Sapropel Deposition Research Articles

Impact of freshwater release in the Mediterranean Sea on the North Atlantic climate

Sediment cores from the Mediterranean Sea have evidenced several periods of Sapropel deposition, which can be explained by events of anoxic bottom conditions. An explanation for such events calls for a very stratified sea, possibly related with freshwater input through increased precipitations and runoff discharges. It has been suggested that such a stratified Mediterranean Sea may in turn weaken the Atlantic meridional overturning circulation (AMOC) through changes in the Mediterranean overflow water (MOW). Nevertheless, models used to establish this result were relatively simple and the mechanisms leading to such an impact remained elusive. To improve on those previous studies, we analyse the impact of different freshwater releases with rates of 0.2, 0.1, 0.05 and 0.02 Sv (1 Sv = 106 m3/s) in the Mediterranean Sea using the IPSL-CM5A-LR model in a few multi-centennial simulations. We focus the analysis on the impact of a decrease in the MOW on the large-scale Atlantic circulation. We find a consistent change in horizontal currents in the upper Atlantic Ocean in all simulations in the first century, with a large enhancement of the northward current west of Rockall in the northeast Atlantic. Concerning the AMOC response, we identify three different processes that impact its fate. The first is related to changes in geostrophic currents at depth induced by the disappearance of the MOW, which tends to weaken the AMOC. On the contrary, the second enhances the AMOC and is associated with the increase in northward currents in the horizontal upper circulation. The last process is due to the spread of surface freshwater anomalies out of the Mediterranean Sea that freshens the North Atlantic convection sites and weakens the AMOC. Depending on the rate of the freshwater release, the strength and balance of these three processes are different. For rates larger than 0.05 Sv, we observe a strong reduction of the AMOC, while for lower rates, we notice an enhancement in the upper cell. The climatic response follows that of the upper AMOC with a warming of the North Atlantic for rates lower than 0.05 Sv and a cooling for higher rates. Given that past estimates of freshwater release in the Mediterranean Sea indicate rates lower than 0.05 Sv, we argue that Sapropel events may have enhanced of the upper AMOC and warmed of the North Atlantic.

An onshore bathyal record of tectonics and climate cycles at the onset of the Early-Middle Pleistocene Transition in the eastern Mediterranean

Early Pleistocene (Calabrian) clays of the Lindos Bay Formation have been uplifted and are exposed today on the eastern coast of Rhodes (Hellenic forearc, Greece). The hemipelagic origin of these sediments and the excellent preservation of the microfossils they contain, make the Lindos Bay Formation a unique element in the eastern Mediterranean, which has the potential to constitute a paleoclimate referential framework of the region for the Early-Middle Pleistocene Transition (EMPT). In this context, 98 samples were collected in the type locality section of the Lindos Bay Formation. Stable isotope (δ18O, δ13C) analyses were carried out on benthic (Uvigerina peregrina) and planktonic (Globigerina bulloides) foraminifera for the interval between ∼1100 ka and ∼858 ka. We identify and continuously reconstruct the climate cycles that occurred between Marine Isotope Stages (MIS) 32 and 21. Counts of planktonic foraminifera were further carried out and allow connecting the conditions occurring locally in the surface waters with the glacial-interglacial cycles during the onset of the EMPT. While (sub)tropical taxa dominate the assemblages, glacials were associated with short-term (<∼10 kyr) influxes of temperate to subpolar species suggesting cooling and/or increasing productivity in the surface waters correlating with the deposition of sapropels in the eastern Mediterranean deep sea. Finally, planktonic/benthic foraminiferal ratios show that the section deposited at upper bathyal depths between ∼450 m and ∼150 m. During the late Early Pleistocene, the eastern coast of Rhodes has locally undergone repeated, transient and possibly very rapid vertical motions which are overprinted on a regional tectonically-forced regressive trend.

Simulating the Occurrence of the Last Sapropel Event (S1): Mediterranean Basin Ocean Dynamics Simulations Using Nd Isotopic Composition Modeling

AbstractSapropels are sediments rich in black, pelagic organic matter which occur mainly in the Eastern Mediterranean, documenting anoxic environments and high biological productivity. The quasiperiodicity of deposition of sapropels ‐over millions of years‐ relates to the Earth's precession cycle, which directly enhances the African monsoon, ultimately increasing freshwater input from the Nile. The last sapropel event, S1, occurred about 10,000 years ago (Early Holocene), when the Mediterranean region was warmer and wetter than today. Several modeling studies reflect the impact of this climate and a stronger Nile influx on Mediterranean oceanic circulation, but the regional models used lacked the spatial resolution necessary to simulate winter intermediate and deep convection. Here, we investigate recently occurring changes in the convective areas in the Eastern Mediterranean, using a regional ocean–atmosphere coupled climate model of high spatial resolution, essential to the simulation of a realistic Mediterranean circulation for present‐day conditions. We focused on the thermohaline circulation and the simulation of neodymium isotopic composition to compare our modeling results to modern data and paleo‐proxies. A sensitivity experiment shows a radical response of the Mediterranean to enhanced Nile discharge, creating the appropriate conditions for sapropel formation. We thus demonstrate that increased discharge of the Nile River can trigger the shutdown of Eastern Mediterranean convection and create conditions favorable to the development of anoxic events.

Organic matter isotopes reveal enhanced rainfall activity in Northwestern Mediterranean borderland during warm substages of the last 200 kyr

In this study we use bulk sediment organic matter stable isotopes (δ13Corg and δ15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (δ13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ∼3‰ values in δ15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represent 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel deposition.

Calcareous nannofossil palaeoenvironmental reconstruction and preservation in sapropel S1 at the Eratosthenes Seamount (Eastern Mediterranean)

The most recent organic carbon-enriched layer (sapropel S1) deposited at the Eratosthenes Seamount has unique features, such as an early lithological interruption, fine light silt laminae and an exceptional vertical extent that is over 25 cm thick. Here we investigate calcareous nannofossil assemblages to reconstruct very high-resolution palaeoenvironmental and palaeoceanographic variations recorded before, during and after the perturbation episode that involved the eastern Mediterranean Sea, due to the massive freshwater discharge via Nile River. Our results show that the deep chlorophyll maximum development, observed in all micropalaeontological groups from previous studies, is a gradual process that started well before the base of sapropel S1. A high-frequency variability in the nutricline depth is evident at millennial- and/or centennial-scale throughout the sapropel deposition time interval. Also we highlight the poor-preservation of delicate tiny holococcolith crystals while anoxia was occurring at the seafloor and we suggest that such a phenomenon may be used to mark the original thickness of sapropel deposition where oxygen re-ventilation fronts were developed. Finally, calcareous nannofossil reworking peaks shed light on the nature of fine silt laminae within the sapropel S1 at the Eratosthenes Seamount, which may be ascribed to fine sediment plumes from the Nile River deposited during exceptional runoff events.

A 10-fold decline in the deep Eastern Mediterranean thermohaline overturning circulation during the last interglacial period

Present-day Mediterranean deep-waters are well oxygenated, but the episodic formation of organic-rich sediments (sapropels) indicates that this pattern was frequently perturbed in the past. Both high export productivity and disruption of the thermohaline circulation, leading to reduced deep-water ventilation, have been proposed to account for sapropel deposition and anoxia. The last interglacial sapropel S5 is considered one of the most strongly developed. Here, we apply the redox-sensitive Mo and U (elemental and isotope) systems to quantify the intensity of anoxic deep-water conditions in the Eastern Mediterranean Sea from ODP core 967 (2550 mbsl). Both U and Mo show strong authigenic enrichment, coupled to progressive increase in δ98Moauth (+1.2–1.8‰ to +2.0–2.3‰) and decrease in δ238Uauth (+0.10‰ to −0.15‰) from the beginning to the end of S5, suggesting increasing water column euxinia and removal fluxes of Mo and U. Based on modern euxinic basins, we show that sedimentary δ238Uauth can be used to derive estimates of water column U depletion and, ultimately, deep-water renewal rates. These principles are first tested on the modern Black Sea, which yields calculated deep-water renewal times of 830+690/−500 yrs, in good agreement with independent estimates. Applying these principles to the end of S5 suggests bottom-water U depletion of ∼50% and deep-water renewal times of 1030+820/−520 yrs. The significantly slower deep-water renewal rates in the Eastern Mediterranean Sea compared to today (∼100 yrs) would have played an important role in the formation of sapropel S5 and are consistent with the proposed suppression of overturning during the last interglacial, due to increased stratification resulting from higher riverine freshwater input under enhanced monsoon forcing.

Contribution of factor analysis to the study of paleoclimatic and paleoceanographic variability in SW Aegean sea, Greece

Factor analysis was subjected to high resolution microfauna data. The microfauna data (planktonic and benthic foraminifera) comes from analyses of two gravity cores selected from SW Aegean Sea and concerns the last 45000yrs. The two cores contain the sapropelic sequence of SI which appears in two layers Sia and Sib and the sapropelitic sequence of S2. The results of the analyses have been high accurate dated by the AMS method. The application of factor analysis to planktonic forams revealed 5 factors. Two of these factors cluster planktonic species which are indicators of temperature changes such as Gs. ruber, Gs. sacculifer, Or. universa and Globigerinella which are indicative of warm and oligotrophic waters and T. quinqueloba and Gr. scitula which are indicative of cold waters. So these factors can be considered as sea surface temperature depended. The variation of their scores compared with the δ1 80 data showed a sequence of palaeoclimatic changes of long and short duration, which may correspond to global events such as Heinrich 1, 2, 3, 4 and Younger Dryas. The other three factors cluster planktonic species which prefer high eutrophicated waters, such as Neogloboquadrina, G inflate and G bulloides. The variation of their scores revealed the periods where the marine productivity was improved by (i) the shoaling of the pycnocline within the euphotic zone and the subsequent development of Deep Chlorophyl Maximum (DCM) layer, (ii) the increase of water outflows and (iii) the well mixed surface waters. Furthermore, factor analysis showed that the organic material of SI is related more to the enhanced river outflows, than that of S2 which seems to be related with the development of DCM layer. The application of the factor analysis to benthonic forams revealed 5 factors, which appear to correspond to five stages of bottom water enrichment. The 1st factor cluster epifauna species with high oxygen demands such as H. elegans, Asterigerinata, Valvulineria, Rosalina, Miliolidae, Nonion spp. and Cassidulina spp. The variation of the scores showed that the last 45kyrs SW Aegean Sea was characterized by well oxidized bottom waters except of the periods of sapropel deposition. When the trophic level of the sea bottom increases such as occurs at the deposition of S2, the dominated benthic fauna is represented by the 2nd factor with species as Cibicides spp., Uvigerina spp. and Cassidulina spp. Species such as Gyroidina spp, Uvigerina spp., Bulimina spp. and Chilostomela spp. are represented by the 4th factor, which indicates environments of reduced oxygen supply and/or increased organic flows, such those which developed during the interruption of the sapropelic deposition of SI. Shallow infauna species such as Bolivina spp. and B. marginata and deep infauna species such as Fursenkoina spp. are represented by the 3th factor, which indicates dysoxic environments such those which developed exactly before the deposition of Sia and Sib. Deep infauna species such as Cassidulinoides spp and Globobulimina spp. are represented by the 5th factor, which shows highly dysoxic, almost anoxic conditions, such those which developed at the begging of Sia and Sib.

Planktonic foraminiferal abnormalities in coastal and open marine eastern Mediterranean environments: A natural stress monitoring approach in recent and early Holocene marine systems

Abstract Marine environmental status can be assessed through the study of bio-indicator species. Here, we monitor natural environmental stress by the occurrence of morphologically abnormal planktonic foraminiferal specimens from a suite of surface sediments in the eastern Mediterranean Sea. We also compare Scanning Electron Microscopy (SEM) abnormality observations from sapropel S1-derived sediments in the Aegean, Libyan and Levantine basins, since they provide a direct record of a natural stress experiment that took place over past time scales. At initial sapropel deposition levels, we observe increased growth asymmetry in Globigerinoides ruber twinned and twisted individuals, possibly associated with eutrophication and anoxia. In modern material, a range of malformations and aberrant morphologies from slight deformity with smaller or overdeveloped chambers to more severe deformity with abnormally protruding or misplaced chambers, distorted spirals, and double tests is also observed, as a result of the hypersaline, oligotrophic and oxygen-depleted nature of the Mediterranean Sea water column. Overall, we highlight the current use of the relative abundance of abnormal tests as a bio-indicator for monitoring natural stress, especially the occurrence of twin specimens as indicative of high-salinity stress conditions, and further illustrate the necessity to map both their spatial and temporal distribution for accurate paleoenvironmental reconstructions. Such an approach presents the advantage to rapidly provide information over wide spatial and temporal scales, extending our ability to monitor a wide variety of environments (from coastal to the open-sea). However, further investigations should extend this approach to test the robustness of our findings in a number of similar oceanic settings.

PRELIMINARY STUDIES ON SEDIMENT CORE FROM THE TYRO BASIN

The present paper summarizes the preliminary results of the mineralogical and micropaleontological analysis conducted on sediment samples from core TYR05 retrieved from the anoxic and hypersaline Tyro basin in the eastern Mediterranean Sea. The core comprises a complex lithostratigraphic sequence attributed to the strong geodynamic regime of the area. The planktonic foraminifera associations present fluctuations which coincide with changes in the lithology of the core. The mineralogical composition of the sediments shows influence from the evaporites developed on the bottom of the basin. The mineral constituents in association to the microfauna assemblages suggest that the sediments include sapropelic layers. Further analyses are needed in order to determine safely the sapropelic deposits.

Late Quaternary Nile flows as recorded in the Levantine Basin: The palynological evidence

This study aims to trace changes in the River Nile flows over the Late Quaternary and is based on palynomorphs which were embedded in the sea floor of the Levantine Basin. The palynomorphs were extracted from two marine sediment cores, which cover the last 86 ka and are located at the two ends of the Levantine Basin: MD-9509, at the southern part of the Levantine Basin, and MD-9501, at its northern part. Core MD-9509 was taken from the Nile cone and is characterized by high sedimentation rates and a good state of palynomorph preservation. The assemblages included palynomorphs which were transported via the Nile headwaters and therefore enabled the reconstruction of the River Nile flows. The results demonstrate that the last glacial period (∼75–16 ka) was characterized by a decrease in Nile water discharge and an increase in sediment influx, while opposite trends were observed prior to the last glacial period as well as during the deglaciation and the Holocene. Based on the study of the spores, it is suggested that during the last glacial, the main contributors of freshwater and sediment load to the Eastern Mediterranean Sea were the Blue Nile and the Atbara and only during interglacials was there a more significant contribution of the White Nile. Within the northern core, MD-9501, pollen was preserved only during the formation of sapropels S3 and S1. The comparison of the sapropelic palynological spectra in both core sites clearly indicates that during sapropel deposition, climate conditions were more humid in the Northern Levant, reflecting the north-south regional Mediterranean climatic moisture gradient. Sapropel formation was a result of the intensification of the monsoonal climate system which was most probably related to the maximum insolation values at 65°N, while, currently, the Atlantic is the main influencing climate system in the region. One of the most interesting observations in this study is that during Heinrich Events H2-H6, which originated in the north Atlantic and were identified in MD-9509 based on minimum arboreal pollen percentages, pollen originating from tropical regions was not embedded in the Levantine Basin. These results lend support to the view that episodes of dryness in tropical/sub-tropical Eastern Africa were associated not only with low-latitude climate controls, but also with high-latitude glacial stress.

Diagenetic transformation of organic matter in the Holocene sediments of the Black Sea: pyrolysis data

The article presents the results of pyrolysis (Rock-Eval) and pyrolysis-gas chromatography-mass spectrometry analysis of organic matter (OM) in the Holocene sediments of the Black Sea. The studied samples represent two parts of the sedimentary column: coccolith ooze with 2.8% Corg at the top and sapropels oozes with 8.5% Corg at the bottom. The deposition of Holocene sediments was inferred to take place under euxinic conditions, which are more favorable for organic-matter preservation and sapropel deposition during the Old Black Sea stage. The results show that the modern processes of microbially mediated methane oxidation in the Old Black Sea oozes are interpreted as the main cause of their apparently lower degree of diagenetic transformation as compared to that of coccolith oozes.

A simple method to model the reduced environment of lake bottom sapropel formation

The Kambala and Barchin brackish lakes (Baraba steppe, southern West Siberia) contain an organic-rich sapropel layer that was formed in oxygen-depleted waters. We measured the bulk sediment elemental composition, the water chemistry and determined the mineralogical composition and predominant biota species (Diatoms and Cyanobacteria in phytoplankton community respectively) in the lakes. The result indicates that the first lake has a siliceous type of sapropel and the second a carbonaceous one. A computer thermodynamic model was developed for chemical interaction in water-bottom sediment systems of the Kambala and Barchin Lakes. The surface sodium bicarbonate waters are supersaturated with respect to calcite, magnesite (or low Mg-calcite), quartz and chlorite with minor strontianite, apatite and goethite (pH 8.9–9.3, Eh 0.3 V). Nevertheless, it is shown that during sapropel deposition, deep silt waters should be anoxic (Eh<0 V). The virtual component CH2O has been used to create an anoxic environment suitable for pyrite formation due to the biotic community impact and abiotic reduction. Thermodynamic calculation has shown that silt water is not necessarily euxinic (anoxic and sulfidic). Depending on Eh, sulfate sulfur can dominate in solution, causing the formation of gypsum together with pyrite. An attempt was made to find a reason for solution supersaturation with respect to Ca and Mg ions due to their complexation with humic acids.

Deep water circulation within the eastern Mediterranean Sea over the last 95 kyr: New insights from stable isotopes and benthic foraminiferal assemblages

The response of eastern Mediterranean Sea (EMS) circulation to climate forcings over the last 95kyr BP was studied using core MD04-2722, collected at 1780m water depth within the Levantine Sea. Foraminiferal stable isotopes and benthic assemblages were combined in order to reconstruct deep water ventilation and oxygenation in relation to surface water freshening. Over the last deglaciation, benthic foraminiferal δ13C values and oxygen index decreased, while the δ18O gradient between benthic and planktonic foraminifera increased. The results, respectively, indicate slower ventilation, bottom water oxygen depletion, and stronger stratification prior to S1 sapropel deposition. A combination of deglacial sea level rise and fresher north Atlantic surface water contributions were determined to be a precondition for S1 formation within the Levantine Sea. Local Nile River freshwater supplied during the African Humid Period further strengthened water column stratification. As for S3 deposit, the central role of monsoonal precipitation was estimated. For the last glacial period, three events at around 53, 46 and 37ka BP were marked by a deep water circulation reduction at the core location. Considering the influence of north Atlantic surface water salinity on Mediterranean Sea circulation, we propose that the 46 and 37ka BP events are responses to Heinrich Events 4 and 5 that supplied fresher surface water to the Mediterranean Sea. Since the ‘53ka event’ was also characterized by the appearance of low oxygen benthic indicators as observed within the S1 and S3 layers, we tentatively attributed it to ‘missing’ sapropel S2. Our results indicate that intense stagnation within the EMS could occur when both the local freshwater supply and fresher north Atlantic surface water are contributors. The influence of north Atlantic conditions was significant to EMS circulation under warm and cold climate conditions.

Eastern-Mediterranean ventilation variability during sapropel S1 formation, evaluated at two sites influenced by deep-water formation from Adriatic and Aegean Seas

Present-day bottom-water ventilation in the Eastern Mediterranean basin occurs through deep-water convection originating from the two marginal basins, i.e. Adriatic and Aegean Seas. In the paleo record, long periods of enhanced deep-water formation have been alternating with shorter periods of reduced deep-water formation. The latter is related mainly to low-latitude humid climate conditions and the enhanced deposition and preservation of organic-rich sediment units (sapropels). This study focuses on sedimentary archives of the most-recent sapropel S1, retrieved from two sites under the direct influence of the two deep-water formation areas. Restricted oxygen conditions have developed rapidly at the beginning of S1 deposition in the Adriatic site, but bottom-water conditions have not persistently remained anoxic during the full interval of sapropel deposition. In fact, the variability in intensity and persistence of sedimentary redox conditions at the two deep-water formation sites is shown to be related to brief episodes of climate cooling. In the Adriatic site, sapropel deposition appears to have been interrupted twice. The 8.2 ka event, only recovered at the Adria site, is characterized by gradually increasing suboxic to possibly intermittently oxic conditions and decreasing Corg fluxes, followed by an abrupt re-establishment of anoxic conditions. Another important event that disrupted sapropel S1 formation, has taken place at ca. 7.4 cal ka BP. The latter event has been recovered at both sites. In the Adriatic site it is followed by a period of sedimentary conditions that gradually change from suboxic to more permanently oxic, as deduced from the Mn/Al pattern. Using the same proxy for suboxic/oxic sedimentary redox conditions, we observe that conditions in the Aegean Sea site shift to more permanently oxic from the 7.4 ka event onwards. However, at both sites the accumulation and preservation of enhanced amounts of organic matter have continued under these suboxic to intermittently oxic sedimentary conditions. It seems thus, that after 7.4 cal ka BP sapropel-like surface or deep-chlorophyll-maximum conditions including enhanced productivity continued, whereas bottom-water conditions were at least intermittently oxic. The latter is related to decreasing precipitation, i.e. run-off, and thus a progressive development and deepening of deep-water formation. The shallower Aegean site, would be affected earlier by such deepening ventilation than the slightly deeper Adriatic site. Finally, termination of sapropel S1 formation as deduced from diminished organic matter contents and Ba/Al, appears to have occurred almost simultaneously in the two areas, namely at 6.6 ± 0.3 and 6.3 ± 0.5 cal ka BP in Adriatic and Aegean sites, respectively.

Hydrochemical characteristic of sapropels in Novosibirsk oblast

Sapropels of lake systems in Novosibirsk oblast were studied to develop a scientific basis for their rational use. Sapropels from lakes in Novosibirsk oblast have been classified based on the ash content, chemical composition, and genesis. Organic–mineral and mineral–organic calcium and mixed sapropels of macrophyte and macrophyte–plankton genesis have been shown to be predominant by far among sapropel deposits in lakes in Novosibirsk oblast.

The timing of Mediterranean sapropel deposition relative to insolation, sea-level and African monsoon changes

The Mediterranean basin is sensitive to global sea-level changes and African monsoon variability on orbital timescales. Both of these processes are thought to be important to the deposition of organic-rich sediment layers or ‘sapropels’ throughout the eastern Mediterranean, yet their relative influences remain ambiguous. A related issue is that an assumed 3-kyr lag between boreal insolation maxima and sapropel mid-points remains to be tested. Here we present new geochemical and ice-volume-corrected planktonic foraminiferal stable isotope records for sapropels S1 (Holocene), S3, S4, and S5 (Marine Isotope Stage 5) in core LC21 from the southern Aegean Sea. The records have a radiometrically constrained chronology that has already been synchronised with the Red Sea relative sea-level record, and this allows detailed examination of the timing of sapropel deposition relative to insolation, sea-level, and African monsoon changes. We find that sapropel onset was near-synchronous with monsoon run-off into the eastern Mediterranean, but that insolation–sapropel/monsoon phasings were not systematic through the last glacial cycle. These latter phasings instead appear to relate to sea-level changes. We propose that persistent meltwater discharges into the North Atlantic (e.g., at glacial terminations) modified the timing of sapropel deposition by delaying the timing of peak African monsoon run-off. These observations may reconcile apparent model–data offsets with respect to the orbital pacing of the African monsoon. Our observations also imply that the previous assumption of a systematic 3-kyr lag between insolation maxima and sapropel midpoints may lead to overestimated insolation–sapropel phasings. Finally, we surmise that both sea-level rise and monsoon run-off contributed to surface-water buoyancy changes at times of sapropel deposition, and their relative influences differed per sapropel case, depending on their magnitudes. Sea-level rise was clearly important for sapropel S1, whereas monsoon forcing was more important for sapropels S3, S4, and S5.

Geochemistry of Aegean Sea sediments: implications for surface- and bottom-water conditions during sapropel deposition since MIS 5

Piston cores collected from the Aegean Sea provide a record of sapropel sequence S1, S3-S5. Primary productivity calculations using the equations of Muller and Suess suggest surface paleoproductivities ranged from 180 to 995 g C m-2 year-1 for sapropels and from 40 to 180 g C m-2 year-1 for nonsapropel sediments with corresponding total organic carbon values of 9%-12% and 1%-3%, respectively. The higher paleoproductivities exceed those in the most fertile modern upwelling zones, so are probably overestimated. Instead, enhanced preservation, particularly for S4 and S5, likely resulted from poor bottom-water ventilation beneath a salinity-stratified water column. If the preservation factor in the equations of Howell and Thunell is increased to account for such conditions, more realistic paleoproductivity estimates ensue. The interpreted presence of a deep chlorophyll maximum layer for S3-S5 within the lower part of the photic zone may account for high marine organic carbon and increased export production. A deep chlorophyll maximum layer is not advocated for S1 because of the presence of N. Pachyderma (d) immediately below S1. The organic geochemical data show that both marine and terrestrial organic matter contributed equally to sapropels S3, S4, and S5. Sapropels S3-S5 were deposited under normal marine conditions with very limited and temporary establishment of near-euxinic bottom-water conditions. Highly depleted and somewhat uniform Δ34S values together with the absence of fully euxinic conditions during sapropel intervals suggest that bacterially mediated sulfate reduction took place consistently below the sediment-water interface. It is believed that climbing levels of primary productivity triggered the onset of sapropel deposition, but that other contemporaneous factors extended and enhanced the conditions necessary for sapropel deposition, including increased nutrient supply from riverine inflow, water column stratification and reduced oxygenation of bottom waters, and buffering of low bottom-water oxygen levels by accumulating terrestrial organic carbon.

Late Quaternary chronostratigraphy of the Aegean Sea sediments: special reference to the ages of sapropels S1–S5

Four sapropel layers (S1, S3, S4, and S5) are identified in five 6-10-m-long piston cores collected from the Aegean Sea basins. A chronostratigraphic framework is established for the last ~130 ka using benthic and planktonic foraminiferal oxygen isotope curves, total organic carbon contents, volcanic ash layers, and limited radiocarbon dates. These data show that the onsets of sapropels S3, S4, and S5 in the Aegean Sea basins were not synchronous, highlighting the heterogeneity of the Aegean Sea basins in terms of rapid versus lagged responses to changing ocean-climate boundary conditions. In all cases, however, the development of sapropels S3, S4, and S5 in the Aegean Sea predate their counterparts in the eastern Mediterranean by several hundred to several thousand years. The onsets of sapropel deposition were abrupt, but sapropel terminations were more gradual, controlled both by the amplitude of paleoclimatic changes and the physiography/location of the basins.

The Uniqueness of Planktonic Ecosystems in the Mediterranean Sea: The Response to Orbital- and Suborbital-Climatic Forcing over the Last 130,000 Years

AbstractThe Mediterranean Sea is an ideal location to test the response of organisms to hydrological transformations driven by climate change. Here we review studies carried out on planktonic foraminifera and coccolithophores during the late Quaternary and attempt the comparison of data scattered in time and space. We highlight the prompt response of surface water ecosystems to both orbital- and suborbital-climatic variations.A markedly different spatial response was observed in calcareous plankton assemblages, possibly due to the influence of the North Atlantic climatic system in the western, central and northern areas and of the monsoon system in the easternmost and southern sites. Orbital-induced climatic dynamics led to productive surface waters in the northern, western and central Mediterranean Sea during the last glacial and to distinct deep chlorophyll maximum layers in the eastern Mediterranean Sea coinciding with bottom anoxia episodes. High-frequency planktonic modifications are well documented in the Sicily Channel and Alboran Sea and highlight the occurrence of different steps within a single stadial (cold phase)/interstadial (warm phase) oscillation.The review of planktonic organisms in the marine sedimentary archive casts light on the uniqueness of the Mediterranean Sea, especially in terms of climatic/oceanographic/biological interaction and influence of different climatic systems on distinct areas. Further research is needed in the eastern Mediterranean Sea where results are obscured by low-resolution sedimentary records and by a strong focus on sapropel deposition dynamics.

Tracking rainfall in the northern Mediterranean borderlands during sapropel deposition

The role of mid-latitude precipitation in the hydrological forcing leading to the deposition of sapropels in the Mediterranean Sea remains unclear. The new GDEC-4-2 borehole, East Corsica margin (northern Tyrrhenian Sea), provides the first precisely dated evidence for enhanced rainfall in the Western Mediterranean during warm intervals of interglacial periods over the last 547 kyr. Comparison of GDEC-4-2 proxy records with pollen sequences and speleothems from the central and eastern Mediterranean reveals that these pluvial events were regional in character and occurred probably in response to the intensification of the Mediterranean storm track along the northern Mediterranean borderlands in autumn/winter. Our dataset suggests that the timing of maxima of the Mediterranean autumn/winter storm track precipitation coincide with that of the North African summer monsoon and sapropel deposition. Besides highlighting a close coupling between mid- and low-latitude hydrological changes, our findings suggest that during warm intervals of interglacial periods the reduced sea-surface water salinities, together with the high flux of nutrient and organic matter, produced by the monsoonal Nile (and wadi-systems) floods, were maintained throughout the winter by the Mediterranean rainfall. This provides an important additional constraint on the hydrological perturbation causing sapropel formation.