Marine Chemistry, 23 (1988) 283-293 283 Elsevier Science Publishers B.V., Amsterdam - - Printed in the Netherlands

ON THE SULPHUR CHEMISTRY OF A SUPER-ANOXIC FJORD, FRAMVAREN, SOUTH NORWAY

LEIF G. ANDERSON, DAVID DYRSSEN and PER O.J. HALL*

Department of Analytical and Marine Chemistry, University of GSteborg and Chalmers University of Technology, S-412 96 GSteborg (Sweden)

(Received June 1, 1987; revision accepted August 1, 1987)

ABSTRACT

Anderson, L.G., Dyrssen, D. and Hall, P.O.J., 1988. On the sulphur chemistry of a super-anoxic fjord, Framvaren, South Norway. Mar. Chem., 23: 283-293.

The concentrations of total carbonate (Ct), sulphate, sulphide, thiols and oxygen, the ratio between the stable sulphur isotopes aS and 328 in sulphate and sulphide, and the density (used to calculate salinity) were determined on samples from the water column of Framvaren, a super- anoxic fjord in southern Norway. From a depth of 18m (the oxic-anoxic boundary) the initial sulphate concentration, ([804Lit), as calculated from salinity, is significantly higher than the sum of the measured sulphur species. This is attributed to a loss of sulphur from the water column. The amount of total carbonate produced, corrected for the initial concentration (C t - 2.4 Sal/35) is found to be proportional to the amount of sulphate consumed, ( [804 ] in i t -- [ 8 0 4 ] ) , according to the following relation C t - 2.4 Sal/35 = 1.84 ([SO4]ini t - [ 8 0 4 ] ). Isotopic fractionation caused by bacterial sulphate reduction in the anoxic part of the water column produces sulphide with a 5~S ~ 40%0 lower than the 5~S for sulphate at corresponding depths. The isotopic fractionation also results in a 5~S value for the remaining sulphate at depths below 80m being considerably higher than the mean value for ocean water, which is close to + 20%0. The $~S values for sulphate at depths between 10 and 50 m were lower than + 20%0 which indicates oxidation of sulphide, which follows upon diffusion of sulphide from deeper parts of the water column and inflow of oxygenated seawater over the sill into the anoxic water of the fjord. A conclusive scenario of the Framvaren sulphur chemistry is presented.

INTRODUCTION

The d i s t r i b u t i o n of c h e m i c a l c o n s t i t u e n t s i n a f jord is d e t e r m i n e d by s e ve r a l

fac tors , e.g. w a t e r e x c h a n g e w i t h the sea, s u p p l y of o r g a n i c m a t t e r , d e c a y r a t e a n d t h e r m o d y n a m i c e q u i l i b r i a . I f t he w a t e r e x c h a n g e is l i m i t e d c o m p a r e d w i t h

the s u p p l y of o r g a n i c m a t t e r a n d decay ra te , a n a n o x i c e n v i r o n m e n t deve lops , u s u a l l y s t a r t i n g i n the deepes t p a r t of the fjord. As a r e s u l t of the de c a y processes the c o n c e n t r a t i o n s of h y d r o g e n s u l p h i d e a n d t o t a l c a r b o n a t e i n c r e a s e a n d pH a t t a i n s a low va lue . O t h e r c o n s t i t u e n t s , s u c h as n u t r i e n t s , wi l l

a lso show i n c r e a s e d c o n c e n t r a t i o n s a n d a d i f f e r en t c h e m i c a l e n v i r o n m e n t is formed.

0304-4203/88/$03.50 © 1988 Elsevier Science Publishers B.V.

2S,I

Under anoxic conditions, the decay process that uses sulphate as electron acceptor will not only result in changes in the concentrations of sulphur, but. also in changes in the isotopic ratios of the sulphur species. The ~3:~:~S value is expressed in per mille according to the following equation.

(~349 :: [ (:~4S/32S)samp]e 1 ] ~ 1000 (%0)

The standard is Canyon Diablo triolite (FeS) with a 34S/3~S ratio of 22.22%0. The percentage ratio of :~/z2S in nature varies around 4.2/95.0, but ocean

waters have a remarkably constant value of 6~4S for sulphate with a mean equal to -~ 20.1%o (Rees, 1973). The sulphate-reducing bacteria metabolize the light isotope at a higher rate than the heavy isotope, giving a lower value of ~4S for sulphide than for sulphate. The fractionation is usually of the order of 206000o (J6rgensen, 1979 and references therein). The oxidation of sulphide to elemental sulphur and to sulphate produces only a very small shift in the isotopic ratio. Likewise, the transformation of sulphide to iron sulphides involves practically no isotopic shift (el. Faure, 1986).

The objective of this study was to investigate what the concentrations and isotopic ratios of different sulphur species can tell us about the biogeochemical processes and hydrographical properties of Framvaren Fjord.

METHODS

Framvaren is a fjord with a shallow sill depth of 2 m and a basin depth of 183 m. Further information about the fjord is given in Skei (1988, this volume).

The seawater samples discussed in this paper were collected in June 1982, September 1983 and February 1985. The samples were analysed for density, sulphate, sulphide and total carbonate. The determination of 3:34S for the 1982 and 1983 samples was performed by Dr Eric Welin (Swedish Museum of Natural History) on precipitates of zinc sulphide or silver sulphide and barium sulphate.

The density was determined at the laboratory in G6teborg, using a PAAR density meter with a precision of 0.05% (relative standard deviation). Sulphate was determined gravimetrically by barium precipitation in the 1982 and 1983 experiments. In the 1985 experiment, sulphate was determined (after acidifi- cation and degasification) by a potentiometric lead ti tration in ethanol medium (precision for duplicate titrations, about _+ 0.05 mmol per kg seawater (mMw)). Sulphide was determined colourimetrically within hours of collection according to Cline (1969) in the 1982 and 1983 experiments, and in 1985 by the potentiometric mercury(II) chloride t i tration of Boul6gue (1981) with a treatment of the data according to Dyrssen and Wedborg (1986). The accuracy of these methods depends mainly on how the samples are handled with respect to air oxidation. Total carbonate (Ct) was calculated from pH and alkalinity. The pH was determined directly after sample collection and the alkalinity was determined by acid ti tration in 1982 and 1983 using potentiometric detection

T A B L E I

285

S u l p h u r c o m p o n e n t s w i t h i s o t o p e a n a l y s e s ( F r a m v a r e n , 20-21 J u n e 1982)

D e p t h Ct a [804 ]init c [804] [02] [S -2 ] [S] (~348 (%0)

(m) (mM) (mM) (mM) (mM) (mM) (mg 1 ' ) ZnS d BaSO4 e

1 0.94 8.93 8.40 0.302 - 0 + 19.5

9 1.36 14.76 14.64 0.333 - 0.09 + 20.3

12 1.62 15.76 15.24 0.229 - - + 19.1

14 1.61 16.39 13.38 0.175 0 - + 14.7

18 2.25 17.28 15.39 0 0.037 - + 21.1

22 2.67 17.42 13.55 0 0.191 - + 21.5

26 3.18 17.34 13.29 0.198 - - 5.8 + 21.7

50 4.06 17.61 13.11 - 0.680 0.05 - 7.5 + 23.2

70 6.04 17.72 15.33 - 0.976 0.30 - 9.9 + 24.8 110 14.27 b 18.73 11.08 5.60 0.04 - 2.4 ÷ 37.9

130 16.37 b 18.87 10.04 7.29 0.13 - 3.5 ÷ 44.2

170 16.78 b 18.97 9.75 - 8.12 0.13 + 2.3 + 46.7

a Ct d e n o t e s t h e t o t a l i n o r g a n i c c a r b o n a t e .

b T h e s e v a l u e s s e e m to be t oo low.

c C a l c u l a t e d f rom t h e r a t i o of s u l p h a t e to c h l o r i n i t y as 0.1400 _+ 0.0014 g k g - ' p e r %0 or 28.73 _+

0.29 m M for a s a l i n i t y of 35 g k g 1. d E s t i m a t e d e r r o r 1-2%0 e3a = 2.1%0.

according to Johansson and Wedborg (1982) and, in 1985, using photometric detection according to Anderson and Wedborg (1983). The precision of the total carbonate determination was + 0.015 mMw. In 1982 and 1983 the samples were brought back to G6teborg in Winkler bottles, but in 1985 the t i trations were carried out within hours of collection. All data, with the exception of the isotope determinations, are given in the Framvaren Data Report (Skei, 1986).

RESULTS AND DISCUSSION

The results are summarized in Tables 1, II and III. A depth profile of the accumulated sulphur species is presented in Fig. 1. The profile shows that below 20 m the sum of sulphate and sulphide is almost constant; however, there is a shift in relative distribution to more sulphide towards the bottom of the fjord. The sulphate concentration calculated from the salinity is higher than the sum of the measured species. This indicates either a loss of sulphur from the water column or a high concentration of an undetected sulphur species. The increasing concentration of sulphide towards the bottom of the fjord is followed by an increased concentration of total carbonate as expected when organic matter is mineralised. The relationship between the excess total carbonate ( C t - 2.4 salinity/35) and the amount of reduced sulphate ( [ S O 4 ] i n i t - [SO4]) is shown in Fig. 2. A straight line with the equation, Ct - 2.4 salinity/35 = 1.84 ( [SO4] in i t -- [ 8 0 4 ] ) , is fitted to the data. This slope (1.84) does

286

TABI, E 11

Data for the 1983 exper iment .

Depth Sa l in i tv [SO,~ I 5~4S [HS] .:~4 ~ S l n l t ,

(m) (mMw) ~' (%) (mMw) (%) (raM,,) (mMwI

0 15.04 19.8 1.09t 12.15

2 14.72 19.6 1.066 1 i.89

4 15•67 20.1 1.091 12.66

6 15.77 19.8 1.092 12.74

8 17.55 13.97 18.0 t.208 14.18

10 18.69 16.6 1.290 15.10

t2 19.02 15.8 1.337 15.37

14 19.46 14.66 16.9 !.416 15.72

16 19.84 15.04 15.8 1.525 16.00

18 19.92 15.06 14.8 0.002 1.522 16.10

20 20.42 16.23 17.8 0.050 1.960 16.50

22 20.85 (18.48) 17.3 0.140 2.209 16.85

25 21.02 17.4 0.175 19.6 2.494 16.98

30 21.23 17.3 0.329 - 19.8 2.621 17.15

40 21.32 (19.29) 16.1 0.604 - 19.4 2.805 17.23

50 21.44 16.9 0.865 17.1 3.403 17.32

80 21.82 13.87 22.9 2.34 11.6 6.907 17.63

110 22.63 28 5.60 - 11.5 15.337 18.29

130 22.86 9.77 30 6.36 - 10.8 17.306 18.47

150 23.04 31 6.63 10.8 18.316 18.62

170 23.02 9.55 30 6.69 - 10.8 18.528 18.60

mM w denotes mmol per kg seawater. b Ct denotes the total inorganic carbonate•

Calculated from the salinity assuming a constant sulphate to salinity ratio of 28.28/35 (mMw).

not agree with the value of 2 which would be expected if only microbial decay, using SO~ as electron acceptor, was responsible for the concentration changes. However, a line with a slope of 2 would go through the two open rings at Ct concentrations of around 10 and 15 mMw. This indicates either a loss of Ct or a consumption of SO~ at depths between 50 and 80 m and below 130 m. We know that there is a chemical formation of CaCO~(s) in the bottom water (Anderson et al., 1987) which could explain the deficit in Ct below 130 m. The deviation from the slope (2) at depths between 50 and 80 m is more difficult to explain, but one plausible cause is the formation of MnCO3(s).

The history of Framvaren is not accurately known; however, around 1850 a channel was made that transformed the 'Lake Framvaren' into a fjord. It is our assumption that at that time a total exchange of water took place and oxic seawater filled the whole fjord. This assumption is strengthened by sediment data, which show a dramatic change in colour from light-coloured to black at a depth corresponding to about the year 1850. Furthermore, no shells of any marine plankton were found in the light-coloured sediments (Skei, 1988, this volume; Skei et al., 1988, this volume). Hence the decay of organic matter since

TABLE III

287

Data for the 1985 experiment

Depth Salinity [SO~] [HS] Thiols C t Sinit a

(m) (mMw) (mMw) (mMw) (mMw) (mM~)

0 5.59 4.27 - - 0.447 4.52 2 11.04 10.11 - - 0.831 8.92 4 11.39 9.88 - - 0.772 9.20 8 14.74 12.58 - - 1.013 11.91

12 15.65 14.78 - - 1.162 12.65 14 18.80 15.53 - - 1.420 15.19 16 18.90 15.55 - - 1.382 15.27 18 20.04 15.97 - - 1.716 16.19 20 20.99 16.18 0 0 1.910 16.97 22 21.34 17.00 0.072 0.005 2.246 17.26 24 21.21 16.74 0.103 0.010 2.286 17.15 26 21.22 16.58 0.173 0.010 2.496 17.16 30 21.70 16.54 0.319 0.010 2.942 17.55 40 21.46 16.51 0.495 0.018 3.467 17.35 50 21.68 15.93 0.670 0.030 4.177 17.54 70 21.75 15.28 1.435 0.025 5.428 17.59 80 - 14.33 1.934 0.074 7.127 - 90 22.56 13.40 3.304 0.126 11.289 18.24

100 1!.95 4.581 0.148 14.468 - 110 22.89 11.42 4.888 0.169 15.726 18.51 130 - 10.38 5.897 0.208 17.680 - 150 23.52 10.05 6.045 0.257 18.167 19.02 160 - 9.95 6.421 0.005 18.528 170 23.81 10.12 5.961 0.227 18.211 19.25

a Calculated from the salinity assuming a constant sulphate to salinity ratio of 28.28/35 (mMw).

1850 is r e s p o n s i b l e f o r t o d a y ' s c h e m i c a l e n v i r o n m e n t m a i n l y t h r o u g h t h e

f o l l o w i n g r e a c t i o n ( n o r m a l i s e d t o o n e p h o s p h a t e ) .

( C H 2 0 ) x ( C H 2 ) y ( N H 3 ) l s H 3 P O 4 + (x/2 + 3y/4 )SO~-

-* 15NH3 + H 3 P O , + (x/2 + 3 y / 4 ) H C O ~ + (x/2 + y /4 )CO2

+ (x/2 + 3 y / 4 ) H S - + (x/2 + y / 4 ) H 2 0

I n t h e a b o v e r e a c t i o n , t h e o r g a n i c m a t t e r is r e p r e s e n t e d b y a m o d e l

s u b s t a n c e h a v i n g a c a r b o h y d r a t e t o l i p i d r a t i o e q u a l t o x/y a n d a n i t r o g e n t o

p h o s p h a t e r a t i o o f 15. T h e s e r a t i o s a r e n o t n e c e s s a r i l y c o r r e c t s i n c e t h e

v a r i a t i o n i n m a r i n e o r g a n i s m s i s l a r g e . I n F r a m v a r e n t h e r e i s a l s o a l a r g e i n p u t

o f t e r r e s t r i a l m a t e r i a l ( t r e e l e a v e s ) , w h i c h is k n o w n t o c o n t a i n m a i n l y c a r -

b o h y d r a t e s . H e n c e i n t h e f o l l o w i n g c a l c u l a t i o n s w e d o n o t k n o w t h e e x a c t

r e l a t i o n s h i p b e t w e e n t h e t o t a l c a r b o n a t e c o n c e n t r a t i o n a n d t h e h y d r o g e n

s u l p h i d e p r o d u c e d . F u r t h e r m o r e , t h e C : N a n d C : P r a t i o s a r e a l s o u n c l e a r f o r

s i m i l a r r e a s o n s ( D y r s s e n , 1985, 1986).

2£S

50

( m m o l e / k g ) 5 10 I _ i

15 20

• 1oo 0

E

"i" l -

w 1 5 0 - a

t F~S~, THIOLS

20oJ Fig. I. A cumulat ive plot of the various sulphur species. The thick line respresents the init ial su lphate concen t ra t ion calcula ted from the sal ini ty using the SO 4/salinity ratio of 28.28/35 mM W .

Even if the limited water exchange with the deep water causes a build-up of hydrogen sulphide, an input of oxic seawater may still occur.

The considerable variation in sulphide concentrations between the different experiments at depths below 50 m (Tables I, II and HI) may be due to analytical difficulties. However, it could also be due to the fact that input ofoxic seawater through the canal is a very irregular process. The reduction of sulphate depends on the bacterial activity and thus the input of suitable food for the bacteria. The rate of bacterial activity (giving the rate of the sulphate reduction) also affects the isotopic fractionation. This may explain the difference between the 1982 and 1983 data for the plot of 5:~4S (sulphate) against #~4S (sulphide) shown in Fig. 3.

One effect of the water input can be seen in Table II from the low values of the isotopic composition of sulphate at depths between 10 and 50m. Oxidation of sulphide to sulphate will shift the isotopic ratio for sulphate to lower values, but it will not significantly alter the ratio for sulphide. The inflow of water to the anoxic layers will result in a reduction in the oxygen content of the inflowing water by the sulphide. Assuming that the inflow has a sulphate concentration of 15mMw and an oxygen content of 0.3-0.4mM,, one should obtain 0.15-0.2mMw of sulphate with a 534S value of -20%0 from the oxidised sulphide. The resulting 534S value for the sulphate of the inflowing water will then be 19.6-19.5 when it is mixed with the anoxic water.

In Fig. 4 the values of 534S in Table II are plotted against the excess of inorganic carbonate. It is seen that a reduction of about 4%o in the 534S value for sulphate is necessary to obtain the measured values at depths between 10

Ct- 2.4.Sal /35 (mM w]

289

15-

10-

5"

• 1983 ()1985

~, 110

[SO,]ini,- [S04] [mMw)

Fig. 2. Relationship between the excess carbonate and the reduced sulphate. The line fitted to the data has a slope of 1.84.

and 50 m. If the inflow of oxic water has been a continuous process since 1850, an annual input of around 6% by volume is necessary in order to reduce the ~ S value for sulphate by 4%o. Of course, in the upper part of this depth interval the low ~34S value for sulphate is obtained by a flux from below, a flux that is a result of the water input to the deeper layers.

Figure 5 shows the isotope data from Table II plotted against depth. This figure also shows the deficit in the ~34S value for sulphate at some depths. Furthermore the high value of 534S for sulphide at 80 m indicates a flux from below.

CONCLUSION

A conclusive scenario is shown in Fig. 6. The surface water containing oxygen is diluted with fresh water from the watershed (which is a granite shield area) as well as from the Lyngdal River which runs into the Hellvik Fjord

290

. (Su phate ( ,: < : , , /.

40-

30- '

2

/ 1983

20 / • • •

~0 r 1

20 10 ,:,

,~s4 S (Sulphide) (o/~)

i

Fig. 3. A plot of ~34S for s u l p h a t e aga i n s t 5~4S for sulphide. The l ine r ep resen t s a f r ac t iona t ion of 40°/~.

outside Framvaren. Below 18m hydrogen sulphide appears. Due to the chemical environment at the interface we assume that FeOOH(s) is trans- formed into FeS(s). Both of these solids may scavenge trace metals. The decay of organic mat ter takes place at all depths, but most of the decay occurs in the redoxcline and at the sediment-water interface (Naes et al., 1988, this volume). Input of oxic seawater to the anoxic layers will oxidise hydrogen sulphide which has a low 534S value. The resulting upward flux will also cause oxidation of hydrogen sulphide by oxygen from the top layer. The oxidation will produce several sulphur species.

Some reactions of special interest, which take place in the whole anoxic water column are

H2S e 0.502-~ S + H~O

S + HS -~ HS2

FeS(s) + H ~ --* Fe 2' + HS

Fe 2~ + HS2 --* FeS2(s) + H '

291

C t [excess]

[mMw]

15-

10

¢

Io

i i

I -20

i i

J i i J i

J i J f i i i

o 8 0 m

J

i

i

J i i

i i J f i

e,

J J i i i J i i

- - - T I -30 -1"0 (] +1'0 +20 +30

( ~ ) 3 4 S ( % 0 )

Fig. 4. The isotope rat io for sulphide (open circles) and sulphate (filled circles) plotted against the excess of inorganic carbonate (Ct - 2.4 salinity/35). The broken lines are parallel with a difference (fractionation) of 40%0.

A

" r

I - n L U O

100 -

200

- 2 0 - 1 0

I I

\ \

\q

0 10 20 30

o l o . I

I

34 32 Fig. 5. Depth profiles of the isotope rat io S/ S. The filled circles are the ~s4S values for sulphide and the open circles the ~s4S values for sulphate. The broken and dotted lines are drawn to fit the broken lines of Fig. 4.

292

v . . . . . . . . . . . . . . . . . % . . . . ,o,o,o ,2::: . . . . . . . . . . . . . . . . .

/

(~ . . . . . . . . , ~ Sso ) / 'i

o E o , Y

Fig. 6. Conclusive scenario of the Framvaren sulphur chemistry.

The last reaction removes sulphide as pyrite. Sulphate with a low 5"~4S value is produced by

HS ~ 2 0 2 ~ SO~ + H

a reaction that only takes place at the anoxic intermediate layer (Fig. 6). In 1982, sulphate with a low g34S value was only found at 14 m (Table I). In 1983 it was found from 8 to 50 m (Table II).

Future research should use meteorological information and temperature and salinity records of the water in the canal to the Hellvikfjord, to account for the inputs of seawater and the entrainment which accompanies these inputs. It should also be possible to obtain more information on the input of organic matter, both with regard to its nature and its seasonal variation.

ACKNOWLEDGEMENT

This work was supported by the Swedish Natural Science Research Council:

REFERENCES

Anderson, L. and Wedborg, M. 1983. Determination of alkalinity and total carbonate in seawater by photometric titration. Oceanol. Acta, 6: 357-364.

Anderson, L.G., Dyrssen, D. and Skei, J., 1987. Formation of chemogenic calcite in super-anoxic seawater .... Framvaren southern Norway. Mar. Chem., 20: 361-376.

Boul6gue, J., 1981. Simultaneous determination of sulfide, polysulfide and thiosulphate as an aid to ore exploration. J. Geochem. Explor., 15:21 36.

Cline, J.D., 1969. Spectrophotometric determination of hydrogen sulfide in natural waters. Limnol. Oceanogr., 14: 454-458.

Dyrssen, D., 1985. Some calculations on Black Sea chemical data. Chem. Scr., 25: 199--205. Dyrssen, D., 1986. Chemical processes in benthic flux chambers and anoxic basin waters. Neth. J.

293

Sea Res., 20: 225-228. Dyrssen, D. and Wedborg, M. 1986. Titration of sulphide and thiols in natural waters. Anal. Chim.

Acta, 180: 473479. Faure, G., 1986. Principles of Isotope Geology, 2nd Edn. Wiley, New York, p. 589. Johansson, O. and Wedborg, M., 1982. On the evaluation of potentiometric titrations of seawater

with hydrochloric acid. Oceanol. Acta, 6: 209-218. JSrgensen, B.B., 1979. A theoretical model of the stable sulfur isotope distribution in marine

sediments. Geochim. Cosmochim. Acta, 43:363 374. Nses, K., Skei, J.M. and Wassman, P., 1988. Total particulate and organic fluxes in anoxic

Framvaren waters. Mar. Chem., 23: 257-268. Rees, C.E., 1973. A steady-state model for sulphur isotope fractionation in bacterial reduction

processes. Geochim. Cosmochim. Acta, 37:1141 1162. Skei, J.M., 1986. The Biogeochemistry of Framvaren. A Permanent Anoxic Fjord Near Farsund,

South Norway. Data Report 1931-1985, Norwegian Institute for Water Research, p. 256. Skei, J.M., 1988. Framvaren - - environmental setting. Mar. Chem., 23: 209-218. Skei, J.M., Loring, D.H. and Rantala, R.T.T., 1988. Partitioning and enrichment of trace metals in

a sediment core from Framvaren, South Norway. Mar. Chem., 23: 269-281.