Geochemical and nannofossil analyses of Cenomanian/Turonian sediments of DSDP Hole 75-530A

One of the key objectives of Deep Sea Drilling Project (DSDP) Leg 75 was to shed light on the underlying causes of Cretaceous oceanic anoxia in the South Atlantic by addressing two major hypotheses: productivity productivity-driven anoxia vs. enhanced ocean stratification leading to preservation of organic matter and black shale deposition. Here we present a detailed geochemical dataset from sediments deposited during the Cenomanian/Turonian (C/T) transition and the global oceanic anoxic event 2 (OAE 2) at DSDP Site 530A, located off-shore Namibia (southeast Angola Basin, north of Walvis Ridge). To characterise the succession of alternating black and green shales at this site and to reconstruct the evolution of their paleoenvironmental setting, we have combined data derived from investigations on bulk organic matter, biomarkers and the inorganic fraction. The location of the C/T boundary itself is biostratigraphically not well constrained due to the carbonate-poor (but organic matter-rich) facies of these sediments. The bulk d13Corg record and compound-specific d13C data, in combination with published as well as new biostratigraphic data, enabled us to locate more precisely the C/T boundary at DSDP Site 530A. The compound-specific d13C record is the first of this kind reported from C/T black shales in the South Atlantic. It is employed for paleoenvironmental reconstructions and chemostratigraphic correlation to other C/T sections in order to discuss the paleoceanographic aspects and implications of the observations at DSDP Site 530A in a broader context, e.g., with regard to the potential trigger mechanisms of OAE 2, global changes in black shale deposition and climate. On a stratigraphic level, an approximation and monitoring of the syndepositional degree of oxygen depletion within the sediments/bottom waters in comparison to the upper water column is achieved by comparing normalised concentrations of redox-sensitive trace elements with the abundance of highly source specific molecular compounds. These biomarkers are derived from photoautotrophic and simultaneously anoxygenic green sulphur bacteria (Chlorobiacea) and are interpreted as paleoindicators for events of photic zone euxinia. In contrast to a number of other OAE 2 sections that are characterised by continuous black shale sequences, DSDP Site 530A represents a highly dynamic setting where newly deposited black shales were repeatedly exposed to conditions of subtle bottom water re-oxidation, presumably leading to their progressive alteration into green shales. The frequent alternation between both facies and the related anoxic to slight oxygenated conditions can be best explained by variations in vertical extent of an oxygen minimum zone in response to changes in a highly productive western continental margin setting driven by upwelling.

Osmium isotope record of Late Pleistocene sediments

We report high temporal resolution osmium isotopes records of bulk sediment and sediment leachates from DSDP Site 480 (Gulf of California) over the last 30 ka; from ODP Site 849 (Eastern equatorial Pacific) from the last 200 ka and from ODP Site 1002C (Cariaco Basin) across the 9-17 ka time interval in order to critically evaluate claims of a global 10% shift in the 187Os/188Os of seawater from glacial to interglacial intervals. We use organic-rich continental margin sites and carbonate-rich pelagic sites to isolate the temporal variations of the osmium seawater isotopic composition. Our results reveal that variations in 187Os/188Os fail to correlate with global changes in temperature across glacials/interglacials cycles as previously claimed. Instead, these results indicate differences of a few percent in the measured 187Os/188Os between each oceanic basin. We argue that these differences strongly suggest that seawater is not well homogenized with respect to its Os isotope composition.

Ingenieros agrónomos, desarrollo rural y cultura política : reflexiones críticas en torno a la práctica de la (ex)tensión rural

En esta nota breve se analiza, desde una perspectiva crítica, la práctica de la extensión rural realizada por profesionales agrónomos en Mendoza, Argentina. En base a una amplia revisión bibliográfica y análisis documental sobre programas de desarrollo rural, analizamos los condicionamientos socio-culturales de dicha práctica, elementos que inciden sobre la concepción, ejecución y orientación de las estrategias de desarrollo promovidas por estos sujetos y sus instituciones. Así argumentamos que a pesar de los profundos cambios ocurridos en el mundo, particularmente los que afectan al mundo rural, las prácticas de los extensionistas y su concepción, continúan arraigadas a los ya viejos supuestos de la modernización y el progreso indefinido. Asimismo sostenemos que la falta de reflexividad crítica institucional retroalimenta la crisis del desarrollo rural con considerables repercusiones en sus promotores, los extensionistas. Finalmente destacamos que la reflexión sobre la práctica de la extensión rural tiene una relevancia social e institucional central, considerando que un análisis crítico de las limitaciones y posibilidades en este campo puede contribuir a orientar -y superar- los desafíos actuales y futuros.

(Appendix 1) Age, Ge/Si ratio, aluminium and opal contents in ODP Sites 175-1081 and 175-1084

As a result of both culture and sediment core studies, the ratio of germanium (Ge) to silicon (Si) in diatom shells has been proposed as a proxy for monitoring whole-ocean changes in seawater Ge/Si, a ratio affected by changes in continental weathering. However, because of the difficulties of extracting and cleaning diatom frustules from deep-sea sediments, only samples from highly pure diatom oozes in the Antarctic region have been previously analyzed. Here we present data on diatom Ge/Si ratios, (Ge/Si)opal, for the time interval between 3.1 and 1.9 Ma from a mid-latitude, coastal upwelling area where significant terrigenous sediment input complicated the sample processing and analyses. In general, our (Ge/Si)opal values show the same decreasing trend after 2.6 Ma than previously measured in Antarctic sediments (Shemesh et al., 1989. Paleoceanography 4, 221-231), but with a noisier background that may reflect the local imprint of proximal continental input superimposed upon global changes in the ocean reservoir. The time of initiation of large-scale North Hemisphere glaciation at ~2.6 Ma is characterized by a declining pattern of diatom Ge/Si ratios, which could have resulted from a global increase in the input of riverine Si due to enhanced silica weathering and/or equatorward (northward) intrusions of subantarctic waters enriched in silica. High (Ge/Si)opal ratios are associated with high opal contents from the same sediment samples and with warm climate as indicated by depleted benthic foraminiferal d18O values from the North and Equatorial Atlantic. Cold periods signified by enriched benthic d18O values, on the contrary, are associated with lower (Ge/Si)opal ratios. We interpret diatom Ge/Si values to reflect the prevailing weathering state on the continents, with greater chemical weathering during warm and wet periods of the Pliocene and less during cooler and drier intervals.

Stable carbon isotope record of Bahama Bank sediments

The carbon-isotopic composition (d13C) of bulk carbonates, obtained from a transect of sites drilled through platform and periplatform sediments of Holocene to Early Miocene age, has been compared to ascertain whether changes in the d13C can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the d13C of the dissolved inorganic carbon in the oceans over this time period. Five of the sites were drilled during Leg 166 of the Ocean Drilling Project (1003-1007) in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of Holocene to Oligocene age that are contained in 17 depositional sequences (A-Q). Two shallow-water sites, Clino and Unda were situated on a extension of the same transect on Great Bahama Bank in a water depth of 10-15 m. With the exception of Unda and Clino, the d13C of the carbonates ranges from +5 per mil in the younger sequences to +1 per mil in the Early Miocene. In each of the sites, the d13C is strongly positively correlated with the percentage of aragonite. As a consequence, the d13C of sequences A through F is strongly correlated, reflecting the decreasing amount of aragonite with increasing depth. In the two platform sites, the d13C is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the Pleistocene. Although the d13C of the individual sequences can be correlated in most instances between the ODP holes, the changes are not related to global changes in the d13C of the oceans which in contrast to the d13C of the platform sediments become isotopically lower towards the present day. Instead variations in the d13C appear to be related to varying mixtures of d13C-rich banktop sediments and pelagic material.

Pequeñas localidades rurales, cambios globales y conocimiento local : Estudios comparados en América Latina

El presente artículo aborda los cambios en los espacios rurales de América Latina y los posibles escenarios futuros a través de estudios de caso desarrollados en el contexto del proyecto "Designing a methology to evaluate local knowlegde on global change and its role in the construction of future land use scenarios by local actors" IAI Institut for Global Change (director Jean Francois Tourrand, CIRAD). Este proyecto tiene un objetivo metodológico principal que consiste en elaborar, testear y validar una metodología para evaluar el conocimiento local sobre el cambio global y cómo se incorpora tal conocimiento en la construcción de escenarios futuros de uso de suelo. La complejidad del problema implica un abordaje interdisciplinario a través de la participación de investigadores de distintos países e instituciones y el desarrollo de trabajos de campo en distintos espacios rurales (1) En lo que respecta a la metodología elaborada, las experiencias permitieron avanzar en la precisión de las informaciones a recabar (factores de cambio, escenarios y condiciones) y fue enriquecedora la experiencia de los trabajos de campo en Uruguay, Brasil y Perú.

Pequeñas localidades rurales, cambios globales y conocimiento local : Estudios comparados en América Latina

El presente artículo aborda los cambios en los espacios rurales de América Latina y los posibles escenarios futuros a través de estudios de caso desarrollados en el contexto del proyecto "Designing a methology to evaluate local knowlegde on global change and its role in the construction of future land use scenarios by local actors" IAI Institut for Global Change (director Jean Francois Tourrand, CIRAD). Este proyecto tiene un objetivo metodológico principal que consiste en elaborar, testear y validar una metodología para evaluar el conocimiento local sobre el cambio global y cómo se incorpora tal conocimiento en la construcción de escenarios futuros de uso de suelo. La complejidad del problema implica un abordaje interdisciplinario a través de la participación de investigadores de distintos países e instituciones y el desarrollo de trabajos de campo en distintos espacios rurales (1) En lo que respecta a la metodología elaborada, las experiencias permitieron avanzar en la precisión de las informaciones a recabar (factores de cambio, escenarios y condiciones) y fue enriquecedora la experiencia de los trabajos de campo en Uruguay, Brasil y Perú.

Pequeñas localidades rurales, cambios globales y conocimiento local : Estudios comparados en América Latina

El presente artículo aborda los cambios en los espacios rurales de América Latina y los posibles escenarios futuros a través de estudios de caso desarrollados en el contexto del proyecto "Designing a methology to evaluate local knowlegde on global change and its role in the construction of future land use scenarios by local actors" IAI Institut for Global Change (director Jean Francois Tourrand, CIRAD). Este proyecto tiene un objetivo metodológico principal que consiste en elaborar, testear y validar una metodología para evaluar el conocimiento local sobre el cambio global y cómo se incorpora tal conocimiento en la construcción de escenarios futuros de uso de suelo. La complejidad del problema implica un abordaje interdisciplinario a través de la participación de investigadores de distintos países e instituciones y el desarrollo de trabajos de campo en distintos espacios rurales (1) En lo que respecta a la metodología elaborada, las experiencias permitieron avanzar en la precisión de las informaciones a recabar (factores de cambio, escenarios y condiciones) y fue enriquecedora la experiencia de los trabajos de campo en Uruguay, Brasil y Perú.

Osmium isotope composition of ODP Hole 121-758A

This study presents osmium (Os) isotope and elemental data for cleaned planktic foraminifera, authigenic Fe-Mn oxyhydroxides and pelagic carbonate host sediments from ODP site 758 in the southernmost reaches of the Bay of Bengal. The Os in the bulk sediments appears to be dominantly hydrogeneous (sourced by carbonate and Fe-Mn oxyhydroxide), but variations in this particular core are controlled by the presence of volcanic ash. Fe-Mn oxyhydroxide leachates (of the bulk sediments) from Holocene samples also yield an Os isotope composition close to that of seawater, but the record diverges from that of foraminifera at a depth corresponding to the oxic/post-oxic boundary, suggesting diagenetic mobilization of Os at depths below this. Holocene planktic foraminifera, cleaned using oxidative-reductive techniques, also give Os isotope compositions indistinguishable from modern seawater, but the record obtained for the past 150 kyr shows strong covaraitions of 187Os/188Os with both the local and global oxygen isotope record, with less radiogenic Os isotope compositions during glacial intervals. These results indicate that foraminifera provide a robust record of seawater Os isotope compositions, and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187Os/188Os over this time interval, while the covariation with oxygen isotopes suggest a process controlling the Os isotope composition that is in phase with global climate cycles. Global excursions to relatively unradiogenic 187Os/188Os during glacial intervals are consistent with decreased input of radiogenic continental material, reflecting cooler temperatures and reduced continental runoff. Modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an ~30% decrease in the global river flux, with an ~5% change in river composition. If the residence time of Os in the oceans is ~5 ka then the post-glacial recovery to present-day seawater values is consistent with a corresponding increase in the river flux of around 30%. However, if the residence time of Os is closer to 40 ka, as is suggested by the global river flux, then this demands either significant changes in both the riverine Os flux and composition of around 40% and 30%, respectively, that closely follow the oxygen isotope record, or else a short-lived post-glacial pulse of weathering some 75% greater than the steady-state flux. In either case, these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial-interglacial timescales.