0!967-0645@5)000&4-4 Deq-Sea Research II. Vol. 43. No. 1. pp. 83-110, 1996

Copyright 0 1996 Elscvier Science Ltd Rinted in Great Britain. All rights reserved

0967-0645/96 IE15.OOCO.00

Spreading of water masses and regeneration of silica and 226Ra in the Indian Ocean

M. DILEEP KUMAR* and YUAN-HUI Lit

(Received 28 June 1994; accepted 5 May 1995)

Abstract-The magnitudes of silica and 226Ra inputs to water (through particle regeneration, in situ, and from sediments) and the validity of observed Si and 226Ra as tracers of water masses and advective processes were examined in the Indian Ocean using the GEOSECS data. The regenerated quantities of these two parameters were calculated as the difference between the observed and the expected concentrations; the latter were estimated from a three end-member mixing model employing potential temperature and salinity as conservative tracers. Here we present results on the quantitative spreading of the Antarctic Bottom Water (AABW); the Modified North Atlantic Deep Water (MNADW, also known as the Circumpolar Water) and the North Indian Deep Water (NIDW)--both these were represented together as High Salinity Deep Waters (HSDW); the Antarctic Intermediate Water (AAIW); the North Indian Intermediate Water (NIIW) and the Central Indian Water (CIW). Our results concur with recent results in the literature. Briefly, the northward flow of the AABW is uneven; the MNADW core layer is found to be closer to the Antarctic that spreads to the north, and AAIW is largely restricted to the Indian Ocean south of 105. Our results also reveal that: roughly 10% more AABW enters the Bay of Bengal than the Arabian Sea; there is greater possibility for deep waters to enter the Central Indian Basin from the Bay of Bengal; CIW occupies a larger part of the Bay of Bengal than of the Arabian Sea; and 10% of the NIIW reaches 305 in the western Indian Ocean.

The regenerations of Si and 226Ra are mainly from the underlying sediments rather than through the dissolution of particles in the water column. The sediments in the northern parts seem to supply 226Ra and Si to the rest of the Indian Ocean. At 10”s there is a subsurface (_ 600 m) maximum in regenerated Si, which is possibly connected to the advection of particles by Indonesian waters. The maxima in regenerated 226Ra and Si contribute about 50% and 30%. respectively, to the observed abundances, suggesting that the observed Si is a more useful tracer of water masses and mixing processes than “‘Ra. Linear relationships were found between regenerated Si and 226Ra, but departures noticed for 226Ra in the eastern Indian Ocean may be attributed to its release from particles transported by the Indian rivers. Diverse regimes with respect to the extent of sources and dissolution of opal were noted in the Indian Ocean: high diatom abundance but low Si regeneration in the Antarctic, high diatom abundance and high regenerated Si in the Arabian Sea, and low diatom abundance but high regenerated Si in the Bay of Bengal.

INTRODUCTION

Silica is one of the bioessential elements in the oceans. The removal of Si from surface waters and its subsequent dissolution at depth is a significant part of the biogeochemical cycle of this element. Radium-226 (half-life of 1622 years) is a daughter of 23c’Th, which in turn is produced from the decay of 234U. Thorium-230 is efficiently scavenged from seawater by particles, facilitating its rapid incorporation into sediments. The general linear relation

*National Institute of Oceanography, Dona Paula, Goa 403 004, India. TDepartment of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii 96822, U.S.A.

83

84 M. Dileep Kumar and Yuan-Hui Li

between Si and 226Ra activities in the water column, away from the sediment-water interface (Ku et al., 1970; Chung, 1980) testifies to their somewhat similar geochemical behaviours in the ocean. As a result, water column concentrations of these two substances are often used to study oceanic mixing processes (Ku et al., 1970; Chung, 1971; Edmond et al., 1979).

The main purpose of the present paper is to evaluate the relative importance of the water mass mixing over that of the regeneration processes in determining the distribution of dissolved Si and 226Ra in the ocean, utilizing primarily the GEOSECS Indian Ocean data. Unless the water mass mixing effects on the distribution of Si and 226Ra are eliminated, actual behaviour and the dominant regions of regeneration in the biogeochemical cycling of these materials in oceans can not be identified. Further, their utility in studying the mixing and spreading of water masses becomes questionable_ if their inputs, through regeneration, occur predominantly within the water column rather than in the sediments.

Study area

The Indian Ocean is much different from the other two major oceans in that it is land- locked to the north. The deep and intermediate waters comprising of the Antarctic Bottom Water (AABW), Circumpolar Water (CW) and Antarctic Intermediate Water (AAIW) are fed from the Indian sector of the Southern Ocean (Wyrtki, 1973). The north-eastern part of the Indian Ocean (Bay of Bengal) receives enormous amounts of dissolved and suspended matter from the rivers flowing through the Indian subcontinent, whereas the north-western part (Arabian Sea) receives highly saline North Indian Intermediate Water (NIIW) because of excess evaporation over the Arabian Sea, the Red Sea and the Persian Gulf.

The northern Indian Ocean in general is highly productive (Krey, 1973; Qasim, 1977). The high productivity contributes to the occurrence of oxygen-deficient conditions and denitrification in the north-western parts (Sen Gupta et al., 1976). The biologically produced siliceous mineral in surface layers of the oceans is opal, SiO2, and is produced by diatoms and radiolarians. Diatoms are abundant in the upwelling areas of the Indian Ocean (Krey, 1973); the Arabia, Somali, equatorial and Antarctic regions. The availability of higher amounts of Si in the upwelled water results in larger incorporation of Si into biota and hence to higher preservation of Si in sediments compared to carbon and phosphorous (C, P and Si of 2%. 2% and 8%, respectively; Baturin, 1983). The opal largely occurs in sediments near the central and eastern equatorial Indian Ocean, in the Antarctic Ocean (Bostrom et al., 1973) and in the Somali basin (Udintsev, 1975). In other areas of the Indian Ocean opal contributes to -=z 2% of the sediments (Broecker and Peng, 1982) including the upwelling zones of the north-western Indian Ocean, probably due to intense remineralization of this mineral, particularly in the north. Dissolved Si in deep water steadily decreases southward but again increases near the Antarctic (Spencer et al., 1982). Significant inputs of 228Ra from sediments are found in the Indian Ocean (Moore and Santschi, 1986). The **‘Ra and Si distributions are used to study the circulation and bottom inputs in the western Indian Ocean (Chung, 1987). In the Bay of Bengal, the Ganges- Brahmaputra river system is estimated to transport 82 x lOI and 5845 x lOi* dpm 226Ra year-’ through the dissolved and suspended phases, respectively (Sarin et al., 1990).

In view of the variable circulation and large changes in primary production and river inputs in both space and time, the Indian Ocean is an ideal area for studying the biogeochemical behaviour of Si and 226Ra and to test their utility as tracers of water

Regeneration of silica and 226Ra in the Indian Ocean 85

circulation. In this study we use data on potential temperature (0) salinity (s), Si and 226Ra obtained during the GEOSECS Indian Ocean Expedition (Weiss et al., 1983; Ostlund et al., 1987) at the stations shown in Fig. 1.

Estimation offractions of water masses

Several water masses occur in the Indian Ocean (Wyrtki, 1973). We considered a total of 21 end member water masses (Fig. 2; Table 1) based largely on the classification of Wyrtki (1973) and on the salinity distribution pattern (Spencer et al., 1982). The Subantarctic Mode Waters (SAMW(E) and SAMW(W)) were selected based on McCartney (1977, 1982). Since

Fig. 1. Station locations covered during the GEOSECS Indian Ocean Programme. Numbers in bold refer to: 1. Owen Fracture zone, 2. Carlsberg Ridge, 3. Ninety-East Ridge, 4. Indo-Pacific

Ridge, 5. Arabian Basin, 6. Somali Basin, 7. Mascarene Basin and 8. Crozet Basin.

86 M. Dileep Kumar and Yuan-Hui Li

3- SALINlTY

EASTEN INDIAN OCEAN

’ 6 1 1 I 1 I 609 400 209 EQ l0.N

x I- L

w 0

SALINITY WESTERN INDlAN OCEAN

6 I I I I I 60. s 40. 20. EQ IO-N

LATITUDE

Fig. 2. Diagrammatic representation of water mass end members in the Indian Ocean (salinity distribution after Spencer et al., 1982). Letters A to U serially refer to water masses given in the first

column of Table 1 (i.e. A to AABW, B to MNADW, . . and U to BBLS).

Water mass

Regeneration of silica and 226Ra in the Indian Ocean

Table 1. Characteristics of various water masses considered

Station No. Depth (m) Theta (0, “C) Salinity Density (as) Si

87

226Ra

AABW MNADW CIW (w) CIW (E) AWW AAIW (D) ITW (W) NIIW AAIW (E) ASW PGW AAIW (E) SAMW (E) SAMW (W) ITW (E) ssw ASHS BBSS NSEHS IW BBLS

430 4709 -0.615 34.659 27.888 137.6 22.0 429 2055 1.769 34.770 27.838 87.2 17.5 423 1383 3.811 34.743 28.638 96.4 19.9 440 993 5.313 34.696 28.437 93.6 21.1 430 119 -1.148 34.025 27.395 45.3 17.9 428 1111 4.509 34.374 27.273 32.9 16.3 422 723 7.415 34.763 27.215 49.1 15.5 417 466 11.483 35.452 27.076 41.5 11.7 433 8 3.334 33.905 27.018 12.0 13.0 431 5 2.349 33.788 27.009 44.6 17.4 416 233 16.965 36.084 26.880 25.7 9.6 429 4 6.492 33.726 26.525 5.4 15.1 434 4 11.754 34.641 26.395 3.7 11.1 428 5 18.569 35.338 25.412 0.6 8.2 440 163 13.842 34.687 26.013 30.4 10.5 436 65 20.432 35.918 25.367 3.8 8.7 416 3 26.345 36.487 24.082 2.0 6.9 446 127 22.345 34.704 23.920 14.8 9.9 441 71 24.775 35.341 23.696 4.3 8.2 440 17 29.130 34.188 21.423 1.1 8.9 446 9 28.107 33.250 21.097 2.1 11.4

AABW-Antarctic Bottom Water; MNADW-Modified North Atlantic Deep Water; CIW(W)-Central Indian Water in the west; CIW(E)---CIW in the east; AWW-Antarctic Winter Water; AAIW(D)--Antarctic Intermediate Water that spreads into deep Indian Ocean from the South Atlantic Ocean; ITWO_Indonesian Throughflow Water in the west; NIIW-North Indian Intermediate Water; AAIW(E)--AAIW near surface in the east; @W-Antarctic Surface Water; PGW-Persian Gulf water; AAIW(W)--AAIW near surface in the west; SAMW(E)-Subantarctic Mode Water in the east; SAMW(W)--SAMW in the west; ITW(E)-ITW in the east; SSW-Subtropical Subsurface Water; ASHS-Arabian High Salinity Water; BBSS-Bay of Bengal Subsurface Water; NSEHS-Northern Southeastern High Salinity Water; IW-Indonesian Water and BBLS-Bay of Bengal Low Salinity Water. For CIW(E), NIIW, AAIW(E), PGW, ASHS. ITW(W) and ITW(E) the 226Ra is taken from depthscloser to those given in column 3. The 226Ra for some end members was obtained as follows: for CIW(W) the average of values around a depth of 14OOm at Stns 421 and 424; for Iv(w) from the same stations at about 750 m; for SAMW(E) average of surface values at Stns 433 and 435 and for SAMW(W) the average of values at 1 and 10 m at 428.Units for Si are pmol kg-’ and for 226Ra are dpm (100 kg-‘).

SAMW is less saline and colder in the east than in the west (McCartney, 1977, 1982), we selected the composition of near surface waters of Stas 434 (SAMW(E)) and 428 (SAMW(W)) as the closest representatives of SAMW, in the respective regions. The deep AAIW (AAIW(D)) was on the basis of subsurface eastward flow of low salinity AAIW from the South Atlantic Ocean (Fine, 1993; Park et al., 1993). The SAMW and the AAIW in the Indian Ocean are also formed by sinking of surface waters in the subantarctic and subtropical frontal regions of this ocean (McCartney, 1977).

Since there occur large variations in the physico-chemical properties of surface waters between the south-eastern and the south-western Indian Ocean (mainly because of the inter- ocean exchange across the Indo-Pacific region south of Australia), the eastern (E) and western (W) end members have been identified for these two water masses. The observed

88 M. Dileep Kumar and Yuan-Hui Li

3.0- NADW

SALINITY

Fig. 3. Three end-member formation of MNADW. Compositions of NADW were obtained from Wyrtki (1973), WSBW from Broecker and Peng (1982), and of ASW (average of summer and winter

values) from Botnikov et al. (1985).

maximum salinity value (34.770, representing NADW), at Sta. 429 was lower than that of the original NADW (S = 34.840) because of the mixing (Fig. 3) of this water mass with the low salinity Antarctic Surface Water (ASW) and the Weddel Sea Bottom Water (WSBW). As the salinity value of 34.770 at Sta. 429 was closer to that of NADW (Fig. 3), we name this core water as the Modified North Atlantic Deep Water (MNADW) hereafter. The deeper regions (~1000 m) of the Indian Ocean are filled by AABW, MNADW, AAIW, Indonesian Throughflow Water (ITW) and NIIW (Wyrtki, 1973; Godfrey and Golding, 1981). Hence, ITW end members were identified in the eastern and western sections (Table 1). The ITW mixes with other water masses in the south Indian Ocean (Godfrey and Golding, 198 1). The western boundary currents in Somali move northward (Schott et al., 1990) carrying this mixture of low salinity that dilutes the northern high salinity intermediate and deep waters. The deep salinity minimum is identified here as Central Indian Water (CIW) that essentially forms due to mixing between ITW and MNADW, as an end member for freshening the deep waters in the north.

In our preliminary analyses four or five end member water masses and an equal number of supposedly conservative properties such as 0, S, Si, PO and NO (the last two as defined by Broecker (1974)) were considered. However, the results mostly yielded negative fractions because of the nonconservative nature of Si and variable Redfield ratios for O/P and O/N in various depth horizons of the Indian Ocean. Variations in these ratios with depth also have been noted by Metzl et al. (1990). In view of these intricacies, we resorted to conserve just 0 and S parameters with a three end-member approach (Fig. 4). Hence, we formed a matrix of three linear equations with two conservative parameters. In Fig. 4,fi are fractions of water masses 1, 2 and 3; S and 0 on the right hand side of the equation represent the observed salinity and potential temperature, respectively, of a sample, A. The end member water masses 1, 2 and 3 were determined from the O-S curve at every station along with the density levels shown in Table 1 and on the geographical location of the stations. For instance, one needs to consider only AABW, MNADW and CIW(W) for deep samples (below MNADW core layer) at Sta. 425 (Fig. l), whereas the end members to be considered for shallow samples are MNADW, CIW(W), AAIW(D), NIIW, SAMW(W), SSW and IW.

Regeneration of silica and 226Ra in the Indian Ocean

1

89

8,f, + B,f, + 9,f, = e*

s, f, + S,f, + S,f, = s A

f + f + f3=1 1 2

Fig. 4. Schematic representation ofcomputational approach for water mass fractions for sample A.

Evaluations were done starting at the bottom and moving up to the surface. Considering the uncertainties in 0 and S for the end member values themselves, the evaluated fractions should be treated to an approximation. At certain locations, four end-member mixing may have been more reasonable. However, the three end-member model generally was found to work well for the entire water column except for the upper 100 m in some cases.

Estimation of expected and regeneratedfractions

The concentration of a dissolved substance at any location (Cobs) in the oceans is the sum of the fractions resulting from its transport by various water masses to that place (Cmix) and from its contribution by biological processes (through uptake during production or regeneration during decomposition) plus sedimentary inputs (C&s), i.e.

Cobs = &ix + Greg (1)

The sum of products of the computed fractions of end member water masses and the respective amounts of Si and 226Ra in the end members yields the expected amounts (Cmix) of these substances for a given sample, at specified depth and location. The difference between the observed and expected amounts represents the Si and 226Ra added due to decomposition of particles (C,,,) within the water column and from sediments. The uncertainties in the estimation of expected silica and 226Ra from water mass mixing were kO.002 pmol kg-’ (at 100 pmol Si kg-‘) and kO.015 dpm (100 kg-‘) (at 25 dpm 226Ra

90 M. Dileep Kumar and Yuan-Hui Li

(100 kg- ‘)), respectively, and the errors involved in the computation of respective regenerated fractions were +_ 0.132 pmol Si kg-’ and f 1 .015 dpm 226Ra (100 kg-‘).

RESULTS AND DISCUSSION

Spreading of water masses

The AABW, MNADW and AAIW form a large portion of deep and bottom waters present in the Indian Ocean (Wyrtki, 1973) and also the Common Water generally present in the Indian and Pacific Oceans (Broecker et al., 1985). In the literature, on the circulation of the Indian Ocean, the term North Indian Deep Water (NIDW) is often mentioned (see Toole and Warren, 1993). Unlike the North Pacific, the North Indian Ocean produces NIDW by the sinking and later mixing of the PGW and the RSW with the circumpolar waters. The NIDW has not been identified as a core layer or a water mass by Wyrtki (1971, 1973) probably because this water does not correspond to a salinity maximum. Nevertheless, a discontinuity is seen in the relationship between 0 and S for Sta. 416 (Fig. 5) with respective values of 1.884”C and 34.767. Hence, the NIDW seems to flow southward overriding the northward-moving polar waters. The composition at the discontinuity in Fig. 5 is closer to that of the MNADW. Consequently, an additional core layer occurs in the north (Fig. 6) corresponding to MNADW composition but actually belonging to the discontinuity observed. For this reason and also because the MNADW and NIDW are relatively high salinity waters, we studied the spreading of these two by referring them, together, as the High Salinity Deep Waters (HSDW).

The northward movement of the AABW is hindered by the bottom topography (Wyrtki, 1973) occurring through the Crozet basin, fractures in the triple junction and in the southwest Indian Ridge and Australian-Antarctic discordance (Park et al., 1993; Toole and

34.04-

34. 82 -

POTENTIAL TEMPERATURE

Fig. 5. Occurrence of a discontinuity in potential temperature (“C) and salinity at Sta. 416.

Regeneration of silica and 226Ra in the Indian Ocean

QEOSECS STATION NUY5EER

I “EAsm I -w W INDIAN OCEAN I

\\\r\\ - \

EASTERN INDIAN OlXAN I Ye HSOW - \

6 I al452 4m 454466 4m 4ss 0 uoul44 u64*1

, 1

91

Fig. 6. (a) Spreading of water masses (%) in the eastern Indian Ocean.

92 M. Dileep Kumar and Yuan-Hui Li

GEOSECS STAT ION NUMBER

4?l a2 433 434 433 438 438 4404442 44Q446

2

I c n. 3 W

0

4

5

??’ NIIW INDIAN OCEAN

6O.S 40. 30 20

LATITUDE

Fig. 6(a). (Continued.)

IO EQ WN

Regeneration of silica and 226Ra in the Indian Ocean 93

OEOSECS STATION NUYDER

V. A40W INDIAN _ ____ 6 I I I ‘6’ I I I I I

0

I

2

I

L l&l 0

4

5

6

GEOSECS STATION NUMBER

203 IQ EQ 209 IO. EQ

LATITUDE

Fig. 6. (b) Spreading of water masses (%) in the central Indian Ocean.

94 M. Dileep Kumar and Yuan-Hui Li

SEOSECS STATION NUMBER

431 430 4284s 427 428 424 421 420 dlS 18 u7 4lS 0 1 1

I v. CIW ‘VYESTENN INOlAN OCEAN I

4

5

8

0

I

2

if a.3 w D

4

5

K HSDW WESTERN WUN OCEAN I

431 430 42s 420 427 421 424 Q) 420 4lS 410 4l7 4H

I

6 ’ I

SO’S 5w 40. 30. XT 10. 20 IO-N

LATITUDE

Fig. 6. (c) Spreading of water masses (%) in the western Indian Ocean.

Regeneration of silica and 226Ra in the Indian Ocean 9.5

QEOSECS STATION NUMBER

WESTERN INDIAN OCEAN

0

I

2

s t-3 0. w 0

I 430 429 428 427 425 424 421 420 419 418 417 4l3

4-

%AAlW WESTERN INDIAN OCEAN 6’ I I I I I I I

60. 30. 40. SD. 20. iw EQ IO’N

LATITUDE

Fig. 6(c). (Continued.)

96 M. Dileep Kumar and Yuan-Hui Li

Warren, 1993, and references therein). The AABW occupies depths > 2000 m in the Indian Ocean (Fig. 6(a)-(c)). Figure 6(a) is in agreement with the drawing of AABW into the Southwest Australian Basin, through the Indo-Pacific Ridge south of Australia, as the abundance of this water mass is relatively higher at the bottom (57%) than at 3600 m (50%) of Sta. 435. As the 50% AABW contour touches the ridge crest, it is also probable that some of this water may flow over it. The AABW is transported to the Pacific Ocean south of the Indo-Pacific ridge (Wyrtki, 1973) (Fig. 6(a)). The AABW contributes >40% to the near bottom waters in the basins west of Australia and to the east of the Ninety-East Ridge. About 40% AABW contour hugs this ridge on the western side, which is probably due to the return flow of this water from the Bay of Bengal into the Central Indian Basin. The flow in the Central Indian Basin is southward in the deeper layers (Warren, 1982). The GEOSECS data indicate that the deep and bottom waters in the Bay of Bengal are relatively colder than those in the Arabian Sea, revealing the comparatively easy accessibility of the former region to AABW than the latter. Figure 7 of Toole and Warren (1993) which depicts the distribution of potential temperature at 4100 m in the central basin water of 1.06”C occurs in the northern part of the basin in the northwest-southeast direction, indicating that these warmer near bottom waters might have their source in the Arabian Sea. Thus this isotherm seems to have been pushed southward, especially along the western flank of the Ninety-East Ridge, by the return flow of the AABW from the Bay of Bengal (Fig. 6(a)). The AABW might have entered the Bay through the passage between the Ninety-East and the Andaman-Nicobar ridge. In the Central Indian Ocean north of the Triple Junction (Fig. 6(b)) the AABW contributes > 40% of the bottom waters.

Spillage of AABW into the Central Indian Basin over the saddles in the Ninety-East Ridge near 1 OS (Warren, 1982) is clearly seen with the 40% contour in Fig. 6(b). This water mass also enters the Central Indian Basin through the passage between the Southeast Indian Ridge and the Ninety-East Ridge (Toole and Warren, 1993).

The spreading of AABW in the west (Fig. 6(c)) is less smooth than in the east. Although the northward moving western boundary current is stronger in the Crozet Basin (Park et al., 1993; Toole and Warren, 1993) the reduced flow of deep and bottom waters through the Atlantis II Fracture Zone results in the decrease of near-bottom %AABW, from 77 at Sta. 428 to 56 at Sta. 427, across the Southwest Indian Ridge. The northward flow of AABW through the narrow passages in ridges across the Crozet, Madagascar, Mascarene and Somali basins is further retarded by the Carlsberg Ridge (Fig. 6(c)). Johnson et al. (1991a,b) have suggested that bottom water from the Somali Basin flows into the Arabian Basin chiefly through the Owen Fracture Zone. Water of 30% AABW hugs the Carlsberg Ridge in the Arabian Basin (Fig. 6(c)). The difference in %AABW between the Arabian Sea and the Bay of Bengal bottom waters at 10”N is about 10.

The NADW is transported to the Pacific Ocean through the circumpolar waters as a part of the global hydrological conveyor belt (Broecker and Peng, 1982; Gordon, 1986). Accordingly, the core layer of the southern HSDW (MNADW) slopes from about 500 m at Sta. 431 to about 2400 m at Sta. 435 (Fig. 6(a)). The most striking feature in Fig. 6 is the occurrence of core layers of HSDW in the north as well, which are related to the discontinuity (Fig. 5) discussed above. Distribution of HSDW in the western section (Fig. 6(c)) is similar to that in the eastern one but for its higher abundance in core layers in the west. The NADW from the South Atlantic Ocean mixes with the Weddel Deep Water and moves eastward along the Antarctic Continent as the Circumpolar Deep Water. In addition, the NADW directly enters the Madagascar Basin through the saddles in the Madagascar

Regeneration of silica and 226Ra in the Indian Ocean 97

Ridge (Toole and Warren, 1993). The northern HSDW (henceforth referred to as NIDW, which corresponds to the composition of the discontinuity in Fig. 5), has its origin in the north-west Indian Ocean. The NIDW has been observed to influence water characteristics in the eastern Crozet Basin (Park et al., 1993), Central Basin and in the eastern Perth Basin (Toole and Warren, 1993). The pattern in Fig. 6 is consistent with the observation of Wyrtki (1971) that salinity at 2500 m in the Bay of Bengal is higher than in the equatorial Central Indian Ocean (Fig. 7).

The volume of the Central Indian Water is greater in the east (Fig. 6(a)) than in the west (Fig. 6(c)), which is at variance with the finding of You and Tomczak (1993). Our analysis suggests that the formation of NIIW and NIDW in the Arabian Sea prevents the northward progress of the CIW. The ASHS and PGW at Sta. 416 pushes the CIW to 1500 m depth in the Arabian Sea (Fig. 6(c)). As a result, CIW at 10”N is 40% at 800 m in the Arabian Sea compared to 60% in the Bay of Bengal. The CIW core layer is shallower in the east (1000 m) than in the west (1300 m) mainly because of the entry of ITW from the east.

LONGITUDE

v .

?Jo’ N SALINITY AT 2!!OOm

Fig. 7. Distribution of salinity and silica (pmol kg-‘) at 2500 m in the North Indian Ocean (after Wyrtki, 1971).

98 M. Dileep Kumar and Yuan-Hui Li

The AAIW originates in the south-west Atlantic Ocean (Piola and Gordon, 1989) or in the south-east Pacific Ocean (McCartney, 1977, 1982). It enters the south-west Indian Ocean generally between 45 and 5O”S, part of which flows into the western and central Indian Ocean and the rest into the Pacific Ocean (Fine, 1993; Park et al., 1993). The AAIW extends northward to about 5”s in all sections in Fig. 6. As this water has a western source, it occupies a larger volume of the South Indian Ocean in the west than in the east. In Fig. 6(a), (c) the southern end of the AAIW core layer lies deeper than in the north because of the southward flow of the more saline northern waters. The AAIW core layer in the eastern section (Fig. 6(a)) exhibits an abundance of 90% that does not seem to belong entirely to the 100% AAIW in Fig. 6(c). This is because the recently ventilated AAIW (< 9 years age), coming from the west into the Antarctic sector of the Indian Ocean, is modified after mixing with the poorly ventilated (< 30 years age) water that re-enters this ocean from the Pacific (Fine, 1993; Park et al., 1993). Hence, a large part of the AAIW spreading to low latitudes in Fig. 6 probably represents that flowing from the Pacific Ocean.

The intermediate waters in the southern Indian Ocean are found to have anticyclonic movement (Wyrtki, 1973). The AAIW and ITW move from the east to the west at low latitudes as a part of major gyre in the south. Recent observations by Fine (1993) show the existence of an additional small scale anticyclonic AAIW gyre in the south-west Indian Ocean. The intermediate and deep waters from the north flow into the Mozambique Basin (Toole and Warren, 1993), and thus the AAIW is found to be warmer and saltier in the South Indian Ocean than in the other two oceans (Taft, 1963). The northward and southward movements of AAIW and NIIW, respectively, are blocked by each other (Taft, 1963) (Fig. 6). Interestingly, AAIW and NIIW have strong sources in the west. Consequently, 10% NIIW contour extends up to 30”s in the west (Fig. 6(c)) but just north of 20”s in the east (Fig. 6(a)). Contours were not shown for Stas 416 and 417 because the NIIW is a mixture of ASHS, PGW and RSW; the last water mass flows at 600-800 m (Wyrtki, 1973), which in the present case occurs south of Sta. 417. Hence, the PGW is used as the high salinity end member in the present computations for Stas 416 and 417. Despite the significant spreading of NIIW in the subsurface layers, this water mass chiefly flows east and south in the upper 400 m (Fig. 6). This analysis suggests that the AABW ventilates into the core layers of the HSDW (comprising of MNADW and NIDW) while HSDW ventilates into AAIW in the south and NIIW in the north. The AAIW and NIIW reach to the upper thermocline waters, in general, and hence will be in exchange with the surface waters in the Indian Ocean. The observed patterns in Fig. 6 are consistent with the net transports estimated by Toole and Warren (1993) across the 32”s: the geostrophic and Ekman transports amount to 7 x lo6 m3 s- ’ southward in the upper 2000 m and to 27 x lo6 m3 s-’ northward below 2000 m. The net southward supply in the intermediate layers could be due to the dominance of ITW, NIIW and SSW over AAIW and SAMW, while the deep northward transport is related to that of AABW and MNADW over NIDW.

Because of the greater volumes occupied by these water masses, the substances transported by these waters are of extreme significance in material cycling in the Indian Ocean. Simple multiplication of the per cent contours at any place in Fig. 6 by the characteristic concentration of a substance in the respective end member water mass will yield the amount of substance transported by that water mass to the specified location. For instance, AABW with an Si of 137.6 pmol kg-’ contributes to 48 pmol kg-’ in the near bottom layers of Sta. 446.

Regeneration of silica and 226Ra in the Indian Ocean 99

Behaviour of silica

The distribution of dissolved Si in the Indian Ocean (Fig. 8(a)-(c)) is characterized by low Si in intermediate layers north of the polar front and near bottom Si maxima at Stas 446 in the east and 429 and 416 in the west. The low Si content between latitudes 30 and 50”s is due to the sinking of surface water (Chung, 1987) together with the transportation of AAIW and NADW from the Atlantic. All sections in Fig. 8(a)-(c) depict tongue-like features extending southward from the Bay of Bengal and the Arabian Sea. Edmond et al. (1979) suggested the southward advective flow, using Si-density plots, from the deep north Indian Ocean. Our results support this view since considerable amounts of Si are added into the overlying

GEOSECS STATICN NUMBER

INDIAN OCEAN

LATITUDE

Fig. 8. Distribution of observed silica (pmol kg - ‘; after Spencer et al., 1982) and 226Ra (dpm (100 kg-‘); after Chung (1987) for the western section) in the (a) eastern, (b) central and (c) western Indian

Ocean.

100 M. Dileep Kumar and Yuan-Hui Li

STATION NO

INDIAN OCEAN 6 I I

LATITUDE

Fig. 8(b).

waters from sediments in the northern Indian Ocean that are subsequently laterally advected towards the south (Fig. 9(a)-(c)).

The water column regeneration of Si is considered to be less important compared to input from underlying sediments, because the sinking Si particles are less decomposed (Broecker and Peng, 1982). An Si,,, of > 40 pmol kg-’ was observed in the deep and bottom waters at

Regeneration of silica and 226Ra in the Indian Ocean 101

GEOSECS STATION NUMBER

INDIAN OCEAN

6’ I I I I, ’ 603 4b 2o” EQ lO”N

LATITUDE

Fig. S(c).

the northernmost Indian Ocean (Fig. 9). Silica seems to be related to the downward diffusion of salt in the North Arabian Sea during which the Si,, from continental margins could be transported to deeper layers. To this diffused Si, more Si,,, is added from the deep sea sediments (Figs 7 and 9). Although upwelling is more prominent in the Arabian Sea than in the Bay of Bengal the observed maxima in Si,, are nearly equal. Probably dissolution of riverine sediment is mostly responsible for the Si,, in the Bay of Bengal, since the maximum of 50 prnol kg-’ occurs at 2300 m at Sta. 446 (Fig. 9(a)).

An important feature in Fig. 9 is the occurrence of an Si,, maximum around 600 m in the water column at 10”s. The AAIW, together with the Indonesian Throughflow Water (ITW), flows from the east to the west (Wyrtki, 1973) in the upper 1200 m, whereas the Indonesian inflow occurs between the surface and 400 m (Godfrey and Golding, 1981). This flow is recognizable up to 70”E, but to the west of this longitude its signature disappears rapidly (You and Tomczak, 1993). The Indonesian region is rich not only in dissolved Si (Wyrtki, 1971) but also in diatom production (Krey, 1973). The westward flow around 10”s probably

102 M. Dileep Kumar and Yuan-Hui Li

SEOSECS SECTION NUlmER

81ssp 433-m 486 48s 0

I - 2.1-

2-

E I- Q 3- w 0

4-

3-

8

INDIAN OCEAN 8 1 I I I I I-

60.5 MT 40. 30’ 20* 10. EQ I0.N

LATITUDE

Fig. 9. Distribution of regenerated silica (pm01 kg-‘) and 226Ra (dpm (100 kg-‘)) in the (a) eastern, (b) central and (c) western Indian Ocean.

transports siliceous particles originating in the Indonesian seas. The abundance of particles might increase considerably in the Indonesian waters because of intense tidal mixing (Wyrtki, 1961) and resuspension of bottom sediment. Thus, the Si,, maxima observed around 600 m at 10”s of the Indian Ocean likely result from the dissolution of particles transported by this westward flow. The relatively higher value of - 30 pm01 kg-’ at Stas 440

Regeneration of silica and 226Ra in the Indian Ocean 103

BEOSECS STAT I ON NUNDER

Si r.9

CENTRAL

I INDlAN DCgAN

265 IDo EO

LATITUDE

Fig. 9(b).

and 450 compared to that at Sta. 424 (N 20 pmol kg- ‘), may be due to decreased transport of particles as the waters moved westward. These observations reveal the significance of suspended particles, transported by the ITW and probably NADW, in the biogeochemical cycling of substances in the Indian Ocean. Broecker and Peng (1982) advanced the idea of regeneration of particles, carried by the hydrological conveyer belt to the Indian and Pacific Oceans, to result in very high dissolved nutrient concentrations in the deep northernmost parts of the respective oceans. The highest 32Si value (Somayajulu et al., 1991) of 145 dpm (100 kg-r) around 2510 m at 38“s (Sta. 428) supports the mechanism of particle transport into the Indian Ocean by NADW. However, Fig. 9(a)-(c) does not exhibit any maximum related to NADW inflow because of the presently selected end member enrichment in Si and 226Ra, compared to that in the original NADW (e.g. Somayajulu et at., 1987), as the NADW mixes with polar and north Indian waters rich in these properties. Moreover, the NADW is younger in the Indian Ocean compared to that present in the Pacific Ocean so that regeneration from NADW-transported particles may not be noticeable in the former region.

The tongue-like feature extending northward from the Antarctic margin to about 60”s around 2000 m in the eastern section could be due to the supply of regenerated Si from slope sediments. These observations agree with those of Abelmann and Gersonde (1992) who reported lateral advection of siliceous particles in waters adjacent to the Antarctic shelf. In the western section (Fig. 9(c)) Si release from sediments at Sta. 429 is calculated to be N 10 pmol kg-’ . Despite the high production of diatoms (Krey, 1973) in the Antarctic waters, the Si,, is low (Fig. 9) due to the faster sinking of siliceous frustules through fecal pellets and aggregate formation (Treguer et al., 1989). This mechanism of transport also is augmented because siliceous suspended particles are minimal in warm deep water (Treguer et al., 1988)

104 M . Dileep Kumar and Yuan-Hui Li

GEOSECS STATION NUMBER

431 430 42S 426 427 426 424 421 420 419 416 417 416

Ra rrl WESTERPi lNDIAN OCEAN

=

sI rrg WESTERN INDIAN OCEAN 6 1 I I I

60.S 40. 209 EQ IO. N

LATITUDE

Fig. 9(c).

and the settling flux of biogenic Si largely accounts for the rate of change in dissolved Si in the upper 100 m (Tsunogai et al., 1986) of the Antarctic Ocean. Hence, in spite of very high production of diatoms in the southern polar waters, Si regeneration is not significant compared to that in the northern parts of the Indian Ocean. Figures 8 and 9 suggest that the fraction of Si transported by water masses is much larger than that contributed by regeneration.

Regeneration of silica and 226Ra in the Indian Ocean 105

Unlike in the Antarctic waters, Si regeneration seems to be prominent in the Arabian Sea, specifically in the margins of the Arabian and off the Persian Gulf (Fig. 7). In the Bay of Bengal, a region where diatoms are less abundant, high Si regeneration could be largely due to inputs from riverine particulates. In spite of the large sedimentation rate in the Bay of Bengal in relation to that in the Arabian Sea, the regenerated Si is only slightly different. Hence, riverine systems must be contributing large amounts of siliceous material of 7.9 g me2 year-’ flux noted at 2000-3000 m in the Bay of Bengal (Ittekkot et al., 1991) while the diatom-rich Arabian Sea has a lower rate of 4.5 g me2 year-i (Nair et al., 1989). This comparison suggests that the opaline material, produced at the surface, could be more efficiently dissolved in the Arabian Sea than in any other part of the Indian Ocean.

Behaviour of “6Ra

Higher activities of 226Ra of 36,35.4 and 30.6 dpm 100 kg- ’ are observed near the bottom in the northernmost parts of the Indian Ocean at Stas 446,447 and 416, respectively (Fig. 8). Chung (1987) presented the distribution of 226Ra in the western Indian Ocean (Fig. S(c)). The distribution of 226Ra is very similar to that of Si (Fig. 8(a)-(c)). Layers of 226Ra-rich water are found near the bottom in the northern regions, and the deepening of 20 dpm (100 kg-‘) contour, around 40-5O”S, is presumably for similar reasons attributed to Si above. However, a contrasting feature is the decreased 226Ra activities in the near-bottom layers around 10”N in the west, although Si distribution remains nearly the same. The observed near-bottom Si at Sta. 447 is lower than that at Sta. 416, but the observed 226Ra is higher at the former station. The difference in the areas occupied by 25 dpm (100 kg-‘) contour from the east to the west is not very large, except for a break at Sta. 424 in Fig. 8(c). Surface layers, in general, contain 226Ra activities lower than 10 dpm (100 kg-‘). South of 40”s latitude the activities of 226Ra are again high.

The distribution of observed 226 Ra is clearly related to that of Si (Fig. 8) leading to similar inferences. Based on these similarities Chung (1980, 1987) related these parameters to the circulation of water masses. There is a difference in the occurrence of maxima at Sta. 447 (Figs 8 and 9); 226Ra peaks at the bottom but Si at about 800 m above the bottom. The mid-column maximum in Si might result from its release from marginal sediments lying further north and its subsequent horizontal diffusion. The near-bottom peak for 226Ra may be due to its release from terrigenous sediments transported by run-off, especially into the Bay of Bengal. In concurrence with this observation, Broecker et al. (1980) observed high nutrient concentrations in a benthic boundary layer in this area and proposed that the deep Bay of Bengal could serve as a source of nutrients to the deep Indian Ocean. A relatively high 226Ra (23 dpm (100 kg-‘)) is correlated with high silica at Sta. 429 (Chung, 1987). The mid-depth highs in 2Z6Ra contours at Stas 425 and 420 (Fig. 8(c)) have been attributed to radium release from ridge flanks and horizontal advection (Chung, 1987). Moore and Santschi (1986) also noted 228Ra maxima in these depths due to horizontal mixing. The general decrease in 226Ra from the north to the south (Fig. 8) agrees with that in the deep Pacific (Ku et al., 1980). The higher 226Ra and Si values in surface waters near Antarctica could be due to upwelling of nutrient-rich deep water (Fig. 8).

The distribution of 226Rare1 (in Fig. 9(a)-(c)) is higher than 2.5 dpm (100 kg- ‘) below 1200 m north of 40% In the depth range between 1200 m and the bottom, Rarei maxima occur close to the sea floor (at the northernmost stations, Fig. 9(b) and (c)). The highest 226Rarel, 18.5 dpm (100 kg-‘), was found at 3300 m at Sta. 446, presumably due to lateral advection

106 M. Dileep Kumar and Yuan-Hui Li

as well as release from the underlying sediments. In the northern Indian Ocean, the 226Rarei decreases westward, with 16.3 and 12.8 dpm (100 kg-‘) values at the bottom of Stas 447 and 416, respectively. Figure 9(c) indeed emphasizes the horizontal advection of 226Rare1 from ridge flanks in the Indian Ocean at 0” and 20”s. Although the maximum released occurs in the east (Fig. 9(a)), the area occupied by the 12.5 dpm (100 kg-‘) isoline is much larger in the central parts than in the east. Between 20”s and the equator the released 226Ra, close to the bottom, is higher in the Central Indian Ocean than in the east or west (Fig. 9). The 226Rare1 of > 12 dpm (100 kg-‘) suggests that this water most likely has a source in the Bay of Bengal. The 12.5 dpm (100 kg-‘) contour hugs the bottom at 10”s in the Central Indian Basin because of the inflow of AABW, with an Rare1 of - 10 dpm (100 kg-‘), across the Ninety- East Ridge. The extent of 226Ra of 2.5 dpm (100 kg-‘) (Fig. 9(a) and (c)) shows that the northern deep waters are more effectively drawn into the South Indian Ocean in the east than in the west. This agrees with the finding that the northern deep waters flow south mostly along the eastern Indian Ocean while the southern waters flow north in the west (Toole and Warren, 1993). Maxima in the released quantities are seen at the southernmost stations in the eastern section between 1400 and 1800 m and 2400 and 2600 m. In the first depth range, high 226Rarel extends to 60”s in the western Indian Ocean. Another higher 226Rarei site is near the bottom at Sta. 429 (Fig. 9(c)).

The 226Rare1 behaviour is similar to that of the 226Ra0r,s . The spatial distribution strongly indicates that the bottom sediments are the major source for 226Rare1, as is the case for Si,,,. Occurrence of the highest 226Rarel at Sta. 446 suggests that the sediments in the Bay of Bengal are the major source of 226Rarel in the Indian Ocean. Despite the higher inputs (Sarin et al., 1990) from rivers for Sta. 446, only a slightly smaller release of 226Ra from sediments at Sta. 447 (16.3 dpm (100 kg-‘)), against that at the former station (18.5 dpm (100 kg-‘)), presumably could be due to increased diffusion because of relatively low sedimentation rates at the latter station (Kadko, 1980). Release of this radionuclide from particles carried by the ITW does not seem to be significant for the water column. It is evident from Figs 8 and 9 that significant amounts of Si and 226Ra are added to the water column of the Indian Ocean from sediments in the north. The maxima in added quantities amount to nearly 50% for 226Ra and 30% for Si of the observed amounts. Hence, Siobs seems to be relatively more useful than 226Ra0bs for studying water mass movements in the oceans. For instance, relatively low levels of Siobs (Fig. 8(a)), Si,,, and 226Rarei (Fig. 9(a)) at the bottom of Sta. 441 (i.e. just east of Ninety-East Ridge) and further east reveal the northward flow of AABW, but those just north of this ridge indicate that the flow is southward from the Bay of Bengal to the Central Basin. This is not as clear from 226Ra0bs (Fig. 8(a)).

Silica and 226Ra relations

Radium-226 is correlated with the dissolution of opal in the Indian Antarctic (Ku et al., 1970) and in the Atlantic Oceans (Broecker et al., 1976). Linear relations occur between the observed Si and 226Ra up to intermediate depths in the Pacific (Chung, 1980) and in the western Indian Ocean (Chung, 1987). However, departures from the trends observed in the upper and intermediate layers seem to occur for the deep and bottom waters, with 226Ra present in excess over that expected from the linear relation. Chung (1980, 1987) related the deviations between Si and 226Ra to variations in source functions. As shown in Fig. 9, contribution from sediments to that of the observed abundance is greater for 226Ra than that for Si, which might account for the departures. These departures generally occur in the

Regeneration of silica and **‘Ra in the Indian Ocean 107

eastern Indian Ocean, and are more significant in the Bay of Bengal (Fig. 10). No clear deviations are noticed in the Arabian Sea despite high Si dissolution. Enhanced release of 226Ra from the river-transported terrigenous material could have resulted mainly in the observed departures in the eastern Indian Ocean. Relationships between Si,, and 226Ra,el for different areas are shown in Fig. 10. At the individual stations we find relations for the regenerated quantities similar to those noted by Chung (1987) for the observed concentrations. The slopes found for relations in Fig. 10 are: 2.1 x lo3 for the north- eastern; 2.0 x lo3 for the north-western; 2.3 x lo3 for the south-eastern; 1.8 x lo3 for the south-western and 2.8 x lo3 dpm mol-’ for the central regions. Our value for the Arabian Sea (2.0 x lo3 dpm mol-‘) is similar to that of 1.3 x lo3 dpm mol-’ calculated by Chung (1987).

CONCLUSIONS

In this paper we have quantified the amounts of Si and 226Ra contributed by water mass mixing and biogeochemical processes in the Indian Ocean. We have used the GEOSECS

20, , 201 .

-IO-+ - - ’ 1 - ’ ’ * -40 -30 -20 -w 0 10 20 30 40 & oo--40 -30 -20 I 0 ’

II

IO

5

0

-5 1

20’

,~ SOUTHWEST

I

REQENERATED SILICA

Fig. 10. Regional relations between regenerated silica (pmol kg-‘) and released 226Ra (dpm (100 kg-‘)) in the Indian Ocean.

108 M. Dileep Kumar and Yuan-Hui Li

Indian Ocean data to document the spreading of AABW, MNADW and NIDW shown together as HSDW, CIW, AAIW and NIIW.-Our quantitative results, which agree with previously published results, suggest that: more AABW enters the Bay of Bengal than the Arabian Sea, northern high salinity waters extend up to 3O”S, and deep waters most likely flow into the Central Basin from the Bay of Bengal. Silica regeneration within the water column is less than that occurring near the bottom. Interestingly, the water column Si,,, is relatively high along 105, at about 600 m. This is probably related to the regeneration from particles transported into the Indian Ocean from the Indo-Pacific. The 226Ra and Si released from sediments in the north seem to be the sources of these materials present in the south Indian Ocean. The 226Rare1 and Si,, exhibit linear relationships in the Indian Ocean but with some departures in the east. Si,,, contributes less than 226Rare1 to the observed values, and consequently the Siobs seems to be a better indicator of mixing processes in the oceans.

AcknoI~ledgements-The senior author acknowledges the Director. NIO, Goa and the Council of Scientific and Industrial Research in India for supporting his stay at the University of Hawaii through the Raman Research Fellowship. He is further thankful to Dr R. Sen Gupta, NIO, Goa for encouragement, to Prof. Klaus Wyrtki for stimulating discussions and the Department of Oceanography, University of Hawaii at Manoa for providing facilities. The authors appreciate the constructive criticism by Dr S. W. A. Naqvi that greatly improved the manuscript.

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