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 ...

Role of sea-level forced sedimentary processes on the distribution of organic carbon-rich marine sediments: a review of the Late Quaternary sapropels in the Mediterranean Sea.

CAPOZZI, ROSSELLA;
2009

Abstract

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 ...
2009
IAS 2009, 27th Meeting, Alghero 20-23 September 2009, Abstract Book,
98
98
Capozzi R.; Negri A.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/78667
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