Abyssal characteristics of the World Ocean waters

Deep-SeaResearch,Vol.30, No. 8A, pp. 805 to 833, 1983. Printedin Great Britain.

0198-0149/83 $3.00 + 0.00 (~ 1983 PergamonPressLtd.

Abyssal characteristics of the World Ocean waters ARNOLD W . MANTYLA* a n d JOSEPH L. REID*

(Received 7 September 1982; in revisedform 15 February 1983; accepted 1 March 1983) Abstract--The abyssal characteristics of the World Ocean, including not only temperature but salinity, density, oxygen, and silica, are displayed on both maps and vertical sections to examine the origins of the waters of some of the major basins. Although the coldest waters that appear at the bottom in each of the oceans have long been known to have come from the Southern Ocean, the characteristics indicate that the major component of the abyssal waters of the World Ocean does not derive directly from the abyssal Antarctic but from the shallower Circumpolar Water (CPW). The CPW is a mixture of Antarctic waters with the warm, saline, oxygen-rich, and nutrient-poor deep waters from the North Atlantic. As the CPW extends northward it is modified by mixing with the overlying waters, which in the North Atlantic and North Indian oceans are more saline and in the North Pacific less saline. Except for the Antarctic area, the northern North Atlantic Ocean is the major source of oxygen to the deep-ocean waters. The abyssal waters of the Northeast Pacific are farthest from regions of ventilation and are the most nearly uniform and may be the oldest of the abyssal waters.

INTRODUCTION

THE purpose of this study is to provide maps of some of the abyssal characteristics for which adequate data exist and to discuss them, both in the light of earlier work and in terms of the new information they provide. Our major result, aside from the maps themselves, is to point out that over most of the ocean north of the Antarctic basins, the abyssal waters do not derive directly from the densest Antarctic waters but from the overlying Circumpolar Water (CPW). While this may not be a new finding to many readers, it merits emphasis. The abyssal waters of the Atlantic, Indian, and Pacific oceans are characterized by low temperatures and are clearly of southern origin. Surface waters of the North Pacific and North Indian oceans do not become dense enough to penetrate to the bottom there, and the dense water from the Norwegian-Greenland Sea is evident at the bottom of the North Atlantic only as far south-as the Grand Banks. Mapping the distribution of various characteristics at the bottom of the ocean identifies the most extreme bottom water values. Such maps have been useful in suggesting the pathways of bottom water spreading. However, their characteristics cannot be traced back to identical values in Antarctic surface water. Instead, they have been modified by subsurface mixtures with waters from other sources. Layers that lie well above the bottom in some areas may fill the abyssal depths in other areas. As a result, the abyssal waters do not derive from a single source, or even from other abyssal domains. The patterns, as will be seen, need not result from successive modifications of an upstream abyssal layer. As the maps are of characteristics at the bottom, alteration of bottom * Marine Life Research Group, Scripps Institution of Oceanography, University of California, San Diego, La Jolta, CA 92093, U.S.A. 805

806

ARNOtA) W, NIANt'YI,A and JOSEPtt L. Rf-ID

water by vertical mixing can take place only by mixing with the overlying waters, In the southern hemisphere, the deep water lying above the bottom water originates from the Circumpolar Current (REID, NOWLIN and PATZERT, 1977). WARREN (1981a) reviewed the history of deep-water studies and provided a background on the development of ideas on abyssal circulation. M a n y maps of bottom characteristics in various oceans and basins have appeared in the oceanographic literature; a few authors have presented bottom maps for the entire World Ocean (W0ST, 1939; LYNN and REID, 1968; OLSEN, 1968). WUST'S (1939) maps of the bottom potential temperature at depths >4000 in in the World Ocean could draw upon only a limited data base, but they compare well with the more recent maps. One major discrepancy in his map is in the a.rea of the Northwest Pacific, where a vessel surveying for telegraph cable had reported temperatures too low and led some investigators to assume that bottom water is formed there or in the Okhotsk Sea. The error was corrected by WOOSTER and VOLKMAN (1960), GORDON and GERARD (1970), and MANTVLA(1975). Another discrepancy is found in Wrist's map of the Indian Ocean; the Ninetyeast Ridge had not been discovered at that time, and the eastern Indian Ocean was not represented well. LYNN and RE1D (1968) mapped typical values of bottom potential temperature, salinity, and potential densities referred to the sea surface (o 0) and to 4000 db (o4),* but they used only a few widely spaced observations and did not attempt to show any detail in the distribution of characteristics. OLSEN (1968) mapped the bottom potential temperature, salinity, potential density (%), and dissolved oxygen in the major basins of the World Ocean, mostly at depths > 5 0 0 0 m. Except for temperature, those maps could not show much detail either. Over 6000 deep oceanographic stations, chosen from N O D C data files and numerous unpublished data reports, were considered during the preparation of the charts used in this paper. There are still large areas of the ocean that are not represented by modern high-quality data, particularly in the case of silica. The lack of sufficient modern data has made it necessary to include data from older expeditions that were less precise and in some cases had systematic errors. An attempt was made to remove such errors by comparison with a few modern large-area expeditions. The results, though usable, are still not entirely satisfactory. Also, the water-column sampling on many modern cruises is surprisingly sparse. The past decade has seen an increased use of electronic continuously recording conductivity, temperature, and pressure devices, often inadequately calibrated, and a decreased water column sampling for dissolved oxygen, nutrients, and salinities analyzed with a laboratory salinometer. The observations span approximately a 50-year period lequivalent to GEORGI'S (1981a) estimate of the time for one circuit of the Antarctic Circumpolar Current around Antarctical. Data from deep and abyssal stations repeated in different years in open-ocean locations mostly agree within the precision of the measurements, lending some confidence for using data from many different years to represent steady oceanic distributions of characteristics.

* To avoid unnecessary repetition of units we list them here and in some cases hereafter may write only the values. The density parameter we use is 04 = p~ - 1000 kg m--~ where P4 is the in situ density in kg m 3 that the water would have if moved adiabatically to the depth where pressure is 4000 db (=40 M Pa). We have used the Knudsen equation of state, which gives specific gravity, a slightly lower number, but we refer to our quantity as density. Potential temperature is expressed as degrees Celsius; Salinity units, which are 10--~ mass mass- ~,will be written simply 10-~, Dissolved oxygen units, which have been expressed previously as milliliters liter-~, or 10 volume volume-~, will be written simply 10--~. Dissolved silica units, which have been expressed as micromoles liter- ~,will be written millimoles m--~,or m molm -~.

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Abyssal characteristicsOfthi~World Ocean waters

813

However, year-to-year changes have been detected near some bottom water source regions. FOSTER and MIDDLETON(1979) reported that the bottom water in the Weddell Sea was colder and more saline in 1976 than in 1975. Changes in the Denmark Strait Overflow Water near Greenland have also been detected between the 1972 GEOSECS expedition and the 1981 Transient Tracers Operation (unpublished data); the bottom water in 1981 was colder and less saline than in 1972. Such year-to-year bottom water variations have not yet been detected far from the source regions. Secular variations in the regions shown here are believed to be less than the contour intervals used. The smoothing during contouring may obscure some real variations near source regions. However, we feel that the maps do portray a realistic picture of the broad-scale bottom oceanic features. The work for this paper was mostly completed before the new Practical Salinity Scale (PSS78) and International Equation of State (EOS80) were recommended for general use; they were not used. The difference between the new and old salinity scale is <0.001 × 10 3 over the abyssal range of salinities and the difference is not significant. The new density equations would have resulted in densities (~4) on the order of 0.06 less than reported here. A simplified bathymetry map with some of the major bathymetric features identified is shown in Fig. 1. The 3.5-km isobath was most useful in depicting the major ridges and basins of the World Ocean. The bottom density, potential temperature, salinity, dissolved oxygen, and silica, generally at depths )>3500 m, are shown in Figs 2a to e. Many of the patterns on the maps simply reflect intersections of the isopleths with the continental slopes or with the ridges. Potential temperature and c 4 change monotonically with depth in the deep and bottom water; such charts are easiest to interpret. Salinity, oxygen, and silica are not monotonic in some basins; they are therefore more difficult to contour and to interpret.

THE ANTARCTIC

ZONE

South of about 50°S, Figs 2a to e are similar to the Antarctic bottom property maps presented by CARMACK (1977) and by SCHLEMMER(1978). Although some new data have been incorporated in the maps shown here, the three studies are based on substantially the same historical deep station array. The three major basins around Antarctica (Atlantic-Indian, South Indian, and Southeast Pacific basins) contain dense waters of distinctly different characteristics; the freshest, coldest, and densest bottom water is in the Atlantic sector, while the most saline, warmest, and least dense bottom water is in the Pacific sector; the South Indian basin contains bottom water of intermediate characteristics. The densest waters are confined, both topographically by the Southern Ocean ridge systems and dynamically by the Antarctic Circumpolar Current, to the deep basins adjacent to Antarctica. FOSTER and CARMACK (1976) proposed that the formation of Antarctic Bottom Water (AABW) in the Atlantic sector takes place in three stages. In the first stage, CPW, characterized in the Weddell Sea by relatively high values of temperature, silica, and salinity and by low oxygen, is modified by the overlying cold, low-silica, low-salinity, and high-oxygen Winter Water (WW) in the southeastern Weddell Sea. In the second stage, modified CPW is carried westward where it mixes with Western Shelf Water (WSW, near freezing temperature, high salinity, high oxygen, and low silica) to form Weddell Sea Bottom Water (WSBW). Weddeil Sea Bottom Water then mixes with CPW as it flows eastward from the northwest corner of the Weddell Sea to form AABW. Thus AABW is roughly made up of 5/8 CPW +

814

ARNOLD W. MAblTYLAand JOSEPH L. REID

1/4 WSW + 1/8 WW; the characteristics of CPW within the WeddeU Sea gyre dominate the AABW found in the WeddeU Sea. MICHEL'S (1978) study of tritium data in the Weddell Sea supported the three-stage mixing concept. As Michel pointed out, tritium was introduced into the atmosphere during the mid-1950s by thermonuclear testing, thus tritium does not occur in subsurface water masses that have not had recent addition of surface water. He found no detectable tritium in the CPW, but small amounts were detected in the WSBW. His mixing ratios based upon the tritium data are about the same as determined by FOSTER and CARMACK (1976) from physical characteristics. WEtSS, OSTLUND and CRAlG (1979) provided further geochemical tracer evidence on the formation of WSBW, and their mixing ratios are also c o n sistent with those of FOSTER and CARMACg (1976). The newly formed WSBW is clearly evident in Fig. 2 at 64°S, 40°W by the high density, high oxygen and low temperature, salinity, and silica lobes extending eastward into the Atlantic-Indian Basin. The changes of the WSBW as it spreads away from the Northwest Weddell Sea are due to increasing admixtures of CPW. Circulation patterns within the Ross Sea are similar to those within the WeddeU Sea (JAcOBS,AMOS and BRUCHHAUSEN,1970). However, the CPW in the Ross Sea is warmer and more saline than in the Weddell Sea. The nearly freezing shelf water found in the western Ross Sea is also slightly more saline than the WSW found in the Weddell Sea, 35.0 vs 34.8 × 10-3. If FOSTER and CARMACK'S (1976) three-stage mixing process also operates in the Ross Sea, then the characteristics of the Ross Sea variety of AABW (lower in density and oxygen, higher in temperature, salinity, and silica than the WSBW, Fig. 2) would be a consequence of the differing characteristics of its two major components, Circumpolar Deep Water and Ross Sea Shelf Water. The variety of AABW in the South Indian Basin is thought to be a mixture of" RSBW, water from the Adtlie Coast (GORDON and TCHERNIA, 1972), and CPW from the west (KoLLA, SULLIVAN,STREETERand LANGSETH, 1976). The coastal configuration adjacent to the South Indian Basin is quite different from that in the Ross and Weddell seas. Although there is a westward-flowing coastal current in the region, high-salinity shelf water does not appear to be present near the South Indian Basin. The CPW is higher in salinity in the Indian Ocean sector than in the Pacific or Atlantic sectors, but the absence of high-salinity shelf water seems to keep the Indian Ocean Basin from producing a major component of bottom water near Antarctica. The Enderby Land-Prydz Bay coast (53 to 73°E) has been identified as another minor source of dense bottom water (JACOBS and GEORGt, 1977); its influence is evident only in Fig. 2d by the 5.8 × 10-3 02 contour near Antarctica between 40 ° and 60°E. The Davis Sea has also been mentioned as a source of dense Antarctic water (TRESHNIKOV,Gins, BARANOV and YEFIMOV, 1973). The low silica and slightly higher oxygen near 90°E may indicate some bottom water formation near the Davis Sea, but its influence is not so strong as the other sources. It is clear that there are several varieties of dense bottom waters in the deep Antarctic basins. Where water masses from different sources occur in a basin, they are frequently separated by an increased density gradient or a maximum in stability (REID et aL, 1977). SCHLEMMER (1978) identified several such stability maximum layers separating dense waters from different regions of Antarctica. The bottom waters closest to the source regions reflect the influence of the near-surface component: cold, higher dissolved oxygen, and slightly reduced silica concentration. The variable salinity of the bottom waters near Antarctica corresponds in general to that of the local CPW component. As the bottom waters spread

Abyssal characteristic.~Ofthe World Ocean waters

815

from source regions into deeper and more remote parts of the Antarctic basins, their characteristics are altered through interaction with the overlying water column and with the bottom sediments. Silica in the bottom water is increased by dissolution of biogenic silica from the ocean floor (EDMOND, JACOBS, GORDON, MANTYLA and WEISS, 1979), and oxygen is decreased by respiration. The highest bottom water silica occurs in the eastern side of the Atlantic-Indian Basin; the eastern Southeast Pacific Basin, and in the central South Indian Basin; all distant from bottom water formation regions. The bottom water speed is very low in the southeastern Bellingshausen Sea (GORDON, 1966). Here, a combination of long transit time of RSBW and sluggish circulation allows longer contact of RSBW with the ocean floor and accounts for the increased silica and decreased oxygen in the corner of the basin most remote from the Ross Sea. Similar mechanisms appear to operate in other Antarctic basins as well. The densest bottom waters in each of the Antarctic basins cannot escape from the basins until they have been mixed with the overlying CPW and reached a density low enough to pass over the sills. The densest waters flowing out of the Ross Sea and Southeast Pacific Basin leave through the Drake Passage, which allows only those waters less dense than about 46.10 in 04 to pass through. REID et al. (1977) identified a stability layer (their No. 7) as separating water entering the Atlantic through the Drake Passage from newly formed WeddeU Sea Water. SCHLEMMER(1978) associated that stability maximum layer with a density of 46.08 in o4 and noted that the layer occurred all around Antarctica. Thus the denser abyssal waters of the Ross Sea do not contribute directly to the Weddeil Sea. Only after their mixture with the overlying waters has reached densities as low as 46.10 do they flow eastward through the Drake Passage. This is most evident in their contribution of high silica into the CPW in the Drake Passage. The high values there are not a continuation from the great silica maximum of the North Pacific Ocean, which lies in a layer of lower density, but trace back directly to the high silica concentrations in the denser waters of the abyssal Southeast Pacific Basin. Without this source, the WSBW, which derives in part from the CPW, would be even lower in silica. As it is, it is lowest in silica of all the Antarctic Basin bottom waters. Classical AABW (0 = -0.7°C, S = 34.66 x 10-3) is found only in the Atlantic-Indian Basin and in the South Sandwich Trench. The term, Antarctic Bottom Water, may not be appropriate in many cases. It is now recognized that there are several dense bottom water types near Antarctica, and the dense bottom waters that extend northward into each of the oceans from Antarctica are actually mixtures of the circumpolar waters with the bottom waters of the various Antarctic basins. The 0--S characteristics of the abyssal waters starting their northward flow into the major ocean basins are in the range 0 = -0.4°C, S = 34.66 × 10-3 to 0 = +0.3°C, S = 34.70 × 10-3. No single abyssal water type exists all around Antarctica, rather each location represents the mixtures of the various circumpolar and Antarctic bottom waters. The above 0-S range occupies a substantial volume in WORTHINGTON'S (1981) volumetric census of the World Ocean, falling within his top 50% grouping of World Ocean volume. WOST (1939) identified seven meridional branches of bottom currents northward from Antarctica. In Fig. 2a, the seven branches can be identified by the 04 lobes projecting northward into (1) the Argentine Basin, (2) Cape Basin, (3) Natal Basin, (4) Madagascar Basin, (5) South Australian Basin, (6) Southwest Pacific Basin, and (7) northern Southeast Pacific (Bellingshausen) Basin. Areas 2, 3, and 7 are dead ends, closed to the north by ridges, and their abyssal waters do not extend farther north. The other four regions allow penetration of dense

8 16

ARNOLD W. MANTYLAand JOSEPH L. Rt~lD

southern bottom waters farther north through a series of deep sills or passages. Each major ocean area will be discussed separately in the following sections. THE A T L A N T I C OCEAN

The densest water leaving the Circumpolar Current enters the Argentine Basin through the South Sandwich Trench. A detailed study of the pathway of bottom water flow and subsequent circulation in the Argentine Basin is given by GEORGI (1981b). Both the low temperature and the low salinity in the Argentine Basin (Fig. 2) reflect the direct influence of the WeddeU Sea, while the lower oxygen and higher silica reflect the influence of the CPW from the Drake Passage and the Pacific Ocean. In vertical sections along the path of the densest bottom water stations in the western Atlantic (Fig. 3), the influence of the CPW is clearly evident in the oxygen minimum and silica maximum, which slope downward from about 60 to 30°S. Over most of the Southern Ocean the CPW has been described as a warm, saline layer of relatively high nutrient content and low oxygen concentration. In the South Atlantic, however, it is separated into an upper and lower layer by the southward penetration of the North Atlantic Deep Water (NADW), which is even warmer and more saline and lower in nutrients and higher in oxygen (REID et al., 1977). The distinction is not so important for the abyssal Atlantic, where only the lower branch, or Lower Circumpolar Water (LCPW), reaches abyssal depths, but as both the remnant salinity maximum of the NADW and the upper branch, or Upper Circumpolar Water (UCPW), affect abyssal depths in some other areas, the distinction is made here also. In the Atlantic section (Fig. 3) the UCPW, which contains the upper minimum in oxygen and maximum in silica, lies above 2000-m depth, and the extrema are only marginally evident here. The oxygen and silica show extrema in the sections because both the overlying NADW and the underlying Weddell Sea Water have been ventilated more recently and are higher in oxygen and lower in silica than the CPW. Corresponding extrema in the conservative characteristics (salinity and potential temperature) do not appear. This is because the LCPW, formed by a mixture of NADW and AABW, is intermediate in these characteristics. The abyssal silica maximum does not extend beyond the Rio Grande Rise, as waters of that density either mix with the overlying waters, losing their identity, or return to the south. The abyssal waters of the Brazil Basin derive from the layers above 4000-m depth in the Argentine Basin, through the gap in the Rio Grande Rise, as seen in Figs 2 and 3 by the o4 --46.12, 0 = 0°C, S = 34.70 × 10-3, and silica = 120 m mol m -3 contours. The map shows a local maximum in oxygen at the bottom of the Argentine Basin (Fig. 2d) and a minimum in the Brazil Basin. The oxygen minimum in the vertical section, evident in the Argentine Basin (Fig. 3), extends for a short distance into the Brazil Basin, but north of about 20°S it is not evident, and oxygen decreases monotonically downward from the NADW maximum. Away from their sources, the abyssal waters reflect the presence of the sills between basins, which may hold back the densest waters, and of vertical mixing with the overlying waters, particularly in the narrower passages. Where layers spill over the ridges, the abyssal waters on the downstream side may derive from the sill-depth waters and bear little relation to the abyssal waters on the upstream side, and the patterns of characteristics at the bottom may appear to be discontinuous, not revealing flow paths. A clear example of such a discontinuity is where the CPW enters the Atlantic through the Drake Passage. Values in c 4 from 46.04 to 46.10 are found at the bottom in the passage and in the northern part of the Scotia Sea, but they do not extend farther east at the

Abyssal characteristicsof the World Ocean waters

8 !7

bottom. They appear again in the abyssal Cape Basin, which has also the same values of potential temperature, salinity, oxygen, and (nearly the same) silica as the Drake Passage. In this case the abyssal waters from the passage have extended eastward, not at the bottom, but above the denser waters from the Weddeli Sea, and lie at the bottom again after crossing the Mid-Atlantic Ridge, which excludes the dense waters from the WeddeU Sea from the eastern South Atlantic. Where the channels between basins are narrow the speed may be higher and the vertical mixing greater. The intensity of the vertical mixing will contribute to the sequence of downstream changes in abyssal characteristics as the waters move northward. Note (Fig. 4) that the bottom 0 - S points in each basin are not found on any part of the 0-S curves in the basins to the south. The northward succession of bottom values indicates that the abyssal waters do not spread northward unchanged, that the successive values at each station do not derive simply from a shallower level on the previous station, and that they cannot derive from vertical mixtures of the water at the previous station alone. Instead, the bottom values must be the result of continuous vertical mixing along the path northward with successively warmer and more saline overlying water. The two major escape routes for dense water from the northern Brazil Basin are through the Romansche Gap and across the broad equatorial sill that separates the Brazil Basin from the Guiana Basin. The higher density and lower temperature of the Guiana Basin indicate that the waters move northwestward with less modification, and probably in greater volume, than through the Romansche Gap into the eastern Atlantic (Sierra Leone, Guinea, and Angola basins). Bottom silica in the Angola Basin is about one-half that of the Brazil Basin, but is still about twice as high as the silica levels of the NADW. A minor source of bottom water into the Angola Basin occurs at the southwestern corner of the basin from the Cape Basin through a channel in the Walvis Ridge (CONNARYand EwINc, 1974), most evident in Fig. 2b. In the western North Atlantic, the densest extension is northward from the Guiana Basin to the Nares, North America, and Newfoundland basins where, near the Grand Banks, it finally meets abyssal water from another source, the Denmark Strait overflow. A side branch of the main bottom water from the south enters the northeastern Atlantic basins through the Vema Fracture Zone (11 o N, 41 o W) (VANGRIESHEIM, 1980) into the Cape Verde Basin. There is little evidence of exchange of bottom water between the Sierra Leone and Cape Verde basins, although HOaART, BUNCE and SCLATER(1975) noted some sedimentary record of such flow. Evidently, the Romansche Gap is the primary passage for bottom water flow into the eastern equatorial and southeastern Atlantic basins, while the Vema Fracture Zone is the main passage for bottom water flow into the northeastern Atlantic basins. Southern ocean water characteristics can be traced northward from the Cape Verde Basin into the Canary, Iberian and West European basins, especially by the relatively high silica fields. Bottom water deriving from the Norwegian-Greenland Sea, where it is evident in the Labrador Sea, is only about 10 m mol m -3 in silica. The bottom water from the Denmark Strait lies too deep to pass eastward through the Gibbs Fracture Zone, and that from the Iceland-Scotland overflow is not dense enough to reach abyssal depths (REID and LYNN,1971; J. H. SWIFT, personal communication). The influence of dense water from the Norwegian-Greenland Sea on the deep and bottom waters of the North Atlantic is clearly evident in the Irminger and Labrador basins by the high oxygen, low silica, and low temperatures (Figs 2 and 3). SWIFT, AAGAARD and I~,IALMBERG (1980) have shown that the primary source of such dense water is from intermediate levels of the Norwegian-Greenland Sea. Apparently the sills between Greenland and Scotland are too

8 18

ARNOLD W. MANTYLA a n d JOSEPH L. REID

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819

Abyssal characteristics of the World Ocean waters

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shallow to permit directescape of the densest water from the deeper parts of those basins.The Denmark StraitOverflow Water is quickly modified, mixing with the immediately overlying warmer water from the Iceland-Scotland overflow (REID and LYNN, 1971; J. H. SWIFT, personal communication) as it flows southward, increasing in potential tcmperaturc from
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KOLLA et aL (1976) discussed the spreading of the denser water in the Indian Ocean, using bottom potentialtemperature, turbiditymeasurements, and bottom sediments. New data have bccn incorporated in Fig. 2b, but the bottom potentialtemperature pattern is essentiallythc same as Fig. 2 in Kolla et aL

Abyssal characteristicsof the World Ocean waters

821

Unlike the spreading of the abyssal water in the Atlantic and Pacific oceans, the denser bottom waters in the Indian Ocean spread far to the north in both the western and eastern sides (WA~,REN, 1981b). For the western Indian Ocean the abyssal waters derive from the Atlantic-Indian Basin, while those in the eastern Indian Ocean derive from the South Indian Basin, passing through a gap in the Southeast Indian Rise south of Australia (Fig. 2). The densest bottom water entering the Indian Ocean is from the eastern Atlantic-Indian Basin, spreading into the Crozet Basin. In the discussions on the Antarctic basins above, it was pointed out that the region has some of the highest silica concentrations observed in the Southern Ocean. The high bottom silica at the beginning of abyssal flow into the Indian Ocean contributes to the high bottom silica concentrations observed throughout the Indian Ocean (> 120 m mol m-3). Overlying waters are lower in silica than the bottom water. The gap between the Crozet Ridge and Kerguelan Plateau both excludes the densest water and causes vertical mixing and thus reduces the abyssal silica concentrations. Likewise, density, potential temperature, salinity, and dissolved oxygen values are altered toward the shallower water-column levels. Vertical sections along the path of the densest bottom water in the western Indian Ocean illustrate the characteristics of the deep water lying above the bottom water (Fig. 5). The salinity maximum and silica minimum from the NADW (entering from the west) slope downward to the north from 47 to 3.0°S. North of 30°S there is a slightly shallower and less dense salinity maximum, originating in the Arabian and Red seas. As the deep-water salinity is thus higher than the bottom water salinity everywhere in the Indian Ocean, vertical mixing can only increase the bottom water salinity along the direction of flow. The Madagascar Basin receives its water from the Crozet Basin through a series of fractures in the Southwest Indian Ridge (WARREN, 1978), and the characteristics change gradually as the water spreads northward from the Madagascar Basin to the Mascarene and Somali basins. Finally, the densest waters of the Arabian Basin enter through the Owen Fracture Zone in the Carlsberg Ridge. In the northern Arabian Basin the bottom oxygen is < 3 × 10 -3 and the bottom silica is <160 m mol m -3. The water is also less dense than incoming bottom water and is probably the source of the high silica, low oxygen, deep waters extending southward (Fig. 5) above the bottom (EDMOND et al., 1979). The deep water is also modified by overlying warm, high-salinity Red Sea Intermediate Water as it spreads southward above the bottom water (WVgTKI, 1971). Although both WeddeU Sea and Circumpolar waters are seen in the western Indian Ocean, the LCPW provides the major component of the abyssal waters of the northern basins. THE EASTERN INDIAN OCEAN

Northward flow through fracture zones near 50°S, 125°E in the Southeast Indian Ridge provides abysffal waters in the South Australian Basin and the eastern Indian Ocean (RODMAN and GORDON, 1982) that are less dense, warmer, and more saline than those at the bottom of the South Indian Basin. The differences reflect the effect of the LCPW. Basins in the eastern Indian Ocean are deeper and the passages connecting them are broader and deeper than similar features in the western Indian Ocean. With fewer barriers to the flow of the densest water, there is less change in the characteristics from the South Australia Basin to the Wharton and Cocos basins. The entrance to the Mid-Indian Basin is a more restrictive saddle in the Ninetyeast Ridge at about 8°S, shown by the 0.2°C rise in bottom potential temperature at that location (Fig. 2b). A minor source of bottom water for the Mid-Indian Basin may occur in the southwest corner of the basin, similar to that observed

822

ARNOLD W . MANTYLA a n d JOSEPH L. REID

~'~ ; - W \

45.72

45.72

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Western Indian Ocean section along the path of the d¢nsest bottom water. Density (04) referred to 4000 db; potential temperature (oC), and salinity (x 103).

Abyssal characteristicsof the World Ocean waters

40"s

823

20os

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Fig. 5b.

Western Indian Ocean section along the path of the densest bottom water. Oxygen (x 103) and silica (m tool m-3).

in the Angola Basin, but the existing data are insufficient to show such a feature. In the eastern Indian Ocean it is mostly LCPW that extends to the Bay of Bengal, where bottom oxygen and silica approach concentrations observed in the northern Arabian Basin. THE P A C I F I C O C E A N

The abyssal circulation of the Pacific Ocean has been inferred from the potential temperature distribution by W0ST (1937), WOOSTER and VOLKMAN (1960), KNAUSS (1962), and MANTYLA(1975). Unlike the other two oceans, there appears to be no direct outlet from the Antarctic basins into the Pacific Ocean. The deepest escape passage from the Pacific sector of the Antarctic, the Southeast Pacific Basin, lies not to the open Pacific but to the east, through the Drake Passage. Lacking recent substantial high-density AABW components, the water entering the open abyssal Pacific (from south of the Campbell Plateau) is the least dense and the lowest in oxygen concentration of the various Antarctic thresholds. Yet this water still has the characteristics of a mixture between CPW and dense bottom water from the Antarctic. As the potential temperature-salinity relation falls in the same range as in the bottom water entering the eastern Indian Ocean, the Antarctic component is presumably the same as in the South Indian Basin.

824

ARNOLD W. MANTYLA and JOSEPH L. REID

2000

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Abyssal characteristics of the World Ocean waters

825

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Fig. 6b. Western Pacific Ocean section along the path of the densest bottom water. Oxygen (x 10~) and silica (m mol m-J).

As the water spreads along the bottom into the Southwest Pacific Basin, it mixes with the overlying LCPW. Characteristics of the shallower water masses are illustrated by the western Pacific vertical sections (Fig. 6). The circumpolar component is most clearly evident by the remnant N A D W salinity maximum and silica minimum sloping downward from about 60°S to near 10°S (WARREr~, 1973), although substantial modification occurs prior to reaching the Pacific sector of the Southern Ocean (CALLAHAt~, 1972). Just as the CPW in the Atlantic appears as oxygen minima and silica maxima because of the contrasting nature of the deep and bottom water there, it appears as a salinity maximum and silica minimum in the South Pacific. No interior extremum from the south appears in the deep oxygen profile because the circumpolar oxygen concentrations, though low, are overwhelmed by the even lower deepwater values from the north. At the northern edge of the Southwest Pacific Basin (near 10°S), the remnant N A D W with extrema in both salinity and silica lies at the bottom, and their horizontal patterns are quite complex in that region. A stability maximum between the deep and abyssal layers in the Southwest Pacific Basin has been reported (REID and LYNN,1971), and a sharp gradient between many deep and bottom water characteristics, including oxygen, has been observed (CRAIG,CHUNG and FIADEIRO,1972). East of the East Pacific Rise the abyssal waters have flowed northward into the Chile Basin

826

ARNOLD W. MANTYLAand JOSEPH L. REID

and on to the Peru and Bauer basins; the Bauer Basin also receives some water directly from the west, across the rise (LoNSDALE, 1976). The densest water flow from the Southwest Pacific Basin to the Central Pacific Basin takes place through the Samoan Passage near 10°S, 169°W (REIo and LONSOALE, 1974). It spreads in three directions from the Central Pacific Basin: to the west into the East Mariana Basin, to the north through the Wake Island Passage into the Northwest Pacific Basin, and to the east into the Northeast pacific Basin (EDMOND, CHUN6 and SCLATER, 1971; MANTVLA, 1975). The Indopac Expedition (ScRIPPs INSTITUTIONOF OCEANOGRAPHY, 1978) helps to define the spreading of deep and bottom waters across a trans-ocean section along 35°N (KENYON, 1978; unpublished data) and in the vicinity of the Philippine Sea (MA~TVLA and REID, 1978; REIn and MArqTVLA, 1980). The temperature (Fig. 2b) most clearly shows the spreading of bottom water from the East Mariana Basin through a gap south of Guam into the Parece Vela and Shikoku basins. The water then enters the northern Philippine Basin through a gap in the Kyushu-Palau Ridge at about 20°N and then spreads southward in the Philippine Basin. As the latter region is a cul-de-sac for the western Pacific bottom water flow, escape of bottom water from the basin must occur by vertical mixing and entrainment with shallower water ( R E I D and MANTYLA, 1980). MORIYASU (1972) noted that the dissolved oxygen at 3000 m was higher inside than outside the Philippine Sea, while at 5000 m, dissolved oxygen is lower in the Philippine Sea than outside. The higher oxygen at 3000 m, well below the oxygen minimum, must come from below. Calculations of the mean oxygen and other characteristics integrated from 2000 m to the bottom at a location in the southern Philippine Sea and at the entrance to the region near Guam result in the same mean water-column con~ centrations, within measurement error. This implies that there has been no significant change in total water-column oxygen content, but apparent changes at 3000 m and the bottom are simply re-distribution by vertical mixing. On the section along 35°N the water with lowest temperature and highest oxygen is found near 165°E, but the ridge extending west along about 40°N from the Emperor seamounts to 160°E prevents it from extending farther north. The density and oxygen patterns (Fig. 2) suggest that the bottom waters farther north, in the Kuril and Aleutian basins, have passed west of 160°E, roughly along the path of the section shown in Fig. I. Within both basins the bottom silica exceeds 200 m molm -3 . Abyssal waters converge in the Northeast Pacific Basin from three sources: along the Aleutian Trench, through the Emperor Seamount Chain near Midway Island, and from southeast of Hawaii (Fig. 2). The Northeast Pacific Basin is believed to be the area most remote from sources of abyssal water (WooSTER and VOLgMAN, 1960; K~At3SS, 1962; MANTVLA, 1975). Bottom silica exceeds 190 m m o l m -3 and bottom oxygen is <3 × 10 -3 (about 40% of saturation) in the Gulf of Alaska. The region also is lower in density than any of the incoming bottom waters and is presumed to be the source of the North Pacific Deep Water compensating return flow to the south (MANTYLA, 1975). The bottom kilometer of the water column in the central North and Northeast Pacific is quite uniform in characteristics and is the region where the world's largest single bivariate 0--S class occurs (1.1 to 1.2°C. 34.68 to 34.69 x 103 S, 25,973 × 1 0 3 kin3; WORTmNC;TON, 1981). The deep salinity maximum in the central South Pacific lies near 45.92 in density (REIn and LYNN, 1971). Although water of this density is found north of the equator the interior maximum in salinity has disappeared. The remnant salinity maximum from the North Atlantic, lying between a layer of less saline water from the Southern Ocean and a layer of

Abyssal characteristics of the World Ocean waters

827

less saline intermediate water, has finally been mixed away. Northward from the equator the salinity decreases upward from the bottom to intermediate depths. The abyssal waters of the North Pacific Ocean have a density corresponding to that of the L N A D W and appear to be made up of that water, some part of LCPW, and some overlying intermediate water (REID, 1965). O C E A N - T O - O C E A N BOTTOM WATER C O N T R A S T S

Figure 7 illustrates the change in the potential temperature-salinity fields along the path of the densest bottom water flow in each of the four major bottom water spreading regions. The downstream changes occur both by exclusion of the densest waters at each ridge and by vertical mixing with the overlying warmer, more saline, and less dense water. Thus, the bottom water to the north in the Atlantic becomes warmer and more saline as far as the Grand Banks, where bottom water of a different source is encountered. In the Indian Ocean, a smaller change in temperature and salinity takes place to the north, probably because the overlying waters are not so extreme as those in the Atlantic. Because the Pacific Ocean does not have a deep connection with the Antarctic basins the incoming waters are the least dense,

,

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Salinity (xlOa ) Fig. 7. Bottom potential temperature vs salinity points along the path of the densest bottom water flow in each of the four major spreading areas: western Atlantic Ocean, western Indian Ocean, eastern Indian Ocean, and western Pacific Ocean.

828

ARNOLD W. MANTYLA and JOSEPH L. REID

Latitude

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Fig. 8a. Bottom water charactcrigtcs vs latitude in the western Atlantic, western Indian, and the western Pacific. Note that rome scai~ are rever~. Potential density (o~), potential temperature (oC), and salinity (x 105).

829

Abyssal characteristics of the World Ocean waters

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60"S

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Latitude Fig. 8b. Bottom water characteristics vs latitude in the western Atlantic, western Indian, and the western Pacific. Note that some scales are reversed. Oxygen (x 103), silica (m mol m-a), and '~C (x 10-3).

830

ARNOLD W. MANTYLAand JOSEPH L. REID

warmest, and most saline of the three oceans. Partly as a result, the ranges of density, poten tial temperature, and salinity are the lowest. The coldest Antarctic waters do not reach the Southwest Pacific Basin, and the North Pacific abyssal waters, like those in the northern Indian Ocean, do not have such warm waters overlying them as do those in the North Atlantic. Salinity increases northward to 10°S, where the CPW salinity maximum reaches the bottom. Northward from there the overlying waters are everywhere less saline up through the intermediate water salinity minimum, leading to a slight northward salinity decrease in the abyssal North Pacific waters. In Fig. 8, the change in bottom characteristics along the densest path of spreading in the western Atlantic, Indian, and Pacific oceans is illustrated as a function of latitude. Data from the eastern Indian Ocean were omitted for clarity; it is similar to the western Indian Ocean. Topographic effects are clearly evident by the change in meridional gradients. Within basins, characteristics change gradually from south to north. Major transitions occur at the prominent sills or passages; the most notable example is evident in the equatorial Atlantic. The greatest difference in bottom densities among the three oceans is seen in the southern hemisphere. The southwestern Atlantic has the most unobstructed link to the dense Antarctic water sources and is denser at the bottom than the other two oceans. Denmark Strait Overflow Water is encountered at 40°N in the western Atlantic and the increase in bottom density there reflects the new source. The bottom potential temperature (Fig. 8) is the same as the bottom density picture, although there is a greater contrast between the potential temperatures in the North Pacific and the North Atlantic. Bottom temperature increases northward everywhere except north of 40°N in the western Atlantic. The bottom salinity increases to the north except in the North Pacific, where the deep-water salinities are lower than the bottom water salinities. The Pacific Ocean, considered to be a low-salinity ocean, has the highest southern hemisphere bottom water salinities as well as the highest temperatures, as it does not receive the densest water from the Antarctic. The Atlantic and Indian salinities and temperatures are alike in the far south. A greater contrast is seen in the northern hemisphere where the Atlantic bottom salinities and temperatures are higher than in the other oceans, reflecting the admixture of the warm, saline NADW. The midlatitude bottom waters in the deeper parts of the Northwest Atlantic and the North Pacific, though different in temperature and salinity, are similar in density when referred to density near the in situ pressure (o 5 = 50.20 + 0.012). The bottom densities in the two regions would be in even closer agreement (by about 0,01) if the salinities were adjusted for the effect of nonconservative ions on conductivity salinities (BREWER and BgAOSHAW, 1975; MtLLERO, FORSHT, MEANS, GmSKES and KENYON, 1978). Both locations have thick bottom layers with low stabilities. Table 1 summarizes the bottom kilometer range of values for examples in the two basins. The uniform values are apparently a basin effect, both deep basins being filled by water over shallower sills or saddles. Bottom oxygen concentration decreases northward in the Indian and Pacific oceans, as might be expected from in situ consumption. Its increase in the western Atlantic, north of the equator, must result by exchange with higher oxygen NADW above: vertical mixing is at least as important as in situ consumption here. Although the oxygen concentrations in the abyssal WeddeU Sea are high, the waters are topographically limited to the far south. The oxygen supply to the abyssal North Indian and North Pacific basins appears to be from the North Atlantic (Figs 2 and 8 and the vertical sections). The bottom silica is surprisingly uniform in the mid-latitudes of the southern hemisphere

Abyssal characteristics of the World Ocean waters

831

Table 1. North Pacific G E O S E C S Sta. 216 40o46'N, 176058'W 4725 to 5837 m E = <0.8 o s = 50.186 to 50.189 0 = 1.10to 1.09"C S = 34.685 to 34.684 x 10 -3

Northwest Atlantic G E O S E C S Sta. 30 31 ° 48' N , 5 0 ° 4 6 ' W 4786 to 5831 m E = <1.7 o s = 50.205 to 50.212 0 = 1.68 to 1.62°C S = 34.866 to 34.857 x 10-3

(Fig. 8); all oceans have more than 120 m mol m -3 in silica. High silica, > 150 m mol m -3, is found in the North Indian and Pacific oceans. The low concentrations in the North Atlantic reflect the near-surface origin of the deep and bottom North Atlantic waters. While it is not yet possible to prepare comparably detailed maps of m4C for the abyssal waters, it is interesting to compare the data that are available, and the near-bottom 14C values reported by STUIVERand OSTLUND(1980) and OSTt.UND and STUIVER(1980) are shown in Fig. 8. Factors that influence 14C levels are complex (CRAIG, 1969), but ~4C is at least a qualitative indicator of the age of abyssal waters (STuxVER, 1976). The lowest levels of 14C ('oldest') are in the North Pacific, while the highest levels of ~4C ('youngest') are in the North Atlantic, confirming implications of bottom water sources and sinks based upon physical characteristics of the bottom water. There is some similarity to the abyssal silica pattern. Surprisingly uniform intermediate 14C levels occur in all three oceans of the southern hemisphere. Although the physical characteristics are different in the three southern oceans, the apparent age is not very different. Also, there is little indication of young water from the Antarctic in the data set. If data for the southwestern Weddell Sea were available, a stronger signal might be seen. However, the major component of WSBW is not surface water but CPW, and the signal might not be so strong as that in the northern North Atlantic Ocean. SUMMARY

Although the densest waters of the World Ocean are formed in the Antarctic and are found at abyssal depths in the basins there, the waters farther north do not derive directly from the abyssal depths of these basins. The densest waters are confined near the Antarctic zone by topography. The waters found farther northward at abyssal depths are less dense and derive largely from waters that lie well above the bottom in the Antarctic, which are mixtures of the Antarctic abyssal waters with water from the North Atlantic. Along the abyssal paths of flow they are further modified by mixing with the overlying less dense waters. The overlying waters are everywhere warmer, and in the abyssal layer both temperature and salinity increase northward along the various channels, except in the North Pacific, where the overlying waters are less saline and the abyssal salinity is reduced in the north. Only in the Northwest Atlantic is the abyssal water derived from another source, the dense saline water overflowing from the Norwegian-Greenland Sea. Such water lies at abyssal depths only north of about 40°N, in the western Atlantic. Its density is reduced by vertical mixing, and south of about 40°N it lies above the abyssal waters from the southern sources. As the warm, saline waters from the North Atlantic extend to the south, they lie between the deeper, denser, less saline waters of the Antarctic and the less dense and less saline overlying

832

ARNOLD W. MANTYLAand JOSEPH L. REID

w a t e r s a n d f o r m a layer o f m a x i m u m salinity t h a t lies well a b o v e the b o t t o m e v e r y w h e r e within the A n t a r c t i c C i r c u m p o l a r C u r r e n t . T h e layer has been called C P W . It is the lower part o f this layer, m a d e up o f w a t e r s f r o m b o t h the n o r t h e r n N o r t h A t l a n t i c and the A n t a r c t i c zone, t h a t fills m o s t o f the a b y s s a l o c e a n .

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