The TiO2Al2O3 ratio in the Cenozoic Bengal Abyssal Fan sediments and its use as a paleostream energy indicator

Marine Geology, 76 (1987) 195-206 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

195

THE TiO2/AI203 RATIO IN THE CENOZOIC BENGAL ABYSSAL FAN SEDIMENTS AND ITS USE AS A PALEOSTREAM ENERGY INDICATOR BIRGER SCHMITZ Department of Geology, University of Stockholm, S-10691 Stockholm (Sweden)

(Received April 2, 1986; revised and accepted November 13, 1986)

Abstract Schmitz, B., 1987. The TiO2/A1203 ratio in the Cenozoic Bengal Abyssal Fan sediments and its use as a paleostream energy indicator. Mar. Geol., 76: 195-206. The TiO2/A1203 ratio in the Cenozoic Bengal Abyssal Fan sediments displays decisive potential as a paleostream velocity indicator, The reason for this is that, even in the finest fan suspendates, hydrodynamic sorting determines the amounts of heavy TiO2-rich minerals relative to lighter A1203-rich clay minerals. In five Deep Sea Drilling Project cores (213 217), which have been recovered in, and south of, the Bay of Bengal, TiO2/A1203 ratios increase linearly with time from the Late Miocene to Recent. The increase reflects fan progradation and intensified bottom current activity on the southern Bengal Fan. These processes are related to the Himalayan elevation by factors such as the maturation of northern India river systems, evolution of orographic monsoon rains and elevation-attributed increases in denudation rates.

Introduction T h e TiO2/A1203 r a t i o in s e d i m e n t s h a s b e e n t e n t a t i v e l y used to deduce p a s t c h a n g e s in s h a l l o w - w a t e r s t r e a m e n e r g i e s (Spears a n d K a n a r i s - S o t i r i o u , 1976). T h i s a p p r o a c h relies on the f a c t t h a t in c r u s t a l r o c k s t i t a n i u m t y p i c a l l y exists in insoluble, h e a v y m i n e r a l s s u c h as ilmenite, rutile, a n a t a s e a n d s p h e n e (Goldschmidt, 1954). D i s a g g r e g a t i o n d u r i n g weathering and subsequent transport with r i v e r w a t e r leads to h y d r o d y n a m i c s o r t i n g of the c r u s t a l m a t t e r , a n d the h i g h e r t h e e n e r g i e s of the s u s p e n d e d m a t t e r , t h e l a r g e r will be the f r a c t i o n of T i - b e a r i n g m i n e r a l s r e l a t i v e to A1rich c l a y m i n e r a l s . L a b o r a t o r y e x p e r i m e n t s p r e c e d i n g this s t u d y (see the A p p e n d i x ) confirm t h a t the TiO2/A1203 r a t i o m a y be a 0025-3227/87/$03.50

too1 as useful as p a r t i c l e size m e a s u r e m e n t s to e s t i m a t e the e n e r g y of a depositional env i r o n m e n t . U n d e r controlled conditions the r e s o l u t i o n of the i n d i c a t o r is v e r y high, and f r a c t i o n a t i o n is easily recordable, e v e n in the particle size r a n g e 1-4 pm. In n a t u r a l environments, on the o t h e r hand, a p p l i c a t i o n of the index m a y be complex due to differences in the c o m p o s i t i o n of s o u r c e r o c k s (see Migdisov, 1960; Bhatt, 1974; S p e a r s and K a n a r i s - S o t i r i o u , 1976). In t h e deep-sea e n v i r o n m e n t t h e use of t h e TiO2/A110 3 s t r e a m e n e r g y index is c o m p l i c a t e d by the u b i q u i t o u s o c c u r r e n c e of TiO2-rich seafloor b a s a l t i c m a t t e r (Correns, 1954; G o l d b e r g a n d A r r h e n i u s , 1958; B o s t r 6 m et al., 1973). F o r i n s t a n c e , G o l d b e r g a n d A r r h e n i u s (1958) s h o w e d t h a t t h e TiO2/A120 3 r a t i o in deep-sea sediments decreases with increasing distance

© 1987 Elsevier Science Publishers B.V.

196

from oceanic basalt islands. The difficulties in determining whether variations in the TiOz/AI/O 3 ratio are due to changes in the rate of basalt influx or changes in bottom cur r ent energies may seem insuperable. However, during a study of the Ba and opaline silica geochemistry of five Deep Sea Drilling Project cores (213-217), drilled on and adjacent to the Bengal Fan in the eastern Indian Ocean (Schmitz, 1987), it was realized that well-mixed, continental crust matter dominates the detritus fraction of the sediments over wide areas in this region. This matter is carried to the Bay of Bengal by the Ganges and B r a h m a p u t r a (G-B) rivers, and is transported by bottom currents at least as far as to 10°S in the central Indian Ocean (see also V e n k a t a r a t h n a m and Biscaye, 1973). If changes in the energy of these bottom streams t h r o u g h o u t the Cenozoic could be revealed with the TiO:/AIEO 3 index, the processes involved in the formation of huge deepsea fans could also be elucidated. Variations in Bengal Fan stream energies may be related to the amount of river water debouching into the Bay of Bengal and to the development of the G-B rivers, including the elevation of their drainage areas. Today these rivers carry the highest sediment load (Holeman, 1968; Milliman and Meade, 1983) of the world's rivers and are the main suppliers of debris to the 3000 km long, 1000 km wide and up to 5 km thick Bengal Fan (Curray and Moore, 1971; Emmel and Curray, 1984).

Samples and m e t h o d s of analysis Samples were obtained from five DSDP cores (213-217) drilled in the north- and centraleastern Indian Ocean (Fig.l). Cores 214, 216 and 217 were drilled on the Ninetyeast Ridge at water depths of 1665, 2247 and 3020 m, respectively. These cores consist mainly of nannofossil and foraminiferal oozes with CaCO 3 contents of more than 8 0 - 9 0 0 . Only core 217 has CaCO3 contents in the range 50-90%, with the lowest concentrations occurring in the younger sediments. Cores 213 and 215 were drilled on the southernmost extremities of the

Fig.1. Map of Bay of Bengal and central-eastern Indian Ocean with DSDP sites marked out. The dotted line shows approximate boundaries of the Bengal and Nicobar Fans (adapted from Curray and Moore, 1971; Emmel and Curray, 1984). The present-day main channel on the fan is also marked out.

Nicobar and the Bengal fans in water depths of 5611 and 5319 m, respectively. The cores are made up of basal Paleogene calcareous oozes, mid-Cenozoic brown clays and uppermost Neogene siliceous oozes. Details on core stratigraphy are given in DSDP Vol.22 (Von der Borch, Sclater et al., 1974). Major element analyses were performed on metaborate digested samples using inductively coupled plasma - optical emission spectrometry (ICP-OES) (Burman et al., 1978, 1981). Calcareous samples were decarbonized in dilute acetic acid (0.5 M, 24 h, 20°C) and analyses were made on the leached residues. Tests showed t hat neither TiO 2 nor A1203 dissolve in the acetic acid. Leaching is necessary since the low content of detrital matter in the oozes prevents high-precision analyses of the detri-

197 t u s - a s s o c i a t e d elements. In I C P - O E S a n a l y s i s , as well as in o t h e r a n a l y t i c a l t e c h n i q u e s , relative analytical errors become enhanced at c o n c e n t r a t i o n s close to t h e d e t e c t i o n limit. All of t h e TiO 2 a n d A120 ~ a n a l y s e s used h e r e h a v e t o t a l u n c e r t a i n t i e s of less t h a n 0.5-4%. T h i s e r r o r e s t i m a t e is d e d u c e d f r o m a n a l y s e s of several international standard rock samples (NIM-G, UB-N, GH, SY-3; G o v i n d a r a j u , 1984), a n d d u p l i c a t e a n a l y s e s of all samples. W h e n c o n s i d e r i n g o n l y the p r e c i s i o n of t h e a n a l y s e s , r e l a t i v e u n c e r t a i n t i e s a r e g e n e r a l l y less t h a n 1.0%. A c c u r a c y u n c e r t a i n t i e s , h o w e v e r , m a y be higher, a n d d e p e n d in p a r t on w h i c h r e f e r e n c e r o c k is c h o s e n as " t r u e " s t a n d a r d .

Results A n a l y t i c a l d a t a for TiO2 h a v e b e e n processed a c c o r d i n g to t h e following c a l c u l a t i o n : TiO2/A1203 x 15 = TiO2* D e t a i l s on this p r o c e s s i n g a r e discussed in Schmitz (1987). B a s i c a l l y , the v a l u e of TiO2* r e p r e s e n t s the TiO 2 c o n t e n t of t h e s e d i m e n t w h e n its A120 3 c o n c e n t r a t i o n is r e c a l c u l a t e d to be 15%. Since m a n y k i n d s of c r u s t a l m a t e r i a l s h a v e A120 3 c o n c e n t r a t i o n s of a b o u t 15% (e.g., T a y l o r , 1964; W e d e p o h l , 1969; T a n a n d Chi-lung, 1970), this p r o c e s s i n g f a c i l i t a t e s c o m p a r i s o n of TiO 2 c o n t e n t in different m a t e r ials. T h e A120 3 (15%)-normalized v a l u e s for TiO 2 in all s a m p l e s a r e g i v e n in T a b l e 1. I n f o r m a t i o n on b u l k g e o c h e m i s t r y of m o s t of the s a m p l e s is g i v e n in S c h m i t z (1987). F i g u r e s 2 a n d 3 d i s p l a y TiO2* v a l u e s p l o t t e d a g a i n s t time. All studied cores h a v e a b a s a l s e q u e n c e of h i g h TiO2* values. T h i s u n i t is followed by a s e c t i o n w i t h v e r y low TiO2* c o n c e n t r a t i o n s . At two sites, 213 a n d 214, t h e t r a n s i t i o n f r o m h i g h to low TiO2* v a l u e s is g r a d u a l , w h e r e a s it seems a b r u p t a t t h e o t h e r sites. M o s t likely this is due to i n c o m p l e t e s t r a t i g r a p h i c r e c o v e r y a t the l a t t e r sites. A n o t e w o r t h y f a c t is t h a t t h e n o r t h e r n m o s t sites, 216 a n d 217, pass from h i g h to low TiO2* v a l u e s a b o u t 20-30 m.y. e a r l i e r t h a n t h e s o u t h e r n sites.

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Fig.2. Distribution of TiO2* in DSDP cores drilled on the sea floor adjacent to the Ninetyeast Ridge. The chronological time scale has been adopted from Berggren et al. (1985a,b), and paleontological biostratigraphy is from Von der Borch et al. (1974). The distribution of data points within each epoch has been deduced by interpolation, using the positions of the samples in the core relative to the epoch boundaries. For the interval Middle Miocene to Middle Eocene in Core 213, the distribution of data points is highly hypothetical since this core section is barren of biostratigraphically indicative fossils. Long intervals without data points represent hiatuses or sections not recovered. Notice the difference in scale regarding the time representation for the late and early Cenozoic Era. For details on core stratigraphy see Table 1 and Von der Borch et al. (1974). Details of the TiO2* distribution are discussed in the text. An i m p o r t a n t c h a n g e in TiO2* d i s t r i b u t i o n o c c u r s in t h e s e d i m e n t s of L a t e M i o c e n e age. In Figs.2 a n d 3 l e a s t - s q u a r e fitted l i n e a r r e g r e s s i o n lines for TiO2* c o n t e n t a g a i n s t t i m e of d e p o s i t i o n h a v e b e e n plotted. At f o u r of the sites t h e lines e n c o m p a s s middle L a t e M i o c e n e to R e c e n t sediments. At Site 213 a b e t t e r fit for the r e g r e s s i o n line w a s o b t a i n e d by considering also t h e e a r l y L a t e M i o c e n e sediments. T h e c o r r e l a t i o n coefficients v a r y b e t w e e n 0.81 a n d 0.89, s u g g e s t i n g a s t r o n g p o s i t i v e r e l a t i o n s h i p . At f o u r of t h e sites the l i n e a r t r e n d s h a v e s i m i l a r slopes, w h i c h s u g g e s t s a c a u s a l relation. Well-developed l i n e a r t r e n d s are p a r t i c u l a r l y a p p a r e n t a t Sites 214 a n d 215. Single d a t a points, h o w e v e r , s h o w a n o m a l o u s c o n c e n t r a tions of TiO2*. F o r e x a m p l e , a t Site 216 a n e x t r e m e l y low TiO2* v a l u e is r e c o r d e d in a

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Pliocene sediment sample (see Fig.3). This may be due to admixture of rhyolitic, low-TiO2* ash layers with the sediments. In the log for Core 216 (Von der Borch et al., 1974) such an ash layer is indicated at a level about 2.7 m below the sample in question. The distinct TiO2* peaks in the Middle and Late Miocene at Sites 214 and 215, respectively, are due to turbidity flows• In the core log for Site 215 a silty turbidite deposit is recorded at the level of the TiO2* peak. No turbidite sequence is registered at Site 214, but this may be due to the masking effect of high CaCO3 content• Another possibility is that the peak at this site reflects a short period of basaltic detritus influx• An additional fact to note is that the proximal Sites 216 and 217 have late Neogene TiO2* trends spanning the interval from about 0.6 in the Late Miocene to Recent values close to 0.75, whereas the distal Sites 213 and 215 show trends from about 0.5 to 0.6. Site 214, situated at about the same latitude as Sites 213 and 215, but at shallower depth, displays Neogene TiO2* values in the range from 0.5 to 0.7. Discussion

Interpretation of the Ti02* distribution In the studied DSDP cores, the basal high TiO2* concentrations can be explained by a

strong input of basaltic matter• Data presented in Tables 1 and 2 show that the basal TiO2* values are mostly almost as high as those of pure basaltic matter• An initial period of basaltic detritus influx is natural since all sites formed in volcanic environments. With increasing age of the sea floor the basalts were covered with sediments, and became less susceptible to abrasion. In connection with the northward movement of the Indian Plate (Norton and Sclater, 1979) the sites, successively, from north to south, passed from high to very low TiO2* values• The minimal TiO2* values are considered to represent extremely fine-particulate, land-distally transported continental detritus• Due to mechanical fractionation this material is depleted in the heavy, TiOE-bearing minerals• A gradual passage from an environment dominated by basalt detritus to one of continental detritus is especially well developed in Core 214. This core is believed to comprise an essentially complete stratigraphic section (Gartner, 1974)• From the beginning of Late Miocene to Recent all five DSDP cores show gradually increasing TiO2* values. This reflects the influx of coarser continental particulate matter, and increasing stream energies at respective site. The validity of this interpretation is greatly corroborated by the TiO2* data compiled in Table 2. The first three values in the table represent a longitudinal profile for the

201

matter causes it to be dispersed over wide areas and diluted with turbidity transported matter before reaching the deep-water sea floor.

TABLE 2 Distribution of TiO2* in crustal matter Typeofmaterial

TiO2* S.D.

Ganges-Brahmaputra river0.790 borne sediments, (n = 56)a Quaternary Bengal Fan 0.724 0.026 sediments, northern sites (216 and 217) (n = 13) Quaternary Bengal Fan 0.617 0.032 sediments, distal sites (213-215) (n = 18) Eocene and Oligocene 0.576 0.040 sediments, Site 217 (n = 16) Eocene and Oligocene 0.486 0.045 sediments, Site 216 (n = 16) Swedish varved clay, summer 0.792 0.014 layer (n = 3) Swedish varved clay, winter 0.754 0.028 layer (n = 3) Phanerozoic clays from Russian 0.795 Platform (n = 10,084) World River suspended matter 0.789 Tiki Basin sediments, southeast 1.319 0.303 Pacific (n = 11) Oceanic basalts from Sites 213 1 . 9 5 6 0.956 216 (n = 22)b Deep-sea sediments from all oceans (n = 35) (Range TiO2* = 0.557-3.588)

References 1 This work

This work

This work This work This work This work 2 3 4 5

6

a Sediment discharge weighted average. b Average of averages for respective sites, n = n u m b e r of samples, S.D. = standard deviation. References: 1= Subramanian et al. (1985); 2=Migdisov (1960); 3 = M a r t i n and Meybeck (1979); 4=Hoffert et al. (1979); 5 = H e k i n i a n (1974); 6 = E1 Wakeel and Riley (1961).

Quaternary. The profile starts at the G-B river system in the north, and runs southwards along the Bengal abyssal cone. The decrease in TiO2* as a function of distance from the river mouth is obvious. One exception from this generalized pattern, however, is the somewhat higher TiO2* values at Site 214 compared to adjacent deep-water sites (see Figs.2 and 3). V e n k a t a r a t h n a m and Biscaye (1973) show t h a t on the Ninetyeast Ridge, the sediments contain more Indonesia-derived, wind-transported, volcanic matter t h a n in the surrounding deepwater areas. This matter may be rich in TiO2*. Possibly the slow settling rate for the eolian

The validity of Ti02* as a stream energy indicator

The use of TiO2* as a stream energy indicator is highly dependent on the assumption t h a t the detritus studied is derived from a single source of constant composition. Considering the Bay of Bengal the influx of detritus from rivers other than the G-B rivers is insignificant. Data presented in Table 2 show that the TiO2* content of suspended matter from the G-B rivers is 0.790 (Subramanian et al., 1985). This value is in excellent agreement with the value of 0.789 for average world river suspended matter (Martin and Meybeck, 1979). The values for suspended matter in their turn are very similar to the average TiO2* value (0.795) for 10,084 clay samples from the Russian Platform (Migdisov, 1960). This similarity between crustal and suspended matter elucidates the effectiveness of natural mixing processes. An additional evidence for this comes from the data for three Swedish varved clays in Table 2. Goldschmidt (1954) proposed t h a t glacial varved clays should be similar to the continental crust in composition. This seems reasonable since a continental ice sheet collects, decomposes and mixes detritus from large areas. The data for the three, randomly selected, Holocene varves show that the summer layers have a mean value (0.792) that coincides with the Russian Platform and suspended matter values. The more fine-grained winter layers, however, exhibit a lower mean value (0.754). The similarity between the TiO2* values discussed above gives credence to the idea that river suspended matter constitutes a representative mixture of well-homogenized continental crust rocks. This also implies that it is reasonable to think of the G-B rivers as a geochemically stable source of suspended matter. Table 2 also shows that the validity of TiO2* as a stream energy indicator is restricted to environments dominated by sedimentation of

202 continental, non-basaltic material. The TiO2* data for 35 deep-sea sediment samples from all oceans (El Wakeel and Riley, 1961) span over a wide range (0.557-3.588%). Obviously the TiO2* values are determined both by variations in stream energy and admixture of basaltic matter. Without knowing the influx rates of basaltic matter at each site, nothing can be said about the stream energies having deposited the sediments. It appears that in the deep-sea, TiO2* content may only provide relevant information about stream energies in areas where the major rivers debouch. Considering the great distances over which riverine sediments are deposited in the north eastern Indian Ocean, this may still represent a substantial part of the present sea floor. In the shelf regions of the oceans the TiO2* index may be particularly applicable. Previously, the grain-size distribution in deep-sea sediments has been used as indicator of bottom water paleovelocities (e.g., Ledbetter, 1979; Ledbetter and Ellwood, 1980). However, it has been pointed out that due to processes such as physicochemical aggregation and fecal pellet formation (see McCave, 1984), only particles in the size range from coarse silts to sands are useful (Anderson and Kurtz, 1985). The results of this study tentatively suggest that the TiO2* stream energy index has potential for revealing the kinetic energy of currents transporting even the most fine-grained matter (see also the Appendix). One reason for this may be that the Ti-carrying particulates have less cohesive surfaces than for example Al-rich clay minerals. Although kinetic energy may be decisive for the TiO2* or particle size distribution in a water mass, the time since the water mass was last exposed to detritus influx may also be of importance. A very '~old" water mass may have a low TiO2* content even though it may have a relatively high energy. At the distal part of the Bengal Fan the most fine-grained particulates are transported with turbid plumes, and variations in the TiO2* content are possibly primarily related to the distance to and the strength of the plume-generating turbidity currents.

Neogene Himalayan orogeny and Bengal Fan progradation The profound Late Miocene to Recent increase in Bengal Fan TiO2* content is indirectly coupled to the elevation of the Himalayas. Studies of the molasse-like Siwalik sediments (Wadia, 1961; Gansser, 1964; Pilbeam et al., 1977), and seismology and mineralogy of Bengal Fan sediments (Moore et al., 1974; Thompson, 1974) suggest that the main phase of the Himalayan uplift occurred in the period between the Late Miocene and Recent. Additional evidence, such as the occurrence of uplifted, Quaternary, lacustrine deposits in Kashmir (Sahni, 1936; Burbank and Johnson, 1983), and fission-track dating of metamorphosed rocks from the Nanga Parbat region (Zeitler et al., 1982), imply that a major stage of orogeny took place during the last few million years. The causal relationship between the Himalayan orogeny and Bengal Fan stream energies is complex, involving several intermediary links. Three such factors, however, may be: (a) progressive maturation of the G-B river system; (b) evolution of North Indian orographic monsoon rains; and (c) progradation of the G-B delta and the Bengal Fan. Considering first the history of the northern India river system, it is recognized that the existence of a major river system along the southern foot of the Himalayas goes back at least as far as to middle Cenozoic time. The oldest fluviatile sediments, the Early to Middle Miocene Murrees, are believed to be derived from a low-relief hinterland (Gansser, 1964). Only in the Middle to Late Miocene do the Himalaya-derived Siwalik deposits begin to form. Pascoe (1919) and Pilgrim (1919) proposed a westward flow for the oldest subHimalayan river, the "Indobrahm" (Pascoe, 1919), whereas the Ganges was thought to have formed later by cutting back and successively capturing the Indobrahm watermasses. Recent studies of sedimentary structures, however, have revealed that even the most westerly Siwaliks formed in an eastward-flowing river

203 system (Johnson et al., 1985). Even though the Indobrahm scenario seems to be incorrect, successive capturing of tributaries by the maturing Ganges and Brahmaputra may have led to increasing water and sediment yields reaching the Bay of Bengal. Another factor of profound influence on Bengal Fan TiO2* values may be the evolution of orographic monsoon rains. With the elevation of the Himalayas, the humid air masses of the southwest monsoon encountered a very lofty orographic barrier. Today mean annual precipitation in the eastern slope regions of the Himalayas (Cherrapunji) is 11 m (Liljequist, 1970). This is among the highest annual precipitation yields on Earth. In conjunction with the Himalayan uplift, annual precipitation may well have increased with a factor of 5-10. This yield is an efficient medium for transport of denudation products from the Himalayas to the Bengal Fan. Moreover, the Himalayan elevation induced an increase in denudation rates (see Corbel, 1959; Donnelly, 1982) which may have further enhanced erosional activity. In northern India today, rivercarried silt load, other than in the monsoon season, is an insignificant fraction of the whole (Menard, 1961). The third factor, delta and fan progradation, is causally related to the two foregoing. However, it is also the factor which is most intimately related to changes in the abyssal TiO2* values. As debris is transported to the sea, a river builds up its delta and fan, leading to progradation and a gross coarseningupwards succession in the fan sediments (Walker and Mutti, 1973). If the assumption is made that the late Neogene increase in TiO2* primarily reflects a period of rapid fan progradation, then the TiO2* values can be used for elucidating progradation rates. For example, the mean difference in TiO2* between the two northern sites and the three southern sites is 0.107 for the Quaternary sediments (see data in Table 2) and the mean distance between the two groups of sites is about 1500 km. Using a progradation model, an increase of 0.007 in distal TiO2* values would (roughly) represent

a progradation of 100 km. Taking Site 215 as representative, TiO2* values have increased by about 0.150 during the Late Neogene. This would correspond to a fan progradation of more than 2000 km, a distance which is more than two thirds of the total length of the present-day fan. When discussing progradation it should be emphasized that this involves the evolution of huge channels on the fan (e.g., Nelson and Kulm, 1973; Normark, 1978; Damuth et al., 1983). As shown for the Amazon cone (Damuth et al., 1983) these channels may simply represent a prolongation of the subaerial river system. The channels show high-sinuosity meandering, which implies a continuous, high volume turbidity flow for long periods of time. On the present Bengal Fan the river discharge passes from the shelf to the abyssal cone via the Swatch of No-Ground Canyon, and then follows a single active channel which continues all the way to the Equator (Emmel and Curray, 1984). Hence, the energies of the distal streams may be directly related to the amount of river discharge debouching into the Bay of Bengal. Considering the smooth TiO2* gradient in Core 215 at the extremely distal fan, it seems that permanent channel turbidity flow may be more important in contributing detritus to the deep-sea than ephemeral, strong turbidity currents. Shephard (1981), however, postulates that such turbidity currents occur with a very high frequency on the sea floor. If this is the case, the TiOz* data may represent the average effect of intermittent turbidity activity throughout the Neogene. A factor that may to some extent have influenced the TiO2* distribution is changes in fan valley courses. The abundant occurrence of abandoned channels on the present Bengal Fan (Emmel and Curray, 1984) suggests that these changes may occur rather often. Within the time-framework of slow depositing deep-sea sediments an averaging effect seems likely. This is substantiated by the great similarity in TiO2* trends for the studied DSDP cores. However, the slight difference in trend for the Nicobar Fan Site 213 may be due to changes in

204

fan valley directions and the shielding effect of the Ninetyeast Ridge. Bowles et al. (1978) suggest that after 5 Ma B.P. the Ninetyeast Ridge has blocked Bengal Fan originated turbidity currents from reaching the Nicobar Fan.

Conclusions In deep-sea environments supplied with continental detritus, the content of TiO2* in the sediments may be used as a stream energy indicator. In the detritus fraction of calcareous and siliceous Cenozoic Bengal Fan sediments, TiO2* displays a profound increase from the Late Miocene to Recent. This increase most probably reflects the progradation of the Bengal Fan, and is related to the Himalayan orogeny by processes such as geomorphological maturation of the G-B drainage system, evolution of orographic monsoon rains and elevation-controlled increases in erosional activity. Further studies of the TiO2* distribution in Recent and Cenozoic sediments in areas adjoining the major abyssal fans may give a new understanding of the processes involved in abyssal fan formation.

Acknowledgements This study was supported by grants from the Swedish Natural Science Research Council to the author and to K. Bostr6m. An additional grant was received from the Hierta Retzius Foundation. The author is indebted to J. Backman, S. Blomqvist, K. Bostr6m, H. Magn6t de Saissy and R. Russell for comments on the manuscript. Thanks are also given to B. BostrSm for advice regarding ICP-OES analysis, to R. L6fvendahl for donating laboratory equipment to the project and to the anonymous referees for their comments. Samples were provided by the Deep Sea Drilling Project (sponsored by the National Science Foundation).

Appendix In o r d e r to t e s t w h e t h e r t h e T i O J A 1 2 0 3 ratio in s u s p e n d e d m a t t e r c a n be u s e d as a s t r e a m e n e r g y or particle size

D

I06m

o~

O7O

'o

HOURS

168

Fig.4. A plot of TiO2* c o n t e n t in s u s p e n d e d v a r v e d clay v e r s u s t i m e of s u s p e n s i o n in a 20 cm w a t e r c o l u m n . S t o k e s ' law c a l c u l a t e d particle sizes a r e g i v e n t o g e t h e r w i t h s u s p e n s i o n time. E s t i m a t e d m a r g i n s of a n a l y t i c a l e r r o r a r e also g i v e n in t h e figure. T h e d a t a p o i n t at t h e far r i g h t in t h e d i a g r a m refers to a s a m p l e t h a t w a s collected from a s e c o n d r u n w h e r e s u b s t a n t i a l l y m o r e clay w a s s u s p e n d e d in t h e w a t e r c o l u m n t h a n in t h e first run. D u e to differences in s o l u t i o n d e n s i t y t h e r e s u l t s from t h e first a n d s e c o n d r u n m a y n o t be a b s o l u t e l y compatible.

gauge, a s a m p l e of Swedish, H o l o c e n e v a r v e d clay w a s h o m o g e n e o u s l y s u s p e n d e d in a w a t e r c o l u m n . Before s u s p e n d i n g t h e clay, it w a s p a s s e d t h r o u g h a 28 ~m sieve and dispersed with Na4P20 7 and ultrasound. Suspended m a t t e r w a s collected at r e g u l a r t i m e i n t e r v a l s w i t h a pipette at a d e p t h of 20 cm in t h e w a t e r c o l u m n . A n a l y s e s of TiO 2 a n d A1203 were p e r f o r m e d as described in t h e m e t h o d s section. TiO2* v a l u e s p l o t t e d a g a i n s t s e d i m e n t a t i o n t i m e a n d S t o k e s ' law c a l c u l a t e d particle d i a m e t e r s a r e g i v e n in Fig.4. T h e r e s u l t s s h o w t h a t TiO2* d e c r e a s e s c u r v i l i n e a r l y w i t h time. T h e r e l a t i o n s h i p b e t w e e n TiO2* v a l u e s a n d particle size c a n be e x p r e s s e d a c c o r d i n g to: TiO2* = a + b In D w h e r e a a n d b a r e c o n s t a n t s d e p e n d e n t on f a c t o r s s u c h as initial c o m p o s i t i o n a n d m i n e r a l o g y of t h e s u s p e n d e d m a t t e r , particle s h a p e s a n d t h e s t a t e of t h e s u s p e n s i o n m e d i u m . D = particle d i a m e t e r . It s e e m s t h a t t h e TiO2* i n d e x m a y be a n as good (or better) i n d i c a t o r of w a t e r m a s s a g e or e n e r g y as particle size. A h i g h r e s o l u t i o n is o b v i o u s w i t h i n t h e clay-size interval; of p a r t i c u l a r i n t e r e s t is t h e fact t h a t TiO2/A120 a f r a c t i o n a t i o n s e e m s to o c c u r e v e n w h e n (Stokes' law c a l c u l a t e d ) particle sizes as s m a l l as 10 Vm are considered. U s i n g t h e TiO2* i n d e x for s t u d y i n g t h e b e h a v i o r of

205 microsuspended matter and Brownian motion-controlled particle settling may prove rewarding.

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