Marine Geology, 110 (1993) 393-402 393 Elsevier Science Publishers B.V., Amsterdam

Neogene oceanographic variations recorded in manganese nodules from the Somali Basin

V . K . B a n a k a r a, R . R . N a i r ~, M. T a r k i a n b a n d B. H a a k e c

aNational Institute of Oceanography, Geological Oceanography Division, Dona-Paula, Goa-403 004, India bMineralogiseh-Petrographisches Institut Universitdt Hamburg, Grindelallee 48, 2000 Hamburg 13, Germany

Clnstitut fiir Biogeochemie und Meeresehemie Universitiit Hamburg, Bundesstrafle 55, 2000 Hamburg 13, Germany

(Received August 20, 1991; revision accepted August 20, 1992)

ABSTRACT

Banakar, V.K., Nair, R.R., Tarkian, M. and Haake, B., 1993. Neogene oceanographic variations recorded in manganese nodules from the Somali Basin. Mar. Geol., 1 I0: 393-402.

A detailed study of a nodule from the Somali Basin dated by 23°The~ccss was correlated with the paleoceanographic events recorded in Site 236 (Leg 24) Deep Sea Drilling Project (DSDP) cores. Tentative indications are that the phase of nodule accretion starting with the development of pillar structure at a depth of 20 mm in the nodule around 13 Ma coincides with increased Antarctic Bottom Water (AABW) flow and an elevated calciumcarbonate compensation depth (CCD).

The Late Miocene lowering of the CCD is represented by the mottled zones between 8 and 18 mm in the nodule is characterised by an abundant silicate component (> 20%) of aeolian origin. The Miocene/Pliocene boundary (5 Ma) occurs at a depth of about 8 mm and is represented by the development of pillar structure and a minimum of aeolian dust (10.3%).

The increased biological productivity of the Somali surface water since the Middle Miocene is demonstrated by the increasing Corg content of the nodule (from 0.11 to 0.19%) towards its surface.

Introduction

The accretion o f manganese nodules and crusts is a very slow process, o f the order o f a few millimetres per 10 6 years (Somayajulu, 1967; Krishnaswami and Cochran, 1978;, K u and Broecker, 1979; Huh and Ku, 1984; Segl et al., 1984; Bernat et al., 1989; Banakar , 1990; Banakar and Borole, 1991). Oceanographic condit ions have been shown to control accret ionary processes and the composi t ion o f ferromanganese precipitates (Halbach and Puteanus, 1984; Segl et al., 1984; Mangini et al., 1990; Hein et al., 1992). A manga- nese nodule is built o f complex but distinct internal microstructures which reflect the environment o f their genesis (Banakar and Tarkian, 1991).

Even though the impor tance o f internal micro-

Correspondence to: V.K. Banakar, National Institute of Ocean- ography, Geological Oceanography Division, Dona-Paula, Goa-403 004, India.

structures o f manganese nodules was recognised and the structures classified two decades ago by Sorem and Foster (1972), their application in interpreting the palaeoceanographic evolution o f nodules has been relatively rare (see Segl et al., 1989; Mangini et al., 1990; Hein et al., 1992). In this paper we at tempt to reconstruct the palaeocea- nographic history o f a nodule f rom the Somali Basin and correlate this history with the history inferred f rom the nearest D S D P Site.

Material and methods

The two manganese nodules studied here were collected using a free fall grab f rom a station (10D) located at 6°S and 52°E in the Somali Basin. The water depth was 4700 m. This station is located about 600 km southwest o f D S D P Site 236 (Leg 24; Fig. 1), which was drilled at a water depth o f 4487 m. This site is used for compar ison of the

0025-3227/93/$06.00 © 1993 - - Elsevier Science Publishers B.V. All rights reserved.

394 V.K. BANAKAR E l AL

1236 v( DSDP SITE) m

50* 55* 60*

Fig. 1. Map showing the locations of the nodules (10D) and DSDP Site 236.

nodule growth history with paleoceanographic events recorded in the sediment cores. The reasons for choosing Site 236 are: (a) the drilling and the nodule sites are located in the deep-sea pelagic environment, (b) the sedimentary environment at both these locations is below the present day CCD, (c) the sediment core recovery at this site was nearly complete and provided a continuous ocean- ographic record and (d) both these locations have come under the equatorial high productivity belt as early as the Early Eocene.

The investigated nodules were highly compact and spherical with a diameter of 68-70 mm. The surface of the nodules was very smooth, fine grained and uniform. The cross-sections of the nodules exhibited fairly well defined internal struc- tures (Figs. 2, 3 and 4).

Each nodule was sliced through the nucleus. The sliced portion was cut into rectangular slabs, vacuum impregnated, polished and studied under a Carl Zeiss ore microscope. Subsamples scraped at depth intervals of 2 mm from one of the nodules (10D A) were analysed for Mn, Fe, Co, Ni and Cu on Perkin Elmer-5000 AAS using 6 M HCI leachates (hydrolysate fraction) and Corg on Carlo- Erba carbon analyser. The mineralogy of the sub-

samples and their residue was determined using a Philips X-ray diffractometer. The other nodule (10D B) was studied only for the internal micro- structures in order to compare these with the structural changes observed in nodule 10D A. A complete transect across nodule 10D B showed the symmetrical internal microstructural zones around the nucleus (Fig. 2). This observation sug- gests that the turnover of this nodule (Krishnas- wami and Cochran, 1978) has not prevented uniform development of internal microstructures on all the sides. The other possibility is that the nodule has grown in a fixed orientation at the sediment-water interface with equal growth rates on all the sides (Banakar, 1990) resulting in a uniform development of microstructures.

The accretion rate of nodule 10D A has pre- viously been estimated by Banakar and Borole (1989). 23°Th ...... and 23°Thexc/232Th methods yielded an accretion rate of 1.5 ram/106 y. This rate has been used to convert depth in the nodule to corresponding age (Fig. 4). We are aware of the limitations in such extrapolation. The stratigraphic boundaries assigned for the structural zones in this nodule are therefore only tentative. The identifica- tion and classification of the observed microstruc- tures were according to Friedrich and Wiechowski (1980) and are presented with their description in Fig. 3.

Results

The total CaCO 3 content in the sediments col- lected around the nodule location varies from 88% at 4100m to 50% at 4500m water depth. The planktonic foraminifera in the former sediment exhibit significant dissolution while in the latter only resistant keels of the foraminifera are present (M.V.S. Guptha, pers. commun., 1992). Based on this observation, we place the present day CCD between 4200 and 4300 m water depth.

Nodule 10D A is composed of poorly crystalline 6-MnO2 identified by broad peaks at 1.4 and 2.4 A in X-ray diffraetograms. The mineral todorokite, which is an indicator of diagenetic precipitation, is absent throughout this nodule and suggests that these nodules are purely of hydrogenetic origin with negligible diagenetic influence. This view is

MANGANESE NODULES FROM THE SOMALI BASIN 395

SCAL E mm

Fig. 2. Cross section of the nodule 10D B showing uniform growth of structural zones around nucleus (portion within dark circular band).

supported by very low Mn/Fe ratios (around 1.0; Table 1).

The residue content varies between 10.3% in the 6-8mm interval and 23.9% in the 14-16mm interval (Table l). The minimum residue recorded is in the zones of pillar structure while the maxi- mum is in the zones of mottled structure (Figs. 3 and 4). In the residue, quartz (identified by 3.3 and 4.3 A reflections) is the most abundant min- eral, followed by feldspar (Table l; Fig. 5).

The transition metal concentrations in the nod- ule are given in Table 1. Mn varies between 13.4 and 16.4%; Fe between 14.3 and 20.8%. Co and Ni show a strong sympathetic variation with Mn (r= > 0.85) while Fe and Cu are poorly correlated with Mn (r = + < 0.4). Antithetic associations are found between Fe and Cu (r=0.39) and Fe and Mn/Fe (r--0.75). The strong sympathetic associa- tion of Co and Ni with Mn provides additional evidence in support of hydrogenetic accretion of these nodules (Halbach et al., 1981; Dymond et al., 1984; DeCarlo et al., 1987). The Mn concentrations in general are low compared to similar hydroge- netic deposits of other basins. This discrepancy might be due to decreasing availability of Mn and its associated elements like Co and Ni in the water column with increasing depth of nodule accretion from the oxygen minimum zone (DeCarlo et al., 1987). Poor sympathetic association of Fe with

Mn might suggest an accretion within a transition zone separating carbonate free and carbonate dominant depositional environment. The low Mn content might therefore be due to the dilution by Fe released in greater amount just below the CCD due to the dissolution of carbonate skeletons (Hal- bach and Puteanus, 1984).

A single diagram (Fig. 4) comprising the mosaic of microstructures with depth in nodule, 23°Thex¢ extrapolated age, generalized CCD fluctuation in the Somali Basin (Leclaire, 1974), the Neogene sedimentation hiatuses recorded at Site 236 (Keller and Barron, 1987), and the variation of major components with depth in the nodule, is presented for an effective comparison of these parameters.

Discussion

The highest residue content of 51.5% below 20 mm depth (Table 1) and the replacement/cavity filling of nucleii by ferromanganese oxyhydroxides (Figs. 3e and 4) suggest that the outermost bound- ary of the nucleii is the base of the pillar structure at a depth of 20 mm. This depth corresponds to an extrapolated age of 13 Ma and marks the beginning of nodule accretion.

The pillar structure between 20 and 18 mm (,-~13-~12 Ma) indicates undisturbed precipita- tion of oxyhydroxides with a lower flux of detritus

396 V K . B A N A K A R ET AL

Fig.3A.

and a steady supply of metals from the overlying water column. The low residue content (18.8%) and higher Fe content compared to the Mn content (17.8 vs. 14.3%) of this zone (Table 1) suggest an elevated CCD associated with increased carbonate dissolution and sedimentation hiatus in a well oxidised bottom environment. An elevated CCD and high carbonate dissolution rate can reasonably be assumed during this period on the basis of relatively less abundant foraminifera than in youn-

ger sediments in DSDP cores of Site 236 (Vincent, 1974). Fe enrichment over Mn could therefore be due to increased supply of Fe to the bottom water as a result of increased dissolution of carbonate tests (Halbach and Puteanus, 1984). Fluctuation in the CCD has been related to surface biological productivity, intensity of corrosive bottom waters and seafloor spreading (Leclaire, 1974). In the present context the effect of seafloor spreading may be ignored because the present configuration of the Somali Basin has developed nearly 60 my B.P. (Schlich et al., 1974; Patriat and Segoufin, 1988) and since then came under the equatorial high productivity belt. Therefore, the major factor influencing the CCD in this basin must be the intensity of bottom water flow. One of the two AABW spreading paths has been traced in the Somali and Central Western Indian Basins (Kolla et al., 1976). In the Somali Basin increased AABW activity was noticed in sediments spanning the Middle Miocene (Vincent, 1974), with associated increase in dissolution of foraminiferal tests

td)

MANGANESE NODULES FROM THE SOMALI BASIN 397

(Leclaire, 1974). The middle Miocene elevated

CCD was also a global p h e n o m e n o n recorded in

all the major ocean basins (Berger, 1972).

The hiatus in deep-sea sedimenta t ion due to the

flow of corrosive A A B W dur ing the Middle Mio-

cene has been used to explain hydrogenous nodule

format ion in the Pacific (Von Stackelberg, 1979). Friedrich and Wiechowski (1980) have shown the

change from pillar to co lumnar structure to be

associated with change from a lower M n / F e ratio

to a higher M n / F e in a Central Pacific manganese

Fig. 3. Internal microstructures of the nodule identified; (A) Nodule 10D A, (B) Nodule 10D B. Magnification = x 250. (a) lamellar structure; in which Fe-Mn oxyhydroxide cusps are arranged in distinct laminae. This structure is observed between surface and 5 mm in nodules. (b) pillar structure; is composed of long (up to 2 mm) slender, regularly spaced and parallely arranged Fe-Mn oxyhydroxide cusps. This structure is characteristic at two depth intervals in the nodules; between 5 and 9 mm and between 18 and 20 mm. (c) mottled structure; is the non-uniform growth of oxyhydrox- ide cusps with abundant detritus. This structure occurs in the 9 to 18 mm depth interval in the nodules. (d) pillar structure (see b for description) which occurs in the 18 to 20 mm depth interval. (e) infilling within or replacement of nucleating material (gray coloured) by Fe-Mn hydroxide precipitate (light coloured). The hexagonal fissure pattern in 3B(e) might suggest that the nucleii for these nodules are completely altered basalt frag- ments(?). The infilling/replacement appears at 32 mm in the nodule and continues up to 20 mm which marks the base of the pillar structure and the boundary between the nucleus and nodule material.

crust (VA 13-2). The base of the pillar structure

and its te rminat ion were dated to be ~ 1 3 and ~ 1 0 Ma, respectively (Segl et al., 1984). The

format ion of this pillar zone was related to

increased Antarct ic Ci rcumpolar Curren t activity.

Similarly, in i t ia t ion of accretion of the ~ 2 6 m m

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MANGANESE NODULES FROM THE SOMALI BASIN

TABLE 1

Compositional variation with depth in nodule RVG-10D A

399

Depth interval Residue content Relative abundance Cot , Transition (mm) (%) of quartz. Max = 1 (%)

Mn (%)

metal concentration

Fe (%) Co (ppm) Ni (ppm) Cu (ppm)

Mn

Fe

0*-2 14.4 0.84 0.19 16.4 15.5 3026 4057 2-4 13.2 0.70 0.15 15.1 14.5 2898 2978 4-6 11.3 0.89 0.15 15.1 16.3 3060 2430 6-8 10.3 0.41 0.13 15.2 15.4 3083 3023 8-10 11.5 0.36 0.13 15.4 14.3 3306 2948 10-12 16.7 0.57 0.12 14.1 15.2 2866 2350 12-14 22.2 0.68 0.13 13.4 16.1 2442 2391 14-16 23.9 1.00 0.I1 13.4 16.5 2417 2212 16-18 22.2 0.59 0.12 16.4 20.8 3306 3626 18-20 18.8 0.41 0.11 14.3 17.8 2731 2940 Nucleus 51.5 nd nd nd nd nd nd.

1740 1719 1116 1466 1403 1490 1364 1294 1342 1408 nd

1.06 1.04 0.93 0.99 1.08 0.93 0.83 0.81 0.80 0.80

*Zero corresponds to the surface of the nodule; nd= not determined.

thick F e - M n crust f rom the Line Island Ridge has been related to enhanced A A B W activity initiated by major global cooling since the Middle Miocene (Halbach and Puteanus, 1984).

In light o f the above studies we propose that the nodule accretion in the Somali Basin began a round 13 Ma as a result o f the global phenomenon related to development o f corrosive A A B W and Neogene sedimentat ion hiatus NH3. Similarly, the Central Indian Basin hydrogenet ic manganese crusts have been shown to be o f Middle Miocene age (Banakar and Borole, 1991). The sharp ter- minat ion o f pillar structure a round 12 M a in the nodules (Fig. 4) might be due to the terminat ion o f N H 3 o f Keller and Barron (1987) and resump- tion o f sedimentation.

The termination o f the pillar structure (at 18 m m depth) in the nodules is followed either by poor ly developed co lumnar structure or by mott led zones (Figs. 3 and 4) until the end o f Miocene (at 9 m m depth in nodules). This period is marked by a gradual lowering o f C C D at Site 236 where the abundance o f planktonic foraminifera in the sedi- ment cores increases to 40% at 6 M a f rom < 5%

at about 11 Ma (Shipboard Scientific Party, 1974). The decrease in carbonate dissolution during this period is demonst ra ted by a lowered Fe concen- trat ion o f the nodules f rom 20.8 to 14.3% (Table 1; Fig. 4). The r andom growth of oxyhydroxide cusps during this period o f lowered C C D is associated with a high flux o f detritus making up to 25% of the surface o f the polished section (Fig. 3).

A further interesting observat ion is the high residue content, reaching up to 24% in the 18 to 12 m m depth interval (period between 12 and 8 Ma). The composi t ion o f this residue is domina ted by quartz and feldspar (Fig. 5). It has been shown that quar tz in the Indian Ocean is derived f rom the surrounding continents and was t ranspor ted to open ocean in the form o f atmospheric dust (Windom, 1975). The content o f quar tz in the pelagic sediments is related to the continental aridity and increased intensity o f winds (Kolla and Biscaye, 1977). They have estimated about 3 to 5% o f quar tz in the surface sediments a round 6°S and 52°E as being due to the increased aridity on the Arabian deserts during last glacial interval. Similar hypotheses o f ice-age aridity and intensified

Fig. 4. Microstructures (1 - - classified according to Friedrich and Wiechowski, 1980), compositional variation in the nodule 10D A compared with the Neogene sedimentation hiatuses (3) recorded at DSDP Site 236 (Keller and Barron, 1987) and palaeo-CCD. Time scale in nodule (2) has been extrapolated using accretion rate of 1.5 mm/106 y (Banakar and Borole, 1989). The blanks in the mosaic column are either due to loss of section while slicing or due to the gaps in photography. The detailed data is presented up to 20 mm depth in nodules.

400 V K BANAKAR EI AL

I I I

30" I I I I I I

2 5 ° 2 0 °

2 e

Fig. 5. X-ray diffractograms of the residue fraction with depth in nodule 10D A; Q=quartz and F= feldspars.

winds during glacial maxima were advanced by others (Sarnthein, 1978; Sirocko and Sarnthein, 1989: Sirocko et al., 1991). It may be mentioned here that the Neogene hiatuses NH3 through NH6 which cover a period between 13 and 6 Ma have witnessed the coldest climates of the Miocene (Barron and Keller, 1982; Keller and Barron, 1983).

The high abundance of quartz and feldspar in the nodule can therefore be interpreted as being the result of intensified aeolian transport of litho- genous material to the Somali Basin from the African and Arabian deserts during the Middle Miocene cooling. This interpretation is based on the facts that, (a) the present continental drainage system was established during the Middle Miocene (Olausson, 1988), (b) the fluvial input to the Somali Basin is negligible due to the absence of a major drainage system along the northeast coast of Africa (Kolla and Kidd, 1982) and (c) such fluvial deposits are limited to the margins of India, Kenya and Oman (Sirocko et al., 1991).

A sharp decrease in the residue content of the nodule around the Miocene/Pliocene boundary (at 8 mm depth, i.e., ~ 5 Ma) might be responsible tbr providing a favourable environment for the formation of the second stage of pillar structures (Fig. 2A). This zone of pillar structure continues upto 5 mm depth in the nodule (i.e., extrapolated age of ~3 Ma; Fig. 4). The minimum residue (10.3%) with minimum quartz content (relative abundance of 0.41: Table l) around the Miocene Pliocene boundary might indicate less intense winds than through the late Miocene. A supporting observation is the record of less severe oceano- graphic conditions with warmer climates during NH7 (4.7 5.2 Ma) than the earlier hiatuses (Keller and Barron, 1983, 1987). In addition, the Miocene/ Pliocene boundary (~ 5 Ma) at Site 236 appears to have experienced a CCD elevation as suggested by the lowest abundance of planktonic foramini- fera ( < 5%) in the sediments (Vincent, 1974). Thus, the Miocene/Pliocene boundary might have wit- nessed a reduced flux of biogenic debris to the seafloor coupled with reduced input of aeolian dust. These conditions probably have culminated in a sedimentation hiatus NH7 and provided favourable conditions for the accretion of pillar structure.

It is quite difficult to relate every individual hiatus from the Late Miocene onwards to the various structural zones in these nodules. However, elevated CCD and a series of Neogene hiatuses (NH4 to NH7 of Keller and Barron, 1987) together with well oxidising bottom environments due to intense AABW flow might have provided pro-

MANGANESE NODULES FROM THE SOMALI BASIN 401

gressively favourable environments for the forma- tion of these nodules.

The increased surface biological productivity, from the Middle Miocene through the Pleistocene in the Somali Basin, is reflected in a gradual increase of the Cor~ content towards the surface of the nodule (0.11 to 0.19%; Table 1). There have been no studies on Corg depth profiles in manganese nodules. However, it is a fact that > 95% of the organic matter in oceanic water has been shown to be oxidised by primary oxidants before reaching the seafloor. The role of secondary oxidants such as MnO 2 is insignificant in the oxidation of organic matter in sediments (Bender and Heggie, 1984). The presence of divalent Mn in nodules (Dymond et al., 1984; Pattan and Mudholkar, 1990) suggests that manganese nodules represent a closed system as far as the oxidation of organic matter is con- cerned. We therefore assume that the Corg content of the nodule at a given depth reflects the surface biological productivity at that time. The increased surface biological productivity through the Neo- gene (Vincent, 1974) is distinctly reflected in the increased abundance of radiolarians and diatoms in the sediments of Site 236 (from < 5 to > 50% and from < 1 to > 20%, respectively). Thus with reasonable approximation the Corg content in the nodule could be used for understanding the biolog- ical productivity of the surface waters above the nodule location during its growth period.

Conclusions

(1) The formation of hydrogenetic manganese nodules in the Somali Basin is favoured by increased carbonate dissolution due to intensifica- tion of AABW flow around 13 Ma.

(2) During the growing stages of nodules corre- sponding to the Middle and Late Miocene mottled structures with high residue content are related to continental aridity and intense aeolian transport on Africa and Arabia due to global cooling. By contrast, the pillar structure with lowest residue content around the Miocene Pliocene boundary correlates with Neogene Hiatus 7 which witnessed warmer climate than the preceeding periods.

(3) The increased biological productivity of the Somali surface waters through the Neogene is

approximately reflected in increasing Corg content of the nodule from the nucleus to the surface.

Acknowledgements

The authors thank Dr. B.N. Desai, Director, NIO Goa, Dr. V. Ittekkot, University of Hamburg, Germany for their interest. Thanks are also due to Mr. P. Stutz and Mrs. B. Cornelisen, Institute of Mineralogy of Hamburg University, for provid- ing polished sections and analytical assistance and to Mr. Prabhu of the National Institute of Ocean- ography, Goa for carrying out the X-ray diffrac- tometer analyses.

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