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Use of clay minerals as proxies for estimation of paleoclimatic conditions in closed basins

Leontopoulou Georgia

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Summary

The aim of this Thesis is the sedimentological and geochemical study of sediments of the southern Aegean Sea. Sediments from two deep-sea bottom cores in SE Aegean (ST5 core) and SW Aegean (KIM4 core), Greece, were examined by quantitative mineralogical and geochemical analysis in order to reveal provenance and paleoenvironmental information of the sediment deposition regime and infer provenance and paleoenvironmental control on sediment deposition.In the framework of research programs of HCMR (Hellenic Center for Marine Research), two sediment cores were recovered from the Aegean Sea. Laboratory analyses mainly included qualitative and quantitative mineralogical analyses, geochemical and grain size analyses. More specifically, the analyses performed are X-ray diffraction (XRD), X-fluorescence (XRF) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), determination of organic carbon (CHNS) and radiocarbon radio (14C) with a mass spectrometry accelerator (AMS).Simultaneously with the evaluation of the mineralogical results, a comparison of two methods of quantitative analysis (Rietveld-Biscaye) of the mineral phases in the samples was performed. In the first approach, all the bulk mineralogical composition of sediments was identified, using the Rietveld method, with the Autoquan software. The results were compared with those obtained from the semi-quantitative Biscaye (1965) approach on oriented clay fractions. The semi-quantitative analysis of Biscaye strongly overestimates smectite content and underestimates the abundances of illite, chlorite and kaolinite. The comparison of the two quantitative methods yielded significant results, such as different trends between the clay minerals. For example, the semi-quantitative mineralogical method yielded different trends between illite and smectite compared to the Rietveld method.The studied sediments consist mainly of mud and a distinct sapropel sequence (sapropel S1, Holocene). The deposition of sapropel S1 was paused as it was evidenced by the appearance of less dark layers with lower organic carbon content. The mineralogical composition comprises mainly calcite, quartz, aragonite, dolomite, clay minerals, feldspars, serpentine and amphibole. Illite and chlorite are the most abundant clay minerals in the sediments. Chlorite dominates in the sediments of ST5 core while illite dominates in the sediments of KIM4 core.The highest carbonate content is observed in the SW Aegean Sea (Myrtoon Basin) whereas quartz and clay contents follow the opposite trend. The sediments are composed of a mixture of biogenic carbonate minerals (main sources of Ca and Sr) and terrigenous clastic materials. The increase of organic carbon in the sapropel sediments is accompanied by similar increases in Ba content. The geochemical results showed that the sediments are enriched in SiO2, CaO, MgO, Sr, Ni and Cr. Sapropel sediments are enriched in Ba, U, V, Cu, Ni and Co and have a significantly higher Ba/Al ratio, compared to the surrounding sediments. The mineralogical and geochemical relationships indicate a strong ultrabasic influence, most probably the Marmaris ophiolite in the Lycian nappes. The age data revealed that the sediments range from 2.086-16.912 yrs BP and reflect sedimentation under depositional conditions from the Late Pleistocene to the Holocene in the study area. The content of organic carbon in the sediment samples ranges from 0.2-2.2 wt%.Clay mineral distribution in SE Aegean suggests that smectite is mainly of volcanogenic origin, illite was supplied by the nearby landmasses of W. Anatolia and the Islands of Rhodes, Tilos and Symi and that the contribution from SE Meditarranean is limited or totally lacking. Clay mineral distribution in SW Aegean revealed that illite dominates in the clay fraction. The southwestern part of the study area is particularly close to the island of Sifnos, which consists of metamorphic rocks. Sifnos is the main source of illite and chlorite (schists) and generally fine-grained clay minerals.The combined use of mineralogical and geochemical analysis of bulk sediments rather than the clay fractions are not only extremely useful in tracing sediment provenance in relatively closed basins, but it also enables a more realistic assessment of the importance of water circulation patterns on sedimentation processes in such environments.

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