni and Zn measured in this study were within its 95% confidence level. Furthermore, this comparison was capable of detecting dissolved Fe and Mn enrichments in Gulf of California waters relative to the North Pacific Ocean concentrations. Calculations of total masses of metals associated with algal biomass on the west coast of the Gulf of California indicated that the lowest masses were represented by Cu (108 ± 25 kg) and Ni (184 ± 52 kg), whereas Pb (1.1 ± 0.6 ton) and Fe (10.9 ± 8.5 ton) were the elements with the highest associated masses.