Manganese concentrations and flux data of sediments from Broken and Ninetyeast Ridge

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene-early Pliocene (6.5-3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086-1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High-resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene-Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3-1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2 deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological productivity.

Regulación de la hidratación y la turgencia foliares por mecanismos evitadores del estrés, y resistencia a déficit hídrico en vid

El transporte del agua en las plantas es impulsado por diferencias de energía libre entre el suelo y la atmósfera, y está regulado por mecanismos biológicos evitadores, como el cierre estomático. La hidratación y la turgencia foliares resultan del equilibrio entre ΨL del apoplasto, el potencial osmótico del simplasto y la elasticidad de los tejidos. Sobre esta base se conjeturó que las interacciones de los mecanismos evitadores del estrés hídrico de la planta tienen un rol clave en la definición de su resistencia a déficit hídrico. Para probar esta hipótesis se construyó un modelo mecanístico basado en las leyes del flujo de savia de Van de Honert, de difusión de Fick, de elasticidad de Hooke, la ecuación de Gardner para el flujo del agua en la rizósfera y el modelo de conductancia estomática (gs) de Buckley. Mediante el modelo se demostró teóricamente que la hidratación y la turgencia foliares dependen de la oferta de agua edáfica (representada por el potencial hídrico del suelo) y de la demanda evaporativa de la atmósfera (representada por la radiación absorbida, la temperatura del aire, la velocidad del viento y el déficit de presión de vapor de la atmósfera). También que los mecanismos evitadores del estrés hídrico -i.e., conductancia hidráulica de la planta, conductancia estomática, elasticidad del tejido y potencial osmótico a turgencia máxima- son todos necesarios para determinar la hidratación y la turgencia foliares. El modelo también demostró que la conductancia hidráulica suelo-hoja (kL) depende de la fracción de agua edáfica transpirable (FTSW) con un patrón de decaimiento sigmoide, a medida que el suelo se seca. Esto implica que las variables que dependen en parte de kL (i.e., gs, transpiración, fotosíntesis y superficie foliar) también dependen de FTSW con el mismo patrón. El modelo se probó experimentalmente a distintos niveles de humedad edáfica (desde déficit hídrico nulo, hasta severo) en cinco variedades de vid y mostró un poder predictivo superior al 90%. En todas las variedades las gs se asociaron linealmente con las kL observadas, al considerar todas las situaciones de déficit hídrico en conjunto, si bien la pendiente de estas relaciones fueron distintas en cada variedad. La contrastación experimental mostró que, en una escala de tiempo de varios meses, las variedades más evitadoras -i.e., Grenache y Cereza- mantuvieron mayor kL, ajuste osmótico y rigidez de los tejidos y una menor pendiente de la relación de gs vs. kL, que las variedades menos evitadoras -i.e., Malbec y Syrah-. La menor pendiente de la relación entre gs y kL, en las variedades más evitadoras, estuvo asociada a una mayor cantidad de estomas, en relación con la cantidad de células epidérmicas. Los variedades más evitadoras bajo déficit hídrico moderado -i.e., con una fracción de agua edáfica transpirable entre 0,6 y 0,4- tuvieron mayor superficie foliar y produjeron más biomasa, favoreciendo raíces profundas y densas, y ahorrando agua. Chardonnay mantuvo una alta hidratación y turgencia a expensas de un alto gasto de agua debido a que privilegiaba una alta kL por sobre el ajuste estomático, por lo que no podría considerarse en forma estricta como muy evitadora.

Particle flux measurements from moorings and surface sediment studies of sediment cores in the SE Pacific Ocean

Flux of siliceous plankton and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples collected in coastal upwelling-influenced waters off northern Chile (30°S, CH site) under "normal" or non-El Niño (1993-94) and El Niño conditions (1997-98). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27° and 43°S off Chile. Regardless of the year, winter upwelling determines the maximum production pattern of siliceous microorganisms, with diatoms numerically dominating the biogenic opal flux. During the El Niño year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicoflagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both sampling periods. Neritic resting spores (RS) of Chaetoceros dominate the diatom flux, mirroring the influence of coastal-upwelled waters at the CH trap site. Occurrence of pelagic diatoms species Fragilariopsis doliolus, members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata, secondary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicoflagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27° and 35°S, the dominance of RS Chaetoceros, Thalassionema nitzschioides var. nitzschioides and Skeletonema costatum reflects strong export production associated with occurrence of coastal upwelling. Both highest biogenic opal content and diatom concentration at 35° and 41°-43°S coincide with highest pigment concentrations along the Chilean coast. Predominance of the diatom species Thalassiosira pacifica and T. poro-irregulata, and higher relative contribution of the silicoflagellate Distephanus speculum at 41°-43°S suggest the influence of more nutrient-rich waters and low sea surface temperatures, probably associated with the Antarctic Circumpolar Water.

Planktonic foraminiferal flux in the Subantarctic

Sediment trap moorings deployed during 1997 and 1998 in the Subantarctic to Polar Frontal regions of the Southern Ocean reveal distinct seasonality in foraminiferal flux. Foraminiferal assemblages vary between each site, yet major species exhibit very similar patterns of seasonal succession which can be associated with changes in mixed layer depth. Enhanced foraminiferal productivity is also associated with periods of high biogenic silica and particulate organic carbon flux. On a broader scale, foraminiferal assemblages are strongly delineated by temperature. Temperature estimates derived from the assemblages using the modern analog technique (MAT) are mostly within 2.5°C of the satellite advanced very high resolution radiometer temperatures observed during the deployment period. This indicates that core top sediments included in the MAT database do reflect modern observed conditions at the sea surface, providing a robust technique for estimating past temperature change from foraminiferal assemblages in Southern Ocean environments.