Eastern Mediterranean Sapropels Research Articles

Organic matter and trace element rich sapropels and black shales: a geochemical comparison

A distinct Pliocene eastern Mediterranean sapropel (i-282), recovered from three Ocean Drilling Program (ODP) Leg 160 Sites, has been investigated for its organic and inorganic composition. This sapropel is characterized by high organic carbon (C org) and trace element contents, and the presence of isorenieratene derivatives. The latter suggests that the base of the photic zone was sulphidic during formation of the sapropel. Combined with evidence of bottom water anoxia (preservation of laminae, high redox-sensitive trace element contents, and the abundance and isotopic composition of pyrite) this leads to the tentative conclusion that almost the entire water column may have been anoxic. This anoxia resulted from high productivity and not from stagnation, because an approximation of the trace element budget during sapropel formation shows that water exchange with the western Mediterranean is needed. Entire water column anoxia has been suggested earlier for several black shales. With regard to the depositional environment and the C org content, however, only the Cenomanian/Turonian Boundary Event (CTBE) black shales appear to be comparable to this sapropel. The proposed trace element removal mechanism of scavenging and (co-)precipitation in an anoxic water column, is thought to be similar for both types of deposits. The ultimate trace element source for the sapropel, however, is seawater, whereas it is hydrothermal and fluvial input for CTBE black shales (because they have a larger temporal and spatial distribution). Nonetheless, the C org-rich eastern Mediterranean Pliocene sapropel discussed here may be considered to be a younger analogue of CTBE black shales.

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.

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.

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 (O 2, NO − 3, Mn 2+ and Fe 2+) 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.

Sulphur Enrichment in Organic Matter of Eastern Mediterranean Sapropels: A Study of Sulphur Isotope Partitioning

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmMS) in eastern Mediterranean sediments containing three sapropels of different ages and with different organic carbon contents (sapropel S1 in core UM26, formed from 5–9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum organic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Sulphur isotopic compositions (δ34S) of the organic matter ranged from -29.5 to +15.8‰ and the atomic S/C ratio was 0.005 to 0.038. The organic sulphur in the sediments is a mixture of sulphur derived from (1) incorporation of 34S-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur with an isotopic signature close to seawater sulphate. The calculated biosynthetic fraction of organic sulphur in non-sapropelic sediments ranges from 68–87%. The biosynthetic fraction of the organic sulphur of the sapropels (60–22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and thus an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.

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 14C nc 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.

Sulphur Isotope Partitioning during Formation of Pyrite and Incorporation of Sulphur into Organic Matter in Eastern Mediterranean Sapropels

The Neogene sedimentary record of the eastern Mediterranean is characterized by the occurrence of organic-rich layers. The formation of these 'sapropels' is thought to have been initiated by palaeoclimatic-related increased productivity in the surface waters and/or enhanced preservation of organic matter (OM) due to oxygen-depletion in stagnant bottomwaters. The sapropels contain high contents of inorganic reduced sulphur species (mainly pyrite) derived from the microbial dissimilatory sulphate reduction (Passier et al., 1998). Under conditions of high abundant dissolved reduced or intermediate sulphur species, OM can be sulphidized leading to a reduction of the lability and thus an enhancement of the overall preservation of OM (e.g. Sinninghe Damst6 and de Leeuw, 1990). Here, we present the contents and sulphur isotopic signatures of pyrite and sulphur in OM. A model for organic sulphur formation and the timing with respect to pyrite formation is proposed.

Pyrite contents, microtextures, and sulfur isotopes in relation to formation of the youngest eastern Mediterranean sapropel

Pyrite within and below sapropels in the eastern Mediterranean is a result of microbial SO42− reduction within the sapropel, and the subsequent reaction of sulfide (HS−) with detrital Fe and Fe2+ diffusing upward from underlying sediments. Below the youngest Mediterranean sapropel, S1, pyrite (as much as 281 µmol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 µmol pyritic S/g) has been detected. Framboidal microtextures indicate pyrite formation at the site of HS− production within the sapropel. Euhedral pyrite, below the sapropel, forms when sulfate reduction in the sapropel outcompetes iron liberation and supply, and HS− diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (−37.3‰ to −38.2‰) as well as below the sapropel (−45.6‰ and −49.6‰), indicating that HS− has formed in a system with abundant SO42− and in the presence of oxidants.

Diagenetic pyritisation under eastern Mediterranean sapropels caused by downward sulphide diffusion

Recurrent organic-rich layers (sapropels) in eastern Mediterranean sediments are enriched in C org, Fe, and S. Sulphur and Fe are enriched in a zone immediately below the sapropels, whereas C org is not. δ 34S values of bulk sediments and simple mass-balance calculations indicate that SO 4 2− reduction has taken place in an open system, with all HS − formed at, or close to, the sediment surface. Formation of pyrite in the sapropel was Fe-limited and consequently, excess HS − was able to migrate downwards (downward sulphidisation). This resulted in the formation of pyrite below the sapropel by reaction of this HS − with solid-phase ferric iron and Fe 2+ diffusing upwards from underlying sediments. The Fe 2+ source probably includes Fe (hydr) oxide layers formed at former oxidation fronts above previously deposited and buried sapropels. This downward sulphidisation mechanism allows accumulation of twice as much S in alternating organic-rich-anoxic/organic-poor-suboxic sediments compared to what is preserved in organic-rich anoxic sediments.

Redistribution and geochemical behaviour of redox-sensitive elements around S1, the most recent eastern Mediterranean sapropel

The youngest eastern Mediterranean sapropel (S1) is generally believed to have formed 7–9 14C ky B.P. The distribution of redox-sensitive elements above, in and below the sharply defined, organic-rich unit reveal that S 1 has already suffered heavy diagenetic alteration. Its present thickness, based on its dark colour, is only a fraction of its original thickness. In one core, visual evidence of S1 with an inferred initial thickness of 6 cm has been destroyed, and a similar thickness appears to have been lost from the tops of another two examples. This is caused by oxidation of the upper reaches of S 1 by a progressive oxidation front which commenced 5.2–5.4 14C ky ago, and still appears to be active in the three cores studied. Diagenetic relocation of many redox-sensitive elements has occurred. Most elements expected to be enriched in sapropels or sulphide-rich sediments already have maximum concentrations above or below, rather than in, the present S 1 layers. Similarities are observed in the geochemical behaviour of As, Fe, and P; of Co, Cu, Mn, Ni, and Zn; and of Cd, Mo, Sb, TI, U, and V. The behaviour of Ba is distinctive, and its immobility as barite makes it an indicator for palaeoproductivity and the original high S1 Corg content. The elements Se and I exhibit particularly large, sharp concentration peaks, and together are proposed as useful indicators of the present boundary between oxic and post-oxic conditions.

Improved preservation of organic matter and pollen in eastern Mediterranean sapropels

Here we investigate the processes leading to high organic content and high pollen concentration in pelagic marine sediments. Three marine cores near the base of the Nile River cone display the classical Quaternary sequence of sapropels and hemipelagic muds. Large variations in total pollen concentration, as well as in percentage composition of the pollen assemblage are clearly linked to the two lithological types. All pollen types have much higher concentration in sapropels than in muds, by 3 or 4 orders of magnitude. Within sapropels, the percentages of Gymnosperms (Pinus and Cedrus) pollen and Pteridophyte spores, are very low and the pollen spectrum is dominated by various Angiosperms. These variations in total pollen concentration as well as in percentages cannot be accounted for by variations in pollen production, or in efficiency of transport, because they similarly involve pollen of different areas and climates, transported to the sea by different means. The cause for these variations can only be a difference in preservation of pollen. Preservation is good in sapropels for all pollen types, but generally poor in muds. We believe that sapropels were deposited under reducing bottom waters which allowed a better preservation of the organic matter including pollen.

Review and new aspects concerning the formation of eastern Mediterranean sapropels

In this paper, it is proposed that the formation of eastern Mediterranean sapropels occurred in an anti-estuarine type of circulation, which was to some degree weakened relative to the present in response to reduction of the eastern Mediterranean excess of evaporation over freshwater input. This reduction of excess evaporation would have been imposed by intensifications of (1) the Indian Ocean summer (SW) monsoon, influencing the eastern Mediterranean via increased Nile discharge, and (2) the system of Mediterranean depressions (an element of the westerly Atlantic system) causing increased precipitation and decreased evaporation. Both the Indian Ocean summer monsoon, and the westerly Atlantic system, would be intensified in response to the occurrence of distinct minima in the cycle of precession.It is demonstrated that reduced excess evaporation, whether or not coinciding with global phases of deglaciation, would lead to reduction of surface water salinities in the eastern Mediterranean, causing “isolation” of previously formed high salinity (cooler) deep water. Thus, mixing was severely reduced, possibly even restricted to eddy diffusion, with only occasional convective events that, because of the existing density gradient, hardly ever would reach the deepest parts of the basin. The consequently diminished oxygen advection down from a few hundred meters in the water column, favoured preservation of the sinking organic matter. This would, however, suffice only to enable the formation of sapropels with low organic carbon contents. The high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production.The described scenario accounts for previously reported (1) increases of organic carbon content with increasing depth of deposition within individual sapropels, and (2) asymmetrical sequences of sapropel deposition, characterized by gradual build-up and rather abrupt ending. It is, furthermore, in agreement with (3) isotopic and faunal reconstructions of the history of exchange transports through both the Strait of Sicily, and the Strait of Gibraltar, and (4) faunal and floral reflections of the presence or absence of a distinct Deep Chlorophyll Maximum with its associated increases in export production, indicating the presence or absence of a shallow pycnocline within the euphotic layer. Moreover, the described scenario (5) is in no way conflicting with the reports of sapropels in the western Mediterranean, and (6) seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation, amidst anoxic (benthic desert) levels, a situation that has been observed in a few Quaternary and Pliocene sapropels.

Modification and complete removal of eastern Mediterranean sapropels by postdepositional oxidation

Research Article| May 01, 1994 Modification and complete removal of eastern Mediterranean sapropels by postdepositional oxidation N. C. Higgs; N. C. Higgs 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom Search for other works by this author on: GSW Google Scholar J. Thomson; J. Thomson 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom Search for other works by this author on: GSW Google Scholar T.R.S. Wilson; T.R.S. Wilson 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom Search for other works by this author on: GSW Google Scholar I. W. Croudace I. W. Croudace 2Department of Geology, The University, Southampton S09 5NH, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information N. C. Higgs 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom J. Thomson 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom T.R.S. Wilson 1Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB, United Kingdom I. W. Croudace 2Department of Geology, The University, Southampton S09 5NH, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (5): 423–426. https://doi.org/10.1130/0091-7613(1994)022<0423:MACROE>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. C. Higgs, J. Thomson, T.R.S. Wilson, I. W. Croudace; Modification and complete removal of eastern Mediterranean sapropels by postdepositional oxidation. Geology 1994;; 22 (5): 423–426. doi: https://doi.org/10.1130/0091-7613(1994)022<0423:MACROE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sapropels, unusually dark, organic-rich sediment layers whose origin is controversial, are widespread in the eastern Mediterranean. The upper surfaces of these layers appear sharp and show little evidence of bioturbation. It has been suggested that postdepositional oxidation may have removed the dark coloration from the upper part of the original layer, so that the top of the visible layer now lies somewhat below the original position of the sapropel top. Such a diagenetic alteration has considerable implications for the interpretation of the sedimentary record, where the tops of dark layers are usually correlated with the cessation of sapropel formation. Geochemical data demonstrate that all visual evidence of a thin example of the most recent eastern Mediterranean sapropel has been removed from the sedimentary record by oxidation processes. Such a process may have thinned all sapropel units by several centimetres. The most recent period of sapropel formation probably ended as recently as 5 ka (cf. cited values between 6.0 and 7.6 ka). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Calcareous nannofossils and the origin of eastern Mediterranean sapropels

A quantitative analysis of calcareous nannofossil assemblages was carried out, with an optical microscope, on four deep‐sea cores raised from the eastern Mediterranean and selected to form a NW‐SE transect. A total of 748 samples were studied, including a very detailed analysis of sapropels S5 and S6 in two, out of the four cores. This paper is restricted to the discussion of the results obtained on Florisphaera profunda which provided important insights on the mechanisms of sapropel formation, an intricated and unravelled puzzle for the scientists of the last decades. On the basis of abundance variations of F. profunda, the only species of calcareous nannoplankton which inhabits the lower part of the photic zone, I suggest that sapropel formation was triggered by increased primary production, either confined to a Deep Chlorophyll Maximum (DCM) (a concept previously introduced by Rohling and Gieskes, [1989]) or extended to a thicker layer of the photic zone. When increased primary production was confined to the DCM (sapropel S3, S4, S5, S7, and S1, this latter only in the eastern part of the basin), the abundance of F. profunda was enhanced greatly with respect to all other species living in the higher part of the photic zone. On the contrary, when the increased primary production was extended to a thicker layer of the photic zone (“glacial” sapropels S6 and S8) the production of most nannoplankton species was favored, decreasing the relative proportion of F. profunda.

Evidence from nitrogen isotope ratios for enhanced productivity during formation of eastern Mediterranean sapropels

THE formation of horizons of organic-rich sediment (sapropels) in the eastern Mediterranean during the Holocene and Upper Pleistocene1 has been ascribed2 to enhanced preservation of organic carbon in bottom waters rendered anoxic by the restriction of deep-water renewal, either by lowered sea level during glacial maximum conditions3 or by the presence of low-salinity surface waters derived from increased runoff2,4. A second possibility is that the sapropels formed as a consequence of higher settling fluxes of organic matter caused by increased primary production connected with the increased runoff5,6. Here we report that in a sediment core from close to the mouth of the Nile, the sapropels have significantly lighter nitrogen isotope ratios (15N/<14N) than the intercalated marl oozes. We adduce evidence that these large differences cannot be caused either by variable mixtures of marine and terrestrial organic matter having different isotopic compositions, or by differences in the extent and type of post-depositional alteration. The differences are consistent, however, with a higher utilization of dissolved nitrate during sapropel formation, implying that the sapropels were ultimately formed as a consequence of high plankton production, which led to high fluxes of organic matter to the sea floor.

Organic carbon accumulation in Bannock Basin: Evaluating the role of productivity in the formation of eastern Mediterranean sapropels

Three sediment cores from Bannock Basin, a hypersaline anoxic basin in the eastern Mediterranean, have been studied to evaluate the importance of productivity on organic carbon accumulation and sapropel formation in this region. Organic carbon accumulation rates were determined for two cores from within the anoxic, hypersaline zone and for one core from the oxygenated portion of this basin. Organic carbon accumulation. Rates are higher by an order of magnitude in the surficial sediments of the anoxic zone due to better preservation. An empirically derived preservation factor, which allows calculation of paleoproductivity during the deposition of the most recent sapropel (SI), indicates that productivity increased by an order of magnitude during the formation of SI. We conclude that increased productivity was coupled with a density stratification and a circulation reversal in the eastern Mediterranean, which together led to sapropel formation.

A Late Quaternary Stratigraphic Framework for Eastern Mediterranean Sapropel S1 Based on AMS 14C Dates and Stable Oxygen Isotopes

Detailed stable oxygen isotope analyses coupled with AMS 14C measurements on an eastern Mediterranean sapropel S1 sequence indicate that adverse bottom conditions persisted for ca 8000 years. AMS dates on additional sequences show that complete bottom anoxia lasted for 300-800 years. The S1 event is not synchronous throughout the eastern Mediterranean, but started earlier in the deeper parts of the basin.

Paleoenvironmental environmental changes from stable isotopes in planktonic Foraminifera from eastern Mediterranean sapropels : Ganssen, Gerald and S.R. Troelstra, 1987. Mar. Geol., 75(1–4):221–230

Paleoenvironmental environmental changes from stable isotopes in planktonic Foraminifera from eastern Mediterranean sapropels : Ganssen, Gerald and S.R. Troelstra, 1987. Mar. Geol., 75(1–4):221–230

The formation of Eastern Mediterranean sapropels

Sapropel formation in the Eastern Mediterranean is explained by an oceanographic approach. It is suggested that sapropels are not only a consequence of eustatic sea-level changes as earlier proposed by Olausson, a model which is widely accepted, but rather a result of a very sensitive reaction of the Eastern Mediterranean to any small climatic amelioration. Present-day aeration of the deep Eastern Mediterranean basin results from circulation of oxygen-rich deep water originating mainly in the Adriatic. Assuming present-day conditions, a climatic amelioration towards more humid and warmer conditions leading to an increase of the water temperature in the Adriatic Basin by 0.7°C or to a decrease of its salinity by only 0.2‰ will cause stratification of water masses and enhance sapropel formation.

Sapropelic deposits in a sediment from the Guinea Basin, South Atlantic

Until recently it has been generally thought that a stagnant water column is a prerequisite to the formation of organic-rich deposits in deep-water marine sediments, a belief best exemplified by studies on the eastern Mediterranean sapropels. Many workers have assumed that, following the hydrological isolation of the deep waters, stagnation of the eastern Mediterranean basins and the concomitant development of anoxic conditions occurred before the deposition of the sapropelic layers1–10. Opposing arguments suggesting a role for increased marine productivity have, however, been put forward, both specifically with respect to the eastern Mediterranean sapropels11,12 and more generally regarding deep-water organic-rich deposits13–15. We report here the discovery of sapropelic layers (>0.5% organic carbon)16 in a recent sediment core from a deep-water site in the Guinea Basin. It is thought most unlikely that stagnant conditions could prevail in the water column at this site, and preliminary analyses of the sediments suggest that the organic-rich deposits have been a direct result of increased productivity in the overlying water column, the most recent occurring about 18,000 yr ago and coinciding with the last glacial maximum.