Paleoceanographic and stratigraphic investigation in two semi-closed basins during Late Quaternary : the southern Red Sea and the Gulf of Corinth

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Σεργίου, Σπυρίδων
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Marginal marine environments with a restricted connection to the open ocean have been considered excellent for tracking the concurrent impact of both regional processes and the influence of the adjoining open sea. Consequently, the investigation of sedimentary successions from such settings provides essential information on the regional paleoclimatic signals and the impact of global sea level changes in hydrographic systems over time. The present thesis is concentrated on core sediments from the southern Red Sea (Saudi Arabia) and the Gulf of Corinth (Greece) aiming to examine the hydrological response of a semi-closed, sill-controlled basin to highly variable climatic conditions and intense sea-level fluctuations during late Quaternary time intervals: the last 30 ka for the southern Red Sea and the period between ca. 130-70 ka (Marine Isotope Stage 5) for the Gulf of Corinth. Chronological framework was built on radiocarbon datings for the Red Sea record while a combination of absolute (Th/U) and relative (biostratigraphic) age markers was applied for the Gulf of Corinth succession. Gravity core FA09 was retrieved from the deep outer part of the southern Red Sea continental shelf within the frameworks of DISPERSE project. The combination of lithological, micropaleontological (planktic and benthic foraminifera), stable isotope (δ18Ο, δ13C), and transfer function (Artificial Neural Networks) analyses of sea-surface temperature and salinity revealed the effects of sea level changes and the South Asian Monsoon System (SAMS) on the oceanographic regime and the seafloor conditions of the area. Moreover, extended comparisons with records from the central and northern Red Sea, the Gulf of Aden, and the northwestern Arabian Sea provided insights into the overall interaction between the Red Sea and the Indian Ocean. Results showed that over the total investigated interval the Red Sea was connected to the Arabian Sea favoring the deposition of hemipelagic beds in the outer shelf of the southern Red Sea. However, a strong glacial/interglacial variability trend was imprinted in the sedimentary succession. The continuous intrusions of inflows from the Gulf of Aden during the glacial section (30–15 ka) induced common sea-surface temperature variations in these regions and prevented hypersalinity in the southern Red Sea, while seafloor conditions were characterized by rather low oxygen levels and high accumulation of terrestrial sediments. The MIS 2 sea-level lowstand (ca. 22-15 ka) was marked by a gradual reduction in planktic foraminiferal abundance together with the highest salinity (~38 psu) and lowest temperature (~23.5 °C) reconstruction estimates, while organic-rich seafloor conditions were indicative of efficient preservation of the organic matter in the oxygen-depleted sediments. Deglacial sea level rise improved the water mass exchange between the northwestern Arabian Sea, the Gulf of Aden, and the entire Red Sea, while in the early Holocene these three areas shared relatively similar surface water conditions. Southwest (summer) Monsoon was dominant during the late glacial and early-middle Holocene and favored a strong coupling between surface productivity levels and organic matter accumulation of both marine and terrestrial sources, thus implying combined effects of nutrient-enriched inflow from the Gulf of Aden and increased riverine runoff in the southern Red Sea. For most of the late glacial era, laminated sediments accumulated in association with oxygen-poor and organic-rich seafloor conditions under a well-stratified water column. Northeast (winter) Monsoon dominated during glacial times and especially around the Heinrich stadials 2 and 1, associated with reduced sea surface temperature and productivity levels in the southern Red Sea. The MIS 5 succession of the Corinth rift sediments corresponds to the subunit 1.3 of hole M0079A, drilled during IODP Expedition 381 in the central basin. Sedimentological (granulometry, composition), micropaleontological (planktic and benthic foraminifera), and isotopic (δ18O and δ13C) analyses were combined with additional data from the expedition overview and records from the surrounding area. The sedimentary succession is characterized by the alternation of a) bioturbated, foraminifera-rich hemipelagites with b) detrital-rich, partly bedded intervals likely corresponding to intrabasinal gravitational mass movements and low oxic sea-surface conditions and c) aragonite-rich, laminated deposits, indicating either marginal conditions between marine and isolated environment or highly stratified water column and anoxic seafloor conditions. Water exchange with the open sea was efficiently established during the MIS 5e and 5c sea level highstands as indicated by the similar planktic faunal biozones between hemipelagic intervals of M0079A and core records from the adjoining Ionian and Adriatic Seas. In contrast, seawater interaction was restricted during the MIS 5b when the sea level fluctuated very close to the sill height. Notably, no imprints of the MIS 5a highstand exist in the studied successions. The combined effects of Ionian Sea inflows and enhanced riverine runoff led to increased water column stratification and low oxygen, eutrophic seafloor conditions in the Gulf of Corinth during times of enhanced humidity and precipitation rates in southern Europe and deposition of sapropel S5 and S4 throughout the eastern Mediterranean (MIS 5e, 5c). However, the high abundance of benthic foraminifera suggests the deposition of sapropel-equivalent sediments rather than sapropel layers within the GoCb. Primary productivity was principally regulated by high riverine runoff in the gulf during S5 whereas at the time of S4 deposition nutrient availability was concentrated in a well-developed DCM layer. Comparing the overall findings of the two records, and despite the several differences in terms of the climatic, oceanographic, and geomorphological regime in the two areas, several similar features have been identified which provide a wide perspective on the environmental response of a northern Hemisphere marginal sea of either tropical or temperate latitudes to sea level fluctuations and modifications in regional climate. 1) Efficient water exchange with the open ocean coincides with the deposition of fine-grained hemipelagites in the semi-closed basin, marked by high numbers of planktic foraminifera. 2) High evaporation, increased sea surface salinity (SSS) and drastic reduction of the planktic foraminifera assemblages are recorded during maximum sea level lowstands when water exchange with the open sea is limited with respect to the sill height. 3) High summer insolation intensity and enhanced humidity lead to a well-stratified water column characterized by low salinity, light δ18O, and elevated productivity sea surface conditions above a low oxygen and organic-rich seafloor. At insolation maxima, the highest stack of planktic foraminiferal numbers is observed in both records driven by increased surficial productivity. 4) Drastic reduction in planktic foraminifera abundance can result from either a) restricted inflows of nutrient-rich waters from the open ocean during sea level lowstands, b) low oxygenated seawater surface, and c) dilution effects triggered by intrabasinal mass transport deposits. 5) Both records are considered highly valuable wider-scale paleoclimatic archives: The southern Red Sea record is excellent for reconstructing past monsoon dynamics while the Gulf of Corinth succession includes imprints of sapropel-equivalent conditions in a semi-closed environment. 6) Several multi-millennial, widespread cooling events of the northern Hemisphere are captured in the investigated records, causing a coupling between cool seawater surface and well-oxygenated seafloor conditions attributed to a temporal breakdown of thermal stratification.
Paleoceanography, Stratigraphy, Red sea, Corinth rift