Eastern Mediterranean Sapropels Research Articles

Low-frequency orbital variations controlled climatic and environmental cycles, amplitudes, and trends in northeast Africa during the Plio-Pleistocene

The eastern Mediterranean sapropels, paced by insolation, provide a unique archive of African monsoon strength over the Late Neogene. However, the longer-term climate of this region lacks characterization within the context of changes in ice volume, sea surface temperature gradients, and terrestrial ecosystems. Here, we examine C28n-alkanoic acid leaf wax hydrogen and carbon isotopes in sapropels, sourced from northeast Africa, along with vegetation-corrected precipitation isotopes, derived from astronomically dated sediment cores from ODP 160 Sites 966 and 967 since 4.5 million years ago. Despite sampling only wet-phase sapropels for African monsoon variability, we find a larger range in hydrogen isotopes than previously published data across wet-dry precession cycles, indicating the importance of long-term modulation of Green Sahara phases throughout the Neogene. An influence of orbital properties on regional monsoonal hydroclimate is observed, controlling up to 50% of total hydrogen isotope variance, but large changes outside of these typical frequencies account for at least 50% of the total variance. This secular trend may track changes in ice volume, tropical sea surface temperature, sea surface temperature gradients, or even lower-frequency orbital cycles. Long-term hydroclimate and environmental shifts provide new contexts for milestone events in northeast African hominin dispersal and evolution.

An exceptional record of millennial-scale climate variability in the southern Iberian Margin during MIS 6: Impact on the formation of sapropel S6

The study of planktic and benthic δ18O at site U1389 in the Gulf of Cadiz allowed us to reconstruct climate variability during the penultimate glacial period at an unprecedented millennial scale resolution. Hereby, a sequence of interstadial-stadial episodes similar to the Dansgaard-Oeschger events recorded during Marine Isotope Stage (MIS)3, were recognized in MIS6. After a detailed correlation with millennial-scale variability recorded along the Iberian margin and the North Atlantic we were able to link the millennial changes next to the Iberian Peninsula with Antarctic climate variability due to the interhemispheric seesaw response of the Atlantic circulation. The straight coupling at site U1389 of a) the planktic δ18O that reflects climate change in the Atlantic with b) the benthic δ18O, which records the temperature and oxygen isotope composition of the mixture of Mediterranean Outflow and Atlantic intermediate water, indicates that millennial climate variability also had a strong impact on the eastern Mediterranean. The detailed analysis of the benthic δ13C and the fine sand content in the sediments from site U1389 led us to recognize the response of the Mediterranean overflow water (MOW) to millennial scale changes in Mediterranean overturning circulation during the penultimate glaciation. Both Mediterranean Overturning Circulation and MOW strength increased at times of cool and arid climates and weakened during warm and more humid episodes, similar to what has been described for MIS3. The tuning of MOW weakening events occurring at times of precession minima with eastern Mediterranean sapropels and enhanced Asian monsoon allowed the elaboration of a new chronology for MIS6. It was compared with the Epica Dome C time scale, which was also based on the tuning of atmospheric δ18O with Asian Monsoons speleothem records.Mediterranean overturning during MIS6 was also strongly affected by freshwater perturbations occurring at times of insolation maxima, leading to the formation of the “glacial sapropel” S6. Two major drops in seawater δ18O centered at 175 and 150 ky were observed near the entrance of the Mediterranean that were related to meltwater released from the ice sheets, coinciding with periods of insolation maxima. The entry of this meltwater anomaly through the Strait of Gibraltar is clearly registered in the western and eastern Mediterranean, especially at the onset of sapropel S6, suggesting that it was a major component of the freshwater that initiated the buoyancy gain preceding deep water stagnation.

Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels

The elemental concentrations and isotope compositions of molybdenum (Mo) and uranium (U) are commonly used for the reconstruction of past global and local redox conditions, and recent studies using both elements have revealed the potential of their paired application. However, such studies have generally focused either on modern marine sediments or on relatively low-resolution reconstructions of deep-time paleo-redox conditions. Here, we present high-resolution profiles (every 0.2-0.3 kyr) of Mo and U elemental and isotope compositions for anoxic organic-rich sediments of Eastern Mediterranean sapropels S5 and S7. The new Mo-U data reveal the processes leading to descent into basinal euxinia in more precise and systematic detail than lower resolution datasets focused on either Mo or U only.During the intensification from anoxic non-sulfidic to persistently euxinic conditions, δ98Mo and δ238U covariation systematics exhibit two stages. We identify the Mo-U isotope signature of the early transition from anoxic non-sulfidic to mildly euxinic conditions in the first stage, characterized by a rise in U isotope ratios (from −0.3‰ to +0.2‰ ±0.05‰) controlled by the depth within the sediment of the uranium reduction-accumulation front. As the water column turns persistently euxinic, δ98Mo values approach the seawater value for both sapropels, but δ238U evolves to different final values in the two sapropels. We interpret these differences as reflecting different redox potentials and/or different degrees of restriction of these two sapropel events, and the more gradual response of U sequestration to redox variation in comparison with threshold behavior of Mo. The findings presented here show temporal patterns in δ98Mo and δ238U on relatively short timescales that suggest the combined use of these proxies at high resolution allows detailed reconstruction of local redox and hydrographic conditions.

Tracing water column euxinia in Eastern Mediterranean Sapropels S5 and S7

Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between δ56Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative δ56Fe - FeT/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (FeHR) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, FeHR/FeT plots show increased strengthening of anoxic water conditions during the passage from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fepy/FeHR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters. Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fepy/FeHR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ56Fe - FeT/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the EMS basin and comcomitant diagenetic sulphidation. Euxinic water column conditions in sapropel S5 and S7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation.

Marine productivity, water column processes and seafloor anoxia in relation to Nile discharge during sapropels S1 and S3

Eastern Mediterranean sapropels S1 (∼10.5–6.1 kyr BP) and S3 (∼85.8–80.8 kyr BP) formed respectively under full interglacial and glacial inception conditions. Consequently, the environmental factors preconditioning and leading to sapropel formation (e.g., global sea level and monsoonal forcing) were different. These factors must have differently influenced processes such as marine productivity, water column processes, and related seafloor anoxia. Here we investigate these differences through an interdisciplinary approach using dinoflagellate cyst and pollen/spore assemblages and sedimentary (redox-sensitive) trace-metal concentrations from a core in the central Nile delta area. Comparing S1 to S3, we demonstrate that (1) Nile discharge appears to be stronger during S3 than S1, as shown by δ18Oresiduals, higher ratio of pollen and spores, and the higher abundance of coastal dinocysts and freshwater palynomorphs, (2) Ba/Al, Corg, and dinocyst accumulation rates indicate that marine productivity was similar at least during the first phase of their deposition and started prior to the onset of both sapropels, (3) bottom water conditions were more reducing during S3, resulting in higher Mo/Al, S, and Corg/Ptot values, but preservation was high and similar for both sapropels, and (4) Sedimentary Mo-U covariation indicates that the depth of water-column ventilation during deposition of S3 was shallower than during S1 (∼1000 m versus ∼1800m, respectively). We attribute the observed differences to slightly enhanced precessional-forced monsoon intensity and potentially lower global sea level, resulting not only in increased North-African run-off, but also in reduced ventilation during S3 compared to S1.

Origin and implications of orbital-induced sedimentary cyclicity in Pliocene well-logs of the Western Mediterranean

The climatic origin of astronomically induced sedimentary cycles in the Mediterranean and adjacent areas during the late Neogene and Quaternary remains puzzling; as cycles have been linked to concomitant but seasonally opposite changes in African summer monsoon precipitation (Eastern Mediterranean sapropels) and Atlantic regulated winter-precipitation (carbonate cycles on the Atlantic side of the Mediterranean). Particularly, little is known about the cyclic sedimentation on orbital time scales in the Western Mediterranean, with the prime exception of the Messinian sapropels from the Sorbas basin (southern Spain).Here we show that regular alternations in Pliocene downhole logs from the industrial drill-site Muchamiel-1, located along the Balearic Promontory in the Western Mediterranean, are related to eccentricity (bundles) and to obliquity and precession cycles (basic meter-scale alternations). We establish an astronomically based age model for the interval between 5.33 and 2.8 Ma, by first correlating cycle bundles to eccentricity and then the basic dominantly precession-related cycles to the 65°N summer insolation of La2004. The striking bed-to-bed similarities between the Muchamiel-1 well-logs and other records from both the Atlantic margin and the Central Mediterranean suggest that the same climatic forcing was responsible for the formation of carbonate cycles across the Western Mediterranean and adjacent Atlantic. We conclude that formation of alternating carbonate-rich/carbonate-poor beds was controlled by Western Mediterranean cyclogenetic mechanisms as well as by peri-Mediterranean precipitation associated with changes in the North Atlantic System (NAS). These findings highlight the importance of peri-Mediterranean precipitation on the sedimentary cyclicity by dictating terrigenous (clay) supply and potentially on the hydrology of the basin by providing additional freshwater required for sapropel formation. Consequently, cyclic sedimentation in the Mediterranean results from the combined effect of precipitation changes driven by (i) the North African monsoon, (ii) the Atlantic system, and (iii) intrabasinal Mediterranean atmospheric dynamics.

Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan.The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle.

Large-scale response of the Eastern Mediterranean thermohaline circulation to African monsoon intensification during sapropel S1 formation

The formation of Eastern Mediterranean sapropels has periodically occurred during intensification of northern hemisphere monsoon precipitation over North Africa. However, the large-scale response of the Eastern Mediterranean thermohaline circulation during these monsoon-fuelled freshening episodes is poorly constrained. Here, we investigate the formation of the youngest sapropel (S1) along an across-slope transect in the Adriatic Sea. Foraminifera-based oxygen index, redox-sensitive elements and biogeochemical parameters reveal – for the first time – that the Adriatic S1 was synchronous with the deposition of south-eastern Mediterranean S1 beds. Proxies of paleo thermohaline currents indicate that the bottom-hugging North Adriatic Dense Water (NAdDW) suddenly decreased at the sapropel onset simultaneously with the maximum freshening of the Levantine Sea during the African Humid Period. We conclude that the lack of the “salty” Levantine Intermediate Water hampered the preconditioning of the northern Adriatic waters necessary for the NAdDW formation prior to the winter cooling. Consequently, a weak NAdDW limited in turn the Eastern Mediterranean Deep Water (EMDWAdriatic) formation with important consequences for the ventilation of the Ionian basin as well. Our results highlight the importance of the Adriatic for the deep water ventilation and the interdependence among the major eastern Mediterranean water masses whose destabilization exerted first-order control on S1 deposition.

Alternating high and low climate variability: The context of natural selection and speciation in Plio-Pleistocene hominin evolution

Interaction of orbital insolation cycles defines a predictive model of alternating phases of high- and low-climate variability for tropical East Africa over the past 5 million years. This model, which is described in terms of climate variability stages, implies repeated increases in landscape/resource instability and intervening periods of stability in East Africa. It predicts eight prolonged (>192 kyr) eras of intensified habitat instability (high variability stages) in which hominin evolutionary innovations are likely to have occurred, potentially by variability selection. The prediction that repeated shifts toward high climate variability affected paleoenvironments and evolution is tested in three ways. In the first test, deep-sea records of northeast African terrigenous dust flux (Sites 721/722) and eastern Mediterranean sapropels (Site 967A) show increased and decreased variability in concert with predicted shifts in climate variability. These regional measurements of climate dynamics are complemented by stratigraphic observations in five basins with lengthy stratigraphic and paleoenvironmental records: the mid-Pleistocene Olorgesailie Basin, the Plio-Pleistocene Turkana and Olduvai Basins, and the Pliocene Tugen Hills sequence and Hadar Basin—all of which show that highly variable landscapes inhabited by hominin populations were indeed concentrated in predicted stages of prolonged high climate variability. Second, stringent null-model tests demonstrate a significant association of currently known first and last appearance datums (FADs and LADs) of the major hominin lineages, suites of technological behaviors, and dispersal events with the predicted intervals of prolonged high climate variability. Palynological study in the Nihewan Basin, China, provides a third test, which shows the occupation of highly diverse habitats in eastern Asia, consistent with the predicted increase in adaptability in dispersing Oldowan hominins. Integration of fossil, archeological, sedimentary, and paleolandscape evidence illustrates the potential influence of prolonged high variability on the origin and spread of critical adaptations and lineages in the evolution of Homo. The growing body of data concerning environmental dynamics supports the idea that the evolution of adaptability in response to climate and overall ecological instability represents a unifying theme in hominin evolutionary history.

Causal links between Nile floods and eastern Mediterranean sapropel formation during the past 125 kyr confirmed by OSL and radiocarbon dating of Blue and White Nile sediments

It has long been hypothesised that beds of highly organic mud or sapropels seen in marine sediment cores retrieved from the floor of the eastern Mediterranean accumulated during times of high Nile fluvial discharge. Our recent fieldwork in the valleys of the Blue Nile, the White Nile and the main Nile has for the first time revealed a sequence of extreme flood episodes synchronous with sapropel units S5 (124 kyr), S4 (102 kyr), S3 (81 kyr), S2 (55 kyr) and S1 (13.5–6.5 kyr). There are more weakly defined links with Nile floods and sapropel units S9 (240 kyr), S8 (217 kyr), S7 (195 kyr), S6 (172 kyr), but the dating error terms are too large to allow us to be too definite. During times of extreme floods over the past 125 kyr, wide distributary channels of the Blue Nile flowed across the Gezira alluvial fan in central Sudan and transported a bed load of sand and gravel into the lower White Nile valley. The sands were reworked by wind to form source-bordering dunes, all of which contain heavy minerals of Ethiopian provenance. These source-bordering dunes were active at 115–105 kyr, 60 kyr and 12–7 kyr, all times of extreme Blue Nile floods. The flood and dune sediments were dated using a combination of optically stimulated luminescence (OSL) and radiocarbon analyses. The Quaternary record of Nile floods discussed here shows a precessional signal and reflects episodes of stronger summer monsoon and more northerly seasonal movement of the ITCZ, linked to times of higher insolation in northern tropical latitudes. Progressive aggradation of Holocene Nile channels in northern Sudan has had a profound influence upon human settlement in the last 8 kyr.

Late glacial initiation of Holocene eastern Mediterranean sapropel formation.

Recurrent deposition of organic-rich sediment layers (sapropels) in the eastern Mediterranean Sea is caused by complex interactions between climatic and biogeochemical processes. Disentangling these influences is therefore important for Mediterranean palaeo-studies in particular, and for understanding ocean feedback processes in general. Crucially, sapropels are diagnostic of anoxic deep-water phases, which have been attributed to deep-water stagnation, enhanced biological production or both. Here we use an ocean-biogeochemical model to test the effects of commonly proposed climatic and biogeochemical causes for sapropel S1. Our results indicate that deep-water anoxia requires a long prelude of deep-water stagnation, with no particularly strong eutrophication. The model-derived time frame agrees with foraminiferal δ(13)C records that imply cessation of deep-water renewal from at least Heinrich event 1 to the early Holocene. The simulated low particulate organic carbon burial flux agrees with pre-sapropel reconstructions. Our results offer a mechanistic explanation of glacial-interglacial influence on sapropel formation.

Sem Petrography of Eastern Mediterranean Sapropels: Analogue Data For Assessing Organic Matter In Oil and Gas Shales

Abstract Examination of organic-matter-hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter (OM) as seen in high-resolution SEM images. Field-emission SEM imaging of eastern Mediterranean sapropels (Pliocene–Pleistocene age from Ocean Drilling Program drill sites) was performed on samples prepared by Ar-ion cross-section polishing. The immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation (hydrocarbon generation). Kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores. The most abundant organic matter is in the form of microns-thick flaky or stringy material, largely nonporous and internally amorphous, and lacking the well-defined shapes of discrete particles. This dominant OM has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt- and clay-size grains. A portion of the larger mineral-hosted pores remain open, unfilled by ductile OM. Minor nanometer-scale pores of uncertain origin occur in the dominant OM. Silt- and clay-size mineral crystals are distributed in organic matter of most types, and it is difficult to fully ascertain if the OM/mineral mixing has occurred during compaction versus during aggregation during gravity settling (marine-snow sedimentation) or through sediment accumulation in OM-rich microbial mats. Results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial. These observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature OM-rich mudrocks. It is clear that petrographic discrimination of diagenetically generated organic matter (bitumen and related OM types) from ductile dispersed amorphous detrital OM (kerogen) using SEM data will be challenging in the absence of compositional information.

Evidence from molybdenum and iron isotopes and molybdenum–uranium covariation for sulphidic bottom waters during Eastern Mediterranean sapropel S1 formation

Redox exerts a critical control on organic carbon-rich sedimentation. This is particularly true for Eastern Mediterranean sapropels where seawater stratification is regarded as a major driving force for oxygen depletion, but in which sulphidic (euxinic) bottom waters occur only sporadically. Here we apply a powerful array of geochemical proxies (Fe and Mo stable isotopes together with Mo/U ratios and redox sensitive trace elements (RSTE)) to the determination of water redox evolution during the deposition of Holocene S1 sapropel and its underlying and overlying sediments (ODP core 967D; 2550 m depth).RSTE are asymmetrically distributed within the sapropel, with peak enrichments occurring in its lower (early) part. Negative correlations are found between δFe57 and both Fe/Al and S wt% in the lower sapropel, and are consistent with the benthic Fe shuttle model Fe enrichment in euxinic basins. MoEF/UEF enrichment factor variations show well defined trends identical to those proposed for open marine settings, in which sub-oxic conditions in the background sediments give way to sulphidic waters at the RSTE peak in the lower sapropel. The most notable features of the Mo isotope profile are ‘atypically’ light values (δMo98/95<−0.7‰) in the lower sapropel. Such light Mo isotope values (relative to sea water δMo98/95=2.3‰) have been related to oxic remobilisation. However, negative correlations between δMo98/95 and Fe/Al, Ba/Al, Mo/Al and S imply that the lowest Mo isotopic compositions are associated with peak reducing conditions. Taken in conjunction with the evidence from the other proxies for a sulphidic water column, the light Mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters ([H2S]aq <10 μM). The data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the Holocene sapropel S1. Molybdenum isotopes and Ba/Al ratios identify a short-lived sapropel re-ventilation event timed to coincide with the 8.2 ka cold climatic Event.

New constraints on climate forcing and variability in the circum-Mediterranean region from magnetic and geochemical observations of sapropels S1, S5 and S6

Eastern Mediterranean sapropels are organic-rich layers whose formation was triggered during boreal summer insolation maxima (precession minima). Sapropel formation is associated with increased precipitation and runoff from Europe and North Africa, although the relative influence of high- and low-latitude climate mechanisms is not fully resolved. Likewise, it remains uncertain how centennial-scale interruptions in conditions that give rise to sapropels may reflect atmospheric circulation changes in low latitudes. We present magnetic, geochemical, and diffuse reflectance spectroscopy data from three sapropels that are representative of glacial (S6) and interglacial (S1, S5) conditions in the eastern Mediterranean Sea to assess environmental changes associated with sapropel formation. The extent of diagenetic magnetite dissolution and authigenic formation of fine-grained greigite in these sapropels and their underlying dissolution intervals are linked to enhanced sulphidic conditions during sapropel formation. Aeolian hematite and goethite are largely unaffected by this reductive dissolution except within the interglacial sapropel S5 and its underlying dissolution interval, which formed under relatively stronger sulphidic conditions. Nevertheless, low hematite contents indicate that the three studied sapropels accumulated under reduced aeolian dust inputs in response to an intensified African monsoon, which resulted in expansion of savannah landscapes into NE Saharan dust source areas. Small variations in goethite contents across sapropels indicate additional aeolian entrainment of goethite that formed under previous wet phases in the NE Sahara or in subtropical savannahs located further south. We link short-lived dust abundance peaks within sapropels S1 and S6 to centennial-scale periods of enhanced bottom-water circulation reported previously for these sapropels. Although these sapropel interruptions are driven by high-latitude cooling events, our results indicate that such centennial-scale episodes of atmospheric reorganization affected not only the eastern Mediterranean northern borderlands, but also subtropical North Africa. Overall, our results point to a dominant low-latitude forcing on sapropel formation via boreal summer insolation maxima and intensification of the African monsoon.

Calcareous Nannofossil and Planktonic Foraminifera Biostratigraphy of selected Piacenzian-Gelasian Laminites from Southern Italy

Here we present the biostratigraphic and chronostratigraphic framework of a number of selected diatomaceous laminated intervals from the Crotone Basin (Calabria, Southern Italy). These layers, which we consider correlative to the Eastern Mediterranean Sapropel Layers, range in age from the early Piacenzian to the Gelasian, and show surprising thicknesses, suggesting that they were probably laid down in a landlocked, overfed basin. Specifically, a thick laminite from the surroundings of Cropani (Catanzaro) can be ascribed to nannofossil biozone MNN 16a and planktonic foraminifera biozone MPL 4b (lower Piacenzian) according to the Mediterranean calcareous plankton biostratigraphic zonations. Two laminites from the “Muto” Section (Botricello-Cutro road, Crotone), belong to MPL 5a and MNN 16b/17 Zones (upper Piacenzian). A very thick laminite from the “Timpone dell’Inferno” badland area (near Termine Grosso, Catanzaro), belong to MPL 5b and MNN 18 Zones (middle-upper Gelasian). These biostratigraphic constraints provide a first contribution to a comprehensive paleoenvironmental reconstruction.

Conflicting coccolithophore and geochemical evidence for productivity levels in the Eastern Mediterranean sapropel S1

Abstract The cyclic development of anoxic conditions in the eastern Mediterranean deep sea waters is one of the most fascinating research topics in paleoceanographic studies. In combination with bottom water stagnation, enhanced primary production is a common explanation for the deposition of organic-rich layers (sapropels). This is supported by extensive evidence from both geochemical and micropaleontological studies. The correspondence of recent sapropel layers with peaks of the lower photic zone coccolithophore species Florisphaera profunda has been interpreted as a proxy for the development of a deep chlorophyll maximum (DCM), due to the pycnocline/nutricline shallowing into the lower part of the photic zone. We present millennial-scale data for coccolithophore assemblages from sediments across the most recent sapropel (S1), in the ODP Hole 964B drilled in the Ionian Sea. Relative and absolute abundances of taxa are compared with selected elemental composition of the bulk sediments. The Mn/Al and Ba/Al profiles are used to determine the original thickness of the S1 interval, and show that the upper part of S1 was affected by post-depositional oxidation of organic matter. The Nannofossil Accumulation Rate, defined by the number of coccoliths·cm− 2·kyr− 1, suggests that there is no evidence of increased productivity within most of the sapropel layer. In fact, coccolithophore production was at its minimum in the lower part. Minimum coccolith concentrations are reached despite the increase in F. profunda in both relative and absolute abundance. We suggest that the DCM deduced from the increased productivity of this species did not significantly contribute to the putative overall increased primary productivity during the deposition of most of the sapropel layer. Within the upper oxidized part of S1, coccolith accumulation was at least five times higher than in the lower part. This period of high coccolith productivity finds a counterpart in the increase of the Ba/Al ratio. The total concentration of coccoliths is again controlled by the amount of E. huxleyi, but it is also supported by concomitant increases in all the other groups, suggesting that coccolithophore productivity increased throughout the year and through the total vertical extent of the photic zone. At site 964, this is apparently the only moment when coccolithophores contributed substantially to the increased primary productivity generally assumed for the S1 layer.

Exploring preserved fossil dinoflagellate and haptophyte DNA signatures to infer ecological and environmental changes during deposition of sapropel S1 in the eastern Mediterranean

In this study we used a comparative multiproxy survey (fossil DNA, calcareous nannofossils, and lipid biomarkers) to test whether preserved genetic signatures provide an accurate view of haptophyte and dinoflagellate populations during deposition of the eastern Mediterranean sapropel S1 and the organic carbon‐depleted oxidized marls flanking the S1 and to see if we could identify important environmental indicator species that did not fossilize and escaped previous microscopic identification. The marls above and below the S1 contained low concentrations of lipid biomarkers diagnostic for dinoflagellates and haptophytes (i.e., dinosterol and long‐chain alkenones), but 500 base pair long ribosomal DNA fragments of these protists were below the detection limit. In contrast, dinoflagellate and haptophyte DNA could be recovered from the organic carbon‐rich S1, but the most abundant sequences did not represent species that were part of the nannofossil (this study) or previously described dinocyst composition. The oldest section of S1 (9.8 to ∼8 14C kyr B.P.) revealed a predominance of dinoflagellate phylotypes, which were previously only detected in anoxic Black Sea sediments. In the same section of the core, the most abundant haptophyte sequence showed highest similarity with uncultivated haptophytes that were previously shown to grow mixotrophically as predators of picocyanobacteria, an adaptation that promotes growth in oligotrophic marine waters. Sequences with highest similarities to clones found in marine surface waters predominated in the S1 after ∼8 14C kyr B.P. We discuss whether the shifts in haptophyte and dinoflagellate populations inferred from the preserved DNA reflect known environmental changes that occurred during the formation of sapropel S1.

Short-time-scale variability in ventilation and export productivity during the formation of Mediterranean sapropel S1

[1] High-resolution laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) scanning of laminated sediments from the Urania basin is used to investigate short-time-scale variability in export productivity and redox conditions during the formation of eastern Mediterranean sapropel S1. Sedimentary enrichments of molybdenum (Mo), vanadium (V), and uranium (U) reflect deep-water redox conditions, most likely those near to the seawater-brine interface, while enrichment of Ba is related to biogenic barite and hence to export productivity. The enrichments of all four elements show strong variability on multidecadal to multicentennial time scales throughout S1. A partial decoupling of export productivity from redox conditions at the height of sapropel formation suggests that hydrographic changes, i.e., a variable ventilation rate of the eastern Mediterranean, played an important role in determining deep-water redox conditions. A pronounced switch is observed in the enrichments of redox-sensitive trace metals, from dominantly 300–600 year variability during early S1 to dominantly 100–300 year variability during late S1, indicating a change in the mean frequency of variability in the ventilation rate. The presence of a similar shift in the frequency of tropical and extratropical climate records at this time suggests that ventilation of the eastern Mediterranean was coupled to global climate variability.

Diagenetic control of nitrogen isotope ratios in Holocene sapropels and recent sediments from the Eastern Mediterranean Sea

Abstract. The enhanced accumulation of organic matter in Eastern Mediterranean sapropels and their unusually low δ15N values have been attributed to either enhanced nutrient availability which led to elevated primary production and carbon sequestration or to enhanced organic matter preservation under anoxic conditions. In order to evaluate these two hypothesis we have determined Ba/Al ratios, amino acid composition, N and organic C concentrations and δ15N in sinking particles, surface sediments, eight spatially distributed core records of the youngest sapropel S1 (10–6 ka) and older sapropels (S5, S6) from two locations. These data suggest that (i) temporal and spatial variations in δ15N of sedimentary N are driven by different degrees of diagenesis at different sites rather than by changes in N-sources or primary productivity and (ii) present day TOC export production would suffice to create a sapropel like S1 under conditions of deep-water anoxia. This implies that both enhanced TOC accumulation and δ15N depletion in sapropels were due to the absence of oxygen in deep waters. Thus preservation plays a major role for the accumulation of organic-rich sediments casting doubt on the need of enhanced primary production for sapropel formation.

Calcareous nannofossil and planktonic foraminiferal distributional patterns during deposition of sapropels S6, S5 and S1 in the Libyan Sea (Eastern Mediterranean)

In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.