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Fig. 7.1. Geography, glaciology and bathymetry of the Ross Sea region. Bathymetry in metres.

The dominant glacial feature in the Ross Sea is the Ross Ice Shelf (Fig. 7.1). It is the glacial terminus of ice draining an area of approximately 2.3 x 106km2, and its drainage basin includes both the east and west Antarctic ice sheets (Fig. 7.1).

Ice flowing into the Ross Sea from East Antarctica must make its way either over or through the Transantarctic Mountains, and is channelled into rapidly flowing ice streams. The largest of these is David Glacier, which extends presently some 70 km off the coast as a vast ice tongue, the Drygalski Ice Tongue (Fig. 7.1). David Glacier is the largest glacial outlet in the Ross Sea. For the most part, the ice sheet in the region is ponded behind the Transantarctic Mountains. Drainage into the Ross Sea is mainly from large valley glaciers. The largest of these include Tucker Glacier, Borchgrevink Glacier, Mariner Glacier, Aviator Glacier, Campbell Glacier, Priestley Glacier and Reeves Glacier (Fig. 7.1).

Because of their much smaller drainage basins and channel characteristics, these glaciers flow into the Ross Sea at slower rates than David Glacier. They therefore form smaller ice tongues. Ice tongues such as the Borchgrevink, Mariner and Aviator are protected from wave erosion by perennial sea ice in the northwestern Ross Sea.

South of the Dry Valleys, the East Antarctic Ice Sheet flows into and becomes part of the Ross Ice Shelf (Fig. 7.1). The Ross Ice Shelf is the largest ice shelf in the world (total surface area is approximately 5.4 x 10s km2). Its grounding line is situated some 960 km inland from its calving line. It produces the largest icebergs on average in the Ross Sea region and its steep calving wall occupies approximately 10% of the Antarctic coastline. The largest icebergs that calve from the Ross Ice Shelf are as large as or larger than the ice tongues of the northwestern Ross Sea and have drafts in excess of 200 m, although shallower drafts may be more common. Some thick icebergs calved from ice tongues, however, have drafts of the order of 300 m (Keys, this volume).

East of the King Edward VII Peninsula, large outlet glaciers flowing down from the mountains of Marie Byrd Land converge to form the Sulzberger Ice Shelf (Fig. 7.1). It is much smaller (1.6 x lO'km2) than the Ross Ice Shelf and its calving wall stands only 6-10 m high, compared to the 20-50 m high calving wall of the Ross Ice Shelf. Icebergs calved from the Sulzberger Ice Shelf therefore have much shallower drafts, probably on the order of 40 to 60 m.

Surface sediments of the Ross Sea region have been examined in detail by Stetson and Upson (1937), Chriss and Frakes (1972), Glasby et al. (1975), Anderson et al. (1980, 1984b), Barrett et al. (1983), Dunbar et al. (1985, 1989), Kellogg and Kellogg (1988), Leventer and Dunbar (1988), Anderson and Smith (1989), Karl (1989) and Harwood et al. (1989). Piston cores from the Ross Sea continental shelf typically terminate in cohesive sediments believed to be basal tills (products of subglacial sedimentation). This interpretation is based on the fact that these deposits are characterized by high cohesive strength, a lack of sorting, lack of stratification, absence of marine fossils (other than reworked fossils), pebbles that are rounded relative to those found in glacial marine sediments, and textural and mineralogical homogeneity within individual units. Kellogg et al. (1979) and Anderson et al. (1980, 1984b) have given more detailed descriptions of these glacial deposits.

Basal tills extend almost to the continental shelf edge. This indicates that the ice sheet was at one time grounded on most of the shelf. In fact, a detailed analysis of the mineralogies and pebble lithologies of Ross Sea basal tills has revealed that distinct petrographic provinces do occur on the continental shelf and these have been used to reconstruct the late Wisconsinan (18,000 yrs B.P.) glacial drainage regime of the marine ice sheet in the Ross Sea region (Anderson et al., 1984b). These data support the paleodrainage model of Denton and Hughes (1981), which calls for ice sheets grounded at the continental shelf edge at this time.

Glacial-marine sediments contain a significant quantity of material deposited from floating ice, either ice shelves, ice tongues or icebergs. This ice-rafted debris is associated with mixtures of pelagic and terrigenous sediments. Variations in the concentration of these different components reflect the relative influence of glacial, biological and oceanographic processes (Chriss and Frakes, 1972; Barrett, 1975; Anderson et al., 1980). Fig. 7.2 shows a surface sediment distribution map for the Ross Sea.

The actual fate of sediment transported to the coast by glaciers is still problematic. What we do know is that the concentration of ice-rafted debris in surface sediments of the continental shelf is highly variable and decreases markedly seaward of the shelf break (Anderson et al., 1979, 1984a). There is a consensus amongst glaciologists that the basal layer of ice shelves (and tongues) is melted before reaching the calving line where icebergs are produced (Robin, 1979; Thomas, 1979a; Drewry and Cooper, 1981; Drewry, 1986). This conclusion is based mainly on theoretical considerations, but is supported by physical oceanographic observations which indicate net melting at the base of the Ross Ice Shelf (Jacobs et al., 1979) and by the absence of a basal debris zone in a hole drilled through the Ross Ice Shelf (Drewry and Cooper, 1981). Furthermore, surface sediments collected near the calving line of the Ross Ice Shelf contain only minor quantities of ice-rafted debris (Dunbar et al., 1985).

Most basal glacial debris probably melts out near the grounding line of ice shelves and glacier tongues (Drewry and Cooper, 1981; Drewey, 1986). Relict sediments formed by melting of the basal debris layer are believed to occur throughout the Ross Sea region (Anderson et al., 1984b) and modern sediments deposited in this manner have been sampled from beneath the Mackay Glacier Tongue in the southwestern corner of the Ross Sea (Macpherson, 1986, 1987). They are similar to basal tills in that they are unsorted to very poorly sorted, generally massive, and display textural and mineralogical homogeneity downcore. They differ from basal tills only in that they are not overcompacted and they frequently contain marine fossils.

Modern glacial marine sediments of the Antarctic continental shelf consist of two distinct groups. The first group includes fine-grained sediments composed of a mixture of terrigenous silt and clay and pelagic material (mostly diatom frustules) and associated ice-rafted debris. These sediments have been termed "compound glacial marine sediment" by Anderson et al. (1980) because they represent a combination of sedimentation by marine currents, pelagic sedimentation, and ice rafting in relatively low energy settings (Chriss and Frakes, 1972). Near the coast, aeolian material may comprise a significant component of surface sediments (Barrett et al., 1983). The proportion of ice-rafted debris in these sediments varies appreciably but is most concentrated in nearshore regions bound by outlet glaciers and ice streams, such as in the westernmost Ross Sea, off the Pennell Coast, and off the Marie Byrd Land Coast (Anderson et al., 1984a). Dirty icebergs and glacial ice show that debris containing layers of ice tongues is not always melted out before their calving zone is reached (J.R. Keys, pers. comm.). During a dozen expeditions to the Antarctic region, I have observed many sediment-laden icebergs on the continental shelf but most were within a few tens of kilometres of the continent, and only rarely were they in the open ocean.

A second group of glacial marine sediments is composed of poorly-sorted mixtures of sand and gravel with various amounts of calcareous, bioclastic sand and gravel (mainly foraminiferal tests and fragments of bryozoans, barnacles, molluscs, corals and echinoderms). These sediments reflect ice-rafting in areas where marine currents are sufficiently strong to remove fine-grained material, either as it settles through the water column or via bottom erosion and resuspension (Chriss and Frakes, 1972; Anderson et al., 1980, 1984a). They are therefore termed by Anderson et al. (1980) "residual glacial marine sediment". Residual glacial marine sediments occur on the outer continental shelf (Fig. 7.2), where impinging deep-sea currents are influential sedimentary agents (Anderson and Smith, 1989) and on relatively shallow (< 300 m) banks and inner shelf regions where wind-driven currents influence sedimentation (Anderson et al., 1984a). Residual glacial marine sediments are typically associated with relatively well-sorted sands, which are the products of bottom current transport. These sediments indicate that relatively strong marine currents are active to depths of up to 300 m (Anderson and Smith, 1989).

Perhaps one of the most intriguing aspects of modern surface sediments of the Antarctic continental shelf is that ice-rafted debris is typically a minor component of these sediments, certainly relative to ancient deposits of the shelf. In fact, siliceous muds and oozes consisting of 10 to 40% biogenic silica, mainly diatom frustules, and less than 10% ice-rafted debris are widespread on the shelf and are the dominant sediment type in shelf basins (Fig. 7.2). Accumulation rates for these siliceous sediments are as high as 2.5 mm/yr in the western Ross Sea, and biogenic silica accumulation in this area is comparable to that of low-latitude upwelling environments (Ledford-Hoffman et al., 1986). In general, the biogenic silica content of surface sediments increases from east to west across the Ross Sea shelf and in an onshore direction (Truesdale and Kellogg, 1979; Dunbar et al., 1985). Siliceous muds and oozes occur as far south as McMurdo Sound (Fig. 7.2). This distribution pattern is attributed to greater productivity in the western Ross Sea, probably due to less persistent sea ice cover there (Truesdale and Kellogg, 1979; Anderson et al., 1984b), and to redistribution of fine-grained sediments by marine currents (Dunbar et al., 1985). Siliceous sediments are also concentrated in glacial troughs off the Pennell Coast (Fig. 7.2). Their occurrence there results from marine currents sweeping fine-grained sediments from the shallower portions of the shelf and into these small basins (Anderson et al.; 1984a).

On the inner shelf of the eastern Ross Sea and in the Sulzberger Bay area, fine-

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•SAMPLE LOCATIONS

Fig. 7.2. Surface sediment distribution map for the Ross Sea region. CGM = glacial marine sediments with a fine-grained matrix, RGM = glacial marine sediments with a coarse-grained matrix, SiM = siliceous mud, SiO = siliceous ooze, S = sand, G =gravel, Z = silt, C = clay, d = diatomaceous, z = silty, c = clayey, s = sandy, and m = muddy. Dots show sample locations.

•SAMPLE LOCATIONS

Fig. 7.2. Surface sediment distribution map for the Ross Sea region. CGM = glacial marine sediments with a fine-grained matrix, RGM = glacial marine sediments with a coarse-grained matrix, SiM = siliceous mud, SiO = siliceous ooze, S = sand, G =gravel, Z = silt, C = clay, d = diatomaceous, z = silty, c = clayey, s = sandy, and m = muddy. Dots show sample locations.

grained sediments (clayey silts) with very little biogenic and ice-rafted material (< 10% of each) are accumulating (Fig. 7.2). These fine-grained sediments are attributed to redistribution of suspended material by marine currents in the absence of significant biogenic sediment input (Dunbar et al., 1985). Nittrouer et al. (1984) argued that the volume of terrigenous sediment accumulating on the Ross Sea floor today is comparable to that of continental shelves which have major fluvial dispersal systems.

In summary, the surface sediment distribution pattern in the Ross Sea indicates that only in nearshore areas adjoining outlet glaciers and ice streams is ice-rafting an important contributor to marine sediments. Ice-rafted debris is a minor component of sediments collected adjacent to the calving wall of the Ross Ice Shelf. Fine-grained sediments consist of both terrigenous material and siliceous biogenic material. These fine-grained sediments are swept from the outer shelf into inner shelf basins where they accumulate rapidly. Shallow banks are covered by coarse-grained residual sediments comprised largely of bioclastic (carbonate) material. These sediments indicate relatively strong marine currents at depths above approximately 300 m.

Surficial sediment dispersal patterns for the remainder of the Antarctic margin of the Pacific are poorly known. The only other detailed work has been conducted in the area of Bransfield Strait at the northern tip of the Antarctic Peninsula (Anderson et al., 1984a) and in Marguerite Bay between longitudes 66°W and 70°W on the Antarctic Peninsula (Kennedy, 1987). In these areas, glacial marine sediments occur on the shallow portions of the continental shelf, and diato-maceous muds and oozes occur in shelf basins and in bays and fjords of the peninsula (Griffiths and Anderson, 1989). Bioclastic sediments, similar to those which blanket the shallower portions of the Ross Sea floor, have not been cored. Recent work in the fjords and bays of the Antarctic Peninsula region has led to the observation that meltwater is a significant factor in the delivery of sediments to the sea only in the South Shetland Islands (Anderson et al., 1987).

Continental Slope Sedimentation

Sedimentation on the Antarctic continental slope has been described in detail by Anderson et al. (1979). Unfortunately, only a few sediment cores have been collected on the Antarctic slope in the Pacific and most of these were collected in the Ross Sea. It is therefore not possible to describe sediment distribution patterns on the slope. However, sediment types, and therefore processes, can be described for the Ross Sea continental slope.

The Ross Sea continental slope is divided into two distinct physiographic portions with Iselin Bank the dividing line (Fig. 7.1). East of Iselin Bank, the slope is relatively gentle (average gradient of 2°) and unbroken by submarine canyons. West of Iselin bank, the slope is steeper with the upper slope averaging 5° and the lower sloper 1.5°, and is dissected by several submarine canyons (Fig. 7.3a).

Surface sediment samples from the upper continental slope of the western Ross Sea (to depths of approximately 1,000 m) consist of sandy, gravelly mud and bioclastic sands and gravels. Bottom photographs from the upper slope also show

Fig. 7.3. Single-channel (sparker) seismic profiles showing (a) a submarine canyon situated on the upper continental slope off Iselin Bank, and (b) large slumps on the continental slope, again off Iselin Bank
Fig. 7.4. Bottom photographs from the western Ross Sea continental slope showing (a) rippled sands (Eltanin camera station 2 in 1,720 m of water), and (b) gravelly lag deposits (Eltanin camera station 7, in 2,040 m of water).

surface lag deposits and current-rippled sands (Fig. 7.4). The upper slope of the Ross Sea is therefore apparently subject to relatively strong bottom currents (Dunbar et al., 1985; Anderson and Smith, 1989). Piston cores from the region have penetrated thin (< 1 m) graded sands. Both detrital and bioclastic carbonate sands occur, and are apparently derived from nearby Iselin Bank. Turbidity currents are therefore important agents on this part of the Ross Sea continental slope. Seismic lines from the slope show huge slumps (Fig. 7.3b), which is another important mechanism of sediment transport on the steep, upper slope.

Only two piston cores have been collected on the upper portion of the eastern Ross Sea continental slope, and these penetrated poorly-sorted sandy, pebbly muds which show no evidence of current winnowing. Marine currents therefore appear to be effective in winnowing sediments on the upper slope but not on the lower slope in this region. Four piston cores collected on the lower, eastern slope penetrated unsorted sediments (diamictons) and laminated silts. Sedimentary properties of these diamictons and displaced foraminifera led Kurtz and Anderson (1979) to conclude that they are debris flows, probably transported down-slope during the last major glacial advance on to the shelf. Laminated silts were possibly deposited by sediment-laden bottom currents. They indicate more sluggish bottom currents than exist on the upper slope in this region today.

ABYSSAL SEDIMENTS Distribution Patterns

The distribution pattern of abyssal sediments on the Antarctic Sector of the Pacific Ocean floor has been mapped and described by several workers (Goodell, 1968,1973; Lisitzin, 1962,1970; Nayudu, 1971; Piper et al., 1985). The sediment distribution maps generated by these authors were used in conjunction with the results of my own examination of surface sediments to compile the surface sediment map shown in Fig. 7.5. The following discussion is based largely on this map.

Terrigenous silts and clays occupy a broad belt extending from the Antarctic continenal slope to well out on to the abyssal floor. These hemipelagic sediments are typically poorly-sorted, polymodal, and include both laminated and massive, frequently bioturbated units. They consist mainly of detrital quartz, and clay minerals are mainly well-crystallized chlorite and illite (Goodell, 1973). Ice-rafted debris, volcanic ash, and ferromanganese micronodules are minor constituents of these sediments.

A belt of terrigenous sediments is also situated offshore of the Peru-Chile margin, but is much narrower than that which occurs off Antarctica. This is largely due to the existence of the Peru-Chile Trench which traps much of the sediment being transported offshore from the continent. To the west, these sediments grade into carbonate ooze. The distribution of these sediment types is regulated by the Carbonate Compensation Depth (C.C.D.), with calcareous ooze occurring above approximately 4,500 m and pelagic clay below this depth. As off

South America, terrigenous sands and silts are virtually confined to the continental shelf and slope of New Zealand. Except for a belt of pelagic clay situated along the northeastern margin of New Zealand, these terrigenous sediments grade offshore into pelagic carbonates.

North of the terrigenous silt and clay belt that surrounds Antarctica, there is a broad belt of siliceous ooze, which consists primarily of diatom frustules. The southern limit of the siliceous ooze belt corresponds approximately with the surface water 0°C isotherm and the northern limit with the Antarctic Convergence (Fig. 7.5; Goodell, 1973). North of the convergence, a relatively narrow belt of mixed diatom-calcareous ooze (foraminiferal ooze) occurs. North of the mixed ooze belt, foraminiferal ooze blankets the abyssal floor to a depth of approximately 4,500 m (the C.C.D.) (Goodell, 1973; Piper et al., 1985). Brown pelagic clays are accumulating in those portions of the Southeastern Pacific Basin and Southwestern Pacific Basin that are situated below the C.C.D. Unlike the terrigenous sediments that occur in belts off Antarctica and South America, these pelagic clays consist almost exclusively of clay minerals, zeolites and detrital grains (Skornyakova and Petelin, 1967; Nayudu, 1971). Montmorillonite is the dominant clay mineral. The dominant zeolite in these sediments is phillipsite, which may comprise upwards of 50% of the sediment by volume, and pyroxene, plagioclase, and opaques are the dominant mineral grains (Nayudu, 1971). These pelagic clays may also contain abundant fish bones (Nayudu, 1971). Hemipelagic clay and calcareous clay also occur in a broad belt west of the South American continent, within the deeper part of the Southeastern Pacific Basin.

Abyssal Sedimentary Processes

A number of factors contributes to the distribution of surface sediments on the deep seafloor of the South Pacific Ocean. Among these are the offshore flux of fine-grained terrigenous sediment from the continents (by ocean currents and turbidity currents), surface productivity of biogenic material, dissolution of biogenic sediments at depth, reworking by marine currents, and, to a much lesser degree, ice-rafting and authigenic mineral formation (Lisitzin, 1962, 1970; Nayudu, 1971; Goodell, 1973).

The Antarctic continent is producing an abundance of terrigenous silt and clay, substantially more than the other continents of the South Pacific. The mineralogies of surface sediments on the seafloor in the area of the Antarctic Peninsula show that the peninsula is a major source of terrigenous sediment for the southeastern Pacific Ocean (Edwards, 1968). The subpolar glacial maritime setting of that region is conducive to glacial erosion and transport of sediment, the occurrence of meltwater being a leading contributor to this supply. South of approximately 65°S, a polar glacial regime exists, and the delivery of sediment to the sea by surface meltwater streams is virtually nonexistent.

It is also noteworthy that the belt of terrigenous sediments surrounding the continent is much broader around West Antarctica than East Antarctica (Goodell, 1973). This is explained, in part, by the fact that the vast ice shelves which drain into the West Antarctic margin have a larger glacial drainage basin, and therefore

Fig. 7.5. Surface sediment distribution map for the South Pacific Ocean. Dots show sample locations. T = terrigenous siltsand clays, Tz = sandy turbidites, DO=diatomaceousoozeand mud, CO=calcareous ooze and mud, PC=pelagic clay, La = gravelly or sandy lag deposits. Bathymetry in metres.

a larger sediment source area, than do glaciers draining into any given portion of the East Antarctic margin. The exception to this is the Amery Ice Shelf and adjacent continental margin. Furthermore, the glacial setting of West Antarctica (wet-based marine ice sheets) is more conducive to sediment erosion and transport than the dry-based terrestrial glacial regime of East Antarctica (Drewry, 1986). The seaward extent of the terrigenous sediment belt around Antarctica is also a function of the efficiency with which these sediments are being transported offshore by marine processes, a subject about which we know relatively little. It is, however, clear that turbidity currents have played a significant role in delivering terrigenous sediments to those portions of the South Pacific seafloor situated near Antarctica, and this mechanism is probably active today (Wright et al., 1985).

The continental slopes and rises of the Amundsen and Bellinghausen Seas are quite extensive and are dissected by numerous submarine canyons (Dangeard et al., 1977). These canyons extend well out on to the abyssal floor and are major conduits through which terrigenous sediments are supplied to the deep (Dangeard et al., 1977). The role of turbidity currents in delivering terrigenous sediments to the deep seafloor is expressed in the form of deep-sea fans that occur on the Bellingshausen continental rise and abyssal plain (Wright et al., 1984). Two large submarine fans, the Charcot and Palmer Fans, have been mapped (Tucholke and Houtz, 1976; Dangeard et al., 1977). These fans systems are apparently inactive today, as turbidites are mainly confined to canyons (Wright et al., 1984). Disorganized gravels and graded gravels and sands cored within these channels consist of glacially-striated grains and include a variety of mineral and rock types. Recent petrographic work by Baegi (1985) has shown that the mineralogical compositions of these deposits vary from canyon-to-canyon, and these data indicate direct input of glacial debris to canyon heads by glaciers during a previous glacial maximum when ice was grounded at or near the shelf break. Piston cores collected from interchannel areas contain finely laminated, very fine sands, silts and clays, interpreted as overbank deposits.

In general, hemipelagic sediments become finer in an offshore direction, away from the Antarctic continent. This simply reflects increasing distance from the source of these sediments. The ice-rafted component of these sediments decreases sharply away from the Antarctic continent (Anderson et al., 1979), and ice-rafted debris is virtually lacking in bottom sediments north of the Antarctic Convergence (Goodell, 1973). This is due to the accelerated decay rate of icebergs as they are subjected to the relatively warm (> 0°C) and rougher surface waters situated just offshore of the continental shelf, and to the tendency of icebergs to drift parallel to the continent once they encounter the circumpolar currents situated seaward of the shelf break.

The siliceous ooze belt shown in Fig. 7.5 results from high surface productivity. It also marks the northern limit of terrigenous sediment transport by turbidity currents. This boundary between siliceous ooze and hemipelagic sediments also corresponds roughly to a major divergence in abyssal circulation. South of this boundary, abyssal currents flow in a westward direction along the Antarctic continental margin while, to the north of this boundary, abyssal flow is more eastward (Heezen and Hollister, 1971). Terrigenous sediments that are trans ported northward from the Antarctic continental margin may be entrained by westward flowing currents, thus restricting their distribution to the north. North of this boundary, siliceous sediments are transported to the east during their final descent to the seafloor.

The northern boundary of the diatomaceous ooze belt marks the northern limit of bottom waters that are undersaturated with respect to calcium carbonate and where calcareous biogenic sediments mask siliceous sediments (Lisitzin, 1970). In the central South Pacific, this boundary corresponds approximately to the position of the mid-ocean ridge, which impedes northward flow of corrosive Antarctic Bottom Water (Heezen and Hollister, 1971).

Nayudu (1971) and Zemmels (1978) carried out detailed geochemical analyses of South Pacific abyssal sediments. Nayudu found that pelagic clays there are enriched in minerals and elements of volcanic origin. He also concluded that the distribution of diatomaceous and terrigenous sediments is influenced by abyssal circulation and that dilution of terrigenous sediment by biogenic phases is the main factor regulating their concentration on the seafloor. Several studies have indicated the possible occurrence of metalliferous sediments of hydrothermal origin at the crest of the Pacific-Antarctic Ridge (Zemmels, 1978; Glasby et al., 1980; Stoffers et al., 1985).

The dispersal of fine-grained sediments once they reach abyssal depths can, in part, be inferred from bottom photographs. A large collection of bottom photographs exists, particularly for the Southeastern Pacific Basin, as a result of early Eltanin and Conrad cruises. These were studied by Goodell (1968) and Heezen and Hollister (1971) for visible evidence of bottom currents, and their combined results are shown in Fig. 7.6. These results show that the Southeastern Pacific Basin may be subdivided into three east-west oriented belts. Photographs taken on the abyssal floor nearest the Antarctic continental margin display evidence for weak bottom currents (Fig. 7.7). This area corresponds approximately to the belt of fine-grained terrigenous sediments situated north of the margin (Fig. 7.5).

North of the low energy depositional belt, bottom photographs display evidence of strong to moderate bottom currents in the form of deflected sessile organisms, scour around pebbles and nodules, current lineation, and ripples. A few photographs taken in the vicinity of oceanic fracture zones and the mid-ocean ridge show exposed basement rocks. This zone of bottom scour also corresponds to an extensive field of manganese nodules (Fig. 7.6). Bottom samples from this region also show rippled foraminiferal sands and gravelly lag deposits. Surface sediment samples from the area reveal that both sediment types contain ferro-manganese micronodules and manganese-coated sand and gravel of ice-rafted origin. Megaripples and ripples in predominantly sand-sized sediments and scour around features on the seafloor imply current velocities of several tens of centimetres per second (Goodell, 1973). Elsewhere, below the C.C.D., this scour zone is characterized by siliceous oozes containing ferromanganese micronodules and manganese-coated ice-rafted grains. Magnetostratigraphic analysis of piston cores from the South Pacific has shown that Brunhes age sediments are thinner, and locally absent, in this sector of the abyssal floor (Goodell and Watkins, 1968).

The South Pacific Scour Zone (Fig. 7.7) is associated with eastwardly transport

Fig. 7.6. Map showing the distribution of scour zones, manganese nodule distributions, and inferred bottom current information in the South Pacific Ocean. Areas in which lag deposits and rippled sands occur is shown with a stippled pattern. The dotted lines show the outer limits of seafloor that is covered by manganese nodules (Piper et al., 1986). The dashed lines show the limits of the South Pacific Scour Zone (from Heezen and Hollister, 1971). Also shown are inferred bottom current directions and speed (from Heezen and Holister, 1971).

Fig. 7.6. Map showing the distribution of scour zones, manganese nodule distributions, and inferred bottom current information in the South Pacific Ocean. Areas in which lag deposits and rippled sands occur is shown with a stippled pattern. The dotted lines show the outer limits of seafloor that is covered by manganese nodules (Piper et al., 1986). The dashed lines show the limits of the South Pacific Scour Zone (from Heezen and Hollister, 1971). Also shown are inferred bottom current directions and speed (from Heezen and Holister, 1971).

Fig. 7.7. Bottom photographs and nephelometer profiles from a transect across the southeastern Pacific Ocean (data from Sullivan et al., 1973). Muddy bottom with tracks and trails occurs north of the South Pacific Scour Zone, and manganese nodules occur within the scour zone. The muddy seafloor near Antarctica is covered by ice-rafted stones. A bottom nepheloid layer extends throughout the area, except in the northern part of the scour zone. Nephelometer data are recorded with a photometering/transmission densitometer. Optical density is converted to log-exposure using a calibration curve constructed from sensitometer patches. The log exposures for two scattered-light strips are averaged and the log exposure for one of the direct strips is subtracted from the average to obtain E/ED See Sullivan et al. (1973) for additional discussion of methods used to obtain and quantify nephelometer measurements.

Fig. 7.7. Bottom photographs and nephelometer profiles from a transect across the southeastern Pacific Ocean (data from Sullivan et al., 1973). Muddy bottom with tracks and trails occurs north of the South Pacific Scour Zone, and manganese nodules occur within the scour zone. The muddy seafloor near Antarctica is covered by ice-rafted stones. A bottom nepheloid layer extends throughout the area, except in the northern part of the scour zone. Nephelometer data are recorded with a photometering/transmission densitometer. Optical density is converted to log-exposure using a calibration curve constructed from sensitometer patches. The log exposures for two scattered-light strips are averaged and the log exposure for one of the direct strips is subtracted from the average to obtain E/ED See Sullivan et al. (1973) for additional discussion of methods used to obtain and quantify nephelometer measurements.

of bottom water through gaps in the Pacific-Antarctic Ridge, along the deepest part of the abyssal plain, and through the Drake Passage (Heezen and Hollister, 1971). North of this scour zone is a belt where bottom photographs reflect weak bottom current energy (Fig. 7.7), and where fine-grained siliceous oozes and pelagic muds blanket the seafloor (Fig. 7.5).

Nephelometer profiles collected along a N-S transect across the eastern edge of the Southeastern Pacific Basin crosses the South Pacific Scour Zone (Fig. 7.7). Bottom nepheloid layers are associated with the two low energy zones, whereas there is no bottom nepheloid layer associated with the northern portion of the scour zone. The nepheloid layer situated nearest the Antarctic continent is associated with westwardly directed bottom currents (Fig. 7.6), and probably consists of fine-grained terrigenous sediments derived from the continent. In the central region, the bottom nepheloid layer is less extensive (Fig. 7.7). This area is associated with eastward flowing bottom currents (Fig. 7.6). The northern sector has a near bottom nepheloid layer that is probably comprised of sediment derived from the South American continent and is associated with bottom currents which flow to the south (Fig. 7.6).

During R.V. Conrad Cruise 15, a series of nephelometer profiles and bottom photographs was gathered along a transect across the northern portion of the South Pacific Ocean (Fig. 7.8). These data, together with the surface sediment map for the region, can be used to illustrate some of the important sedimentological provinces and processes in an east-west transect across the basin. The seafloor in the northeastern part of the region (between 150° and 140°W) is floored by manganese nodules (Fig. 7.8), and there is no bottom nepheloid layer in this area. The transect crosses an area of basalt outcrop at 138°W, which separates the manganese nodule fields to the west from a relatively featureless seafloor (Fig. 7.8) which has a near bottom nepheloid layer (Fig. 7.8). Sediments in this area consist of pelagic clays (Fig. 7.5). Between approximately 130° and 90°W, the seafloor is covered with tracks and trails (Fig. 7.8) and the bottom sediments consist of calcareous clay and ooze (Fig. 7.5). Rock outcrops occur where the transect crosses the mid-ocean ridge (Fig. 7.8). There is no near bottom nepheloid layer in this central province (Fig. 7.8). A bottom nepheloid layer occurs within about 500 m of the bottom in the eastern portion of the East Pacific Basin, and coincides approximately with the boundary between calcareous clays and calcareous ooze (Fig. 7.5). This nepheloid layer probably consists of terrigenous sediments derived from the South American continent.

More recently, Schmitz et al. (1986) have determined sediment accumulation rates in the Southwestern Pacific Basin and demonstrated the importance of erosion and sediment focusing by Antarctic Bottom Water (A.A.B.W.) there.

Ferromanganese Deposits

The distribution and abundance of manganese nodules on the floor of the Antarctic Sector of the Pacific have been mapped by a number of investigators (Goodell, 1968, 1973; Goodell et al., 1971; Glasby, 1976a; Piper at al., 1985). Geochemical studies of nodules from this region have been reported by Goodell et al. (1971), Glasby (1976b) and Meylan and Goodell (1976).

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