Alteration of volcanic matter in deep sea sediments: evidence from the chemical composition of interstitial waters from deep sea drilling cores

Grochvnku PI Cnumochtm#cu Acre Vol. 45. pp. 1687 to 1703. 1981 Prmted in Great Br~tam. All rights reserved

0016-7037~81 101687-17M2.00’0 Copynghc 0 1981 Pergrmon Press Ltd

Alteration of volcanic matter in deep sea sediments: evidence from the chemical composition of interstitial waters from deep sea drilling cores JORISM. GEXE~ knpps

Institution of Oceanography,

University of California, San Diego, La Jolla, CA 92093. U.S.A. and JAMES R. LAWRENCE

Lamont-Doherty

Geological Observatory, Palisades. NY 10964, U.S.A.

(Rewired 26 March 1979: accepted in recised,form 11 May 1981)

Abstract-Six Deep Sea Drilling Project (DSDP) Sites (252. 285, 315, 317, 336, 386) were examined for the chemical composition of the dissolved salts in interstitial waters. the oxygen isotopic composition of the interstitial waters. and the major ion composition of the bulk solid sediments. An examination of the concentration-depth profiles of dissolved calcium, magnesium, potassium. and Hz*‘0 in conjunction with oxygen isotope mass balance calculations confirms the hypothesis that in DSDP pelagic drill sites concentration gradients in Ca. Mg. K. and Hz “0 are largely due lo alteration reactions occurring in the basalts of Layer 2 and to alteration reactions involving volcanic matter dispersed in the sediment column. Oxygen isotope mass balance calculations require substantial alteration of Layer 2 (up to 25% of the upper 1000 m). but only minor exchange of Ca, Mg, and K occurs with the overlying ocean. This implies that alteration reactions in Layer 2 are almost isochemical.

Si;Al ratios of the bulk solid phases. This correlation was noted particularly in sediments rich in biogenic DURING the twelve years of drilling of the Deep Sea carbonate and opaline silica. LAWRENCEet al. (1975), using mass balance conDrilling Project (DSDP), an intensive program of siderations, argued that carbonate diagenesis and sampling and analysis of interstitial waters from the reactions involving opaline silica, particularly reconretrieved cores has been carried out on a routine stitution of opal-A to opal-CT or quartz, could not basis. Several explanations have been offered for the explain the large decreases in 6’*0 of the interstitial observed changes in the concentrations of the major constituents of seawater, focusing attention on diswaters. They suggested that alteration reactions of solved calcium and magnesium (MANHEIM and volcanic rocks of Layer 2, or of volcanic material disSAYLES.1974; SAYLE~ and MANHEIM,1975: LAWRENCE persed in the sediment column, were the most likely et al., 1975: LERMAN,1975, 1977; MANHEIM. 1976; causes for the observed concentration changes. MCDUFF and GIESKES, 1976: DONNELLY and MERStudies of the oxygen isotopic composition of interRILL, 1977). stitial waters and silicates, as well as mineralogical studies (PERRYet al., 1976) indicated that alteration of In their summary of the data collected during the first 23 legs of DSDP. SAYLES and MANHEIM (1975) volcanic material in the lower parts of Site 149 (Venezuela Basin) were responsible for the calcium and argued that increases in calcium and decreases in magnesium concentration, particularly in biogenic magnesium concentration gradients in this site. GIE.WE~ et al. (1975) and MCDUFF and GIESKE~ oozes, can be understood in terms of the formation of (1976) studied the interstitial water chemistry and the high-magnesium calcites and/or dolomite. This would chemistry and mineralogy of the solid phases of Site explain the almost equal changes in calcium and magnesium observed in many sites. In certain cases, how245 (Madagascar Basin). They showed that the ever. particularly in silicate-rich sediments, silicate gradients in calcium and magnesium were essentially reconstitution reactions were also considered to be conservative in nature and could be explained in important. terms of mass transfer, through the sediments, An alternative explanation for the observed concenbetween the underlying basal sediments and/or tration changes was offered by DONNELLY and MER- basalts and the overlying ocean. Our subsequent studies of the oxygen isotopic composition of two RILL(1977), who suggested that magnesium uptake on opaline silica as a result of ion exchange with silanol interstitial water samples again supported the concept groups was responsible for the decreases in dissolved of alteration of volcanic material (6l*O = - l.l”,., at magnesium. Dissolution of calcium carbonate would 60 m: - 2.0”,,, at 240 m). then serve lo buffer any increases in hydrogen ions. KASTNERand GIESKE~(1976) studied the interstitial These authors were led to this conclusion by the water chemistry and mineralogy.of Site 323 (Bellinggenerally close correlation between the MgiAl and shausen Abyssal Plain) and concluded that calcium, INTRODUCYION

1687

J. M. GIESKES and J. R. LAWRENCE

1688

magnesium. and potassium gradients could, in part. be understood in terms of reactions associated with a silicification front in the sediments. These reactions included: dissolution of opaline silica. dissolution of plagioclase feldspar. dissolution of calcareous nanno fossils. formation of smectites and of authigenic feldspars. as well as formation of opal-CT. Subsequent detailed studies of the oxygen isotopic composition of the interstitial waters and of the solid phases again have confirmed these observations (LAWRENCErt crl.. 1979). The detailed studies of the sites discussed above have helped to strengthen the hypothesis that. at least in sites away from areas of rapid sediment accumulation. a uniform mechanism is responsible for the observed gradients in dissolved calcium and magnesium, as well as for the decreases in S”O of interstitial waters in DSDP cores (LAWRENCErt al.. 1975). Alteration reactions, involving volcanic or igneous material underneath or dispersed in the sediment column. would lead to enrichments in dissolved calcium. depletions in dissolved magnesium. and decreases in the J’*O of the Interstitial waters. In order to distinguish between those sites where underlying basement rocks are chiefly responsible for the observed gradients and those in which reactions in the sediment column are of importance. MCDUFF and GESKES ( 1976) developed an advection-diffusion model which included the variation of diffusion coefficients as a function of depth, temperature, and lithology. The model was specifically designed to test whether a concentration-depth profile in a DSDP site could be understood in terms of transport processes between two boundaries: the overlying ocean and the zones of reaction below the sampled section. A number of sites were identified in which no significant contributions by reactions in the sediment column could be detected (MCDUFF, 1978). In a separate contribution MCDUFF (1981) considers the importance of these observations to the chemical exchange between the oceans and Layer 2 of the oceanic crust. Many DSDP sites do show more complex gradients in major cations and 6’“O. e.g. Site 323 in the Bellingshausen Abyssal Plain (LAWRENCE er (I/.. 1979). In such sites it is of importance to understand the relative contributions of reactions taking place in the sediments and those occurring in the basalts of Layer 2 of the oceanic crust. A major aim of the present paper is to investigate this problem using data on the chemical and oxygen isotopic composition of interstitial waters as well as the chemical composition of the solid phases. SOCRCE OF DATA

Interstltlal water data

for Sites 251 and 185 were obtained by methods described by GIESKES (1974). Data are tabulated in Appendi\- I. Interstitial water data for Sites 315 and 317 (GIESKES. 1976). Site 336 (GIESKES er nl.. 1978). and Site 386 (MILLER zr rrl.. 1979) have been published in the ‘lnitlal Reports of the Deep Sea Drilling Project’. Oyy-

gen isotope data ~111 be pubhshed elsewhere (L.\MR~\(-E and GIESKES. 1%Ii. Chemical dnal)ses of rhe sediments were cart-led out at the Grant Institute of Geology. University of Edmburgh. using methods described by HARVEY Y( (I/. (1973). Bulk chemical analysis was carried out on sediments of Sites 257.285. 336. and 386. ds well as for the bottom portlon of Site 317. Samples \\ere dried at I IO C. but the data are not corrected for loss on ~gmtlon. so that H20( c) and CO? contents as well as those of Na,O are not accounted for. In addition. though correctlons were made for pore water contents oi Mg. Ca. and K (via chlortde content). the data are not reported on a salt free basis. Thus the sum of the oxIdes does not total IOO”,,. Our main Interest was m the elemental atomic ratios. and here the data compare favorably with those of other workers (see site descriptionsl. Chemtcal analyses of samples of Sites 3 IS and 3 17 were made on carbonate free samples ldlssolutlon m a sodium acetate-acetic acid buffer at pH = 5). Both XRF analyses (HARVEY sr al.. I9731 as well as wet chermcal analyses usmg the method of DO~SELLY and WALLACE (1976a. b) were carried out. The data are presented in Appendix 3. Treatment with NaAc HAc can, in principle. affect the exchangeable ions in the carbonate free fraction. However. we assume that such effects on relattvely immobile species such as Ti. Al. Si are negligible. and are small for species such as Mg and K. Because of the extremely low noncarbonate fraction in Site 317 it was necessary to dissolve relatively large amounts of sediments. Whether this treatment did have an adverse effect on the analysis 1s not certam. but the oxide totals appear rather low for Sne 317 samples. whereas the data for Site 315 protide acceptable totals. Ratios of Ti Al and Si Al In Site 317. however. are m good agreement with the rattos calculated from the data of D~VNELL~ and WALLACE (1976a). Addittonal data on the soltds were obtained from DONUELLY and WALL.U-E I I976al and from Dr T. W. Donnelly (Site 3861. Data on the mterstltlal water compositions and sedtment chemistry are presented m Fig l-6. Note that all raflos tJ aluminum are atomic ranos.

DESCRIF’TIOh OF SITES ASD INTERSTITIAL WaTER CHEMISTRY

The
Alteration of volcanic matter in deep sea sediments meq/

I

mM 20 25 .

1689

mM

Y)

E I

r

c,

Sulfate

%c

Calcium

8 Ma&sum

mM

mM

F /O

/” htg++

P

I

‘i/Al

Mognesium

Site 252 Fig. 1. lnterstltial water and sediment chemistry. Site 252. Lithology in Table 1. Data with + are from FLEETand KEMPE(1974). Elemental ratios are atomic ratios.

deep-seated reactions were responsible for the observed concentration changes. and diffusion were the only important transport process. linear concentration depth profiles would occur. Therefore. the upward curvature can have two possible causes: (1) upward advection of pore waters occurs from deeper strata: or (2) reactions occur throughout the sediment column. Measurements of surface heat flow in an area a few degrees east of Site 252 have been interpreted by ANDERSONer al. (1979) in terms of convective cells in the sediment column. It is. therefore, possible that upward advection may occur in this site of unusually high porosities (SO’, at 250 m). The small curvature would imply, only low advective velocities. On the other hand, the non-hnear correlation of the changes in dissolved calcium and magnesium (Fig. 7) indicates that transport processes alone cannot account for the observed concentration changes. Thus, notwithstanding the possibility of a contribution of advective transport, reactions occurring in the sediment column must. at least in part, contribute to the observed changes in calcium. magnesium, and strontium. No systematic increase in the Mg/AI ratio is observed in

the bulk solids that indicates substantial uptake of magnesium in the sediments. The Mg/Al ratio closely parallels that of the Ti/Al ratio and may reflect different percentages of volcanic material in the sediments. Changes in 6*“0 correlate well with those in calcium or magnesium (Fig. 8). The data quality, however. does not allow a decision of which correlation is better. Sire 2X3-South Fiji Basin. 26’4gS, 175”4BE This site is characterized by a high volcanic input below 62 m sub-bottom depth (ANDREWSet 41.. 1975). Sedimentation rates have been slow during the last 10Myr ( -2 m/Myr). below whidh (62-135 m) sedimentation rates of 40 m/Myr and below 135 m of as high as loo0 m/Myr have been inferred (ANDREWSet al., 1975). X-ray mineralogical data (ZEMMELS et ol., 1975) indicate smectite as the dominant clay mineral, but also the presence of augite between 0 and 350 m. Clinoptilolite is a major component between 0 and 20m and below 350 m. The volcanic material, with many glass shards in various stages of alteration, has a low Ti/Al ratio and has been described as

J. M.

1690

meq/

and J. R. LAWRENCE

GIESKES

mM

I

mM

0 10 20 30 40 50 60 MQ 0 20 40 60 60 loo 120140 C(j 0 /-----/% &

co o\ 90 Colclum ,9 Magnesium

a

k

Sulfate

‘F

0

Mq

\

mM

oo

0.1

0.2

0

mM

I 2 3 4 5

200

0

40;M *@y

T

E “\ s” K/AI 400 .‘-I- ?

Si/Al

Site FIN. 2. lntersutial

water and sediment

6’5

t-

H,SiO,

285

chemistry. Site 285. Ltthology atomic ratios.

intermediate to acidic m nature. with as a possible ortgm volcanism associated with the Lau Ridge (ANDREWS er ul.. 1975). Shipboard observations indicate the almost complete absence of biogenic silica, so that the Si/AI ratio of about 3.8 and the dissolved silica values of about 650 pmol (Fig. 2) probably represent the chemistry of the volcanic material and the solubility of volcanic glass. Carbonate contents range between 55 and 75”,, in the upper 6 m. but are generally low (6-16”,) below these depths (CAMERON. 19751. The concentration depth profiles of calcium and magnesium are non-linearly correlated (Fig. 7). and uptake reactions of magnesium are largely taking place in the upper portion of Unit IV. The dissolved calcium concentration depth profile indicates that reactions in the underlying basalts are an important source of calcium (MCDUFF. 1981) and that reactions in the sediments contribute to a lesser extent to the calcium Increases. Dissolved potassium decreases rapidly. but no data are available for the lower part of this site. The same is true for dissolved strontium. which shows a small increase with depth. Studies of the “‘Sr:“Sr ratio of dissolved strontium Indicate values of as low as 0.7074 at 250m IH. ELDER-

m Table

I. Elemental

ratios

are

FIELD. personal commumcatlon). values distmctl) lower than of contemporaneous upper Miocene seawater. These lower values can be understood in terms of alteration reactions involvmg strontium release or exchange with volcamc matter (HAWKESWORTH and ELDERFIELD. 1978: LAWRENCE Cf ui.. 1979). Decreases m 3’*0 are well correlated with dissolved calcium (Fig. 8) and thus if calcium has a significant source In the underlying basalts. the latter must also constitute a significant sink for lRO.

SITES Sire 3/j

Line Iltirld

315 AND

317

C/I&I. 1 /O..?O’N. ISX _1/.?4’W

.Thls sate was spot cored to a depth of 7lOm. with less than IO”,, of the drilled section recovered. Below this. the sediment was almost continuously cored to basement. To a depth of 710 m. the sediments are characterized b) nanno-oozes and chalks. with a relatively large radiolarian siliceous component. Rates of deposition have ranged between Z&50 m Myr. Below 710 m limestones. cherts. and claystones characterize the sediment to a depth of 834 m.

1691

Alteration of volcanic matter in deep sea sediments mM

meql I

mM 0

IO 203040

2

4

6

Ma

mM

mM oO

50

6

IO

12

E

?’

I-

BOO

fm

Rnte

Potassium

Ti/AI

I

Mq/Al

I

Magnesium PM

Site 315 Fig. 3. Interstitial water and sediment ~carbonate-free) chemistry. Site 315. Lithoiogy in Table 1. Elemental ratios are atomic ratios. .

with very low water contents in places (SCHLANGER er al., 1976). Estimated accumulation rates for this section are as low as 2 m/Myr. From 844 to 996 m claystones and volcaniclastic sediments. with volcanic glass in various stages of alteration, overlay igneous basaltic flows. The relatively large radiolarian component in the upper 710 m is reflected in the high %/Al ratio as well as the high dissolved silica concentrations (Fig 3). Siliceous material may comprise as much as SO”, of the noncarbonate fraction of these sediments. Carbonate contents range between 88 and 95”” KAMEROK. 1976) and thus biogenic components are the domjnan~ fraction of these sediments. The presence of volcanic material is evident from the reported small amounts of volcanic glass (SCHLANGER ef al., 1976) as well as the sometimes slightly elevated Ti/AI ratios (Fig. 3). The clay minerals (COOKand ZEMMELS.1976) in the upper section consist of illite. smectite and minor amounts of

palygorskite. Of course, only about 8”, of the sediments were recovered and thus extrapolations of the above information to the entire sediment column is questionable. Site 3~7-Munihiki blamer. Ii-(i.i’S. 162 lS.XW The upper 310m of sediments of Site 317 consist of rapidly accumulated nanno oozes and nanno-chalks (5 10 m/Myr). Opaline silica disappears below 250 m as a result of the transformation of biogenic silica to opal-CT and quartz (SCHLANGERet al.. 1976). Carbonate contents are high at 95-993; CaCO, (CAMERON,1976). Between 500 and 6OOm (Unit I1 C: cf. Table I) accumulation rates appear to have been slow at I m!Myr, but this may be partly due to compaction (SCHLANGER et al., 1976). A minimum in porostty of about 30”, centers around 620 m. This is. in part. due to silicification processes and, as a result. digusive communication with underlying sediments should

1692

J. M. GIESKES

and J. R. LAWRENCE

Table 1. Lithoiogic and age informatton on Sites 752. 285. 315. 317. 336 and 386

252 37'3's. 59'14'E 2% 25°49'S,1750aB'E

315 3olo',N,158o32'ji

AGE

LITHOLOGY

SITE

I. Pyrite-rich radiolarian-detrital I;; Radiolarian-detrital clay I: Zeolitlc

clay

clay

iI: III: IVa: IVb: V:

Nanno ooze Nanno ooze; siliceous fossils Glass-shard nanno ooze C-20 CaC03i Sandy slltstone; nannos and glass shards Basalt

I: IIa: 5: c:

Cyclic nanno ooze \ianno foram ooze tianno foram chalk Nanno foram chalk

- radlolarlans

and

chert

III: Claystone. lime packstone; cherts IV: Volcaniclastic sandstones/claystones i: Ferruginous claystones and volcaniclastic sandstones V:: Easalts 317 11°0'S,162016'W

Ia: Nanno

336 7 47'ti

>

and

jleistocene allocene Upper 'liocene l:pper '4locene Lilper Yiocene Y!elstocene-?liocene tipper+liddle Pliocene Lower Miocene Lioper OligoceneEocene Eocene-Upper Cretaceolis Campanian

3irlstocene-

chalk-radlolanans

b: :Ia: 5: c:

:ianno cnalk - radiolarlans gone Foram ooze and chalk; chert :10 recovery Foram chalk and cnerr !palygorsiite

iii:

with K-feldspar) Volcanlclastic sandstones

Jpper Lower

Oligocene Sligocene

Eocene ?aestr:iht1ar: :!banlar Aotlan

(:

Sasalt

IV: 63'21'U

ooze

Pleistocene ',uper '4locene

1, Grey mua, sandy muo. ciay Ii: Olive and dusky yellow mud, clay mudstone clay, cldys:one. III: Red, red-black volcanic rubble IV: Basalts Yarly nanno and foran ooze I;; Zeolitic clays and nanno ooze !II: 'Yolcaniclastic turbidites Siliceous turbidites IVa turbidites b. Calcareous Va: Cherty-claystones mudstones b: Radiolarlan

:

.jIa: Rea

claystones

b: Banded reolltic claystone claysrone 'YII' Greenish-black 1111:

Altered

and

;le:stocene Jpper Sllgocene and jpoer :~ladle Eocene Eocene

/I,

.J*er Pleistocene ,pper-Lower Xiocene 'jllgocene. ?pper Eocene Jpper-$Mldd:e Eocene 'liodle Eocene "ioale-Lower Focene Lower Eocene, lpper Paleocene jpper-Hiadle Yaestrlcntian :oper Cenomanldn .opep Cenomanian, Lower Albian

basalt

be slow. These lower sechments constst of volcanogemc material overlying basaltic basement at 910 m. Shipboard reports indicate a substantial siliceous contrabution to the upper sediments, which is borne out in the high Si’AI ratios (DONNELLY and WALLACE,1976a; Fig. 4). Carbonate-free fractions indicated the presence of green and brown volcanic glass (SPARTAKCHOCHOV.personal communication) and elevated Ti/AI ratios support the contention that volcanic material is probably the most important non-biogenic component of these sediments. X-ray mineralogical data indicate that smectite is the most abundant clay mineral though some illite and quartz represent terriginous components. Sediments at the 590 m level are characterized by assemblage of authigenic quartz (chcrt). palygorskite and K-feldspar (c.f. high K/AI. Fig. 4). The underlying volcanic sediments show Ti/Al ratios representative of materials of basaltic origin (ENGEL er al., 1965). These sediments have undergone hydrothermal alteration (JENKYNS,1976). which can explain the high MgiAl ratios (Fig. 4). This hydrothermal activity presumably took place during the early history of the site. In both Sites 315 and 317 a zone of very low porosity consisting of limestones and cherts overlies a section of volcanogenic sediments (Unit III in Site 315; Unit II C in

Site 317). These low porosity zones act as almost complete diffusion barriers for calcium. magnesium, strontium. potassium, and Hz “0 as IS evident from the concentration-depth profiles. Clearly. some communication occurs across this barrier. but the diffusion barrier 1s quite effective. This allows the consideration of two main reaction zones in these sites: II 1the basalts and volcanogenic sediments overlying basement: and (2) the carbonate-rich sediments above the chert-limestone zones. Data on dissolved calcium, magnesium. potassium and S’*O (Site 317) indicate a strong influence of reactions taking place tn the volcanic section of these sites. On the other hand, curvature in the concentration profiles in the upper carbonate sections also indicates that reactions occur affecting dissolved calcium, magnesium. strontium, and the d’*O of the pore waters, as welt as dissolved sulfate and alkalinity Concentration changes in Site 315 are larger than in Site 317 for these upper sediments. which can be understood in terms of the much higher accumulation rates in Site 315. as discussed by GIESKES(1976). SAYLES and MANHEIM(1975) established a rather close correiation between changes in strontium concentrations and sedimentation rates for a number of carbonate sites. At higher sedimentation rates. concentration changes will become more noticeable because diffusive exchange processes are not able to dissi-

1693

Alteration of volcanic matter in deep sea sediments TM

meq/l

mM 0 IO 20

3040

5060

UP

1

t17.7 Wote

mM VJ 0 I

a2

0.3 0.4 0.5 0.6 01

200 E

a8

Ox\,

400 600

._-----O

600

%

Strontium

r---

1000

mM

E

1

5 Mg++nO Magnesium rM

E

Site 317 Fig. 4. Interstitial water and sediment (carbonate-free) chemistry, Site 317. Lithology in Table 1. Si/AI data: O-DONNELLY and WALLACE(1976); O---this work. Elemental ratios are atomic ratios.

pate concentration changes caused by reactions in the sediment column (GIESKES,1981). The correlation between dissolved calcium and 6’*0 (Fig. 8) for Site 317 indicates decreases in Hz’*0 less than expected from similar correlations for Sites 252, 285, 336 and, perhaps, Site 315. Similar observations were made in Site 253 (McDuff. 1978). This can be understood in terms of alteration reactions in the basal sediments and/or basalts having taken place at elevated temperatures. thus, yielding high dissolved calcium concentrations. but relatively small decreases in 6’eO (LAWRENCE er 01.. 1975). This would agree with the evidence for hydrothermal alteration of these sediments as noticed by SCHLANGERer ai. (1976). Thus the increases in calcium conantration and the decreases in 6’sO may signal reactions that have occurred in the past, and the concentration gradients in Site 317 are essentially dissipative in nature. Site 336Iceland-Faroe

Ridge. 63”21J’N,

07”47.(TW

The upper 16Om of the sediments consist of rapidly deposited Plio-Pleistocene clays and muds with considerable

glacial contributions (TALWANIet al., 1976). Average accumulation rates have been 56m/Myr. Below this section, accumulation rates have varied between 6 and 25m/Myr and the sediments consist largely of mudstones and claystones. Directly over basaltic basement is a unit consisting of basaltic rubble which shows signs of subaereal alteration (TALWANIer al., 1976). Biogenic-siliceous input has been relatively high during the Plio-Pleistocene, whereas below 2OOm the sediments are virtually barren of biogenic silica. This is clearly reflected in the low Si/AI ratios below 200m (Fig. 5) as well as by the low dissolved silica values. Volcanic glass is present throughout the sediment column and ash layers are quite common (TALWANIet al., 1976). The bulk chemistry of the sediments is characterized by rather high Ti/Al ratios in the bulk sediments (Fig. Sk indicating an alkaline basaltic origin (ENGELet al, 1965). Pt?ttav et al. (1978) noted that expandable illite-smectites (75-looo/,) occur mainly in the Eocene section of this site, suggesting that alteration of volcanic material is mainly restricted to these greater depths.

1694

J. M. GIE~KESand J. R. L.~WRENCE

o/oo

r mM

O

4

6

mM 8 P

IO 0 0

.dO@

I/ 0'

potassium

0.0 02 0.4 0.6

0

20

40

60

o*) P PC8 P

E f

Magnesium r-

Site 336 Fig. 5. Interstitial water and sediment chemistry, Site 336. Lithology m Table I. Elemental ratios are atomic ratios. Quaternary sedimentation rates in this site have been very rapid and this is clearly reHected in the minimum m dissolved sulfate and the maximum in alkalinity in the upper Unit I of the sediment column. At an average accumulation rate of 56m/Myr and a sediment diffusion coefficient of 2-3 x IO-’ cm’:‘sec (McDL’FF, 1978) the diffusion scale length (GIEEKES,1975: LERMAN.1977) between 100 and 160 meters. i.e. about the thickness of Unit I. Biogenic silica contents m Unit I are relatively high (compare, for instance, dissolved silica values and Si/Al ratios in Fig.5) and reactive organic matter contents are probably also higher than in the underlying sediments. With the relatively short diffusion length and enhanced Pleistocene sulfate reduction rates, it is not surprising to observe a sulfate minimum (GIESKES.1975). The correlation between calcium and magnesium concentration changes (Fig. 7) is non-linear, which can be understood either in terms of calcium carbonate precipitation in the sulfate reduction zone. or in terms of enhanced magnesium uptake in this zone. Correlation plots between calcium and dim0 (Fig. 8). as well as between calcium and potassium (not shown) indicate very good linear correlations. well within the accuracy of the data. If.

therefore. the calcium concentration-depth profile is mostly processes, then we conclude that the nonlinear calcium-magnesium correlation is the result of magnesium uptake in the sediments. The upward curvature of the magnesium profile supports this conclusion. Dissolved strontium data imply production of strontium in the sediment coiumn. HAWKESWORTH and ELDEaFtELD(19781studied the a’Srinb Sr isotopic ratio of the dissolved strontium and concluded that considerable exchange of strontium with volcanic material must have occurred in this site. Whether these exchange reactions involve also the neoformation of clays and exchange of magnesium. calcium. and ‘*O between the solid phases and interstitial waters is not known. though these processes are likely to be closely linked to the above strontium exchange reactions. A major problem of course. is the quantitative importance of these reactions vts-a-vis those taking place in the underlying basalts. due to transport

Sire _3Y~Bermuda

Rw.

3 1~1 I.?N.

64’14.YW

Site 386 has a complex history of deposition. The lithology has been described extensively by TUCHOLKEer al. (1979) and is also summarized in Table 1. During the last

Alteration of volcanic matter in deep sea sediments meW I

mM

1695

mM

mM

-

600 600

F

Ti/AI

1000

Mg/Al

OF14.,

E

oE

Ma&esium

8 _-a 0

IO00

I-

’ 4

K/AI

Site

386

Fig. 6. Interstitial water and sediment chemistry, Site 386. Lithology in Table 1. Data for Mg” with 0 obtained by atomic absorption methods. Data with x from T. W. DONNELLY (personal communication).

40 Myr accumulation rates have been low. less than 10 m/Myr and only during the Quaternary have they been as high as 35 m/Myr (TUCHOLKE er al..1979). Of particular interest in this site is the section of volcaniclastic turbidites of Unit III, which shows a distinct chemistry as evidenced especially by the high ‘E/Al and Mg/Al ratios (Fig. 6). This chemistry is typical of Bermudaderived volcanic material (T. W. DONNELLY, personal communication). Directly underlying these turbidites is a section of siliceous turbidites with associated high Si/Al ratios and high dissolved silica values. In the deeper sections of this site. high illite contents have been interpreted as diagenetic products of smectite alteration (TUCHOLKEet al.. 1979). Interstitial water profiles in this site are of particular interest because of the concentration extrema which occur at about 325 m depth, both in dissolved calcium and dissolved magnesium. Associated with these extrema is a small but discernible change in the 6’sO profile of a nature similar to that reported for Site 323 by LAWRENCEer al. (1979). The correlation diagram between calcium and magnesium (Fig. 7) does not allow a judgement on linear or

non-linear correlation. and the correlation between calcium and a’*0 (Fig. 8) is distinctly non-linear. An analysis of the concentration profile of dissolved magnesium indicates that the minimum occurs approximately at the boundary between the volcanic turbidites of Unit III and the siliceous turbidites of Unit IVa. The solid Mg/Al ratios in the uppermost part of Unit IVa are slightly enhanced over those found at the bottom of this unit. However, it will be very difficult to distinguish whether this increase in Mg/Al is actually a result of magnesium uptake reactions. The latter observations, together with the observed curvature of the magnesium profile in Unit III, strongly suggest that magnesium uptake reactions occur in the volcanic sediments. The very high Mg/AI ratios in these sediments. however, are primarily due to the high inherited magnesium concentrations in these volcanic materials (T. W. DONNELLY. personal communication). The large deviation from linearity in the calcium6is0 plot (Fig. 8) is mainly the result of the relatively large calcium signal in the reaction zone of Unit III. The concentration ratio of calcium in the fluids versus that in the solids is about 0.015 in this unit, whereas that of oxygen in the fluids versus solids is

1696

J. M.

mM

GIESKES

mM

mM

317

386

and J. R. LAWRENCE

\

'ZCi

315

Fig. 7. Correlation of concentrations of dissolved calcium and magnesium, Sites 252. 285, 336, 315. 317, and 386. Data with o are data below concentration extrema in Site 386. close to unity. Thus a small change in the solids could easily be reflected in a large change in the fluids for a constituent such as dissolved calcium: on the other hand. any detectable change in 6’*0 must involve a relatively large amount of reaction (LAWRENCE er al.. 1975). Associated with the calcium maximum is a dissolved strontium maximum. Further studies of strontium isotopes will be necessary to establish whether this strontium release is primarily a result of exchange with volcanic materials or due to recrystallization of carbonates. The dissolved potassium concentration profile has been shown to be conservative by MILLER et aI. (19791, thus implying a main sink for potassium in the lower sediment column or in the underlying basalts. Slight increases with depth have been observed in the K.AI ratio. but it is uncertain whether these increases are related to the potassium profile.

components occur out of or into the basalts of Layer 2 of the oceanic crust (McDurr. 1978). Often a major problem has been to assign the observed concentration changes in the interstitial waters to chemical reactions taking place in the sediments and,‘or in the underlying basalts. Only in very few cases have actual reaction products, consistent with observed interstitial water chemistry, been identified in the sediments. e.g. in the zone of silicification of Site 323 (KASTNER and GIESKES, 1976). For these reasons interpretations have often relied on correlations between concentration changes of interstitial water constituents (SAYLES and MANHEIM. 1975: LAWRENCEet ul.. 1975) or between changes in interstitial water constituents and changes in the bulk chemical composition of the solid phases (DONNELLYand MERRILL. 1977). In the following we intend to use similar correlations and we will base our arguments in particular on oxygen isotope mass balance considerations. The latter type of calculations does not only set limitations to the type of reactions which may occur. e.g. carbonate recrystallization, silica diagenesis. alteration of volcanic material but also can help to evaluate the relative importance of reactions in the sediment column versus those taking place in the underlying basal&. OXYGEN

MASS

BALANCE

For oxygen isotope mass balance calculations a knowledge is required of the initial isotopic composition of the reactants. the equilibrium fractionation factors between the authigenic product and the interstitial water, as well as the temperature at which the

DISCUSSION A cursory inspection of the nature of the concentration gradients for dissolved calcium, magnesium. strontium, potassium, sulfate, as well as of alkalinity, indicates that for most of these components reactions in the sediment column must, at least in part, be responsible for the observed concentration changes. In all sites, calcium and magnesium concentration changes show a nonlinear correlation (Fig. 7). a relationship required if only deep-seated reactions in the underlying basalts would be the cause of the concentration variations (MCDUFF, 1978, 198I). However. the profiles of dissolved magnesium, but particularly those of calcium, potassium, and 6”‘O do indicate continuing gradients to the base of the sediment column. This implies that significant fluxes of these

ISOTOPE

1”:

t 252

‘X

285

336

1

1

L

315

317

386

Fig. 8. Correlation of concentrations of dissolved calcium and 6’80 interstitial waters. Sites 252. 285. 315. 317. 336 and 386.

Alteration of volcanic matter in deep sea sediments reactions are taking place. The details of this type of calculation have been discussed by LAWRENCEer al. (1975. 1979). Because of the relative dearth of information on oxygen isotope fractionation factors LAWRENCE er al. (1975) only considered the following transformation reactions: biogenic silica to chert; fossil carbonate to recrystallized carbonate; and basalt or volcanic ash to montmorillonite. Though clearly other reaction products can occur, both in the sediments and in the basalts. as a result of alteration reactions, we estimate that the above mentioned reations are representative for the main reaction types that would affect both the oxygen isotope and the chemical concentration changes in the interstitial waters. LAWRENCEet al. (1975) showed that recrystallization of carbonates or conversion of biogenic opal-A to quartz could not account for the large depletions in HZ’*0 such as observed in the present study. Even in Site 315. with more than 90?; CaCO, in the upper sediment section, and with biogenic silica contents of perhaps more than 5%, complete recrystallization of all carbonate at relatively low temperatures, would be required to cause the observed depletion in 6’sO. There is no evidence for such extensive recrystallization in this site. If significant recrystallization were to occur below 300 m. i.e. at temperatures of above 15”C, increases in a’*0 might well result (LAWRENCEet al., 1975). In Site 317 depletions in 6”O are clearly caused in the volcanic section of this site (Fig. 4). Conversion of opal-A to quartz, especially because of the relatively low opaline silica concentrations in these sediments, also cannot account for the observed HZ”0 depletions in Site 315. For the same reasons the process of magnesium absorption on opaline silica as suggested by DONNELLYand MERRILL(1977) is not likely to explain the oxygen isotope data, unless such adsorption processes would be accompanied by unrealistically large fractionation factors of oxygen isotopes. Although carbonate recrystallization reactions may to some extent affect the profile of dissolved calcium, magnesium, and strontium in Sites 315 and 317, these reactions cannot account for the observed depletions in 6”O. In addition in both sites gradients in calcium and magnesium are to a large extent influenced by concentration changes in the volcanic sediment sections. In the other DSDP sites discussed in this paper carbonates and opaline silica are generally minor constituents, thus ruling out their importance in reactions affecting the oxygen isotopic composition of the interstitial waters. Therefore, in the following only the alteration of volcanic material is considered in the mass balance calculations of the oxygen isotopes. For each site two separate calculations have been carried out (Table 2). The first mode1 assumes closed system alteration of basalt or basaltic ash to smectite progressively through the history of the site. If diffusion of HZ’*0 were sufficiently slow, such that the influx of HZ’*0 did not occur, the model would give a reasonable approximation of the amount of alter-

1697

ation that must have occurred at each site. Because diffusion has occurred, however, these calculations can only give the minimum amount of alteration. The second model allows for an open system influx of H2”0. MCDUFF (1981) considers a model for the diffusive flux through an accumulating, unreactive sedimentary layer supported by continued reaction in Layer 2. The time history of the diffusive flux is difficult to establish in an exact fashion (MCDUFF.personal communication) and for these reasons we have assumed an average gradient equal to 500,< of the present average gradient. This procedure does allow an estimate of the importance of diffusive exchange with respect to mass balance estimates. Because the calculation is simply the material balance of “0 needed to maintain an “0 flux through time, the calculations are directly sensitive to the values assumed for the diffusion coefficients, the 6”O gradients. and the time period over which the influx has been considered operative. Of course, in principle, the calculated thicknesses of the reaction zones must be added to those estimated in the first model. In Site 252 ash alteration in the sediments alone cannot be the cause of the entire H2’*0 depletion, especially because the total amount of alteration calculated for ash exceeds the thickness of the sediment column. Therefore, basalt alteration reactions in Layer 2 must be of importance. The available data however, do not allow a quantitative estimate of the relative importance of basalt alteration versus ash alteration. Site 285 indicates a strong correlation between 6”O and dissolved calcium (Fig. 8). As discussed previously, the gradient of dissolved calcium implies a significant flux of dissolved calcium from Layer 2 into the sediments. The same argument, of course, holds true for gradients in 6180. Again a quantitative estimate of the relative importance of ash alteration visa-vis basalt alteration is not possible. Both Sites 315 and 317 show a partial diffusion barrier in the limestone-chert zone as has been discussed previously. Especially in Site 315 the sediments above the complex appear to contribute significantly to the observed 6”O depletion (c.f. Fig. 3). Alteration of volcanic ash in the upper 700m of the sediment column, amounting to less than 5% of the sediments, would be necessary to explain the observed depletion in 6’*0 in the upper 700m. The depletion in 6’*0 observed in Site 317 (Fig. 4) must be entirely due to reactions in the basal sediments and/or the underlying basalts. The data presented in Table 2 suggest substantial alteration of volcanic material in the basal sediments and the underlying basalts, amounting to as much as 150meters of basalt. Of course, the zone of low porosity does appear to act as a diffusion barrier, so that the estimated amount of alteration may be a high estimate. In Site 336 data for 6”O and dissolved calcium are strongly correlated (Fig. 8). X-ray mineralogical analysis of the sediments (PERRYet al., 1978) indicates

1698

J. M. GIESKESand J. R. LAWRENCE

Table

2. Reactlon

zone thickness

from .qc

Sasefvent Ueothizl

Aqe

Site

!m)

('yi

252

375

25

x5

565

20

315

398 1300Jf 900

:ic

317

oxygen

’ 20

336

3"5

15

386

374

'05

'pore

Fluid

:g

2orositva PI=-c.02552 G5.6 "z=-0.05:3z a?.1 oz= o.oi5z +ao.o Pz=-0.0667z +BO 0-675~ D*= 6'1 Pz=-!l&Iz CO.!3

::eans 06

Eouations

%v-eFIUldS

=-O.O12!4z -0.06 o.Jm072~ 'z= -0.02 ~,=-0.30213z co.07 =-O.')O23lz ' co.40 ~z=-O.O1OIRz +O.!O .,=-il.034261 +?.!4

-7. 31:

Isotope Volumea Fluld

matertal 06

Reaction

IQ

:edlments x.305

balance Zone

--Ti aaimed

." Iniila?

’ ”

:sh

Basalt

;sh

W!

101

24

462

164 OP 93

z

c

Thickness

-2.:3

36.740

.x5

59

2c

123

-0.61

29,973

.ii;b

?1

5

13c

-0.55

50,920

-1.77

23

-:.'7

49.195

. : i)o

Basalt

.ci:

12

5

211

l5C

.16Y

36

'7

349

?4c:

.76"

54

25

330

14':

,‘This equation was derived by a least-squares analysis of the available porosity data from the appropriate lnitlal Report of the Deep Sea Drilling Project. Integration of the equation over the depth interval from zero depth to the basement depth yielded the total volume V in cm3 of fluid in the sediment column ‘cm’ in area. b This equation was derived by a least-squares analysis of the 6’sO analyses of the pore fluids. Z is meters depth m the sediment column. ‘Using the ci’“O and the porosity equations at each and every meter depth interval from zero depth to the basement depth and the total column of fluid in the sediment column a mean ii’*0 for the pore fluids was calculated

d The values m meters determined by the relationship [X,~V.‘100(1 - .~,)m,][1,~1OO~P - 1) + I] represent the ti-uckness of a totally reacted zone including inherent porosity in the sediment or basalt column needed to balance the observed mean depletion in d’*O in the pore Buids. The reaction was assumed to be basaltic ash or basalt altering to smectite. The model in Table 5 of LAWRENCErl (11.(1979) was used to calculate values of XI. the mole fraction of oxygen in the solid phase. The 6’sO of the reactant, 6a. was set equal to 60“W,. The initial SIB0 of the pore fluid. 6h. was set equal to -0.3’:,,. The fractionation factor z> was allowed to vary as a function of temperature from 6 C (no sediment cover) to the value at the sediment-basalt interface presently, assuming a geothermal gradient of 5’C:lOO m. The number of steps used in the reaction sequence. n. was set equal to 10. The value of XI was determined by repeated trlals until the observed 6”O of the pore fluid. dd, taken from this table was obtained. Using (1) this value of X,. (2) the volume of the pore fluid ( V). (3) the oxygen content of water. m,, 0.05555 mol of oxygen per cm’ and of basalt. r+,. 0.08078 mol per cm3 and (4) the porosity. (P) in ‘,,, the thickness of the reaction zone was determined. A porosity of 60”,, was assumed for the ash reaction except for site 252 where 80”, was used. A porosity of 15”” was assumed for the basalt reaction. ‘The values given represent the thickness of a reaction zone including inherent porosity in the sediment or basalt column needed to balance the intlux of ‘*O by diffusion. The calculations were made assuming a diffusion coefficient for Hz”0 of 6 x IO-“cm’ set for an average temperature in the sediment of I5 C. The gradient in c~‘~O used was so”,, of the present gradient. The ci”O shift for the reaction. basaltic -+ ash smectite. was assumed to be 21.3”,,,, (see f II belowl. The ri’“O shift for the reaction. basalt -+ smectite. was assumed to be 14.1’*,,,, (see (21 below). The reaction was assumed LO have been going on since the time the first sediments were deposited on the basement. i.e. the basement age. The oxygen content of the solids was assumed to be O.OgO78 mol,cm’. The porosities assumed were those given m the above footnote. ’ The ci’#O shift was calculated using a revised closed system model (LAWRESCE rt ~1.. 1979) where ricr = 6”,,, j/I = -0.3”,,, and ,I = IO. The fractionation factor for smectite-H,O (YEH and SAWK 19771 r‘d was allowed to decrease progressively through the temperature range 2-22 C. X, was determined assuming a porosity of 50”,, with replacement of the pore Huid and by diffusion. The ci”O shift in an open system at the average temperature of I2 C would be + 22.4”,,,,. ’ The J’“O shift was calculated as m footnote (I). except that the temperature range used was 2-72 C. the porosity was

assumed to be IS’,, and the basalts were assumed to be IO”,, altered. The ii”0 shift m an open system at the average temperature of 37 C would be + 17.2”,,,, ’ Basement depth at this site was assumed to be XOOm because the low porosity basal ash below 8COm was for ‘$0 material balance purposes the same as basaltic basement.

that alteration reactions in the sediment column are of quantitative importance only in the lower sediment column. and. therefore, the large amount of alteration required to explain the ~5~~0 depletion (Table 2) must be due to reactions in the underlying basalts, rather than in the lower sediment column. In Site 386 a somewhat more complex ii”0 gradient occurs. Whereas extrema occur in the calcium. magnesium. and strontium concentration-depth profiles. the shape of the (iI profile (Fig. 6) suggests only a small change in ci’“O associated with these extrema. The main sink for “0 is located below the

sampled interval. presumably in the underlying basalts. In general the mass balance calculations presented in Table 2 suggests that in all sites considered in this paper considerable alteration of volcanic material. mostly of basalts of Layer 2. has contributed to the Hz”0 depletions of the interstitial waters. The two models used in the calculations yield very different results, the first model yielding minimum estimates, and the second model emphasizing the importance of diffusive exchange between the sediments and the overlying ocean. The results of the model calculations

Alteration of volcanic matter in deep sea sediments

1699

increases are usually accompanied Jy both decreases in alkalinity and in total dissolved carbon dioxide. Though in carbonate sediments, carbonate recrystallization can lead to increases in dissolved calcium, such increases are often too high to be accounted for by this process. As was already suggested above, the strong correlation between calcium and cS”O (Fig. 8) indicates the importance of basalt alteration in estabCHEMICAL MASS BALANCE lishing the often large concentration changes in calAssociated with the alteration processes implied in cium. Many authors have noticed the loss of calcium the above discussions are observed changes in disduring basalt alteration (e.g. DONNELLYet al., 1979). solved magnesium. potassium. calcium and strontium. Increases in dissolved strontium. particularly in Changes in 6’“O generally correlate best with those carbonate rich sediments have often been accounted in dissolved calcium. (Fig. 7) and thus. if the major for in terms of carbonate recrystallization reactions sink for lRO is located in the basalts of Layer 2, dis(SAYLESand MANHEIM,1975: GIESKES,1976: BAKERet solved calcium must have its major source in the al.. in preparation). However, especially in carbonate basalts. For potassium and magnesium the case is less poor sediments. such as Sites 252, 285, and 336. data clear and the nonlinear correlation between calcium on a?Sr,‘s?Sr isotopes of the dissolved strontium, do and magnesium. as well as the shape of the concentraclearly indicate that volcanic materials in the sedition-depth profiles of magnesium. suggest that uptake ment column also contribute to the strontium concenof magnesium in the sediments is of importance. Distration changes in the interstitial waters (HAWKESsolved strontium concentration-depth profiles indiWORTHand ELDERFIELD, 1977). cate that this constituent is produced mainly in the An important question arises with respect to the sediments. whether by carbonate recrystallization or quantitative importance of fluxes of the chemical eleby alteration of volcanic material. ments considered in this section compared with the Volcanic matter alteration. both in the sediments amount of alteration required by the oxygen isotope and in the basalts. often results in the formation of mass balance considerations. Rather than consider smectites. a ready sink for magnesium PERRY et al.. each site in detail we will focus attention on Sites 285 1975: KASTNERand GIESKES.1976: LAWRENCEet ai., and 315. In the former site reasonable estimates are 1979). Though there is evidence for a possible loss of available of the amount of basalt or ash alteration magnesium during low temperature alteration of (Table 2) and in Site 315 much of the present flux of basalt in contact with percolatmg seawater (DCIN- calcium and magnesium as well as of H2'‘0 is associNELLYer al.. 1979). the basalts constitute a sink for ated with reactions in the upper sediment column. magnesium after burial beneath a sediment cover, The calculations are presented in Table 3. Present presumably as a result of changes in- solution chemiconcentration gradients have been assumed to be cal conditions. An additional sink for magnesium in operative over the last lOMyr, which may result in carbonate sediinents is recrystallized calcium carbonoverestimates. particularly for Site 285. ate. which usually has higher magnesium concenIf reactions were restricted to the sediments only in trations than the original carbonate (MATTER et al.. Site 285 and if the sediments were affected uniformly, 1975). Of course. we previously pointed out that, with only small concentration changes in the solids would the possible exceptions of Sites 315 and 317. the latter result (Table 3). These changes would not be detectprocess cannot be of importance in the sites conable easily in the bulk chemical composition. If the sidered in this paper. flux of calcium were principally due to basalt alterSinks for potassium in the sediments are less well ation and the basalt would lose all its calcium, only defined. though KASTNERand GWKES (1976) found about 3.5 m of basalt would be involved. On the other authigenic K-feldspar associated with opal transformhand, if between 30 and 1OOm of basalt has been ation processes. In addition illite-smectite is conaltered to cause the H2t80 depletion (Table 2) the sidered an important sink for potassium upon deep basalts would lose on the average between 0.75 and burial (PERRYand HOWER. 1970). This process could 0.2:<, calcium, or between 9 and 2.590 of the original calcium contents. Calcium plagioclase is rather resistserve as a sink for potassium in the deeper sections of Site 386 (TUCHOLKEet al.. 1979). In basalts celadoant to weathering, but less calcium rich phases such nites are a typical sink for potassium and magnesium as olivines are more subject to alteration (ANDREWS, (KASTSERand GIESKES.1976: LAWRENCEet al.. 1979: 1980). Magnesium and potassium fluxes also are relaASDREW. 1980). In addition authigenic K-feldspar tively minor (MCDUFF, 1981). serves as a sink for potassium in basalts (KASTNER In the sediments of Site 315 a change of only 0.04~, and GIESKES. 1976). Typically. altered basalts are in magnesium would be expected if a sediment enriched in potassium (DONNELLYer al., 1980). column of 250m thick were to absorb the magnesium The often large increases in dissolved calcium are flux. Of course, if carbonates and biogenic silica were not likely to be due to carbonate dissolution reactions not to serve as a sink for this magnesium this number in the sediments. especially because the concentration would amount to over 2q, of the remaining solid imply substantial alteration of basaltic material. amounting to as much as 20”,, in the upper 1000 m of Layer 2. In a separate contribution we consider in greater detail the consequences of such material balance calculations with respect to the oxygen mass balance in the oceans (LAWRENCE and GIESKES,1981).

J. M. GIE~KE~ and

1700 Table 3.

Chemical mass balance calculations, Site

Average

Time

J. R. LAWRENCE

285

-

Sites285 and

Calcium, magnesium, potassium

gradients:

Cd ?D mM/lOOm '10 30 */lOOm < 5 mM/lOOnz

of diffusion:

10‘ years

Diffusion coefficient: (McOuff, in press)

2 x 10-e cm';sec 5 x lo-* cm:/sec

Fluxes

48 grams Ca/cd

over

10. years:

43 qrams 30 warns 500 meters Average

sediment,

oorosity

60

:

subtraction/addition:

S:te

Time

Ca:

0.1,

%I:

0.09.

loss

0.06

(2 of average total calcium, Appendix 2) (7.5 of average total maqnesium) qa?n (6' of total potassium) nain

5 fl/lOOm

of diffusion:

10' years

Diffusion

coefficient:

2 I( 10el

Flux

10' years:

7.3 orams/cm‘

250 meters Average

sediment,

solids

315 - i'aonesium

gradient.

over

(Ca. Ma) (K)

Mq/cm; K/cm-

50,000 Grams

i:

Average

315

oorosity

70':

addition:

phases (assuming 987” biogenic contents). Changes in the Mg/AI ratio have been observed in the carbonate free fraction (Fig. 3). but much of this magnesium increase may be due to absorption of magnesium on the biogenic opal as discussed by DONNELLY and MERRILL (1977). Only 10’; of the sediments in the upper 700 m of this site were recovered, so that reaction zones may have been missed. The above calculations do indicate that although fluxes in Hz’*0 suggest substantial alteration of volcanic material in the sediments and/or basalts, the fluxes of calcium, magnesium, and potassium are relatively small. For these reasons studies of the bulk chemical composition of sediments or basalts are not likely to yield quantitative information on the degree of alteration, with the possible exception of oxygen isotope studies. Thus much of the minerals produced during basalt alteration must derive their elemental constituents from the altering basalts themselves and much less by exchange with the ocean via the sediment column. CONCLUSION

Investigations of the chemical composition of the interstitial waters of Deep Sea Drilling Cores obtained in Sites 252. 285, 3 15. 317, 336, and 386, as well as oxygen isotope mass balance calculations based on observed depletions in HZ’*0 of the interstitial waters have confirmed the hypothesis that in pelagic sediments, well removed from areas of rapid terriginous detrital sediment deposition, concen-

18.750

cm-/set

qrams

'3.P

tration changes in dissolved calcium, magnesium. potassium, and H2 I80 are primarily due to the alteration of volcanic material dispersed in the sediment column as well as due to alteration of basalts of Layer 2 of the oceanic crust. Superimposed on these processes may be reactions involving the recrystallization of calcium carbonate or transformation of opaline silica, but these reactions appear to be of relatively minor importance compared to reactions involving alteration of volcanic materials. Alteration of volcanic matter dispersed in the sediment column appears to have only a minor effect on H2180 depletions. but can affect dissolved constituents (especially Mg and Sr) noticeably. This is particularly well demonstrated in Site 386 (Bermuda Rise) in which extrema in the concentration-depth profiles of calcium, magnesium, and strontium have been observed, with only a minor change in the 6180 profile. Oxygen isotope mass balance calculations indicate that low temperature basalt alteration amounting to lO-20”,, of the first 1OOOmof Layer 2 must occur to account for the observed H2180 depletions of the interstitial waters. Associated with these alteration reactions are relatively minor fluxes in dissolved calcium, magnesium. and potassium. Much of the alteration reactions in Layer 2 imply little exchange of elements with the overlying ocean with the exception of “0 exchange. .4cknowlrdgements-We are grateful to Drs H. ELDERFIELD and T. W. DONNELLY for providing some of their data. Special thanks are due to Drs N. BRIAN PRICE. GEOFFREY

Alleration of volcanic matter in deep sea sediments

1701

FITTOS.EDU’ARDSHOLI(O~ITZ.and Mr GEOFFREYAYGELL. EN(~ELA. E. J.. ES(;EL C. G. and HAVENSR. G. (1965) Chemical characteristics of oceanic basalts and the who introduced JMG to the X-ray fluorescence methodupper mantle. G&. Sec. Am. Bull. 76, 719-734. ology during his stay in Edinburgh. Drs J. 1. DREVERand FLEETA. J. and KEMPED. C. R. (1974) Preliminary geoT. W. DONNELLYcr&cally reviewed an earlier version of chemical studies of the sediments of Leg 26. Southern this communication. Indian Ocean. In Iniriul Reporrs 01 the Deep Seu Dri/lim/ We wish to acknowledge the support by the Natlonal Projecr 20. pp. 541-551. U.S. Govt Printing Office. Science Foundation through its funding of the Deep GIESKF~J. M. (1974) Interstitial water studies. Leg 25. In Sea Drilling- Proiect and by grants DES-72-01410. _ Irlirirll Rcptwrx if the Deep Sea Drilling Project 25. pp. OCE-?6-20151. and OCE-77-2ilO_ IO JMG and grants 361-394. U.S. Govt Printing Oflice. OCE-76-81952 and OCE-77-24819 to JRL. GIESKESJ. M. (1975) Chemistry of interstitial waters of marine sediments. Ann. Ret.. Earth Planer Sci. 3. 433453. GIESKESJ. M. (1976) Interstitial water studies. Leg 33. In fniriul Rcpwrz of r/w Deep Sea Drill&\ Projecr 33. pp. REFERENCES 563-570. U.S. Govt Printing Office. GIESKESJ. M. (1981) Deep sea drilling interstitial waters: ANDERSOPI‘ R. N.. HOBARTM. A. and L,~NGSETHM. G. Implications for chemical alteration of the oceanic crust. (1979) Geothermal convection through oceanic crust and Layers I and II. SEPM. Spec. Pub/. (in press). sediments in the Indian Ocean. Sc,ierlce 204, 828-830. GIESKESJ. M. and LAWRENCE J. R. (1976) Interstitial water ANDRE~S A. J. (1980) Saponite and celadonite in Layer 2 studies. Leg 35. In Initial Rcport.s ofthe Deep Sro Drilling basalts. DSDP leg 37. Conrrih. Minc,ra/ Per& 73. Project 35. DD. 407-424. U.S. Govt Printinn Office. 323-340. GIES&S J. M.: ‘KASTNERM. and WARNERT-B. W. (1975) A~DREWSJ. E.. PACKMASG. et (11. (1975) Site Report 285. Evidence for extensive diagenesis. Madagascar Basin. Initial Reporr.s of rhe Deep Sea Drillirlg Projecr 30. pp, DSDP site 245. Geochint. Cosnlochinl. Acta 39, 27-67. U.S. Govt Printing Office. 13x5- 1393. BAKERP. A.. GIESKESJ. M. and ELDERFIELD H. (1981). The GIESKES J. M.. LAWRENCE J. R. and GALLEISL;I’G. (1976) distribution of Sr’durmg diagenesis of carbonate Interstitial water studies, Leg 38. In lnirial Rrporr.\ (If fhe rocks: experimental and natural evidence. J. Sediment. Deep Sea Drilling Project 38, (Overflow Vol.). pp. Perrol. (submitted). 121-133. U.S. Govt Printing Office. CAMEROND. (1975) Carbon and carbonate analyses. Leg HARVEYP. K.. TAYLORD. M.. HEYDRYR. D. and BAS30. In Initial Reporr\ of the Deep Sea Drilling Prqjecr 30. CROFT F. (1973) An accurate fusion method for the pp. 687-688. U.S. Govt Printing Office. analysis of rocks and chemically related materials by CAVEROND. (1976) Carbon and carbonate analyses. Leg X-ray fluorescence spectrometry. X-ra.v Specrrom. 3, 33. In Iniriul Reporr.s qf rhe Deep Seu Drilling Prqjecr 33. 33-44. pp. 959-963. U.S. Govt Printing Office. HAWKESWORTH C. J. and ELDERFIELD H. (1978) The stronCOOK H. E. (19761 Sedimentary stratigraphic framework tium isotope composition of interstitial waters from sites along the Line Islands. equatorial Pacific. In Inirral 245 and 336 of the Deep Sea Drilling Project. Earth Reporrs of r/se Deep Seu Drilling Prolecr 33. pp. 849-853. Pluner Sri. Lerr. 40, 423-432. U.S. Govt Printing Ofice. JEN~~VNS H. C. (1976) Sediments and sedimentary history of COOK H. E. and ZEMMELSI. (19761 X-ray mineralogy data the Manihiki Plateau. South Pacific Ocean. In Initial from the Central Pacific. Leg 33 DSDP. In Itdial Repor!\ of rhr Deep Sea Drilling Project 33. pp. 873-890. U.S. Govt Printing Office. . Report.7 of rhe Deep Sea Drilling Pwjecr 33. pp. 539-555. KASTUERM. and GIESKESJ. M. (1976) interstitial w’ater U.S. Govt Printing Office. profiles and sites of diagenetic reactions. Leg 35. DSDP. COOK H. E.. ZEMMELSI. and MATTI J. C. (1974) X-Ra) Bellingshausen Abyssal Plain. Eurrh P/uwr. SG. Len. 33, mineralogy data. Southern Indian Ocean--Leg 26. 1I-20. DSDP. In-lnirial Rep0rr.r ql rhe Deep Sea Drilling P&jecr LAWHENCE J. R. and GIESKESJ. M. (1981) Constraints on -76. DD. 573-592. U.S. Govt Printine Office. water transport and alteration in the oceanic crust from DAVI&‘T. A.. LUYENDIJKB. P. e/ a/.(1974) Initiul Reports the isotopic composition of pore water. J. Geoph.~x Res. yf the Deep Seu Drilling Pwjecr 26. pp. 135-152. U.S. (submitted). Gov! Printing Office. LAWRENCE J. R.. GIESKESJ. M. and BROECKER W. S. (1975) DOSNELLYT. W. (I 980) Chemical composition of deep sea Oxygen isotope and cation composition of DSDP waters sediments--Sites 9 through 425, Legs 2 through 54, and the alteration of Layer 11 basalts. Earth Pluner. Sci. Deep Sea Drilling Project. In Initial Report.\ of rhe Deep Lerr. 27, I-IO. Seu Drilling Pyject 34. pp. 899-949. U.S. Govt Printing LAWREY~ J. R.. DRE~ER J. I.. ANDERSONT. F. and Office. BRUECKNERH. K. (1979) Importance of volcanic alterDC%NELLY T. W. and MERRILLL. (1977) The scavenging of ation in the sediments of site 323: chemistr). O’*/O”‘. magnesium and other chemical species by biogenic opal SrR- ‘Sr”(‘. Geochinl. Cosniochitn. Acfa 43, 573-588. in deep sea sediments. Cllern. Geol. 19, 167-186. LERMASA. (1975) Maintenance of steady state in oceanic DONNELLYT. W. and WALLACEJ. (1976a) Major element sediments. Am. J. Sci. 275, 609635. chemistry of the tertiary of site 317 and the problem of LERMANA. (1977) Migrational processes and chemical the origin of the nonbiogenic fraction of pelagic sedireactions in interstitial waters. In The Sea (eds E. D. ments. In Initial Reports of rhe Deep Sea Drilling Prqjecr Goldberg et al.). Vol. 6. pp. 695-738. Wiley. 33. pp. 557-562. U.S. Govt Printing Office. MANHE~MF. T. (1976) Interstitial waters of marine sediDONSELLYT. W. and WALLACEJ. (1976b) Major and ments In Chemicul Oceumxgraph~~ (eds J. P. Riley and R. minor element chemistry of Antarctic clav rich sediments. In ltliriul Reporrr of the Deep Seu Drj’//i,tg Project Chester). 2nd edn. Vol. 7. pp. 115-186. Academic Press. MANHEIME. T. and SAYLESF. L. (1974) Composition and 3.5. pp. 427-446. U.S. Goal Printing Office. DOSNELLYT. W.. PRITCHARDR. A.. EMMERMAN~R. and origin of interstitial waters of marine sediments based on PUCHELTH. (19801 The aging of oceanic crust: mineraldeep sea drill cores. In The Sea (eds E. D. Goldberg ef u/.1. Vol. 5. pp. 527-568. Wiley. ogic and chemical synthesis of the results of legs 51-53. MATTERA.. DOIIGL~SR. G. and PERCH-NIELSEYK. (1975) In Iniriul Reporrs qf the Deep Seu Drilling Projecr 51-S3. Fossil preservation. geochemistry. and diagenesis of pp. 1563-1577. U.S. Govt Printing Office.

J. M.

1702

GIESKES

and J. R.

carbonates from the Shatsky Rise, Northwest Pacific. In Inirial Reports of the Deep Sea Orilling Project ?I?, pp. 891-921. U.S. Govt Printing Office. MCDUFF R. E. (1978) Conservative behavior of calcium and magnesium in the interstitial waters of marine sediments: identification and interpretation. Ph.D. Thesis. Univ. of California (San Diego), 183 pp. MCDUFF R. E. (1981) Major cation gradients in DSDP interstitial waters: the role of diffusive exchange between seawater and upper oceanic crust. Geochim. Cosmochim. Acta 45, 1705-1713. MCDUFF R. E. and GIESKE~ J. M. (1976) Calcium and magnesium profiles in DSDP interstitial waters: diffusion or reaction? Earth Planet Sci. Lefts. 33, l-10. MILLERR. S., LAWRENCE J. R. and GIEZ.KE.TJ. M. (1979) Interstitial water studies, sites 386 and 387, Leg 43. In Initial Reports of the Deep Sea Drilling Project 43, pp. 669-674. U.S. Govt Printing Office. PERRY E. and HOWER J. (1970) Burial diagenesis in gulf coast pclitic sediments. Clays Clay Miner. 18, 165-177. pelagic

PERRY E. A., GIESKES J. M. and LAWRENCE J. R. (1976) Mg, Ca and O’8/O’6 in the sediment-pore water system.

hole 149, DSDP. 413-423.

Acta

Geochim. Cosmochim.

40,

PERRY E. A.. GRADY S. J. and KELLYW. M. (1978) Miner-

LAWREYCE

alogic studies of sediments from rhe Norwegian-Greenland Sea (Sites 336. 343. 344, 345. and 348). In ItGrid Reports 4 the Deep Sru Drilling Projecrs. Suppl. 38-41. pp. 135-139. U.S. Govt Printing Office. SAYLESF. L. and MAxHEIh~ F. T. (1975) Interstitial solutions and diagenesis in deeply burled marine sediments: results from the Deep Sea Drilling Project. Geochitn. Cosmochim. Acra 39. 103-127. SCHLANGER S. 0.. JACKSON E. D. YC(11.(1976) Site 315. In fnirial Reporrv of‘ rite Deep Sea Drilling Projrcr 23. pp. 37-104 U.S. Gobt Printing Office. SCHLANGER S. 0.. JI\CKSONE. D. er d/. (1976) Site 317. In Initial Reports II/ the Deep Sru Drilling Pro~rcr ZZ. pp. 161-300. U.S. Gobt Printing Office. TALWANIM., UDINTSEVG er ctl. (19761 Site Report 336. In fnirial Rrporrs of the Derp Sea Drilling Projecr 38. pp. 23-l 16. U.S. Govt. Printing Office. TKJCHOLKE B. E.. VOGTP. R. et 01.11979) Site 386: Fracture valley sedimentation on the central Bermuda Rise. In Iniriul Reports of rhr Deep Seu Drilling Projecr 43, pp. 195-32 1. U.S. Govt Printmg Office. ZEMMELSI.. CWK H. E. and MATTI J. C. (19751 X-ra> mineralogy data. Tasman Sea and far Western Pacific, Leg 30 DSDP. In fnirid Rrports of the Deep Sea Drilling Project 30, pp. 603-616. U.S. Govt Printing Office.

1

APPENDIX

Interstitial water chemistry ‘, Sites 252 (37-3’s. 59 14’E) and 285 (26 49’S 17.548’E) Site 252

Depth P,

COW

9 54 103 149

b-3-144/150 7-3-1441150

195 252 23 62.5

2R5

Z-3-144/150 4-5-144/150

285Fi

l-3-114/150 3-l-145/150 5-2- j2/53' 6-Z- ;0/736-i-lCC/lS~ i-3- j9/61Y 7-4-162/150

135 245 45j j12 517 j58

562

’ Methods used as described data. ’ Determined

from

Sr m::

1.53

14.0 30.4 4;,2 45.9 56.1 53.5

36.4 28.; j:,; 2.4 3.; 21

.:c ,i; ..~ '; ." i; !7

--__ -__ --

2.44 1.20

14.6 2".

iii.2 co.1

.3 .:;

..: :

27 Z.j i

0.5; 1 33 __ __

46.4 58.6 79.3 -' c ,L_.,

.'L .?: --

t: : 5.4 --

3j.j 1ti.j

a.39 __

__

;7.5 9.7 6j -__

74.1

:

__

Cd mM

1

3.10 2.20 2.42 1.70

2-j-144/155 3-5-1441150 4-5-144/150 j-2-144/150

post-crtuse

50.

% wnP

Alkalinity fiwqi

1.ic

1

.,'A

__

.PI'

23.3 23.3 21 4

22.: 23.2 23.3

’

_.

0 53 by GIESKES (1974): samples

(adjusted

APPENDIX

alkalinltj

from

shipboard

for evaporatwn~.

2

Chemical composition of bulk sediments (corrected for pore fluid contents). Data are m weight percent; method used XRF-analysis (HARVEYtv ul. 1977) Sample

Depth m

Si02

TiO,

4:

1.09 1.29 1.21 1.37 1.52

Al,03

M90

Cd0

K20

P,O,

Cl

6.95

3.03 2.79 2.47 2.75 2.07

2.35 3.18 2.69 2.76 3.81

2.19 2.14 2.08 1.76 1.69

0.179 0.174 0.197 0.209 0.236

2.00 2.90 2.00 1 .Q4 2.00

252-2-5-144/150 -3-5-144/150 -4-5-144/150 -S-2-144/150 -6- 3- 144/ 150 -J-3-144/150

103 149 195 242

55.2 50.1 56.0 52.2 54.0 55.7

1.37

10.35

7.23

2.67

3.61

2.19

0.222

1.51

'a""~~-;-;;'~~ _ _ /

455 350

56.2 56.1

3.58 0.68

12.15 12.77

5.40 7.02

2.09 1.53

7.23 7.19

I.37 :.29

0.392 0.117

5.73 1.03

512 560

53.2 55.2

0.71 0.67

12.58 12.11

6.85 6.69

2.11 2.62

8.33 7.05

1.22 1.23

0.128 0.094

0.32 0.37

727 765 795 832 a72

50.6 52.2 48.2 47.1 48.6

1.31 0.86 1.08 0.89 1.16

18.23 14.87 14.11 13.02 14.19

10.89 11.32 12.18 11.51 12.58

4.43 6.32 10.01 11.89 i.76

1.92 1.85 1.76 1.76 1.30

1.25 1.81 2.65 1.78 2.85

0.009 0.005 0.096 0.021 0.034

0.26 0.24 0.16 0.12 0.30

-6-2-70/73 -J-3-17/19 3174-ZO-2-119/121 -23-3-76178 -25-3-lOQ/lll -27-3-59/61 -29-4 53/55

12.00

Fe,O,

10.17

6.55

9.98 12.55 10.92

6.44 6.93 7.39

CI~IU~~W~

OII m.vr ,uqc

Alterationof volcanic matter in deep

1703

sea sediments

Appendix 2-+ontinued) Cl

Depth m

SiO,

TiO,

AlzOj

Fe203

Ma0

CaO

K,O

pzo5

6 43

1.61 1.54 1.90 1.07 1.55

9.08 9.05 11.29 6.22 11.51 8.65 8.32 12.10 11.88 14.98 13.67 16.44

3.30 3.38 4.05 2.34 2.67 2.57 2.02 3.22 4.71 4.33 3.22 2.09

5.08

i-:5 2124 2.40 2.35 2.27 1.64

12.68 11.89 13.11 11.36 7.35 11.21 15.21 16.61 14.38 14.74 15.80 16.19

:?i 5:14 3.90 2.25 1.09 1.44 1.69 1.96 1.92 1.57

1.75 1.78 1.46 2.13 2.40 1.73 3.52 2.73 2.63 2.16 1.71 0.19

0.179 0.158 0.218 0.149 0.154 0.080 0.108 0.149 0.172 0.192 0.183 0.062

0.46 0.47 0.57 0.26 0.40 1.83 0.88 0.83 0.35 0.50 0.54 0.23

0.41 0.87 4.64 3:i8 2.86 2.39 2.67 0.32 0.31 0.34 0.34 0.30 0.26 0.61 0.60 0.45 0.58 0.80 0.50

9.39 17.02 10.75 9.86 9.66 8.26 6.54 6.95 6.54 7.56 8.20 7.43 5.67 14.38 13.64 10.07 13.98 18.69 11.69

4.12 7.35 13.40 11.75 10.22 8.58 8.09 2.73 2.45 2.76 3.12 2.76 1.79 5.68 4.52 3.42 5.39 7.52 7.36

1.96 2.79 10.20 11.09 3.91 3.43 4.06 1.54 1.51 1.77 1.87 1.62 1.13

28.96 1.55 13.35 8.00 12.35 17.53 24.75 10.33 9.96 9.71 11.59 12.91 17.34 1.53 6.67 8.77 5.60 1.12 0.83

2.17 1.90 1.24 1.52 2.47 2.11 1.69 1.10 1.04 1.17 1.25 1.10 0.76 2.05 1.93 1.60 2.25 3.24 2.06

0.110 0.330 0.928 0.804 0.749 0.660 0.562 0.071 0.066 0.062 0.073 0.069 0.089 0.071 0.041 0.078 0.062 0.186 0.133

0.52 0.54 0.28 0.78 0.50 0.29 0.19 0.97 0.87 0.95 0.69 0.61 0.60 0.21 0.25 0.26 0.15 0.10 0.13

336-l-4-144/150 -5-5-142/150 -8-4-143/150 -ll-4-143/150 -16-4-144/150 -18-4-143/150 -22-5-140/150 -27-5-141/150 -3O-4-142/150 -33-5-140/150 -35-3-144/150 -39-3-144/1!0

1:: 184 203 243 292 336 395 430 479

58.1 60.8 51.3 61.5 59.0 53.9 48.1 46.4 44.8 42.2 42.3 43.1

386-l-4-144/150 -4-4-144/15D -7-2-140/150 -ll-l-140/150 -13-3- 79/81 -13-3- 97199 -l&3-148/150 -14-l- 17/19 -14-l- 49/51 -14-l- 98/100 -14-5-140/150 -15-5-140/150 -17-5-140/150 -25-l-140/150 -29-3-140/150 -31-4-140/150 -34-5-140/150 -36-4-140/150 -4O-2-140/150

62 156 185 262 312 312 312 329 329 329 337 356 395 480 518 547 613 651 718

18.5 49.6 37.9 36.0 41.1 35.0 28.3 62.0 60.7 60.9 56.8 56.4 55.1 61.3 57.8 61.0 57.2 51.6 57.8

APPENDIX

:+

1:94 3.j8 2.42 1.84

3

Chemical composition of carbonate free sediments of Sites 315 and 316. Methods used are similar to those of DONNELLY and WALLACE (1976a. b) Depth m

SiOz

TiO:

A1?03

Fe:03

MqO

CaO

K20

P,05

4.5 62.5

60.3 71.0

0.36 0.28

9.48 6.99

5.06 3.22

1.39 1.49

3.04 2.17

0.92 0.64

0.94 0.64

315A-3-5-144/150 150 -4-P-144/150 259 -5-l-144/150 372 -6-l-144/150 467 -7-l-143,'150 514 -8-Z-144/150 592 -9-l-144/150 704 781 -15-2- O/6

82.2 78.5 8i.l 82.2 94.8 74.9 77.4 54.1

0.37 0.16 10.21) '0.13 0.21 0.27 0.33 0.56

3.14 4.23 3.75 3.78 3.87 3.95 5.27 9.28

2.80 1.92 2.47 1.53 1.99 1.46 3.65 4.80

0.98 0.93 0.66 0.83 1.04 2.02 2.96

4.04 1.60 2.03 0.97 2.39 2.59 3.29 2.14

0.40 0.35 0.40 0.47 0.33 0.33 0.59 2.31

2.22 0.69 0.73 0.41 1.08 1.10 1.51 1.42

317A-l-l- 75/77 -3-1-144/1SO -3-z-144/150 -j-4-144/150 -5-l-107/109

47.3 49.4 40.4 57.1 49.4

0.78 0.67 0.56 0 59 0.68

7.7ti 11.45 8.75 6.90 12.89

8.39 5.86 4.20 4.53 7.81

6.33 2.34 1.64 1.56 3.02

4.48 2.39 2.45 3.33 2.39

0.80 3.84 3.24 3.29 3.96

1.44 0.25 __

31.0 43.0 57.6 68.7 64.4 43.0 38.7

0.98 0.67 0.38 0.22 0.33 0.44 0.53

9.64 6.75 5.50 3.65 5.65 9.11 7.54

11.74 5.68 4.50 2.54 3.40 5.48 6.53

2.32 2.25 2.09 1.46 2.24 5.01 3.23

2.59 3.18 7.53 4.34 5.07 2.63 2.70

0.71 0.63 0.54 0.42 1.07 2.00 1.36

--------

Sample 315-l-3-144/150 -4-c-144/150

402 5E! 567 568 577

317B-l-O-144/150 0 -g-2- o/10 74.5 -16-5-144/150 147 -22-5-144/150 204 -27-4-144/150 250 -32-4-144/150 298 -37-3-1441150 344

0.96

0194