Solar irradiance over and under seasonal land-fast sea ice off Barrow, Alaska, in 2010

The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along transects under undeformed land-fast sea ice at Barrow, Alaska (March, May, and June 2010). The measurements were performed with a spectral radiometer mounted on a floating under-ice sled. The objective was to quantify the spatial variability of light transmittance through snow and sea ice, and to compare this variability along its seasonal evolution. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt causes as much relative spatial variability of light transmittance as the contrast of ponded and white ice during summer. Both before and after melt onset, measured transmittances fell in a range from one third to three times the mean value. In addition, we found a twentyfold increase of light transmittance as a result of partial snowmelt, showing the seasonal evolution of transmittance through sea ice far exceeds the spatial variability. However, prior melt onset, light transmittance was time invariant and differences in under-ice irradiance were directly related to the spatial variability of the snow cover.

Geochemistry of the oldest Atlantic oceanic crust

Controversy has surrounded the issue of whether mantle plume activity was responsible for Pangaean continental rifting and massive flood volcanism (resulting in the Central Atlantic Magmatic Province or CAMP, emplaced around 200 Ma) preceding the opening of the central Atlantic Ocean in the Early Mesozoic. Our new Sr-Nd-Pb isotopic and trace element data for the oldest basalts sampled from central Atlantic oceanic crust by deep-sea drilling show that oceanic crust generated from about 160 to 120 Ma displays clear isotopic and chemical signals of plume contamination (e.g., 87Sr/86Sr(i) = 0.7032-0.7036, epsilonNd(t) =+6.2 to +8.2, incompatible element patterns with positive Nb anomalies), but these signals are muted or absent in crust generated between 120 and 80 Ma, which resembles young Atlantic normal mid-ocean ridge basalt. The plume-affected pre-120 Ma Atlantic crustal basalts are isotopically similar to lavas from the Ontong Java Plateau, and may represent one isotopic end-member for CAMP basalts. The strongest plume signature is displayed near the center of CAMP magmatism but the hotspots presently located nearest this location in the mantle reference frame do not appear to be older than latest Cretaceous and are isotopically distinct from the oldest Atlantic crust. The evidence for widespread plume contamination of the nascent Atlantic upper mantle, combined with a lack of evidence for a long-lived volcanic chain associated with this plume, leads us to propose that the enriched signature of early Atlantic crust and possibly the eruption of the CAMP were caused by a relatively short-lived, but large volume plume feature that was not rooted at a mantle boundary layer. Such a phenomenon has been predicted by recent numerical models of mantle circulation.

Particulate and phytoplankton absorption during POLARSTERN cruises ANT-XXVI/3 and ANT-XXVIII/3

The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.

Las voces del discurso crítico en la escuela. Parte II. 16H227

Este Proyecto se propuso continuar las indagaciones ligadas a la teorización educacional crítica y el enfoque constructivista y su vigencia en diversas materializaciones del contexto escolar actual. Durante el desarrollo del trabajo investigativo titulado “Las voces del discurso crítico en la escuela (2003-2007)” estudiamos la puesta en acción de dichos enfoques a través de diversas formas de mediación del discurso escolar. En el actual proyecto nos ocupamos de continuar dichas indagaciones y profundizar los campos de análisis elegidos. Observamos algunas prefiguraciones de dichos enfoques y discursos en la diversidad de voces que convocan las elaboraciones Curriculares Oficiales para acreditación de los I.F.D.C. y las evaluaciones finales de la Especialización en Alfabetización Intercultural. Según lo analizado en la variedad del escenario escolar, el discurso crítico y la perspectiva constructivista asumen múltiples sentidos en donde se conjugan experiencias pedagógicas que ponen en cuestión la matriz monológica de la educación tradicional junto con planteos que enfatizan la lógica prescriptiva centrada en los mandatos del “deber ser”. Hemos analizado y puesto en tensión dichas materializaciones en diversos contextos de producción del discurso pedagógico: normativas, prescripciones, fundamentos pedagógicos, concepciones de enseñanza y aprendizaje, modalidades de implementación de propuestas, etc. Nos ocupamos de desplegar algunos aspectos en un nivel teórico tratando de situar la problemática de los enfoques presentados desde diversos campos del conocimiento. Y además indagar las implicaciones de estas perspectivas ya sea en las materializaciones mencionadas como así también sus consecuencias prácticas. Desplegamos las estrategias analíticas del discurso que tales documentaciones requieran, y sus cotejos como voces de esa toma de postura crítica. Así, en relación con los procedimientos metodológicos, nos proponemos explorar distintas versiones discursivas de dichas esferas del discurso. La línea de investigación es de tipo cualitativa, basada en la interpretación y estudio de los procesos de mediación y sus configuraciones en los dispositivos escolares y en la documentación institucional disponible.