Περίληψη:
The Eastern Mediterranean Transient (EMT) represents a recent (early 1990s to today) water circulation phenomenon, during which more arid climatic conditions in the Mediterranean lead to the production of very dense, surface water masses in the north Aegean Sea. The produced north Aegean dense water (NADW) sinks to seafloor, flows to the south Aegean Sea and, then, spills into the Eastern Mediterranean Sea through the Cretan Straits, contributing to the rejuvenation of the bottom water. A large dataset of vintage high-resolution seismic-reflection profiles indicates that the floors of the basins in the southwest Aegean Sea are characterised by late Quaternary erosional gullies, abraded surfaces, and channel-related sediment drifts. Such structures provide evidence for the presence of strong bottom currents, which originate from the southward propagation of NADW masses. The basins have depths of 300 m to 1000 m and are connected to each other by narrow (a few km) straits and sills. The dominance of abraded surfaces and erosional gullies on the northern flank of the basins and in the narrow straits/sills between them provides evidence that the southward propagation of the NADW occurs by the filling and flooding of each basin by dense water, which then spills and cascades into the next basin southward. Seismic stratigraphy suggests that the formation of the drifts started at marine isotope stage (MIS) 11 (420 ka BP). The coincidence of at least three erosional surfaces within the sediment drifts with interglacial MIS 11, 5, and 1, respectively, suggests that bottom currents were mostly active during high sea-level stands and sluggish during glacial low sea-level stands. A 90-100 m drop of the sea level below the current level would have resulted in the emergence of the eastern Cyclades Plateau and, thus, in the disconnection of the southwest Aegean Sea from the north Aegean Sea. Several (0.05-0.5 m thick) beds of gravelly sand with algal and mollusc debris, laminated to mottled sand, and mottled mud are observed in the sediment cores analysed in this paper; these beds can be confidently correlated from core to core, and are interpreted as bottom-current deposits. The separation of these beds by hemipelagic sediments indicates that bottom-current activity in the southwest Aegean is not permanently established but episodic. Six episodes of intense bottom-current activity are recorded in the MIS 2-4 (10-80 ka) sediments and they appear to correlate well with the cold and dry Younger Dryas and Heinrich 3, 4, 5, 6, and 7 events. In contrast, at least three episodes of intense bottom-current activity are recorded in the cores during the Holocene. The higher frequency of strong bottom-current episodes in the southwest Aegean Sea during the Holocene is attributed to the higher sea-level stand that makes the circulation of water masses in the Aegean Sea more sensitive to climatic changes. © 2015 Elsevier B.V.
Συγγραφείς:
Tripsanas, E.K.
Panagiotopoulos, I.P.
Lykousis, V.
Morfis, I.
Karageorgis, A.P.
Anastasakis, G.
Kontogonis, G.
Λέξεις-κλειδιά:
Climate change; Erosion; Glacial geology; Landforms; Oceanography; Sea level; Seismology; Stratigraphy; Stream flow; Surface waters, Bottom current; Confined basins; Dense waters; Eastern Mediterranean; Sediment drift, Sediments, alga; bottom current; bottom water; climate conditions; climate variation; flooding; gully erosion; Holocene; mollusc; oceanic circulation; paleoceanography; Quaternary; seismic stratigraphy; water depth; water mass; Younger Dryas, Aegean Sea; Mediterranean Sea; Mediterranean Sea (East), algae; Mollusca
