摘要:

Oceanography, and marine geophysics in particular, has always been as much a science of exploration as of experimentation. Even our most carefully planned and supposedly controlled experiments as often as not result in serendipitous but new physical insights into how the earth works. Of late, the profession has been dominated more and more by the invention of new technology, for example, ways of seeing deeper into the earth with greater and greater resolution. Satellite measurements of gravity, multichannel seismic profiling, the deep diving submersibleAlvin, and remote survey systems such as Seamark, Deeptow, Gloria, and Angus come quickly to mind.

ISSN:0148-0227    

DOI:10.1029/JB088iB02p00993

年代:1983

数据来源: WILEY

摘要:

Heat transfer processes at the mounds area of the Galapagos Spreading Center at 86°W are revealed by temperatures measured at ≈ 10‐m intervals in the 30±10m sediment at each of 12 holes at DSDP Leg 70 Sites 506–509 and by temperatures of up to five thermistors on eleven 8–12 m long piston cores. The 325 needle‐probe values show a significant linear increase of thermal conductivity with depth in each core. About half of the temperature‐thermal resistance profiles are nonlinear and are fit to a steady state, vertical pore water advection model. Results indicate high and variable total heat flow and localized hydrothermal discharge at ≈ 10−8m/s, associated with individual mounds. Recharge is indicated at similar rates in the low heat flow belt ≈ 5 km south of the mounds and is suggested at slower rates in the intermediate heat flow (0.17–0.42 W/m2) belt surrounding the mounds heat flow high. Possible slow entrained recharge within ≈ 100 m of discharging mounds is suggested. Also suggested is strong local discharge along the major fault bounding the mounds crustal block to the north. About 95 km north of the spreading axis, at DSDP Site 510, temperatures in the 114‐m sediment cover on 2.7‐m.y. crust are linear, consistent with the suggestion that the hydraulic resistance of this layer is sufficient to seal off free hydrothermal exchange between basement and bottom water. The combination of heat flow data and the physical properties data of Karato and Becker (this issue) suggests that ≈ 50 m of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement, where the topography is smooth. We suggest that the formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment, as its thickness ap

ISSN:0148-0227    

DOI:10.1029/JB088iB02p00995

年代:1983

数据来源: WILEY

摘要:

Density and porosity of sediments were measured on DSDP legs 69 and 70 samples from the Galapagos spreading center. Permeability and the hydraulic impedance of each sediment layer were estimated from measured values of porosity. The gradients of porosity and density with depth where sediment layers are thin ( ≲50 m thick ) are anomalously high compared with those of other areas and with the upper part of thicker sediment layers in this area. A good correlation was found between the anomalous porosity and density gradients and the present‐day heat flow. We interpret these observations to suggest that these high gradients may be due to active hydrothermal circulation through a thin sediment cover, which is inhibited by a thicker sediment layer, and that the pattern of hydrothermal circulation may be essentially fixed with the moving plate. Hydraulic impedance of the sediment layer was estimated from the observed depth variation of porosity and was shown to increase rapidly with its thickness. Our interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields after an average permeability of the underlying basement layer of about 3 −6×10−14m2( 30–60

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01009

年代:1983

数据来源: WILEY

摘要:

Nineteen new heat flow measurements made across deformed oceanic lithosphere in the Central Indian Ocean, and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region. Many of the temperature‐depth profiles are nonlinear. Upward convection of water is the most likely explanation for the curvature of the temperature profiles, since other possible causes, including variations in bottom water temperatures, conductivity changes with depth in the sediments, and experimental error, can be eliminated. This interpretation reguires water velocities of the order of 7×10−8m/s, which is unusual because the lithosphere is relatively old (72–82 m.y.) and a thick sedimentary cover (1−2.5 km) is present. These observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment‐water interface. We infer that extra heat is being generated at shallow depths (perhaps less than 35 km) in the plate, although the specific mechanism by which deformational energy is converted into heat is difficult to

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01018

年代:1983

数据来源: WILEY

摘要:

The heat flow distribution at the Galapagos Spreading Center is compared to results of two‐dimensional numerical models for the hydrothermal convection through oceanic crust. The model calculations are based on the equations for fluid flow through porous media adapted for the situation at spreading oceanic ridges. The temperature‐ and pressure‐dependent thermodynamic characteristics of water were used in the fluid flow equations. Models with average permeabilities of approximately 5×10−15m2and penetration depths between 2 and 5 km produce heat flow distributions compatible with the observations at the Galapagos Spreading Center. Because of the convective heat loss, temperatures within the hydrothermal layer are significantly lower than for conductively cooling crust. Two different types of convection cells develop. The one or two cells closest to the ridge axis are fixed in location with respect to the ridge axis. Convection there is characterized by high temperatures (>300°C), rapid flow rates, and low water to rock ratios (∼1). These cells remove most of the heat associated with the intrusion process at the ridge axis. Cells farther away from the ridge axis move with the moving plate and serve to prevent the oceanic crust from reheating. Temperatures there typically are moderate to low (<200°C), and flow velocities are lower than those in the axial cell, but water to rock ratios can be very high in

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01033

年代:1983

数据来源: WILEY

摘要:

On DSDP Leg 70,Glomar Challengerpiston cored hydrothermal MnO2‐encrusted nontronite mounds and adjacent pelagic sediments through to basement. Pore waters were collected by centrifuging, squeezing, and in situ sampling; analyses are presented here for Ca, Mg, Si, NH3, Mn, and Fe. Our results confirm Maris and Bender's (1982) conclusions that hydrothermal solutions enriched in Ca by 1–2 mM and depleted in Mg by ∼2 mM are upwelling through the mounds and the surrounding pelagic sediments. Si, NH3, and Mn2+concentrations generally increase upcore, reflecting addition of products of metabolic reactions to upwelling hydrothermal solutions. Pore water iron concentrations decrease upcore, probably as a result of oxidation and precipitation of upwelling hydrothermal iron. The formation of nontronite (Fe(III)4Si8O20(OH)4) involves oxidation of dissolved Fe2+. Several models, constrained by the electron balance, are proposed to explain the process of nontronite formation. The stratigraphy of the mounds (thick nontronite covered by a thin MnO2crust) may be explained by postulating Fe2+oxidation by MnO2 and replacement of MnO2by nontronite at the base of the MnO2crust, followed by upward migration of Mn2+and precipitation of MnO2at the sediment water inte

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01049

年代:1983

数据来源: WILEY

摘要:

We report 48 new heat flow measurements on the western flank of the East Pacific Rise at 21°N. The stations were taken in 1 to 30 m of sediment on 0.4 to 1.4 m.y. old crust. The low average measured value of ≥173 mW/2(including 12 tilted minimum values) is about 1/3 the mean heat flow predicted for crust of this age range by cooling plate theory. This may be partly due to a bias of measurement locations in sediment ponds. Nevertheless, 4‐km running averages of measured heat flows, projected normally to the axis, display an oscillatory trend suggestive of cellular hydrothermal circulation in the basement. We sampled no more than one cycle of the apparent heat flow modulation, which is of ≈ 15‐km wavelength and ≈ 200‐mW/m2amplitude. This variation appears to be uncorrelated with the topographic variation of ≈ 10‐km wavelength, Assuming that the porosity sensitive electromagnetic results of Young and Cox (1981) reflect a permeability distribution that restricts the depth of circulation to ≈ 1.4 km, average temperatures of circulation may be ≈ 50–60°C. Deeper, higher‐temperature circulation is allowable if a more uniform permeability

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01057

年代:1983

数据来源: WILEY

摘要:

A detailed study of a 60×150 km area at 60°W, 24°N at the eastern end of the Nares Abyssal Plain indicates that hydrothermal circulation is still active in the 80 m.y. B.P. oceanic crust. The 58 heat flow measurements made at five stations in the area have revealed (1) constant heat flow over the abyssal plain (56 mW m−2), (2) a cyclic heat flow over the abyssal hills (mean of 77 mW m−2), and (3) a large anomaly of 710 m W m−2over one of several small domes which protrude from the abyssal plain. The domes are 0.5–1.0 km in diameter near the top and rise 50 m above the level of the abyssal plain. They are recognized from surface echo sounders by an abrupt disappearance in the abyssal plain subbottom reflectors, but on near‐bottom pinger records they appear as steep‐walled structures which are covered by ∼10 m of sediment (compared to ∼75 m on the surrounding abyssal hills). From analogy with active ridge crests, these features are probably small volcanoes. The heat flow anomaly over one of the domes is matched well by a finite element convection model with the following characteristics: (1) recharge at one basement outcrop and discharge at another, (2) 300 m of sediment fill between outcrops, and (3) permeabilities of 10−10cm2for basalt and 10−13cm2for sediment. In other words, we believe that there is very effective convective heat transfer within the crust and out of the relatively permeable, thinly sedimented basement dome, resulting in the local high heat flow. Overall, the results from the Nares survey vividly show the age independent muting effect of sediment on the surface manifestation of crustal convection. In our survey area the mode of heat transfer varies from purely conductive in the more thickly sedimented abyssal plain areas (∼300 m sediment cover) to moderate amplitude convection pattern beneath the abyssal hills (∼75 m sediment cover) to a very large thermal anomaly over the small dome or ‘chimneylike’ structure (∼10 m sediment cover). The domes are possibly active analogues to the presently inactive basement c

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01067

年代:1983

数据来源: WILEY

摘要:

Thermal gradient measurements, consolidation tests, and pore water compositions from the Mariana Trough imply that water is moving through the sediments in areas with less than about 100 m of sediment cover. The maximum advection rates implied by the thermal measurements and consolidation tests may be as high as 10−5cm s−1but are most commonly in the range of 1 to 5×10−6cm s−1. Theoretical calculations of the effect of the highest advection rates upon carbonate dissolution indicate that dissolution may be impeded or enhanced (depending upon the direction of flow) by a factor of 2 to 5 times the rate for diffusion alone. The average percentage of carbonate is consistently higher in two cores from the area with no advection or upward advection than the average percentage of carbonate in three cores from the area with downward advection. This increase in average amount of carbonate in cores with upward moving water or no movement cannot be attributed solely to differences in water depth or in amount of terrigenous dilution. If the sediment column acts as a passive boundary layer, then the water velocities necessary to affect chemical gradients of silica are in the range 10−9to 10−10cm s−l. However, if dissolution of silica occurs within the sediment column, then the advection velocities needed to affect chemical gradients are at least 3×10−8cm s−land may be as high as 3×10−6cm s−l. This order of magnitude increase in advection velocities when chemical reactions occur within the sediments is probably applicable to other cations in addition to silica. If so, then the advection velocities needed to affect heat flow (>10−8cm s−1) and pore water chemical gradients are much nearer in magni

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01075

年代:1983

数据来源: WILEY

摘要:

The heat flow into 7000‐year‐old Crater Lake caldera is dominated by hydrothermal processes. The average of the conducted heat flow value is 138±121 mW/m2(3.3±2.9 HFU). The total heat flow is estimated to be between 669 and 1381 mW/m2(16 and 33 HFU). Hydrothermal fluids penetrate into the lake floor to a depth of at least 1.5 to 2.0 km and possibly much deeper. Two thermal spring areas were discovered on the deep lake floor. These springs, and probably other as yet undiscovered ones, have a dramatic effect on the water temperature, circulation, and stratification of the lake. Dense thermal waters pond in the southwest basin. The deep waters of the east and northwest basin are warmed to the point that they convect, causing the lake to be vertically mixed. A hyperadiabatic temperature gradient results from this convection and possibly the stagnating effects of small quantities of suspended solids. Thermal fluids carrying dissolved solids cause anomalously high concentrations of chloride and sulfate in the relatively pure lake

ISSN:0148-0227    

DOI:10.1029/JB088iB02p01094

年代:1983

数据来源: WILEY