33. SEDIMENTS AND INTERSTITIAL WATER AT SITES 582 AND 584, THE NANKAI TROUGHAND THE JAPAN TRENCH LANDWARD SLOPE1

Hodaka Kawahata, Marine Geological Department, Geological Survey of Japanand

Kantaro Fujioka and Toshio Ishizuka, Ocean Research Institute, University of Tokyo2

ABSTRACT

Two trenches off Japan were explored during DSDP Leg 87. One is the Nankai Trough and the other is the JapanTrench; Site 582 is located on the floor of the former and Site 584 is situated on the deep-sea terrace of the latter. Coresfrom Site 582 and 584 consist mainly of hemipelagic sediments and diatomaceous silts and mudstone, respectively. Inthis report we analyze the chemistry of the interstitial water and sediments, as well as the sediment mineralogy.

Sulfate reduction is accompanied by the production of secondary pyrite, which is rich in the sediment at both sites.Dissolved Ca concentration is relatively low and changes only slightly at both sites, probably because of the formationof carbonate with high alkalinity. Concentrations of dissolved Mg decrease with depth at Site 584. The dissolved Mg de-pletion probably results from the formation of Mg-rich carbonate and/or ion exchange and reaction between interstitialwater and clay minerals. Higher Si/Al values are due to biogenic opal in the sediments and roughly correlate with highervalues of interstitial water SiO2. Increases in dissolved Li concentrations may be related to its release from clay minerals,to advection that results from dewatering, and/or to fluid transport.

INTRODUCTION

The chemical composition of interstitial waters of ma-rine sediments may be modified by many processes, par-ticularly diagenetic reactions and diffusive and advec-tive transport of dissolved constituents. The qualitativeand quantitative estimation of the above-mentioned pro-cesses is difficult because of the complexity and varia-tion of the reactants, the multitude of possible reactions,and the scatter of sediment physical properties data. OnlyPerry and others (1976) and Lawrence and others (1975)clarified the relation between interstitial water and sedi-ments with respect to Mg, Ca, and oxygen isotope bymeans of the precise analysis of both phases. In addi-tion, a few studies have been done on samples from ac-tive margins, where sediment accumulation rates are veryhigh and many faults and fissures exist (Moore and Gies-kes, 1980).

DSDP Leg 87 offered an opportunity to study thediagenesis of terrigenous sediments along an active mar-gin. This report centers on the interstitial water chemis-try of Sites 582 and 584 (Fig. 1) and incorporates infor-mation regarding sediment chemistry and mineralogy.

SAMPLING AND ANALYTICAL PROCEDURES

Sampling

Samples of interstitial water were collected at all of the Leg 87 sitesby standard shipboard squeezing techniques, and several samples werecollected by means of the IPOD in-hole sampler, a device that collectsboth water for chemical analysis in a stainless tubular coil and water forgas analysis in a connected copper coil.

Kagami, H., Karig, D. E., Coulbourn, W. T., et al., Init. Repts. DSDP, 87: Washing-ton (U.S. Govt. Printing Office).

2 Addresses: (Kawahata) Marine Geological Department, Geological Survey of Japan 1-1-3 Higashi Yatabe, Tsukuba, Ibaraki 305, Japan; (Fujioka and Ishizuka) Ocean Research In-stitute, University of Tokyo, Minamidai, 1-15-1, Nakanoku, Tokyo 164, Japan.

40°N —

30

130°E 135° 140°

Figure 1. Location of DSDP Sites 582, 583, and 584, as well as loca-tion of earlier, related DSDP sites.

Analysis of Interstitial Water

Interstitial waters were obtained from the drilled hole(s) and ana-lyzed on board for pH, alkalinity, chlorinity, and salinity. On the basisof concentrations obtained by these shipboard analyses, we decided tocarry out more complicated studies on samples obtained at Sites 582and 584.

The concentrations of Li, Na, K, Ca, and Mg were measured by aHitachi 170-50A type atomic absorption spectrometer. All solutionswere adjusted to 4000 ppm LaCl3 solution to prevent interference byco-existing elements. The Na concentration was calculated from chargebalances.

865

H. KAWAHATA, K. FUJIOKA, T. ISHIZUKA

The colorimetric method was employed to test for silica (Matsui,1963). Ammonium molybdate is added to the sample solution, result-ing in the production of silicomolybdic acid. The color of this solu-tion is yellow. Next, the reducing agent is added at regular intervalsuntil molybdenum-blue appears. The concentration of silica is deter-mined by measuring the absorbance of 800-µm wave-length light bythis blue color. The Hitachi 101 spectrophotometer was used for mea-surements of absorbance.

Sulfate analyses were carried out using a titration technique.

Analysis of Sediments

Bulk chemistry. The sediment samples used for chemical and min-eralogic analysis are the same sediment plugs squeezed on board forinterstitial water extraction. One method of analysis employs a Ion-Coupled Plasma (ICP) Emission Spectroscope (Javrel-Ash model 975)located at Tsukuba University. In this analysis, the JB-1, JB-2, JA-1,and JG-1 (JGS standard rocks) and the G-2, AGV-1, and BCR-1(USGS standard rocks) served as standards. In ICP analysis, 100 mgof sample powder is dissolved with 10 ml of fluoric acid and 5 ml ofsulphuric acid. The solution is diluted with nitric acid and double dis-tilled water to make a total volume of exactly 100 ml. The relative ac-curacy of this method is approximately 5% for each element (Notsu,1980).

In order to normalize the variability for the nonbiogenic sediments(removing biogenic carbonate or opal), values of Mg, K, Ca, Na, andZn are commonly reported as the atomic ratios Mg/Al, K/Al, and soforth. In addition Al should be the least mobile major element in-volved in any alteration or diagenetic processes.

Mineral identification. X-ray diffraction was used to identify theminerals existing in the starting materials and in the reaction products.We used the Rigaku Denshi diffractometer and CuKα radiation.

Identification of clay minerals. Clay minerals in remnant sedi-ments were concentrated by decantation in distilled water and centri-fuged. These clay minerals were identified by X-ray diffraction pat-terns before and after treatment by ethylene-glycol.

GENERAL CHARACTERISTICS OF SITES 582AND 584

Site 582 is located on the floor of the Nankai Troughabout 2 km south of the deformation front (Fig. 1). Thesediments are dark olive gray and gray turbidites andhemipelagic clays and silts (Fig. 2). Coarse sand turbi-dites decrease in frequency with sub-bottom depth. Trenchturbidites include volcanic glass, lithic fragments, heavyminerals, red chert, well-preserved mixtures of shallowand deep benthic foraminifers, and both marine and non-marine diatom assemblages. Site 582 is characterized byextremely high rates of accumulation during the Quater-nary caused by the large input of continental-derivedsediment. Biostratigraphic and paleomagnetic data sug-gest reduction of rates from near 900 m/Ma before 0.4Ma ago or earlier to less than 300 m/Ma after 0.4 Ma(Leg 87 Scientific Party, 1983; Karig et al., 1983).

Both the turbidites and the hemipelagic sediments ofthe axial deposits have an organic carbon content from0.5 to 0.7%, decreasing to less than 0.5% in the hemipe-lagites beneath. Hydrocarbon gases (mainly methane withvery minor CO2, C2, C3, and i-C4, as well as traces ofH2S, isopentane, and neopentane) are present through-out the section as gas pockets. No solid gas hydrate wasencountered (site chapters, Sites 582 and 584, this volume).

Site 584 is situated on the deep-sea terrace of the trenchslope (Fig. 1) where 954 m of the sedimentary sectionwere penetrated. The oldest sediment cored was middleMiocene. Four lithostratigraphic units are recognized (sitechapter, Site 584, this volume). The uppermost 4 m atHole 584 is Pleistocene sediment (0 to 4 m sub-bottom

depth). A second lithologic unit (4 to 231 m sub-bottomdepth) is lower Pliocene diatomaceous mud and mud-stone. A third lithologic unit (231 to 537 m sub-bottomdepth) is also a diatomaceous mudstone, but is distin-guished by fine sand and silt beds, seaward-dipping stra-ta, and markedly higher induration than in the overlyingsediments. The Pliocene/Miocene boundary occurs with-in that unit, near 564 m sub-bottom depth, but no litho-logic contrast marks the boundary. A bioturbated mud-stone with a much-reduced diatom content constitutesUnit 4 (Fig. 2). Rates of sediment accumulation were es-timated based on selected datum levels and are highest(200 to 700 m/Ma) for the early Pliocene and latest Mi-ocene, a 7-Ma time span (Leg 87 Scientific Party, 1983;Karig et al., 1983).

SITE 582 RESULTS FOR INTERSTITIAL WATERAND SEDIMENT CHEMISTRY AND

MINERALOGY

Pore-Water Chemistry

Salinity, pH, and chlorinity are nearly constant withrespect to sub-bottom depth at this site (Table 1). Alka-linity reaches a maximum of 45 meq/1 at 200 m sub-bot-tom then remains virtually constant at about 25 mmoles/1beneath 280 sub-bottom depth (Fig. 3A).

The SO4 concentration decreases to 5 mmoles/1 with-in the upper 100 to 200 m, increases to about 25 mmoles/1to a total depth of 670 m (Fig. 3B).

The Mg concentration decreases to 42 mmoles/1 with-in the upper 50 m of sediments, remains constant to200 m, then decreases to a total depth of 690 m (Fig. 3B).

The dissolved Ca is constant at 5 mmoles/1, a concen-tration of about half that of bottom seawater (Fig. 3C).

The K concentration remains fairly constant through-out the section (Fig. 3C).

Dissolved Sr shows only a slight peak around 200 msub-bottom and increases below 400 m sub-bottom(Fig. 3D).

Dissolved Li concentration increases slightly betweenthe seafloor and 550 m sub-bottom in Lithologic Unit 1and then increases rapidly below that depth in Unit 2(Fig. 3E).

The SiO2 concentration is nearly constant through-out at about 0.6 mmoles/1 (Fig. 3F).

Sediment Chemistry

The bulk chemistry of the remnant sediment samplesfrom Sites 582 and 584 is presented in Table 2. At Site582, the atomic ratios for Si/Al, Mg/Al, Na/Al, K/Al,and Li/Al remain low and fairly constant (Fig. 4). TheCa/Al and Sr/Al ratios are scattered but decrease withdepth (Fig. 4C, D), and, conversely, the Zn/Al ratio in-creases with depth by nearly a factor of 2 (Fig. 4H).

Sediment Mineralogy

Smear-slide estimates for Site 582 indicate that quartzand feldspar concentrations decrease gradually with depthas clay concentrations gradually increase (Fig. 5). Clayminerals present include kaolinite, illite, and smectite.Pyrite is found in all the sections at concentrations of2-9%.

866

SEDIMENTS AND INTERSTITIAL WATER

Holes 582, 582A, 582B

100

150

200

250

300

350

400

450 I-

450

500 -

550 -

600 -

650 -

700 -

750

Age

1 '

'Qua

tern

ary

' '

uppe

Hol

e

B

V

CDUO

I

Uni

t

1

Core

434445464748495051525354555657585960

"64~65

6667686970717273

• ü

• •

• •

Lith-ology

Hmmiii

T.D. = 749.4 m

Holes 584, 584A

U

50

100

150

1 200.cQ.Φ

"D

O

Z 250

CO

300

350

400

450

CD

α>cα>

ioc

α.

$αQ

-

-

α>

Ho

i

-BA

H. KAWAHATA, K. FUJIOKA, T. ISHIZUKA

Table 1. Interstitial water data, Leg 87.

Core-Section(interval in cm)

Hole 582

1-3, 143-1503-5, 120-130

Hole 582A

1-5, 0-102-5, 0-10

Hole 582B

2-2, 140-15014-5, 138-15018-5, 140-15023-1, 140-15030-5, 138-15033-4, 140-15038-3, 140-15050-3, 135-15054-2, 135-15066-2, 135-15061-2, 135-150

Hole 584

1-4, 140-1502-3 140-1503-3, 140-1505-5, 140-1506-2, 140-1507-1, 140-1509-1, 140-15010-3, 140-15012-5, 140-15016-2, 135-15019-6, 135-15022-3, 135-15027-4, 135-15032-2, 135-15037-5, 135-15042-4, 135-15047-3, 135-15051-2, 135-15057-3, 135-15062-1, 135-150

Sub-bottomdepth(m)

4.526.9

35.144.8

61.2181.5217.0252.2335.4362.5408.3522.3559.3674.8626.7

4.014.824.546.351.259.378.591.0

113.0147.0182.1206.6255.8300.3352.6398.9445.5482.3541.1590.1

PH

7.797.19

7.507.71

7.817.987.787.867.857.957.767.897.917.647.70

7.277.427.637.417.577.387.397.617.517.527.507.587.797.747.707.747.807.637.717.90

Salinity(‰)

33.8033.00

33.6033.80

33.8034.6034.1033.6033.0032.2033.8033.6033.3033.0033.30

36.3035.5036.6036.5035.8035.5035.8035.8035.5035.8034.6034.4034.6033.6033.0032.4033.0033.0033.0033.00

Chlorinity(‰)

19.7819.99

19.7819.07

19.5819.6819.3719.3419.1018.8919.1019.0719.3718.0419.20

19.1418.5819.3120.0119.6119.3419.4419.4819.4119.2119.3119.4419.7819.3119.1718.9119.0119.3119.2119.17

Alkalinity(meq/1)

21.1216.26

19.5324.64

28.3345.6236.4323.4820.3620.2120.0821.3425.3618.0020.62

3.723.644.975.615.965.856.597.567.60

10.3311.9614.5320.3523.6419.9319.9821.7919.6719.8718.14

Mg

48.0342.91

42.0743.20

43.2146.3945.0240.5634.6836.3436.5528.2628.6325.1727.81

52.6451.1552.0652.7552.5052.1852.2751.8250.8952.1350.5748.9748.4342.3637.1833.3729.3929.5127.3323.51

Ca

6.925.65

4.984.75

4.465.147.696.395.115.654.817.615.759.236.47

10.6710.4610.9711.5811.6211.5811.4511.8511.2711.449.348.476.165.565.645.977.156.645.926.46

Sr

0.070.07

0.080.08

0.080.090.090.080.070.070.070.080.090.110.11

0.080.080.080.080.080.080.080.080.080.080.070.080.080.060.070.100.140.090.090.10

Na

481.12494.14

481.67465.53

483.69484.61477.29477.38475.49369.92485.02496.98517.22489.76536.41

457.77445.17462.93481.88473.25461.89465.33466.65466.23449.89456.93468.62478.15486.54473.05472.34474.13481.69485.05488.62

K

13.3612.95

12.2511.78

12.6912.8812.4011.9111.6011.6911.1510.9712.2111.8113.00

13.1813.1013.5913.0613.2511.8912.3812.8612.7611.4311.3610.859.53

10.0010.8310.6910.5110.169.99

10.09

Li

0.010.01

0.010.01

0.010.010.010.010.010.020.010.020.030.040.05

0.020.020.020.020.020.020.020.020.020.030.020.030.030.030.050.070.090.080.090.09

SO4

12.7312.13

5.365.41

5.620.006.257.183.856.35

10.1010.3113.3321.8628.00

27.0726.9626.5526.8627.9025.0525.1924.9924.1918.2215.8215.829.37

12.104.504.270.000.000.000.00

Silica

0.630.66

0.600.60

0.550.430.550.540.610.470.600.600.590.680.67

0.850.921.130.990.940.520.800.890.930.841.021.050.491.000.990.780.770.770.560.74

Note: pH, salinity, chlorinity, and alkalinity data are from site chapters for Sites 582 and 584, this volume. Concentrations of elements and silica are inmmoles/1.

Dissolved Sr remains constant in the upper 200 m ofsediments, reaches a minimum concentration at about300 m, then increases to 594 m (Fig. 6D).

The dissolved Li concentration is low and relativelyconstant at 0.02 mmoles/1 in the upper 300 m of sedi-ment, but then increases steadily to about 0.08 mmoles/1by 450 m, remaining nearly constant at this concentra-tion to a total depth of 594 m (Fig. 6E).

Although dissolved silica analyses show a scatteraround 0.8 mmoles/1, there is an overall decrease withdepth (Fig. 6F).

Sediment Chemistry

Sediments remain low and nearly constant in theirMg/Al, and Sr/Al ratios throughout the entire sedimentsequence (Fig. 7E, 7D). Si/Al, Ca/Al, Na/Al and K/Alratios generally decrease with depth (Fig. 7A, 7C, 7E,7F). Li/Al ratios are scattered (Fig. 7G), and Zn/Al ra-

tios increase with depth by nearly a factor of 3 (Fig. 7H).Other elements show no marked depth-related trends.

Sediment Mineralogy

Smear-slide observations indicate that diatom concen-trations are high (60%) in the upper 200 m of the sedi-ments, but decrease gradually to 10% at total depth(Fig. 8).

Other biogenic silica microfossils (such as sponge spi-cules, radiolarians, and silicoflagellates) are common atSite 584, but biogenic carbonate is scarce.

Quartz and feldspar concentrations gradually increasewith depth to 30%. Also, clay concentrations graduallyincrease, despite considerable scatter within the data. Clayminerals present include kaolinite, illite, and smectite.

Although pyrite and other opaque minerals are foundin all the sections, the pyrite concentration is variablefrom 1 to 6%.

868

SEDIMENTS AND INTERSTITIAL WATER

o

i 1 1 1 1 1 1 1 1 1 1 1 r

i i i i i

I L I I I I I I I I I I

I I I I I I I I I I

I 1 I I I I

I I I I I I I I I I I I 1

1 1 1 1 1 1 1 1 1 1 1 1

1.0

8 0.8oε 0.603

| 0.4CO

0.2

100 200 300 400 500Sub-bottom depth (m)

- F

-

- d

-

-

i—

I

I

1

1

- i

0

1

1—

0— -

" 1 —

0— • -

1

- 1

0

—

1

T

o

1

~T r

—o

1 1

—i 1 r

0 — o ^

1 1 1

—1

-

O- 0 "

-

:

-

600 700 100 200 300 400 500Sub-bottom depth (m)

600 700

Figure 3. Site 582 interstitial water chemistry. Alkalinity data are from on-board analyses (site chapter, Site 582, this volume). Labels A-F are fortext references.

DISCUSSION

Site 582

Sulfate reduction is accompanied by the productionof secondary pyrite at low temperature, probably owingto reduced sulfur and ferrous iron formation during theproduction of biogenic methane. The sediments have anorganic content between 0.5 and 0.7°7o and hydrocarbongases, mainly methane with very minor CO2, C2, andC3, and traces of H2S are present (site chapter, Site 582,this volume). Pyrite occurring in cores below the sulfate-reducing zone are probably relicts of a time when thosestrata were within 300 m of the seafloor.

High alkalinities are accompanied by low concentra-tions of dissolved sulfate and calcium at Site 582, sug-gesting precipitation of calcium carbonate in relation tosulfate reduction.

Pore-water concentrations of dissolved Mg decreaseby a factor of 0.85 within the first 4.5 m below the sea-water/sediment interface and then decrease by a factorof 0.5 to a depth of 680 m. Although this trend must beexplained in terms of interstitial water interaction withsediments, the Mg/Al in sediments is quite constant. Evi-

dently, no large quantity of Mg ions is added to the sedi-ment phase. Probably the sediments of Unit 1 at Site582 are so young (Quaternary) that the interstitial waterhas not yet reacted with the sediments.

Dissolved K does not indicate a sink in the section,also probably a result of incomplete reaction with theserelatively young sediments.

Site 584Sediments from Sites 438, 439, 440, and 441 of Leg

57 have an average organic carbon content of about 0.8%(Rullkötter et al., 1980; Sato, 1980). Despite the lack ofLeg 87 shipboard analyses of organic carbon, the sedi-ments at Site 584 are estimated to have high organic car-bon content. Sulfate reduction is accompanied by theproduction of pyrite and a large increase in alkalinity,which is responsible for the precipitation of calcium car-bonate, as reflected in a minimum dissolution calciumat about 300 m.

There are one or more possible reasons for the in-crease in dissolved Sr with depth: (1) carbonate recrys-tallization processes; (2) alteration of volcanic matter;and (3) ion exchange and reaction between the intersti-

869

H. KAWAHATA, K. FUJIOKA, T. ISHIZUKA

Table 2. Chemical analyses of sediments, Sites 582 and 584.

Core-Section(interval in cm)

Hole 582

1-3, 143-1503-5, 120-130

Hole 582A

1-5, 0-102-5, 0-10

Hole 582B

2-2, 140-15014-5, 138-15018-5, 140-15023-1, 140-15030-5, 138-15033-4, 140-15050-3, 135-15054-2, 135-15066-2, 135-150

Hole 584

1-4, 140-1502-3, 140-1503-3, 140-1505-5, 140-1506-2, 140-1507-1, 140-1509-1, 140-15010-3, 140-15012-5, 140-15016-2, 135-15019-6, 135-15022-3, 135-15027-4, 135-15032-2, 135-15037-5, 135-15042-4, 135-15047-3, 135-15051-2, 135-15057-3, 135-15062-1, 135-150

SiO 2

59.5961.39

67.6159.80

59.0557.9058.8560.9760.1165.9562.2861.1859.57

66.2970.4271.7868.7166.2571.2172.4468.8168.7471.8971.9971.8174.6269.7967.1066.3269.1766.7865.8163.51

A1 2O 3

16.1315.20

13.7716.08

16.0916.3315.8116.3215.7814.3516.1016.3316.84

8.577.767.786.99

10.547.837.059.428.696.836.246.406.579.79

11.3011.7710.5212.2412.8014.37

FeO*

6.085.61

4.516.14

6.326.696.205.986.004.815.947.236.46

3.052.832.792.414.412.562.423.493.202.652.252.262.443.684.244.604.054.745.316.05

MgO

2.922.46

1.972.72

2.723.002.662.562.512.032.622.942.84

1.400.991.261.111.171.201.211.551.511.131.101.101.051.651.921.971.752.302.512.79

CaO

1.411.92

1.951.84

2.232.352.521.322.032.301.210.981.77

3.932.390.990.642.570.700.631.200.990.731.501.020.720.780.960.820.530.891.141.46

N a 2 O

3.493.10

2.752.92

2.892.982.992.922.773.022.852.852.50

2.962.933.032.863.173.283.043.042.862.942.662.692.502.862.973.012.802.862.783.06 :

K2O

2.882.82

2.512.94

2.912.752.622.932.832.342.902.972.99

.591.501.31.22.11.55.31.58.62.58.37.40.28.49.83.94.80.92.94

2.13

TiO 2

0.720.68

0.560.74

0.740.740.710.710.720.570.760.740.76

0.320.260.290.240.360.270.260.380.340.230.210.220.220.390.450.490.420.580.620.72

P2θ5

0.130.14

0.100.15

0.150.180.170.140:i50.110.150.120.11

0.060.030.020.010.040.020.010.040.040.020.020.010.020.040.050.060.040.070.070.11

MnO

0.090.10

0.090.10

0.110.100.100.080.100.080.090.100.10

0.040.040.040.030.060.040.030.040.040.030.030.030.030.050.050.160.050.060.060.10

Li

77.061.4

52.768.0

66.074.953.761.860.441.058.361.571.0

48.645.234.034.429.844.244.040.743.247.553.352.840.437.243.249.452.847.255.254.1

Sr

149168

176162

174190196153165191147140156

18213310085

1248686

11510078

1029182

106121122102129141160

Co

13.0116.24

9.9717.29

18.3121.4516.3416.5016.0811.4817.3419.3818.37

5.963.734.570.725.830.862.687.168.505.656.156.264.706.309.559.309.47

11.558.54

19.02

Ni

47.2944.98

25.5750.89

48.0351.8944.1948.2652.3345.1653.2048.7756.42

26.3026.8324.3320.8819.4323.2521.5030.5836.8825.3831.1127.3826.3830.4651.6249.6233.8741.0033.5775.19

Zn

50.3366.32

53.9565.48

60.9683.9973.0077.4673.9357.03

121.78102.22114.62

19.5818.4021.0415.4626.3419.6915.9426.2432.4724.9117.0521.0815.9833.0353.7758.6045.6271.4373.2585.85

LOI

6.566.58

4.186.57

6.806.987.366.057.014.455.104.556.06

11.7910.8610.7215.7710.3111.3411.6110.4611.9711.9812.6313.0710.559.489.118.858.887.566.965.69

Note: SiO2, Al2θ3, FeO*, MgO, CaO, Na2O, K2O, TiO2, and P 2 θ5 concentrations are expressed in %; other concentrations are in ppm.iron is expressed as FeO*. LOI is loss on ignition (i.e., loss on heating to 1000°C).

indicates that total

tial water and clay minerals. Although it is difficult tospeculate about the processes because of the absence ofSr isotope data at this time, we postulate that the Srsource is associated with the recrystallization reactions.The reason is that the dissolved Ca profile, like the Srprofile, shows a minimum at 300 m sub-bottom. In con-trast, the alkalinity profile shows a maximum at the samedepth.

Dissolved Mg appears to be consumed throughout thesediment column during diagenesis. Identification of new-ly formed silicate minerals is hampered, however, becauseof dilution by similar detrital minerals. At Site 584 de-trital materials are relatively fresh and authigenic calcit-ic and dolomitic concretions occur (Matsumoto et al.,this volume).

Usually decreases in dissolved Mg are associated withthe alteration of volcanic matter in pelagic sediments(Perry et al., 1976). In a continental margin setting, how-ever, where rapid accumulation rates and high organiccarbon contents lead to very high alkalinity values, car-bonate precipitation and dolomitization may be the causeof the observed magnesium depletions. But Mg-rich and

Fe-rich carbonate concretions are found in discreet depthintervals. Another possibility is that dissolved Mg deple-tion results from ion exchange and reaction between in-terstitial water and clay minerals.

The distribution of pore-water Na values is reflectedin the sediment chemistry. Dissolved Na concentrationsincrease with depth, whereas the Na/Al ratio decreasesin the sediment column. Despite no clear evidence, theformation of secondary minerals probably controls thistrend.

RELATIONS BETWEEN INTERSTITIAL WATERAND SEDIMENTS

Dissolved Silica Content

Dissolved silica analyses for Site 584 are scatteredaround 0.8 mmoles/1, a typical value for sediment con-taining both clay minerals and biogenic silica. Diatomfrustules occur throughout, even at 590 m (the deepestinterstitial water sample). We found no secondary silicaminerals at this site, perhaps because of the low geo-thermal gradient.

870

SEDIMENTS AND INTERSTITIAL WATER

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and 7). Although increases in dissolved Li concentra-tions may be related to its release from clay minerals,there is another related possibility. Dewatering structuresare observed below Unit 4 at Site 584 (site chapter Site584, this volume). At Site 582 dewatering veinlets arenot common, but faults and fissures exist and their per-meability is much lower than that of normal sediments.The high and constant Li concentrations may be relatedto advection that results from dewatering processes andfluid transport (Kinoshita et al., this volume).

SUMMARY AND CONCLUSIONS

Both the Nankai Trough and the Japan Trench wereexplored during Leg 87. Site 582 is on the floor of theNankai Trough and Site 584 is situated on the deep-seaterrace of the Japan Trench. Our investigation of thechemistry of the interstitial water and sediments, as wellas the sediment mineralogy, reveals that:

1. Sulfate reduction is accompanied by the produc-tion of secondary pyrite, which is rich in the sedimenta-ry column at both sites.

2. Dissolved Ca concentrations are relatively low andchange slightly at both sites, probably because of theformation of carbonate with high alkalinity.

3. Concentrations of dissolved Mg decrease with depthat Site 584. Dissolved Mg depletion probably results fromthe formation of Mg-rich carbonate and/or ion exchangeand reaction between interstitial water and clay minerals.

4. Higher Si/Al values caused by biogenic opal inthe sediments correlate roughly with higher values of in-terstitial water SiO2.

5. Increases in dissolved Li concentrations may be re-lated to its release from clay minerals and/or to advec-tion that results from dewatering processes and/or fluidtransport.

ACKNOWLEDGMENTS

We wish to thank Drs. M. Nohara and S. Wakamiya for reviewingthe manuscript. The ion-coupled plasma measurements were carriedout in the laboratory of associate Professor N. Notsu at Tsukuba Uni-versity. All figures showing concentrations with depth were plotted bythe GEOCAPS program (Yoshii and Sato, 1983) in the laboratory ofDr. Yoshii at the Geological Survey of Japan. We are also grateful tothem.

REFERENCES

Karig, D. E., Kagami, H., and DSDP Leg 87 Scientific Party, 1983.Varied responses to subduction in Nankai Trough and JapanTrench fore arcs. Nature, 304(5922): 148-151.

Lawrence, J. R., Gieskes, J. M., and Broecker, W. S., 1975. Oxygenisotope and cation composition of DSDP pore waters and the al-teration of layer 2 basalts. Earth Planet. Sci. Lett., 27:1-10.

Leg 87 Scientific Party, 1983. Leg 87 drills off Honshu and SW Japan.Geotimes, 28(1): 15-18.

Matsuii, Y., 1963. Analytical and geochemical investigation of vol-canic rocks. Pap. Inst. Therm. Spring Res. Okayama Univ., 32:1-85(in Japanese).

Moore, G. W., Gieskes, J. M., 1980. Interaction between sediment andinterstitial water near the Japan trench, Leg 57, Deep Sea DrillingProject. In Scientific Party, Init. Repts. DSDP, 56, 57, Pt. 2:Washington (U.S. Govt. Printing Office), 1269-1276.

Notsu, K., 1980. Analytical method of ICP for geochemistry. Kagakuno Ryoiki, 127:217-230 (in Japanese).

Perry, E. A., Gieskes, J. M., and Lawrence, J. R., 1976. Mg, Ca, and18Q/16Q exchange in the sediment-pore water system, Hole 149,DSDP. Geochim. Cosmochim. Acta, 40:413-423.

Rullkötter, J., Cornford, C , Flekken, P., and Welte, D. H., 1980. Or-ganic geochemistry of sediments cored during Deep Sea DrillingProject Legs 56 and 57, Japan Trench: organic petrography and ex-tractable hydrocarbons. In Scientific Party, Init. Repts. DSDP, 56,57, Pt. 2: Washington (U.S. Govt. Printing Office), 1291-1304.

Sato, S., 1980. Diagenetic alteration of organic matter in Leg 57 sedi-ments, Deep Sea Drilling Project. In Scientific Party, Init. Repts.DSDP, 56, 57, Pt. 2: Washington (U.S. Govt. Printing Office)1305-1312.

Yoshii, M., and Sato, T., 1983. An outline of GEOCAPS, a geochemi-cal data analysis program system in Basic. Jpn. Soc. Geol. DataProcessing, 8:21-40. (in Japanese with English abstract)

Date of Initial Receipt: 6 July 1984Date of Acceptance: 30 November 1984

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