Sapropel Formation Research Articles

Hydrogen isotopic compositions of long-chain alkenones record freshwater flooding of the Eastern Mediterranean at the onset of sapropel deposition

Sapropels are organic-rich sediment layers that were intermittently deposited in the Mediterranean Sea, especially in its eastern basin, during the last 10Myr. The associated anoxic events that gave rise to sapropel formation resulted indirectly from the impact of African monsoon maxima on the basin's hydrography. Sharp shifts in oxygen isotopes ( δ 18O) to values more depleted in the heavy isotope ( 18O) in carbonates from surface dwelling planktonic foraminifera, slightly preceding sapropel deposition, suggest that the Mediterranean was flooded by large amounts of freshwater leading to the development of a low salinity of the surface water and a strong density stratification of the water column. However, the degree of freshwater flooding and concomitant drop in sea surface salinity (SSS) remain elusive. Recent work has shown that the hydrogen isotope ( δD) values of long-chain alkenones produced by haptophyte algae depend mainly on the δD of the water and on salinity, and may therefore offer a new tool for salinity reconstructions. Our analysis of the δD of alkenones from last interglacial sapropel S5 from the Aegean Sea shows a large decrease in δD of 25‰ at the onset of sapropel formation, suggesting a drop in SSS of 6, from 39 to 33. Although the absolute SSS estimates should be interpreted with care as they are subject to relatively large uncertainties, the estimated SSS values appear quite reasonable as they, for example, yield SSS before sapropel deposition similar to that of the present day Aegean Sea. To reduce uncertainties in SSS estimates, the δD-salinity relationship has to be better constrained with cultures and also tested in field studies. However, our results do illustrate the promise of a combined use of δD of alkenones, U 37 k′ of alkenones, and δ 18O of surface dwelling planktonic foraminifera, for SSS reconstructions.

New radiocarbon dates from sapropels in three Holocene lakes of the Coorong coastal plain, southeastern Australia

The Holocene calcareous mudstone successions of numerous shallow coastal lakes in the Coorong region of southeastern South Australia include prominent organic-rich intervals (0.01 – 1 m thick; total organic carbon >2%), herein termed sapropels. New radiocarbon dates (n = 25) on bulk organic matter in single cores from three such lakes indicate that these sapropels are older in North Stromatolite Lake, part of the Salt Creek lake chain, than 100 km further south in the Robe Range at Old Man Lake and Lake Amy. The onset of sapropel deposition in North Stromatolite Lake at ca 6400 cal y BP marked its isolation from the open Coorong Lagoon, whereas in Old Man Lake, the onset of sapropel formation at ca 4000 cal y BP coincided with the transition from lagoonal to perennial lacustrine conditions as the sea withdrew from the interdunal Robe – Woakwine corridor. In contrast, the depocentre at Lake Amy was never directly connected to the marine system. Here, organic matter from near the base of the lowermost sapropel, dated at ca 4100 cal y BP, provides a minimum age for the onset of lacustrine conditions. Sapropel deposition recorded thus far in these lacustrine sequences ceased earlier in North Stromatolite Lake (ca 5000 cal y BP) than in Old Man Lake (ca 2600 cal y BP) and Lake Amy (ca 1900 cal y BP). This may simply reflect a marked local climatic gradient (drier in the north, wetter in the south). Alternatively, it may indicate that these shallow lacustrine sapropels, unlike their marine and deep-lake counterparts, do not form only during times of high precipitation but may be triggered by other processes such as the enhanced aeolianite influx associated with aridification.

Organic-rich sediments in brine-filled Shaban- and Kebrit deeps, northern Red Sea

The element compositions Si, Ca and Al of up to 21.1 ka old sediments in about 10 m long cores from the southern basin of the Shaban and Kebrit deeps in the northern Red Sea allowed a classification of major sediment types in carbonate sands and -muds and siliceous oozes. A FeOOH-enriched sediment horizon and a few samples with high Zn values in the Kebrit core indicate a hydrothermal origin probably near the brine–sea water interface with subsequent transport of hydrothermal compounds into the deep sediments. High organic carbon contents up to 8.4% are positively correlated with the Ba concentrations, which suggests that high bioproductivity and rapid deposition ( 14C dating suggests a sedimentation rate near 70 cm/ka) led to the formation of sapropelic sediments between 11.8 and 13.6 ka (Younger Dryas). Organic petrological observations showed that the sediment organic material largely consists of < 20 μm-sized roundish fecal pellets (intimate mixtures of organic matter and inorganic constituents) and bituminite. Terrestrial organic matter (pollens of land plants, fusinite etc.) is very rare in the sediment cores from both deeps. Organic-geochemical investigations of kerogens and organic extracts show that a significant (hydrothermal) hydrocarbon production did not occur in near-surface sediments of the Shaban and Kebrit deeps. Rock Eval pyrolysis of kerogens characterised the organic matter to be of type II quality. The δ 13C values of the kerogens from the most prominent sapropel in the Shaban deep indicate an enrichment of ( 12C-rich) nutrients in the water column during postglacial sapropel formation in the Younger Dryas. The n-alkane spectra are dominated by short chain lengths between n-C 15 and n-C 25. Prevailing n-C 15 to n-C 25 alkanes in low mature sediments are indicative of algal and microbial source. Pristane/phytane ratios are generally low (< 1 to ∼ 1) which suggests that anoxic conditions prevailed within the anaerobic brine-filled deeps for the whole time covered by the sediments. This again indicates that sapropel formation was caused by high bioproductivity in the northern Red Sea rather than episodic stagnation with better preservation of the organic matter. Long-chain alkenones and sterols are the dominating compounds of the lipid fraction. Cholesterol contents in the sediment cores reflect phases of eukaryotes production in the water column, whereas the positive correlations of dinosterol with TOC and the amounts of total extract suggests that the major organic carbon source in the northern Red Sea during postglacial high-productivity stages were dinoflagellates. Another important carbon source, however, is indicated by the occurrence of 22,29,30-trisnorhopan-21-one (TNH). Although the formation of TNH from its precursors is not fully understood, this compound probably results from microbial degradation of intact bacteriohopanepolyols (BHP), which can be used as indicators for bacterial abundances and phyla. TNH is most likely produced at the brine–sea water interface where sedimenting organic matter accumulates and, if the redoxcline corresponds to the density gradient, the organic matter is subjected to efficient aerobic bacterial degradation processes. However, during high bioproductivity stage (Younger Dryas) the redoxcline was probably higher in the water column and thus, a significant TNH production at the brine–sea water interface did not occur at times of sapropel formation in the northern Red Sea deeps.

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.

217 000‐year‐old DNA sequences of green sulfur bacteria in Mediterranean sapropels and their implications for the reconstruction of the paleoenvironment

Deep-sea sediments of the eastern Mediterranean harbour a series of dark, organic carbon-rich layers, so-called sapropels. Within these layers, the carotenoid isorenieratene was detected. Since it is specific for the obligately anaerobic phototrophic green sulfur bacteria, the presence of isorenieratene may suggest that extended water column anoxia occurred in the ancient Mediterranean Sea during periods of sapropel formation. Only three carotenoids (isorenieratene, beta-isorenieratene and chlorobactene) are typical for green sulfur bacteria and thus do not permit to differentiate between the approximately 80 known phylotypes. In order to reconstruct the paleoecological conditions in more detail, we searched for fossil 16S rRNA gene sequences of green sulfur bacteria employing ancient DNA methodology. 540 bp-long fossil sequences could indeed be amplified from up to 217 000-year-old sapropels. In addition, such sequences were also recovered from carbon-lean intermediate sediment layers deposited during times of an entirely oxic water column. Unexpectedly, however, all the recovered 16S rRNA gene sequences grouped with freshwater or brackish, rather than truly marine, types of green sulfur bacteria. It is therefore feasible that the molecular remains of green sulfur bacteria originated from populations which thrived in adjacent freshwater or estuarine coastal environments rather than from an indigenous pelagic population.

Cyanobacteria as a trigger for increased primary productivity during sapropel formation in the Baltic Sea—a study of the Ancylus/Litorina transition

A sediment sequence from the Gotland Basin, representing the Ancylus Lake/Litorina Sea transition, was analysed for pigments, stable isotopes (δ13C and δ15N), diatoms and carbon and nitrogen content. The negative correlation between the cyanobacterial specific pigment zeaxanthin and δ15N supports the hypothesis that cyanobacteria in the early Litorina Sea indeed were nitrogen fixers. Their incorporation of nitrogen could have acted as a trigger for eutrophication during the Litorina Sea stage of the Baltic Sea. As cyanobacteria normally flourish in eutrophicated waters, the increasing concentrations of zeaxanthin during early Litorina Sea also corroborate that high primary production was an important part of the formation of organic rich (sapropel) sediments in the Baltic Sea. The first occurrence pre-dates the formation of laminated sediments, but the peaks of both zeaxanthin and organic carbon are within laminated sequences. This implies that the oxygen conditions of the bottom water also play a major role in the formation of sapropel sediments.

The co-evolution of Black Sea level and composition through the last deglaciation and its paleoclimatic significance

The Black Sea was an inland lake during the last ice age and its sediments are an excellent potential source of information on Eurasian climate change, showing linkages between regionally and globally recognized millennial-scale climate events of the last deglaciation. Here, we detail changes from the last glacial maximum (LGM) through the transition to an anoxic marginal sea using isotopic (strontium and oxygen) and trace element (Sr/Ca) ratios in carbonate shells, which record changing input sources and hydrologic conditions in the basin and surrounding region. Sr isotope records show two prominent peaks between ∼18 and 16 ka BP cal, reflecting anomalous sedimentation associated with meltwater from disintegrating Eurasian ice sheets that brought Black Sea level to its spill point. Following a sharp drop in Sr isotope ratios back toward glacial values, two stages of inorganic calcite precipitation accompanied increasing oxygen isotope ratios and steady Sr isotope ratios. These calcite peaks are separated by an interval in which the geochemical proxies trend back toward glacial values. The observed changes reflect negative water balance and lake level decline during relatively warm periods (Bølling–Allerød and Preboreal) and increasing river input/less evaporation, resulting in higher lake levels, during the intervening cold period (the Younger Dryas). A final shift to marine values in Sr and oxygen isotope ratios at 9.4 ka BP cal corresponds to connection with the global ocean, and marks the onset of sedimentation on the Black Sea continental shelf. This date for the marine incursion is earlier than previously suggested based on the appearance of euryhaline fauna and the onset of sapropel formation in the deep basin.

Sapropel burn-down and ichnological response to late Quaternary sapropel formation in two ∼ 400 ky records from the eastern Mediterranean Sea

Two approximately 400 ky long sedimentary records from the eastern Mediterranean Sea were used to study the factors determining why some sapropels are more or less completely erased while others are preserved. X-ray radiographs, bulk density, organic carbon content, Fe / Al and Mn / Al ratios were used to reconstruct initial thickness and burn-down ratios of sapropels S1 to S11. The results show that, at the low-sedimentation rate, open ocean settings studied, initial carbon content of the sapropel, as well as post-sapropel sedimentation rate and carbon flux have had only limited influence on the preservation of sapropels. In contrast, the regional position relative to the deep-water source controlling bottom water current intensity and bottom water oxygenation played a major role in the preservation of the sapropels. Furthermore, orbitally controlled variations in the intensity of subsequent summer insolation maxima also seem to play an important role in the preservation potential of sapropel layers. Subsequent strong insolation maxima resulting in thick sapropels tend to act as a lid, preventing further burn-down of the older sapropels below. The long and well-dated records also offer an opportunity to study the response of the benthic fauna to the drastic environmental changes related to the formation of sapropels. The sediment in the two cores is generally characterized by mottled burrows, with only scarce occurrences of distinct trace fossils such as Scolicia, Thalassinoides, Chondrites, and Trichichnus. The meager ichnofauna is attributed to the well-oxygenated and warm bottom waters in an oligotrophic environment typical for non-sapropel times. The sparse ichnofauna and the lack of tiered ichnocoenosis render existing models of bioturbational response to changes in bottom water oxygenation unsuitable for this kind of environments. However, observations on individual traces support earlier studies that suggest that the trace fossils Trichichnus and Chondrites are the result of chemosymbiotic activities that benefit from reducing conditions in the pore water, and that the producers of Scolicia have a preference for well-oxygenated conditions and sandy substrates.

Productivity-generated annual laminae in mid-Pliocene sapropels deposited during precessionally forced periods of warmer Mediterranean climate

Paleoproductivity estimates for a sequence of five mid-Pliocene Mediterranean sapropels illustrate the importance of insolation maxima in enhancing organic carbon accumulation. Well-laminated sapropelitic intervals in the Northern Apennines have been studied by a combination of sedimentological and micropaleontological analysis, detailed electron microscope description, bulk chemical composition, carbon-isotopic composition and elemental ratios. Each sapropel, formed during precessional minima, lasted 7.5 to 10 kyr, which is the same duration calculated for coeval counterparts in ODP sites in the eastern Mediterranean. The organic carbon mass accumulation rates of the studied sapropels show the same values of those calculated in the coeval Mediterranean sapropels, suggesting that the same productivity conditions were controlling sapropel formation in the whole Mediterranean, despite the differences in depositional setting and the strong variations in sedimentation rate. Mat-forming diatoms play an important role in increasing the settling velocity, allowing rapid sinking of organic matter and preventing bacterial remineralization in the water column. The consequent partial oxygen depletion at the seafloor increases preservation, which is therefore considered an effect, rather than a cause of the organic carbon accumulation.

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.

Detecting missing beats in the Mediterranean climate rhythm from magnetic identification of oxidized sapropels (Ocean Drilling Program Leg 160)

Eastern Mediterranean sapropels are organic-rich sediments whose formation is related to variations in the Earth's orbit. They are therefore important for reconstructing past climatic variations and for producing astronomically tuned geological timescales. Previous studies have suggested that the distinctive magnetic properties of sapropels, which result from non-steady-state diagenetic reactions related to degradation of organic matter, might be used for identifying sapropels that have escaped visual identification after being completely erased during post-depositional oxidation. We present a high-resolution multi-proxy magnetic, geochemical and paleontological data set from selected intervals of Ocean Drilling Program Sites 966 and 967. Our results demonstrate that magnetic properties can be unambiguously used for identifying oxidized sapropels, and also for determining whether suspected intervals actually correspond to oxidized sapropels, because they enable detection of the former presence of organic matter and of climatic and oceanographic conditions suitable for sapropel formation. Systematic application of high-resolution magnetic analyses to future coring efforts in the eastern Mediterranean should allow determination of the original distribution of sapropels through long sedimentary sequences, which will improve our knowledge of paleoceanographic and paleoclimatic conditions that led to their formation.

Phyto_ and zooplankton paleofluxes during the deposition of sapropel S1 (eastern Mediterranean): Biogenic carbonate preservation and paleoecological implications

The relative and absolute abundances and accumulation rates of foraminifera and calcareous nannofossils were quantified in a box_core containing sapropel S1 from the Florence Rise area (SE of Cyprus; 2302 m water depth). The main objective of this study was to reconstruct in detail variations in paleoecological conditions of water masses during the deposition of this sapropel. In particular, we qualitatively evaluated the importance of carbonate dissolution on planktonic assemblages to better interpret the abundance profiles obtained from the two investigated taxa. Selective carbonate corrosion in the core is shown by 1) the decrease in shell mass weight of selected species of planktonic foraminifera, 2) the decrease in accumulation rate of dissolution_susceptible holococcoliths and coccolith species, and 3) the absence of pteropods. However, the presence of other relatively dissolution_susceptible coccolith species throughout all of the S1 interval suggests moderate dissolution. Florisphaera profunda shows a marked increase in paleofluxes within the sapropel coupled with a decrease in the accumulation rate of the upper_middle photic zone coccoliths, suggesting an ecological depth_separation of the water column, probably characterised by higher nutrient availability at depth and nutrient_depleted surface waters between ∼ 10 and 6.5 kyr BP. In the same interval Turborotalita quinqueloba and Globigerina bulloides, two foraminiferal species frequently occurring during periods of high fertility, increase in relative abundance. The maximum increase in relative abundance of Globigerinoides ruber (var. alba and rosea) marks the climatic optimum phase and the maximum stratification in surface water that occurred at the beginning of sapropel S1 deposition when the bottom waters were anoxic. An important change in foraminiferal assemblages occurs at ∼ 8 kyr BP and corresponds with a negative shift in CaCO 3, Ba and C org contents. This short interval marks the establishment of relatively less anoxic conditions in the bottom water, introducing the last phase of sapropel formation. After ∼ 6.5 kyr BP, a progressive re_establishment of normal oceanographic conditions occurred before the real end of the sapropel S1. This transition is well recorded by the reoccurrence and major accumulation rate of the mixing indicator foraminiferal species Globorotalia inflata and by the gradual decrease in abundance of F. profunda.

Trans-Mediterranean comparison of geochemical paleoproductivity proxies in a mid-Pleistocene interrupted sapropel

Sapropel layers in the sediment record of the Mediterranean Sea reflect periods of climate-induced elevated marine production associated with precessional minima. Some of these layers feature interruptions that represent centennial-to-millennial returns to the predominantly low-productivity conditions of this sea. We have measured multiple geochemical paleoproductivity proxies in the interrupted sapropel layer that corresponds to insolation cycle 90 (955 ka) at 1-cm intervals in a four-site transect from the Balearic Basin to the Levantine Basin to assess similarities and differences in its expression at the four locations. The interrupted sapropels are similarly divided by an organic-carbon-poor interval into an upper layer in which organic carbon concentrations reach 2–7 wt.% and a lower layer in which they peak between 1 and 3 wt.%. The interruption was essentially synchronous in the Balearic Basin, the Tyrrhenian Basin, and the Ionian Basin, but it was delayed and shorter in the Levantine Basin. Lighter nitrogen isotope compositions consistently accompany higher organic carbon accumulation rates in the sapropel layers, which implies a shift from coccolith-dominated organic matter production towards a mode of primary production in which cyanobacteria were important during times of sapropel deposition. Organic carbon isotopic compositions become heavier as organic carbon accumulation increases at three of the locations, which is consistent with elevated productivity. However, they become lighter at the Levantine Basin site, which suggests greater delivery of isotopically light fluvial waters to this part of the Mediterranean Sea. The local paleoclimate conditions conducive to sapropel formation evidently were not completely uniform across the Mediterranean region but their occurrences were synchronous. Moreover, the co-existence of an essentially same-age interruption of the sapropels at the four locations shows that climate could change rapidly and concordantly across the region at much less than precessional intervals.

Marine records of Holocene climatic variations

Abstract Holocene millennial climate variability is smaller than that of the last glaciation, due to the disappearance of large unstable ice sheets. Marine records show that the sea-surface temperature (SST) exhibited small variations, mainly in the high and low latitudes. They may be interpreted as a linear response to the mean annual insolation. Major changes in the hydrological cycle have been evidenced in the Asian and African monsoon area, resulting in enhanced precipitation and large river outflow in the Bay of Bengal and the Gulf of Niger. Enhanced rainfall over the Mediterranean Basin resulted in a weak circulation and sapropel formation below 800-m water depth in the eastern Mediterranean Sea. Finally small changes in the thermohaline circulation and the warm North Atlantic Drift have been detected in the Nordic Seas. The Holocene climatic variability is therefore similar to that of the Quaternary, but with small amplitude, while continents experienced large rainfall variations. To cite this article: J.-C. Duplessy et al., C. R. Geoscience 337 (2005).

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.

Florisphaera profunda and the origin and diagenesis of carbonate phases in eastern Mediterranean sapropel units

High relative concentrations of the lower photic zone nannofossil Florisphaera profunda have been reported in all recent eastern Mediterranean sapropels. In the most recent sapropel (S1), high bulk sediment Sr/Ca ratios occur along with high F. profunda contents toward the base of the unit, exemplified here in four cores from 1.5–3.5 km water depth. Co‐occurring biogenic carbonates contain insufficient Sr to account for these high Sr concentrations, and X‐ray diffraction and selective leaching show that the high Sr/Ca ratios are due to aragonite, the CaCO3 polymorph that is rarely preserved in deep marine sediments, with ∼1 wt % Sr. The possible sources of this aragonite include (1) precipitation with surface ocean production, (2) detrital input from shallow‐water sediments by high continental runoff, or (3) postdepositional diagenetic formation driven by increased pore water alkalinity resulting from sulphate reduction. The third formation mechanism for the aragonite is favored, in which case the similarity in the positions of the aragonite and F. profunda abundance maxima in sapropels is probably related to Corg accumulation and resulting sulphide diagenesis that produces high pore water alkalinity. There is clear micropaleontological evidence that dissolution of the less soluble biogenic low‐Mg CaCO3 is occurring, or has occurred, during early diagenesis in these sediments despite the coexistence of the more soluble high‐Mg calcite and aragonite polymorphs. Similar Sr/Ca maxima are also found associated with older sapropels, always located close both to local minima in surface ocean δ18O that signal maximum monsoon‐driven runoff and to maxima in diagenetic sediment sulphide contents. High freshwater flows from monsoons are believed to drive eastern Mediterranean sapropel formation through water column stabilization that favors F. profunda production and later through development of deep water column dysoxia/anoxia because of reduced ventilation. The relative abundances of F. profunda are high between 5 and 11 14C kyr B.P. with a maximum at ∼9 14C kyr B.P. The surface ocean production changes marked by F. profunda therefore begin earlier and finish later than the formation of the S1 sapropel, which only develops between 6 and 10 14C kyr B.P.

Different nutrient sources forcing increased productivity during eastern Mediterranean S1 sapropel formation as reflected by calcareous dinoflagellate cysts

Comparison of calcareous dinoflagellate cyst assemblages with Ba, Al, Mn, and Fe records from three sediment cores collected in the eastern Mediterranean Sea indicate that calcareous dinoflagellate cysts are generally resistant to postdepositional dissolution. Cyst association changes during and after sapropel S1 formation can therefore be closely related to variability in surface water productivity. Two groups of cysts are defined: those having highest abundances within the sapropelic and postsapropelic sediments. The temporal cyst distributions suggest increased freshwater input mainly from the Nile and a shallowing of the pycnocline as the most important processes increasing nutrient concentration in the photic zone, thus leading to increased productivity and organic carbon fluxes during sapropel formation. Furthermore, a general warming trend at the beginning of S1 formation and a slight salinity decrease are reconstructed.

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).

A hydraulic box model study of the Mediterranean response to postglacial sea‐level rise

This paper quantifies the role of changing sea level in affecting Mediterranean stratification using a new model of the strait‐basin system, which allows for explicit time dependence. During the last deglaciation, Fairbank's Meltwater Peak 1B leads to rapidly rising global sea levels and a freshening of the oceans and hence increasing influx of ever freshening Atlantic water into the Mediterranean basin. Owing to long residence times, the salinity of the deep and intermediate water reservoirs would have decreased more slowly, so that the basin was more stably stratified for a period, with possible implications for sapropel formation. Our model is used to estimate the size of this reservoir effect in comparison to the freshwater influx from the opening of the Black Sea at the Bosphorus. It is found that this mechanism affects the stratification to a similar degree as the Black Sea opening. Other climatic mechanisms have also affected the freshwater budget of the Mediterranean at this time, but their sizes are more difficult to quantify.