Hydrobiologia 235/236 : 205-217, 1992 .B. T. Hart & P. G. Sly (eds), Sediment/ Water Interactions .© 1992 Kluwer Academic Publishers . Printed in Belgium .

Sulphur isotope ratios in sulphate and oxygen isotopes in water from asmall watershed in Central Sweden

Per Andersson,** Peter Torssander & Johan Ingri*Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden ; *present

address: Department of Economic Geology, Lulea University of Technology, S-951 87 Lulea, Sweden**Present address : Division of Geological and Planetary Sciences, California Institute of Technology,Pasadena, CA 91125, USA

Key words: Sulphur isotopes, oxygen isotopes, precipitation, streamwater, lakewater, sulphatereduction

Abstract

During 1988-89 water samples for sulphur and oxygen isotope measurements were collected in the LakeMjtsjtn watershed (7 .3 km2), central Sweden. Samples included : precipitation, throughfall, lakewater,shallow groundwater and inlet and outlet streams .The S34 S of sulphate in precipitation ranged from + 6 .41%. in winter to + 3 .88%. in summer, the

higher winter values attributed to seasonal differences in the kinetic and equilibrium isotope fractionationduring oxidation of atmospheric sulphur dioxide to sulphate . The S34S in rain samples and in pine andspruce throughfall were similar, indicating no gain of sulphur from the trees . In the inflowing stream,the S 34 S value increased as discharge decreased, from + 5 .57%. in spring to + 26.21%. in summer,indicating bacterial sulphate reduction . The fluctuations in the inlet water were damped by the lake andin the outlet water, only a small decrease in the 6 345 value during spring discharge was observed .

During winter 1988-89, the near surface waters in the lake showed the same 534S as snow indicat-ing that meltwater governs the isotopic composition . During the winter, the 634S in the near bottomwaters increased while oxygen decreased due to bacterial sulphate reduction in the sediments . This alsocaused an increase in the alkalinity in the near bottom waters .

Based on the 5 18o data the water within the watershed is derived largely from meteoric water . Duringspring discharge, meltwater governs the inflow and outflow stream while additional groundwater influ-ences occurred during the drier period . Most sulphur is derived from atmospheric deposition and theS34S in sulphate increased during passage through the watershed due to bacterial sulphate reduction .

205

Introduction

The study of sulphate in coniferous forest eco-systems has grown during recent years mainlybecause it is the major anion in areas with acidrain, i .e. northern Europe and eastern NorthAmerica. Most studies of sulphur in coniferouswatersheds concern input and output studies toassess man-made influences on forest ecosystems

(Likens et al., 1977; Johnson, 1984) . However, inrecent years the study of stable isotopes in eco-systems has become more common as a usefulcomplement to mass balance studies, since iso-tope data may indicate source-sink relations andbiogeochemical processes (Peterson & Fry, 1987) .

In most forest ecosystems dissolved sulphur isderived mainly from the atmosphere (Waring &Schlesinger, 1985) . During its passage through a

206

terrestrial watershed the sulphate is exposed tovarious geochemical processes which are con-trolled mainly by residence time and redox state .The 34S/32S isotope ratio in sulphate is largelyaltered by biogeochemical processes (Kaplan,1975). This isotope ratio constitutes the key forstudy of processes influencing sulphate in a co-niferous watershed .The 180/160 isotope ratio in water has been

extensively used as a tracer in hydrogeologicalstudies (Fontes, 1980) . The 180/160 in precipita-tion is basically controlled by temperature, withlower values during winter and higher values dur-ing summer . The interpretation of temporal vari-ations in the 180/160 ratio in a catchment in lightof the ratios in groundwater and precipitation,can be used to apportion stream and lakewaterinto their precipitation and groundwater compo-nents .

This paper discusses an isotopic survey of asmall coniferous watershed in Central Sweden .The study examined snow, rainwater, pine andspruce throughfall, streamwater, lakewater andshallow groundwater during 1988-89 . The totalsulphur concentration and the 34S/32S ratio of thesulphate were determined in order to study thepathways and biogeochemical processes of sul-phate. The 180/160 ratio in water was used toreveal the origin and pathways of that water .

Experimental details

Study area

Figure 1 shows the location of the Lake MjOsjdnwatershed, about 10 km from the Gulf of Bothniain Central Sweden (N 62'39' : E 17'46'). Thetotal watershed area is 7 .3 km2 , including a lakearea of 0.5 km2 . The elevation ranges between116 and 240 m.(a.s .l .) . The Precambrian bedrockis dominated by gneisses, mostly metamorphosedgreywackes and other sediments (Lundqvist,1990). Outcrops of bedrock are numerous onridges and slopes, while Quaternary deposits are0-5 m thick in low-lying areas . The soils are ofglacial origin, mainly composed of coarse silt, finesand and till of local provenance. After the de-

glaciation, some 7700 B .P., the area was coveredby the sea, the highest marine shoreline of thearea being 270 m above present sea level (Lun-dqvist, 1987) . Therefore, the soil within the wa-tershed has been reworked by wave-action, dur-ing the isostatic uplift of the area .

Peat and sphagnum mats up to several metresthick occur in the basin (Lundqvist, 1987) ; other-wise the basin is covered with pine and spruce,with some minor clearcuttings . The mean annualprecipitation is about 650 mm and the mean an-nual temperature + 4 .4 ° C (SMHI, 1988). Theclimate is characterized as boreal forest with amaritime influence . During winter, approximatelyNovember-December to March-April, themonthly mean temperature is below 0 'C . At thattime the area is blanketed with snow and the lakesare covered with ice . Lake Mjosjon is an olig-otrophic forest lake, slightly acidified, with a max-imum depth of 10 m, a mean depth of about 4 .5 mand an area of 0 .22 km2 .

Sampling

The locations of the sampling sites are indicatedin Fig. 1 . Bulk precipitation was sampled using a10 L polyethylene bottle and a funnel . The upperand lower openings of the funnel are 21 cm and2 cm, respectively . The upper opening was cov-ered with a nylon screen (1 x 1 mm mesh) . Sim-ilar types of sampling equipment is commonlyused in Sweden (Ross, 1986) . Experiments haveshown that alteration of deuterium due to evap-oration from this type of sampling device is in-significant (Arnason, 1976) . The equipment wasplaced on a wooden stand 1 m above the groundin a glade about 100 m from the nearest tree .During winter, bulk snow was sampled accordingto the procedure described in Andersson et al.(1990). Throughfall (TF) was collected with sam-plers that consisted of 21 cm funnels with nylonscreens which drained into 2 L polyethylene bot-tles. These were placed under both pine andspruce. In order to obtain enough sulphate foraccurate isotopic determinations, water fromthree TF-samplers under each tree was com-posited .

LEGEND

Streamwater was sampled in 10 L polyethylenebottles and filtered, within 2 h of collection . Lake-water was sampled from the ice with a battery-powered peristaltic pump attached to a siliconerubber tube, which was lowered to the appropri-ate depth . All samples were filtered immediatelyafter collection .Groundwater was collected from a shallow

well, which was set in glaciofluvial material, lo-cated near the shore of the lake .The well is 5 mdeep with a diameter of 1 m covered with a locked

km

207

Fig. 1 . Map of the Lake MjOsjOn watershed . Sampling stations indicated : R, rainwater ; TF, pine and spruce throughfall ; S1 andS2, inflow and outflow stream ; DP, lake-water depth profile ; GW, groundwater .

cap preventing evaporation. The well is used everyday and therefore it was not necessary to purgeit prior to sampling . The sample was obtainedwith a plastic bucket and transferred to a 10 Lpolyethylene bottle for filtering within 2 h .

Analytical methods

All water samples were filtered through 0 .45 immembrane filters (Schleicher& Schtill ®) mounted

125 CONTOURS AT 25m INTERVALS

STREAM

WATERSHED BOUNDARY

SMALL ROAD

LAKES

0J

V SAMPLING STATION

208

in a polycarbonate holder . Filtered water for sul-phur isotopic determination was stored in 5 Lpolyethylene bottles . A 20 mL aliquot of eachsample was used for measurement of the oxygenisotopic composition of the water .

The pH was measured with a Metrohm® E 604pH-meter equipped with a glass combinationelectrode. The error in the pH measurements wasestimated to be ± 0 .1 pH unit. The alkalinity wastitrated directly in the field using 0 .32 M H 2SO4and a pH-meter. Oxygen saturation was mea-sured with a dissolved oxygen meter (YSI ® model58) giving oxygen saturation in % with an accu-racy of + 0.3% . Total S and Na concentrationswere determined with optical emission spectrom-etry using an inductively coupled plasma as ex-citation source (ICP-AES) .

The low sulphate concentrations in the waterrequired preconcentrating the sulphate beforeBaSO4 precipitation . An ion exchange system(Fig. 2) modified from Nehring et al. (1977) andHesslein et al. (1988) was used. The ion exchangecolumn (Dowex AGO 1-X8, 50-100 mesh, chlo-ride form) was placed in two sections, with a10 mL chromatography column on top of a1 x 20 cm column. The upper column was packedwith 2 mL resin and connected to the lower col-umn, via a silicone tube . The lower column waspacked with 12 mL resin . A three-way stopcockwas placed on the top, with one connection to thesample reservoir and one to the eluation fluidreservoir . The upper resin, which was discardedafter each sample treatment, was used as a trapfor dissolved organic material which binds verytightly to the resin. During the summer, whenstream water was high in organic content, thelower section of the resin also became stained andwas replaced after each sample . The column wasconditioned with 200 mL 0.5 M NaCl and100 mL distilled water . The eluation flow wasabout 5 mL min -1 using 200 mL 0.5 M NaCl .The NaCl solution was acidified (pH : 1) andheated to boiling . Thereafter, 25 mL of 0 .25 MBaC12 were added and the precipitated BaSO 4was left to settle for 2 h in a water bath at 80-90 °C . The BaSO 4 was then recovered on a0.4 µm membrane filter (Nucleopore ®), dried

SAMPLEINPUT

ION EXCHANGERESIN

15 cm

2-WAY VALVE

i FROM 500 mL0PTFE

RESERVOIR'- TUBE

3-WAY VALVE

PTFE } TUBE

9cm

20 cm

1TO COLLECTIONBEAKER

Fig . 2 . Ion exchange system for sulphate determination .

overnight at 60 'C, weighed and stored in a glasstube until analysis .The BaSO4 was mixed with V205 and Si02

and burned under vacuum at a temperature of900 °C (Yanagisawa & Sakai, 1983) . The SO2formed was measured for its isotopic composi-tion with a rebuilt and modernized VG Micro-mass 602 at the Laboratory of Isotope Geology,Museum of Natural History in Stockholm. The180/160 ratios were measured in CO2 equilibratedwith the H2O (Burgman et al., 1981). The analy-ses were done on a VG ISOTECH SIRA 9 at the

Department of Physical Geography, Division ofHydrology, University of Uppsala . The stableisotope composition is given in the conventional8 notation, in parts per thousand (%.) :

S sample

Rsample - 11 x 10 3standard

where R is the abundance ratio (34S/32S or 180/160). The standard for sulphur is Canon DiabloTroilite (CDT) and for oxygen Standard MeanOcean Water (SMOW) . The accuracy of the sul-phur and oxygen isotope measurements is within± 0.2%, and ± 0.1%, 7 respectively .

Results and discussion

Precipitation and throughfall

The 834S in precipitation ranged from + 6 .41%during winter to + 3 .88%. in summer (Table 1,Fig. 3). The total sulphur concentration in TFwas about four times higher than in precipitation,but there was no significant difference in 8 34S(Table 1, Fig . 3). Lindberg & Garten (1988) con-cluded from radiosulphur studies of mature trees

May

I

June

1989Fig. 3 . Seasonal variations in b 34 S (%,) of sulphate in precipitation, pine and spruce throughfall during 1989 .

Feb . March

I April

I

209

in the field that dry deposition wash-off accountedfor 86 to 98% of the sulphate enrichment in TF .Hence, the similarity in 6 145 between precipita-tion and throughfall in our data suggest that mostsulphur in TF was derived from atmospheric drydeposition .

Increased 8 34S in winter precipitation com-pared to summer precipitation has also been ob-served in other studies from similar areas in NorthAmerica (Nriagu & Coker, 1978 ; Caron et al.,1986). Several processes have been suggested toexplain this increase ; (a) a temperature dependentisotope fractionation during oxidation of SO 2 tosulphate; (b) seasonal variability in homogeneousand heterogeneous atmospheric oxidation reac-tions of SO 2 to sulphate; (c) seasonal variationsin the sources contributing to atmospheric sul-phur. The temperature effect has been thoroughlyreviewed by Caron et al. (1986). They suggestedthat the seasonal variations of the sulphate 8 34 5may be due to preferential extraction of 34 S fromthe atmosphere in the winter compared to sum-mer. It has been shown that the reaction :

SO2 (g) + H2O (1) •--HSO3 (aq) + H + (aq) (1)

fractionates sulphur (14S/32 S) because the iso-tope fractionation factor, a(xso, -so,) is 1 .0 109 at

July August Sept.

210

Table 1 . Total S concentration, pH and isotope data for bulk precipitation, throughfall, streamwater and groundwater in the LakeMjosjon watershed.

25 ° C (Eriksen, 1972) . This factor is temperaturedependent. A decrease in temperature has beenestimated to cause an increase_ in 6 345 of about0.08-0.145%° ° C -1 (Caron et al., 1986) .The 6 180 in precipitation generally exhibits

seasonal variations correlated with the tempera-ture, so that precipitation during the winter isdepleted in the heavy isotope (Gat, 1980) . Ourdata indicated that 8 180 was lowest in precipita-tion during winter, becoming progressivelyheavier towards summer (Table 1) .

In Fig. 4a, b 6 34S in precipitation was plottedversus both monthly mean temperature and 6 180in precipitation for the sampled period . Theseplots clearly show a good correlation betweenmonthly mean temperature and 6 34S, with heaviersulphur isotopic composition during the winter .The mean monthly temperature increased about21 ° C between February (-6.9 'C) and June(+ 14.2 °C) while 534S decreased 2.5%° . Thetemperature dependence obtained from our data(about 0.12% °C -1 ) is in good agreement with

Sample Date pH S(mg L

_ 1)b34S(%)

b 18o(%o)

Precipitation, RSnow 890225 4 .22 0 .67 +6.41 -15.84Snow 890418 5 .08 0 .28 +4.48 -11.74Snow and rain 881017-890520 4.36 1 .26 +4.24 -11.19Rain 890520-890626 - 0.56 +3.88Rain 890626-890905 4.34 0.89 +3.88 -10.31

Throughfall, TFPine 881017-890518 4.03 3 .23 +3.88 -9.76Pine 890518-890625 4.49 1 .74 +4.00 -10.69Pine 890626-890905 4.01 2.72 +3.68 -10.59Spruce 881017-890518 3.88 5 .50 +3.44 -10.08Spruce 890518-890625 4.19 2.55 +4.03 -10.98Spruce 890626-890905 3.95 3 .92 +3.61 -10.99

StreamwaterInflow, S 1 890225 4.86 2 .62 +9.56 -13.29Inflow, S 1 890418 4.63 2 .48 +5.57 -12.60Inflow, S 1 890518 4.62 2 .24 +6.73 -11.92Inflow, S 1 890625 5 .34 1 .11 +12.47 -12.20Inflow, S 1 890726 5 .74 1 .54 +26.21 -12.88Inflow, S 1 890905 5 .46 2 .80 +21.46 -12.90Inflow, S 1 891222 5 .50 3 .87 +17.22 -12.97Outflow, S2 890223 6.02 2 .60 +7.54 -12.32Outflow, S2 890416 5.68 2 .46 +6.05 -12.55Outflow, S2 890518 5.98 2.09 +6.15 -11.99Outflow, S2 890625 6.02 2 .10 +6.20 -11.28Outflow, S2 890726 6.35 2 .60 +6.13 -13.20Outflow, S2 890905 6.20 2.29 +7.13 -12.43Outflow, S2 891222 6.22 2 .38 +7.24 -10.62

GroundwaterShallow well, GW 890726 5 .61 2 .41 +4.71 -12.54Shallow well, GW 891222 6.22 2.89 +5.92 - 11 .82

-10 0

10Temperawre °C

20

0 .08-0.145% ° C- 1 reported by Caron et al.(1986) .

Saltzman et al . (1983) suggested that the SO 2oxidation rate varies seasonally, with faster oxi-dation during warmer months . They also foundthat SO2 oxidation in the atmosphere during thesummer is a fast homogeneous reaction involvinga small kinetic isotope effect, which will cause aslight enrichment of 32 S in the sulphate formed .During winter, the oxidation is much slower andheterogeneous involving both equilibrium and ki-netic effects on the isotopic composition . Thisleads to larger S34S values than in summer . Asboth homogeneous and heterogeneous reactionsare important, this model is consistent with ourS34S values .

The variation in S 34S could also be influencedby varying supplies of sulphate from sea waterand/or the burning of fossil fuels . A dominatingsource, such as sea water, would weigh the 8 345in precipitation toward larger values . We havecalculated the contribution of sea salt followingKrouse (1980) and Hitchcock & Black (1984),with Na as a solely seawater (SW) derived com-ponent, assuming an (S/Na)sw ratio of 0 .084(Krauskopf, 1982) and a S34S for sea water of

2 1 1

Fig. 4 . (a) S34S%° of sulphate in precipitation versus monthly mean temperature for the study period, (b) S 34S% O of sulphate inprecipitation versus 5 180% ° in precipitation .

+ 20%, The concentration balance is given byEqn. 2 and the isotopic balance by Eqn . 3 :

Stot - Sexcess + (S/Na)sw x Na

Stot534 'Stot

Sexcess 534 Sexcess+ ((S/Na) 5W x Na)834 S SW (3)

where Stot is total sulphur and Sexcess is non-seawater derived sulphur . The seawater contri-bution never exceeded 4 % in the sampled pre-cipitation and was even lower in throughfall (Ta-ble 2). Thus, seawater sulphur contribution is nota plausible explanation for the observed trend,with heavier sulphur isotopic composition in pre-cipitation during winter.

Fossil fuels have a large span in S34S, from- 8%o to + 32% (Faure, 1986). Regionally, itis therefore difficult to evaluate the effect of burn-ing of fossil fuels on the S34S value in precipita-tion .

Another possibility is emission of reduced sul-phur species enriched in 32S from the biosphereduring summer. Winner et al. (1981) reported thatgrowing plants released H 2S with a low PS. Thesignificance of this contribution can not be fullyevaluated in this study, but appears unlikely to be

(2)

212

Table 2 . Sulphur excess calculations .

Total measured sulphur concentrations (S tot) have been used to calculate the non seawater sulphur excess (Sex) and the sulphurderived from seawater (S, W) in precipitation using formula (2) and (3), see text. The b 34S prefix refers to the same as for con-centrations .

important as precipitation and throughfall exhibitsimilar b34S values .

Stream water

Total dissolved sulphur concentration, 534S , 518o

and pH for the inflow and outflow streams arelisted in Table 1 . These plus discharge and alka-linity are plotted in Fig. 5a-f. Discharge from theoutflow, which drains the whole watershed(Fig. 5c, d), was about two orders of magnitudelarger than that for the inflow, which drains about16% of the watershed into Lake MjOsjtn . How-ever, the flow patterns are similar with peak dis-charge during the melting of snow in spring. Thesummer and autumn of 1989 were extremely dryand the streams were almost completely dry dur-ing the summer .

The inflow stream showed a large variation intotal sulphur concentration with a sharp decreaseduring summer (Fig . 5a). The 6 34S in inflowingwater (Fig. 5a) displayed the lowest value,+ 5.57%., during discharge in April . This valuewas close to that of snow, indicating that most ofthe sulphate in inflow water during spring wasderived from meltwater . The 6 34S in inflow in-

creased from + 5.57% in spring to + 26 .21%during summer, with a corresponding decrease inthe total dissolved sulphur concentration(Fig. 5a). During the summer, inflow changedfrom a murmuring stream to a line of pools withstagnant water . The decrease in the total sulphurconcentration and the increase in b 34S were prob-ably related to bacterial sulphate reduction in ananaerobic environment . Fixation of 32S as sul-phides in the stream sediment (cf. Goldhaber &Kaplan, 1974) could explain the increased b34Svalue during summer .

Alkalinity and pH in the inflow stream dis-played a clear seasonal trend with low values dur-ing spring discharge (Fig . 5e). During summerboth pH and alkalinity increased contemporane-ously with increasing 534S and decreasing totalsulphur concentration . Most probably the pH andalkalinity increase were related to the sulphatereduction (see lakewater discussion below) .

The 6 180 in the inflow stream showed a clearseasonal trend, similar to precipitation, i .e. lowwinter values increasing during summer (Fig . 5e) .Thus, the summer increase could be caused byheavier precipitation or evaporation .

The total sulphur concentration in the outflowstream (Fig . 5b) was fairly constant with only a

Sample Date Na(mg L - ')

S tot(mg L - ')

Sex(mg L - ')

Ssw(%)

b 34 S tpt(%,)

b 34 SeX(%e)

b34S . .

(% )

Precipitation, RSnow 890225 0 .28 0.67 0 .65 3 .1 +6.41 +5 .92 0.49Snow 890418 0 .11 0.28 0 .27 3 .7 +4.48 +3.94 0.54Snow and rain 881017-890520 0.34 1 .26 1 .23 2 .4 +4.24 +3 .87 0.37Rain 890626-890905 0.20 0.89 0.87 2 .3 +3.88 +3 .58 0.30

Throughfall, TFPine 881017-890518 0.64 3 .23 3 .18 1 .6 +3.88 +3.61 0.27Pine 890518-890625 0.52 1 .74 1 .70 2 .4 +4.00 +3.59 0 .41Pine 890626-890905 0.56 2 .72 2.67 1 .9 +3.68 +3.39 0.29Spruce 881017-890518 1 .51 5 .50 5 .37 2 .4 +3.44 +3.05 0.39Spruce 890518-890625 0.71 2 .55 2.49 2 .4 +4.03 +3.65 0 .38Spruce 890626-890905 0.98 3 .92 3 .84 2 .1 +3.61 +3.26 0 .35

Inflow stream

small decrease in early summer . In contrast to theinflow, outflow water was never fully stagnantand showed smaller variations in both the totalsulphur concentration and V'S . However, therewas a clear decrease of b34S from + 7 .54%. inFebruary to around + 6%, during spring . Thisindicated that during the winter, the lake water

Outflow stream

21 3

Fig. 5 . Stream water, inflow (S 1 ; open symbols) and outflow (S2 ; filled symbols) sampled at six occasions during 1989, (a, b) Totaldissolved sulphur concentration and 6 345 versus time (note different scale for the 5 34S axis), (c, d) Water discharge and 6180 versustime, (e, f) alkalinity (HCO3 ) and pH versus time .

governed the b34S signal while meltwater withlower b34S depressed the isotopic ratio of sulphurduring spring discharge . The pH variations in theoutflow were smaller than in inflow. The alkalin-ity displayed almost the same pattern as inflowwith a peak during summer. The outflow streamfrom Lake MjSsjbn depicts a slightly more com-

2 14

plicated pattern than the inflow, although thesame general trend with higher values duringsummer can be noticed (Fig . 50. The low 5 180 inthe outflow during July was coupled with lowdischarge (Fig. 5d) probably originating from agroundwater store with lower 5 180 than surfacewater (Table 1) .

Lake water

Lakewater was sampled from the ice of LakeMjOsjtn in November 1988, December 1988 andApril 1989. The ice had formed approximatelyone week before the measurement started in No-vember, which was close to the autumn overturn .

b34 s+5

+7

+90

2

4

6

8

10

Fig . 6 . Vertical lake-water profiles sampled at three occasions during the winter 1988-89, (a) d 34S%0 versus depth, (b) total dis-solved sulphur versus depth, (c) oxygen saturation (%) versus depth, (d) pH versus depth, (e) alkalinity (HCO3 ) versus depth .

Total S mg/l,

Oxygen saturation %2 .5

3.0 20 40 60 80 100

The thickness of the ice increased from 10 cm inNovember to about 1 m in April . From a well-mixed and almost totally oxygen saturated watercolumn (90 %) in November 1988, oxygen satu-ration decreased with depth to a minimum ofabout 0.9% in the near bottom water in April1990 (Fig. 6c) .

Shortly after autumn overturn the 6 34S valuewas almost constant throughout the water col-umn (Table 3, Fig . 6a). During winter the sulphatein the water column below 2 m became progres-sively enriched in 34S with the largest increaseclose to the bottom. The total sulphur concentra-tions in the water did not vary significantly withdepth or between the sampling occasions (Fig . 6b)except for the near surface water sample in late

pH

Alkalinity mM6 .0

7.0

0.10 0.20 0.30

\

- + - 881109- .0- 881219--t-- 890417

b

10*

I

II

i

i '

c

I

r'

i'

d

+\

\

i;

i

\

e

I

I

F e,I s

j ~

! I

lI I I

i

1 I I iI I I

t I I I

=j I ,

i 1

I

1 =I 1

i,I

f1

I

I,

_

I

%

II

I

It

I \

% 1 I 1_ S 1 \

\ I \

Table 3 . Total S concentration, pH and isotope data from theLake MjbsjSn, winter 1988-89 .

winter. This increase probably originated fromatmospheric deposition of sulphate, which alsoexplains the low 5 34S in the near surface water inApril .

Dissimilative sulphate reduction caused bybacterial activity in the uppermost sediment prob-ably increased the 5 34S in the near bottom sam-ples . In the bottom water sample from April, asmall decrease in total sulphur was observed .Deposition of 32S enriched sulphides in the sedi-ment can explain the lowered total sulphur valueand the high 634S of sulphate (Nakai & Jensen,1964). Adsorption on sediment has also beenshown to selectively retain 32 S in sediments(Nriagu, 1974) . Groundwater in the area showed634S below + 6% (Table 1), therefore groundwa-ter seepage can be excluded as a major factor toexplain the increased 534S in bottom water dur-ing winter .

2 1 5

The near bottom water in April showed an in-crease in pH and alkalinity (Fig . 6d, e) . Sulphatereduction is a hydrogen ion consuming processwhich leads to an increase in alkalinity (Stumm& Morgan, 1981). Increased production ofHCO3 as a result of anaerobic bacterial sulphatereduction in the top sediments has been shown byBerner (1971). This observation also confirms theresults from Cook et al. (1986) which shows thatdissimilative sulphate reduction in sedimentscould be a significant pH buffering process inlakes receiving acid deposition .

A small but significant increase in 3 34S in sul-phate occurred at intermediate depths as shownby increased values during winter. This shift couldnot be detected in our total sulphur analyses(Fig. 6a, b) . The uniform increase of 5 345 duringwinter in intermediate water may be caused bymigration of 34S enriched bottom water .

The 8 180 in the lake showed a homogeneouswater column with slightly lower 8 180 in April1989 compared to December 1988 (Table 3) . Thisdecrease was probably caused by inflow of melt-water with lower 8 180 during winter .

Groundwater

The two samples from the shallow well, collectedin the winter and summer, showed approximatelythe same total sulphur concentration as lake water(Table 1) . The 8 180 values in water from the shal-low well were lower than in summer precipitation(Table 1) . The 534S values were similar to precip-itation but generally significantly lower thanstream- and lakewater . Both the 5345 and 6 180values indicate that groundwater had formed fromrapidly percolating rain- and meltwater duringspring. Bacterial sulphate reduction appeared tohave little or no influence on the 5 34S in the shal-low groundwater .

Conclusions

Seasonal differences in the importance of kineticand equilibrium isotope fractionation during at-

Sample Date pH S(mg L - ')

b34s(%)

b 180(%°)

Lake waterDepth profiles, DP1.0 m 881109 6.53 2.30 +5.545 .0 m 881109 6.32 2 .31 +5.977 .0 m 881109 6.10 2 .25 +6.229.0 m 881109 6.39 2 .28 +5.94

0 .5 m 881219 6.06 2.44 +6.18 -11.871 .0 m 881219 6.08 2 .38 +6.35 - 11 .833 .0 m 881219 5 .93 2.29 +6.23 - 11 .765 .0 m 881219 5 .93 2.40 +6.20 - 11 .816.0 m 881219 5 .83 2.39 +6.83 - 11 .627.0 m 881219 5 .91 2.30 +6.68 - 11 .858.0 m 881219 5 .88 2.27 +6.81 - 11 .839.0 m 881220 5 .79 2 .30 +7.03 -11.929.8 m 881220 5 .86 2 .22 +7.48 - 12 .10

1 .0 m 890417 5 .08 2 .87 +6.12 -12.723.0 m 890417 5 .64 2 .35 +6.61 -11.975.0 m 890417 5 .66 2 .35 +6.69 - 11 .856.0 m 890417 5 .64 2 .35 +7.08 -12.047.0 m 890417 5 .63 2 .30 +6.89 -12.088.0 m 890417 5 .63 2 .26 +7.14 -12.069.0 m 890417 5 .69 2 .47 +6.90 -12.099.8 m 890417 6.10 2 .23 +8.89 - 12 .13

216

mospheric oxidation of SO 2 to sulphate wereprobably the reason for the temporal variation inS34 S, with heavier 534S in precipitation sulphateduring winter. The coniferous trees act as sinksfor atmospheric deposition of sulphate with littleor no contribution of sulphur from the trees them-selves .

The main sulphur source in the watershed wasatmospheric deposition . During the passagethrough the watershed, bacterial sulphate reduc-tion in stream and lake sediments altered the S34 Stowards heavier values . This indicates that 32S isretained in the watershed most probably fixed assulphide in the sediments. The reduction causedan increase in alkalinity production in streamsand lake .

Acknowledgements

This study was made possible thanks to grantsfrom the Swedish Natural Science ResearchCouncil (NFR). We thank Prof. S. Claesson atthe Laboratory of Isotope Geology, Museum ofNatural History in Stockholm for using their massspectrometer equipment, M. Hedberg and C-M .MSrth for technical assistance with sulphur iso-tope determination. F. Westman at Uppsala Uni-versity performed the oxygen isotope analyses .Dr. R. LOfvendahl, Prof. K. Bostrom and K .Bishop critically reviewed the manuscript . We aregrateful to two anonymous referees for valuablecomments on the manuscript .

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