Seafloor Sites Research Articles

Precise GPS/Acoustic positioning of seafloor reference points for tectonic studies

Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy). The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of ±0.8 cm north and ±3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island.

Broadband seismology and noise under the ocean

Most of our understanding of the Earth's interior has been derived from measurements from global seismic networks, although no network has ever been truly “global” because some 71% of the Earth's surface is underwater. The resulting gaps in coverage produce a biased and incomplete image of the Earth. Work has begun toward establishing permanent observatories on the deep seafloor, although the technical difficulties remain severe. Will these stations be useful, and where and how shall they be established? Data from seafloor observatories will be of poorer quality than continental site data because the sea surface is an important and local source of broadband noise. This noise is derived from wind and waves through direct forcing at long periods and by nonlinear coupling to elastic waves at short periods. Our understanding of the generation and propagation of seismic noise and of wind and wave climatology can be used to predict the temporal and geographical variability of the noise spectrum and to assess likely sites for permanent seafloor observatories. High noise levels near 1 Hz may raise detection limits for short‐period, teleseismic arrivals above mb = 7.5, limiting the usefulness of many seafloor sites. Noise levels in deep boreholes will be 10 dB quieter than those at the seafloor, but sensors buried short distances below the seafloor may also provide comparable noise levels and fidelity. The retrieval of data from permanent seafloor observatories remains an unsolved problem, but long‐term temporary arrays of ocean bottom seismometers are now being used in regional scale experiments using earthquakes as sources. Such experiments are likely to be less successful in the Pacific basin than in either the Indian Ocean or North Atlantic Ocean because of higher noise levels.

Fine-scale heat flow, shallow heat sources, and decoupled circulation systems at two sea-floor hydrothermal sites, Middle Valley, northern Juan de Fuca Ridge

Fine-scale heat-flow patterns at two areas of active venting in Middle Valley, a sedimented rift on the northern Juan de Fuca Ridge, provide thermal evidence of shallow hydrothermal reservoirs beneath the vent fields. The extreme variability of heat flow is explained by conductive heating immediately adjacent to vents and shallow circulation within sediments above the reservoir. This secondary circulation is hydrologically separated from the deeper system feeding the vents by a shallow conductive lid within the sediments. A similar separation of shallow and deep circulation may also occur at sediment-free ridge-crest hydrothermal environments.

Phytodetritus at the abyssal seafloor across 10° of latitude in the central equatorial Pacific

Fresh phytoplankton detritus (or phytodetritus) has been reported from numerous deep seafloor sites in the temperate North Atlantic and Pacific Oceans following seasonal phytoplankton blooms. Here we report the first strong evidence for abyssal accumulations of phytodetritus in the tropics, in the central equatorial Pacific. In November–December 1992 we obtained photographs and/or sediment-core samples from 61 abyssal stations (water depths of 4280–5012 m) between 12°S and 9°N along ∼ 140°W. Greenish flocculent material was recovered from the top of multiple-core samples from 5°S to 5°N; this material was most abundant from 2°S to 2°N, in some areas forming continuous layers at least 5 mm thick, and individual aggregates > 1 cm in diameter. The greenish material was clearly visible in bottom photographs as a green veneer that covered >95% of the seafloor near the equator, and as individual cm-scale aggregates covering <1% of the seafloor. Occasionally, thick accumulations of cm-scale aggregates occurred in biogenic pits. Cleared trails and feeding traces suggest that surface-deposit-feeding holothurians and echiurans grazed the greenish material. Microscopic examination of greenish material recovered from core tops and a burrow lumen revealed relatively intact diatoms (including Rhizosolenia sp.) and other microalgae with chloroplasts containing chlorophyll. The greenish material was 1–12.5% organic carbon by weight, i.e. 5–39 times richer than associated seafloor sediments. It also contained high excess activities of 234Th, suggesting arrival from the water column in the previous 100 days. Samples of the greenish flocculent material from 0° and 5°N incubated at simulated environmental pressure and temperature with 14C-labeled glutamate exhibited ⩾ 5-fold higher rates of microbial activity than underlying sediments or brown floc from 9°N. Surface-sediment samples (which included the greenish flocculent material) from 5°S to 5°N also contained significant concentrations of chlorophyll a and other chloropigments; the chloropigment concentrations were roughly comparable to deep-sea phytodetritus collected in the North Atlantic. We conclude that fresh, organic-rich phytodetritus was present on the seafloor from 5°S to 5°N along 140°W in November–December 1992, with highest concentrations within 2–3° of the equator. This material is likely to be a concentrated, high-quality food resource for deep-sea microbes and metazoans. We estimate an upper limit for the standing stock of this phytodetritus to be ∼2.6 mmol C org/m 2; this corresponds to ∼3% of the annual flux of organic carbon to the seafloor at these latitudes in 1992. Because the degradation rate of this material appears to be very high, its presence at the seafloor for several months per year could yield significant phytodetrital contributions to the annual seafloor organic-carbon budget. We also suggest that the phytodetrital aggregates are formed at intense convergence zones resulting from seasonal passage of tropical instability waves within 5° of the equator; if so, phytodetrital accumulations are likely to recur seasonally over broad areas of the abyssal equatorial Pacific.

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A. Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust. Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.

The electrical conductivity of the oceanic upper mantle

SUMMARY Previous inversions of sea-floor magnetotelluric (MT) sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest, and controlled source-field electromagnetic (CSEM) soundings require a lithospheric mantle conductivity of 3 x lop5 Sm-’, which is so low that the electromagnetic (EM) coast effect would produce more anisotropy in MT soundings than is observed. We address these issues by constructing an olivine mantle model for conductivity and examining the inversion of MT data from three sea-floor sites, and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered. Our mantle model is based on recent measurements of olivine conductivity, the conductivity of tholeiite melt, a thermal model for the upper mantle based on lithospheric cooling and the temperature of a+ a + /3 olivine transition, the pyrolite model of mantle petrology, and conductivities derived from CSEM sounding. We propose Archie’s Law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity, and Archie’s Law with exponent 1.5 as the preferred estimate. The 1-D forward response of this model is not compatible with observed sea-floor MT data. Three data sets presented by Oldenburg (1981) pass a test for one-dimensionality based on the size of the residuals when fit with Parker’s D+ algorithm, but two of the three soundings fail a test for independence of residuals. We also find that the presence of the upper mantle ‘high-conductivity zone’ previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount. Re-inversions of the MT data produce models which are incompatible with our petrological model of mantle conductivity. However, by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3-D problem using thin-sheet analysis we predict MT responses which are distorted in a manner that is remarkably similar to the observed data.

ODP Leg 136 sets seismograph test site

On the evening of March 2, 1991, the research vessel JOIDES Resolution departed from Honolulu harbor for a 17‐day mission that reflects an important new trend in the international Ocean Drilling Program (ODP). The primary objective of ODP Leg 136 was to prepare a seafloor site for future experiments needed to develop the Ocean Seismographic Network (OSN). The focus of OSN is much deeper than the strata reached by the drill bit of the Resolution, and this cooperative effort will enable the drilling ship's resources to be shared with a community whose interests extend to the inner core of the Earth.During Leg 136, ODP Hole 843B (proposed Site OSN‐1) was established on the Hawaiian Arch, approximately 225 km south‐southwest of the island of Oahu (Figure 1). The hole was drilled through 242 m of sediment and 70 m into the basaltic basement. It will provide a site for performing borehole seismometer experiments that include noise measurements, recording data from teleseismic events for comparison with an existing high‐quality station on Oahu, and testing new broad‐band sensors and other instrumentation for long‐term deployment. A significant part of this report will be devoted to providing the reader with the background and objectives of the OSN.

The electrical conductivity structure of the oceanic lithosphere beneath the Tasman Sea

Seafloor magnetotelluric observations of natural fluctuations of the Earth's magnetic and electric fields provide information about the electrical structure of the suboceanic lithosphere and upper mantle. However, such measurements may be distorted by three-dimensional induction effects in the overlying ocean, associated with the geomagnetic coast-effect and changes in bathymetry. In the present paper electromagnetic three-dimensional modelling is applied to the Tasman Sea. A thin-sheet algorithm is used, and calculated magnetotelluric parameters are compared to observations made during the Tasman Project of Seafloor Magnetotelluric Exploration (TPSME). The model calculations suggest that, away from coastlines, three-dimensional induction in the ocean distorts the electromagnetic signature of the sub-oceanic lithosphere and upper mantle in a frequency-dependent but smooth manner. The model calculations thus permit the observed TPSME responses to be corrected for distortion, to give improved estimates of the electromagnetic response of the solid-Earth underlying the observation sites. Inversion of such (corrected) response estimates for one-dimensional electric structure reveals that the structures beneath 55 and 70 Ma seafloor sites are very similar. Furthermore, constraints imposed by the thin-sheet modelling indicate that to replicate the TPSME observations, a poorly conductive (less than 10-3 S/m) upper lithosphere is required. This result implies that electric currents which are induced and 'trapped' in the ocean may contaminate magnetotelluric observations far inland.

Electrical conductivity structure Beneath the Ryukyu trench-arc system and its relation to the subduction of the Philippine Sea plate.

Seafloor measurements of geomagnetic variations were undertaken at six sites in the Ryukyu trench-arc system in order to investigate the electrical conductivity structure of the crust and upper mantle with special reference to the subduction of the Philippine Sea plate. In addition to the conventional analysis in which transfer functions for the vertical component are derived, we show the effectiveness of another type of transfer functions through which seafloor horizontal fields relate to horizontal fields simultaneously recorded on an island near seafloor sites. Because of the high attenuation of geomagnetic variations at periods shorter than 30min at seafloor sites, analysis was possible only for the periods longer than 30min. We estimated both types of transfer functions for the period range of 30 to 240min. We attempted a two-dimensional modeling for a profile orthogonal to the strike of the Ryukyu trench-arc system, using a finite element method. The comparison of transfer functions derived from observations and calculations yields a model showing two columnar conductors in the mantle wedge; one at the depth range from 20 to 60km in the forearc region and the other at the depth range from 60 to 130km or more beneath the northwestern margin of the Okinawa Trough behind the arc. According to recent mineralogical studies, dehydration of some minerals in hydrated peridotite at depths of about 100km and 150km plays an important role on magma genesis in subduction zones. The columnar conductor found below the forearc region is likely to be a zone containing water released through dehydration of some minerals in the hydrated peridotite, whereas the columnar conductor beneath the northwestern margin of the Okinawa Trough seems to correspond to a zone of partial melting which may be triggered by water released through dehydration of some other minerals.

Geomagnetic induction in the Tasman Sea and electrical conductivity structure beneath the Tasman Seafloor

SUMMARY During the Tasman Project of Seafloor Magnetotelluric Exploration recordings were made of the natural magnetic and electric field variations along a line of seafloor sites crossing the Tasman Sea. Using these data and additional magnetic field recordings made on the Australian continent the pattern of geomagnetic induction in the Tasman Sea Region is examined. An investigation of the seafloor magnetotelluric impedance tensor indicates the geomagnetic induction is influenced by the large-scale 3-D conductivity structure of the region. This result is confirmed by a study of impedance estimates obtained using the vertical gradient sounding method (in which the seafloor impedance is determined from the attenuation of the horizontal magnetic field variations through the ocean). Inversion of the E-polarization impedance component from sites in the central Tasman Sea indicates the conductivity structure at depths greater than about 100 km is laterally homogeneous in this region. It is probable that the conductivity structure at these sites includes a high-conducting layer similar to one observed in the Pacific Ocean. One seafloor site, lying on the Tasmantid Seamount Chain, exhibits an increased conductivity at depths of less than 100 km. The increased conductivity may be due to heating of the lithosphere by a hotspot, one proposed source of the seamount chain.

Auriferous chert, banded iron formation, and related volcanogenic hydrothermal alteration, Atik Lake, Manitoba

Small-scale alteration pipes and stratiform alteration in Archean glomeroporphyritic tholeiitic basalts at Atik Lake, Manitoba, stratigraphically underlie silicate-oxide banded iron formation (BIF) and auriferous sulfide-bearing chert. The auriferous chert is locally interbedded with graphitic argillite, indicating euxinic conditions during deposition. Cordierite–gedrite rocks formed by recrystallization of alteration assemblages during the lower amphibolite-facies metamorphism (T = 550 °C, P = 2.5 kbar). Al2O3, TiO2, Zr, and Nb, which were relatively immobile during alteration, have been used to monitor igneous differentiation and alteration. Volcanogenic hydrothermal alteration resulted in depletion of Ca, Si, Mg, Na, and Sr in the altered basalt and the addition of K, Fe, Rb, and Ba. This was accompanied by mass and volume losses of up to 25%. The mineralizing fluid was reducing and somewhat acidic. Rare-earth-element (REE) profiles of BIF and graphitic argillite, normalized to Archean shale, are less steep ((La/Lu)N = 0.51 and 0.49 respectively), than those of both mineralized chert ((La/Lu)N = 0.04) and recent sea-floor, siliceous, gold-enriched massive sulfides ((La/Lu)N = 0.11). REE profiles and Boström's plot suggest that the auriferous, sulfide-bearing chert formed by mixing of hydrothermal and detrital components. The overall chemical changes in the Atik Lake alteration system are comparable to those in Noranda-type massive-sulfide deposits. The trace-metal association in the auriferous chert is similar to that at some modern sea-floor hydrothermal sites.

The Tasman Project of Seafloor Magnetotelluric Exploration: experiment and observations

A major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between Australia and New Zealand; simultaneous observations were made by land magnetometers on the Australian continent. The large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea. Parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites. The Parkinson arrows show a strong coast effect near the Australian continent and a weaker coast effect over the submerged Lord Howe Rise. The occurrence of smoothly varying arrows across the Tasman Sea suggests a simple, large-scale pattern for electromagnetic induction in the Tasman Sea. The interpretation of Parkinson arrows at seafloor sites is shown to require special care, due to the attenuation and phase-shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations. The magnetotelluric impedances exhibit a strong anisotropy at all sites in the Tasman Sea. The orientation of the impedance axes is constant across the Sea and the anisotropy may be attributed to the two-dimensional shape of the Tasman Sea. Calculations based on the distance scale of the magnetotelluric anisotropy, interpreted as an ocean-continent boundary effect, give a lower bound for the integrated resistivity of the Tasman Sea oceanic lithosphere of order 10 7 Ω m 2, corresponding to an estimate of 2 × 10 2 Ω m for its average resistivity.

Hydrocarbon gas in sediment of the Southern Pacific Ocean

Methane, ethane, ethene, propane, and propene are common hydrocarbon gases in near-surface sediment from offshore areas in the southern Pacific Ocean near Papua New Guinea, the Solomon Islands, Vanuatu, Tonga, New Zealand, and Antarctica. Sea floor sites for sampling of sediment were selected on the basis of anomalies in marine seismic records, and the samples were intentionally biased toward finding possible thermogenic hydrocarbon gases. In none of the areas, however, were thermogenic hydrocarbons clearly identified. The hydrocarbon gases that were found appear to be mainly the products ofin situ microbial processes.

The geomagnetic response across the continental margin off Vancouver Island: Comparison of results from numerical modelling and field data.

Geomagnetic variation data have been measured along a profile from central Vancouver Island, across the continental shelf to the deep seafloor on the Juan de Fuca plate. As expected, the maximum response in the geomagnetic coast effect occurs near the shelf-edge and the fall-off inland and seaward is observed. The direction and magnitude of the real part of the induction arrows vary slightly with period between 30 and 103 minutes. The induction arrows are co-linear at all sites and are aligned approximately normal to the shelf-break trend. At the seafloor sites, the imaginary part of the induction arrows has a large variation with period, reversing its direction at periods less than 35 minutes. At the land sites, the imaginary part increases in magnitude with increasing period.The responses of several numerical models were calculated using the finite difference algorithm of Brewitt-Taylor and Weaver. The comparison of the calculated and observed responses shows that a good conductor is required at sub-lithospheric depths beneath the profile. The addition of a conductive wedge of sediments beneath the continental shelf improves the fit. The best-fitting model incorporates this wedge and a conductive, shallow-dipping slab representing the Juan de Fuca plate subducting under Vancouver Island.

Hydrogen and oxygen isotope evidence for sea-water-hydrothermal alteration and ore deposition, Troodos complex, Cyprus

Synopsis A model is presented for the genesis of the Cyprus massive cupriferous sulphide deposits, their associated stockwork mineralization and the metamorphism of the Troodos ophiolite complex based on D/H, 18 O/ 16 O and 13 C/ 12 C ratio analyses, where applicable, of 94 whole rocks and minerals, chemical, mineralogical and sulphur iso tope data. The upper 3–5 km of the hot Troodos crust — pillow lavas, Sheeted Intrusive Complex, trondhjemites and upper gabbros — were metamorphosed to zeolite facies, temperatures of about 0 to 250° C, and greenschist-amphibolite facies, temperatures of about 250 to 450–550° C, during interaction with a deep circulating sea-water geothermal system. Minimum isotopic temperatures of 320–400°C are given for the epidote-bearing vein-halo assemblages in the upper gabbros, trondhjemites and Sheeted Intrusive Complex. If temperatures at the base of the Sheeted Intrusive Complex were higher than ∼ 500° C, either a magmatic or metamorphic water component must have been present in the hydrothermal fluids. The magmatic water could be of sea-water origin, being derived from the melting or assimilation of sea-water-hydrothermally altered roof rocks overlying the gabbroic magma chambers. Minimum water/rock ratios during pervasive alteration decrease from &gt; 1 by weight in zeolite facies pillow lavas to ∼ 0.1 in the underlying green-schist facies rocks, but were an order of magnitude larger in the veins and fractures. In the stockwork zones, which underlie the sea-floor sites of massive sulphide orebodies, isotopic temperatures declined from initial values greater than 300° C (with water/rock ratios &gt; 1–5, by weight) to less than 220° C during post-sulphide later-stage vein quartz formation; these temperatures were higher than temperatures in the predominantly zeolite facies country rocks. The stockwork zones are interpreted to represent the discharge zones of the seawater-hydrothermal fluids responsible for the regional-scale mass transfer and the mineralization. At least some of the sulphide sulphur in the stockworks and orebodies is of seawater origin. Formation of massive sulphide deposits requires the coincidence of several factors, including the penetration of a brine to deep levels, where temperatures were greater than 300° C in an active spreading centre, leaching of ore metals, and discharge of the hot springs into basins where both sulphide deposition and localization could occur.

Electric and Magnetic Fields Induced by Deep Sea Tides

A lunar semidiurnal variation (12·4206 h period) induced by tides in the deep ocean is demonstrated in (i) a month's observations of the horizontal electric field and magnetic declination at a sea floor site located 600 km off the California coast in 4·4 km of water, in (ii) a month's observations of the vertical magnetic field at a coastal site near Cambria, California, and in (iii) two years' observations of the vertical magnetic field at the island magnetic observatory on San Miguel, Azores. Comparison of the sea floor, coastal, and island observations with simultaneous continental magnetic observations permits an estimation of that part of the variation due to a lunar ionospheric oscillation. The observed oceanic induced variation at the sea floor and coastal sites are found to agree well with tidal induced fields computed for a model of the Earth's electrical conductivity and lunar semidiurnal tide. The model assumes (i) a flat semi-infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (ii) non-conducting atmosphere, continent and upper mantle, and (iii) superconducting mantle at a uniform depth beneath the ocean. The observed oceanic induced vertical magnetic field at San Miguel is found to agree qualitatively with tidal induced fields computed for an island model. This model assumes (i) a uniformly conducting small circular island, (ii) a flat infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (iii) a non-conducting atmosphere and upper mantle, and (iv) a superconducting mantle at a depth much greater than the size of the island.