Sediment transport and distribution in the Argentine Basin. 2. Antarctic bottom current passage into the Brazil Basin

2 SEDIMENT TRANSPORT AND DISTRIBUTION IN THE ARGENTINE BASIN. 2. ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN By XAVIER LE PICHON,* MAURICE EWING a n d MAREK TRUCHAN

CONTENTS Introduction I. Bathymetry General bathymetry Morphology of Vema Channel

31 32 32 33

1I. Seismic profiler evidence

36

III. Further evidence: Surface sediments Temperature distribution Nepheloid layer

38 38 40 44

Discussion

45

Conclusions

46

Acknowledgements

47

References

47

* Now at CNEXO, 39 av. d'Iena, Paris 17, France.

2 SEDIMENT TRANSPORT A N D DISTRIBUTION IN THE ARGENTINE BASIN. 2. ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASINt By XAVIERLE PICHON, MAURICE EWING and MAREK TRUCHAN

ABSTRACT

The Rio Grande Rise is a major boundary between the Argentine Basin and the Brazil Basin The only topographic break between Brazil and 30°W is a narrow gap along 40°W, the Rio Grande Gap. Within the gap, a 650-kin long channel, called the Vema Channel, is cut as much as 700 m below the adjacent sea floor and provides a 4700 m deep passage for the Antarctic Bottom Current (AABC). The channel has been cut by the AABC and its history goes as far back as Late Mesozoic when the present regime of bottom water circulation was probably established. The velocity and transport of the AABC through the channel are computed and lead to an estimate of the transport of sediment in the AABC from the Argentine Basin into the Brazil Basin.

INTRODUCTION

In Part 1, we have presented evidence that the Antarctic Bottom Current (AABC) in the Argentine Basin is a major agent of erosion and of control of sedimentation. These findings support the conclusions of W0ST (1957) that its velocity may be as high as half a knot or more, and its transport is comparable to the Gulf Stream. The previous report examined the area corresponding to the entrance of the AABC into the southern Argentine Basin, where the flow of the current has to pass through a system of basement ridges before entering into the Argentine Basin. It was shown that the channelling of the current and its sudden release into the open basin resulted in large-scale erosional and depositional sedimentary features. Similar effects, but on a smaller scale, can be observed where the AABC leaves the Argentine Basin to enter into the Brazil Basin through a gap in the Rio Grande Rise. The interpretation of hydrographical data by Wiist in 1957 showed that just north of the Rio Grande Rise the volume of water flowing in the AABC was only a small portion of the 60 x 106 m~/sec measured at the section along 35°S. However, we will present bathymetric, sedimentary and hydrographical evidence which supports the hypophesis of the passage of a still well defined and strong AABC through a topographic gap which is situated near 40°W and 30°S. The channelling of the current has resulted in the erosion of a deep sea channel which is maintained for a distance of at least 350 km north of the gap. t Lamont-Doherty Geological Observatory, Columbia University, Palisades, N.Y. Contribution No. 1426. 31

32

XAVIER LE PICHON, MAURICE EWING

and

MAREK TRUCHAN

The topographic gap was first discovered by the Meteor expedition and reported by MArinER and STOCKS(1933) as the Rio Grande Rinne. Consequently we follow their nomenclature and use the name of Rio Grande Gap. As this report gives the first detailed description of the channel cut into the sedimentary layer within the Rio Grande Gap and since most of the survey was done by the RV Vema, we propose to call this channel the Vema Channel. The Vema Channel was traversed during cruise 12 of the Vema in 1956. Further traverses were made during cruises 15 and 16 and cruise 238 of the RV Atlantis. A special survey was made during cruise 22 of the Vema with the specific purpose of investigating the relation between this gap and the passage of the AABC. In early 1967, during cruise 11, RV Robert D. Conrad crossed four times the northern extension of the channel into the Brazil Basin. This survey confirmed that the Vema Channel continues into the open basin for more than 300 km beyond the Rio Grande Gap as a deeply cut, sinuous trench. A total of 5500 km of sounding lines, including 14 crossings of the channel and 2000 km of seismic profiler lines, including 9 crossings of the channel were used to construct a bathymetric map (Fig. 1), profiler sections (Figs. 2 and 3) and topographic sections (Figs. 4 and 5) of the channel. The unit used in the maps is the nominal fathom, based on a sea-water sound velocity of 800 fm/sec, as this unit was and still is used for all the recordings and working maps. Depths given in meters are corrected according to Matthews tables.

I. BATHYMETRY EWING et al. (1964) and EWING and EWlNG (1965) had shown that the large body of sediments of the Argentine Basin progressively thins toward the north on the southern slope of the Rio Grande Rise. The steep scarp forming the northern limit of the Rio Grande Rise along 29°S is a major boundary between the Argentine Basin and the Brazil Basin. The only topographic break existing between the Brazilian continent and 30°W is a narrow gap situated along 40°W. General Bathymetry Figure 1 shows the bathymetry of the area, and Figs. 2 and 3 are reproductions of the profiler records along the tracks shown in Fig. 1. An examination of the figures reveals that the topographic gap is due to a break in the basement topography. The presumably basaltic basement culminates at 700 fm (1300 m) on the east beyond the part of the profiler reproduced and 1800 fm (3300 m) on the west but is 2800 fm (5100 m) deep in a 100-km wide section between these two highs. It is through this basement gap that a long channel has been cut by the current into the overlying sediment layer, which is generally more than 1 km thick. The channel is first recognized as a broad trough at 32°40'S. It develops rapidly into a narrow trough cut more than 800 m below the adjacent sea floor, where it is constricted by the topographic highs on each side. Its general trend is slightly east of north, but it has developed a sinuosity with a wavelength of about I00 kin. Figure 2 shows that this curvature, within the gap, is closely

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FIG. 1. Bathymetry at a 100 fm (183 m) contour interval based on Vema 22 (solid line), Conrad 11 (dashed line) and other cruises (dotted line). The letters refer to profiler crossings in Figs. 2 and 3. Stations identified will be discussed later. controlled by the existence of basement walls which first divert the channel toward the west near 31°30'S and then toward the east near 30°30'S. North of 29°30'S, in the Brazil Basin, the channel is still meandering with the same wavelength, but this meandering is not controlled any more by basement structure (see Fig. 3). While the terminations at both ends are not known, it is probable that the channel progressively broadens until it loses its identity.

Morphology of Vema Channel Figures 4 and 5 show tracings of the precision depth recorder (PDR) records along five crossings of the channel made during cruise 22 of the Vema (Fig. 4)

34

XAVIER LE P1CHON, MAURICE EWING

and MAREK TRUCHAN

and four crossings made during cruise 11 of C o n r a d (Fig. 5). Table 1 lists the main characteristics of the channel at each of thirteen topographic sections. The progressive narrowing of the channel as it is constricted against the basement wall is well demonstrated in Fig. 4, in contrast to its broad and gentle section on the southernmost profile. The steepest wall of the channel is always •\

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ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

35

situated on the eastern side. The western side is consistently higher and has a much gentler slope. Exceptions exist in sections BC and AB (Fig. 2) but are explained by an outcrop of basement on the western wall. This asymmetry between the two walls is to be expected from the northward flowing AABC. Such a deep cold western boundary current is characterized by iso-velocity surfaces which slope toward the east and velocities which increase toward the bottom. Thus, a higher velocity is found on the western side of the channel than on the eastern at the same depth. Prevention of deposition or even erosion is therefore to be expected further up and away from the axis of the channel on the western side, resulting in a more gentle western wall. The Coriolis acceleration acting on turbidity currents flowing through the channel from south

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T A B L E 1. P A R A M E T E R S OF V E M A C H A N N E L

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36

XAVIER LE PICHON, MAURICE EWING and MAREK TRUCHAN

to north would produce a higher bank on the western side (MENARD, 1955). However, the centrifugal acceleration in the bends of the channel is likely to become significant for the faster flowing turbidity currents. Near 29°S, the shortest radius of the bend is about 35 kin. With such a radius, in a bend toward the left, the Coriolis acceleration would be balanced by the centrifugal accelerations at velocities of 250 cm/sec, that is about 5 knots. These velocities are below those generally quoted for turbidity currents in deep sea channels. Therefore, if the channel were created by turbidity currents one would expect the asymmetry between the walls of the channel to reverse itself between profiles E'F' and A'B'. As the channel turns toward the left, the centrifugal acceleration acts against the Coriolis acceleration instead of reinforcing it. An examination of Fig. 3 and Fig. 5 shows that there is no major difference between profiles E'F' and A'B'. A more powerful argument against attributing the channel to turbidity current action is the absence of a significant regional gradient along the 655 km of the surveyed part of the channel. Between the two extreme profiles, there is an average slope of about 1 part in 6000, compared with 1/1000 for Cascadia Channel for example (HURLEY, 1964). However, the main indication of Table 1, clearly illustrated in Fig. I0, is that the axis of the channel maintains a depth close to 4750 m throughout its entire length and that this depth bears no relation to the depth of the adjacent floor. The main topographic accident near km 200 is related to a basement high (profile CD of Fig. 2). The remarkable leveling action of the agent of formation of the channel is not surprising if this agent is the AABC. Therefore, a study of the morphological characteristics of the Vema Channel leads us to three main conclusions: the channel is created by flow coming from the south; this flow does not reach velocities larger than about 6 knots; this flow is not maintained by an overall regional slope of the channel axis. Yet, the agent of formation of the channel is powerful as this feature has an average cross section of 10 kmL a total length of more than 655 km and a total volume below the adjacent sea floor exceeding 5000 km 8. The only likely agent of formation, powerful and consistent enough to create and maintain the Vema Channel is the AABC. Two other interesting morphological details should be mentioned here. Note that the small scale roughness of the sea floor seen along sections EF and DE (Fig. 4) vanishes as the channel becomes narrower and more deeply cut, probably corresponding to an increase in velocity of the flow. Along section BC, a smaller flat bottom channel has been cut within the main one (see also Fig. 2). II. S E I S M I C P R O F I L E R E V I D E N C E

EW1NG et al. (1964) had shown that layer A, a major level intermediate reflector which extends over the whole Argentine Basin, can be followed upslope on the southern flank of the Rio Grande Rise. This reflector has been tentatively dated as marking the boundary Mesozoic-Cenozoic, and its slope upward on the flank of the rise was interpreted as indicating a post-Mesozoic uplift of at least 1 km. Further work by EWING et aL (1966) and LE PICHONet al. (1966) has confirmed that the whole structure of the Rio Grande Rise points

ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

37

to a major phase of uplift during the Cenozoic era. This phase of uplift must have strongly affected the flow of the AABC through the Vema Channel and we might expect to find evidence pointing to such an uplift in the sedimentary record left by the passage of the current through this gap. The first point revealed by the examination of the seismic profiler records shown in Fig. 2 and Fig. 3 is that the Vema Channel is primarily an erosional feature. However, this examination shows that the geological history of the Vema Channel is complex, and not one of a single cutting of a channel through a uniformly stratified sedimentary layer. On the contrary, there is much evidence of earlier buried channels, disturbed stratification near the channel, and faulting in the basement which has strongly affected the sedimentary cover. It is consequently difficult to discern in this complex picture the features which are due to the fluctuations of the AABC and the features due to the tectonic history of the area. The clearest indication of tectonic control of the channel is found along profile E-F (Fig. 2). The strong intermediate reflector at a depth of about 0.6 sec below the sea floor (6.5 sec below the sea surface) has been identified by continuity with other profiles as layer A. The plane continuous dip of layer A west of the seamount is in contrast with its distorted surface on the Rio Grande Rise side of the seamount. A major step offsets the level of layer A on the east side of the channel and this step is clearly due to basement faulting posterior to the deposition of A. This faulting is such as to produce a differential upward movement on the Rio Grande Rise side. This uplift by an amount of nearly one kilometer must have had considerable effect on the circulation of the bottom water; and the location of the channel along section F-E has probably been determined by this faulting (see also section D-E). It can then be concluded that major tectonic movements, posterior to the deposition of layer A, have distorted its surface, produced a differential upward movement of the Rio Grande Rise side and at least partly controlled the location of the channel. It is more difficult to identify positively reflector A in sections AB to CD within the Rio Grande Gap but there is little doubt that "A" corresponds to the major intermediate reflector at the same depth of 6.5 sec below the sea surface in profile A'B' (Fig. 3). Using as a criterion the greater acoustical transparency of the layer between horizon A and the basement, it becomes possible to trace horizon A, by continuity, throughout the entire length of the channel. The amount of distortion and uplift is very large within the Rio Grande Gap (sections AB to EF, Fig. 2), but horizon A is essentially undeformed north of the gap, except on reaching the base of the Rio Grande Rise (near A' and F', Fig. 3) where it has been tilted upward toward the east. This suggests that postMesozoic tectonic activity has been minimal in the basin north of the gap. Therefore, it is in the part of the Vema Channel situated within the Brazil Basin that we have a better opportunity to obtain information about the history of the AABC during the Tertiary. The only possibility of obtaining a complete record of the geological history of the passage of the AABC through this area would be by deep-sea drilling. However, Fig. 2 and Fig. 3 provide a few important conclusions: The most startling is that the history of the passage of the AABC reaches as far back as the "A" horizon age (end of Mesozoic). In sections A'B', C'D' and E'F' (Fig. 3) a buried channel can be recognized at the surface or just below the surface

38

XAVIER LE PICHON, MAURICE EWING

and

MAREK TRUCHAN

of layer A, under the present western wall of the channel. This feature is more evident in section C'D'. This buried channel can also be recognized in section AB and perhaps also in section EF of Fig. 2. In sections BC and CD, layer A seems to terminate against a rise of the basement, at a depth of 6.5 sec below the sea surface, about 20 km west of the present axis of the channel. The exact date of formation of the channel is of course impossible to determine. It may be argued that the channel was cut through layer A quite a long time after layer A was deposited, and that this fossil channel was later filled in. However, a second deep undisturbed intermediate reflector overlies the buried "A" channel in section A'B' of Fig. 3 and a thickness of more than 500 m of sediments has since covered this buried "A" channel in section AB of Fig. 2. It is more probable that the circulation of the AABC began at the time A horizon was deposited and that, in fact, the great change in acoustical properties of sediments that occurred at this time is due in some way to the beginning of large scale abyssal circulation. This change in type of sediments, described by EWlNG et al. (1964) and discussed at length in the third report, is most clearly displayed in section A'B'. The sediments are acoustically transparent below A, but somewhat fuzzy with many ripple-like undulating reflectors above A. HOUTZ et. al. (1968) have shown that this change in acoustical transparency at "A" horizon is accompanied by a step increase in compressional velocity, which can best be explained by a change in the type of sediments. The greater resistance of the sediments forming the layer below horizon A is also suggested by the apparent outcrop of layer A in the crossing A'B' (Fig. 3 between the arrows). At this point, a high point in layer A seems to have resisted erosion resulting in a small saddle at the axis of the channel. As mentioned above, the sedimentary column, above horizon "A", is characterized by "fuzzy" reflectors with an undulating surface of the type associated with most of the "post-A" Argentine Basin sediments (see Part 3). This surface has been smoothed by stratified sediments which sometimes have filled large depressions, as in section AB (Fig. 2). An indication about the nature of the sediments is given by the thick graded layers of foraminiferal sands found at $75 and 9 (Fig. 1). It suggests that this smoothing process might correspond to the spreading of turbidity currents originating on the slopes of the adjacent Rio Grande Rise which has a thin cover of foraminiferal sand. These turbidity currents should have been more frequent during the Late Cenozoic phase of uplift postulated by LE PICHON et aL (1966). A final remark should be made about the steep slope found at many locations on the east wall of the channel. This slope attains 14° in section AB, E'F' and A'B', indicating the strong cohesiveness of the sediments. III. FURTHER

EVIDENCE

Surface Sediments

The distribution of the surface sediments inside and outside the Vema Channel gives important indications about the present distribution of the bottom current. Furthermore, during the Conrad cruise 11, oriented bottom photographs in the channel provided unmistakable evidence of strong northward flowing current.

39

ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

During the Vema 22 survey, four stations were occupied in the area, three over

the channel, $76 through 78 and one just outside the channel, $75 (see Figs. 1, 4, and 6 for locations). Four cores have been obtained in this area during earlier cruises (numbers 1 t h r o u g h 4 in Fig. 1 and Fig. 6), two o f them within the channel. During cruise 11 o f the Conrad five stations were occupied in the area,

two of them inside the channel (Figs. 1 and 5). Table 2 lists the locations of the stations and the measurements made at these stations• Figure 6 represents schematically the lithology o f the twelve cores obtained• A striking point 43 27

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34 :36

40

XAVIER LE PICHON, MAURICE EWING

and

MAREK TRUCHAN

revealed by Fig. 6 is that all the cores taken in the channel are short while those taken on the adjacent sea floor are several times longer. The three cores coming from the narrowest section of the channel ($77 and 1 and 2) are very short as the corer was rapidly stopped by a thick crust of manganese overlying very compact noncalcareous lutite. The existence of a crust of manganese covering the floor of the channel at its narrowest and shallowest section indicates that the bottom current at this location is fast enough to prevent any deposition of sediment. Similarly, at $5, further north within the Vema Channel, a thick crust of manganese stopped the core during two attempts to sample layer A (cores 38 and 39, Table 2). At $76 (Fig. 6), below 32 cm of uncompacted terrigenous lutite, the corer hit a concentration of manganese nodules overlying much more compact material, similar to the one found in $77. One can then conclude that, since the deposition of the layer containing the manganese nodule concentration, some slowing of the current has occurred at $76, permitting more recent sediments to cover this layer. This station, however, is situated downslope from a high along the channel axis, at km 200 in Fig. 10. In contrast, farther south, in the wide portion of the channel, uncompacted terrigenous lutite was found at $78 and 4. A layer of quartz sand is present at the surface of the sediments at station 4. Both cores are unfossiliferous. Cores taken outside the channel consist of much less compact foraminiferal lutite, as indicated by the greater penetration of the corer. The presence of a manganese crust overlying compact sediments within the main portion of the channel is in contrast with the absence of manganese where the channel begins to develop. This is consistent with an acceleration of the current as it is constricted within the narrow cross-section of the channel. An important point to make is that nowhere within the channel did we recover what could be interpreted as terrigenous turbidites. In fact, the only layer of quartz sand was found at $4. This is another argument against turbidity current origin for the channel. The results of the oriented bottom photographs taken at station 5 through 9 during cruise 11 of the Conrad will be discussed in relation with the general circulation of the bottom water in Part 3. Here, we just mention briefly the indications they give about the circulation within the channel area. No indication was found at $8, about 40 km east of the channel. Otherwise every photograph shows evidence of strong northerly current, especially within the channel. The results are summarized by the small arrows in Fig. 6. Figures 7 and 8 are samples of the two stations taken within the channel. Both show unmistakable evidence of very strong current and, in the case of Fig. 7, the current is flowing due north. Therefore the bottom photograph evidence confirms the conclusion reached earlier that there is a fast northern current within the channel.

Temperature Distribution The Meteor had obtained two hydrographic stations situated 470 km apart on each side of the Vema Channel near 32°40'S. However, these stations, 86 and 85, did not reach a depth greater than 4000 m. Similarly, it had occupied two stations, 44 and 43, situated 160 km apart in the shallower part of the basin on each side of the channel near 30°S. However, one of the stations did

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not reach a depth greater than 3700 m whereas we have shown that the channel is generally more than 4700 m deep. It is thus not surprising that these stations failed to reveal a sizeable flow of AABW. In fact, in view of the small width of the passage and of its depth greater than the adjacent sea floor, only stations reaching to the bottom in the deepest part of the channel would be able to detect this flow. During cruise 22 of the Vema, measurements were obtained of the continuous temperature structure of the water column at stations $75 and $76 with a Ewing

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FIG. 9. Continuous temperature profiles obtained with a Ewing thermograd at station $75 (inside the channel) and $76 (on the eastern bank of the channel). thermograd (LANGSETH, 1965; see Table I and Figs. 1 and 5). The temperature profiles are shown in Fig. 9. While the relative accuracy has been shown to be quite good (LANGSETH, 1965), the absolute values had to be adjusted to the nearby Meteor station 43 making the minimum temperature in the Antarctic Intermediate Water Mass equal to 2.77°C. T27 at $75 was obtained at a distance of 22 km east of the axis of the channel, whereas T28 at station 76 was taken over the axis of the channel. Notice that while the temperature is the same at a depth of 3.3 km (2.40°C), it is equal to 0.98°C at the sea floor for T 27, whereas at the same water depth over the channel for T28 it is only 0.25°C. The horizontal difference in temperature of 0.75°C between $75 and

ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

43

$76 indicates qualitatively a current flowing toward the NW. However, the presence of a topographic obstacle along the western edge of the channel indicates that the true flow must be toward the north along the direction of the channel. It is thus permissible to project these two stations along a section perpendicular to the axis of the channel reducing the distance between them from 68 to 22 km. The constant temperature of 0.25°C (--0.10°C potential temperature) within the 700 m deep channel indicates that the water flowing through the channel is AABW. If we had information about the distribution of salinity it would be possible to compute the geostrophic current using the level of no motion assumed by DEFANT (1941) which is 1800 m here. This level of no motion has been shown to provide satisfactory results. In the absence of salinity measurements, we assumed that the salinity associated with a given temperature would be the same as that at the nearby Meteor station. This assumption cannot lead to large errors here as the maximum variation over the total thickness of deep water from 1800 m to the sea floor does not exceed 0 . 6 ~ (from 34.63 to 34.93 g/liter). Using 1800 m as the level of no motion one obtains a current flowing toward the south from 2500 to 3500 m with a maximum velocity of about 15 cm/sec at 2800 m. Then the current reverses itself and reaches a velocity of 25 cm/sec toward the north at 4200 m in depth. These results are quite reasonable as they give the North Atlantic Deep Water (NADW) flowing toward the south while the AABW flows toward the north. It is not possible to obtain velocities below 4200 m as this is the depth of the sea floor at $75, but an extrapolation of the difference of dynamic height curve would lead to a velocity of about 40 cm/sec at the bottom of the channel. According to these computations, between stations $76 and $75, a total volume of 2.1 x 10e m s is flowing each second toward the north between 3800 m and the level of the sea floor adjacent to the channel (4240 m). Assuming that the velocity stays constant at 25 cm/see within the channel (cross-section of 12.4 km2), another 3.1 x 10a m 3 of Antarctic Bottom Water is flowing per second toward the north. Thus, the total northward transport of AABW which could not be detected by the Meteor stations is at least 5.2 x 10e m3/sec. This figure does not take into account a probable increase of the velocity of the current with depth in the channel, and it ignores whatever amount of water is flowing between $75 and the eastern wall of the gap and $76 and the western wall of the gap. It is thus reasonable to assume that the total northward transport of AABW through this gap is of the order of 10 x 106 m3/sec. This figure should be remembered when trying to estimate the global budget of the deep water masses in the Atlantic Ocean. This crude estimate obviously does not take into account the nongeostrophic portion of the current which might be affected in particular by friction. However, it is probably conservative as the geological evidences of deep erosion of semiconsolidated sediments and total prevention of any sedimentation at the axis of the channel certainly argue for bottom water velocity at least as high as the half a knot computed from the temperature sections. Also, the continuation oi this erosional feature as a well defined channel for more than 200 km north of the gap, far into the open basin, indicates that this current is sufficiently fast and concentrated to maintain a "jet-like" character far out of the restraining basement walls.

44

XAVIER LE PICHON, MAURICEEWING and MAREK TRUCHAN

Nepheloid Layer The large total transport of the AABC through the Vema Channel could be quite significant for the distribution of the nepheloid layer in the Brazil Basin, if a dense nepheloid layer exists in its core while flowing through the channel. The nephelometer measurements (EWlNG a n d THORNDIKE, 1965) obtained during cruise 22 of the Vema at stations 75, 76, 77 and 78 (L.S. 36 through 39, see Table 2) were not of high quality, an instrument problem being responsible for a large background noise. Since then, nephelometer measurements of good South

North

?,o,~ ,o , = ~ I

0

I

I

2o 4o 6o 8o ioo

E ~

o. o t:)

I

I

I

I

I

I

0

I00

200

300

400

500

Distonce AionQ

Chonnel,Krn

F1o. 10. Longitudinal topographic profile along the axis of the Vcma Channel and the adjacent sea floor. The four solid curves are the nephelometer profiles plotted in

arbitrary units (see text). The intersection of the curve with the bottom is at the station location. The wavy line refers to the upper boundary of the AABC as defined by the water temperature profiles. The arrow refers to the direction of flow of the AABC. Vertical exaggeration ~ 100:1. quality have been obtained during cruise 11 of the Conrad, which will be discussed in Part 3. We will just present here briefly Vema cruise 22 nephelometer results which indicate the presence of a strong nepheloid layer flowing with the AABC through the Vema Channel. The results of these light scattering measurements are shown in terms of relative transparency of the negative film, as measured by an optical densitometer. Giving an arbitrary number of 100 to the clearest water at the station (in this case, the core of the North Atlantic Deep Water Mass), the relative clearness of the water at the four stations is

ANTARCTIC BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

45

shown in Fig. 10. The large fluctuations found in the upper layer of water at stations 75 and 76 (L.S. 36 and 37) may not be significant, but the overall consistency of these measurements indicates that a nepheloid layer is indeed found in the core of the AABC flowing in the Vema Channel. The layer roughly corresponds to the part of the water column below the thermocline separating the core of the AABW from the NADW. No strong nepheloid layer was detected at station 75 (L.S. 36). If we had an estimate of the concentration of particulate matter corresponding to this measurement of scattering, we could evaluate the importance of this transport of sediments in the sedimentary budget of the Argentine Basin and the Brazil Basin. The actual measurement of this particulate matter density is difficult and perhaps unreliable because of its minute concentration. At each of the three V22 stations in the channel a sample of 200 liters of bottom water was obtained (EWING and TI~ORNDIK~., 1965); the centrifugation of the water coming from these water barrel samplings 23, 24 and 25 yielding respectively 29.9, 6.1, and 23.7 mg of dry matter. If these measurements are meaningful, an estimate of the concentration of the water in lutite of 0.1 mg/liter is probably of the right order of magnitude. From considerations of the average residence time in the nepheloid layer, the average concentration of particulate matter in it is estimated to be between 0.03 and 0.3 mg/liter in Part 3. Taking as 5 x 10~ m~/sec, the transport of water carrying a nepheloid layer through the Vema Gap, the corresponding transport of particulate matter is 1.5 to 15 x 105 g/see or 0.47 to 4.7 x 1013g/year. These figures are to be compared to a total amount of 3 x 10X3g of lutite deposited in the Argentine Basin every year. Thus, unless the estimate of sediment concentration in the nepheloid layer is grossly in error, the amount of sediment transported through the Vema Channel is significant in the sedimentary budgets of the Argentine Basin and the Brazil Basin. DISCUSSION

The findings reported in these two first papers have many important implications concerning the abyssal circulation, paleoclimates and processes of deep sea sedimentation. A first result is that the transport of the AABC toward the west in the southern Argentine Basin is about 80 to 100 x 106 m3/sec; that it is about 60 x 10e mS/see at the latitude of 42°S but that only about I0 x 106 m3/sec pass through the Rio Grande Gap into the Brazil Basin. We do not know how much of the 80 x 10e m3/sec measured north of the Falkland Plateau comes from south of the Argentine Basin through the Falkland gap, and how much might be due to recireulation of water around the Argentine Basin. But the very fast bottom currents measured south of the Falkland Fracture Zone and the enormous sedimentary structure associated with the passage of the current through the Falkland gap suggests that the contribution from the south is large. Thus we are faced with a major water budget problem, which can be solved only by assuming some large scale upwelling or some unknown escape of AABW toward the east. The second related problem is the nature of the mechanism by which the AABC loses maybe as much as 90 ~o of its water during its traverse from the Falkland Plateau to the Rio Grande Rise. As the AABC transports a dense nepheloid

46

XAVIER LE PICHON) MAURICE EWING

and MAREK TRUCHAN

layer, water budget and sediment budget are closely linked and their solution might give the key to the understanding of the sedimentary processes in the Argentine Basin. This will be the subject of Part 3. The transport of significant amount of sediment into the Argentine Basin and then from the Argentine Basin into the Brazil Basin provides the mechanism to explain the dominant influence of the AABC on the distribution of clay minerals in the southwestern Atlantic Basin (BISCAYE, 1964). A second result is that the initiation of the circulation of the AABC occurred probably at the time "horizon A" was deposited, that is, somewhere at the end of Mesozoic and that since this time it has dominated the process of sedimentation in this area. No major sedimentary feature can be related to the Pleistocene glacial ages and, as far as we can tell, the circulation of the abyssal water has been essentially the same in this part of the ocean for the last 60 million years. This startling conclusion implies that the abyssal circulation was not affected in any major way by the progressive cooling of the climate characteristic of the Cenozoic, suggesting that the amount of heat exchange between the equator and the poles through the abyssal circulation has not greatly changed. In other words, while there might have been a general cooling of the earth, the climatic zonation must have been large enough to maintain an important difference in temperature between the equator and the poles. Finally, this obviously indicates no large change in the geographic poles, and equator position throughout the Cenozoic era. Any possible change in the relative positions of continents apparently did not affect the circulation of the AABC.

CONCLUSIONS

We have used evidences from morphology, sediment stratification, surface sediment nature, oriented bottom photographs and water temperature profiles to prove that the Vema Channel is due to the flow of the powerful AABC flowing from the Argentine Basin into the Brazil Basin. The direction of flow toward the north is proven in particular by the asymmetry between the two banks of the channel, the lineations seen on bottom photographs and the geostrophic calculations from the temperature profiles. The absence of a regional slope of the channel floor and the absence of terrigenous turbidites within the channel area rule out a turbidity current origin. It can thus be said that the Vema Channel, a deep sea channel more than 650 km long, situated in a topographic gap in the Rio Grande Rise provides a 4700 m deep passage for the AABC from the Argentine Basin into the Brazil Basin. This channel has been eroded by the Antarctic Bottom Current, sometimes as much as 700m below the adjacent sea floor. Together with the Falkland Channel in the south Argentine Basin, they are major deep sea topographic features, and the largest ones ever attributed to deep sea current erosion. Thus, evidence has been obtained that the AABC is sufficiently fast to be an important agent of erosion. These findings support the interpretation given by W0sx (1957) of the bottom water circulation in the southwestern Atlantic, based on the level of no motion of DEFANr (1941). However, an estimate of the total transport of AABW through the Rio Grande Gap, based on geostrophic computation, gives 107 mS/sec, which is

A N T A R C T I C BOTTOM CURRENT PASSAGE INTO THE BRAZIL BASIN

47

larger than found by Wiist. A similar estimate of the transport can be reached on the basis of probable bottom velocities and total channel sections. And the general consistency of these results lends strong support to the reality of this estimate. Such a transport is of great importance when establishing the budget of the deep water mass in the Atlantic Ocean. While this transport is large, it is less than the transport measured by Wiist at the latitude of Buenos Aires, and this implies that most of the water does not leave the Argentine Basin through the Rio Grande Gap. The existence of a strong nepheloid layer in the core of the AABC leads to the conclusion that it transports a significant amount of sediment from the Argentine Basin into the Brazil Basin which might be of the order of 1013g/year. Finally, a comparison should be made between the Vema Channel and the Falkland Channel. Both channels have been eroded where the AABC is constrained by the basement topography. In both cases, evidence of earlier buried channels are numerous thus pointing to the existence of this current during a great part of the Cenozoic. The current transports significant amounts of sediment through both channels from one basin to the other. However, while a large sedimentary feature, the "Zapiola Ridge" has been created at the opening of the Falkland Channel, no such feature is directly related to the Vema Channel. The depositional "Zapiola Ridge" is due to a special hydrodynamical configuration which allows the current to deposit part of its sedimentary load. This configuration does not exist in the Vema Channel.

ACKNOWLEDGEMENTS Many co-workers of Lamont Geological Observatory participated in the cruises used in this study. In particular, A. Lonardi was chief scientist during the latest Conrad cruise 11. M. Langseth provided us with the thermograd data. B. C. Heezen pointed out that the Rio Grande Gap had been described by Maurer and Stocks. T. Ichiye and A. Gordon read the manuscript. J. Ewing supervised the seismic profiler program a n d provided constant advice. The U.S. Navy, under contract Nonr 266 (48), Naval Ships System Command, and the National Science Foundation supported this work.

REFERENCES BIscAYE, P. E. (1964) Mineralogy and sedimentation of the doep-sea sediment fine fraction in the Atlantic Ocean and adjacent seas and oceans. Geochemistry Tech. Pep. No. 8, Yale Univ., Ph.D. dissertation. D~FA~rr, A. (1941) Die absolute Topographie des physikalischen Meeresniveaus und der Druckflachen, sowie die Wasscrbewegungen im Atlantischen Ozean. Wiss. ERA,. Deutsche Atlantische Exped. "Meteor" 1925-1927, vol. 6, Part 2, Sect. 5, pp. 191-260. EwINr, M., En'rRv.~, S. L., EWlNO,J. I. and LE PICHO~,X. (1970) Sediment transport and distribution in the Argentine Basin, Part 3. Nepheloid layer and processes of sedimentation. This volume, pp. 49-77. Ewr~o, M. and EWING,J. (1965) The sediments of the Argentine Basin, Symp. on Oceanography of the Western South Atlantic. Ann. Acad. Bras. Cienc., Rio de Janeiro, BraMl, 37, 31-61.

EWING, M., LUDWIG,W. J. and EWING, J. (1964) Sediment distribution in the oceans: the Argentine Basin. J. Geophys. Res. 69 (10), 2003-2032.

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XAVIER LE PICHON, MAURICEEWlNG and MAREK TRUCHAN

EWING, M., Lv. PICHON, X. and EwING, J. (1966) Crustal structure of the mid-ocean ridges. 4. Sediment distribution in the South Atlantic Ocean and the Cenozoic history of the mid= Atlantic Ridge. J. Geophys. Res. 71 (6), 1611-1636. EwrNG, M. and THORNDIKE,E. M. (1965) Suspended matter in deep ocean water. Science, 174 (3663), 1291-1294. GROOT, J. J., GROOT, C. R., EWING, M., BtmCKLE, L. and CONOLLY,J. R. (in press) Spores, pollen, diatoms and provenance of the Argentine Basin sediments. Syrup. on the Quat. Hist. of Ocean Basins. Hotrrz, R., EWINGJ. and LE PtCHON, X. (1968) Velocity of dcep-sea sediments from sonobuoy data J. Geophys. Res., 73 (8), 2615-2642. HURLEY,R. J. (1964) Analysis of flow in Cascadia deep-sea channel. Papers in Marine Geology. Shepard Commem. Vol. New York, 531 pp. LANGSETH,M. G. (1965) Techniques of measuring heat flow through the ocean floor. Terrestrial Heat Flow Geophys. Mono. 8, Am. Geophys. Un. LE PICHON, X., EITI~EIM,S. L. and LuDwig, W. J. (1970) Sediment transport and distribution in the Argentine Basin, Part 1. Antarctic bottom current passage through the Falkland fracture zone. This volume, pp. 1-28. LE PtCHON, X., SAITO,T. and Ew~o, J. (1966) Mesozoic and Cenozoic sediments from Rio Grande Rise. Abstract, GSA Annual Meeting, San Francisco, Calif. MATI't4EWS,D. J. (1939) Tables of the velocity of sound in pure water and sea water for use in echo sounding and sound ranging. Admiralty, London, 52 pp. MENARD, H. W. (1955) Deep-sea channels, topography and sedimentation. Bull. Amer. Assoc. Pet. Geol. 39, 236-255. MAURUR, H. and STocIcs, T. (1933) Wissenschaftliche Ergebnisse der Deutschen Atlantischen Expedition auf dem forschungs und Vermessungsschiff "Meteor" 1925-1927, vol. 11, 309 pp. Berlin. TUR~IAN, K. K. (1964) Clay and carbonate accumulation rates in three South Atlantic deepsea cores. Science, 146 (3640), 55-56. WOs% G. (1957) Stromgeschwindigkeiten und Stromengen in den Tiefen des Atlantischen Ozeans. Wiss. Ergeb. Deutschen Atlantisehen Expedition "Meteor" 1925-1927, vol. 6, part 2, pp. 261--420.