Anoxic Basin Research Articles

Tethyan–Mediterranean organic carbon‐rich sediments from Mesozoic black shales to sapropels

AbstractThe Jurassic to Holocene record of black shale deposition in the Tethys‐Mediterranean region is unrivalled by that of any other ocean basin, either in land sections or drill cores. The term ‘black shale’ is used here broadly for sediments with elevated organic carbon concentrations (> 1%), including the Pliocene to Recent sapropels. Most of the black shales are devoid of benthonic organisms, are laminated, and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic. The Tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales. These records have been central in understanding climate control on ocean dynamics and biogeochemical cycles. The Mesozoic black shales were deposited within well‐defined time envelopes of around 0·5 to 1 Myr. These black shales, which were confined to certain chronostratigraphic intervals in the Jurassic and Cretaceous, were recognized as expressions of global Oceanic Anoxic Events in the mid‐1970s and subsequently named after prominent researchers (Bonarelli, Selli, Goguel). The black shale episodes were dated by biostratigraphic methods and by high‐resolution chemostratigraphy and cyclostratigraphy. Mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate. Research into younger (Pliocene to Pleistocene) sedimentary cycles (including black shales, termed sapropels) exposed in land sections, or found in pelagic and hemipelagic marine sediment cores of Late Quaternary age, started in the 1950s. Main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes, palaeoclimate evolution of the circum‐Mediterranean area and biogeochemical dynamics in anoxic basins. After 50 years of intensive research, the Tethyan and Mediterranean black shales remain subjects of fascination in Earth Science. Tracing the origins and building upon recent progress, the current hypotheses on their formation are reviewed here. It is a panorama of complex interplays between global and regional tectonics, climate dynamics during both Ice House and Greenhouse states of global and regional climate, oceanographic responses to these climate changes, and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity.

Fish activity: a major mechanism for sediment resuspension and organic matter remineralization in coastal marine sediments

We quantify sediment resuspension due to groundfish activity in a partly anoxic basin using acoustic backscatter sensors, transmissometers, and remotely operated cameras on stationary and mobile platforms. Where these fish were present, a distinct benthic nepheloid layer (BNL) devel- oped despite minimal bottom currents ( 100 disturbances m -2 d -1 resulting in complete surface reworking every 2.5 d and a daily resuspension of 1.3 ± 0.7 l bulk sediment m -2 d -1 . Preliminary geochemical measurements suggest substantial impact of fish resuspension activity, the immediate effect being an instantaneous increase in nutrient concentration in the benthic boundary layer and a drop in oxygen concentration. Over longer time scales (hours to days), the freshly exposed organic matter is oxidized and additional nutrients (silica, ammonium, phosphate) are regenerated and released to the water. The increase in benthic oxygen demand suggests that fish activity reduces organic carbon sequestration by 9 ± 5 mmol C m -2 d -1 , equivalent to ~40% of its downward flux in this basin. To date, these processes are missing from geochemical models and require further investigation, particularly considering the depletion of groundfish stocks and the likely effects on global biogeochemical cycles.

Protistan community patterns within the brine and halocline of deep hypersaline anoxic basins in the eastern Mediterranean Sea

Environmental factors restrict the distribution of microbial eukaryotes but the exact boundaries for eukaryotic life are not known. Here, we examine protistan communities at the extremes of salinity and osmotic pressure, and report rich assemblages inhabiting Bannock and Discovery, two deep-sea superhaline anoxic basins in the Mediterranean. Using a rRNA-based approach, we detected 1,538 protistan rRNA gene sequences from water samples with total salinity ranging from 39 to 280 g/Kg, and obtained evidence that this DNA was endogenous to the extreme habitat sampled. Statistical analyses indicate that the discovered phylotypes represent only a fraction of species actually inhabiting both the brine and the brine-seawater interface, with as much as 82% of the actual richness missed by our survey. Jaccard indices (e.g., for a comparison of community membership) suggest that the brine/interface protistan communities are unique to Bannock and Discovery basins, and share little (0.8-2.8%) in species composition with overlying waters with typical marine salinity and oxygen tension. The protistan communities from the basins' brine and brine/seawater interface appear to be particularly enriched with dinoflagellates, ciliates and other alveolates, as well as fungi, and are conspicuously poor in stramenopiles. The uniqueness and diversity of brine and brine-interface protistan communities make them promising targets for protistan discovery.

Sulfur- and oxygen-isotopes in sediment-hosted stratiform barite deposits

Sulfur- and oxygen-isotope analyses have been obtained for sediment-hosted stratiform barite deposits in Alaska, Nevada, Mexico, and China to examine the environment of formation of this deposit type. The barite is contained in sedimentary sequences as old as Late Neoproterozoic and as young as Mississippian. If previously published data for other localities are considered, sulfur- and oxygen-isotope data are now available for deposits spanning a host-rock age range of Late Neoproterozoic to Triassic. On a δ 34S versus δ 18O diagram, many deposits show linear or concave-upward trends that project down toward the isotopic composition of seawater sulfate. The trends suggest that barite formed from seawater sulfate that had been isotopically modified to varying degrees. The δ 34S versus δ 18O patterns resemble patterns that have been observed in the modern oceans in pore water sulfate and water column sulfate in some anoxic basins. However, the closest isotopic analog is barite mineralization that occurs at fluid seeps on modern continental margins. Thus the data favor genetic models for the deposits in which barium was delivered by seafloor seeps over models in which barium was delivered by sedimentation of pelagic organisms. The isotopic variations within the deposits appear to reflect bacterial sulfate reduction operating at different rates and possibly with different electron donors, oxygen isotope exchange between reduction intermediates and H 2O, and sulfate availability. Because they are isotopically heterogeneous, sediment-hosted stratiform barite deposits are of limited value in reconstructing the isotopic composition of ancient seawater sulfate.

Chronology of Atmospheric Deposition of Arsenic Inferred from Reconstructed Sedimentary Records

Vertical distributions of As, Fe, and organic C were determined in dated (210Pb and 137Cs) sediment cores obtained from two adjacent basins (one perennially oxic and the other seasonally anoxic) of an oligotrophic headwater lake where atmospheric deposition is the only input of anthropogenic As. Despite similar sources in the two basins, the As profiles in the sediments differed markedly. Differences include the following: (i) As concentrations increased sharply upward close to the sediment surface in the perennially oxic basin due to scavenging of upward diffusing As by Fe oxyhydroxides, whereas they decreased in the seasonally anoxic basin where As scavenging by Fe oxyhydroxides did not occur, and (ii) the magnitude and position of major subsurface As maxima differed between the two basins. We applied a one-dimensional transport-reaction equation to porewater As concentration profiles obtained at the two sites to estimate sedimentary As concentrations at the time of deposition as well as subsequent addition or removal of As at various sediment depths. By multiplying As concentrations at the time of deposition by sediment mass accumulation rates, we were able to estimate variations in As fluxes at the sediment surface over the last two centuries. These fluxes were then transformed into atmospheric As deposition fluxes by applying a correction for basin-specific processes using the ratio of expected from atmospheric deposition to measured unsupported 210Pb inventories at the sampling sites. The resulting chronological profiles of atmospheric fluxes of As deposition are similar in both basins, and are consistent with both the history of specific markers for coal combustion and direct historical measurements of As in dry and wet atmospheric deposition in rural areas of North America. We conclude that the history of As inputs can be reconstructed from As sedimentary records using appropriate corrections for diagenesis and basin-specific processes.

Black Mudrocks: Lessons and Questions from the Mississippian Barnett Shale in the Southern Midcontinent

Although largely unstudied until recently, the Barnett Shale of the Ft.Worth Basin,Texas, has become one of the better known black mudrock successions in the world. Emerging results of new research on these rocks, based mostly on subsurface data, offer intriguing insights into fundamental aspects of mudrock formation. Among these insights is the recognition that mudrock facies are probably not widely continuous, that organic matter conversion to hydrocarbon may control distribution and abundance of nanoscale pores and thus permeability, that fracture distribution may be affected by mineralogical variations along a proximal-distal gradient, and that, based on trace element chemistry, the Barnett ocean may have been much more stratified than most other anoxic basins. Developing concepts of processes and products of mudrock formation being derived from Barnett research offer great potential for comparison to other mudrocks worldwide.

Heavy-metal pollution of sediments from the Polish exclusive economic zone, southern Baltic Sea

Analysis of 59 surface sediment samples from the Polish exclusive economic zone (EEZ) shows that Szczecin Lagoon sediments are the most polluted by heavy metals and that the degree of heavy-metal pollution decreases substantially on passing from the Szczecin Lagoon to the Pomeranian Bay and the inner shelf area and then on passing to the Bornholm Deep and Slupsk Furrow. Heavy-metal pollution in the sediments of the western part of the Polish EEZ therefore appears to follow the dispersion of the Oder River. Fluffy material from the Oder estuary appears to be the main source of heavy metals in the muddy sediments of the Bornholm Deep. The formation of sulphides is therefore not the principal factor controlling the enrichment of heavy metals in the sediments of this anoxic basin, although it may be responsible for the uptake of Mo, Sb and As. Two main factors control the distribution of the rare earth elements (REE) in sediments of the Polish EEZ: the input of Fe-organic colloids from rivers and the presence of detrital material in the sediments.

Lithological-mineralogical zonality of sulfide cyclothems in the Yaman-Kasy and Saf’yanov massive sulfide deposits (Urals)

Formulation of the general concept of hydrothermal-sedimentary massive sulfide formation is related to the development of models of Red Sea brines and Black Smokers with different lithological interpretations of the deposition mechanism for stratified sulfide sediments. The first model is based on the precipitation of hydrothermal sulfide particulates from waters of an anoxic basin and the formation of stratified ore sediments [3, 5]. According to the second model, stratified ores are formed during the submarine destruction of hydrothermal edifices and transport of the ore clast material by turbid flows to flanks of hydrothermal fields [1]. In ancient massive sulfide deposits, stratified sulfide ores have generally lost the primary features of chemogenic or clastogenic sediments due to diagenetic or metamorphic transformations. Elucidation of signatures of the diagenesis of clastogenic sulfide beds would make it possible to choose the optimal model.

Multiple Sources of Metals of Mineralization in Lower Cambrian Black Shales of South China: Evidence from Geochemical and Petrographic Study

AbstractBlack shales of the Lower Cambrian Niutitang Formation in southern China (Huangjiawan mine, Zunyi region, northern part of the Guizhou Province) host regionally distributed stratiform polymetallic Ni‐Mo‐platinum group elements (PGE)‐Au phosphate‐ and sulfide‐rich ores. These are confined to a ≥0.2‐m thick ore horizon composed of mineralized bodies of algal onkolites, phosphate nodules, and sulfide and shale clasts in a mineralized phosphate‐ and organic matter‐rich matrix. Compared to footwall and hanging wall shales, the ore bed is strongly enriched in Ni (up to 100‐fold), As (up to 97‐fold), Mo (up to 95‐fold), Sb (up to 67‐fold), Rh (up to 49‐fold), Cu (up to 37‐fold), Pd (up to 33‐fold), Ru (up to 24‐fold), Zn (up to 23‐fold), Pt (up to 21‐fold), Ir (up to 15‐fold), Co (up to 14‐fold), and Pb (up to 13‐fold). Even footwall and hanging wall black shales are significantly enriched by Mo (21‐fold) and Ni (12‐fold) but depleted in Cr in comparison to average Cambrian black shale. Organic matter is represented by separate accumulations dispersed in the rock matrix or as biotic bitumen droplets and veinlets in ore clasts. Similar organic carbon (Corg) values in an ore bed and enclosing footwall and hanging wall shales of little mineralization indicate that metal accumulation was not controlled only by biogenic productivity and organic matter accumulation rate. Evaporitic conditions during sedimentation of the basal part of the Niutitang Formation were documented by an occurrence of preserved Ni‐, V‐, Cr‐, and Cu‐enriched phosphate‐rich hardground with halite and anhydrite pseudomorphs on the paleosurface of the underlying Neoproterozoic carbonates. Neoproterozoic black shales of the Doushantuo Formation are characterized by increased metal concentrations. Comparison of metal abundances in both hardground and Doushantuo black shales indicate that black shales could have become a source of metal‐rich hardground during weathering. The polymetallic Ni‐Mo‐PGE sulfide‐rich ore bed is interpreted to represent a remnant of shallow‐water hardground horizon rich in metals, which originated in a sediment‐starved, semi‐restricted, seawater environment. During the Early Cambrian transgression an influx of fresh seawater and intensive evaporation, together with the hydrothermal enrichment of seawater in a semi‐restricted basin, resulted in the formation of dense metalliferous brines; co‐precipitation of metals together with phosphates and sulfides occurred at or above the oxic–anoxic sediment interface. Metal‐enriched hardground was disintegrated by the action of waves or bottom currents and deposited in a deeper part of the anoxic basin. Contemporaneously with the formation of a polymetallic Ni‐Mo‐PGE‐Au sulfide ore bed, economic sedimentary exhalative (SEDEX)‐type barite deposits were forming in a stratigraphically and geotectonically similar setting. The results of geochemical study at the Shang Gongtang SEDEX‐type Ba deposit indicate that concentrations of Ag, As, Cr, Cu, Fe, Mn, Ni, Pb, Sb, V, Zn and other metals decrease from top of the barite body toward the hanging wall black shale. Lower Cambrian black shales of the Niutitang Formation above the barite body also display similar element abundances as Neoproterozoic black shales of the Doushantuo Formation, developed in the footwall of the barite body. But the geochemical composition of the sulfide layer is different from the Ni‐Mo ore bed, showing only elevated Pb, Cu, Ni and Mo values. It is suggested that hydrothermal brines at Shang Gongtang might have leached metals from footwall Neoproterozoic sequences and became, after mixing with normal seawater, an additional source of Ag, Cr, Cu, Pb, Sb, Zn, Ni, PGE, V and other metals.

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans

The increase in atmospheric oxygen ca. 2.4 Ga had a significant impact on the geochemical cycling of Fe. The history of environmental oxygenation may be recorded in the Fe isotope composition of Archean and Proterozoic sediments, but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations. Here we present Fe isotope data and iron/aluminum ratios from the Black Sea oxic shelf and euxinic basin. The isotope data demonstrate that shelf Fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin. We propose that there is net transport of isotopically light Fe from sediments of the shelf to those of the distal, anoxic basin, consistent with enrichments in reactive Fe seen in the deep basin. The low δ 56 Fe benthic Fe flux is generated during the coupling of microbial Fe(III) reduction or sulfidization with Fe 2+ aq oxidation. Low δ 56 Fe values reported previously from Late Archean sedimentary pyrites may be an isotopic fingerprint of analogous Fe redox cycling in the Late Archean oceans. This interpretation implies shallow-water Fe redox recycling in the Late Archean. We predict that the light isotopic compositions of the Late Archean will prove to be distinct from those of the Early Archean, before Fe redox cycling became an important process, and we infer that this difference may be related to the presence of oxygen in the surface ocean.

The Rhyolite-Hosted Volcanogenic Massive Sulfide District of Cuale, Guerrero Terrane, West-Central Mexico: Silver-Rich, Base Metal Mineralization Emplaced in a Shallow Marine Continental Margin Setting

The Cuale mining district, situated in the Cordillera Madre del Sur, ~30 km southeast of Puerto Vallarta, Mexico, comprises a number of small, polymetallic Pb, Zn, Cu, volcanogenic massive sulfide deposits (VMS), and vein and stockwork-hosted deposits with low-sulfidation epithermal characteristics. Massive sulfide mineralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massive sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed the Cuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterrane belonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane hosting the majority of VMS deposits in Mexico. The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit intense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within the Cuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyolite flows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartz and plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite and subordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated with rhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence for shallow submarine or subaerial deposition. The rhyolites of the Cuale volcanic sequence are calc-alkaline with only limited compositional variations. However, the trace element signatures reveal a weak trend from the footwall rhyolites, which are characterized by low Zr/Y (1‐1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), to slightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6‐7.6 and chondrite normalized La/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasing degrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphibole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as the volcanic edifice was built. Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using isotope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of a minor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale represents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between 151.3 and 138 Ma. Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late lowsulfidation epithermal vein and stockwork mineralization characterized by high Au (1.89 ppm) and Zn (2.35%) but low Cu (0.2%) contents. The VMS mineralization is contained in more than 15 small orebodies, which exhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork vein networks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and vertically into sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhyolite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated or brecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed in some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins adjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediate footwall.

Molecular isotopic and dinoflagellate evidence for Late Holocene freshening of the Black Sea

The Black Sea is the world's largest anoxic basin with oxygen-free conditions below water depths of approximately 100 m resulting from strong density stratification. The salinity of its surface water likely varied substantially over time due to variations in freshwater input from large rivers and in the saline bottom water of Mediterranean origin coming across the shallow sill of the Bosporus. However, long-term reconstructions of surface water salinities are lacking. The invasion of the coccolithophorid Emiliania huxleyi in the Black Sea at approximately 2720 a, responsible for a marked change in sediment composition (lithology), has been commonly attributed to salinity levels rising above 11. We analyzed the δD values of long-chain alkenones produced by haptophyte algae, mainly E. huxleyi, in a core from the eastern basin of the Black Sea to reconstruct past variations in sea surface salinities, and combined this with relative salinity changes generated from organic-walled dinoflagellate cyst (dinocyst) distributions from the same core. Combined results indicate a substantial freshening of Black Sea surface waters in the last 3000 years, suggesting that sea surface salinity was substantially higher than the present-day salinity of approximately 18 at the time E. huxleyi invaded the Black Sea.

Geochemical and anthropogenic enrichments of Mo in sediments from perennially oxic and seasonally anoxic lakes in Eastern Canada

We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, 210Pb and 137Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition. The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment–water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1–2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment–water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm 3 mol −1 s −1. Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment–water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3–16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.

Twentieth century proxy records of temperature and soil organic matter input in the Drammensfjord, southern Norway

The Drammensfjord, in southern Norway, is an enclosed anoxic basin with a laminated sedimentary record. The fjord is a transition area between the marine and terrestrial environments and thus its sedimentary archive contains information on both environments. In order to reconstruct recent environmental changes in southern Norway, two sediment cores covering the last 90 years were analyzed. We used the Branched versus Isoprenoid Tetraether (BIT) index as an indicator of terrestrial organic mater input and compared it to the stable carbon isotopic composition of total organic carbon ( δ 13C TOC). The δ 13C TOC and BIT index show opposing trends, with δ 13C TOC indicating an increase in terrestrial input while the BIT index suggests a decrease towards the present. This contradiction is probably due to the different sources reflected by δ 13C TOC (vegetation, soil and anthropogenic input) and the BIT index (only soil organic matter input). In addition, an increase in the recycling of respired CO 2 within the fjord’s water column could have depleted the δ 13C TOC of the marine component towards more negative values, thus seemingly indicating an increase in terrestrial input, whilst the BIT index could have been affected by a reduction of riverine input and/or an increase in archaeal biomass production. The TetraEther indeX of lipids with 86 carbons (TEX 86) was determined as a proxy for past water surface temperatures. While some of the signal could be coming from the chemocline, our results suggest that TEX 86 indicates surface (upper 25 m) temperatures. Moreover, TEX 86 temperature patterns seem to follow variations in the historical record of averaged spring to fall Oslo air temperature relatively closely, suggesting the signal is produced during this period. Therefore, TEX 86 temperature records of fjord sediment cores can potentially be a useful tool in reconstructing past variations of coastal climates.

Oceanic Methane Biogeochemistry

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Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe 7S 8, and its absolute magnetic susceptibility is 2.10 − 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism. The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, ( 206Pb/ 204Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member ( 206Pb/ 204Pb ∼ 18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits. The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO 2 conditions and neither a high temperature nor a low aH 2S (below 10 − 3 ) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds. Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.

Viral infection plays a key role in extracellular DNA dynamics in marine anoxic systems

To investigate the role of viruses in extracellular DNA production through cell lysis, we selected two systems where viruses are expected to be the main component controlling prokaryote dynamics. These systems include anoxic subsuperficial coastal sediments and water and sediments of two deep‐hypersaline anoxic basins (i.e., DHABs) of the eastern Mediterranean. Viral production was high in both places. Viruses were responsible for 10–60% of prokaryote mortality in anoxic sediments and up to 100% in deep‐anoxic waters. The daily contribution of DNA released by viral lysis to the total extracellular DNA pool was 2–11% in anoxic sediments and more than 100% in the brines of both deep‐sea basins. Extracellular DNA released by viral infection was rapidly degraded by deoxyribonucleases (DNases), which were also high in permanently anoxic conditions. Overall, our data suggest that DNA released by viral lysis, because of its high lability and fast turnover, may represent an important mechanism of trophic supply for prokaryotes, particularly in systems characterized by limited availability of external trophic sources.

Deep sea turbidites in brine-filled anoxic basins of the Mediterranean Ridge. Depositional models based on sedimentological and geochemical characterization

The detailed sedimentological and geochemical study of four long piston cores collected from the Bannock Libeccio, Bannock Scirocco, Tyro and Urania basins (eastern Mediterranean) lead to the distinction of two basic types of muddy turbidites, one of grey color, the other of green. Individual turbidites are several meters thick, up to 13.30 meters for the mid Holocene Homogenite related to the Santorini tsunami. A conceptual model was presented soon after the first discovery of the deep-seated brines related to submarine dissolution of Messinian evaporites, some twenty years ago: the turbid flow triggered by seismic events and/or by seismically induced tsunami, when reaching the pycnocline that separates the high density anoxic brines from normal sea-water, was split in two parts, one running above the interface, the other breaking it and descending along the basin slope. The model was tested with flume experiment in a tank, and applies to both types of turbidites. The green turbidites are richer in organic C, rare and trace elements and in siliceous plankton, and prevail in the lower portion of the cores, pre-dating the Homogenite event. The grey turbidites are characteristic of the Homogenite and of the lower part of the cores: they record a strong mixing of fine sediments whose source area was above the brine top layer and the bacterial pellicles that characterize the interface. An interplay ofa) the original composition of the source sediment,b) the density and chemical composition of the fluid, andc) of the basin size and configuration controls the depositional models in this complex part of the Mediterranean Ridge. The core collected from the deepest part of the Urania Basin (pit) is strongly different. Early diagenesis, encrustations of fossil shells, and sulphur nodules characterize this core. The anomaly is accounted for by the expulsion of fluids of deep origin along a fault plane. The very high temperatures measured in the Urania pit support this interpretation.

Distribution of abundant prokaryotic organisms in the water column of the central Baltic Sea with an oxic–anoxic interface

Anoxic marine deeps are characterized by their chemically stratified water columns. This has implications for the vertical distribution of microbial assemblages, but knowledge of this distribution is still poor. To further evaluate these systems we investigated the Faro Deep (central Baltic Sea), which was anoxic in its bottom waters in August 2003. To resolve the vertical distribution of the dominating prokaryotic microorganisms along the physico-chemical gradients, we used 16S rRNA or 16S rRNA gene-based single strand conformation polymorphism (SSCP) and denaturing gradient gel electrophoresis (DGGE). Altogether, 43 microorganisms were differentiated by sequenc- ing of gel bands; 55% of these belonged to Phylum Proteobacteria, 24% to Phylum Bacteroidetes, 7% to Class Actinobacteria, 5% to Phylum Cyanobacteria, 2% were distantly related to the genus Nitro- spina and 5% were identified as Phylum Crenarchaeota. Only 10% of the investigated assemblages were identified by both fingerprinting methods used. However, both methods revealed dominant 16S rRNA bands, the sequences of which were related to the following organisms: a phototrophic Synechococcus in the photic zone, an aerobic and heterotrophic Pseudoalteromonas in the oxic to suboxic zone, and an anaerobic and autotrophic nitrate-reducing Epsilonproteobacterium in the anoxic to sulphidic zone. The latter 2 bacterial taxa have also been reported from other anoxic basins and seem to be characteristic of marine pelagic redoxclines, indicating that these environments could harbor similar microbial communities.