Enrique Barba y el orbe colonial rioplatense. Balances y proyecciones

La Universidad Nacional de La Plata y los estudios históricos en la Facultad de Humanidades y Ciencias de la Educación se construyeron con la impronta de Ricardo Levene, la cual dejó su estela por años en muchos de sus discípulos. Dentro de esa escuela histórica se desarrolló y consolidó la figura de Enrique Barba en la ciudad de La Plata y en su Universidad. Sus libros, folletos y colaboraciones suman más de 50 trabajos a lo largo de 55 años de labor, sin embargo en estas líneas que siguen a continuación nos referiremos a su veta menos divulgada y es ella el aporte historiográfico que realizó Enrique Barba para la construcción del pasado colonial rioplatense como también quiénes fueron los que continuaron por el camino que Barba trazó en el campo de los estudios coloniales.

Flow separation and roughness lengths over large bedforms in a tidal environment

This study characterises the shape of the flow separation zone (FSZ) and wake region over large asymmetric bedforms under tidal flow conditions. High resolution bathymetry, flow velocity and turbulence data were measured along two parallel transects in a tidal channel covered with bedforms. The field data are used to verify the applicability of a numerical model for a systematic study using the Delft3D modelling system and test the model sensitivity to roughness length. Three experiments are then conducted to investigate how the FSZ size and wake extent vary depending on tidally-varying flow conditions, water levels and bathymetry. During the ebb, a large FSZ occurs over the steep lee side of each bedform. During the flood, no flow separation develops over the bedforms having a flat crest; however, a small FSZ is observed over the steepest part of the crest of some bedforms, where the slope is locally up to 15°. Over a given bedform morphology and constant water levels, no FSZ occurs for velocity magnitudes smaller than 0.1 m s**-1; as the flow accelerates, the FSZ reaches a stable size for velocity magnitudes greater than 0.4 m s**-1. The shape of the FSZ is not influenced by changes in water levels. On the other hand, variations in bed morphology, as recorded from the high-resolution bathymetry collected during the tidal cycle, influence the size and position of the FSZ: a FSZ develops only when the maximum lee side slope over a horizontal distance of 5 m is greater than 10°. The height and length of the wake region are related to the length of the FSZ. The total roughness along the transect lines is an order of magnitude larger during the ebb than during the flood due to flow direction in relation to bedform asymmetry: during the ebb, roughness is created by the large bedforms because a FSZ and wake develops over the steep lee side. The results add to the understanding of hydrodynamics of natural bedforms in a tidal environment and may be used to better parameterise small-scale processes in large-scale studies.

Enrique Barba y el orbe colonial rioplatense. Balances y proyecciones

La Universidad Nacional de La Plata y los estudios históricos en la Facultad de Humanidades y Ciencias de la Educación se construyeron con la impronta de Ricardo Levene, la cual dejó su estela por años en muchos de sus discípulos. Dentro de esa escuela histórica se desarrolló y consolidó la figura de Enrique Barba en la ciudad de La Plata y en su Universidad. Sus libros, folletos y colaboraciones suman más de 50 trabajos a lo largo de 55 años de labor, sin embargo en estas líneas que siguen a continuación nos referiremos a su veta menos divulgada y es ella el aporte historiográfico que realizó Enrique Barba para la construcción del pasado colonial rioplatense como también quiénes fueron los que continuaron por el camino que Barba trazó en el campo de los estudios coloniales.

Enrique Barba y el orbe colonial rioplatense. Balances y proyecciones

La Universidad Nacional de La Plata y los estudios históricos en la Facultad de Humanidades y Ciencias de la Educación se construyeron con la impronta de Ricardo Levene, la cual dejó su estela por años en muchos de sus discípulos. Dentro de esa escuela histórica se desarrolló y consolidó la figura de Enrique Barba en la ciudad de La Plata y en su Universidad. Sus libros, folletos y colaboraciones suman más de 50 trabajos a lo largo de 55 años de labor, sin embargo en estas líneas que siguen a continuación nos referiremos a su veta menos divulgada y es ella el aporte historiográfico que realizó Enrique Barba para la construcción del pasado colonial rioplatense como también quiénes fueron los que continuaron por el camino que Barba trazó en el campo de los estudios coloniales.

Chemical composition of rocks from the Marcus-Necker Rise

The Marcus Necker Rise has two geologically distinct parts: the western one is composed of the Marcus Wake seamounts and the eastern one of the Wake-Necker seamounts. The first contain differentiated alkali basaltoids, while the second show a distinctive prolonged and complicated history. The latter contain Early Cretaceous olivine basalts and Late Cretaceous alkali and hornblende basaltoids.

(Table 2) Integrated concentration, production and dissolution of silica in the water column of the southern Pacific

An intense diatom bloom developed within a strong meridional silicic acid gradient across the Antarctic Polar Front at 61°S, 170°W following stratification of the water column in late October/early November 1997. The region of high diatom biomass and the silicic acid gradient propogated southward across the Seasonal Ice Zone through time, with the maximum diatom biomass tracking the center of the silicic acid gradient. High diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of January 1998 (ca. 70 d) driving the gradient over 500 km to the south of its original position at the Polar Front. The bloom consumed 30 to >40 µM Si(OH)4 in the euphotic zone between about 60 and 66°S leaving near surface concentrations <2.5 µM and occasionally <1.0 µM in its wake. Integrated biogenic silica concentrations within the bloom averaged 410 mmol Si/m**2 (range 162-793 mmol Si/m**2). Average integrated silica production on two consecutive cruises in December 1997 and January 1998 that sampled the bloom while it was well developed were 27.5±6.9 and 22.6±20 mmol Si/m**2/d, respectively. Those levels of siliceous biomass and silica production are similar in magnitude to those reported for ice-edge diatom blooms in the Ross Sea, Antarctica, which is considered to be among the most productive regions in the Southern Ocean. Net silica production (production minus dissolution) in surface waters during the bloom was 16-21 mmol Si/m**2/d, which is sufficient for diatom growth to be the cause of the southward displacement of the silicic acid gradient. A strong seasonal change in silica dissolution : silica production rate ratios was observed. Integrated silica dissolution rates in the upper 100-150 m during the low biomass period before stratification averaged 64% of integrated production. During the bloom integrated dissolution rates averaged only 23% of integrated silica production, making 77% of the opal produced available for export to depth. The bloom ended in late January apparently due to a mixing event. Dissolution : production rate ratios increased to an average of 0.67 during that period indicating a return to a predominantly regenerative system. Our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around Antarctica also occur in the deep ocean near the Polar Front. The bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of Antarctica implying a role for such blooms in the formation of those sediments. Comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the Southern Ocean, which have been corrected for sediment focusing, indicate a burial efficiency of <=4.6% for biogenic silica. That efficiency is considerably lower than previous estimates for the Southern Ocean.