Composition and growth history of surficial and buried manganese nodules in the Penrhyn Basin, Southwestern Pacific

Marine Geology, 114 (1993) 133-153

133

Elsevier Science Publishers B.V., Amsterdam

Composition and growth history of surficial and buried manganese nodules in the Penrhyn Basin, Southwestern Pacific A . U s u i a, A . N i s h i m u r a a a n d N . M i t a b

aMarine Geology Department, Geological Survey of Japan, Higashi, Tsukuba, lbaraki, Japan bGeochemistry Department, Geological Survey of Japan, Higashi, Tsukuba, Ibaraki, Japan (Received March 9, 1993; revision accepted June 2, 1993)

ABSTRACT Usui, A., Nishimura, A. and Mita, N., 1993. Composition and growth history of surficial and buried manganese nodules in the Penrhyn Basin, Southwestern Pacific. Mar. Geol., 114: 133-153. A comparative study of occurrence and composition of hydrogenetic manganese nodules and their relation to sediment lithology were carried out on material from the Penrhyn Basin in the Southwestern Pacific. The regional distribution of the nodules and compositional variations within nodules are closely correlated to the sedimentary history of the basin. Radiochemical and fossil data indicate that the nodules started to grow after the initiation of sea-floor spreading in Cretaceous time or later. Initially, Co-poor nodules grew on pelagic clay sediments as a result of continuous uplifting, although some of them were left behind within the sediment (old generation) through Paleogene time. More abundant Co-rich nodules followed during or after a hiatus through to the present and formed the large manganese nodule provinces. These young generation encrusted old small nodules, stiff pelagic clay sediments, hydrothermal manganese deposits, and fossils. AABW played an important role in the formation of the young generation of nodules. It is inferred that the AABW flows through the Aitutaki Passage into the Penrhyn Basin and runs along the western margin of the Basin. Scattered thick young sediments in places in the survey area have prevented the growth of nodules.

Introduction R e c o n n a i s s a n c e surveys o f m a n g a n e s e n o d u l e d e p o s i t s o f the P e n r h y n Basin a r e a s in the S o u t h Pacific have been c a r r i e d o u t since 1970 (B/icker et al., 1976; L a n d m e s s e r et al., 1976; M o n z i e r a n d Missegue, 1977; G l a s b y , 1981; M e y l a n et al., 1990). T w o cruises o f R / V Hakurei-maru II were also c a r r i e d o u t in the Exclusive E c o n o m i c Z o n e o f the C o o k I s l a n d s to survey e c o n o m i c - g r a d e n o d u l e s ( M M A J , 1986, 1987; C r o n a n et al., 1991). These surveys revealed a b u n d a n t d i s t r i b u t i o n o f m a n g a nese n o d u l e d e p o s i t s in the western P e n r h y n Basin, a l t h o u g h their g r o w t h h i s t o r y a n d local v a r i a t i o n p a t t e r n s are n o t well u n d e r s t o o d . We have studied v a r i a t i o n s o f the m a n g a n e s e n o d u l e d e p o s i t s on a r e g i o n a l a n d s m a l l e r scale in c o m p a r i s o n with s e d i m e n t l i t h o l o g y a n d a c o u s t i c s t r a t i g r a p h y in the western p a r t o f the P e n r h y n Basin. M a t e r i a l was collected d u r i n g Cruises 0025-3227/93/$06.00

G H 8 3 - 3 a n d GH80-1 o f R / V Hakurei-maru I c o n d u c t e d by the G e o l o g i c a l Survey o f J a p a n (GSJ) as p a r t o f G S J ' s 5-year p r o g r a m " M a n g a n e s e N o d u l e I n v e s t i g a t i o n in the C e n t r a l Pacific O c e a n ( 1 9 7 9 - 1 9 8 3 ) " on the W a k e - T a h i t i T r a n s e c t (Fig. 1). Details o f the geological a n d g e o p h y s i c a l d a t a as well as field d e s c r i p t i o n s will be p u b l i s h e d as a GSJ Cruise Report. A regional d i s t r i b u t i o n m a p o f m a n g a n e s e nodules in the a d j a c e n t a r e a c o m p i l e d f r o m earlier r e c o n n a i s s a n c e surveys is s h o w n in Fig. 1 t o g e t h e r with o u r r e g i o n a l data. In this p a p e r , m i n e r a l o g y , g e o c h e m i s t r y a n d internal structure o f m a n g a n e s e nodules a n d crusts are studied in o r d e r to elucidate the processes a n d history o f f o r m a t i o n the f e r r o m a n g a n e s e d e p o s i t s in a typical pelagic e n v i r o n m e n t . Very few c o m p a r ative studies o f n o d u l e d e p o s i t s in relation to a s s o c i a t e d s e d i m e n t core l i t h o l o g y have been carried out, a l t h o u g h n o d u l e s are believed to be

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

134

A. USUI ET AL.

160°W

140ow

20°N

~

,'e

O

•

•

~o

•

•

•

-

0°

i I ii.,' •

a

®

Penrhyn ,

v

¥1

';/

l

20°S

)

(_t

IHYN BASIN

'ZA~"

"l []

+

L---"

,TP ~XI\

°:,%'

[]

~"~ ,kD$Z, ',' ' ' ~y;.'~..".~.~ ("~V 4~, GH83-aArea

~AMOA ,5°S B ~ Nx

B."~'~

'

•

<:/~ftd

/;;.~

/

i

o

o

o d f

v

-@ O

~. . . . .

7 ""

% °

)

•

,<165"w,4J :::h°°

) ~/~

•

)~

... #0 ~-' f" /_U~" ~ ~ ' . ~ ; ~2

•

~

o

•

'

o ~

Oi

6~-, cJ-Ai~otokit

,-'/-'.".2&C,,~~ ~ ~'~-

\'" "

i/i

i

"-

ABUNDANCECOVEFIAGE >~kg~ • >9o~ 10-20 • 50-9O ~ 5-10 • 20"50 i ~ 1<15 [] abundant

no

nodule [3 p~Sont

Fig. 1. Regional distribution of manganese nodule deposits in the vicinity of survey area. Abundance and coverage data are from this study (Cruises GH83-3, GH80-1), Backer et al. (1976), Landmesser et al. (1976), Monzier and Missegue (1977), MMAJ (1986, 1987), Cronan et al. (1991) and Grau and Kudrass (1991). Solid and hachured lines are ship tracks of DSDP Leg 33 and HakureiMaru GH80-1. Base map modified from Chase et al. (1977). Upper left index map show the GH83-3 survey area (solid box) and previous areas (open boxes and two lines) by GSJ.

135

MANGANESE NODULES: PENRHYN BASIN, SW PACIFIC

uplifted and kept on the sediment surface during slow sedimentation most likely to be supported by activity of benthic organisms (Von Stackelberg, 1984; Sanderson, 1985; McCave, 1988). We have attempted to correlate the growth history recorded in the nodule interior to the sedimentary history of the cores with special reference to the history of uplifting of manganese nodules.

Methods of field survey and analysis A reconnaissance acoustic survey and bottom sampling were carried out during Cruise GH80-1 along the Wake-Tahiti Transect and during the first leg of GH83-3 in the Penrhyn Basin. During the second leg of GH83-3, detailed nodule and sediment sampling was carried out in the survey area using free-fall grabs, deep-sea cameras, piston corers, box corers and dredges and a geophysical study undertaken. The smallest station interval in the detailed survey area is around one kilometer which is nearly equal to the maximum of ship positioning error. Methods of sampling, sea-bed photography, and on-board description employed are detailed in previous GSJ Cruise Reports (Nakao, 1986; Usui, 1992). Nodule samples were described and split for chemical analysis, ore microscopy, scanning microscopy (SEM), powder X-ray diffraction (XRD) and radiochemical dating. One half of large nodules was polished for ore microscopy and the other half ground into powders in acetone for XRD, atomic absorption spectroscopy (AA), and water content determination. 169 powders were prepared from 90 stations in order to describe the regional and local variations of bulk chemical composition and the layer-by-layer variations inside nodules. About 80 powders were made from several entire nodules so that they may represent approximate mean bulk characteristics. A number of 2 to 10mm thick subsamples were separated from nodule surface to nuclei. Nine elements (Mn, Fe, Cu, Ni, Co, Pb, Zn, Si and A1) were determined by AA using the analytical methods of GSJ (Terashima, 1978; Mochizuki and Terashima, 1983) with reference to USGS Rock Standards Nod-A-1 and NodP-1. Water contents were determined as H 2 0 + by

the Penfield Method and H / O - by drying in an oven at l l0°C for 3 hours. Mineral composition was determined by XRD and/or microscopy. XRD was carried out on aliquots of each air-dried powder for AA on the constant measuring conditions in air at room temperature by means of a Rota-Flex Type RADrA diffractometer (Rigaku Denki Co. Ltd.). Samples were not heated to avoid mineral transformation due to dehydration (Usui et al., 1989). Peak heights were measured at 10 and 2.4 d-spacings for semi-quantitative determination of ferromanganese minerals. Accessory silicate minerals, phosphates, carbonates were examined in comparison with ASTM and JCPDS data files. Microscopic identification was also used to identify ferromanganese minerals and for description of internal microstructure. The criteria for mineral identification were those of Usui (1979a) and Usui et al. (1989).

Topography and geology of survey area The GH83-3 area (12°00'S-14°00'S, 158°00'W 160°00'W) is located in the western part of the Penrhyn Basin to the east of the Manihiki Plateau. The area is characterized by fiat but sometimes undulated deep-sea floors at water depths between 5100 and 5300 m. The most prominent topographic feature is a N S trending asymmetric deep trough nearly at the center of the survey area. The western basin of the survey area is generally fiat with some scattered deep-sea hills up to 400 m in height and depressions up to 200 m in depth. The eastern basin, by contrast, forms more rugged topography with higher associated by deep-sea hills and some depressions. The highest elevation (1100 m) is encountered at 12°17'S, 159°00'W in the eastern basin. The detailed survey area (12°30'S-12°55'S, 159°00'W-159°35'W) was selected near the center of the survey area just to the west of the trough, and seismic reflection and bathymetric surveys were carried out along lines at intervals of two nautical miles. The surface sediment is zeolitic deep-sea clay throughout the survey area except for one calcareous clay sediment at the shallowest depth of 4714 m (box core B97). No calcareous or siliceous

136

sediments were found on the basin floor sediment cores. The absence of biogenic sediments indicates that the basin area is below carbonate compensation depth (CCD). Pautot and Melguen (1979) assumed that the present CCD is 4800 m or deeper up to 5000 m. A comparative study of 13 sediment cores and 3.5 kHz subbottom profiles (SBP) revealed three characteristic lithological units I, II and III (Nishimura et al., in press; Nishimura and Saito, in press) in the survey area. Unit I (highly transparent) is unconsolidated reddish pelagic-zeolitic brown clay sediment, Unit II (semi-opaque) is semiconsolidated dark brown pelagic clay sediment, and Unit III (opaque, acoustic basement on SBP records) is alternation of semiconsolidated pelagic clay and yellowish brown stiff claystone. Nishimura and Saito (in press) found pre-Miocene (probably Cretaceous-Oligocene) microfossils from Unit II and Unit III, and assume a longterm sedimentary hiatus between Units I and II. Sedimentological and acoustic investigation revealed sparse distribution of outcrops of the Unit II or possibly Unit III sediments, suggesting a dominant long-term sedimentary hiatus and erosion in the area. Occurrence and morphology of surficial manganese nodules and crusts

Manganese nodules and/or crusts were recovered from the sea floor at 179 locations out of 198 sampling locations during the Cruise GH833. Surficial nodules and crusts were also identified at 115 sea-floor photographs taken by a camera mounted on the box corer or free-fall grab out of 134 successful photographs during the cruise. According to morphological classification of nodules of Moritani et al. (1977), all surficial nodules and sub-surface nodules of the area have a smooth surface and are of hydrogenetic origin (type s). The surface color is generally brownish black. The surface is sometimes granular consisting of 0.5 to l mm growth cusps, but the mineralogy and microtexture indicate the occurrence of hydrogenetic vernadite as shown in later sections. A rough-surface type nodule (type r) typical of highNi and Cu diagenetic deposits is not found in this

A. U S U I ET AL.

area. The surface features and morphology of the nodules of this area are remarkably uniform when compared with the Central Pacific nodules (cf. Usui et al., 1987). Most of the surficial nodules are exposed to overlying sea water. Plots of nodule coverage against abundance show a significant correlation with no data on the abundance axis (Fig. 2), which is consistent with exposed nature of surficial nodules. The exposed nature is typical of hydrogenetic nodules, whereas diagenetic nodules are often covered with thin surface sediments (Felix, 1980; Usui et al., 1987). Nodule shape is generally spherical but is sometimes flattened and broken, whereas the size is variable ranging from less than 1 cm to 8 cm along long axis. Irregularly-shaped nodules generally have a large nucleus or several nuclei. A frequency diagram of averaged thickness of encrusting ferromanganese oxide layers for each station (Fig. 3) demonstrates a bimodal distribution pattern. This clear separation in thickness of encrusting layers is in agreement with the occurrence of smaller nodules (1.2-2.2 cm) and larger nodules (2.4-4.0 cm). The bimodal pattern probably represents two growth generations of nodule deposits if assumed constant growth rates. Abundance ranges from 0 to 40 kg/m 2 and is greater than 20kg/m 2 in a third of the total sampling sites with the average of 13.9 kg/m 2. Sea 5O E

LU

o

Z

i

rn < HJ

•

0

i

e • 0

~ 0

i

I

i.,

I I I

:

• • "

| I •

| • •

I

I

•

• 50

,00

SEA FLOOR COVERAGE (area %)

Fig. 2. Plots of nodule abundance recovered versus sea-floor coverage from sea-bed photos for all stations during the Cruise GH83-3. Note a linear relation of coverage to maximum abundance. Wide range of abundance is due to variable size of nodules with sites.

MANGANESE NODULES: PENRHYN BASIN, SW PACIFIC

137

40

30 >O 20 0 kid

Em 0

4

S

12

16

20

24 28 32

36

40 44

48

AVEP AGE THICKNESS OF ENCRUSTING LAYER (ram) Fig. 3. Frequency distribution of average thickness of encrusting ferromanganese layer of the nodules at each site. Frequency axis in number of sites. Note a clear bimodal distribution.

floor coverage ratio ranges from 0 to 90% with the average of 57%. When compared with the nodule abundance in the Northeast Pacific Manganese Nodule Belt (9.2kg/m 2 in average; 25.9 kg/m 2 in maximum) and the coverage (43% in average; 70% in maximum; Piper et al., 1979; Halbach and Puteanus, 1989), the values are rather greater in the Penrhyn Basin. The abnormally high abundance of sea-floor nodules and closely packed nature of large nodules have been previously reported only in areas of strong bottom current such as Samoan Passage floors in the Southwestern Pacific (Bficker et al., 1976; Lonsdale, 1980). Another typical feature of manganese deposits is the frequent occurrence of manganese crusts which cover the outcrop of consolidated old clayey rocks of Units lI or III. Similar manganese crusts are known on some volcanic pinnacles on the Manihiki Plateau to the west of the survey area (Grau and Kudrass, 1991). Based on the general description of the nodules and crusts, the manganese nodule/crust facies are classified as follows. Facies A (small, low-abundance nodules): Many small nodules (up to about 2 cm in diameter) are dispersed on the sea bed. Thickness of ferromanganese oxide layer is around 4 to 16 mm (Fig. 3). Abundance of nodules ranges from less than 1 to about 5 kg/m z. The nodules are generally discoidal and frequently polynucleated. Facies B (large, high-abundance nodules): Large nodules (2-12 cm in diameter) sometimes closely packed on the sea bed. Abundance ranges from

10 to 40 kg/m 2. The nodules are generally spherical in the case of a small nucleus, but sometimes elongated or flattened with a large block as nucleus. The thickness of the ferromanganese oxide layer is usually from 20 to 30ram up to 40mm. An outermost thin brownish layer of 1 to 2 mm thickness which entirely covers the internal nodule is common. The outer layer is similar to the outer encrusting layers of small nodules of Facies A. Small Facies A nodules are sometimes associated with this facies at a single site. Facies C (manganese crust): Encrustations on outcrops of hard rocks or large flattened slabs. The occurrence is mainly observed on sea-bed photographs, but a few crust samples were recovered. These facies are illustrated in Fig. 4. The areal distribution of these three facies is patchy even within the detailed survey area (50 km x 60 km).

Regional and local variation of manganese nodule facies During the 30-nautical mile grid survey, no significant difference in regional variation pattern between the eastern and western basins in the GH83-3 survey area (Fig. 5) was found, except that nodules are rare on the bottom of the trough. Nodule facies is most variable on a scale of several kilometers rather than on regional scale. In the detailed survey area (12°54'S-13°10'S, 159°07'W -159°22'W), Facies A, B and C are irregularly distributed despite monotonous flat topography with depths between 5100 and 5250 m. Facies B is relatively dominant, and the three facies occupy specific areas on the basin floor. We find no clear relationship between nodule facies and topography, but a relationship to substrate lithology which is variable on a scale of kilometers within the area. A plot of nodule abundance superimposed on distribution of lithological units of the sea floor (Fig. 5) demonstrates the relationship of nodule facies to subbottom sediments. In Fig. 6, Unit I (more than 20 m thick) displays low abundances of small manganese nodules ( = Facies A), whereas outcrops of Unit II without detectable Unit I has abundant large-nodule deposits (=Facies B).

A. USUI ET AL.

138 --

tl. ~ ' A

• o.

J iJ

FG663

to I : • : ~ - l q F l , ~ " ~ ' e ' ' : . oii, t

I

-"

, ,t1,,

t

I ~

I

~ o~

Ft I" " 6 - ,

I l l

I

_'~e

l ~ l l

~ 41P

a,=;,~. ~l.l--e'

":/Facies A ]

-:/ ID'i

'e

o'0."I, :~ ~41m ~ illO e ,, i l l

4, 4J I '~J~ I . lip ~ - , q e e 4J o - -_ ~ ~

I 0 1

I I

Composition of nodules and crusts

i 4k 1

_ _e l l l ~

Mineralogy

u

Facies B

_ . as mO

!

049

qlF g p

eOe m 6 Oo 'qP _ ~ Q u"

,ooo6o®;

/ °ee

tinuous pelagic sedimentation of Unit I may have prevented the formation of abundant nodules.

liiOoll~..q....lo/ aS 0 6 0

•

"

Q OqOOo °eee , , o 6 ® e e FG667

Facies C

4 o" . . . .

r" - ~Ocm

Fig. 4. Typical nodule morphology of Facies A, B, and C. (A) 13°05.35'N, 159°07.70'W, 5202 m water depth, 4.4 kg/m 2 abundance, 25% coverage, zeolite-rich clay surface sediment, (B) 13°18.61'N, 159°27.6YW, 5220 m, 19.4 kg/m 2, 80%, pelagic clay, (C) 13°05.52'N, 159°11.77'W, 5237 m, 17.8 kg/m 2, 40%, zeolite-rich clay.

Outcrops of Unit III are usually covered with manganese crusts, indicating no sedimentation or erosion over a long geologic period, most probably since Neogene time (Nishimura and Saito, in press). This relationship between nodule facies and sediment lithology is again illustrated along line profiles showing topography and 3.5 kHz SBP (Fig. 6). The plots of nodule abundance and thickness of Unit I (Fig. 6, upper part) also demonstrate the relationship of nodule facies to sedimentary conditions and show that a thick young (Neogene-Quaternary) sediment is not favored for abundant nodule growth. This indicates that con-

As reported by earlier workers, mineralogy is one of the most important criteria to discriminate origins of marine ferromanganese deposits in the ocean. The deposits are composed of three principle mineral components of hydrogenetic, diagenetic and hydrothermal origins (Burns et al., 1983; Stouff and Boul6gue, 1988; Usui et al., 1989). However, crystallographic characterization of marine manganese minerals is still controversial, due to their low crystallinity, submicroscopical size, intergrowth, and hydrous nature (Burns and Burns, 1977; Giovanoli, 1980; Burns et al., 1983; Ostwald, 1984). Terminology is thus often complicated in the literature. In this article, the terms, buserite (diagenetic), todorokite (hydrothermal), and vernadite (hydrogenetic) are used. The former two minerals, which are both 10 A manganates, are distinguished as a phyllomanganate (buserite) and a tunnel-structure todorokite by thermal treatment at ll0°C in air (Usui et al., 1989). Vernadite is a low-crystallized iron-manganese oxide mineral. Semi-quantitative XRD analysis shows that vernadite is the major manganese mineral of GH83-3 nodules, whereas huserite is a minor component. Hydrothermal todorokite was identified in the nuclei of FG692 and FG693 nodules. Buserite is detectable on the XRD diagrams of some nodules but its content is generally less than a few percent, as revealed by microscopical observations. On polished sections, buserite occurs occasionally as thin concordant layers, replacement of biogenic structure, or filling of voids or veins (Fig. 7) inside the nodules but not on the recent surface. The 7 A mineral (socalled birnessite), common in hydrothermal deposits, was not detected from any nodules in this area. Manganese-free accessory minerals in the nodules are quartz, plagioclase, phillipsite and smectire. Quartz and phillipsite are most dominant, and plagioclases and smectite are less. Silicate minerals

MANGANESENODULES: PENRHYN BASIN,SW PACIFIC 160°O0'W

139

3o'

159°00'W

3~'

I

158°00'W

I

I

OL_

510km

12°O0'S

5300 --

5100

3~

_

_

13°00'S

f~;;'.'-"2, 0.5 o o~;~/,,

0

,2. ~-

•

~z

C

~-.(~9~ L~-~

,_. _z. . . .

--

o

,Q

14°00'S

o

/HI

/1~1\~1\\\\\~

O.O,Ooo

,[

\

I

I

3

Oll(llgmlQ

13°00'S ,

3.5 KHz SBP

09

~ O

~UNIT [~

~UNIT I

"

~

~oL~ •

-

• • ~oo0 o ~~~!~i 10"

•

20'

I

UNIT II III

NODULE ABUNDANCE

•... ~o ~o,~,o, tun

crust/rock

159°10'W

Fig. 5. Topography and distribution of manganese nodules in GH83-3 area (upper part). Small dots are sampling locations. Numbers and C with dots mean the nodule abundance in kg/m 2 and occurrence of crusts. Contours are in meters. The lower close-up map is nodule abundance on sea floor lithology by 3.5 kHz SBP showing relationship of nodule facies to variation of substrate. Numbers on map are line numbers.

140

A. USUI ET AL.

40-

b-z

30-

LL

O

m w 20

oo

•

•

anlo •

ooo

Z o o o o

O t

I--

lO

e0o

oo

•

•

m~

10

Line

20

FG641

FG642

FG643

FG644

FG645

FG646

38.3

--

14,2

I I. 3 k g / m 2

~.~

~.~

3g

10~ I cm

kg/m 2

85 ~

5g ~

40

2.7

3 or*

1.8

om

,,

40

5o

60

NODULE ABUNDANCE (kg/m2)

4 kg/m 2

30

•

kg,.'m 2

om

1.6

cm

kg/m 2

-

FG649

kg/m 2 8.~ ~

lore

kg/m 2

5~

1.4cm

FG648 ~.5

kg/m 2

5~ 1.5cm

,..

5200

Y300

Fig. 6. Example of variation patterns of nodule facies with substrate. Uppermost transparent layers are Unit I. Plots of thickness of Unit I (3.5 kHz SBP) versus nodule abundance during the cruise indicate the absence of nodules on thick Unit I sediments.

MANGANESE NODULES: PENRHYN BASIN,SW PACIFIC

141

FG6

__.13

bar: 0.2 mm Fig. 7. Variation of microstructure within a nodule (13°00.85'N, 159°17.50'W, 5214 m water depth). The dotted line on photo D is the boundary of old and young generations. Bright part (b) are buserite and other darker part without note is vernadite.

142

occur as clay-size particles within oxide layers or as nuclei and inclusions. As shown in a later section, many of large nodules of Facies B contain two discrete generations of growth. The soft, fragile and porous nodules of 1 to 2 cm diameter are distinguishable on polished sections as an old-grown nodule at the center of a large nodule, while outer layers are dense (Fig. 8). Despite a similar composition of major ferromanganese mineral, the inner old nodule is enriched in smectite and often in quartz, but lacks phillipsite. The nodules are markedly similar in minor mineral composition to those of soft nodules buried within Unit II in sediment cores.

Chemistry The bulk chemical composition of the GH83-3 nodules is characterized by similar concentrations of Mn and Fe (Mn/Fe ratio: approximately 1.2), low Cu (<0.7%), Ni (<0.9%) and Zn (<0.1%), and by moderately enriched Co (average: 0.42%) and Pb (average: 0.08%) (Table 1). The compositional characteristics are common to hydrogenetic nodules from other areas (Glasby, 1982; Exon, 1983) and are more typical to deep-sea manganese crusts (Friedrich and Schmitz-Wiechowski, 1980; Puteanus et al., 1989; Pattan and Mudholkar, 1991). Among these deep-sea manganese nodules and crusts, the Penrhyn Basin nodules are of most typical hydrogenetic origin but least affected by diagenetic effect from surface sediments. As revealed by mineralogical analysis, the major manganese mineral in GH83-3 nodules is vernadite. The major element composition of the nodules is consistent with the earlier idea that the major host phase of Fe, Co, and Pb is vernadite while for Cu, Ni, Zn, it is buserite (Usui, 1979b; Halbach et al., 1982). Minor occurrence of buserite accounts for significant amounts of Cu, Ni and Zn in the nodules. As shown in Fig. 9, concentrations of the elements are linearly correlated with buserite content. The ternary diagram Mn-Fe-(Cu + Ni + Zn) also demonstrates that the region of plots of the GH83-3 nodules mainly falls near a combined line of vernadite and buserite slightly biased to Fe (Fig. 10). The correlation

A. USUI ET AL.

coefficients for elements in surficial nodules (n = 147) distinguish the following three phases (Table 2): vernadite accommodating Fe, Mn, Co and Pb; buserite accommodating Mn, Cu, Ni, Zn and probably Mg; and detrital aluminosilicate minerals with Na, K and part of Mg. Ca is not well correlated with other elements probably due to the presence of phillipsite. Minor compositional variations were observed between top (sea water side) and bottom (sediment side) of large flattened nodules. Cu, Ni, Zn are slightly more enriched in the bottom part of the nodules, while Co is enriched in the top part. Similarly, smaller nodules tend to contain more buserite than larger nodules in a single site. This is probably the result of an early diagenetic process of surface sediments. The most marked compositional variation within surficial nodules of Facies B is the difference in Co concentration between younger generations (approximately 1 to 2 cm depths from nodule surface) and innermost old soft nodules. In many of the large nodules, the Co concentration drops from 0.5-0.6 wt.% at the surface to 0.1 wt.% at the center (Fig. 8). Harada et al. (in press) also point out the depletion of Co concentration (0.08-0.12%) at the innermost part of large nodules of 6.4cm diameter and enrichment (0.44-0.61%) in the outermost layer of the nodules after layer-by-layer chemical analysis and microstructural observations. A significant deletion of Co with the depth of the surface of a hydrogenetic manganese deposit is also reported in a 20-40 cm thick manganese crust from the Central Pacific sea floor (water depth: 4830 m) varying from 0.45 to 0.08% (Friedrich and Schmitz-Wiechowski, 1980), which reminds us of global change of deep water circulation and consequent influence on depositional process of manganese deposits.

Mineralogy of nuclei The bulk mineral composition of nuclei of 24 nodules from 21 locations were determined by XRD. Most of nodule nuclei are composed of consolidated clayey sediments or rocks of buff, brown, and gray color. Shark teeth (ranging less than 1 cm to about 8 cm across), highly weathered

MANGANESE

NODULES:

PENRHYN

BASIN,

143

SW PACIFIC

FG640

Co phiI/smt/qtz

omm

0.62

,/4/o

0.13

0/21/0

0 i

i

i

i

i

J

i

0.7% Co =

d

FG644 2O

0.55

4/14/0

0.18

0/44/9

0.54

0/0/0

0.60

0/16/0

0.13

0/34/9

/9

~

~

6

k

FG654 ~ ..... --. I I

o

3(3

FG659 o

0.61 0.52

0/0/0

r~

0.31

034/0

o

0.52 0.62 0.62 0.50 0.35 0.12

0/0/0 9/0/0 7/16/0 0/43/0 0/44/6 0/31/9

0/0/0

"S~

~... .................

G681

~o

9

....

I

ft porous part

I

I

I

I

() peak height on XRD 1 kcps

2 cm Fig. 8. Variations of Co concentration (in wt.%) and mineral contents (calculated from X-ray reflection at a diagnostic peak on arbitrary scales) within nodules. A dashed line in cross section is the boundary between young and old generations (soft porous part marked with waved lines in each column). Note a depleted Co in the old generation and nice stratigraphic correlations between nodules from different sites.

144

A. U S U I E T A L .

TABLE 1 Average chemical composition of manganese deposits from the Penrhyn Basin with other areas (in wt.%) Mn nodules

Mn crusts

Area origin: depth (m): source:

GH83-3 (bulk) hydrogenetic 5100-5300 This study

Central Pacific Basin hydrogenetic 5000-5500 Usui and Moritani (1992)

Mn nodule Belt diagenetic 4400-5200 Haynes et al. (1986)

Johnston Is. hydrogenetic 1900-2500 Hein et al. (1990)

Central Pacific hydrogenetic 4830 Friedrich and Wiechowski (1980)

n

147

314

234-2277

97

6

Mn Fe Mn/Fe Cu Ni Co Zn Pb

17.3 16. I 1.20 0.22 0.43 0.42 0,059 0.084

19.3 14.5 1.48 0.38 0.56 0.32 0.065 0.080

25.4 6.9 4.10 1.02 1.28 0.24 0.140 0.045

19.2 13.7 1.40 0.08 0.35 0.58 0.055 0.170

24.6 20.6 1.19 0.20 0.37 0.29 0.064 -

25

0.8 0.7-

~#-.....

200.6-

.415-

~ 0.5• , •

.~ 0 . 4 -

t3

0

•

0.3-

,i|:V

:...:b;:;':

•

i!r~':~

0.2-

Sea-bed nodule (bulk) Buriednodule (Mio.-Quat.) Buried nodule (Pa eogene or olde( Innermost part of large nodule Hydrothermal Mn oxide as nuclei

i

•

V~,,: o

0.10

~10-

•

]

I

I

I

10 210 30 40 X - r a y I N T E N S I T Y at 1 0 A

50 0

50

I

I

I

10

20

3O

40

Mn (wt,%)

0.7

0.7 =

I

•

0.6-

0.6-

0.5-

0.5-

v v,'lv. ",.

• :.. ~,:.. •

o~'0.4-

:b'" ". • v"

0

00. 2 0.1

V o• V v o V °v°

0

~t) +

•

v®V•

V~

I

I

I

J

I

1

2

3

4

5

AI (wt.%)

•. , . ? ,,.?-

0.1

w

0

.~- .:

o .~..*-"... : "~ 0.30 0.2-

• -% , ,

+

.

• ..,...~:-.'..

0.4-

"~,.., ; . ,

~.~.0.3-

•

0 0

I

[

i

I

5

10

15

20

25

Fe (wt.%)

Fig. 9. Correlation plots between elements and buserite content on a relative scale expressed as reflected X-ray intensity at d = 10 ~,.

MANGANESE NODULES: PENRHYN BASIN, SW PACIFIC

145

Comparison of surfieial nodules and buried nodules in cores

(Cu+Ni+Zn)*lO

• • v o

Fe

Sea-bednodule (bulk) Buried nodule (Mio.-Quat.) Buriednodule (Paleogeneor older) In~rmost part of large nodule rothermalMn oxideas nuclei

vQ

Manganese nodules sometimes occur within sediments beneath the sea floor, although occurrence is much less frequent than surficial nodules (Aumento and MacGillivray, 1975; Menard, 1976; Banerjee et al., 1991; De Carlo and Exon, 1992). Glasby (1978) first argued the possibility of stratigraphic record of buried manganese nodules in sediments. Among 17 piston cores in the survey area, 14 buried nodules occur within 10 sediment cores. These nodules are not altogether strata-bound deposits, but their occurrence are spread over various horizons of sediments even in the same core. The buried nodules occur mostly within Unit II and two within Unit I (Fig. 11). Nodules in Unit II are typically soft, fragile, and porous. The diameter ranges 1 to 2.5 cm, and their surface often show a submetallic sheen. These characteristics are unique to Unit II nodules. In contrast, the two dense hard nodules in Unit I of Core P406 are similar in appearance to surficial nodules but are smaller. Bulk chemical analyses of buried nodules from Units I and II show differences in Co concentrations (Table 3). The Unit II nodules are significantly depleted in Co ranging from 0.06 to 0.17%, though the Co concentrations of surficial nodules

Mn

Fig. 10. T e r n a r y d i a g r a m o f M n / F e / ( C u + N i + Zn). S y m b o l s b a n d v d e n o t e ideal c o m p o s i t i o n s o f b u s e r i t e ( d i a g e n e t i c ) a n d vernadite (hydrogenetic).

basaltic rocks, and fragments of silica minerals occasionally serve as nuclei of nodules. These rocks were classified on the basis of mineralogical composition into stiff pelagic clay (smectite and quartz rich) and altered volcanic rocks (plagioclase and phillipsite rich). The stiff pelagic clay is very similar in composition and structure to Unit II clay sediments in cores but different from phillipsite-rich surface sediment. This occurrence means that the nuclei have been brought in from ancient sea floor during nodule growth and slow sedimentation of Unit I.

TABLE 2 C o r r e l a t i o n m a t r i x f o r e l e m e n t s in surficial n o d u l e s f r o m G H 8 3 - 3 a r e a . S y m b o l s : *, + a n d - d e n o t e less t h a n 0.3 a b s o l u t e value, between +0.3 and +0.5, and between -0.5 and -0.3, respectively Mn

Zn

Ni

Cu

Fe

Co

Pb

Si

A1

Na

K

Ca

Mg

1

0.47 1

+ 0.86 1

+ 0.79 0.91 1

* -0.78 -0.75 1

* -0.60 -0.69 0.69 I

* 0.64 +

* + + -0.62 -0.64

* +

* * + + -0.66 -

* * + + -0.72 -0.71

+ * * * +

+

.

.

1

.

1

0.61 0.68 -0.81 -0.78 .

0.86 1

+

Mn Zn Ni Cu Fe Co Pb Si A1 Na

+

K

0.87 0.91 0.87 -0.67 -0.65

*

0.78 0.73 I

0.90 0.88 0.85

*

1

-

+ 0.63

1

1

Ca Mg

146

A. USUl ET AL. P406

P402

.

P407

P412

1/11/1111 ~

~

~

P404

0.06Co

M

o.14Co 0.07Co

"I Ir S

-I'1 !

-

,

Fig. 11. Comparison in Co concentration between surficial nodules and buried nodules in Unit II (hatched; probably Cretaceous to Oligocene). Note a low Co concentration in the buried nodules (underlined). Mesh scale is 2.5 cm with photos.

and the Unit I nodules are within the same range (0.3-0.5% Co). Furthermore the Unit II nodules are rich in smectite and quartz but lack phillipsite, while the Unit I nodules are rich in phillipsite. The buried nodules are considered to have been left behind during uplifting, and kept buried in the sediments while most nodules have been continuously uplifted during sedimentation (Von Stackelberg, 1984). The question arises as to whether it is possible to correlate older buried nodules to uplifted surficial nodules on the modern sea floor. As the growth rate of deep-sea nodules is usually several millimeters per million year, the larger nodules should have a longer growth history. Larger nodules often have a more complicated internal structure than smaller nodules even at a single site. Figure 7, for example, shows the textural variation from surface to nucleus. Similar

stratigraphic change was observed in the internal textural pattern among large nodules from different sites. Figure 8 demonstrates common stratigraphic variation of texture and marked change in Co and smectite contents of surficial nodules. The inner soft nodules are always depleted in Co (0.12-0.31%) and associated with abundant smectite and lack phillipsite. The outer surface of the inner soft nodule is also surrounded by very thin buserite layer as observed under microscope (Fig. 7). The chemical and mineralogical composition of the internal section of the surficial nodules is remarkably similar to the buried nodules in Unit II sediments. A comparison of the internal variation in minor mineral composition (Fig. 8) with downcore mineralogical variation in piston core P406 also supports the idea of uplifting of nodules (Table 4). Higher

169 47 9 6 6 1

169 47 9 6 6 1

169 47 9 6 6 1

Total Surficial (bulk) Buried (Unit I). Buried (Unit II) Surficial (inside) Hydrothermal

Total Surficial(bulk) Buried (Unit I) Buried(UnitlI) Surficial (inside) Hydrothermal

Total Surficial (bulk) Buried (Unit I) Buried (Unit II) Surficial (inside) Hydrothermal

1.33 1.39 1.48 1.71 1.31 1.87

Mg

7.13 7.11 8.41 7.45 9.90 1.11

Si

17.3 17.6 15.9 17.2 16.3 39.4

Mn

n Ave.

0.25 0.25 0.37 0.25 0.13 -

1.76 1.25 3.39 1.13 0.94 -

2.6 1.8 4.1 0.9 2.5 -

s.d.

2.14 1.95 2.12 2.14 1.50 1.87

13.74 10.44 13.74 8.96 10.78 1.11

39.4 22.1 19.6 19.0 21.0 39.4

max.

0.97 1.00 1.13 1.43 1.12 1.87

1.11 4.57 4.35 5.81 8.47 1.11

6.4 13.6 6.4 16.6 14.0 39.4

min.

1.70 1.71 1.83 1.56 1.44 1.21

Na

2.93 2.98 3.35 2.69 3.43 0.67

AI

15.9 15.6 14.7 16.1 15.3 2.6

Fe

Ave.

0.19 0.15 0.37 0.07 0.15 -

0.68 0.51 1.06 0.25 0.45 -

2.8 2.3 4.3 1.4 2.2 -

s.d.

2.70 2.16 2.70 1.67 1.63 1.21

5.63 4.38 5.63 2.97 4.09 0.67

24.0 21.3 18.6 17.8 17.1 2.6

max.

1.21 1.42 1.47 1.47 1.30 1.21

0.67 1.45 2.35 2.35 2.85 0.67

2.6 11.5 4.5 14.2 11.3 2.6

min.

0.87 0.86 1.08 1.14 1.11 1.26

K

0.059 0.061 0.056 0.083 0.062 0.039

Zn

0.22 0.25 0.28 0.22 0.16 0.17

Cu

Ave.

0.30 0.29 0.50 0.15 0.16 -

0.011 0.009 0.015 0.020 0.010

0.09 0.09 0.19 0.08 0.04 -

s.d.

2.13 1.84 2.13 1.43 1.33 1.26

0.122 0.085 0.085 0.122 0.075 0.039

0.73 0.52 0.73 0.35 0.22 0.17

max.

Averages of element concentration for surficial and buried nodules in wt. % except for Mn/Fe ratio

TABLE 3

0.44 0.49 0.60 1.02 0.94 1.26

0.035 0.044 0.036 0.068 0.053 0.039

0.07 0.12 0.16 0.14 0.12 0.17

min.

1.20 1.16 1.31 1.08 1.11 14.97

Mn/Fe

0.080 0.081 0.062 0.042 0.061 0.029

Pb

0.43 0.49 0.42 0.53 0.47 0.16

Ni

Ave.

1.11 0.25 0.95 0.14 0.38 -

0.027 0.027 0.031 0.015 0.011

0.14 0.15 0.17 0.22 0.16 -

s.d.

14.97 1.80 3.72 1.34 1.85 14.97

0.208 0.208 0.095 0.062 0.075 0.029

0.93 0.81 0.75 0.93 0.66 0.16

max.

0.35 0.77 0.35 0.93 0.86 14.97

0.002 0.043 0.002 0.017 0.044 0.029

0.13 0.20 0.23 0.31 0.30 0.16

min.

0.28 0.19 0.48 0.17 0.27

0.11 0.07 0.18 0.04 0.10

s.d.

0.71 0.80 0.76 0.83 0.69 0.37

0.23 0.24 0.36 0.32 0.20 -

Cu + Ni + Zn

1.93 1.93 1.86 1.52 1.52 0.91

Ca

0.40 0.40 0.36 0.12 0.20 0.11

Co

Ave.

1.57 1.40 1.57 1.40 0.95 0.37

3.16 2.70 2.39 1.67 1.90 0.91

0.62 0.52 0.55 0.17 0.35 0.11

max.

0.25 0.37 0.43 0.52 0.47 0.37

0.91 1.54 1.02 1.28 1.32 0.91

0.06 0.24 0.10 0.06 0.12 0.11

min.

z

148

A. U S U I ET AL,

TABLE 4 M i n e r a l c o m p o s i t i o n o f p i s t o n core s e d i m e n t s s h o w i n g a distinct difference between U n i t s I a n d lI No.

Unit

bsf (cm)

description

phlp

smt

qtz

K-fsp

plc

-

x

Mnl0

Mn7

P406 A

I

1

dark brown clay

x

x

x

B

I

45

dark brown clay

x

x

x

x

-

C

I

225

dark brown clay

x x

x

x

x

D

I

405

dark brown clay

x x

x

x

x

-

-

E

I

495

dark brown clay

x x

x

x

-

-

-

F

1I

545

dark brown clay

-

x x

x x

x

-

-

G

II

685

dark brown clay

-

x x

x x

x

-

H

I1

754

dark brown claystone

x x

x x

x

-

-

x

x

x

-

x x

x x

-

P412 A

1

8

dark brown clay

B

II

80

dark brown clay

x

tr x

C

II

146

dark brown clay

-

x

x

-

D

Ill

225

buff claystone

-

x x

-

x x

-

-

E

III

250

black clay

×

tr

x

-

-

F

III

258

buff claystone

-

x x

-

x x

G

III

275

buffclaystone

-

x x

-

x x

H

III

295

black clay

-

I

llI

332

" b u f f clay, h e t e r o "

-

Mineralogy: phlp=phillipsite, 10 ,~ m a n g a n a t e ,

Mn7=7/~

smt=smectite, manganate,

qtz=quartz,

x x =abundant,

x x

-

-

-

-

x x

x

-

x x

K-fsp=potash

-

×

-

-

feldspar, plc=plagioclase,

x =common,

tr=traceable,

=not

Mnl0 =

detected, bsf =

d e p t h f r o m sea f l o o r .

contents of smectite and quartz and absence of phillipsite in Unit II sediments are remarkably similar to those within nodules (Fig. 12). The comparison reveals that the small soft nodules had been formed during deposition of Unit II (Oligocene-Cretaceous) before a long-term hiatus between Units I and II. During and after the hiatus, abundant growth of nodules has taken place. Most of the old nodules have been uplifted and covered with younger layer, but others were left behind in the sediment. 1°Be data (T. Inoue and A. Usui, unpubl, data) suggest a very slow growth rate of i.1 mm/Ma (Fig. 13) for a spherical nodule (6.5 cm in diameter) from an adjacent site (13°47.15'S, 159°28.26'W: 5147 m water depth) during the Cruise GH80-1. If assumed the constant growth rate and extrapolated to the nucleus, the age of the nodule can go back to Oligocene or older. A nucleus of a FG692 nodule yields abundant siliceous microfossils (Dictyomitra-type radiolarian and others) which is totally cemented by pure hydrothermal todorokite crystals (Fig. 14). The

0

P406

200 •

phlp

smt

400 i

~

-60C

8C~

.

,

.

10

, 2O

.

,

.

30

, 40

Unit I

&

'"[i ii'ii'"

.

, 5O

qtz

plc

K-fsp

. 6O

X-ray intensity F i g . 12. D o w n c o r e

v a r i a t i o n in m i n e r a l o g i c a l c o m p o s i t i o n

of

U n i t I a n d II s e d i m e n t s . N o t e a s i g n i f i c a n t d i f f e r e n c e o f u p p e r and

lower

sediments bounded

by a d o c u m e n t e d

h i a t u s by

N i s h i m u r a a n d S a i t o (in press). M i n e r a l c o n t e n t is o n a r b i t r a r y scales e x p r e s s e d as r e f l e c t e d X - r a y i n t e n s i t y a t e a c h d i a g n o s t i c d-spacing.

MANGANESENODULES:PENRHYNBASIN,SWPACIFIC 10-7 • FG219-2 nodule

o~

~ ; " ~

10-8

Quat. i

I

Plio.

J

Oligocene?

at 32ram(nucleus)

Mio.

i

DEPTH FROM NODULE SURFACE (mm) Fig. ]3. Growth rate of a nodule from the survey area (T. [noue, unpub], data). Least square regression indicates a slow growth rate since Miocene or older.

age of radiolarian shows that a low-temperature hydrothermal activity took place in Cretaceous time or later, probably related to mid-oceanic spreading which occurred on the Manihiki Plateau about 100-110 Ma or at a later stage at the triple junction between the Pacific, Farallon and Antarctic Plates (Jackson and Schlanger, 1976).

Environment and growth history of nodules Radiochemical data of one nodule profile and biostratigraphy in the nodules constrain the onset of growth of nodules in this area. A 1°Be measurement estimates a slow and constant growth rate of 1.1 mm/Ma for the outer 6-mm layer of the nodule. The age of the nucleus can be extrapolated to 29 Ma (Oligocene), assuming a zero age for the nodule surface and a constant growth rate. Many of the large shark teeth which serve as nodule nucleus are identified Megalodon carcharodon of Oligocene to Miocene age. The shark teeth are always encrusted by 1 - 2 c m thick manganese layers. Dictyomitra-type radiolarians which are cemented in a nucleus by hydrothermal manganese mineral also suggest a Cretaceous age. Other evidence for nodule age is the presence of the inner most old nodule that is textually, mineralogically and compositionally correlated to the nodule buried within Unit II sediments (Cretaceous-Oligocene; Nishimura and Saito, in

149

press). The correlation suggests that the age of large nodules goes back to Oligocene or older. It is inferred from these data that the nodules started to grow after the sea floor spreading in the western part of the Penrhyn Basin in the Cretaceous time. The volcanic and hydrothermal activity provided volcanic rocks and hydrothermal manganese deposits as nuclei for the nodules. During followed deposition of Unit II in the Paleogene time, some of the old nodules continued to grow, but some were left behind during uplifting and remained buried within the sediments. However, the old generation of growth appears less active than the later generation because of less frequent occurrence of nodules. Uplifted nodules have continued to grow during the Neogene through the Quaternary and formed large spherical nodules (Facies B) on Unit II sediments, where sedimentation was very slow or absent. Some nodules probably started to grow from that time, because stiff clay sediments which is closely similar to Unit II sediments often serve as nuclei together with shark teeth. Small nodules on thick Unit I sediment (Facies A) are probably formed as younger generation. The distribution of manganese nodules deposits relative to sediment lithology (Fig. 5) and the plots of nodule abundance and thickness of Unit I (Fig. 6) demonstrate the relationship of nodule facies to sedimentary history. They show that a thick younger (Neogene-Quaternary) sediment is not favorable for abundant nodule growth. Continuous pelagic sedimentation prevents abundant nodule formation while scarce or no deposition of sediments yields abundant nodule fields. These relationships are most probably caused by increased bottom currents of the Antarctic Bottom Water (AABW) since Paleogene time or older after deposition of Unit II. Schmitz et al. (1986) and Mangini et al. (1990) found a close relationship between abundant nodule province and slow sedimentation rates caused by AABW in the Southwestern Pacific Basin. Our data of nodule facies and sediment distribution strongly support the idea that a long-term continuous flow of oxygenated water promoted abundant manganese nodule fields over the survey area. Pautot and Melguen (1976) assumed a northeast branch of

150

A. USUI ET AL

FG692

2 cm

SEM

bar: 0.2 mm Fig. 14. Hydrothermal manganese deposits as nodule nucleus Cementing fossil radiolarians. Left grey part in upper right photo is vernadite growing on hydrothermal todorokite. Other bright part in reflecting micrographs and a SEM photo is todorokite.

MANGANESE NODULES: PEN RHYN BASIN, SW PACIFIC

AABW flow into the Penrhyn Basin through the Aitutaki Passage on the basis of previous bottom water temperatures. Exon (1983) explained the regional distribution patterns of nodules in the South Pacific based on the northward flow of AABW. Bottom water temperature measured by Yamazaki (in press), by contrast, suggests the clockwise flow of AABW around the Manihiki Plateau and a southward or eastward branch into the Penrhyn Basin. The abundant and continuous nodule deposits extending from the Samoa Basin, the Aitutaki Passage to the western margin of the Penrhyn Basin (Fig. 1) may prefer the former idea. The AABW possibly has been flowing into the Penrhyn Basin through the Aitutaki Passage and running northward as western boundary currents for a long geologic time. These currents may have provided an optimal environment for growth of nodules and crusts under the condition of no sedimentation or partly erosion. However, the modern active formation of manganese deposits is unclear, since the present-day increased inflow of AABW into the Penrhyn Basin is not well demonstrated.

Summary Manganese nodules of this area are generally composed of typical hydrogenetic vernadite together with minor amount of diagenetic buserite. The composition is similar in chemistry and mineralogy to manganese crusts of the Central Pacific particularly to those from the deep sea. Abundant nodule fields have been formed in the western part of the Penrhyn Basin since Oligocene or Cretaceous times under the influence of AABW in the western margin of the basin possibly through the Aitutaki Passage. Considering nodule composition in relation to sediment stratigraphy indicates that most of the nodules have been uplifted without burial in the associated sediments and that others were left behind in sediments. Variations in mineralogy, texture, and composition within the individual nodules and material of nuclei often record some environmental change or events on the sea floor such as hydrothermal activity. The nodules began to grow as Co-poor vernadite after the Cretaceous sea-floor spreading or later

151

during deposition of Unit II clay sediments (old generation). Some of the nodules were buried and others continued to be uplifted. The formation of Co-rich vernadite which encrusted the nuclei, such as uplifted old nodules, consolidated Unit II clay sediments, and fossil shark teeth, followed and produced abundant nodule deposits (young generation). Continuous pelagic sedimentation of Unit I prevented nodule formation, whereas low sedimentation rates or sediment erosion encouraged abundant nodule growth.

Acknowledgements This study was partly funded by Special Program of the Agency of Industrial Science and Technology, MITI, Japan. We acknowledge the captain and crew of Hakurei-Maru for their technical assistance and help during the cruise. Dr. A. Mizuno of Ehime University, Dr. T. Moritani of Sumitomo Construction Co., and Dr. S. Nakao of GSJ are thanked for useful discussions and suggestions. Dr. T. Inoue of Daiichi Radioisotope Laboratory kindly analyzed the growth rate of the nodule. The SOPAC Technical Secretariat in Fiji and Metal Mining Agency of Japan kindly provided us with an opportunity of referring to unpublished official reports on marine mineral resources exploration in the Cook Islands area. Special thanks are due to Dr. G.P. Glasby for his critical reading of the manuscript.

References Aumento, F. and MacGillivray,J.M., 1975. Geochemistryof buried Miocene-Plioceneferromanganese nodules from the Antarctic Ocean. Init. Rep. DSDP, 28: 795-803. B/icker, H., Glasby, G.P. and Meylan, M.A., 1976. Manganese nodules from the SouthwesternPacificBasin. N.Z. Oceanogr. Field Rep., 6, 88 pp. Banerjee, R., Iyer, S.D. and Dutta, P., 1991. Buried nodules and associated sediments from the Central Indian Basin. Geo-Mar. Lett., 11: 103-107. Burns, R.G., Burns, V.M. and Stockman, H.W., 1983.A review of the todorokite-buserite problem: implicationto the mineralogy of marine manganese nodules. Am. Mineral., 68: 972-980. Burns, R.G. and Burns, V.M., 1977. Mineralogyof Manganese Nodules. In: G.P. Glasby (Editor), Marine Manganese Deposits. Elsevier,Amsterdam, pp. 185 248. Chase, T.E., Menard, H.W. and Mammerickx, J., 1977. Topography of the South Pacific. Scripps Inst. Oceanogr.,

152 Inst. Mar. Res., Univ. California, La Jolla. (Bathymetric Map.) Cronan, D.S., Hodkinson, R.A., Miller, S. and Hong, J., 1991. Part I: An evaluation of manganese nodules and cobalt-rich crusts in South Pacific Exclusive Economic Zones--Nodules and crusts in and adjacent to the EEZ of the Cook Islands (the Aitutaki-Jarvis Transect). Mar. Mining, 10: 1-28. De Carlo, E.H. and Exon, N.F., 1992. Ferromanganese deposits from the Wombat Plateau, north west Australia. Proc. ODP Sci. Results, 122: 335-345. Exon, N.F., 1983. Manganese nodule deposits in the Central Pacific Ocean and their variation with latitude. Mar. Mining, 4: 79-108. Felix, D., 1980. Some problems in making nodule abundance estimates from seafloor photographs. Mar. Mining, 2: 293 302. Friedrich, G. and Schmitz-Wiechowski, A., 1980. Mineralogy and chemistry of a ferromanganese crust from a deep-sea hill, central Pacific, "Valdivia" Cruise VA 13/2. Mar. Geol., 37: 71-91. Giovanoli, R., 1980. On natural and synthetic manganese nodules. In: I.M. Varentsov and Gy. Grasselly (Editors), Geology and Geochemistry of Manganese. Hungarian Acad. Sci. Publ., I, pp. 159-202. Glasby, G.P., 1978. Deep-sea manganese nodules in the stratigraphic record: Evidence from DSDP cores. Mar. Geol., 28: 51-64. Glasby, G.P., 1981. Manganese nodule studies in the Southwest Pacific 1975-1980: A Review. South Pacific. Mar. Geol. Notes, 2(3): 37-46. Glasby, G.P., 1982. Manganese nodules from the South Pacific: an evaluation. Mar. Mining, 3:231 270. Grau, R. and Kudrass, H.R., 1991. Pre-Eocene and younger manganese crusts from the Manihiki Plateau, South Pacific Ocean. Mar. Mining, 10: 231-246. Halbach, P. and Puteanus, D., 1989. Distribution of ferromanganese deposits. In: P. Halbach, G. Friedrich and U. von Stackelberg (Editors), The Manganese Nodule Belt of the Pacific Ocean. Ferdinand Enke, Stuttgart, pp. 10-16. Halbach, P., Giovanoli, P.R., Von Borstel, D., 1982. Geochemical processes controlling the relationship between Co, Mn, and Fe in early diagenetic deep-sea nodules. Earth Planet. Sci. Lett., 60: 226-236. Halbach, P., Friedrich, G. and Von Stackelberg, U. (Editors), 1988. The Manganese Nodule Belt of the Pacific Ocean. Ferdinand Enke, Stuttgart, 254 pp. Harada, K., Higashitani, K., Choi, J.-H. and Usui, A., 1993. Chemical and textural variations within manganese nodules of the Penrhyn Basin, South Pacific (GH83-3 Area). Geol. Surv. Jpn. Cruise Rep., 23, in press. Haynes, B.W., Law, L.L. and Barron, D.C., 1986. An elemental description of Pacific Manganese nodules. Mar. Mining, 5: 239-276. Hein, J., Kirshenbaum, H., Schwab, W.C., Usui, A., Taggert, J.E., Stewart, K.C., Davis, A.S., Terashima, S., Quinterno, P.J., Olson, R.L., Pickthorn, L.G., Schultz, M.S. and Morgan, C.L., 1990. Mineralogy and Geochemistry of Corich Ferromanganese Crusts and Substrate Rocks from Karin Ridge and Johnston Island, Farnella Cruise F7-86-HW. U.S. Geol. Surv. Open File Rep., 90-298, 32 pp.

A. USUl ET AL. Jackson, E.D. and Schlanger, S.O., 1976. Regional synthesis, Line Islands chain, Tuamotu Island chain, and Manihiki Plateau, Central Pacific Ocean. Initial Rep. DSDP, 33: 915-927. Landmesser, C.W., Kroenke, L.W., Glasby, G.P., Sawtell, G.H., Kingan, S., Utanga, E., Utanga, A. and Cowan, G., 1976. Manganese nodules from the South Penrhyn Basin, Southwest Pacific. South Pacific Mar. Geol. Notes, 1(3): 17-40. Lonsdale, P., 1980. Manganese-nodule bedforms and thermohaline density flows in a deep-sea valley on Carnegie Ridge, Panama Basin. J. Sediment. Petrol., 50: 1033-1048. Mangini, A., Segl, M., Glasby, G.P., Stoffers, P. and Pliiger, W.L., 1990. Element accumulation rates in and growth histories of manganese nodules from the Southwestern Pacific Basin. Mar. Geol., 94: 97-107. McCave, I.N., 1988. Biological pumping upwards of the coarse fraction of deep-sea sediments. J. Sediment. Petrol., 58: 148 158. Menard, H.W., 1976. Time, chance, and the origin of manganese nodules. Am. Sci., 64: 519-529. Metal Mining Agency of Japan, 1986. Ocean Resources Investigation in the sea area of CCOP/SOPAC: Report on the Joint Basic Study for the Development of Resources, Sea Area of Cook Islands. JICA-MMAJ, Tokyo, 1, 86 pp. Metal Mining Agency of Japan, 1987. Ocean Resources Investigation in the sea area of CCOP/SOPAC: Report on the Joint Basic Study for the Development of Resources, Sea Area of Cook Islands. JICA-MMAJ, Tokyo, 2, 136 pp. Meylan, M.A., Glasby, G.P., Hill, P.J., McKelvy, B.C., Walter, P. and Stoffer, P., 1990. Manganese crusts and nodules from the Manihiki Plateau and adjacent areas: Results of HMNZS Tui Cruises. Mar. Mining, 9: 43-72. Mochizuki, T. and Terashima, S., 1983. Chemical analysis of manganese nodules. Methods Chem. Anal. G.S.J., 53: 1-36. Monzier, M. and Missegue, F., 1977. Polymetallic nodules sampling in the Cook Islands archipelago: "Danaides 2" and "Geotransit 2" Surveys. Preliminary Rep. ORSTOM. (Unpubl.) Moritani, T., Maruyama, S., Nohara, M., Matsumoto, K., Ogitsu, T. and Moriwaki, H., 1977. Description, classification, and distribution of manganese nodules. Geol. Surv. Jpn. Cruise Rep., 8: 136-158. Nakao, S. (Editor), 1986. Marine Geology, Geophysics and Manganese Nodules Around Deep-sea Hills in the Central Pacific Basin. Geol. Surv. Cruise Rep., 21,257 pp. Nishimura, A. and Saito, Y., 1993. Deep-sea sediments in the Penrhyn Basin, South Pacific (GH83-3 Area). Geol. Surv. Jpn. Cruise Rep., 23, in press. Nishimura, A., Okuda, Y. and Usui, A., 1993. Subsurface acoustic stratigraphy on 3.5 kHz SBP records in the Penrhyn Basin, South Pacific (GH83-3 Area). Geol. Surv. Jpn. Cruise Rep., 23, in press. Ostwald, J., 1984. Ferrugenous vernadite in an Indian Ocean ferromanganese nodule. Geol. Mag., 121(5): 483-488. Pattan, J.N. and Mudholkar, A.V., 1991. M6ssbauer studies and oxidized manganese ratio in ferromanganese nodules and crusts from the Central Indian Ocean. Geo-Mar. Lett., 11: 51-55. Pautot, G. and Melguen, M., 1979. Influence of deep water

MANGANESE NODULES: PENRHYN BASIN, SW PACIFIC

circulation and sea floor morphology on the abundance and grade of Central South Pacific Manganese Nodules. In: J.L. Bischoff and D.Z. Piper (Editors), Marine Geology and Oceanography of the Pacific Manganese Nodule Province. (Mar. Sci. Ser., 9.) Plenum, London, pp. 621-649. Piper, D.Z., Leong, K. and Cannon, W.F., 1979. Manganese nodule and surface sediment compositions: DOMES Sites A, B, and C. In: J.L. Bischoff and D.Z. Piper (Editors), Marine Geology and Oceanography of the Pacific Manganese Nodule Province. (Mar. Sci. Ser., 9.) Plenum, London, pp. 437-473. Puteanus, D., Glasby, G.P., Stoffer, P., Mangini, A. and Kunzendorf, H., 1989. Distribution, internal structure, and composition of manganese crusts from seamounts east of the Teahitia-Mehetia hot spots, Southwest Pacific. Mar. Mining, 8:245 266. Sanderson, B., 1985. How bioturbation supports manganese nodules at the sediment-water interface. Deep-Sea Res., 32: 1281 1285. Schmitz, W., Mangini, A., Stoffers, P., Glasby, G.P. and Plfiger, W.L., 1986. Sediment accumulation rates in the Southwestern Pacific Basin and Aitutaki Passage. Mar. Geol., 73: 181-190. Stouff, P. and Boul6gue, J., 1988. Synthetic 10-.~ and 7-,~ phyllomanganates; their structures as determined by EXAFS. Am. Mineral., 73: 1162-1169. Terashima, S., 1978. Atomic absorption analysis of Mn, Fe, Cu, Ni, Co, Pb, Zn, Si, AI, Ca, Mg, Na, K, Ti, and Sr in manganese nodules. Bull. Geol. Surv. Jpn., 29: 401-41l. Usui, A., 1979a. Minerals, metal contents, and mechanism of

153 formation of manganese nodules from the Central Pacific Basin (GH76-1 and GH77-1 areas). In: J.L. Bischoff and D.Z. Piper (Editors), Marine Geology and Oceanography of the Pacific Manganese Nodule Provinces. (Mar. Sci. Ser., 9.) Plenum, London, pp. 651 679. Usui, A., 1979b. Nickel and copper accumulation as essential elements in 10 ,~. manganite of deep-sea manganese nodules. Nature, 279: 411-413. Usui, A. (Editor), 1992. Marine Geology, Geophysics and Manganese Nodules Deposits in the Southern Part of the Central Pacific Basin. Geol. Surv. Cruise Rep., 22, 276 pp. Usui, A. and Moritani, T., 1992. Manganese nodule deposits in the Central Pacific Basin: Distribution, geochemistry, mineralogy, and genesis. In: B.H. Keating and B.R. Bolton (Editors), Geology and Offshore Mineral Resources of the Central Pacific Basin. Springer, New York, 14, pp. 205-223. Usui, A., Nishimura, A., Tanahashi, M. and Terashima, S., 1987. Local variability of manganese nodule facies on small abyssal hills of the Central Pacific Basin. Mar. Geol., 74: 237-275. Usui, A., Mellin, T.A., Nohara, M. and Yuasa, M., 1989. Structural stability of marine 10 ,~ manganates from the Ogasawara (Bonin) Arc: Implication for low-temperature hydrothermal activity. Mar. Geol., 86:41 56. Von Stackelberg, V., 1984. Significance of benthic organisms for the growth and movement of manganese nodules, Equatorial North Pacific. Geo-Mar. Lett., 4: 37-42. Yamazaki, T., 1993. Bottom temperature in the Penrhyn Basin, South Pacific (GH83-3 Area). Geol. Surv. Jpn. Cruise Rep., 23, in press.