31. THE CHEMISTRY OF INTERSTITIAL WATERS, DEEP SEA DRILLING PROJECT LEG 861

Russell E. McDuff, University of Washington2

ABSTRACT

Interstitial water samples from Leg 86 were analyzed for Cl ~, SO 4

2 ~, C a 2 + , Mg2 + , K + , L i + , Sr2 + , M n 2 + , Si,N O 3 ~ , N H 4

+ , and δ 1 8 θ . The principal findings are as follows. (1) The chloride profiles consistently show a ~ 1 . 5 %downcore increase to about 40 m sub-bottom depth. This increase is consistent with model profiles generated by consid-ering the effects of variations in seawater salinity resulting from Pleistocene glaciation on pore-water composition. δ 1 8 θprofiles support this model. This agreement of model and observation precludes advection of the pore fluids at ratesgreater than 1 mm/yr. (2) C a 2 + and Mg 2 + gradients are small relative to other Deep Sea Drilling Project sites, likelyresulting from low diffusive fluxes through underlying chert horizons. (3) Other constituents exhibit profiles consistentwith previous observations.

INTRODUCTION

Under what conditions does advection of pore fluidsin deep-sea sediments occur? Flow has been convincing-ly documented in young ridge-flank sediments by usingvarious thermal and chemical tracers (Maris and Bend-er, 1982; Sayles and Jenkins, 1982). Anderson et al.(1979) proposed on the basis of thermal data that flowoccurs even in very old sediments. The global distribu-tion of chemical anomalies in Deep Sea Drilling Project(DSDP) cores is consistent with a model in which flow isactive until the upper basalts are capped by a sufficientthickness (~ 100 m) of relatively impermeable sediment(McDuff, 1981).

With the question of advection in mind, we analyzedpore fluids sampled during Leg 86 of DSDP in the north-west Pacific. We investigated the chemistry of the sedi-ments at sites of similar crustal age but of varying sedi-ment type and thickness, in order to refine the model ofMcDuff (1981). The geologic settings of the sites occu-pied are summarized in Table 1. All sites except Site 577are below the carbonate-compensation depth (CCD) andconsist mainly of pelagic clays, siliceous clays, and sili-ceous oozes (see Site chapters, this volume). Site 577 con-sists of nannofossil oozes. Sedimentation rates vary wide-ly between the sites and increased with the onset of Pleis-tocene glaciation. Leg 86 samples were attractive for twoother reasons. First, if flow occurs, one would expectrather small chemical anomalies because of the limitedtime available for chemical reaction to take place. It isdifficult to document these small anomalies in rotary drillcores because of contamination by surface seawater, thedrilling fluid. The samples from Leg 86 were collectedwith a hydraulic piston corer (HPC). Second, at Sites576 and 578, multiple complete sequences were cored byHPC. These sequences provided sufficient material toallow much more closely spaced sampling (~ every 10 m)

Heath, G. R.( Burckle, L. H., et al., Init. Repts. DSDP, 86: Washington (U.S. Govt.Printing Office).

2 Address: School of Oceanography, University of Washington, Seattle, WA 98195.

than the 30- to 50-m spacing more typical in DSDP drillholes.

METHODS

Samples were collected aboard the Glomar Challenger during Leg86 by expressing pore fluids in a Manheim-type stainless steel squeez-er. No special precautions were taken with respect to temperature ofsqueezing artifacts (Sayles and Manheim, 1975). Three samples weretaken at Site 576 with the Barnes in situ sampler; however, these sam-ples were excessively contaminated with seawater and are not consid-ered further.

Samples were analyzed aboard ship for alkalinity, Cl, Ca, Mg, sa-linity, and pH following the standard protocol (see Introduction andExplanatory Notes, this volume). In our laboratory we repeated theanalyses of Cl, Ca, and Mg. In addition, we determined K, Li, Sr, Mn,S O 4

2 - , Si, N O 3 - , and N H 4

+ and, for Sites 576 and 578, δ 1 8 θ of thewater. The analytical methods used are summarized in Table 2.

The method for chloride analysis requires some explanation. Chlo-ride was determined by titration with AgNO3 with electrochemical end-point detection. The electrochemical cell used was Ag,AgCl| Ag+ |I Ag + (0.165 M) I Ag(s). The Ag.AgCl electrode is placed in the sampleand the Ag wire electrode is imbedded in the reservoir of a micrometerburet. The liquid junction is established across the buret tip, which issubmersed in the sample, with Ag+ controlled by the titrant strengthon one side of the junction and by the progress of the titration on theother. The potential of the cell is balanced by means of a potentiome-ter. The course of the titration is followed in terms of ΔmV/Δ volumeversus volume added. Control of quantities is the ultimate limit to pre-cision; the 0.2-g sample is gravimetrically controlled to ± 0.0001 g andthe delivery of ~0.7 ml titrant is known to ±0.0002 ml. Precisions of± 0 . 0 5 % (lσ) are routinely obtained. The titer is determined by titra-tion of IAPSO standard seawater.

The shipboard (Site chapters, this volume) and shore-based chlo-ride and calcium determinations agreed within the precision of the me-thods employed. Our shore-based magnesium data are offset + 0 . 9 %relative to the shipboard values, which have been converted to gravi-metric units assuming a density of 1.022 g/cm3. The reason for theoffset is not readily apparent. The correction equations of Gieskes andLawrence (1976) for reducing Ca and Mg data have not been appliedto our data. The uncertainty in the correction is large for the small dif-ferences that exist in these samples relative to the composition of thestandard. We found that for standards containing 46.0 mmol/kg Mgand 15.0 mmol/kg Ca we obtain better accuracy using the IAPSO titerthan if we had applied the Gieskes and Lawrence correction. However,the correction is small relative to the observed offset and is of thewrong sign.

RESULTS AND DISCUSSION

Shore-based data are summarized in Table 3 and shownfor Sites 576-580 in Figures 1-5.

675

R. E. McDUFF

Table 1. Geologic settings, Leg 86 sites.

Site Location

Waterdepth

(m) Sediments

Pliocene-Pleistocenesedimentation

rate(m/m.y.)

Cenozoicsedimentation

rate(m/m.y.)

576

577

578

579

580

581

32°21.4'N,164°16.5'E

32°26.5'N,157°43.4'E

33°55.6'N,151°37.7'E

38°37.7'N,153°5O.3'E

41°37.5'N,153°56.6'E43°55.6'N,159°47.8'E

6217

2675

6010

5737

5375

5476

Cenozoic pelagic clayCretaceous carbonate turbi-

dite/pelagic clayCenozoic and Cretaceous

nannofossil ooze (Mio-cene hiatus)

Pleistocene siliceous clayPliocene siliceous ooze, clayMiocene clayPaleogene pelagic clayMiocene-Pliocene-Pleistocene

siliceous clay and ooze,with ash layers, pyritezones

Pliocene-Pleistocene siliceousclay and ooze

Washed to 181 mPliocene-Miocene siliceous

clay and ooze overlyingchert

Basaltic basement at 344 m

7

13

27

34

48

65

0.9

1.8

3

Not reached

Not reached

4

Table 2. Methods used in Leg 86 interstitial water analyses.

Component Method Reference

ci-S θ 4 2 ~

Ca 2 +

Mg 2 +

K +

Li +

Sr2 +

Mn 2 +

Si

NOfNH^f"5 l 8 O

Titration, electrometnc end pointTitration

Titration

Titration

Atomic absorption

Atomic absorption

Atomic absorption

Atomic absorption

Colorimetric

Colorimetric

Colorimetric

Mass spectrometry

Methods section, this chapterGieskes, 1974

Gieskes and Lawrence, 1976

Gieskes and Lawrence, 1976

Gieskes, 1974

Gieskes, 1974

Gieskes, 1974

Gieskes, 1974

Mann and Gieskes, 1975

Gardner et al., 1976

Gieskes, 1974

Epstein and Mayeda, 1953

Chloride

Background

Chloride is normally thought of as a conservative con-stituent of the interstitial waters of marine sediments.Yet, beginning with the onset of distinct glacial-inter-glacial cycles in the late Pliocene-early Pleistocene, sealevel has fluctuated considerably as a consequence of wa-ter storage in continental ice sheets. Concomitantly, meanseawater chlorinity must have changed, perhaps as muchas 5% if one accepts the upper end of the range of esti-mated sea-level variations (-200 m). How would suchchloride fluctuations be recorded in deep-sea sediments?

Model

To model the potential influence of changes in deep-water chloride concentrations on interstitial-water pro-files, we have adopted the following assumptions.

Assumption 1. The history of deep-water chlorinitycan be represented by the pattern shown in Figure 6,which parallels the δ18θ record and assumes chloride is3.7% elevated at maximum glaciation.

Assumption 2. Initially (2 m.y. ago), chloride is con-stant with depth.

Assumption 3. Transport of chloride in the sedimentsis by diffusion alone.

Assumption 4. Chloride is considered to be unreac-tive in the sediment column.

With these assumptions, the mathematical model thatfollows is a simple one:

de

t z2

subject to the initial condition:

c(z, 0) = c0

and the boundary conditions:

c(0,t) = f{t)

c(zh,t) = cb

(1)

(2)

(3)

(4)

where c(z,t) is the chloride concentration, t is time, z issub-bottom depth, and f(t) is defined by Figure 6. Thedepth -zb corresponds to the depth of the sediment sec-tion and is taken as 200 m although any value will do aslong as it is large relative to the depth to which the sig-nal propagates. The parameter cb, the concentration ofchloride at the base of the sediment, is an additionalvariable as moderate increases in chloride are occasion-ally observed in drill cores, apparently supported by aflux from basaltic basement (McDuff, 1978). This set ofequations is solved by a Crank-Nicholson scheme to yieldchloride versus sub-bottom depth curves every 2000 yr.on a 10-m grid.

Plots of chloride versus time at sub-bottom depths of10, 30, and 60 m are shown in Figure 7 for a case with cb

676

CHEMISTRY OF INTERSTITIAL WATERS

Table 3. Interstitial water data, Leg 86.

Sample(interval in cm)

Site 576

576A-1-4, 140-150576A-2-4, 140-150576A-3-5, 130-140576A-4-4, 140-150576A-5-4, 140-150576A-6-5, 130-140576A-7-4, 130-140

Site 577

577-2-5,140-140577-6-4, 140-150577-10-5, 140-150577A-4-5, 140-150577A-8-5, 140-150577A-13-5, 140-150

Site 578

578-2-5, 140-150578-5-6, 140-150578-8-5, 140-150578-11-6, 140-150578-14-3, 140-150578-17-3, 140-150

Site 579

579-2-5, 140-150579A-5-5, 140-150579A-8-5, 140-150579A-11-4, 140-150579A-14-4, 140-150

Site 580

580-2-5, 140-150580-5-5, 140-150580-8-5, 140-150580-11-4, 140-150580-14-5, 140-150580-17-5, 140-150

Site 581

581-5-3, 140-150581-8-4, 140-150

Sub-bottomdepth (m)

6152634435462

1450903674121

12426999

128152

165988

115143

11396895

124153

214244

K +

(mmol/kg)

11.312.011.611.810.712.411.6

12.511.810.011.910.010.1

13.412.411.610.78.79.5

12.411.312.212.211.1

13.213.012.311.911.811.7

12.010.3

Mg2 +(mmol/kg)

51.5751.9051.4451.0151.19

_51.59

51.9452.2753.0652.3552.9052.06

50.4449.2748.76

_49.6347.66

51.0549.5548.5047.8047.64

50.3948.7347.8246.4546.6145.42

47.5848.30

Ca2 +

(mmol/kg)

10.2210.3510.6210.7810.80

10.60

9.7910.3810.459.97

10.8010.78

10.9511.8112.45

_13.9314.59

10.5811.3111.7412.0812.44

10.4010.8411.0611.4211.6511.80

11.4811.58

Sr2 +

(µmol/kg)

90909090909090

115155125125145135

909590909090

9090959090

909090909590

9090

Li +

(µmol/kg)

28323133333737

252322222225

252730394546

2744536371

273035414963

6068

Mn2 +

(µmol/kg)

523000020

1130300

1551302426

235245

13511011092

120

20012097938777

81180

Cl"(mmol/kg)

540.4541.7542.7545.4546.6543.9539.8

543.9545.6541.9544.6543.0540.2

542.6544.7540.9542.8533.4539.0

545.6551.2547.6550.2548.3

540.4546.1547.0549.0546.1546.1

545.3542.1

S O 4

2 "(mmol/kg)

28.928.028.427.427.826.027.1

29.127.727.327.527.727.3

28.428.027.727.225.323.2

27.025.925.224.924.1

27.024.923.822.422.321.

23.922.9

Si(µmol/kg)

450_

260215180_190

185200155195205225

520275430500425215

290410470425535

335385595485500540

670725

(µmol/kg)

500

24603

2627

02720

0

1351601259241

3

160—

265275225

200395475525530515

275160

NO 3 " a

(µmol/kg)

55445558586052

38

11430

10041

1834

50

1396

76847

11

1610

« I 8O(‰)

+ 0.13+ 0.26+ 0.27+ 0.14+ 0.19

0.00+ 0.05

——

—

—

+ 0.13+ 0.11

—-0.43-0.68-0.77

_

————

_

—————

_

—

Note: — means not determined.a NOJ" determinations when NH41" is appreciable are considered unreliable.

set to the present interglacial chloride concentration. Threeregimes are evident. Near the sediment surface (10 m),the forcing function is quite obvious. At intermediatedepths (30 m), the concentration is dampened somewhatand has achieved a value near the mean of the forcingfunction. At greater depth (60 m), the concentration isconsiderably dampened and has a value somewhat be-low the mean value of the forcing function, that is, thesignal has just begun to arrive.

Thus, in a present-day profile, one would expect tofind a maximum at about 30 m depth (Fig. 8). Alsoshown are cases in which cb has been increased and de-creased 10%. With cb increased, at present chloride ispredicted to increase monotonically with depth. With cb

decreased, a subsurface maxium is still predicted; how-ever, the shift relative to bottom water is smaller than inthe case with cb at the present interglacial chloride con-centration.

I emphasize here that, although the signal is causedby glacial-interglacial cycles, it is not stratigraphic. Ra-ther, it has penetrated from the sediment/water inter-face to a depth characteristic of a diffusive process act-ing over ~ 1 m.y.

Observations

The Leg 86 chloride data, normalized to the present-day sediment/seawater interfacial values, are shown inFigure 9. The pattern predicted by the model is evident.Chloride increases by about 1.5% by 40 m depth. Belowthat depth the concentration levels are variable, consis-tent with various cb values.

Isotopic data confirm that this agreement is the resultof glacial-interglacial phenomena. Profiles of δ 1 8θ ofthe pore fluids at Sites 576 and 578 are shown in Figure10. A maxium is apparent in both profiles. It is some-what attenuated at Site 578. This attenuation is causedby the gradient of δ 1 8 θ supporting a flux toward base-ment (Lawrence and Gieskes, 1981), analogous to thelow chloride case shown in Figure 8.

Pore-Water Migration

Steady pore-water movement should displace the peak.Some model curves incorporating an advective term inEquation 1 are shown in Figure 11. The position of thepeak is sensitive to velocities on the order of 0.1 mm/yr., and the existence of the signal at Leg 86 sites sug-

677

R. E. McDUFF

Calcium(mmol/kg)

•

0

I 20

tom

dep

tho

AZ 60

an

3 1 1t 'o

o

o

o

o

o

13 IE

-

-

10

j= 20 -

E 40 "

6 0 -

80

Potassium

(mmol/kg)

11 12 13

t

o

o

o

o

o

o

1

o

Sulfate(mmol/kg)

25 27 29

E 20 -

Chloride(mmol/kg)

535 540 545 550

20

Lithium(µmol/kg)

30 40 0

Manganese(µmol/kg)

20 40 60

Figure 1. Interstitial water data, Site 576. Arrows indicate bottom-water concentration.

gests that advection must be occurring at rates certainlyless than on the order of 1.0 mm/yr.

Historic Data

It is difficult to tell whether other existing DSDP da-ta show the same feature, since most data sets includelow-precision chloride determinations and samplingdepths are generally widely spaced. One approach is toconstruct a composite profile. This has been done forthe subset of sites identified by McDuff (1981) as havinggradients of calcium and magnesium indicative of nofluid advection. Figure 12 shows the data displayed intwo forms. The primary data (Fig. 12A) appear at firstglance to form a shotgun pattern. However, averages overvarious depth intervals (Fig. 12B) show a distinct pat-

tern, which we interpret in the following way. From thesediment/water interface to about 40 m depth there is aclear increase in the mean value. Note the very tightstandard deviation in the 20- to 40-m depth interval andthat no value between 20 and 40 m falls within 1% ofpresent-day bottom water. Below this depth the mean re-mains about constant, however, the standard deviationsgenerally increase. This is consistent with inclusions ofsites with a flux from basement of chloride; that is, aprofile of the mean less standard deviation correspondsto the no basement flux case (Fig. 8) and a profile of themean plus the standard deviation corresponds to the base-ment flux case (Fig. 8). A similar result is found for δ 1 8 θ(Fig. 13), although the subset of sites considered is dif-ferent (those included in table 1 of Lawrence and Gies-

678

CHEMISTRY OF INTERSTITIAL WATERS

Calcium(mmol/kg)

9 11 13 48

Magnesium(mmol/kg)50 52 54

140

-ó 100 "

120 -

140

26

Sulfate(mmol/kg)

28 30 535

Chloride(mmol/kg)

540 545 5500

20

40

60

80

100

120

un

-

Δ

-

-

O

-

Δ

to

-

o-

Δ

-

-

— 4 0 -

C 8 0 -

2100 -

1 2 0 "

140

Manganese(µmol/kg)

10

Figure 2. Interstitial water data, Site 577. Symbols: diamond = Hole 577, triangle = Hole 577A. Arrowsindicate bottom-water concentration.

kes, 1981). A more comprehensive synthesis, includingthe implications for excess ice volume-δ1 8θ coupling, isbeing prepared.

Calcium and Magnesium

The gradients of calcium and magnesium encounteredduring Leg 86 are small relative to the range observed inDSDP cores. Site 578 exhibits the largest value, -0.03mmol/m, which can be contrasted with gradients as highas 0.2 mmol/m at other DSDP sites (McDuff, 1981).

Changes in the concentrations of calcium and mag-nesium with sub-bottom depth are generally indicativeof reaction of volcanic material, either basement basaltsor volcanic ash that is dispersed within the sediment col-umn (Gieskes and Lawrence, 1981; McDuff, 1981). Atthe three sites with measurable gradients (Sites 578-580,

Site 581 is excluded because of lack of data), Ca and Mgare linearly correlated, a feature usually resulting fromconservative behavior of calcium and magnesium withinthe sediment column and gradients supported by diffu-sive fluxes from Layer 2 (McDuff and Gieskes, 1976;McDuff, 1978).

McDuff (1981) has presented a model for control ofCa and Mg gradients. To summarize, Ca is added to andMg removed from the pore fluids of the basalts of Layer2 by chemical reaction and lost or gained, respectively,by exchange with seawater. The thickness of sedimentaccumulated is critical in determining the mechanism ofthis exchange. One regime is characterized by thin sedi-ment cover and lack of a Ca gradient. Two possible mech-anisms can be imagined. One, previously presented, isthat circulation of seawater through the sediment con-

679

R. E. McDUFF

Calcium

(mmol/kg)

11 13 15

Magnesium(mmol/kg)

49 51 53

160

120 -

160

E 40 -

Chloride(mmol/kg)

530 535 540 545 5500

160 160

20

E 40 "

Lithium(µmol/kg)

30 40 50 0 50

Manganese(jumol/kg)

100 150 200 250

160 160

Figure 3. Interstitial water data, Site 578. Arrows indicate bottom-water concentration.

tinues at rates sufficient to obliterate gradients. There isstrong evidence that this occurs in very thin sedimentsections (Maris and Bender, 1982; Sayles and Jenkins,1982). Although a moderately thin sediment cover maybe sufficiently impermeable to stop fluid movement, ano-ther possible mechanism is that Layer 2 pore fluids maycontinue to be exchanged with seawater at basement out-crops. The profile of Ca in the sediment pore waterswould then be controlled by diffusion between two sea-water-dominated end members. The other regime is cha-racterized by thicker sediments and the presence of Cagradients. Transport through the sediment column is bydiffusion (McDuff and Gieskes, 1976). Oxygen-isotopemass balances (Lawrence and Gieskes, 1981) and physi-cal factors (McDuff, 1981) suggest that convection with-in Layer 2 continues to support these diffusive fluxes.

Some aspects of Leg 86 results are difficult to inter-pret within this framework. Relevant parameters are sum-

marized in Table 4. Drilling generally penetrated a smallfraction of the sediment cover inferred from seismic re-sults, with the exception of Site 579. The seismic resultscommonly contain evidence of chert horizons, sampledat several of the sites. The very low rate of diffusion ofsolutes through these horizons makes meaningful com-parison of the observed fluxes to DSDP sites difficult.To some extent the concentration of solute in deeper lay-ers will increase in response to the reduced transportthrough chert horizons stabilizing the flux; however, sol-ubility prevents this concentration feedback from occur-ring on a compensating basis. The net result is a lowergradient in the upper sediments relative to sites withoutsilicification. A good example of this phenomenon isthe Ca profile at Site 356 (McDuff, 1978).

The chemistry observed at Leg 86 sites is quite similarto that of other DSDP sites. Figure 14 shows the valuesof ΔCa/ΔMg as a function of the Ca gradient and, for

680

CHEMISTRY OF INTERSTITIAL WATERS

Magnesium

(mmol/kg)

47 49 51

160

ε 40 •

160

120 "

160

Potassium

(mmol/kg)

11 12 13

Sulfate(mmol/kg)

24 25 26 27 28 29

160

560

160

Lithium(µmol/kg)

20 40 60

Manganese(µmol/kg)

0 40 80 120 160

160 160

Figure 4. Interstitial water data, Site 579. Symbols: diamond = Hole 579, triangle = Hole 579A. Arrowsindicate bottom-water concentration.

comparison, the linear relation found for Legs 1-53 (Mc-Duff, 1981). At the extremely low fluxes characteristicof Leg 86 sites, Ca increases are much smaller than Mgdecreases. This is consistent with the concept that, atlow rates of basalt hydrolysis, magnesium must be sup-plied to counterbalance release of both excess calciumand sodium (McDuff, 1981).

Redox Species

The discussion of redox species is necessarily limitedsince shipboard participation would be required to pro-duce reliable data for the full suite of important redoxspecies. Our results include measurement of NO 3 ~,N H 4

+ , M n 2 + , and SO 4

2~. Some artifacts may be intro-duced by analyzing for NO 3~ and N H 4

+ approximately1 yr. after sample collection. There is no visual evidence

for extensive biological uptake in the samples; however,some bacterial oxidation of N H 4

+ present at several hun-dred micromoles per kilogram in many of the samples isthe probable source of non-zero NO 3~ determinations.

Recently there has been much progress in understand-ing the sequence of redox reactions that occurs in theupper tens of centimeters of marine sediments (Froelichet al., 1979; Jahnke et al., 1982). This sequence is drivenby the relatively constant flux of organic carbon to thesediments. Generally the utilization of terminal electronacceptors follows the order of free energy that can bederived, that is O 2 before NO 3 ~, before or concurrentwith MnO2, before FeOOH, before S O 4

2 " , before CO 2 .At high sedimentation rates, the complete sequence canbe found in a short core, whereas at lower sedimenta-tion rates O 2 may be present even to the base of the sam-

681

R. E. McDUFF

£ 40 -

3> 1 2 0 -

160

ε 40 -

Magnesium(mmol/kg)

47 49 51

eo 120 -

160

Potassium(mmol/kg)

12 14

160

Sulfate(mmol/kg)

21 23 25 27 29

Chloride(mmol/kg)

535 540 545 5500

J 40 -

160

120 -

160

20

Lithium(µmol/kg)

40 60 80 0 50

Manganese(µmol/kg)

100 150 200 250

160 160

Figure 5. Interstitial water data, Site 580. Arrows indicate bottom-water concentration.

pled interval. In cores of similar sedimentation rate, sec-ondary processes such as bioturbation can play an im-portant role (Emerson et al., 1985).

The rate of sedimentation, and proportionally the sup-ply of organic material, varies significantly in drill cores.Accordingly, while this sequence of utilization will befollowed at a particular horizon, historic variations insedimentation rate will influence the level within the se-quence attained. At most Leg 86 sites, sedimentation hasincreased dramatically in Pliocene-Pleistocene times, asindicated in Table 1. Thus, one would expect tne great-est progress through the sequence in the youngest sedi-ments. An additional factor in older sediments is diffu-sion from either above or below, which may modify thesignal. Other factors may also be of importance, espe-cially for manganese. Gieskes (1981) noted a dissolvedmanganese-dissolved silica correlation which he attrib-

utes to either association of Mn-oxides with siliceous ma-terial or availability of organic carbon in siliceous hori-zons. Boyle (1983) discussed the role of carbonate coat-ings in Mn cycling, especially in immobilizing M n 2 +

released during suboxic diagenesis.Most features of the Leg 86 results can be understood

in the above context. (1) NO 3 ~ is enriched relative tobottom seawater in Site 576 sediments, a sign of oxicdiagenesis and consistent with the slow rate of sedimen-tation. At all other sites NO 3~ is depleted in the upper-most sediments. Because of the high N H 4

+ concentra-tion at deeper levels, it would be hazardous to assignsignificance to other results except to note that concen-trations are probably very low in situ. (2) Mn 2 + is strong-ly enriched in the upper sediments at all sites except Site577. This is consistent with the suggestion of Boyle (1983)that carbonates can serve to immobilize M n 2 + . At deeper

682

CHEMISTRY OF INTERSTITIAL WATERS

565 560 r

5253 2Time (m.y. ago)

Figure 6. Chloride as a function of time used in model calculations.The curve parallels δ 1 8 θ as a function of time assuming that chlo-ride is elevated 3.7% at maximum glaciation (~ 150 m of sea-levelchange). The δ 1 8 θ record used is a composite of the smoothedstacked record of SPECMAP (Imbrie et al., 1984) for the interval0-0.8 m.y. ago and the measurements of Shackleton and Cita(1979) for the interval 5.0-0.8 m.y. ago. The Shackleton and Citameasurements have been converted to the standard deviation scaleusing the mean and standard deviation of their measurements inthe interval 0-0.8 m.y. ago and have been interpolated betweenpoints to give a spacing of 2000 yr. No correction has been made forpossible bottom-water temperature differences, which may be sig-nificant in the early parts of the record. These will have little effecton the computed chloride profiles, however.

levels Mn2+ enrichment is generally lower or absent, theexception being the substantial increase in Mn2 + con-centration at the base of Site 578. Note at this horizonthe very sharp gradient of sulfate. There are few data tosupport the Mn-Si association noted by Gieskes (1981).For example, at Site 578 the lowest Mn2 + concentrationsoccur at the level with the highest Si concentrations. (3)The SO4

2~ profiles are unremarkable. Gradients are gen-erally steeper at sites with higher sedimentation rates.An interesting feature appears in the sulfate profiles atSites 576 and 577. The sedimentation rate is lowest atthese two sites. Sulfate values at both sites are in excessof bottom water in the uppermost sample. This may re-present the same kind of effect as seen in the chloridedata, recognizing that the diffusivity of sulfate is muchlower so that the maximum is found closer to the sedi-ment/water interface and will decay more slowly fromthe latest influence of glacial conditions. This requiresconservative behavior of sulfate in these shallow sedi-ments, consistent with the low sedimentation rate.

Other Constituents

Strontium

Only carbonate-bearing sediments from Site 577 ex-hibit a gradient of Sr, reaching a maximum value ~ 1.8 ×seawater at 50 m depth. This shallow maximum doesnot conform to the models of Gieskes (1981) and Bakeret al. (1982). First, the maximum occurs nearly a factorof ten shallower than the communication length (ratiodiffusion coefficient to sedimentation rate, Gieskes, 1981).To some extent this may be the result of the Pliocene-

530

560

530

560

5303 2Time (m.y. ago)

Figure 7. Model chloride curves versus time at (A) 10 m, (B) 30 m, and(C) 60 m sub-bottom depth computed as described in text. Notethat the magnitude of the shift depends on the assumption madeas to sea level. See caption of Figure 6.

Pleistocene increase in sedimentation rate. Second, theSr/Ca ratio achieved is much below the equilibrium ra-tio of ~5 × seawater for these pore waters (Baker etal., 1982).

LithiumGieskes (1981) noted an association of Li enrichment

with Si enrichment and has commented on the contrast-ing chemistries of Li and K. The data for Sites 578-580generally agree with these ideas. Site 577, with predomi-nantly carbonate sediment, shows no significant Si en-richment and slightly decreasing Li values. Site 576 ex-hibits a much different pattern. Si is significantly ele-vated in the upper sediment with little influence on Li.As depth increases, Si decreases while Li increases. Drill-ing was terminated at a chert horizon, perhaps support-ing a downward flux of Si and exclusion of Li as silicifi-cation proceeds.

ACKNOWLEDGMENTS

I thank Ms. Barbara Paul and Mr. Ron Citterman for assistance inthe laboratory. Dr. Peter Grootes and Dr. Paul Quay of the Quater-nary Isotope Laboratory, University of Washington facilitated the ox-ygen-isotope determinations. Ms. Trudy Rasp and Ms. Shirley. Patter-son prepared versions of the manuscript. This manuscript was re-viewed by James Lawrence and Joris Gieskes. This research was

683

R. E. McDUFF

Chloride (mmol/kg)

520 560 600

I 80 -

120 -

160 -

200

Figure 8. Model chloride curves versus sub-bottom depth at the presenttime for three different values of c b. Note that the magnitude ofthe shift depends on the assumption made as to sea level. See cap-tion of Figure 6.

supported by Sandia National Laboratories (Contracts 26-6612 and47-5696). This is University of Washington School of OceanographyContribution No. 1427.

REFERENCES

Anderson, R. N., Hobart, M. A., and Langseth, M. G., 1979. Geo-thermal convection through oceanic crust and sediments in the In-dian Ocean. Science, 204:828-832.

Baker, P. A., Gieskes, J. M., and Elderfield, H., 1982. The distribu-tion of Sr 2 + during diagensis of pelagic carbonate rocks—isotop-ic, chemical, and experimental evidence. /. Sediment. Petrol., 52:71-82.

Boyle, E. A., 1983. Manganese carbonate overgrowth on foraminiferatests. Geochim. Coschim. Acta, 47:1815-1819.

Emerson, S., Fischer, K., Reimers, C , and Heggie, D., 1985. Organiccarbon dynamics and preservation in deep sea sediments. Deep SeaRes., 32:1-22.

Epstein, S., and Mayeda, T., 1953. Variations in the 1 8O content ofwater from natural sources. Geochim. Cosmochim. Acta, 4:213-224.

Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, N. A.,Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., and Hart-man, B., 1979. Early oxidation of organic matter in pelagic sedi-ments of the eastern equatorial Atlantic: suboxic diagenesis. Geo-chim. Cosmochim. Acta, 43:1075-1090.

Gardner, W. S., Wynne, D. S., and Dunstan, W. M., 1976. Simplifiedprocedure for the manual analysis of nitrate in seawater. Mar.Chem., 5:393-396.

Gieskes, J. M., 1974. Interstitial water studies, Leg 25. In Simpson, E.S. W., Schlich R., et al., Init. Repts. DSDP, 25: Washington (U.S.Govt. Printing Office), 361-394.

, 1981. Deep-sea drilling interstitial water studies: implica-tions for chemical alteration of the oceanic crust, layers I and II.Soc. Econ. Paleontol. Mineral. Spec. Publ, 32:149-167.

Gieskes, J. M., and Lawrence, J. R., 1976. Interstitial water studies,Leg 35. In Hollister, C. D., Craddock, C , et al., Init. Repts.DSDP, 35: Washington (U.S. Govt. Printing Office), 407-424.

, 1981. Alteration of volcanic matter in deep sea sediments:evidence from the chemical composition of deep sea drilling cores.Geochim. Cosmochim. Acta, 45:1687-1703.

Imbrie, J., Hays, J. D., Mclntyre, A., Mix, C , Morley, J. J., Martin-son, D. G., Pisias, N. G., Prell, W. L., and Leinen, M., 1984. Theorbital theory of Pleistocene climate: support from a revised chro-nology of the marine δ 1 8 θ record. In Berger A., Imbrie, J., Hays,J., Kukla, G., and Saltzman, B. (Eds.), Milankovitch and Climate:Dordrecht (Reidel).

Jahnke, R., Emerson, S., and Murray, J. W., 1982. A model of oxygenreduction, denitrification, and organic matter mineralization in ma-rine sediments. Limnol. Oceanogr., 27:610-623.

Lawrence, J. R., and Gieskes, J. M., 1981. Constraints on water trans-port and alteration in the oceanic crust from the isotopic composi-tion of pore water. /. Geophys. Res., 86:7924-7934.

McDuff, R. E., 1978. Conservative behavior of calcium and magne-sium in the interstitial waters of marine sediments: identificationand interpretation [Ph.D. dissert.]. University of California, SanDiego.

, 1981. Major cation gradients in DSDP interstitial waters:the role of diffusive exchange between seawater and upper oceaniccrust. Geochim. Cosmochim. Acta, 45:1705-1713.

McDuff, R. E., and Gieskes, J. M., 1976. Calcium and magnesiumprofiles in DSDP interstitial waters: diffusion or reaction. EarthPlanet. Sci. Lett., 33:1-10.

Mann, R., and Gieskes, J. M., 1975. Interstitial water studies, Leg 28.In Hayes, D. E., Frakes, L. A., et al., Init. Repts. DSDP, 28:Washington (U.S. Govt. Printing Office), 805-814.

Maris, C. R. P., and Bender, M. L., 1982. Upwelling of hydrothermalsolutions through ridge flank sediments shown by pore water pro-files. Science, 216:623-626.

Sayles, F. L., and Jenkins, W. J., 1982. Advection of pore fluids throughsediments in the equatorial east Pacific. Science, 217:245-248.

Sayles, F. L., and Manheim, F. T., 1975. Interstitial solutions and dia-genesis in deeply buried marine sediments: results from the DeepSea Drilling Project. Geochim. Cosmochim. Acta, 39:102-127.

Shackleton, N. J., and Cita, M. B., 1979. Oxygen and carbon isotopestratigraphy of benthic foraminifers at Site 397: detailed history ofclimatic change during the late Neogene. In von Rad, U , Ryan, W.B. F., et al., Init. Repts. DSDP, 47, Pt 1: Washington (U.S. Govt.Printing Office), 433-445.

Date of Initial Receipt: 23 November 1983Date of Acceptance: 13 July 1984

684

CHEMISTRY OF INTERSTITIAL WATERS

5300

Chloride (mmol/kg)540 550 560 δ1 8O{%o)

-0.5 0.0 0.5

160

Figure 9. Composite of chloride versus sub-bottom depth for Leg 86.In accord with the model curves (Fig. 8), chloride increases ~ 1 °7oby 40 m depth before assuming values controlled by diffusion frombelow. Note that the concentration scale is considerably expandedas compared to Figure 8.

160

Figure 10. δ 1 8 θ versus sub-bottom depth for Sites 576 and 578. Sym-bols: diamond = Site 578, triangle = Site 576. Arrow indicatesbottom-water value.

685

R. E. McDUFF

5300

40 "

£ 80 -

ot

JI 120 -

160 -

Chloride (mmol/kg)540 550 560

-

/ /

/

1

(

( \

\

>

\

\

x0.25mm/yr.-

^1.0 mm/yr.

v 5.0 mm/yr. ~

200

Figure 11. Model curves of chloride versus sub-bottom depth at thepresent time for different values of upward advection rate. Veloci-ties of the order 1.0 mm/yr. shift the maximum upward and reduceits magnitude. Higher velocities eliminate the signal entirely.

540

Chloride (mmol/kg)550 560 570 540

Chloride (mmol/kg)550 560 570

200 200

BFigure 12. Composite of chloride versus sub-bottom depth for DSDP sites summarized by McDuff (1981)

(A) Raw data. (B) Means and standard deviations in 10-m depth intervals. Note especially the tightstandard deviation at about 40 m depth and that no value falls below 550 mmol/kg between 20 and 50m depth. Typical deep waters have Cl = 541 mmol/kg (arrow).

686

CHEMISTRY OF INTERSTITIAL WATERS

δ 1 8 θ (%o)

u

100

2

I

-

—

-

_

I

I

I

0

r

-

_

-

Figure 13. Composite of δ 1 8 θ versus sub-bottom depth for DSDP sitessummarized by Lawrence and Gieskes (1981) and expressed as meansand standard deviations in 20-m depth intervals. Note the slightmaximum at 20-40 m depth. Arrow indicates bottom-water value.

Table 4. Parameters relating to Ca and Mg, Leg 86.

Deepestsample ΔCa ΔMg

Site (m) (mmol/kg) (mmol/kg)Ca gradient

• ΔCa/ΔMg (mmol/kg/m)

576577

578

579

580

62121

152

143

153

00

+ 5

+ 2

+ 2

00

-5

-5

-7

——1.00.4

0.3

00

0.03

0.02

0.01

0.00 0.05 0.10

ΔCa/Δz (mmol/kg) /m

0.15 0.20

Figure 14. - ΔCa/ΔMg versus ΔCa/Δz. Points are for sites with gradi-ents, see Table 4. Line is trend for DSDP Legs 1-53 summarized byMcDuff (1981) (sites with gradients, Ca and Mg linearly correlated).

687