| Abstract |
The Mediterranean Sea is an ideal ‘natural laboratory’, for the study of environmental and climatic changes that took place in the marine environment. In this enclosed environment, abrupt climatic changes occur more intensively and in time intervals much shorter than those of the open ocean. During the late 80’s to early 90’s, significant changes have been observed in the Eastern Mediterranean circulation, with more predominant the shift of region forming deep waters from the Adriatic Sea to the Aegean Sea. The causes of this rearrangement, that have high impact for the climatic and ecological balance of the Mediterranean Sea (Klein et al., 1999), have not completely clarified until today. The Mediterranean basin exhibit occasionally in the past dramatic changes in the deep-water ventilation, on longer time scales, which are witnessed in the sedimentary record by the presence of sapropels. Sapropels are dark layers of sediments rich in organic content (> 2% Corg), and have been found in several sites in the Eastern and Western Mediterranean Sea. They were formed throughout the past 7 Million years (Myr), with at least 11 formed during the last 450 thousand years (ka), (reviewed by Rohling, 1994). The precise mechanisms leading to this unusual past accumulation of organic matter in the Mediterranean Sea is still a mater of debate. Their formation is related to slow-down of deep-water ventilation in response to climate related reductions in buoyancy loss. These reductions were, in most cases, caused by changes to much wetter climatic conditions at times of increased Northern Hemisphere Insolation (precession cycle minima, Berger and Loutre, 1994). Together, the timing and the apparent rapidity of past changes in the Eastern Mediterranean suggest a direct atmospheric link between the Eastern Mediterranean hydrographic regime and the global glaciation/ deglaciation phases. The present work is an organic geochemical study of the last 20,000 yrs B.P., focused on the North Aegean Sea, and aims to determine the environmental conditions responsible for the deposition of sapropel S1. More specifically, the study of organic biomarkers (n-alkanes, n-alkanols, sterols) in a molecular level offered us valuable information that concerns the sources of organic matter (terrestrial and marine), the nature of transformation/ mineralisation processes that occured in the marine water column but also in the sediment, and in the paleoenvironmental/ paleoclimatic conditions prevailed in the Aegean at the end of Last Glacial and Holocene periods. From our results we conclude that the deposition of sapropel S1 is related to an important increase in the deposition of organic material during the period 9,800 –6,500 yrs B.P. This organic material has terrestrial and marine origin. More specifically, the distribution of n-alkanes is characteristic of land plant sources, dominating by the long chain odd carbon number homologues, which originate from the epicuticular waxes of higher plants. This is also confirmed by the high values of the CPI index (Carbon Predominance Index) for the homologous C21 - C36. Similar distributions have been observed for n-alkanols, with predominance of the long chain even carbon number n–alkanols, which as the n-alkanes originate from land plants. The important contribution of terrestrial organic origin can be attributed to enhanced rainfalls and river discharge. Furthermore, an increase of biological productivity in surface waters during the formation of S1 can be verified from the high abundance of sterols, which are characteristic of marine planktonic organisms. This trend is characteristic of a stratified water column (Casford et al., 2003). The occurence of dysoxic and/or anoxic conditions due to slow-down of deep-water ventilation allowed the preservation of organic material leading to the deposition of sapropel. The relative contribution of marine and terrestrial sources of organic material appears from the values of Marine Sterols/Terrestrial Alcohols and Marine Sterols/Terrestrial Alkanes ratios. The high values of these ratios indicate an important input of marine organic matter, without nevertheless the contribution of land material to be of minor importance. Finally, the cross-correlation of our results with those of Casford and his colleagues (Casford et al., 2002), permitted us to present a more complete picture of the climatic and oceanographic (biogeochemical and physical) changes that occurred during the last 20.000 years in the North Aegean Sea.
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Οργανική Γεωχημική Μελέτη των Αιφνίδιων Κλιματικών Αλλαγών στην Περιοχή του Βορείου Αιγαίου τα τελευταία 20.000 χρόνια
