Regions Of Seafloor Research Articles

Fluxes of fluid and heat from the oceanic crustal reservoir

Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.

Interaction between the Mid‐Atlantic Ridge and the Azores hot spot during the last 85 Myr: Emplacement and rifting of the hot spot‐derived plateaus

Multiple‐ and single‐beam bathymetric data are compiled over the Azores plateau to produce a 1 km × 1 km grid between latitudes 32°N and 49°N and longitudes 22°W and 43°W. Mantle Bouguer anomalies are then calculated from this grid and the satellite‐derived gravity. These grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis. The elevated seafloor of the Azores plateau is interpreted as resulting from the interaction of a mantle plume with the Mid‐Atlantic Ridge (MAR). The presence of a large region of elevated seafloor associated with a thick crust between the Great Meteor Seamounts and the Azores platform on the Africa plate, and less developed conjugate structures on the North America plate, favors genetic relations between these hot spot‐derived structures. This suggests that a ridge‐hot spot interaction has occurred in this region since 85 Ma. This interaction migrated northward along the ridge axis as a result of the SSE absolute motion of the Africa plate, following a direction grossly parallel to the orientation of the MAR. Kinematic reconstructions from chron 13 (∼35 Ma) to the present allow a proposal that the formation of the Azores plateau began around 20 Ma and ended around 7 Ma. A sharp bathymetric step is associated with the beginning of important melt supply around 20 Ma. The excess of melt production is controlled by the interaction of the ridge and hot spot melting zones. The geometry and distribution of the smaller‐scale features on the plateau record episodic variations of the hot spot melt production. The periodicity of these variations is about 3–5 Myr. Following the rapid decrease of widespread volcanism, the plateau was subsequently rifted from north to south by the Mid‐Atlantic Ridge since 7 Ma. This rifting begins when the MAR melting zone is progressively shifted away from the 200‐km plume thermal anomaly. These results bear important consequences on the motion of the Africa plate relative to the Azores hot spot. They also provide an explanation to the asymmetric geochemical signature of the Azores hot spot along the Mid‐Atlantic Ridge.

Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan

The Zaire deep-sea fan is one of the largest fans in the world still affected by turbidite sedimentation along a unique active meandering turbidite channel. This active channel is fed by turbidity currents, which are generated at the Zaire River mouth, and flow via a deeply entrenched canyon across the shelf and the continental slope. Based on a detailed study of the morphology and architecture of the present Zaire Canyon/Channel, several main zones can be defined (the canyon, the upper-fan valley, the upper and the lower channel-levee system leading into distal lobes). They are characterised by different behaviours in terms of erosion, transport and sedimentation within the canyon/channel. An important characteristic of the Zaire Canyon/Channel is the deep incision of the thalweg, well below the regional sea floor along its whole path. The origin of this entrenchment may be linked to the incision of the Zaire Canyon back across the shelf during the last sea-level rise. This incision of the canyon has allowed the continuity of turbidite activity during the Holocene, in maintaining the connection between the canyon head and the river mouth (in contrast to most of other large deep-sea fans, which are generally inactive during highstands). The entrenchment of the Zaire Canyon/Channel limits the overflow of turbidity currents and the turbidite sedimentation over levees, and prevents avulsion along the upper part of the Zaire Channel. Most of the sediment transported in turbidity currents are probably led down to the lower channel-levee system and the distal lobes.

An epeiric ramp: low‐energy, cool‐water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia

The Murray Supergroup records temperate‐water carbonate deposition within a shallow, mesotrophic, Oligo‐Miocene inland sea protected from high‐energy waves and swells of the open ocean by a granitic archipelago at its southern margin. Rocks are very well preserved and exposed in nearly continuous outcrop along the River Murray in South Australia. Most facies are rich in carbonate silt, contain a background assemblage of gastropods (especially turritellids) and infaunal bivalves, and are packaged on a decimetre‐scale defined by firmground and hardground omission surfaces. Bioturbation is pervasive and overprinted, resulting in rare preservation of physical sedimentary structures. Facies are grouped into four associations (large foraminiferan–bryozoan, echinoid–bryozoan, mollusc and clay facies) interpreted to represent shallow‐water (<50 m) deposition under progressively higher trophic resource levels (from low mesotrophy to eutrophy), and restricted marine conditions from relatively offshore to nearshore regions. A large‐scale shift from high‐ to low‐mesotrophic conditions within lower Miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow, land‐locked sea. Overall, low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone, a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity. Inshore, heterozoan facies became progressively muddier and restricted towards the shoreline. In contrast, periods of wet climate led to rising trophic resource levels, resulting in a shallowing of the euphotic zone, a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids. Rates of carbonate production and accumulation were relatively low. The Murray Basin is best described as an epeiric ramp. Wide facies belts developed in a shallow sea on a low‐angled slope reaching many hundreds of kilometres in length. Grainy shoal and back‐barrier facies were absent. Internally generated waves impinged the sea floor in offshore regions and, because of friction along a wide and shallow sea floor, created a low‐energy expanse of waters across the proximal ramp. Storms were the dominating depositional process capable of disrupting the entire sea floor.

A shallow, chemical origin for the Marquesas Swell

Young hotspot volcanoes within plate interiors are frequently surrounded by smooth, broad regions of shallow seafloor termed midplate swells. These swells are typically hundreds of kilometers wide and can be more than a kilometer in elevation. The most frequently invoked explanation for these swells is that they represent the thermal and dynamic surface uplift from rising mantle plumes. Here we argue that buoyancy of a volcanic unit underplating the Marquesan volcanoes just below the Moho, as imaged by a seismic refraction experiment, is capable of producing much, if not all, of what has been previously interpreted as a thermal swell in both the bathymetry and the geoid. The shallow compensation depth previously calculated for the swell based on geoid observations is thus expected given that the compensation resides at 20‐km depth. The volcanic unit underplating the islands would also be expected to have a thermal effect, but its contribution to swell uplift is never more than 10% of the chemical contribution. The predicted increase in heat flow, on the other hand, is very large, but the thermal transient rapidly decays in the first 2 million years after emplacement. If similar large volumes of underplated material underlie other hotspot volcanoes, problems in explaining the lack of heat flow anomalies at later times and negligible rates of swell subsidence can be alleviated. Although volcanic underplating is unlikely to explain the entire swell signature for particularly large and wide swells, the results from the Marquesas suggest that at least some portion of hotspot swells may not be thermal in origin. After removing the geoid signature from the Marquesas volcanoes and swell, what remains in the geoid are stripes of amplitude 1 m and wavelength 650 km trending normal to the fracture zone. The anomalies are presumably relict from when this crust was created, but their amplitude is too large to be caused by variations in isostatically compensated crustal thickness.

Origin of intraplate volcanoes from guyot heights and oceanic paleodepth

The height of a guyot as measured from the surrounding regional sea floor to the volcano's slope break records the water depth at the time the guyot submerged. Thus guyot heights may be used as indicators of the paleodepth of the surrounding ocean floor. We compile data on the heights of 68 intraplate guyots and atolls in the Pacific Ocean as well as 46 volcanic islands in the Pacific, Atlantic, and Indian Oceans. We find that guyot heights generally increase with the age of the lithosphere upon which they were emplaced, although there is a large amount of scatter. In nearly all cases, seamount height, and thus seafloor paleodepth, is less than expected of normal seafloor. These results suggest that most of the volcanoes in this study formed on anomalously shallow seafloor, consistent with formation at hotspots. To characterize thermal anomalies associated with these hotspot swells, we model guyot heights by calculating the isostatic uplift predicted for normal lithosphere that has been partly reheated and is underlain by anomalously hot mantle. This model is able to explain the anomalous water depth at most of the seamounts with hotspot thermal anomalies of 100°–300°C. The heights of a few volcanic chains, however, are not anomalously low, suggesting that these volcanoes are not associated with hotspots. In addition, the observed trend of Hawaiian‐Emperor guyot heights as well as the subdued morphology and gravity signature of the oldest Emperor seamounts supports our hypothesis that Cretaceous age Meiji seamount may have formed on or near a spreading center.

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.

Reheating of old oceanic lithosphere: Deductions from observations

Deep, wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy. When the depths of these basins are corrected for the effect of sediment accumulation, and variation in crustal thickness, the principal non-thermal factors have been eliminated. We collect the most precise and reliable values of heat flow for the same basins, from multi-penetration measurements with in situ thermal conductivity, or deep sea drilling thermal gradients backed up by surface surveys. The 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere. When considered without regard to age, all the points fall at greater depths than predicted by the ‘plate’ models with constant temperature lower boundaries, and remarkably close to boundary-layer cooling with parameters determined from the pre-80 Ma depth and heat flow history of the ocean floor. They are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age. Modeling of discrete reheating events shows that near boundary layer conditions are re-established after about 40 Myr, but corresponding to a younger-than-real age. The data therefore favor discrete reheating events rather than a continuously hot basal boundary, as implicitly assumed by the plate model. Lithospheric reheating appears to start only on ocean floor > 100 Ma. The data alone cannot discriminate between a few discrete reheating events due to convective peel-off at the base of the lithosphere or one or more catastrophic events. However, the distribution of points from the Blake-Bahama basin is more consistent with distal reheating associated with the Bermuda hotspot than local convective peel-off.

Comparison of along‐track resolution of stacked Geosat, ERS 1, and TOPEX satellite altimeters

Cross‐spectral analysis of repeat satellite altimeter profiles was performed to compare the along‐track resolution capabilities of Geosat, ERS 1 and TOPEX data. Geophysical Data Records were edited, differentiated, low‐pass‐filtered, and resampled at 5 Hz. All available data were then loaded into three‐dimensional files where repeat cycles were aligned along‐track (62 cycles of Geosat/Exact Repeat Mission; 16 cycles of ERS 1, 35‐day orbit; 73 cycles of TOPEX). The coherence versus wave number between pairs of repeat profiles was used to estimate along‐track resolution for individual cycles, eight‐cycle‐average profiles, and 31‐cycle‐average profiles (Geosat and TOPEX only). Coherence, which depends on signal to noise ratio, reflects factors such as seafloor gravity amplitude, regional seafloor depth, instrument noise, oceanographic noise, and the number of cycles available for stacking (averaging). Detailed resolution analyses are presented for two areas: the equatorial Atlantic, a region with high tectonic signal and low oceanographic noise; and the South Pacific, a region with low tectonic signal and high oceanographic variability. For all three altimeters, along‐track resolution is better in the equatorial Atlantic than in the South Pacific. Global maps of along‐track resolution show considerable geographic variation. On average globally, the along‐track resolution (0.5 coherence) of eight‐cycle stacks are approximately the same, 28, 29, and 30 km for TOPEX, Geosat, and ERS 1, respectively. TOPEX 31‐cycle stacks (22 km) resolve slightly shorter wavelengths than Geosat 31‐cycle stacks (24 km). The stacked data, which are publicly available, will be used in future global gravity grids, and for detailed studies of mid‐ocean ridge axes, fracture zones, sea mounts, and seafloor roughness.

Computer model simulations of reverberation from the ARSRP acoustics experiment

Detailed reverberation measurements were made from the mid-Atlantic ridge region during the 1993 Acoustics Experiment, Acoustic Reverberation Special Research Program (ARSRP). The reverberation scenario (designated as ‘‘Seg 076’’) that ensonified a segment of sedimented seafloor overlying an anelastic subbottom has been examined. Computer simulations of the reverberant field were generated and compared with the ARSRP measured reverberation. Time domain and cw computer simulations were made using high-resolution ocean acoustic–seismoacoustic computer models. Several realizations of the rough range-dependent sediment and sub-bottom were used (from the Webb–Jordan sediment distribution model and the Goff–Jordan fractal seafloor/basement model). Computer simulations allowed ‘‘numerical experiments’’ to be performed whereby parts of the environment were changed or eliminated. Simulation results that help identify those seafloor regions responsible for the reverberant returns are presented. [Work supported by ONR, Acoustic Reverberation SRP, and the High Performance Computing DoD Shared Resource Center: U.S. Army Corps of Engineers Waterways Experiment Station (CEWES) HPC Center, Vicksburg, MS.]

Comparison of acoustic model predictions with reverberation data from the ARSRP reconnaissance experiment

Reverberation measurements from the mid-Atlantic ridge region were made during the Acoustic Reverberation Special Research Program (ARSRP) 1991 reconnaissance cruise. Ocean acoustic models have been used to investigate acoustic backscatter from a sedimented seafloor overlying an anelastic subbottom typical of this seafloor region. Computer simulations of the reverberant field were generated and compared with the ARSRP measured reverberation. Computer simulations were made using high-resolution ocean acoustic and seismoacoustic cw computer models. The parabolic equation model, fepe, was used to propagate the long-range acoustic fields to and from the vicinity of the mid-Atlantic ridge region, and the seismoacoustic finite-element model, safe, was used to determine the scattered fields from the rough seafloor and anelastic subbottom. Time-domain calculations were obtained from FFTs of the backscattered cw fields. Several realizations of the range-dependent sediment and subbottom were used. The bathymetric data for the sedimented regions were generated by the Webb–Jordan sediment distribution model. The bathymetric data for the subbottoms were generated by the Goff–Jordan fractal seafloor/basement model. The envelope of the simulated reverberation compared favorably with the measured reverberation data. Results support the conclusion that seafloor reverberation can be accurately simulated using high resolution ocean acoustic-seismoacoustic computer models together with high-resolution seafloor bathymetric data obtained from geomorphological models. [Work supported by the Office of Naval Research, Acoustic Reverberation SRP.]

Applied marine geodetic research in polar regions

At the Alfred Wegener Institute for Polar and Marine Research, several geodetic methods are used to map the seafloor in polar regions. Multibeam bathymetric surveys are regularly conducted in polar regions by the institute's icebreaker Polarstern. These multibeam data are combined with bathymetric data from other sources to create bathymetric maps. The Hydrosweep multibeam system on the Polarstern is presently being modified to have a wider survey swath and the ability to collect sidescan and backscatter data. These modifications will enable researchers to survey the seafloor more efficiently and will provide important information about the seafloor materials. Satellite altimetry data is also used to supply the correlation between the earth's geopotential field and the seafloor topography. This correlation can be used to guide bathymetric contouring in areas with little or no bathymetric data. Current efforts involve the study of ERS‐1 altimetry data from the Weddell Sea, Antarctica, where the altimetry d...

Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine

Research Article| January 01, 1994 Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine Joseph T. Kelley; Joseph T. Kelley 1Maine Geological Survey, Station 22, Augusta, Maine 04333 Search for other works by this author on: GSW Google Scholar Stephen M. Dickson; Stephen M. Dickson 1Maine Geological Survey, Station 22, Augusta, Maine 04333 Search for other works by this author on: GSW Google Scholar Daniel F. Belknap; Daniel F. Belknap 2Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar Walter A. Barnhardt; Walter A. Barnhardt 2Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar Mark Henderson Mark Henderson 2Department of Geological Sciences, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (1): 59–62. https://doi.org/10.1130/0091-7613(1994)022<0059:GSBPEF>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph T. Kelley, Stephen M. Dickson, Daniel F. Belknap, Walter A. Barnhardt, Mark Henderson; Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine. Geology 1994;; 22 (1): 59–62. doi: https://doi.org/10.1130/0091-7613(1994)022<0059:GSBPEF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Circular depressions, or pockmarks, cover the sea floor in many estuarine regions of the western Gulf of Maine. In Belfast Bay, Maine, they are found in densities up to 160/km2, are up to 350 m in diameter and 35 m in relief, and are among the largest and deepest known. The pockmarks appear to form from the escape of biogenic natural gas and pore water and are far larger than features associated with thermogenic gas elsewhere. These pockmarks are thought to have formed (1) catastrophically during an earthquake, tsunami, or storm, or (2) slowly over thousands of years. Recent observations of bubble releases suggest continuing activity and a potential geologic hazard. The pockmarks involve a poorly documented coastal process of sediment redistribution and methane release, largely unrecognized in the rock record but widespread in middle- to high-latitude embayments. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Superswell and Mantle Dynamics Beneath the South Pacific

The region of sea floor beneath French Polynesia (the "Superswell") is anomalous in that its depth is too shallow, flexural strength too weak, seismic velocity too slow, and geoid anomaly too negative for its lithospheric age as determined from magnetic isochrons. These features evidently are the effect of excess heat and extremely low viscosity in the upper mantle that maintain a thin lithospheric plate so easily penetrated by volcanism that 30 percent of the heat flux from all hot spots is liberated in this region, which constitutes only 3 percent of the earth's surface. The low-viscosity zone may facilitate rapid plate motion and the development of small-scale convection. A possible heat supply for the Superswell is a mantle reservoir enriched in radioactive isotopes as suggested by the geochemical signature of lavas from Superswell volcanoes.

Early Tertiary basalts from the Labrador Sea floor and Davis Strait region

Fine- to medium-grained, phyric and aphyric basalt samples from ODP Leg 105, site 647A, in the Labrador Sea show little evidence of alteration. Chemically, these rocks are low-potassium (0.01–0.09 wt.% K2O), olivine- to quartz-normative tholeiites that compare closely with the very depleted terrestrial Paleocene volcanic rocks in the Davis Strait region of Baffin Island and West Greenland. However, differences exist in the Sr–Nd isotope systematics of the two suites; the Labrador Sea samples have ε Nd values (+9.3) indicative of a more depleted source, and are higher in 87Sr/86Sr (0.7040), relative to the Davis Strait basalts (ε Nd +2.54 to +8.97; mean 87Sr/86Sr 0.7034). The higher 87Sr/86Sr in the Labrador Sea samples may reflect seawater exchange despite no petrographic evidence for significant alteration. The Labrador Sea and early Davis Strait basalts may have been derived from a similar depleted mantle source composition; however, the later Davis Strait magmas were generated from a different mantle. None of the Baffin Island, West Greenland, or Labrador Sea samples show unequivocal geochemical evidence for contamination with continental crust.

High-resolution seismic-reflection interpretations of some sediment deposits, Antarctic continental margin: Focus on the western Ross Sea

High-resolution seismic-reflection data have been used to a varying degree by geoscientists to interpret the history of marine sediment accumulations around Antarctica. Reconnaissance analysis of 1-, 3.5-, and 12-kHz data collected by the U.S. Geological Survey in the western Ross Sea has led to the identification of eight echo-character facies and six microtopographic facies in the sediment deposits that overlie the Ross Sea unconformity. Three depositional facies regions, each characterized by a particular assemblage of echo-character type and microtopographic facies, have been identified on the continental shelf. These suites of acoustic facies are the result of specific depositional processes that control type and accumulation of sediment in a region. Evidence of glacial processes and products is uncommon in regions 1 and 2, but is abundant in region 3. McMurdo Sound, region 1, is characterized by a monospecific set of acoustic facies. This unique assemblage probably represents turbidity current deposition in the western part of the basin. Most of the seafloor in region 2, from about latitude 77°S to 75°S, is deeper than 600 m below sealevel. The microtopographic facies and echo-character facies observed on the lower slopes and basin floor there reflect the thin deposits of pelagic sediments that have accumulated in the low-energy conditions that are typical of deep-water environments. In shallower water near the boundary with region 3, the signature of the acoustic facies is different from that in deeper water and probably indicates higher energy conditions or, perhaps, ice-related processes. Thick deposits of tills emplaced by lodgement during the most recent advance of the West Antarctic Ice Sheet are common from latitude 75°S to the northern boundary of the study area just south of Coulman Island (region 3). The signature of microtopographic facies in this region reflects the relief of the base of the grounded ice sheet prior to decoupling from the seafloor. Current winnowing and scour of shallow parts of the seafloor inhibits sediment deposition and maintains the irregular, hummocky relief that characterizes much of the region. Seafloor relief of this type in other polar areas could indicate the former presence of grounded ice.

Stochastic modeling of seafloor morphology: A parameterized Gaussian model

Stochastic methods of analysis are useful for quantifying ensemble properties of small‐scale bathymetric features such as abyssal hills. In this paper we model the seafloor as a stationary, zero‐mean, Gaussian random field completely specified by its autocovariance function. We formulate an anisotropic autocovariance function that has five free parameters describing the amplitude, anisotropic orientation and aspect ratio, characteristic length, and Hausdorff (fractal) dimension of seafloor topography. Parameters estimated from various seafloor regions by an inversion of Sea Beam data indicate that the seafloor exhibits a wide range of stochastic characteristics within the constraints of the model. Synthetic topography can be generated at arbitrary scale and resolution from the Gaussian model using a Fourier method. Color images of these synthetics are useful for illustrating the stochastic behavior of the model.

The ICE-MOSES experiment: Mapping permafrost zones electrically beneath the Beaufort Sea

A novel variation of the geophysical technique known as MOSES, for Magnetometric Off-Shore Electrical Sounding, has been developed to map the electrical properties of the sea floor in Aretic regions. The particular target is the permafrost layer under the Beaufort Sea, a layer containing frozen or partially frozen sediment from 100 to 600 m thick underlying shallow sea water, typically 10 to 100 m deep, and several tens of metres of soft sediment. A detailed knowledge of the location and physical properties of the permafrost layer is essential for accurate interpretation of reflection seismic data. The permafrost can contain pockets, regions or layers of gas hydrate. The latter is both a possible resource and a hazard to drilling operations or hydrocarbon production. A local map of the permafrost zone is essential geotechnical information required prior to the construction of an offshore structure or pipeline.

Vent-type taxa in a hydrocarbon seep region on the Louisiana slope

Recent discoveries on the northern Gulf of Mexico continental slope have altered our understanding of biological and chemical processes occurring in the deep ocean. A biological community of hydrothermal vent-type organisms was recently discovered at the base of the Florida Escarpment1, where the fauna are apparently nourished by hydrogen sulphide-rich hypersaline water seeping out onto the sea floor. Dense colonies of deep living chemosynthetic benthic organisms were first discovered during investigations of warm water anomalies along the axis of the Galapagos Rift in the Pacific Ocean in 19772—4, and this first discovery of clusters of clams, tube worms and other filter feeders in the immediate proximity of warm water vents has been followed by the discovery of a number of other hydrothermal vent sites, for example Guaymas Basin, East Pacific Rise at 21° N. The dense population assemblages at these sites are apparently restricted to small areas of the ocean floor where hydrogen sulphide-rich water is escaping from spreading centres, but the Florida Escarpment discovery indicates that these communities can also exist on passive margins. Here we report the discovery of dense biological communities associated with regions of oil and gas seepage on the Louisiana continental slope. These communities of large epi- and infaunal organisms are similar to those associated with the vents of the Pacific and the hypersaline brine seeps of the Florida Escarpment. Carbon isotope analyses suggest that these communities are chemosynthetic and derive their energy from hydrogen sulphide and/or hydrocarbons. Similar communities may be widely distributed on the sea floor in other oil-producing regions of the ocean.

Major determinants of the biogeographic pattern of the shallow-sea fauna

The benthic shallow-sea is defined as the region of sea floor lying between the supralittoral zone at the shoreline and the impingement of the thermocline separating a warm shallow and variable portion of the water column from rather homogeneous and constant cooler waters beneath. Three types of shallow-sea provinces can be recognized: (1) one-dimensional, linear shelves; (2) two-dimensional shelves; and (3) scattered islands in two-dimensional arrays. Dispersal powers of marine invertebrates vary with developmental mode, and patterns of dispersal, endemism and speciation vary among the different provincial types. Invertebrate developmental modes vary systematically with geography, and presumably are adaptive to environmental conditions. Clades with only a single mode of development tend to be restricted to regions appropriate to that mode, significantly affecting their biogeographic patterns. The consequences of geographic and other environmental changes are reviewed.