Optimization and application of the itraxtm core scanner for diagenetic, paleoclimatic and pollution studies in marine sediments

Resumo

This thesis is focused on different applications of the ItraxTM Core Scanner (Itrax) in palaeoclimatic, diagenetic and pollution studies in marine sediments. The elemental composition of these sediments are usually obtained by techniques (as spectrometry (ICP-OES, ICP-MS) or conventional X-ray fluorescence (XRF)) that need the preparation of discrete samples, implying several working days. Several XRF core scanners (as the Itrax) have been developed as an alternative for these techniques, providing high-resolution geochemical data directly from sediment cores in a few hours. Even though the Itrax has been used in many scientific studies there is still not a common protocol to select the best measurement parameters in each case. For that reason, one of the objectives of this thesis is the optimization of the measurement conditions of this equipment and the data processing for getting better results according to the objectives of each study. To achieve that, several sedimentary cores were selected in deep waters (Galician Continental Margin and Alborán Sea) and coastal zones (Rías Bajas and Ría de Huelva), including a wide diversity of sedimentary environments, physico-chemical properties, textural characteristics and even different environmental problems. Some factors that affect the measurements as the tube used, the X-ray exposure time, and the storage time were evaluated in these cores. Moreover, different data processing were tested and they were compared with quantitative data obtained from conventional XRF, ICP-OES and ICP-MS analyses. These data were combined with other conventional sedimentologic and geochemical tools for detecting diagenetic processes, Heinrich Events and marine pollution, allowing the monitoring and the evaluation of the bioavailability of these pollutants. The results demonstrated that the best balance between data quality and the core preservation are obtained by using a X-ray exposure time of 20 seconds. The best option for data processing was the application of running-means on the re-evaluated data with the same depth discrete sampling interval. The combination of the Itrax geochemical profiles with pore-water analyses, colour data and SEM analyses allowed the detection of diagenetic processes and the location of the oxidation front in the sediment cores. Moreover, the combination of these profiles with a robust chronological model and with magnetic susceptibility profiles allowed the detection of Heinrich Events. Increasing the exposure time to X-ray it was possible the detection of trace metal pollution (as Hg and Cd), including rare earth elements in very low concentrations. These data allowed the extrapolation of the pollution levels from some ICP-OES analyses, the determination of its origin according to the relationships between different elements, and the identification of the levels where these pollutants could be more bioavailable (according to the inc/coh ratio). The pollution was monitored with time through the profiles and the sedimentation rates were estimated considering the pollutants peaks as time markers, obtaining similar results as those obtained from 210Pb and 137Cs dating. Moreover, Itrax data complemented the radionuclides analysis by minimizing the matrix effect in the sediments. In summary, the combination of the ItraxTM Core Scanner with other techniques of chemical analysis allowed minimizing the analysis time. In this way, it represents a powerful complementary tool of environmental diagnosis in palaeoclimatic, diagenetic and pollution studies.