Seafloor Conditions Research Articles

Geochemistry of surface sediments in the Archipelago Sea, SW Finland: a multiparameter and multivariate study

This study investigates the geochemistry of soft, organic-rich brackish-water surface sediments in the Archipelago Sea (SW Finland). The area is one of the world’s largest archipelagos, and, although it is scarcely populated, frequent ship and boat traffic along with fish farming occurs. From 76 sites, 47 chemical elements were determined after aqua regia digestion. Other parameters determined were grain size, water, organic and nitrogen contents. At sampling, the sediments were visually classified into groups representing oxic, anoxic and fluctuating (intermediate) oxic seafloor conditions. The results show that the element concentrations are quite similar to other studies in the region but with clear signs of lower anthropogenic loading. Point source pollution was identified in one sample taken nearby a fish farm and a cable ferry. This site was strongly enriched in Cu, Zn and Sn derived from chemicals used in either, or both, activities at the site. Overall, the sediments can be divided into two groups that separate chemically and geographically. One group contains more fine-grained sediments that occur near shore, while the other group contains more anoxic, organic and sulfur-rich sediments occurring more offshore. The second group is interpreted to reflect material reworked from previous deposits as a result of shoaling of the area due to glacio-isostatic land uplift. The visual classes were included in both partial least squares regression (PLSR) and PLS discriminant analysis. These analyses showed that the oxic and anoxic seafloor sediments can be predicted from the chemical variables and grain-size distribution.

PALEOECOLOGY OF THE MIDDLE CAMBRIAN EDRIOASTEROID ECHINODERM TOTIGLOBUS: IMPLICATIONS FOR UNUSUAL CAMBRIAN MORPHOLOGIES

The spheroidal edrioasteroid echinoderm Totiglobus, preserved in the middle Cambrian Chisholm Shale of eastern Nevada, was one of many interesting morphologies to develop during the period of rapid evolutionary diversification in the early and middle Cambrian. To understand more about how the unusual morphology of Totiglobus evolved, a detailed examination of fossil specimens and the strata in which they are preserved was performed. A total of 263 specimens were examined and placed into taphonomic categories based on their preserved orientation relative to the seafloor. Seventy-two wellpreserved specimens were used for statistical analysis. Of these, 49% were preserved with the aboral surface down. The preference for preservation in this category, with the aboral surface (which contains a suctorial attachment disc) oriented directly on top of the sediment, was statistically significant (p , 0.025). These data support the hypothesis that Totiglobus lived attached to the seafloor through suction. Rock samples were also collected from seven localities in the Chisholm Shale and the bioturbation levels were determined using the ichnofabric index (ii) method. A total of 48 samples, comprising 4.09 m of strata, were collected and x-radiographed. Ichnofabric index (ii) data recorded from these rocks revealed extremely low bioturbation levels (,ii 5 1) with no mixed layer development (1.01 average ii). These results likewise support the sediment attachment hypothesis for Totiglobus. This study indicates that some unusual Cambrian morphologies, including that of Totiglobus, evolved partly in response to the presence of nonactualistic seafloor conditions and are therefore not simply early evolutionary experiments.

Seafloor alteration of basaltic glass: Textures, geochemistry, and endolithic microorganisms

The incipient low‐temperature alteration of glassy margins of recent seafloor lavas from the Mohns Ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements. These investigations are designed to evaluate the endolithic microbial biomass, to identify relations between alteration textures and microorganisms, and to identify the chemical processes that take place during alteration. The basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned, yellow‐brown amorphous gel‐palagonite. In most fractures microorganisms are observed at the outer surfaces of palagonite rims, within porous zones of the rims, and frequently also at the glass‐palagonite interfaces. The cells act as nucleation sites for precipitation and become encrusted and embedded in palagonite with time. Zones of porous palagonite containing numerous hollow, fossilized cells alternate with zones of compact palagonite lacking distinct cell structures, which together indicate that the microbial growth is discontinuous. The palagonite has an average organic carbon content of 0.9 wt% (δ13Corg: −22‰), which derive from both living and fossilized biomass. The microbial growth and biomineralization are major controls on the porosity and texture of the palagonite and thus likely on the chemical exchange between glass and seawater. Pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting. Elemental and isotope data show that the transformation of glass to gel‐palagonite at seafloor conditions with high water/rock ratios results in near complete loss of Si and alkali elements to seawater; formation of Fe‐ and Al‐oxyhydroxides; complete exchange of alkaline earth elements like Sr with seawater; retention of V, Cu, Y, Pb, Th, U, and trivalent REEs, accompanied by a strong gain of these and other elements from seawater. Higher Fe/Ti ratios in the gel‐palagonite compared to the parental glass suggest that the palagonite represents a mixture of Fe‐Ti oxyhydroxides derived from both the glass and seawater.

The short-term fate of organic carbon in marine sediments: Comparing the Pakistan margin to other regions

Pulse-chase experiments with isotopically labelled phytodetritus conducted across the Pakistan margin reveal that the impact of biological activities on benthic C-cycling varies markedly among sites exhibiting different seafloor conditions. In this study, patterns of biological C-processing across the Pakistan margin oxygen minimum zone (OMZ) are compared with those observed in previous tracer studies. Variations in site environmental conditions are proposed to explain the considerable variations in C-processing patterns among this and previous studies. Three categories of C-processing pattern are identified: (1) respiration dominated, where respiration accounts for >75% of biological C-processing, and uptake by metazoan macrofauna, foraminifera and bacteria are relatively minor processes. These sites tend to show several (although not necessarily all) of the properties of being cold and deep, and having low inputs of organic carbon to the sediment and relatively low-biomass metazoan macrofaunal communities; (2) active faunal uptake, where respiration accounts for <75%, and metazoan macrofaunal, foraminiferal and bacterial uptake each account for 10–25% of biological C-processing. This type is further split into metazoan macrofaunal- and foraminiferal-dominated situations, dictated by oxygen availability; and (3) metazoan macrofaunal uptake dominated, characterised by metazoan macrofaunal uptake accounting for ∼50% of biological C-processing, due to unusually large biomasses of the phytodetritus-consuming animals. Total respiration rates (of added C) on the Pakistan margin fell within the range of rates measured elsewhere in the deep sea (∼0.1–2.8 mg C m−2 h−1), and seem to be dominantly controlled by seafloor temperature. Rates of metazoan macrofaunal uptake of organic matter (OM) on the Pakistan margin are larger than those in most other studies, and this is attributed to the large and active metazoan macrofaunal communities in the lower OMZ, characteristic of OMZ boundaries. Finally, biological mixing of Pakistan margin sediments was reduced compared to that observed in comparable tracer studies on other margins. This probably reflects faunal feeding and burrowing strategies consistent with low oxygen concentrations and a relatively abundant supply of sedimentary OM.

Anaerobic pyrite oxidation rates determined via direct volume-loss measurements: a Vertical Scanning Interferometric approach

AbstractPresented here are quantitative dissolution rate data (volume of pyrite lost/time) for the inorganic oxidation of pyrite in synthetic, anaerobic and acidic (pH 2) hydrothermal vent fluids (HVF) from experiments where the volume loss was measured directly via Vertical Scanning Interferometry (VSI). The VSI-derived reaction rate was 2.12x10-10±1.14x10-1 mol/m2/min, which is ∼2 to 4 orders of magnitude slower than pyrite oxidation rates previously determined using traditional batch experiments where rates are calculated based on changes in solution chemistry. This lower rate stems primarily from differences in experimental conditions (i.e. water to rock ratios, vigorous vs. gentle stir rates, grain-size effects, time), yet the rates derived here are believed to be more representative of pyrite oxidation in natural environments where more static and high solution to solid rate conditions prevail, such as seafloor conditions or acid-mine-drainage environments.

What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean–Continent Transition

The break-up of continental lithosphere in magma-poor margins is accompanied by the exhumation of mantle and crustal rocks in the footwall of large-scale detachment faults. Although these structures have been described from many modern and ancient margins, little is known about how they accommodate strain and evolve in time and space during continental break-up. The Tasna Ocean–Continent Transition (OCT) in southeastern Switzerland is one of the rare examples where such detachment faults are exposed and can be observed on a kilometre scale. In this paper we describe the deformation structures and their evolution observed along detachment faults in the Tasna OCT. Our results show that continental break-up was attained by a series of detachment faults. These detachment faults accommodated extensional strain in fault zones formed by a localized core zone that is surrounded by a several tens to hundred metres wide damage zones. The core zone corresponds to the zone of highest strain and is well defined structurally by the occurrence of gouges and/or foliated cataclasites and physically by separating a hanging wall from a footwall. Deformation in the fault zones occurred under greenschist facies to seafloor conditions and within the stability field of serpentine. U/Pb ages on zircon from a garnet-bearing pegmatite cross cutting high-temperature shear zones (upper amphibolite facies and higher) provide Carboniferous ages and demonstrate that these shear zones are neither kinematically nor genetically related to the detachment faults observed in the Tasna OCT. Ar/Ar ages on phlogopite from spinel websterite suggest that mantle exhumation occurred during Middle Jurassic time. Our data show that the detachment faults observed in the Tasna OCT formed during latest rifting, post-date major thinning of the crust and onset of mantle serpentinization. These results compare well with those from the deep Iberia margin. Our observations support the idea that rifting leading to continental break-up is a multi-phase process, and that serpentinization is the consequence rather than the reason for strain localization at non-volcanic margins. Apart from the more general implications for the tectonic evolution of continental break-up, our results have some important consequences for the palaeogeographic reconstruction of certain Alpine domains and question the existence of an independent Early Cretaceous Valais ocean in the Alpine realm.

Comparison and implications from strikingly different authigenic carbonates in a Nyegga complex pockmark, G11, Norwegian Sea

Authigenic carbonates sampled in a complex pockmark (G11) from the Nyegga area, south western Vøring Basin, reveal strikingly different morphologies. A comparative study of macro and micro features of these neighbouring samples was approached in order to attempt a reconstruction of the pockmark dynamics. The carbonate morphologies include thin (∼ 1 cm) platy carbonate crusts, blocky and massive carbonate ridges up to several metres in size, and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity. Our observations indicate that three distinct mechanisms of fluid seepage and carbonate precipitation occurred within the pockmark. Petrography studies reveal that the samples contain three generations of predominantly aragonitic cements. In addition, zoned crystals with coexisting Mg-calcite and dolomite are abundant in one sample suggesting a trend of gradual increase of the sulphate concentrations in the pore fluids during carbonate precipitation. Isotope measurements on bulk carbonate cements show consistent values with δ 13C as low as − 52.1‰, δ 18O as high as 6.4‰, and 87/86Sr of 0.7092. Anaerobic oxidation of methane close to seafloor conditions can explain these isotope data. The carbonates have a common carbon source despite the differences in morphology. Methane probably originated from the dissolution of gas hydrates as supported from calculated 18O enrichment in the pore fluids and the methanogenic carbon. We propose a model where the differences in bulk carbonate morphology are related to local sediment properties, depth of carbonate precipitation and the flow rate of the seeping fluids.

Paleoenvironmental change at the Danian–Selandian transition in Tunisia: Foraminifera, organic-walled dinoflagellate cyst and calcareous nannofossil records

In the present study, we document paleoenvironmental change across the Danian–Selandian transition (planktic foraminiferal interval P2–P3b; calcareous nannofossil Zone NP4, Subzones NTp6–NTp8A; 61–59 Ma) in NW Tunisia. Diversifications of Paleogene planktic foraminifera with the evolution of the muricate and photosymbiotic lineages Morozovella, Acarinina and Igorina and of the biostratigraphically important nannofossils genus Fasciculithus are recorded within this interval. The present study aims to understand early Paleogene environmental changes in the southern Tethys, by analyzing the evolution of surface-water and–to a lesser extent–seafloor conditions. Three localities were investigated: Ain Settara, Elles and El Kef, all representing outer neritic deposition in the same basin, the Tunisian Trough. Paleoenvironmental changes are explored by combining planktic foraminiferal, organic dinocyst and calcareous nannofossils assemblages and several proxy parameters (planktic/benthic ratio, numbers of planktic foraminifera per gram, peridinioid/gonyaulacoid ratio; terrestrial/marine palynomorph ratio). In addition, also some geochemical parameters (calcite content and stable isotopes) are examined. Our records indicate that the environment evolved from an initially oligotrophic, open marine, deep outer neritic setting in P2–P3a towards a shallower and nutrient-rich setting from the base of Subzone P3b. This change is seen in the foraminiferal assemblages, with the substitution of Praemurica by Morozovella among the planktic foraminifera and an upward decrease in deeper benthic taxa. Also the organic-dinocyst assemblages show a peak of peridinioid cysts ( Cerodinium and Lejeunecysta). Associated to these dinocyst assemblages, the lowest occurrence of Apectodinium is recorded, which seem to have evolved in this region, possibly in response to enhanced nutrient levels on the shelf. Additionally, a distinct change in calcareous nannofossil assemblages is also described, marked by the lowest appearance of Chiasmolithus edentulus, the lowest consistent occurrence of Fasciculithus and a slight increase in near-shore taxa (essentially Pontosphaera). This project provides an accurate understanding of paleoenvironmental change across the Danian–Selandian transition in Tunisia. Especially, integrating different proxies demonstrates a paleobathymetric shallowing from the Danian to the Selandian, associated to increase surface paleoproductivity. Furthermore, the results are compared with those from other localities along the Southern Tethyan margin (Egypt and Jordan) and a more regional paleoclimatic/paleoceanographic perturbation in the Southern Tethys is suggested.

Lower Eocene carbonate-cemented “chimney” structures (Varna, Bulgaria) — Control of seepage rates on their formation and stable isotopic signature

The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions ( δ 18O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ 13C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column ( δ 13C = − 8‰) and raised precipitation temperatures ( δ 18O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.

Late Pliocene benthic foraminifera and mollusks from the Atsipades Section, Central Crete; Palaeoecological distribution and use in palaeoenvironmental assessment

Changes in benthic foraminiferal and mollusk assemblages from the lower part of the Upper Pliocene of the Atsipades Section (Iraklion Basin, central Crete) were studied. The Atsipades Section represents a shallowing-upward sequence from outer shelf blue–gray clays at the bottom of the sequence, deposited below the storm wave base, to shallow inner shelf deposits affected by storm waves at the top. The foraminiferal assemblage at the bottom of the sequence is dominated by Bolivina spathulata, Bolivina dilatata and Uvigerinidae, a microfossil assemblage corresponding to the deepest deposits formed under dysoxic sea-floor conditions. Foraminiferal assemblages of the middle part of the section are highly diversified, predominantly Haynesina depressula, Cassidulina carinata and Reusella spinulosa. The top of the section is mainly characterised by Asterigerinata planorbis, Bolivina pseudoplicata, Cibicides lobatulus and Elphidium sp., a typically epiphytic foraminiferal assemblage which can be correlated with the presence of an algal covered sea-bottom. Within this general environmental trend, a minor shallowing cycle can be differentiated. The boundaries of this cycle can be inferred, based on a substantial microfossil assemblage change and on the coincidence of species diversity maximum and a planktonic/benthic (P/P + B) ratio peak. Nonetheless, upwelling currents and/or over-abundance of nutrients due to continental outflow could also contribute to increased diversity and P/P + B ratio. The character of the mollusk assemblages is in accordance with these trends. Moreover, the increase in diversity and in sculpture constitutes a clear indication of an increase in hydrodynamic energy related to a shallowing-upward trend.

A Mid-Cretaceous Association of Shell Beds and Organic-rich Shale: Bivalve Exploitation of a Nutrient-Rich, Anoxic Sea-floor Environment

The Toolebuc Formation (Late Albian) is a thin (<40m), very widely distributed unit marking the maximum deepening of the Cretaceous epicontinental sea recorded by the infill of the Great Artesian Basin, eastern Australia. It consists of organic-rich shale, with TOC ranging to 35%, and limestone as laminae and thin beds comprised of Inoceramus sutherlandi McCoy and, less commonly, Aucellina hughendenensis Etheridge. Finely interlayered organic-rich shale and coquina are typical of the formation, resulting in a distinctively black-and-white, thinly bedded to laminated rock. Inoceramus valves are commonly disturbed by breakage, reorientation, and imbrication, and, in many cases, have disintegrated into prism horizons. Sedimentary laminae show that the sea floor was subject to some current activity, but benthic scavengers are considered to have been active agents of shell disturbance. Although shelly substrate suitable for encrusting epibenthos or colonization by endoliths was available in abundance, other benthic elements, inclusive of trace fossils, are very poorly represented. The formation contains diverse planktonic, pelagic, and nektonic fossil remains attesting to deposition beneath a water column of normal salinity, supporting a complex food chain. It displays a negative δ13Corg excursion, considered to relate to sea floor bacterial reworking of detrital organic material derived from plankton. In the context of the Toolebuc Formation, Inoceramus sutherlandi and Aucellina hughendenensis represent ecological specialists, with shell-growth strategies designed to cope with soft substrates and physiologies that were tolerant of oxygen-poor bottom conditions. Their intimate association with organic-rich shale suggests a trophic link with sea-floor bacterial productivity, supported from the organic-rich substrate. The lateral extent of Toolebuc coquinas suggests that Inoceramus sutherlandi and Aucellina hughendenensis were filter feeders rather than dependent on chemotrophic symbionts. Fine-scale interlayering of coquina and organic-rich shale attests to frequent alternation of sea-floor conditions conducive to colonization by bivalve communities and those that were not. Alternation is attributed in part to an autocyclic mechanism, with sea-floor accumulation of shelly debris having progressively isolated bivalve communities from trophic support, causing their episodic demise.

The dynamics of submarine geothermal heat pipes

To better understand natural two‐phase hydrothermal systems, we have constructed one‐dimensional heat‐pipe solutions for NaCl‐H2O fluids and explored the effects of basal heat flux and permeability on their behavior. For seafloor conditions, saline brines form quickly at the base of the heat pipe; and in some cases halite is precipitated. NaCl‐H2O heat pipes may become liquid or vapor dominated but, in contrast to their pure‐water counterparts, often do not achieve steady state. When steady state solutions do exist, they are characterized either by broad, weak counter‐flow or by vigorous counter‐flow across a thin layer. The latter behavior may be analogous to that occurring in the Salton Sea Geothermal System, California.

Growth and demise of Permian biogenic chert along northwest Pangea: evidence for end-Permian collapse of thermohaline circulation

The Permian Chert Event (PCE) was a 30 Ma long episode of unusual chert accumulation along the northwest margin of Pangea, and possibly worldwide. The onset of the PCE occurred at about the Sakmarian–Artinskian boundary in the Sverdrup Basin, Canadian Arctic, where it coincides with a maximum flooding event, the ending of high-frequency/high-amplitude shelf cyclicity, the onset of massive biogenic chert deposition in deep-water distal areas, and a long-term shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A similar and coeval shift is observed from the Barents Sea to the northwestern USA. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. Biotically impoverished and increasingly narrow cold-water carbonate factories (characterised by non-cemented bioclasts of sponges, bryozoans, echinoderms and brachiopods) were then progressively replaced by silica factories. By Late Permian time, little carbonate sediments accumulated in the Barents Sea and in the Sverdrup Basin, where the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. By that time, biogenic silica sedimentation was common throughout the world. Silica factories collapsed in the Late Permian, abruptly bringing the PCE to an end. In northwest Pangea, the end-Permian collapse of the PCE was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas during the early Middle Triassic (Anisian) after a 8–10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwestern and western margin of Pangea, and possibly throughout the world oceans. These conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions, hindering silica dissolution, while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. This process started near the Sakmarian–Artinskian boundary, intensified throughout Middle and Late Permian time and ceased suddenly in latest Permian time. It led to oceanic conditions much colder than normally expected from the palaeolatitudes, and the influence of cold northerly-derived water was felt as far south southern Nevada. The demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.

A new experimental facility for investigating the formation and properties of gas hydrates under simulated seafloor conditions

A seafloor process simulator (SPS) has been developed for experimental investigations of the physical, geochemical, and microbiological processes affecting the formation and stability of methane and carbon dioxide hydrates at temperatures and pressures corresponding to ocean depths of 2 km. The SPS is a corrosion-resistant pressure vessel whose salient characteristics are: (i) an operating range suitable for study of methane and carbon dioxide hydrates; (ii) numerous access and observation ports, and (iii) a large (0.0722 m3) internal volume. Initial experiments have shown that the SPS can be used to produce large amounts of high-purity methane hydrate over a wide range of experimental conditions.

Overview of SAX99: environmental considerations

A 1-km/sup 2/ area located 2 km off the Florida Panhandle (30/spl deg/22.6'N; 86/spl deg/38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18-19 m of water, is characterized by 1-2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics.

DEEP-SEA BENTHIC FORAMINIFERA FROM THE CENTRAL RED SEA

The distribution of living (Rose Bengal-stained), dead and fossil benthic foraminifera was investigated in six short cores (multicores, 30–32 cm total length) recovered from the central Red Sea. The ecological preferences as well as the relationship between the live and dead/fossil assemblages (preserved down-core) were examined. The sites, located along a W-E profile and between the depth of 366 and 1782 m, extend from the center of the oxygen minimum zone (OMZ, ~200–650 m), through its margin at ~600 m, and down to the well-aerated deep-water environment. Live (Rose-Bengal stained) and coexisting dead foraminifera were studied in the upper 5 cm of each of the sites, and the fossil record was studied down to ~32 cm. Q-mode Principal Component Analysis was used and four distinct foraminiferal fossil assemblages were determined. These assemblages follow different water mass properties. In the center of the OMZ, where the organic carbon content is highest and the oxygen concentration is lowest (≤0.5 ml O 2 /1), the Bolivina persiensis-Bulimina marginata-Discorbinella rhodiensis assemblage dominates. The slightly more aerated and lower organic-carbon-content seafloor, at the margin of the OMZ, is characterized by the Neouvigerina porrecta-Gyroidinoides cf. G. soldanii assemblage. The transitional environment, between 900–1200 m, with its well-aerated and oligotrophic seafloor, is dominated by the Neouvigerina ampullacea-Cibicides mabahethi assemblage. The deeper water (>1500 m), characterized by the most oxygenated and oligotrophic seafloor conditions, is associated with the Astrononion sp. A- Hanzawaia sp. A assemblage. Throughout the Red Sea extremely high values of temperature and salinity are constant below ~200 m depth, but the flux of organic matter to the sea floor varies considerably with bathymetry and appears to be the main controlling factor governing the distribution pattern of the benthic foraminifera. Comparison between live and the dead/fossil assemblages reveals a large difference between the two. Processes that may control this difference include species-specific high turnover rates, and preferential predation and loss of fragile taxa (either by chemical or microbial processes). Significant variations in the degree of loss of the organic-cemented agglutinants were observed down core. This group is preserved down to 5–10 cm at the shallow OMZ sites and down to greater depths at well-aerated and oligotrophic sites. The lower rate of disintegration of these forms, in the deeper locations of the Red Sea, may be related to low microbial activity. This results in the preservation of increasing numbers of organic-cemented shells down-core.

The behaviour of deep-sea sediments under the impact of nodule mining processes

Abstract The complex processes of the suspension, resettling and consolidation behaviour of deep-sea sediments caused by deep-ocean mining were investigated on board the German RV Sonne and in the laboratory using sediment from the uppermost semi-liquid layer of the Peru Basin. On-board tests were accomplished directly with undisturbed sediment samples recovered by the maxicorer, a sediment sampling device especially developed for such studies. The impact was simulated by the use of a propeller-generated water jet. The results were used to prepare a substitute for the semi-liquid top layer for large-scale laboratory tests to simulate the sea-floor conditions to be expected during manganese nodule mining. In line with the design of most nodule miners, these tests were performed with the use of water jets. The amounts of displaced and suspended sediment were determined as was the resettling behaviour. The tests revealed that a mean value of 16% of the sediment was suspended by the water jets. The remainder was remoulded and laterally displaced for a short distance, but not suspended into the water column. The results of the on-board tests were confirmed by the laboratory tests and have great significance in the prediction of sediment clouds generated by ocean-mining activities. Additionally, effects on the resettling behaviour and formation of aggregates due to temperature and particle concentration were investigated. It was demonstrated that high particle concentration and high temperature within the suspension accelerate the aggregation process and thus lead to enhanced resettlement of sediment particles. Finally, the fabric of the resettled sediment was evaluated by its void ratio determined by the consolidation process. It is estimated that it will take at least 20 years for the sediment to regain its original density.

Olenid trilobites: the oldest known chemoautotrophic symbionts?

Late Cambrian to early Ordovician trilobites, the family Olenidae, were tolerant of oxygen-poor, sulfur-rich sea floor conditions, and a case is made that they were chemoautotrophic symbionts. Olenids were uniquely adapted to this habitat in the Lower Paleozoic, which was widespread in the Late Cambrian over Scandinavia. This life habit explains distinctive aspects of olenid morphology: wide thoraces and large numbers of thoracic segments, thin cuticle and, in some species, degenerate hypostome, and the occasional development of brood pouches. Geochemical and field evidence is consistent with this interpretation. Olenids occupied their specialized habitat for 60 million years until their extinction at the end of the Ordovician.

The late Paleocene event and a potential precursor compared: First results from Egypt

Background characteristics. ‐ During the early Paleogene an extensive epicontinental basin covered Egypt, reaching maximum southward extension into northern Sudan during the late Paleocene. Basinal deposits consist of monotonous marls and occasionally limestones. Near local highs and along the basin ’ s margins, these interfinger with platform carbonates and deltaic deposits. The marls which are generally low in TOC contain rich planktic foraminifera and calcareous nannoplankton assemblages, pointing to open connections to the Tethys. Planktic foraminifera assemblages (Zones P3‐P5) are diverse, composed of the genera Morozovella, Acarinina, Igorina, Globanomalina, and Subbotina. Benthic assemblages, characterized by Midway (neritic) and mixed Midway-Velasco (bathyal) type faunas, are highly diverse, increasing with paleodepth. These assemblages indicate well to moderately oxygenated conditions at the sea floor. P/B ratios, generally vary between 60 and 95%, but wherever there is evidence of severe post-mortem dissolution these may be much reduced. Physical discontinuities in the monotonous successions are rarely apparent, but are indicated by detailed biostratigraphic investigations. Late Paleocene event. ‐ Quite a different picture emerges in various Egyptian sections where a c. 1-m thick interval correlates with the LPTM. In the sections situated in the deeper parts of the basin, dark brown, finely laminated marl beds, often overlain by foraminifera-rich calcarenitic marl beds mark the interval comprising the CIE and BEE (see Speijer & Schmitz this volume). The laminated beds are enriched in TOC (2‐3%) and contain abundant peloids ‐ indicating condensation ‐ and fish scales. In some profiles, the contact between this lithologic unit and the underlying monotonous marls is marked by bioturbated omission surfaces or channel fill, indicating discontinuities at the base of the CIE. The microbiota indicate anomalous paleoenvironmental conditions. Planktic foraminifera assemblages in the laminated interval are almost exclusively composed of Acarinina species. Morozovella allisonensis is the only common representative of its genus. The benthic assemblages in the same interval are oligotaxic, composed of a few opportunistic taxa capable of ‐ occasionally? ‐ populating a sea-floor under oxygen stress (see Speijer & Schmitz this volume). These taxa invaded the entire basin from the shallower shelf. P/B ratios are extremely high (up to 99.9%), a feature common to many Neogene to Holocene sapropels and attributable to the hostile sea-floor conditions.

Recent Research on Cable Jet Burial

Perry Slingsby Systems have recently initiated a new cable burial Research &amp; Development project as part of an overall cable burial system development program. The project sets out to investigate the effect of water jetting on stiff to very stiff clay soils. To date, such soils have generally been beyond the capabilities of the industry’s existing cable burial and maintenance ROV fleet. However, Perry Slingsby Systems have found that it is becoming increasingly important for users to be able to extend the range of seafloor conditions in which burial can effectively take place. Perry Slingsby’s intention is that the project will eventually lead to the delivery of a new range of high capable jet burial equipment.This technical note describes the background and methodology to the first stage of the test program. It also details the selection of suitable soil test beds which were used to model stiff clay seafloor conditions. The test apparatus and procedures are also described and indications of the type of results obtained are given.