Browsing by Author "Ghaleb, Bassam"
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- First isotopic records from stalagmites in Algarve (South Portugal)Publication . Veiga-Pires, C.; Moura, Delminda; Luis, Joaquim; Varela, J.; Ghaleb, Bassam; Hélie, Jean-François; Hillaire-Marcel, C.Os espeleotemas representam registos paleoclimáticos continentais de alta resolução e estão a ser cada vez mais estudos para fins de reconstrução climática de períodos não afectados por acções antropogénicas. Na região do Algarve (Sul de Portugal), a existência de formações carbonatadas do Jurássico e Cretácico permitiu um desenvolvimento de sistemas cársicos onde se encontram actualmente estruturas estalagmíticas. Neste estudo preliminar realizado sobre estas estalagmites da região do Algarve, foram realizadas datações U/Th e análises de isótopos estáveis que demonstram pela primeira vez a possibilidade de obter registos isotópicos continentais representativos do paleoclima do extremo Sudoeste da Península Ibérica.
- A first last glacial maximum stalagmite record from southern PortugalPublication . Veiga-Pires, C.; Ghaleb, Bassam; Hélie, Jean-François; Moura, Delminda; Luis, Joaquim; Hillaire-Marcel, C.A newly launched research program permitted the sampling of speleothem deposits in the Algarve area (Southern Portugal). Unfortunately, this stalagmite yielded very small amounts of U (~ 20 ppb), making the setting of a U-series chronology difficult due to both the relative importance of a detrital contaminating fraction and the lack of precision on overall 230Th measurements. Nevertheless, based on the top and most recent sample, we extracted a first order estimate for the 230Th/232Th ratio of the contaminating fraction and for its 234U/232Th and 238U/232Th ratios as well, assuming a secular equilibrium between 234U and 238U in this fraction.
- A first last glacial maximum to younger dryas stalagmite record from southern PortugalPublication . Veiga-Pires, C.; Ghaleb, Bassam; Hélie, Jean-François; Moura, Delminda; Luis, Joaquim; Hillaire-Marcel, C.A newly launched research program permitted the sampling of speleothem deposit in the Algarve area (Southern Portugal) with the primary objective of constraining the ages of past humid intervals in the area from U-series measurements.
- Plataformas litorais no sul de Portugal: uma tentativa de datação com U/Th em cracasPublication . Veiga-Pires, C.; Ghaleb, Bassam; Campo, Leif; Albardeiro, L.; Moura, Delminda; Machado, NunoNa costa sul portuguesa, mais especificamente na zona central algarvia, foram identificadas várias plataformas litorais situadas acima da presente faixa intermareal.
- Studying the past of Mediterranean outflow based on 230th excess inventories and contouritesPublication . Veiga-Pires, C.; Ghaleb, Bassam; Voelker, Antje; Abrantes, FatimaThe Mediterranean Outflow water (MOW) comes out from the Mediterranean Sea and then contours the northern slope of the Cadiz Gulf. Along its way to the southern Portuguese Margin, it divides itself into three levels flowing at different depths, 400 m, 800 m and 1200 m, respectively. These different pathways induce a series of contourites along the Cadiz slope as well as some sedimentary drifts, such as the Faro Drift. Based on the assumption that the sedimentologic characteristics of these contourites should give some light on the history of MOW velocity and intensity variability, two long sedimentary cores collected during the Marion Dufresnes 114/Images cruise in 1999 have been studied. The sampling sites of these two cores, MD99-2336 and MD99-2339, located in the Cadiz Gulf at 690 and 1177 m water column depths respectively, are thus, actually, below the first level and in the main core of the MOW third level. Along time, variations in these current levels, parallel to the slope, should then influence the existence and characteristics of contourites in both sedimentary records. For this purpose, thorium-230 (230Th) as well as granulometric and micropaleontologic analysis have been undergone at high resolution on the 4 uppermost meters spanning MIS1 to LGM times. The referred current prints can be detected by analysing surface and down core sediment for its 230Th content. This radioisotope is produced by the radioactive decay of uranium-234 which content in oceanic waters is known. Therefore, its production rate in the water column can be estimated as a linear function of the water depth (~ 2.6 dpm/cm2.ka for 1 km water depth). As 230Th is almost insoluble, it will sink to the oceanic floor together with the settling particles. This vertical flux to the underlying sediment is considered, in a first order approximation, equal to its production rate in the water column. On this basis, the 230Th excess in the sediment becomes a proxy for sedimentation versus erosion processes accordingly to the sign of the difference between the total and the vertical 230Th flux, i.e. if it is, respectively, positive or negative. With this method it is then possible to extrapolate on the location of the high velocity core area and whether its intensity changed or not looking at the inventory of excess 230Th in the contourite units. We acknowledge FEDER and OE that financed this study through the Portuguese Foundation for Science and Technology (PDCTM/PP/MAR/15297/1999).