Doctoral Thesis 1996: 200 D

Suspended particulate matter, sedimentationand early diagenetic processes in the Kalix River

estuary, northern Sweden

Anders Widerlund

Division of Applied GeologyDepartment of Environmental Planning and Design

Lule% University of TechnologyS97187 Lule& Sweden

Lule3 1996

Key words: Kalix River, estuaries, Gulf of Bothnia, suspended particulatematter, Fe oxides, Mn oxides, organic matter, trace metals

ABSTRACT

Many important reactions controlling the transfer of elements from thecontinents to coastal waters and the oceans are taking place in estuaries. Formany elements, estuaries can act as filters, capable of reducing the river load ofdissolved and particulate elements to the oceans. Although sedimentation is animportant process in estuaries, early diagenetic processes in sediments may beresponsible for the remobilization of redox-sensitive elements back intoestuarine waters.

This thesis focuses on the behaviour of suspended and settlingparticulate Fe - Mn oxides and organic matter in the stratified, low-salinity (< 3o/00) Kalix River estuary. In addition, the early diagenetic cycling of Fe, Mn,organic C and trace metals was also studied.

Samples of the dissolved (c 0.45 pm) and suspended phase werecollected along vertical profiles at three stations in the estuary. Settlingparticulate matter was collected by the use of sediment traps. Pore-water andsolid phase sediment data were used to study early diagenetic processes.

The estuary appears to be relatively inefficient as a trap for rivertransported, particulate, oxide and organic-associated elements. Suspended,slow-settling particles rich in Fe - Mn oxides and organic matter to a largeextent are flushed through the estuary in the seaward flowing surface water.Organic matter is an important carrier of Cu, which appears to be due in part tobiological uptake of Cu. Flocculation of dissolved Fe, Si and Al is of little or nosignificance, while Mn is desorbed from suspended particles early in theestuarine mixing of freshwater and brackish water.

A study of the varved, undisturbed sediments deposited in the centralpart of the estuary shows that the deposition of trace metals has decreased sincethe late 1970’s.

Despite active post-depositional redox cycling of Fe and Mn, efficientoxidation processes in the surface sediment prevent diffusion of dissolved Feand MI-I back into the water column. Reduction/dissolution of Fe oxides is aslow process, resulting in burial of Fe oxides in permanently deposited, anoxicsediments.

Post-depositional mobilization of trace metals in the bottom sediment iscontrolled by the type of carrier phase and the redox conditions in thesediment. The post-depositional mobility of trace metals is low - moderate,except for As, which is associated with redox-sensitive Fe oxides. Copper isassociated with with labile organic matter, and early diagenetic remobilizationof Cu appears to be entirely controlled by oxic decomposition of organic matterclose to the sediment - water interface.

1

SUSPENDED PARTICULATE MATTER, SEDIMENTATION AND EARLY

DIAGENETIC PROCESSES IN THE KALIX RIVER ESTUARY,

NORTHERN SWEDEN

CONTENTS

LIST OF PAPERS

INTRODUCTION

BackgroundEstuarine processes - a brief outlineScope of the thesis

STUDY AREA

The Kalix River drainage areaThe Kalix RiverThe Kalix River estuary

METHODS

RESULTS AND DISCUSSION

Dissolved and suspended particulate matterSedimentationEarly diagenetic processes

SUMMARY OF THE MAIN FINDINGS OF THE PRESENT WORK

ACKNOWLEDGEMENTS 16

REFERENCES

PAPERSI-IX I

APPENDIX A

2

3

335

5

567

9

10

101112

14

17

APPENDIX B

APPENDIX C

2

LIST OF PAPERS

This thesis is based on the nine papers listed below, in the following referred toby their Roman numerals:

I

II

III

IV

V

VI

VII

VIII

IX

Widerlund, A., 1996. Export of river-borne particulate iron andmanganese from the Kalix River estuary, northern Sweden.

Widerhmd, A., 1996. Dynamics of suspended particulate organic matterand copper in the Kalix River estuary, northern Sweden.

Widerlund, A. and Roos, P., 1994. Varved sediments in the Kalix Riverestuary, northern Sweden. Aqua Fennica, 24 (2): 163 - 169.

Widerlund, A. and Ingri, J., 1996. Redox cycling of iron and manganesein sediments of the Kalix River estuary, northern Sweden. (Submittedto Aquatic Geochemistry).

Widerlund, A. and Roes, P., 1996. Early diagenetic trace metal mobilityin sediments of the Kalix River estuary, northern Sweden.

Widerlund, A. and Ingri, J., 1995. Early diagenesis of arsenic insediments of the Kalix River estuary, northern Sweden. ChemicalGeology, 125: 185 - 196.

Widerlund, A., 1996. Early diagenetic remobilization of copper in near-shore marine sediments: a quantitative pore-water model. MarineChemistry, 00: 000 - 000.

Ingri, J. and Widerlund, A., 1994. Uptake of alkali and alkaline-earthelements on suspended iron and manganese in the Kalix River,northern Sweden. Geochim. Cosmochim. Acta, 58: 5433 - 5442.

Ingri, J., Widerlund, A. and Land, M., 1996. Dissolved major elementratios as tracers for hydrological compartments and flowpathsin the Kalix River catchment, northern Sweden. (Submitted to Journal ofHydrology).

INTRODUCTION

Background

Since the 1970’s, several research projects with the aim of studying

geochemical processes in natural waters, soils and sediments have been carriedout at the Division of Applied Geology, Lulel University of Technology.

Studies of freshwater environments have included lakes within the Kalix River

drainage area (Ponter et al., 1992; Peinerud, 1995), the Kalix River and other

North Swedish rivers (Burman, 1982; Ponter et al., 1990; Paper VIII; Ingri, 1996)

and Lake Imandra, Kola Peninsula (Peinerud et al., 1996). Weathering processes

in till (ohlander et al., 1991; Land et al., 1996) and mine tailings (Ljungberg,

1995) have also been studied. Several projects have dealt with the brackish

water environment of the Gulf of Bothnia - Baltic Proper (Bostrom et al., 1978;

1981; 1982; 1983; Ingri, 1985a; Ingri and Ponter, 1986; Ingri et al., 1991; &lander

et al., 1993), and other marine environments (Ingri, 1985b).

In order to better understand the estuarine processes affecting the river

transport of suspended major and trace elements to the Gulf of Bothnia, the

present investigation was directed towards the Kalix River estuary in the

northern Gulf of Bothnia. The study was carried out in 1991 - 1993 within theframework of the Gulf of Bothnia Year 1991, a joint Finnish - Swedish research

programme focusing on the marine environment of the Gulf of Bothnia (Perttilg

and Rhlin, 1993).

Estuarine processes - a brief outline

The question of how the dissolved concentrations of trace elements aremaintained at relatively constant levels in natural waters has received much

attention during the last decades. Sill& (1961) discussed the composition of

seawater in terms of an equlibrium model, where dissolved concentrations

were assumed to be regulated by solubility equilibria with marine sediments,

specifically oxides and carbonates. However, progress in analytical chemistry

and better knowledge of oxide and carbonate equilibrium constants showedthat, for most trace elements, seawater is undersaturated with respect to any

reasonable pure solids. Consequently, the idea of solubility equilibriaregulating the dissolved concentrations had to be abandoned.

4

The search for other processes controlling the dissolved trace element

concentrations has continued with the development of models based on

adsorption/desorption equilibria and element uptake into and release frombiota (e. g. Schindler, 1975; Balistrieri et al., 1981; Bourg, 1983; Turner and

Whitfield, 1983; Morel and Hudson, 1985; Fox, 1989; Stumm, 1992). These so-

called scavenging processes are taking place also in estuaries, where the mixing

of fresh and saline waters results in changes in electrolyte concentration and

pH (Burton, 1976; Bowden, 1980). Salinity and pH both are fundamentalparameters in adsorption/desorption models. Many estuaries are characterized

by high primary production, which may be expected to promote the uptake of

biorective elements into biota (Wollast and De Broeu, 1971; Shiller and Boyle,1991; Szefer, 1991). Moreover, in estuarine environments a fraction of the

dissolved (< 0.45 pm) river load often is transferred to the particulate phase due

to flocculation of colloids (Boyle et al., 1977; Ma&in and Aller, 1984).

In the above-mentioned processes, particulate Fe - Mn oxides and living

or dead particulate organic matter (POM) have been found to play important

roles as carrier phases for major and trace elements being scavenged from the

dissolved phase. This transfer of dissolved species into settling particles is nowbelieved to be of fundamental importance for the regulation of dissolved trace

element concentrations in natural waters. Due to settling of particulate matter,

estuaries often act as filters capable of reducing the river load of dissolved and

particulate elements to the oceans.

Refractory particulate matter such as inorganic detrital material is, in

general, effectively trapped in estuaries and permanently buried in bottom

sediments (Postma, 1980). In contrast, the estuarine behaviour of redox-

sensitive Fe - Mn oxides and POM is usually more complicated. Due to theirsmall size and/or low density, these particles are less susceptible to

sedimentation, and may to some extent be exported from estuaries (Coonley et

al., 1971; Turekian, 1977; Bostrom et al., 1981; Sundby et al., 1981; Forsgren and

Jansson, 1992). The fraction of suspended Fe - Mn oxides and POM that does

settle out in estuaries is subject to changing redox conditions and degradation

during early diagenesis in bottom sediments. Particulate, oxide- and organic-associated elements thus are remobilized into dissolved forms, which may

result in a diffusive benthic efflux of mobilized elements back into the water

column (Emerson et al., 1984; Santschi et al., 1990).

5

Scope of the thesis

The Kalix River and the Kalix River estuary offer a unique environment

to study estuarine processes at very low salinities (< 3 o/00) (cf. Morris et al.,

1978; Morris, 1986). Of particular interest is the flocculation/sorption of river-

transported major and trace elements. Iron and manganese oxides and

particulate organic matter are probably involved in the regulation of the

dissolved concentrations of several elements. Knowledge of the estuarinebehaviour of these compounds is therefore fundamental for a better

understanding of the estuarine cycling of oxide- and organic-associated trace

elements.

The main objectives of the present work were to:

* Determine the extent of flocculation/sorption of dissolved (< 0.45 pm)

Fe, Mn, Si and Al at low salinities (c 3 0/00).* Study the estuarine behaviour of suspended Fe - Mn oxides and POM in

a low-salinity, stratified estuary, where tidal mixing is absent.* Identify reaction zones in the bottom sediment where early diagenetic

transformations of organic C, Fe, Mn and trace metals are taking place.* Estimate the diffusive benthic effluxes of dissolved Fe, Mn and trace

metals back into the water column.

STUDY AREA

The Kalix River drainage area

The Kalix River drainage area is situated in the boreal zone in northern

Sweden, and stretches from the Scandinavian Caledonides in the northwest to

the Gulf of Bothnia in the southeast (Fig. 1 in Paper IX). The total area of the

watershed is 23 600 km2, of which 55 - 65 % is covered by coniferous forest.

Peatland and lakes cover 17 - 20 % and 4 % of the area, respectively, and less

than 1% is farmland (Hjort, 1971). During the period 1990 - 1992, the annual

precipitation in the area varied between 517 - 642 mm yr-’ (Ingri, 1996).

6

In the northwest, mica schists, quartzites, amphibolites and carbonate

rocks of the ca. 400 Ma old Scandinavian Caledonides cover approximately 5 %

of the drainage area. Towards the southeast, Precambrian (1750 - 2800 Ma)

granites and acid, intermediate and basic volcanic rocks predominate (ijdman,

1957; Gaal and Gorbatschev, 1987; Perdahl, 1995).

The Quaternary deposits in the area consist primarily of till showing

well developed podzol profiles, and mixed mires (Fromm, 1965).

The Kalix River

The Kalix and Tome Rivers, connected by the Tirendo bifurcation, are

among the last major unregulated river systems in Europe (Fig. 1 in Paper IX).

For the period 1975 - 1993, the mean annual discharge in the Kalix River was

296 m3 s-* (Ingri, 1996). During the same period, winter baseflow (January -

April) varied between 35 - 70 m3 s-l. In May, the water discharge increases

sharply by a factor of 20 - 30. The peak flow during spring flood is reached

during the period mid-May to late June, when the maximum discharge

normally varies between 1100 - 1600 m3 s-l.

For the period 1 September 1991 to 31 August 1992, the average

concentration of total dissolved solids (TDS; defined as dissolved major

elements except organic carbon) was 32.6 mg 1-l (II@, 1996). During the same

period, the concentration of total suspended matter (TSM; defined as inorganic

+ organic material) varied between 0.8 - 6.3 mg l-l, with a weighted average of

2.0 mg 1-l. Organic matter constituted approximately 30 % of average TSM

(Ingri, 1996). In major world rivers, the average concentrations of TDS and TSM

are approximately 110 mg 1-l and 350 mg l-l, respectively (Milliman and

Meade, 1983; Bemer and Bemer, 1987). The concentrations of TDS and TSM in

the Kalix River thus are low in comparison with major world rivers.

Suspended matter transported by the Kalix River is characterized by

unusually high concentrations of non-detrital Fe and Mn, which reach 45 wt %

and 2.5 wt % of ashed suspended load, respectively. Non-detrital Fe and Mnare interpreted to occur in the form of Fe and Mn oxides (Ponter et al., 1992;

Paper VIII).

The Kalix River estuary

The Kalix River estuary is situated in the northernmost Gulf of Bothnia

(Fig. 1 in Paper I), which comprises the northern part of the Baltic Sea (Voipio,

1981). According to the classification of Pickard and Emery (1990), the estuary

is of the salt wedge - highly stratified type, and tidal mixing is absent. The

upper and central parts of the estuary (RepskIrsfjirden) are relatively sheltered

from wave action by surrounding land areas. Towards the southeast, shallow

water areas form an irregular sill, and the central part of the estuary can be

characterized as a flat-bottomed, shallow basin. The water depth generally

ranges between 10 - 20 m.Below the seaward flowing surface water, the salinity reaches up to

approximately 3.2 Q/00 in the bottom water in the lower estuary. During the

summer months, the thermocline coincides with the permanent halocline

which, depending on the river discharge, is situated at a water depth of 2 - 8 m

(Appendix A and B). This density stratification is likely to reduce the

sedimentation of river suspended matter, and to minimize the effect of wind

induced turbulence in the bottom water. The water column is always well

oxygenated (02 saturation > 80 %), and pH normally varies between 7.0 - 7.8

(Appendix B). During the period December to May, the estuary is ice-covered.

Despite the well oxygenated bottom water, bioturbation seems to be virtually

absent. This is most probably a reflection of the scarcity of burrowingorganisms in the low-productive waters of the northern Gulf of Bothnia

(primary production 10 - 30 g C mm2 yr-‘; Elmgren, 1984).

Several of the above-mentioned factors (sheltered position, ice-cover,

density stratification and absence of bioturbation) probably contribute to the

fact that virtually undisturbed, varved sediments are deposited in the central

part of the estuary (Fig. 1 and Paper III).

The Kalix River estuary and nearby areas of the Gulf of Bothnia have

been subject to a few earlier studies that mainly focused on the sediments and

sedimentary processes (Anonymous, 1974; Mellin, 1976; Bostrom et al., 1978;Ingri, 1985a; Foberg and Kautsky, 1992; Jonsson and Blomkvist, 1992).

8

Fig. 1. Varved sediment from the Kalix River estuary (Paper III). This core wascollected at Station 2 in April 1991 by using an in-situ, core-freezing technique(Renberg, 1981). The brown, flocculent surface layer is deposited during theperiod June - April, and is relatively rich in organic matter and settled Feoxides. Below this layer follow thin, diagenetically precipitated Mn oxide (darkbrown) and Fe oxide (orange-brown) layers. Grey layers are deposited duringthe spring flood in May, and are dominated by minerogenic material (quartz,feldspar and mica). In the anoxic zone, formation of black Fe-sulfide phasesbegins at a depth of 3.5 - 4 cm. The preservation of brown Fe oxide layers in theanoxic zone shows that reduction/dissolution of Fe oxides is a slow process inthis sediment. Match is 49 mm.

METHODS

In the Kalix River, samples of the dissolved and particulate phase were

collected at a station situated ca. 30 km upstream from the estuary

(Kamhmgeforsen). During the period April 1991 to September 1992, sampling

was performed twice weekly from May to October, and weekly during the

remaining months of the year. River discharge data were obtained from the

Swedish Meteorological and Hydrological Institute (Station 4 - 17 Riktfors). In

the estuary, samples of the dissolved and particulate phase were collectedalong vertical profiles at three stations by using a peristaltic pump. Sampling

was performed in late June and August 1991 and March (two stations) and

early June 1992 (Fig. 1 in Paper I and Appendix A).

In the river as well as in the estuary, water temperature, conductivity,

salinity, pH and dissolved O2 were measured in-situ with a Surveyor II sonde(Appendix B).

The particulate and the dissolved phase were separated by filtration.

Samples for particulate C and N were collected on glass fibre filters (pore size

0.7 pm). For all other major and trace elements, membrane filters (pore size 0.45

urn) were used (adman et al., 1996a; 1996b). Individual suspended particles

were characterized by scanning electron microscopy. As a complement to field

data, a laboratory mixing experiment was carried out to study the

sorption/desorption behaviour of Fe, Mn, Si and Al during the mixing of

freshwater and brackish water.

Settling particulate matter was collected at Stations 2 and 3 in the estuary

(Appendix A) by using sediment traps consisting of paired cylinders (Larsson

et al., 1986). A Kajak gravity corer (Blomqvist and Abrahamsson, 1985) and an

in-situ core-freezing technique (Renberg, 1981) were used to collect sedimentcores at Stations 1 and 2 (Appendix A). Pore-water was isolated from the

gravity cores under oxygen-free conditions by using a modified Reeburghsampler (Reeburgh, 1967). Sediment accumulation rates were obtained by

varve-counting and by using 137Cs maxima from the Chernobyl accident and

nuclear weapons tests as time horizon markers (Appendix C).

More detailed descriptions of the sample collection and the analytical

methods used are given in Papers I, II, VIII and IX (Kalix River) and Papers I -

VII (Kalix River estuary).

10

RESULTS AND DISCUSSION

Dissolved and suspended particulate matter

Weathering and podzolization of till is the ultimate source for much of

the dissolved and suspended non-detrital Fe and Mn occurring in the Kalix

River. Contrary to all major dissolved (operationally defined as c 0.45 pm)

elements, dissolved Fe and Mn show maxima during high-discharge periods

such as spring-flood (Fe and Mn) and storm events (Fe) (Papers I, VIII and IX).

The use of dissolved major element ratios as tracers suggests that water being

discharged during these high-flow periods to a large extent originates from

peatland or the upper sections of till in the woodland (Paper IX). It may

therefore be speculated that run-off via peatland may be a major pathway for

much of the dissolved Fe and Mn (including colloids) reaching the Kalix River.

Although the suspended particulate load (> 0.45 pm) in the Kalix River

is, in general, subordinate to the dissolved load (Ingri, 1996), the composition

and estuarine behaviour of particulate matter is of crucial importance for the

formation of sediments in the Kalix River estuary and the Gulf of Bothnia. The

fact that the concentrations of non-detrital Fe and Mn and organic C in

suspended matter are significantly higher than in settling material collected in

sediment traps suggests the presence of slow-settling (small and/or low-

density) particles rich in Fe - Mn oxides and organic matter. These particles to a

large extent appear to be flushed through the estuary in the seaward flowing

surface water (Papers I and II).

The operational definition of dissolved and particulate phases based onfiltration through 0.45 pm filters does not take into account the presence ofcolloidal particles (size range 1 nm - 1 pm; Lyklema, 1991). Depending on their

size, Colloids and sub-micrometer particles therefore may be somewhat

arbitrarily defined as either “dissolved” or “particulate” matter when filtration

is used for the separation. During the transfer of dissolved species into

particulate forms, colloidal intermediates are believed to play important roles

(O’Melia, 1980; Morel and Gschwend, 1987; Vezina and Cornett, 1990; Dai et al.,

1995). According to the “Brownian-pumping” model of Honeyman and

Santschi (1989), dissolved elements adsorb rapidly to colloids, which thenaggregate more slowly into particles that can be collected on 0.45 pm filters.

11

This implies that the rate of colloid aggregation into macroparticles controls thetransfer of elements into the particulate phase, before their removal from thewater column by sedimentation.

In this study, field data and laboratory mixing experiments both showthat flocculation - sorption of dissolved (c 0.45 arm) Fe, Mn, Si and Al appearsto be of little or no significance at salinities < 3 o/g0 (Paper I). Approximately 55% of the total river inflow to the Gulf of Bothnia enters via the low-salineBothnian Bay, the northern basin of the Gulf of Bothnia (Ehlin, 1981). A majorfraction of this river inflow can be assumed to reach open sea areas of theBothnian Bay before the salinity exceeds 3 o/00 (Paper I; Kullenberg, 1981). Ifthe absence of flocculation at salinities < 3 o/0Q is a general phenomenon in theBothnian Bay, large amounts of dissolved and/or colloidal (< 0.45 lun) Fe, Mn,Si and Al probably enter open sea areas of the northern Gulf of Bothnia.Considering the large river load of trace elements on the Bothnian Bay (Ahl,1977), this is probably true also for several trace elements. Due to their highspecific surface area and potential adsorptive capacity, colloidal particles thusmay play an important role in the biogeochemical cycling of trace elements andnutrients in the northern Gulf of Bothnia.

The absence of Fe flocculation appears to be confined to the low-saline,northern part of the Gulf of Bothnia (Bothnian Bay). In the &e estuary, situatedin the central Gulf of Bothnia, Forsgren and Jansson (1992) and Forsgren et al.(1994) found that dissolved (c 0.45 - 0.7 pm) Fe flocculated rapidly and wasefficiently removed from the water column. In this estuary, the slightly higher

salinity (4 - 5 o/00) and high concentrations of suspended matter (5 87 mg 1-i)

are important factors that both promote the aggregation and sedimentation ofFe.

Sedimentation

All sediment trap fluxes reported in this work (Papers III - VII )represent gross sedimentation rates, which include resuspended material. In anextensive data series, Blomqvist and Larsson (1994) showed that resuspendedmaterial commonly exceeded 50 % of the material collected in sediment traps ina coastal area of the Baltic Sea. For geochemical mass balance studies (e. g.Bri.igmann, 1986), sediment trap fluxes must be corrected for the resuspended

12

portion of the settling material. In the present work, however, the quantitative

use of trap flux data is restricted to measurements of gross sedimentation rates.

Blomqvist and Larsson (1994) also showed that settling flux rates may

vary considerably between seasons and years. In addition, factors such as

trapping efficiency, the presence of swimming zooplankton in the traps, effects

of added poisons/preservatives and sample integrity (affected by e. g.

biological decomposition, grazing and leaching) further complicate the use of

sediment traps (Knauer et al., 1984; U. S. GOFS, 1989; Gundersen and

Wassmann, 1990). At Station 2 in the estuary, where virtually undisturbed,

varved sediments are deposited (Fig. 1 and Paper III), the reliability of the trap

flux measurements was checked by comparing the trap fluxes of dry sedimentand the refractory elements Al and Ti with the accumulation rates of these

components in the underlying sediment. Unfortunately, this comparison is

complicated by the fact that the sediment accumulation rate appears to vary

over short distances at Station 2 (Appendix C). However, the trap flux of

sediment was in good agreement (+ 15 %) with the accumulation rate in thefreeze-core from Station 2 (Papers III and V).

Because sediment traps collect settling particles, they are biased towards

the larger and denser grain size fractions of suspended particulate matter. In

the Kalix River estuary, inorganic detrital material dominates the trap flux of

settling particles (Fig. 13 in Paper I), while this fraction contributes less to the

“standing stock” of suspended particles (Fig. 2 in Paper 11).

Early diagenetic processes

Papers IV - VII deal with early diagenetic transformations of organic C,

Mn, Fe and trace metals taking place close to the sediment - water interface.

In natural waters, very sharp gradients in physical, chemical and

biological properties are often found at the sediment - water interface. In this

environment, the succesive utilization of 02, N03-, MI-I and Fe oxides and

S042- as electron acceptors is driven by the early diagenetic respiratory

degradation of organic matter (e. g. Froelich et al., 1979; Santschi et al., 1990;

Henrichs, 1992; Burdige, 1993; Thamdrup et al., 1994). Primary production in

coastal marine waters is, in general, relatively high (Bemer and Bemer, 1987),

and controls the export of reactive organic C from the euphotic zone to the

13

sediments (Nixon, 1981; Wassmann, 1990; Jonsson and Carman, 1992). Because

many early diagenetic chemical transformations are fueled by this supply of

organic C, early diagenetic recycling of redox-sensitive elements may be intense

in coastal sediments. This cycling also involves oxide- and organic-associated

trace metals and nutrients (Toth and Lerman, 1977; Shaw et al., 1990; 1994;

Sholkovitz et al., 1992; Sundby et al., 1992; Lapp and Balzer, 1993). These early

diagenetic processes may result in redistribution of mobilized elements within

the sediment (Papers IV and VI), or diffusion of released species back into theoverlying water column (Paper VII). For some elements, benthic flux from

sediments may represent a significant input to the water column, that must be

considered in mass balance calculations for coastal waters (e. g. Elderfield et al.,

1981; Brugmann, 1986; Paulson et al., 1988). It has also been proposed that

recycling from coastal sediments followed by lateral transport may be an

important mechanism for the supply of Mn, Cu and possibly Fe to open sea

areas and the deep ocean (Sundby et al., 1981; Heggie et al., 1987; Landing and

Bruland, 1987).

Compared with most other coastal waters, the primary production in the

Bothnian Bay is low (10 - 30 g C mm2 yr-‘; i. e., on the order of 10 % of that in the

Baltic Proper (Elmgren, 1984)). The resulting low settling flux of reactive,

autochthonous organic C contributes to the fact that the bottom waters of the

Bothnian Bay are well oxygenated. This, in turn, results in a permanently

oxygenated surface layer of the sediments of the Bothnian Bay and the Kalix

River estuary (Ingri and PontQ, 1986; Paper VI). The presence of this oxic

surface layer is probably the reason why benthic effluxes of oxide-associated

elements like Mn, Fe, As, Co and MO appear to be of little or no significance inthe Kalix River estuary (Papers IV, V and VI). This study therefore suggeststhat recycling from near-shore sediments is not the source of Mn and Fe

occurring in nodules and concretions in the open Gulf of Bothnia (Paper IV).

Also for Cu, which is partly associated with labile organic matter decomposing

close to the sediment - water interface, benthic efflux in the estuary is low

(Paper VII).

However, due to the absence of resuspension, the sediment investigated

may not be representative for near-shore sediments of the Gulf of Bothnia.

Resuspension events are common in near-shore areas of the Gulf of Bothnia

(Brydsten, 1993), and will increase diffusive benthic effluxes to the water

14

column (Vanderborght et al., 1977).

Large amounts of As are stored in the surface sediments of the Bothnian

Bay and the Bothnian Sea (Borg and Jonsson, 1992; Jonsson and Blomkvist,

1992; Leivuori and Niemistii, 1993). Well oxygenated bottom waters appear to

be of crucial importance to prevent release of this As into the water column

(Paper VI; Holm, 1988). Accelerated eutrophication or restricted water

circulation leading to oxygen deficiency in the bottom waters may therefore

result in a considerable increase in internal loading of As from sediments.

Scavenging of dissolved P by suspended Fe oxides (including colloids)

(Fox, 1989), followed by settling and burial of Fe oxides in the anoxic zone of

sediments (Fig. 1; Papers I and IV; Ingri and Ponter, 1986) may be an efficient

mechanism for the permanent removal of dissolved P from the water column of

the Bothnian Bay. Such a mechanism is likely to contribute to the fact that P is

the limiting nutrient in this part of the Baltic Sea (Graneli et al., 1990; Wulff et

al., 1994).

Signs of eutrophication (increasing concentrations of dissolved N03-

and increasing total biomass of the benthic fauna) are apparent in the Bothnian

Bay since the 1960’ s. The concentrations of dissolved P have, however,

remained constant at a low level (Kuparinen et al., 1994). Thus, the

eutrophication of the oligotrophic Bothnian Bay still appears to be relatively

limited, resulting in well oxygenated bottom waters.

SUMMARY OF THE MAIN FINDINGS OF THE PRESENT WORK

In the Kalix River, suspended Fe oxides are an important carrier of non-detrital Ca, Mg and Sr, while non-detrital Ba is associated with suspended Fe

and Mn oxides. In the estuary, organic matter is a major carrier of non-detrital

Cu. The association of Cu with organic matter appears to be due in part to

biological uptake of Cu. The estuarine behaviour of these redox-sensitive

carrier phases may control whether associated elements are remobilized mainly

under oxic (organic-associated) or anoxic (oxide-associated) conditions (Papers

I, II, IV, V, VI, VII and VIII).

Dissolved major element ratios (Si/Mg and Ca/Mg) in the Kalix River

can be used as tracers for waters originating from the woodland and from the

15

Scandinavian Caledonides. Furthermore, major element ratios can be used to

identify run-off from peatland (S/M& and from the upper sections of till

(Ca/Mg and K/M@ (Paper IX).

In the Kalix River estuary, suspended matter is richer in Fe, Mn and

organic C than settling material collected in sediment traps. This is interpreted

as a result of the presence of slow-settling (small and/or low-density) particles

rich in Fe - Mn oxides and organic matter. Due to the stratification of the

estuary, these slow-settling particles to a large extent tend to be flushed

through the estuary in the seaward flowing surface water. The estuary thus

appears to be relatively inefficient as a trap for river-transported suspended

matter (Papers I and II).

Flocculation and/or sorption of dissolved (< 0.45 ltm) Fe, Si and Al

appears to be of little or no significance at salinities < 3 Q/g.,. Manganese is

desorbed from suspended particles at low salinities early in the estuarine

mixing of freshwater and brackish water (Paper I).

Varved, undisturbed sediments suitable for studies of early diagenetic

processes and interpretation of the historical sedimentary metal record are

deposited in the central part of the estuary. Two sediment cores suggest that

the deposition of trace metals has decreased since the late 1970’s (Papers III and

V).

Iron and Mn oxides form substantial fractions of the total settling flux of

Fe and Mn. Despite active post-depositional redox cycling of Fe and Mn,

efficient oxidation processes in the surface sediment prevent diffusion ofdissolved Fe and Mn back into the water column. Reduction/dissolution of Fe

oxides is a slow process, resulting in burial of Fe oxides in permanently

deposited, anoxic sediments (Paper IV). This process may be important as asedimentary sink for P associated with Fe oxides, and is likely to contribute to

the fact that P is the limiting nutrient in the Bothnian Bay.

Post-depositional mobilization of trace metals in the bottom sediment is

controlled by the type of carrier phase and the redox conditions in the

sediment. In general, post-depositional mobility of trace metals is low in the

16

sediment investigated (Paper V).

Arsenic is associated with Fe oxides and is very mobile in the uppermost

10 - 15 cm of the sediment. The high mobility results in a surficial enrichment of

As, which complicates the interpretation of the sedimentary record of As.

Accelerated eutrophication leading to oxygen deficiency in the bottom waters

of the Bothnian Bay may result in a considerable increase in internal loading of

As from sediments (Paper VI).

Due to the association of Cu with labile organic matter, early diagenetic

remobilization of Cu appears to be entirely controlled by oxic decomposition of

organic matter in the surface layer of the sediment (Paper VII).

ACKNOWLEDGEMENTS

I am grateful to Johan Ingri and Bjiim ohlander for initiating this researh

project, and for introducing me into the field of geochemistry. The

encouragement and support provided by Johan Ingri, my supervisor, is greatly

appreciated. Per Roos, Lund University, and friends and colleagues at the

Division of Applied Geology are acknowledged for cooperation during field

work and stimulating discussions. Special thanks to Milan Vnuk, who skilfully

drafted the figures. Without the excellent laboratory facilities and the support

provided by Svensk GrundHmnesanalys AB and its staff, the completion of thiswork would not have been possible. I also thank the staff at the Erikoren pilotstation, Kalix, for their generous assistance in the field. The major financial

support provided by the Swedish Natural Science Research Council, the

Swedish Environmental Protection Agency and the County Board of

Norrbotten is gratefully acknowledged.

17

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