A critical evaluation of depositional parameters controlling the variability of organic carbon in Arabian Sea sediments

Marine Geology, 107 (1992) 213-226 Elsevier Science Publishers B.V., Amsterdam

213

A critical evaluation of depositional parameters controlling the variability of organic carbon in Arabian Sea sediments A n i l L. P a r o p k a r i , C. P r a k a s h B a b u a n d A n t o n i o M a s c a r e n h a s National Institute of Oceanography, Dona Paula, Goa 403 004, India

(Received October 4, 1991; revision accepted January 9, 1992)

ABSTRACT Paropkari, A.L., Prakash Babu, C. and Mascarenhas, A., 1992. A critical evaluation of depositional parameters controlling the variability of organic carbon in Arabian Sea sediments. Mar. Geol., 107:213-226. Organic carbon distribution in the surficial sediments of the Arabian Sea to some extent mirrors the surface productivity, being both high in the peripheral portions and low in the central region, suggesting that the productivity is a primary source of organic matter. However, a critical examination reveals that the organic .enrichment on the slope of the Indian margin is two to four fold higher (max. 16.71%) than on the slope of the Arabian Peninsula (max. 7.54%) while the productivity in the former region is three to four times lower than the latter. This observation suggests that the influence of productivity on organic enrichment is of secondary importance. High concentrations (> 4%) of organic carbon exactly coinciC.e with the oxygen minimum zone (150-1500 m water depth) evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon. Incidently, the organic-rich band along the: western Indian slope is wide, long and highly concentrated as compared to that of the slope of the Arabian Peninsula and no organic enrichment is found on other continental slopes of the Arabian Sea although an equally intense oxygen minima impinge on the floor of the slope. Such a variabilit~ of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments, rates of fine ter~x!genoussediment deposition, shelf width, slope gradient, bottom currents and adsorption capacity of individual clay minerals. All these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunctio:" with various depositional parameters which determine the "degree of preservation".

Introduction A n u m b e r of biological, physical and chemical factors such as p r i m a r y productivity o f overlying waters, dissolved oxygen in b o t t o m waters, texture of sediments and rate of sedimentation govern the preservation of organic matter in marine sediments (Trask, 1939; Bordovskiy, 1965; Muller and Suess, 1979; Demaison a n d Moore, 1980; P a r o p k a r i et ai., 1987; Pedersen a n d Calvert, 1990; Stein, 1990). However, in any given situation, the importance o f one factor over the other is debatable since varied and complex environments are often encountered in the oceanic realm. After an extensm

Correspondence to: A.L. Paropkari, National Institate of Oceanography, Geological Oceanography Division, Dona Paula, Goa 403 004, India.

0025-3227/92/$05.00

ive survey of the world ocean data, Demaison and M o o r e (1980) emphasised the anoxic character of b o t t o m waters as a principal factor for o~ganic enrichment in sediments. This view is also supported by the D S D P d a t a which have established a synchronous occurrence of anoxic events on a global scale during the Middle Cretaceous when organic-rich marine black shales were formed (Demaison and Moore, 1980; Stein, 1990, and references therein). Therefore, the existing notion is that the black shales formed under conditions of water column anoxia (Demaison and Moore, 1980). In contrast, Calvert (1987), Pedersen and Calvert (1990) and Calvert et al. (1991) suggested that high primary production and not watercolumn anoxia provides a first-order control on accumulation of organic-rich facies in Cretaceous,

© 1992 w Elsevier Science Publishers B.V. All rights reserved

214

Quaternary and modern oceans. This shows that a key problem in studies of modern and ancient organic carbon-rich sediments is whether the enrichment of organic carbon is caused by an increased preservation rate of organic matter under oxygen-deficient deep water conditions, or by an increased production of organic matter, or both. The debate on "productivity vs anoxia" can be addressed by taking the Arabian Sea as an example since this sea exhibits the following characteristics: (1) the Arabian Sea is recognised as one of the zones of highest seasonal productivity in the world (Qasim, 1977), (2) the entire Arabian Sea is characterized by a permanent intense oxygen minimum zone between depths of 150-1500 m (Wyrtki, 1971; Von Stackelberg, 1972).

Distribution of organic carbon Several studies have reported organic carbon contents in the surficiai sediments of the Arabian Sea from a large number of stations (Fig. I). The distribution of organic carbon (Fig. 2) shows that the sediments of the western and northern shelves and the northern slopes of the Arabian Sea (Wiseman and Bennett, 1940; Stewart et al., 1965; Kolla et al., 1981; Hermelin and Shimmield, 1990; Shimmield et al., 1990) are poor in organic carbon ( < 1-2%), while the sediments from a large part of the Arabian Peninsula slope and a part of the Pakistan slope have moderate concentrations (2-4%). Within this, significantly higher concentrations (>4%, up to 7.54%) are noticed on the slope off the Arabian Peninsula whose occurrence has been identified as a short and narrow band, and as a small patch for the first time during this investigation. In comparison, organic carbon in the sediments along the Indian margin is very high (Marchig, 1972; Paropkari et al., 1987; Rao and Rao, 1989; Ramamurty and Murty, 1989; Naidu et al., 1991). In this region, the inner shelf sediments contain up to 4% of organic carbon while on the outer shelf it is low (< 1%). In general, high organic carbon values ( > 2%) are observed on the slope while very high concentrations [above 4% and up to 12% (Paropkari et al., 1987) and up to 16.7.1% (Ramamurty and Murty, 1989)] are found at mid-slope as a long and wide band and

A.L. PAROPKARi ET AL.

also as small patches off Saurashtra and the southern tip of India (Fig. 2). This organic-rich ( > 4 % ) band occurs continuously between Bombay and the southern tip of India and is relatively wide between Ratnagiri and Mangalore, and south of Cochin. The sediments of the lower slope, continental rise and peripheral deep Arabian Sea, except for the northern portion, indicate low to moderate levels (1-2 and 2-4%) of organic carbon. The areal coverage of these concentrations in the western Arabian Sea is wider compared to the eastern part. The vast area covered by the deeper Arabian basin exhibits very low concentrations (generally < ! % ) of organic carbon (Wiseman and Bennett, 1940: Stewart et al., 1965; Marchig, 1972; Udintsev, 1975; Kolla et al., 1981). Three important observations can be made from the above. (l) Overall, the organic carbon content is higher in peripheral portions and exhibits a decreasing trend towards the center of the basin. Within this distribution pattern, significant differences are also noticed. (2) High organic enrichments (>4%) are observed only along the mid-slope of the Arabian Peninsula and western India. (3) The sediments of the deeper Arabian Sea are poor in organic carbon ( < 1%). With this background in view, we felt a necessity to reevaluate the role of depositional factors which govern the variable distribution of organic carbon in the Arabian Sea. For this purpose, we have utilised published data from the entire Arabian Sea which include data from 454 closely spaced samples from the western margin of India, analysed at the National Institute of Oceanography (NIO), Goa. In addition, unpublished data from 150 stations (available with the authors) covering the entire western continental margin of India, collected during various cruises of NIO's research vessels Gaveshaniand SagarKanya, is also incorporated. This is a first attempt to solve the ongoing debate on "productivity vs anoxia" with special reference to the Arabian Sea.

Discussion The distribution of organic carbon (Fig. 2) to some extent mirrors the surface productivity

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(Fig. 3) in the Arabian Sea, both being high in the peripheral portions compared to the central part. This similarity in distributions suggests that ~urface productivity is the main source of organic matter in the sediments of the Arabian Sea. This view is also confirmed by various other studies, such as C/N ratios (Paropkari et al., 1987), 61aC values (Fontugne and Duplessy, 1986) and organic geochemical investigations (Caratini et al., 1981). However, an in-depth comparison reveals that the regions of high biogenic productivity (> 0.75 gC/ m2/day) such as those off the Arabian Peninsula, Iran and Pakistan are underlain by sediments having low to high concentrations (< 1-7.54%) of organic carbon. Comparatively, the productivity along most of the western Indian margin is almost

threefold lower and yet the overall organic carbon contents are higher by a factor three to five. A critical examination of primary productivity versus organic enrichment along the western Indian margin shows absolutely no definite correspondence between the two (Paropkari et al., 1987)• Despite the fact that productivity (Fig. 3) is high (>0.75 gC/m2/day) south of Cochin and north of the Gulf of Kutch as compared to most of the western coast of India (between the Gulf of Kutch and Cochin) (< 0.75 gC/m2/day), even then the organic enrichment (Fig. 2) is uniformly high along most of the inner shelf (1-4%) and slope (4-16%) and is very poor throughout the outer shelf (< 1%). These observations show that the organic carbon distribution does not strictly reflect the surface produc-

216

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tivity pattern. Therefore, surface productivity cannot be taken as a first-order control on organic enrichment in the bottom sediments of the Arabian Sea. Similarly, Moore (1975) ruled out primary productivity as the main cause of organic enrichment in the bottom sediments of the world oceans. In view of the above, it is the crucial role played by oxic and anoxic bottom waters which needs prime attention as far as the distribution of organic carbon in the Arabian Sea is concerned. Other factors such as texture of sediments, rates of sedimentation, shelf width, slope gradient, prevailing current patterns and clay mineralogy also have to be considered. Oxic environment

The water at shallower zones (up to 150 m) and at greater depths (below 1500 m) in the Arabian

Sea are oxic (dissolved 02 = > 0.5 to 5 ml/1; Fig. 4; Wyrtki, ! 971; Von Stackelberg, 1972). Oxic shelves and upper slopes ( < 150 m depth)

The low organic content (< 1 and 1-2%; Fig. 2) of sediments on the oxic shelves aud upper slopes of Somalia, the Arabian Peninsula, lran, Pakistan and India (except a part of inner shelf), irrespective of primary productivity variation (Fig. 3), is mainly ascribed to decomposition of organic matter in contact with oxygenated bottom waters. The presence of refractory organic matter on the shelf of Somalia, Arabian Peninsula, lran and India is evidenced by the higher C/N ratios (10-12) (Wiseman and Bennett, 1940; Marchig, 1972; Kolla et al., 1981; Paropkari et al., 1987; Shimmiled et al., 1990) and supports the view that the organic matter here is subjected to oxidation. In addition,

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the moderate productivity (0.25-0.75 gC/m2/day; Fig. 3) in coastal region of Somalia and most of the Indian shelf may account for the low organic content of sediments. The textural characteristics considerably influence the organic content of the sediments of the Arabian Sea. On the shelves and upper slopes, predominant coarse sediments (Stewart et al., 1965; Schott, 1968; Von Stackelberg, 1972; Nair, 1974; Udinstev, 1975; Paropkari, 1990; Hermelin and Shimmield, 1990; Shimmield et al., 1990) aid the effective destruction of organic matter by permitting easy diffusion of free oxygen and oxidising salts in the sub-surface levels resulting in a lower organic carbon content. Further, the occurrence of residual/relict sands in these areas suggests deposition under high-energy environments

whereby fresh organic matter is removed by the winnowing action of currents thus decreasing the organic content. This process is also aided by the narrow shelves, especially in the western Arabian Sea (almost absent at places along the Arabian Peninsula), which favour cross shelf transport thereby diverting the finer fraction towards the deeper sea. Dilution of organic content by lithogenic components in the shelf sediments of the northern Arabian Peninsula (off Oman), Iran and western Pakistan is evident from the high lithogenic content (>60%; Sirocko and Sarnthein, 1989), low to moderate CaCOa (10-3f~%) and low opal content (KoUa et al., 1981). The lower concentration (< 1% organic carbon) on the shelf north of Saurashtra is m~inly ascribed to the hinterland

A.L. PAROPKAR!ET AL. 218

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VARIABILITY OF ORGANIC CARBON IN ARABIAN SEA SEDIMENTS

which is arid with no significant fluvial discharge. In addition, a part of the sediments discharged from the Indus, which are transported to this part of the shelf under the influence of southerly currents (Nair et al., 1982), are coarse sandy silts in nature (Von Stackelberg, 1972) and poor in organic content. However, the lower organic content (< 1% organic carbon) in the shelf region between Saurashtra and Bombay is attributed to dilution by fine-grained terrigenous sediments (clays and silty clays) owing to the high fluvial discharge (about 58 m.tonnes/yr) from the Narmada-Tapti. On the contrary, a higher organic content (1-4%; Fig. 2) is invariably associated with sediments of the inner shelf between Bombay and Cochin. This is attributed to higher organic input, mainly of terrigenous nature as indicated by C/N ratios (10-30), protection of organic matter from oxidation by the fine nature of the sediments and higher rates of fine terrigenous deposition which lead to the preservation of organic carbon (Paropkari et al., 1987). Oxic lower slopes (> 1500 m depth) In the lower slope of part of Somalia and Arabian Peninsula the organic carbon content is of the order of 1-4% while that of Somalia, Iran and Pakistan generally ranges between 1 and 2% and sometimes even < 1%. Very low concentiations (< 1%) off Somalia are the result of moderate surface productivity and oxidative destruction of organic matter. However, the very low content of organic carbon ( < 1%) on the lower slopes of eastern Iran and western Pakistan and the Oman basin reflects the combined effect of two processes: high aeolian input (Sirocko and Sarnthein, 1989; Sirocko and Lange, 1991) and oxidation. In the western Arabian Sea, the surface productivity is very high (>0.75 gC/m2/day) which also influences the sedimentation of this region as revealed by the high content of biogenic components (Sirocko and Sarnthein, 1989). Perhaps the influx of organic matter on the bottom exceeds the rate of its destruction by oxidising bottom waters resulting in moderate (1-4%) organic emrichment. The zone of these moderate concentrations on the lower slope occurs as a wide band parallel to the coasts. This is mainly attributed to

219

the extension of high productivity zone (>0.75 gC/m2/day) far offshore (Fig. 3). The terrigenous supply from the Indus river is barred from reaching the deeper parts in the Owen and Oman basins by the Owen-Murray ridge (Shimmield et al., 1990). This damming of terrigenous material prevents the dilution of organic matter and is perhaps another cause for the occurrence of a moderate concentration of urganic carbon in a wider portion along the western and northern Arabian Sea. Moderate organic carbon content (1-4%) on the lower slope of India can be attributed to low to moderate productivity (Fig. 3) which leads to low organic input to the bottom sediz,~ents inspite of greater preservation by the fine nature of sediments (clays, silty clays) (Schott, 1968; Von Stackelberg, 1972; Kolla et al., 198 I), moderate rates of terrigenous sediment deposition (Fig. 5) and gentler slope and rise morphology. However, the strip of this :moderate organic content on the lower slope of I:~dia is narrower than that on lower slopes of the western and northern Arabian Sea, perhaps due to the large supply of terrigenous sediments discharged by the Indus River as well as those derived from the Indian land mass. Oxic deeper Arabian Sea In the central open ocean conditions, low organic carbon content (< 1%) can be ascribed to various depositionai parameters. The organic matter in this deeper portion is of both marine and terrigenous nature as evidenced by the 61ac (Fontugne and Duplessy, 1986) and the C/N ratios (Wiseman and Bennett, 1940; Kolla et al., 1981; Shimmield et al., 1990). The decrease in productivity away from peripheral land masses (Fig. 3) and scavenging and decomposition of organic fallout during its transit through a high water column reduces the marine biogenic supply to bottom sediments. Aeolo-marine sediments provide the major part of sediments in the central Arabian Sea (Sirocko and Lange, 1991) and induce dilution of organic carbon. The poor supply of organic matter from the Indus (Ittekot and Arain, 1986) may be one of the causes for the low organic content on the deeper Indt, s cone. The sedimentation rates in this region are very low (Fig. 5), leading to longer exposure of organic material to the oxygenated

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Fig. 5. Holoccnc sedimentation rates at various locations in the Arabian Sea. Numbers in brackets denote water oopth in meters. [Data sources: Hcyc, 1970; Zobcl, 1973; Pcng ct al., 1977; Prell ct ai., 1980; Gupta and Hashimi, 1985; Fontugnc and Duplcssy, 1986; Borolc, 1988; Sirocko, 1989; Naidu, 1991; Paropkad et aI., 1991; Sarkar and Bhattacharya, 1991; Paropkari (enpub]. data); R.R. Nair (pcrs. ¢ommun., 1991).] Open circles represent sediment trap locations o6 Ramaswamy ct al. 119911.

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shelf is extremely narrow and the slope is very steep which may preclude the southward extension of this organic-rich band. Surprisingly, the organic content of the anoxic slopes of eastern Iran and western Pakistan (west of Karachi) is very low (< 1-2%; Fig. 2). These slopes are gentler (but uneven), the productivity is high (>0.75 gC/m2/day; Fig. 3) and equally intens~ oxygen minima impinge on the floor of the slope (Fig. 4; Wyrtki, 1971) but the extremely poor organic content is intriguing. In this case, low organic content is ascribed to its dilution by aeolian terrigenous component (Stewart et al., 1965; Sirocko and Sarnthein, 1989). Sediments like silts and clayey silts imply the winnowing and removal of fine biogcnic particles. Thus, these adverse conditions cause low organic carbon ~:ontents on slopes of these regions in spite of very high biological fluxes and favourable anoxic conditions. Along eastern Pakistan (eart of Karachi), the sediments of the anoxic slope exhibit moderate concentrations of organic carbon (2-4%; Fig. 2) The productivity of surface waters is high (> 0.75 gC/m2/day; Fig. 3) and the sedimentation rates are also relatively high (5-15 cm/kyr; Fig. 5) mainly caused by large terrigenous discharge (Nair. 1984) of fine sediments derived from the Indus river. In spite of these favourable environments, the organic enrichment is moderate perhaps due to excessive deposition of terrigenous load masking the organic content. The low carbonate content (< 36%; Kolla et al., 1981) of sediments of this region supports this view.

Indian slope In the northern most anoxic slope between the Indus mouth and Porbandar (Saurashtra) (Fig. 2) the organic content is low (< !% and !-2%), except for a small patch of moderate values (2-4%) off the Gulf of Kutch, despite the fact that the productivity (0.5-1.0 gC/m~/day; Fig. 3) and sedimentation rates (4-8 cm/ky:; Fig. 5) are moderate to high. It is well known that most of the Indus discharge is transported to the deep sea through the Indus canyon ,~,hl.]~, a part of it is deposited on the Saurashtra slop~ (Nair et al., 1982), which accounts for the obser.~ed low o=ganic content. Similarly, a small portio::: of the anoxic slope along

A.L. PAROPKARI ET AL.

the southern Indian tip is also poor in organic content (1-2%) even though the productivity of the surface waters is very high (> 1.0 gC/me/day). In this area, the lithogenic component is high (> 45%) and the sedimentation rates are also very high (51.25 cm/kyr, the highest in the Arabian Sea; Fig. 5) (Sirocko and Lange, 1991). Therefore, the observed low-to-moderate organic content is attributed to a continuous addition of terrigenous material. In contrast, most of the anoxic slope of India presents an altogether different scenario. The organic matter present on the anoxic slope is primarily of marine origin (Fontugne and Duplessy, 1986; Paropkari et al., 1987) and derived from the productivity of overlying waters which varies from low to moderate (0.25 to 0.75 gC/m2/ day; Fig. 3). The overall productivity is on average almost three told lower than that of other coasts of the Arabian Sea. Nevertheless, moderate to very high concentrations of organic carbon (2-4 and 4-16%; Fig. 2) are invariably associated with the entire slope sediments, forming a long and wide band in contact with oxygen minima from Saurashtra to the southern tip of India. It may be noted that the highest organic carbon values (10-16%; Paropkari et al., 1987; Ramamurthy and Murty, 1989) occur between Ratnagiri and Mangalore, precisely where the productivity is very low (0.25-0.50 gC/m2/day; Fig. 3). This mis-match of productivity and organic enrichment is a classic example which clearly demonstrates that the productivity of suface waters, although aa important contributor of organic matter, does not necessarily car.se ~e organic enrichment of underlying sedimc nts. characteristic feature of this band is the occurrence of the highest values in the middle part alcJng the entire length which c~,n only result from the lowest oxygen content (often undetectable) w:,thin the oxygen minimum zone. The sediments from this depth have also been found to emit a str,;i~g rI2S odour. Therefore, we attribute this high oIganic content (>4%) along th.~ Indian margin to the impingement of the oxygen minimum zone on the floor of the slope which reconfirms earlier reports (Marchig, 1972; Kolla et al., 1981; Slater and Kroopnick, 1984; Paropkari et al.,

223

VARIABILITY OF ORGANIC CARBON IN ARABIAN SEA SEDIMENTS

1987). Observations in other places have shown a similar significant organic enrichment wherever the regional oxygen minimum zone intercepts the continental shelf and slope (Premuzic et al., 1982). The band of organic rich sediments (> 4%; Fig. 2) on the slope between Ratnagiri and Mangalore and the slope off south of Cochin is broader with a gentler slope morphology (about 135 km wide) thus augmenting the areal extent of intercept of the anoxic layer on the floor of the slope. In addition to bottom water anoxia, other sedimentological factors also assist the preservation of organic carbon in this region. The reported high sedimentatio.,~ rates (generally 6-19 cm/kyr; Fig. 5) is the result of deposition of terrigenous material comprising fine silts and clays (a clay fraction of 40-70%; Schott, 1968; Von Stackelberg, 1972; Udintsev, 1975; Rao et al., 1983; Paropkari et al., 1987; Ramamurthy and Murthy, 1989; Sirocko and Lange, 199 l). Large and rapid lithogenic inp,~t enhances the preservation of organic carbon by burying it and protecting it from destruction. The other factors leading to the orgatfic enrichments along the Indian slope also need attention. The importance of cross shelf transport for organic enrichment on slopes has been stressed by Premuzic et al. (1982). On the western Indian slope smectite is the abundant clay mineral derived from the Deccan Traps occurring between Saurashtr:~ and Mormugao. In addition, kaolinite and gibbsite, originating from peninsular rocks of southern. India, occur on the southern Indian slope. The presence of these clay minerals (Paropkari, 1983; Rao et al., 1983; Rao, 1991) clearly indicates that cross shelf transport is an important phenomenon. This process also diverts the settling biogenic flux on the wider shelf (about 350 km off nolth of Bombay aad about 60 km off Cochin) along the Indian margi~ which is subsequently winnowed and exported towards the slope thus enriching the organic content. The mineralogy of the clays also play a significant role in the organic contents o~i the Indian slope. Indus derived illite- chlorite is abundant off Saurashtra (Guptha and i-Iashimi, 1985; Rao, 199 l). These clay minerals exhibit poor adsorption capacity for organic materials and hence are responsible for low organic content of this region. . _

In comp~wison, river discharged smectite is a prominent cla~mineral on most of the slope of western India (Paropkari, 1983; Rao et al., 1993). This mineral has a relatively high reactive surface area and is thus capable of adsorbing more dissolved organic substances. This view is also supported by laboratory experiments of Hedges (1978). Therefore, smectite along the Indian margin may have aided preservation by adsori~ing organics, which has also been noted by Kolla eta!. (1981). Further, the smectite on the Indian slope is the product of the tropical humid conditions which prevail along the Indian coast and therefore have some terrestrial organics bound to them. These organics, though insignificant, can contribute to the total organic content of this slope region. The above discussion clearly demonstrates the role of anoxic bottom waters as a fundamental cor~trol in the preservation of organic carbon in the Arabian Sea. According to Kolla et al. (198 l) the degree of preservation of organic matter along the Indian continental slope is 15-80 times higher than that off the Arabian Peninsula. These ~bservations unequivocally suggest that it is the interplay of various depositional parameters such as surface productivity, texture of sediments, rates of fine terrigenous deposidon, bottom currents, shelf width, slope gradient and clay mineralogy in an exclusively ano×ic environment which determines the "degree of preservation".

Summary and conclusions (1) The Arabian Sea provides an unique situation to test the theories regarding the enrichment of organic carbon in marine sediments since it has a high surface productivity and is characterised by the presence of an oxygen minimum zone in the entire region. (2) The organic carbon distribution to some extent mirrors tl~e surface productivity, both being high in the periphera~ portions and low in the open ocean, implying that the organic carbon is mainly of marine oiigin. This view is also supported, by the C/N ra~ios, 6 JaC values and organic geoche~ical studies. HoweveJ', a critical examination reveals that there is rc direct correspondence between the inteasity of p~mary productivity and

224

the organic enrichment in the sediments beneath. This suggests that the primary productivity is not the fundamental co)trol of the organic enrichment in the Arabian Sea. (3) This study has clearly identified two organicrich bands: one a'ong the slope of the Arabian Peninsula and the other along the western slope of India. A perfeo: coincidence of these organic enrichments with the oxygen minimum zone (1501500 m) clearly demonstrates the influence of anoxic waters on organic enrichment. (4) The organic-rich band along the Arabian Peninsula is short, narrow and comparatively less enriched (max. 7.54%) inspire of high sedimentation rates. This has been attributed to the absence of fine-grained terrigenous deposition, relatively coarse texture, narrower shelf, steeper slope gradient and presence of wind derived illite, chlorite and quartz. On the contrary, very high organic enrichment (max. 16.71%) occurs as wider and longer band along the mid-slope of the western margin of India which is ascribed to moderate to high rates of fine terrigenous deposition, gentler slope gradient, wider shell cross-shelf transport and increased adsorption capacities of smectite. (5) Bottom water anoxia, though a principal factor for organic enrichment on the slopes, and its interplay with other depositional parameters cumulatively determines the "degree of preservation" and fixes the location of organic enrichment along the slopes. Acknowl~gements The authors thank Dr. B.N. Desai, Director and Mr. P.S.N. Murty, former Head, Geological Oceanography Division for their encouragement. They are grateful to Mr. R.R. Nair, Head, Get, logical Oceanography Division fol .his keen interest and critically reviewing the manuscript. Dr. V. Purnachandra Rao is also thanked for constructive comments. The help rendered by Miss Maria d'Cruz in digitizing the data and getting the computer plots is highly appreciated. The authors thank Mr. Uday Kumar Javali for the drawings. They also thank two anonymous reviewers for their constructive comments which helped to improve the manuscript.

A.L. PAROPKAR! ET AL.

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