geologija. 2008. Vol. 50. No. 4(64). P. 275–289 DOI: 10.2478/v10056-008-0053-y © lietuvos mokslų akademija, 2008 © lietuvos mokslų akademijos leidykla, 2008 © Vilniaus universitetas, 2008 Sedimentation and near-bottom currents in the South- Western Atlantic

Emelyan M. Emelyanov Emelyanov E. M. Sedimentation and near-bottom currents in the South-Western Atlantic. Geologija. Vilnius. 2008. Vol. 50. No. 4(64). P. 275–289. ISSN 1392-110X The aims of the paper are: 1) to study the bottom relief and Late Quaternary bottom sediments of the South-Western Atlantic from the Amazon cone to the Vema Channel and Rio Grande Rise, and 2) to reconstruct recent and palaeo-Antarctic near-bottom currents (AABW). For this purpose, we used three main Parasound seismic profiles: 30 cores (up to 500 cm in length), the nanoplankton stratigra- phy of 9 cores from the Brazilian lithological profile (along 24 °W), and literature sources. No soft sedimentes were found in the Vema channel; the bottom of the channel is acoustically “hard”. Our geological data confirm that AABW flows mainly through this channel. The velocity of this flow should be higher than 100 cm. s–1. Only this strong current is able to rewash not only soft Holocene sediments, but also consolidated Quaternary deposits. Soft layered sediments occur at a depth less than 4200 m in the Hunter channel. Consequently, the AABW is able to flow from the Argentine Basin to the Brazil Basin only at a depth of more than 4200 m in this channel. Brown red clay or yellowish gray miopelagic clay prevail in the Brazil Deep. The age of red clay in the cores is different: Early or Late Pleistocene, or Holocene. Clay was rewashed and re-deposited in many areas of the deep. This means that the hydrodynamics sometimes was very active at a depth of 4000–5000 m in the Brazil Deep. The presence of conturite and turbidite interlayers in the red clay of the S. America continental base confirms the occurrence of a strong jet of the AABW (Deep Western Boundary current – DWBC) here. Antarctic and other diatoms were brought by AABW from Antarctica up to 10–5 °S. An unusual Pleis- tocene Ethmodiscus rex ooze was discovered at the latitude of 20 °S. Our data confirm the occurrence in the area between 10–5 °S of two mid-oceanic channels, one of them (EMOC) being located on a large sedimentary swell. The AABW in the cross-section from the Amazon River to the MAR flows through the Nara (depth 4640–4660 m) plain. This flow was confirmed by hydrochemical data. The AABW started to appear in the Rio Grande Rise region, about 50–30 mill. years. Cyclic events of glaciation and interglacial transitions throughout the Miocene–Pleistocene is a mechanism that caused the AABW currents to become more intensive or passive, with the result that the intensity of the influx of these waters from the Brazilian Basin into the Guiana Basin also changed from strong to weak. Key words: South-western Atlantic, Brazil Basin, near-bottom currents, sediments, palaeorecon- struction Received 24 July 2008, accepted 25 September 2008 Emelyan M. Emelyanov. P. P. Shirshov Institute of Oceanology of the RAS, Atlantic Branch, Pros­pect Mira 1, 236000 Kaliningrad, Russian Federation. E-mail: [email protected]

Introduction nental slope and the Mid-Atlantic Ridge (MAR) mountains. This area is located between the point M near the Recifi port (05°00’ Cold and heavy sea water sinks into the polar regions of the S; 35°00’ W) and the point RM on the western flank of the MAR Earth to the deep basins of the ocean and as deep and near-bot- (01°33’5 S; 30°00’ W) (Fig. 2). The deep (more than 100 m) tom currents flows to the equator (Warren, 1981) (Fig. 1). Equatorial Mid-ocean channel (EMOC) had been found on this Sources of cold and heavy water in the Southern hemisphere profile earlier (Damuth, Gorini, 1976; Belderson, Kenyeon, 1980; are located in the shelf and slope of the Antarctic, i. e. in the Emery, Uchupi, 1984; Emelyanov, 2005). This channel suppos- Weddel Sea. edly serves as the main one for near-bottom water. Also, other The main exchange of near-bottom water between the North channels were found in this profile. In lithological and geomor- and the South Atlantic occurs in the area between the Brazil phological investigations (echosounding and seismic researches (depth 4500–5500 m) and the Guiana (depth 4500–5000 m) with the Parasound profiler were performed and sediment cores Basins, i. e. in the narrowest place between the Brazilian conti- were collected. 276 Emelyan M. Emelyanov

The grain size and chemical analysis of the sediments was The bottom relief of the Hunter channel is hilly (site 22), done, and in the cores of the Bk-Bk profile micropaleontological with sedimentary strata among the hills. The upper part of the (nano) stratigraphy was investigated (Svalnov et al., 2007). strata is represented by low-calcareous terrigenous mud with The main goal of this paper is to describe the bottom relief coccoliths and forams. The mud is light gray. and the structure of the sedimentary strata, to get more detailed We could’t obtain sediment samples from the Hunter information concerning the features of the circulation of near- Channel itself. The core AI-1054 was obtained outside of the bottom water on the basis of sedimentary research, and to search channel in a small pond. for near-bottom current indicators. The work was focused on the The southern and eastern flanks of the Hunter Channel geological consequences of the Antarctic bottom water (AABW) (sites 27 and 28) are steep: in the base of these slopes, at a depth and North-Atlantic deep water (NADW) in the B–RGR, M–RM of 4220 m, layered sedimentary strata 50 m thick occur. This and A–MAR profiles (Fig. 2). means that in the sediments there are no evidences (indicators) of near-bottom currents at a depth of 4220–4250 m. Bottom relief and structure of the sedimentary strata Profile M–RM is 450 miles long (Fig. 5). Its main part ex- The South America continental slope in the B–RGR profile tends from point 578. The floor of the ocean crossed by this pro- (Fig. 3) is acoustically “hard”, without soft sediments at a depth file is 4600 to 4900 m deep. The foot of the volcanic mountain up to 3730 m, or “transparent”, i. e. covered by soft sediments. RM (010°32’ S, 23°00’ W) is the deepest point of the profile. Below 3750 m the bottom is “hard” again. Such a character is Three main channels (cuts) are recognized on the sub- marked in the Parasound acoustic profiles up to a hard terrace meri­dional (M–RM) profile (Fig. 4): 1) Equatorial mid-Ocean (depth 4580–4590 m) of the Vema channel. The depth of this Channel (EMOC); 2) unnamed channel AI (“Akademik Ioffe”) in channel in the B–RGR profile is 4640 m. Its left (western) slope points 592–593 (part 4) on the profile, the relative depth 116 m is very high (4160 m) and precipitous. Consequently, both the (132 m relative to the northern wall) (the depth of the water col- shape and the depth of the Vema channel in the B–RGR profile umn from the floor up to the ocean surface is 4892 m); and 3) are different from those in Fig. 4 according to Johnson (1984). channel B (Brazilian) in the southernmost part of the profile, the Johnson’s profiles cross the channel at the latitude 29 °S, and our relative depth 70 m (130 m relative to the northern wall) (Fig. 6). profile crosses it at the latitude 31 °S. The rest channels are significantly smaller and their relative Sediments on the Rio Grande Rise (a layer thicker than depth is 30 to 40 m. Thus, channel AI (4892 m) is the deepest 3–5 cm) were found in small depressions. In one of these depres- one (relative to the ocean surface) in the study area. Its depth is sions, at a depth of 1626 m, mixed sand–silt shelly-foraminiferal very similar to that of a trough-like trench in the north, which is sediments found. referrend to as NT (4943 m).

Fig. 1. Schematic presentation of water mass distribution and general circulation terns in the Atlantic Ocean after, Dietrich & Ulrich, 1968; Dietrich et al., 1975, (see Bleil and Thiede, 1990, p. 735). AABW: Antarctic Bottom Water; AAIW: Antarctic Intermediate Water; CDW: circumpolar Deep Water; MW: Mediterranean Water; NADW: North Atlantic Deep Water; SIW: Sub-Arctic Intermediate Water; PF: Polar Front; SC: Subtropical Convergence. RA – research area 1 pav. Vandenų masės pasiskirstymo schema Atlanto vandenyne pagal Dietrich & Ulrich, 1968; Dietrich et al., 1975 (žr. Bleil, Thiede, 1990, p. 735) Sedimentation and near-bottom currents in the South-Western Atlantic 277

Fig. 2. GEBCO bathymetric chart of the South-Western Atlantic (200; 4000 and 5000 isobaths, m). Legend: 1–3 – Echosounding and Parasound profiles (research vessels – 1–2 – Akademik Ioffe, 8 and 11 cruises; 3 – Akademik Sergey Vavilov, 17 cruise); 4 – sites of Parasound profiles (and their numbers), investigated in details; 5–10 – geological stations of research vessels: 5 – Piotr Lebedev (PL); 6 – Professor Shtokman (PSh); 7 – Dmitry Mendeleev (DM); 8 – Akademic Sergey Vavilov (ASV); 9 – Akademic Kurchatov (AK); 10 – Akademik Ioffe (AI); 11 – drill holes of Glomar Challenger (GCH); 12 – drill holes of Joides Resolution (JR); 13 – isobaths, m; 14 – Antarctic diatoms in sediments. Cut-in A: Canyons and channels in the S–W Atlantic (after Emery a. Uchupi, 1984). Position of the profile M-RM is shown. Abreviations on the charts: MAR – Mid-Atlantic Ridge; RGR – Rio Grande Rise; B – Brazil; E. ch. – EMOC channel (8); AI–AI channel (4); B – Brazil canyon (or channel); T – trench. Lithological profiles Bk–Bk; B–B and I–I are shown on the bathymetric chart 2 pav. Pietvakarių Atlanto vandenyno batimetrinis žemėlapis (200, 4000 ir 5000 m izobatės, geologinės stotys ir litologiniai profiliai) 278 Emelyan M. Emelyanov

Fig. 3. The schematic bottom relief through the Vema and Hunter channels and Rio Grande Rise (RGR). Positions of the obtained cores were shown 3 pav. Schematizuotas dugno reljefas Vimos kanale ir rytinėjė terasoje (pagal Jonson, 1984)

Fig. 4. Seismic profiles across Vema Channel­­ axis and eastern terrace near 29 ºS (after Johnson, 1984) Most crossings of the channel show an ir­ regular acoustic basement with up to several hundred meters of relief, and a strong, relatively smooth mid-section ref­ector near 6.5 s (Refector A. This refector probably denotes the onset of AABW fow through the Vema Channel) 4 pav. Seisminis profilis per Vimos kanalo ašį ir rytine terasą 29 °S rajone (pagal Jonson, 1984)

It is very significant that the EMOC lies in the highest part The northern slope of the channel gradually falls north- of the ocean floor on the M–RM profile, to say nothing about wardly from its highest mark to 4940–4950 m (01°31’ : 947 S; single volcanic mountains. This elevated part of the ocean bot- 29°58’ : 810 W). Such factor as stretching of the EMOC chan- tom on the M–RM profile represents the central sedimentary nel over a strong swell makes it clearly distinguished from swell (CSS). This swell supposedly has the NW–SE trending, channels AI, B and others, which have no such swells in their just like the EMOC channel. The CSS is 180–190 m in height. grounds. The EMOC channel is asymmetric: its northern wall is 59 m The cross-sectional profile of channel B (Brazilian) is very higher than the southern one, and its depth is 4564 m (Fig. 6). similar to that of the EMOC channel (Fig. 6). The right-hand Sedimentation and near-bottom currents in the South-Western Atlantic 279

(northern) wall of the channel is oval in shape and rises 59 m above the southern wall, i. e. like in the case of the EMOC channel. The form of the cross-section of the channel AI is different from that of the Al (Academic Ioffe) Channel; Ioffe) (Academic Al

– EMOC and the B channels: steep (cut) is the southern but not the northern wall. It should be noted that the southern wall rises only 9 m over the northern one. Around the M mountain (in the south- ernmost point of the M–RM profile) there are depressions on the bottom surface Ch. Al. Channel; Equatorial – (Fig. 5), caused by the non-deposition (or Ch weak deposition) of sediments. Such deep channels and canyons have not been found in the sub-latitudinal profile A–MAR (Fig. 7). The deepest point of the profile (4765 m) lies in the plain between volcanic mountains (05°13’ N, 38°45’ W). In volcanogenic mountain; E. mountain; volcanogenic

– general, in the Nara plain the depths range mostly within 4660–4640 m (Fig. 8), i. e. they are about 100 m smaller than those in the southern part of the M–RM profile and 300 m than the passage near the base of the RM mountain in the northern part (NT trough) of this profile. As evidenced by a bathymetric map (Fig. 2), in the Searra plain the depth ranges from 4300 to 4500 m. In the A–MAR profile, the maximum depth of this plain is 4423 m (see point 1540 in Fig. 7). To the south-west of the Searra rise, which is a natural obstacle for the distal part of the Amazon River cone, the depths are as large as 4200–4150 m (Fig. 8) and are lo- RM (Ridge mountain). After Emelyanov, 2005. Depth in m, VM m, in Depth 2005. Emelyanov, After (Ridgemountain). RM

– cally found throughout the extension of the A–MAR sub-latitudinal profile, and these depths are 400–450 m smaller than those of the Nara plain.

Lithology and stratigraphy of sedimen- tary strata Mixed terrigenous-foraminiferal (or fo- raminiferal–terrigenous) mud which con-

tains 30–50% of CaCO3 is distributed at the base of the Rio Grande Rise and of the Brazil continental slope (Fig. 9). The core AI-1054 (depth 4370 m) from the Hunter channel is represented by terrigenous low-

calcareous (10–30% CaCO3) aleuro-pelitic

mud and calcareous (30–50% CaCO3) foraminiferal-terrigenous mud. The cal- careous part of the mud is represented by foraminiferal shells and coccoliths. The mud is layered or weakly layered. This feature means that the hydrodynamic and (mountain) M profile submeridional the on Atlantic Equatorial Western the of relief Bottom maybe chemical environments underwent points, for which coordinates are given. are which coordinates for – points, numbers without circles figures: on the separate shown for which sedimentary – part are Numbers in circles Channel; strata – Brazilian Ch. on the profile, B. Fig. 5. Fig. mountains sedimentary6 – volcanogenic with turbidities; surface strata bottom 5 – layered occur; banded; 3 – intermediate surface 2 – bottom strata is smooth, bottom hard; 1–6 – types surface: of the bottom strata surface bottom 1 – bottom is rough, zonoje ekvatorinėje vandenyno Atlanto Vakarų reljefas dugno M–RM submeridianinio profilio 5 pav. changes. 280 Emelyan M. Emelyanov

Fig. 6. Channels and canyons in the Equatorial Atlantic. After Emelyanov, 2005. 8 – Equatorial Mid-Ocean Channel (EMOC) between Northern and Southern Atlantic at part 8 (points 615 and 617) of M–RM profile. MA – proposed border of Amazon Messinian. Area Q of cross-section of the Channel is 0.780 km2. 3 – and 4 – profiles of B and A. I. channels at parts 3 and 4 of M–RM profile (see Fig. 5) 6 pav. Kanalai ir kanjonai ekvatorinėje Atlanto van- denyno dalyje Sedimentation and near-bottom currents in the South-Western Atlantic 281

The surface sediments at a depth of 4194–5602 m in the

Brazil Basin are represented by pelagic red clay (<10% CaCO3).

The calcareous (30–50% CaCO3) and carbonate (>50% CaCO3) nano-foraminiferal oozes, covering only the elevations and hills of the Brazil Deep, occur on the flank of the MAR (depth 2899–5000 m) also (Emelyanov et al., 1975). Such a distribution of the sediments allows to conclude that carbonate compensation depth (CCD) in the Brazil Basin occurs a little more higher than in the other regions of the Atlantic. The CCD level in the Brazil Deep varies within 4400–5000 m. It is higher on the South Americal side (4400 m) where the AABW stream is much stronger than on the MAR side (4900–5200 m, Lisitzin et al., 1979). The colour of the red clay in the Brazil Basin changes from strictly brown to greenish-gray or dark grey. There were bed- dings of these two types of clay in the cores. The core ASV-1535 (Fig. 10) contained Pleistocene–Holocene clay corresponding to the fifth nanoplankton zone (Svalnov et al., 2007). The redeposited Paleogene–Pliocene Discoasters, ceratholites and other remains were found in all the core. There was red clay (Emelyanov et al., 1975) or miopelagic clay (according to Svalnov et al., 2007) in the core ASV-1536. The

clay contained the minimal quantities of CaCO3 (0.09–0.91%)

and Corg (0.03–0.33%). The FeMn nodules were found on the surface of the clay. There were a lot of Mn microconcretions in the clay. The age of the clay was not determined. The fine (pelitic) light brown red clay was obtained in the core

ASV-1537 (0–27 cm). It contained 0.09–1.25% CaCO3 and 0.05– –4 0.28% Corg. and up to 329 · 10 % arsenic. Brown-red clay with

thin laminae of diatomic (Ethmodiscus) ooze (0.05–0.54% CaCO3,

0.06–0.21% Corg.) occurred in the 247–309 cm horizon. The thick- ness of the laminae was 3–10 mm, and their colour varied from greenish-grey to dark grey. Siliceous remains were represented mainly by Ethmodiscus rex. The layer 309–407 cm was represented

by Ethmodiscus (rex) ooze which contained up to 74% of SiO2total. Formation of siliceous ooze is going on due to episodi- cal development of Ethmodiscus diatoms during glacial stages (mainly in the near-shore upwelling zone of Africa, Emelyanov et al., 1989) and their transporation to the pelagic equatorial ar- eas of the ocean. During transporation, they settle on the bot- tom and are resuspended many times. The Etmodiscus ooze in the core ASV-1537 was supposedly formed during the Late Pleistocene. The microlayering of the oozes was formed during the changes of the physical and chemical environments. Active deposition of diatomic oozes in the Equatorial Atlantic began in the Cenomanian (Emelyanov et al., 1989). Their deposition was most active in the Eocene. These oozes formed not only at the high, but also at the middle and low latitudes. The core ASV-1538 contained pelagic red clay (0.07–0.32%

CaCO3; 0.07–0.14% Corg). There were FeMnN (about 2 cm in diame- ter) in the upper part of the core and a lot of Fe–Mn microconcre- tions in clay of all the core. Zeolites and bones also occurred in it. The same clay was characteristic of the core ASV-1539

(0.16–0.84% CaCO3; 0.05–0.35% Corg.). There were also remains of Radiolaria, spiculae and fish bones.

Dark brown-red clay (0.16–1.00% CaCO3; 0.08–0.35% Corg.) was discovered in the layer 0–160 of the core ASV-1540. There were

°N. After Emelyanov, 2005 Emelyanov, °N. After along 5–4 Ridge) profile (Mid-Atlantic A–MAR on the longitudinal profile relief bottom 7. The Fig. koordinates platumos reljefas pagal 5–4º š. kalnagūbris) dugno Atlanto profilio A–MAR (Vidurio 7 pav. some small greyish-yellow lenses in the clay (horizon 135–160 cm). 282 Emelyan M. Emelyanov

Fig. 8. Structure of sedimentary strata at sites 211 and 1450 of the A–MAR profile (see Fig. 7). 211 – the lower distal part of the Amazone cone 1450 – structure of the sedimentary strata in the Nara Plane. There is sedimentary evidence of AABW stream with the lowest temperature and salinity in the right – hand part of the Nara Plane 8 pav. Nuosėdų struktūra 211 ir 1450 taškų rajonuose, A–MAR profilyje (žr. 7 pav.) Sedimentation and near-bottom currents in the South-Western Atlantic 283

Fig. 9. Surficial (0–5 cm) bottom sediments in the area of Rio Grande Rise (RGR).

Legend: 1–5 – sediment types: 1 – sand; 2 – coarse aleurite; 3 – fine-aleuritic mud;4 – aleuro-pelitic mud; 6–8 – content of CaCO3 in the sediments: 6 – <10% (terrigenous); 7 – 30% (terrigenous- low calcareous); 8 – 50% (border between mixed calcareous terrigenous and carbonate); 9 – hydrological stations; 10 – geological stations; 11 – DSDP drill holes; 12 – position of Parasound profile B-RGR with the numbers of seismic sites; 13 – polygon of Vema channel; 14 – direction of AABW fow 9 pav. Paviršinės (0–5 cm) dugno nuosėdos Rio Grande pakilumos (kalnagubrio) akvatorijoje

The layer 160–1998 cm was represented by greyish-yellow red Clay of the core ASV-1542 is Pleistocene in age (Pseudoeuno­ clay (0.20–0.69% CaCO3; 0.18–0.14 Corg) with dark-brown lens- tia doliolus zone). It was deposited during the last 700,000 years. es. The border 198–218 cm was sharp and rough. Yellowish red The lithology of the cores of B–B and I–I profiles was almost clay occurred in the horizon 218–260 cm (0.57–7.06% CaCO3; the same as of the cores in Bk–Bk profile, but there was no dia-

0.15–0.16 Corg). The lower horizon (260–510 cm) was repre- tomic ooze in these cores. sented by yellowish-grey and yellow red clay containing 17.59– The core AK-414 in the profile I–I penetrated as deep as

26.49% CaCO3; 0.12–0.40% Corg). Grey and dark-grey interlayers 265 cm from the sea-bed surface and revealed continuous de- of red clay with an elevated content of hydroxides occurred in posits of brown red clay without any signs of layering or coars- the horizon 418–450 cm. er sediments (Fig. 9). This clay is fair, fine and contains 79.34 to

Redeposited coccoliths (Discoaster brouweri, D. tamalis, 82.12% of fraction <0.01 mm. The content of CaCO3 in this clay

D. pentaradialis), prevailed in the red clay of the core ASV-1540. is 0.00–0.23% (1.75% in the 0–6 cm horizon); Corg 0.21–0.45%;

A lot of them were found in the horizons of 260 and 470 cm. The Fe 5.28–5.56%; Mn 0.06–1.43%; P 0.06 0.07%; SiO2am 0.68–1.71%. Miocene species of Discoaster quinqueramus Gart., D. kugleri It is a typical composition of the red clay of the Atlantic. Hay and others were also present. All these facts show that sedi- Station AK-416 (depth 4570 m) is located at the smallest ments were rewashed and redeposited actively from the nearest distance from the acoustic profile. This core revealed very dense hills of the bottom, whose depth was below the CCD level. The brown red clay. Core AK-416 is actually found below the level of red clay of the core ASV-1540 was dated to the lower part of the CCD. Here, clay is non-carbonaceous, and a 270 cm-long sediment Late Pleistocene (Svalnov et al., 2007). core is composed of clay with distinct interlayers and lenses of Red clay of brown colour was discovered in the core sand and coarse aleurite (silt). These interlayers are found in the

ASV-1541 (0.05–0.25% CaCO3; 0.05–0.13% Corg). High FeMnN horizons 91–94, 104–112, 157–160, 240–241, 268–270 cm (Fig. 2). levels were found in the upper layer of the clay and small nod- Coarser and well-sorted sand is found in the horizon 104–112 cm. ules in the clay strata. There were grains of philipsite, frag- Core AK-419 from the continental slope (depth is 2760 m). ments of bones, radiolarians, and spicules of sponges. It comprises the only interlayer of sand (horizon 41–47 cm). The The core ASV-1542 was represented by: horizon 0–125 cm – remainder of the core is represented by gray terrigenous mud. dark brownish-grey siliceous-clayey mud (0.16–6.40% CaCO3; The lithology of the three described cores implies that a

0.19.55% Corg), horizon 15–478 cm – red clay of mixed colour strong jet of AABW flew just in the region where core AK-416 with abundant radiolarians and diatom frustules (0.16–6.40% was collected. Sand and aleuritic interlayers present in the

CaCO3; 0.19–0.55% Corg). According to author’s new data, the sedimentary cores suggest that the sand-aleuritic material was content of CaCO3 in the horizon 55–65 cm is more than 10% and transported both along and from the side of the continental reaches 11.85%. It contains 20–23% of diatoms and up to 25% of slope, because there are no signs of sorting of coarse interlayers; radiolarians (from the bulk sediment) in the layer 15–478 cm. also, no sand interlayers were found in core AK-419. Diatoms (about 50 species) are represented by Azpeitia nodulif­ The upper layer of red clay present in the vicinities of sta­tions er, Rhizosolenia alata, Rh. styliformis, Hemidiscus cuneiformis. AK-416 and AK-414 is thought to have deposited during the HI Red clay in the horizon 30–80 cm is calcareous (16.19–30.89% or at the very end of the Wisconsin, while clays with coarse in-

CaCO3; 0.30–0.60% Corg). Its colour changes from grey to light terlayers (91–270 cm) have deposited in the Wisconsin when the yellowish grey, with patches of dark grey spots. near-bottom current AABW was much stronger than in the HI. 284 Emelyan M. Emelyanov

Fig. 10. Schematic bottom relief and lithological profiles A, B and C in the central part of Brazil Basin. CCD – carbonate compensation depth (Hl – in Holocene; Plt – in Pleistocene). 1–5 – granulometrical sediment types: 1 – sand; 2 – coarse aleurite; 3 – fine-aleuritic mud; 4 – aleuropelitic mud; 5 – pelitic mud (clay); 6–9 – types of pelagic clay: 6 – brown, reddish, red clay; 7 – light yellowish, yellowish gray, gray; 8 – brown or reddish-brown with the patchiness or lenses of light colour; 9 – light ( osvetlionnaya), brown clay with the patchiness of brown colour ( relict brown red clay); 10 – siliceous (Ethmodiscus) ooze; 11 – nano-foraminiferal ooze (>50% CaCO3): 12–16 – remnants in the clay: 12 – forams; 13 – diatoms; 14 – pteropods; 15 – shelly; 16 – vegetal; 17 – ferro-manganese nodules; 18 – FeMn microconcretions; 19 – iron-manganese interbeds or lenses; 20 – sharp border; 21 – gradual border 10 pav. Brazilijos baseino vidurinės dalies dugno reljefas ir litologiniai profiliai A, B ir C Sedimentation and near-bottom currents in the South-Western Atlantic 285

Terrigenous coarse aleurite with abundant remnants of Brazil Basin) present at a depth more than 4800 m is character- chilly material and thin sandy interlayers (conturites or tur- ized by a high concentration of silica – more than 110 μmol · kg. bidites?) lies on the bottom of the continental slope (stations The isoline 34.8 psu and the isotherm 1.8 °N occur at a depth AK-405, –403 and – 402-1). of 3500 m in the Vema and the Hunter channels (Fig. 12). This Station AI-43 (depth 4534 m) lies on the B–B profile from is the highest layer of the AABW. The temperature of the near- the MAR side. The core is represented by the foraminiferal (aleu- bottom water in the both these channels (3900–4000 m) is fall ro-pelitic) ooze. This means that the AABW does not flow here up to 0.2 °C, and salinity – up to 34.7 psu. This means that the and the CCD position is lower by some hundreds meters than in main flux of the AABW goes through the Vema and the Hunter the base of the continental slope of South America. According to channels. However, the bottom structure and the sediments our previous data (Lisitzin et al., 1977), the CCD nowadays oc- don’t confirm this proposal: the occurrence of the muddy Late cur at a depth of 5200 m in the latitude zone 10 °S–10 °N of the Quaternary sediments with abundant calcareous foramini­ Western Atlantic. fera shells in cores AI-1044 and AI-1047 (depth 4370–4113 m) The core PL-20 (5000 m) was obtained near the eastern part (Fig. 2) indicates that at a depth of 4100–4300 m the flux of the of the Northern trough NT (28 miles to the west from the point AABW in the Hunter channel, if this water occurs there, is very 644 on the profile M–RM which extends along the meridian, weak. According to Speer’s and Zenk’s data (1993), the cold near- 30 °W) (Fig. 2). The 170-cm-long core is represented by grey ter- bottom water flows deeper than 4400 m in the Hunter channel. rigenous mud with numerous sandy interbeds and lenses. This However, no sediments from this depth were obtained. evidences that the area was exposed to active hydrodynamical The absence of soft, layered bottom sediments in the Vema processes; turbidity currents are believed to be responsible for channel and its acoustically hard bottom say that the strong- the well-sorted layers. Unfortunately, the Parasound profile is est current of the AABW occurs here. This current is so strong located very far (30 km) from the station, and we couldn’t see that it rewashes soft sediments completely and does not allow turbidity interlayers on the seismogrammes. to deposit them even during weak stages of the AABW in this channel. The Vema channel has an asymmetric cross-section Near-bottom currents, resuspension and redeposition of (its right slope is steeper). This phenomenon allows to suppose sediments that the right slope is mostly eroded. The current being closer to Antarctic bottom water (AABW) in the Brasil Basin (Figs. 1 the right slope is due to Ekman’s law (Morozov, 2007). Notably, and 2) moves northward as a continuous flow 1000 km wide warming of the AABW takes place during the last decades. This and about 700 m thick (Fig. 11). This water is found between warming of the cold stream of the AABW could be see not only 4500 m and the bottom (Andrie et al., 1998). According to Hall’s in the Vema channel itself, but also along the channel at a dis- data (Hall et al., 1977), the AABW occurs between 4500 km and tance of 700 km: from 0.135 °N in the south to 0.084 °C in the the bottom in the latitudes 2 °S–2 °N. The top of these waters is north. This warming began in 1972 and is going on till now found due to the isotherm 0 ≤ 1.8 °C. The AABW at 4 °30'S (the (Morozov, 2007).

Fig. 11. Sketch of the circulation patterns of AABW in the Brazil Basin. The hatchet arrows indicate infow inferred from others’ studies. Thin lines delineate the limits of the Deep Western Boundary Current (DWBC) and the return fow. After Madron De a. Weatherly, 1994. RGR – Rio Grande Rise; ch – channel. 1 – resuspension and redeposition of sediments; Pl – Pleistocene; 3 – redeposited Antarctic diatoms (AABW indicators) in the sediments (after Johnes and Johnson, 1984); 4 – contourites; 5 – vegetal remnants in the sediments; 6 – turbidities; 7 – fows of AABW (arrows added by author); ER – erosion of the bottom 11 pav. Brazilijos baseino AABW cirkuliacijos modelių schema 286 Emelyan M. Emelyanov

Fig. 12. Schematic illustration of the mean zonal cur- rent fow in the Vema Channel region, based on the hydrography and current measurements near 31 ºS and at other transects crossing the channel axis (after Johnson, 1984). 5 – strong current to the north (V > 20 m/sec); 6 – weak current to the north; 7 – weak current to the south; 8 – weak current to the unknown direction 12 pav. Apibendrinta vandenyno masių srovių schema Vimos kanalo akvatorijoje netoli 31º pietų platumos. 1 – kanalo ašis, 2 – vakarų terasa, 3 – rytinė terasa, 4 – Rio Grande pakiluma (pagal Jonson, 1984). Ant terasų banguota linija parodytas banguotas dugno paviršius (nuosėdinės bangos)

To the north from the Vema channel and Rio-Grande Rise, by a large concentration of silica, low temperature and other pa- the AABW seems to be driven to the base of the Brazil continen- rameters (Andrie et al., 1998, p. 919). tal slope formed near the slope of the Deep Western Boundary In part, AABW moves northwestward, as well as along the current (DWBC) (Fine, Johns, 1992; Molinari et al., 1992; northwestern flank of the CSS parameters (Andrie et al., 1998. Madron De, Weatherly, 1994) (Fig. 11). The main stream of the p. 918). The part of the AABW shown as a thin northward-ori- DWBC at the latitude 10°30’S, where the 5000 m isobaths are ented arrow in Fig. 11 (Madron De, Weatherly, 1994, p. 635) en- closest to the base of the continental slope as evindenced by the ters channel B; its formation is very similar to that of the EMOC: composition of the sediment cores, moves mostly between sta- the southern (left-hand) wall of channel B is erosive, while its tion AK-414 (depth 4570 m) and the 4500 m isobath. higher right-hand wall is accumulative (Fig. 6). We found no clear lithological-geochemical indicators of the The suspended particulate matter (SPM) transported by AABW in the sediments. Only presence of Antarctic diatoms in near-bottom currents, and the SPM which is re-suspended by the sediments of the western part of the Brazil Basin, such as the erosion of the bottom and the slopes of the channels, are Nitzschia kerguelensis, Eucampia antarctica, Coscinodiscus len­ dispersed over the large equates outside of the channels. The tiginosus and others, shows the path of this water (Fig. 11). there most intensive deposition of this material is going on along the are more diatoms in the interglacial sediments than in the sedi- main ocean channel EMOC where the central sedimentary swell ments of the glacial stages (Johnes, Johnson, 1984), showing that (CSS) is forming. The nearer to the channel the higher the rate of during the glacial stages the AABW flow was weaker. sedimentation and the CSS is becoming bathymetrically higher. The character of the surface of the bottom relief forms and The layering of the acoustically transparent sediments and non- the presence of layering of sediments and its features confirm the transparent borders between them occurs on both sides of the presence of active hydrodynamics near the bottom in the South- channel. There is no deposition in the channels themselves: the Western Atlantic, including the central part of the Brazil Basin, bottom of them is represented by acoustically non-transparent where the bottom sediments (clay) were sometimes rewashed or deposits (pre-Quaternary rocks). The channels cut the sedimen- redeposited. Consequently, the AABW flows over the bottom be- tary strata as deep as 120–160 m from the sea bottom. We can came stronger or weaker. The occurrence of channels such as the see that the layers of the sediments are interrupted on the steep EMOC evidenced strong near-bottom currents (“strong under- slopes, they are not bent down as its characteristic of the less water rivers”) and the transport to the Northern hemisphere not steep and higher SSW slopes of the EMOC and B channels. only of Antarctic water and heat (or cold), but also of suspended This means that the SSW slopes are cutting and the NNE particulate matter. In the deep mid-ocean channels, there are slopes are growing. This growth is going on due to deposition places of non-deposition of sediments (zero deposition) and of the flux of the sedimentary material by the valley near-bot- places of bottom erosion or deposition–erosion. tom flows. The character (features) of the layered sediments at In the region of 5 °S, the AABW current is estimated at 4.7 Sv the base of the right (northern) channel slope indicates that (Rhein et al., 1995). Two-thirds of these waters move eastward, the stream (flow) in the EMOC valley consists of two levels: the mainly into the Romanche trench, or they are involved once first (not so fast) at 4610–4720 m (the upper part of the V-shape again into circulation in the Brazil Basin (Madron De, Weatherly, cross-section of the channel) and the second, narrower but very 1994; Rhein et al., 1995): in this basin at 04°30’S, a portion of fast (strong) at a depth of 4730–4753 m, i. e. over the bottom of the AABW now turns eastward and southward (Fig. 11). The the valley. The fast stream, like a saw-tooth, cuts the bottom of remainder of the flow continues northward. In the region near the channel’s valley 500–700 m wide and throws to the foot of 05°00’ S and 31°30’ W, this northward flow merges with the the northern (right) slope the sedimentary matter transported EMOC (Rhein et al., 1995). by the stream. The scheme of the hydrodynamics, characteristic The AABW flow occurs in the deepest part of the EMOC of the upper (main) part of the EMOC, is repeated: the accumu- (depth 4550 m) at the point of 00o19’N. The AABW can be recog- lation of sedimentary matter to the northern (NNE) side of the nized in the deepest part of the EMOC (depth 4450 m), 00°39’N, valley, i. e. on the right slope (Fig. 6). Sedimentation and near-bottom currents in the South-Western Atlantic 287

Approximately the same structure occurs in channel B, but network in South America during the Late Miocene (Cisielski, channel B lacks the lower (valley) layered sedimentary strata like Weaver, 1983) when ice cover was formed in the Antarctic. in the EMOC, because the cross-dimension of channel B is much The AABW started to appear in the Rio Grande Rise region in narrower: the base of the channel is only 600–800 m wide. the Paleogene, about 50–30 mill. years BP (Johnson, Rasmusen, We may suppose that streams cut the EMOC and B chan- 1984, p. 255–256). The erosional Vema channel was formed at nels. Channel AI was cut by the strong turbidity currents flowing the same time. down from the South American continental slope. Cyclic events of glaciation and interglacial transitions throu­ The construction of channel AI is somewhat different from ghout the Miocene-Pleistocene are the mechanism that caused that of the EMOC and B channels. In channel AI, erosive is the the AABW currents to become more intensive or passive, with northern wall but not the southern one. Also, on the northern the result that the intensity of the influx of these waters from the wall of this channel, a levee is absent, and on the southern wall Brazilian Basin into Guiana Basin also changed from strong (the such levee is rather low (relative to the right wall) – only 10 m depth of the roof of these cold water masses decreased by 300– high. This shows that turbidity currents move through the AI 400 m) to weak. This suggests that the passage of these waters over channel and the concentrations of suspended matter in turbidity a sill represented by the Searra and the Nara plains was facilitated currents in this channel are lower than those in currents flowing in times of sea-level changes during glaciation. For example, dur- through the EMOC and B channels. ing the last glaciation (about 23,000 years ago), the roof of cold, The forms on the ocean floor of the Northern Trench (NT) deep water present in the Southern Atlantic was by 400 m shal- and the depression near the base of RM (Fig. 5) evidence a lower than at present (Emelyanov, 2005; Emelyanov et al., 1989). strong bottom current, supposedly eastward. It is clear that the main masses of cold AABW are transported from the Brazil Conclusions Basin eastward, into the Angola Basin, through the Romanche trench. The trench with horizontal bottom is periodically filled The upper part of the Brazil continental slope (up to 2930 m), due up with turbidites. This hypothesis is confirmed by the core to active hydrodynamics, is “hard” with no soft sediments. The wa- PL-20 which consists of terrigenous turbidites and sandy- ter strata between 2930 and 3600 m are more or less calm. aleuritic lenses with remnants of vegetation (Emelyanov, 2005). At a depth of 3770 m, the bottom is “hard” again: soft sedi- Also, strong currents flowing in the northwestward direction ments occur in the depressions only. The bottom is rewashed by are bathymetrically higher than the NT floor, i. e. their depth is part of the AABW flow. no more than 4860 m (Fig. 5). At a depth of 4860–4750 m they The Vema channel (depth 4230–4590 m) is rewashed by the come in contact with the floor of CSS or, more precisely, with strong AABW stream. The velocity of this stream sometimes ex- the bottom surface of its northern slope which is reduced by ceeds 100 m · s–1. Only these strong currents could fully rewash the EMOC. In this part of the ocean floor, the current is clearly soft sediments and weak consolidated pre-Quarternary depo­sits. turbulent; bottom currents of such type give rise to the undu- The soft layered sediments occur on the slopes of the Hunter lating topography of the ocean bottom. The development of this channel. The bendt sediments allow to conclude that bottom “turbulence” is related to events involving contact between the currents are not constant here. AABW current and the lower parting of the CSS. At a depth of Our data confirm previous conclusions that the AABW flows 4500–4570 m (the upper part of CSS), high velocities of bottom mainly through the Vema channel. The upper layer of this water currents preclude the deposition of layered sediments (or any lies at a depth of 3500 m. The NADW occurs above this level. sediments at all) on the ocean floor. These are exposed to bot- Brown red or yellowish grey miopelagic (layered) clay pre- tom erosion. Sites 5, 14, 2, 1 and other parts of the M–RM profile vails in the Brazil Deep. Nano-plankton remains show that the (Fig. 5) experience the same processes of bottom erosion. age of red clay in the cores is different (early or Late Pleistocene, The construction of the channel B is approximately the same or Holocene). Clay was rewashed and re-deposited in many ar- (Fig. 6). The difference is that, unlike the EMOC channel, layered eas of the Deep. This means that hydrodynamics sometimes was sedimentary strata on the channel bed are absent, possibly be- very active at a depth of 4000–5000 m in the Brazil Region. cause of the diminished cross-section of channel B: its floor is The presence of conturite and turbidite interlayers in the red only 600–800 m wide. clay of the S. American continental base indicate that a strong jet The geographical position of the channels EMOC and B, their of the AABW (Deep Western Boundary current – DWBC) oc- extension and cross-sectional profiles, and also the presence of a curred here. sedimentary levee on the right-hand sides (to the north) evidence Antarctic diatoms Nitzshia kerguelensis antarctica, Eucampia that turbidity currents (“underwater rivers”) move in the south- antarc­tica, Coscinodiscus lentiqinosus and others were brought north­ward direction, i. e. these channels are the pathways for the in by the AABW from the Antarctic up to 10–5 °S. AABW. An unusual Pleistocene Ethmodiscus rex ooze was discow- ered in the core ASW-1537 (m) at the latitude 20 °S. The origin Some aspects of palaeooceanology of the ooze is unknown. Circulation of near-bottom waters between the Northern and the Our data confirm the occurrence in the area between Southern Atlantic, through the “Equatorial Gates”, is thought to 10 °S–5 °S of two mid-oceanic channels, one of them (EMOC) be- have started during formation of a deepwater passage between ing located on a large sedimentary swell. The AABW in the cross- Africa and South America (Eocene?). This was the time of the section from the Amazon River to the MAR flows through the striking of the Andes mountains and rearrangement of the river Nara (depth 4640–4660 m) plain and later to the Guiana Basin. 288 Emelyan M. Emelyanov

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The deep west- sound“ buvo gauti trijų pagrindinių profilių – (1) Brazilija – Rio Grande ern boundary current in the tropical north Atlantic ocean. pakilimas–Hantero kanalas (B–RGR); (2) Brazilija (Resifi uostas) (NNE) Deep Sea Res. 39. 1967–1984. (Brazilijos profilis M–RM) ir (3) nuo Amazonės žiočių į rytus iki Atlanto 16. oudot C., Morin P., Baurand F., Wafar M., Corre P. le. vidurio kalnagūbrio (MAR) (Amazonès profilis) reljefo echogramos ir 1994. Distribution of silicate, phosphate and nitrate in the geofiziniai nuosėdų storymės pjūviai. Iš B–RGR profilio ir Bk–Bk lito- Equatorial Atlantic Belt. Deep Sea Res. 1(45). 873–902. loginio profilio Brazilijos įduboje bei kituose rajonuose buvo paimta 30 17. Rhein M., Stramma L., Send U. 1995. The Atlantic Deep nuosėdų kolonėlių (iki 2–5 m ilgio). Bk–Bk profilio kolonėlėse, remiantis Western Boundary Current: water masses and transport nanoplanktono duomenimis, buvo išnagrinėta biostratigrafija. near the equator. J. Geoph. Res. 100. 2441–2457. Geologiniai ir geofiziniai duomenys patvirtino Vimo kanale stiprios 18. Supko P. R., Perch-Nielsen K. 1977. General synthesis of antarktinių vandenų srovės (AVS arba AABW) egzistavimą. Srovės grei- central and south drilling results. Leg. 39. Deep Sea Drilling tis – apie 20 cm · s–1, bet epizodiškai jis turėtų viršyti 100 cm · s–1, ka- Sedimentation and near-bottom currents in the South-Western Atlantic 289 dangi tik tokia srovė gali eroduoti konsoliduotas nuosėdines kvartero осадков (длиной 2–5 м). В колонках профиля Бк–Бк изучена стра- ir prekvartero uolienas. тиграфия осадков по данным нанопланктона. Hantero kanale AVS aptinkamos tik didesniame negu 4200–4300 m Геолого-геофизические данные подтвердили наличие сильно- gylyje, mažesniame AVS neaptikta. го потока Антарктической донной воды АДВ (или ААBW) через Nano planktono reliktai patvirtina stiprių srovių egzistavimą vi- канал Вима. В канале Вима скорость потока обычно составляет durinėje Brazilijos įdubos dalyje. Srovės išplauna ir perneša pelaginius около 20 см · с–1, но эпизодически эта скорость должна превышать raudonuosius molius (PRM), neleisdamos jiems susikloti ant dugno. 100 см · с–1, так как только такая скорость способна эродировать Daugelyje įdubos vietų nuosėdose aptinkami pernešti ir susikloję pleis- кон­солидированные четвертичные и дочетвертичные породы. В toceno, neogeno ir paleogeno diskoasterai. Daugelyje kolonėlių PRM ка­нале Хантер потоки АДВ (AABW) обнаруживаются на глубинах yra pleistoceno amžiaus. более 4200–4300 м. Они не наблюдаются на глубинах менее 4200 м. Pietų Amerikos panuovalio papėdėje rasti konturitų sluoksneliai Нанопланктонные остатки в осадках Бразильской котловины (išrūšiuotas aleuritai ir smėlis), kurie susiklostė giluminės vakarų srovės подтверждают наличие сильных потоков придонных вод в цент­ (DWBC) dėka. Pietų Amerikos žemyninio panuovalio apatinėje dalyje ральной её части. Эти потоки либо размывают ранее нако­пившиеся vienoje kolonėlėje rasti turbiditai. Turbiditų pėdsakai su augalų reliktais красные глубоководные глины (КГГ), либо не позволяют им aptikti P1-20 kolonėlėje į vakarus nuo Romanšo lūžio. Mūsų duome- накапливаться в настоящее время. Во многих местах котловины nys patvirtina Ekvatorinio vidurio vandenyno kanalo EVVK (angliš- обнаруживаются переотложенные плейстоценовые, неогеновые и kai – EMOC) egzistavimą, taip pat dar du panašius kanalus – B ir AI. B ir палеогеновые дискоастеры. В большинстве колонок в Бразильской EVVK kanalų sandara panaši. EVVK kanalu į šiaurę teka AABW vande- котловине КГГ являются плейстоценовыми. nys. AI Kanalas greičiausiai yra ne Vidurio vandenyno kanalas, o kanjo- У подножья материкового склона Южной Америки в КГГ об- nas – vieno Brazilijos žemyninio panuovalio kanjono tęsinys. Antarktikos наружены прослои контуритов, представленных сортированны- priedugnio vandenys (APV) išilgai žemyninio panuovalio perneša ми песчано-алевритовыми прослоями. Это результат воздействия Antarktidos diatomėjas iki 10–5°S. ASV-1537 kolonėlėje (gylis 5000 m) на дно Глубинного Западного течения (DWBC) – западной ветви rasta diatominių (Ethomidiscus rex) dumblių. Per A–MAR Amazonės АДВ. В нижней части материкового склона Южной Америки в profilį APV nuteka į Gvianos įdubą tiktai per mažą slėnį Naros lygumoje одной из колонок обнаружены турбидиты. Следы турбидитов (с (gylis 4620 m). Tą patvirtina ir hidrocheminiai duomenys. растительными остатками) обнаружены также у западной части Searros kalnagūbrio (pakilimo) rajone, taip pat Amazonės išnašų разлома Романш (колонка ПЛ-20). konuse APV pėdsakų neaptikta. Подтверждены наличие в западной экваториальной части Ат­ Antarktikos vanduo Rio Grande pakilumos rajone pasirodė pa­ лантики ранее открытого Срединно-Океанического канала (ЭСОК), leogene (eocene), tuo metu pradėjo formuotis Vimos Kanalas. Mak­si­ а также наличие еще двух подобных каналов В и АИ. Канал В имеет ma­lios APV srovės buvo prieš apledėjimo epochas (ypač per izotopines сходное с ЭСОК строение. По каналу ЭСОК текут АДВ на север. Ка­ 7/6 ir 3/2 stadijas). нал АИ, очевидно, не является собственно срединно-океаническим. Mesinos gliacialiniai įvykiai (prieš 7,3–5,5 mln. m) sąlygojo (?) Он предположительно является продолжением одного из каньонов, kritinio karbonatų lygio (angliškai – CCD) ciklus, kurie tęsėsi (su 30– берущих начало на материковом склоне Бра­зилии. 50 tūkst. m. periodais) ir kvartero metu. АДВ переносят антарктические виды диатомей вдоль запад- Amazonės išnašų kūgis (konusas) pradėjo formuotis apytikriai ного склона Южной Америки до 10–5о ю. ш. На широте 20о ю. ш. prieš 15 mln. metų. Per tą laiką į Atlanto vandenyną buvo atnešta maž- (станция АСВ-1537, глубина 5000 м) обнаружены плейстоценовые daug 5 · 105 mlrd. tonų išnašų. Susiformavęs milžiniškas kūgis užtvėrė диатомовые (Ethmodiscus rex) илы. kelią priedugnio vandeniui tekėti į vakarus nuo Searros kalnagūbrio. АДВ на Амазонском профиле А-MAR (от Амазонки до САХ) перетекают в Гвианскую котловину только по одной долинке рав­ Емельян м. Емельянов нины Нара с глубинами 4620 м. Это подтверждено и гидрохими­ чес­кими данными. В районе возвышенности Сеарра, а также Осадконакопление и придонные течения в конуса выносов Амазонки геологических следов перетока АДВ в Юго-Западной Атлантике Гвианскую котловину не обнаружено. Резюме Антарктическая вода в районе возвышенности Рио-Гранде На основании собственных и литературных данных по рельефу стала появляться в палеогене (эоцен). В это время стал фор­ми­ дна и строению осадочной толщи выявлены литологические ро­ваться и канал Вима. Потоки АДВ были мак­си­маль­ными во приз­наки придонных течений в Юго-Западной Атлантике (пре­ времена, предшествовавшие сильнейшим эпохам оле­де­нения имущественно в Бразильской котловине). (осо­бенно в изотопные стадии 7/6 и 3/2). В экспедициях на судах „Академик Иоффе“ и „Академик Сер­ Мессинские гляциальные события (7,3–5,5 млн. ЛН) обусло- гей Вавилов“ выполнен эхолотный промер и изучено строение вили цикличность вертикальных колебаний критической глуби- осадочной толщи с помощью сейсмопрофилографа „Парасаунд“ ны карбонатонакопления (КГК). Эти колебания продолжались и в на трех основных профилях: 1) Бразилия – возвышенность Рио- четвертичное время с периодичностью в 30–50 тыс. лет. Гранде – канал Хантер (профиль Б-ВРГ); 2) от Бразилии (порт Конус выносов Амазонки стал образовываться примерно Ресифи) на ССВ (Бразильский профиль M-RM)); 3) от устья Ама­ 15 млн. л. н. За это время в Атлантический океан было вынесено зонки на восток до САХ (Амазонский профиль А–МАR). На про- около 5 · 105 млрд. т твердого вещества. Конус приобрел колоссаль- филях Б-ВРГ и литологическом Бк–Бк, а также в других районах ные размеры и преградил путь придонным водам к западу от воз- Бразильской котловины было отобрано и изучено 30 колонок вышенности Сьерра.