Paleoceanographic Setting and Preservation of Buried Manganese Deposits in DSDP/ODP Cores

1. Introduction

Cores obtained by the Deep Sea Drilling Project(DSDP) and the Ocean Drilling Program (ODP) show,in addition to surficial manganese nodules from the sed-iment/water interface and crusts on the seamount sur-face, buried fossil manganese nodules and crusts.Although the evidence that the buried materials are notmixing or contamination by surficial nodules duringdrilling but in situ deposits had remained inconclusive,analysis of these fossil manganese nodules and crustswas expected to assist in reconstructing the changes inthe deepwater environment throughout the geologictime scale. Glasby (1978) examined the formative agesof fossil manganese nodules and crusts from DSDPLegs 1–41 and found that the deposits occur with partic-ularly high frequency after middle Miocene sediments,suggesting that formation is closely related to the devel-opment of oxic deepwater circulation such as theAntarctic Bottom Water (AABW). Usui and Ito (1994)studied the fossil manganese nodules and crusts fromDSDP/ODP Legs 1–126 and revealed that the composi-

tion and structure of the deposits are identical to thoseof modern deposits, confirming that such nodules andcrusts are good indicators of oceanographic conditionsat the time of formation. Most of the fossil manganesenodules and crusts were found to occur at or immediate-ly above sedimentary hiatuses, or in sediments deposit-ed with slow sedimentation rates (similar to moderndeposits) overlain by rapidly deposited sediments. Ito etal. (1998) confirmed that most of the buried nodules arefossil nodules in situ with strontium isotopic ratios con-sistent with the depositional age of the host sediments.

During the last decade and a half, many ODP coreshave been recovered, and further practical informationhas been accumulated. However, the relationshipsbetween the occurrences of fossil manganese nodulesand crusts and changes in the deepwater environmenthave not been examined in detail. In the present study,the occurrence, age, stratigraphic position, lithology anddepositional setting of fossil manganese nodules andcrusts on Legs 123–210 are investigated, and the forma-tive deepwater environment and preservation are dis-cussed as an extension of the work of Usui and Ito (1994).

In addition to fossil manganese nodules and crusts,

RESOURCE GEOLOGY, vol. 56, no. 4, 457–470, 2006

457

Paleoceanographic Setting and Preservation ofBuried Manganese Deposits in DSDP/ODP Cores

Takashi ITO and Kosei KOMURO*

College of Education, Ibaraki University, Mito, Ibaraki 310-8512, Japan [e-mail: [email protected]]* Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Ten’nodai, Tsukuba 305-8572, Japan

Received on September 22, 2006; accepted on November 9, 2006

Abstract: The occurrence, lithology, and stratigraphic setting of buried manganese deposits and associated host sediments incores obtained on Legs 123–210 of the Ocean Drilling Program (ODP) are examined in order to establish the formative envi-ronment and conditions of preservation. Fossil manganese nodule and crusts are found to have formed or deposited through-out the period from 100 Ma to the present, with an additional example of formation near 137 Ma, suggesting that the deep-sea environment has been oxic and suitable for the formation of manganese nodules and crusts since the Cretaceous. Manymanganese nodules and crusts occur on horizons corresponding to hiatuses in sedimentation or periods of slow sedimenta-tion, consistent with the environment in which modern nodules form (sedimentation rate less than 10 m/m.y.). Sedimentsoverlying the fossil nodules and crusts are oozes or biogenic sediments with sedimentation rates of 1–18 m/m.y. Low totalorganic carbon (<0.1 wt%) in the overlying sediments and high sulfate content (>25 mM) in interstitial water around themanganese horizon suggest that no strong reduction occurred within the overlying sediments. Coverage by biogenic sedi-ments containing only small amounts of organic matter is therefore considered important for the preservation of manganesenodules and crusts. Manganese carbonate occurs sporadically as nodules, concretions or thin layers in various host sedi-ments, including clay, calcareous ooze and siliceous ooze with sedimentation rates of 6–125 m/m.y. Hiatuses are rare aroundthe host sediments of manganese carbonate. Higher total organic carbon (0.2–1.8 wt%) in the host sediments and lower sul-fate content (0–25 mM) in interstitial water around the manganese carbonate horizon suggest that reduction in associationwith decomposition of organic matter would have proceeded in the host sediments.

Keywords: Paleoceanographic setting, preservation, manganese nodule, manganese crust, manganese carbonate,rhodochrosite, Ocean Drilling Program (ODP), hiatus, sedimentation rate, reduction, diagenesis

manganese carbonate nodules, concretions and beds havebeen also found in DSDP/ODP cores. Matsumoto (1992)reported the reprecipitation or deposition of rhodochrositewithin the shallow suboxic sulfate reduction zone, and themanganese in rhodochrosite accumulated initially on thesea floor as manganese oxyhydroxides and subsequentlydissolved and mobilized. Information on manganese car-bonate is therefore expected to be important in under-standing the behavior of manganese under depositionaland diagenetic environments. The occurrence, age, strati-graphic position, lithology and depositional setting ofmanganese carbonate and host rock are thus also exam-ined in the present study of cores from Legs 123–210.

The accretionary complexes of the Japanese Islandscontain several types of manganese mineralizations con-sidered to have been formed in a deep-sea environment.Stratiform manganiferous iron deposits and stratiformmanganese deposits occur conformably in chert-clasticsequences, and are composed mainly of manganese car-bonates and silicates (e.g., Yoshimura, 1969). It has gen-erally been accepted that these deposits were initially pre-cipitated as manganese oxides and/or oxyhydroxides bydeep-sea hydrogenous or hydrothermal activity, changingto manganese carbonates and silicates under subsequentdiagenetic and metamorphic conditions (e.g., Sato andKase, 1996). In addition to such deposits, manganese car-bonate nodules are also present, found mainly in siliceousshale, and are considered to have been formed under earlydiagenetic conditions. The similarities and dissimilaritiesbetween these deposits and manganese deposits inDSDP/ODP cores are discussed.

2. Methods

The descriptions of buried manganese deposits in theinitial reports for ODP Legs 123 to 210 were examinedcarefully using the search engine for the ODP databasethrough the Oceanic Drilling Program Science Operator(Ocean Drilling Program, 2006). Indices of the initialreports and scientific results were also investigated. Thesearch terms used were "manganese nodule", "Mn nod-ule", "manganese crust", "Mn crust", "manganese carbon-ate", "Mn carbonate" and "rhodochrosite". As to the fossilmanganese nodules and crusts, through careful examina-tion of the description for each record, manganese nodulesand crusts larger than 2 mm in diameter or thickness werepicked out and listed, while smaller, disseminated, andstained manganese oxide, manganese nodules and crustsof unreliable in situ occurrence and of distinct hydrother-mal origin were omitted. For the listed buried manganesedeposits, age, stratigraphic position, lithology and deposi-tional setting were examined.

3. Results

3.1. Fossil manganese nodules and crusts

The sites of ODP cores containing fossil manganesenodules and crusts are shown in Figure 1, and the detaileddata are listed in Table 1. The depth, age, type of host sed-iment, and size of fossil manganese nodules and crusts aresummarized in Table 2, based primarily on the Proceed-ings of the Ocean Drilling Program, Initial Report (Ship-

458 RESOURCE GEOLOGY :T. ITO and K. KOMURO

120°E 180° 120°W 60 °W 0° 60 °E

60°S

40°S

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760

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699

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1207

121612171222

1221 1218

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1141

962

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10901089

10931092

913

1149

881

799

Fig. 1 Locations of ODPSites bearing manganesedeposits. Black squares:ODP sites bearing fossilmanganese nodule orcrust deposits. Black cir-cles: ODP sites bearingmanganese carbonate.Gray squares: data report-ed by Usui and Ito (1994).

board Scientific Party, 1990a, 1992, 1993a-f, 1996a, b,1998a-e, 1999b, e, 2000a, 2001a, b, 2002a-f; Bogdanov etal., 1995; Murdmaa et al., 1995; Skornyakova andUspenskaya, 1995; Watkins et al., 1995). The mode ofoccurrence of fossil manganese nodules and crusts aredescribed for each site in Appendix A1.

Fossil manganese nodules and crusts were found incores located geomorphologically in deep-sea basins,and on seamounts, plateaus and continental slopes. Thedeposits occur at depths of 0–930 m below the sea floor(mbsf). The stratigraphic setting, age and features of theoverlying sediments are summarized below.

3.1.1. Stratigraphic setting: The stratigraphic position of

fossil manganese nodules and crusts areshown against sedimentation rate on the geo-logical time scale in Figure 2. Many of thefossil manganese nodules and crusts occur athiatuses or in sediments deposited with lowsedimentation rates, as pointed out in Usuiand Ito (1994). This is consistent with the dis-tribution pattern of modern nodules andcrusts, which occur on sea floor with lowsedimentation rate (<10 m/ m.y.).

3.1.2. Age: The stratigraphic position of fos-sil manganese nodules and crusts, and possi-ble period of growth on the geological timescale, are shown in Figure 3. The fossil man-ganese nodules and crusts were predominant-ly formed or deposited from 100 Ma to thepresent, although one instance of formationfrom around 137 Ma (Valanginian toBerriasian) was observed (oldest among theDSDP/ ODP cores). These results indicatethat manganese nodules and crusts wereformed even before the late Cretaceous.

Recently, Klemm et al. (2005) analyzedosmium isotopic ratios for a manganese crustfrom a seamount of the Karin Ridge, south ofthe Hawaiian Ridge in the Central Pacific,and revealed that the crust appears to haveundergone continuous formation from theCampanian to the present, with no-growth orerosion between 47 and 43, 36 and 33, and29.5 and 13.5 Ma. The present data suggestthat manganese nodules and crusts formedeven during the no-growth or erosional peri-ods identified by Klemm et al. (2005), sug-gesting the occurrence of local paleoceano-graphic events.

3.1.3. Overlying sediments: Many of theoverlying sediments are oozes or biogenicsediments with sedimentation rates of 1–18m/m.y. The average total organic carbon

(TOC) content in the overlying sediments is low, in manycases lower than 0.1 wt% (Fig. 4), while the sulfate con-tent of interstitial water is high, typically >25 mM (Fig. 5),suggesting that no strong reduction would proceed in theoverlying sediments. Coverage with biogenic sedimentscontaining a small amount of organic matter with weakdiagenetic reduction is thought to be important for preser-vation of the manganese nodules and crusts.

3.2. Manganese carbonate

The sites of ODP cores containing manganese carbon-ate are shown in Figure 1, and the detailed data are listedin Table 3. The depth (mbsf), age and type of host sedi-

vol. 56, no. 4, 2006 Paleoceanographic Setting of Mn Deposits in DSDP/ODP Cores 459

Table 1 Site data for ODP cores bearing manganese nodules and crusts.

Leg Site Geographical Hole Latitude Longitude Water depthsetting (m)

Atlantic Ocean 159

962 Continental slope 962C 3°15.057'N 3°10.943'W 4628 171 1049 Plateau flank 1049A 30°08.5436'N 76°06.7312'W 2656

1050 Plateau flank 1050B 30°05.9981'N 76°14.0958'W 2300 1051 Plateau flank 1051A 30°03.1740'N 76°21.4580'W 1983 1052 Plateau flank 1052A 29°57.0906'N 76°37.5966'W 1345

1052B 29°57.0791'N 76°37.6098'W 1357 1052C 29°57.0798'N 76°37.6104'W 1345 1052D 29°57.0773'N 76°37.6123'W 1343 1052F 29°57.0794'N 76°37.6098'W 1342

1053 Plateau flank 1053A 29°59.5385'N 76°31.4135'W 1630 1053B 29°59.5391'N 76°31.4141'W 1630

177 1090 Ridge flank 1090B 42°54.821'S 8°53.984'E 3699 1090C 42°54.812'S 8°53.990'E 3704 1090D 42°54.814'S 8°53.998'E 3702 1090E 42°54.818'S 8°53.994'E 3704

Indian Ocean123 765 Continental slope 765C 15°58.54'S 117°34.49'E 5718 183 1141 Ridge crest 1141A 32°13.6'S 97°07.7'E 1197

Pacific Ocean138 854 Deep-sea basin 854A 11°13.433'N 109°35.652'W 3568

854C 11°13.431'N 109°35.649'W 3580 143 865 Seamount 865A 18°26.410'N 179°33.339'W 1518

865B 18°26.415'N 179°33.349'W 1516 144 873 Seamount 873A 11°53.796'N 164°55.188'E 1335

873B 11°53.838'N 164°55.230'E 1334 874 Seamount 874A 12°00.216'N 164°56.388'E 1375

874B 12°00.228'N 164°56.388'E 1375 875 Seamount 875C 12°00.756'N 164°56.466'E 1409 876 Seamount 876A 12°14.796'N 164°55.908'E 1399 877 Seamount 877A 12°01.146'N 164°55.326'E 1355 878 Seamount 878A 27°19.143'N 151°53.028'E 1323

878B 27°19.143'N 151°53.028'E 1323 878C 27°19.143'N 151°53.028'E 1323

181 1121 Plateau foot 1121A 50°53.876'S 176°59.862'E 4492 1121B 50°53.876'S 176°59.862'E 4488

191 1179 Deep-sea basin 1179B 41°04.7887'N 159°57.7879'E 5564 192 1184 Ridge crest 1184A 5°0.6653'S 164°13.9771'E 1662 198 1207 Rise crest 1207A 37°47.4287'N 162°45.0530'E 3101 199 1216 Deep-sea basin 1216A 21°27.1629'N 139°28.7904'W 5153

1217 Deep-sea basin 1217A 16°52.0133'N 138°05.9981'W 5342 1218 Deep-sea basin 1218B 8°53.3777'N 135°21.9995'W 4828 1221 Deep-sea basin 1221A 12°01.9987'N 143°41.6514'W 5175 1222 Deep-sea basin 1222A 13°48.9780'N 143°53.3477'W 4989


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Table 2 Summary of descriptions of manganese nodules and crusts.Leg Hole Core No. mbsf Ocean Buried age Overlying sediment Remarks

Atlantic Ocean159 962C 1R 73.1 North Atlantic Unknown Claystone Mn hardground (6 cm in thickness)171 1049A 2X 0 North Atlantic - - Mn nodule above hiatus since M. Eocene171 1050B 1X 0 North Atlantic - - Mn nodule above hiatus since M. Eocene171 1051A 2H 0 North Atlantic - - Mn nodule above hiatus since M. Eocene171 1052A 1X 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1052B 1X 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1052C 1H 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1052D 1H 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1052F 1H 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1053A 1H 0 North Atlantic - - Mn nodule above hiatus since L. Eocene171 1053B 1H 0 North Atlantic - - Mn nodule above hiatus since L. Eocene177 1090B 8H 66 South Atlantic E. Pliocene Foraminifer nannofossil ooze Mn nodule above hiatus since E. Miocene (4 cm in diameter)177 1090C 8H 61 South Atlantic E. Pliocene Foraminifer nannofossil ooze Mn nodule above hiatus since E. Miocene (3 cm in diameter) 177 1090D 7H 63 South Atlantic E. Pliocene Foraminifer nannofossil ooze Mn nodule above hiatus since E. Miocene (5 cm in diameter)177 1090E 7H 58 South Atlantic E. Pliocene Foraminifer nannofossil ooze Mn nodule above hiatus since E. Miocene (3-5 cm in diameter)

Indian Ocean123 765C 24R 570.55 Indian E. Campanian Claystone Black manganiferous horizon (1.5 cm in thickness)123 765C 61R 919.3 Indian L. Berriasian- Claystone Cauliflower-shaped Mn hardground and Mn nodule

Valanginian (<2 cm in diameter)123 765C 62R 927.7 Indian L. Berriasian- Calcareous silty claystone Mn nodule (<1 cm in diameter)

Valanginian183 1141A 11R 99 Indian E. Miocene Foraminifer nannofossil ooze Mn crust above hiatus since L. Eocene (1 cm in thickness)

Pacific Ocean138 854A 2H 15.9 North Pacific L. Pliocene Foraminifer nannofossil ooze Small Mn concretion138 854A 2H 18.4 North Pacific L. Pliocene Foraminifer nannofossil ooze Stiff black Mn oxide layer (3 cm in thickness)138 854C 5H 32.1 North Pacific L. Miocene Clayey nannofossil ooze Mn nodule138 854C 6H 41.6 North Pacific L. Miocene Zeolitic clay Small Mn nodule143 865A 16R 139.7 North Pacific E. Paleocene Foraminifer nannofossil ooze Mn crust above hiatus since L. Albian143 865A 17R 139.7-144.7 North Pacific E. Paleocene Foraminifer nannofossil ooze Mn crust above hiatus since L. Albian143 865A 19R 149.4-159.1 North Pacific E. Paleocene Foraminifer nannofossil ooze Mn crust above hiatus since L. Albian143 865B 16X 141.5 North Pacific E. Paleocene Foraminifer nannofossil ooze Mn oxide coating above hiatus since L. Albian143 865B 19X 155.8-165.5 North Pacific E. Paleocene Foraminifer nannofossil ooze Mn crust above hiatus since L. Albian144 873A 1R 54.3-59.8 North Pacific E. Miocene Phosphatized limestone Mn crusts and nodule above hiatus since M. Eocene (< 1.5 cm

conglomerate in thickness)144 873A 2R 59.8-69.3 North Pacific E. Miocene Skeletal floatstone Mn crust above hiatus since M. Eocene144 873B 7H 54-58 North Pacific E. Miocene Chalk Mn crusts above hiatus since M. Eocene144 873B 8N 58 North Pacific L. Paleocene Pelagic foraminifer packstone Mn crust above hiatus since Maastrichtien (< 3 cm in thickness)144 873B 9N 62.5 North Pacific L. Paleocene Skeletal grainstone Mn crust above hiatus since Maastrichtien (< 1.5 cm in thickness) 144 874A 1R 0 North Pacific - - Mn crust above hiatus since Maastrichtien144 874B 1R 0 North Pacific - - Mn crust above hiatus since Maastrichtien (< 1.5 cm in thickness)144 875C 1M 0 North Pacific - - Mn crust above hiatus since Maastrichtien (< 1 cm in thickness) 144 876A 1R 0 North Pacific - - Mn crust above hiatus since Maastrichtien (< 3 cm in thickness)144 877A 1R 0 North Pacific - - Mn crust above hiatus since Maastrichtien (3 cm in thickness) 144 878A 1R 0-1 North Pacific E. Pleistocene Foraminifer nannofossil ooze Mn nodule (1-3 cm in diameter) and Mn crust (0.75 cm in thickness)144 878A 1R 1.1 North Pacific L. Pliocene Foraminifer nannofossil ooze Mn nodule (<3 cm in diameter)144 878B 1R 0.1 North Pacific E. Pleistocene Pelagic and neritic carbonates Mn nodule and crust144 878C 1R 0.1 North Pacific L. Pliocene Foraminifer limestone Mn nodule181 1121A 1H 4.5 South Pacific L. Pliocene Silt-bearing clay Mn pavement181 1121B 1H 0.2 South Pacific L. Pleistocene Silty sand Mn nodule above hiatus since L. Paleocene181 1121B 1H 3.1 South Pacific L. Pliocene Silty clay Mn nodule above hiatus since L. Paleocene181 1121B 1H 3.4 South Pacific L. Pliocene Silty clay Mn nodule above hiatus since L. Paleocene181 1121B 1H 4.3 South Pacific L. Pliocene Silty sand Mn nodule above hiatus since L. Paleocene181 1121B 1H 5.2 South Pacific L. Pliocene Silty sand Mn nodule above hiatus since L. Paleocene191 1179B 2H 14 North Pacific L. Pleistocene Radiolarian diatom ooze Mn nodule192 1184A 9R 201.1 South Pacific E. Miocene Foraminifer nannofossil ooze Mn crust above hiatus since M. Eocene (1 cm in thickness) 198 1207A 18H 163.2 North Pacific M. Miocene Nannofossil clay Mn crust above hiatus since Campanian (1 cm in thickness) 199 1216A 5H 39.6 North Pacific M. Eocene- Clay Discontinuous Mn hardground

L. Pleistocene199 1217A 1H 0 North Pacific - - Mn nodule199 1218B 3H 15.9 North Pacific Pleistocene- Zeolitic clay Mn nodule (3 cm in diameter)

L. Miocene199 1221A 3H 21.86 North Pacific L. Eocene Clayey radiolarian ooze Fe and Mn oxide concretion199 1221A 3H 27.5 North Pacific M. Eocene Clayey radiolarian ooze Fe and Mn oxide concretion199 1221A 4H 30.85 North Pacific M. Eocene Radiolarian ooze Mn nodule (<5 mm in diameter)199 1221A 4H 30.9 North Pacific M. Eocene Radiolarian ooze Mn nodule (<5 mm in diameter)199 1221A 4H 31.76 North Pacific M. Eocene Radiolarian ooze Mn nodule (<5 mm in diameter)199 1221A 5H 38 North Pacific M. Eocene Radiolarian ooze Mn nodule (<1 cm in diameter)199 1221A 9H 76 North Pacific M. Eocene Radiolarian ooze Mn nodule (1-2 cm in diameter)199 1222A 3H 20.2 North Pacific E. Pliocene Zeolitic clay Botryoidal Mn nodule199 1222A 4H 32.4 North Pacific M. Miocene Zeolitic clay Mn nodule (1 cm in diameter)199 1222A 5H 35.2 North Pacific M. Miocene Zeolitic clay Mn nodule199 1222A 5H 35.9 North Pacific M. Miocene Zeolitic clay Mn nodule


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ment of manganese carbonate are summarized in Table 4,based primarily on the Proceedings of the Ocean DrillingProgram, Initial Report (Shipboard Scientific Party,1990a, b, 1993g, 1995, 1999a, c, d, 2000b), and Urbat andPletsch (2003). The mode of occurrence of manganese

carbonate is described for each site in Appendix A2. The manganese carbonate is found in cores located

geomorphologically in deep-sea basins and marginal seas,and on seamounts, plateaus, and continental slopes. Thedeposits occur at depths of 16–893 mbsf. The occurrenceof manganese carbonate, and the lithology and age of hostsediments are summarized below.

3.2.1. Occurrences: Manganese carbonate occurs sporad-ically as nodules, concretions or thin layers throughout thehost sediments, similar to other carbonates (such asdolomite and calcite) and consistent with formation dur-ing diagenesis after burial. Matsumoto (1992) analyzedthe oxygen isotopic ratios of carbonate concretions forSite 799 from the Japan Sea and determined an early dia-genetic period of formation in the sulfate reduction zone(<50–100 mbsf). The manganese in rhodochrosite was


Num

bers

of s

ampl

es

6

4

2

00 0.5 1.0 1.5 2.0

TOC (%)Fig. 4 Histogram of TOC content. Values are averages of

data for each sediment. Solid bar: sediment overlyingfossil manganese nodule and crust horizon at >30 mbsf.Open bar: host sediment of manganese carbonate.

Num

bers

of s

ampl

es

0

2

4

6

8

0 302010sulfate (mM)

Seawater value

Fig. 5 Histogram of sulfate concentrations in interstitialwater. Abbreviations are as shown in Figure 4.

Table 3 Site data for ODP cores bearing manganese carbonate.

Geographical Water Leg Site

settingLatitude Longitude depth

(m)Atlantic Ocean151 913 Deep-sea basin 75°29.35'N 6°56.83'E 3318177 1089 Ridge flank 40°56.18'S 9°53.64'E 4621177 1092 Rise crest 46°24.70'S 7°4.79'E 1974177 1093 Rise flank 49°58.59'S 5°51.93'E 3626

Indian Ocean123 765 Continental slope 15°58.54'S 117°34.49'E 5718

Pacific Ocean145 881 Deep-sea basin 47°6.14'N 161°29.49'E 5531185 1149 Deep-sea basin 31°20.10'N 143°21.81'E 5818

Sea of Japan128 799 Back arc trough 39°22.05'N 133°86.67'E 2072

Table 4 Summary of descriptions of manganese carbonate.

Leg Site mbsf Ocean Age Sediment type Remarks

Atlantic Ocean151 913 501-674 North Atlantic M. Eocene-E. Laminated and massive clay Large microspherules (200-600 µm) of

Oligocene rhodochrosite (Chow et al., 1996)177 1089 16 South Atlantic L. Pleistocene Mud-bearing siliceous Purple bands of rhodochrosite

nannofossil ooze177 1092 118-128 South Atlantic L. Miocene Nannofossil ooze Rhodochrosite and siderite fronts177 1093 444-453 South Atlantic L. Pliocene Diatom ooze and nannofossil Pink and green laminations of

diatom ooze rhodochrosite and sideriteIndian Ocean123 765 740-893 Indian Valanginian-E. Reddish brown claystone Rhodochrosite sediments and rhodo-

Aptian and calcareous claystone chrosite microconcretions (Compton, 1992; Dumoulin and Brown, 1992)

Pacific Ocean145 881 23-277 North Pacific L. Miocene- Clayey diatom ooze and Pockets of dolomite/rhodochrosite,

Pleistocene diatom ooze alternation of dolomitic/rhodochrositic ooze and clayey diatomaceous ooze

185 1149 28 North Pacific E. Pleistocene Biogenic silica-bearing clay 30 cm rhodochrosite rich zone (Urbat and Pletsch, 2003)

Sea of Japan128 799 100-730 The Sea of M. Miocene- Diatomaceous clay, Solid nodules and thin layers of

Japan Pleistocene diatomaceous ooze and rhodochrosite (Matsumoto, 1992)siliceous claystone

inferred to initially have accumulated onthe sea floor as manganese oxyhydrox-ides, with subsequent dissolution andmobilization.

3.2.2. Lithology of host sediments:Many of the host sediments are clays,calcareous oozes and siliceous oozes.Distinct hiatuses are not recordedaround the host sediments, and sedimen-tation rates are 6–125 m/m.y. The aver-age TOC content predominantly falls inthe range of 0.2–1.8 wt% (Fig. 4). Thesulfate content of interstitial water is10–25 mM (Fig. 5). At Site 799 (Sea ofJapan), the sulfate content of interstitialwater is lower (0.05 mM) near the sedi-ments hosting manganese carbonate.These data suggest that reduction inassociation with the decomposition oforganic matter occurred in the host sediments.

3.2.3. Age of host sediments: The host sediments aredated as late Pleistocene to Valanginian (early Creta-ceous), consistent with the range of fossil manganese nod-ules and crusts (Fig. 6). Many manganese carbonatedeposits are reported in cores of the middle Miocene tothe present.

4. Discussion

4.1. Formation of manganese nodules and crusts

Glasby and collaborators (Glasby, 1978, 1986, 1988;Glasby et al., 1982) emphasized that the formation andgrowth of manganese oxide are closely related to thedevelopment of the AABW on the basis of the distribu-tion of surficial and fossil manganese nodules with respectto the stratigraphy of the sedimentary column. It is reason-able to consider that the inflow of oxic water to the deep-sea floor, as is the case with the AABW, produces favor-able geochemical and environmental conditions for man-ganese oxide formation. Nevertheless, the present resultsshown in Figure 3 demonstrate that manganese nodulesand crusts have formed or deposited throughout the periodfrom 100 Ma to the present, representing a time consider-ably earlier than the Eocene-Oligocene boundary at whichthe AABW has been inferred to have formed in associa-tion with Antarctic cooling. This result suggests that thedeep-sea environment has been oxic and suitable for theformation of manganese nodules and crusts since the lateCretaceous. This is supported by the seawater Ce anomalycurve reported by Liu and Schmitt (1996), which indicatesthat seawater from 120 Ma to present has been depleted inCe relative to other rare earth elements (REEs) due toselective adsorption of Ce4+ to manganese oxides (Fig. 3).Although the origin of oxic deepwater before Antarctic

cooling is not fully understood, thermohaline circulation(Manabe and Bryan, 1985) or inflow of warm saline deep-water (Kennett and Scott, 1990) are possible processes bywhich oxic water can be conveyed to depth.

Distinct fossil manganese nodules and crusts have hith-erto not been found in deep DSDP/ODP cores during theValanginian and middle Albian (137 to 100 Ma). In thisperiod, black shales are dispersedly deposited in the NorthAtlantic basin (e.g., Jansa et al., 1978), some of whichhave been designated as being the result of Cretaceousoceanic anoxic events (OAEs) in the Tethys and Atlantic(Erba, 2004). Fossil manganese nodules and crusts wouldnot have formed under such stagnant oxygen-poor deep-water conditions, and the widespread absence of man-ganese nodules and crusts in the deep-sea environmentduring this period might suggest that deepwater condi-tions throughout the ocean were not sufficiently oxic toallow manganese oxide to form.

As noted previously, many of the fossil manganesenodules and crusts occur at hiatuses or in sediments withlow sedimentation rates, showing that the formation ofmanganese nodules and crusts is locally governed by thesupply of detrital material. Considering that the mineralo-gy, composition and structure of fossil nodules and crustsare similar to those of modern nodules and crusts (Usuiand Ito, 1994), the mode of formation, growth rate anddepositional setting are regarded as being essentially iden-tical to those of modern nodules and crusts.

4.2. Preservation and dissolution of manganese deposits

The preservation, diagenetic modification and/or disso-lution of manganese nodules and crusts after burial areprimarily controlled by the early diagenetic redox envi-ronment, which in turn is governed by the organic mattercontent of the overlying sediments, as summarized in


0 10 20 30 40 50 60

1089

1092

913

125

1093

Atlantic Ocean

16

? ?

4

250 80

88146

1149 18

Pacific Ocean

Sea of Japan

?71 ?799

6

Hiatus

Sedimentation rate (m/m.y.)

Legend

Manganese carbonate

0 10 20 30 40 50 60

PlePlio Miocene Oligocene Eocene PaleoceneL E Late Late Late Late EarlyEarlyEarlyEarlyMiddle Middle

DSDP ODP Site

(Ma)

(Ma)Fig. 6 Age of host sediments of manganese carbonate in ODP cores. Age of

manganese carbonate from Site 765 (Leg 123) is shown in Figure 3.

Figure 7. Modern manganese nodules are generally classified into

three types on the basis of mineralogy and occurrence:hydrogenous, oxic diagenetic, or suboxic diagenetic(Dymond et al., 1984). The majority of manganese crustsare in the category of hydrogenous type. Hydrogenousnodules and crusts are composed of vernadite, and arefound on deep-sea red clay displaying a sedimentation ratelower than 2 m/m.y. and on exposed rocks on seamounts.The manganese in hydrogenous nodules and crusts essen-tially originates from seawater. Oxic diagenetic nodules,on the other hand, mainly consist of buserite and developin radiolarian ooze with a sedimentation rate of lower than1–2 m/m.y. The manganese in oxic diagenetic nodules isderived diffusively from the underlying sediments, whereburied manganese nodules dissolve by reduction in associ-ation with the decomposition of organic matter in the earlydiagenetic environment. Suboxic diagenetic nodules arecomposed of todorokite, and are distributed in hemipelag-ic environments, where clay deposits as fast as 5 m/m.y.The manganese in suboxic diagenetic nodules is alsoderived diffusively from underlying sediments, however,the degree of reduction of the underlying sedimentsaffording these deposits is considered to be strong (up tosulfate reduction). It should be noted that manganese nod-ules cannot be preserved under diagenetic environments inwhich oxic and suboxic nodules are well developed due todissolution by reduction with burial.

Assuming that manganese nodules and crusts areabruptly covered by appropriate sediments of a certainthickness in association with a change in the sedimentary

environment, preservation and dissolution of buried man-ganese deposits may occur as follows. As the sedimentson the hydrogenous nodules and crusts contain very smallamounts of organic matter, the manganese oxides are notdissolved and are preserved under early diagenetic condi-tions to form fossil manganese nodule and crusts. This isconsistent with the observation that many of the fossilmanganese nodules and crusts in the DSDP/ODP coresare composed exclusively of vernadite (Usui and Ito,1994). On the other hand, the sediments on oxic and sub-oxic nodules, containing greater amounts of organic mat-ter, become reductive due to decomposition of organicmatter, resulting in dissolution of the manganese nodulesand the subsequent upward diffusion of manganese toform manganese nodules at the new sediment/water inter-face. Manganese oxides cannot be preserved as fossils insuch environments, as supported by scarcity of fossilmanganese deposits containing buserite and todorokite(Usui and Ito, 1994). In these environments, carbonateoriginates from the oxidative decomposition of organicmatter. If the carbonate and manganese contents in theinterstitial water exceed the solubility product ofrhodochrosite, nodules or concretions of rhodochrositewill form in these environments. Preservation and dissolu-tion of manganese nodules and crusts are thus concludedto be primarily controlled by the sedimentation rate andthe reductive capacity of the overlying sediments.

4.3. Implications for manganese mineralization in accre-tionary complexes

Several types of manganese mineralization occur in the


Seamount

Sediments

Basement rock

Sulfatereductionzone

Hiatus

Hydrogenous

Oxic diagenetic

Suboxic diagenetic

Surfical manganese nodules

Manganese crust

MMn2+2+ Mn oxidedissolution

Mn oxidepreservation

Organic Cnic Cdecomposition

HCHCO -33

Manganese carbonate

Biogenic ProductivityHigh Low

Fossil manganese nodule

Terrigenous detritus

Fossil manganese crust

Mn2+

Mn2+

Fig. 7 Model for preservation and dissolution of marine manganese deposits after burial.

accretionary complexes of the Japanese Islands, includingstratiform manganiferous iron deposits, stratiform man-ganese deposits, and manganese carbonate nodules. Thelithology of the host rocks indicates that such depositsformed in a deep-sea floor environment. To better under-stand the genesis of these deposits, the similarities anddissimilarities between manganese deposits in theDSDP/ODP cores and those in the accretionary complex-es of the Japanese Islands (excluding Hokkaido) areinvestigated.

The stratiform manganiferous iron deposits are associ-ated with basic volcanic rocks and chert in the Chichibu,Shimanto and northern Kitakami Terranes, and are com-posed mainly of hematite, braunite, rhodonite andrhodochrosite. Recently, Kato and collaborators, in stud-ies dealing with the chemical composition of ferroman-ganese ore and associated volcanic rock from the latePermian Kunimiyama deposit in the Chichibu Terrane,revealed that on the basis of the similarity of chemicalcomposition the stratiform manganiferous iron depositsare a type of umber deposit, which is found extensivelyaround mid-ocean ridges (Kato et al., 2005; Fujinaga andKato, 2005; Nozaki et al., 2005). Although hydrothermalmanganese deposits lie outside the scope of the presentpaper, hanging-wall red cherts, probably formed diagenet-ically from radiolarian ooze deposited in pelagic environ-ments with low organic content, may play an essentialrole in the preservation of such manganese deposits.

Stratiform manganese deposits are hosted in chertsequences in the Tanba, Mino, Ashio, northern Kitakamiand Chichibu Terranes, and are composed mainly of man-ganese carbonate and a wide variety of manganese sili-cates formed during diagenesis and/or metamorphismfrom manganese carbonate. The distinct stratigraphic suc-cession near the manganese horizon in such deposits,from lower bedded chert, through black shale, massivechert, manganese ore, and upper bedded chert, suggestsdeposition in a pelagic environment (e.g., Komuro andWakita, 2005; Komuro et al., 2005). The formative age ofthese deposits is restricted to two or more horizons of thelate Triassic and middle Jurassic (Sato and Kase, 1996;Nakae and Komuro, 2005; Kuwahara et al., 2006).Komuro and Wakita (2005) and Komuro et al. (2005)proposed that these deposits formed diagenetically frommanganese oxides deposited initially in association withthe inflow of oxygenated deepwater into stagnant bottomwater. This result is supported by lithological, chemicaland isotopic evidence. Fujinaga et al. (2006) suggestedthat the deposit associated with basic volcanic rock of theAnanai deposit in the Chichibu Terrane formedhydrothermally due to hot-spot volcanism. Thick carbon-ate layers and specific stratigraphic succession similar tothose of the stratiform manganese deposits in the accre-tionary complexes are not recognized in the ODP cores,

suggesting that carbonate layers did not form in theCenozoic oxic pelagic environment. The stratiform man-ganese deposits may instead be related to a major geologi-cal event associated with changes in the mode of deepwa-ter circulation, as suggested out by Hori (1993), Komuroand Wakita (2005) and Komuro et al. (2005).

Manganese carbonate nodules are found in Jurassicsiliceous mudstones of the Mino, Ashio, northernKitakami and Chichibu Terranes, and are characterized bywell-preserved microfossils (mainly radiolarian). Thesecarbonate nodules have been the subject of extensivepaleontological study (e.g., Yao 1997; Hori and Wakita,2005). The presence of radiolarians suggests that the car-bonate nodules formed under early diagenetic conditions(Yao, 1997), while the lithology suggests formation underhemipelagic conditions. The mode of occurrences of man-ganese carbonates and the lithology of the host sedimentsare similar to those of the ODP cores, and this correlationis supported to a certain extent by Henrich (1989), whoreported the preservation of delicate structures of siliceousmicrofossils in rhodochrosite concretions from ODP Site643, Leg 104.

5. Conclusions

Buried manganese deposits in cores from Legs 123–210 of the Ocean Drilling Program were carefullyexamined in order to understand the paleoceanographicsetting and preservation of such manganese deposits.The principal results are summarized as follows:

(1) Many of the fossil manganese nodules and crustsoccur at hiatuses in sedimentation or in sedimentary hori-zons with low sedimentation rates, consistent with previ-ous reports (Usui and Ito, 1994) and the environment ofmodern nodule formation (sedimentation rate, <10m/m.y.).

(2) The stratigraphic position of fossil manganese nod-ules and crusts was clarified, and the periods of growthwere estimated. The fossil manganese nodules and crustsare inferred to have formed or deposited predominantlysince 100 Ma, although one example around 137 Ma(Valanginian to Berriasian) was also identified.

(3) The sediments overlying fossil manganese nodulesand crusts are oozes or biogenic sediments with sedimen-tation rates of 1–18 m/m.y. The overlying sediments havecharacteristically low TOC (0–0.3 wt%) and high sulfate(>25 mM) in interstitial water, suggesting that no strongreduction occurred within the overlying sediments.Coverage by biogenic sediments with low organic mattercontent is considered to be important for preservation ofthe manganese nodules and crusts.

(4) Manganese carbonate occurs sporadically as nod-ules, concretions or thin layers throughout the host sedi-ments, which are predominantly clays, calcareous oozes


and siliceous oozes. The formative age of this manganesecarbonate is within the period of fossil manganese noduleand crust formation. Hiatuses are not conspicuous amongthe sites containing manganese carbonates. The sedimen-tation rates are in the range of 6–125 m/m.y. The TOCcontent is in the range of 0.2–1.8 wt% and the sulfate con-tent of interstitial water at many sites is 0–25 mM, sug-gesting that reduction in association with decompositionof organic matter occurred in the host sediments.Acknowledgments: This research was partly supportedby JSPS through Grant-in-Aid Nos. 14740297 and18253006 to TI. Critical review of the early manuscriptby Y. Kato and an anonymous referee was very con-structive.

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APPENDIX

A1. Occurrence of Fossil Manganese Nodules and Crustsat Each ODP Site

A1.1. Atlantic Ocean

Fossil manganese nodules and crusts were found inthree localities; Legs 159, 171 and 177.

A1.1.1. Site 962 (Leg 159): This site is located at a depth of4628 m on the western continental slope of the AfricanContinent. A manganese hardground of 6 cm in thicknessoccurs at 73.1 m below the sea floor (mbsf) in palygorskiteclaystone and silty sandstone of unknown age, overlying lateAlbian to Cenomanian chert and porcellanite and overlain byclaystone containing radiolarians and diatoms of earlyMiocene age. Sediment accumulation rates for the overlyinglower Miocene strata are estimated to be 38–42 m/m.y. TheTOC contents in the host sediments and overlying claystoneare 0.02–0.04 and 0.08–0.14 wt%, respectively. Sulfate con-centrations in interstitial water are near 20 mM.

A1.1.2. Sites 1049–1053 (Leg 171): These sites are located atdepths of 2656, 2300, 1983, 1342–57 and 1630 m, respective-ly, on the flank of the Blake Plateau to the east of the NorthAmerican Continent. The nodules occur in the uppermostportion of the cores over foraminifer nannofossil ooze ofmiddle to late Eocene, indicating an extraordinarily long hia-tus between deposition of the footwall oozes and the nodules.

A1.1.3. Site 1090 (Leg 177): This site is located at depths of3699–3704 m on a flank of the Agulhas Ridge, south of theAfrican Continent. Many fossil manganese nodules of 3–5cm in diameter were found in foraminifer nannofossil ooze


of early Pliocene around 60 mbsf, overlying early Miocenemuddy diatom ooze separated by a hiatus of up to 15 m.y.The sediment accumulation rate for the overlying lower earlyMiocene strata is estimated to be 18 m/m.y. The TOC con-tent in the host foraminifer nannofossil ooze is 0–0.22 wt%,and sulfate concentrations in interstitial water around thenodule horizon are close to 26 mM. Manganese carbonatefound at Site 1089 of this leg is described below.

A1.2. Indian Ocean

Manganese nodules and crusts were detected in twolocalities; Legs 123 and 183.

A1.2.1. Site 765 (Leg 123): This site is located at a depth of5718 m on the northwestern continental slope of Australia.Manganese nodules and crusts were found in two stratigraph-ic horizons, 571 and 920–927 mbsf, and manganese carbon-ates were found at 740–893 mbsf (described later). The man-ganese oxide bed at 571 mbsf is 1.5 cm in thickness andoccurs within early Campanian claystone with a sedimenta-tion rate of 1.7 m/m.y. TOC in the host clay is 0–0.03 wt%.The manganese hardground and nodules at 920–927 mbsfoccur in brown to red silty claystone of late Berriasian toValanginian and are overlain by claystones and turbiditychalks with sedimentation rates of approximately 6 m/m.y.The TOC content in the host clay is 0–0.05 wt%. The sulfateconcentration in interstitial water at this site decreases expo-nentially with depth from the concentration of seawater at 0mbsf to 7 mM at 441 mbsf. Sulfate increases sporadicallybelow 441 mbsf to a maximum of 16.9 mM at 801 mbsf, andthen decreases to 11.7 mM at 912 mbsf.

A1.2.2. Site 1141 (Leg 183): This site is located at a depthof 1197 m on Broken Ridge in the eastern Indian Ocean. Amanganese crust of 1 cm in thickness was found at 99mbsf within early Miocene foraminifer nannofossil oozewith a sedimentation rate of 6 m/m.y. The foraminifer nan-nofossil ooze overlies late Eocene sandy foraminifer nan-nofossil limestone separated by a hiatus of up to 20 m.y.The TOC content in the host foraminifer nannofossil oozeis 0–0.02 wt%.

A1.3. Pacific Ocean

Many manganese nodules and crusts were found in deep-sea basins (Legs 138, 191 and 199), and seamounts or plateaus(Legs 143, 144, 181, 192 and 198) in the Pacific Ocean.

A1.3.1. Site 854 (Leg 138): This site was drilled at a depthof 3568–3580 m in a deep-sea basin to the north of theClipperton fracture zone, southwest of Mexico. The coresin this site are composed of metalliferous clay, clayey nan-nofossil ooze, clay of late Miocene and late Pliocene age,and foraminifer nannofossil ooze of late Pliocene andPleistocene age, in ascending order. A distinct hiatus isrecognized in the clay between the late Miocene andPliocene. The sedimentation rates of the sediments beforeand after the hiatus are 8 and 6 m/m.y., respectively. Themanganese oxides are embedded in late Miocene clayeynannofossil ooze and also in late Pliocene foraminifer nan-nofossil ooze. TOC contents in the host sediments are

0.25–0.50 wt%, and the sulfate concentrations in intersti-tial water are 28.2–29.2 mM throughout the column.

A1.3.2. Site 865 (Leg 143): This site is located at a depth of1516-1518 m on a summit of the Allison Guyot in the Mid-Pacific Mountain. Manganese crusts were identified at 140–166 mbsf on late Albian phosphatized limestone, overlainby early Paleocene foraminifer nannofossil ooze displaying asedimentation rate of 3.5 m/m.y. A long hiatus of ca. 35 m.y.is recognized between the late Albian and early Paleocenedeposits. The TOC content for the foraminifer nannofossilooze above the crust horizon is 0.17 wt%. The sulfate concen-tration of interstitial water around the horizon is 28.4 mM.

A1.3.3. Sites 873–878 (Leg 144): These sites are located atdepths of 1323–1409 m in the Wodejebato Guyot near theMarshall Islands. At Site 873, manganese crusts occur ontwo horizons. The upper manganese crusts are formed inphosphatized limestone which is covered by foraminifer oozeof middle to early Miocene age with a sedimentation rate of4 m/m.y. The TOC content in the phosphatized limestone is0.025 wt%, and the sulfate concentration in interstitial wateris 28 mM. The lower manganese crusts occur onMaastrichtian limestone and are covered by late Paleocenephosphatized limestone. At Sites 874–877, manganese crustsof <3 cm in thickness are found in the uppermost portion ofthe cores on Maastrichtian platform carbonates, with a longhiatus of up to ca. 65 m.y. At Site 878, the manganese nod-ules and crusts are embedded in the surficial 1 m of the corewithin foraminifer nannofossil ooze of early to late Plioceneage displaying a sedimentation rate of 0.4 m/m.y.

A1.3.4. Site 1121 (Leg 181): This site is located at a depth of4490 m at the foot of the Cambell Plateau, south of NewZealand. Many manganese nodules occur at depths of 0.2–5.2 mbsf in silty clay and silty sand of late Pliocene to latePleistocene age. The host material is deposited on siliceousooze, nannofossil-bearing ooze, and chalk of Paleogene age,indicating a hiatus of ca. 50 m.y. The sedimentation rate ofthe overlying silty clay and silty sand is 1 m/m.y.

A1.3.5. Site 1179 (Leg 191): This site is located at a depthof 5564 m in a deep-sea basin of the northwestern Pacific.A manganese nodule was found at 14 mbsf in a latePleistocene radiolarian diatom ooze with sedimentationrate of 8 m/m.y. The TOC content in the host radiolariandiatom ooze around the nodule horizon is 0.14–0.31 wt %,and the sulfate concentration in interstitial water aroundthe horizon is 28.0–29.4 mM.

A1.3.6. Site 1184 (Leg 192): This site was drilled at a depthof 1662 m on an eastern salient of the Ontong Java Plateau.A manganese crust of 1 cm in thickness was found on middleEocene tuff, covered by early Miocene foraminifer nannofos-sil ooze with a hiatus of 20 m.y. at the manganese crust hori-zon. The foraminifer nannofossil ooze exhibits a sedimenta-tion rate of 17 m/m.y.

A1.3.7. Site 1207 (Leg 198): This site is located at a depthof 3101 m on the Northern High of the Shatsky Rise in thenorthwestern Pacific. A manganese crust of 5 cm in thick-


ness was found at a depth of 163.2 mbsf on Campaniannannofossil ooze, covered by middle Miocene nannofossilooze and clay with a sedimentation rate of 10 m/m.y. Ahiatus of up to 57 m.y. is indicated at the manganese crusthorizon. The sulfate concentrations in interstitial wateraround the horizon are 25.0–25.5 mM.

A1.3.8. Sites 1216–1222 (Leg 199): These sites were drilledat a depth of 4828–5342 m in a deep-sea basin of the easternequatorial Pacific. Manganese nodules and hardground wererecognized at depths of 0–40 mbsf in clay. Index fossils werefew at Sites 1216, 1217 and 1218. At Site 1221, fossil man-ganese nodules were found at 22–38 mbsf and 76 mbsf in anEocene radiolarian ooze exhibiting a sedimentation rate of3–18 m/m.y. The TOC content of the host radiolarian ooze is0.00–0.01 wt%, and sulfate concentrations in interstitialwater of the host radiolarian ooze are 29.2–29.4 mM. At Site1222, fossil manganese nodules occur at 20–36 mbsf inzeolitic clay of middle Miocene to Pliocene age exhibiting asedimentation rate of 1.3–6 m/m.y. The TOC content in thehost radiolarian ooze is 0.00–0.09 wt%, and sulfate concen-trations in interstitial water of the host radiolarian ooze are29.7–30.2 mM.

A2. Occurrence of Manganese Carbonates at EachODP Site

A2.1. Atlantic Ocean

Manganese carbonate was found in two localities oncontinental slopes and flanks, Legs 151 and 177.

A2.1.1. Site 913 (Leg 151): This site is located at a depth of3318 m in the Greenland Basin. Rhodochrosite nodules orconcretions were frequently found with wide dispersion at501 to 674 mbsf in laminated or massive clay of middleEocene to early Oligocene age exhibiting a sedimentationrate of 21.5 m/m.y. The sulfate concentrations in interstitialwater decrease from 13.6 to 6.5 mM with depth in the lami-nated or massive clay. The TOC contents range from 0 to 5.3wt%. Chow et al. (1996) concluded that the carbon inrhodochrosite was mainly derived from the oxidation ofmarine organic matter in bacterial sulfate-reduction zonesduring the early stages of methanogenesis based on the lowδ13C values of rhodochrosite from this site.

A2.1.2. Sites 1089, 1092 and 1093 (Leg 177): These sites arelocated south of the African Continent. In addition to the fos-sil manganese nodules in Site 1090 described earlier, man-ganese carbonate occurs in cores obtained at these threeSites. Site 1089 was drilled at a depth of 4621 m on a flankof the Agulhas Ridge. The cores are composed of alternatingbeds of nannofossil ooze and mud-bearing nannofossil-poordiatomous ooze. A rhodochrosite band occurs at a depth of16 mbsf in a Pleistocene nannofossil ooze with a sedimenta-tion rate of 125 m/m.y.

Site 1092 is situated at a depth of 1974 m on the northernMeteor Ridge. Rhodochrosite bands were found at depths of118–128 mbsf in a late Miocene nannofossil ooze with a sed-imentation rate of 16 m/m.y. The sulfate concentrations ininterstitial water around the rhodochrosite horizon are close

to 25 mM. Site 1093 is located at a depth of 3626 m to the north of

the Shona Ridge. Pinkish rhodochrosite lamination isembedded at depths of 444–453 mbsf in diatom and nan-nofossil oozes of late Pliocene age displaying a variablesedimentation rate of 80–250 m/m.y. The TOC contentsare 0.7–0.8 wt%, and the sulfate concentrations in intersti-tial water are in the range of 13–17 mM.

A2.2. Indian Ocean

Manganese carbonate is only reported in Leg 123.

A2.2.1. Site 765 (Leg 123): This site was drilled at a depth of5718 m on the northwestern continental slope of Australia. Inaddition to the manganese oxides described earlier,rhodochrosite nodules or concretions were found embed-ded at depths of 740–893 mbsf within reddish brown andcalcareous claystones of Valanginian to early Aptian agedisplaying a sedimentation rate of 7.6 m/m.y. TOC con-tents are 0–1.5 wt%, and the sulfate concentration in inter-stitial water decrease from 17 to 11.7 mM. Dumoulin andBrown (1992) found many manganese carbonate micro-grains in smear slides and thin sections of clay-richbiosiliceous sediments. Compton (1992) suggested on thebasis of the geochemical profiles of interstitial water thatrhodochrosite was formed through sulfate reduction fol-lowed by recrystallization during methanogenesis.

A2.3. Pacific Ocean

Manganese carbonate is described in two sites on thedeep-sea basins.

A2.3.1. Site 881 (Leg 145): The site is drilled at 5531 m on adeep-sea basin, northwestern Pacific. Rhodochrosite nodulesor concretions occur at 23–277 mbsf in diatom ooze of lateMiocene to late Pliocene and clayey diatom ooze of latePliocene to Pleistocene, the sedimentation rates of which are34-84 m/m.y. TOC contents of the host rocks are 0-0.5 wt%.The sulfate concentration of interstitial water is about 24mM.

A2.3.2. Site 1149 (Leg 185): The site is located at a depth of5818 m in the Nadezhda Basin of the northwestern Pacific.Minor deposits of rhodochrosite were found at a depth of 28mbsf in 30 cm-thick biogenic silica-bearing clay of earlyPleistocene with a sedimentation rate of ca. 18 m/m.y.

A2.4. Sea of Japan

A2.4.1. Site 799 (Leg 128): The site was drilled at a depth of2072 m in the Kita Yamato Trough of the Sea of Japan.Nodules and thin layers of rhodochrosite are embedded indiatomaceous clay, ooze and siliceous clay of Miocene toPleistocene age displaying sedimentation rates of 15–91m/m.y. (av. 65 m/m.y.). The TOC contents are in the rangeof 0–6.36 wt%, and the sulfate concentrations in interstitialwater are 0–0.4 mM. On the basis of isotope geochemicalstudy of carbonates from Site 799, Matsumoto (1992)showed that rhodochrosite was reprecipitated or depositedwithin the shallow suboxic sulfate reduction zone during theearly diagenetic process.


Documents

Paleoceanographic Setting and Preservation of Buried Manganese Deposits in DSDP/ODP Cores