Abrantes, Fatima

Several locations along the North Atlantic margins are investigated to provide high-resolution paleoceanographic reconstructions based on diatom records for the last 2000 yrs: the Tagus pro-delta (SW Portuguese margin), the Skagerrak (NE North Sea) and the Laurentian fan (South of Newfoundland). The North Atlantic Oscillation (NAO) is considered as the main climatic factor controlling the northeastward heat flux associated with the Gulf Stream, the wind pattern and the distribution of the precipitation over the North Atlantic realm. The major climatic periods of the last 2000 yrs, the Dark Ages (DA), the Medieval Warm Period (MWP) and the Little Ice Age (LIA), are identifiable in the sedimentary sequences and the climatic events identified correspond to NAO type situations, even at the Tagus pro-delta, the oceanic response is not in phase with the others locations during the DA, showing that the NAO mechanism by itself is not sufficient to explain all the observed oceanographic variability. Besides, the centennial frequencies and timing of the cooling indicated by the diatom record over the Laurentian fan suggest the impact of the solar forcing.

The main objectives of this thesis were 1) to calibrate the first regional coastal upwelling transfer function for the western Iberian margin and 2) to investigate centennial- to millennial-scale hydrographic and productivity variations along this margin. To improve the knowledge on coastal upwelling areas like the western Iberian margin is of crucial social and economic importance. Upwelling regions act as high accumulation areas allowing high-resolution reconstructions of past climatic changes, and as sinks for CO2 and thus possible climate regulators.The Present day record and the past oceanographic and productivity conditions on the Iberian margin were investigated through a multi-proxy study (planktonic and benthic foraminifera abundances and stable isotopes, planktonic foraminifera associations, detrital grain abundance (IRD), grain size analysis, carbonate content) of surface sediments and sediment cores.The distribution of the most abundant planktonic foraminifera species on surface sediment samples (134) collected along the Iberian margin clearly mirror the specific hydrographic conditions in this area. Globigerina bulloides and Neogloboquadrina pachyderma (dextral) reflect the seasonal (May to September) coastal upwelling north and south of LisbonÃ ï¿½Ã ´s latitude, respectively, evidencing the two sources of upwelled water: subpolar Eastern North Atlantic Central Water (ENACW) in the north, and subtropical ENACW in the south. However, Neogloboquadrina pachyderma (dextral) in association with Globorotalia inflata also seems to mark the descending branch of the North Atlantic Drift (Portugal Current). The tropical-subtropical species Globigerinoides ruber (white), Globigerinoides trilobus trilobus, and Globorotalia inflata are related to the winter-time eastern branch of the Azores Current (Portugal Coastal Countercurrent). This data set and an eighteen year integration of daily satellite SST were used to calibrate and define a regional transfer function, which was then used to compare the results from two commonly different transfer function techniques, Imbrie&Kipp and SIMMAX. The simultaneous application of two different computational approaches allowed the validation of the modern samples and the selection of SIMMAX as the technique to be used to reconstruct sea surface temperature (SST) and export productivity (Pexp) in sediment cores from this area. This regional data set was also included into the North Atlantic MARGO data set contributing a considerable number of analogs from one upwelling region.During the Late Holocene, productivity and surface and deep-water hydrography were estimated by studying 6 box/multi-cores recovered along two transects along the southwestern (off Sines, 37˚ 50Ã ï¿½Ã ´N) and southern (off Albufeira, 8˚ 15Ã ï¿½Ã ´W) Portuguese margin. Results from productivity proxies show that productivity conditions off Sines were more stable during the major historical warm European North Atlantic periods, i.e. the Roman Warm Period and the Medieval Warm Period, than during the cold ones (Dark Ages and Little Ice Age). The opposite was observed off Albufeira. Alternate intensification of northerly (upwelling along the southwest margin) and westerly winds (upwelling along the south margin) during the summer of warm and cold climatic periods during the last 2500 yr, could explain such results. However, the southwestern transect shows higher productivity than the southern transect through time, which is attributed to the stronger and more persistent upwelling influence on the western Portuguese margin such as today. No major changes between warm and cold climatic periods were observed in both transects for the intermediate and deep bottom currents (Mediterranean Outflow Water and North Atlantic Deep Water).The impact of millennial-scale climatic variability on the Iberian margin in terms of oceanographic and productivity conditions was investigated back to 150 ky using three deep-sea cores located along a North - South transect (43Ã ï¿½Ã º12Ã ï¿½Ã ´N - 37Ã ï¿½Ã º48Ã ï¿½Ã ´N). In all cores, coldest summer SSTs and lowest Pexps are found during the Heinrich events. Only in the southernmost core, the lowest Pexp did not always coincide with these events. At the glacial/interglacial time scale, SST increases from North to South through time, while Pexp shows the opposite trend during interglacials and no significant variation during the glacial periods.The northernmost core reveals the highest SST amplitude over time, in agreement with the strongest influence of glacial polar front movements. The southermost core shows the highest Pexp amplitude indicating that the Sines upwelling filament was present during most of the studied interval although affected in its intensity by the southward penetration of subpolar to polar water masses.

Abrantes, Fatima

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