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1.
Inversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple `Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple `flood' geometry. Complete burial, with a typical sediment density of 2300 kg m–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m.  相似文献   

2.
On the basis of the generalization of the concept of standard ocean to the Black Sea, we deduce practical formulas for the conversion of pressure into depth and vice versa depending on the latitude of the place with regard for the equation of state for 1980. The error of these relations for the standard Black Sea (whose salinity is equal to 22.2 at a temperature of +9°C from the surface to the bottom) does not exceed ±0.2m and ±0.2dbar. The difference between the practical and actual depths in winter and summer periods does not exceed ±0.35m for depths varying within range 0–2000m. The proposed practical formulas enable one to simplify the procedure of rapid evaluation of depth (or pressure) in real time by excluding the procedure of integration over a specific volume.  相似文献   

3.
The Ninetyeast Ridge north of the equator in the eastern Indian Ocean is actively deforming as evidenced by seismicity and its eastward subduction below the Andaman Trench. Basement of the ridge is elevated nearly 2 km with respect to the Bengal Fan; seismic surveys demonstrate continuity of the ridge beneath sediment for 700 km north of 10° N where the ridge plunges below the Fan sediment. The ridge is characterised by a free-air gravity high of 50 mgal amplitude and 350 km wavelength, and along-strike continuity of 1500 km in a north-south direction, closely fringing (locally, even abutting) the Andaman arc-trench bipolar gravity field. Regression analysis between gravity and bathymetry indicates that the ridge gravity field cannot be explained solely by its elevation. The ridge gravity field becomes gradually subdued northwards where overlying Bengal Fan sediments have a smaller density contrast with the ridge material. Our gravity interpretation, partly constrained by seismic data, infers that the ridge overlies significant crustal mass anomalies consistent with the hot spot model for the ridge. The anomalous mass is less dense by about 0.27 g cm–3 than the surrounding oceanic upper mantle, and acts as a cushion for isostatic compensation of the ridge at the base of the crust. This cushion is up to 8 km thick and 400–600 km wide. Additional complexities are created by partial subduction of the ridge below the Andaman Trench that locally modifies the arc-trench gravity field.  相似文献   

4.
The Central Spreading Ridge (CSR) is located in the central part of the North Fiji Basin, a complex back-arc basin created 12 Ma ago between the Pacific and Indo-Australian plates. The 3.5 Ma old CSR is the best developed, for both structure and magmatism, of all the spreading centers identified in the basin, and may be one of the largest spreading systems of the west Pacific back-arc basins. It is more than 800 km long and 50–60 km wide, and has been intensively explored during the French-Japanese STARMER project (1987–1991).The CSR is segmented into three first order segments named, from north to south, N160°, N15° and N-S according to their orientation. This segmentation pattern is similar to that found at mid-ocean ridges. The calculated spreading rate is intermediate and ranges from 83 mm/yr at 20°30 S to 50 mm/yr at 17°S. In addition, there is a change in the axial ridge morphology and gravity structure between the northern and southern sections of the CSR. The axial morphology changes from a deep rift valley (N160° segment), to a dome split by an axial graben (N15° segment) and to a rectangular flat top high (N-S segment). The Mantle Bouguer Anomalies obtained on the northern part of the CSR (N160°/N15° segments) show bull's eye structures associated with mantle upwelling at the 16°50S triple junction and also in the middle of the segments. The Mantle Bouguer Anomalies of the southern part of the ridge (N-S segment) are more homogeneous and consistent with the observed smooth topography associated with axial isostatic compensation.At these intermediate spreading rates the contrast in bathymetry and gravity structure between the segments may reflect differences in heat supply. We suggest that the N160° and N15° segments are cold with respect to the hot N-S segment. We use a non-steady-state thermal model to test this hypothesis. In this model, the accretion is simulated as a nearly steady-state seafloor spreading upon which are superimposed periodic thermal inputs. With the measured spreading rate of 50 mm/yr, a cooling cycle of 200,000 yr develops a thermal state that permits to explain the axial morphology and gravity structure observed on the N160° segment. A spreading rate of 83 mm/yr and a cooling cycle of 120,000 yr would generate the optimal thermal structure to explain the characteristics of the N-S segment. The boundaries between the hot N-S segment and its cold bounding segments are the 18°10 S and 20°30 S propagating rifts. A heat propagation event along the N-S segment at the expense of the adjacent colder failing segments, can explain the sharp changes in the observed morphology and structure between the segments.  相似文献   

5.
Total magnetic intensity and bathymetric surveys were carried out in the northern Bay of Bengal between 6° to 11° 45 N latitudes and east of 84° to 93° 30 E longitudes. The hitherto known 85° E Ridge is characterised as a subsurface feature by a large amplitude, positive magnetic anomaly surrounded by Mesozoic crust. A newly identified NE to NNESSW trending magnetic anomaly between 7° N, 87° 30 E and 10° 30 N, 89–90° E may be one of the unidentified Mesozoic lineations in the northern Bay of Bengal. The Ninetyeast Ridge is not associated with any recognizable magnetic anomaly. The Sunda Trough to the east of the Ninetyeast Ridge is characterised by a positive magnetic anomaly. A combined interpretation, using Werner deconvolution and analytical signal methods, yields basement depths ~ 10 km below sea level. These depths are in agreement with the seismic results of Curray (1991).Deceased 24 December 1991  相似文献   

6.
Hekinian  R.  Juteau  T.  Gràcia  E.  Sichler  B.  Sichel  S.  Udintsev  G.  Apprioual  R.  Ligi  M. 《Marine Geophysical Researches》2000,21(6):529-560
The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37N–25° 27W and 1° N–27° 42W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25W (¡3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.  相似文献   

7.
Mercury, cadmium, arsenic, and antimony were analyzed in cores sampled on the Azores-Iceland Ridge. High values of 780 g · kg–1 for Hg, 1.7 g · g–1 for Cd, 87 g · g–1 for As, and 8.1 g · g–1 for Sb occur in the rift valley and transform faults. These enrichments, strictly linked to the ridge, could not have an allochtonous origin. A local hydrogenous flux may explain this phenomenon. These metallic enrichments may be connected to a hydrothermal activity extended between 43° N and 47° N.  相似文献   

8.
Analyses of about 6000 km of processed magnetic data in the central Bay of Bengal using Analytical Signal Processing and Werner Deconvolution techniques revealed that the depth to top of the magnetic basement varies between 5 and 12 km from the sea surface, where the water column thickness is about 3.4 km. These inferred depths are comparable to the reported acoustic basement depths. The basement map derived from magnetic interpretation defines the general configuration of the central Bay of Bengal. The N10–12° W trending subsurface 85° E Ridge buried under 2 to 3 km thick sediments is a prominent tectonic feature. Offshore basins characterised by deeper magnetic basement (9 km) and 100–200 km wide are present on either sides of the ridge. These basins were filled with 6–8 km thick lower Cretaceous to recent sediments. Integrated geophysical study depicts that the magnetic basement is characterised by NW-SE, NE-SW, NNE-SSW, N10-12° W and E-W trending structural features that are associated with the lower Cretaceous ocean floor. The Analytical Signal Processing and Werner Deconvolution techniques proved to be effective in determining the depth to the basement in areas covered by thick sediment overburden and characterized by a complex geologic/tectonic framework.  相似文献   

9.
Ishiwatari  R.  Hirakawa  Y.  Uzaki  M.  Yamada  K.  Yada  T. 《Journal of Oceanography》1994,50(2):179-195
Organic geochemical study of bulk organic matter (OM), hopanoid hydrocarbon and normal hydrocarbon (C23C35) was conducted for a 936-cm-long sediment core sample from the Oki Ridge of the Japan Sea (Core KH-79-3, C-3; 37°03.5 N, 134°42.6E, water depth 935 m). Stable carbon isotopic ratios were also measured for both bulk OM and individual hydrocarbons. The following results were obtained: (1) The weight ratios of total organic carbon to total nitrogen range from 6.2 to 9.4 in the core. The 13C values of bulk OM range from –25.1–20.7%.. The 13C values of OM in the sections of 140190 cm are lower (–25–24) than those in the other sections (–23–21). This result indicates that OM in the core except for the 140190 cm sections is essentially of marine origin. (2) The 13C value of diploptene (a hopanoid hydrocarbon) in the last glacial maximum (LGM), is –66.3 (vs. PDB), which indicates it originating in methanotrophic bacteria. This result provides evidence to support for the previous ideas (Oba et al., 1980, 1984; Masuzawa and Kitano, 1984) that the bottom waters in the Japan Sea were anoxic in LGM. (3) Long chain (C23C35) n-alkanes of higher-plant wax origin were found throughout the core. Their concentration is high in 140190 cm in depth, suggesting that eolian dust load was high in LGM. (4) The n-alkane/TOC ratio increases with decreasing 13C values of bulk OM. This result indicates that the load of terrestrial (probably eolian dust-derived) OM to the Japan Sea became higher in colder climates. (5) The CPI values of long-chain n-alkanes are different in different 13O stages of paleoclimate, probably reflecting variations in species of terrestrial higher plants as a result of climatological adaptations.  相似文献   

10.
Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constrain the width of the zone of active faulting (plate boundary zone) to be 90 km (0.8 Ma) wide along the East Pacific Rise 8° 30N – 10° 00N. Fault scarps, identified on the basis of contoured, shaded relief and slope intensity maps of bathymetry, are measured. These scarp measurements, used in conjunction with data from a separate near-axis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of 100 m at 0.8±0.2 Ma, 45±10 km from the ridge axis. Beyond this distance, there is no significant increase in scarp height. Earlier studies had suggested that the width of the zone of active faulting for outward-dipping faults might be significantly narrower than for inward-dipping faults. A lower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to 20–200 km from the ridge based on previously published lithospheric models. Such a decoupling zone may explain why outward-dipping faults continue to be active as far off-axis as inward-dipping faults. If the width of the zone of active faulting is controlled by the width of a lower crustal decoupling zone, our observations predict an 90 km wide decoupling zone in the lower oceanic crust at this location.  相似文献   

11.
Chlorofluorocarbons (CFC-11 and CFC-12) in the intermediate water having between 26.4 and 27.2 were determined at 75 stations in the western North Pacific north of 20°N and west of 175.5°E in 1993. The intermediate water of 26.4–26.6 was almost saturated with respect to the present atmospheric CFC-11 in the zone between 35 and 45°N around the subarctic front. Furthermore, the ratios of CFC-11/CFC-12 of the water were also of those formed after 1975. These suggest that the upper intermediate water (26.4–26.6) was recently formed by cooling and sinking of the surface water not by mixing with old waters. The water below the isopycnal surface of 26.8 contained less CFCs and the area containing higher CFCs around the subarctic front was greatly reduced. However, the CFC age of the lower intermediate water (26.8–27.2) in the zone around the subarctic front was not old, suggesting that the water was formed by diapycnal mixing of the water ventilated with the atmosphere with old waters not containing appreciable CFCs, probably the Pacific Deep Water. The southward spreading rate decreased with depth and it was one sixth of its eastward spreading rate of the North Pacific Intermediate Water (NPIW).  相似文献   

12.
PCBs, DDT compounds and HCH isomers were detected in the air and surface waters of the North Pacific and Indian Oceans, including the Bering Sea, East China Sea, South China Sea, Bay of Bengal and the Arabian Sea. The general concentrations of each chlorinated hydrocarbon were as follows: water PCBs 0.1 to 1.0, DDT 0.01 to 1.0, HCH 1.0 to 10 ngl –1; air DDT 0.01 to 1.0, HCH 0.1 to 10 ng m–3. PCB concentrations in surface waters were slightly lower than those of the North Atlantic and North Sea previously reported, while DDT concentrations in the air and water were higher. Remarkably high concentrations of DDT and HCH were found in the air off the western coast of India. Also in the Pacific site off Central America, a fairly high concentration of DDT was observed in an air sample. These data suggest that large amounts of DDT and HCH are being used in the tropical zone, especially in southern Asia. Furthermore, high concentrations were observed both in the air and water of the Northwest Pacific between 30°N and 40°N latitude. There is a possibility that both pesticides are not only still being used in lower latitude countries but also in the mid-latitude ones of the Asian continent excluding Japan. In addition to this atmospheric circulation may also contribute to the concentration of these pesticides in the mid-latitudinal zone.  相似文献   

13.
High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m2 in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.  相似文献   

14.
We study examples of surveys of the Black Sea performed with the help of a SeaWiFS satellite instrument. It is shown that the results of measurements carried out in spectral channels of 510 and 555nm give information on the basic specific features of the space and time variability of the characteristics of absorption and scattering of light in seawater. The use of a spectral channel of 412nm is inefficient due to the low reliability of the results of the procedure of atmospheric correction performed according to standard algorithms. The results of model calculations demonstrate that the predominant contribution to the absorption of light in the Black Sea in summer is made by the yellow substance.  相似文献   

15.
A study of Sea Beam bathymetry and SeaMARC II side-scan sonar allows us to make quantitative measures of the contribution of faulting to the creation of abyssal hill topography on the East Pacific Rise (EPR) 9°15 N–9°50 N. We conclude that fault locations and throws can be confidently determined with just Sea Beam and SeaMARC II based on a number of in situ observations made from the ALVIN submersible. A compilation of 1026 fault scarp locations and scarp height measurements shows systematic variations both parallel and perpendicular to the ridge axis. Outward-facing fault scarps (facing away from the ridge axis), begin to develop within 2 km of the ridge and reach their final average height of 60 m at 5–7 km. Beyond these distances, outward-dipping faults appear to be locked, although there is some indication of continued lengthening of outward-facing fault scarps out to the edge of the survey area. Inward-facing fault scarps (facing toward the ridge axis), initiate 2 km off axis and increase in height and length out to the edge of our data at 30 km, where the average height of inward fault scarps is 60–70 m and the length is 30 km. Continued slip on inward faults at a greater distance off axis is probable, but based on fault lengths, 80% of the lengthening of inward fault scarps occurs within 30 km of the axis (>95% for outward faults). Along-strike propagation and linkage of these faults are common. Outward-dipping faults accommodate more apparent horizontal strain than inward ones within 10 km of the ridge. The net horizontal extension due to faulting at greater distances is estimated as 4.2–4.3%, and inward and outward faults contribute comparably. Both inward- and outward-facing fault scarps increase in height from north to south in our study area in the direction of decreasing inferred magma supply. Average fault spacing is 2 km for both inward-dipping and outward-dipping faults. The azimuths of fault scarps document the direction of ridge spreading, but they are sensitive to local changes in least compressive stress direction near discontinuities. Both the ridge trend and fault scarp azimuths show a clockwise change in trend of 3–5° from 9°50 N to 9°15 N approaching the 9° N overlapping spreading center.  相似文献   

16.
Temperature and currents were measured 100 m south of the Pipe Organ vent site on the Juan de Fuca Ridge during a 5-day interval in September 1997 to examine the temporal variability of hydrothermal heat signals close to a source. Temperatures were sampled at three depths every 15 s while current speed and direction were sampled at a single depth hourly. Results show that rapid potential temperature () changes of as much as 0.02°C occurred at all three depths, partly caused by changes in current speed and direction. Spectra of showed no prominent spectral peaks, however. An analytic model of heat transport under variable currents was used to help put observed temporal variability into context and to point out problems with estimating hydrothermal heat fluxes from mooring data. Model distributions, when contoured on a horizontal plane and animated, demonstrate the pooling of effluent over vents at various times, the streaming of effluent as a very narrow plume away from vents at times of large current speed, and the spawning of boluses of heated fluid and their transport away from venting regions when a pooling period is followed by a streaming period. Model results also show that estimated heat flux based on mooring data is strongly dependent on analysis assumptions and suggest that vents are capable of causing measurable anomalies as far as several kilometers downstream.  相似文献   

17.
Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30 N.The two northernmost segments (38°20 N and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and 38°20 N segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.  相似文献   

18.
Dependences have been determined which connect the parameters of the dispersion relation of the lowest mode of internal waves with the integral characteristics of the seasonal thermocline when 10 min30 min, 20 mh150 m, and 0·4 m2/s2 Q5·2 m2/s2.Translated by Mikhail M. Trufanov.  相似文献   

19.
The reflection and breaking of internal waves on a sloping beach were studied in a small wavetank filled with water and petroleum. The dependence of the reflection coefficient of the internal waves on wave steepness and on beach slope is found to be very similar to that of surface waves. The reflection coefficient is small for the very gentle slope, increases rapidly as the slope increases, and becomes almost constant for the steep slope. The reflection coefficient decreases with increase of the wave steepness. Also, the transition slope at which the coefficient curve has the maximum gradient increases with increase of the wave steepness. Breaking pattern of the internal waves is classified into four types; breaking, semi-breaking, wrinkle-generating, and non-breaking. Their dependence on beach slope and wave steepness is examined. The regular sequence of the four breaking types from breaking to non-breaking is observed with decrease of wave steepness or with increase of beach slope.  相似文献   

20.
In 1983 a combined SeaMARC I, Sea Beam swath mapping expedition traversed the East Pacific Rise from 13°20 N to 9°50 N, including most of the Clipperton Transform Fault at 10°15 N, and a chain of seamounts at 9°50 N which runs obliquely to both the ridge axis and transform fault trends. We collected temperature, salinity and magnetic data along the same track. These data, combined with Deep-Tow data and French hydrocasts, are used to construct a thermal section of the rise axis from 13°10 N to 8°20 N.Thermal data collected out to 25 km from the rise axis and along the Clipperton Transform Fault indicate that temperatures above the rise axis are uniformly warmer by 0.065°C than bottom water temperatures at equal depths off the axis. The rise axis thermal structure is punctuated by four distinct thermal fields with an average spacing of 155 km. All four of these fields are located on morphologic highs. Three fields are characterized by lenses of warmed water 20 km in length and 300 m thick. Additional clues to hydrothermal activity are provided in two cases by high concentrations of CH4, dissolved Mn and 3He in the water column and in another case by concentrations of benthic animals commonly associated with hydrothermal regions.We use three methods to estimate large-scale heat loss. Heat flow estimates range from 1250 MW to 5600 MW for one thermal field 25 km in length. Total convective heat loss for the four major fields is estimated to lie between 2100 MW and 9450 MW. If we add the amount of heat it takes to warm the rest of the rise axis (489 km in length) by 0.065.°C, then the calculated axial heat loss is from 12,275 to 38,525 MW (19–61% of the total heat theoretically emitted from crust between 0 and 1 m.y. in age).  相似文献   

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