Compositional changes of surface sediments and variability of manganese nodules in the Peru Basin

Deep-Sea Research II 48 (2001) 3523–3547

Compositional changes of surface sediments and variability of manganese nodules in the Peru Basin Vesna Marchiga,*, Ulrich von Stackelberga, Heinz Hufnagela, Goran Durnb a

Bundesanstalt fur . Geowissenschaften und Rohstoffe, Alfred-Bentz-Haus, Postfach 510153, 30631 Hannover, Germany b Faculty of mining, Geology, and Petroleum Engineering, Zagreb, Croatia

Abstract Two types of manganese nodules were observed in the Peru Basin: large botryoidal nodules in basins and small ellipsoidal nodules on slope positions. The sediment in areas with large botryoidal nodules contains a thinner and weaker oxidation zone than the sediment under small ellipsoidal nodules, indicating that diagenetic processes in the sediment, which supply manganese nodules with metals for their growth, are stronger in sediments on which large botryoidal nodules grow. Organic matter, which activates remobilization of metals, occurs mostly in the form of refractory lipidic compounds in the inner capsule of radiolaria. This material needs bacterial degradation to act as a reducing agent. Easily oxidizable organic components could not be found in the sediments. Other changes in sediment composition do not have a link to manganese nodule growth. Biogenous components (radiolarians, organogenic barite and apatite) increase towards the equatorial high-productivity zone. Authigenous clay minerals (nontronite as well as montmorillonite with high Fe+3 incorporation on positions of ochtaedral Al) increase with distance from the continent. The assessment of environmental impacts will have to take into account the regional differences in sediment composition and the small-scale variability of manganese nodules. r 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction The Peru Basin manganese nodule field was discovered in 1978 on an East Pacific N–S transect during R.V. Sonne cruise SO 04 (Thijssen et al., 1981). Following this discovery, cruise SO11 investigated this new field in detail. Nodules of unusually large size were sampled and their growth rates determined, being two orders of magnitude higher than the growth rates in previously documented manganese nodule fields (Reyss et al., 1982). The predominantly diagenetic supply of *Corresponding author. Tel.: +49-511-643-2792; fax:+49-511-643-2304. E-mail address: [email protected] (V. Marchig). 0967-0645/01/$ -see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 5 5 - 8

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metals for the growth of these nodules causes extreme enrichment in manganese compared to iron (Marchig and Reyss, 1984). In the late eighties, the Peru Basin was selected for technical tests by German industry and as an experimental field for investigations of disturbance of the natural sea floor environment in event of manganese nodule mining (Thiel and Schriever, 1990). Within this program two cruises of R.V. Sonne investigated surface sediments and manganese nodules (SO 79 in 1992 and SO 106 in 1996). The aim of this paper is the basic description of the environment in which manganese nodule mining will take place. The large-scale changes in sediment supply and the small-scale changes in manganese nodule coverage are going to dictate changing assessments of environmental impacts in case of manganese nodule mining.

2. Geological setting The Peru Basin is situated in the eastern tropical Pacific, south of the equatorial highproductive zone (Wyrtki, 1974). Two sub-areas were investigated in detail, the western one named ‘‘SEDIPERU’’ (Fig. 1a) and the eastern one named ‘‘DISCOL’’ (Fig. 1b). In both areas, abyssal hills alternate with small basins, resulting in large variations in water depth (Wiedicke and Weber, 1996). The winnowing of the fine fraction of sediments on the hills and the focusing of this sediment into the basins create large variations in the thickness of Quaternary sediment cover (between 5 and 14 m). The lysocline (presently at 3700 m water depth) and the CCD (presently at 4250 m water depth) are situated within the range of water depths investigated. The amount of calcite therefore varies from a few % to over 80% (Weber et al., 1995). The sediment samples and manganese nodules in these two areas were obtained from water depths between 3833 and 4329 m. Four box cores were sampled off these two areas, on two opposite margins of the nodule field with the aim of serving as comparison with box cores from the SEDIPERU and DISCOL areas (287KG and 178KG north of the SEDIPERU area; 255KG and 252KG east of the DISCOL area).

3. Sampling and analytical methods The sediment was sampled using a 50
50 cm short box corer that collected up to 0.5 m of sediment with a reasonably well-preserved semi-liquid surface and manganese nodules in their original orientation. The relevant data on box cores investigated are listed in Table 1. Before sampling the sediments, manganese nodules were collected in the oriented position from the surfaces of the box cores. They were measured, weighed, and described. From the data, different parameters were calculated that are relevant for understanding the genesis of the nodules (von Stackelberg, 1997): (a) abundance (in kg/m2), (b) number of nodules per square meter (n/m2 ), (c) size distributions of the nodules,

V. Marchig et al. / Deep-Sea Research II 48 (2001) 3523–3547

3525

Fig. 1. (a) Bathymetric map of the SEDIPERU area with positions sampled during cruises SO 79 and SO 106. Symbols for sampling devices: KG, box corer; MX, maxi corer; BG, boomerang grab; GA, TV grab; KD, dredge. Symbols for different types of size distribution of manganese nodule assemblages: (filled circles) large botryoidal nodules; (filled triangles) small ellipsoidal nodules; (open triangles) intermediate nodule population; (open circles) no nodules recovered. Small numbers near sample number indicate: (1) ts/bs (number of nodules with smooth top versus number of nodules with smooth bottom); (2) w=n (average weight of nodule in grams). (b) Bathymetric map of the DISCOL area (SO 106) with sampling positions. For explanation of symbols see Fig. 1a. Lists 1 and 2 are within the DISCOL disturbance area (Ahnert and Schriever, 2001; Bluhm, 2001). The nodules from sampling stations given in lists 1 and 2 are all of large botryoidal nodules type.

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Fig. 1. (continued )

(d) median nodule weight (in grams) ðw=nÞ; (e) number of nodules with smooth top surface compared with number of nodules with smooth bottom surface (ts/bs). Part of the nodules were then cut vertically to the sea floor. The cut surface was polished and the growth zones observed under a binocular microscope. The data on nodules are compiled in Table 2. The sediment was sampled at 50 mm intervals within the core. For chemical analyses, the sediment was freeze-dried and ground in a shatter box with agate inlay. X-ray fluorescence was performed using Phillips PW 1400 and PW 1480 instruments. Analytical precision was better than 2% for major elements and better than 5% for trace elements. For calibration of sediment analyses, international standards were used. Synthetic standards were used for those elements or ranges of concentrations not covered by international standards. CaCO3 content was calculated from the Ca content analyzed by means of X-ray fluorescence. Taking into account the fact that the non-carbonate fraction contains 1.6% CaO, this method is more precise than the volumetric CO2 determination method, which is specific for carbonate.

Table 1 Composition of surface sediments and characteristics of manganese nodules from the same sampling stationsa Sediment

Manganese nodules Redox boundary (cm)

Abundance (kg/m2)

Station

Latitude

Longitude

Water depth

Carbonate content (%)

Equatorial high production zone

287KG

5106.500 S

90114.850 W

3940

55.8

8.5

7.5

178KG

5130.100 S

90131.010 W

3967

48.6

9.6

10.0

9.0

24

A

SEDIPERU

179KG 183KG 185KG 186KG 234KG 236KG 237KG 264KG 292KG

6131.510 S 6133.400 S 6128.700 S 6130.810 S 6129.990 S 6129.540 S 6134.050 S 6132.050 S 6134.070 S

90124.810 W 90131.100 W 90129.010 W 90129.810 W 90127.910 W 90125.590 W 90127.950 W 90129.540 W 90128.010 W

4146 4106 4111 4132 4082 4174 4127 4092 4127

17.6 24.8 23.3 23.1 21.8 15.1 17.4 26.1 21.3

14.8 17.8 14.9 12.9 11.7 12.2 10.9 14.6 12.0

7.5 17.5 12.5 17.5 12.5 12.5 22.5 12.5 17.5

28.0 16.8 26.0 25.2 14.0 8.0 21.6 20.4 F

36 292 204 280 184 28 324 344 F

A B B B B A B B B

DISCOL

190KG 193KG 216KG 290KG 515KG 537KG 556KG 558KG 563KG

7103.320 S 7106.410 S 6158.980 S 7104.420 S 7107.480 S 7104.290 S 7104.230 S 7104.320 S 7104.220 S

88128.200 W 88122.700 W 88124.500 W 88127.550 W 88127.000 W 88127.920 W 88127.940 W 88127.850 W 88127.920 W

4178 4293 4007 4170 4169 4154 4140 4155 4153

3.8 2.5 14.2 3.9 (6.6) (4.8) (4.2) (3.9) 4.4

14.2 15.0 9.7 12.7 ? ? ? ? 11.9

7.5 7.5 17.5 7.5 ? 12.5 12.5 17.5 12.5

21.0 2.4 40.8 22.0 ? ? 0.4 31.2 3.2

32 32 172 24 32 40 4 52 16

A A B A A A A A A

Eastern area Eastern area, upwelling influenced

252KG 255KG

6134.290 S 5130.080 S

86111.430 W 85122.370 W

4105 4084

2.9 3.8

10.1 10.2

12.5 7.5

21.6 0

32 0

A

a

0

Number (n/m2)

Type

0

V. Marchig et al. / Deep-Sea Research II 48 (2001) 3523–3547

Opaline silica (%)

Area

Values in brackets are from 5 cm depth under the sediment surface. Type: A poorly sorted (big caulyflower); B well sorted (small elipsoidal). 3527

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Table 2 Characteristics of manganese nodules from box cores (0–50 cm) recovered during cruises SO79 and SO106a Area

Station

Water depth (m)

Histogram type

High productivity

178KG 271KG 284KG 596KG 597KG

3967 3944 3939 3942 3939

A A A A A

SEDIPERU

01KG 10KG 25KG 40BG 42BG 44BG 52KG 55KG 59KG 63KG 74KG 81KG 86BG 87BG 89BG 90BG 91BG 97KG 101BG 106KG 109BG 110BG 179KG 183KG 185KG 186KG 234KG 236KG 237KG 238MX 264KG 265KG 292KG 590KG 591KG 592KG

4119 3961 4170 3833 3896 3937 4006 4119 4339 4257 3878 4067 3896 3961 4051 4119 4143 4148 4313 4025 4032 4033 4146 4106 4111 4132 4082 4174 4127 4264 4092 4057 4127 4152 4152 4151

A B A B/A B/A B B/A A A A B B/A B/A B/A B B B A A B/A B/A B/A A B B B B A B A B B B A A A

DISCOL

188KG 190KG 193KG 194KG 196KG 197KG 201KG 202KG

4172 4178 4293 4126 4085 4211 4260 4254

A A A A A A A A

n/m2

kg/m2

w=n (g)

ts/bs

24 8 4 12 4

9.0 F F F F

375 F F F F

1/2 F F F F

84 392 48 135 203 910 40 48 104 40 260 144 225 45 601 985 489 64 165 172 30 188 36 292 204 280 184 28 324 F 344 332 F 36 32 40

32.8 19.2 17.2 1.5 0.9 10.1 F 46.8 7.6 49.6 9.6 13.4 3.8 1.5 12.0 15.4 16.9 44.2 6.8 13.4 0.7 9.8 28.0 16.8 26.0 25.2 14.0 8.0 21.6 F 20.4 13.2 F F F F

390 49 358 11 4 11 F 975 73 1240 37 93 16 33 20 16 34 691 41 78 25 52 778 57 127 90 76 286 66 F 59 40 F F F F

0/5 4/5 0/1 F F F F 0/1 0/13 0/3 F 0/4 7/0 1/0 0/0 3/2 0/0 0/3 0/0 0/0 0/0 9/0 0/6 2/2 0/1 1/3 13/2 0/5 24/6 1/4 38/0 3/0 F F F F

8 32 32 56 48 52 F 40

6.0 21.0 2.4 58.0 17.2 12.0 F 1.6

750 656 75 1036 358 231 F 40

F 0/2 0/2 2/4 0/3 0/1 0/1 1/2

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V. Marchig et al. / Deep-Sea Research II 48 (2001) 3523–3547 Table 2 (continued) Station

Water depth (m)

Histogram type

n/m2

kg/m2

204KG 207KG 212KG

4059 4247 4086

A A B

72 8 224

23.0 F 23.6

319 F 105

0/1 F 0/4

DISCOL

215KG 216KG 220KG 221KG 223MX 226KG 227KG 230KG 231KG 246KG 247KG 248KG 260KG 262KG 277KG 290KG 514KG 515KG 517KG 518KG 527KG 528KG 529KG 532KG 533KG 534KG 537KG 538KG 539KG 540KG 541KG 543KG 544KG 549KG 556KG 558KG 559KG 563KG 564KG 565KG 579KG

4004 4007 4260 4225 4115 4036 4159 4311 4120 4140 4106 4004 4208 4058 4158 4170 4168 4169 4172 4167 4150 4164 4166 4171 4173 4170 4154 4157 4160 4162 4158 4151 4168 4161 4140 4155 4171 4153 4150 4155 4147

B B A A B B A A A B A B A B A A A A A A A A A A A A A A A A A A A A A A A A A A A

300 172 16 20 F 576 76 36 16 364 36 148 4 148 56 24 20 32 20 24 28 28 8 20 12 20 40 44 28 36 8 20 16 12 4 52 16 16 16 8 28

25.6 40.8 F 12.0 F 20.4 36.8 3.2 25.6 23.6 11.0 29.8 F 25.2 2.2 22.0 F F F F F F 7.6 F F F F F F F F F 7.6 F 0.4 31.2 F 3.2 F F F

85 237 F 600 F 35 484 89 1600 65 305 201 F 170 39 916 F F F F F F 950 F F F F F F F F F 475 F 100 600 F 200 F F F

12/2 9/0 F 0/2 33/1 90/4 4/2 0/3 0/0 1/17 2/0 12/7 F 4/5 1/2 0/2 0/1 0/3 0/1 F 0/2 0/1 F F 0/2 0/1 0/4 0/6 0/1 0/2 F 0/1 F F F 0/5 0/3 0/2 0/1 F 0/2

Continent near

252KG

4105

A

32

21.6

675

0/4

Area

a

w=n (g)

ts/bs

Note: A, nodules with a poorly sorted size distribution; B, nodules with a well sorted size distribution; n; number of nodules; w=n; average weight of nodules in g; ts, number of nodules with smooth top surface; bs, number of nodules with smooth bottom surface; KG, box corer; MX, maxi corer; BG, boomerang corer.

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The determination of opaline silica was performed by the automated leaching method using . NaOH (Muller and Schneider, 1993). We used the ‘‘kink’’ in the curve of determination during continuous measurement of Si in the leacheate as the change of the above method. It improved the accuracy to better than 5%, as the ‘‘kink’’ occurs at the moment when all the opaline silica is dissolved and the poorly crystallised neo-formed clay minerals still supply the leacheate with further Si but at a lower speed. Organic carbon was analyzed with a LECO CS 344 furnace after removing CO2 from carbonate with hydrochloric acid. The method was calibrated with synthetic standards. Its accuracy was better than 3%. The analytical data for the sediment are compiled in the attached CD-ROM. Representative portions of the sediments were prepared for the organic–petrographic studies, embedding the sediment in araldite and making polished sections. The studies were carried out by a LEICA DMR microscope using incident normal light and under fluorescence illumination with UV and blue light excitation (barrier filter 530 nm). Compared to palynological preparations, which use chemically treated material, this procedure has the advantage that neither a selection of organic material nor a loss of components occurs during sample preparation. In addition, the spatial relations of organic and inorganic constituents in the sediment can be observed in its original particle distribution. The lower limit of the microscopic detection of the particles is somewhat less than 1 mm. The classification of the microscopically visible organic matter follows the classical organic– petrographic nomenclature (Stach et al., 1982), which has been described in more detail in Hufnagel and Porth (1989), but modified for young, unconsolidated marine sediments. The data on the composition of the organic matter are shown in Table 3. They are based on an estimation procedure (Hufnagel and Porth, 1989) and are given in volume percent, so that all components of particulate organic matter in the sample total 100. Components present in very minor amounts, less than 1%, generally were not registered but nevertheless were noted as present (p). The part of metals bound to the sediment in oxide form was determined using reductive leaching after Chester and Hughes (1967), followed by determination of metals in the leacheate by AAS. The results of these analyses are presented in the attached CD-ROM. For X-ray diffraction, the sediment was sampled separately, the soluble salts were removed, and fractions >63, and o2 mm were prepared. Washed bulk samples as well as the grain size fractions o2 and >63mm were analyzed using a Phillips X-ray diffractometer with CuKa radiation generated at 34 kV and 50 mA. Prior to separation of grain size fractions, the samples were treated to remove (i) carbonates (maintaining pH>4), and (ii) iron and manganese oxides (as described by Mehra and Jackson, 1960). The o2 mm fractions were spread on glass slides and X-rayed after each of the following treatments: (i) Mg-saturation, (ii) Ksaturation, (iii) Mg-saturation and glycol solvation, (iv) K-saturation and dimethyl-sulfoxide solvation, and (v) heating to 5501C. Powder washed free of salts and the fractions >63 mm were scanned from 2 to 70 (12y). Oriented samples of the o2 mm fraction were scanned from 2 to 40 (12y ). IR absorption spectra of the o2 mm fraction were recorded on a Perkin Elmer Spectrum 1000 spectrometer over the range 4000–200 cm1 using 13 mm diameter KBr pressed disks containing 1 mg of sample. The compiled results of semi-quantitative mineral analyses are shown in Table 4. Montmorillonite, presented in Table 4, in which octahedral Al is partly replaced by

V. Marchig et al. / Deep-Sea Research II 48 (2001) 3523–3547

3531

Fe3+, was recognised by the presence of the OH deformation band of the AlFe3+OH groupings at around 890 cm1, 675 cm1 and 290 cm 1 (Russel, 1987). Data on the Mn oxides presented in Table 5 are based on X-ray diffraction of hand-picked micronodules from the >63 mm fraction. They consist of two crystalline phases, the dominant phase being birnessite and the minor phase todorokite.

4. Results and discussion 4.1. Manganese nodules We distinguished two main types of nodules in Peru Basin: large botryoidal nodules (so-called ‘‘cauliflower-nodules’’) and small ellipsoidal nodules. Botryoidal nodules are poorly sorted, with a wide range of diameters and a low number of nodules per m2 (n/m2=4–165). The small ellipsoidal nodules are well sorted and show a distinct maximum between 2 and 4 cm diameter in high numbers (n/m2=148–985). Botryoidal nodules typically occur in basins, whereas ellipsoidal nodules are found on slopes and topographic highs (Figs. 1a and b; Table 2). Manganese nodule abundance is the result of two processes: the supply of nuclei, mainly from elevated areas, and the growth rates of the nodules. Maximum abundances occur in the vicinity of the CCD, i.e. around 4150 m water depth, and coincide with the highest rates of diagenetic nodule growth (von Stackelberg, 2000). We deduce the high growth rates from the observation that the inner structure of concerned botryoidal nodules (cauliflower nodules) contains thicker diagenetic layers (von Stackelberg, 1997, 2000). Several radiometric determinations of growth rates confirm the extremely high growth rate of this type of nodules, up to 200 mm/106 yr (Reyss et al., 1982, . . 1985; Bollhofer, 1996; Bollhofer et al., 1996). Ellipsoidal nodules grow preferentially on slopes or on topographic highs. In these settings rock fragments are frequent and serve as nodule nuclei. It should be emphasized that nodule abundance (kg/m2 from Table 2) is not lower in these settings, although small, ellipsoidal manganese nodules grow with lower rates than large botryoidal nodules (concluded from thinner and more rare diagenetic layers in their interior structure). Rather, we infer that small nodules have been initiated significantly earlier than the large nodules. This is in accordance with growth . . rates of several millimetres per million years (Reyss et al., 1982, 1985; Bollhofer, 1996; Bollhofer et al., 1996). Thus, the different types of nodules are mainly due to variations in growth environments at different water depths. The different nodule growth environments also can be illustrated by median nodule weight, which can roughly be set in connection with the growth rate of the nodules. A maximum of median nodule weight values was found at 4150 m water depth. Weber et al. (2000) and von Stackelberg (2000) found that this water depth also coincides with maximum organic carbon in surface sediments. This conclusion could not be confirmed with short box cores described in this paper. Another observation helps to differentiate between diagenetic and hydrogenetic growth. Predominant hydrogenetic growth produces (small ellipsoidal) nodules with smooth tops, and predominantly diagenetic growth produces (large botryoidal) nodules with smooth bottoms. The ratio of nodules with smooth top to the ones with smooth bottom is given in Table 2. This ratio is

3532

Station

Depth (cm)

OM

High-productivity 287KG 30 2.5 178KG 30 3.5 252KG 30 3 SEDIPERU 179KG 179KG 179KG 179KG 179KG 179KG 179KG 179KG 179KG 179KG

3 5 10 15 20 25 30 35 40 45

2 3 2.5 2 3 2.5 3 2.5 2.5 3

186KG

30

3

DISCOL 290KG 290KG 290KG 290KG 290KG 290KG 290KG 290KG 290KG 290KG

0–1 5–6 10–11 15–16 20–21 25–26 30 35–36 40–41 45–46

3 2.5 2.5 2 2 3 2.5 3.5 3 3

FluorRadiol.

Fecal Pellets

98 84 99

1 p p

85 95 95 98 97 97 84 98 100 99

15 5 3 2 2 2 10 1

Bituminite

Liptinite

1 15 p

Inertinite

Huminite

For.

Rad.

1.5 1 2

2 2 2

0 3 0

1.5 5 1

p p

0 0 0 0 2 0 0 p p 0

4 3 2 1 0.5 0.5 0.5 0.5 0.5 0.5

1 2.5 1.5 1.5 1.5 1.5 2 2 1.5 1.5

1 1 0 p 0 0 2 0.5 0 0.5

1 1 1 1 3 2 2 1 1 1

3 2 2.5 3 3 2.5 3 3 3 3

0

3.5

2

0

p

2

0 0 p p 0 0 0.5 p 0 0

4 4 3.5 3.5 2.5 1 0.5 0.5 p p

p p p 1 1.5 2 1.5 2 2 1.5

0 0 0 0 0 0 0 p 2 p

0 0 0 0 0 0 0 0 0 0

3 1 1 1 2 0.5 0.5 1.5 1.5 p

1 p p 1

1

p p

p

p

97

P

3

?

p

98 90 95 96 98 100 100 100 100 100

2 10 5 4 2 p p p p 0

p

p p 0

p 0 p

p 0 p p

p p p p p p

p

Diatoms

1.5 0.5 1.5

p

p p

FMR

0.5 p 0.5

1 6

0 0 p

Fe-(OH)x

p 1 1

2 p

Pyrite

p

V. Marchig et al. / Deep-Sea Research II 48 (2001) 3523–3547

Table 3 Results of the organic-petrographic studiesa

3.5 3.5 3 3 3 3 2.5

100 100 100 97 98 100 96

Continent-near 255KG 0 255KG 5 255KG 10 255KG 15 255KG 20 255KG 25 255KG 30 255KG 35 255KG 40 255KG 46

2.5 2.5 2.5 2 2 1.5 2 2 1.5 1.5

252KG

3

30

p

p p p p p 1

0 0 0 0 0 0 0

4.5 4.5 4 4 3.5 3.5 2.5

0.5 0.5 0.5 1.5 1.5 1.5 2.5

1.5 0.5 0.5 1 1 1 2.5

2 1 1 1 2 1.5 1

1 1 1 1 1 1.5 2.5

p p p p 2 p p p p p

0 0 0 (p) p 0 1 p 0.5 0.5

4.5 4 1.5 1.5 0.5 0.5 1 0.5 0.5 0.5

1 0.5 1.5 1.5 2.5 2.5 2 2.5 2 1.5

0 0 p p 1.5 2 2.5 2 1.5 1.5

0.5 0 1 1 0 0 0 0 0 0

1.5 0.5 0.5 1.5 2 1.5 1 1 0.5 1.5

1

0.5

1.5

2

2

0

1

p 3 2 p p

4

97 99 100 100 98 100 80 100 100 100

p p 0 0 p p 5

p 0 0 0 0 0 5

p p

p

99

p

p

p

3 1 p p p p 10 p

Explanations: OMFparticulate organic matter; fluor.Ffluorescing; Rad.FRadiolaria; Fe(OH)xFferro-magnanese hydroxides; FMRFfaunal-mineral remains; ForFforaminifera; (fluorescing radiolaria+fecal pellets+bituminite+liptinite+inertinite+huminite100 vol.%); Abundance of particulate organic matter pyrite etc.: 0Fbarren; 1Frare; 2Fseldom; 3Fcommon; 4Fabundant; 5Fvery abundant/predominant; pFpresent. a

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0 5 10 15 20 25 30

216KG 216KG 216KG 216KG 216KG 216KG

3533

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Table 4 Semiquantitative X-ray diffractometry determination of mineral components in the sedimenta Area

Core

Depth (cm)

Ca

Q

P

Ba

I

Kl

Ch

S

Bi

Kf

AM

High Productivity

178KG 178KG

0–10 20–30

d p

a a

a a

a a

m c

a a

a a

p p

a a

? ?

c c

SEDIPERU

186 186 216KG 216KG 255KG 255KG

0–10 20–30 0–10 20–30 0–10 20–30

p c P m c p

a a m m m m

a a a a a a

a a a a a a

c c c c p c

a a m m m m

a a m m m m

p p p p p p

a a a a a a

? ? ? ? ? ?

c c c c c c

DISCOL Continent-near a

Note: CaFcalcite; QFquartz; PFplagioclase; BaFbarite; IFIllitic material; KlFkaolinite; ChFchlorite; SFdioctahedral smectite (montmorillonite in which there is some replacement of octahedral Al by Fe3+ and nontronite); BiFbirnessite (dominant) and todorokite, (minor) as crystalline mineral phases and X-ray amorphous oxides of Fe and Mn; KfFK-feldspar (Only one characteristic reflection of K-feldspar was observed and this is not enough for recognition of this mineral. However, this is the strongest reflection of this mineral. If it is K-feldspar, than its content in the bulk sample is o1%); AMFopal. From >50 >25 >10 >5

To

Code

Name

p50 p25 p10 p5

d p c m a

dominant plentiful common minor accessory

important where growth types changed with time, like on station 246KG from the DISCOL area, on which small elipsoidal nodules later developed rough tops and smooth bottoms typical for diagenetic growth. 4.2. Metals as oxides Manganese, iron, copper, zinc, nickel and cobalt were leached and determined in sediments from 12 box cores (Table in the attached CD-ROM). They are all enriched in the upper brown oxidation zone, between 50 and 200 mm thick, and depleted in the underlying sediment. Manganese shows the strongest difference between the oxidation zone and underlying sediment, and nickel is strongly correlated with manganese (r ¼ 0:96). Less enriched is iron, and copper correlates well with iron (r ¼ 0:92). Co and Zn show the weakest enrichment. The thickness of the brown, manganese oxide-enriched zone is on average 150 mm in the SEDIPERU as well as in the DISCOL area. The box cores outside of the manganese nodule field contain thin (50–100 mm) but distinct enrichment zones. The metals bound as oxides in the sediment can be reduced and dissolved under sub-oxic conditions, first manganese and then iron. Reprecipitation within the oxidized sediment-water interface produces the typical dark brown oxidation zone in the top of a sediment profile. Some of

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Table 5 Average composition of metals present in oxide form in sediment and in manganese nodules (calculated to sum=100%)

Reprecipitated oxides on sediment/water interface (n ¼ 26) Reprecipitated oxides in 15 cm depth in the sediment (n ¼ 5) Dendritic diagenetic layers in manganese nodules (n ¼ 11)a Laminar diagenetic layers in manganese nodules (n ¼ 10) a

MnO2 (%)

Fe2O3 (%)

Co (%)

Cu (%)

Ni (%)

Zn (%)

80.1 89.2 89.6 98.2

17.3 8.9 7.45 1.12

0.12 0.16 0.037 0.016

0.95 0.61 0.85 0.20

1.21 0.87 1.79 0.40

0.39 0.26 0.27 0.078

von Stackelberg (1997).

the Cu, Zn, Ni, and Co follow this remobilization and migration. They need not be reduced to become mobile, since they are bound to manganese and iron oxide and dissolve with them (Koschinsky et al., 2001). The most prominent indicator for remobilization of oxides is manganese, making up between 76% and 91% of the remobilized metals. The manganese maximum at the sediment surface is followed in some cases by another maximum with depth. The content of reductively leached manganese in this lower maximum of metal oxides increases from 80% on the top to 89% in a depth of 15 cm under the sea floor. The remobilization and reprecipitation of Co, Ni, Cu, and Zn is always weaker than the remobilization of Mn, showing that the mobility of Mn is greater and that a larger proportion of Co, Ni, Cu, and Zn resists remobilization. This explains why strongly corroded buried manganese nodules in deeper parts of Quaternary sediments have relative enrichments of Ni and Cu compared to Mn (von Stackelberg, 2000). Only a small part of bulk iron from the sediment can be remobilized (Stummeyer and . Marchig, 2001). Konig et al. (1997) explained that a part of the iron bound to smectite can be reduced under suboxic conditions, but it does not transfer to pore water because it remains insoluble in the smectite. Only that part of the iron occurring in the sediment as iron oxide can be mobilized. Fig. 2 shows the Mn/Fe ratio in the oxide phase of eight box cores in which the second maximum of oxides within the sediment was observed. The most striking feature is the increase of Mn/Fe ratio in the deeper oxide maximum compared to the maximum at the sediment surface. Table 5 compares the composition of the oxide phase in the sediment/water interface with the composition of Mn enrichment in box cores at 5 to 15 cm depth in the sediment (the second oxide maximum) and with the composition of two types of diagenetic layers of manganese nodules. In order to be compared, contents of the metal oxides are summed to 100%. The deeper oxide phase (at the lower limit of the oxidated layer) is probably precipitated following weak remobilization in the deeper parts of the sediment. Manganese is preferentially mobilized, and it does not travel very far during its upward transport but is reprecipitated at the edge of the oxidation zone. The oxide phase in the higher levels of the sediment could be precipitated following stronger remobilization that attacks not only manganese, but also iron and heavy metals. However, it could also be a residual oxide phase enriched in iron and heavy metals as the manganese is selectively leached.

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Fig. 2. Changes of the Mn/Fe ratio of the oxide phase with depth in the core for the cores that contain additional maximum in oxides between 5 and 15 cm under the sea floor.

4.3. Chemical composition of the bulk sediment 4.3.1. Opaline silica The content of opaline silica in the sediment varies between 6% and 26%. All the investigated box cores show an increase of opaline silica with core depth. The amount of opaline silica decreases, when normalised to Al (i.e. the dilution with carbonate is removed by calculation), from maximum contents in the high productivity area, to SEDIPERU and DISCOL areas, to minimum amounts in the easternmost areas. This shows that opaline silica (composed of

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radiolarian and diatom tests) can be used as indicator for bio-production, being highest in amount in the high productivity zone, lower in SEDIPERU, and even lower in DISCOL. Samples east of the DISCOL area all contain the lowest amount of opaline silica relative to Al. For these samples, the high amount of detritus rather than lower production also could be responsible for the decrease in the amounts of bio-indicators relative to aluminium. The amounts of opaline silica in the sediment are integrated in the bulk chemical analyses in the attached CD-ROM. 4.3.2. Total organic carbon content in sediment profiles The amount of total organic carbon increases steadily from a minimum of 0.52% in the highproductivity zone to an average of 0.58% in the SEDIPERU area, 0.64% in the DISCOL area, and 0.85% near the South American continent. In most cases the content of total organic carbon exhibits a characteristic distribution with core depth, with a minimum at about 10 cm depth and a maximum at about 30 cm in the core (Fig. 3). 4.3.3. Carbonate The carbonate content varies between 2% and 60%, the highest values observed in the cores from the equatorial high-productivity zone. Vertical profiles of carbonate and total organic carbon contents exhibit opposite trends in cores with higher carbonate contents, the maximum carbonate at 10 cm depth in the sediment corresponding to minimum organic carbon, and the minimum carbonate at about 30 cm depth in the sediment coinciding with maximum organic carbon. In the cores with low carbonate contents this trend is not observed. The carbonate content in sediment surface samples is also opposite to the content of organic carbon; the highest carbonate contents and lowest organic carbon contents can be found in surface sediment of the cores from the high productivity zone. The other end of the negative correlation are continent-near cores with highest organic carbon content and lowest carbonate content in the sediment surface samples. Carbonate, although composed of biogenous tests, does not contribute to the organic content in the sediment, but rather depletes the organic matter by dilution. It is also of no use as an indicator for bio-production because planktonic tests are significantly dissolved. 4.3.4. Composition of the non-carbonate phase Element associations in the non-carbonate fraction of the sediment were calculated by means of a correlation matrix and rotated factor analysis. They show the following groups of elements: (a) The principal factor consists of Si, Al, Ti, Na, K, Mg, Rb, Fe, Ga, Sc, and Zr (Si was calculated after eliminating opaline silica Si from bulk Si). They are enriched in sediments from the DISCOL area and east of it. This factor is representative of silicate phase in the sediment. (b) The second factor consists of opaline silica, P, Y, and Ba. These elements are enriched in the southern margin of the high production zone and in the SEDIPERU area. This group represents that phase of sediment controlled by bio-productivity; P and Y are representative of biogenic apatite and Ba of biogenic barite. Weber et al. (2000) have used the ratios of P, Ba,

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Fig. 3. Changes of content of organic carbon with depth in the core arranged in groups for investigation areas.

and opaline silica to Al as bio-indicators. Al is thought to be representative of the detrital phase. The results show that the high productivity zone together with the SEDIPERU area are characterized by higher productivity and the DISCOL area by lower productivity. 4.3.5. Particulate organic matter in the sediment The sediment samples contain very few particles that can be classified as common organic components like liptinite (remains of algae or pollen), inertinite (coal-like carbonized fragments mostly of plants) or huminite (humic plant remains). Their proportion in the sediment, however, is extremely low so that their contribution to the total organic matter in the sediment can only be

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minor, compared to the analyzed contents of total organic carbon. Another source of organic matter must therefore exist in the sediments. Likely candidates are the fluorescing remains of numerous radiolarians. Radiolaria contain a chitinous central capsule that divides the cytoplasm into an extracapsular and intracapsular section (Schussnig, 1954; Campbell and Moore, 1954) (Fig. 4). The true nature of this organic material is unknown. Cytoplasm and other organic substances of the radiolaria are immediately degraded after their death by various biological processes, but the organic constituents of the central capsule are apparently more resistant and may persist in the early stages of diagenesis. Our observations suggest that the siliceous skeleton may contain additional organic substances that increase the fluorescence intensity. The latter disappears on ignition of the shells at a temperature of 9001C, indicating that the organic substances are completely oxidized. The amount of the organic components in these shells is relatively low. The total organic carbon-value of isolated radiolaria and some foraminifera of a selected sample is slightly higher (0.66% TOC) to that of the bulk sample (0.59% TOC), supporting the assumption that the shells contain organic substances. A group of the Radiolaria, the Phaeodorina, contains a pigmented mass in their cytoplasm, the phaeodium, which is also interpreted as metabolic excretion. Based on its microscopical appearance it is possibly a source of bituminite. Based on microscopic studies, the particulate organic material in the sediments is classified into the following groups: fluorescing radiolaria, bituminite, faecal pellets, liptinite, and inertinite. Fluorescing radiolaria appear mostly as 10–50 mm-sized fragments, or less frequently as larger chambered shells up to several 100 mm in diameter (Fig. 4). Their fluorescence intensity varies depending on the amount of organic material present. The central capsule displays the strongest fluorescence as a result of the highest contents of organic compounds. Bituminite consists of very fine-grained to amorphous brown to yellowish-brown organic material with varying fluorescence intensities. Bituminite occurs in various concentrations intergrown with the clayey-marly matrix of the sediment, occasionally together with liptodetrinite and faecal pellets. Bituminite is derived from lipidic components and originates from algal or faunal source material through various microbial activities in the water column, (e.g., as marine snow; Wehner and Hufnagel, 1986) or on and within the sediment.

Fig. 4. Fluorescing radiolaria from sample 178KG, depth in the core 30 cm; (left: focus at cortical shell; right: focus at central medullary shell).

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Faecal pellets are globular or, infrequently, elliptical brownish objects that appear singly or in clusterlike aggregations in young unsolidified sediments. Their size generally varies from some mm up to 10 mm and occasionally 20 mm. Their structure is very finegrained and they show weak to medium fluorescence. They occur rarely but are enriched in some layers. The source of the faecal pellets is unknown, but they are probably derived from copepods, although the size of the faecal pellets from copepods is mostly >20 mm. Within elliptical aggregates in the size range of 40–70 mm they are often associated with bituminite, resembling excretion pills from worms or other organisms. Faecal pellets of the described size have been found in young marine sediments from the Red Sea, where they form the principal component of the particulate organic matter (Maroske et al., 1999), and other marine areas, (e.g., on the Blake Plateau, Wehner et al., 2000). The components of the liptinite group are resistant to degradation. The liptinites are represented by forms derived from planktonic algae mostly as smooth thin-walled globular bodies, 5–30 mm in diameter, but with some small rods or globules about 3 mm in size. The latter are normally called liptodetrinite, but they are most probably small algae belonging to the picoplankton. Thin, often folded, membranes derived from larger algae or from faunal elements like sheets of eggs occur rarely. In one sample, from core 255KG, even a tricolpate pollen from a land plant was observed. The elements of the inertinite group are of relatively heterogeneous composition. In part, they are derived from oxidized remains of terrigenous plants as fusinite (strongly reflecting) and semifusinite (less reflecting); small rods of graphite occur occasionally, which derive from eroded metamorphic or crystalline basement complexes. The size of all these particles is normally less than 10 mm. Aggregates, about 50 mm in diameter and consisting of several cells, also occur rarely. Higher reflecting faunal remains are found regularly as up to 100 mm large membranes with and without ornamentation. They are thought to stem from chitinous sheets of eggs or arthropod remains (Stach et al., 1982). Fig. 5 shows the composition of particulate organic matter in the investigated cores or parts of cores. The dominant components in all samples are the fluorescing remains of the radiolaria (Fig. 4). Other components are present only in minor amounts. The particulate organic matter of all samples is predominantly marine in origin. The proportion of terrigenous material, mainly fusinite and semi-fusinite together with graphite, is very low, even in the cores east of the DISCOL area, which are nearest to the South American continent. The contribution of the phytoplankton is only locally important, and its content is estimated to be very low. This can be explained by the fact that the proportion of the heterotrophic communities (zooplankton, protozoa and bacteria) exceeds that of the autotrophic biomass, the phytoplankton, in the oligotrophic open ocean (Gasol and del Giorgio, 1997). The contribution from benthos to the organic content of the sediment is very probably also low. Possibly parts of the bituminite and of the faecal pellets are derived from such organisms. The mineral substance of the sediments is relatively rich in biogenic components consisting of shells from radiolaria (inorganic, non-fluorescing components), diatoms and foraminifera (Table 3). Phosphatic fragments, such as scales, teeth, and bone remains, also occur. The well-developed oxic surface layer is illustrated by the occurrence of micronodules, containing manganese oxides and iron-hydroxide minerals (Fig. 6).

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Fig. 5. Composition of microscopically determined particulate organic matter; changes with core depth. The sum of individual components is set to 100%. Hatched: fluorescing radiolaria; dotted: fecal pellets; bit.: bituminite; in.: inertinite; li.: liptinite.

The determination of the vitrinite reflection as a standard parameter for the maturity or diagenesis (Stach et al., 1982) was not possible due to the lack of suitable material. The reflectance of the oxidized semi-inertinitic fragments, which reflect more than vitrinite, varies from 0.5% to 1%. This and the fluorescence properties of the liptinite indicate immature, early diagenetic organic material. 4.3.6. Mineral composition of the sediment Semi-quantitative X-ray diffraction analyses of bulk samples show that all the samples contain calcite, quartz, plagioclase, barite, K-feldspar, clay minerals and amorphous matter (see Table 4). Analysis of the o2 mm fraction gives the composition of clay minerals in more detail. These results are based on: (i) X-ray diffraction of the o2 mm fraction, and (ii) IR absorption

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Fig. 6. Occurrence of iron hydroxide (ferro-manganese micronodules) and pyrite plotted against core depth (0, barren; 1, rare; 2, seldom; 3, common; 4, abundant; 5, very abundant/predominant).

spectroscopy of the >2 mm fraction. There is more quartz, illite, kaolinite and chlorite in the sediment cores from DISCOL area and east of the DISCOL area than in sediment cores from the equatorial high-production zone and the SEDIPERU area. Two types of smectite, montmorillonite with partial replacement of octahedral Al by Fe+3, and nontronite are enriched in the sediments from SEDIPERU area and from equatorial high-production zone relative to sediments from DISCOL and east of the DISCOL area. Quartz, illite, kaolinite, and chlorite are more abundant near the continent and in the DISCOL area, which shows their predominantly detrital origin. Smectites, i.e. montmorillonite in which there is some replacement of octahedral Al by Fe3+ and nontronite, show the opposite trend. They are enriched in the SEDIPERU area and in the zone of high biogenic productivity. It is not possible to distinguish which one of these two phases (montmorillonite or nontronite) predominates. However, due to their very small particle size and enrichment away from the continent, both mineral phases are considered to be authigenic. We plotted all the elements from group (a) formed in course of statistical evaluation of the noncarbonate phase against Al (indicative for detritus) to see if the differences in mineral composition influence the chemical composition of the silicates. The elements Ti, Si, and Ga do not show changes in their behaviour ratio to Al with increasing distance from the continent. The elements Fe, Mg, Sc, Zr, and Na are enriched against aluminium in areas distant from the continent. The enrichment must be caused by authigenic clay mineral formation, as these are the components found in higher quantities in the areas remote from the continent. Potassium and rubidium also correlate well with aluminium but display different behaviour. Sediments from the DISCOL as well as the SEDIPERU and the equatorial high-productivity areas show constant ratios to aluminium, whereas the cores near the continent show clear depletion of potassium against aluminium and a little depletion of Rb. It is possible that halmyrolysis introduces these elements from sea water to the weathering material on the sea floor and that this process is less advanced in the sediments near the continent.

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The SEDIPERU and DISCOL areas are located close to each other, and their abrupt change from detritus-dominated to authigenic-dominated is not likely if we assume a constant sediment transport. Near-bottom current measurements in the Peru Basin manganese nodules field (Klein, 1993) show a change of direction of near-bottom currents exactly between the SEDIPERU and DISCOL areas. DISCOL gains its sediment supply by near-bottom currents from SEE (continental influence), and SEDIPERU from NNW (influence of deep-sea environment). Near-bottom currents in the Peru Basin manganese nodules field, however, have been observed only for a short period of time and additional measurements are needed. However, if this trend is proved by additional measurements, it could provide an explanation for the twofold division of sediment types between the SEDIPERU and DISCOL areas. 4.3.7. Comparison of manganese nodules with underlying sediment In previous paragraphs, we have described the variations in sediment type between the eastern and western portions of the Peru Basin (silicate composition, carbonate content, opaline silica, organic matter). Manganese nodules occur in similar abundance and morphological configuration throughout the Peru Basin manganese nodules field; therefore we can not expect that sediment type controls the diagenetic growth of the nodules. In contrast to other factors, the distribution of the oxide phase in the sediment under large botryoidal nodules is different from the sediment under small ellipsoidal nodules. At locations with small ellipsoidal manganese nodules (great number of well sorted, mainly small nodules) the manganese-rich brown surface layer is 162 mm thick on average (Fig. 7). Sediments below this redox boundary contain between 1 and 0.5% MnO2. Characteristic of this distribution of the oxide phase is the second maximum of MnO2 at the redox boundary, which is not followed by a maximum of iron and heavy metals (see Table 5). Assemblages of large botryoidal nodules are underlain by a brown surface layer only 104 mm thick on average and poorer in MnO2 than the first one (Fig. 7). The second MnO2 maximum at the redox boundary is absent, and the sediment under the redox boundary contains only about 0.3% MnO2. At locations beyond the manganese nodule field (255KG in a continent-near area and 287KG at the border of the equatorial high-productivity zone), the brown surface layer is even thinner and the redox boundary is as shallow as 75 mm below the sea floor. The MnO2 content in the sediment under the brown surface layer is also very low, mostly below 0.25%. The distribution of MnO2 in the sediments is closely linked to the distribution of dissolved Mn2+ and oxygen in the pore water (Haeckel et al., 2001). These parameters govern the growth of nodules. In sediments with a higher redox boundary there is a better chance for nodules to reach the level of optimum growth immediately above the redox boundary. As a result, large cauliflower nodules develop. The leaching of manganese in the reduced part of the sediment is also further advanced in these positions. In sediments with a lower redox boundary, there is less chance for manganese nodules to be in contact with the level of optimum growth at the bottom of the oxidation zone. Slowly growing, small ellipsoidal nodules can be found in these situations. The leaching of manganese in the reduced part of the sediment also is not so advanced, and a higher ratio of remobilized metal precipitates are disseminated within the sediment. At locations beyond the manganese nodule field, the metal remobilization is even more pronounced, and upward diffusion exports part of the metals into the water. Only a thin brown

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Fig. 7. Mean values (triangles) and standard deviations (shaded areas) of bulk manganese content in sediments of box cores. The three groups of sediment are formed due to the following criteria: (a) sediment with scarce large cauliflower nodules on the top, (b) sediment with frequent small elipsoidal nodules on the top, (c) sediment without nodules on the top. Dashed line: redox boundary.

surface layer is preserved, which probably also is reduced as a result of high flux of organic matter. This is the reason why no manganese nodules can grow at these locations. Dymond et al. (1984) postulated that the level of the redox boundary in the Guatemala Basin changes such that during cold periods, enhanced production and accumulation rates of organic matter lift the redox boundary almost to the sediment surface. As a consequence, nodules can grow. We can confirm this hypothesis in so far that the situation described brings faster diagenetic growth for manganese nodules in the Peru Basin, but large botryoidal manganese nodules reach the level of the redox boundary with optimum growth rate even during warm periods, such as the Late Holocene. The organic matter content in the sediment is responsible for the growth of manganese nodules as it enables the remobilization of metals and diagenetic growth of nodules. It is therefore unexpected that the particular organic matter in our sediments is refractory organic matter that has survived various biogenic degradation processes. We also could not observe systematic differences in organic matter in sediment under positions with large nodules compared to organic matter under small nodules. Theoretically other forms of organic material, bacterial communities, etc. might exist in the near-surface sediment, which are not recorded by microscopic analysis. From chemical and organic-petrographic data, however, we conclude that if such organic substances are present, the

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amount must be very low. This means that only a very small amount of the in situ organic matter might contribute to the chemical processes regulating manganese nodule formation and destruction. Our investigations show that fluorescing radiolaria are the main carrier of organic carbon in the sediments. It reacts due to microbial mediated processes via the acetate fermentation or CO2 reduction gradually utilising the organic substances incorporated in the siliceous skeletons. This seems to be the only acceptable way for the observed decrease of redox and mobilization of metals.

5. Summary and conclusions The link between sediment type and manganese nodule population concerns different types of nodule growth connected with different conditions of metal remobilization within the sediment. Metal oxides in the sediment are mobilized in the deeper part of cores and reprecipitated in the top sequence of the sediment. In basins near the CCD, large ‘‘cauliflower’’ nodules rest at the sediment surface; leaching of metals from the sediment is stronger, and the redox boundary is shallower (mean depth 104 mm). As a consequence, a high ratio of mobilized metals reprecipitates on the surface of the manganese nodules, increasing their growth rates. At slope positions, where small ellipsoidal nodules occur, the mobilization from the sediment is weaker, the redox boundary is deeper in the sediment (average 16.2 cm under the sea floor), and large parts of the mobilizedmetal precipitates are disseminated in the sediment. Therefore, the growth rate of small ellipsoidal nodules is two orders of magnitude lower than that of nodules in basin sediments. At two diametrical positions, at which sediment void of manganese nodules was sampled, the redox boundary in the sediment is extremely high (at depth of 75 mm in the sediment column). Under slightly changed conditions, like the last glacial biogenic optimum, the position of the redox boundary should have risen to the overlying sea water, thus hindering the formation of manganese nodules, and even dissolving pre-existing manganese nodules. There is no link between the amount and type of organic matter and type of manganese nodules. The amount of organic matter is low, and preliminary microscopic investigations show that organic matter in Peru Basin consists predominantly of refractory lipidic components from radiolarian skeletons. The most probable process lowering the redox potential and thus remobilizing metals from the sediment is slow bacterial disintegration of lipidic components. There is no link between the changes in composition of clay minerals and manganese nodule growth. Clay minerals are divided into two due to different supply areas: In the SEDIPERU area and on the border of the high productivity zone the sediment is enriched in authigenic clay minerals (nontronite and montmorillonite with replacement of octahedral Al by Fe3+). In the DISCOL area and to the east more detrital clay minerals have been found (illite, kaolinite, and chlorite together with quartz enrichment). The biogenous content is also independent from manganese nodule type, coverage being regulated by primary production; it is higher in the equatorial high-productivity zone and decreases in the direction of the South American continent. Best indicators for bioproductivity are opaline silica (radiolarians and diatoms), organogenic barite, and apatite. Foraminifera are not suitable as bio-indicators because their concentration is ruled by dissolution. There is also a vertical trend with depth in the core for all bio-indicators: the short (max. 50 cm) cores extend into

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the last glaciation period, which has been preserved in the sediment as optimal for biogenic production. The data from various regions throughout the German mining claim demonstrate the smallscale changes in quality and quantity of manganese nodules. Environmental impact assessments must take into account this variability. Acknowledgements The material for this publication was gained during two cruises of R.V. Sonne, both funded by the Bundesministerium fur . Bildung und Wissenschaft, (projects no. 03G0079 and 03G0106). The responsibility for the contents of this publication is solely with the authors. We thank Sally Mamann for the patient correction of English in different versions of the manuscript. Appendix. Data on CD-ROM included in this volume (a) chemical composition of the sediment including TOC and opaline silica (b) chemical composition of the oxide phase from the sediment.

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