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A one-dimensional ecosystem numerical model is used to simulate the ecosystem changes that could have occurred in the open ocean areas of the Eastern Mediterranean Sea during the Climatic Optimum interval (9500-6000 B.P., Mercone et al. [Mercone, D., Thomson, J., Croudace, I.W., Siani, G., Pateme, M., Troelstra, S., 2000. Duration of S1, the most recent sapropel in the eastern Mediterranean Sea, as indicated by accelerator mass spectrometry radiocarbon and geochemical evidence. Paleoceanography 15, 336-347]). In this period the S1 sapropel was deposited. S1 is the most recent sapropel in the succession of organic carbon-rich layers intercalated in normal Neogene sedimentary sequences. Different theories have been invoked in order to explain the deposition of this peculiar layer. Our simulations seem to indicate that the modified thermohaline circulation, supplying oxygen only in the first 500 m of the water column, is responsible for the sapropel deposition when higher productivity is allowed in the euphotic zone. The model shows the importance in this process of bacteria that consume oxygen by decomposing the Particulate Organic Matter (POM) produced in the upper water column. The sinking velocity of POM partially regulates the timescale of the occurrence of anoxia at the bottom and in the whole water column, allowing the relatively rapid onset of sapropel deposition. (c) 2005 Elsevier B.V. All rights reserved.

Sapropels are dark, organic matter rich, often laminated sediments that have cyclically been deposited in the Mediterranean area starting from the late Miocene. A general agreement has emerged that sapropel deposition is accompanied by conditions of anoxia or dysoxia and that the recurrence of these layers is related to the Milankovitch precessional cycles. Organic matter accumulation results from the combination of several favoring factors that are, first of all, high primary productivity and enhanced preservational conditions but is also modulated by other factors intrinsic to sedimentation rate, such as dilution, time of residence of the organics at the sediment surface, water depth, detrital or biogenic mineral phases and distance from the shore. Literature dealing with the role of sea-level variations on deposition of Cenozoic sapropels is not abundant and so far limited to the Middle Pliocene, whereas it is more abundant for older successions, generally due to the fact that Paleozoic and Mesozoic black shales are objects of hydrocarbon exploration. The aim of our study is to assess the role of sea-level variations in modulating the depositional record in the different physiographic basin settings (platform, slope, abyssal plain) that characterize siliciclastic systems, since these systems react to sea-level oscillation by changing the balance between the rate of sediment supply and the rate of creation of accommodation space. The 124 ka to present sapropel S5-S1 series in the eastern Mediterranean Sea, correspond to the last Glacial – Interglacial period, and permit detailed investigation of 5th order sedimentary frequencies. The Sapropel layers are clustered in high-frequency Milankovian cycle that occur within sequence stratigraphic 4th order cycles of late Pleistocene-Holocene sea-level oscillations. The sea-level changes had an amplitude exceeding 120 m and have been correlated to the glacial-interglacial cycles modulated by the 100 ka eccentricity. Using published data we examined occurrences of the complete S5-S1 series in the Mediterranean Sea and then we focussed on two stratigraphic sequences in the central Adriatic shelf and Middle Adriatic Depression where sapropels S5 and S1 were deposited during periods of sea-level rise recorded by the Transgressive Systems Tract (Marine Isotopic Stage 5 and 1, respectively). Sapropels S4 and S3 were deposited during the Highstand Systems Tract, that developed throughout MIS 5 of the late Pleistocene sequence. Sapropel S2 should have been deposited during the warm MIS 3.3 and followed the MIS 4 sea-level drop when sediment supply was enhanced during the late highstand and falling stage of sea-level of the late Pleistocene sequence. The pronounced MIS 2 cold stage led to a lowstand and to the regional erosional surface (Sequence Boundary) . We also compared the Adriatic sapropel S5 - S2 sequence (4th order) to the Mid-Pliocene cluster “O” sequence (3rd order). In both cases, sapropels are well formed before the increase of sediment supply due to progradation of the shelf wedge, that occurs in the late high stand and initial falling stage in sea-level. Finally, we considered the influence of sea-level lowering which could cause a weakening of North Adriatic Deep Water formation, therefore influencing the Mediterranean deep water circulation. This condition would favour oxygen depletion, but it does not correspond to a well developed sapropel (as in the case of S2). Based on the assumption that productivity increases related to insolation play a dominant role in their sedimentation, sapropel S2 is the result of low amplitude insolation maxima, that were unable to generate a productivity increase leading to the burial of conspicuous amount of organic matter. Therefore, in the Adriatic Sea the absence of the S2 sapropel is correlated to the high sedimentation rate diluting the signal, whereas in the eastern Mediterranean basin this sapropel is virtually absent because ...

The Mid-Pliocene deposition in the Northern Apennines includes finely laminatedsapropelitic horizons (insolation-cycles i-292 to i-282; 3.058 to 2.983 Myr) intervening within theslope-basin deposition of the foredeep, during a late highstand dated to the Globorotalia bononiensis zone. An integrated sedimentological, geochemical and micropaleontological study of the sapropel-bearing succession allowed the estimation of the time involved in the sapropel layer deposition, andto define the origin of the organic matter and the contribution of the primary productivity. The calcu-lated average sedimentation rate of the section, including the intervening bioturbated beds, wasabout 0.4 mm yr –1 —similar to the rest of the pelitic deposition during the whole Pliocene. Within indi-vidual sapropels, recurrently-spaced 0.3 to 0.6 mm thick laminae consisting of calcareous and/orsiliceous plankton tests, alternating with pyrite- and organic-rich mudstones, were interpreted as therecord of annual variation in the supply. We could thus calculate the duration of individual sapropelsby simply counting the number of couplets occurring in the layers: these spanned between 7500 and10000 yr. The kerogen content of the sapropels is ≥65% marine origin, whereas the same componentin the intervening bioturbated intervals is ~10%. This supports the hypothesis that the sapropeldeposition is linked to significant primary productivity peaks. In contrast, changes in the terrigenoussupply or strong disoxia were not recorded at the bottom. Taking the nutrient supply as constant, pro-ductivity peaks could then be forced mainly by variations in the intensity of the insolation. The latter,is linked to the precession minima and has a wide oscillation; this oscillation occurred even at theend of the Mid-Pliocene warm climate, which was mainly characterized by a progressive climatedeterioration that heralded the Northern Hemisphere glaciation.KEY WORDS: Mid-Pliocene climate · Sapropel deposition · Sedimentation rate · Organic matter ·Primary productivity

Paleoproductivity estimates for a sequence of five mid-Pliocene Mediterranean sapropels illustrate the importance of insolation maxima in enhancing organic carbon accumulation. Well-laminated sapropelitic intervals in the Northern Apennines have been studied by a combination of sedimentological and micropaleontological analysis, detailed electron microscope description, bulk chemical composition, carbon-isotopic composition and elemental ratios. Each sapropel, formed during precessional minima, lasted 7.5 to 10 kyr, which is the same duration calculated for coeval counterparts in ODP sites in the eastern Mediterranean. The organic carbon mass accumulation rates of the studied sapropels show the same values of those calculated in the coeval Mediterranean sapropels, suggesting that the same productivity conditions were controlling sapropel formation in the whole Mediterranean, despite the differences in depositional setting and the strong variations in sedimentation rate. Mat-forming diatoms play an important role in increasing the settling velocity, allowing rapid sinking of organic matter and preventing bacterial remineralization in the water column. The consequent partial oxygen depletion at the seafloor increases preservation, which is therefore considered an effect, rather than a cause of the organic carbon accumulation.