Present status of oceanic heat-flow measurements

P R E S E N T S T A T U S OF O C E A N I C HEAT-FLOW MEASUREMENTS* R. P. VoN HERZEN'~ University of California, San Diego, Marine Physical Laboratory of the Scripps Institution of Oceanography, San Diego, California and

M. G. LANGSETH Lamont Geological Observatory of Columbia University, Palisades, New York

CONTENTS 1. Introduction

367

2. Techniques of Measurement A. Instruments 1. Temperature gradient measurement 2. Conductivity measured with the heated needle probe technique 3. Heat-flow measurements at the Mohole test site 4. Errors of heat-flow measurements B. Effects of the environment

368 368 369 372 372 374 375

3. Distribution and Results of Heat-flow Measurements A. Distribution of values B. Ridge measurements C. Basin measurements D. Special regions I. West of North America 2. Trenches E. Summary

378 378 379 382 384 384 386 386

4. Indian Ocean Measurements A. Distribution of measurements

388 388

* Contribution from the Scripps Institution of Oceanography, University of California, San Diego; and the Lamont Geological Observatory, Columbia University, Palisades, New York. ? Present address: Office of Oceanography, UNESCO, Place de Fontenoy, Paris 7, France. 365

366

R. P. VON HERZE/qand M. G. LANGSETH

B. Measurements by the different techniques

396

C. Correlation of heat flow with physiographic features in the Indian Ocean 1. Description of ridges and basins 2. Heat-flow measurements on ridges 3. Low values associated with ridges 4. Trench values D. Mean heat flow of the Indian Ocean

399 399 399 402 402 402

5. Future Investigations

405

Acknowledgments

406

References

406

1. INTRODUCTION The outward flow of heat through the earth's surface is a measurable geophysical quantity from which deductions can be made on the amount and distribution of heat sources in the interior, the mode of heat transfer from the interior, and the thermal history of the earth. Unfortunately, deductions about these quantities are not independent of one another, nor are they uniquely determined by measurements of surface heat flow alone. When interpreted in the light of other geophysical and geochemical investigations, however, the surface heat flow becomes an important constraint for most theories concerning the constitution and history of the earth. The measurement of terrestrial heat flow in the oceanic regions was suggested and initiated by Sir Edward Bullard during a visit to the Scripps Institution of Oceanography in 1949, and the first measurements were made about a year later. On the whole, the measurements on land are considerably more difficult and tedious to make than those at sea, the latter being facilitated by the large thermal inertia of the deep-sea waters and the relatively soft muds on the bottom. Most of the major oceanographic institutions with geophysical programs are now making heat-flow measurements at sea, which presently number about 103; there is every reason to expect that the number will increase to the order of 104 in the next decade with the increase in oceanographic survey expeditions and availability of reliable instrumentation. Perhaps the most important discovery of the ocean investigations has been that the steady heat flow per unit area through the ocean floor is nearly equivalent to that through the surface of the continental regions. This equality of heat flow has special significance as a boundary condition for the distribution of heat sources and the mode of heat transfer in the mantle. Despite this similarity, the values of oceanic heat flow vary by more than one order of magnitude between extremes. These variations are frequently correlated with major geologic structures of the ocean floor, especially the high values associated with oceanic rises. Some of the variations, however, especially some low values, apparently result from configurations and processes peculiar to the environment of the sea floor; these values may not represent the heat flow from the deep interior. This paper discusses these and other results of the measurements in the oceanic regions up to the present time. New measurements from the Indian Ocean are presented and compared with results from other oceanic areas.

367

368

R. P. VON HERZENand M. G. LANGSETH 2. TECHNIQUES OF MEASUREMENT

A. Instruments Approximately 900 measurements of terrestrial heat flow through the floors of the earth's oceans have been made to date (W. H. K. LEE, 1964, personal communication). About 600 of these measurements were made with the Bullardtype cylindrical probe (BULLARD, 1954; VON HERZEN et al., 1962) developed at Scripps Institution of Oceanography (SIO), and roughly 200 with the Ewingtype instrument with outrigged probes (GERARD et aL, 1962) developed at the Lamont Geological Observatory (LGO). The remainder of the measurements were made with similar instruments used by Cambridge University, England (LISTER, 1963), the Earthquake Research Institute in Tokyo (UYEDAet al., 1961) and Woods Hole Oceanographic Institutiort, U.S.A. Both the cylindrical and the outrigged instruments measure the vertical component of the temperature gradient in the upper several meters of sediment by driving into the bottom a probe with two or more thermal elements set a known distance apart. The thermal element temperature or temperature difference is recorded versus time while the probe is in the sediment. Since the probe is heated by friction during penetration of the bottom, it must remain undisturbed in the sediment sufficiently long to allow a large fraction of the initial heat to dissipate. Only one or two minutes is required for probes 0.3 cm in diameter to dissipate most of their excess heat, whereas larger probes up to 2-7 cm in diameter may require up to 40 min. The time required in the bottom is roughly proportional to the square of the probe diameter (BULLARD,1954). Knowing the vertical temperature gradient at a locality, the heat flow is then determined from the thermal conductivity of the material in which the temperature gradient has been measured Q = K(dT/dz). For this purpose, gravity or piston core samples of the sediment are obtained at or near the site of the gradient measurement. The thermal conductivity may be determined by steady state techniques such as the divided bar method (KATCUFFE, 1960) or by the transient heated-probe technique (VON HERZEN and MAXWELL, 1959). In addition, a good estimate of the conductivity can be made from a measurement of water content. RATCLIFFE,(1960) and BULLARDand DAY (1961) show that the variation of thermal conductivity depends principally on the water content of the sediment. Most of the measured values of conductivity obtained on sediments recovered from the ocean floor are within +25 per cent of 2.0 x 10- 3 cal/cm °C sec. The measurements of heat flow in the Indian Ocean which are reported in detail in this paper were made with the two-meter cylindrical type probe developed at SIO and with the outrigged probe instrument attached to a piston coring device developed at LGO. All of the thermal conductivities were measured by the transient heated-needle method soon after the sediment cores were brought aboard. Since these techniques have been used for the majority of the heat-flow measurements made from oceanographic vessels, they will be described in somewhat more detail below.

Present Status of Oceanic Heat-Flow Measurements

369

1. Temperature gradient measurement. The cylindrical probe shown in Fig. 1A, has two thermal elements (pairs of thermistors) spaced a fixed distance apart (1.7 m) in a hollow stainless steel tube. The probe is two meters in length and 1.9 cm in diameter. The top of the probe is rigidly attached to the recorder pressure vessel which has sufficient weight to drive the probe completely into the soft sediment in a vertical position. RECOR1DERUNIT.

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370

R. P. VON HERZEN and M. G. LANGSETH

0"001°C. Typical records made for high, normal and low sediment gradients are shown in Fig. 1C (VON HERZEN and UYEDA, 1963). Temperature gradients are determined from such records by fitting the observed values of temperature difference versus time to the theoretical curve expressing the cooling of a cylinder in an isotropic medium (BULLARO, 1954). L

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The outrigged probe technique uses very small diameter probes (0-3 cm) mounted on a standard piston coring device. Generally four thermistor probes are used: three on the pipe which penetrates the sediment, and the fourth, or water probe, on the recorder pressure vessel. The water probe has a very small diameter and its primary function is to measure the water temperature during lowering and hoisting of the instrument. A typical arrangement of the probes and the recorder pressure vessel on the Ewing piston corer is shown in Fig. 2. The piston corer can penetrate up to 20 m into soft sediment so that relatively

371

Present Status of Oceanic Heat-Flow Measurements

large separations of thermal elements may be used. Unlike the cylindrical probe technique, the outrigged method collects a core sediment sample for thermal conductivity measurements simultaneously with the gradient measurement. In the Ewing instrument the temperature at each probe is measured by placing each thermistor sequentially into a simple fixed-resistor Wheatstone bridge

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372

R. P. VoN HEl~ZENand M. G. LANGSETH

Ewing instrument, and the temperature difference between the sediment/water interface and the probes in the sediment as deduced from the record, are shown in Fig. 3B and C, respectively. The use of three or more probes allows the comparison of gradients over two or more intervals in the sediment. Some of the ocean bottom measurements with the outrigged instrument have shown that the heat flow is not constant with depth. Such measurements are believed to be in areas where transient thermal conditions exist near the sediment surface. Measurements in these areas do not necessarily represent the regional heat flow. Therefore gradient measurements over two intervals in the sediment are of considerable value in that they enable one to detect transient or disturbed thermal conditions. 2. Conductivity measured with the heated needle-probe technique. Before making conductivity measurements aboard ship, sediment samples are allowed to remain in a room of nearly constant temperature for about an hour. Point measurements with the needle probe are usually made at regular intervals along the core. The effective conductivity of the sediment column over the interval of gradient measurement is taken as the reciprocal of the average of individual determinations of thermal resistivity. (Resistivity equals conductivity-~.) Determining the conductivity in this way does not introduce a great uncertainty unless the core is made up of many layers of different type sediment and different thicknesses, a situation which frequently creates difficulty for measurements on land (BmCH, 1954). The probe used for the conductivity measurements of ocean-floor sediment (Fig. 4) is about 6-3 cm long and about 0.08 cm in external diameter. The probe contains an internal heater wire and a small thermistor midway along its length. The probe is inserted into the soft sediment and power is supplied to the heater wire at a constant rate (0-7 W of power are commonly used). Due to the thinness of the probe, its rise of temperature closely approximates to that of a line source soon after power is applied. Thereafter the temperature of the probe is proportional to the logarithm of time. The relation is: T = (q/4rcK) In t + C where T is the probe temperature (°C), q is the heat per unit length per unit time (cal/cm sec), K is the conductivity (cal/cm °C sec), t is time (sec), C is a constant. To determine the conductivity with the needle probe it is only necessary to measure the rise of temperature with time and the power applied. In Fig. 4, a circuit to record the temperature of the needle probe is shown together with details of the needle construction, and plot of probe temperature versus the logarithm of time for a typical measurement. Some comparisons of the needleprobe method with the steady-state method have shown good agreement (Von HERZ~N and MAXWELL,1959). 3. Heat-flow measurements at the Mohole test site. Several holes were drilled about 170 m deep in the sediment near Guadalupe Island during a preliminary phase of the Mohole project. Temperature measurements were made in one of the holes by a special probe (VoN HERZE• and MAXWELL, 1964). The probe used

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thermal conductivity (from VaN H~ZEN et al., 1962). is 1.5 m long and 1-27 cm in diameter at the tip, containing a thermistor element in the lower end. The thermistor is part of a resistance-sensitive oscillator that is in a pressure vessel at the top of the probe. The output of the oscillator is carried to the surface by a two-conductor cable. Therefore the sediment temperature is measured aboard the ship by determining the oscillator's frequency which is calibrated versus probe temperature. The probe was lowered down the drill stem while the drilling operation was

374

R. P. VON HERZEN and M. G. LANGSETH

temporarily stopped. Hydraulic pressure applied on a piston on top of the probe drove the thermistor into the sediment more than a meter beyond the drill bit. This was far enough to remove the thermistor from thermal disturbances caused by the drilling operation. The probe was left in the sediment approximately 20 rain to come to equilibrium with the sediment. Conductivity measurements on the sediment samples obtained during the drilling were made using the transient heated needle technique. A very important result of these measurements was the direct observation that the heat flow is constant with depth within instrumental error over the two intervals of measurement, from the sea floor to 44 m depth and from 44 m to 154 m depth. BULLARDand DAY (1961) claimed to have measured a small but significant increase of heat flow with depth in the Atlantic, using a probe which contained three sets of thermal elements. The great majority of measurements by L G O investigators in all oceans with the Ewing instrument show that the heat flow is constant with depth where three probes have penetrated the sediment. 4. Errors of heat-flow measurements. The temperature difference between thermal elements 1.7 m apart is measured by the cylindrical probe technique with a precision of about 0.001°C. Hence, for a normal gradient of 0.06°C/m, the precision of temperature difference measurement is about 1 per cent. However, additional uncertainty is introduced in the extrapolation of the record of temperature versus time to obtain the undisturbed geothermal gradient. Where the gradient is normal the correction is about 20 per cent of the last observed temperature difference (VON HERZEN and U~DA, 1963), whereas for very low gradients the correction can be 100 per cent or more. Therefore the error of determination depends on the value of the geothermal gradient. For normal gradients of about 0.06°C/m the uncertainty of the extrapolated temperature difference is thought to be 3 per cent or less (BULLARD, 1954). Other uncertainties in the measurement can arise if the probe does not penetrate vertically or completely. Auxiliary devices were used at all stations in the Indian Ocean to indicate the attitude of the probe in the bottom and completeness of the penetration (VON HERZEN et aL, 1962). Except for a few cases which are mentioned in the results, the probe penetrated completely and vertically, so that errors from this source should be negligible. The error of heat-flow measurements by the cylindrical probe method must also include errors of conductivity measurement which are discussed below. With the multi-probed Ewing-type thermoprobe a more direct method to estimate the error of heat-flow determination may be used. The heat flow measured over two intervals in the sediment at the same station can be compared. The temperature difference between the interface and the points in the sediment, and the conductivities determined at points along the core sample for two stations are shown in Fig. 5. A comparison is made between the heat flow calculated for the upper and lower intervals for other stations made with the outrigged probe apparatus in the Indian Ocean under Table 3. These stations were made in the deep ocean basins which are thought to be free from transient thermal disturbances. For the 23 values the mean of the deviation from zero

Present Status of Oceanic Heat-Flow Measurements

375

difference is 7 per cent. This value is probably a good measure of the uncertainty of a heat-flow measurement with the outrigged probe instrument for a measurement interval of 4m. For a measurement interval of 8m the uncertainty is probably closer to 5 per cent. The precision of temperature difference measurement in the Ewing instrument is about 0.01 °C, which results in about 4 per cent uncertainty for a gradient measurement over a 4m interval in an area of average geothermal flux. The remainder of the uncertainty is thought to be due to the errors of conductivity determination. The comparison of heat flow over two intervals does not test for possible systematic errors in conductivity measurements; however, these are not thought to be greater than 5 per cent (VoN HERZEN and MAXWELL, 1959). Measurements with the needle-probe technique on substances of known conductivity by BULLARD et al. (1956) gave results within 3 per cent of the listed values in the critical tables. Measurements on cores which have very uniform sediments along their length, yield conductivities which show very little variability. (See for example the measurements of V19--68 shown in Fig. 5.) In cores which have layers of different sediment type and thickness there is a sampling problem when making a finite number of point measurements. Alternating layers of turbidites and lutites are common in cores from abyssal plains and deep sea trenches. The variability of conductivity measurements in this type of core is exemplified by values at station V19-89 shown in Fig. 5. If only a few measurements are made in such cores uncertainties of 15 per cent may result. The majority of the cores are homogeneous deep-sea lutites and the effective thermal conductivity is determined to within 5 per cent. Another possible source of error is the physical distortion of coarse-grained or inhomogeneous sediments by the corer, depending on the coring technique used and on the competence of the various layers. Thus the error of heat-flow measurements at individual stations varies with the geothermal gradient, the sediments covering the bottom and the techniques used. It is therefore impossible to give an over-all error associated with heat-flow determinations. Where the geothermal gradient is average (0.06°C/m) and the bottom sediment is uniform with depth, the error is not more than 10 per cent for a good station. Where the gradient is low or the sediment type varies greatly with depth, errors of up to 20 per cent are possible. B. Effects o f the Environment There are several possible effects from the environment around and near a measurement site which may prevent the measured value from being representative of the steady heat flow from the deep interior. The assumptions of heat flow through plane horizontal layers bounded above by a plane of constant temperature are only partially fulfilled at most stations. A discussion of the effect of possible past changes in bottom water temperature has been given by BULLARDet al. (1956), who concluded that the effect was negligible. However, some recent measurements in the Pacific by one of us (R. V. H., in preparation) have demonstrated the presence of adiabatic and super-adiabatic temperature

376

R . P. V O N H E R Z E N a n d M. G. L A . X O S e r H

gradients near the bottom at some stations. If such temperature gradients are quasi-stable, the temperature of the water near the sea floor could change periodically by several hundredths °C. Such variations would have a larger effect on heat-flow measurements with a short probe than with a long one, which may explain the greater variability of values with the short cylindrical probe than with the outrigged probes (see Section IV). The heat flow could be reduced by rapid sedimentation at the ocean floor. For steady sedimentation, the effect becomes important for rates of the order of 0.1 cm/yr (Vow HeRZrN and UYEDA, 1963), which, except for a few specialized regions, is greater than prevailing rates for most of the deep-sea floor. However, Vow Hr~zEN and UVrDA suggested that some of the isolated low heat-flow OIFFERENCE

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Present Status of Oceanic Heat-Flow Measurements

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values could be explained by local sediment slumping or landslides, combined with the geometric effect of the local topography. The effect of low-conducting sediments in a steep-sided basin could reduce the heat flow by 1/3 or more. The bottom or sub-bottom topography is usually not well enough known around a station to make a quantitative estimate of the topographic effect. The movement of interstitial water in the sediments, either by convection or squeezing out of the water by compaction, was estimated by several authors (BULLARD et al., 1956; VON HERZEN and UYEDA, 1963) tO be negligible unless vertical water velocities of the order of tens of centimeters per day were achieved. However, a more exact solution (LUBIMOVAet al., 1964) has shown that velocities two orders of magnitude lower (few mm per day) can have a significant effect on the heat flow. BULLARD and DAY (1961) postulated that some slow movement of water in the sediments might account for some of their anomalous results. In spite of this, the general constancy of heat flow with depth measured by the outrigged probe apparatus and the measurements at the Mohole site described above imply that this effect is generally negligible. A few measurements with the outrigged probe show, when three probes have penetrated the sediments, that the heat flow is not constant with depth (see Table 2). Frequently these measurements have been made in relatively shallow water where temperature fluctuations may be expected near the bottom. On rare occasions even negative heat flows have been. observed (temperature decreasing with depth in the sediment) (MSN-43 and V19-107, Table 2). The latter may be the result of recent slumping or turbidity flows from higher to lower sea-floor elevations. It is only seldom, if ever, that a systematic and quantitative study of these environmental effects can be made, and only a very few stations reported in the literature have been corrected for these disturbances. The scatter of values from repeated or closely spaced measurements is probably due in large part to these effects of the environment. The standard deviation of values from some repeated measurements off the west coast of North America (VON HERZEN 1964) was about 0.3-0.4 /~cal/cm 2 sec,* or about 25-30 per cent of the mean heat flow in the oceanic regions; a similar variation.is reported below (Section IV) for some nearby measurements in the Indian Ocean. On the other hand, the variability of values obtained from some areas (REITZEL, 1963) seems hardly greater than the instrumental error. It is difficult to determine if the environmental effects have any systematic effect on the mean heat flow from the ocean floors. However, investigators systematically tend to locate stations on relatively smooth areas of the ocean floor, in order to insure sufficient sediment for penetration of the temperature gradient probe. The geometric effect of local flat basins filled with sediment, and the transient effect of possible sediment slumps into the smoother areas, may cause the heat flux near the surface to be less than that from the deep interior in such areas. The present mean heat flow of the ocean basins, then, might be considered a minimum estimate, if these environmental effects prove to be significant. * 1/a.cal = 1 x 10-~ cal. P.P.C.E. VOL. VI~N.

378

R. P. VON HEgZEN and M. G. LANGSETH 3. D I S T R I B U T I O N AND RESULTS OF HEAT-FLOW MEASUREMENTS

A. Distribution of Values As mentioned above under Section 2, nearly 1000 measurements of heat flow have been made in the oceanic regions over the past 15 years. The measurements are distributed over all the major ocean areas, including some recently reported values in the Arctic Ocean (LACHENBRUCHand MARSHALL, 1964). As research vessels are now able to reach nearly every area of the earth's surface which is covered by ocean waters, the distribution of measurements in the oceans is more uniformly distributed than those on land, which have been generally restricted to pre-existing boreholes or mines constructed for economic purposes. Measurements in the high-latitude areas of the oceans are somewhat less dense than in equatorial regions, due to the limitation of weather conditions on research vessel operations. LEE (1963) has recently made a complete compilation and statistical summary of the data. A useful representation of the distribution of the data is given by the average of values for each 5 ° × 5 ° area of the earth's surface. Figures 6A and 6B show the average and number of measurements for each such area in the major ocean areas of the earth; the new data for the Indian Ocean is discussed below in Section IV. The 5 ° × 5 ° representation attenuates the bias produced by greater or fewer numbers of measurements in certain areas and also reduces the range of heat-flow values by combining local anomalous measurements with others nearby. A sufficient number of 5 ° × 5 ° areas with measurements exists to make meaningful statistics. To construct Figs. 6A and 6B, stations ___I0 km distant from one another have been combined into one value. Some of the authors' unpublished data has been included in Figs. 6A and 6B, especially for some areas of the western and southwestern Pacific, the south Atlantic, and the Caribbean Sea. The curving dashed line shows the crest of the major mid-oceanic ridge of the ocean basins. Two histograms of values for 5 ° × 5 ° areas of the oceans are shown in Fig. 7, the dashed histogram representing all values and the solid-line histogram representing only those values vchich were obtained from more than one measurement. These histograms are quite similar to that obtained by LEE (1963, fig. 2C) for individual measurements, with a peak around I.I to 1-2/lcal/ cm 2 sec and a long tail extending to higher values. That the range of values is considerably reduced by considering only areas with more than one measurement probably indicates that the very high values ( > 5 #cal/cm z sec) and the very low values ( < 0 . 2 #cal/cm 2 sec) are not representative of areas as large as 5 ° × 5 ° on a side (30,000 km2). Further results of the 5 ° × 5 ° values are summarized in Table 1. The average for the Atlantic is somewhat lower than that for the Pacific, but this difference is probably not significant with the present distribution and number of measurements. The decrease of standard deviation when only 5 ° x 5 ~ areas with more than one measurement are considered shows the effect of the extreme values.

Present Status of Oceanic Heat-Flow Measurements

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TABLE 1 SUMMARY OF OCEAN HEAT FLOW IN 5 ~ 3< 5 ~' AREAS

5° × 5° areas with more than 1 measurement

All 5~ x 5° areas Standard Average* deviation Atlantic Ocean Pacific Ocean Indian Ocean All Oceans

1.40 1.53 1.43 1.48

0.99 0.84 0.73 0.85

No. areas 91 203 87 381

Standard Average* deviation 1.34 1.55

1-44 1-47

0.73 0.81 0.50 0.74

No. areas 52 113 47 212

* 10-6 cal/cm -°sec. These averages compare with those given by LEE (1963) for the Atlantic of 1.36/~cal/cm 2 sec and for the Pacific of 1.71 #cal/cm 2 sec, based on individual measurements available at that time. Apparently the higher value for the Pacific resulted from the concentration of measurements on the East Pacific Rise. We should point out that a straight average of 5 ° x 5 ° areas tends to overweigh the values at high latitudes, since the area of 5 ° x 5 ° squares decreases with the cosine of the latitude. Nevertheless, this is probably not a serious omission with the present distribution of measurements. The over-all average, 1.48 #cal/ cm 2 sec, is nearly the same as that given by Lee from fewer measurements, and does not differ significantly from the average obtained from continental regions.

B. Ridge Measurements The development of instrumentation over the past dozen years to measure heat flow on the ocean bottom has been accompanied with a rapidly increasing knowledge of the structure and topography of the ocean floor. A fundamental discovery during recent years has been that all the major ocean basins have large features designated as mid-oceanic ridges. Although the classic mid-Atlantic ridge has been known for some time, it has only been established during the past several years that the corresponding feature in the Pacific, the East Pacific Rise, is a continuous ocean-spanning feature (MENARD, 1960) and that both ridges connect with corresponding features in the Indian Ocean (EwIN6 and HEEZEN, 1960). Indeed, it appears that some of the designated ridges may connect to span the whole earth as a nearly continuous feature (H~ZEN and EWING, 1963). The mid-ocean ridges are recognized by the great magnitude of their dimensions. Their crests frequently range in elevation from 1 to 2 k m above the surrounding sea floor; widths typically extend 1000 to 3000 k m ; and lengths, at least several times the widths, or more. Hence, in profile, these are very gently doping features on the average, although the local topography may be

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extremely rough. Heights are comparable to mountain ranges on continents but horizontal dimensions are many times greater. Frequently the crests of the ridges lie near the center of the ocean basins, nearly equidistant from the bordering continents (MENARD, 1958). Also the crests of the mid-oceanic ridges correspond closely with the foci of earthquakes (EwING and HEEZEN, 1956). The first heat-flow measurements on these ridges indicated values several times higher than normal on both the east Pacific rise (up to 5.25/~cal/cm 2 sec, Btn.LARO et al., 1956) and the mid-Atlantic ridge (6.5/~cal/cm 2 sec, BULLARO and DAY, 1961). Subsequently a considerable amount of attention has been focused on measurements on oceanic ridges. On the mid-Atlantic ridge, another comparable high value (> 6.2/tcal/cm 2 sec) was obtained farther north by REITZEL (1961). Several measurements extending up the west flank to the crest in the north Atlantic (GERARDe t al., 1962) indicated the heat flow increased gradually from the lower flank up to the crest of the ridge. A pattern contrasting

Present Status of Oceanic Heat-Flow Measurements

381

somewhat with this was found in a profile of measurements across the north Atlantic by NASON and LEE (1962) which showed the highest values (2.86"5 pcal/cm 2 sec) confined to a region close to the crest, with lower than average values (0.3-0.7 #cal/cm 2 sec) measured on the lower flanks and adjacent parts of the deep basins. A similar pattern has also been found in a series of shorter profiles on the south mid-Atlantic ridge (VACQUIER and VON HERZEN, 1964), with most of the highest values (up to 8-1 #cal/cm: sec) located within a narrow band 200 km wide centered on the ridge crest, and low values found at distances of 300 to 600 km from the ridge crest. In summary, the high heat flow on the mid-Atlantic ridge appears generally in a relatively narrow region near the crest of the ridge, and contrastingly low heat-flow values over a somewhat broader region several hundred km distant from the crest, generally on the lower flanks and adjacent basins. In the Pacific, a somewhat similar picture, but different in detail, is found on the east Pacific rise. The first measurements on the rise (BULLARDe t aL, 1956) followed by other widely spaced measurements (VoN HERZEN, 1959), indicated high values (up to 8.1 /acal/cm 2 sec) comparable to the mid-Atlantic ridge and distributed over a considerable distance along the crest of the rise. The detailed structure of the high heat-flow anomaly was investigated by relatively closely spaced measurements in a restricted region near the crest (VON HERZEN and UYEDA, 1963), indicating two narrow bands of high heat flow near the crest, separated by a few hundred kilometers in which values are only moderately high. A recently detailed crossing to the south (LANGSETH et al~, 1964) also indicates the bands are quite narrow, perhaps less than 30-50 km wide and that there may be more than two. The rise has been traced northward into the North American continent (MENARD,1960), and high values north of the Mendocino scarp (discussed below) indicate it trends northward somewhat seaward of the coast of the Pacific northwest. Contrasting with measurements on the mid-Atlantic ridge, some crossings of the east Pacific rise near the equator (VON HERZEN and UYL~A, 1963; LANGSETH et al., ] 964) have indicated a zone of moderately high heat flow (2-3 pcal/cm 2 sec) of considerable width, perhaps approaching 1000 kin. These investigations have shown that near the equator the high heat flow probably extends east of the Galapagos Islands, perhaps to the Central and South American continental borders. Contrasting also with the Atlantic picture, the low heat-flow bands originally indicated adjacent to the east Pacific rise (VON HERZEN, 1959) now appear to be more localized, but still substantial, equidimensional areas near the equator (VON HERZEN and UYEDA,1963); these are discussed further below. Thus far we have discussed heat-flow values on the seismic mid-oceanic ridges of the Atlantic and Pacific. Some measurements have also been obtained on the smaller aseismic ridges of these oceans, although not nearly as numerous as the former. Some brief examples of these ridges are given by EWlNG and H~ZEN (1956). In the Atlantic, a measurement reported (VACQUIERand VON HERZEN, 1964) on the Walvis ridge gave a value of 2.17/acal/cm 2 sec, somewhat higher than normal and higher than measurements to each side. In the southeast

382

R. P. VON HERZEN and M. G. LANGSETH

Pacific, a normal value was reported (sta. 11', VON HEI~ZEN, 1959) on the ridge extending from the east Pacific rise toward the southern part of Chile. Farther north, measurements on and near the Carnegie and Cocos ridges near the equator (VON HERZEN and UVZDA, 1963; LANGSETHet al., 1964) give generally high values, although a few values on the seaward extremity of the Cocos ridge were low. These high values appear to be representative of a large region of high heat flow extending from the east Pacific rise to the American continent, however, rather than that of a local anomaly associated with the topographic expression of these ridges. The Tuamotu Archipelago, an island ridge in the south-central Pacific, shows about normal heat flow from several measurements on the deeper-lying regions of this feature (VON HERZEN and UVEDA, 1963). Similarly, recent values around the Hawaiian Island ridge (RHEA et al., 1964) show normal heat flow, perhaps surprising in view of the volcanic activity there. Other unpublished measurements by one of us (R. V. H.) on some of the numerous island ridges of the western Pacific in general give values about normal. MENARD (1964) proposes that these old island ridges of the south Pacific were once part of an active oceanic ridge system, which has since subsided; the lack of high heat flow seems consistent with this idea. Discussed below in Section IV are new measurements in the Indian Ocean on both seismic and aseismic types of ridges. Some of the variations in Japan have been correlated with geologic units and past orogenic activity on these islands (UYEDA and HORAI, 1964). Several measurements across the Caribbean arc indicated slightly higher than normal heat flow near the ridge (VACQLaERand VON HERZEN, 1964). Other investigations are discussed below under trench measurements, since these features are frequently associated with island-arc structures. C. Basin Measurements

To contrast with the foregoing discussion of ridge measurements is a discussion.of measurements made in the deep basins of the oceans. In nearly all cases, the basins are geographically confined by the ridges or continents which surround them and are defined as the areas so enclosed. In the Atlantic, the basins are well defined by the bordering continents and the mid-Atlantic ridge. The floors of the basins are generally much smoother than the ridge topography ('HEEZENet al., 1959), which is almost certainly due to the greater thickness of sediment accumulation in the basins. The deeper parts of many of these basins are occupied by abyssal plains, resulting from the outpouring of sediments from higher areas by turbidity flows. In general, the heat-flow values from basins also show less variation than those from ridges. The first measurements from the north Atlantic basins were obtained by BULLARD(1954), five measurements giving a range of values from 0"58 to 1"42 Ftcal/cm 2 sec. The principal error in these early measurements was in the thermal conductivity but was probably less than about 10 per cent; additional measurements (BULLARDand DAY, 1961), and a correction for in

Present Status of Oceanic Heat-Flow Measurements

383

situ values of thermal conductivity, changed only slightly the range of 20 values

for the northeast Atlantic basin from 0.54 to 1.34 #cal/cm 2 sec. In the northwest Atlantic basin, GERARD et al. (1962) reported no values less than 1 #cal/cm 2 sec for the first measurements with outrigged probes in a wide range of basin environments; the average, nevertheless, appeared similar to that of the northeast Atlantic basin. Two additional measurements by REITZEL (1961) in the western basin of the north Atlantic and one in the eastern basin supported these results. A recent detailed study (REITZEL, 1963) of a sizeable area of the northwest Atlantic basin east of the Bahama Islands shows the uniformity of the heat-flow field in some basin areas. The average of sixteen values over an area of nearly 106 km 2 is 1.14 pcal/cm z sec with a standard deviation of only about 5 per cent. In the northeast Atlantic basin, a detailed study (LISTER, 1963) of ten measurements over a much smaller area also showed a similar uniformity of values, with differences ascribed only to instrumental error and local topography. Contrasting somewhat to the above discussion, NASON and LEE (1962) made a profile of stations across the north Atlantic which showed considerable but systematic variations among the values. Very high values were observed close to the crest of the mid-Atlantic ridge, as discussed above in the section on ridges; but values lower than average were observed on the flanks and in the basins adjacent to the mid-Atlantic ridge. Normal values were observed close to the uniform area of the northeast basin studied by LISTER (1963), however. The lower values on flanks and in basins close to ridges were substantiated by a south Atlantic study (VACQtnER and VON HERZEN, 1964) of profiles across the ridge crest. That some of the low values may be due to local processes or configuration of the ocean floor was suggested by a large variation among five values taken over a local region of the sea floor east of the mid-Atlantic ridge. In the Pacific basin, perhaps due to its larger size and complexity, the distribution of values appears more variable than in the Atlantic. The first measurements obtained in the Pacific (R~VELLE and MAXWELL, 1952), over a relatively small area west of the Hawaiian chain, were similar in average and range to those first made in the Atlantic. Recent unpublished results by one of us (R.V.H.) to the southwest of Hawaii indicates the region west and southwest of these islands, extending to the Marshall-Gilbert-Ellice Island chains, may be one of the more uniform heat-flow regions in the Pacific basin. An expansion of the area of measurements to the south Pacific widened the range of values from 0"25 to 2.43/~cal/cm 2 s e c (BULLARD et al., 1956) away from ridges. Some of the higher values in the western Pacific were attributed to being near to or within the continental side of the "andesite line". Further measurements in the eastern Pacific basin, to each side of the east Pacific rise, gave an even greater range of values and showed the existence of significant regions of lower than normal heat flow (VON HERZEN, 1959; VON HERZEN and UYEDA, 1963). Consistently low heat-flow values obtained from equi-dimensional areas to each side of the rise in the east Pacific near the equator seem to indicate significantly lower heat flow for areas as large as several

384

R. P. VON HERZEN and M. G. LANGSETH

million square kilometers. Recent measurements (LANGSETH e t aL, 1964; and unpublished) appear to substantiate the large area of low heat flow to the west of the rise (area " A " ) ; 37 measurements in this area give an average of about 0.73/~cal/cm 2 sec, about half the oceanic average. To the east of the rise, area " B " appears smaller than originally proposed (VoN HERZEN and UYEDA, 1963) and is perhaps restricted to the Guatemala basin and trench off Central America, which is discussed further below. The measurements presented by Von Herzen and Uyeda strongly suggested that many of the isolated low-heat-flow values were due to local effects from the environment on the sea floor, and hence are not representative of the regional flux. The results of detailed profiles over the east Pacific rise showed the lack of systematically low heat-flow regions near or on the flanks, as was found in the Atlantic. Over the eastern part of the north Pacific between Hawaii and west of the continent of North America, values appear more uniform and average to about 1.4 #cal/cm 2 sec (RHEA et al., 1964). High values and greater variations are observed locally near the coasts and along the Mendocino fracture (FOSTER, 1961; VON H~rtZEN, 1964). The measurements by Foster in the Bering Sea showed a small variation and averaged to about 1.I #cal/cm 2 sec. As these measurements are on the continental side of the "andesite line", the normal values contrast with those also on the continental side of the "andesite line" in the western Pacific, mentioned above. The Bering Sea may correspond more closely to the Caribbean Sea, in which the values obtained are also close to average (GERARD et al., 1962; VACQUmR and VoN HEgZEN, 1964; LANGSETH and GRIM, unpublished). In the northwestern Pacific, measurements are lacking over a very large area, between Hawaii and the Asian continent. Only a group of variable values close to the Japanese islands have been recently reported (YAstn et aL, 1963).

D. Special Regions l. West of North America. More detailed measurements exist in some special regions off the west coast of North America. The Mendocino scarp extending west of the United States has been studied by measurements on both sides of the scarp out to about 151°W longitude (VoN HERZE~, 1964). The values obtained showed a large variation, from 0-1 to 5.8 #cal/cm z see, with an apparent systematic distribution in zones extending offshore (Fig. 8). The highest values were measured north of the Mendocino fault close to shore, an area which is a possible extension of the east Pacific rise (MENARD, 1960). The lowest values were measured adjacent to the high values, suggesting a pattern similar to that on the mid-Atlantic ridge discussed above. Another nearby region studied in more detail is that of the southern California borderland and Gulf of California to the south. Very high values have been measured in and near the Gulf of California (VoN HERZEN, 1963), which is probably part of the active rift and ridge system of the earth; the Gulf of Aden has similar structure and heat flow. The heat-flow values in the borderland west

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Present Status of Oceanic Heat-Flow Measurements

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of southern and Baja California are somewhat above normal (VONHERZEN, 1964). This area may be a transition region from the Gulf of California to the normal values ( ~ 1.4/~cal/cm 2 sec) in the deep basin between the continent and Hawaii (RHEA et al., 1964). However, the heat flow decreases irregularly to the west in the continental borderland, and there is no sharp boundary in heat flow at the continental slope. 2. Trenches. The oceanic trenches have been subjects of geophysical investigations and speculations since the classic gravity surveys of VENING-MEINESZ (1934) in these narrow and deep arcuate features of the East Indies. With the hypothesized genetic connection of trenches with mantle convection patterns, heat-flow measurements in trenches assumed special interest. The first measurements in and near the middle American trench (BULLARD et al., 1956) gave low values and appeared to support the idea of a downward-moving limb of a convection cell beneath this trench. This appeared also to hold up for subsequent measurements in the South American trench (VON HERZEN, 1959), and that these trenches are elongated in a direction roughly parallel to the axis of the east Pacific rise was cited as additional supporting evidence. Subsequent measurements in these and other trenches, however, have resulted in values which are not always low. Vorq HERZEN and UYEDA (1963) verified one of the previous low values in the trench off Peru, but failed to find any abnormally low values farther north in the same trench. Normal values have also been reported from the Puerto Rico trench (GERARD et al., 1962) and the Japan trench (UV'EDA et al., 1962). Although a low value was reported from a bench on the continental side of the Aleutian trench (Fosx~R, 1962), similar to the pattern observed in the Japan trench, a value from the deepest part of the Aleutian trench is higher than normal. Some unpublished measurements around some of the numerous island-arc structures in the west and southwest Pacific show that the heat flow seems to depend more on particular geographic location than on position relative to a given island-arc structure. Whereas some values in and surrounding a given trench are generally low, another trench area may give high values. Therefore, it seems premature to draw any general conclusions about the heat flow associated with trenches on a world-wide basis. Some of the complications observed in the Java trench are discussed below with the Indian Ocean measurements.

E. Summary The heat-flow measurements in the world's ocean areas frequently show correlations with the largest structural features of these regions, i.e. oceanic ridges and basins. Most of the high values of heat flow are measured on or near oceanic ridges. In the Atlantic and Pacific, measurements at 22 different localities along the crest of the mid-Atlantic ridge and east Pacific rise give at least one value which is greater than 2/~cal/cm 2 sec. There are only five localities near the crests of these ridges where measurements give only values less than 2 pcal/cm z sec, and on these the possibility exists that narrow regions of high

Present Status of Oceanic Heat-Flow Measurements

387

heat flow were missed. Therefore we conclude that most of the crest of these oceanic ridges is the locus of high heat flow. However, the detailed pattern of the high heat flow differs from one locality to another. In many places the high heat flow is confined to a relatively narrow strip at the crest, less than a few hundred kilometers wide. In a few places this strip has been further resolved into several narrow bands of very high heat flow only a few tens of kilometers wide, suggesting the recent intrusion of linear magma bodies (dikes) into the crust or upper mantle. In other areas, the high heat flow appears over a broad zone 1000 km or more wide, but always including the crest of a major ridge. On the smaller aseismic ridges of the ocean basins, normal or only slightly higher heat flow is frequently observed. The excess heat flow could be explained by the fact that such ridges are frequently underlain by thick crusts with possibly greater concentrations of radioactivity. On the other hand, some of these ridges in the Pacific are believed to have been part of an ancient major ridge which may have had high heat flow in the past (MENARD, 1964). In contrast, the ocean basins show a much smaller range of heat flow, and except for an occasional high value which probably indicates local magmatic activity on or beneath the sea floor, the average is about 1-1 to 1.2 pcal/cm 2 sec (BuLLARD and DAY, 1961). No significant difference appears between the averages for different ocean basins. Some of the very low values of heat flow in the basins and on the ridge flanks are likely the result of thermal disequilibrium within the sediments at the sea floor or to unusual topography at or beneath the sea floor. Some regions of basins show systematically low heat flow, in the Atlantic close to the flanks of the mid-Atlantic ridge and in the Pacific close to the equator to each side of the east Pacific rise. The general difference of heat-flow patterns associated with the mid-Atlantic ridge and east Pacific rise is brought out by Fig. 9 taken from calculations by LE PICnON (personal communication, 1964). This representation clearly shows that the low heat flow occurs close to the mid-Atlantic ridge crest but not to the east Pacific rise crest. The basin values in the Pacific show a greater variability than those in the Atlantic; this may be partly due to the low heat-flow areas in the Pacific basin. Another possibility is that the more uniform values in the Atlantic result from a greater thickness of the upper sedimentary layer in the Atlantic (EWlNG and EWING, 1959; KAITT, 1956) which covers to a greater depth the topographic irregularities of the underlying "volcanic" layer (EWlNG and EWING, 1963). Some special regions in the ocean areas off the North American coast have been studied in more detail and show a large range among the values obtained. Many of these values fit into a pattern which apparently shows a small-scale systematic variation with geographic location. Since these special regions are frequently only more detailed studies of the larger features, the local variability is indicative of the complexity of the distribution of heat-flow values. At present, this variability appears to over-ride any adequate representation of the global heat flow by low-order harmonics (LEE and MACDONALD, 1963). The

R. P. VoN HERZEN and M. G. L^NGSEXH

388

differences between successive representations, as more data are accumulated, are large; and the initial agreement between the heat-flow fields and gravity fields is questionable. Some tests o f the representation are given below in the discussion o f the Indian Ocean data.

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EAST PACIFIC RISE FIe. 9. Heat flow vs. distance from the axes of the mid-Atlantic ridge and the east Pacific rise. Circled points show heat-flow values at the 75, 50, and 25 percentile points of measurements in zones 0-100 kin, 100-300 kin, 300-600 kin, and greater than 600 km distant from the crest of the ridges. Small figures associated with each zone show number of measurements included. 4. I N D I A N O C E A N M E A S U R E M E N T S

A. Distribution of Measurements Heat-flow measurements have been carried out in the Indian Ocean during the past several years as part o f the International Indian Ocean Expedition (1960-1965). The geographical distribution o f heat-flow measurements in the Indian Ocean is shown in Fig. 10. The measurements are distributed over a sizeable fraction o f the ocean basin, extending completely across the Indian

Present Status of Oceanic Heat-Flow Measurements

389

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Ocean in an east-west direction, and from the Gulf of Aden in the northwest to nearly 50°S latitude in the southeast. Therefore, although this ocean was the last to be investigated in a systematic manner, there is a more uniform and complete coverage of measurements here than presently exists in the Atlantic and Pacific. The measurements by SIO, utilizing the cylindrical probe technique, and those by LGO, using outrigged thermistors on a core barrel (see Section II), were made independently of one another. In Table 2 are listed the measurements obtained by the cylindrical probe and outrigged probe methods in the Indian Ocean. Some of the values by the cylindrical probe technique were obtained where the cylindrical probe achieved only partial penetration, such that only one thermistor position penetrated the sediment. These values, followed by " p p " are listed only where the extent of penetration was sufficiently well determined (usually from the bend in the probe) to obtain a reasonably accurate value of the temperature gradient (_+20 per cent). The values for the outrigged probe measurements are followed by a number indicating the number of thermistor probes penetrating the sediment. Some of these values are subsequently followed by a " T " indicating possibly transient thermal conditions in the sediment. These conditions are indicated either by considerable non-linearity of heat flow over different vertical intervals between thermal probes, or by a non-linearity suggested by a discrepancy between the observed temperature at the sea floor and that deduced from extrapolation of the sediment temperature gradient to the surface. For all outrigged probe measurements, the heat flows were computed using the tern perature difference between the two probes in the sediment with the largest vertical separation. For some of the outrigged probe measurements of Table 2, three probes penetrated the sediment so that two intervals were made available for comparison of the temperature gradient at different depths. Table 3 summarizes the results for these measurements made in the deep basins of the Indian Ocean which are thought to be free from transient thermal disturbances. A plus sign has been placed in front of the difference if the heat flow in the upper interval is greater than that in the lower interval, and a minus sign is in front of the difference if the reverse is true. Of the 23 entries in the table there are four more positive than negative differences. The predominance of positive values for stations V19-97 through V19-108 may be significant and result from heat flow which is not constant with depth in the sediment. These stations are taken in relatively shallow water in the Mozambique channel and near the east coast of Africa. Aside from these stations, there appears to be no systematic difference in heat flow over the two intervals of measurement. Stations denoted by an asterisk (*) in Table 2 were made at locations where the echo sounder indicated the local bottom relief did not exceed 20 m within a few kilometers of the station. VON HERZEN and UYEDA (1963) had previously noted a correlation between some low heat-flow values and such locally flat topography. If such flat topography extended for more than 10 km to each side of a station along the ship's track, the station number is further designated by a superscript "A".

Present Status of Oceanic Heat-Flow Measurements

391

TABLE 2

INDIAN OCEAN HEAT-FLOW MEASUREMENTS

Position Station No.

E. Latitude

Water depth, m

Thermal cond.t

Heat flows

2.09 2-02 1.77 1.65 1.63 1.85 1.87 1.88 1"99 1 "63 1.83 1.71 1.55 2.06 1.99 (1.99) 2,26 1.98 2-19 2-08 2.08 (2-0) (2-0) (2.0) (2.0) 1.93 1.78

1 "69 1"7 1 "12 1 "05 0"39 1 "48 1 "87 0"48 1 '56 1"0 1 '63 1 "73 I "20 0"14 2-78 2"19 0"34 4"91 0"91 1"38 1 "67 Neg. 0.5 0.7 1.3 0,76 1.04 1"3

Longitude

MONSOON EXPEDITION MSN-12* MSN-15 MSN-16 MSN- 17 M S N - I 8" MSN-20 MSN-21 *A MSN-23 MSN-24 MSN-28 MSN-29 MSN-30 MSN-32* MSN-33* MSN-34* MSN-35* M SN-36* MSN-38 MSN-40 MSN-41 MSN-42 MSN-43 MSN-44 MSN-45 MSN-46 MSN-47 MSN-48 MSN-49

9°14'S 7°46'S 11°58'S 12°48'S 10°ll'S 13°19'S 11°39'S 8°49'S 12°21'S 16°59'S 18°14'S 15°51'S 14°05'S 14°56'S 16°25'S 16°58'S 17°48'S 26°22'S 33°20'S 37°44'S 42°09'S 39°50'S 38°26'S 37°50'S 37°18'S 36°19'S 39°18'S 49°31'S

127030 , 121014 , 115026 , 115°2a ' 115019 ' 109034 ' 109o35 ' 109°36 ' 101°25 ' 93o29 ' 86°42 ' 81010 ' 72015 , 70°lY 66001 ' 64o46 ' 62040 , 74008 , 72037 ' 71o47 ' 70037 ' 75003 ' 79034 ' 85°22" 90°42 ' 98°41 ' 119°52 ' 132o14 '

3300 4840 5010 5400 4330 4630 4605 3300 4745 5230 4455 5000 5200 4460 3660 4055 3740 4130 4220 4260 4200 3780 3410 3600 3855 4375 4895 3500

(1-8)

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12°27"N 12°57'N 13°17'N 12°54'N 12°25'N 9o08'N 9°09'N 9°16'N 9°34'N 9°32'N 9°34'N 9°40"N 9°48"N 9°50'N 9°56"N 9°59"N

47°07 ' 48o16 , 49°15 ' 49°38 ' 50°33 ' 54042 ' 57°30 ' 59000 ' 59052 ' 61024 , 63006 " 66~19 ' 69015 ' 71050 ' 73008 " 74°50 ,

1820 2205 2425 2200 2420 3705 3265 3200 3895 4580 4505 4450 4550 2370 1925 2285

2"03

(I-92) 1"81 (1.92) 2.02 2'11

(2'01) 1"91 (2'01) 2'10 (2.25) 2-30 2.17 2'21 2.09 1 "92

5"98 3"62 3"22 2"47 3"09 1 "66 1 '37 1 "74 1 '68 0"95 0"23pp 0"8pp 1 '49 1 "29 1 '70 1-57

R. P. VON HERZEN and M. G. LANGSETH

392

TABLE

2--continued.

INDIAN OCEAN HEAT-FLOW MEASUREMENTS

Position Station No.

E. Latitude

Water depth, m

Thermal cond.*

Heat flow.+

1.44 I '6pp 1 "lpp 1'8pp 1"51 1 "92 0"57 0"30 1 "67 3'8pp 1 "62 1 '23 1 '54 1 '56 0'92 1.44 1.00 1 "32 0.90 1 '21 0"91 0-7 0"42 0"68 3-7pp 1 "49 0"38 2'03 0.88 1 '22 1 79 1.15 1.39 1.30 3"Opp 1.15 1.14 0"93 1.00 2-22 0.82 1.45 0.04

Longitude

LUSIAD EXPEDITION LSDA-I LSDA-2 *A LSDA-3*A LSDA-.4 LSDA.5 *A LSDA-6* LSDA-7 LSDA-8* LSDA-9 LSDA-10(A) LSDA-10(B) *A LSDA-I 1 LSDA-12(A)* LSDA-12(B)* LSDA- 13 LSDA-14 LSDA-15 *A LSDA-16*x LSDA-17 LSDA- 18 LSDA-19 LSDA-20 LSDA-21(B) LSDA-22 LSDA-23(B) LSDA-24 LSDA-25 LSDA-26 LSDA-30(A) LSDA-30(B) LSDA-32*-', LSDA-33 LSDA-34 LSDA-35 LSDA-36 LSDA-37 LSDA.38 *A LSDA-39 *A LSDA-50 *A LSDA-51 * LSDA-52* LSDA-53 LSDA-54 *A LSDH-I LSDH-2 LSDH-3* LSDH-4

8°13'N 3°57'N 0°05'S 2°40'S 5°21 'S 5°23'S 5°40'S 5°52'S 5°34'S 5°26'S 5°25'S 5°30'S 9°57'S 9°56'S 10°21'S 10"34'S 13°42'S 17°20'S 22°01'S 24°34'S 26°5YS 29°5YS 31°25'S 32°55'S 39°44'S 44°36'S 35o47"S 36o52'S 31o29'S 31o27,S 29o42'S 25o03,S 16o25"S 13o48,S 13o09,S 14o56'S 13"46'S 13o31"S 30°08'S 31°04"S 31°39'S 32°14'S 32°22'S

70039 , 70o49 , 71050 , 73°16 ' 75°08 , 72047 ' 70°17 ' 66°36 , 63042 , 59014 ' 59"13' 57*56' 57°07 ' 57°07 ' 58031 ' 59051 ' 59o42 ' 57042 ' 57034 " 57026 ' 58012 ' 61"52' 61056 ' 62*25' 63*56" 70°57 ' 73037 ' 76022 " 114025 ' 114024 ' 111°30 ' 104012 ' 89019 ' 90050 ' 93°13 ' 108°09 ' 115°32 ' 118c29 ' 37°47 ' 36°40 ' 35*57' 34°16 ' 32*47"

4145 4130 4200 2980 5220 2530 3935 4370 4210 3980 3980 2525 4040 4050 3575 2315 3900 4145 4750 5000 5540 4620 4420 4745 4810 3580 4380 3925 3730 3750 5340 5100 5625 5200 5230 5580 5680 5680 4990 4535 2545 2660 3560

2"03 1 "91 2"15 2'28 1"64 2"28 1 "88 (1 "90) 1'91 1 '97 1 '97 2"O2 (2.03) (2.03) 2.02 2'04 2.00 2.21 1-77 1.57 1 "58 1.70 1-73 1.59 (2.J8) 1.89 1.93 2.17 2.04 2"04 (2-18) 1-63 1.64 1-59 1.64 1-70 1 '65 1.64 1-97 (2"26) 2.40 (2-30) 2-12

9°07'N 9°03"N 7°24'N 5°22'S

72°59 " 73°10 ' 70040 ' 74"17'

2135 2110 4110 4780

2-08 2"08 2"19 1"64

1-61 1'18 1.44pp 1 '88

Present Status of Oceanic Heat-Flow Measurements TABLE

393

2--continued.

INDIAN OCEAN HEAT-FLOw MEASUREMENTS

Position )

Station No.

LSDH-5 * LSDH-6* LSDH-7* LSDH-8*A LSDH9(A) LSDH-9(B) L S D H - 11 LSDH-13* LSDH-14 LSDH-15(A) LSDH-15(B) LSDH-18(A) LSDH-18(B) LSDH-20 LSDH-21 LSDH-22 LSDH-23 * LSDH-24* LSDH-25 LSDH-26 LSDH-27 LSDH-28 LSDH-29 LSDH-30* LSDH-32 LSDH-33 LSDH-34*^ LSDH-35 LSDH-36* LSDH-37 LSDH-38* LSDH-39 LSDH-40 LSDH-43 * LSDH-44 LSDH-45 LSDH-46*A LSDH-47 *A LSDH-48*^

Latitude

E. Longitude

5°40'S o 53 , S 5o31'S 5°28'S 5°26"S 5°26'S 4°10'S 9°49'S 10°05'S 10°30'S 10°30'S 31°14'S 31°14'S 33°16'S 39°54'S 40°47'S 40°58"S 40°19'S 36°05'S 37°21'S 32°58'S 32°06'S 32°45'S 32°59'S 33°01'S 32°17'S 29°16'S 25°40'S 24°33'S 20°11"S 14°12'S 13°39'S 13°23'S 14°06'S 14°56'S 14°58'S 14°13'S 13°09'S 13°41"S

69°40 , 65°57 ' 63004 ' 60002 , 59029 ' 59°29 ' 57°15 ' 56028 , 57o53 ' 59°23 ' 59023 , 62o57 ' 62°58 ' 61°43 ' 67053 , 72o46 ' 75°08 ' 76°32 ' 75°59 ' 76o35 ' 96°02 ' 100020 ' 102045 , 103033 , 111°11 ' 113058 ' 110042 ' 105°22 ' 103°39 , 96°22 ' 89050 , 91°31 ' 92°32 ' 101°22 ' 107016 ' 109012 , 114o54 ' 116029 , 117023 '

Water depth, m

Thermal cond.t

Heat flow~

3815 4260 4255 4100 3945 3960 3765 3885 3935 2870 2845 5065 5060 4695 4065 4000 4030 3020 3290 3380 4030 2450 4760 5130 4390 4190 5550 4830 5400 4910 5315 5150 5200 5110 5805 5630 5670 5670 5715

2-00

0-00 1.16 2.26 1.54 4.6pp 3.3pp 1.9pp 0.27

1.90 ( 1"94) (1.97) 2"02 2.02 2.03 2-03 (2"02) 1-94

1-94 1-60 1.60

1.56 2.18 2.30 2.16 2.25 (2-17) 2"10 (2'1) 2"37

1-71 1"70 (2"26) 2"26 2'18

(1.63) (1.63) 1 '59

1.29 1,22 1-34 0.24 0.19 1.77 0-00 0-40 0.54 2.12

1.74 0.92 0.01 2"9pp 0"93

1.27 5"3pp 0-99 2.0pp 1.13 1.04

(1.59)

1-05 1.07 1.48

1.73

3-20

1-63

1.62

1.81 1.37 1.13 1.02

1.6o (1.62)

1.11 0.94

2"42 2"52 2'23 1 "68 1 "92 1 "99 2"67 2'74 1 "75

1.53(3) 1.57(2) 1.65(3) 1.81(2) 1.67(2) 1.46(3) 0.43(2) > 2-64(2)

1-63

1.74 (1.74)

V E M A C R U I S E 18 VI8-54*A V18-55 V18-58 VI8-59*A V18-60*a VI 8-61 *A V18-63" V 18-67 V18-69

36°55'S 38°59'S 31°12'S 26°42'S 23°59'S 21°26'S 20°35'S 25°29'S 25°47'S

23°24 ' 29056 ' 48005 ' 50028 ' 51011 , 51037 ' 63032 , 85009 ' 93043 '

5064 4202 4395 5266 4928 4959 3296 4559 4435

1.30(2)

R. P. VON HERZEN and M. G. LANGSETH

394

TABLE

2--continued.

INDIAN OCEAN HEAT-FLow MEASUREMENTS Position Station No.

E. V18-70 V18-71 V18-72 V18-73 V18-74 VIS-76*A

Latitude

Longitude

25o46"S 25°41'S 25°41'S 27°59'S 36°07'S 37°27'S

95058 • 99004 , 101°56 , 108°40 , 118°47 , 133°40 ,

Water depth, m

Thermal

4937 5365 4720 5148 4590 5576

1"81 i "76 1"82 2"00 2"19 2"22

condA"

Heat flows

1.35(2) 1.20(2) 1.54(I) 1'26(2) 1.02(2) >1'15(2)

V E M A C R U I S E 19 V19.54 *,~ V19-55 V19-57 V19-58 V19-59 V19-60 V19-61 V19-64 V19-65 V19-66 V19-67 V19-68 V19-69 V19-70 V19-72 V19-73*A V19-74 V19-75 V19-76*A V19-78 V19-79 V19-80 V19-82 V19-83' V19-84 V19-85 V19-87.-~ V19.88*x V19-89 V19-90 V19-91 V19-92*A V19-93 V19-94 V19-95 V19-96 V19-97 V19-98 V19-100*A Vl9-101*a V19-102

7°43'S 7°16'S 14°31'S 16°20"S 18°II'S 19°02'S 20°56'S 18°2YS 16°11'S 14°11'S 12°44'S 10°lYS 7°54'S 7"04'S 7°07'N 7°35'N 8°07'N 8°09"N 8°09'N 8°07'N 7o26'N 6o42'N 7°04'N 6°52'N 6°37'N 6°I0'N 4°43'N 2°29'N 0°29'S 2°40'S 3°34'S 3°24'S 3°II'S 3°43'S 4°lYS 5o20'S 6°59'S 9°28'S 13°08'S 14°53"S 16°56'S

103°15 ' 102°02 ' 101o21 , 100°33 ' 99°24 ' 97°15 ' 91012 ' 82°08 " 82°06 ' 82°08 " 82o01 " 81°37 ' 81°25 ' 80°46 ' 76o33 ' 74013 ' 73015 ' 70°38 ' 69°15 ' 62047 ' 61o04 ' 59020 ' 60o55 ' 60042 ' 59048 ' 57o10 ' 52°05 ' 51°28 " 53°41 ' 54°45 ' 51°51 ' 48046 ' 45o49 ' 43°52 ' 41°3Y 40°26 ' 41011 , 43°19 ' 44°09' 42051 " 41006 '

6411 5663 5363 5906 5754 5500 4840 5224 5380 4798 5107 5229 5045 1770 2769 2186 4128 4650 4325 3605 2857 2680 3356 2923 4128 5111 5095 4857 4186 5056 4987 4607 4089 2722 1863 3369 3643 3548 3250 2548

2-03

1.89 1-69 1.86 1.81 1.91 1.87 1.63 1-77 1.84 1.68 1-63 1.76 1-79 2.22 2-t5 2.32 2.36 2.11 2.32 2.50 2.30 2.02 2.41 2.33 2.31

1.96 1.78 1.92 2.30 (1-89)

1.99 1.90 2.32 2.44 2.34 2.39 2-18 2.10 2.30 2.51

1.95(2) 1.72(3) 1.20(3) 1"12(2) 1.26(3) 1.70(2)

1.55(3) 1"38(3)

0.66(2) 1.36(2)

1.44--2.6(1) 1"58(3, T) 1.02(3) 1'38(3, T)

1.09(3, T) 1-72(3) 1-65(3, T)

1-80(3) 1.90(2) 1.13(3) 2.98(2) 0-64(3, "I) 1.23(3) 0.61(3, T) 2.12(3)

1.16(3, T) 1.05(3) I'12(2) 1-78(3) 1.71(3) 1.66(3) 1'1-1"2(1, T) 1-15(3) 1.30(3) 1.27(3) 1.72(3) 1.48(3) 1.50(3) 1.37(2) 1.33(3) 0.72(1)

Present Status o f Oceanic Heat-Flow Measurements TABLE

395

2--continued.

INDIAN OCEAN HEAT-FLOw ME,t~UREMENTS Position Station No. E.

V19-103 V19-106 V19-107 V19-108 V19-109 V19-1,10 V19-111 V19-I12 V19-114 V19-115 *A V19-116.^

Latitude

Longitude

17°54'S 22°57'S 22°58'S 23°11 'S 23o22'S 23o31'S 25o20'S 31o42'S 34°24'S 35o30'S 35o55'S

29030 ' 42°10' 41 °22' 39°58 ' 38o51 ' 37~51 ' 36o47 ' 38o10 ' 31 °25' 29057 ' 27045 '

Water depth, m

Thermal cond.t

Heat flow~

2314 3175 3885 3345 3087 2903 2203 5018 4124 4565 4656

2"23 2"18

1.12(3, T) 1.40(3, T) Neg. (1, T) 1 54(3) 1.44(2) 1-60(2) t .32(2) 1.20(3) 1-1-1.9(1, T) 1-32(3) 1.68(3, T)

2"19 2"36 1 "99 2"34 2"03 2"23 2"49 2"67

.* 10-6 cal c m --° $ec -1.

'f 10- 3 cal °C-~ cm -1 sec-L

TABLE 3. COMPARISON OF HEAT-FLOW MEASUREMENTS IN THE UPPER AND LOWER INTERVALS AT STATIONS WHERE

THREE l~oBr.s PgNFn~TED ~-m S E D ~ r r

Station number

V19-57 V19-59 V19-60 V19-61 V19-68 V19-70 VI 9-73 VI9-75 V19-78 V19-89 V19-90 V19-93 V19-94 V19-97 V19-98 VI9-100 V19-101 V19-108 V19-112 V19-115 V18-54 V18-58 V18-61

Heat flow p.eal/cm 2, sec upper interval

lower interval

1 "32 1 '21 1 "85 1 "62 1"47 1 '46 1 "74 1"80 1 "13 1"80 1"64 1"15 1"26 1 '59 1"58 1 '37 1"38 1 "67 1 '23 1"32 1 "50 1 "65 1 "42

1 "18 1 "28 1 "66 1 "47 1 "58 1 "38 1 '63 1 "80 1 "24 1 '72 1 '79 1 "23 1-31 1 '33

1 "41 1 "35 1-24 1 "45

1"15 1 "32 1 "6t 1 "65 1 "51

Difference between intervals in the sediment in % o f over-all gradient +12 --

5

+11 +10 --7 +6 +6 0 --9 +4 --9 --7 --4 +17 +11 + 1 +10 +14 +7 0 --7 0 -- 6

R. P. VON HERZEN and M. G. LANGSETH

396

B. Measurements by the Different Techniques As the measurements in Table 2 were made independently by two different techniques, it is instructive to compare their results independently. The average of 5 ° x 5 ° areas in which measurements were made by the cylindrical probe technique (SIO) is 1.44/~cal c m - z sec- t ; for those in which the outrigged probe technique (LGO) was used, it is 1.40/~cal c m - 2 sec-1. The difference is hardly significant; since many of the measurements by the different techniques were made in different parts of the ocean, the agreement may indicate relative uniformity of heat flow over the Indian Ocean. An alternative possibility is that systematic differences between the measurement techniques were compensated by differences in heat flow over the respective areas of measurements. To investigate this, five sets of nearby measurements made by the two different techniques were compared, four in the Indian Ocean and one in the Pacific. Table 4 shows that the agreement is only fair within the TABLE 4 COMPARISONOF NEARBYMEASUREMENTSBETWEENTHE CYLINDRICALPROBEAND OUTILIGGEDPROBETECHNIQUES

Set

Station

Distance between Thermal measurements (kin) conductivity*

LSDA-1 V19-75 LSDH-1 LSDH-2 V19-74 MSN-30 V19-65 LSDH-43 V19-57 RIS-16t V18-114-~ * 10-3 cal/°C cm sec. ? 10-s cal/cm z see.

7

Heat flow?

2.03 2.36 2-08 2.08 2-32

1.65

106

1.71 1.77

1.73 0.66

46

1.63 1.69

1.81 1.20

(1.99) 2.19

0.91 0.92

115 105

2O

1-44 1.80 1-61

1.18

i VON HEP.ZENand UYEDA~1963). LANGSETHet al. (1964).

sets. Some of the discrepancies may be due to differences in values of thermal conductivity. For example, at the set of measurements only 7 km apart (set I), the temperature gradients are similar but the computed heat flows differ by 22 per cent of the mean value, most of which is due to the difference in conductivity of the two measurements. At the station which gave the higher value (V19-75), the sedimentary section obtained in the core consists of 2 m of relatively low-conducting material underlain by higher conducting material; however, the higher conductivity material does not appear to reflect a lower gradient compared to the nearby station LSDA-I. It is possible that the different coring apparatus used by the different techniques (2-m gravity core with the cylindrical

Present Status of Oceanic Heat-Flow Measurements

397

probe, 12-m piston core with outrigged probes which samples deeper in the sediment column) could account for some of the differences in thermal conductivity. For sets 3 and 4, the piston core technique indicated significant increase of conductivity with depth in the sediment. Also, station LSDH-43 in set 4 was located only about 1 mile from an 800 fathom high seamount, which may account for the relatively high heat-flow value there. On set 2, thermal inhomogeneities in the relatively shallow water may explain the different results; station V19-74 in this set is listed in Table 2 as indicative of transient thermal conditions. The standard deviation for all sets of nearby measurements in Table 4 is 0.40 p cal/cm z sec, which is close to the value obtained for repeat sets of measurements over a restricted area of the eastern Pacific Ocean (VoN HERZEN, 1964). The results of a study of the thermal conductivity of the cores recovered from the Indian Ocean floor are shown in Fig. 11. Here is plotted a histogram of individual measurements of thermal conductivity by the needle-probe method of SIO (dotted), LGO (dashed), and the total for both institutions (solid). All individual measurements have been approximately corrected to ambient conditions by reducing the measured values by a constant 4 per cent. The independent results of both institutions show a noticeable, and probably significant, tendency for the distribution to be bimodal. It seems likely, from the distribution of sediments in the Indian and other oceans, that the lower peak (I-6--1.8 x 10 - 3 cal/°C cm sec) is representative of the deep-sea lutites, which have a relatively high water content, and that the upper peak (2-0-2-3 x 10-3 cal/°C cm sec) results from measurements on calcareous sediments having relatively low water contents. The valley between the peaks in the distributions may result from measurements on sediments which are a mixture of these two primary components. An immediately noticeable discrepancy in Fig. 11 is that the peaks for the distributions are not at the same conductivity values for the two independent sets of measurements at SIO and LGO. The peaks for the histogram of values made at SIO are centered at conductivity values about 5 per cent less than the distribution obtained at LGO. This difference is sufficient to nearly eliminate the bimodal nature of the distribution when the results from the two institutions are combined. We believe it is unlikely that a systematic error of this magnitude exists in the measurements of either or both institutions, although this possibility is not excluded. Some alternative explanations are: (1) The measurements were made on cores obtained from systematically different environments of the ocean floor by each institution. Heat-flow measurements were mostly made in the deep basin floors of the Indian Ocean by LGO, whereas the SIO measurements include a greater proportion of shallow basin and ridge environments. As topography often controls the type of sediment accumulation at any locality, this possible cause of a systematic difference may be plausible. (2) The measurements at L G O were made on cores obtained deeper in the sediment column than those measured at SIO. If there is a significant increase of conductivity with depth (see, for example, BULLARDand DAY, 1961) then the peaks at smaller

398

R. P. V o ~ HERZEN and M. G. LANGSETH

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Present Status of Oceanic Heat-Flow Measurements

399

values of conductivity for SIO measurements than those at LGO may be accounted for. Nevertheless, systematic differences in conductivity do not appear along many of the long cores obtained in the Indian Ocean (riz., see Fig. 5). Whatever the cause of the discrepancy, we do believe that the bimodality of the distribution is a real feature of the Indian Ocean sediments. A similar distribution seems likely for the sediments of other oceans also, which should be expected to correlate with the different sediment types.

C. Correlation of Heat Flow with Physiographic Features in the lndian Ocean 1. Description of ridges and basins. The major topographic features of the Indian Ocean may be shown by the contour map in Fig. 12. This map is only slightly revised from one originally prepared by STOCKS (1960). As Stocks pointed out, the Indian Ocean has a well developed basin and ridge structure. Topographic surveys since 1960 have shown continuity of the Ninety-East Ridge on the basis of several crossings and the potential temperature of the bottom water, and the continuity of the southwest branch of the Mid-Indian Ocean Ridge (MIOR) (EWINGand HEEZEN,1960). The dominant topographic feature of the Indian Ocean is the MIOR System which is believed to be part of the seismically active world-wide rift system. This feature begins in the north as the Carlsberg Ridge and extends from the Gulf of Aden south east to the equator. South of the equator the MIOR runs nearly due south passing to the east of the Mascarene Ridge. At about 20°S the ridge branches into two separate ridges, one of which runs southwest around the southern tip of Africa to join the mid-Atlantic ridge, whereas the other branch runs south east below Australia and is continuous with the ridge of the southwestern Pacific Ocean. This ridge system divides the Indian Ocean into western, eastern, and southern units. The western and eastern units have been well charted by recent work during the International Indian Ocean Expedition. However, topographic surveys in the southern unit are still sparse. The three major units are subdivided into basins by minor ridges which, in general, extend from the MIOR system toward the continental land masses. Notable among these are the Madagascar Ridge, the Laccadive Ridge, the Kerquelen Ridge and the Ninety-East Ridge. For the most part these ridges are seismically inactive and vary in width from 150 to 300 km. The Mascarene Ridge extends from the Seychelles Islands at its northern extremity to Mauritius Island in the south, apparently not a physically connected branch of the MIOR. The Seychelles Islands group is notable for its granitic composition (REED, 1949, p. 544), suggesting it may once have been part of a continental region. There may be other ridges of smaller extent which are as yet unrecognized or undiscovered; obviously, the topography and structure of the Indian Ocean floor is complex. 2. Heat-riow measurements on ridges. The correlation of high heat flow with the crests of mid-ocean ridges in other regions of the earth has made them areas

4O0

R. P. VON HERZEN and M. G. LANGSETH

Fro. 12. Structural features of the Indian Ocean. Depth contours in kilometers. of unusual interest. Three crossings of the ridges in the northwestern Indian Ocean between 5½°S to 10°N latitude have been made, with the heat-flow results and the topography shown in Fig. 13; the profiles in the figure extend from the margin of the African continent in the west to the Laccadive Ridge in the east. The seismically active Carlsberg Ridge is about 500 km wide on these profiles with very rugged topography. Contrasting with this picture, the aseismic Laccadive and Mascarene ridges have higher elevations and smoother crests, perhaps due to accumulation of coral growth on the tops of these ridges. This may imply the aseismic ridges are older structural features than the seismic ridges. Evidence for an old age of the aseismic ridges also derives from the well-known occurrence of pre-Cambian granitic rocks on the Seychelles, although seismic refraction results (SHOR, 1963) indicate the granitic rocks may be confined only to the northern part. In general, higher-than-normal heat-flow values were observed on both the seismic and aseismic ridges, as shown in Fig. 13. Seven values on the Laccadive

,COl

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FIG. 13. Sections of heat flow and topography across the northwest Indian Ocean. Location of sections given on fig. 10. Vertical scale H in Fcal/cm ~ sec; horizontal dashed line indicates average heat flow for Indian Ocean. Depth scale D in kilometers.

Ridge range from 1.18 to 1-92/~cal/cm 2 sec, with most of them being near the upper end of this range. East of the Seychelles (See. C-C'), three closely-spaeed measurements gave very high values; however, this anomaly appears localized by surrounding values which are near average. Three excellent values north and west of Seychelles average about 1.7/~cal/cm: sec compared with somewhat lower values in the basin to the north and west. On crossing A-A', moderately high values were observed near the crest of the Carlsberg Ridge. Seven values across the Carlsberg Ridge at 6½°N latitude (Sec. B-B') range from 0.61 to 2.98 #cal/cm 2 sec, but show no clear pattern with respect to the ridge axis. On crossing C-C' no measurements were made near the crest of the Carlsberg Ridge. Farther south, high heat-flow values were measured on the MIOR near 17°S, and an isolated very high value near 26°S,

402

R. P. VON HERZEN and M. G. LANGSETH

74°E on the southeast branch of this ridge. The location of the ridge crest is uncertain in this region. South of Australia a normal value (MSN-49) was measured on the ridge crest. 3. Low values associated with ridges. Contrasting with these generally high values near the crest of the ridges, low values were measured in the basin to the east of the Carlsberg Ridge on crossing A-A'. However, on two of these measurements, partial penetration of the probe caused greater uncertainty of these values. In crossing C-C', three low values on the sides of the Carlsberg Ridge were also made. On the western flank of the MIOR east of Mauritius Island (station MSN-36 and V18-63) low heat flow was also measured. However, as denoted in Table 2, these stations were taken in the floor of small flat valleys similar to those which have been correlated with local values of low heat flow in the Pacific (VON HERZEN and UYEDA, 1963), and hence may not be representative of the regional heat flow. Several values (LSD-A-17 to 19) slightly lower than normal south of Mauritius Island may indicate all of these measurements are located in the same low heat-flow area. Another area of low heat flow may be partially defined by the group of low values near 30 ° to 35°S latitude, east of the southwest branch of the MIOR. No measurements have been made on the crest of the ridge in this area due to the extremely rough topography. In the vicinity of Amsterdam-St. Paul Islands, which are located near the crest of the southeast MIOR, some low values were obtained in the basin and on the flanks up to the crest west of this ridge. The situation is more confused there, however, because these values appear to be intermingled with some normal and even high values. To the east of St. Paul, two values of low heat flow have been measured (MSN-44 and 45). 4. Trench values. A few measurements have been made in the deep-sea trench on the convex side of the Indonesian Archipelago, the only trench in the Indian Ocean. Two low values at stations MSN-18 and MSN-23 were measured in the inner trench south of Java, where there are two such parallel structures separated by a deep ridge between. South of Sumatra two relatively high values (1.72 and 1-95 #cal/cm 2 see) were measured in the outer trench. Another moderately high value (1.87 #cal/cm 2 sec, station MSN-21) was measured 30 km seaward of this trench south of Java. Hence, from these limited measurements, it seems possible that the inner trench may be a region of low heat flow and the outer trench a region of moderately high heat flow. This tentative conclusion is tempered by one of the low values (MSN-18) being measured in an area of locally flat bottom topography; more detailed measurements will be necessary to elucidate the significant variations here.

D. Mean Heat Flow of the lndian Ocean The average of heat-flow values for each 5 ° x 5 ° area is also a useful number for statistical analysis in the Indian Ocean and for comparison with other ocean areas. Figure 6 shows both the average heat flow and the number of measurements for each 5° × 5° area in the Indian Ocean where measurements exist.

Present Status of Oceanic Heat-flow Measurements

403

All the measurements of Table 2 are included, except the two stations resulting in negative heat-flows values (VI 9-107 and MSN-43) at which the gradients have been disturbed. There are 87 5 ° × 5 ° areas in which at least one measurement exists, out of a total of about 300 possible such areas in the Indian Ocean. The average of the values for all 5 ° x 5 ° areas in the Indian Ocean is given in Table 1 as 1.43 (S.D. = 0.73) #cal/cm 2 sec. Also, the median value of all measurements in Table 2 is 1.38 pcal/cm 2 sec. These averages are not significantly different from that of the earth as a whole, about 1.5 /~cal/cm 2 sec (LEE, 1963), nor from the Atlantic and Pacific averages in Table 1. Analyses of the terrestrial heat-flow distribution over the earth by LEE and MACDONALD (1963), which has utilized a part of the data in Table 2, show a general north-to-south decrease of heat flow in the Indian Ocean (Figs. 4 to 7, LEE and MACDONALD, loc. cit.). To test this representation, zonal averages of heat flow from 5 ° x 5 ° areas (Fig. 6) were computed for the different latitude sectors over 10 ° intervals of latitude, with the result given by Fig. 14. Except

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Present Status of Oceanic Heat-Flow Measurements

405 ,

point up the difficulties in attempting to represent the heat flow in a region without measurements (or even in one with a few measurements). From the present distribution of Indian Ocean measurements, it seems difficult to conclude that there are any regional north-south or east-west variations in heat flow over this ocean area as a whole. 5. F U T U R E I N V E S T I G A T I O N S With the increasing emphasis on oceanographic research, it seems likely that the number of oceanic heat-flow measurements could increase by a factor of 5 or 10 in the next decade. Such an increase will naturally lead to a more complete areal coverage of measurements in oceanic regions, and to the establishment of a mean heat flow with greater confidence. With sufficient numbers of measurements, analyses of the variation of heat flow can give important new information on the composition and rheology of the earth's interior, principally the upper mantle. Whenever possible, emphasis should be given to more measurements in high latitude areas, which at present are relatively sparse. The ratio of measurements per unit area at sea to those on the continents will undoubtedly continue to increase, due to the inherent difficulties of making measurements on continents. Therefore the largest gaps in regional coverage of the earth will be on the continents, especially Asia, Africa, and South America, and it is to be hoped that special efforts can be given to measurements in these regions. An understanding of the variability of the oceanic heat-flow values is at present of great interest. Undoubtedly, much of the variability is real, such as the high values associated with the crests of oceanic ridges. Nevertheless, the local variability in some regions, as deduced from measurements at repeated and nearby stations, is poorly understood, although it is likely caused by effects from the local environment as discussed in Section II. This local variability makes it difficult to establish minimum distance scales of regional variations in some areas, unless large numbers of measurements are made to average out the local variations. To establish causes of the local variations, it seems necessary to have better control of the position, topography, and geologic setting at stations than have been made in the past; moored buoys will probably be most useful in such investigations. Equipment should be made to penetrate as deep as possible into the bottom, and it is desirable that the temperature gradient be measured over at least two, and preferably more, vertical intervals. The outrigged thermal probes on a core barrel (Fig. 2) seem most suitable for this task. Important information on the manner of heat transfer into the deep ocean waters may be obtained by measuring temperature and temperature gradients in the water above (several hundred meters) and close to (few meters) the bottom, perhaps with apparatus similar to that developed by GAMUTILOVet al. (1960) for the latter purpose. Near-bottom water samples also could be obtained with this apparatus for chemical and physical analysis to determine possible interactions of the water with the sediments. To determine the geologic setting

406

R. P. VON HERZEN and M. G. LANGSETH

around a station, precise surveys with an echo sounder and continuous subbottom reflection equipment will be required. Deep manned submersibles would be useful for many detailed investigations, although it seems likely that their operations in the deep sea will be expensive compared with those for surface ships.

ACKNOWLEDGMENTS

The SIO measurements in the Indian Ocean on Expedition Monsoon were supported by contract Nonr 2216(01) of the Office of Naval Research and on Expedition Lusiad by grants NSF-G22255 and G19239 from the National Science Foundation, U.S.A. We are indebted to R. L. Fisher as director of the Indian Ocean project at SIO; and to A. H. Giobbi for making the measurements on Monsoon and to K. Rhea and D. Keith for many of the measurements on Lusiad. The LGO measurements in the Indian Ocean on Vema cruises 18 and 19 were supported by contract ONR 26648 with the Office of Naval Research. The help of P. Grim, K. Griffith, and A. Lowrie in making the measurements is gratefully acknowledged.

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