Guillermo Francés

The last 100,000 years in the western Mediterranean: sea surface water and frontal dynamics as revealed by coccolithophores

A quantitative analysis was carried out on coccolith assemblages from two Pleistocene cores (K1 and K10) from the western Mediterranean. The distribution of selected coccolithophore species provides new paleoclimatic and paleoceano-graphic data. A continuous sequence from the top of Isotope Stage 5 to the Holocene was recorded. The reversal in dominance between Gephyrocapsa muellerae and Emiliania huxleyi was dated in both cores at ca. 73 ka. At about 47 ka, E. huxleyi shows a regular increase, whereas G. muellerae progressively decreases in abundance. During interglacial periods, high concentrations of coccoliths are observed, whereas in glacial times, coccoliths are more diluted and the percentage of reworked forms increases as a consequence of the higher terrigenous input. After taking careful account of the dilution factor, we conclude that the production of coccolithophores was higher during warm periods. Maxima in coccolith concentrations coincide with highstand episodes, probably as a result of the intensification of the Atlantic flux into the Mediterranean across the Gibraltar Strait. This intensification could have produced an increase in nutrient content in the surface Mediterranean waters. During cold periods, the western Mediterranean front underwent a reduction in activity, probably due to an increase in the saline and/or thermal gradients between the superficial waters, and intermediate waters in the Liguro Provencal basin.

Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Abstract Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.

Orbitally-controlled oscillations in planktic communities and cyclic changes in western Mediterranean hydrography during the Messinian

We use quantitative analyses of the planktic foraminiferal assemblages and stable isotope analyses of the Sorbas section in the western Mediterranean to reconstruct the cyclical changes in surface and deep water hydrographic conditions during the Late Miocene prior to the Messinian salinity crisis. Oscillations in winter and summer temperatures linked to cyclical fluctuations in the hydrographic conditions and nutrient availability were the main mechanisms driving the cyclical changes in the planktic foraminiferal assemblages during the Messinian. The winter intensification and southward expansion of the northern cool and dry winds during precession maxima lowered sea surface temperatures (below≃14–15°C), favoring water convection and the upward mixing of nutrients with the consequent increase in the proportion of cold, eutrophic water foraminifera. The low summer temperatures (below 24°C) inhibited the growth of warm, oligotrophic water foraminifera, as occurs today in the Mediterranean. By contrast, at times of precession minima the low influence of the high latitude air masses caused winter temperatures to remain relatively high which, combined with the lower surface salinities, led to the formation of a permanent pycnocline. Winter conditions prevented the growth of cold, eutrophic water foraminifera, while high summer temperatures and the formation of a permanent pycnocline stimulated the growth of warm, oligotrophic water foraminifera. The prevalence of stable water stratification during winter due to the large density gradient between the surface and intermediate waters prevented deep water formation and slowed down the rates of oxygen supply to the bottom, resulting in the formation of the sapropels. A sharp decrease in δ13C of benthic and planktic foraminifera that occurred between 6.8 and 6.7 Myr is related to an increase in the residence time of Mediterranean waters. Because similar changes have been observed in other regions of the Mediterranean, we conclude that this change records a significant reduction in the Atlantic–Mediterranean water exchange at that time. Mediterranean climatic amplification increased throughout the Messinian as the Atlantic–Mediterranean water exchange was progressively more limited and oscillations in the ratio of planktic/benthic foraminifera and of warm-oligotrophic/cold-eutrophic species are good records of this increasing amplification, which culminated at 5.95 Myr with the deposition of gypsum–pelitic layers that mark the onset of the Messinian salinity crisis.

DISTRIBUTION OF BENTHIC FORAMINIFERA IN COARSE SEDIMENTS, RÍA DE VIGO, NW IBERIAN MARGIN

The distribution of live benthic foraminifera (>63 μm) in box core samples from coarse sediments of the Ria de Vigo (NW Iberian margin) is mainly related to food availability and bottom current range. Live benthic foraminifera are dominated by small forms (63–125 μm). Faunal densities and diversity values suggest that very coarse substrates provide a favorable settlement for live benthic foraminifera. It is suggested that microalgae or bacterial biofilms that colonize shell surfaces are the main food resource for foraminifera. During upwelling events, the food supply to the bottom increases and the environment is colonized by opportunistic species. Moderate- to low-velocity bottom currents favor such pathways. The upwelling not only enhances the density of benthic foraminifera, but also allows the arrival of new species from the shelf. Attached forms are related to relatively high-velocity bottom currents and free living forms to moderate- to low-velocity bottom currents. Live benthic foraminifera inhabit a wide range of sediment depths and are not restricted to the uppermost centimeter. Under relatively high-velocity bottom current regimes, the vertical distribution of live benthic foraminifera is controlled by current strength. In coarse sediments, attached forms commonly considered strict epifaunal, and inhabiting above the disturbance depth, are considered “pseudoepifauna.” Below the disturbance depth, the sediment is not affected by currents and a “true” infauna is recognized.

Palaeoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidence

The Holocene palaeoclimatic history of the Galician continental shelf has been investigated through the analyses of diatom remains, other siliceous compounds, biogenic silica (BSi) and metals content in a multiproxy approach to a gravity core recovered from the Galicia Mud Patch, NW Iberian Peninsula, covering the last 5000 years. Downcore changes in diatom assemblages composition and abundance reflect changes in diatom production related to long-/short-term variations in climate, regional oceanography, upwelling strength and river influx off the coast of NW Spain and Portugal. Palaeoclimatic variability was related to the relative strengths and position of the Azores High and Iceland Low pressure cells. Metals and microflora fluctuations are interpreted as changes in the riverine influence and upwelling intensity paced by oceanographic, atmospheric and climatic changes. Lack of diatoms between 4700—3300 and 1800—1200 cal. yr BP could be linked to early diagenetic processes taking place in the sediment after burial. Biogenic barium (Baexcess), metals and excess diatom assemblages, show a general increase of marine productivity for the last 1200 cal. yr BP. Between 800 and 500 cal. yr BP high production of the microflora is triggered by influx of river-derived nutrients under conditions of SW winds and storms resulting from a NAO negative-like phase. The biosiliceous and geochemical signatures of sediments from the last 500 cal. yr BP indicate conditions of enhanced upwelling and increased phytoplanktonic production associated with the intensification of northerly winds. Upwelling strengthening in this area, attributed to recent global warming, could provoke an increase in phytoplankton biomass with consequent biological, climatological and socioeconomical impacts. The imprint of anthropic activities has been recorded by the increasing Pb/Al ratios for the last 400 cal. yr BP.

El efecto de exposición en el patrón sedimen- tario del sector submareal de tres playas en la Ría de Muros y Noia (NO de España)

En este trabajo se presenta la comparacion de los sedimentos superficiales de la plataforma proximal de tres complejos playeros situados en las inmediaciones de la embocadura de la Ria de Muros y Noia. En cada uno de ellos, se han identificado las distintas poblaciones sedimentarias en funcion de los analisis granulometricos realizados. La aplicacion de un analisis estadistico de componentes principales a las fracciones granulometricas, contenido en carbonatos y materia organica, ha permitido diferenciar poblaciones de muestras y la interpretacion sedimentaria de las mismas. Las diferencias observadas en la distribucion de las poblaciones sedimentarias de los tres complejos estudiados son una consecuencia del grado de exposicion de cada complejo al oleaje dominante y de la presencia de afloramientos rocosos sumergidos que ejercen un efecto barrera.