Final Report Summary - PALEO CRETAN BASIN (An integrated paleoceanographic-sedimentological study of the Cretan Sea, South Aegean Sea)
Changes in the ocean circulation and climate have been brought to the focus of research mainly during the last 30 years, due to their correlation to human activities. A dramatic hydrological change occurred during the last 20 years in the Eastern Mediterranean, where there has been a switch in the formation of new deep water from the Adriatic Sea to the Aegean Sea, and is named as the Eastern Mediterranean Transient (EMT). This is attributed to the increased salinity of the surface waters of the Mediterranean Sea due to river damming and decreased precipitation. The impact of this phenomenon in the hydrology and ecological system of the Eastern Mediterranean has puzzled the scientific community. The limitation of detailed hydrological data for the past 50 years does not allow numerical simulations to make any accurate long-term predictions, whereas conventional paleoceanographic studies fail to capture similar short-lived events of the past due to their low time resolution (about 0.2 - 0.5 ky).
This project proposes the application of an integrated paleoceanographic-sedimentological study of a large set of sediment cores and ultra-high-resolution seismic-reflection profiles from the Cretan Basin, South Aegean Sea. Throughout the duration (25 May 2009 - 25 June 2012) of the project, all of the major proposed goals have been accomplished. All of the data used in this project have been acquired during a series of 10 cruises from 1986 - 1992, with the R/V Sonne 41 and Aegean, and are included in an open file (final report) published by the Hellenic Center for Marine Research.
The first step of this project concerned the location, collection, and organisation of the data. Then, detailed sedimentary descriptions were done in 47 sediment cores, trying to identify the sedimentary processes that dominated during the late Quaternary in the South Aegean Sea. The stratigraphy of in the cores was established by identification of characteristic chronostratigraphic horizons, such as sapropels and tephra layers. Sapropels were identified by their sedimentological characteristics and their content in organic carbon, whereas the tephra layers based on their geochemical composition. All information extruded at this stage was then verified by published palaiontological and isotopic analysis performed in five stratigraphic cores from the South Aegean Sea (personal communication with Maria Geraga and Assy Antonarakou).
The quality of all the ultra-high-resolution seismic-reflection (3.5 kHz sub-bottom profiler) profiles was very poor, and thus we were forced to limit our focus to the analysis of only the high-resolution seismic-reflection (40 inch3 airgun) profiles. Sediment failures and gravity flows obscure the stratigraphy in initially proposed area of study and, for this reason, we expanded our effort to the north and west. The shallow seismic stratigraphy (upper 200 - 300 ms) in this study was established using progradational units and erosional surfaces related to sea-level fluctuations, whereas the already existing seismic stratigraphy was used for the identification of deeper stratigraphic reflectors. This allowed us to expand our study from the 70 ka captured in the sedimentary record of the gravity cores to the entire quaternary (about 1.8 My). The main outcomes of this work are briefly discussed in the paragraphs below.
Wide-spread erosional features (moats) and lenticular sediment bodies (sediment drifts) in the upper 300 ms of high-resolution seismic profiles indicate that bottom-current activity was a significant sedimentary process in the South Aegean Sea during late quaternary. However, it is striking that all of these features do not occur around the same isobath like most sediment drifts around the world, but appear to spill over from North Aegean to the Cyclades Plateau, then to cascade into the Myrtoon Basin to the east, fill it up, and then to spill-over to the western Cretan Basin through the southwest Aegean Rise. Therefore, these bottom currents are interpreted to originate from the formation of cold, dense water in the northern Aegean Sea and Cyclades, which through the Aegean thermohaline circulation flows to the south and spills over to the Eastern Mediterranean through the Cretan Straights. A correlation of the moats with the progradational units in the area indicate that thermohaline circulation was intensified during high sea-level stands, whereas they are first observed at MIS 11 (about 400 ka B.P.). The occurrence of distinct sandy and silty beds, interpreted as contourites support the interpretation of bottom-currents activity in the area. In addition, all of these beds occur at similar stratigraphic levels in the sediment cores, with ages that correlate them with the Heinrich events H1, H4, H6, and H7. This indicates that the thermohaline circulation in the Aegean Sea is sensitive to millennial-scale climatic variability in the Aegean Sea and is intensified during cold and dry events. The above argument provides solid clues that the recently observed EMT might not be an isolated event, but it has most likely occurred repeatedly in the past, being possibly the predecessor of an upcoming cold and dry time period, such as that of the Heinrich events.
In Christiani and Central Cretan basins, several incoherent seismic packets are observed to be interbedded in well-stratified strata in the upper 300 ms (250 m) of the seismic-reflection profiles. The age of the lower incoherent packet ranges from 250 - 600 ka, and thus, they are interpreted to represent gravity flows, originating from pyroclastic flows triggered by Santorini Volcano eruptions. The organisation of the incoherent packets into two discrete groups, an upper and a lower group, is most likely related to the two eruptive cycles of the Santorini Volcano. The larger thickness of the lowest gravity-flow deposits suggests that the catastrophic Minoan eruption, which resulted in the destruction of an entire civilisation, is not the strongest volcanic eruption that Santorini Volcano has experienced in its geological history.
Another aspect of this progress concerns the re-integration and incorporation of the beneficiary into the European scientific community. Dr Tripsanas through chats with his colleagues in the Hellenic Center of Marine Research (HCMR), visits to people in academia, and his participation in national and international conferences has managed to exchange ideas with European geo-scientists, and in some occasions to develop collaboration with them.
This project proposes the application of an integrated paleoceanographic-sedimentological study of a large set of sediment cores and ultra-high-resolution seismic-reflection profiles from the Cretan Basin, South Aegean Sea. Throughout the duration (25 May 2009 - 25 June 2012) of the project, all of the major proposed goals have been accomplished. All of the data used in this project have been acquired during a series of 10 cruises from 1986 - 1992, with the R/V Sonne 41 and Aegean, and are included in an open file (final report) published by the Hellenic Center for Marine Research.
The first step of this project concerned the location, collection, and organisation of the data. Then, detailed sedimentary descriptions were done in 47 sediment cores, trying to identify the sedimentary processes that dominated during the late Quaternary in the South Aegean Sea. The stratigraphy of in the cores was established by identification of characteristic chronostratigraphic horizons, such as sapropels and tephra layers. Sapropels were identified by their sedimentological characteristics and their content in organic carbon, whereas the tephra layers based on their geochemical composition. All information extruded at this stage was then verified by published palaiontological and isotopic analysis performed in five stratigraphic cores from the South Aegean Sea (personal communication with Maria Geraga and Assy Antonarakou).
The quality of all the ultra-high-resolution seismic-reflection (3.5 kHz sub-bottom profiler) profiles was very poor, and thus we were forced to limit our focus to the analysis of only the high-resolution seismic-reflection (40 inch3 airgun) profiles. Sediment failures and gravity flows obscure the stratigraphy in initially proposed area of study and, for this reason, we expanded our effort to the north and west. The shallow seismic stratigraphy (upper 200 - 300 ms) in this study was established using progradational units and erosional surfaces related to sea-level fluctuations, whereas the already existing seismic stratigraphy was used for the identification of deeper stratigraphic reflectors. This allowed us to expand our study from the 70 ka captured in the sedimentary record of the gravity cores to the entire quaternary (about 1.8 My). The main outcomes of this work are briefly discussed in the paragraphs below.
Wide-spread erosional features (moats) and lenticular sediment bodies (sediment drifts) in the upper 300 ms of high-resolution seismic profiles indicate that bottom-current activity was a significant sedimentary process in the South Aegean Sea during late quaternary. However, it is striking that all of these features do not occur around the same isobath like most sediment drifts around the world, but appear to spill over from North Aegean to the Cyclades Plateau, then to cascade into the Myrtoon Basin to the east, fill it up, and then to spill-over to the western Cretan Basin through the southwest Aegean Rise. Therefore, these bottom currents are interpreted to originate from the formation of cold, dense water in the northern Aegean Sea and Cyclades, which through the Aegean thermohaline circulation flows to the south and spills over to the Eastern Mediterranean through the Cretan Straights. A correlation of the moats with the progradational units in the area indicate that thermohaline circulation was intensified during high sea-level stands, whereas they are first observed at MIS 11 (about 400 ka B.P.). The occurrence of distinct sandy and silty beds, interpreted as contourites support the interpretation of bottom-currents activity in the area. In addition, all of these beds occur at similar stratigraphic levels in the sediment cores, with ages that correlate them with the Heinrich events H1, H4, H6, and H7. This indicates that the thermohaline circulation in the Aegean Sea is sensitive to millennial-scale climatic variability in the Aegean Sea and is intensified during cold and dry events. The above argument provides solid clues that the recently observed EMT might not be an isolated event, but it has most likely occurred repeatedly in the past, being possibly the predecessor of an upcoming cold and dry time period, such as that of the Heinrich events.
In Christiani and Central Cretan basins, several incoherent seismic packets are observed to be interbedded in well-stratified strata in the upper 300 ms (250 m) of the seismic-reflection profiles. The age of the lower incoherent packet ranges from 250 - 600 ka, and thus, they are interpreted to represent gravity flows, originating from pyroclastic flows triggered by Santorini Volcano eruptions. The organisation of the incoherent packets into two discrete groups, an upper and a lower group, is most likely related to the two eruptive cycles of the Santorini Volcano. The larger thickness of the lowest gravity-flow deposits suggests that the catastrophic Minoan eruption, which resulted in the destruction of an entire civilisation, is not the strongest volcanic eruption that Santorini Volcano has experienced in its geological history.
Another aspect of this progress concerns the re-integration and incorporation of the beneficiary into the European scientific community. Dr Tripsanas through chats with his colleagues in the Hellenic Center of Marine Research (HCMR), visits to people in academia, and his participation in national and international conferences has managed to exchange ideas with European geo-scientists, and in some occasions to develop collaboration with them.