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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/194761
Holocene Climate variability in the Western Mediterranean Sea: an integrated oceanic and atmospheric perspective
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[eng] The Mediterranean region is very sensitive to modern climate variability. Located in a temperate subtropical climatic zone and surrounded almost entirely by large continental masses, the Mediterranean region climate and oceanography are largely controlled by the climatic variability of the subtropical and subpolar atmospheric systems. The western Mediterranean Sea exchanges seawater through the Strait of Gibraltar with the Atlantic Ocean, where inflowing surface waters from the Atlantic Ocean enter this region. In contrast, Mediterranean waters outflow into the Atlantic at depth, a net exchange of heat and salts that is very sensitive to present and past climatic changes. This thesis presents a new approach to studying and understanding past natural climatic variability of the Mediterranean region by analysing the changes in the ocean and the atmosphere during the Holocene. This thesis aims to improve our understanding of the mechanisms and climatic/oceanographic responses that have modulated the southern Iberian Peninsula climate during the last 11.700 years. In addition, relevant methodological advances in
«state-of-the-art» geochemical tools are implemented here to study past hydrological changes and, for the first time, these methods are validated using a modern time series record in the Alboran Sea. This thesis has been built on four main key points:
First, a new high-resolution deglacial and Holocene Sea Surface Temperature (SST) reconstruction is presented for the Alboran Sea (western Mediterranean), based on Mg/Ca ratios measured in the planktonic foraminifera Globigerina bulloides. This new SST record is evaluated by comparison with other Mg/Ca–SST records and previously published alkenone–SST reconstructions from the same region. The comparison shows a high degree of coherence between the different Mg/Ca–SST records but strong discrepancies compared to the alkenone– SST records. We argue that these discrepancies arise from differences in the sensitivity of the proxy–response to environmental changes. A working hypothesis in this thesis is that the discrepancy in SSTs reflects a resilience strategy of planktonic foraminifera G. bulloides, shifting its main growing season into the colder SST upwelling season (spring) thus explaining the shorter deglacial warming in Mg/Ca records compared to alkenones. Interestingly, Holocene short–term SST variability is larger in the Mg/Ca–SST record than in the alkenone–SST records. It is proposed that the larger Mg/Ca–SST variability reflects spring temperature variability, while the smoothed alkenone–SST variability represents averaged annual temperatures. The Mg/Ca–SST record differentiates the Holocene into three periods: (1) the Early Holocene (11.7 – 9 kyr BP) with the warmest SST values recorded; (2) the Middle Holocene (9 – 4.2 kyr BP) characterised by a continuous cooling trend that culminated in the coldest Holocene SST at around 4.2 kyr BP;
(3) the Late Holocene (4.2 kyr BP to present) characterised by the lack of clear cooling/warming trends but enhanced millennial-scale variability. This SST evolution is discussed in the context of the changing physic-chemical properties of inflowing Atlantic surface water associated with changes in the North Atlantic circulation conditions as well as with regional hydrographical and atmospheric changes. It is proposed that a close mechanistic link between North Atlantic
circulation patterns and the inflow of surface waters into the Mediterranean played a crucial role in controlling the Holocene climatic variability of this region.
The second key point of this thesis is based on developing a new fast, efficient and reliable method to prepare lithogenic sediment samples for the radiogenic isotope analysis (87Sr/86Sr, 143Nd/144Nd, 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb). Two new methods are developed: the first is targeted to achieve a total digestion of the lithogenic fraction, whereas the second is aimed at separating and purifying the different isotopic systems (Sr, Nd, Pb) in the lithogenic fraction by using a one single pass chromatographic column with the DGA resin. The new chromatographic separation method for the Sr, Nd and Pb is developed in a single step using a controlled-pressure vacuum system applied to each column.
The third key point of this thesis is based on a novel approach to studying the provenance of the lithic sediment fraction captured by a sediment trap between 1997 and 1998 in front of the Malaga coast. This sediment trap captured the sinking lithogenic particles in the Alboran Sea with an approximately bi-weekly resolution for a complete year; thus, an annual record of changes in the amount and nature of particles was obtained. A systematic provenance study is developed using isotope geochemistry techniques to characterise the source area of the lithogenic particles arriving in the Alboran Sea. In particular, the radiogenic isotopes of 87Sr/86Sr, 143Nd/144Nd, 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb were measured in the lithogenic sediment fraction, and the measured isotopic ratios signal were compared with the potential source areas and their corresponding isotopic endmember values. In this sense, due to the singular location of the Alboran Sea, lithogenic particles are typically introduced by both from fluvial discharges and/or atmospheric dust plumes from the Sahara desert. These new Sr and Nd isotope results clearly indicate that river discharge is the primary source of sediments arriving into the sediment traps. Even though there is evidence for a degree of presence of Saharan dust signal in all the samples, this endmember is generally significantly diluted by the sheer mass of sediments transported by rivers. Only during periods of the year with minimum river runoff, it is possible to clearly distinguish the Sahara isotopic signature in the lithogenic sediment samples. A comparison of the newly generated radiogenic isotope records with instrumental rainfall gauge records in Malaga and the Guadalhorce river discharges (located in front of the sediment trap) indicates that the observed variations in the isotopic composition of terrigenous materials (particularly Sr isotope ratios) are closely linked to the rainfall and river discharges. Therefore it is now possible to clearly differentiate through the 87Sr/86Sr ratio those events when the first rains occurred after the dry season (autumn and early winter), especially those associated to the torrential events and also those periods at the end of the rainy season followed by dry summer conditions (late winter to autumn).
The fourth point takes advantage of the mechanistic changes described in the radiogenic isotope measurements in present-day sediment traps as a response to changes in rainfall patterns in the western Mediterranean Sea. Using these new results as a present-day analogue, it is possible now to extrapolate this isotopic mixing model to reconstruct past changes in rainfall
patterns during the Holocene. With this goal in mind, in this thesis has been measured 87Sr/86Sr, 143Nd/144Nd, 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb isotope ratios in the lithogenic fraction of marine sediments, at the same core site located in the Alboran Sea where the Mg/Ca-SST were measured. By applying the radiogenic isotope mixing models developed for the sediment traps, it is shown that a relatively constant fluvial discharge source of sediments during the Holocene at this location. Moreover, the elemental composition of the bulk sediment measured on the XRF- core scanner is statistically treated using the principal component analyses. This treatment reveals that Zr and K are potentially affected by hydrological changes. Based on the new measurements of radiogenic isotopes in lithogenic sediments as well as bulk XRF analysis, we can clearly subdivide the Holocene into three main climatic periods as defined by changes in the rainfall patterns: (1) the Early Holocene (11.7 – 7.6 cal kyr BP), described as relatively constant rainfall regime, reduced erosion and transport of fluvial sediments towards the sea; (2) the Middle Holocene (7.6 – 4.2 cal kyr BP) which is described as a transitional period marked by a progressive increase in erosion and sediment transport by rivers, potentially as a response to a regional aridification increase and/or enhanced torrential character of the rains, reaching a maximum expression in the sedimentary record at around 4.2 cal kyr BP. (3) the Late Holocene period (4.2 – 0 cal kyr BP) is punctuated by stronger millennial-scale variability compared to the previous intervals. It is argued that this last period corresponds to the establishment of rainfall patterns analogue to present-day conditions.
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CATALÀ I CAPARRÓS, Albert. Holocene Climate variability in the Western Mediterranean Sea: an integrated oceanic and atmospheric perspective. [consulta: 30 de novembre de 2025]. [Disponible a: https://hdl.handle.net/2445/194761]