Pliocene Sapropels Research Articles

Biomarker evidence for the occurrence of anaerobic ammonium oxidation in the eastern Mediterranean Sea during Quaternary and Pliocene sapropel formation

Abstract. The eastern Mediterranean Sea sedimentary record is characterised by intervals of organic-rich sapropel sediments, indicating periods of severe anoxia triggered by astronomical forcing. It has been hypothesised that nitrogen fixation was crucial in injecting the Mediterranean Sea with bioavailable nitrogen (N) during sapropel events. However, the evolution of the N biogeochemical cycle of sapropels is poorly understood. For example, the role of the complementary removal reactions like anaerobic ammonium oxidation (anammox) has not been investigated because the traditional lipid biomarkers for anammox, ladderane fatty acids, are not stable over long periods in the sedimentary record. Using an alternative lipid biomarker for anammox, bacteriohopanetetrol stereoisomer (BHT isomer), we present here for the first time N removal throughout the progression, e.g. formation, propagation, and termination, of basin-wide anoxic events. BHT isomer and ladderanes were analysed in sapropel records taken from three eastern Mediterranean sediment cores, spanning S1 to Pliocene sapropels. Ladderanes were rapidly degraded in sediments, as recently as the S5 sapropel. BHT isomer, however, was present in all sapropel sediments, as far back as the Pliocene, and clearly showed the response of anammox bacteria to marine water column redox shifts in high-resolution records. Two different N removal scenarios were observed in Mediterranean sapropels. During S5, anammox experienced Black Sea-type water column conditions, with the peak of BHT isomer coinciding with the core of the sapropel. Under the alternative scenario observed in the Pliocene sapropel, the anammox biomarker peaked at onset and termination of said sapropel, which may indicate sulfide inhibition of anammox during the core of sapropel deposition. This study shows the use of BHT isomer as a biomarker for anammox in the marine sediment record and highlights its potential in reconstructing anammox during past anoxic events that are too old for ladderanes to be applied, e.g. the history of oxygen minimum zone expansion and oceanic anoxic events.

Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

AbstractThe sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.

Sea surface temperatures and environmental conditions during the “warm Pliocene” interval (~ 4.1–3.2 Ma) in the Eastern Mediterranean (Cyprus)

Organic geochemical (alkenones) and micropaleontological (nannofossil) data from the Pissouri South section (PPS) in the island of Cyprus provided a detailed description of the paleoclimatic (sea surface temperature-SST) and paleoenvironmental conditions during the “warm Pliocene” (c. 4.1–3.25Ma) in the Eastern Mediterranean. We found that the suite of sapropel events recorded in the studied interval took place under conditions of increased SST, enhanced water column stratification and development of a productive deep chlorophyll maximum (DCM), as witnessed by the dominance of Florisphaera profunda species. Such conditions are similar to those prevailing during Quaternary sapropel formation, triggered by freshwater discharges from the N. African margin due to insolation-driven intensification of the African monsoon. The absence of F. profunda in Pliocene sapropels from central Mediterranean records highlights the sensitive response of the eastern basin to freshwater perturbations. Comparisons between alkenone and calcareous nannofossil assemblage patterns infer Pseudoemiliania lacunosa as the main alkenone producer in sapropel layers; yet Reticulofenestra spp. contribution cannot be ruled out.The first Pliocene alkenone-SST record in the E. Mediterranean presented here documents the “warm Pliocene” period (~4.1–3.25Ma) characterized by mean SST of c. 26°C. Distinct SST minima at ~3.9Ma, 3.58Ma and between 3.34 and 3.31Ma, correspond to the MIS Gi16, MIS MG12 and MIS M2 global cooling episodes, before the onset of the Northern Hemisphere glaciation. Our findings imply that the peak of the MIS M2 cooling in the Eastern Mediterranean may be up to ~40kyrs older than the age attributed before to benthic stable oxygen isotopes records of this event.

Micro-trace fossils reveal pervasive reworking of Pliocene sapropels by low-oxygen-adapted benthic meiofauna.

Animal burrowers leave an indelible signature on the sedimentary record in most marine environments, with the seeming exception of low-oxygen environments. In modern sedimentary settings, however, sub-millimetre-sized benthic animals (meiofauna) are adapted to low oxygen and even sulfidic conditions. Almost nothing is known about their impact on ancient marine sediments because they leave few recognizable traces. Here we show, in classic Pliocene-aged anoxic facies from the Mediterranean, the first reported trace fossil evidence of meiofaunal activity and its relation to changing oxygenation. A novel approach utilizing electron imaging of ion-polished samples shows that meiofauna pervasively reworked sediment under oxygen-depleted conditions that excluded macrofauna, fragmenting organic laminae and emplacing 15- to 70-μm-diameter faecal pellets without macroscopically influencing the fabric. The extent of reworking raises the question: how pervasively altered are other sediments presently assumed to lack animal influence and how far into the geological record does this influence extend?

Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels: paleoceanographic and paleoecological significance

Abstract. We have studied a suite of 35 sapropel sequences from a transect of four ODP sites across the Eastern Mediterranean to explore for paleoproductivity patterns and provide new insights on ecological changes during their deposition. Paleoproductivity variations were identified using TOC and Babio mass accumulation rates and δ15Ntotal and δ13Corg values. Elevated Ba/Al and TOC mass accumulation rates record periods of basin-wide amplified productivity. Our data further support that sapropels were formed by cyclic increases in primary production of marine organic matter largely sustained by N-fixing bacteria. This productivity increase was triggered by climate factors leading to increased fluvial discharge and amplified nutrient input that also favored the establishment of N-fixing bacteria. Enhanced productivity led to depletion of deepwater dissolved oxygen and consequently improved organic matter preservation. Primary production was more intense during the middle to Late Pleistocene compared to Pliocene equivalents, coinciding with increasing total sedimentation rates. δ15N values are dramatically lower in the sapropels than in TOC-poor background sediments, indicating a major contribution from nitrogen-fixing bacteria to the higher productivity during sapropel deposition. Additionally, different degrees of denitrification occurred as a consequence of water column oxygenation which in turns evolved from stagnant anoxic bottom waters during Pliocene sapropels to oxygen depleted and sluggish circulation in late Quaternary layers. These differences between sapropel layers provide new evidences for the general evolution of the Eastern Mediterranean basin during the last 3 Mys in terms of paleoceanographic conditions and the intensity of climate variability leading to sapropel deposition.

Trace element indicators of increased primary production and decreased water-column ventilation during deposition of latest Pliocene sapropels at five locations across the Mediterranean Sea

Compositions of productivity- and redox-sensitive trace-metals in two Upper Pliocene sapropel sequences from western Mediterranean ODP Sites 974 and 975 and eastern Mediterranean Sites 964, 969, and 967 provide information about the conditions leading to sapropel deposition. Except at Site 975, closest to the Atlantic Ocean, all sapropels display elevated element/Al ratios for Ba, Re, Cd, Mo, U, Cr, Cu, V, and Zn. High total organic carbon (TOC) concentrations and Ba/Al ratios point to increased primary productivity during sapropel deposition. Enrichments of redox-sensitive trace elements and elevated Re/Mo ratios indicate water-column oxygen depletion during sapropel formation. Anoxia likely was caused by increased fluxes of organic matter to the seafloor that would have enhanced oxygen demand at depth. One of the sapropel sequences investigated in this study, insolation-cycle 178, contains a zone having low TOC, low Ba/Al ratios, and low trace-metal contents. These features identify this interval as an interruption of the increased productivity associated with sapropels and of the sub-oxic/anoxic conditions that are fully-developed in the upper half of this sapropel layer. Significant differences in trace-metal enrichments between the eastern and western Mediterranean indicate that the eastern basin was a focal point for sapropel initiation. In contrast, Site 975 appears to have been heavily influenced by the Atlantic Ocean during i-cycles 178 and 180 and was not strongly influenced by the paleoceanographic changes affecting the rest of the Mediterranean.

Patterns of organic carbon and nitrogen isotopic compositions of latest Pliocene sapropels from six locations across the Mediterranean Sea

Inter-layered in the organic-poor sediments that comprise most of the sedimentary record of the Mediterranean Sea are organic-rich sapropels that represent periods of wetter climate, increased primary productivity, and suboxic to anoxic conditions at depth. In order to explore processes important in the spatial and temporal variability of sapropel-forming conditions, we selected sedimentary sequences containing two latest Pliocene sapropels at ODP Sites 974 and 975 in the western basin, the Vrica Plio-Pleistocene type-section, and ODP Sites 964, 967, and 969 in the eastern basin. Our main tools are nitrogen and organic carbon isotopes, which are diagnostic of changes in nutrient origins and availability and in primary productivity. Extremely low δ 15N values (to − 5.1‰) indicate high rates of nitrogen fixation during sapropel formation stimulated by nearly complete consumption of nitrate in the water column due to exceptionally high rates of denitrification. Interruptions in sapropels represent brief intervals of low productivity and diminished nitrogen fixation. The geographical distribution of the interruptions and the presence of spatial patterns in TOC contents, C org/N tot ratios, and δ 15N values suggest a central role of the eastern basin in initiating sapropel formation. The eastern Mediterranean evidently experienced higher rates of primary productivity during sapropel deposition and probably more intense dysoxic conditions in the water column. Organic carbon isotopes present an intriguing geographic trend across the eastern Mediterranean that is probably related to non-biological processes including variations in water temperature, salinity, and amounts of local fluvial runoff.

Membrane tetraether lipids of planktonic Crenarchaeota in Pliocene sapropels of the eastern Mediterranean Sea

The distribution of glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominantly derived from planktonic Crenarchaeota, were studied in three age-equivalent Pliocene sapropels as well as in the homogeneous intervals, below and above the sapropel. In both the homogeneous intervals and sapropels the dominance of GDGT-0 and crenarchaeol among the GDGTs indicated their origin from planktonic Crenarchaeota as it was found in many other marine sediments. In the homogeneous intervals highest GDGT abundances (normalized to TOC) were measured at the easternmost study Site 967 of the eastern Mediterranean Sea and decreased at the central and western Sites 969 and 964. Within the three studied sapropels large variations in GDGT abundance were observed. The newly established sea surface temperature (SST) proxy, the TEX 86 index, derived from GDGTs of planktonic Crenarchaeota was used to estimate past SST. Comparison with previous obtained SST data using the alkenones from haptophyte algae revealed substantial differences. Whereas the U 37 K′-based SSTs were almost constant at ca. 25 °C in both the homogeneous intervals and the sapropels, the TEX 86-based SSTs were 26–29 °C in the homogeneous intervals and decreased to 15–17 °C in the sapropels. The TEX 86-based SST values outside the sapropel probably reflect summer SST based on a comparison with the present-day Mediterranean Sea. The surprisingly low TEX 86-based SST estimates for the sapropels showed similarities with those obtained for the contemporary euxinic Black Sea. The distribution of marine Crenarchaeota in the modern Black Sea, i.e. thriving at the deeper and colder chemocline, suggests that this leads to a reduction in TEX 86, resulting in artificially low SST estimates. A similar situation is inferred for the Pliocene eastern Mediterranean during sapropel deposition since the SST anomaly co-occurs with the build-up of a shallow chemocline.

The molecular composition of kerogen in Pliocene Mediterranean sapropels and associated homogeneous calcareous ooze

The organic matter of three time-equivalent Pliocene sapropels was characterized as being predominantly Type II kerogen using RockEval pyrolysis. It is dominated by amorphous organic matter, originating mainly from marine organisms, as indicated by the relatively high abundances of algaenan- and chlorophyll-derived pyrolysis products in the Curie-point pyrolysates of the kerogen of the sapropels. However, a small amount of terrigenous organic matter was also observed in the form of pollen and spores that most probably yielded the oxygenated aromatic pyrolysis products known to derive from sporopollenin. Lignin pyrolysis products, such as methoxyphenol and catechol (1,2-benzenediol), commonly used as unequivocal evidence for a contribution of terrestrial higher plants in the form of woody material to the kerogen, were absent from the kerogen pyrolysates. No major spatial and temporal differences in the kerogen composition of the three time-equivalent sapropels were observed. Palynological studies revealed that organic matter in the homogeneous intervals, below and above the sapropels, consisted of pollen, spores and amorphous organic matter. This was confirmed by Curie-point pyrolysis – GC/MS results showing a relatively high abundance of algaenan-derived compounds and the presence of oxygenated aromatic pyrolysis products. The differences observed between the kerogen pyrolysates of the sapropels and their homogeneous intervals are suggested to be governed primarily by enhanced preservation conditions, resulting in increased preservation of marine organic matter in the sapropels.

Identification of carotenals in sediments

High performance liquid chromatography with photodiode array UV–Vis detection and atmospheric pressure chemical ionization mass spectrometry was used to analyze sediment samples from the meromictic Ace Lake (Antarctica), as well as a Pliocene sapropel from the Mediterranean Sea and a Miocene marl from the Vena del Gesso formation (Italy). The Ace Lake samples contained intact chlorobactene, isorenieratene, γ- and β-carotene and two series of carotenoid breakdown products. The first and more abundant series was identified as aryl (ϕ)-carotenals of increasing chain length (C 13, C 15, C 18, C 20, C 22, C 25, C 27, C 30, and C 32) on the basis of their UV–Vis and mass spectra. They appear to be formed from chlorobactene through oxidative cleavage of the double bonds of the polyene chain. The minor series (β-carotenals) was also identified, and may be formed from γ-carotene in a similar manner. Intact isorenieratene and the ϕ-carotenal series were also found in the Mediterranean Sea sapropel. Only a trace of the C 32 ϕ-carotenal could be detected, suggesting isorenieratene as the predominant source of these ϕ-carotenals. Additionally, intact β-carotene and C 13–C 30 β-carotenals were found in the sapropel. The absence of the C 32 β-carotenal strongly suggests these breakdown products are derived from β-carotene. The Vena del Gesso marl contained both intact isorenieratene and the C 13–C 25 ϕ-carotenals. The absence of the longer chain length ϕ-carotenals from this sample may indicate more advanced oxidation of isorenieratene. The carotenals are likely formed during storage of samples and/or extracts under non-ideal conditions, i.e., not frozen and exposed to oxygen although they may also be formed by diagenetic processes. They can be relatively abundant and sometimes be an order of magnitude higher in concentration than the parent carotenoid. Hence, not including the ϕ-carotenals could lead to an underestimation of the persistence of photic zone euxinia.

Higher plant vegetation changes during Pliocene sapropel formation

The δ 13 C values of higher plant wax C 27–33 n -alkanes were determined in three, time-equivalent Pliocene (2.943 Ma) sapropels and homogeneous calcareous ooze from three different sites forming an east-west transect in the eastern Mediterranean Basin in order to study the composition of the vegetation on the continents surrounding the Mediterranean Sea. A two-end member mixing model transformed the measured δ 13 C values into the contribution of C 4 plants to the terrestrial vegetation. These calculations indicated a high C 4 plant contribution (i.e. 40–50%) in the periods just before and just after sapropel formation. During sapropel deposition the C 4 plant contribution increased by up to 20% at all sites. This is interpreted to record the increased overall plant coverage of the Mediterranean borderlands resulting from the change in formerly barren desert areas into C 4 grass-dominated savannahs as a response to the wetter climate during sapropel deposition. Enhanced accumulation rates (ARs) of long-chain n -alkanes (C 27–33 ) and n -alkan-1-ols (C 26–30 ) towards the middle of the sapropel in concert with a decrease in the Ti/Al ratio confirm an increased delivery of terrigenous organic matter at all sites. These biomarkers were probably predominantly fluvially transported to the Mediterranean Sea, not only by the Nile but by fossil wadi river systems on the northern African continent.

Higher plant vegetation changes during Pliocene sapropel formation

The δ 13C values of higher plant wax C 27–33 n-alkanes were determined in three, time-equivalent Pliocene (2.943 Ma) sapropels and homogeneous calcareous ooze from three different sites forming an east-west transect in the eastern Mediterranean Basin in order to study the composition of the vegetation on the continents surrounding the Mediterranean Sea. A two-end member mixing model transformed the measured δ 13C values into the contribution of C 4 plants to the terrestrial vegetation. These calculations indicated a high C 4 plant contribution (i.e. 40–50%) in the periods just before and just after sapropel formation. During sapropel deposition the C 4 plant contribution increased by up to 20% at all sites. This is interpreted to record the increased overall plant coverage of the Mediterranean borderlands resulting from the change in formerly barren desert areas into C 4 grass-dominated savannahs as a response to the wetter climate during sapropel deposition. Enhanced accumulation rates (ARs) of long-chain n-alkanes (C 27–33) and n-alkan-1-ols (C 26–30) towards the middle of the sapropel in concert with a decrease in the Ti/Al ratio confirm an increased delivery of terrigenous organic matter at all sites. These biomarkers were probably predominantly fluvially transported to the Mediterranean Sea, not only by the Nile but by fossil wadi river systems on the northern African continent.

Controls on phosphorus regeneration and burial during formation of eastern Mediterranean sapropels

The carbon (C) and phosphorus (P) geochemistry of sapropels from four sites in the eastern Mediterranean Sea was determined to obtain more insight into the role of differences in sediment accumulation rates and bottom water anoxia on P regeneration and burial in sediments. Sediment C org/P org (with org=organic) ratios above Redfield indicate enhanced regeneration of P relative to C from organic matter during formation of the most recent sapropel S1 and a Pliocene sapropel (ODP-site 969E; i-282c). Release of P from Fe-oxides was relatively unimportant. Increased burial of Ca-P (authigenic carbonate fluorapatite and/or biogenic hydroxyapatite) occurred but was of minor importance compared to the enhanced P release from organic matter. For sapropel S1, C org/P org and C org/P reac, (with reac=reactive, defined as the sum of organic, Fe-bound, authigenic and biogenic Ca-P) ratios decreased with increasing sedimentation rate and oxygen exposure and decreasing water depth. Whether the water column in the deep basin was dysoxic/semi-euxinic (S1) or euxinic (Pliocene) does not appear to affect the estimated burial efficiencies of P org and P reac. Enhanced C org burial under the euxinic water column explains the much higher C org/P org and C org/P reac ratios in the Pliocene sapropel (on average ∼2860 and ∼760, respectively) compared to sapropel S1 (on average ∼530 and ∼160, respectively).

Reconstruction of changes in export productivity during Pliocene sapropel deposition: a biomarker approach

Biomarkers (alkenones, loliolide/isololiolide, dinosterol and C 30 1,15-diol/keto-ol) representative of four major classes of marine primary producers (haptophytes, diatoms, dinoflagellates, eustigmatophytes, respectively) from three sites of an east–west transect spanning the entire eastern Mediterranean basin were studied for changes in phytoplankton export production before, during and after deposition of a Pliocene, total organic carbon-rich (up to 27%) sapropel (2.943 Ma). Biomarker contents showed substantial differences between non-sapropelic and sapropelic sediments. In non-sapropelic sediments, small contents are observed reflecting extensive oxic degradation in oxic pore waters during deposition. In the sapropels, biomarker accumulation rates showed a substantial increase from the base to the centre, likely resulting from increased biological productivity since the bottom waters were continuously anoxic and preservation conditions are similar. The diatom biomarker, loliolide/isololiolide, became distinctly more abundant in the sapropels indicating that the eastern Mediterranean basin was enriched in nutrients. This caused enhanced export production by the major phytoplankton classes leading to periods of sapropel deposition. Alkenone-based sea surface temperature reconstructions indicated that this change in phytoplankton composition was not influenced by changes in sea surface temperature, but was mainly nutrient-controlled. Compound-specific δ 13C measurements did not provide additional evidence for substantially increased primary production rates: the δ 13C values of C 37:2 alkenones and loliolide/isololiolide showed a slightly negative shift instead of the anticipated positive shift. This was probably caused by the enrichment of dissolved inorganic carbon in the upper water column, caused by enhanced recycling of respired CO 2 due to the shallow chemocline.

Pliocene sequence stratigraphy, climatic trends and sapropel formation in the Northern Apennines (Italy)

The Pliocene stratigraphy of the Romagna Northern Apennines foredeep has been reconstructed through mapping and measuring the different lithostratigraphic units. The physically correlated sedimentary bodies have been chronologically calibrated through biostratigraphic analyses. Analysis of facies and fossil content allowed reconstructing the main depositional systems and their evolution. The co-occurrence of shallow- to deep-water environments and carbonate to siliciclastic systems allowed understanding fully the role of eustasy and climate vs. continuous tectonic deformation controlling the accommodation and basin morphology. Following a limited sea-level lowering at 4.2 Ma, associated with the first signal of cooling in the Early Pliocene, a fast growing subtropical type carbonate platform developed, in response to warm and oligotrophic surface waters. Furthermore, a significant drop in sea-level at 3.75 Ma points to an important Pliocene continental glaciation in the Northern Hemisphere. During the subsequent transgression, warm and possibly humid climate favoured the deposition of the first Pliocene sapropel around 3.3 Ma. The growth of a second generation of carbonate platform between 3.3 and 3.042 Ma suggests a renewed warm period and limited runoff. At the top of this highstand, thick sapropels developed, which can easily be correlated across the Mediterranean. The platform abruptly drowned, was overwhelmed by bio-erosion and was covered by iron–phosphate crusts; whereas in the adjacent basins numerous sapropel layers occurred, rich of biosiliceous forms, documenting the shift from C carb to C org deposition. Strong oscillations of climate, due to precessionally controlled seasonality, and increased precipitation and runoff are considered as the main causes of the increase of surface water fertility. After the 2.87 Ma sea-level drop, again very significant, the sapropel rapidly disappears, in conjunction to the development of the lowstand wedge. After this cold period, which ended around 2.6 Ma, carbonate platforms never recovered due to the enduring cooler surface waters and the onset of an active drainage on the Apenninic range. This study documents the role of the eustatic variations on the Pliocene deposition of the actively deforming Northern Apennines foredeep. The abrupt lithostratigraphic changes appear also controlled by palaeoproductivity variations associated with climate and runoff.

Geochemical evidence for paleoclimatic variations during deposition of two Late Pliocene sapropels from the Vrica section, Calabria

The near-shore paleosetting of the Vrica Pliocene–Pleistocene boundary section provides an expanded sedimentary sequence that is ideal for investigating small-scale variations in sapropel deposition. We analyzed a suite of geochemical parameters in samples from a freshly drilled core that includes sapropelic layers c (1.851 Ma) and d (1.829 Ma) to characterize their depositional variation. Both sapropelic layers differ from background sediment by having higher total organic carbon (TOC) contents, lower δ 15N values, and higher contents of redox-sensitive and chalcophile elements. Layer c consists of a lower sub-layer in which TOC content is less than that of the upper sub-layer. Layer d exists as three sub-layers that are separated by low-TOC sediment. The lowermost sub-layer is weakly expressed and lacks chalcophile element enrichment. The other two sub-layers are strongly developed and illustrate that sapropel-forming conditions can be quickly established and interrupted. The two interruptions within layer d indicate a decrease in continental runoff, verifying the importance of humid conditions over continental borderlands and freshening of Mediterranean surface waters in forming sapropels. Although organic matter production was elevated during deposition of layers c and d, barium contents of sapropels and background sediment are similar, which shows that this paleoproductivity proxy is not suitable for shallow settings like Vrica. Our results confirm that considerable paleoclimatic variation can occur during deposition of an individual sapropel layer and in closely spaced sapropel events.

Development of photic zone euxinia in the eastern Mediterranean Basin during deposition of Pliocene sapropels

Carotenoids were analysed in ca. 1-cm thick subsamples of three laterally time-equivalent sapropels from a west–east transect of the eastern Mediterranean Basin to study euxinic periods during Pliocene sapropel formation. The amount of intact isorenieratene (summed all- trans and cis isomers), ranged from non-detectable at the base and top of a sapropel up to 140 μg/g sediment in the central parts. Isorenieratene accumulation rates at the central and western site are remarkably similar and increase sharply to levels of up to 3.0 mg m −2 yr −1 in the central part of the sapropel and then drop to low levels. This pattern indicates an expansion of euxinic conditions reaching into the photic zone, followed by deepening of the chemocline during deposition of this Pliocene sapropel. The sapropel from the easternmost site of the basin, which contains less organic carbon, shows much lower isorenieratene accumulation rates and even absence of isorenieratene in the central part of the sapropel. Ba/Al ratios indicate enhanced palaeoproductivity during sapropel formation, supporting previously proposed models, according to which increased productivity is the driving force for the generation of euxinic conditions.

Combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a Pliocene sapropel from the eastern Mediterranean Sea

Layers of organic-carbon-rich sapropels in the sediment record of the Mediterranean Sea give evidence of repetitive changes in regional Plio-Pleistocene climate. Results from biomarker molecule and major and trace element analyses of closely spaced samples are used to reconstruct the conditions leading to deposition of a Pliocene sapropel at Ocean Drilling Program (ODP) Site 969 on the Mediterranean Ridge. Organic carbon concentrations increase from 0.2% outside the sapropel and peak to more than 30% within it. Major and trace elemental composition and biomarker-derived parameters indicate elevated productivity, depletion of water-column dissolved-oxygen content, and changes in sediment provenance in response to climatic changes. Budgets of rhenium, thallium, and other trace metals indicate that deep-water exchange between the Mediterranean subbasins and the Atlantic Ocean was not completely interrupted during sapropel formation. Enrichment factors of redox-sensitive and sulfide-forming trace metals as well as the presence of isorenieratene derivatives and high stanol/sterol ratios point to an extended zone of anoxic water masses. Depth profiles of biomarker compositions (sterols, long-chain alkenones, alkandiols and -ketols, fatty acids) indicate great floral diversity during deposition of a single sapropel and highlight the sensitive response of the marine community to variable environmental conditions. Changes in water mass circulation and eolian transport can be reconstructed by use of both lithogenic elements and average chain lengths of n-alkanes (ACL index).

Trace metal signatures of eastern Mediterranean sapropels

Sapropel samples of Pliocene to Pleistocene age recovered during ODP Leg 160 from Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 966, 967 (top and northern slope of the Eratosthenes Seamount, south of Cyprus) and 968 (Cyprus margin) were analysed for major and trace elements. Eastern Mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox-sensitive and/or sulphide-forming trace metals (Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V). Enrichment factors relative to ‘average shale’ are comparable to those found in CTBE black shales. The Re content is extremely high in the sapropels studied with maximum values up to 1000 ng/g. Re/Mo ratios averaging 2.7×10 −3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to an anoxic water column during sapropel formation. Additionally, enhanced Ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes. Trace metal signatures of sapropels originating from different drill sites on a transect across the eastern Mediterranean are not only influenced by water depth but also by their location in the eastern Mediterranean. Metal enrichments seem to be higher at the western sites compared with the eastern sites. This probably reflects a change in the circulation pattern during periods of sapropel formation. Pliocene sapropels exhibit elevated trace metal contents in comparison with Pleistocene examples. Therefore, different palaeoceanographic conditions may have developed during the Pleistocene compared with the Pliocene. Altering palaeoceanography was probably controlled by climatic change, which may be related to the onset of glaciation of the northern Hemisphere. Decreasing summer precipitation throughout the late Pliocene and the Pleistocene may also have influenced sapropel deposition.

Geochemical constraints on Pliocene sapropel formation in the eastern Mediterranean

Abstract Seventeen eastern Mediterranean Pliocene sapropels from ODP Sites 964, 966, 967 and 969, some of which are coeval, have been analysed for their geochemistry. The sapropels are characterized by very high organic carbon contents (up to 30%) which are reported to be the result of both increased productivity and improved preservation. Although the organic matter in the sapropels is mainly of marine origin, the δ 13 C org values and C/N ratios appear “terrestrial”. This is the result of anaerobic organic matter degradation which preferentially removed nitrogen- and 13 C -rich organic components. A comparison with Ti/Al profiles, which mimic the precession index, and a calculation of organic carbon accumulation rates indicate that sedimentation rates were at most 30% lower or at most 50% higher during sapropel formation. Thus, sapropel formation lasted from between 2000 and 10,000 years at Site 964 to between 4500 and 12,000 years at Site 967. A synthesis of new data and a comparison with existing models indicates that productivity, which increased due to extra nutrients supplied as a result of winter mixing and as a result of enhanced input by the Nile, was the driving mechanism behind sapropel formation. The resulting sapropel formation was simultaneous at different depths, but lasted longer in the part of the basin closest to the Nile.