Cambios climáticos y ambientales durante la transición Pleistoceno-Holocenonuevos datos palinológicos procedentes de las Rías Baixas (NO ibérico)

Resumo

Palaeoenvironmental studies allow us to understand the ecosystem responses to past climate changes and other environmental impacts; this is essential to develop strategies for the management and conservation of sensitive ecosystems, and also to predict the effects of future environmental changes. The research carried out by the XM-3 group (Basin Analysis group) of the University of Vigo aims to reconstruct the environmental changes that affected the Galician rias during the Quaternary. The main tool is the multidisciplinary analysis of stratigraphic sequences obtained in shallow marine sediments. The rias are ancient river valleys which have been flooded by the sea after the last deglaciation. The Rías Baixas are located at the Atlantic margin of NW Iberia, and they are very sensitive to climatic and oceanographic changes affecting the NE Atlantic. Therefore, they comprise environments that are very suitable for obtaining relevant sedimentary sequences for palaeoenvironmental studies. In this context, this thesis has been designed to increase the knowledge on the palaeoecosystems occurred in the Rías Baixas during the Last Glacial-Interglacial Transition (LGIT). In this report, detailed reconstructions of the environmental changes occurred in the coastal ecosystems of two rias during the MIS-3, the Lateglacial and the Early Holocene are presented. This study is mainly based on the interpretation of three palynologycal sequences coming from three sedimentary cores which have been obtained in the shallow environments of the Ría de Vigo (B5 and MVR-3 cores) and the Ría de Arousa (A14 VC15 core). Analyses of pollen, dinoflagellate cysts and some non pollen palynomorphs (NPP), such as fungal and freshwater algal spores have been carried out. The chronologies of the sedimentary sequences are supported by radiocarbon dating. Also, palynologycal data are correlated with other sedimentary data: lithology, grain-size distribution, elemental analyses (C, N, Al, Pb, Fe, Ca, etc.) and seismic stratigraphy, following a multi-disciplinary perspective. New data presented reveal that the ecosystems of the rias have been sensitive to the main climatic and oceanographic oscillations that affected the NE Atlantic during the MIS-3 and the Lateglacial/Holocene transition. They include some of the climatic substages of cooling which have been previously described in the isotopic records from the Greenland ice-cores and in some sedimentary sequences from the mountains of NW Iberia. Such variations have modified the hydrology, the littoral configuration, the sedimentary patterns and the coastal vegetation dynamics of the rias. The comparison of the biostratigraphy described for the Rías Baixas with other pollen sequences available in NW Iberia has revealed the existence of a high diversity of ecosystems during the LGIT. This is mainly related to the topographic and mesoclimatic features, and especially to the different degree of oceanic influence between distinct territories. In the marine realm, a high diversity of environmental conditions within the rias has been also detected. This may be attributable to changes in river and oceanic influence. Furthermore, palynologycal data from shallow marine environments support the existence of glacial refugia for mesophilous vegetation in NW Iberia, particularly in the littoral area of Galicia; they reveal that Carpinus betulus has been an important component of the regional forests of the Ría de Vigo during the MIS-3, and that it persisted in the region until at least the Early Holocene. This thesis also collects two additional studies of the palynomorphs distribution in modern sedimentary environments and in a Late Holocene high-resolution sequence. These studies provided data which have been used as reference to discuss and interpret the stratigraphic sequences of the LGIT more objectively and precisely. The first study includes the palynologycal analyses of 40 surface samples obtained from different coastal habitats from the Ría de Vigo. The palynologycal fingerprints typical of different habitats studied have been described. Also, their relationship with the modern vegetation and sedimentary environments is discussed. Moreover, it is presented a high resolution palynologycal study for the upper levels of one of the sediment cores obtained from the Ría de Vigo (B5). Isotopic 210Pb analyses support the chronology of this Late Holocene sequence. The data obtained allowed to reconstruct the hydrological and vegetation changes occurred in the Ría de Vigo during the last two centuries. Its comparison with other environmental data coming from instrumental records and indirect evidence from other sedimentary proxies supports the interpretations and allowed to assess the usefulness of certain palynomorphs as possible palaeoenvironmental indicators. During the last ~180 years, relative sea-level (RSL) rise and climatic fluctuations have had important effects on the hydrology and the landscape of the Ría de Vigo. Besides, the ecosystems of the ría have been strongly modified by human interactions, mainly from the second half of the 20th century, when water eutrophication and damages of certain coastal habitats are detected. In summary, this thesis represents an unpublished contribution to the knowledge of paleoclimatology and paleobotany of the LGIT in NW Iberia. It is presented new data on the past and current distribution of some floristic taxa (e.g. Carpinus betulus) and dinoflagellate cysts (e.g. Peridinium ponticum), which may be of particular interest in paleoenvironmental and biogeographical studies.