Origin and transformation of organic matter in Pliocene–Pleistocene Mediterranean sapropels: organic geochemical evidence reviewed

Drilling of deep-water post-Messinian sedimentary sequences by ODP Legs 160 and 161 in the Mediterranean Sea has shown that occurrence of organic-carbon-rich sapropels and sapropel-like sediments extends from the Levantine Basin westward into the Alboran Basin. In the eastern Mediterranean, sapropel deposition started in the Early Pliocene, whereas in the Western Basin the onset of sapropel formation occurred later, in the Early Pleistocene. Precessional cycles are apparently the primary external forcing for sapropel formation. Nevertheless, the pattern of sapropel occurrence suggests that the precessional influence is modulated by the glaciation cycles. Large differences were observed in the organic carbon contents of sapropels recovered in the eastern and western Mediterranean. Correspondence between organic carbon contents, Rock-Eval hydrogen index values and elemental C/N ratios indicate that both variations in the production and preservation of marine organic matter have led to the accumulation of high amounts of organic matter in sapropels. Molecular organic geochemical compositions of sapropels from the eastern Mediterranean further confirm that the major fraction of organic matter in sapropels is derived from marine algal sources and has undergone variable oxidation. Enhanced marine productivity and improved preservation of organic matter is central to sapropel formation. Accumulation of increased amounts of land-derived material at times of sapropel formation is also evidenced, supporting the hypothesis of significant periodic freshwater discharges.     

The occurrence and significance of Pleistocene and Upper Pliocene sapropels in the Tyrrhenian Sea

Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many “pelagic sapropels” have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (Proc. ODP, Sci. Results 107, 1990) indicates that sapropel occurrences coincide with negative δ¹⁸O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several “anoxic events” in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.     

Sedimentology and geochemistry of an exceptionally preserved last interglacial sapropel S5 in the Levantine Basin (Mediterranean Sea)

A combined study of lithological, geochemical and physical sediment properties is reported from a completely laminated S5 sapropel, recovered in three gravity cores (M40-4 SL67, M51-3 SL103, M51-3 SL104) from the Pliny Trench region of the eastern Mediterranean. The thickness of the studied sapropel S5 varies between 85 and 91 cm and tops most S5-sapropels in the Mediterranean. Based on optical features like color and thickness of laminae, the sapropels were subdivided into thirteen distinct lithostratigraphic zones. These zones, as well as the finer layering pattern within them, could be followed exactly among the three cores, indicating that the processes responsible for this variation acted at least on a regional scale. The sapropel sediment is characterized by exceptionally high porosity, which is strongly correlated with Si/Ca. This relationship implies that the sapropel is in essence an organic-matter rich diatomite and its exceptional thickness can be explained by preservation of diatoms forming a loosely packed sediment fabric. Compared to other S5 sapropels, the preservation of diatoms has apparently led to a twofold increase in the thickness of the sapropel layer. Relative abundances of 10 elements were determined at ultra-high resolution (0.2 mm) by XRF-scanner over the complete length of each sapropel including several cm of enclosing marl. An analysis of the chemical data indicates that the lowermost 13 cm of the sapropel is chemically more similar to the underlying marl and that the sediment chemistry shows different signals at different scales. The strongest pattern is the contrast between the sapropel and the surrounding marl, which is accentuated in elements indicative for redox conditions as well as terrigenous sediment input and productivity. Within the sapropel, a mm- to cm-scale layering is observed. The abundances of many elements are systematically linked to the pattern of these layers, indicating a common origin, related to productivity and/or terrigenous sediment and/or redox conditions. This pattern indicates a link to a regional climatic process, making the S5 sapropel horizon in M40-4 SL67, M51-3 SL103 and M51-3 SL104 a potential high-resolution archive of climatic variability during the last interglacial in the Mediterranean Sea and its adjacent landmasses.     

Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea

Diagenetic dissolution of magnetic minerals has been widely observed in organic-rich sediments from many environments. Organic-rich sediments from the eastern Mediterranean Sea (sapropels), recovered during Leg 160 of the Ocean Drilling Program, reveal a surprising catalogue of magnetic properties. Sapropels, from all sites studied across the eastern Mediterranean Sea, are strongly magnetic and the magnetization is directly proportional to the organic carbon content. The magnetization of the sapropels is dominated by a low-coercivity, probably single domain magnetic mineral (with an inverse magnetic fabric) that exhibits a clear decay in magnetic properties when exposed to air. During heating, the magnetic particles irreversibly break down between 360 and 400°C. The contrast between the magnetic properties of sapropels and surrounding sediments is marked, with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments. The contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non-steady-state diagenesis: sulphate-reducing conditions dominated during sapropel deposition, while overlying sediments were deposited under oxic conditions. The mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate-reducing conditions that existed during times of sapropel formation. Attempts to identify this mineral have been unsuccessful, but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase. The influence of diagenesis on the magnetic properties of cyclically-deposited eastern Mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments.     

Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate

Cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre-evaporitic Messinian in a marginal basin of the western Mediterranean. The fluctuations coincide with a dominantly precession-controlled sedimentary cyclicity (sapropels). During sapropel deposition, high planktic foraminiferal diversities are indicative of relatively stable marine conditions, while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable, more saline surface water conditions. The dominance of a precession-related signal indicates that regional climate oscillations rather than (obliquity-related) glacio-eustatically controlled influxes of Atlantic and/or Mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity. Our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run-off along the western Mediterranean at times that perihelion occurred in Northern Hemisphere summer. Less favourable, highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes. The cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins.     

The Messinian event in the Paratethys: Astronomical tuning of the Black Sea Pontian

This study presents new data on the orbitally calibrated Maeotian/Pontian and Pontian record of the Black Sea Basin (Paratethys) obtained by time-series analysis of magnetic susceptibility (MS) data from relatively deep-water Upper Miocene sediments exposed in the Zheleznyi Rog section (Taman Peninsula, Russia). In the studied interval, a ∼145-m-long sedimentary sequence, spectral analysis revealed statistically significant signals with 6.1–8.2 m and 3.0–4.0 m wavelength. These signals correspond to the obliquity and precession cycles, respectively. This study correlates the main steps of Messinian Salinity Crisis (MSC) of the Mediterranean to the Black Sea Pontian record based on astronomical tuning of the study sequence and evaluation of integrated biostratigraphic, paleomagnetic and sedimentological data. Based on cyclostratigraphic results, Maeotian/Pontian beds with Actinocyclus octonarius accumulated from ∼6.3 to 6.1 Ma. Most of the Novorossian sediments correspond to the first MSC step. The TG 22 (5.79 Ma) and TG 20 (5.75 Ma) glacial events occur in the uppermost Novorossian record and are marked by extraordinary high values of MS. The Portaferian, dated at the base as ∼5.65 Ma and the top as ∼5.45 Ma, corresponds to the second MSC step. The Novorossian/Portaferian transition is marked by the hiatus of approximately 150–160 kyr, which agrees well with the concept of the intra-Pontian unconformity in the Black Sea Basin and a sea-level drop in the Mediterranean from 5.6 to 5.46 Ma. The ages for the base and the top of the Bosphorian were estimated as ∼5.45 Ma and ∼5.27 Ma, respectively. The base of the Bosphorian corresponds to the third Lago Mare episode caused by the high sea-level connection between the Mediterranean and Eastern Paratethys.     

Neogene sapropels in the Mediterranean: a review

For 50 years the existence of sapropels (organic-carbon-rich sediments) deposited within Plio–Pleistocene sediments of the Mediterranean Sea has been known. Initially, research concentrated on material recovered in relatively short gravity/piston cores taken from the eastern basins where sequences were found to be well developed/preserved and had extensive spatial coverage. In the main, previous studies concentrated upon establishing a workable stratigraphy, spatial correlation of individual layers and determining the probable depositional mechanisms. However, despite a plethora of research papers, some issues still remain unresolved. This is in part due to a lack of agreement between investigators; sampling and analytical short comings, restricted sample size and the fact that, in many instances, like was not being compared with like. Recently, the limit of sapropels in the western basin has been further extended. As a result, the palaeoceanographic/palaeoclimate models which had previously been developed for deposition of sapropels in the eastern basin have been modified. Most recently, strong links have been established between astronomical cyclicity and sapropel formation. This review paper provides a summary of sapropel research to date, and ongoing sapropel research in the Mediterranean, some of which appears in this thematic issue of Marine Geology. It is fitting that this thematic issue of Marine Geology be dedicated to the memory of Colette Vergnaud-Grazzini and Rob Kidd who in many ways helped to initiate the resurgence in sapropel studies in the 1970s in the Mediterranean —perhaps in 50 more years we will know all of the answers!     

A synthetic pollen record of the eastern Mediterranean sapropels of the last 1 Ma: implications for the time-scale and formation of sapropels

The Quaternary climate of southern Europe (south Italy and Greece) is investigated by pollen analysis of the sapropels which were deposited in the deep eastern Mediterranean Sea during the last 1 million year (Ma). The time-scale of core KC01b in the Ionian Sea has been established by tuning its oxygen isotopic record to the ice volume model of Imbrie and Imbrie (1980). For the last 250,000 year (250 ka), the previous pollen studies and astronomical tuning have been confirmed. Sapropels were deposited under a large range of Mediterranean climates: fully interglacial, fully glacial, and intermediary, as revealed mainly by the balance between the respective pollen abundances of oak (Quercus) and sage-brush (Artemisia). The high value of the oak reveals the warm and wet climate of an Interglacial, and the high value of the sage-brush, the dry and cold climate of a Glacial. Whereas the Mediterranean climate is directly related to the variation of the high-latitude ice sheets, the deposition of sapropels is not so. In contrast with the wide climatic range, sapropels were deposited only when summer insolation in the low latitudes reached its highest peaks. However, between 250 ka and 1 Ma, that stable pattern is not yet established. Only six sapropels are observed, many expected ones do not appear, even as ghosts signalled by peaks of barium abundance, that remain after the post-deposition oxidation of organic matter. The pattern of sapropel formation in stable and direct relationship to highest insolation does not seem to apply. For five of those sapropels, neither climate extremes are observed; they mainly formed during intermediary types of Mediterranean climate. In contrast, one sapropel (and one ghost) relates to a relatively low peak of insolation, and its climate is of a unique, composite type not seen later. This might suggest an unsuspected, more complex pattern linking the formation of Mediterranean sapropels to the astronomical configuration.     

Middle to Late Cenozoic tectonic events in south and central Palawan (Philippines) and their implications to the evolution of the south-eastern margin of South China Sea: Evidence from onshore structural and offshore seismic data

Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.     

Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence

The sediments of the eastern Mediterranean basin contain C_org-enriched layers (sapropels) interbedded with the C_org-poor sediments which form by far the greater part of the record. While it is generally appreciated that different surface ocean productivity and bottom water conditions are necessary for the formation and preservation of these two sediment types, less attention has been paid to diagenetic effects which are an expected consequence of transitions between dramatically different bottom water oxygenation levels. A geochemical interpretation has emerged of post-depositional oxidation of the most recent sapropel (S1), initially based on the relationship of the Mn, Fe, C_org and S concentration/depth profiles observed around S1, and the characteristic shapes of these elemental profiles known from other situations. This indicates that post-depositional oxidation has removed approximately half of the visual evidence of the sapropel (∼6 cm from a total of ∼12 cm in the deep basin). The oxidation interpretation from redox-sensitive element redistribution profiles has subsequently been consolidated with evidence from pore water (O₂, NO⁻₃, Mn²⁺ and Fe²⁺) studies, from characteristic solid phase Ba profiles which yield palaeoproductivity records, and from oxidation-sensitive indicator trace elements (I and Se). So far, these geochemical observations have been concentrated in the deeper central parts of the basin, where sediment accumulation rates are lower than on the basin margins, and radiocarbon dating indicates that S1 formation occurred between 5.3 and 9.0 ky (uncorrected conventional radiocarbon time). It remains to be demonstrated whether or not these times are applicable to the entire E. Mediterranean basin. The implications of these findings to guide sampling in future work on the S1 productivity episode and on older sapropels for palaeoenvironmental investigations are discussed.     

Late Miocene stable isotope stratigraphy of SE Atlantic ODP Site 1085: Relation to Messinian events

High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ¹⁸O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ¹⁸O variations is followed by a period marked by maximum in the δ¹⁸O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ¹⁸O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage.     

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.     

Lower Miocene structural evolution of the central Vienna Basin (Austria)

The tectonic evolution of the Vienna Basin overlying the Alpine-Carpathian fold and thrust belt includes two stages of distinct basin subsidence and deformation. The earlier phase contemporaneous with thrusting of the Alpine-Carpathian floor thrust is related to the formation of a wedge-top basin (“piggy-back”), which was connected to the evolving foreland basin (Lower Miocene; c. 18.5–16 Ma). This stage is followed by the formation of a pull-apart basin (Middle to Upper Miocene; c. 16–8 Ma). Sediments of the latter unconformably overly wedge-top basin strata and protected them against erosion.     

Benthic foraminiferal successions across Late Quaternary Mediterranean sapropels

Benthic foraminiferal records across Late Quaternary Mediterranean sapropels S6, S5 and S1 are reviewed and re-considered in the light of recent advances in foraminiferal ecology. It is suggested that the main factor controlling the foraminiferal successions that occur immediately before and after sapropel deposition is the amount of time involved in the onset of anoxic conditions and in the re-oxygenation of the benthic environment. Faunas dominated by deep infaunal taxa such as Globobulimina and Chilostomella reflect a very gradual decrease or increase of bottom water oxygenation. Post-sapropel faunas dominated by small biconvex, trochospiral taxa, which are inferred to have a much more opportunistic life strategy, are typical of a very rapid re-oxygenation of the benthic environment after sapropel deposition. The time involved in re-oxygenation depends upon the mechanism causing sapropel formation. The faunal succession found above sapropel S5 suggests a basin with a strong density gradient in the upper part of the water column during sapropel deposition, followed by a rapid turnover of the following column and quick re-oxygenation of the benthic environment. In contrast, the faunal succession above sapropel S6 suggests a water column with a more gradual density gradient deeper in the basin, a progressive deepening of the halocline, and a very slow increase of bottom water oxygenation.     

Evidence for the evolution of an oxygen minimum layer at the beginning of S-1 sapropel deposition in the eastern Mediterranean

The sediments of the eastern Mediterranean basin contain a series of organic-rich sapropels intercalated with organic-poor nannofossil oozes. Until recently the timing of the onset of sapropel formation was not known accurately because of the low resolution achievable by conventional radiocarbon dating. Compilation of all available ¹⁴C-AMS dates show that the base of S-1 (the most recent sapropel) was initiated 8800 years B.P. (¹⁴C age corrected by 400 years for reservoir effect) under a 500 m water column and moved progressively into deeper water reaching depths of 3500 m at 8200 years B.P. The linear correlation between the age of S-1 onset and water depth

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suggests that formation of sapropels moved into deeper water at a rate of 1000 m/200 year. A model is suggested in which export production which sank below the well-mixed surface layers (500 m) was respired consuming dissolved oxygen in the Levantine deep water until a threshold value was reached when sapropels began to be preserved in the sediment. This resulted in a progressively deepening oxygen minimum zone with time until eventually the entire deep water in the basin was oxygen depleted. Assuming that the threshold value for sapropel formation was complete anoxia, it was calculated that primary productivity in the basin during the deposition of S-1 was a factor of 5 greater than that found at present.     

Eastern Mediterranean sapropel S1 interruption: an expression of the onset of climatic deterioration around 7 ka BP

We discuss the palaeoclimatic interpretation of unprecedented high-resolution micropalaeontological studies of short-term (2 to 4 centuries) interruptions within early Holocene organic-rich layer (sapropel) S1 from the eastern Mediterranean. Results for cores from the Adriatic and Aegean seas that contain `double' S1 sapropels indicate that these interruptions, which are centred roughly around 7000 years <sup>14</sup>C<sub>nc</sub> BP, are genuine and related to climatic deterioration. This interpretation is endorsed by a coeval dry event recorded in terrestrial records and indications of climatic deterioration affecting human migration patterns and early societies in Egypt. The presence of sapropel interruptions in the two major source areas of deep water for the entire eastern Mediterranean likely implies that similar intervals may be found throughout the basin, provided that sedimentation rates and sampling resolutions allow the detection of events with a duration of only several centuries. Moreover, our results show that the `sapropel mode' of circulation comprises a delicate balance between reduced ventilation and enhanced productivity, which is easily disturbed through surface water cooling triggering a short time of improved deep water ventilation.     

Carbon and nitrogen isotope excursions in mid-Pleistocene sapropels from the Tyrrhenian Basin: Evidence for climate-induced increases in microbial primary production

The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems. Resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that corresponds to 1001 to 946 ka. Organic δ¹³C values systematically increase from − 26‰ to − 21‰ and δ¹⁵N values systematically decrease from 4‰ to < 0‰ as organic carbon mass accumulation rates increase in the sapropel layers. The increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total marine productivity. The precessional minima with which sapropels coincide were times of wetter climate that stratified the surface Mediterranean Sea, increased delivery of soil-derived phosphorus, and evidently amplified microbial primary production. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode that suggest that sapropel deposition was a climate-sensitive surface-driven phenomenon that was not accompanied by basin-wide stagnation.     

Mediterranean functioning and sapropel formation: respective influences of climate and hydrological changes in the Atlantic and the Mediterranean

During Ocean Drilling Program (ODP) Legs 160 and 161, sapropels were recovered both in the western and eastern Mediterranean. This obliges to a reassessment of the previous studies focused on sapropels from only the eastern Mediterranean, and to consider the changes which occurred in the Mediterranean climate but also in the water characteristics both in the Atlantic and in the western Mediterranean. In the North Atlantic, the position of the polar front which migrated southwards during glacial times and the melting of northern ice caps during interglacial periods, together with the convection in the Labrador and Norwegian Seas, appear essential to control the salinities of the waters facing the Strait of Gibraltar. The salinities of the surface and intermediate layers constitute the first driving force of the Mediterranean dynamics, the second driving force being the Mediterranean climate. The stagnation of deep waters leading to sapropel deposition in the western Mediterranean may be explained by a drastic weakening of the density difference between Mediterranean outflow and Atlantic intermediate waters facing the Strait of Gibraltar. This weakening was induced primarily by the salinity decrease of Atlantic surface water and secondly by a rather high salinity in the Atlantic intermediate layer, rather than by a drastic deterioration of the Mediterranean climate. This scenario probably concerns most of the sapropel events and it may be used for the knowledge of Atlantic and Mediterranean functioning over climatic changes.     

Oscillating Quaternary water levels of the Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea–Black Sea drainage corridor

Detailed interpretation of single-channel air-gun and deep-tow boomer profiles demonstrates that the Marmara Sea, Turkey, experienced small-amplitude (∼70 m) fluctuations in sea level during the later Quaternary, limited in magnitude by the sill depth of the Strait of Dardanelles. Moderate subsidence along the southern shelf and Quaternary glacio-eustatic sea-level variations created several stacked deltaic successions, separated by major shelf-crossing unconformities, which developed during the transitions from global glacial to interglacial periods. Near the Strait of Dardanelles, a series of sand-prone deposits are identified beneath an uppermost (Holocene) transparent mud drape. The sandy deposits thicken into mounds with the morphology and cross-sectional geometries of barrier islands, sand waves, and current-generated marine bars. All cross-stratification indicates unidirectional flow towards the Dardanelles prior to the deposition of the transparent drape which began ∼7000 years BP, in strong support of the notion that the Marmara Sea flowed westwards into the Aegean Sea through the Dardanelles at times of deglaciation in northern Europe. The global sea-level curve shows that, at ∼11,000 and ∼9500 years BP, sea level rose to the sill depths of the Straits of Dardanelles and Bosphorus, respectively. The effect from ∼11,000 to ∼9500 years BP was seawater incursion into the Marmara Sea, drowning and formation of algal-serpulid bioherms atop lowstand barrier islands, and transgression of shelves and lowstand deltas. At ∼9500 years BP, glacial meltwater temporarily stored in the Black Sea lake, developed into a vigorous southward flow toward the Aegean Sea, forming west-directed sandy bedforms in the western Marmara Sea and initiating deposition of sapropel S1 in the Aegean Sea. This strong outflow persisted until ∼7000 years BP, after which a mud drape began to accumulate in the Marmara Sea and euryhaline Mediterranean mollusks successfully migrated into a progressively more saline Black Sea where sapropel deposition began. Most eastern Mediterranean sapropels from S1 to S11 appear to correlate with periods of rising sea level and breaching, or near-breaching, of the Bosphorus sill. These events are believed to coincide with times of vigorous outflow of low-salinity (?fresh) surface waters transiting the Black Sea–Marmara Sea corridor, and ultimately derived from melting of northern European ice sheets.     

Origin and differentiation of clay minerals in pelagic sediments and sapropels of the Bannock Basin (Eastern Mediterranean)

Two cores from an anoxic basin of the southeastern Mediterranean Ridge were investigated to compare the clay mineralogy of pelagic sediments and of the interbedded sapropels. The sediments of Core BAN 84-02, raised from the basin floor, and those of Core BAN 84-08, from the eastern plateau of the Bannock Basin, provide evidence for different sedimentary environments. The anoxic conditions, which are still present near the bottom, produce an important decrease in smectite crystallinity (Core 02), whereas well-organized smectite persists in the normally oxygenated sediments (Core 08). Detrital clay minerals from various sources were deposited in the basin and no appreciable diagenesis was recognized downcore.

The clay mineralogy of the sapropels shows remarkable differences compared to the pelagic sediments. The changes observed are dependent on aggressive chemical reaction and on the sudden input of detrital crystalline sediments into the stagnant environment. A climatic curve registers the variable degree of clay mineral hydrolysis in continental areas and exhibits good correspondence with an already published oxygen isotope curve for the area.