Modes of sapropel formation in the eastern Mediterranean: some constraints based on pyrite properties

Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO²⁻₄ reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(II) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. δ³⁴S values of pyrite are −33‰ to −50‰. Below the sapropels δ³⁴S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels.     

Cyclic variations in the chemical composition of eastern Mediterranean Pliocene sediments: a key for understanding sapropel formation

Two Pliocene Mediterranean sediment cores from Ocean Drilling Program (ODP) Leg 160, Sites 964 (Pisano Plateau, Ionian Basin) and 967 (lower northern slope of Eratosthenes Seamount, south of Cyprus) have been analyzed for major and minor elements (Si, Ti, Al, Fe, Mg, Ca, K, Ba, Cr, Ni, V, Zn, Zr) by X-ray fluorescence analysis at a depth resolution of 3 to 4 cm (a total of 430 samples). Core 964C-9H comprises five sapropels with up to 9.7% total organic-carbon (TOC), one 2.5-m-thick turbidite, and a 50-cm segment of a second turbidite, whereas Core 967B-9H contains eight sapropels with TOC contents of up to 6.7%. Cyclic variations in carbonate content and detrital-matter composition are evident at both sites. At Site 964 (Pisano Plateau) terrigenous detrital-matter chemistry reflects periods of stronger Saharan dust input (higher Si/Al and Zr/Al ratios) alternating with fluvial input from the northern borderlands (higher Mg/Al ratios). Cyclicity is not well-expressed at this location because turbidites and winnowing occur. Site 967 (Eratosthenes Seamount) sediments are strongly influenced by discharge from the Nile River during humid periods (low K/Al and Mg/Al ratios). At Site 967 sapropels occur only during periods of enhanced Nile discharge with correspondingly lower Si/Al, Ti/Al, Mg/Al, K/Al, and Zr/Al ratios. Accumulation rates of carbonate and terrigenous detrital matter were 30% lower during such episodes because of the reduction in eolian input and carbonate production and/or carbonate dissolution. Periodical Ba enrichments, which are not only seen in the sapropels, but in every K/Al minimum, document the association of enhanced bioproductivity and sapropel formation. As calculated from excess Ba concentrations, productivity increased by up to a factor of five during intervals of intensified Nile runoff. Fluviatile discharge seems to be responsible for an enhanced input of nutrients and suspended matter containing low Mg/Al and K/Al ratios. Based on organic-carbon concentrations, episodes of sapropel formation lasted at least from 1000 to 4000 years at Site 964, and from 2000 to 6000 years at Site 967. Periods of enhanced bio-productivity, as defined by Ba enrichments, lasted from 8000 to 12,000 years at Site 967. Episodes of high river discharge and corresponding nutrient input, as well as conditions of sapropel formation, seem to have lasted longer in the eastern part of the basin (Site 967) than at the Pisano Plateau (Site 964).     

Middle Miocene (Langhian) sapropel formation in the easternmost Mediterranean deep-water basin: Evidence from northern Cyprus

Mid-Miocene (Langhian; ∼15.4 Ma) sapropels formed within the easternmost Mediterranean basin, now uplifted in northern Cyprus. These sapropels represent the oldest known sapropels in a predominantly marl succession. Six well-developed sapropels were studied. Strontium isotope dating of twelve samples gave a preferred age of ∼15.4 Ma (Langhian); i.e. during the final phases of the Middle Miocene Climatic Optimum (MCO). The age of the best-preserved nannofossil assemblage (Langhian) is close to the strontium ages. The Langhian strontium ages are preferred over an alternative early Serravallian age for less well-preserved nannofossil assemblages. Total organic carbon contents in the sapropels reach maximum values of 3.9 wt.%. Relative to the host marls, the sapropels show enrichments in terrigenous-derived minerals and related major and trace elements. Sedimentological evidence indicates that the terrigenous sediments were eroded from the northern borderlands of the deep-water basin under warm, humid conditions. High fresh-water run-off from surrounding landmasses is likely to have promoted a low-salinity lid to the eastern Mediterranean deep-water basin. This, in turn, would have restricted deep-water ventilation and promoted widespread anoxia. Exceptionally high concentrations of chalcophile elements (e.g. Cu, Ni and Zn) are consistent with anoxic conditions. Abundant nutrient-rich fresh-water input is also likely to have stimulated siliceous productivity (although any siliceous microfossils did not survive diagenesis). A significant role for diagenesis in sapropel formation is indicated by the mobilisation of Ba from sapropels to marl directly beneath. Orbitally induced dry–wet oscillation, the mechanism invoked to explain the Pliocene to Holocene sapropels, apparently was already in place during the latest stages of the MCO when the Langhian sapropels accumulated. These sapropels accumulated immediately after the Middle Miocene closure of the Southern Neotethys when the Eastern Mediterranean Sea apparently became more sensitive to orbital cyclicity. The development of a semi-enclosed deep-water basin was, therefore, a prerequisite for sapropel formation.     

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.     

长链烯酮在西北大西洋重建全新世气候变化的研究进展

大量研究表明,长链烯酮不饱和度(UK37?指数)可用于重建过去的海面温度(sea surface tempera-ture,SST),然而是否存在其他因素导致实际温度与重建的SST之间存在差异,以及重建的温度是代表年均SST还是季节性温度都仍有争议.除了反映温度,在受季节性海冰影响或SST较低的区域,长链烯酮C37:4...     

Deposition of sapropel S1 sediments in oxic pelagic and anoxic brine environments in the eastern Mediterranean: differences in diagenesis and preservation

Sediments from a boxcore in the previously anoxic brine-filled Poseidon Basin, eastern Mediterranean, have been studied and compared to sediments deposited in a `normal' eastern Mediterranean environment. The boxcore can be divided into three main sedimentary intervals based on AMS-radiocarbon ages, foraminiferal and geochemical zonations. From the base of the core upwards these are: (1) 12.3–31.2 cm, organic-rich sediments redeposited from within the brine; (2) 6.6–12.3 cm, sediment containing a `cold' foraminifera fauna redeposited from above the brine into the basin while the brine was still present; (3) 0–6.6 cm, oxic pelagic sediment accumulated since the reoxygenation of Poseidon Basin which occurred ∼1800 yrs BP. Near the base of the latter unit, a Mn-oxide peak has formed and it marks the present boundary between oxic and suboxic environments. A progressive downward oxidation front, which is usually found in `normal' sapropel S1 sediments, has never formed in Poseidon Basin sediments. This has resulted in the preservation of the relationship between organic carbon and organic-related trace elements, e.g. Se, in the organic-rich sediments of Poseidon Basin, whereas such a relationship has been obliterated in `normal' sapropel S1 sediments. On the basis of the carbonate content as well as the Sr/Ca ratio, preservation of carbonates appears to be better in the brine sapropel sediments of BC15 than it is in `normal' sapropel S1 sediments. The high opal content of BC15 shows that biogenic opal is also much better preserved. The overall lower C<sub>org</sub>/Ba ratio in BC15 suggests a better preservation of barite relative to that of organic carbon in shallow brine sediments, but is as yet inconclusive for the organic carbon preservation potential of brine relative to `normal' unoxidised sediments.     

Physical and hydrological characteristics of climatic frontal zones in the Tropical Atlantic

Climatic frontal zones are selected in the thermohaline fields of the Tropical Atlantic by analyzing the many-year-average seasonal database reduced to the nodes of a one-degree grid. We determine physical characteristics of the frontal zones, study their spatial and temporal variability, and reveal basic regularities of the appearance of frontal zones in the fields of thermohaline characteristics. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

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.     

Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river

The polychaete fauna of muddy bottoms off the Rhone delta (NW Mediterranean) was seasonally sampled at two sites at 70 m depth, from 1993 to 1996. During this period, five severe flood events occurred. A clustering analysis (distance coefficient of Whittaker) and three way fixed factor ANOVAs (site x season x year) showed strong year-to-year changes in species density and community structure, changes that masked seasonal variations. Total density increased by a factor of 3 and density of most species significantly increased following the first flooding event. Changes in the community structure were due to the successive dominance of a few species. The opportunistic species, such as Cossura sp., Mediomastus sp. and Polycirrus sp., exhibited peaks in density 1-3 months after the flood. These peaks were followed by a drastic decline. For species with a long life span, such as Sternaspis scutata, a slower but continuous increase in density was observed which was maintained for several months. The density fluctuations of these species exhibited a good correlation with river flow with time lags of 1-2 years. The successional dynamics observed are explained according to the geographical origin of the floods and the biology and feeding ecology of species.     

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.     

Specific features of the 2010 summer heat formation in the European territory of Russia in the context of general climate changes and climate anomalies

Specific features of the extreme summer heat of 2010 in the European part of Russia are analyzed against the background of global and regional climate changes taking into account antropogenic influences and natural anomalies related, in particular, to the El Niño/La Niña phenomena. The tendencies of the characteristics of the activity of blocking anticyclones (blockings) responsible for the formation of drought regimes and the increase in the fire hazard at midlatitudes are estimated in connection with climate changes.     

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.     

Arsenic in sediments of the North Atlantic Ocean and the Eastern Mediterranean Sea

Chemical extraction techniques show that the majority of the arsenic in North Atlantic deep-sea sediments is associated with an iron phase compositionally similar to that found in deep-sea ferromanganese nodules (As/Fe ～ 11 · 10^?4) and is probably of seawater origin. Some sediments also contain As associated with Fe oxides produced by continental weathering. A minority (～8%) of the arsenic is of detrital origin but is not associated with Fe or Mn oxides; it has a content (1.7 ppm) similar to the average crustal abundance. In the Eastern Mediterranean Sea, near-shore sediments contain As associated with land-derived Fe oxides (As/Fe ～ 2 · 10^?4), but As/Fe ratios increase to ～ 13 · 10^?4 in deep-sea sediments as the contribution of seawater derived arsenic becomes dominant. Arsenic is enriched in metalliferous sediments (As/Fe ～ 20?50 · 10^?4) but As/P ratios of metalliferous sediments, deep-sea ferromanganese nodules and deep-ocean water are all similar. Although a hydrothermal contribution cannot be discounted, it is likely that the arsenic is also of seawater origin, suggesting that hydrothermal iron oxyhydroxides remove As more efficiently from seawater than do iron phases (goethite) in deep-sea sediments and nodules. Arsenic accumulates in deep-sea sediments (～ 6 μg cm^?2 10^?3 yr^?1) at sediments (～ 120 μg cm^?2 10^?3 yr^?1) at rate sufficient to balance river input input (～3 · 10¹⁰ g yr^?1). These estimates give an oceanic residence time for arsenic of 1–2 · 10⁵ yr.     

Habitat changes in coastal wetlands associated to hydrological changes in the watershed

In recent decades wetlands associated with the Mar Menor lagoon (SE Spain) have undergone several hydrological and biological changes as a consequence of increased water inputs from agricultural drainage off the watershed. Changes in the vegetal communities were studied by means of supervised classification of Landsat images, and TM and ETM+ sensors during the period 1984–2000, and a series of land cover maps was created. Changes in the wetland area during this period were analysed and a good fit was found with the total area of irrigated lands in the watershed. The series of land cover maps also allowed the study of the internal composition of the vegetal communities in each wetland. Important changes were detected, with an expansion of reedbeds and a significant reduction in the area occupied by priority habitats according to the Habitats Directive.     

Metabolism of Centropages species in the Mediterranean Sea and the North Atlantic Ocean

Information on the metabolism rates of Centropages typicus and congeneric species (C. hamatus, C. furcatus, C. brachiatus and C. abdominalis) in neritic areas of the Mediterranean Sea, the North Atlantic Ocean and the Pacific Ocean are reported here. Respiration rates and excretion rates are strongly influenced by abiotic (i.e. temperature, salinity) and biotic factors (i.e. food availability and composition). Differences in the response of respiratory rates to temperature of acclimated, acclimatized and adapted individuals are clearly observed among regions of the Mediterranean Sea and the West and East shores of the Atlantic Ocean. Food supply also strongly affects respiration and excretion rates, as well as the size, sex and stage development of the individuals. The co-measurement of these two rates allows confirmation of the omnivory or carnivory oriented feeding habits of these species. The role of this neritic genus in coastal environment is also discussed.     

Palaeoproductivity and post-depositional aerobic organic matter decay reflected by dinoflagellate cyst assemblages of the Eastern Mediterranean S1 sapropel

In the reconstruction of bioproductivity in surface waters the extent to which a proxy has been diagenetically altered is often a matter of debate. Here we investigate how organic- and calcareous-walled dinoflagellate cysts can be used for separately estimating bioproductivity and oxygen related diagenesis. This is achieved by studying the cyst content of the most recent Eastern Mediterranean sapropel S1, that is thought to have been deposited under conditions of increased primary production in surface waters and possible anoxia in the bottom waters. Based on chemical evidence, it has been shown that the visible sapropelic layer represents only the residual lower part of what was initially a much thicker sapropel, as a result of post-depositional decay of organic matter related to oxygen penetration into the sediments. The effect of aerobic organic matter decay on the cyst associations is studied through the comparison of the unaffected, lower part of the initial sapropel and the ‘oxidised’ upper part. Comparing the unaffected sapropelic sediments with pre- and post-sapropelic material gives insight into the relationship between fossil cysts assemblages and palaeoproductivity.Impagidinium aculeatum, Impagidinium patulum, Operculodinium israelianum, Polysphaeridium zoharyi and probably Impagidinium spp., Impagidinium paradoxum and Nematosphaeropsis labyrinthus are very resistant against aerobic decay and their accumulation rates appear to be primarily related to productivity in surface waters. Protoperidinium and Echinidinium species, on the other hand, are shown to be very sensitive and can be used to recognise oxygen-related decay. The calcareous-walled dinoflagellate cysts seem to be unaffected by oxic organic matter decay in Mediterranean sediments.