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Reconstituição paleoclimática e paleoambiental em estuários com base no registo micropaleontológico de foraminíferos : relação com indicadores de escala local, regional e global
Publication . Moreno, João; Fatela, Francisco Manuel Falcão, 1964-; Soriano, Eduardo Leorri; Bilbao, Alejandro Cearreta
In recent years, many studies have sought to produce high-quality paleoclimatic reconstructions for the late Holocene based on high-resolution (annual or decadal) biological and/or chemical proxies, both providing a great amount of information on climate variability and change over the last two millennia. In this context, salt marsh sedimentary records have previously proved to be particularly suitable in unravelling anthropogenic impacts on coastal ecosystems from those caused by climate variability and other natural stressors, at local and regional scales, with intertidal benthic foraminifera providing consistent responses to the latter, specifically to relative sea-level change. This dissertation focuses on showing evidence of climatic variability and change from a collection of multiple proxy studies combining marsh benthic foraminifera assemblages, wine production (WP), grape harvest dates (GHD), bromine (Br) contents in tidal marsh sediments, and different datasets ranging from the last decades, using instrumental (temperature–precipitation) records, to the past two millennia, by means of reconstructions (temperature, Total Solar Irradiance – TSI and North Atlantic Oscillation – NAO) and regional model (temperature–precipitation) simulations, with the aim of reconstruct the paleoclimate history of the west coast of Portugal for the past 2000 years. Simultaneously, the sensitivity of marsh benthic foraminifera to climate change induced by solar activity (SA) – via hydrological balance (controlling the salinity baseline of the high marsh environment) – is assessed, to show their reliability as a paleoclimatic proxy to be integrated in forthcoming studies. For that, the foraminiferal records from two dated sedimentary cores collected in the northwest (Caminha salt marsh; Minho River) and southwest (Casa Branca salt marsh; Mira River) Portuguese coast have been carefully examined and compared. Since the marshes’ development in the 1300s, two compositionally distinct tidal marsh foraminiferal assemblages have been found, dominated by Haplophragmoides manilaensis and Trochamminita salsa/irregularis in the NW (Caminha) and by Jadammina macrescens and Trochammina inflata in the SW (Casa Branca), in accordance with their respective climate types. A number of revealed common key changes in the main assemblages’ composition has been attributed to larger scale climatic shifts, particularly as regards the transitions firstly from the Medieval Climatic Anomaly (MCA) to the Little Ice Age (LIA), and next from the LIA to the Current Warm Period (CWP) in the Iberian Peninsula, as well as to major temperature–precipitation excursions throughout the LIA and directly correlated with sustained negative phases of the NAO index in periods of lowest SA, known as Grand Solar Minima. It is also found, throughout the application of spectral and cross wavelet methods, that in the time span analyzed (from the 1300s to present), the signals of solar forcing in both foraminiferal and paleoclimatic records were intermittent, with the regional climate modulated by the solar secular Gleissberg cycle, especially after AD 1700–1750, following the Maunder Minimum (1645–1715). Besides, an important – and unexpected – feedback is achieved concerning Br in the sedimentary record from the two salt marshes. It is found that Br enrichment peaks, and its connection with organic matter (OM) – promoted by the climate-driven marsh dynamics involving the production/release of volatile Br compounds (as methyl bromide – CH3Br) to atmosphere – can be diagnostic, to some degree, of Grand Solar Minima and enhanced volcanic activity during those Grand episodes of SA. The CH3Br has an ozone (O3) depleting potential of 0.6, and belongs to Class I O3 depleting substances. Its multiple anthropogenic sources were subjected to phase-out under the Montreal Protocol and its amendments, the reason why natural emissions, as the ones from salt marshes, have become progressively more important contributors to stratospheric O3 loss processes. According to the results of the current work, based on the available knowledge about Br– OM relationships in terrestrial and marine ecosystems and moreover compatible with a wider recognized latitudinal (North-to-South) rising gradient on the salt marshes’ CH3Br emissions, it is plausible to assume (i) a switch in the marshes’ Br source/sink dynamic in response to future major excursions of SA and (ii) an increased total flux of CH3Br under a continued global warming trend. Overall, the novelty of this PhD dissertation lies in showing a broader role of marsh benthic foraminifera as indicators of changes in the climate system triggered by SA. Therefore, and not disregarding the great amount of effort still required in their improvement, the results presented here have some meaningful implications, namely on (i) a better understanding of patterns and causes of late-Holocene climatic variability in the west Iberian margin, (ii) the estimate of local and regional changes in hydro-climate and temperature in response to future solar variability, despite the current scenario of anthropogenic climate change, (iii) the research of trends and exposure of naturally produced brominated compounds in salt marshes worldwide, and (iv) the setting up of Br as a paleoclimate proxy linked to SA.
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Fundação para a Ciência e a Tecnologia
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SFRH
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SFRH/BD/87995/2012