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Marine Chemistry, 15 (1984) 193--201 193 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands A LARGE VOLUME SEDIMENT SQUEEZING SYSTEM FOR THE ANALYSIS OF RARE EARTH ELEMENTS IN DEEP OCEAN PORE WATERS P.S. RIDOUT Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB (Gt. Britain) R.M. PAGETT Department of Earth Sciences, University of Leeds, Leeds LS2 9JT (Gt. Britain) (Received January 3, 1984; revision accepted May 14, 1984) ABSTRACT Ridout, P.S. and Pagett, R.M., 1984. A large volume sediment squeezing system for the analysis of rare earth elements in deep ocean pore waters. Mar. Chem., 15: 193--201. The measurement of rare earth elements (REE) in pore waters raises a number of analytical problems. Relatively high sample volumes are required to obtain sufficient concentrations for measurement along with a strict control over temperature and oxygen contact. As part of a study of REE in marine systems, we have developed a sampling technique which has proved its capability to provide good quality samples of sufficient size for this work. INTRODUCTION In marine geochemistry the chemical similarity of the rare earth elements (REE) has proved invaluable in constraining the likely sources of REE to sedimentary deposits, acting as water mass tracers and markers of oceanic change generally. Thus, it is important to clarify the cycling of the rare earths within the world's oceans. The REE are known to enter the oceans by two major routes: the rivers and the atmosphere (Martin and Maybeck, 1979). Their subsequent distri- bution and partition in the water column and sediments has yet to be thoroughly elucidated; however, it is known that the dissolved REE occur in very low concentrations in the water column. Despite reported enrichments in surface and deep waters, maximum values seem unlikely to exceed 100 × 10-a2 mol kg-1 (Elderfield and Greaves, 1982). It has been suggested that the enriched bottom water values are a result of an REE flux out of the under- lying sediment (Elderfield and Greaves, 1982). There is some evidence that deep-sea sediments are enriched in rare earths (Bonatti et al., 1971). It remains, therefore, to demonstrate that there are high REE concentrations in the pore waters and that they could support a flux which would satisfy the observed bottom water values. Work on the water column has been carried out using sample sizes of about 501 {Elderfield and Greaves, 1982). Although recent efforts have reduced 0304-4203/84/$ 03.00 © 1984 Elsevier Science Publishers B.V.

A large volume sediment squeezing system for the analysis of rare earth elements in deep ocean pore waters

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Page 1: A large volume sediment squeezing system for the analysis of rare earth elements in deep ocean pore waters

Marine Chemistry, 15 (1984) 193--201 193 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

A L A R G E V O L U M E S E D I M E N T S Q U E E Z I N G S Y S T E M F O R T H E A N A L Y S I S O F R A R E E A R T H E L E M E N T S IN D E E P O C E A N P O R E W A T E R S

P.S. RIDOUT

Institute of Oceanographic Sciences, Brook Road, Wormley, Surrey GU8 5UB (Gt. Britain)

R.M. PAGETT

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT (Gt. Britain)

(Received January 3, 1984; revision accepted May 14, 1984)

ABSTRACT

Ridout, P.S. and Pagett, R.M., 1984. A large volume sediment squeezing system for the analysis of rare earth elements in deep ocean pore waters. Mar. Chem., 15: 193--201.

The measurement of rare earth elements (REE) in pore waters raises a number of analytical problems. Relatively high sample volumes are required to obtain sufficient concentrations for measurement along with a strict control over temperature and oxygen contact. As part of a study of REE in marine systems, we have developed a sampling technique which has proved its capability to provide good quality samples of sufficient size for this work.

INTRODUCTION

In mar ine g e o c h e m i s t r y the chemica l s imilar i ty of the rare ear th e l emen t s (REE) has p r o v e d invaluable in cons t ra in ing the l ikely sources o f REE to s e d i m e n t a r y depos i t s , ac t ing as wa t e r mass t racers and marke r s o f oceanic change general ly . Thus , it is i m p o r t a n t to c lar i fy the cycl ing of the rare ear ths wi th in t he wor ld ' s oceans .

The R E E are k n o w n to en te r the oceans by t w o m a j o r routes : the rivers and the a t m o s p h e r e (Mart in and M aybeck , 1979) . The i r subsequen t distri- bu t i on and pa r t i t i on in the wa te r c o l u m n and sed imen t s has ye t to be t h o r o u g h l y e luc ida ted ; however , i t is k n o w n t h a t the dissolved R E E occur in very low c o n c e n t r a t i o n s in the wa te r co lumn. Desp i te r e p o r t e d e n r i c h m e n t s in sur face and deep waters , m a x i m u m values seem unl ike ly to exceed 100 × 10-a2 m o l kg-1 (Elderf ie ld and Greaves, 1982) . I t has been suggested t ha t the enr iched b o t t o m w a t e r values are a resul t o f an R E E f lux o u t o f the under- lying s e d i m e n t (Elderf ie ld and Greaves, 1982) . The re is some ev idence t ha t deep-sea sed imen t s are enr iched in rare ear ths (Bona t t i e t al., 1971) . I t remains , t he re fo re , to d e m o n s t r a t e t ha t the re are high R E E c o n c e n t r a t i o n s in the p o r e wa te r s and t h a t t h e y cou ld s u p p o r t a f lux which wou ld sat isfy the obse rved b o t t o m wa te r values.

Work on the w a t e r c o l u m n has been carr ied ou t using sample sizes o f a b o u t 501 {Elderf ield and Greaves, 1982) . A l t hough r ecen t e f for t s have r educed

0304-4203/84/$ 03.00 © 1984 Elsevier Science Publishers B.V.

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16 23 24 27 26 25

, I ! : /

22 I

Fig. 1. Scale drawing of sediment squeezer unit.

this to 1--21 (Klinkhammer et al., 1983), with a corresponding loss in analytical precision, it remains essential that volumes of pore water, for REE analysis, should be as large as possible. Despite the potentially high concen- tration of rare earths in pore waters, it is necessary to envisage a sample size of the order of 200--500 ml so that the generated REE values are statistically above the noise of the reagent and analytical blanks. The determination of accurate REE values in pore waters depends upon a strict sampling and treat- ment regime. It is essential to minimise any temperature increase in the sediment from its in situ temperature of 2--4°C (Mangelsdorf et al., 1969; Bischoff et al., 1970; Fanning and Pilson, 1971). Additionally, oxygen con- tact with anoxic sediments may have subtle ye t profound effects on element mobil i ty (Bray et al., 1973; Troup et al., 1974) and therefore obscure the distribution of metals, in this case rare earths, in sediments and pore waters. It is essential, also, to minimise the storage time of the core prior to squeezing, to reduce the possibility of vertical mixing (Bischoff and Ku, 1971).

Various systems have been developed for pore water extraction. They include leaching, centrifugation, liquid/gas displacement, low and medium pressure gas/mechanical and high pressure hydraulic/mechanical squeezers (Ridout , 1981, and references therein). In more recent years, in situ samplers have been used for pore water extraction from deep ocean sediments (Sayles et al., 1973; Sayles et al., 1976). The majori ty of the systems do not fulfil all the major requirements for REE analysis, of large volume production in

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T A B L E I

C o m p o n e n t s list (see Fig. 1)

No. Desc r ip t ion Size Material

1 Cyl inder base OD 7 in Rigid PVC 2 Cyl inder OD 4.92 in Rigid PVC

ID 3 .940 in 3 Re t a in ing collar OD 7 in Rigid PVC

ID 4 .125 in 4 P i s ton OD 3.93 in Rigid PVC 5 Gauze 400 p m mesh Nylon (Ni tex) 6 F i l t e r W h a t m a n 542 Paper 7 O-ring 50 - -245 Imper ia l N e o p r e n e 8 Wing nu t 1 /4 in BSW St. s teel 9 S tudd ing 1/4 in dia. St. s teel

10 O-rings 7 1 - - 1 0 0 0 met r i c N e o p r e n e 11 Out le t po r t 3 /32 in C' bo red to - -

5 /32 in 12 Hydrau l ic jack 1.5 t on - - 13 Tie rod 3/8 in dia. Mild steel 14 S u p p o r t p la te 3 in dia. Mild steel 15 T h r u s t pad 3.5 in dia. at p i s ton Rigid PVC 16 Pressure gauge 0 - - 5 0 0 0 psi 6 0 m m dia. - - 17 Hydrau l ic p ipe 6 m m dia. 1 /2 ha rd coppe r 18 E lbow 90 ° Plain brass 19 Base pla te 8 .75 x 8.75 x 0 .375 in Mild steel 20 Hexagon head screw 1/4 in BSF x 1/2 in St. s teel 21 Top p la te 8 .75 x 8 .75 x 0 .375 in Mild steel 22 Hexagon nu t 3 /8 in BSW Mild steel 23 Ch. Hd. M/C screw and washer 2 BA Plain brass 24 C o n n e c t o r c l amp 25 wg Plain brass 25 Male a d a p t o r S t ra igh t Plain brass 26 Sealing washer F o r 1/8 in BSP th read A l u m i n i u m 27 Pressure gauge c o n n e c t o r 1 in A / F hexagon Plain brass

addition to low temperature and oxygen exclusion during squeezing, and minimal trace metal contamination. Kalil and Goldhaber (1973)described a system suitable for 7--10-cm sections of compact marine sediments, but it would not be suitable for unconsolidated sediments or for thin core sectioning (1--2 cm) as used in a detailed pore water sediment depth profile study. Sasseville et al. (1974) designed a large-volume system for lacustrine sediments, but it allows sediment--air contact to take place and also has a number of metal parts which are not appropriate for trace metal work.

M E T H O D S

The squeezing units

A scale drawing of the unit is shown in Fig. 1. A full, detailed components list is provided in Table I. The sediment is contained within the PVC unit

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which comprises a base (1), cylinder {2), securing ring (3), and a piston (4). Nitex nylon gauze (5) and a filter paper (6) located on the base, are held in position by the O-ring {7). The cylinder is fixed in place and the securing ring is t ightened down using the wing nuts (8}, on the stainless-steel studs (9). The piston presses into the barrel, sealing on its two O-rings (10). A syringe (60 ml) is inserted into the out le t por t (11) of the base. The complete PVC assembly is positioned on the hydraulic jack (12), inside the steel frame (13), so that the support plate (14) locates in a recess in the PVC base. The thrust pad (15) is placed on top of the piston. The pressure in the system is measured by a gauge (16) connected to the base of the jack (17, 18).

The PVC construct ion allows no metal contact with either the sediment or the pore water. All mild-steel parts are epoxy coated before use. The Nitex gauze facilitates lateral flow to the outlet port which has a minimal dead volume. The filter papers used (Whatman 542) have good retent ion and wet strength properties. The cylinders should be no more than ~-filled with sediment for efficient squeezing.

The steel frames are mounted on a wooden support, in a domestic chest freezer (13.7 c u f t ) modified to run between -- 5 and 10°C with a circulation fan in the lid. The inner walls of the freezer are sprayed with vinyl to prevent corrosion. The hydraulic units can be operated wi thout much disturbance of the cool air in the cabinet, using a modified handle.

Sample collection and storage

Sediment was sampled using a box corer (Peters et al., 1980) to provide a large volume of sediment with an undisturbed interface, minimal contami- nation and effective buffering against an increase in temperature. Four sub- samples were taken from the box core sediment by the insertion of precleaned, butyric core liners, taking care to avoid the extreme edges of the corer. Each tube was flushed with nitrogen and capped at the top. The box corer, with its shovels open, was lifted away to leave the block of sediment with the inserted tubes on a plastic tray. The tubes were quickly dug out, capped at the bot tom, transferred to a cooler cabinet (4°C) and stored upright prior to sectioning.

Sectioning sub-core and transfer to squeezers

Sectioning and transfer of sections to the squeezer units was carried out in a perspex, nitrogen-filled, glove box (Fig. 2). The glove box was completely purged with nitrogen, which was cont inuously supplied to the base from a nitrogen generator. The atmosphere in the cabinet was moni tored with a portable oxygen meter. The sub-core was sealed into the base of the glove box using a flange plate with an O-ring seal. The core and caps were removed and a PVC piston inserted at the base. The core was extruded using a scissor jack and a series of PVC spacing blocks. The ext ruded cores were sectioned

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Gloves

Access Plote

Air Lock

Gloves / ~ , / • ~ N2 inlet

Sub Core of Sediment ~ ~ for Sectioning

PVC Piston / ~ Steel Frame

PVC Spacer Scissors Jack

Fig. 2. Perspex glove box used for core sectioning and transfer to squeezer units.

(normally 2 cm) using a plastic spatula, each section was wrapped in "cling- fi lm", and stored in airtight polythene containers.

The sections were transferred to prepared, precooled, squeezer units in the glove box immediately prior to squeezing. Although the glove box was operated at normal laboratory temperature, its ease of operation for core sectioning and transfer allowed the separate sections to be returned to the cooler wi thout suffering a substantial increase in temperature. However, for some work it may be necessary to house the cabinet in a low-temperature room.

Squeezing the sediment

After allowing time for the units to come to temperature, each one was pressurised to a reading in the range 2000--4000 psi (the reading is dependent on sediment type). The pressure gauges provide a good indication of squeezing efficiency and serve to warn the operator of over-pressure and leakage. The first few ml of water were discarded to flush the dead volume and the filter assembly, before a plastic syringe (60 ml) was inserted into the outlet port. The pressure was maintained by further pressurisation at approxi- mately 5-min intervals. The bulk of the sample (generally 100--200 ml) was

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collected within half an hour. The syringes were capped and stored in the cabinet. By running four squeezers at a time, the four sections from each horizon were run at once. The water collected in the syringes was trans- ferred, in the nitrogen cabinet, through a 0.45-pm Swinex filter assembly, into acid-cleaned PTFE bottles, spiked with concentrated nitric acid (2 ml), and stored at 2--4°C prior to analysis. The squeezed sediment cakes were pressed out of the units, transferred to polythene bags and stored, frozen, for mineralogical analysis. The squeezer units were washed in warm Decon solution, rinsed twice in distilled water and quartz-distilled water, then finally in ethanol (100%). When dry, the units were reassembled in the nitrogen cabinet.

ANALYSIS OF PORE WATERS

The chemical separation of the REE in pore waters was based on a method used recently in the separation of REE in seawater (Elderfield and Greaves, 1982). This latter method was based on the principles established by Hooker et al. (1975).

The pore waters were analysed for Yb, Er, Dy, Eu, Sm, Nd, Ce and La by solvent ion-exchange and mass spectrometric isotope dilution. Both auto- matic and manual mass spectrometers, incorporating on-line computer facilities, were used. Based on Nd determinations, the sample-to-blank ratio was approximately 35. Precision for most elements was generally better than 8% while for La it was typically 10--12%. Analyses were carried out on sample volumes of 150--300 ml.

RESULTS AND DISCUSSION

Sample collection and storage, as described above, have been routinely carried out on a number of IOS cruises. If rigorously cleaned materials are used and care is employed during coring operations, then high quality cores can be obtained. Sectioning these cores and transferring the sections to the squeezer units potentially offered contact of the sediment surfaces with the atmosphere, which could affect anoxic sections of the core; careful operation of air locks and seals reduced this problem to a minimum. The PVC units avoid sediment/metal contact, but this does not eliminate all possible contamination and careful cleaning is necesary between operations (Robertson, 1968).

Some contamination from the precleaned squeezers and syringes might still be possible. To reduce this possibility, the residence time of the extracted pore water in contact with these components was kept to a m i n i m u m - - n o longer than 2h. Potential contamination from squeezers, syringes and filters was assessed by leaving them in contact with sub-boiling quartz-distilled water for several hours and then analysing the leachate for Nd.

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TABLE II

Component sources for the total Nd blank (Nd concentration of pore water sample = 22.3 × 10-12mol kg -1)

Component Nd (10-12 tool kg- 1 )

Quartz-distilled water 0.0019 Squeezer unit leachate 0.0700 Syringe and filter leachate 0.0700 Concentrated nitric acid (2 ml) 0.0180 Column and reagent blank 0.4800

0.64

TABLE III

Measurements of REE in North Atlantic Ocean water (Elderfield and Greaves, 1982) and squeezed pore waters

Concentration ( 10-12 tool kg- 1 )

REE N. Atlantic water Squeezed pore water a

Yb 5.16 6.25 Er 5.34 3.63 Dy 6.83 5.66 Gd 8.27 NM b Eu 1.22 1.53 Sm 8.25 4.35 Nd 45.8 22.3 Ce 55.1 28.1 La 54.4 16.4

aSampling site at Great Meteor East, N. Atlantic (IOS Cruise 129). bNot measured.

Table II shows the to ta l Nd blank and its c o m p o n e n t sources. The sample Nd is app rox ima te ly 35 t imes the blank which, considering the relatively small pore water volumes, is qui te adequate . Using Nd as a typical indica tor for the REE, there seems to be little evidence for any significant contami- na t ion ei ther f r o m the materials or inheren t in the ex t rac t ion process. Table I I I compares the REE values o f a seawater sample with those o f a pore water ex t rac ted by the m e t h o d descr ibed above. The results suggest little contami- na t ion f r o m either a seawater source or f rom the ex t rac t ion process. The squeezed sed iment sample r epor t ed here was taken f rom the near surface sed iment layers which compr i sed a marl o f app rox ima te ly 50% calcium ca rbona te (S. Colley, personal c o m m u n i c a t i o n , 1984) . This sect ion o f the core was oxic; however , deeper layers compris ing anoxic turbidi tes also gave sensible values for the REE. The comple te profi le for this core is inappro-

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X

. . j = -- P o r e w a t e r

-- S e a w a t e r

I I I I I ! I I La Ce Nd Sm Eu Gd Dy Er Yb

Fig. 3. Normal i sed rare ea r th e l e m e n t e n r i c h m e n t p a t t e r n for N o r t h A t l an t i c seawater and squeezed pore water .

priate in this technique description but the preliminary data are to be pub- lished elsewhere (Elderfield et al., 1984). Figure 3 shows the shale normali- sation pat tern (Haskin and Haskin, 1966) for squeezed pore waters compared with those values reported for North Atlantic bo t tom water (Elderfield and Greaves, 1982). Although the seawater samples and squeezed pore waters were taken from different areas of the North Atlantic, it can be seen that the enrichment patterns are relatively smooth and show no evidence of light REE enrichment that would typi fy significant contamination.

C O N C L U S I O N

The use of this system for extracting large volume pore waters from marine sediments for REE analysis has proved successful in the production of good quality samples of sufficient size.

A C K N O W L E D G E M E N T S

The assistance of Dr. T.R.S. Wilson (IOS), participants on Discovery Cruise 129, H.A. Kennedy, J. Mott (Leeds), and R. Potter (IOS) for design and construction work, is gratefully acknowledged. This work was partly supported by NERC Grant GST/02/03.

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Bischoff, J.L., Greer, R.E. and Luistro, A.O., 1970. Composition of interstitial waters in marine sediments: temperature of squeezing effect. Science, 167: 1245--1246.

Bonatti, E., Fisher, D.E., Joensu, U.O. and Rydell, H.S., 1971. Post-depositional mobility of some transition elements P, Ur, and Ti in deep sea sediments. Geochim. Cos- mochim. Acta, 35: 189--201.

Bray, J.T., Bricker, O.P. and Troup, B.N., 1973. Phosphate in interstitial waters of anoxic sediments: oxidation effects during sampling procedure. Science, 180: 1362--] 364.

Elderfield, H. and Greaves, M.J., 1982. The rare earth elements in seawater. Nature (London), 296: 214--219.

Elderfield, H., Kennedy, H.A., Pagett, R.M. and Ridout, P.S., 1984. A preliminary study of the rare earth element chemistry of deep ocean pore waters. Department of Environment Report, in preparation.

Fanning, K.A. and Pilson, M.E.Q., 1971. Interstitial silica and pH in marine sediments: some effects of sampling procedures. Science, 173: 1228--1231.

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Hooker, P.J., O'Nions, R.K. and Pankhurst, R.J., 1975. Determination of rare earth elements in U.S.G.S. standard rocks by mixed solvent ion exchange and mass-spectro- metric isotope dilution. Chem. Geol., 16: 189--196.

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