Paleoceanographic and paleoclimatic evolution of the Aegean Sea since the last interglacial (~130 ka BP) with special emphasis on sapropel formation

Isler, Ekrem Bursin (2012) Paleoceanographic and paleoclimatic evolution of the Aegean Sea since the last interglacial (~130 ka BP) with special emphasis on sapropel formation. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Five long piston cores collected from different subbasins of the Aegean Sea constitute the primary source of data for this PhD thesis. This study is the first to document a continuous paleoceanographic and paleoclimatic record of the Aegean Sea since the last interglacial. The chronostratigraphic reconstructions of the cored sediments based on organic carbon contents, stratigraphic position of known ash layers and oxygen isotopic curve matching collectively demonstrate the presence of sapropel S1 and MISS sapropels S3, S4 and S5 in the Aegean Sea subbasins. Generally, the organic carbon (TOC wt%) contents in sapropels range between 0.8% and 2% with highest concentrations of 9-13% in sapropels S4 and S5. Average sedimentation rates range between 4.7 and 11.8 cmlka with highest rates being observed in Euboea and North Ikaria basins (9.8 and 11.8 cm lka, respectively). The timing of the onset of sapropels S4 and S5 mostly predate those in the eastern Mediterranean with ages ranging from 106.4-105.6 and 128.6-128.4 ka BP, respectively. On the other hand, the initiation of the onset of sapropel S3 (i.e., 83.2-80.4 ka BP) seems to agree with its Mediterranean counterparts, which highlights the heterogeneity of the Aegean Sea subbasins in terms of rapid vs. lagged response to changing climatic conditions. The sapropel initiations appear to be synchronous across the Aegean Sea; whereas, the terminations display a wider temporal variability implying that the cessation of sapropels is controlled both by the amplitude of paleoclimatic changes and the physiography/location ofthe subbasins. Quantitative variations in the planktonic faunal assemblages exhibit a sequence of bioevents during the last -130,000 years which allow identification of four major biozones. The distributional patterns of the most significant taxa demonstrate similar trends among all core localities suggesting that the major changes in the planktonic foraminifera assemblages have taken place rather synchronously in the Aegean Sea. Sapropels S3, S4 and S5 were deposited under similar hydrographic conditions during which a distinct deep chlorophyll maximum (DCM) layer was established. This situation points to a stratified water column and increased export productivity during times of sapropel formation. On the other hand, the faunal contrast between Sl and older sapropels indicates that the former was developed in the absence of a DCM layer, lacking a deep phytoplankton assemblage. Under such conditions, oxygen advection via intermediate water flow must have been significantly reduced which implies significant stagnation. Sapropels are interpreted to have been deposited under normal marine conditions with temporary establishment of semi-euxinic bottom water conditions. Both marine and terrestrial organic matter contributed equally to MISS sapropels. In addition, organic carbon isotopic values across sapropels are more depleted than those in the eastern Mediterranean which, in tum, suggests enhanced riverine input during their deposition. Primary productivity calculations show that, particularly for sapropels with very high TOC values, both preservation and increased productivity are imperative in order to deposit sapropels with very high organic carbon contents (i.e., up to 13%).

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