Eastern Mediterranean Sapropel S1 Research Articles

Evidence from molybdenum and iron isotopes and molybdenum–uranium covariation for sulphidic bottom waters during Eastern Mediterranean sapropel S1 formation

Redox exerts a critical control on organic carbon-rich sedimentation. This is particularly true for Eastern Mediterranean sapropels where seawater stratification is regarded as a major driving force for oxygen depletion, but in which sulphidic (euxinic) bottom waters occur only sporadically. Here we apply a powerful array of geochemical proxies (Fe and Mo stable isotopes together with Mo/U ratios and redox sensitive trace elements (RSTE)) to the determination of water redox evolution during the deposition of Holocene S1 sapropel and its underlying and overlying sediments (ODP core 967D; 2550 m depth).RSTE are asymmetrically distributed within the sapropel, with peak enrichments occurring in its lower (early) part. Negative correlations are found between δFe57 and both Fe/Al and S wt% in the lower sapropel, and are consistent with the benthic Fe shuttle model Fe enrichment in euxinic basins. MoEF/UEF enrichment factor variations show well defined trends identical to those proposed for open marine settings, in which sub-oxic conditions in the background sediments give way to sulphidic waters at the RSTE peak in the lower sapropel. The most notable features of the Mo isotope profile are ‘atypically’ light values (δMo98/95<−0.7‰) in the lower sapropel. Such light Mo isotope values (relative to sea water δMo98/95=2.3‰) have been related to oxic remobilisation. However, negative correlations between δMo98/95 and Fe/Al, Ba/Al, Mo/Al and S imply that the lowest Mo isotopic compositions are associated with peak reducing conditions. Taken in conjunction with the evidence from the other proxies for a sulphidic water column, the light Mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters ([H2S]aq <10 μM). The data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the Holocene sapropel S1. Molybdenum isotopes and Ba/Al ratios identify a short-lived sapropel re-ventilation event timed to coincide with the 8.2 ka cold climatic Event.

Conflicting coccolithophore and geochemical evidence for productivity levels in the Eastern Mediterranean sapropel S1

Abstract The cyclic development of anoxic conditions in the eastern Mediterranean deep sea waters is one of the most fascinating research topics in paleoceanographic studies. In combination with bottom water stagnation, enhanced primary production is a common explanation for the deposition of organic-rich layers (sapropels). This is supported by extensive evidence from both geochemical and micropaleontological studies. The correspondence of recent sapropel layers with peaks of the lower photic zone coccolithophore species Florisphaera profunda has been interpreted as a proxy for the development of a deep chlorophyll maximum (DCM), due to the pycnocline/nutricline shallowing into the lower part of the photic zone. We present millennial-scale data for coccolithophore assemblages from sediments across the most recent sapropel (S1), in the ODP Hole 964B drilled in the Ionian Sea. Relative and absolute abundances of taxa are compared with selected elemental composition of the bulk sediments. The Mn/Al and Ba/Al profiles are used to determine the original thickness of the S1 interval, and show that the upper part of S1 was affected by post-depositional oxidation of organic matter. The Nannofossil Accumulation Rate, defined by the number of coccoliths·cm− 2·kyr− 1, suggests that there is no evidence of increased productivity within most of the sapropel layer. In fact, coccolithophore production was at its minimum in the lower part. Minimum coccolith concentrations are reached despite the increase in F. profunda in both relative and absolute abundance. We suggest that the DCM deduced from the increased productivity of this species did not significantly contribute to the putative overall increased primary productivity during the deposition of most of the sapropel layer. Within the upper oxidized part of S1, coccolith accumulation was at least five times higher than in the lower part. This period of high coccolith productivity finds a counterpart in the increase of the Ba/Al ratio. The total concentration of coccoliths is again controlled by the amount of E. huxleyi, but it is also supported by concomitant increases in all the other groups, suggesting that coccolithophore productivity increased throughout the year and through the total vertical extent of the photic zone. At site 964, this is apparently the only moment when coccolithophores contributed substantially to the increased primary productivity generally assumed for the S1 layer.

Exploring preserved fossil dinoflagellate and haptophyte DNA signatures to infer ecological and environmental changes during deposition of sapropel S1 in the eastern Mediterranean

In this study we used a comparative multiproxy survey (fossil DNA, calcareous nannofossils, and lipid biomarkers) to test whether preserved genetic signatures provide an accurate view of haptophyte and dinoflagellate populations during deposition of the eastern Mediterranean sapropel S1 and the organic carbon‐depleted oxidized marls flanking the S1 and to see if we could identify important environmental indicator species that did not fossilize and escaped previous microscopic identification. The marls above and below the S1 contained low concentrations of lipid biomarkers diagnostic for dinoflagellates and haptophytes (i.e., dinosterol and long‐chain alkenones), but 500 base pair long ribosomal DNA fragments of these protists were below the detection limit. In contrast, dinoflagellate and haptophyte DNA could be recovered from the organic carbon‐rich S1, but the most abundant sequences did not represent species that were part of the nannofossil (this study) or previously described dinocyst composition. The oldest section of S1 (9.8 to ∼8 14C kyr B.P.) revealed a predominance of dinoflagellate phylotypes, which were previously only detected in anoxic Black Sea sediments. In the same section of the core, the most abundant haptophyte sequence showed highest similarity with uncultivated haptophytes that were previously shown to grow mixotrophically as predators of picocyanobacteria, an adaptation that promotes growth in oligotrophic marine waters. Sequences with highest similarities to clones found in marine surface waters predominated in the S1 after ∼8 14C kyr B.P. We discuss whether the shifts in haptophyte and dinoflagellate populations inferred from the preserved DNA reflect known environmental changes that occurred during the formation of sapropel S1.

Short-time-scale variability in ventilation and export productivity during the formation of Mediterranean sapropel S1

[1] High-resolution laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) scanning of laminated sediments from the Urania basin is used to investigate short-time-scale variability in export productivity and redox conditions during the formation of eastern Mediterranean sapropel S1. Sedimentary enrichments of molybdenum (Mo), vanadium (V), and uranium (U) reflect deep-water redox conditions, most likely those near to the seawater-brine interface, while enrichment of Ba is related to biogenic barite and hence to export productivity. The enrichments of all four elements show strong variability on multidecadal to multicentennial time scales throughout S1. A partial decoupling of export productivity from redox conditions at the height of sapropel formation suggests that hydrographic changes, i.e., a variable ventilation rate of the eastern Mediterranean, played an important role in determining deep-water redox conditions. A pronounced switch is observed in the enrichments of redox-sensitive trace metals, from dominantly 300–600 year variability during early S1 to dominantly 100–300 year variability during late S1, indicating a change in the mean frequency of variability in the ventilation rate. The presence of a similar shift in the frequency of tropical and extratropical climate records at this time suggests that ventilation of the eastern Mediterranean was coupled to global climate variability.

Abundant Sr-rich aragonite in eastern Mediterranean sapropel S1: Diagenetic vs. detrital/biogenic origin

CaCO 3 contents in eastern Mediterranean sapropels are generally lower than in the enclosing marl sediments. In sediments of a transect from ∼ 650 to 2100 m water depth at the Sirte continental slope, the opposite feature was observed. Although the increased Sr/Ca ratios observed in these sediments agree with such feature observed in other eastern Mediterranean S1 sediments, their absolute magnitude is much higher. The enhanced Sr/Ca ratio in these sediments is associated with their aragonite content, which for the Sirte transect reaches levels of up to ∼ 40 wt.%. The aragonite content and Sr/Ca ratios in the S1 sediments of this transect decrease with increasing water depth and decrease with distance to the African coast for all eastern Mediterranean cores. In view of the lack of a coherent relationship with sedimentary reduced sulphur contents and porosity, and considering the major amount of aragonite found specifically in the sediments of the Sirte transect, authigenic precipitation can contribute only a limited fraction at most. SEM observation and electron-microprobe analyses showed that the needles and needle clusters are morphologically aragonite, contain the highest Sr/Ca ratio, and are similar to skeleton fragments of the green alga Halimeda. Consequently, a detrital/biogenic source is the likely mechanism for the major part of the aragonite enrichments found in S1 sediments. Possibly, offshore-directed surface water flows related to wind stress and/or enhanced run-off (consistent with enhanced precipitation during sapropel S1) may have assisted in the transport of near-coastal aragonitic organisms to more coast-remote areas.

Changes in Magnetic Parameters After Sequential Iron Phase Extraction of Eastern Mediterranean Sapropel S1 Sediments

Iron is distributed over different minerals (i.e. silicates, pyrite, detrital oxides) that are present in a sediment sequence that formed under anoxic conditions. After post-depositional re-oxidation of the sediments pyrite is no longer present and diagenetic iron phases constitute an important portion of the iron in the oxidised part of the sapropel. They are very fine-grained making them amenable to analysis by means of sequential extraction and mineral-magnetic methods. The sequential extraction shows that besides iron in silicates, iron mainly occurs in ‘amorphous’ oxides in the oxidised part of the S1 sapropel. Pyrite constitutes an important fraction in the still reduced part of the S1 sapropel. Some silicon is dissolved during the extraction for the ‘amorphous’ oxides, suggesting that ‘amorphous’ iron also occurs as ferro-silicate coatings. Mineral-magnetic analysis involved component analysis of the isothermal remanent magnetisation (IRM) and hysteresis loop measurements. Three coercivity phases could be identified in the IRM component analysis; these were interpreted as ‘detrital’ magnetite, hematite, and biogenic magnetite. The diagenetically formed iron phases influence the parameters of the IRM components. Hysteresis measurements together with the IRM component analysis, indicate the importance of bacterial magnetite in the oxidised sapropel, particularly in the lower part of the active oxidation zone.

Black Sea impact on the formation of eastern Mediterranean sapropel S1? Evidence from the Marmara Sea

Water exchange between the Black Sea and the Mediterranean Sea has been a major focus of the paleohydrography of the eastern Mediterranean. Glacial melt water released from the Black Sea is a potential factor in the formation of sapropel S1, an organic-rich sediment layer that accumulated during the Early Holocene. A high-resolution study done on sediments from the Marmara Sea, the gateway between the Mediterranean and the Black Sea, sheds light on the Holocene exchange processes. Past sea surface temperature and sea surface salinity (SSS) were derived from stable oxygen isotope ratios (δ 18O) of foraminiferal calcite and alkenone unsaturation ratios (U k′ 37). Heavy δ 18O values and high SSS in the Marmara Sea suggest absence of low salinity water from the Black Sea during S1. The comparison with data from the Levantine Basin and southern Aegean Sea outlines gradients of freshening in the eastern Mediterranean Sea, whereby the major sources of freshwater were closer to the Levantine Basin. It is thus concluded that the Black Sea was not a major freshwater source contributing to formation of S1. Given the absence of a low salinity layer, the deposition of organic-rich sediments corresponding to S1 in the Marmara Sea is likely the result of the global transgression and the concomitant re-organization of biogeochemical cycles, leading to enhanced productivity as shown by Globigerina bulloides.

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

Abstract 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.

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.

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.

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.