20. INTERSTITIAL WATER STUDIES, LEG 631

Joris M. Gieskes, Brad Nevsky, and Aileen Chain, Scripps Institution of Oceanography, La Jolla, California

ABSTRACT

Interstitial water studies of drill sites off the California Continental Borderland (Sites 467, 468, 469) reveal thatfresh water can penetrate through the sediments as far as the San Miguel Gap (Site 467). In the rapidly depositedsediments of Site 467, decreases in magnesium concentrations can be related to the formation of dolomite in theclaystone unit of the sediment column. Decreases in magnesium and increases in calcium at Site 469 at the base of thePatton Escarpment are probably the result of alteration of volcanic matter in the sediments and/or the underlyingbasalts.

Studies of the gradients of calcium, magnesium, potassium, strontium, and lithium at Site 471 indicate that a150-meter-thick porcellanite-claystone-limestone complex acts as an almost complete barrier to diffusive exchange withthe underlying sediments. Uptake reactions involving potassium and magnesium may still occur in the Porcellanite com-plex.

At all sites, with the exception of Site 467, gradients of calcium show concentration increases with depth andsimultaneous decreases in dissolved magnesium—a pattern commonly observed in open-ocean environments.

INTRODUCTION

During Leg 63 of DSDP 11 holes at 7 sites were drilledalong the west coast of southern California and BajaCalifornia. The locations of the sites are presented inFigure 1.

Interstitial waters were obtained in each hole drilled,and the shipboard analyses included pH, alkalinity, cal-cium, magnesium, chloride, and salinity. On the basisof concentration depth profiles obtained by these ship-board analyses we decided to carry out more completestudies on samples obtained at Sites 469, 470, 471, 472,and 473. In addition, sulfate analyses were carried outfor Sites 467 and 468.

Methods are similar to those used by Gieskes (1974),Gieskes and Lawrence (1976), and Gieskes and Johnson(in press a). Shipboard analyses were carried out by Mr.Richard Myers.

The data are presented in Table 1, as well as inFigures 2 through 6.

DISCUSSION

Site 467

Site 467, drilled at the San Miguel Gap in the outerCalifornia Continental Borderland, is characterized byextremely high rates of accumulation during the Quater-nary (~75 m/m.y.) and the Pliocene (100-150 m/m.y.).For these reasons it is not surprising to observe veryhigh values of alkalinity and the complete depletion ofdissolved sulfate below a sub-bottom depth of 50 meters(Fig. 2). Typically the high alkalinity values are accom-panied by a minimum in dissolved calcium, indicatingprecipitation of calcium carbonate (Lawrence, 1973;Gieskes, 1975).

Initial Reports of the Deep Sea Drilling Project, Volume 63.

Shipboard reports indicate the presence of microcrys-talline calcite and dolomite rhombs in Unit 2 (367-699.5m sub-bottom depth), which can explain the rapiddecrease in dissolved magnesium to the top of this unit.Note that no increase in dissolved calcium accompaniesthis sharp decrease in magnesium; also, alkalinity valuesonly start to decrease below 400 meters. Usually de-creases in dissolved magnesium have been associatedwith the alteration of volcanic matter in pelagic sedi-ments (Gieskes and Lawrence, in press; Gieskes, inpress). In a continental margin setting, however, whererapid accumulation rates and high organic carbon con-tents lead to very high alkalinity values, carbonateprecipitation and dolomitization may be the causes ofthe observed magnesium depletions.

The data on dissolved chloride do suggest a trendtoward lower chlorides with depth, which can be under-stood in terms of fresh-water aquifers penetrating intothe continental margin sediments (Manheim and Sayles,1974; Gieskes, in press). Unfortunately no interstitialsamples were obtained between 500 and 800 meters, sothat a minimum in chloride, as suggested by the data,could not be verified.

Sedimentation rates below 500 meters have beenmuch slower. Thus the elevated value of sulfate at 800meters may be real—a result of incomplete sulfatereduction.

Site 468

Site 468 is located on the Patton Escarpment. Ac-cumulation rates during the last 10 m.y. have been low(~5 m/m.y.), and thus diffusive communication withthe overlying ocean is easily established. Provided reac-tions do take place at relatively slow rates, diffusion willannihilate any gradients caused by such reactions(McDuff, 1978). For these reasons no depletion is ob-served in dissolved sulfate, and only a very slight in-crease in dissolved calcium and a small decrease in

623

J. M. GIESKES, B. NEVSKY, A. CHAIN

120°

115°

110°

25°

Figure 1. Map of area drilled by DSDP Leg 63.

dissolved magnesium occur (Fig. 3). The causes of thechanges in calcium and magnesium are difficult toestablish.

Shipboard observations indicate the presence of dol-omitic claystones in Cores 21 to 26, Hole 468 (184-241m), and in Cores 18 to 37, Hole 468B (235-416 m). Sedi-mentation rates below about 60 meters sub-bottomdepth (i.e., in the sediments older than 13 m.y.) havebeen relatively rapid, so that much of this dolomitiza-tion may have occurred during an early stage of deposi-tion similar to that observed in Site 467. During the last13 m.y., however, as a result of slow sedimentationrates and the occurrence of hiatuses, the average sedi-mentation rate has been less than 5 m/m.y., which mayhave allowed the dissipation of any of the gradients, in-cluding that of sulfate. Thus, if in the past, uptake ofmagnesium from pore waters has been important in theformation of these dolomites, this process must at pres-ent have little quantitative significance.

Site 469This site is located at the foot of the Patton Escarp-

ment and is characterized by sedimentation rates ofabout 27 m/m.y. in the upper 90 meters, below whichhiatuses occur to a depth of 170 meters (13 m.y.). Belowthis depth, accumulation rates have been moderatelyfast—at 60 m/m.y.

The dissolved sulfate profile indicates a rapid de-crease in dissolved sulfate to about 20 mM in the first100 meters (i.e., the recently deposited Quaternary toupper Pliocene section) (Fig. 4). The sulfate reductionprocess is also reflected in the maxima in alkalinity andin ammonia.

Below 225 meters, low dissolved silica values reflectthe increased induration of the sediment and the dis-appearance of biogenic opal-A. Increases in dissolvedcalcium and decreases in dissolved magnesium are prob-ably related to reactions taking place in the sediments ofUnit 3 as well as in the underlying basalts. These reac-tions may well be the result of alteration reactions in-volving volcanic material, much like the reactions iden-tified by Kastner and Gieskes (1976), which were associ-ated with a silicification front in Site 323 (Bellings-hausen Abyssal Plain). Additional work on the 18O/16Ocomposition of the interstitial waters as well as that ofthe strontium isotopic composition of dissolved stron-tium (Hawkesworth and Elderfield, 1978) will be usefulto unravel these processes. Dissolved strontium has amaximum at 190 meters (i.e., in the carbonate section ofthis site). However, isotopic studies of this dissolvedstrontium would allow us to distinguish the various con-tributors to dissolved strontium, that is, carbonate re-crystallization versus alteration of volcanic material.

Dissolved potassium does show a decrease withdepth, but data quality does not allow an analysis ofzones where major sinks may occur. Dissolved lithiumhas a source in the upper siliceous section, probablyrelated to release by siliceous tests (Gieskes, in press).

624

INTERSTITIAL WATER STUDIES

pH Alkalinity (meq/l)6 7 8 20 40 0

J 200

3QE 400

600-

800 -

. 1

•

2

3

4

•

f

"j1

• \" \

\\1•

Sulfate(m/W)10 20 30 0

j I l •

Calcium and Magnesium (mM)10 20 30 40 50

Chloride and Salinity (%o)28 30 32 34

J8 19 20 21

-§ 200

faE 400S

600

800u

Vc,2

Figure 2. Interstitial water chemistry, Site 467. (Lithology: 1—clay; 2—claystone; 3—tuff; and 4—claystone.)

pH Alkalinity (meq/l) Sulfate (mM) Calcium and Magnesium (mM)6 7 8 2 4 6 0 10 20 30 0 10 20 30 40 50

£100

f5QE200S

400

• B

Ca Mg'©

iFigure 3. Interstitial water chemistry, Site 468. (Lithology: 1—ooze; 2—claystone. All data are combined; Hole 468B [B] is

only indicated for alkalinity.)

Site 470

This site is very close to the first experimental MoholeSite, east of Guadalupe Island. Analyses of calcium andmagnesium (Table 1) show minor increases in dissolvedcalcium and decreases in dissolved magnesium. Theseobservations are in good agreement with data obtainedat the Mohole Site (Siever et al., 1965), as is shown inFigure 5.

Most other constituents show little change withdepth, and generally the interstitial waters appear littleaffected by reactions in the sediments.

Site 471This site is characterized by a thick 150-meter sec-

tion of porcellanites, clay stones, and dolomites, whichyielded very little core recovery. Above the porcel-lanites, sedimentation rates have been 20 m/m.y. orless, whereas below the Porcellanite zone sedimentationrates have been extremely rapid at more than 250m/m.y.

The most significant observation at this site is thelarge effect exerted by the Porcellanite zone on thevarious pore-water constituents. Low porosities must

625

J. M. GIESKES, B. NEVSKY, A. CHAIN

Sulfate and Ammonia

pH Alkalinity (meq/l) ° 2 0 0 «0(µ/l*NH 4 )7 8 9 2 4 6 15 2 0 25(µ/WSO4) 0

1

2

-

• 3

4

III

\

- II

Silica (µM)400 800

.§100 -

Q)

QE200

I 300

400

• ©

i /• Θ

S O 4 # 0 N H 4

Calcium and Magnesium20 40

Strontium (µM)100 200 300

Lithium and Potassium0 4 8 12 (mJUKI0 20 40 60(µMLi)

100-

200-

300-

400 -

/©'Mg

\\

1 ""ė

•

•\•1

/

KG

o

s y •• Θ

y4Figure 4. Interstitial water chemistry, Site 469. (Lithology: 1—clay; 2—clay and ooze; 3—claystone; 4—diabase; claystone;

basalt.)

have caused low diffusivities, and thus large gradientsoccur across the Porcellanite zone. This is particularlyevident for sulfate, magnesium, strontium, and potas-sium (Fig. 6). Yet the calcium and lithium profilesespecially suggest some communication across thisfairly efficient diffusion barrier. Similar observationswere made at Site 462 of Leg 61 (Gieskes and Johnson,in press b).

Below the Porcellanite zone little change occurs indissolved magnesium, but the gradient in dissolved cal-cium is large and probably is due to reactions in theunderlying basement. Charge balance calculations indi-cate that the calcium flux implied by the calcium gra-dient below the Porcellanite complex is balanced by asodium flux into the basalts, a phenomenon previouslynoticed by McDuff (in press). Similarly, potassium con-centrations seem little affected below the porcellanites.Perhaps significant uptake of both potassium and mag-nesium occur in the Porcellanite zone, but data qualitydoes not allow an analysis of this problem similar to thetreatment given to the Site 462 data (Gieskes and John-son, in press b).

Dissolved lithium appears to increase gradually withdepth, probably partially as a result of lithium release inthe lower sediment column. The source of this lithium isunclear at this stage.

Site 472

Only three analyses were carried out for this site(Table 1). Dissolved calcium data show a slight increasewith depth, whereas dissolved magnesium values de-crease. Dissolved potassium concentrations decreaserapidly with depth in this site. These observations arecommon in open-ocean, pelagic sediments (Gieskes, inpress).

Site 473

This site, on the Rivera plate, south of the TresMarias Islands, is characterized by rapid Quaternarysedimentation rates (60 m/m.y.) and less rapid Pliocenethrough late Miocene sedimentation (33 m/m.y.).

As a result of rapid sedimentation rates dissolved sul-fate values decrease rapidly with depth, and alkalinitydata show a maximum at about 50 meters sub-bottomdepth (Fig. 7). Dissolved ammonia values increase toabove 1 mM. Organic carbon contents lie between1.69% at the top to 0.25% at the bottom of the hole.Sulfate is never completely depleted in this hole, andtypically no gas production has been observed.

Notwithstanding the high sedimentation rates and theproduction of alkalinity, no minimum has been ob-served in dissolved calcium, which increases gradually

626

INTERSTITIAL WATER STUDIES

Table 1. Interstitial water data, Leg 63.

Sample(interval in cm)

Hole 467

2-3, 144-1507-1, 144-15013-5, 144-15018-5, 144-15025-4, 140-15030-2, 140-15036-3, 140-15041-2, 140-15048-1, 140-15054-1, 140-15085-1, 130-150

Hole 468

1-1, 144-1505-4, 144-15010-4, 140-15018-4, 140-150A-2-3, 144-150B-4-1, 144-1509-3, 144-15014-4, 129-13921-2, 140-15025-1, 140-15031-1, 140-150

Hole 469

2-1, 144-1507-5, 144-15012-4, 144-15020A 140-15028-1, 54-6436-2, 0-943-2, 140-150

Hole 470

2-2, 144-1508-2, 144-15012-7, 144-15017-2, 144-150A-l-3, 144-1504-6, 144-1506-2, 140-150

Hole 471

3-3, 144-1508-5, 144-15013-6, 144-15038-1, 138-15044-2, 140-15050-3, 140-15057-4, 140-15063-2, 140-15070-3, 140-15075-2, 140-150

Hole 472

2-4, 144-1507-2, 144-15012-1, 144-150

Hole 473

2-4, 144-1507-2, 144-15013-3, 144-15018-2, 144-15024-1, 144-150

Sub-bottomDepth

(m)

15.563

122158225272329377443500795

23584

1631247

100182260300341

9.062.5

109185255332390

12.570

1131555285

165

23.574

123355412470538592660705

11.556

102

6.551

110156202

pH

7.237.297.057.136.996.947.087.007.036.857.56

7.637.737.447.517.436.916.937.257.647.877.57

7.927.197.067.218.068.057.93

8.067.477.967.477.877.44

8.017.337.267.908.048.267.71—

8.037.95

7.287.487.49

7.737.607.677.637.47

Alk(meq/1)

14.033.730.050.344.039.843.342.837.933.9

1.1

2.482.942.862.083.243.083.734.211.801.411.75

4.675.002.942.850.72—

0.37

3.343.322.892.172.142.80

6.155.314.58—

0.72——

—

2.733.102.63

10.2216.8410.177.956.17

SO4

(raJM)

16.801.4000.21.4

1.01.0

11.0

27.6—

28.627.6————

(15.0)28.027.6

25.020.019.318.921.321.321.9

28.928.529.428.028.428.227.6

24.423.524.0

1.6

—2.2

—

27.928.1—

20.15.89.63.72.4

NH4

(µM)

170308462396363319220

163117584466658

217

2264748

8381190141013971480

SiO2(µM)

480780760

111622215654

337919791900618866618

295960

1129

93

118120

438830718

510495238786577

Ca(mM)

6.364.238.548.569.458.898.508.568.62

10.1122.46

10.1710.4110.4912.3810.4110.3911.5215.9911.8812.2814.74

9.9410.0412.6418.9330.237.941.5

13.4315.3515.2314.6014.4614.90

10.4511.8212.1424.5029.4535.843.650.264.678.1

11.7512.9513.01

10.1010.6112.8214.9316.55

Mg(mM)

50.846.545.544.536.532.026.020.617.015.015.0

50.552.351.851.352.552.152.248.152.053.050.2

50.746.442.635.424.318.419.3

48.349.149.251.048.149.5

49.349.650.211.911.110.311.412.612.98.3

51.551.250.8

49.442.542.637.535.8

Sr(µM)

101160180220170144162

8781

11296

11510485

92110100

630

776804454—

808797

7371849215

Mn(µM)

23.04.6

25.30

16.832.7

8.2

75.213.75.9

17.014.312.66.7

511010

10

137

125.154.222.6

54.543.937.247.738.1

Li(µM)

26394540474556

26282728312628

30.929.638.2

190—294294

340

262626

27478896

102

K(mM)

9.812.510.17.67.04.74.5

12.910.510.89.5

10.910.69.0

11.09.28.2

1.3

1.71.62.4—

7.96.76.2

7.95.13.62.01.2

Cl(‰)

19.1419.1819.1419.3418.9819.0119.0118.8818.6518.2818.28

19.1719.6119.3119.3419.3819.3819.3419.2419.1419.4819.27

19.219.419.319.318.918.919.4

19.2719.3419.4119.5119.5119.44

19.419.619.518.818.818.919.318.919.019.3

19.119.219.1

19.219.319.219.119.1

S(%) Remarks

34.434.434.434.934.134.133.833.633.032.225.6 Contamination from rock saw?

35.236.036.836.635.236.035.835.5 H2S smell?35.536.335.2

35.234.934.434.934.434.434.6

35.535.835.535.5 Basalt contact36.035.835.5 No good—chunks etc.

35.535.535.231.9 Mostly cuttings31.931.932.432.733.634.4

34.235.235.2

34.934.134.133.032.7

Note: — indicates no data available.

with depth. The dissolved magnesium profile indicatesuptake of this element in the sediments, as does thedissolved potassium profile. We postulate that thesechanges in magnesium and potassium are associatedwith the alteration of volcanic matter in these siliceoussediments. Again, studies of the oxygen isotopic compo-sition of the pore waters and the strontium isotopiccomposition of the dissolved strontium should help to

unravel this problem. In Site 474 of Leg 64 a goodcorrelation between changes in magnesium, calcium,and 87Sr/86Sr was found (Gieskes et al., in press).

ACKNOWLEDGMENTS

We thank Richard Myers for his careful analytical work on boardship. This work was supported by NSF Grant OCE 77-24102. Drs.Russell McDuff and Kenneth Pisciotto reviewed the manuscript.

627

J. M. GIESKES, B. NEVSKY, A. CHAIN

Calcium(mM)

10 20 40

Magnesium

{mM)50 60

40 -

E 80

I

1 2 0 -

. λ

A

i

A

*

à

A

4

»

A

A

1

à

>

i l i l

\ {I

4

A

i

A'

-

A

'A

»

A

A ?

•

• = Site 470A = Mohole

i i i i i

160 -

Figure 5. Interstitial water chemistry, Site 470 and Mohole Site(calcium and magnesium).

REFERENCES

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.

, 1975. Chemistry of interstitial waters of marine sediments.Annu. Rev. Earth Planet. Sci., 3:433-453.

., in press. Deep Sea Drilling interstitial water studies: impli-cations for chemical alteration of the oceanic crust, Layers I andII. Spec. Publ. Soc. Econ. Paleontol. Mineral.

Gieskes, J. M., Elderfield, H., Johnson, J., et al., in press. Interstitialwater chemistry and geochemistry of sediments, Leg 64. In Cur-ray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64: Washing-ton (U.S. Govt. Printing Office).

Gieskes, J. M., and Johnson, J., in press a. Interstitial water studies,Leg 59. In Kroenke, L, Scott, R., et al., Init. Repts. DSDP, 59:Washington (U.S. Govt. Printing Office), 627-630.

, in press b. Interstitial water studies, Leg 61. In Larson,R. L., Schlanger, S., et al., Init. Repts. DSDP, 61: Washington(U.S. Govt. Printing Office).

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), 407-423.

, in press. Alteration of volcanic matter in Deep Sea sedi-ments: evidence from the chemical composition of interstitialwaters from Deep Sea Drilling Cores. Geochim. Cosmochim. Acta.

Hawkesworth, C. J., and Elderfield, H., 1978. The strontium isotopecomposition of interstitial waters from Sites 245 and 336 of DSDP.Earth Planet. Sci. Lett., 40:423-432.

Kastner, M., and Gieskes, J. M., 1976. Interstitial water profiles andsites of diagenetic reactions, Leg 35, DSDP, BellingshausenAbyssal Plain. Earth Planet. Sci. Lett., 33:11-20.

Lawrence, J. R., 1973. Interstitial water studies, Leg 15—stable oxy-gen and carbon isotope variations in water, carbonates, and sili-cates from the Venezuela Basin (Site 149) and the Aves Ridge (Site148). In Heezen, B. C , MacGregor, I. D., et al., Init. Repts.DSDP, 20: Washington (U.S. Govt. Printing Office), 891-899.

McDuff, R. E., 1978. Conservative behavior of calcium and mag-nesium in interstitial waters of marine sediments: identificationand interpretation [Ph.D. thesis].,University of California, SanDiego.

, in press. Major cation gradients in DSDP interstitialwaters: the role of diffusive exchange between seawater and theupper oceanic crust. Geochim. Cosmochim. Acta.

Manheim, F. T., and Sayles, F. L., 1974. Composition and origin ofinterstitial waters of marine sediments based on deep sea drillcores. In Goldberg, E. D. (Ed.), The Sea (Vol. 5): New York(Wiley-Interscience), pp. 527-568.

Siever, R., Beck, K. C , and Berner, R. A., 1965. Composition ofinterstitial waters of modern sediments. / . Geoi, 73:39-73.

628

INTERSTITIAL WATER STUDIES

pH Alkalinity (meq/l) Su If ate (mM) Silica (µM)7 8 9 2 4 6 8 0 10 20 30 0 400 800 1200 0

Calcium and Magnesium (mM)20 40 60

- 2 0 0

400

|J? 600

a800

~i 1 1 1—y—r

PMg*• Ca

Strontium (µM)400 800 0

400

600

800

Lithium (µM) Potassium (mM)200 400 0 4 8 12

>

Figure 6. Interstitial water chemistry, Site 471. (Lithology: 1—clay and ooze; 2—Porcellanite; 3—claystone; 4—diabase.)

pH

7 8 9

£100o

j>200 -

300 -

1

-

2

V V V V V

3

"1

Alkalinity {meq/l)5 10 15

Sulfate and Ammonia (mM)0 1 2 3 N H 4

20

Silica(µM)200 400 600

100

3 200 -

3 0 0 -

Calcium and Magnesium (mM)20 40

Lithium (µM)50 100 150 0

Potassium (mM)4 8 12

. \*\• Ca

\ #

* \

/

/OMg

1Θ

\' ' '*\\\\*\

i

• ^

•/•J7

Figure 7. Interstitial water chemistry, Site 473. (Lithology: 1—clay; 2—claystone; 3—diabase; basalt.)

629