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Clay mineralogy and sedimentation in the western Indian ocean
Deep-SeaResearch, 1976,Vol.23, pp. 949 to 961. PergamonPress. Printedin GreatBritain.
Clay mineralogy and sedimentation in the western Indian Ocean* VENKATARATHNAM KOLLA,t~ LAWRENCE HENDERSON~ a n d PIERRE E. BISCAYE~ (Received 24 November 1975; in revised form 30 January 1976; accepted 7 February 1976)
Abstract--The clay mineralogy of 235 surface sediment samples from the western Indian Ocean reveals many sedimentary sources and processes: (1) some areas, mainly in the central Indian Ocean, where very little sediment transport is involved, have smectite-rich clays derived from the alteration of in situ submarine basalts. (2) In many other areas, the mineralogy is significantly influenced by several modes of long-distance sediment transport, and the continental climate or the southern ocean volcanism. These areas consist of: (a) smectite-rich sediments in the Crozet and Madagascar basins and in the eastern Arabian Sea; (b) illite-rich sediments in the Indus Cone, the Agulhas Basin, and the ocean adjacent to Africa west of the Agulhas Plateau; (c) illite-palygorskite-rich sediments on the Carlsberg Ridge, Owen Ridge and in the westernmost Arabian Sea; (d) illite&aolinite-rich sediments in the Mozambique Basin and in the regions between Madagascar and Africa; (e) kaolinite-rich sediments adjacent to central Africa and Madagascar. (3) A third category of areas has mixed type sediments with no specific mineral in characteristically high amounts. Mixing of sediments derived from the alteration of in situ submarine basalts and/or from other areas rich in different minerals resulted in this group. The mineral distributions suggest that the Antarctic Bottom Water circulation is responsible for the sediment dispersal in the Crozet, Madagascar, Agulhas and Mozambique basins. Although the aeolian process could be effective throughout the Arabian Sea, it is dominant only in the western Arabian Sea.
INTRODUCTION SEVERAL p a p e r s (STEWART, PILKEY a n d NELSON,
port. Surface water circulation is also in part meridional, especially near the continents, and in part latitudinal (Fig. 1). Although the importance of northward-moving AABW in sediment transport has been mineralogically shown in the South Atlantic (e.g. BISCAYE, 1965), no such role of bottom currents has been demonstrated in the Indian Ocean, nor is the role of other water masses in sediment transport known. In view of the complex physiography (Fig. 1), the potential influence of AABW, and other currents that need not flow latitudinally, combined with a variety of probable sediment source regions, the near-perfect latitudinal clay mineral zonation established by RATEEV,GORBUNOVA,LISITZINand Nosov (1969) (on the basis of about 100 samples
1965; GORBUNOVA,1966; GRIFFIN, WINDOM and 1968; RATEEV, GORBUNOVA, LISITZIN and Nosov, 1969; GOLDBERGand GRIFFIN, 1970) have dealt with clay mineralogy in the western Indian Ocean. The studies were based on relatively few samples and have provided only a broad understanding of Quaternary sedimentation. More recent investigations resulting from the Deep Sea Drilling Project Legs 23, 24, 25, and 26, have contributed much to the knowledge of the pre-Quaternary mineralogy and sedimentation but have added little or no detail on the Quaternary sedimentation. W~ST (1938), LE PICHON (1960), WYRTKI(1971, 1973), WARREN (1974), and KOLLA, SULLIVAN, STREETER and LANGSETH (1976) discussed the * Lamont-Doherty Geological Observatory Contribution spreading pattern of Antarctic Bottom Water No. 2363. t Previous publications by this author have appeared under (AABW) on the floor of the Indian Ocean. This the name Kolla (or K) Venkatarathnam. flow is essentially meridional (Fig. 1) and is a Lamont-Doherty Geological Observatory of Columbia potentially important means of sediment trans- University, Palisades, New York 10964, U.S.A. 949 GOLDBERG,
950
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERRE E. BISCAYE
20
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°
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~ 80 °
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Fig. 1. Circulation patterns of the Antarctic Bottom Water and North Atlantic Deep Water (taken from KOLLA, SULLIVAN, STREETERand LANGSETH, 1976), and of surface water (taken mainly from DOING, 1970) in the western Indian Ocean. The surface circulation in the northern ocean is strongly seasonal; the 'February' circulation, which is less prevailing than the 'August' one, is shown by dashed arrows only in a few places to avoid cluttering of symbols. C.I. = Crozet Island-Plateau re#on; K.1. = Kerguelen Island-Plateau re#on; O.R, = Owen Ridge in the Arabian Sea adjacent to the Arabian Peninsula; C.R. = Carlsberg Ridge.
Clay mineralogy and sedimentation in the western Indian Ocean
covering the whole Indian Ocean including the Southern Ocean) may not be valid. Also because of low sampling densities, we suspect that several potentially important sediment contributions from the vast African continent, which is characterized by arid climates in the northern and southern regions and tropical humid climates in the central regions, were not adequately identified in previous studies. We report here clay mineral analyses of 235 core-top (Quaternary age) sediment samples from the western (west of about 80 °E) Indian Ocean. Largely because of increased sampling density, we have been able to distinguish numerous sediment sources and processes that support the inferences of previous workers in some cases but differ in others. METHODS OF STUDY The top 2 to 5 cm of trigger weight and piston cores were sampled and analyzed to determine the distributions of fine fraction minerals. Samples were prepared as described by BISCAYE (1965) except that the separated size fractions (< 2 lam) were freeze-dried and two or three aliquots of each sample were made into pastes with water and smeared rather than pipetted on glass slides (GIBBS, 1965). The clays were analyzed on a Siemens X-ray diffractometer with a scan speed of 1.0 °20 rain-1. The major clay mineral groups-smectite, kaolinite, chlorite, and illite--were identified and their weighted peak-area percentages were estimated using the same criteria as those of BISCAYE(1964, 1965). The analytical precision is estimated to be between 5 and 10~o. A sharp peak at 10.4 to 10.6h on X-ray diffractograms was considered to be indicative of palygorskite; this was confirmed by electron microscopy. The abundance of palygorskite relative to that of illite was estimated by measuring the peak heights of palygorskite and illite and calculating their ratios. RESULTS AND DISCUSSION Quaternary clay mineral distributions from the analysis of about 200 samples are shown in Figs. 2, 3, 4, 5, 6, and 7. Because of the non-
951
availability of significant numbers of cores in the Arabian Sea, the analyses by GOLDBERG and GRIFFIN (1970) of 35 samples from that region have been collated with our data. Their methodology was similar to ours. We have also included some data from VENKATARATHNAMand BISCAYE (1973) for the regions just east of 80 °E. On the basis of clay mineral distributions (Figs. 2 to 7) and the generalized oceanic circulation (Fig. 1), many sedimentary provinces (Fig. 8) and processes have been inferred. We believe that these sedimentary provinces are caused primarily by the derivation of sediments from different sources and subsequent sediment dispersals. The distinction of these provinces is based only on fine sediment (< 2-~tm size) mineralogy. Some of these provinces will no doubt change when more detailed data become available in the future. However, description and discussion of the observed distributions are best made here in terms of the provinces instead of mineral by mineral. Some provinces appear to be of mixed type where no specific mineral is present in characteristically high amounts. The fine-grained sediments of such provinces are dominated by the alteration products of in situ basalts and/or sediments from other adjacent provinces. These mixed type provinces will not be further discussed. Smectite-rich provinces Several mineral provinces are characterized by abundant smectite ( > 609/0) and are distinguished primarily geographically. The smectite-rich area in the eastern Arabian Sea adjacent to India (designated as Deccan province) is similar to that shown by GOLDBERGand GRIFFIN (1970). As in the case of the smectite province in the western Bay of Bengal (GOLDBERG and GRIFFIN, 1970; VENKATARATHNAM and BISCAYE, 1973), the source of the smectite-rich sediments in the Arabian Sea is in the soils developed on Deccan volcanics in an arid climate. The Narmada and Tapti rivers (Fig. 1), which drain the Deccan soils, supply the smectite to the eastern Arabian Sea. The dispersal of these sediments in the Arabian Sea is primarily brought about by the prevailing surface currents (Fig. 1),
952
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60 °
80 °
100 °
80*
]t~60o q 100 °
60*~1
F 20*
40*
60*
Fig. 2. Smectiteweightedpeak-area percentage. which flow southward along the continental margin. We recognize some areas, mainly in the central Indian Ocean, where smectite-rich clays appear
to be derived mainly from the alteration of in situ submarine basalts and the associated volcanic products, analogous to that described by GRIFFIN, WINDOM and GOLDBERG (1968) for the central
Clay mineralogyand sedimentationin the western Indian Ocean
20 ° h
40 ° A
60 ° Zh
80 ° /~
953
/00" l~
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~
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~>50
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_J r--6OO 7 20 °
V 40 °
V 60 °
V 80*
IOO"
Fig. 3. Illiteweightedpeak-area percentage. South Pacific. Except east of 80 °E, volcanic ash layers are absent from the cores of these provinces. However, some smectite contribution from the alteration of fine, dispersed ash coming from the global fallout of volcanic dust (VENKATARATHNAM
and BISCAYE,1973) cannot be excluded. In general, transport for only short distances is involved in these provinces~ although east of 80 °E, transport of Indonesian tephra by the northeast trade winds and equatorial surface currents becomes signi-
954
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60*
80 °
100 °
Z>2o o
KAOLINITE [ ~ ' f f J <'I0
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/
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66
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/
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/
/
wlr'~60o
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~7
20 °
40*
60 °
80 °
100 °
Fig. 4. Kaolinite weighted peak-area percentage.
ficant (VENKATARATHNAM and BISCAYE,1973). The Antarctic-Crozet smectite province occupies the northeasternmost region of the IndianAtlantic and Crozet basins, part of the MidIndian Ridge (50 to 63 °E and 30 to 25 °S), and
the eastern sections of the Madagascar and Mascarene basins (Figs. 2 and 8). GRIFFIN, WINDOM and GOLDBERG (1968) showed high smectite concentrations in these areas similar to our Antarctic-Crozet Province, but did not dis-
955
Clay mineralogyand sedimentation in the western Indian Ocean
20 °
40 °
60 °
80 o
~0
o
C>2o o
CHLORITE
/ /
/
/ /
/
/
U/(/" /
,=,l t ~ 6 o o q 20 °
40 °
60 °
80 °
tOO°
Fig. 5. Chlorite weighted peak-area percentage. cuss the details of the sedimentary processes bringing about the distribution. RATEEV,GORBUNOVA, LISITZIN and Nosov (1969) did not show this smectite pattern at all. The effects of northward-moving AABW on the
sea floor in the Crozet Basin are reflected in erosional-depositional forms, current lineations, and relatively high bottom water turbidity (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976). The Antarctic-Crozet Province appears to
956
VENKATARATHNAMKOLLA, LAWRENCEHENDERSON and PIERRE E. BISCAYE
40* 50"
50*
60*
70*
80* 30"
20*
20*
10"
10"
0 a
40* Fig. 6.
50*
60*
70 °
0° 80*
Distribution of palygorskite/illite peak-height ratios in the Arabian Sea. Solid dark line is a traverse along which data for Fig. 7 were taken.
be influenced mainly by the AABW transport of smectite-rich clays from the areas of southern ocean volcanism--the Crozet Plateau-Island and Kerguelen Plateau regions. The altered products of the Mid-Indian Ridge volcanics and the sediments from the Madagascar Province (to be described later) could also locally influence the Antarctic-Crozet Province. The Crozet Basin has 300- to 500-m thick sediments above the acoustic basement (EwING, EITTREIM, TRUCHAN and EWING, 1969), suggesting that the AABW has been active in the Crozet Basin since preQuaternary times. The high smectite zone extending continuously
from the Crozet Basin to the Madagascar Basin (Figs. 2 and 8) suggests that part of the MidIndian Ridge at 50 to 63 °E serves as a sill or sills for AABW flowing from the Crozet Basin into the Madagascar Basin. A sill passage at 29 to 26 °S and 60 to 64 °E was first suggested by WARREN (1974) on the basis of the temperaturesalinity characteristics of the Madagascar and Crozet basins. The low carbonate content (KOLLA, Bk and BISCAYE, 1976) and the distributions of Antarctic diatoms (BuRCKLE, VENKATARATHNAMand BOOTH, 1974), smectite (Fig. 2), and potential temperature (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976; WYRTKI, 1971)
Clay mineralogy and sedimentation in the western Indian Ocean
957
extending from the Crozet Basin through this illite-chlorite-rich sediments from the Antarctic region of the Mid-Indian Ridge into the margin of the Indian Ocean sector; (2) because of Madagascar Basin all broadly confirm WARREN'S their coarse size, illite and chlorite would be (1974) suggestion. Insufficient data in the im- quickly dropped to the sea floor; instead, the mediate vicinity of the sill, however, preclude its finer grained smectite available around the Crozet Plateau-Kerguelen Island volcanic regions, would precise geographic location. be picked up by AABW and transported north into the Crozet Basin. 1000-
l llite-rich provinces We define five illite-rich areas in the western 3000Indian Ocean--the Antarctic, South African and Zambezi provinces in the southwest Indian 4000Ocean, and the Arabian and Indus provinces in the northern ocean (Figs. 3, 5, 6, and 8). 5000 The South African Province occupies the African continental margin and abyssal areas between the Agulhas Plateau and South Africa. It apparently derives its sediments from South African soils considered to be rich in illite (VAN 2.0 DER MERWE and HEYSTEK, 1955, 1956; GRIFFIN, WINDOM and GOLDBERG, 1968; RATEEV, GOR1.0 BUNOVA,LlSITZINand Nosov, 1969). The Zambezi Province with 25 to 60% illite and 15 to 25% 0.0 ' 6' 5 ' 45 8'o kaolinite (usu ally with some gibbsite) occupies the 45 0 55 6' 0 70 LONG/TUDE ( o f f ) region between Madagascar and Africa, the Fig. 7. Palygorskite/illite peak-height ratios versus water Mozambique Basin, and to some extent the depth and longitude of samples along a traverse (dark solid Madagascar Ridge where it is diluted by line in Fig. 6) in the Arabian Sea. Madagascar Province days. The Antarctic illite province has 30 to 600/0 illite, but in contrast to We have no samples from the southernmost the Zambezi Province, it is lower in kaolinite and areas of the Southern Ocean. RATEEV, GOR- higher in chlorite (Figs. 4 and 5). This province BUNOVA, LISITZlN and Nosov (1969) noted high occupies the Agulhas and northwest part of the amounts of illite-chlorite on the Antarctic con- Atlantic Indian basins. tinental margin of the Indian Ocean sector. HowThe Weddell Sea-derived AABW spreads from ever, our studies further north, around 50°S in the Atlantic-Indian Basin into the Agulhas and the southern Crozet Basin, which are based on the Mozambique basins through two passages in the analysis of more samples than those reported by Mid-Indian Ridge--one at 20 to 25 °E and the the above authors, show abundant smectite. If the other at 35°E (HEEZEN, THARP and BENTLEY, high amounts of illite reported by RATEEV, 1972; KOLLA,BURCKLE,BISCAYEand HENDERSON, GORBUNOVA, LISITZINand Nosov (1969) for the 1975; KOLLA, SULLIVAN, STREETER and LANGsouthernmost ocean are correct, these illite-rich SETH, 1976). The passage at 35 °E appears to be sediments are not transported very far north for reflected in the distributions of illite (Fig. 3) and two possible reasons: (1) the AABW that travels Antarctic diatoms (KOLLA, BURCKLE, BISCAYEand north through the Crozet Basin is Weddell Sea- HENDERSON, 1975), although these distributions derived (e.g. KOLLA, SULLIVAN, STREETER and are based on the analysis of samples from the LANGSETH, 1976) and could not have transported deep basins with no representation on the inter2000-
958
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60 °
BO °
1000
~20 °
LEGEND [ ] I GROUP
?
4" P R O V .
I ~ IT GROUP [ ] 1TrGROUP
(:..
C E N TRA L ~AFRICAA/
.~N D'O~ ~.S'YAiV "/~ VOLOXI NlC: : PROVINCE:
...............
.'W: : : ; :
~ : " :~ :N:S: 7: r:O: : :.:-
6 0 o.= 20 °
PRO V I N C E
?
,,
F
'~US T ~
?
? t~.
:: ::
,
V
~
40 °
60 °
80 °
"60
o
I00"
Fig. 8. Mineral provinces in the western Indian Ocean. I Group. Provinces rich in smectite derived from the alteration of in situ submarine basalts and with little or no effects of long-distance sediment transport• II Group• Provinces rich in different minerals derived from continents or southern ocean volcanic areas and with significant effects of several modes of long-distance sediment transport• III Group• Mixed type provinces with no minerals in characteristically high amounts. The sources of sediments for areas with question marks are not known.
Clay mineralogy and sedimentation in the western Indian Ocean
vening ridge at 50°S and 35 °E. However, the Antarctic illite province as a whole has probably been affected by the Antarctic Bottom Water traveling through the above two passages. Icerafting of some illite in these areas is also possible. The ultimate source of this illite must be somewhere in the Antarctic continental margin although some of it could be derived from the Zambezi Province or the South African Province. The Zambezi is the only major river draining from Africa into the Indian Ocean and contributes about 100 million tons of sediment per year (RATEEV, GORBUNOVA, LISlTZIN and
Nosov,
1969). The day mineral composition of the Zambezi River sediments is not known. However, the climate of the river-drainage basin suggests that the Zambezi sediment should contain fairly high amounts of kaolinite in addition to illite. Transport of Zambezi sediments to the south (Madagascar Basin and in part Madagascar Ridge) is accomplished in part by the Agulhas surface current system. The nature of weathering in the Zambezi River drainage areas during glacial times might have differed from that of the present, producing more illite in the past. During times of lowered sea levels, turbidity currents must have been the dominant transport mechanism that deposited sediments with thickness exceeding 1 km in the Mozambique Basin (EwING, EITrRF.IM, TRUCHAN and EWIN6, 1969). Some patches of high amounts of illite within the Zambezi Province may be due to the analysis of pre-Holocene turbidite sediments exposed on the sea floor in places. In addition to the Agulhas surface and turbidity currents, the AABW and North Atlantic Deep Water (NADW) play important roles in the transport of Quaternary Zambezi sediment or the redistribution of it (Fig. 1). However, no clear distinction can be made between various modes of transport in the Zambezi Province with the available data. The northern illite-rich areas in the Arabian Sea--the Indus and Arabian provinces (Figs. 3, 5 and 8)--also have high amounts of chlorite. The Indus Province is mainly confined to the Indus Cone (central regions of the Arabian Sea with sediment thickness exceeding 2 km), whereas the
959
Arabian Province is confined to the western Arabian Sea, especially the Carlsberg Ridge (northwest section of the Mid-Indian Ridge) and the Owen Ridge adjacent to the Arabian Peninsula. The two provinces appear to overlap in the southeastern Arabian Sea. The chief distinction between the Arabian and Indus provinces is that the former has high to moderate amounts of palygorskite whereas the latter has little or none of it (Fig. 6). Although palygorskite is present in a suite of samples from a broad band extending from Arabia to the southern tip of India, its abundance is high in the western Arabian Sea adjacent to the Arabian Peninsula. The palygorskite/illite ratios versus longitude and water depth are shown in Fig. 7 for a group of samples along a traverse indicated in Fig. 6. There is a good correlation ofpalygorskite highs with topographic highs. A similar relationship was noted by GOLDaERG and GRIFFIN (1970) on the basis of fewer samples. A relationship of this type reflects aeolian processes. GOLDBERGand GRIFFIN(1970) showed an illiterich band extending continuously from about the region of the Indus River confluence through the Arabian Sea and eastern equatorial Indian Ocean all the way to the Indonesian Archipelago and attributed it to the aeolian process. VENKATARATHNAM and BISCAYE (1973), on the basis of higher sample density, have concluded that the eastern equatorial Indian Ocean illite is derived primarily from the Ganges source. Absence of palygorskite in the eastern Indian Ocean sediments (Fig. 6) confirms this. An aeolian component would certainly be more dominant in the Arabian Sea than in the eastern equatorial Indian Ocean because the former is surrounded by arid to semi-arid land masses influenced by strong monsoon winds. In the Indus Province, even though the amount of aeolian component may be considerable, we suspect that sediment derived from the Indus River would be more significant because of the huge amounts of sediment (400 million tons per year) delivered to the ocean by this river (RATEEV, GORBUNOVA, LISlTZIN and Nosov, 1969). The Indus sediment is believed to have been dispersed largely by the prevailing
960
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
surface circulation (Fig. 1) and by turbidity currents, which were more active at times of lowered sea levels during the Pleistocene. Available data on bottom water temperature and turbidity, as well as bottom photographs, suggest no bottom current activity except possibly in the southernmost deep area of the Arabian Sea (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976). Unlike in the Indus Province, the aeolian component appears to be dominant in the Arabian Province. The Arabian Province sediments are derived from the deserts of Somalia and Arabia by prevailing monsoon and northwesterly winds as implied by HEEZEN, NESTEROFF, OBERLIN and SABATIER (1965) and GOLDBERG and GRIFFIN (1970). Kaolinite provinces
The kaolin-rich Central African, Zambesi (described under illite-rich provinces), and Madagascar provinces are adjacent to Africa and Madagascar (Figs. 4 and 8). The Australian Province (Fig. 8) is a continuation of the kaolinite province in the southern Wharton Basin and on the adjacent Ninety East Ridge reported by GRIFFIN, W1NDOM and GOLDBERG (1968) and VENKATARATHNAM and BISCAYE (1973). This Australian Province kaolinite has been transported by the southeast trade winds from the deserts of western Australia. Most of the African continent from about the equator to about 30°S contributes fairly high amounts of kaolinite, which are delivered to the ocean by rivers. However, the sediments supplied from Africa at about 10 °S (adjacent to the Central African Province) and from Madagascar (adjacent to the Madagascar Province) seem to have the highest amounts of kaolinite. The dispersal of African kaolinites within the ocean may be affected in part by surface circulation and in part by deep currents, for which no details are available. It is possible that fairly high amounts of smectite besides high kaolinite could have been supplied from Madagascar. Smectite, being more fine-grained, could have been transported farther
than kaolinite, thus leaving behind the kaoliniterich clays in the Madagascar Province. The smectite transported farther could add to the clays in the eastern Madagascar Basin that are believed to be derived by AABW transport from the Crozet Basin. CONCLUSIONS
From detailed clay mineral analyses of Quaternary sediments from the western Indian Ocean, we draw the following main conclusions: 1. The clay mineral distributions in the western Indian Ocean are influenced not only by the continental climate and geology but also by the complex physiography of the ocean, submarine volcanism, and several modes of sediment transport; hence, the near-perfect latitudinal distributions of clay minerals shown by RATEEV, GORBUNOVA, LISITZINand Nosov (1969) for the Indian Ocean are not generally valid. 2. Some areas mainly in the central Indian Ocean consist of fine-grained smectite sediments derived from the alteration of in situ submarine basalts. Little or no long-distance sediment transport has affected these areas. 3. Sediments in many other areas of the western Indian Ocean are significantly affected by the continental climate and geology or southern ocean volcanism, and by long-distance sediment transport through oceanic surface circulation, Antarctic Bottom Water movements, turbidity currents, or aeolian processes. These sediments are rich in smectite, illite, illite-kaolinite, illitepalygorskite, or kaolinite. Distributions of smectite and illite in the southwestern regions of the Indian Ocean, supplemented by those of diatoms, calcium carbonate, and bottom water characteristics reported by previous workers, suggest two passages--one at 60 to 64 °E and the other at 35 °E in the Mid-Indian Ridge, which serve for the AABW northward flow and sediment transport from the southern ocean. 4. In a third group of areas, mixing of sediments derived from in situ basalt alterations, from other adjacent areas mentioned above, or both, has resulted in sediments with no minerals in characteristically high amounts.
Clay mineralogy and sedimentation in the western Indian Ocean
961
Acknowledgements--We thank Drs. J. E. DAMUTH, G. Ross
hebdomadaires S~ances de I'Acad~mie des sciences, Paris,
HEATH, D. A. MCMANUS, L. BURCKLEand E. BONATTI for reading the manuscript critically and offering many helpful suggestions. The work was supported by the Office of Naval Research Contract N00014-75-C-0210 and the National Science Foundation Grants IDO-71-04204-A04 or OCE-7519627 (CLIMAP). Grants from ONR (00014-75-C-0210) and NSF (DES72-01568-A03) have supported the shipboard coring operations and curatorial services to preserve the sediment cores at Lamont-Doherty Geological Observatory. We appreciate the help of Miss PHYLLIS HELMS in providing samples from the Scripps Institution of Oceanography Core Library.
260, 2819-2821. HEEZEN B. C., M. THARP and C. R. BENTLEY (1972) Morphology of the Earth in the Antarctic and Subantarctic.
REFERENCES BISCAYE P. E. (1964) Distinction between chlorite and kaolinite in recent sediments by X-ray diffraction. American Mineralogist, 49, 1281 1289. BISCAYE P. E. (1965) Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Bulletin of the Geological Society of America, 76, 803-832. BURCKLEL. n., K. VENKATARATHNAMand J. D. BOOTH (1974) Sediment transport by Antarctic Bottom Water in the western Indian Ocean. (Abstract). Transactions. American Geophysical Union, 55, 312. D131NG W. (1970) The monsoon regime of the currents in the Indian Ocean, East West Center Press, 68 pp. FWING M., S. EITTREIM, M. TRUCHAN and J. EWING (1969) Sediment distribution in the Indian Ocean. Deep-Sea Research, 16, 231 248. GIBBS R. J. (1965) Error due to segregation in quantitative clay mineral X-ray diffraction mounting techniques. American Mineralogist, 50, 741-751. GOLDBERG E. D. and J. J. GRIFFIN (1970) The sediments of the northern Indian Ocean. Deep-Sea Research, 17, 513 537. GORBUNOVAZ. N. (1966) Clay mineral distribution in Indian Ocean sediments (English translation). Oceanology, 6, 215 221. GRIFFIN J. J., H. WINDOM and E. D. GOLDBERG(1968) The distribution of clay minerals in the world ocean. Deep-Sea Research, 15, 433-459. HEEZEN B. C., W. D. NESTEROFF,A. OBERLIN and G. SABATIER (1965) Drcouverte d'atlapulgite darts les s~diments profonds du Golf d'Aden et de la Mer Rouge. Comptes rendus
Antarctic Map Folio Series, American Geographical Society, Folio 16. KOLLA V., L. BURCKLE, P. E. BISCAYE and L. HENDERSON (1975) Clay mineralogy, diatoms and sedimentation in the western Indian Ocean. (Abstract). Transactions. American Geophysical Union, 56, 370. KOLLA V., L. SULLIVAN, S. STREETER and M. LANGSETH (1976) Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom water potential temperature, turbidity, and sea-floor photography. Marine Geology, 21, 171 189. KOLLA V., A. W. H. Bf~ and P. E. BISCAYE(1976) Calcium carbonate distribution in the surface sediments of the Indian Ocean.flournal of Geophysical Research, 81, 2605-2616. LE PICHON X. (1960) The deep water circulation in the southwest Indian Ocean. Journal of Geophysical Research, 65, 4061-4074. RATEEV M. A.. Z. N. GORBUNOVA. A. P. LISITZIN and G. L. NoSOv (1969) The distribution of clay minerals in the ocean. Sedimentology, 13, 2143. STEWART R. A., O. PILKEY and B. W. NELSON (1965) Sediments of the northern Arabian Sea. Marine Geology, 3, 411-427. VAN DER MERWEC. R. and H. HEYSTEK(1955) Clay minerals of South African soil groups: III. Desert and adjoining semi-arid regions. Soil Science, 80, 479-494. VAN DER MERWEC. R. and H. HEYSTEK(1956) Clay minerals of South African soil groups: IV. Soils of the temperate regions. Soil Science, 81, 399 414. VENKATARATHNAMK. and P. E. BISCAYE(1973) Clay mineralogy and sedimentation in the eastern Indian Ocean. Deep-Sea Research, 20, 727-738. WARREN B. A. (1974) Deep flow in the Madagascar and Mascarene basins. Deep-Sea Research, 21, 1 21. ~h3sw G. (1938) Bodentemperatur und Bodenstrom in der atlantischen, indischen und pazifischen Tiefsec. Bcitraqe zur Geophysik, 54, 1-8. WYRTRI K. (1971) Oceanographic atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, D.C. WVRTKIK. (1973) Physical oceanography of the Indian Ocean, In: The biology of the Indian Ocean, B. ZEITZSCHEL, editor, Springer-Verlag, pp. 18-36.
Clay mineralogy and sedimentation in the western Indian Ocean* VENKATARATHNAM KOLLA,t~ LAWRENCE HENDERSON~ a n d PIERRE E. BISCAYE~ (Received 24 November 1975; in revised form 30 January 1976; accepted 7 February 1976)
Abstract--The clay mineralogy of 235 surface sediment samples from the western Indian Ocean reveals many sedimentary sources and processes: (1) some areas, mainly in the central Indian Ocean, where very little sediment transport is involved, have smectite-rich clays derived from the alteration of in situ submarine basalts. (2) In many other areas, the mineralogy is significantly influenced by several modes of long-distance sediment transport, and the continental climate or the southern ocean volcanism. These areas consist of: (a) smectite-rich sediments in the Crozet and Madagascar basins and in the eastern Arabian Sea; (b) illite-rich sediments in the Indus Cone, the Agulhas Basin, and the ocean adjacent to Africa west of the Agulhas Plateau; (c) illite-palygorskite-rich sediments on the Carlsberg Ridge, Owen Ridge and in the westernmost Arabian Sea; (d) illite&aolinite-rich sediments in the Mozambique Basin and in the regions between Madagascar and Africa; (e) kaolinite-rich sediments adjacent to central Africa and Madagascar. (3) A third category of areas has mixed type sediments with no specific mineral in characteristically high amounts. Mixing of sediments derived from the alteration of in situ submarine basalts and/or from other areas rich in different minerals resulted in this group. The mineral distributions suggest that the Antarctic Bottom Water circulation is responsible for the sediment dispersal in the Crozet, Madagascar, Agulhas and Mozambique basins. Although the aeolian process could be effective throughout the Arabian Sea, it is dominant only in the western Arabian Sea.
INTRODUCTION SEVERAL p a p e r s (STEWART, PILKEY a n d NELSON,
port. Surface water circulation is also in part meridional, especially near the continents, and in part latitudinal (Fig. 1). Although the importance of northward-moving AABW in sediment transport has been mineralogically shown in the South Atlantic (e.g. BISCAYE, 1965), no such role of bottom currents has been demonstrated in the Indian Ocean, nor is the role of other water masses in sediment transport known. In view of the complex physiography (Fig. 1), the potential influence of AABW, and other currents that need not flow latitudinally, combined with a variety of probable sediment source regions, the near-perfect latitudinal clay mineral zonation established by RATEEV,GORBUNOVA,LISITZINand Nosov (1969) (on the basis of about 100 samples
1965; GORBUNOVA,1966; GRIFFIN, WINDOM and 1968; RATEEV, GORBUNOVA, LISITZIN and Nosov, 1969; GOLDBERGand GRIFFIN, 1970) have dealt with clay mineralogy in the western Indian Ocean. The studies were based on relatively few samples and have provided only a broad understanding of Quaternary sedimentation. More recent investigations resulting from the Deep Sea Drilling Project Legs 23, 24, 25, and 26, have contributed much to the knowledge of the pre-Quaternary mineralogy and sedimentation but have added little or no detail on the Quaternary sedimentation. W~ST (1938), LE PICHON (1960), WYRTKI(1971, 1973), WARREN (1974), and KOLLA, SULLIVAN, STREETER and LANGSETH (1976) discussed the * Lamont-Doherty Geological Observatory Contribution spreading pattern of Antarctic Bottom Water No. 2363. t Previous publications by this author have appeared under (AABW) on the floor of the Indian Ocean. This the name Kolla (or K) Venkatarathnam. flow is essentially meridional (Fig. 1) and is a Lamont-Doherty Geological Observatory of Columbia potentially important means of sediment trans- University, Palisades, New York 10964, U.S.A. 949 GOLDBERG,
950
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERRE E. BISCAYE
20
40 °
°
60"
~'~,o~"
I00 o A
80 °
~0
.•ID,. AUG SURFACE
r~
CURRENTS =.FEB. SURFACE - - ~CURRENTS [ I ~ ANTARCTIC BOTTOM WATER ] l ~ b NORTH ATLANTIC DEEP WATER
14
iLL--
-
•
•
" - ~ "
C.l. °"
4
k,
0
(?. ,
"
-ATLANTIC
-
°
RISE/
I.~5/
•
••
~
•
~
/NO/AN
BASIN
r'.- 6 0 o P' 20"
V 40"
V 60 °
~ 80 °
I00 °
Fig. 1. Circulation patterns of the Antarctic Bottom Water and North Atlantic Deep Water (taken from KOLLA, SULLIVAN, STREETERand LANGSETH, 1976), and of surface water (taken mainly from DOING, 1970) in the western Indian Ocean. The surface circulation in the northern ocean is strongly seasonal; the 'February' circulation, which is less prevailing than the 'August' one, is shown by dashed arrows only in a few places to avoid cluttering of symbols. C.I. = Crozet Island-Plateau re#on; K.1. = Kerguelen Island-Plateau re#on; O.R, = Owen Ridge in the Arabian Sea adjacent to the Arabian Peninsula; C.R. = Carlsberg Ridge.
Clay mineralogy and sedimentation in the western Indian Ocean
covering the whole Indian Ocean including the Southern Ocean) may not be valid. Also because of low sampling densities, we suspect that several potentially important sediment contributions from the vast African continent, which is characterized by arid climates in the northern and southern regions and tropical humid climates in the central regions, were not adequately identified in previous studies. We report here clay mineral analyses of 235 core-top (Quaternary age) sediment samples from the western (west of about 80 °E) Indian Ocean. Largely because of increased sampling density, we have been able to distinguish numerous sediment sources and processes that support the inferences of previous workers in some cases but differ in others. METHODS OF STUDY The top 2 to 5 cm of trigger weight and piston cores were sampled and analyzed to determine the distributions of fine fraction minerals. Samples were prepared as described by BISCAYE (1965) except that the separated size fractions (< 2 lam) were freeze-dried and two or three aliquots of each sample were made into pastes with water and smeared rather than pipetted on glass slides (GIBBS, 1965). The clays were analyzed on a Siemens X-ray diffractometer with a scan speed of 1.0 °20 rain-1. The major clay mineral groups-smectite, kaolinite, chlorite, and illite--were identified and their weighted peak-area percentages were estimated using the same criteria as those of BISCAYE(1964, 1965). The analytical precision is estimated to be between 5 and 10~o. A sharp peak at 10.4 to 10.6h on X-ray diffractograms was considered to be indicative of palygorskite; this was confirmed by electron microscopy. The abundance of palygorskite relative to that of illite was estimated by measuring the peak heights of palygorskite and illite and calculating their ratios. RESULTS AND DISCUSSION Quaternary clay mineral distributions from the analysis of about 200 samples are shown in Figs. 2, 3, 4, 5, 6, and 7. Because of the non-
951
availability of significant numbers of cores in the Arabian Sea, the analyses by GOLDBERG and GRIFFIN (1970) of 35 samples from that region have been collated with our data. Their methodology was similar to ours. We have also included some data from VENKATARATHNAMand BISCAYE (1973) for the regions just east of 80 °E. On the basis of clay mineral distributions (Figs. 2 to 7) and the generalized oceanic circulation (Fig. 1), many sedimentary provinces (Fig. 8) and processes have been inferred. We believe that these sedimentary provinces are caused primarily by the derivation of sediments from different sources and subsequent sediment dispersals. The distinction of these provinces is based only on fine sediment (< 2-~tm size) mineralogy. Some of these provinces will no doubt change when more detailed data become available in the future. However, description and discussion of the observed distributions are best made here in terms of the provinces instead of mineral by mineral. Some provinces appear to be of mixed type where no specific mineral is present in characteristically high amounts. The fine-grained sediments of such provinces are dominated by the alteration products of in situ basalts and/or sediments from other adjacent provinces. These mixed type provinces will not be further discussed. Smectite-rich provinces Several mineral provinces are characterized by abundant smectite ( > 609/0) and are distinguished primarily geographically. The smectite-rich area in the eastern Arabian Sea adjacent to India (designated as Deccan province) is similar to that shown by GOLDBERGand GRIFFIN (1970). As in the case of the smectite province in the western Bay of Bengal (GOLDBERG and GRIFFIN, 1970; VENKATARATHNAM and BISCAYE, 1973), the source of the smectite-rich sediments in the Arabian Sea is in the soils developed on Deccan volcanics in an arid climate. The Narmada and Tapti rivers (Fig. 1), which drain the Deccan soils, supply the smectite to the eastern Arabian Sea. The dispersal of these sediments in the Arabian Sea is primarily brought about by the prevailing surface currents (Fig. 1),
952
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60 °
80 °
100 °
80*
]t~60o q 100 °
60*~1
F 20*
40*
60*
Fig. 2. Smectiteweightedpeak-area percentage. which flow southward along the continental margin. We recognize some areas, mainly in the central Indian Ocean, where smectite-rich clays appear
to be derived mainly from the alteration of in situ submarine basalts and the associated volcanic products, analogous to that described by GRIFFIN, WINDOM and GOLDBERG (1968) for the central
Clay mineralogyand sedimentationin the western Indian Ocean
20 ° h
40 ° A
60 ° Zh
80 ° /~
953
/00" l~
t>eo ;
ILLITE ~---~
r ~lO-20
m :o
F7--~ 20" 30 ~-~
30-40 m
~
40-50 z
~>50
60
_J r--6OO 7 20 °
V 40 °
V 60 °
V 80*
IOO"
Fig. 3. Illiteweightedpeak-area percentage. South Pacific. Except east of 80 °E, volcanic ash layers are absent from the cores of these provinces. However, some smectite contribution from the alteration of fine, dispersed ash coming from the global fallout of volcanic dust (VENKATARATHNAM
and BISCAYE,1973) cannot be excluded. In general, transport for only short distances is involved in these provinces~ although east of 80 °E, transport of Indonesian tephra by the northeast trade winds and equatorial surface currents becomes signi-
954
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60*
80 °
100 °
Z>2o o
KAOLINITE [ ~ ' f f J <'I0
~ ~ ~
"o m
~-20~ 20-30 30-40m
/
•
66
•
/
" //
/
/
wlr'~60o
F
~7
20 °
40*
60 °
80 °
100 °
Fig. 4. Kaolinite weighted peak-area percentage.
ficant (VENKATARATHNAM and BISCAYE,1973). The Antarctic-Crozet smectite province occupies the northeasternmost region of the IndianAtlantic and Crozet basins, part of the MidIndian Ridge (50 to 63 °E and 30 to 25 °S), and
the eastern sections of the Madagascar and Mascarene basins (Figs. 2 and 8). GRIFFIN, WINDOM and GOLDBERG (1968) showed high smectite concentrations in these areas similar to our Antarctic-Crozet Province, but did not dis-
955
Clay mineralogyand sedimentation in the western Indian Ocean
20 °
40 °
60 °
80 o
~0
o
C>2o o
CHLORITE
/ /
/
/ /
/
/
U/(/" /
,=,l t ~ 6 o o q 20 °
40 °
60 °
80 °
tOO°
Fig. 5. Chlorite weighted peak-area percentage. cuss the details of the sedimentary processes bringing about the distribution. RATEEV,GORBUNOVA, LISITZIN and Nosov (1969) did not show this smectite pattern at all. The effects of northward-moving AABW on the
sea floor in the Crozet Basin are reflected in erosional-depositional forms, current lineations, and relatively high bottom water turbidity (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976). The Antarctic-Crozet Province appears to
956
VENKATARATHNAMKOLLA, LAWRENCEHENDERSON and PIERRE E. BISCAYE
40* 50"
50*
60*
70*
80* 30"
20*
20*
10"
10"
0 a
40* Fig. 6.
50*
60*
70 °
0° 80*
Distribution of palygorskite/illite peak-height ratios in the Arabian Sea. Solid dark line is a traverse along which data for Fig. 7 were taken.
be influenced mainly by the AABW transport of smectite-rich clays from the areas of southern ocean volcanism--the Crozet Plateau-Island and Kerguelen Plateau regions. The altered products of the Mid-Indian Ridge volcanics and the sediments from the Madagascar Province (to be described later) could also locally influence the Antarctic-Crozet Province. The Crozet Basin has 300- to 500-m thick sediments above the acoustic basement (EwING, EITTREIM, TRUCHAN and EWING, 1969), suggesting that the AABW has been active in the Crozet Basin since preQuaternary times. The high smectite zone extending continuously
from the Crozet Basin to the Madagascar Basin (Figs. 2 and 8) suggests that part of the MidIndian Ridge at 50 to 63 °E serves as a sill or sills for AABW flowing from the Crozet Basin into the Madagascar Basin. A sill passage at 29 to 26 °S and 60 to 64 °E was first suggested by WARREN (1974) on the basis of the temperaturesalinity characteristics of the Madagascar and Crozet basins. The low carbonate content (KOLLA, Bk and BISCAYE, 1976) and the distributions of Antarctic diatoms (BuRCKLE, VENKATARATHNAMand BOOTH, 1974), smectite (Fig. 2), and potential temperature (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976; WYRTKI, 1971)
Clay mineralogy and sedimentation in the western Indian Ocean
957
extending from the Crozet Basin through this illite-chlorite-rich sediments from the Antarctic region of the Mid-Indian Ridge into the margin of the Indian Ocean sector; (2) because of Madagascar Basin all broadly confirm WARREN'S their coarse size, illite and chlorite would be (1974) suggestion. Insufficient data in the im- quickly dropped to the sea floor; instead, the mediate vicinity of the sill, however, preclude its finer grained smectite available around the Crozet Plateau-Kerguelen Island volcanic regions, would precise geographic location. be picked up by AABW and transported north into the Crozet Basin. 1000-
l llite-rich provinces We define five illite-rich areas in the western 3000Indian Ocean--the Antarctic, South African and Zambezi provinces in the southwest Indian 4000Ocean, and the Arabian and Indus provinces in the northern ocean (Figs. 3, 5, 6, and 8). 5000 The South African Province occupies the African continental margin and abyssal areas between the Agulhas Plateau and South Africa. It apparently derives its sediments from South African soils considered to be rich in illite (VAN 2.0 DER MERWE and HEYSTEK, 1955, 1956; GRIFFIN, WINDOM and GOLDBERG, 1968; RATEEV, GOR1.0 BUNOVA,LlSITZINand Nosov, 1969). The Zambezi Province with 25 to 60% illite and 15 to 25% 0.0 ' 6' 5 ' 45 8'o kaolinite (usu ally with some gibbsite) occupies the 45 0 55 6' 0 70 LONG/TUDE ( o f f ) region between Madagascar and Africa, the Fig. 7. Palygorskite/illite peak-height ratios versus water Mozambique Basin, and to some extent the depth and longitude of samples along a traverse (dark solid Madagascar Ridge where it is diluted by line in Fig. 6) in the Arabian Sea. Madagascar Province days. The Antarctic illite province has 30 to 600/0 illite, but in contrast to We have no samples from the southernmost the Zambezi Province, it is lower in kaolinite and areas of the Southern Ocean. RATEEV, GOR- higher in chlorite (Figs. 4 and 5). This province BUNOVA, LISITZlN and Nosov (1969) noted high occupies the Agulhas and northwest part of the amounts of illite-chlorite on the Antarctic con- Atlantic Indian basins. tinental margin of the Indian Ocean sector. HowThe Weddell Sea-derived AABW spreads from ever, our studies further north, around 50°S in the Atlantic-Indian Basin into the Agulhas and the southern Crozet Basin, which are based on the Mozambique basins through two passages in the analysis of more samples than those reported by Mid-Indian Ridge--one at 20 to 25 °E and the the above authors, show abundant smectite. If the other at 35°E (HEEZEN, THARP and BENTLEY, high amounts of illite reported by RATEEV, 1972; KOLLA,BURCKLE,BISCAYEand HENDERSON, GORBUNOVA, LISITZINand Nosov (1969) for the 1975; KOLLA, SULLIVAN, STREETER and LANGsouthernmost ocean are correct, these illite-rich SETH, 1976). The passage at 35 °E appears to be sediments are not transported very far north for reflected in the distributions of illite (Fig. 3) and two possible reasons: (1) the AABW that travels Antarctic diatoms (KOLLA, BURCKLE, BISCAYEand north through the Crozet Basin is Weddell Sea- HENDERSON, 1975), although these distributions derived (e.g. KOLLA, SULLIVAN, STREETER and are based on the analysis of samples from the LANGSETH, 1976) and could not have transported deep basins with no representation on the inter2000-
958
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
20*
40 °
60 °
BO °
1000
~20 °
LEGEND [ ] I GROUP
?
4" P R O V .
I ~ IT GROUP [ ] 1TrGROUP
(:..
C E N TRA L ~AFRICAA/
.~N D'O~ ~.S'YAiV "/~ VOLOXI NlC: : PROVINCE:
...............
.'W: : : ; :
~ : " :~ :N:S: 7: r:O: : :.:-
6 0 o.= 20 °
PRO V I N C E
?
,,
F
'~US T ~
?
? t~.
:: ::
,
V
~
40 °
60 °
80 °
"60
o
I00"
Fig. 8. Mineral provinces in the western Indian Ocean. I Group. Provinces rich in smectite derived from the alteration of in situ submarine basalts and with little or no effects of long-distance sediment transport• II Group• Provinces rich in different minerals derived from continents or southern ocean volcanic areas and with significant effects of several modes of long-distance sediment transport• III Group• Mixed type provinces with no minerals in characteristically high amounts. The sources of sediments for areas with question marks are not known.
Clay mineralogy and sedimentation in the western Indian Ocean
vening ridge at 50°S and 35 °E. However, the Antarctic illite province as a whole has probably been affected by the Antarctic Bottom Water traveling through the above two passages. Icerafting of some illite in these areas is also possible. The ultimate source of this illite must be somewhere in the Antarctic continental margin although some of it could be derived from the Zambezi Province or the South African Province. The Zambezi is the only major river draining from Africa into the Indian Ocean and contributes about 100 million tons of sediment per year (RATEEV, GORBUNOVA, LISlTZIN and
Nosov,
1969). The day mineral composition of the Zambezi River sediments is not known. However, the climate of the river-drainage basin suggests that the Zambezi sediment should contain fairly high amounts of kaolinite in addition to illite. Transport of Zambezi sediments to the south (Madagascar Basin and in part Madagascar Ridge) is accomplished in part by the Agulhas surface current system. The nature of weathering in the Zambezi River drainage areas during glacial times might have differed from that of the present, producing more illite in the past. During times of lowered sea levels, turbidity currents must have been the dominant transport mechanism that deposited sediments with thickness exceeding 1 km in the Mozambique Basin (EwING, EITrRF.IM, TRUCHAN and EWIN6, 1969). Some patches of high amounts of illite within the Zambezi Province may be due to the analysis of pre-Holocene turbidite sediments exposed on the sea floor in places. In addition to the Agulhas surface and turbidity currents, the AABW and North Atlantic Deep Water (NADW) play important roles in the transport of Quaternary Zambezi sediment or the redistribution of it (Fig. 1). However, no clear distinction can be made between various modes of transport in the Zambezi Province with the available data. The northern illite-rich areas in the Arabian Sea--the Indus and Arabian provinces (Figs. 3, 5 and 8)--also have high amounts of chlorite. The Indus Province is mainly confined to the Indus Cone (central regions of the Arabian Sea with sediment thickness exceeding 2 km), whereas the
959
Arabian Province is confined to the western Arabian Sea, especially the Carlsberg Ridge (northwest section of the Mid-Indian Ridge) and the Owen Ridge adjacent to the Arabian Peninsula. The two provinces appear to overlap in the southeastern Arabian Sea. The chief distinction between the Arabian and Indus provinces is that the former has high to moderate amounts of palygorskite whereas the latter has little or none of it (Fig. 6). Although palygorskite is present in a suite of samples from a broad band extending from Arabia to the southern tip of India, its abundance is high in the western Arabian Sea adjacent to the Arabian Peninsula. The palygorskite/illite ratios versus longitude and water depth are shown in Fig. 7 for a group of samples along a traverse indicated in Fig. 6. There is a good correlation ofpalygorskite highs with topographic highs. A similar relationship was noted by GOLDaERG and GRIFFIN (1970) on the basis of fewer samples. A relationship of this type reflects aeolian processes. GOLDBERGand GRIFFIN(1970) showed an illiterich band extending continuously from about the region of the Indus River confluence through the Arabian Sea and eastern equatorial Indian Ocean all the way to the Indonesian Archipelago and attributed it to the aeolian process. VENKATARATHNAM and BISCAYE (1973), on the basis of higher sample density, have concluded that the eastern equatorial Indian Ocean illite is derived primarily from the Ganges source. Absence of palygorskite in the eastern Indian Ocean sediments (Fig. 6) confirms this. An aeolian component would certainly be more dominant in the Arabian Sea than in the eastern equatorial Indian Ocean because the former is surrounded by arid to semi-arid land masses influenced by strong monsoon winds. In the Indus Province, even though the amount of aeolian component may be considerable, we suspect that sediment derived from the Indus River would be more significant because of the huge amounts of sediment (400 million tons per year) delivered to the ocean by this river (RATEEV, GORBUNOVA, LISlTZIN and Nosov, 1969). The Indus sediment is believed to have been dispersed largely by the prevailing
960
VENKATARATHNAMKOLLA,LAWRENCEHENDERSONand PIERREE. BISCAYE
surface circulation (Fig. 1) and by turbidity currents, which were more active at times of lowered sea levels during the Pleistocene. Available data on bottom water temperature and turbidity, as well as bottom photographs, suggest no bottom current activity except possibly in the southernmost deep area of the Arabian Sea (KOLLA, SULLIVAN, STREETER and LANGSETH, 1976). Unlike in the Indus Province, the aeolian component appears to be dominant in the Arabian Province. The Arabian Province sediments are derived from the deserts of Somalia and Arabia by prevailing monsoon and northwesterly winds as implied by HEEZEN, NESTEROFF, OBERLIN and SABATIER (1965) and GOLDBERG and GRIFFIN (1970). Kaolinite provinces
The kaolin-rich Central African, Zambesi (described under illite-rich provinces), and Madagascar provinces are adjacent to Africa and Madagascar (Figs. 4 and 8). The Australian Province (Fig. 8) is a continuation of the kaolinite province in the southern Wharton Basin and on the adjacent Ninety East Ridge reported by GRIFFIN, W1NDOM and GOLDBERG (1968) and VENKATARATHNAM and BISCAYE (1973). This Australian Province kaolinite has been transported by the southeast trade winds from the deserts of western Australia. Most of the African continent from about the equator to about 30°S contributes fairly high amounts of kaolinite, which are delivered to the ocean by rivers. However, the sediments supplied from Africa at about 10 °S (adjacent to the Central African Province) and from Madagascar (adjacent to the Madagascar Province) seem to have the highest amounts of kaolinite. The dispersal of African kaolinites within the ocean may be affected in part by surface circulation and in part by deep currents, for which no details are available. It is possible that fairly high amounts of smectite besides high kaolinite could have been supplied from Madagascar. Smectite, being more fine-grained, could have been transported farther
than kaolinite, thus leaving behind the kaoliniterich clays in the Madagascar Province. The smectite transported farther could add to the clays in the eastern Madagascar Basin that are believed to be derived by AABW transport from the Crozet Basin. CONCLUSIONS
From detailed clay mineral analyses of Quaternary sediments from the western Indian Ocean, we draw the following main conclusions: 1. The clay mineral distributions in the western Indian Ocean are influenced not only by the continental climate and geology but also by the complex physiography of the ocean, submarine volcanism, and several modes of sediment transport; hence, the near-perfect latitudinal distributions of clay minerals shown by RATEEV, GORBUNOVA, LISITZINand Nosov (1969) for the Indian Ocean are not generally valid. 2. Some areas mainly in the central Indian Ocean consist of fine-grained smectite sediments derived from the alteration of in situ submarine basalts. Little or no long-distance sediment transport has affected these areas. 3. Sediments in many other areas of the western Indian Ocean are significantly affected by the continental climate and geology or southern ocean volcanism, and by long-distance sediment transport through oceanic surface circulation, Antarctic Bottom Water movements, turbidity currents, or aeolian processes. These sediments are rich in smectite, illite, illite-kaolinite, illitepalygorskite, or kaolinite. Distributions of smectite and illite in the southwestern regions of the Indian Ocean, supplemented by those of diatoms, calcium carbonate, and bottom water characteristics reported by previous workers, suggest two passages--one at 60 to 64 °E and the other at 35 °E in the Mid-Indian Ridge, which serve for the AABW northward flow and sediment transport from the southern ocean. 4. In a third group of areas, mixing of sediments derived from in situ basalt alterations, from other adjacent areas mentioned above, or both, has resulted in sediments with no minerals in characteristically high amounts.
Clay mineralogy and sedimentation in the western Indian Ocean
961
Acknowledgements--We thank Drs. J. E. DAMUTH, G. Ross
hebdomadaires S~ances de I'Acad~mie des sciences, Paris,
HEATH, D. A. MCMANUS, L. BURCKLEand E. BONATTI for reading the manuscript critically and offering many helpful suggestions. The work was supported by the Office of Naval Research Contract N00014-75-C-0210 and the National Science Foundation Grants IDO-71-04204-A04 or OCE-7519627 (CLIMAP). Grants from ONR (00014-75-C-0210) and NSF (DES72-01568-A03) have supported the shipboard coring operations and curatorial services to preserve the sediment cores at Lamont-Doherty Geological Observatory. We appreciate the help of Miss PHYLLIS HELMS in providing samples from the Scripps Institution of Oceanography Core Library.
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