An automated leaching method for the determination of opal in sediments and particulate matter

Deep-SeuRe~earchI ~ol 411 ",do 3 pp 4Zr~...4A41o¢3

0q67--llO37/q3.~lNl+llflO ~ 1~3 PergamonP~ss Ltd

Printed m Great Brztaln

An automated leaching method for the determination of opal in sediments and particulate matter PETER J. MULLER* a n d RALPH SCHNEIDER"

(Received 13 September 1991. m revised form 14 Aprd 1992: accepted 28 Aprd 1992)

A b s t r a c t - - A n automated leaching method for the analysis of b,ogeme slhca (opal) zn sediments and parueulate matter ts described The opal,ne material ,s extracted with I M N a O H at 85°C m a stainless steel vessel under constant stirring, and the increase m dissolved slhea IS continuously momtored For th,s purpo.,e, a minor port=on of the leach,ng solution ts e3,eled to an autoanalyzer and an,dyzed for d l s s o h e d sdlcon by molybdate-blue spectrophotometr) The resulting absorbance versus t=m¢ plot Js then e,.aluated according to the cxtrapolat,on procedure of DEMASTER (1981) The method hi,,, been tc,.tcd on ,,l~mge spicule',, rad=olar=an tests. Recent dfld Phoeene dl,~tomaceou'-, ooze samples, cla~ mineral,., and quartz. ,irtlli¢lal ,,edlmenl ml~ittlreS, anti on v;.irlol.is pl,mkton, s e d , n c n t tr,,p and sedm,ent sample,, The re,.ults ,,how that the relevant forms of b~ogcmc opal m Quaternary sethments are qu,mtltat~vely recovered The tmle required h)r an ,m,dy,qs l,, dependent on the ~alllrll¢ t~pe. ranging from II) t~ 21)m m for pl,,nkton and sediment trap n|;.itcrl,ll .,nd up to 4tl-.4~11 mm for Qn.,tcrnar,, sethment,, The silica co-cxtr,|ctcd fronl sd=catc minerals ts largely eompen,,atcd lor by the ,tpphed extrapol:ttton tcchmque. The remaining degree ot tmcertamty zs on tile order oftl 4 wt% SIO_, or Its,,. depending on the clay mmer:d composition ,llld content.

INTRODUCTION

BIOGENICsilica (opal) produced by diatoms, radiolaria, sponges and silicoflagcilatcs is a major constituent of marine sediments and an important parameter for geochemical and paleoceanographic studies. The flux rates of diatoms from time series sediment trap experiments (TAKAHASrtl, 1986) and the abundance pattern of opal in deep-sea sediments (CALVERt, 1983; LrINZn et al., 1986) are both closely related to surface productivity. The spatial and temporal distribution of biogenic opal in marine sediments thus may be used to reconstruct changes in paleD-productivity (LvLE et ill., 1988; MORtLOCKet al., 1991). Several techniques have been employed to determine biogenic opal in marine sediments. These include infrared spectroscopy (CHESTERand Et,DErEtELD, 1968; FROHLICH, 1989), direct X-ray diffraction of opal (EtSMA and VAN DER GAAST, 1971; HEMPELand BOHRMANN, 1990) or after conversion to cristobalite (GOLDaERG, 1958; ELLISand MoosE, 1973). elemental normative partitioning (LEINEN, 1977; BrEwsrea, 1983), microfossil counting (LEtNEN, 1985; POKRAS, 1986), density separation (BoImMANN, 1988) and wetchemical leaching (KAMATANh 198(I; EGG,MANNet al., 1980; DEMASTER, 1981; HUrD, * Fachbcreich Gcowi,,,,cn,,ehaftcn. Umvcr,.=tat Bremen. W-28(X) Bremen, Germany. 425

426

P. J MULLERand R. Scu~.eloEa

1983; MORTLOCKand FROELICH, 19891. Although each of these methods may have its specific application, none is universally accepted. Principal problems include matrix effects, incomplete opal recovery, and contamination by non-biogenic silica (LeINeN. 1985; VAN DER GAAST. 1991). The potentially most sensitive technique for determining biogenic silica is wet-chemical leaching. The advantages and limitations of the most common extraction procedures have recently been discussed by DEMASTER (19911. These methods in~ob, e the extraction of biogenic silica using hot alkaline solutions, measurement by colometry or ICP-ES (e.g. FtSCHER, 1989), and then correction for non-biogenic silica released from coexisting alumino-silicates and quartz. EGGIMANNet al. (1980) and N[ORTLOCKand FROELICH(1989). for example, used dissolved aluminum and germanium measurements, respectively, to estimate the amount of clay-derived silica. In applying a sequential leaching technique. DEMASTER (1979. 198 t) proposed an extrapolation procedure to compensate for the nonbiogenic sihca. In order to minimize contamination of biogenic silica by clays, weak bases like sodium carbonate solutions are used for leaching in most applications. This. however, involves thc risk of incomplete opal recovery. There is good evidence that solution-resistant forms of opal such as radiolarian tests, aged diatoms and sponge spicules are not completely recovered by the common sodium carbonate leaching techniques (DeMasTEr. 1979; EGGIMANN et al., 1980; KAMATANI. 198(I; SCHLUIEg, 19911). According to MoRrtOCK and Froet.lCH (1989), t,p to 50% of the radiolarian test,, in scdimcnt samples may escapc dissolution in 2 M N;.I2CO3 at 85°C. For samplcs rich in radiolarlan~, they suggested a very rigorous treatment (2 M NaOH, 85°C, 5-8 h) of thc residual solids m the coarse fraction of the tirst extraction to cn~urc complete opal solution. Another problem is that the susceptibility of biogcmc opal to solution decreases during dmgcnesis depending on poorly dclincd factors such as aging, incipient cry~talhnity, surface coatings, or aluminum content (tIuRD, 1973. 1983, EG(,IMANNet al., 19811; KAMAIANIel al.. 1988: VAN 1]| NNI"KOMet al , 19891. l lcncc, it appears that sothunl carbonate leaching techniques may result in low estimates of the biogemc opal content ot marine sediments. The automated extraction method of this study considers both aspects, a complete opal recovery and a correction for the non-biogcn~c silica. The method is an improvement on the manual sequential leaching method of DcM,xsrer (1981) and uses the same correction procedurc. An advantage of this approach is that only one element (Si) hits to be determined and that no assumption with respect to the sediment composition hits to be made. A weak point in the manual technique is that the linear slope and the extrapolated intercept value are only based on very few nlcasurcmcnts, usually three to four. With such a low number of data points, potential outlicrs arc difficult to identify and the extrapolated biogenic silica wdue may have a largc error. A large degree of uncertainty in determining the intercept can bc overcome by applying the automated technique presented below.

DESCRIPTION OF THE METHOD I n s t r u m e n t a t i o n attd o p e r a t t o n

The automated system for determining biogcnic silica consists of two major components: the extraction apparatus (extraction vcsscl, rod stirrer, water bath) and a simple

A u t o m a t e d leaching method for determination ot opal m sediments

427

EXTRACTION APPARATUS

From AUTOA,NALYZEI:I 0 SOl myron ~

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Schematic ~tc~ o1" the c'qract~on appar,du', (not true to ,,¢a1¢), Sc¢ text for description

autoanalyzer for continuous flow analysis ot dissolved silicon by molybdate-blue spectrophotometry (pcn.,,taltlc pump, analytical manifold, flo~-through photometer, recorder). The extraction is earned out in a cylindrical stainles'~ stccl vessel (height 15 cm, diameter 6.5 cm). wluch is placed m a water bath maintained at a constant temperature of 85 _+ 0.2°C (Fig. 1). Prior to ats first use. the vc,,sel shoukl bc scoured in a dilute H,O,/FlCI-solution. The top of the vessel i.,, scaled by a removable but tightly fitting Plexiglas cover to min0mize evaporation during analysi,,. The cover must have three small holes for the in,,crtion of a starrcr rod (,,tainle.,,s ,,teel) and two capillary tubes. These tubes allow a continuou.,, cycle of a small portion of the leaching ,,olution to the autoanalyzer and back. To prevent particulate m:attcr Irom entering the analytical lines, the solution is pa.,,scd through a small paper filter (c, 1.5 cm in diameter), punched front blue ribbon liltcr paper (c.g Schlclchcr and Schull. rcl. No. 3111121111. This liltcr type has no defined pore ,,izc but i,, ,,pecdicd to retain particle,, > I-2 , m The filter i,; held by a self-made Teflon screw cap liltcr holder littcd to the mlct ot the .sampling tube and is replaced with a n e w o n e after each r u n . Continuous agitation of the sample material by stirring and rapad temperature adjustmcnt within the stainless steel vessel result in a haghly clficlcnt extraction system. For a leaching solution added to the vessel at room temperature, it takes about 4 rain to heat to 8(1°C. and a total of only 8 rain to reach the final temperature. Di,,solvcd silicon analysis is performed using a simple continuous flow system applying a modified version of the atttomated molybdate-blue method described by GraSSHOt:F et al. ( 19831. The manifold and autoanalyzcr set-up is ~hown in Fig. 2. The indicated pumping rates (in ml man- a) arc the nominal rates lk)r calibrated tygon tubes at standard pumping spccd (e.g. sctting 4 for the ISMATEC MPI3 G J-4 peristaltic pump). Mixing coils and tran.,,mi~.,,ion tubes have an mrlcr diameter of 1.2 mm. For analysis, the sampling tube from the extraction vc,,scl is connected to the corresponding pumping tube (0.6 ml rain- t). The alk:dine solution is then passed to a de-bubbler where the stream is split. The minor portaon (0.096 ml rain- t) i~ directed to the manifold for ~ilicon analysis, while the major fraction i.,, recycled to the extraction vessel.

428

P J ML LLER and R Sell%EIDER

The sample split branched off for silicon analysis is segmented by air bubbles (0.032 ml mm -l) and acidified b~ admixing 0.088 M HzSO.~ at a rate of 0.6 ml mm -I. At the gb, en pumping rates, th=s acid strength is sufficient to acidif} a 1 M NaOH leaching solution to a pH of about 2. The acid strength has to be adjusted accordingly if the molarity of the leaching solution is changed. The acidified sample solution is then successively mixed with the molybdate, oxalic acid and aseorbic acid reagents (each pumped at 0.6 ml rain -x) and finally passed to the photometer. The absorbance is measured at a wavelength of 660 nm using a photometer (e.g. S K A L A R 6100) equipped ~ith an automatic 1 cm flow-through cuvette (with fixed de-bubbler) and recorded on a strip-chart recorder (e g. L A U M A N N D [ H 250). The continuous absorbance versus time plot (Fig. 3) is then evaluated according to the extrapolation procedure of DEMASTER (1981) as detailed below.

Reagents The reagents are prepared in 2 I batches from analytical grade chemicals and silica-free distilled or high quality de-ionized water and stored m polyethylene or polypropylene containers. They are stable for several weeks, but are usually consumed ~ithin a few days of continuous operation. NaOH, !.0 M (leach#tg sohttion). Dissolve 8t1 g sodium hydroxide pellets m about 151111 ml of demtneralized water (under a hood), allow the ',olutlon to cool to room temperature, dilute to 21X)0 ml and store in a polyethylene container. As an alternative, a commercially available standardized I M NaOH solution may be used. Sulphttrtc acid, ().1)88 M. Dilute 352 ml of ,,tandarthzed 115 M sulphuric acid with dcmineralized water to 2111111ml. At the given punlpmg rates (see Fig. 2), th=s acid strength is required to acidify a I M NaOtl solution to a pl-I of about 2. The acid strength has to be adjusted accordingly if the molarity of the leaching solution is changed. Molyhdate reagent. Dissolve 14 g of sod,urn molybdate dihydrate (Na_,MoO4 x 2ti20) m about 14111)ml of demincraliTcd water, add 374.4 ml of standardized 0.5 M tleSOa and make tip to 21X111 nil.

AUTOANALYZER SET-UP

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Automated leaching method for determination of opal m sedtments SiL,eeous hemlpeLo(j~c mud (Kontjo Fan)

CaLcareous ooze (C-umea R~clge)

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Fig. 3 anginal recorder output (absorbance versus time) for sthcon standard soluuons v,lth sdJca dassolutton curves of a calcareous ooze (core GeoB 11141-3,58 cm, 6t) 7 % CaCao. I 79 % Co,_~)and a sthceous mud (core GeoB I1Xl8-3.218 cm. 4.8 % CaCao. 4 ll2 % Co,g) sample The sample v.ctghts used for analysts were 42.0 and 21) 2 mg. re,.pcctlvely Opahne sdtca (here 5.9 and 16 11wt% SiO,, respectively) Is calculated from the intercept ab',,orbance~,alucobtained by hnear extrapolation (A). back to tmle zero (B) C repre,,ent,, the ,qhta rclca,,ed tram alummo-sdtcate,, and quartz (,,co text tar more detad,,)

Orafc acid reagent. Dis,,olve 16 8 g of oxalic acid (C_,fl,,Oa x 2I-1:O) in dcmineralizcd water to make 21RlI) ml. Ascorbic acid reagent. Dissolve 32 g of ascorbic acid (C, tlsO,) in dcmincralized water to make 2(100 ml. Add 2 ml of a suitable surfactant (25% Brij 35 solution or L E V O R IV). Stlicon .~tandard solttttons. A silicon standard concentrate (e.g. SiCI 4 pro-dissolved in 14% NaO! I, Titrisol. M E R C K No. 9947) is diluted with sihca-frec demincralized water to obtain a concentration of I mg Si m l - i. Working standard ,;ohttions containing 10.20, 30, 4(1 ;.rod 511 mg Si I- t are prcparcd tram this stock solution by diluting 1,2, 3, 4 and 5 ml to 100 ml using the actual N a O f l leaching solution as dilttcnt (~tlibratton After assembly, the analyzer is calibrated and checked for linearity by running the blank leaching solution and the working standard solutions alternately The resulting concentration versus absorbance plot should show a straight line up to about 45 mg Si !-t and pass through the origin. Note that the alkaline sample solution is diluted during acidification by a nominal factor of 7.25 as defined by the respective pumping rates (Fig. 2). Hence, the reaction is actually linear up to a concentration of about 6 mg Si 1-1 (or about 210pmol Si I-t). In a second step. the calibration and performance of the extraction system are tested by analyzing samples with known amounts of pure biogenic silica (e.g. purified sponge spicules). These test runs should yield average results within _+1% of the theoretical (water-free) silica value. Under normal operating conditions, only a few daily calibration runs are required. Usually, an appropriate workmg standard solution is run twice, before and after thc daily

430

P J MUt.LEXand R. SCrlNt'IDER

sample series (e.g. Fig. 3). These daily calibration runs are necessary, because the response factor may change slightly due to alterations of pumping tubes and reagents. However. standard solutions need not be placed into the water bath as the samples are. since there is no measurable temperature effect. This is because the diluted sample stream cools to near room temperature before the molybdate reagent is admixed.

Analytical procedure The following measurement procedure was developed based on the tests described later in this paper. The analyzer is allowed to warm up and stabilize while running the blank sodium hydroxide leaching solution. This solution (not water) should always be run as an intermediate wash solution in order to avoid concentration fronts and to maintain baseline stability. When the baseline has stabilized, the zero settings of the photometer and recorder are adjusted and the blank leaching solution is replaced by an appropriate working standard solution. When the absorbance increase emerges on the recorder (after about 7 rain), the standard solution is again replaced by the blank leaching solution. The first sample is then placed into the extraction vessel with exactly I00 ml of the leaching solution. The maximal sample weight to bc used with this volume is determined b~ the linear range of the molybdate-blue reaction, i.e. about 10 mg for pure opal or 100 mg for samples with an opal content of 10 wt"/,,. The vessel is scaled by the Plcxiglas cover (with the stirrer rod and sampling tube inserted) and placed into the preheated (85°C) water bath. The sampling tube is immcdiatcly attached to the corresponding pumping tube and the stirrer motor ts connected to the rod and started. At this time some air enters thc tube. After the air has passed the de-bubbler and reached the outlet of the recycling tube (flus takes about 2 rain), this tube is inserted into the third boring of the extraction ~es,,cl's cover. Thc alkaline solution is then recycled and the analyzer may operate unattended until the next sample is to be started After each run, the paper tilter tit the mitt of the samphng tube is replaced. Attcr the la,,t sample, the working standard solution of the first cahbration is run again to tcst tk~r any drift (Fig. 3). Usually both calibration runs agree within _0.5% and arc ave ragcd. Fmally, all reagent lines tire switched to a common water reservoir and the tubing is rmscd with dcmincralized water for about 15 min at elevated pumping speed.

Evahtation of the absorbance versus time plot To illustrate the performance of the analyzer and the evaluation procedure, Fig. 3 shows the original records for three calibration runs followed by the dissolution curves of two natural sediment samples and finally, another calibration run. Both sediment samples arc from the Angola Basin in thc South Athmtic Ocean, and represent two different sediment facies. One is a calcareous ooze from the Guinea Ridge (core GeoB l(~tl-3, 58 cm) and the other is a carbonate-poor siliceous mud from the Congo Fan (core GeoB 1008-3, 218 cm, see Fig. 12). Data on the bulk chemical composition of these samples are included in the caption to Fig. 3. Both extraction curves show an early rapid absorbance increase due to the preferential

Automated leachmgmethod for determinationof opal m sediments

43 [

dissolution of amorphous silica (opal) followed by a slower, linear increase representing the dissolution rates of the silicate minerals present. This final slope is higher for the Congo Fan sample, reflecting its higher clay mineral content. Opaline silica is determined from these plots by extrapolating the linear portion of the dissolution curve back to zero extraction time adopting the procedure of DEMAS'rER (1981). The following relationship is then used to calculate the biogenic silica content of a sample: wt% SiO2 = ABSwt x Si,ttt × 21.39 × mV,pa ABS,to × Wt × m V , t d

(1)

where ABS,p~ is the extrapolated intercept value (extraction time = 0) for the sample (taken in absorbance or arbitrary recorder paper units). ABS,td is the absorbance for the working standard solution, Sista is the silicon concentration of the standard solution in mg Si I-l, Wt is the sample weight in mg, the factor 21.39 is the molar SiO2/Si ratio (60.09/28.09) multiplied by a scaling factor of 10. and mV,vt and mV,ttl are the gam settings on the recorder for the sample and standard runs, respectively. Usually, the same gain setting is used for both standards and samples. The intercept value ABS,t,j may be determined directly on the recorder output strip (Fig. 3) or by linear regression analysis of the respective data. The results obtained by both approaches are virtually identical. Wc therefore prefer the straightforward graphical evaluation. Unless otherwise stated, biogcnic opal is reported as wt% SiO, to avoid a correction for the bound water content of the opal, which is dffticult to predict. The water content of biogenic opal may range from 2 to 15 wt% depending on the type and age of the material (! IurD, 1983; Mort l ocK and FroEt I( u, 1989).

Samph" withdrawal and evaporatum cJJects With the dcscrihed automated technique, a sample allowed to react Ik~r60 rain wtll lose about 6 ml vol by sample withdrawal (0.096 ml rain - l x 60 rain) and 3 ml by evaporation of water through the borings of tile extract.on vessel's cover. The latter value is based o n a linear evaporation rate of 0.05 ml H , O nun- t t h a t was determined lor our device by running a working standard solution in place of a sample and monitoring the silica concentration increase in the vessel. However. these losses do not signiticantly affect the concentration changes m the vessel or tile extrapolated intercept values (Fig. 4). For this diagram, it was assumed that I m g of sdica d~ssolves in a constant volume of 100 ml at a first-order reaction rate of 0.165 rain- t and that tile linal concentration is attained after 40 min. This "closed system" case is represented by curve D and the dotted hne. Curve A of Fig. 4 indicates the w~lume loss due to sample withdrawal (0.096 rain- I), expressed in per cent of the initial volume (100 nd), and.curve B the resulting sihca loss, expressed in per cent of the total silica to be dissolved (1 rag) (right scale). Because volume and silica are removed in a nearly constant proportion after the bulk of the silica has been dissolved, the effect on the concentration changes in the vessel is negligible. Actually, the deviation from "'closed system" concentrations due to sample withdrawal (curve E) is so small that it is difficult to resolve in Fig. 4. Evaporation, on the other hand, increases the concentration of dissolved silica (curve F. Fig. 4) by removing only water vapor volume (curve C). The following relationship could be used to correct the absorbance values for this effect:

432

P. J ML LLER and R

SCHNEIDER

A B S c . r r = ABSmea~ x (1 - t x

re~V).

(2)

where ABS~,,~ and ABSm~., are the corrected and measured absorbance readings at time t (in min). m is the evaporation rate (e.g. 0.05 ml min-i for our extraction vessel), and V is the initial volume of the leaching solution (usually 100 ml). However. as illustrated in Fig. 4. the applied extrapolation technique largely compensates for the evaporationdependent concentration mcrease. At the indicated low evaporation rate. the effect on extrapolated intercept values is on the order of 0.1 wt% StO_, or less, and thus can be neglected. TESTING THE METHOD

The materials used in the development of this method include purified sponge spicules. radiolarian tests. Recent and Pliocene diatomaceous ooze samples, various clay minerals, feldspar and quartz, artificial sediment mixtures, as well as a variety of plankton, sediment trap and sediment samples. Descriptions of some of the more significant results follow.

Sponge spictdes Pure biogenic s d i c a w a s prepared from a deep-frozen siliceous sponge from the Antarctic Ocean (kindly provided by G. Bohrmann AWl. Brcmcrhaven). The sponge w a s broken into pieces, washed to remove sediment material, treated three times ovcrnight with an acithc solution of 10"/,, If:O, to oxithzc the organic material, freezeSample withdrawal and evaporation effects

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Dmgram dlustratmg ,,,,topic v, zthdrawal and ¢~,aporatzon eflccts on ,,zltca dt,,,,ohmon curves +end¢xtn,polated intercept salues, a-,,',,ummg tin arbitrary tiP, t-order dw,.olution rate ,end a fin,d ',dica concentration o t l mg/llXI ml after 4(1 nun (curve D. closed system) Curve A indicates the volume loss by sample withdrawal (% of ,nmal volume+ right scale), curve El the assoc,atcd 1o,,,, of di,,solvcd sd,ca (% of total sdlca, right scale) and curve E the (ncghgible) effect on the selica concentration of the solution rcnlammg in the vcs,,cl. Curve C represents the water vapor lossdue to evaporation (% of mmal volume, right ,,talc) ,rod cur~e F the re,,ultmp ,ncrc;P.c of tile dt~.soDed ,,d,c,, concentration Note that the cxtrapol,,tcd intercept value,, ,,re not affl.'cted by either of the procc,,~¢,, (,,ce text for further dctad,,)

Automated leaching method for determination of opal m sediments

433

Sponge spicules 100 3 A C


k- 4O 0 i T OM NaOH 'j • I LU I

0

20

40

60

1

80

EXTRACTION TIME (mm.)

Fig 5 Effectof ~odlum h~dro,clde strength on the dissolution of purified sponge opal The theoretical ,,rhea content of this material is 91 wt% $10., (see te,ct) In thl', and other comparable figures, dissolution cur~es are md,catcd by symbols(m general taken m 5 nun intervals) to slmphfy plott,ng. Note that the dissolution process ;s actuall) recorded contmuously (,,ee F~g.3) dried and ground. The purified matcrml contained 9 _ (}.2 wt% of bound water (determined ttl duphcate by roasting at 10()()°C for 24 h) and only traces of organic matter (1).1 wt"/, organic carbon, determined by dry combustion at 1050°C using a C H N analyzer). Hence the nominal silica content of the purilicd sponge opal is taken as 91 ___().2 ~ t'¼, S l O , .

The effect ol dllfcrent sodium hydroxide ~trcngths on the tlis,,oluttotl rate ol the puriticd sponge opal i~ .qlown in Fig. 5. Evidently, a (1.1 M NaOI'I solution (pll 13) is too weak to dissolve the sponge opal within a reasonable time at the given temperature ot 85°C. The SiO2 recovery after 6(I min was only about 43 %. l lowcvcr, at higher p! I v:ilucs, using il.5 and 1.0 M NaOI I, complete solution was achieved within 4(1 rain. This is in accordance with the results reported by KaM,XraNI (198(I) tor silicilicd sponge spicules. The fact that the silica concentration continues to increase linearily beyond 40 rain in Fig. 5 is the result of evaporation (.,,co above) and not duc to undi.~olvcd rc~idu:d solids in the vessel or on the filter. The effect of varying temperature on the dissolution rate of the sponge opal is illustrated in Fig. 6. At a water bath temperature of 70°C (lower curve) it would take about 100 mm to dissolve completely the sponge material, compared with about 35 minutes at 85°C. To come up with a reasonably low extraction time, it is therefore necessary to ensure a rapid heat transfer from the water bath to the extraction solution. It i~ mainly for this reason that the extraction vessel should be made of stainless steel rather than of plastic material which has a low heat conductivity. The temperature could, of course, be increased beyond 85°C to further reduce the extraction time, but this would result in unfavorable water bath conditions. Radtolartan lesIs

The next step was to test the dissolution characteristics of radiohtrian tests, which also are known to bc solution-resistant. They arc not completely recovered by the techniques

434

P. J

MULLER and

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Sponge spicules 100 - 8

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using sodium carbonate solntion,~ (e.g. MORTLOCKand FROCLICH, 1989). A concentrate of radiolarmn tests was prepared from an Antarctic shelf sediment with an original opal content of about 8 wt%. The sediment was washed through a 4()/im sieve and the fraction >4ll t t m was repeatedly slurried with water in a glass beaker. The grains adhering to the glass wall wcrc spilled off and dried. Microscopic inspection revealed that this concentrate consisted predominantly ot radiolarian test,,, qtiartz grains and a few dark minerals. The concentrate ~as divided in half, and one half wa,, ground (using a mortar and pestle) bttt not pulverized. A~ would be expected, the ground radiolarian tests dissolved faster than the whole tests and both dissolved faster m I M NaOt I than in 11.5 M NaO! I (Fig. 7). While it took about 311 rain to dissolve completely the ground tests in 1 M NaOI !, about 45 min were required h~r the whole tests (Fig. 7A). Using 0.5 M NaOH, the respective times were considerably Ioqgcr, i c. 55 and 1311rain (Fig. 7B). The whole tc,,t sample of Fig. 7B was actually run fl~r a longer period 12311 rain) than shown in the diagram, in order to confirm the final linear slope. Despite the different NaOH concentrations and dissolttt~on rates, the font experiment,, yielded virtu~,ll} itlentlcal biogcnic silica values (mean _+ (I: 66.8 __. (1.7 wt% SiO2). The remaining 33% can bc attributed to bound water and to undissolved minerals, since the residual solids consisted essentially of quartz grains and dark minerals. Thus, we can conclude that the radiolarmn tests were quantitatively dissolved by our tcchmque. DltllO/HlICCOllS OOZC

While "fresh" diatom skeletons arc known to dissolve rapidly in alkaline solutions "aged" tests may show considerably reduced solubilities (e.g. EGC,,IMANNet al., 19811). To evaluate this clfcct, the dissolution characteristics of Recent versus Pliocene diatomaceous ooze sample,, wcrc compared (Fig. 8). The Recent sample it a surface sediment from the thattmlacctms ooze belt in the South Pacilic (core KN7812-! !, (1-2 cm; 63°19'3 S, 169°43'8 W) ;and tile Plioccnc sample (3.8-4.11 M;.I) is froth tile Maud Rise (core 1467) in the South Atlantic. The Plioccne sample is composed of pure opal as verified by X-ray

Automated leachingmethod for determnnatlonof opal m sediments

.J.35

Radiolarian concentrate

o

•

o o

4O

o

A

1 M NaOH < .J

E3 LU I-O

t ~ 0

Who~ I--'-

•

8O

60 o

LU 40 o o

20

•

0 5 M NaOH

o

' Ground

W'c,ole t e ~

0

EXTRACTION TIME (rain)

Fig 7 Slh¢,lth~,'.,olutxorlcur~,cxIor ground and ,.~,hol¢radioLiri,m tc',t'~( A , I M NaC)tl. ,~¢"C. I~. 0 5 M N,l()t I. ,";5'(') l Nmg I M Na(-)tI. grL~umlIt2sl~.,ire qlt,ttltltdtl~,cjy ths',olved ~lthlrl ~lt'~out3() nlll}

analysis (G. [~OIIRMANN, pcr,,onal conununicatuon) and was collected from a thick, mono,,pecilic diatom ooze layer conM~tillg Of tIle species l ' . ' t h , u ~ d t s c , s r e x (AIII x MANN Ut al., 1990). The restnltn of .solution experiment.,, with these samples .,,how Ill;It the Recent diatom opal is qtumtitutivcly dissolved within ;.ll')ottt 2(I nlln (u*,,Ing I M N a O t l at 85"~C) (Fig. 8). Experiments with various dlatoxn;aceotns plankton and sediment trap samples yielded similarly shaped dnssolulion curve.,, and even lugher dis.solution rates (Fig. 11 ). In contr;lst, about 8(1 rain were required to quuntit;ilivcly dissolve the Plnocene /~'lhmodLsctLs rex skeletons using l M NaO! I, and 120 nun I'or (1.5 M N a O t l (Fig. 8). Four rephc;ate un;llyses ol tile E. re.t" sample yielded a mean value ( _+ ~I) ol 95.(I _+ !. 1 wt'/,, SiO> indicating an opal recovery close to 1(1{1%. The remaining 5% may well be ;.IccOtLnted lor by the bound water content of the opal The "'overshoot.'," m tile dt.nsolution curves o| Fig. 8 require an explanatkm. They represent temporally limited periods ol diseqtfilibrium between the leaching solution m the extraction vessel and the fraction cycled to the autoanalyzer. This effect is caused by opal particles adhering to the filter surfilce at the inlet of the sampling tube. Dissolution of these particles results ,n a temporary increase of the silica concentration in tile tubing used to cycle the leaching solution. After dissolution is complete, equilibrium between the filtered and non-liltered portions of the leaching solution is quickly re-established. It is also evident from FLg. ~ that both the width and temporal occurrence of the dissolution maxima are a function of tile solubihty of the opahne material. The most pronounced "'overshoots" have been observed for opal-rich diatomaceous plvnkton and .sediment trap

436

P. J ,~ILLLER a n d R. SCH,'~EIDER

Diatomaceous ooze ",_==:=;===:_=__=

Recent

A

<

~

0

u.l

moil

o x w

•

8o

[3

o

[3 60 •~ 40

•:

Ethmodiscus

rex

Phocene

20

EXTRACTION TIME (mm) ~1~ ~ ~lltCd dl,,,,(.)ltltlt)n ~:ur~,c,, |or d Recent ,in(.[ ,I |)llo~:cn~: ~JhltOlll,|LCt~tl'~ OOZ~ '~:Ullpl¢ |r*~nl the Ant,tr~.t,¢ Oct,ill "I hu R¢~.¢nt dl,itom tu.~t,~tllZ,llltttdhvcl ~ dl',',ol~,cd ~ttillrl ;.ll~ot|t 2() mirl t|'qng l ~| ,%,=Otl [it .~5"C. ~.~,IHIc .=l'~ut ,~() nun v.:rc required to d=,,,,(H'.c thu I'h~)cum.. l'rhmr,d~i(.~ r('r ',kclcton'., ~cc text |or ,in c,[pl,zn.tt]on ol the "'o',cr,,h¢~ol,,"

Clay minerals

?

1 M NaOH. 85 °C

Montmorillomte

te I---

O

EXTRACTION TIME (m.n.) F=g ¢) S=1=¢.=extraction cur'.,¢... (1 .%1 N a O I I . 85°C) f~.)r v.zrlou~ reference ¢l.=y rnmcrdls ',,h()~mg II1.=I clay minerals qu=ckly ;.(tam hm:;,r d=sscHut=¢m r;.t~:,, The m~t=al n(m-hnc,=r mcr¢;.~ ..,, probably c;.u,,,.:d by a prcfcrcnl=,d d=',,.¢~lu(]¢)n ¢)f ultr..-tim: part=tic,, "Flit ch.y miner.d,, w.Jrc kindly pr,wldt:d h~ ~l Zuthcr (G~:-w=,,scnsch.=ften. Umversltat Bremen) Cl.=y mmur,d pr.w:n;,ncc' m(mtm¢)rllh~n.tc (h*:n..mllc) N¢). 26. Clay Spur~,VY U S A . =lille No 35. F=lhlun/IL U S A . k;.¢Hm=l¢ !1-7. B..lh/~>C. U S.A.: chlor.h:. D;,Ik,=rshcrg. Swt:dcn

A u t o m a t e d leaching m e t h o d for d e t c r m m a u o n of opal an ~ d t m e n t s

437

samples (Fig. 11). It should be emphasized, however, that the extrapolated intercept values, and thus the biogenic silica results, are not affected to a measurable degree, because the bulk of the dissolved silica is recycled to the extraction vessel. Clay contamination

From the foregoing experiments it was evident that solution-resistant forms of biogenic opal, e.g. sponge spicules, radiolarian tests and aged diatoms, were quantitatively recovered by the automated leaching method. The next step, then, was to evaluate the potential contamination by silica co-extracted from alumino-silicates and quartz. The standard reference minerals for these experiments were kindly provided by M. Zuther (Geowissenschaften. Universit,'it Bremen). The dissolution curves (1 M NaOH, 85°C) of four different clay minerals are shown m Fig. 9. Based on the linear portions of the curves, the silica release rates were highest for montmorillonite (0.092 wt% SiO, min-t) followed by illite (0.035), kaolinite (0.007) and chlorite (0.002). The dissolution rates of feldspar and quartz (not illustrated) also were found to be in a low range, near that for kaolinite. The same order of susceptibility to solution for these minerals was reported bv E~ImANy et al. (1980) who experimented wtth sodium carbonate solutions of different strengths. From Fig. 9. it also can bc noted that the clay minerals quickly attain linear dissolution rates. Hence, the basic presupposition of the correction procedure of DEMASTER (1981) appears to bc fultillcd. The remaining unccrta,nty lies in the extent of the initial nonlinear increase of the silica concentration that causes a positive intercept value on the ordinate. This value is highest fl)r montmorillomtc (0.7 wt% SIO2), slightly lower for illitc (0.5 wt%). and considerably lower for kaohmtc ((I.2 wt%) and chlorite (0.1 wt%). The non-linear increase of the dissolved silica conccntrahon during the initial extraction period is probably not the result of parabolic tlissolution kinetics. The experimental studies of BI;RNI R (1981) and KNAUSS anti Wol IRY (1988), rather, suggcst a grain size

Matrix effect % Silica Added/Found .___e_----e----e- 910/943 ~10

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4 70/4 95

,,°,s I-

4

~

2 4412 79

~

0 55/O 9O

0 00/0 39 r

~

~

;

EXTRACTION TIME (man.)

Fsg 10. Sdtca cxtractlon curves for an artdict,d ,,edlment mixture (30% montmordlomtc. 30% dhte. 211% quartz. 20% calcmm carbonate) spiked with dill'trent amount.,, of purified sponge op:,l The silica relea,,cd from the mineral matrix and not compensated for by the cxtral'~datlon lcchnique a m o u n t s to a,, much a.,, 0 4 wt% S,O z

438

P. J MULLERand R. SCHNEIDER

Particulate matter 9O ! 80 ~

~

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Plankton

.

++- +,.

Tr.p

30

.'.rap (KG85#5) I

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.

15

.

20

,

25

30

35

EXTRACTION TIME (ram) F:g 11. Sd,ca d~ssolutton curves (I M NaOH. 85°C) for an op~,l-r,ch surface plankton sample (No, M8 I01-38. fr,actton <75 m. spht 7. Drake Passage. WEFERel' al 1982) and two sediment trap samples from the King George Basra, Bransfield Stra:t. Antarctic (KG3 No I. 687 m water depth. Dec. 19,'~4. KG85 No 5, 170 m. Nov. 1985. FmCHER. 1989). The sample amounts used f o r extractton were 1 15.6 5(1 and 5.56 mg. respectively See the "'D~,,tomaceous ooze" sect,on for an explanat,on ol the "'over,,hoots'" ,

effect, i.e. a prefercntml dissolution of ultra-fine particles during the initial extraction period. DEMasr~ et (d. (1983) also noticed this effect and attributed the initial release of silica after grinding to the creation of highly reactive surface sites. Hence. as a precaution, clay-bearing sediments shottld not be pulverized by excessive grinding if they are to be analyzed for biogenic silica by wet-chemical methods. For this reason, the samples of the present ~tutly were ground and homogenized in a mortar using a pestle, but not pulverized. To further qu;mtify the potential contamination by clay minerals+ artificial sediment mixtures were prepared from montmorillonite, illite, kaolinite, quartz, calcium carbonate and pure opal+ and extracted as described above. The results are listed in Table 1, and illustrated m part in Fig. 10. For samples with blogcnic sihca contents >5 wt%, the deviation between nominal and measured silica values was <4% of the nominal values, which is well within the overall analytical precision of the method (see below). Samples with Im+er biogcnic sihca contents (<5 wt%), however, appear to be biased by 0.2-0.4 wt% S~O,. "['he "blank" value for the unspikcd matrix was found to be 0.39 wt% SiO2. Phtnkton and .~edtment trap material The automated method also has been tested on wtrious plankton and sediment trap samples A few typical dissolution curves are shown in Fig. 11. The st, trace plankton sample is from the Drake Passage ~,nd the two sediment trap samples are from the King George Basin m the Bransfield Strait. Antarctica (see figure caption for more details). These samples were treated in the same manner as the sediment samples of this study. They were freeze-dried and homogenized in a mortar, and then extracted using 1 M NaOH at 85°C. "['he dissolution curves in Fig. 11 show that the opaline plankton and sediment trap material r~,pidly dissolved within 10 rain. Comparable high dissolt, tion rates

Automated leaching method for determlnauon of opal in sediments

439

Table 1. Recovery of blogemc sdwa from arufictal sediments spiked with purified sponge opal

Blogenlc silica Sample

Matnx

Added Found (wt% StO_.)

Deviation Abs Iwt% )

I

A

0 00

0 39

0 39

2 3 4 5 6 7 8 9 111

A A A A A A A B A

0 55 [.08 2 07 2 44 3 01 4 70 9 Ill 22 80 26 I(I

0 90 1 29 2 22 2.79 3 06 4 95 9 43 23 511 25 211

0 35 0 21 0 15 0 35 0 05 0 25 0 33 I1 70 -(I 90

lI

B

45 50

45.~1

0 I0

12

B

68 30

66 8(1

- 1.511

Rel (% ) 63 64 19 44 7 25 14.34 I 66 5 32 3 63 3 I17 -3 45 0 22 -2 211

A 30% Montmonllomte. 30% ,lhle. 20% quartz. 20% calcium carbonate B 4(10,/,,Montnlordlomte. 4(|"~, kaohmtc. 20% quortz were observed for plankton and sediment trap material of tropical and subtropical provenance. A prctrcatmcnt to oxidize organic material does not appear to be necessary. Potential organic coatings are at least partly hydrolyzed by treatment with hot sodium hydroxide, thereby enhancing opal dissolutkm. The biogenic silica content obtained for the plankton sample of Fig. 11 is 69.8 wt% SiO,. This value is about 5 wt% lower than the biogenic silica values determined by WEFt:r et al. (1982) for two different splits of this sample applying the elemental partitioning method of SvEss and UNGERER(1980). The two sediment trap samples shown in Fig. l I had been previously analyzed by FiSCIIER (1989) who u';cd the mantml sequential leaching technique of DI:MAsTr-r (198l). A g r e e m e n t between the results obtained by the manual and automated techniques is excellent ,n the case of sample K G 8 5 # 5 (22.2 versus 22.0 wt% SiO_,) and fair in the case of sample K G 3 # 1 (45.9 versus 49.4 wt% SiO.,). Most likely, the differences arc due to mhomogeneities a m o n g the splits or the small sample amounts used for analysis, rather than reflecting the precision of the different analytical techniques. These results indicate that the automated method is also suitable for determining biogenic silica in particulate matter samples. The required extraction time is dependent on the sample type. Plankton and sediment trap samples usually attain plateau values very quickly, so that the extraction can be terminated after about 15 min. Quaternary sediments will require a total analysis time of up to 60 rain to verify the final slope of the dissolution curve. However, since the progress of dissolution is continuously monitored, each sample can be treated individually and optimally. Precision antl accuracy

The precision of the analytical system is better than _+ 0.5% based on rephcate measurements of silicon standard solutions. The reproducibility of the overall method was

P. J MULLERand R. SC~'~EtDER

4~1)

Table 2

Examples o[ prectston o f btogemc sthca e~tmtates [or selected samples t, tth low and htgh opal contents Mean S D (~, t% SlOe)

Sample

Hem,pelagic mud. off North Angola (GeoB 11116-3.33 cm) Calcareous ooze. Gu,nea Ridge (GeoB 11141-3.58 cm) Hemlpelagtc mud. Congo Fan (GeoB |~,~--3. 158 cm) Sdlceous ooze. Maud Rise (PS1585-3.4 cm) Rad~olanan te,,ts (concentrate) Ethmod:~ut rer. Phocenc Purtficd ,,ponge ,,ptcules S.D

C V ( o, )

N

22

(I 2

9 1

10

54

02

37

5

98

0 4

4 1

17

5(I 4

(I 4

0 S

3

¢~6 8

II 7

I0

4

t~5 1! ~l 3

I 1 11 9

l I [ 11

4 6

',tandard de~,latton. C V . coetficacnt of ~,armtlon. N. n u m b e r of

a ll dl v~.~'*,

estimated from replicate analyses of pure opal and of artificial and natural sediment samples wtth dilferent opal content,,,. Each of the,,e mea,,urements was performed on a different day. The rc,,ults arc listed in Table 2. Six analy,,cs of the purilicd sponge opal gave a mean vahtc and sta,ldard deviation of 911.3 + 0.9 wt% SiO_,, closely matching the value of 91 +_.I).2 wt'Y,, SiO 2 dctermincd by roasting (see abovc). The respective values for the Phocene E. rex sltmple and for the radiolarian concentrate arc 95.0 + !. 1 wt% SiO 2 (n = 4), and 66.8 + 0.7 wt% SlOe. These vahtes suggest a precision of about 1% for purc opal. Based on replicate am|lyses of various siliceous ooze samples from the Antarctic (e.g. thc Mattd Rise sltmple in "Fable 2), the rcproducibihty Ior high opal sediments is its good its it wits lbr pure opal. l-lowcvcr, the homogeneity of the sample material is important in determining precision because small m:tsscs (< 1(I--20 rag) must be used to analyze material of htgh (>5(1%) opaline content. To estimate the long-term precision for sediments with relatively low opal contents (< 10 wt%), thrcc scdimcnt .,,ample,,,were analyzed repeatedly (Table 2). Thc samples arc lrom the contincntal slope off Angola (core GcoB 1016-3, 33 cm), thc Guinea Ridge (GeoB 1041-3, 58 cm) and the Congo dccp-sca fan (GcoB 1008-3, 158 cm). The resulting mean values and standard deviations arc 2.2 + (I.2 wt% (n = 10), 5.4 _.+0.2 wt% (n --- 5), and 9.8 __. 0.4 wt% SiO 2 (n = 17), respectively, indicating a relative precision of 4-10% depending on the opal content. The most abundant clay mineral in these sedtments is k.'|olimtc, followed by montmordlonitc and ilhte (VAN OER GAAST and JANSEN, 1984; GINGEr.E, 1992). The accuracy of the method is more diflicult to assess because there are no certified biogcnic silica standards available. Therefore, our estimates arc based on analyses of the puriticd sponge opal and the artifici:d sediment mixtures described above. As shown by the data in Table I, the measured biogenic silica values for samples with
Automated leaching method [or determmatton of opal m sediments

44 [

systematic error. For most of these samples, however, the deviations are well within the overall analytical precision of the method. Hence. only the values obtained for samples with very low opal contents (<1 wt%) appear to be biased by more than 20%. In conclusion, then. the relative precision of the automated leaching method is better than 2% for pure opal and opal-rich samples and 4-10% for samples with 10--2 wt% biogenic silica, respectively. The accuracy is comparable to the precision for samples with more than 2 wt% opal. The results for samples with lower opal contents may be biased towards higher values depending on the amount and composition of the clay minerals present.

Application example The method described in this paper was applied to Late Quaternary sediment cores from the Angola Basin in the South Atlantic Ocean to study glacial to interglacial variations in paleoproductivity (Sc.NE(DEX, 1991). These results will be discussed in detail elsewhere. For the purpose of the present study, only two of the obtained high-resolution opal records are compared (Fig. 12) to illustrate the potential of the method. These cores were recovered from the Congo deep-sea fan (Core GeoB 1008-3, 3124 m water depth) and from the continental nse off North Angola (GeoB 1016-3.34ll m). The age scale given in Fig. 12 is based on oxygen isotope measurements on the planktonic foraminifcr Globtgerinoides rzzber (pink) (ScHNE[OER. 1991). applying the chronostratigraphy of IMaR[E et al. (1984). At both sites, the sediments are fine-grained, hcmipclagic muds and have a low carbonate content (generally <6 wt%). Only the interglacial intervals in core GeoB 1016-3 show t,p to 2(I-30 wt% carbonate. Both c~)res reveal pronounced organic carbon cycles, with high contents (up to 5 wt% C,,r~) in sediments deposited during glacial L

30 ¸

25 20. v

w, m 01

~

15"

GeoB1008.3

0

"E 'tO.

&

o

m

~oB1016-3

5"

0

20

40

60

80 100 120 140 160 180 200 220 240 260 280 300 AGE (ky BP)

Fig 12.

B,t)gentc',dtca content', ver,,us age for two,,ediment ct)re~ from the ea',tern Angola B~.~m

(SCHNEII)FR. ltY')l ). Core GeoB IIM18-3 Congo deep-sea fan. (16°34.9'S, 1(1°19.I'E, 3124 m water depth, core GcoB 1[)16-3. con(mental rest: off North Angola. 11°46 2'S, 11"46.9'E, 3411 m water depth.

4-12

P. J ML'LLERand R ~CH",/EIDER

and cold interglacial periods (e.g. oxygen isotope stages 5.2 and 5.4) and lower values ( 1-2 wt%) in interglacial sediments. The principal clay minerals of the sediments in this region are kaolinite, smectite and illite (GRIFFIN et al.. 1968" VAN OER GAAST and JANSEN. 1984: GINCELE. 1992). The two records shown in Fig. 12 display quite different opal distribution patterns. Core GeoB 1008-3 shows pronounced opal cycles with high contents (up to 25 wt%) in sediments deposited during glacial stages 2. 3 and 6. and generally lower values for interglacial sediments. Oxygen isotope stage 5.5 marks the most pronounced minimum in opal content (3-5 wt% SiO_-). with the next lowest at the younger Holocene (7-8 wt% SiO-,). Core GeoB 1016-3. on the other hand. had consistentl) low opal contents, ranging from 0.6 to 4.7 wt% SiO:. The average value for this core is 2.2 wt%. As in the Congo Fan core. however, sediments deposited during oxygen isotope stage 3 also showed the highest values. The maximal opal content of about 25 wt% SiO. found in the glacial sediments of core GeoB 1008-3 is considerably lower than the values (35-65 wt%) determined by VAN OER GAAST and JANSEN (1984) for sediments from this region applying the direct X-ray diffraction technique (EISMA and VAN DER GAAST, 1971). In that method, opal is determined from the opal btdge in the XRD patterns. To check our measurements, eight samples of core GeoB 1~)8-3 with opal contents between 6 and 23 wt% were also analyzed by the above XRD technique as modified by FI[MrH and BOHRMANN(1990). These analyses were performed and kindly placed at our disposal by G. Bohrmann. The average difference between the results obtained by both methods (assuming a bound water content of 10%) was 2 wt% opal and the maxmlal dcvmtion was 3 wt'Y,,,, thu', confirming the values obtained by the automated wet-chemical method. Moreover. the XRD opal measurements nl;Adc by GIN(;EI.t: (1992) on 33 samples of a ',,econtl ,~cdimcnt core (GcoB 10118-4) from flus statnon cnt)~cly m.'ttch the opal distrnbutton in core GcoB 1008-3 and show c',,,cntially the same range of values (5-25 wt'Y,,), i lcncc, ot, r results do not corroborate the very high op;.tl contents rcportcd by VAN DLR GA.xSl and JANSl N (1984) for the Congo Fan region although wc cannot complctcl~ rule out the po,,,,d~ulnty of small-scale spatn,d variations.

Finally. it should bc recognized that. ju',t a', ~lth all other instrumental and wctchcmtcal methods lor determining biogcmc sdlca, the automated leaching technique riot,, not thscriminatc between blogcnlc opal and w~lcanic glass. II the presence ot ,,ignilicant proportions of w~lcanic gla,,s is suspected, addittonal mncro,,copic in,,pcctton may be nccessary. Acl~nowh'dgements--We would like to thank Peter Vcttcrs tor technical a,,,,i,,tan~.c and suggc,,tlons ~on~.efnlug the mstrumentai set-up We arc especudly Indebted to Gcrd Bohrmann for providing the unique Ethmodt*cus rex sample and for perfornung XRD control anal.~,ses, and to Gerd Fischer lot preparing the ptirlhed sponge opal We also thank Michael Zuther for providing the relcrenc.e ,.Ivy minerals Walter Hale crltt|.ally read the manuscript. We have also bcnehted considerably lrom critical comments ol two anonymous rc~,le~,ers. This research was funded by the Deutsche Forschung,,gcn|cmschaft (Sonderlorschungsberct~.h 261 at Bremen University. Contribution No 33) ,rod the Bundesmmtster lur ['orschung und Technologic (BM[:T). Bonn

REFERENCES ABt LMnNN A . R GEXso'~OLand V. Srtt..',s (19~.~J) Phoecne-Plelst~n:ene Palcoceanography in the Weddel[ Sea--sll,CCOUS mlcrofossd evidence [rl (;,'oh,¢ual htwor* o [ t h e I'¢dar (hccm* Ar¢ttc versu~ Antarett¢.

Automated leachmg method for determmat~on of opal m ~d~ments

443

NA TO ASI Sertes C. Mathemattcal and Ph.~tcal Scwnces. U. BLEIL and J. THIEDE. edRors. Kluwer Academic. Dordrecht. pp. 729-759 BER'~ER R A. (198!) Kinetics of weathenng and dlagenesls In: Kmettcs ofgeochemwalprocesses, rev:ews m mineralogy. A C LASAG~and R J KIRgr~rntcg. editors. Vol 8. pp 11|-[35 BOHaMANS G. (1988) Zur Sedtmentatlonsgeschlchte yon btogenem Opal im nordhchen Nordatlanttk und dem Europalschen Nordmeer (DSDP/ODP-Bohrungen 4(18.6.42. 643. 644.646 und 647). Benchte Sonderforschungsberetch 3|3. Umbers|tat Ktel, Nr 9.22! pp BREWSTER N A (1983) The determmatton of btogemc opal m htgh latitude deep-sea sedtments. In. Sdtceo,~ depostts m the Pactfic regton, developments m sed:mentology. A IUtMA.J R. HEIN and R. StEVER.edttors, EIse~ter. New York. Vol 36. pp 17-33! CALVEgl"S. E (1983) Sedimentary Geochemtstry of Sdlcon In. Sthcon geochemtstrv and blogeochemtst~. S. R ASTOr. editor. Academic Press. London. pp 143--186 CHESTER R and H ELOEkFtELD(1968) The mfrared determmatton of opal m sdiceous deep-sea sediments Geochtmwa et Co~moc htmtc a Acta, 32, [ 128--1140 DEMAS'rEx D J (1971J) The marine budgets of sdtca and St-32 Ph.D Dtssertatton. Yale UmversRy. New Ha~en. L-q-. 3(18 pp DEMAsTER D J (1981) The ~upply and accumulatton of sthca m the m a n n e environment Geochmuca et Cosmochtmtta A(ut. 45, 17 L¢~--1732. DEMAsTER D J (191~11 Mea~urmg btogemc sdtca m marine sedtments and suspended matter. In: Marine parttth's anal~ ~t~ and chara¢tert;'.atton. D C. H t Ro and D W. SPENCER. edRors. Geophysteal Monograph 63. American Geophy,qcal Unton. Wa,,hmgton. DC. pp 363-367 DFM ~STtR D J . G B K s a r r and C A NrrrRouEx (1983) Btologtcal uptake and accumulation o f s t h c a on the Anhtzon continental ,,helf Gem htn,ca et Co~tnoc htmt(a Acta. 47. 1717,---1723 E¢.¢aMa.,,n D W , F T MANItlIM and P R BFTZFR (It~81|) Dt~olutton and analyst~ of a m o r p h o u s sthca m re,trine ,,¢thment,, Journal o] Sedtmentary I'etroh~gv. 511, 215-225 Ei'~Ma D and S J van DrR G,~asr (1971) Detcrmmatton o! opal m marine sethment'~ by X-ray dfffractton ,Vetherhtnd~ Jottrnal o] 3~'a Re~eam h. 5. 382-3811 l,'l t t,. D B dftd T C Nh,~rt (It~83) Calclttllt t.trbottat¢, op,tl anti quartz m tlolocenc pelagtc sedtments and the t,tltll:e conlpensation level m the South Atlanhc f)ccan. Journal o] Martne Re~earch. 31, 2111--227. ['1~,[ tt! R G ( It)S~,1) Stabdc Kohlcn,,to[t-I,,otope m parhkularer orgam,,cher Sub.,tanz atp. d c m Sudpolarmeer (Atl,tntr, thcr Scktor) D~,,,,crtatttm Bcr~chtc. [ acht~re~ch Gcow~,.,,cn,,chaftcn. Univcr,,ttat Bremen. Nr. 5. 161 pp [ RIHII l( It I' (I~lStJ) l)ccp-,,ca btogcntc ,,d~ea new ,,tructural ,rod anal~ttcal data from mfr;trcd ,maly,,t,,--gcoh~g~c,d mq~htation,, Terra Nova, I. 267-273 (; ~ ~.1 S J VAN Dt R ( I11~1) Mtncr,t[ogtcal anaty',v, ol m,trmc part~cle,, by X-ray powder thtlract~on In Martne partt~ le~ analv~t~ and ~hara~ t~'rt2atton. D (" | h~Rl~ and l) W. SPt N{ t-R, ethtors, f.;eophy~tcal Monograph 63, Amer~c.m Gcophy,,tcal U m o n , Wa,,hmgton, DC, pp 343,-362. (i,~,,,t S J v~,'~ DtR aitd J 11 I' JA',.'~tN (It,S4) Mineralogy. opal, and m:mgane,,e ol Mtddle and Late Ouaternary ,,ethment,, of the Z,nre (Congt~) decp-,,ca fan" ortgm and chmattc v:triation. Netherland~ Journal o] Sea Re~ear~ It. 17.313--34 I ( h ' ~ . t t t_ [: (I11~2) Zur khnhtgc,,teucrten lhldttng b~ogener und terrtgener Sedm~entc untl threr Veranderung tlttr~.h the t-rtththagcnc',¢ mt zcntralcn und ~,,thchen Sudatl,mt~k Dt,,scrtatton. Univcrsttat Bremen. 202 PP Gt~t Dt~! R~0E D 11958) Dctcrntmatitm o( opal in marine ,,¢thmcnt,, Journal o] ~,lartne Re~ear¢h, 17, 178---182. GaAS,,,tt~! I- K . M Ettatt ~at~t and K Kat ~1 t'~¢,, ethtor,, (1983) Method~ o[ ~eawater analy~t~. 2nd edn, Verlag Chenue, W c m h c m t . 417 pp G~Htt',. J J , [I WI~D~..t and E D Got t~t~ R~, (1~)68) The dt,,trtbntton of clay mineral,, m the World Ocean /h'q,-Sea Re,cart It. 15. 433--459 I It ~,tt.t t P and G BoHR',~~,',.,',, (19911) Carbonate-lrcc ,.ed~rncnt component,, and a,,pects of sihca dtagenests at .,~tc,, 7117. 7119 ,rod 711 (Leg 115. Wc,,tcrn Ind~,m O t e a n ) In R A. DUNCAN. J. BACIZ.Mas and L C. PI t l a,,o~, et al , I'ro¢ ODI', & t Re,~tdt~. 115, College St,tt~on, Texas. pp 677--698 tlt:~D D C (1~73) Intcrattton of btogcntc op,d. ,.edm~cnt and ,,eawater m the Central Equatorml Pactfic (h,ochlmt¢ a e t (,'o~mo¢ htmt¢ a a ¢ at, 37. 2257-2282 lit Rt~ D C (1983) Phy',~cal and chcmtcal propertte,, of ,,d.ceou.~ ,.kclcton., In Sthcon geochemt~trv and htogeo[hemt~trv. S. R. A,,ro~. cdttor. Acadcnuc Pro',,,, London. pp 187-244 IMttRtt J ,J D I'1~',. D G MARtIN',O:,,. A M~l~rvRt . A C Nhx. J . J MORt Ev. N G Ptstas. W. L PRELt, and

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