Browsing by Author "Luis, Joaquim"
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- Algarve: a study case for global vs regional climatic changes in mid latitude estuaries during the HolocenePublication . Veiga-Pires, C.; Mestre, Nélia; Moura, Delminda; Luis, Joaquim; Boski, T.Located at the extreme southwestern end of the Iberian Peninsula, in the South of Portugal, the Algarve province is bordered to the north by the Alentejo province, to the east by the Guadiana River, and to the west and south by the Atlantic Ocean. Its coastal region can be divided into two different sectors: the eastern sector, which is mainly made of barrier island and lagoon system named the Ria Formosa, and the western sector mainly represented by clastic and carbonated cliffs. In each sector, we sampled long and short sedimentary cores (ca. 30m and 3m, respectively) in two different estuaries. Therefore, the different sedimentary records obtained from the four estuaries should permit to identify global vs. west-eastern regional climatic changes through the observation of several proxies. Two of these proxies are sediment color and magnetic susceptibility, which have been acquired every five centimeters along core depth, using the Colortron II spectrophotometer and SM-20 magneto-susceptometer, respectively. Sediment surfaces of short cores were also digitized using the Mustek 1200 A3 PRO Scanner. Furthermore, short cores have been sampled every five centimeters along depth in order to analyze the sediment for its major and minor elements content, its organic carbon content, and its granulometry and mineralogy. Some shell fragments have also been sampled and sent for radiocarbon radiometry or AMS dating, allowing therefore constructing some absolute chronologies for the several cores. Some relative chronologies based on digital color profiles are also being used in order to correlate sedimentary sequences between cores. All the cores are mainly composed of clay, with the existence of soil horizons at the top of the cores and frequent sand layers towards their base. Several element profiles along depths are compared, namely those regarding Sr, Ca, F, Ba, Al and Ti, as well as their ratios. First results indicate that geochemical and color data allow to identify similar sequences from west to east probably related to major climate episodes. Although, pronounced differences in mineralogy and accumulation rates from on site to another are probably due to differences, respectively, in the geochemistry of the drainage basin substratum and in the flow rate of the rivers, and therefore to local precipitation. We acknowledge FEDER and OE that financed this study through the Portuguese Foundation for Science and Technology (FCT) (POCTI/CTA/39733/2001).
- An attempt to develop spectrophotometry as a proxy for sedimentary studies in estuarine environmentsPublication . Veiga-Pires, C.; Mestre, Nélia; Moura, Delminda; Luis, Joaquim; Boski, T.In deep-sea sediment studies, reflectance has been used as a proxy for high vs. low productivity intervals for several years. This value of the white-black scale is obtained through the use of spectrophotometers when using the CIE Lab color system, which also gives values on green-red and blue-yellow scales. In estuarine/continental sediment studies, color data are still often based on Munsell Table which does not allow to obtain numeric and continuous data and which is biased by the observer’s choice.
- An open source geophysical software for studying speleothem growth bands and comparing with a reference curve: MironePublication . Veiga-Pires, C.; Coelho, Catarina; Moura, Delminda; Luis, JoaquimSpeleothem growth bands are commonly referred as one of the parameters that are used for paleoclimate reconstructions. Accordingly, this work presents a new tool for detecting these bands based on the gray-scale image of the speleothem using the Mirone open source geophysical software. This program has initially been developed for working on georeferenciated images for geophysical studies, as for instance for recognizing paleomagnetic inversions from deep-sea sediments. This recognition based on gridded images is very similar to what is needed for the recognition and detection of speleothem growth bands.
- Connections between upwelling patterns and phytoplankton variability under different coastal regimes in SW Iberia PeninsulaPublication . Krug, Lilian; Silvano, Kathleen M.; Barbosa, Ana B.; Domingues, Rita B.; Galvão, Helena M.; Luis, Joaquim; Platt, Trevor; Relvas, Paulo; Sathyendranath, ShubhaThe region off southwestern Iberia (NE Atlantic) encompasses a wide variety of oceanographic regimes, including differently (geographic) oriented coastal areas impacted by upwelling, riverine inputs and submarine groundwater discharge, submarine canyons and seamounts, and open ocean waters, thereby potentially promoting zone-specific phytoplankton dynamics. Overall, this heterogeneous region is classified as being very sensitive to climate change, and climate-driven alterations (e.g., sea surface warming, changes in upwelling patterns and intensity) have been recently reported for the area. The present study aims to understand the contribution of upwelling to seasonal and interannual variability of coastal phytoplankton, using a remote sensing-based approach. Phytoplankton variability was evaluated using satellite-derived chlorophyll-a (Chl-a), as a proxy for phytoplankton biomass, and primary productivity (PP). Chl-a were obtained from merged SeaWiFS (Seaviewing Wide Field-of-view Sensor), MeRIS (Medium Resolution Imaging Spectrometer) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer) sensors at Globcolour portal. PP data at 9.25 km resolution were derived from Eppley’s Vertically Generalized Production Model, based on SeaWiFS and MODIS-Aqua and available at the Ocean Productivity site. Upwelling intensity was estimated using the difference in sea surface temperature (SST) between off and nearshore zones. Advanced Very-High Resolution Radiometer (AVHRR) 4 km SST were obtained from Pathfinder database. Other phytoplankton environmental drivers, such as local (e.g., river flow) and global (e.g., North Atlantic Oscillation - NAO) climate variables, were also analysed. The study area was divided into subareas differently impacted by upwelling and riverine flow, and satellitederived data was averaged for each zone. Seasonal and interannual variability covering a 14-year time series (1998- 2011) for each variable/region were explored. Chl-a at offshelf locations was significantly lower than coastal areas, and exhibited a fairly stable unimodal annual cycle, with maximum during March. Coastal locations displayed more variable annual patterns, with spring and summer Chl-a maxima, reflecting the impact of upwelling events and freshwater inputs. In respect to interannual variability, NAO index and coastal Chl-a were negative and significantly correlated, with 1-month lag. Chl-a interannual trends were also correlated to local climate variables, namely riverine flow for the easternmost coastal zone. The correlation between upwelling intensity and phytoplankton off SW Iberia is region-dependent being less strong within regions dominated by riverine influence.
- Detection of speleothem growth bands with an open source geophysical softwarePublication . Veiga-Pires, C.; Moura, Delminda; Luis, JoaquimSpeleothem growth bands are commonly referred as one of the parameters that are used for paleoclimate reconstructions. Accordingly, this work presents a new tool for detecting these bands based on the gray-scale image of the speleothem using the Mirone open source geophysical software.
- 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.
- Importance of the mesoscale in the decadal changes observed in the northern Canary upwelling systemPublication . Relvas, Paulo; Luis, Joaquim; Santos, A. Miguel P.Analysis of sea surface temperature (SST) time series since 1960 from existing data bases shows a generalized warming trend in the northern Canary upwelling system. The field of the satellite-derived SST trends off Western Iberia was built at the pixel scale (4 x 4 km) for the period 1985-2008, revealing significant spatial differences in the warming rates. Weaker warming trends fit to the known upwelling pattern off the southern part of the Western Iberia, pointing out the intensification of this feature since 1985, particularly during the peak summer months. A more regular behavior is found further north suggesting significant decadal changes in the mesoscale patterns of the northern Canary upwelling system. Citation: Relvas, P., J. Luis, and A. M. P. Santos (2009), Importance of the mesoscale in the decadal changes observed in the northern Canary upwelling system, Geophys. Res. Lett., 36, L22601, doi:10.1029/2009GL040504.
- Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone arrayPublication . Goslin, J.; Lourenço, Nuno; Dziak, R. P.; Bohnenstiehl, D. R.; Haxel, J.; Luis, JoaquimThe seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002-09/2003), the northern 'SIRENA' network, deployed between latitudes 40 degrees 20'N and 50 degrees 30'N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0-5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35-4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of main shock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.