Late glacial-holocene evolution of the Aegean Sea
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- Tez No: 35786
- Danışmanlar: PROF. DR. MUSA KAZIM DÜZBASTILAR
- Tez Türü: Doktora
- Konular: Su Ürünleri, Aquatic Products
- Anahtar Kelimeler: Ege Denizi, Evrim, Geç buzul dönem, Aegean Sea, Evolution, Late glacial period
- Yıl: 1994
- Dil: İngilizce
- Üniversite: Dokuz Eylül Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
Four hemipelagic lithofacies are identified in the late glacial to Holocene sediments in the Aegean Sea. These lithofacies are silty-clay and clay in texture and include an admixture of (i) loose terrigenous sediments directly supplied from the adjacent landmass, (ii) gravel- to sand-sized cemented carbonate clasts and nodules, (iii) sand- to silt-sized air-born volcanic debris and (iv) various biogenic remains. Sediment dispersal is mainly controlled by the fluvial discharge rates, the sea- level variations during the last ça 15,000 years, which determined the proximity of the Aegean basins to river mouths and the prevailing oceanographic conditions in the Aegean Sea. Cemented carbonate clasts and nodules occur throughout the cores except within the sapropel layer SI. They are well- to friably- cemented, mottled to peloidal micrite or microcrystalline inter -granular cement between terrigenous and bioclastic debris, and are composed of carbonate, in the form of 10 to 11 mole % magnesian calcite. Aragonite, in the form of pteropods and microcrystalline aragonite is present throughout the cores, and together with the cemented carbonate clasts and nodules, both in surface and subsurface sediments, indicate that shallow pore waters in the Aegean Sea are now and have been for at last 15,000 years, saturated to supersaturated with respect to both aragonite and magnesium calcite. XRD results show that Facies C (sapropel SI) is characterized by lower smectite, higher illite abundances, with notable increases in chlorite and a reciprocal decreases in kaolinite abundances. Lower smectite/illite and kaolinite/chlorite ratios are a function of increased supply of illite and chlorite and decreased supply of smectite and kaolinite during the deposition of this facies. Late glacial to Holocene paleoclimatic and paleoceanographic changes are examined using records of calcareous and organic -walled marine microfossils, pollen and terrestrial spores and oxygen isotope data in cores from theXIII Aegean Sea basins. Planktonic foraminiferal, coccolith and dlnoflagellate data show that the last glacial - Holocene transition in the region was associated with a considerable warming in surface water temperatures. Paleotransfer function applied to planktonic foraminiferal assemblages show 5-10°C increase in surface water temperature from -14,000 to -9,600 yrBP. Estimates of surface water oxygen isotopic composition (518Ow) derived from“ planktonic foraminiferal oxygen isotopic and transfer function data indicate that this warming was associated with a 2.0 to 2.5 °/00 reduction in 6lB0”. Mediterranean -bas e d transfer function results indicate corresponding 1.0 to 1.5 °/oo salinity reductions between -9,600 and 6,400 yrBP throughout the Aegean Sea. The early Holocene excess fresh water originated from rapid melting of the northern European and Siberian ice sheets, supplied primarily from the Black Sea by the opening of Bosphorus and Dardanelles Channels, during the post glacial sea- level rise, supplemented by major rivers that flow into the Aegean Sea. Continuous outflow of fresh water into the Aegean Sea provided a low salinity surface lid, preventing the ventilation of the deep water. Benthic foraminiferal data shows a remarkable turn- over in the benthic assemblage indicative of very low dissolved oxygen levels, however, anoxic bottom water conditions did not occur. Stagnant deep water conditions in isolated depressions, together with the increased input of terrigenous organic matter and primary productivity, as indicated by pollen and dinocysts, subsequently resulted in the formation of sapropel level Si. The surface water temperature and salinity reached present-day values at -6,400 yrBP, and very little change occurred since, despite the major deforestation onshore. Sapropel SI occurs as a 25-35 cm-thick black, weakly laminated muds in Aegean Sea cores. AMS dates show that SI was deposited between 9600 and 6400 yrBP, about 1000 years after the last glacial- interglacial transition. SI was deposited during a period of isotopically depleted (1.5-2.0 °/oo) andXIV relatively cool surface waters. Isotopic, microfaunal and floral data indicate a major reduction in surface waters salinity during the deposition of SI and distribution maps of S180w show a northerly fresh water source. Relatively light S13Corg and high pollen and spore concentrations in Si suggest increased influx of terrestrial organic carbon, probably supplied by major rivers draining into the northern Aegean Sea. Benthic foraminiferal data indicate high nutrient - low oxygen bottom waters during this time, and together with silt-sized hematite and various levels of manganese coatings suggest that during the deposition of SI surface sediments were oxic. Visual and XRD identification of pyrite in SI together with considerable enrichments in S, Cu, Zn, As, Ni, Cr and Fe suggested that subsurface conditions were sufficiently reducing in nature for S0A2" reduction to occur within SI, probably taking place by diffusion from surface oxic into subsurface anoxic sediments. Palynomorphs in Si show large increases in terrestrial pollen and spores, with the floral assemblage indicating significant northern European and minor African components. No significant increase in dinoflagellate cyst abundance in SI suggests the absence of major upwelling in the region. These data suggested that the evolution of SI in the Aegean Sea largely resulted from stagnation of the surface waters during the disintegration of the continental ice sheets, rather than an increase in primary productivity and the associated increase in the preservation of organic carbon on the sea floor.
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