Seafloor Processes Research Articles

Evolution and variability of the Asian monsoon system: state of the art and outstanding issues

The Asian monsoon is comprised of the Indian and East Asian subsystems. These two components are linked to one another in varying degrees by regions of strong sensible heating (Indo-Asian landmass) and strong latent heat export (the Western Pacific Warm Pool and the southern subtropical Indian Ocean). Variability within the Indian and East Asian subsystems, interactions among them, and the extent to which they interact with other climate phenomena (e.g., ENSO) are current topics of modern and paleoclimate research. This work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records. While past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales, additional efforts are required to make further progress in two critical areas. (1) Additional efforts are needed in terms of proxy development and evaluation, requiring concerted efforts at long-term sediment trap deployments in key monsoon-influenced regions as well as development of adequate and widely available core-top databases. These are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies. (2) Additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change. These records should have high sedimentation rates (including varved sections). This requires substantial survey support to identify the most appropriate coring and drilling targets.

Ocean Disposal of CO2: Conditions for Producing Sinking CO2 Hydrate

Carbon dioxide sequestration at intermediate ocean depths in the form of a sinking CO2 stream has been shown to be technically feasible. Buoyancy characteristics of CO2 hydrate particles obtained from a coflow reactor operating under conditions that produce negatively buoyant CO2 hydrate composite particles were studied. Using a simplified process model, the limiting ratios of CO2 to water to produce sinking hydrate composite streams were established for pressure and temperature conditions of intermediate ocean depths. A kinetic model involving a mass transfer step and a reaction step was proposed to explain experimental data using a seafloor process simulator. Results suggest that the reactor operates under mass‐transfer controlled conditions. Although the density of freshwater is only a few percentage units less than that of seawater, operating conditions derived from freshwater‐based studies are significantly different for ambient seawater. Owing to the higher density of seawater, greater conversion of CO2 to hydrate is needed to obtain a sinking CO2 composite stream than at the same depth of freshwater.

Geohazard studies offshore the Faroe Islands: slope instability, bottom currents and sub-seabed sediment mobilisation

Prior to the 1990s only few geological investigations of the seabed and the shallow geology around the Faroe Islands had been undertaken (Waagstein & Rasmussen 1975; Nielsen et al. 1981). However, in the 1990s marine geological and in particular seismic investigations were markedly intensified. Since 1993 several studies on the structure of the Faroe Islands margin and seafloor processes have been funded by the European Union, namely the ENAM (European North Atlantic Margin) project I and II (1993–1999) and the STRATAGEM (Stratigraphy of the Glaciated European Margin) project (2000–2003), and these have provided significant new information on the mechanisms shaping the Faroe Islands margin (e.g. Boldreel et al. 1998; Kuijpers et al. 1998a; Nielsen & van Weering 1998; van Weering et al. 1998). Due to the expertise and regional geological knowledge obtained during these projects the Geological Survey of Denmark and Greenland (GEUS) became involved in socalled ‘geohazard’ seabed studies of the Faroe–Shetland Channel in 1997. These investigations were financed by the petroleum industry that had begun to show significant interest in exploration of the Faroe–Shetland Channel area. The studies focused on possible natural risks that would affect submarine structures, such as slope instability and strong bottom currents, and included both shallow seismic data acquisition and sediment core analyses. Most of the work at sea was undertaken with the Russian research vessel Prof. Logachev, and carried out within the framework of the international, UNESCO-supported ‘Training-Through-Research’ (TTR) programme co-ordinated by Moscow State University, Russia. Since 1997, more than three million DKK have been granted for various projects and this work has been documented in 14 classified reports. This paper presents some of the main results from these ‘geohazard’ studies, in particular with respect to the sediment instability affecting the western flank of the Faroe–Shetland Channel, the occurrence of very strong bottom currents in the channel, and the newly discovered mud diapirs at the northern entrance of the channel (Fig. 1).

Life in the Deep Sub-Seafloor Biosphere

The interest in the origin of life on earth and the prospects of life on other planets have accelerated in recent years and have gained great public awareness. Whereas life has been explored and found everywhere on the surface of our own planet, the deep subsurface was until recently considered to be influenced only by abiotic processes. The discovery of micro-organisms in several million year old sedimentary deposits and even in basement rock has profoundly changed our perspective of the limits of living organisms. It is now clear that processes in the geosphere may provide the driving force for life and that, vice versa, the sub-surface biosphere has a large impact on present geological processes. The emerging fluids of cold seeps in subduction zones along continental margins are chemically modified by deep bacteria, just like the fluids of hydro-thermal vents along mid-oceanic ridges are modified by geo-thermal processes. Both fluid flows have a major influence on modern ocean chemistry. The slow biological degradation of organic carbon in the deep sub-surface and the formation of huge quantities of methane have a large-scale impact on sea-floor processes, such as the accumulation of gashydrates or the formation of carbonate mounds.

Caco 3 precipitation kinetics in waters from the great Bahama bank: : Implications for the relationship between bank hydrochemistry and whitings

The source of whitings on the Great Bahama Bank and their relationship to major changes in the chemistry of Bank waters have been among the longest and most hotly debated topics in carbonate geochemistry. In this paper, we demonstrate that the reaction kinetics of calcite with Bank waters for a given saturation state are similar to, but somewhat slower (2 to 3 times) than with Gulf Stream water. The interpretation of the reaction kinetics of suspended Bank sediment with Bank water requires that the precipitating phase be about twice as soluble as aragonite. Good agreement at equivalent saturation states was found between experimental precipitation rates and those calculated for the rate of change of Bank water chemistry in the region of whitings. These results indicate that the dominant mode of carbonate removal is via precipitation on resuspended sediments rather than the rapid pseudo-homogeneous precipitation of calcium carbonate in the water column resulting in the formation of a whiting. Estimates indicate that single aragonite needles may be resuspended many times over a period of decades during which they experience repeated overgrowth. A major portion (>98%) of suspended calcium carbonate is outside the visually dramatic whitings. Thus, as visually spectacular as they are, whitings do not represent a short-term locally massive precipitation of carbonate on the Great Bahama Bank, nor are they even likely to be the dominant sites of carbonate removal in this region. Although future refinements are needed that include seafloor processes, we have at this point arrived at a mechanistic kinetic model that provides a reasonably quantitative explanation for the hydrochemistry of the carbonate system on the northern Great Bahama Bank.

Effects of early sea-floor processes on the taphonomy of temperate shelf skeletal carbonate deposits

Cool-water shelf carbonates differ from tropical carbonates in their sources, modes, and rates of deposition, geochemistry, and diagenesis. Inorganic precipitation, marine cementation, and sediment accumulation rates are absent or slow in cool waters, so that temperate carbonates remain longer at or near the sea bed. Early sea-floor processes, occurring between biogenic calcification and ultimate deposition, thus take on an important role, and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool-water carbonate deposits. The physical breakdown processes of dissociation, breakage, and abrasion are mediated mainly by hydraulic regime, and are always destructive. Impact damage reduces the size of grains, removes structure and therefore information, and ultimately may transform skeletal material into anonymous particles. Abrasion is highly selective amongst and within taxa, their skeletal form and structure strongly influencing resistance to mechanical breakdown. Dissolution and precipitation are the end-members of a two-way chemical equilibrium operating in sea water. In cool waters, inorganic precipitation is rare. There is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor, but test maceration and early loss of aragonite in particular are reported. Dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter, strong bioturbation, and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits. Laboratory experiments demonstrate that surface-to-volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution. Biological processes on the sea floor include encrustation and bioerosion. Encrustation, a constructive process, may be periodic or seasonal, and can be reversed. It produces both information and material. Bioerosion, in contrast, is destructive and permanent. In temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor, on the order of hundreds to thousands of years. Overall, processes on the temperate sea floor may combine to destroy more carbonate than they produce, and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected. Where preservation does occur in such a destructive regime, the effects of early sea-floor processes will be key determinants of the deposit, resulting in a “taphofacies” characteristic of temperate shelf carbonate sediments.

In situ acoustic and laboratory ultrasonic sound speed and attenuation measured in heterogeneous soft seabed sediments: Eel River shelf, California

We compared in situ and laboratory velocity and attenuation values measured in seafloor sediments from the shallow water delta of the Eel River, California. This region receives a substantial volume of fluvial sediment that is discharged annually onto the shelf. Additionally, a high input of fluvial sediments during storms generates flood deposits that are characterized by thin beds of variable grain-sizes between the 40- and 90-m isobaths. The main objectives of this study were (1) to investigate signatures of seafloor processes on geoacoustic and physical properties, and (2) to evaluate differences between geoacoustic parameters measured in situ at acoustic (7.5 kHz) and in the laboratory at ultrasonic (400 kHz) frequencies. The in situ acoustic measurements were conducted between 60 and 100 m of water depth. Wet-bulk density and porosity profiles were obtained to 1.15 m below seafloor (m bsf) using gravity cores of the mostly cohesive fine-grained sediments across- and along-shelf. Physical and geoacoustic properties from six selected sites obtained on the Eel margin revealed the following. (1) Sound speed and wet-bulk density strongly correlated in most cases. (2) Sediment compaction with depth generally led to increased sound speed and density, while porosity and in situ attenuation values decreased. (3) Sound speed was higher in coarser- than in finer-grained sediments, on a maximum average by 80 m s −1. (4) In coarse-grained sediments sound speed was higher in the laboratory (1560 m s −1) than in situ (1520 m s −1). In contrast, average ultrasonic and in situ sound speed in fine-grained sediments showed only little differences (both approximately 1480 m s −1). (5) Greater attenuation was commonly measured in the laboratory (0.4 and 0.8 dB m −1 kHz −1) than in situ (0.02 and 0.65 dB m −1 kHz −1), and remained almost constant below 0.4 m bsf. We attributed discrepancies between laboratory ultrasonic and in situ acoustic measurements to a frequency dependence of velocity and attenuation. In addition, laboratory attenuation was most likely enhanced due to scattering of sound waves at heterogeneities that were on the scale of ultrasonic wavelengths. In contrast, high in situ attenuation values were linked to stratigraphic scattering at thin-bed layers that form along with flood deposits.

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.

Seabed mapping and seafloor processes in the Kish, Burford, Bray and Fraser Banks area, South‐Western Irish Sea

Abstract Mapping of the seabed and sub‐seabed strata in an area comprising offshore banks and intervening sediments in outer Dublin Bay is presented. Bathymetric comparisons suggest that the offshore banks are quasi‐stable over time probably maintaining their position due to the interaction between wave and current regimes. Seven acoustic seabed facies are defined on the basis of side‐scan sonar characteristics reflecting differences in bedforms and bottom types. Sediment waves indicative of a mobile substrate are common both on and between banks. Maximum sediment wave development occurs on bank flanks and outer limits. The effects of wave action on seabed morphology are clearly discernible in the structure and appearance of the bank crests. Grain‐size data and bedform interpretations suggest a northerly sediment transport system with gravel dominant in the south of the area (Bray Bank) grading to sands in the north (Kish and Burford Banks). Sub‐bottom profiling reveals a consistent upper unit overlying a...

Fluids Transport Optimization Using Seabed Separation

Terrain slugging is extremely likely to occur sometime during the life of a deep subsea pipeline/riser system. But, can the terrain slugs be mitigated? In this study, we performed transient multiphase flow simulations to explore the feasibility of sea floor separation to mitigate terrain slugging and to optimize fluid transport from wellhead to downstream processing. We considered the effects of separator location and separation efficiency on production system behavior. We found that there is an optimal amount of gas underflow into the liquid line and there is an optimum location for subsea separator. Our study also covered how the operational transient processes, such as startup and shutdown, would affect the performance of the pipeline/separator system. It is necessary to have a good understanding of the performance of subsea separation system before a reliable one can be designed. The results of this study are aimed toward improving the design of sea floor processing for ultra-deepwater production. [S0195-0738(00)00403-9]

Seafloor mapping and 3-D visualization: An integration of technologies providing new insights into seafloor processes

The past few years have seen rapid advancements in sonar development, in vessel orientation and positioning technology, and in our ability to manage and visualize the massive amounts of data that modern seafloor sonars are capable of collecting. The confluence of these technologies has resulted in revolutionary changes in our ability to map and visualize the seafloor. Until recently, maps of the seafloor were based on the interpolation (typically through the construction of contours) of sparsely spaced discrete soundings or lines of soundings. With the advent of multibeam or swath-sonar technology, we can now produce near-100% coverage of large areas of the seafloor with high resolution (both vertical and horizontal) in a timely and cost-effective manner. More recently multibeam sonars have also begun to collect coregistered acoustic backscatter information (and inteferometric sidescan sonars have begun to collect coincident bathymetric information), potentially providing information on the composition (texture or material type) as well as the morphology of the seafloor. When combined with modern interactive 3-D visualization tools, these new data sets result in a radically different representation of the seafloor than that provided by traditional echo sounders that can lead to unprecedented levels of understanding of seafloor processes.

Shallow-water imaging multibeam sonars: A new tool for investigating seafloor processes in the coastal zone and on the continental shelf

Hydrographic quality bathymetry and quantitative acoustic backscatter data are now being acquired in shallow water on a routine basis using high frequency multibeam sonars. The data provided by these systems produce hitherto unobtainable information about geomorphology and seafloor geologic processes in the coastal zone and on the continental shelf. Before one can use the multibeam data for hydrography or quantitative acoustic backscatter studies, however, it is essential to be able to correct for systematic errors in the data. For bathymetric data, artifacts common to deep-water systems (roll, refraction, positioning) need to be corrected. In addition, the potentially far greater effects of tides, heave, vessel lift/squat, antenna motion and internal time delays become of increasing importance in shallower water. Such artifacts now cause greater errors in hydrographic data quality than bottom detection. Many of these artifacts are a result of imperfect motion sensing, however, new methods such as differential GPS hold great potential for resolving such limitations. For backscatter data, while the system response is well characterised, significant post processing is required to remove residual effects of imaging geometry, gain adjustments and water column effects. With the removal of these system artifacts and the establishment of a calibrated test site in intertidal regions (where the seabed may be intimately examined by eye) one can build up a sediment classification scheme for routine regional seafloor identification. When properly processed, high frequency multibeam sonar data can provide a view of seafloor geology and geomorphology at resolutions of as little as a few decimetres. Specific applications include quantitative estimation of sediment transport rates in large-scale sediment waves, volume effects of iceberg scouring, extent and style of seafloor mass-wasting and delineation of structural trends in bedrock. In addition, the imagery potentially provides a means of quantitative classification of seafloor lithology, allowing sedimentologists the ability to examine spatial distributions of seabed sediment type without resorting to subjective estimation or prohibitively expensive bottom-sampling programs. Using Simrad EM100 and EM1000 sonars as an example, this paper illustrates the nature and scale of possible artifacts, the necessary post-processing steps and shows specific applications of these sonars.

Hydrothermal venting and geothermal heating in Cascadia Basin

Observations in Cascadia Basin on the eastern flank of the northern Juan de Fuca Ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting. Seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small (∼1 km2) isolated igneous basement outcrops. In the northern sector of the plateaulike basin, the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250‐ to 350‐m‐thick “geothermal boundary layer” characterized by anomalously high temperature, reduced vertical stability, and high dissolved silicate concentration. Using a basinwide average lithospheric heat flux of 0.3 W m−2 and the observed thermal anomaly structure of the water column, we obtain a mean residence time of 1 to 2 years for the deep water over Cascadia Basin. Detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north‐central sector of the basin indicate that local bottom‐water heating arises from low‐temperature venting through the summit and flanks of the outcrops. Near the smallest edifice, especially well‐defined layers of anomalously warm, particle‐laden water were found within ±20 m of the outcrop summit depth of 2610 m. Maximum anomalies of temperature, light attenuation coefficient, and dissolved silicate concentration in the layers were 0.040°C, 0.015 m−1, and 5 μmol L−1, respectively. We estimate the local heat flux, Fo′ from the smallest outcrop to be (2.4±0.8)U × 109 W, where U (m s−1) is the mean horizontal current at the venting depth. For reasonable mean currents in the range 10−3 to 10−2 m s−1, we find Fo′ ≈ 0.2 to 2.4 × 107 W. Assuming that the depressed conductive heat flow of −0.05 W m−2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop, the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km.

The Deer Cove deposit, Baie Verte Peninsula, Newfoundland, a Paleozoic mesothermal lode-gold occurrence in the northern Appalachians

The Siluro-Devonian Deer Cove deposit, Baie Verte Peninsula, Newfoundland, is hosted by volcanic cover rocks of the Point Rousse Complex ophiolite. Mineralization consists of quartz vein lodes with gold, pyrite, lesser chalcopyrite, and minor arsenopyrite. Gold occurs as relatively pure gold intergrown with pyrite, and as solitary grains within the quartz gangue. Host rocks include basalt and gabbro at greenschist-facies metamorphic grade. The volcanic rocks have a general calcalkaline affinity, with (anhydrous) SiO2, TiO2, MgO, Al2O3, and Zr contents of 34–62%, 0.36–0.9%, 2.8–9%, 13.4–18.5%, and 28–48 ppm, respectively; Mg # ranges from 37 to 61. The host basaltic rocks were the products of island-arc or back-arc volcanism. Well-developed alteration haloes surround the quartz veins; the alteration grades from quartz–chlorite–carbonate in the veins, through sericitic wall rock, into propylitic (chlorite–epidote–carbonate–leucoxene) host rock. Trace-element geochemistry indicates that the ore fluids had large-ion lithophile element relationships similar to average crustal values. δ13C and δ18O values of carbonate separates range from −7 to −8‰ and 10 to 12‰, respectively. On the basis of geochemical, alteration, and isotopic data, the Deer Cove deposit should be classified as a typical mesothermal lode-gold occurrence. The veins apparently formed from mesothermal fluids with average crustal compositions that flowed along brittle fracture systems within the cover sequence during Siluro-Devonian Acadian deformation. As such the veins were unrelated to either sea-floor processes or Taconic ophiolite obduction. The vein systems apparently formed during transpressive tectonism which followed ocean closure and ophiolite obduction.

Polar continental margins: Studies off East Greenland

The passive continental margin off east Greenland has been shaped by tectonic and sedimentary processes, and typical physiographic patterns have evolved over the past few million years under the influence of the late Cenozoic Northern Hemisphere glaciations. The Greenland ice shield has been particularly affected.GLORIA (Geological Long Range Inclined Asdic), the Institute of Oceanographic Sciences' (IOS) long‐range, side‐scan sonar, was used on a 1992 RV Livonia cruise to map large‐scale changes in sedimentary patterns along the east Greenland continental margin. The overall objective of this research program was to determine the variety of large‐scale seafloor processes to improve our understanding of the interaction between ice sheets, current regimes, and sedimentary processes. In cooperation with IOS and the RV Livonia, a high‐quality set of seafloor data has been produced. GLORIA'S first survey of east Greenland's continental margin covered several 1000‐ × 50‐km‐wide swaths (Figure 1) and yielded an impressive sidescan sonar image of the complete Greenland Basin and margin (about 250,000 km2). A mosaic of the data was made at a scale of 1:375,000. The base map was prepared with a polar stereographic projection having a standard parallel of 71°.

Early proterozoic continental tholeiites from western bergslagen, Central Sweden, I. Petrology, geochemical petrogenesis and geotectonic setting

The Early Proterozoic Bergslagen Supracrustal Series (BSS) of western Bergslagen consists of a pile of felsic volcanites and intercalated carbonates, with in the upper part an alternating succession of volcaniclastics and basic flows. This pile is intruded by a bimodal sequence of granites and basic sills and dikes. Major- and trace-element data reveal the influence of several processes that have acted upon the basic magma. Crustal contamination or supra-subduction zone (SSZ) modification, fractional crystallization and potassic and sodic alteration due to (sub-) seafloor processes are recognized. Using geochemical and petrological criteria the basic rocks are divided in two least-altered (LA) and two alkali-altered (AA) sets. Geochemically, the least-altered basalts resemble Phanerozoic continental within-plate tholeiites and thus contradict the interpretation by many previous authors of Bergslagen as an active margin volcanic arc. Alkali-alteration leads to pseudo calc-alkaline or pseudo alkaline signatures. However, a SSZ-component, observable in part of the LA set, indicates a relation in space and time to a convergent margin.

A multiple fluid history recorded in Alpine ophiolites

Petrographic and cathode luminescence studies of Alpine ophicalcites are combined with stable isotope data of pelagic sediments and associated ophiolite relicts, to document multiple phases of fluid-rock interaction related to sea-floor processes and Alpine orogeny. Internal sediments and primary carbonate microstructures in ophicalcites provide evidence for early sea-floor fragmentation, cementation and fluid activity in the Jurassic Tethys. Ca-carbonate cementation is comparable to cementation in modern transform setting analogues, where alkaline and Ca-enriched fluids are associated with serpentinites. Local mineralization and isotopic compositions of serpentine in ophicalcites are interpreted as signs of hydrothermal activity at temperatures of 100–150 °C. Differences in the degree of oxygen isotope re-equilibration reflect differences in fluid/rock interaction during Alpine accretionary tectonics and continent-continent collision. Regional-scale homogenization of oxygen isotopes in carbonates and cherts indicate the presence of pervasive metamorphic fluids in the pelagic sediments during early Alpine recrystallization. In contrast, a lack of oxygen isotope re-equilibration in serpentine may reflect lower permeabilities and more limited fluid flow in serpentinites during Alpine metamorphism. Cathode luminescence and preliminary oxygen isotope data of late vein-forming phases suggest that late metamorphic fluids were channelled along discrete brittle fractures in a closed, rock-dominated system.

The use of REMOTS ® technology in monitoring coastal enrichment resulting from mariculture

REMOTS ® technology is a combination of sediment-profile imagery and computer image analysis. With it, one can deduce the dynamics of biological and physical seafloor processes from imaged structures. The equipment is easy to deploy and broad areal coverage can be accomplished in a very short time. The data return is equally rapid, allowing the implementation of management decisions which based on ‘real time’ information rather than the ‘after the fact’ remedial actions imposed by the more traditional surveying/monitoring methods.

Sea floor gouges and pits in deep fjords, Baffin Island: Possible mammalian feeding traces

Pisces submersible dives within Baffin Island fjords have revealed the common occurrence of pits on the sea floor, at water depths between 40 and 326 m. The size of these pits are in the decimeter to meter range. Through indirect evidence (by comparison of morphologic features to pits or gouges of known origin) they are believed to be feeding traces of narwhal, beluga, or bowhead whales. If so, they are the deepest mammalian feeding traces yet reported. Bioerosion by large foraging mammals may be a more common sea floor process than previously thought.

An underwater camera-TV system for the study of sea floor profile

A ship deployed remote control underwater camera closed-circuit TV system was developed for observation of sea floor dynamics and processes, microgeomorphology and sediment-water profile of siltclay sediment, water visibility, activities of benthic organisms, etc.