Sr and Nd composition, stable oxygen isotope record and lithogenic flux in eastern Mediterranean sediments

Isotopic ratios of Sr and Nd from lithogenic components of three isochronous core sections recovered from an east-west transect in the Eastern Mediterranean Sea (EMS) have been analyzed. The data are used for a quantitative estimate of the temporal and spatial variation of detrital flux to the EMS, assuming Saharan dust and Aegean/Nile particulate matter as dominant end members. It was established that the carbonate-free Saharan dust flux during deposition of the nonsapropel layers of marine oxygen isotope stage 5.4 (MIS 5.4) was similar to the present flux. During the deposition of sapropels S5 and S6, however, the Saharan dust input was drastically reduced and was not balanced by a change in the riverine influx at this time. Denser vegetation cover during more humid conditions may have reduced physical erosion and sediment removal in the source area. During marine oxygen isotope stage 6.2 (MIS 6.2) a pronounced increase of Saharan dust and detrital influx from the Aegean region is evident and implies more arid conditions in the southern and northern catchment areas. During this period, intersite variations are interpreted in terms of their geographic location relative to the seaways connecting the Aegean Sea and EMS. The width of the straits and hence the amount of sediment entering the eastern basins may have been affected by a low sea level that impeded interbasin sediment dispersal.

Sr- and Nd-isotopic composition of bulk lithogenic sediments from the Eastern Mediterranean Sea (Table 1)

In order to characterize the provenance of lithogenic surface sediments from the Eastern Mediterranean Sea (EMS), residual (leached) fraction of 34 surface samples have been analysed for their 143Nd/144Nd and 87Sr/86Sr isotope ratios. The sample locations bracket all important entrances of riverine suspended matter into the EMS as well as all sub-basins of the EMS. The combined analyses of these two isotope ratios provide a precise characterization of the lithogenic fraction of surface sediments and record their dilution towards the central sub-basins. We reconstruct provenance and possible pathways of riverine dispersal and current redistribution, assuming more or less homogenous isotopic signatures and flux rates of the eolian fraction over the EMS. Lithogenic sediments entering the Ionian Sea from the Calabrian Arc and the Adriatic Sea are characterized by high 87Sr/86Sr isotope ratios and low epsilon-Nd(0) values (average 87Sr/86Sr=0.718005 and epsilon-Nd(0)=-11.06, n=5). Aegean Sea terrigenous sediments show an average ratio of 87Sr/86Sr=0.713089 (n=5) and values of epsilon-Nd(0)=-7.89 (n=5). The Aegean isotopic signature is traceable up to the southwest, south, and southeast of Crete. The sediment loads entering the EMS via the Aegean Sea are low and spread out mainly through the Strait of Casos (east of Crete). Surface sediments from the eastern Levantine Basin are marked by the highest epsilon-Nd(0) values (-3.3, n=6) and lowest 87Sr/86Sr isotope ratios (average 0.709541, n=6), reflecting the predominant input of the Nile sediment. The influence of the Nile sediment is traceable up to the NE-trending, eastern flank of the Mediterranean Ridge. The characterization of the modern riverine dispersal and eolian flux, based on isotope data, may serve as a tool to reconstruct climate-coupled variations of lithogenic sediment input into the EMS.

AMS 14C and calibrated ages of sediment core GeoB10426-1

Newly acquired bathymetric and seismic reflection data have revealed mass-transport deposits (MTDs) on the northeastern Cretan margin in the active Hellenic subduction zone. These include a stack of two submarine landslides within the Malia Basin with a total volume of approximately 4.6 km**3 covering an area of about 135 km**2. These two MTDs have different geometry, internal deformations and transport structures. The older and stratigraphic lower MTD is interpreted as a debrite that fills a large part of the Malia Basin, while the second, younger MTD, with an age of at least 12.6 cal. ka B.P., indicate a thick, lens-shaped, partially translational landslide. This MTD comprises multiple slide masses with internal structure varying from highly deformed to nearly undeformed. The reconstructed source area of the older MTD is located in the westernmost Malia Basin. The source area of the younger MTD is identified in multiple headwalls at the slope-basin-transition in 450 m water depth. Numerous faults with an orientation almost parallel to the southwest-northeast-trending basin axis occur along the northern and southern boundaries of the Malia Basin and have caused a partial steepening of the slope-basin-transition. The possible triggers for slope failure and mass-wasting include (i) seismicity and (ii) movement of the uplifting island of Crete from neotectonics of the Hellenic subduction zone, and (iii) slip of clay-mineral-rich or ash-bearing layers during fluid involvement.

(Table 3) Mediterranean core-top d18O values for various planktonic foraminiferal species

A box model is presented to simulate changes in Mediterranean long-term average salinity and d18O over the past 20,000 years. Simulations are validated by comparison with observations. Sensitivity tests illustrate robustness with respect to the main assumptions and uncertainties. The results show that relative humidity over the Mediterranean remained relatively constant around 70%, apparently narrowly constrained to the lower end of the range observed globally over sea surfaces by the basin's land-locked character. Isotopic depletion in run off, relative to the present, is identified as the main potential cause of depletions in the Mediterranean d18O record. Also, slight increases in relative humidity (of the order of 5%) might have caused very pronounced isotopic depletions, such as that in sapropel S5 of the penultimate interglacial maximum. The model shows distinctly non proportional responses of d18O and salinity to environmental change, which argues against the use of isotope residuals in Mediterranean paleosalinity reconstructions.

Mean redox potential values in surface sediments along the transect D in the Cretan Sea (Table 1)

The seasonal, spatial and bathymetric changes in the distribution of chloroplastic pigments (Chl a, phaeopigments and CPE), TOC, TON, ATP, bottom water nutrient content and the main biochemical classes of organic compounds (lipids, proteins and carbohydrates) were recorded from May 1994 to September 1995 over the continental margin of northern Crete. The concentration of chloroplastic pigment equivalents (CPE) was always low, dropping dramatically along the shelf-slope gradient. Microbial activity (ATP) also dropped sharply beyond the continental shelf following a distribution pattern similar to TOC and TON. Lipid, protein and carbohydrate concentrations, as well as biopolymeric carbon were comparable to those reported for other more productive areas, however, the quality of the organic matter itself was rather poor. Thus, carbohydrates, the dominant biochemical class, were characterised by being highly (80-99%) refractory, as soluble carbohydrates represented (on annual average) only 6% of the total carbohydrate pool. Protein and lipid concentrations strongly decreased with depth, indicating depletion of trophic resources in the bathyal zone. Proteins appeared to be the more degradable compounds and indeed the protein to carbohydrate ratios were found to decrease strongly in the deeper stations. Organic matter content and quality decreased both with increasing distance from the coast and within the sediment. All sedimentary organic compounds were found to vary between sampling periods, with the changes being more pronounced over the continental shelf. The different temporal patterns of the various components suggest a different composition and/or origin of the OM inputs during the different sampling periods. The amount of material reaching the sediments below 540 m is extremely low, suggesting that most of the organic material is decomposed and/or utilised before reaching the sea floor. In conclusion, the continental shelf and bathyal sediments of the Cretan Sea can be considered, from a trophic point of view, as two different subsystems.