Sapropel Layers Research Articles

Middle-Late Quaternary paleoclimate of northern margins of the Saharan-Arabian Desert: reconstruction from speleothems of Negev Desert, Israel

Speleothems in arid and hyper-arid areas of Negev Desert, Israel, are used in paleoclimate reconstruction of northern margins of Saharan-Arabian Desert, focused on the following objectives: 1) precise U–Th dating of the timing of speleothem growth as an indicator of periods of humid climate, i.e. positive effective precipitation; 2) the origin of rainfall using the speleothem δ 18O and changes in spatial pattern of speleothem deposition and speleothem thickness along a north–south transect; 3) changes of vegetation cover based on speleothem δ 13C variations. During the last 350 ka major humid periods, referred to herein as Negev Humid Periods (NHP), occurred in the central and southern Negev Desert at 350–310 ka (NHP-4), 310–290 ka (NHP-3), 220–190 ka (NHP-2), and 142–109 ka (NHP-1). NHP-4, NHP-2 and NHP-1 are interglacial events, whereas NHP-3 is associated with a glacial period. During NHP-1, 2 and 3 the thickness and volume of the speleothems decrease from the north to the south, and in the most southern part of the region only a very thin flowstone layer formed during NHP-1, with no speleothem deposition occurring during NHP-2 and 3. These data imply that the Eastern Mediterranean Sea was the major source of the rainfall in northern and central Negev. More negative speleothem δ 18O values, relative to central parts of Israel (Soreq Cave) are attributed to Rayleigh distillation because of the increasing distance from the Mediterranean Sea. Speleothem deposition during the NHP-4 in the southern Negev was more intensive than in most of the central Negev, suggesting the prominence of the tropical rain source. Decrease in speleothem δ 13C during NHP events indicates growth of the vegetation cover. Nevertheless, the ranges of δ 13C values show that the vegetation remained semi-desert C4 type throughout the NHPs, with an additional significant carbon fraction coming from the host rock and the atmosphere. These observations, together with small thickness of the speleothem layers, favor that NHP events consisted of clusters of very short humid episodes interspersed with long droughts. NHP events were contemporaneous with climate periods with monsoon index of ≥51 (cal/cm 2 × day) and with the formation of sapropel layers in the Mediterranean Sea. Such simultaneous intensification of the monsoon and Atlantic-Mediterranean cyclones is probably related to the weakening of the high pressure cell above sub-tropical Atlantic Ocean, which enabled more rainfall to penetrate into the Saharan-Arabian Desert from the north and south. The contemporaneous occurrence of the NHP events and the increased monsoon rainfall could have opened migration corridors, creating climatic “windows of opportunity” for dispersals of hominids and animals out of the African continent.

Trace-elemental derived paleoceanographic and paleoclimatic conditions for Pleistocene Eastern Mediterranean sapropels

Eastern Mediterranean sediments are characterized by cyclic deposition of organic-rich sediments known as sapropels. Enhanced primary productivity combined with bottom water oxygen depletion are thought to be the main drivers for sapropel deposition. We selected sapropel layers from a suite of ODP-Leg 160 cores, and applied a set of geochemical proxies to determine paleo-productivity variations, redox conditions of the water column during deposition, and provenance of detrital material. High sedimentary Ba/Al and C org contents indicate enhanced primary production, whereas the sedimentary La/Lu ratio, points to an enhanced contribution from a North African riverine source, during sapropel formation. These features are especially pronounced on Sapropels S5 and S7, deposited during a particularly warm climatic interval. This indicates a more intense North African drainage/weathering and consequently run-off for those sapropels that have the most enhanced expression of productivity too. Correspondingly, the latter has also resulted in bottom water redox conditions that have been more severe during these sapropels than during others. Deepwater formation from Adriatic and Aegean areas, thought to be mainly controlled by sustained cooling of preconditioned surface waters, triggers the onset of bottomwater ventilation, thus sapropel duration. Our data, therefore, suggest that the intensity of sapropel formation is determined by the North African monsoonal system, whereas their duration is directed by northern borderlands climatic conditions.

The precession phase of the boreal summer monsoon as viewed from the eastern Mediterranean (ODP Site 968)

The astronomical timescale of the Eastern Mediterranean Plio–Pleistocene builds on tuning of sapropel layers to Northern Hemisphere summer insolation maxima. A 3000-year precession lag has become instrumental in the tuning procedure as radiocarbon dating revealed that the midpoint of the youngest sapropel, S1, in the early Holocene occurred approximately 3000 years after the insolation maximum. The origin of the time lag remains elusive, however, because sapropels are generally linked to maximum African monsoon intensities and transient climate modeling results indicate an in-phase behavior of the African monsoon relative to precession forcing. Here we present new high-resolution records of bulk sediment geochemistry and benthic foraminiferal oxygen isotopes from ODP Site 968 in the Eastern Mediterranean. We show that the 3000-year precession time lag of the sapropel midpoints is consistent with (1) the global marine isotope chronology, (2) maximum (monsoonal) precipitation conditions in the Mediterranean region and China derived from radiometrically dated speleothem records, and (3) maximum atmospheric methane concentrations in Antarctica ice cores. We show that the time lag relates to the occurrence of precession-paced North Atlantic cold events, which systematically delayed the onset of strong boreal summer monsoon intensity. Our findings may also explain a non-stationary behavior of the African monsoon over the past 3 million years due to more frequent and intensive cold events in the Late Pleistocene.

Hydrology in the Sea of Marmara during the last 23 ka: Implications for timing of Black Sea connections and sapropel deposition

[1] Sediments deposited under lacustrine and marine conditions in the Sea of Marmara hold a Late Quaternary record for water exchange between the Black Sea and the Mediterranean Sea. Here we report a multiproxy data set based on oxygen and strontium isotope results obtained from carbonate shells, major and trace elements, and specific organic biomarker measurements, as well as a micropaleontological study from a 14C-dated sediment core retrieved from the Sea of Marmara. Pronounced changes occurred in δ18O and 87Sr/86Sr values at the fresh and marine water transition, providing additional information in relation to micropaleontological data. Organic biomarker concentrations documented the marine origin of the sapropelic layer while changes in n-alkane concentrations clearly indicated an enhanced contribution for organic matter of terrestrial origin before and after the event. When compared with the Black Sea record, the results suggest that the Black Sea was outflowing to the Sea of Marmara from the Last Glacial Maximum until the warmer Bølling-Allerød. The first marine incursion in the Sea of Marmara occurred at 14.7 cal ka B.P. However, salinification of the basin was gradual, indicating that Black Sea freshwaters were still contributing to the Marmara seawater budget. After the Younger Dryas (which is associated with a high input of organic matter of terrestrial origin) both basins were disconnected, resulting in a salinity increase in the Sea of Marmara. The deposition of organic-rich sapropel that followed was mainly related to enhanced primary productivity characterized by a reorganization of the phytoplankton population.

Integrated stratigraphy and astronomical tuning of lower–middle Pleistocene Montalbano Jonico section (southern Italy)

Astronomical calibration of the lower–middle Pleistocene Montalbano Jonico section located in the Lucania Basin (Southern Italy) is presented. Previous papers widely discussed the integrated stratigraphy (calcareous nannofossils, sapropel stratigraphy, benthic and planktonic oxygen stable isotopes) and the paleoenvironmental features of this section and its potential suitability for the selection of the Middle Pleistocene Global Stratotype Section and Point (GSSP). In this study, new planktonic δ 18O data, additional biostratigraphical constraints and new tephrochronology on volcaniclastic layers occurring within the studied record are reported. The new chronostratigraphic framework provides a robust base for correlation of the oxygen isotope stratigraphy for the Montalbano Jonico section with the glacial and interglacial fluctuations of the Oceanic and Mediterranean δ 18O reference deep-sea records. Specifically, the lower part of the Montalbano Jonico section (Interval A) provides correlation of the planktonic and benthic δ 18O cycles to Marine Isotope Stage (MIS) 36 to MIS 23. Interval A includes a distinct peak of left-coiled neogloboquadrinids, the Globoratalia crassaformis influx, the First Occurrence of Gephyrocapsa omega, and the First Common Occurrence and Last Common Occurrence of Reticulofenestra asanoi. These stratigraphical constraints support the tuning of five sapropel layers included in this part of the section to insolation cycles i-112, i-104, i-102, i-90 and i-86. The upper part of the section (Interval B), which includes the temporary disappearance (td2) of G. omega and tephra layer V5, Ar/Ar age dated at 719.5 ± 12.6 ka, is consistent with identification of MIS 22 to MIS 16 in the planktonic δ 18O pattern. The δ 18O time series of the whole section was reconstructed using the midpoints of individual sapropels and their correlative precession minima, visual comparison of the δ 18O pattern with the record available at the Mediterranean ODP Site 975, and, in the upper part of the section, the Ar/Ar age of tephra V5. The developed astronomical tuning revealed that the Montalbano Jonico section covers an interval from 1240 ka to 645 ka. A significant change in sedimentation rate occurs between Intervals A (0.53 m/ky) and Interval B (0.91 m/ky) at about 870 ka and is consistent with a sea-level drop from a bathyal to a circalittoral environment. Bioevents recognised in the Montalbano Jonico section have been dated according to the astronomical calibration, and age assignments of tephra V1–V4 and V6–V9 are also proposed. The Montalbano Jonico section fills the gap between the top of the Vrica section and the base of the Ionian informal Middle Pleistocene stage, and represents a Mediterranean reference section for the Mid-Pleistocene transition.

The geochemical behavior and isotopic composition of Hg in a mid-Pleistocene western Mediterranean sapropel

The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average δ 202Hg value of −0.91‰ ± 0.15‰ ( n = 5, 1 SD) and Δ 199Hg value of 0.11‰ ± 0.03‰ ( n = 5, 1 SD). Background sediments have an average δ 202Hg of −0.76‰ ± 0.16‰ ( n = 5, 1 SD) and Δ 199Hg of 0.05‰ ± 0.01‰ ( n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.

Benthic environmental changes in the Eastern Mediterranean Sea during sapropel S5 deposition

High resolution benthic foraminiferal analyses of sediment cores collected at different sites in the Eastern Mediterranean Sea revealed that profound changes occurred in deep-sea ventilation over the past 124–119 ka BP. The three cores (SIN97-GC01, BAN89-GC09, BD02-GC01) were selected in order to investigate middle and deep bathyal ecosystems in the Eastern Mediterranean basin. At site SIN97-GC01 (933 m water depth, Urania Basin), the presence of benthic fauna in the sapropel layer, and the gradual increase in benthic foraminiferal abundance from the middle part of the sapropel, suggest that the sea-floor was partially ventilated and that re-oxygenation had increased considerably during the late phase of S5 deposition. Cores BAN89-GC09 (2011 m water depth, Napoli Dome) and BD02-GC01 (2470 m water depth, Ionian Sea) show a different pattern. During sapropel S5, benthic abundance and diversity strongly decrease and microfauna even disappears in some levels, suggesting the establishment and maintenance of stagnant and anoxic conditions at the seafloor until the end of S5 deposition. In the deepest part of the basin, the re-establishment of the benthic foraminiferal community at the top of the S5 layer indicates a relatively slow bottom re-oxygenation. At the southernmost site, the foraminiferal assemblage records a short oxygenation pulse during the deposition of sapropel S5 linked to a short cold spell. The results of this study suggest that the evolution of the dysoxic–anoxic conditions, as well as the re-oxygenation pattern at the end of the stagnant period, were characterised by spatial and temporal variability, possibly controlled by basin physiography.

Geochemical evidence for high-resolution variations during deposition of the Holocene S1 sapropel on the Cretan Ridge, Eastern Mediterranean

Major and minor element distributions and solid phase phosphorus contents were measured in a sediment core from the Cretan Ridge that contains the Holocene S1 sapropel. Micro-XRF ultra-high resolution analysis reveals multiple Mn peaks in the oxidized upper portion of the sapropel, which implies either a non-steady-state upward remobilization of Mn or not constant downward diffusion of bottom water oxygen. Sequential extraction allowed the identification of different phosphorus phases. Detrital phosphorus increases in the sapropel layer, suggesting enhanced delivery from land during sapropel deposition. Higher organic carbon to organic phosphorus ratios in the sapropel indicate enhanced regeneration of phosphorus relative to carbon under low-oxygen conditions. Fourier analysis on the ultra-high resolution XRF data reveals frequencies that are provisionally assigned to millennial to decennial solar cycles. Although these cycles have been reported from several continental and marine archives, they have never been documented from Sapropel S1 in the Eastern Mediterranean.

Mud volcanoes and gas hydrates in the Anaximander mountains (Eastern Mediterranean Sea)

Detailed multibeam, sedimentological, and geophysical surveys provide ample new data to confirm that the Anaximander Mountains (Eastern Mediterranean) are an important area for active mud volcanism and gas hydrate formation. More than 3000 km of multibeam track length was acquired during two recent missions and 80 gravity and box cores were recovered. Morphology and backscatter data of the study area have better resolution than previous surveys, and very detailed morphology maps have been made of the known targeted mud volcanoes (Amsterdam, Kazan and Kula), especially the Amsterdam “crater” and the related mud breccia flows. Gas hydrates collected repeatedly from a large area of Amsterdam mud volcano at a sub-bottom depth of around 0.3–1.5 m resemble compacted snow and have a rather flaky form. New gas hydrate sites were found at Amsterdam mud volcano, including the mud flow sloping off to the south. Gas hydrates sampled for the first time at Kazan mud volcano are dispersed throughout the core samples deeper than 0.3 m and display a ‘rice’-like appearance. Relative chronology and AMS dating of interbedded pelagic sediments (Late Holocene hemipelagic, sapropel layer S1 and ash layers) within the mud flows indicate that successive eruptions of Kula mud volcano have a periodicity of about 5–10 kyrs. New mud volcanoes identified on the basis of multibeam backscatter intensity were sampled, documented as active and named “ Athina” and “ Thessaloniki”. Gas hydrates were sampled also in Thessaloniki mud volcano, the shallowest (1264 m) among all the active Mediterranean sites, at the boundary of the gas hydrate stability zone. Biostratigraphical analyses of mud breccia clasts indicated that the source of the subsurface sedimentary sequences consists of Late Cretaceous limestones, Paleocene siliciclastic rocks, Eocene biogenic limestones and Miocene mudstones. Rough estimations of the total capacity of the Anaximander mud volcanoes in methane gas are 2.56–6.40 km 3.

Role of sea-level forced sedimentary processes on the distribution of organic carbon-rich marine sediments: A review of the Late Quaternary sapropels in the Mediterranean Sea

The 124 ka to present sapropel S5–S1 series in the eastern Mediterranean Sea, correspond to the last Glacial–Interglacial period, and permit detailed investigation of 5th order sedimentary frequencies. The Sapropel layers are clustered in high-frequency Milankovian cycle that occur within sequence stratigraphic 4th order cycles of late Pleistocene-Holocene sea-level oscillations. The sea-level changes had an amplitude exceeding 120 m and have been correlated to the glacial-interglacial cycles modulated by the 100 ka eccentricity. Using published data we examined occurrences of the complete S5–S1 series in the Mediterranean Sea and then we focussed on two stratigraphic sequences in the central Adriatic shelf and Middle Adriatic Depression where sapropels S5 and S1 were deposited during periods of sea-level rise recorded by the Transgressive Systems Tract (Marine Isotopic Stage 5 and 1, respectively). Sapropels S4 and S3 were deposited during the Highstand Systems Tract, that developed throughout MIS 5 of the late Pleistocene sequence. Sapropel S2 should have been deposited during the warm MIS 3.3 and followed the MIS 4 sea-level drop when sediment supply was enhanced during the late highstand and falling stage of sea-level of the late Pleistocene sequence. The pronounced MIS 2 cold stage led to a lowstand and to the regional erosional surface (Sequence Boundary). We also compared the Adriatic sapropel S5–S2 sequence (4th order) to the Mid-Pliocene cluster “O” sequence (3rd order). In both cases, sapropels are well formed before the increase of sediment supply due to progradation of the shelf wedge, that occurs in the late high stand and initial falling stage in sea-level. Finally, we considered the influence of sea-level lowering which could cause a weakening of North Adriatic Deep Water formation, therefore influencing the Mediterranean deep water circulation. This condition would favour oxygen depletion, but it does not correspond to a well developed sapropel (as in the case of S2). Based on the assumption that productivity increases related to insolation play a dominant role in their sedimentation, sapropel S2 is the result of low amplitude insolation maxima, that were unable to generate a productivity increase leading to the burial of conspicuous amount of organic matter. Therefore, in the Adriatic Sea the absence of the S2 sapropel is correlated to the high sedimentation rate diluting the signal, whereas in the eastern Mediterranean basin this sapropel is virtually absent because of a weak orbital forcing combined with oxidation that erased its record.

Paleoceanographic implications of nitrogen and organic carbon isotopic excursions in mid-Pleistocene sapropels from the Tyrrhenian and Levantine Basins, Mediterranean Sea

Comparison of the total nitrogen and organic carbon isotopic compositions of sediment sequences from the Tyrrhenian and Levantine Basins that contain three same-age sapropels reveals evidence of both similarities and differences in mid-Pleistocene sapropel-forming conditions in the western and eastern Mediterranean Sea. Total δ 15N values systematically decrease at both locations from 5‰ in background sediments to − 1‰ in the sapropels. The lower δ 15N values signify that nitrogen fixation was important to the increased marine productivity implied by the higher organic carbon concentrations, which is consistent with widespread salinity stratification of surface waters when sapropels accumulated. Organic δ 13C values systematically increase from − 25‰ in the background sediments to − 22‰ in the sapropels at the Tyrrhenian site, which confirms that increased marine productivity accompanied sapropel deposition. In contrast, δ 13C values decrease in the sapropel layers in the Levantine Basin even though organic carbon concentrations are higher in these sapropels. The different types of δ 13C excursions associated with increased organic matter production at the two locations indicates a difference in the dissolved carbon pool from a combination of greater fluvial delivery of continental carbon and stronger recycling of organic matter in the Levantine Sea than in the Tyrrhenian Sea. This difference implies that the shifts to wetter climate during times of sapropel deposition were stronger in the eastern than in the western Mediterranean.

Analysis of foraminiferal paleocoenoses from the upper Ypresian sapropelic layers of the northeastern Peri-Tethys

A series of layers rich in organic matter (sapropelites) is known in the upper Ypresian deposits of the northeastern Peri-Tethys. Study of formainifers and quantitative analysis of their different ecological groups showed that proportion of infauna is considerably grater in sapropelic layers than in host deposits. In distinction from infauna, tests of species not burrowing into sediments (epifauna) are partially dissolved. The analyzed paleocoenoses of benthic foraminifers imply that sapropelites have been formed most likely in dysoxic environments, under conditions of the bottom waters contamination by carbon dioxide in response to a high Corg influx, weakened water circulation and low oxygen concentration. The comparison of foraminiferal assemblages with paleocoenoses studied previously in the Paleocene/Eocene boundary sapropelic interbed suggests that deposits rich in Corg originated under the same hydrogeochemical environments.

Climatic cycles as expressed in sediments of the PROMESS1 borehole PRAD1‐2, central Adriatic, for the last 370 ka: 1. Integrated stratigraphy

A multiproxy integrated chronological framework, based on oxygen and carbon stable isotope stratigraphy, biostratigraphy (foraminifera and nannoplankton bioevents and foraminifer assemblage‐based climate cyclicity), magnetostratigraphy, sapropel stratigraphy, and 14C AMS radiometric dates, has been achieved for borehole PRAD1‐2, collected in 185.5 m water depth in the central Adriatic. This work was carried out within the European Community project Profiles across Mediterranean Sedimentary Systems (PROMESS1). The 71.2 m long borehole spans a time interval between late MIS 11 and MIS 1 (the last 370 ka), showing a chronological resolution of 500 and 250 years per cm during interglacial and glacial intervals, respectively. At present, this record is the most expanded and continuous marine record available for the Adriatic Basin. Several orbital cycles can be recognized in the PRAD1‐2 record: the 100 ka glacial‐interglacial fluctuations and the 23 ka precession‐related cycles, which in turn control the deposition of sapropel layers. An integrated analysis of short‐term oscillations within the Last Glaciation interval (MIS 4–MIS 2) allowed the identification of the Adriatic signature of Dansgaard‐Oeschger events, showing the potential to achieve a more refined chronostratigraphic framework for the top part of the PRAD1‐2 record. Finally, the age model obtained by this study allowed the chronological integration of the main foraminifera bioevents detected in the borehole as well as of the volcanoclastic layers present in the upper part of the record. Despite its proximal location, PRAD1‐2 presents a continuous record and shows the potential to be consistently correlated both with deep‐sea and continental records in the Mediterranean region and beyond.

Two Distinct Photobacterium Populations Thrive in Ancient Mediterranean Sapropels

Eastern Mediterranean sediments are characterized by the periodic occurrence of conspicuous, organic matter-rich sapropel layers. Phylogenetic analysis of a large culture collection isolated from these sediments revealed that about one third of the isolates belonged to the genus Photobacterium. In the present study, 22 of these strains were examined with respect to their phylogenetic and metabolic diversity. The strains belonged to two distinct Photobacterium populations (Mediterranean cluster I and II). Strains of cluster I were isolated almost exclusively from organic-rich sapropel layers and were closely affiliated with P. aplysiae (based on their 16S rRNA gene sequences). They possessed almost identical Enterobacterial Repetitive Intergenic Consensus (ERIC) and substrate utilization patterns, even among strains from different sampling sites or from layers differing up to 100,000 years in age. Strains of cluster II originated from sapropels and from the surface and carbon-lean intermediate layers. They were related to Photobacterium frigidiphilum but differed significantly in their fingerprint patterns and substrate spectra, even when these strains were obtained from the same sampling site and layer. Temperature range for growth (4 to 33 degrees C), salinity tolerance (5 to 100 per thousand), pH requirements (5.5-9.3), and the composition of polar membrane lipids were similar for both clusters. All strains grew by fermentation (glucose, organic acids) and all but five by anaerobic respiration (nitrate, dimethyl sulfoxide, anthraquinone disulfonate, or humic acids). These results indicate that the genus Photobacterium forms subsurface populations well adapted to life in the deep biosphere.

Organic geochemical evidence of Late Glacial–Holocene climate instability in the North Aegean Sea

Sedimentary molecular biomarker compositions, organic carbon and CaCO 3 concentrations, and bulk organic δ 13C and total δ 15N values document a series of paleoclimatic changes that have affected organic matter production and sedimentation in the North Aegean Sea from the termination of the latest glacial maximum through the middle Holocene. Continental environmental conditions have been inferred from land–plant biomarkers. Paleoproductivity patterns of diatoms, prymnesiophytes, dinoflagellates, and nannoplankton were discerned from marine-derived biomarkers. Although organic matter is predominantly marine, variations in the proportion of marine and land contributions and in its general preservation are evident. Rapid changes in the Eastern Mediterranean thermohaline circulation evidently occurred in parallel to sea surface temperature oscillations. Enhanced deep-water ventilation accompanied cold intervals in the late Last Glacial, the Younger Dryas, and during an interruption of deposition of the S 1 sapropel, probably in response to strengthening of north winds over the region. In contrast, decreased thermohaline circulation occurred during the warm interval associated with the S 1 sapropel. During this period, increased freshwater inputs diminished ventilation of deep-water masses and enhanced organic matter production and preservation. Intriguing fluctuations in sea-surface temperatures and the concentrations of marine and terrestrial biomarkers within the sapropelic layer, most prominently in the interruption from 8.2 to 7.8 calibrated radiocarbon conventional kyr BP, support the hypothesis that several distinct events of deep water reoxygenation occurred in the North Aegean Sea during deposition of the S 1 sapropel. These events reflect centennial excursions to the more common low-productivity and well ventilated conditions of the Mediterranean that may possibly be related to North Atlantic Oscillation cycles.

Opal phytoplankton assemblages of the Late Quaternary sapropel layers S5 and S7 from the southeastern Mediterranean Sea (“Meteor”-Cruise 40/4, Site 67)

Well-preserved opal planktic assemblages containing diatom and silicoflagellate species were discovered in the finely laminated sapropels S5 and S7 from Site 67, core GeoTüKL51 during “RV Meteor”-Cruise 40, Leg 4 (34° 48 N; 27° 17 E). The siliceous microflora, composed quasi-exclusively of warm water species, is here studied for taxonomy, stratigraphy and ecology, as well as for quantitative distribution. The diatom assemblage is very diverse and dominated by rhizosolenids, which are generally characteristic of stratified, oligotrophic waters. A significant shift to higher abundances of Pseudosolenia calcar-avis is observed in the upper part of Sapropel S5, with a simultaneous decrease in the abundances of Thalassionema bacillaris/frauenfeldii, T. nitzschioides, T. oestrupii and Chaetoceros sp., which is probably due to nutrient – poor conditions in the surface waters.

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.

Pliocene–Holocene evolution of depositional conditions in the eastern Mediterranean: Role of anoxia vs. productivity at time of sapropel deposition

A multiproxy geochemical study of nine sapropel layers from ODP Hole 964A, ODP Leg 160 in the eastern Mediterranean spanning the Pliocene–Holocene time interval provides new insights into the evolution of sapropel deposition. Paleoenvironmental proxies were used for reconstruction of productivity (Ba derived from marine barite), oxygen conditions (trace metal ratios) and sedimentary regime (clay minerals, detrital elements). These proxies reveal a significant increase in river runoff relative to decreasing aeolian input during sapropel deposition over the whole time interval. Ba excess supports the argument that a significant increase in export productivity is the main triggering mechanism for sapropel deposition, although preservation also played an important role. Furthermore, major differences exist in depositional conditions, including both oxygenation and productivity since the Pliocene. Productivity fluctuated substantially and was higher during the Pliocene and Pleistocene than during the Holocene; at the same time decreasing oxygen availability parallels the enhanced productivity. Dysoxic to anoxic conditions appear to coincide with marine productivity maxima, thus suggesting that oxygen depletion may be linked to greater consumption rather than restricted circulation. This correspondence supports the hypothesis that productivity fluctuations resulting from climate oscillations were the main cause of enhanced organic matter contents and also a main controlling factor for reduced oxygen availability.

Mid-Pliocene warm climate and annual primary productivity peaks recorded in sapropel deposition

ABSTRACT: The Mid-Pliocene deposition in the Northern Apennines includes finely laminatedsapropelitic horizons (insolation-cycles i-292 to i-282; 3.058 to 2.983 Myr) intervening within theslope-basin deposition of the foredeep, during a late highstand dated to the Globorotalia bononiensis zone. An integrated sedimentological, geochemical and micropaleontological study of the sapropel-bearing succession allowed the estimation of the time involved in the sapropel layer deposition, andto define the origin of the organic matter and the contribution of the primary productivity. The calcu-lated average sedimentation rate of the section, including the intervening bioturbated beds, wasabout 0.4 mm yr –1 —similar to the rest of the pelitic deposition during the whole Pliocene. Within indi-vidual sapropels, recurrently-spaced 0.3 to 0.6 mm thick laminae consisting of calcareous and/orsiliceous plankton tests, alternating with pyrite- and organic-rich mudstones, were interpreted as therecord of annual variation in the supply. We could thus calculate the duration of individual sapropelsby simply counting the number of couplets occurring in the layers: these spanned between 7500 and10000 yr. The kerogen content of the sapropels is ≥65% marine origin, whereas the same componentin the intervening bioturbated intervals is ~10%. This supports the hypothesis that the sapropeldeposition is linked to significant primary productivity peaks. In contrast, changes in the terrigenoussupply or strong disoxia were not recorded at the bottom. Taking the nutrient supply as constant, pro-ductivity peaks could then be forced mainly by variations in the intensity of the insolation. The latter,is linked to the precession minima and has a wide oscillation; this oscillation occurred even at theend of the Mid-Pliocene warm climate, which was mainly characterized by a progressive climatedeterioration that heralded the Northern Hemisphere glaciation.KEY WORDS: Mid-Pliocene climate · Sapropel deposition · Sedimentation rate · Organic matter ·Primary productivity

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.