Palaeo-environmental variations in eastern Mediterranean sediments: a multidisciplinary approach in a prehistoric setting

Progress in Oceanography 44 (1999) 369–386

Palaeo-environmental variations in eastern Mediterranean sediments: a multidisciplinary approach in a prehistoric setting Gert J. De Lange a,*, P.J.M. Van Santvoort a, C. Langereis a, J. Thomson b, C. Corselli c, A. Michard d, M. Rossignol-Strick e, M. Paterne f, G. Anastasakis g a

Department of Geochemistry, Institute Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands b Southampton Oceanography Centre, Southampton, UK c Department Scienze della Terra, University of Milan, Italy d GJosciences de l’Environnement, Aix-en-Provence, France e University P. & M. Curie, Paris VI, France f CNRS, Gif-sur-Yvette, France g National and Capodistrian University of Athens, Greece

Abstract Not only the occurrence of distinct organic-rich intervals (sapropels), but also the relative contents of key major and minor elements and isotopes in the sediments of the eastern Mediterranean, appear to be cyclic and to be astronomically associated. Interpretations of the environmental conditions leading to sapropel formation are based on results from sedimentological, micropalaeontological and geochemical studies of the dark-coloured layers and the cream/brownish sediments that occur above and below them. Part of the signal may be removed by early diagenetic processes. The extent and direction of these processes are ultimately controled by the amount and reactivity of organic matter. The interval of dark colour associated with a sapropel is often somewhat thicker than that defined by the ⬎2% Corg definition and usually has a sharp colour change at upper and lower boundaries. A grey so-called ‘proto-sapropel’ layer of variable thickness underlies most sapropel layers. A few centimeters above the most recent sapropel S1, is usually found a clear darkbrown layer 2–3 cm thickness is usually found, which has a diffuse, often mottled, upper boundary and a relatively abrupt colour transition at its lower boundary. The colour is charac-

* Corresponding author. Tel.: +30-535034. E-mail address: [email protected] (G.J. De Lange) 0079-6611/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 9 9 ) 0 0 0 3 7 - 3

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teristic of Mn oxyhydroxide enrichments. In the interval from the dark-brown layer to the visible upper S1 boundary, there is usually an increasingly red-brownish colour. The distinct upper manganese Marker-Bed has been related to the Santorini (Minoan) eruption in 3356±18 BP (Bruins, H. J., & Van Der Plicht, J. (1996). The Exodus enigma. Nature, London, 382, 213–214), but is more likely to be associated with a Basin-wide re-ventilation event induced by changing climatic (humidity) conditions. Using barite-Ba as a paleo-productivity indicator, enhanced fluxes, and hence increases in accumulation rates of organic carbon to the seafloor, must have occurred from approximately 9 to 5 ky BP. The perfect correlations between observed Corg and calculated Corg from the Corg/Ba relation in the visible S1 interval, and the total lack of such correlation in the interval between the darkbrown layer and the visible S1 layer are remarkable. It seems, therefore, that S1 deposition lasted from 9 to 5 ky BP but the organic carbon for the upper part has been removed. The double barium peak visible in a number of sediments studied in high-resolution, may be related to the ‘Flooding’ of the Black Sea that occurred around 7150 BP (Ryan, W. B. F., Pittman, W. C., Major, C. O., Shimkus, K., Moskalenko, V., Jones, G. A., Dimitrov, P., Gorur, N., Sakinc, M., & Yuce, H. (1997). An abrupt drowning of the Black Sea shelf. Marine Geology, 138, 119–126). Using various ‘proxies’, the (initial) occurrence of eastern Mediterranean sapropels appears cyclic and to be strongly related to the Monsoonal/Insolation Index. However, the relative value of an insolation maximum and initial Corg content of the corresponding sapropel are not always related in a simple way. This again points to additional (possibly hydrographic) factors determining (the intensity of) sapropel formation. Understanding the mechanisms of sapropel formation and their subsequent preservation is not only necessary if our reconstructions of palaeoenvironmental conditions in the Eastern Mediterranean are to be improved, but may also assist in understanding the present-day situation and in forecasting possible future developments. The observed paleoenvironmental variations are discussed in a geochemical and environmental context, and are illustrated using typical examples from the eastern Mediteranean.  1999 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

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Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

3. Results and discussion . . . . . . . . 3.1. Manganese marker bed . . . . . 3.1.1. Steady-state diagenesis . . . . 3.1.2. Major hydrothermal eruption 3.1.3. Major re-ventilation event . . 3.2. Double barium peak . . . . . . . 4.

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

1. Introduction In the deep marine environment, the organic carbon content (Corg) preserved in a sediment is primarily a balance between the initial input flux of organic matter and its subsequent oxidation by bottom water oxygen. Most marine sediments have Corg contents in the range 0.2–2%. The Eastern Mediterranean sedimentary record, therefore, comprises an intercalation of unusually Corg-rich sediments (the sapropels) within Corg-poor sediments, caused by repeated fluctuations through time in either (or both) the Corg flux from surface ocean export production or in dissolved bottom water O2 concentrations. This intercalation is reflected by alternating layers of (light) brown hemipelagic organic-poor and dark olive-green organic-rich sediments. The occurrence of sapropels appears to have been associated with climatic changes. Cycles in the obliquity of the Earth’s axis and the precession of the equinoxes are the underlying controling variables that influence the global climate through their effects on planetary insolation (Berger & Loutre, 1992). The timing of sapropel formation coincides with the times of maximum summer insolation, and seems to be related to the (African-Indian) Monsoon intensity (e.g. Rossignol-Strick, 1983; Hilgen, 1991; Lourens, Hilgen, Zachariasse, Van Hoof, Antonarakou, & Vergnaud-Grazzini, 1996; Langereis, Dekkers, De Lange, Paterne, & Van Santvoort, 1997; Rossignol-Strick, Paterne, Bassinot, Emeis, & De Lange, 1998). Sapropels have not been formed during every maximum in the insolation curve. This lack of formation may have resulted from variations in climatic conditions during the maxima such that sapropels did not always form. Alternatively, sapropels initially deposited may subsequently have disappeared as a result of ongoing oxidation reactions (e.g. De Lange, Middelburg, & Pruysers, 1989; Pruysers, De Lange, & Middelburg, 1991; Pruysers, De Lange, Middelburg, & Hydes, 1993; Thomson, Higgs, Wilson, Croudace, De Lange, & Van Santvoort, 1995; Van Santvoort et al., 1996). Enhanced levels of barite-barium, a proxy which is thought to be related to primary productivity in the surface waters of the ocean (e.g. Dymond, Suess, & Lyle, 1992; Francois, Honjo, Manganini, & Ravizza, 1995) are usually found at the most emphatic maxima in the insolation curve (e.g. Van Santvoort, De Lange, Langereis, Dekkers, & Paterne, 1997). However, the origin of the ‘bio-barite’ proxy is not yet fully understood, and seems to reflect an association between photic zone primary productivity and the quantities of barium in settling organic debris. Highly reducing conditions may lead to the incomplete preservation of barite-Ba in the reduced sedimentary interval (e.g. Church, 1979; Brumsack, 1986; Van Os, Middelburg, & De Lange, 1991; Torres, Brumsack, Bohrmann, & Emeis, 1996). Because of the rapid alternation of distinct reduced sediment intervals with (sub)oxic intervals for some sapropels, the barite-barium has been redistributed over rather short distances, whereas for others it appears to have remained in place. As a conse-

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quence, the enhanced productivity ‘event’ can still be detected, or in the case of immobility can be used to detect the initial limits of the interval of enhanced productivity even when the Corg signal has been totally or partially removed. (e.g. Van Santvoort et al., 1996, 1997). The origin and mode of sapropel formation has been disputed ever since their first recovery in bottom sediments from the eastern Mediterranean (e.g. Kullenberg, 1952; Olausson, 1961; Cita, Vergnaud-Grazzini, Robert, Chamley, Ciaranfi, & D’Onofrio, 1977; Rossignol-Strick, Nesteroff, Olive, & Vergnaud-Grazzini, 1982; RossignolStrick, 1983; Calvert, 1983; Anastasakis & Stanley, 1986; Howell, Thunnell, Tappa, Rio, & Sprovieri, 1988; Rohling & Gieskes, 1989; Troelstra, Ganssen, Van Der Borg, & De Jong, 1991; Fontugne & Calvert, 1992; Calvert, Nielsen, & Fontugne, 1992; Van Os, Lourens, Hilgen, & De Lange, 1994; Thomson, Higgs, Wilson, Croudace, De Lange, & Van Santvoort, 1995; Van Santvoort et al., 1996; Strohle & Krom, 1997). Two processes enhanced primary productivity leading to enhanced organic matter fluxes to the seafloor, and stagnation leading to enhanced preservation of organic matter fluxes at the seafloor, have most frequently been cited as resulting in their formation acting either separately or in combination. Using barite-barium as a productivity proxy, it is beyond doubt that productivity has been enhanced during periods of sapropel formation, although the exact magnitude of the increase can yet not be determined with certainty, because of the lack of calibrating parameters for such alternate environmental conditions as those occuring in sapropel periods in the eastern Mediterranean (Dymond et al., 1992; Francois et al., 1995; Dymond & Collier, 1996). There are some convincing indications that anoxic bottom water, and possibly even (lower) photic zone anoxic conditions coincided with deposition of at least some of the sapropel intervals (Passier, Bosch, Nijenhuis, Lourens, Boettcher, Leenders et al., 1998). Paleoclimatic humidity variations and the related fluctuations in the density and temperature of outflowing Mediterranean deep water has lead to speculations as to the possible effect of these variations on the Global Conveyer Belt, in particular on the Gulfstream in the North Atlantic (Reid, 1979). Similarly dramatic changes in temperature and salinity (density) of deep Mediterranean waters seem to be forced at present, largely by anthropogenic influences, such as the great reductions in inflows of water from rivers reaching the Mediterranean as a result of the Aswan High Dam in the Nile, and the enhanced use of water for irrigation. This has lead to a renewed speculation on the possible imminent consequences for the global, but in particular the European, climate. Johnson (1997), for example, has suggested the building of a dam to control the in- and outflow of water through the Straits of Gibraltar so as to prevent a shift in the climates of Canada and Europe in the near future. If such a dam were to be constructed, it might perhaps conserve global climate, but it would have dramatic environmental consequences for what would then be a poorly flushed Mediterranean. Clearly, it is of vital importance to know and to understand the paleoceanography in connection to paleo-environmental conditions of the (eastern) Mediterranean. In the present paper some aspects will be discussed in relation to observations on eastern Mediterranean sediments deposited during the

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last 10 000 years, which may be linked to some prehistoric events. Understanding the mechanisms of sapropel formation and their preservation is not only a basic necessity for improved reconstructions of palaeoenvironmental conditions in the Eastern Mediterranean, but may also assist in understanding the present-day situation and possible future developments (e.g. Bethoux, Gentili, Raunet, & Tailleze, 1990; Bethoux & Gentili, 1996; Roether et al., 1996).

2. Material and methods Core material discussed here was collected during cruises of: R/V Marion Dufresne in 1991 (MARFLUX-1) and R/V Urania in 1994 (PALAEOFLUX-3). The sample locations are shown in Fig. 1. Oxygen concentrations and redox potentials were measured onboard as soon as possible after core recovery. Pore waters were obtained by squeezing. The shipboard routine has been described in detail elsewhere (De Lange, 1992). Box cores were extruded at 1 cm resolution and squeezed in a glovebox with a high-quality nitrogen

Fig. 1. Eastern Mediteranean sample locations discussed in the text: boxcore UM26, and Long Pistoncore KC01B.

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atmosphere. Sampling and pore water extraction took place at in situ temperatures (13°C). Profiling of dissolved oxygen in sediments was carried out using a new robust oxygen electrode (Cussen, Braithwaite, & Wilson, 1994). The stiffness of the sediment and the relative size of the core tubes to the oxygen probe presented problems when forcing the probe to depths ⬎15 cm. Hence the profiles were measured in two or three 15 cm sections, each time removing the upper 10 cm of the sediment that had already profiled. The redox potential was measured on split subcores as soon as possible after collection by a punch-in Ingoldt Eh electrode with a separate reference electrode. A Zobell standard solution (Eh =215 mV at 13°C) was used for calibration. Sediment samples were freeze dried and finely ground in an agate mortar, then digested in a mixture of hydrofluoric, nitric and perchloric acids. The final solutions were analysed by ICP-AES, 1 N HCl on a Perkin Elmer Optima 3000. The quality of the analyses was monitored by the inclusion of laboratory and international standards. The error in the analyses was ⬍4% for all elements. Organic carbon (Corg) was determined on a Fisons Instruments NCS NA 1500 analyzer using dry combustion at 1800°C. Inorganic carbon, as carbonate, was removed before analysis by shaking the sample for 24 hours in 1 N HCl. This procedure was repeated to ensure that all inorganic carbon had been removed. After drying at 80°C, the sample was ground in an agate mortar. International and in-house standards were used to check the accuracy of the method. Standard deviations were always ⬍2%. 3. Results and discussion The usual sequence of colours in alternating intervals of sediments deposited during the last 10 000 years in the eastern Mediterranean as found in a ‘typical’ core is as follows: For sediments from waterdepths shallower than 3 km, the top few mm of sediment consists of a pteropod ooze, which overlies a 1–2 cm layer that is slightly more brownish than the layer of ca. 10 cm below it. About 12–13 cm below the surface interface, a marked dark-brown bed of 3–4 cm thickness is usually found. It has a diffuse, often mottled upper boundary and a relatively abrupt colour transition at its lower boundary (Fig. 2). This dark brown colour is typical of enrichment with Mn oxyhydroxide; this characteristic bed is usually referred to as the (manganese) Marker Bed (e.g. De Capitani & Cita, 1996). At 20–25 cm depth a dark olive-green interval occurs which has a sharp colour change at its upper and lower boundary. This is usually referred to as the most recent sapropel S1, but in fact represents only the residual lower part of what was initially a much thicker sapropel S1 deposit (see Introduction and discussion below; e.g. De Lange et al., 1989; Higgs, Thomson, Wilson, & Croudace, 1994; Thomson et al., 1995; Van Santvoort et al., 1996; Thomson et al., 1999). Between the lower manganese Marker Bed boundary and the upper dark olive-green boundary, there is subtle change in colour from (light) brown to a darker reddish-brown. The S1 sapropel overlies a dark grey interval which is usually

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Fig. 2. Colours for sediments deposited in the last 10 000 years in a core typical for the eastern Mediterranean (site UM26). For the same period some reference has been included to (pre)historic events, some of which may be related to features observed in the sediments (see text).

referred to ‘proto-sapropel’, but which is thought to have formed during or slightly after sapropel formation (Passier, Middelburg, Van Os, & De Lange, 1996). Below the ‘proto-sapropel’ the colour usually changes to light-grey and subsequently to light-brownish again. The boundaries of the initial sapropel interval can be detected accurately using the barite-barium solid-phase profile (Fig. 3). If sampled in high-resolution, this bar-

Fig. 3. Oxygen, solid-phase Manganese, Org.C, Barium, and redox potential for boxcore UM26. The grey interval is the presently visible organic-rich S1 sapropel interval; dashed line indicates level of manganese-rich Marker Bed and the inferred initial ending of the S1 interval (figure after Van Santvoort et al., 1997). For colours, see Fig. 2.

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ium depth profile usually shows a double-peak. Accumulation of the organic rich sapropel is thought to have occurred during the relatively wet climatic interval from 9 to 5 kyrs BP. The ending of sapropel accumulation appears to have coincided with the formation of the manganese Marker Bed. Subsequent oxidation by oxygen diffusing down from the sediment interface has resulted in the partial or even complete removal of the initial sapropel organic matter and its pyrite contents (e.g. De Lange et al., 1989; Higgs et al., 1994; Thomson et al., 1995; Van Santvoort et al., 1996). It has also resulted in the slight increase in reddish (Fe-oxyhydroxides) and brownish (Mn-oxyhydroxides) colours going from the lower boundary of the Marker Bed to the upper boundary of the dark olive-green interval. In the next sections the possible origins of the manganese Marker Bed and of the double nature of the barite peak will be discussed. 3.1. Manganese marker bed The distinct dark-brown interval, commonly refered to as (manganese Marker Bed) appears to occur in sediments throughout the eastern Mediterranean. 14C-dated or extrapolated datings of this interval all come out around 5 kyr BP (e.g. Van Santvoort et al., 1996; De Lange et al., personal communication). This clear layer, therefore, indicates an important basin-wide process during which large amounts of Mn-oxyhydroxides have been deposited on the seafloor. Only three general geochemical processes can account for such deposition: 1) steady-state diagenesis; 2) a major hydrothermal eruption; and 3) a major re-ventilation event. 3.1.1. Steady-state diagenesis During steady-state diagenesis in hemipelagic suboxic sediments a distinct peak of Mn-oxyhydroxides may develop at the boundary of upward diffusing dissolved Mn and of downward diffusing dissolved oxygen (e.g. Froelich et al., 1979; Burdige & Gieskes, 1983). In the 3 cores studied thus far at high resolution, the oxidation front, i.e. the boundary where upward moving dissolved Mn and downward moving dissolved oxygen meet, appears to be located at the upper boundary of the dark olive-green interval (see Van Santvoort et al., 1996). Consequently, the oxidation front occurs well (ca. 7 cm) below the upper manganese Marker Bed. Therefore, the Marker Bed in the cores is unlikely to be associated with the diagenetic steady state formation of a Mn-peak at an oxidation-front moving upward in pace with the sedimentation rate. 3.1.2. Major hydrothermal eruption Hydrothermal vents are known to produce enormous amounts of dissolved manganese which subsequently oxidize and become distributed over large areas of the ocean (e.g. Klinkhammer, Bender, & Weiss, 1977). Hydrothermal Mn-rich layers or crusts have been described for localities associated with mid-ocean ridges and island arcs (e.g. Hoffert et al., 1978) and in the Eastern Mediterranean (Varnavas, Papapioannou, & Catani, 1988; Cita et al., 1989). One of the major catastrophic eruptive events that occurred in the Mediterranean region during the last 5000 years is the Santorini

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‘Minoan’ eruption. This eruption has been held responsible for a major tsunami wave which affected the whole of the eastern Mediterranean, and resulted in the deposition of a ‘homogenite’ in the deeper parts of the basin (Cita et al., 1984). The Minoan eruption was likely not only to have caused a tsunami, but also probably generated an enormous dust cloud that probably extended over the entire eastern Mediterranean and beyond. It has been suggested (e.g. Vitaliano, 1973; Stanley & Sheng, 1986) that this eruption coincided with the Exodus of the Israelites from Egypt: ‘plague of darkness’ (EXODUS 10:21), and ‘the retreat and reflooding of the water while crossing the sea’ (EXODUS 14: 21–31). The Exodus was followed by 40 years of roaming through the desert (NUMERI 14:33–34), and the destruction of Jericho (JOSHUA 6: 1–25). The time difference between the averaged 14C datings of the Santorini eruption (3356±18 yr BP) and the destruction of Jericho (3311±13 yr BP) is surprisingly close to that 40 years period (Bruins & Van Der Plicht, 1996, and refs therein). In view of present-day and inferred past watermasses circulation patterns, a massive emission of dissolved manganese and subsequent Mn-oxyhydroxides related to such Minoan eruption, would have spread rapidly throughout the eastern Mediterranean. However, there seems to be one problem in linking the formation of the manganese Marker Bed with the Minoan Eruption, namely the timing of the event(s). If one corrects the age found for the Marker Bed for ‘reservoir age’, and if one assumes that a 3356 BP Mn-event of 15% Mn in a 4 mm thick layer (corresponding to the highest Mn content observed for a ‘marker bed’, found near Crete by De Capitani & Cita, 1996) was followed by bioturbation of the 3–5 surficial cm of the sediment, then one finds that: 1) the Mn profile versus depth is similar to those observed (e.g. Fig. 4); and 2) the modelled age is similar to the corrected Marker Bed age. Consequently, it is hypothetically possible that the Santorini Minoan eruption is linked to the manganese Marker Bed in eastern Mediterranean sediments. Nonetheless this option seems less likely, because: 1) up to now all Marker Beds except the one found immediately south of Crete (which has not been dated) demonstrate a rather uniform thickness and Mn content (1–1.2%); and 2) several of the older sapropels also seem to have been followed by a deposition of Mn-rich interval (marker bed), and for most others similar such initial Mn-rich layer can be suspected (e.g. Van Hoof, Van Os, Rademakers, Langereis, & De Lange, 1993; De Lange et al., 1994; Van Santvoort et al., 1997; Van Santvoort et al., personal communication). Since formation of sapropels has been reported to be closely associated with astronomical parameters, it seems unlikely that the Mn-rich intervals that formed subsequent to a sapropel, would in general be associated with hydrothermalism. In addition, the preserved double-Ba peak (see below) and the Gaussian shaped barium curve for the S1 sapropel all indicate that sediment mixing as a result of bioturbation has been relatively minor. Consequently, despite being a feasible explanation for the observations of events associated with the formation of the last sapropel S1, and also attractive from a prehistorical perspective, the hydrothermal option is unlikely to provide an adequate explanation for the occurrence of the upper manganese Marker Bed in most eastern Mediterranean sediments.

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Fig. 4. Scenario of the hypothesized instantaneous (hydrothermal) deposition of the manganese Marker Bed and its subsequent downward mixing.

3.1.3. Major re-ventilation event On the basis of enhanced barite-barium content, enhanced primary productivity has been reported for the 9 to 5 kyr BP period (e.g. Thomson et al., 1995; Van Santvoort et al., 1996). In addition, there are a number of strong indications that bottom waters must have been anoxic during formation of sapropel S1, namely: 1) the absence of benthic organisms; 2) enhanced pyrite content with a relative light δ34S (-40%o) indicating ‘open’ conditions during sulphate reduction, i.e. upper cms or even bottom water are anoxic; and 3) the presence in some sapropels of isorenieratane or its derivatives, which are related to photic zone green algae exclusively living in an anoxic environment, such as occurs in the present-day Black Sea at 80 m waterdepth (e.g. Passier et al., 1998, and refs therein). It seems, therefore, well established that not only the sediments but possibly also the bottom water were anoxic during deposition of most sapropels. In the more extreme case, large part of the water column may have been suboxic, with enhanced levels of dissolved Mn2+, which upon a re-ventilation/re-oxidation

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event would suddenly precipitate. This mechanism has been proposed to explain the Mn (and Th/Pa) spike observed at the glacial/interglacial transition in some Pacific sediments (Mangini, Eisenhauer, & Walter, 1991; Mangini, Rutsch, Frank, Eisenhauer, & Eckhardt, 1994). A similar reventilation event, not necessarily invoking a suboxic watercolumn, could explain the basin-wide simultaneous deposition of a Mnrich layer. The subsequent gradual downward oxidation of the previously anoxic organic-rich sapropel sediments is in line with this scenario. In addition, the enhanced freshwater flux into the eastern Mediterranean during the ‘wet period’ (e.g. Rossignol-Strick et al., 1982; Rossignol-Strick, 1983, 1997) from 9–5 kyr BP must have resulted not only in enhanced nutrient supplies but must also have increased water column stability, i.e. developed stagnant conditions. The distinct decrease in such freshwater inputs at the end of the wet period, perhaps accompanied by different climatic conditions such as wind directions, may have destabilized the water column. At present the different climatic, and consequently hydrographic, conditions result in low productivity, but have not led to the development of a stagnant and stable water column. Such variations in climatic conditions have occurred as a result of the cyclic precession-dominated climatic pattern in the Eastern Mediterranean (Fig. 6; e.g. Hilgen, 1991; Hilgen, Lourens, Berger, & Loutre, 1993; Lourens et al., 1996; Langereis et al., 1997). At the end of most sapropel periods, there is proof for the (initial) presence of a Mn-rich layer (e.g. Van Hoof et al., 1993; De Lange et al., 1994; Van Santvoort et al., 1997). Peaks of enhanced Ba (primary productivity), coincide with peaks of low ARM intensities (relatively low amounts of certain Fe-oxides, i.e. possibly reduced redox conditions), and peaks of relatively negative δ18O (enhanced freshwater). It appears, therefore, that sapropel formation only occurs during relatively ‘wet’ periods when ‘low-oxygen’ conditions prevailed, whereas during the ‘dry’ periods formation of a sapropel has never occurred. In summary, of the three potential processes that could lead to the formation of a basin-wide Mn-rich sediment interval, a major reventilation with concomitant change in redox conditions is the one that best fits most observations. 3.2. Double barium peak Barium has been reported to be a useful proxy for surface water primary productivity (e.g. Dymond et al., 1992; Von Breyman, Emeis, & Suess, 1992; De Lange et al., 1994; Francois et al., 1995). Although barite-Ba may be removed during highly anoxic conditions, the Ba in the sediments studied appears to have remained inplace. Using barite-barium as a productivity proxy it is no doubt that productivity had been enhanced during periods of sapropel formation. Enhanced levels of Ba are usually found at the most emphatic maxima in the insolation curve (e.g. Van Santvoort et al., 1997), corresponding to periods of enhanced surface water productivity, and enhanced river discharge and consequently nutrient input. However, the lack of calibrating parameters for the dramatically changing environmental conditions during and in between sapropel formation in the eastern Mediterranean, means that it is not possible to quantify exactly the prevailing primary productivity

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(Dymond et al., 1992; Francois et al., 1995; Dymond & Collier, 1996). Usually enhanced freshwater and nutrient outputs from the Nile, associated with variations in the Monsoonal index have been reported to have resulted in increases in organic matter accumulation during the sapropel S1 period (e.g. Rossignol-Strick et al., 1982; Rossignol-Strick, 1983). Recently, in a few high-resolution sediment studies and in some studies in sediments with high-accumulation rates (10–20 cm/kyr) a double peak feature in the sedimentary Ba profile and the organic-matter content has been observed (Rohling, Jorissen, & De Stigter, 1997; J. Thomson, personal communication; this study). The calibrated age for the mid-S1 low organic carbon interval in the Adriatic Sea is 7470 BP with an estimated ⬎300 yr deviation because of interpolations these authors had to use (Rohling et al., 1997). The saddle between the maxima in the cores of this study (2 subcores from the same boxcore at site ABC26; and core UM26) is dated at 7000±300; the high error is mainly caused by interpolation and comparison between 14C dated intervals and assumed timing of ‘events’ like age of ‘Marker Bed’ (see above), age of beginning and ending of initial S1 formation (Van Santvoort et al., 1996; De Lange et al., personal communication.) These ages are relatively close to the flooding of the Black Sea which is hypothesized to have occurred approximately 7150 BP ±100 (Ryan et al., 1997). Flooding of the Black Sea is thought to have occurred as a result of the continuous rise in sea level at the end of the last Glacial and resulted in the overflow and subsequent erosion of the sill at the Bosporus. The latter authors demonstrated that previously the Black Sea was fresh water and nearly 130 m lower than at present and was catastrophically flooded in a relatively brief time period. They report possible inflows of 50 km3 of water per day, filling the lake initially at a rate of 10s of cm per day. Any population living on the fertile plains surrounding this freshwater lake, must have suffered catastrophic flooding possibly as described in the Gilgamesh epic and the Noah Flood story (GENESIS 7: 11–24). As a consequence, around 7150 BP farming spread rapidly inland into southeastern Europe (e.g. Hodder, 1990), and light plough and irrigation techniques appear suddenly in the Transcaucasus (Glumac & Anthony, 1991; ref. in: Ryan et al., 1997). The flooding of the Black-Sea freshwater lake will have resulted in a major outflow of freshwater enriched in nutrients; consequently, it will have ‘boosted’ the primary productivity that had probably been decreasing as a result of the declining waterflow from the Nile. This may have generated the double primary peak in productivity recorded in the sediment as the double Ba peak (Figs. 3 and 5). Taking an estimate of the time period (500–1500 yrs) that the influence of the Black Sea freshwater outflow may have persisted in the eastern Mediterranean, it is possible to compare this flux to that of the hypothesized (10× Present) Nile input during sapropel times. Various estimates have been reported for the contemporary input of freshwater from the Nile (e.g. from 900 to 2600 m3/s; Frost, O’Nions, & Goldstein, 1986; Emelyanov & Shimkus, 1986). We adopt here the higher value of 2600 m3/s, which will, if the input was tenfold greater, flux during sapropel times and which indicates the flux was 800 km3/yr. The volume of freshwater in the Black

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Fig. 5. Double Barium peak feature and inferred relative impacts on eastern Mediterranean primary productivity by ‘Nile’ and ‘Black Sea Flooding’ periods. Barium results from two different boxcores taken from the UM26 site during two different cruises, 7 years apart. (*) : Ba/Al ratio multiplied with average Al content of core; (**): Ba* corrected for non-bio-barite. Lightly shaded area assumed to be the result from Nile- induced primary productivity, whereas the black area is assumed to be the result from the Black Sea Flooding.

Sea during the ‘preflood’ era is estimated as follows: total present-day volume is 537.103 km3 (Emelyanov & Shimkus, 1986), during the Flooding the water level rose approx. 100 m (Ryan et al., 1997). Correcting for the shelf areas which are shallower than 100 m and for the difference in water-level, results in an estimated pre-Flood volume of freshwater in the Black Sea to be ca. 500.103 km3. This would result in an estimated freshwater input into the eastern Mediterranean of 1000–333 km3/yr throughout the period of 500–1500 years we have assumed the input to have persisted. If this hypothesis of subsequent freshwater inputs from Nile and Black Sea is correct, then the shape and width of the Ba curve (Figs. 3 and 5) indicates that the effect of such Black Sea flooding is likely to have influenced the eastern Mediterranean rather during a period of 1500 than of 500 years. Similar to the increase in Nile input, the pulse of freshwater input from the Black Sea will have resulted in large increases in nutrients supply, and concomitantly enhanced primary productivity and hence the rate of barium-accumulation in the sediments (Fig. 6). Clearly, future research on sediments from the Aegean Sea should be targeted at clarifying the relative importance of the Black Sea Flooding event to eastern Mediterranean paleoceanographic conditions.

Fig. 6. Profiles versus age in core KC01(B) for Ba/Al, the LA90 insolation curve, ARM-intensities, Susceptibility and δ18O (see also Van Santvoort et al., 1997); magnetic reversals, and the visible evidence for sapropels have been indicated in columns on the left and in the middle of the figure respectively. The total length of this core is over 35 m (figure after Van Santvoort et al., 1997).

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4. Conclusions 앫 The ‘Monsoonal system’ is closely associated with the humidity of the Mediterranean climate; 앫 Sapropel formation only occurs during humid (pluvial) periods associated with times of maximum isolation; 앫 Sapropel formation is always accompanied by enhanced freshwater input which leads to enhanced Ba fluxes to the sediments as a result of basin-wide increases in primary productivity; 앫 The end of sapropel formation is generally marked by the formation of a manganese Marker Bed, in which the enhanced levels of manganese, probably indicate a major change in redox conditions, i.e. a reventilation event; 앫 The occurence of a double peak in barium is consistent with the catastrophic flooding of the freshwater Black-Sea lake around 7150 BP (Ryan et al., 1997) 앫 Mediterranean sediments have excellent time control due to the regular presence of ash-layers and sapropel intervals. 앫 Mediterranean sediments appear to be excellent recorders of the variations in Global paleoclimate which result in monsoonal cycles.

Acknowledgements Captains and crews, and technicians from NIOZ, CNR, and TAAF, are thanked for their cooperation during cruises with R/V Tyro (1991, 1993), R/V Urania (1993, 1994) and Marion Dufresne (1991). H.C. de Waard, D. van der Meent, R. Alink, G.N. Nobbe are acknowledged for their analytical assistence. Two anonymous persons are acknowledged for their thorough reviews. This research was in part supported by the MAST contracts #MAS1-CT90-00022C (MARFLUX), MAS2-CT93005 (PALEOFLUX), MAS3-CT95-0043 (CLIVAMP), MAS3-CT96-00137 (SAP), GdL by SOZ/NWO grants and by GOA grant #750.00.620-7290. This is publication # 990606 of the Netherlands School of Sedimentary Geology. M. Paterne, responsible for the oxygen isotopes, is a participant from the CLIVAMP Mast-3 project, all the other authors are participants from the SAP Mast-3 project. The speculative part in this paper is the responsibility of the first author alone.

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