Early diagenesis of molybdenum in estuarine sediments

Marine Chemistry, 16 (1985) 213--225 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

213

EARLY DIAGENESIS OF MOLYBDENUM IN ESTUARINE SEDIMENTS S.J. MALCOLM Scottish Marine Biological Association, Dunstaffnage Marine Research Laboratory, P.O. Box 3, Oban, Argyll PA34 4AD (Gt. Britain) (Received July 16, 1984; revision accepted January 14, 1985) ABSTRACT Malcolm, S.J., 1985. Early diagenesis of molybdenum in estuarine sediments. Mar. Chem., 16: 213--225. A study is reported on the geochemistry of molybdenum in the organic-rich sediments of Loch Etive on the west coast of Scotland. The pore waters show profiles of sulphate, sulphide and dissolved organic matter (measured by pore-water absorbance at 280 nm) typical of those found in anoxic marine sediments where sulphate reduction is a major process. Mo and Mn are enriched in the surface sediments due to the reduction of oxides of Mn, with Mo in association. Pore-water profiles of Fe and Mn are typical of anoxic sediments, showing concentration maxima in the surface sediments. Mo in the pore waters contrasts with Fe and Mn by increasing with depth to a maximum and then decreasing. In the upper part of the profile there is a strong association of dissolved Mo with DOM, indicated by a positive linear correlation with absorbance at 280 nm and by measurement of a frac.tion of the dissolved Mo, 'non-labile' with respect to Chelex-100 under defined operational conditions. Mo is very mobile in these sediments and is cycled through the upper part of the sediment, to be fixed in a sulphide phase at depth. INTRODUCTION M o l y b d e n u m is e n r i c h e d in s e d i m e n t s f o r m i n g u n d e r a n o x i c c o n d i t i o n s (Gross, 1 9 6 7 ; B e r t i n e a n d T u r e k i a n , 1 9 7 3 ; Berrang a n d Grill, 1 9 7 4 ; J o n e s , 1 9 7 4 ; P i l i p c h u k a n d V o l k o v , 1 9 7 4 ) a n d in a n c i e n t s e d i m e n t s t h o u g h t to h a v e f o r m e d u n d e r a n o x i c c o n d i t i o n s ( B r u m s a c k , 1 9 8 0 ; Vine a n d T o u r t e l o t , 1 9 7 0 a n d o t h e r s ) . T h e n a t u r e o f t h e e n r i c h m e n t m e c h a n i s m has received c o n s i d e r a b l e a t t e n t i o n a n d it is n o w clear t h a t several p r o c e s s e s a s s o c i a t e d w i t h r e d o x c h a n g e in s e d i m e n t s result in m o l y b d e n u m a c c u m u l a t i o n . T h e p r o c e s s e s include: (i) scavenging b y m a n g a n e s e o x i d e s p r e c i p i t a t i n g in t h e w a t e r c o l u m n o r a t t h e s e d i m e n t / w a t e r i n t e r f a c e ( B e r r a n g a n d Grill, 1 9 7 4 ) ; (ii) c o - p r e c i p i t a t i o n w i t h iron sulphides ( K o r o l e v , 1 9 5 8 ; Bertine, 1 9 7 2 ; P i l i p c h u k a n d V o l k o v , 1 9 7 4 ) ; a n d (iii) a d s o r p t i o n b y organic m a t t e r (Szilagyi, 1 9 6 7 ; B e r t i n e , 1 9 7 2 ; Calvert a n d Morris, 1977). R e c e n t studies o f m o l y b d e n u m in s e d i m e n t p o r e w a t e r s have s h o w n t h e i m p o r t a n c e o f organic m a t t e r o n t h e cycling o f m o l y b d e n u m in r e c e n t s e d i m e n t s ( C o n t r e r a s et al., 1 9 7 8 ; B r u m s a c k a n d Gieskes, 1 9 8 3 ) . H o w e v e r , t h e diagenetic p a t h w a y w h i c h leads to t h e close a s s o c i a t i o n o f m o l y b d e n u m w i t h organic m a t t e r a n d / o r p y r i t e in a n c i e n t s e d i m e n t s has n o t b e e n clearly d e m o n s t r a t e d . M o l y b d e n u m is t h e m o s t a b u n d a n t t r a n s i t i o n e l e m e n t in s e a w a t e r 0304-4203/85/$03.30

© 1985 Elsevier Science Publishers B.V.

214 (Bruland, 1983) and is unusual in that it appears to behave conservatively (Sugawara and Okabe, 1966; Head and Burton, 1970; Riley and Taylor, 1972; Morris, 1975). The conservative behaviour is perhaps surprising considering that Mo is contained in some important biological enzyme systems (e.g., nitrogenase and nitrate reductase (Bray, 1975)) and that other elements of biological importance show a 'nutrient-type' behaviour (Bruland, 1983). The concentration of Mo in p h y t o p l a n k t o n is low in comparison to other transition elements (Martin and Knauer, 1973; Bowen, 1 9 7 9 ) s o it might be speculated that utilisation of Mo by enzyme systems requires only a small fraction of the available Mo in seawater and that this may also apply to other elements incorporated in enzymes. The 'nutrient type' of behaviour is probably due to adsorption of elements on rather than incorporation in marine organisms (P.J. Wangersky, personal communication, 1984). Of possible particular relevance in anoxic sediments is that Mo-containing proteins have been isolated from sulphate-reducing bacteria (Moura et al., 1978). This paper reports a study of the distribution of m o l y b d e n u m in anoxic marine sediments from a fjordic estuary on the west coast of Scotland. Both solid sediment and interstitial water data are presented to elucidate the early diagenetic cycling of molybdenum. A particular feature of the study is the use of an operationally defined speciation technique to further examine the importance of dissolved organic matter in m o l y b d e n u m geochemistry. MATERIALS AND METHODS Loch Etive is oligotrophic, due principally to the large freshwater input which also results in a significant proportion of the organic matter in the sediments being of terrigenous rather than marine origin (Malcolm and Price, 1984). The inner basin stagnates, due to its particular hydrography (Edwards and Edelsten, 1977), b u t never becomes completely anoxic. Sediment cores for this study were collected in May 1982, at two sites in Loch Etive, E6 in the outer estuarine basin and E9 in the inner stagnant basin (Fig. 1). The samples were collected using a gravity corer designed to collect the sediment/water interface (Pedersen et al., 1985). After subsampling on a centimetre basis, the sediments were handled in an O2-free nitrogen-flushed glove bag to minimise oxidation of the anoxic samples and also to minimise contamination. The sediments were loaded into centrifuge bottles which were sealed and briefly removed from the glove bag for centrifugation at 5000 rpm for 15 min. The samples were returned to the glove bag and the supernatant (pore water) was removed carefully by disposable syringe (with a teflon plunger) and then filtered through an in-line 0.4-pm Nuclepore polycarbonate membrane filter. Several aliquots were collected for different analyses, One 5-ml aliquot was collected in a glass vial containing 1 ml of 10% (w/v) CdNO3 solution to fix the dissolved sulphide as CdS. A n o t h e r was collected for pH and UV absorption measurements and the remainder was retained for m o l y b d e n u m analysis.

215

Fig. 1. Location of sampling stations in Loch Etive.

Sulphate was determined by an atomic absorption m e t h o d measuring excess Ba in the sample after BaSO4 precipitation from a known addition of BaC12. UV absorbance at 2 8 0 n m was measured in l c m quartz cells against distilled water as a measure of dissolved organic matter (DOM) (Krom, 1976; Krom and Sholkovitz, 1977; Bricaud et al., 1981). The pH was determined using a small glass combination electrode and a Beckman pH/mV meter. Preparation of samples for m o l y b d e n u m analysis was started directly after filtration of the pore water samples. The sample at its natural pH was passed over a column of Chelex-100 in the H+-form to remove labile Mo from solution (Ternero and Gracia, 1983). The effluent from this column was digested by boiling with 0.5 ml of concentrated Aristar HNOa to destroy organic matter (this procedure reduced the UV absorbance of the sample to the blank value) (Florence and Batley, 1975). The digested sample was adjusted back to a pH of 6 by cautious addition of an NH4OH solution and passed over a column of Chelex-100 in the H+-form to collect the 'non-labile' Mo. Mo was eluted from the column with NH4OH solution. This solution was evaporated to dryness on a laminar-flow, clean-air bench and taken up in I N HNOa, with a concentration factor of x 10 if required. Analysis was by flameless atomic absorption spectrophotometry using a Pye Unicam SP9 furnace system fitted to an SP9 spectrophotometer. The analytical program and instrumental parameters axe shown in Table I. Pyrolitically coated graphite tubes were used with argon as the purge gas. The surface of the tubes was coated with lanthanum by injecting a suspension of La2Oa into the furnace. Reproducibility was 14% for triplicate injections of pore-water extracts and the detection limit was 7 #g 1-1 in the extract. Fe and Mn were determined by direct injection into the graphite furnace.

216 TABLE I Analytical parameters for determination of Fe, Mn and Mo using an SP9 graphite furnace (pyrolytically coated tubes were used (coated with La for Mo analysis); 20-pl injections were dried at 110 C for 40s; temperature ramp rates were chosen for optimum per. formance, atomlse ramp was ) 2000 C s ) •

o

o

Ash Element

Purge gas

Mo Mn Fe

Argon Argon Argon

-1

Atomise

T(aC)

t(s)

T(°C)

t(s)

1250 900 1100

40 40 40

2800 2500 2450

3 3 3

Detect. limit (ppb)

Reproducibility (%)

7 3 5

14 8 10

C o n t a m i n a t i o n is a severe problem in any trace analytical procedure. In this study all e q u i p m e n t was prepared by e x t e n d e d soaking in 10% HNO3 prior to use. At every stage, f r om core slicing to final analysis, all practical steps were taken to avoid contamination. Initial handling was in an enclosed glove bag and subsequently m uch use was made of a Class-100 clean-air bench. Blank levels were below the detection limit for the entire analytical p r o c e d u r e and th e data are internally consistent. S e d i m e n t samples were freeze dried and finely ground in an agate ball mill prior to total digestion using a mixture of HF, HNO3 and HC104 in teflon beakers. Mo, Fe and Mn were det er mi ned by flame atomic absorption spectrophotometry. RESULTS AND DISCUSSION

Sediments Iron, manganese and m o l y b d e n u m c o n c e n t r a t i o n profiles in the sediments o f t h e two stations are shown in Fig. 2. T he Fe concentration, 4.8--5.0 wt% at E9 and 4 . 5 - 4 . 8 wt% at E6, shows no systematic variation with depth at either site. T h e data are similar to those previously r e p o r t e d for L och Etive using an X R F te c hni que (Malcolm, 1981) and for L och Duich (K rom and Sholkovitz, 1978). Manganese is enriched in the surface sediments at bot h stations. Th e m a x i m u m c o n c e n t r a t i o n is in the 0--2 cm dept h sections but is higher (8600 ppm) at E9 than at E6 (2450 ppm) (Malcolm, 1981; Malcolm et al., 1985). Mn c o n c e n t r a t i o n decreases rapidly with depth, reaching a baseline c o n c e n t r a t i o n which is also higher at E9 ( 2 0 0 0 p p m ) than at E6 ( 6 0 0 p p m ) . M o l y b d e n u m is enriched in the surface sediments, highest c o n c e n t r a t i o n s being in the 0--2 cm samples and, like Mn, the highest c o n c e n t r a t i o n {16.5 ppm) occurring at E9. Unlike Mn, the c o n c e n t r a t i o n o f Mo does vary with dept h below t he surface enrichment. Mo c o n c e n t r a t i o n decreases to a m i ni m um before increasing to a m a x i m u m of 8 - - 1 0 p p m

217 Mo

ppm

10 b

Mn 20

u

2

4

ppm xlO 3 6

Fe

8

10

,

3

wt~/o

4 i

5

'-;o

10 . (~)0 2O

o 0

30

0

0 0 0 0

o o 0

o o 0 o

5O

o o

0 0 0 0

60

0 0

0 0

0 o o 0

0 0

0 o

4(]

o

0 70

o

o

E9 1 ~

0 10

O0

20

0 :

'U

30 . o 4O

50

o 0 o

'

u

o 0 0 o

0 0 0 0

0 o

o

0

0 0

0 0

60

o o

70

2 o

0

0 0

0

o

0 0 o

0 o

E6

0

Fig. 2. Profiles versus depth of total Mo, Mn and Fe in the sediments o f Loch Etive stations E9 and E6.

which is maintained to the b o t t o m of the cores, below 40 cm depth at E9 and below 60 cm depth at E6. The profiles suggest that Mo is involved in the reductive dissolution of oxides in the surface sediments, but that unlike Mn much of the Mo is retained in the sediments as an enrichment at depth. It is clear t h a t there are profound changes in the partition of Mo in the sediments as t h e y undergo diagenetic changes. Bertine and Turekian (1973) show an association of Mo with Mn in oceanic sediments, but an examination of the surface sediments of Loch Etive failed to reveal a clear association of Mo with Mn (S.J. Malcolm, unpublished data). The lack of correlation in Loch Etive may be explained in the varying redox status of the sediments within the loch associated with varying hydrography, which allows part of the loch to become stagnant. Within the profiles reported here the association of Mo with Mn is clear; however, the association of Mo with other components of the sediments below 10 cm is n o t clear. Studies of both recent and ancient sediments have suggested that Mo associates with the sulphides and/or organic matter. It is likely t h a t this is the state in the Loch Etive sediments.

218

Interstitial water

Sulphate and sulphide Sulphate concentration shows little systematic variation in the top 15 cm at E9 and 25 c m at E6 (Fig. 3). This has b e e n interpreted to represent a

zone of bioturbation (irrigation) in these sediments (Malcolm, 1981), but t h e d e p t h is m u c h greater than t h e z o n e s o f b i o t u r b a t i o n n o t e d in o t h e r n e a r s h o r e m a r i n e s e d i m e n t s ; t h e s e are l i m i t e d to t h e t o p 10 c m o f s e d i m e n t

(e.g., Goldhaber et al., 1977). Elderfield et al. (1981) show profiles from one site (JN) where bioturbation extends to the depths seen in these sediments. The concentration of sulphate in this upper zone is higher than that in the overlying water; this is probably due to the oxidation of sulphide produced during sulphate reduction (Goldhaber and Kaplan, 1980). The potential for s u l p h a t e r e d u c t i o n in these s e d i m e n t s is d e m o n s t r a t e d by l a b o r a t o r y incub a t i o n of the surface sediments (Malcolm et al., 1985). Below the zone of

bioturbation, sulphate is depleted from the pore waters, the concentration being near z e r o b e l o w 40 and 6 0 c m at E9 and E6, respectively. S u l p h i d e

Sulphate mM 0

5

10

15

10

20

Sulphide mM 25

1 w

Absorbance 280nm

2 r

3

05

10

CY O

20

O

O

O

O

30

O

0

O

O

0 0

0

40 0

O O

0

D 50 0 :) 60 )

0

O O

0 0

70 D

,

,

0

O O

0 0

O O

E9 2 l

,~,

=

111

0

2~

0

O

0

0

3G

0 0

0 0

4C

0

O 0

5C 60 ~

70 ~

0 0

0 0

0

O

0

O

0 0

0 0

0 0 0 0

E6 Fig. 3. Profiles versus depth of SO~-, ~H2S and absorbance at 280 nm (Abs2s0) in the interstitial water of sediments from Loch Etive stations E9 and E6.

219

varies at low to undetectable concentrations to 20 cm depth at E9 and to 30 cm depth at E6, approximately coinciding with the depths of the m o o t e d bioturbation zone. These sulphide profiles are consistent with the interpretation of sediment biogeochemical structure given above. Sulphide concentration rapidly increases with depth, reaching a m a x i m u m at 45 and 60 cm at E9 and E6, respectively, then decreasing. The m a x i m u m concentrations are 2.1 mM at E9 and 2.6 mM at E6, suggesting that the sedimentation rate at E6 is higher than that at E9 (Goldhaber and Kaplan, 1975). The initial rate of sulphate reduction with depth may be used to estimate the sediment accumulation rate (Berner, 1978). The sediment accumulation rate by this m e t h o d is 0 . 5 1 c m y -1 at E9 and 0 . 4 9 c m y -1 at E6, rates indistinguishable from each other. A bsorbance and p H Absorbance at 280 nm increases with depth at both sites. (Fig. 3), as seen in other nearshore sediments (Krom and Sholkovitz, 1977; Elderfield, 1981). The increase is most rapid in E9 sediments but decreases below an inflexion point at 30 cm depth. The pore waters at E9 contain more dissolved organic matter at depth than those at E6; this was confirmed by measurements made on a few samples by a photo-oxidation method. The concentration of dissolved organic carbon is 1.5 mg l-1 in the overlying water at both stations and increases to a m a x i m u m of 90 and 74mg1-1 at E9 and E6, respectively. The pH decreases from 7.8 in the overlying water to 7 . 4 - 7 . 5 in the surface sediments. The pH remains virtually constant with depth. Iron and manganese The profiles of Fe and Mn in the interstitial waters are shown in Fig. 4. The data are consistent with the known mobility of both Fe and Mn under the reducing conditions f o u n d in nearshore organic-rich sediments and are similar to those reported from other sites (e.g., Calvert and Price, 1972; Holdren et al., 1975; Aller, 1978; Elderfield et al., 1981 and others). The sediments become anoxic within a couple of centimetres below the sediment/ water interface, resulting in the rapid reductive dissolution of Fe and Mn o x y h y d r o x i d e s and release some of the Fe and Mn into the pore water. Substantial concentration gradients are developed with resulting diffusion of reduced Fe and Mn upward into the water column, where oxidation and precipitation will occur, and downward to be removed into a solid phase. Assuming that the upper zone of both cores is subject to major irrigation, the rate of the diagenetic reactions resulting in the redox change responsible for the Fe and Mn solubility is faster than the rate of bioturbation. This is in contrast to the rate of sulphate reduction, which is slower than the rate of bioturbation. There is a major turnover of Mn in the surface sediments, maintaining the enrichment of Mn at the sediment/water interface. There is no similar

220 MO 5O

,o

ppb 100

Mn 2O0 i

150

400 J

ppb

' 0

Fe

800 o J

_%8 0 0

0 0

0 0

30

0

0

0

0 .0

0

0

0 0 0

00

2

0

.0 O 0

0 0

70

°O 0 0

0 0

0 0

0

50

2000

0

20

40

ppb

1000

E9 200

400

o 10 0 20

0

.0%0,

0 .0

0

30

0

0 O

0 }

40

0 0

0 0

3 )

0 D D D

D 3 ) )

0 JO

D )

0

0 50

0 0

60

0 0

70

0

E6

Fig. 4. Profiles versus depth of total dissolved Mo, Mn and Fe in the interstitial water of sediments from Loch Etive stations E9, E6. enrichment of Fe in the surface sediments due to the faster kinetics of reactions o f Fe compared to Mn, i.e., precipitation reactions are fast enough to permit little transport o f Fe in pore waters. It is difficult to detect changes in the solid-phase Fe distribution because the bulk of the Fe is present in alumino silicates and n o t digenetically available. For example, a sample from the E9 core which had a total concentration of 4.97 wt% Fe contained only 0.45 wt% 1 N HCl-leachable Fe.

Molybdenum The interstitial water profiles of Mo are shown in Fig. 4. The profiles contrast with those of Fe and Mn in that there is no sharp maximum near the sediment/water interface. The concentration of Mo increases from below that in the overlying water ( ~ 1 0 p p b ) to maximum concentrations of 170 and 130 ppb at E9 and E6, respectively. These concentrations represent considerable enrichments with respect to seawater and are similar to the few reports o f pore water Mo in the literature (Contreras et al., 1978; Lyons et al., 1980; Brumsack and Gieskes, 1983). Maximum concentrations are reached at 25 cm in E9 sediments and at 35 cm in E6 sediments; below these

221

depths the concentration of Mo decreases, rapidly at E9 and more slowly at E6. This marked decrease with depth is not observed in data from the Gulf of California (Brumsack and Gieskes, 1983) nor in the Gulf of Maine sediments (Contreras et al., 1978). Brumsack and Gieskes (1983) show a positive correlation between the pore-water concentrations of Mo and the UV absorbance of the pore water measured at 375 nm, t h o u g h t to correlate with low-molecular weight organic m a t t e r and shown to be linearly related to dissolved organic carbon (DOC). Figure 5 is a plot of total dissolved Mo versus absorbance at 280 nm. There is a good linear relationship between Mo and Abs2s 0 up to absorbance values of 0.6, suggesting that in these pore waters there is a close association of Mo with dissolved organic matter (DOM). At the higher concentrations of DOM (Abs2s 0 ~ 0.6) the relationship breaks down. The same breakdown of the association of Mo and DOM is seen in Fig. 6a of Brumsack and Gieskes {1983) but at much higher Mo concentrations ( ~ 600 ppb). An a t t e m p t was made to investigate the association of Mo with DOM in these pore waters further by using an operational 'speciation' technique which measured a 'labile' fraction with respect to a chelating resin (Chelex100, Biorad) and a 'non-labile' fraction. The fraction which was non-labile was subsequently digested with HNO3; it was found that all of the Mo then become labile, that is, extractable on the Chelex-100 (Florence and Batley, 1975). The extraction of Mo from seawater is based on the methods of Riley and Taylor (1968) and Ternero and Gracia {1983). Samples of pore water from all depths in the two cores investigated were analysed using the speciation procedure and the detailed profiles shown in Fig. 6. In the upper portion of both cores, where total Mo is increasing, all of the dissolved Mo is present in the 'non-labile' fraction. As this is likely to be organically associated Mo, these data confirm the close association of Mo with DOM in marine pore waters. Below the peak of concentration an increasing proportion of the dissolved Mo is present in the 'labile' fractions. Elderfield (1981) suggests that an observed decrease in organically associated metals in

0

0

0 0

0

O.8 E o O

~0.4

o O o

0.2

o

•

:.

'

~E6

o •

0

o~,

I0

'

,~

'

do

'

,~'o

'

M o ppb

Fig. 5. Total dissolved M o

versus a b s o r b a n c e at 2 8 0 nrn (Abs2s0) in the interstitial water.

222 Mo

50

Fog 2O

(o)

ppb

100

150

°/o

non ~iabile

20o

20

40

60

(b) 80

100

o

0

•

0

0

0 0

3O

0

•

4f

0

0

•

•

0

0

0

g •0 N

0

O•

0 0 0

E9 %

10 i ~ ) ~ 20

• 0

oo

o

30

0

40

00 0

50 6G

Oo

0 0 0

70 CI) 0

•

•

0 0 0 non labile

0

0

• labile

0

E6 Fig. 6. Profiles versus depth of (a) labile (e) and non-labile (©) Mo and (b) percentage non-labile contribution to the total dissolved Mo concentration in interstitial waters of sediments from Loch Etive stations E9 and E6.

Narragansett Bay pore waters is due to dilution by the addition of metalpoor organics with depth in the sediments. This is clearly not the case in these pore waters where Mo is being removed from the Mo-organic material and is precipitating either associated with Fe sulphides or as discrete Mo sulphide. The decrease in the percentage of 'non-labile' Mo, together with the increase in dissolved sulphide concentration, may suggest that transfer of Mo from pore water to sediment is achieved by competition between sulphide (and other soluble reduced-sulphur compounds) and the DOM. Lyons et al. (1980) suggest the possible importance of Mo polysulphides in the pore waters of sulphide-containing sediments. Further speculation is, however, dependent on greater knowledge of the nature of the Mo-organic binding in the pore waters. It appears that much of the Mo entering a sediment is eventually retained in association with the sulphide fraction, although apparently entering in association with Mn. The difference in the behaviour of Mn, which is predominantly recycled in the surface sediments, and Mo may perhaps be explained by the differing reaction with DOM in the pore waters. After reduction, Mn is predominantly present in an ionic form which may readily diffuse out of the sediment (with or without advection due to bioturbation), while Mo, which is tied to relatively high-molecular weight dissolved organic material, will have a much lower diffusion coefficient and consequently inhibited transport. Elderfield {1981) shows the diffusion of organically

223

associated metals is not likely to be a major factor in metal transport in sediments. In conclusion, this work has shown that Mo is significantly enriched in the surface sediments of Loch Etive, in association with Mn. The possibility of scavenging of Mo from the overlying water by reductive adsorption onto organic matter (Szilagyi, 1967; Contreras et al., 1978; Brumsack and Gieskes, 1983} cannot be discounted. In the pore waters Mo is associated with DOM, which is probably HMW organic material being created de novo from small organic molecules and may be the precursor of sedimentary humates. The Mo mobilised in the surface sediments is removed from the association with DOM and transferred to a solid sulphide phase at depth. Mo has a very active geochemistry in anoxic sediments. ACKNOWLEDGEMENTS

The author would like to thank the Captain and crew of the R.V. "Calanus" for assistance in collecting the samples from Loch Etive. Miss A.T. MacDonald assisted with some of the field operations and sample preparation. This is a contribution from the Sediment Processes Group of the Scottish Marine Biological Association. The Scottish Marine Biological Association is a grant-aided b o d y of the Natural Environment Research Council. REFERENCES Aller, R.C., 1978. Experimental studies of changes produced by deposit feeders on pore water, sediment and overlying water chemistry. Am. J. Sci., 278: 1185--1234. Berner, R.A., 1978. Sulphate reduction and the rate of deposition of marine sediments. Earth Planet. Sci. Lett., 37: 492--498. Berrang, P.G. and Grill, E.V., 1974. The effect of manganese oxide scavenging on molybdenum in Saanich Inlet, British Columbia. Mar. Chem., 2: 125--148. Bertine, K.K., 1972. The deposition of m o l y b d e n u m in anoxic waters. Mar. Chem., 1: 43--53. Bertine, K.K. and Turekian, K.K., 1973. Molybdenum in marine deposits. Geochim. Cosmochim. Acta, 37: 1415--1434. Bowen, H.D.M., 1979. Environmental Chemistry of the Elements. Academic Press, London, 333 pp. Bray, R.C., 1975. Enzymes, Vol. 12. 3rd edn., Academic Press, New York, pp. 299--419. Bricaud, A., Morel, A. and Prieur, L., 1981. Absorption by dissolved organic matter o f the sea (yellow substance) in the UV and visible domains. Limnol. Oceanogr., 26: 43--53. Bruland, K.W., 1983. Trace elements in Seawater. In J.P. Riley and R. Chester (Editors), Chemical Oceanography, Vol. 8. Academic Press, London, pp. 157--220. Brumsack, H.J., 1980. Geochemistry of Cretaceous black shales from the Atlantic Ocean (DSDP Legs 11, 14, 36 and 41). Chem. Geol., 31: 1--25. Brumsack, H.J. and Gieskes, J.M., 1983. Interstitial water trace-metal chemistry o f laminated sediments from the Gulf of California, Mexico. Mar. Chem., 14: 89--106. Calvert, S.E. and Morris, R.J., 1977. Geochemical studies of organic rich sediments from the Namibian Shelf. II. Metal--organic associations. In: M.V. Angel (Editor), A Voyage of Discovery. Pergamon Press, New York.

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