Paleoceanography and paleoclimatic evolution of the Galicia Interior Basin (NW Iberian Peninsula) during the past 60 ka.

Resumen

The climatic changes study during the Quaternary in marine records is reaching, in the last years, a significant development and is achieving very good results which allow to obtain an accurate knowledge about the climate system running and the oceanographic evolution of the Earth in the last tens of thousands of years. In the research group XM-1, Oceanografía xeolóxica e bioxeoquímica of the Departamento de Xeociencias Mariñas e O.T. from the Universidade de Vigo, during the last years, were developing a detailed multi-topic analysis of the climatic and oceanographic evolution of the rias, continental shelf and continental slope of the NW Iberian Peninsula. This Thesis represents a new contribution in this way and amplifies the study area beyond the continental slope with the new data from the Galicia Interior Basin (GIB), poorly studied until now. The development of these studies is based on marine record from the different sedimentary environments of the Galicia continental margin, specially the Atlantic margin. This Thesis focus on the study of the abrupt climatic changes occurred during the past 60000 years and in their processes and products. The general objective of this Thesis is addressed to explain the abrupt climatic changes recorded from Galicia Interior Basin during the past 60000 years by means of a multi-proxy approach including sedimentary, oceanographic and climatic approaches. The study of abrupt climatic changes plays an important role due to their similarity with the current Climate Change that the Earth climatic system is experimenting. This study is specially focused in the Heinrich Events (HE), the more significant abrupt climatic changes occurred during the last glaciation and showing a clear signal in the Galicia margin attending to the oceanography and sediment features. This study is made following the new trend in the sedimentary record analyses based on the use of different proxies, in this case, chemical, physical and micropaleontological, in the same records. In the central chapters of this Thesis methods used in this study will be shown and explained. These methods, taking into account news in the studies of marine sedimentary records, combine the use of classical methods, as the planktonic foraminifera assemblages or the physical and chemical properties of the sediment, with new trace elements studies in the shells of the planktonic foraminifera. Besides, this thesis shows a novel method, the use of the Computed Tomography (CT) in the study of marine records. Taking in mind the described multidisciplinary approach to the paleoceanography of the Galicia Interior Basin during the past 60000 years, the main specific objectives of this Thesis are: -Characterization of the sedimentary records and filling of the Galicia Interior Basin. -To establish a chronoestratighraphy frame for the Galicia Interior Margin -Characterization of the sedimentary environments and processes occurred during the past 60000 years. -Analysis of the planktonic foraminifera species recorded in the Galicia Interior Margin and characterization of their assemblages. -Identification, description and characterization of the climate changes which affected to the Galicia Interior Basin during the past 60000 years. This study is focused on the abrupt climate changes and their effects in the oceanography of the study area. -Estimation of the sea surface temperature using different methods (transference equations, oxygen isotopes, trace elements) and comparison of these results with other sea surface temperature records from the Iberian margin, establishing a global climatic evolution for the Galicia Interior Basin. -Development and application of the Computed Tomography in sedimentary records of the Galicia margin as a new sedimentological method. -Establishment of a protocol of Computed Tomography use. -Design and development of specific software to optimize the study of the records using Computed Tomography. -Discussion of the integral paleoceanographic and paleoclimatic evolution of the Galicia margin during the last climatic cycle. Attending to these objectives and the developed research, the Thesis is structured according the following schema: Chapter I: Introduction The chapter includes a summary and description of the study area from geologic, climatic and oceanographic point of view. The aim of this chapter is addressed to depict the global and regional oceanographic and climatic contexts related to the study area and time considered. Chapter II: Sedimentary evolution of the Galicia Interior Basin during the past 60 ka: Sedimentation processes and palaeoceanographic significance A multi-proxy analysis for six gravity cores retrieved along an E-W transect in the Galicia Interior Basin (GIB";" NW Iberian Peninsula) was carried out with the aim of reconstructing the sedimentary, palaeoceanographic and palaeoclimatologic evolution at the Galicia Atlantic Margin during the last 60 ka. Sedimentation rates are higher during the glacial period than in the Holocene, especially during extreme cold episodes (Heinrich Events). Besides high sedimentation rates, glacial periods were dominated by along-slope currents in the eastern part of the Galician Interior Basin that favoured the building up of contourite bodies in the slope, and the pelagic and hemipelagic sedimentation took place in the central parts of the basin. Lower sedimentation rates characterized the Holocene. Variations in sedimentation rates result from an interaction between a) sedimentary processes that affected to GIB (contouritic, turbiditic and pelagic/hemipelagic sedimentation), and b) variable sediment supply to the basin due, mainly, to sea level oscillations that affected to shallow areas such as the Galicia Bank and the Galicia continental shelf. In general, the changes in the sedimentary processes that concerned the basin were controlled by the evolution of the oceanography. Heinrich Events (HE) are the most distinctive feature of the basin filling, and are easily identifiable and correlateted in the different sedimentary environments present at the GIB. The intensity and the range action of the HE are not uniform in the whole basin, causing differences that modulate the processes present in the glacial times in the GIB. Chapter III: CT-scan as a new sedimentological method: Tomographic characterization of the GIB and the development of anidoC as specific software. The non-destructive techniques of core analysis, especially of marine cores, are being broadly employed for sedimentary, paleoceanographic and paleoclimate research. The Computer Tomography scan (CT-scan) allows acquisition of 3D and 2D images, according to desired planes and, thus, the identification of sedimentary structures, large grains and their distribution, as well as direct measurements of densities. The most significant contribution of this technique is the possibility of getting results without opening the core. In this work CT-scan data obtained for five cores from the Galicia Interior Basin (NW Peninsula Iberia) are presented and discussed, focussing on (1) methodology of the CT-scan use, (2) tomography description of sedimentary facies identified in the GIB, (3) development of specific software to analyses the tomographic data (anidoC) and (4) comparison of tomographic data with data obtained by other methodologies. A preliminary protocol of use of the CT-scan was developed for core study according to balance the higher resolution and the most manageable files obtained. Specific tomographic features for each facies were described in based on visual and numerical data to present a tomographic catalogue of the facies in the GIB, with the possibility of using it as a predictive tool for future studies. The development of the anidoC program aims to provide tool able extracting selected radio-densities from the whole volume of CT-scan data and of profiling the radio-density for each core. The most singular feature of GIB cores is the presence of Ice Rafted Debris (IRD) deposited during Heinrich Events (HE), which can be easily recognized using the CT-scan and this software by high radio-density grains immersed in a low radio-density muddy matrix. So, the software allows a quick and accurate recognition of the IRD radio-density without requiring to observe and process images. ComparingCT-scan data with analytical sedimentary data and HE proxies of the cores validates the CT-scan as a sedimentary method that provides a powerful tool to improve correlations, identify well constrained events and make more accurate basin reconstructions without opening the cores. Chapter IV: Planktonic foraminifera analysis and surface oceanographic evolution of the Galician Interior Basin for the past 60 ka. Detailed analysis of quantitative and qualitative planktonic foraminifera assemblages of three cores placed in the centre and in E and W extremes of the basin were studied. Analyses were completed with SST reconstruction using the Modern Analogue Technique and the monospecific Globigerina bulloides, ?18O and ?13C measurements. All the analyses allow the reconstruction of sea surface configuration of the GIB during the past 60 ka based on similarities and differences between the GIB domains. HE characterize the main part of the records displaying an alternation of the HE conditions and the no-HE conditions. The planktonic foraminifera assemblages, the SST grouping and the Q mode factor analysis establish sea surface conditions and configurations based on the presence of the HE, and the prevalence in the no-HE intervals. These water column conditions could be related with current states, characterized by how stratified the column is and productivity of waters in a similar basin behaviour. Chapter V: Multi-proxy SST reconstruction from the NW Iberian Margin during the last 60 ka BP: decoding the seasonal impact of the Heinrich Events The paleoceanography of the past 60 ka in the Galician Interior Basin (GIB, NW Iberian peninsula) based on multi-proxy analysis which include Sea Surface Temperature (SST) reconstructions (Mg/Ca and MAT-SST), ?18Osw and HEs markers is discussed in this work with particular focus on Heinrich Events (HE) 1 and 4 as the strongest HE in the region. Mg/Ca-SST record of the planktonic foraminifera Globigerina bulloides from the GIB is interpreted to reflect mostly conditions of the upwelling-cold season while Modern Analog Technique (MAT)-SST from the GIB and published UK’37-SST from the SW Iberian peninsula reflect annual mean temperatures. The comparison of obtained Mg/Ca-SST data for the GIB and published Mg/Ca-SST data from the SW Iberian margin shows an extraordinary parallelism, indicating a broad regional impact of the major climatic and oceanographic changes. Detailed multi-proxy analysis for H1 and H4 reveals relevant differences between both. During H1 local iceberg melting occurred later and did not correlate with the lowest SST and salinity values. This suggests that the main cause of the cooling and freshening was a change in surface currents rather than a local iceberg melting. During H1 calcification in G. bulloides occurred during “warmer” season, close to its thermal tolerance, although the conditions were extremely cold. This fact is related to a strong oceanographic impact based on icebergs discharge and polar waters invasion along whole year. In contrast, H4 cooling may be related to two massive events of local iceberg melting. The lower abundance of G. bulloides during H1 than during H4 suggests that the cooling of H4 was not as extreme as during H1. Although IRD counts during H4 indicate iceberg melting and cool SST are supported by the MAT reconstructions, the Mg/Ca suggest a warmer season conditions for G. bulloides calcification without the action of surface stratification due to iceberg melting. Chapter VI: Synthesis and conclusions. This chapter expounds a general synthesis and the main conclusions of the results and discussions obtained from each previous chapter. This synthesis is divided in the two principal topics of the Thesis: (1) contribution to the CT-scan method and (2) the evolution of the Galicia Interior Basin extracted from the sedimentary filling, the water column evolution andthe surface circulation changes and their implication in the planktonic foraminifera species and assemblages. Annexe I: how to use the anidoC: instructions. This annexe shows the frame of the anidoC in the Galicia Interior Basin and the main features of its use. The annexe corresponds to the manual that was sent together with the software for registration in the intellectual property list. Annexe II: Registration characteristics of anidoC and programming code. This annex shows the authors, ownership and registration of technique characteristics of anidoC. Programming code is presented to include it as free software for its use and distribution. Attending to the structure of the Thesis and considering the different employed methods, the chapters that form the core of the Thesis (Chapter II, Chapter III, Chapter IV and Chapter V), are presented according to a conventional scientific conventional paper structure. This structure was chosen due to the different topics tackled in each chapter, although they conserve an evident relation are focused to the same general objectives. Each chapter present the methods, results and discussion, but for adjusting to the normative, the Thesis presents a global view of the project with a general synthesis and conclusions. The references are included separately at the end of each chapter, to make easier reading and searching.