Geochemistry of sediments from the mid-oceanic Kolbeinsey Ridge

In order to assess recent submarine volcanic contributions to the sediments from the active Kolbeinsey Ridge, surface samples were analyzed chemically. The contribution of major and trace elements studied differ within the study area. A statistical analysis of the geochemical variables using factor analysis and cluster method allows to distinguish possible sample groups. Cluster method identifies three distinct sediment groups located in different areas of sedimentation. Group 1 is characterized by highest contents of Fe2O3, V, Co, Ni, Cu and Zn demonstrating the input of volcaniclastic material. Group 2 comprises high values of CaCO3, CaO and Sr representing biogenic carbonate. Group 3 is characterized by the elements K, Rb, Cs, La and Pb indicating the terrigenous component. The absolute percentage of the volcanic, biogenic and terrigenous components in the bulk sediments was calculated by using a normative sediment method. The highest volcanic component (> 60% on a carbonate free basis) is found on the ridge crest. The biogenic component is highest (10-30%) in the eastern part of the Spar Fracture Zone influenced by the East Iceland Current. Samples from the western and southeastern region of the study area contain more than 90% of terrigenous component which appears to be mainly controlled by input of ice-rafted debris.

Chemical composition of Ninetyeast Ridge basalts

On Leg 121 of the Ocean Drilling Program, we recovered basaltic rocks from a total of three basement sites in the southern, central, and northern regions of Ninetyeast Ridge. These new sites complement the previous four basement holes drilled during Legs 22 and 26 of the Deep Sea Drilling Project, and confirm the predominantly tholeiitic, light rare earth element-enriched character of the basalts that cap the ridge. The basalts show marked iron enrichment; ferrobasalts occur at Sites 214 and 216 and oceanic andesites at Site 253. All of the basalts recovered during Leg 121 are altered, and range from aphyric olivine tholeiites (Site 756), to strongly plagioclase-phyric basalts (Site 757). Basalts from Site 758, which were clearly erupted in a submarine environment (pillow basalts are present in the section), are sparsely to strongly plagioclase-phyric. The basalts recovered at any one hole are isotopically homogeneous (except for the basalts from Site 758, which show a range of Pb isotopes), and it is possible to relate the magmas at any one site by high-level fractionation processes. However, there are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. Such variation, in view of the large volume of magmatic products that form the ridge system, is not surprising. There is not, however, a systematic variation in basalt composition along the ridge. We agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge (e.g., Iceland, Galapagos Islands, and St. Paul/Amsterdam islands). The absence of recovered alkalic suites is inconsistent with an intraplate setting, such as the Hawaiian Islands or Kerguelen Island. Thus, the major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelen plume and mid-ocean Ninetyeast Ridge basalt mantle.

Las empresas recuperadas por sus trabajadores en la RMBA : ¿Una anomalía en la dinámica de metropolización propia del modelo neoliberal impuesto en la Argentina en la década del 90?

Fil: Cirio, Gastón. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Las políticas públicas de empleo : De los 90 a la postconvertibilidad

El proceso de activación de las políticas de empleo surge junto con el paradigma neoliberal que se instala a partir de mediados de los 70 como modo de acumulación a nivel mundial. El presente trabajo busca hacer una revisión sobre el surgimiento y los trasluces de dicho proceso, para luego observar cómo se desarrolló en nuestro país. En este contexto, con el presente trabajo se busca echar luz sobre las políticas públicas de empleo impulsadas por el Estado a partir de la postconvertibilidad, haciendo un estudio comparativo entre aquellas políticas de empleo desarrolladas durante el período menemista

Strontium and Oxygen isotope compositions from DSDP Holes 54-424A, 54-424B and 70-509B

Strontium and O isotope compositions of green clay minerals from sediment cores of three boreholes drilled into (sites 424A and 509B) and close to a hydrothermal mound (site 424B) near the Galapagos Spreading Center (DSDP Legs 54 and 70) were determined. The green clays consist mostly of a transition from Fesmectite (nontronite) to glauconite. 87Sr/86Sr ratios were measured on clay size-fractions after gentle acid leaching and on the recovered leachates from different samples. The 87Sr/86Sr ratios of the clay residues from both the 424A and B sites are well below the modern seawater value, which points consistently to precipitation from hydrothermal fluids that contained variable amounts of seawater, even away from mound. However, most of the clay residues from mound site 509B have 87Sr/86Sr ratios significantly above the seawater value, suggesting the occurrence of a detrital component together with the new authigenic particles. The clay minerals of the hydrothermal mound are mixed with detrital components, and that of the sample taken outside but near the mound as a reference for the surrounding oceanic environment, yields a hydrothermal signature. Crystallization temperatures of the clays range from 32 to 63 °C assuming a d18O value of +2.2 per mil for the mineralizing fluids. Hydrothermal fluids generated in the underlying oceanic crust, mixed in varied proportions with ambient seawater and migrated into beds of the mound in a sequence of recurrent processes that ultimately resulted in the formation of the observed clay minerals. No significant temperature differences were detected for crystallization of the K-rich glauconite and K-depleted nontronite. The 87Sr/86Sr ratios of the Sr leached off the clay particles are near the value of modern seawater, inferring a progressive replacement of the hydrothermal fluids by seawater in the pore space of the mound sediments.

Bibliotecarios, Tecnologías de la Información, Matemática, y la Ley del 90-10

Fil: Archuby, César Orlando. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Strontium isotopic composition of interstitial waters from DSDP sites 25-245 and 38-336

Measurements of 87Sr/86Sr ratios of interstitial waters from leg 25, site 245 and leg 38, site 336 of the Deep Sea Drilling Project show that the enrichment of Sr[2+] with depth is caused both by the alteration of volcanic material and by the introduction of strontium derived from calcium carbonate. 87Sr/86 Sr ratios range from 0.70913 to 0.70794 at site 245 and from 0.70916 to 0.70694 at site 336. The low ratios compared with contemporaneous seawater reflect the release of Sr from a volcanic source having, according to material-balance calculations, a 87Sr/86 Sr ratio of about 0.7034 at site 336. At this site the source appears to be volcanic ash and not basaltic basement which acts as a sink for Sr[2+] during in situ low-temperature weathering. The volcanic contribution to the strontium enrichment in the basal interstitial waters varies from <10% at site 245 to >50% at site 336. The remaining Sr[2+] is derived from Sr-rich biogenic carbonate during diagenetic recrystallization to form Sr-poor calcite.

De corceles y de aceros : El 25 de Mayo y las Guerras de la Independencia en manuales del tercer ciclo de la Escuela General Básica de la 'transformación educativa' de los años 90

Este artículo analiza los cambios en el relato de la independencia de Argentina que se intentaron introducir en los manuales escolares destinados al tercer ciclo de la Educación General Básica, que conformaba el nuevo trayecto obligatorio de la educación definido por la reforma de los años 90. Estos cambios abarcaban una actualización de los conocimientos que se transmitían en la escuela y las habilidades que la Historia debía promover en las aulas.

Laura Ruiz, Voces ásperas. Las narrativas argentinas de los 90

Fil: Sager, Valeria. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Transformaciones y conflictos en el agro chaqueño durante los ´90. Articulaciones territoriales de una nueva racionalidad productiva

Las transformaciones ocurridas en el sector agrícola del Chaco en los ´90, constituyen un ejemplo concreto de procesos de desarrollo geográfico desigual. A partir de 1999 esta provincia, dejó de ser la principal productora algodonera argentina para incorporarse a la siembra de soja genéticamente modificada, convertida en el principal cultivo nacional. El reemplazo de una lógica productiva que sustentó la organización económica y social provincial durante más de cuatro décadas por otra que privilegió la eficiencia y los menores costos comparativos, suscitó conflictos y reacciones diferenciales en el sector según la vulnerabilidad selectiva de los actores involucrados. El objetivo del trabajo es profundizar en ese proceso de reemplazo, estableciendo las principales características de las transformaciones acontecidas y dimensionando tanto sus efectos, como las respuestas de los distintos agentes del sector (una mayoría de pequeños productores tradicionales apoyados por organizaciones no gubernamentales, una minoría de medianos y grandes productores empresarios y el gobierno provincial)