Six cores were taken from the southern part of the Fert}o-Hans ag area using a ..... a corresponding increase in magnetisation (Schwarz 1975, Dekkers 1988). If.
Acta Geod. Geoph. Hung., Vol. 35(3), pp. 255{264 (2000)
SOME MAGNETIC PROPERTIES OF SEDIMENTS FROM LAKE FERTO} (NEUSIEDLERSEE) REGION, AUSTRIA-HUNGARY A Jelinowska1 , C Carvallo1, V Wesztergom2 , P Tucholka1, M Menvielle3 , L Szarka2, F Kohlbeck4 , J J Schott5
Manuscript received January 29, 1999] This paper presents the results of a magnetic susceptibility and thermomagnetic behaviour study from 6 sediment cores from the Lake Fert}o area (Hungary/Austria boundary). These data indicate a very low content of magnetic particles in the sediment, which represents late Holocene. In studied bulk sediment samples we observed four characteristic types of thermomagnetic behaviour. Some of these behaviours indicate chemical transformations of non magnetic iron minerals: siderite and iron and sulphur bearing mineral into magnetite and magnetic iron monosulphide respectively during heating. The observed minerals are independent of lithology, which suggests their post depositional origin related to the anoxic early diagenetic conditions in the sediments. The presence of siderite and iron and sulphur bearing minerals in the sediment can re ect variation in the salinity of waters in the lake and thus can give information about the development of the basin in terms of the hydrological system (relation between evaporation and alimentation) of the lake. Keywords: core sample Lake Fert} o magnetic properties palaeoclimate
Introduction
The presence of magnetic minerals, their concentration and size in lake sediments depend on the circumstances surrounding the deposition of this sediment and post-depositional processes. The allogenic fraction is supplied from the catchment area by waters and wind and depends on the detrital input to the lake, which carries varying quantities of magnetic minerals (e.g. iron oxides: magnetite Fe3 O4 , maghemite Fe2 O3, hematite Fe2 O3 , and iron hydroxides: goethite FeOOH). It also carries paramagnetic (e.g. iron-bearing clay) and diamagnetic (e.g. Ca/Mg carbonates, quartz, organic matter . . . ) minerals and materials. The post-depositional processes include the dissolution of detrital iron minerals, the authigenic production of iron sulphides (e.g. greigite Fe3 S4 , pyrite FeS2 ), oxides (e.g. magnetite), carbonates (e.g. siderite FeCO3 ) and phosphates (e.g. vivianite Fe3 (PO4)2 8H2 O) minerals, and biogenic mineralisation (e.g. magnetite, greigite). These processes can Laboratoire de G eophysique, Universit e Paris Sud, Bat. 504, F{91405 Orsay Cedex, France Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, H{4901 Sopron, POB 5, Hungary 3 Centre d'Etude des Environments, Terrestre et Planetaires, F{94107 Saint Maur des Fosses, 4 Avenue de Neptune, France 4 Technische Universit at Wien, Abt. Geophysik, A{1040 Wien, Gusshausstrasse 27{29, Austria 5 Ecole et Observatoire de Physique du Globe de Strasbourg, France 1 2
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alter the mineral magnetic assemblage of detrital origin in the sediments (Berner 1980, Berner 1981, Curtis 1987, Mann et al. 1990). Thus investigations of magnetic minerals, their concentration and size are extremely useful in studies of lake sediments: their uctuations depend on the environmental and climatic changes in the cachment area of the lake. The composition, grain size and concentration of magnetic minerals in the sediment can be detected using di erent magnetic parameters of the bulk sediment (Thompson and Oldeld 1986, Verosub and Roberts 1995). Several parameters (magnetic susceptibility, various remanent magnetisations, magnetic hysteresis parameters) depend mainly on the concentration and grain size of ferromagnetic (sensu lato) minerals. Low eld magnetic susceptibility depends also on the concentration of paramagnetic and diamagnetic materials. Analysis of the thermomagnetic behaviour with determination of the Curie temperature of magnetic minerals allows their identication (Curie temperature is a critical temperature below which ferromagnetic materials carry a remanent magnetisation above this temperature they are paramagnetic Curie temperature is unique for each specic magnetic material). It also gives information about the possible transformations with respect to temperature of the various iron-bearing materials. Combined interpretation of all these parameters provides valid information about the magnetic fraction, even when the concentration of magnetic material in the sediment is very small, to the point that it is undetectable by other methods such as X-ray di raction (XRD). In 1997, in the Lake Fert}o region, physical properties of lake sediments measured from the earth's surface and directly from sediment cores were investigated by a Hungarian-Austrian-French geophysical team. Under this project 6 sediment cores were collected and geoelectrical soundings were carried out from the surface around the drill holes. Electric properties of sediments deduced from soundings were compared to those measured directly on cores (Kohlbeck et al. 1998, 2000). In this paper we present some magnetic parameters studied in sediment cores. These parameters provide information on changes in the lake system during the late Quaternary period covered by this sediment sequence. Lake Fert}o (Neusiedlersee) (309 km2 ) is located on the Austro-Hungarian border south (20 km) of the Danube. It is a shallow (average water depth 50{60 cm) water body with large water level uctuations over the historical period. More than onethird of its surface is overgrown with potuberant ulignant plants. Lake Fert}o is the terminal lake of the Vulka and Rakos rivers and without an outlet in its natural condition. Water can be discharged through the Hansag main-channel. The lake is supplied by local rainfall (78%) and the Vulka and Rakos rivers (20%) groundwater feed represents only 2%. The age of the lake is estimated to be nearly 20 kyrs. Its origin is related to the simultaneous formation of two basins (Hansag and Fert}o). The gravel barrier separating these basins was formed by large depositions from the Danube and Raba rivers at the end of Pleistocene. The two basins were frequently connected during high water periods.
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Core collection and sediment description Six cores were taken from the southern part of the Fert}o-Hansag area using a Wright piston corer. The length of the cores varies from 100 cm to 350 cm. Hard sediments or liquied sands prevented collection of longer cores using this method. One core was taken from Lake Fert}o at Fert}orakos, and the following ve, from the dried out part of the area eastward, in order to obtain sediments from the Fert}o and Hansag basins (Fig. 1). The collected sediment presents the youngest part of the Quaternary, most probably late Holocene. The Lake Fert}o sediment in the core Fert}orakos (Fig. 2) consists of brown, downwards grey clay, overlying grey, ne grained sand. Below the sand, sandy clay and clay occur, with oxidation traces in its upper part, and grey colour downwards. Sediments from ve other cores are characterised by similar lithological units (clay, sand and intermediate phases) of brown, grey or yellow (rarely) colour and of di erent quantities in each core. There are also gravel levels. As these sediments were taken from the dried out part of the Fert}o-Hansag area, their upper parts consist of the organic matter accumulation due to the soil development. Variations of the lithology observed in studied cores show the stages of the evolution of each studied part of the basin: 1. clay sediments indicate the presence of the water body with low transport energy, most probably in lacustrine conductions, 2. sand and gravel suggest high transport energy and are attributed to the uvial deposits, 3. soil development indicates dry episodes.
Magnetic mineral sampling and measurements Standard sampling methods were employed to obtain semi-continuous coverage of each core. The cores are preserved in plastic tubes which were sliced into two parts (one half kept as an archive and the second used for sampling). We pushed standard 22 22 22 mm perspex cubes into the sediment, after cleaning the surface of a half-core. 260 samples were obtained from all studied cores. Low eld susceptibility ( = M=H , where M is the magnetisation induced in a material by an applied eld H , of a strength less than 1 mT) was measured with a Bartington Instruments MS-2 susceptibility bridge. The thermomagnetic behaviour, with the Curie point of the magnetic components of the sediment was determined on a horizontal force translation balance (Curie balance) in air atmosphere. Mineralogy of sediments was determined by X-ray di raction (XRD) and microprobe.
Results Magnetic low eld susceptibility (Fig. 2) presents low values in all measured samples. This is due to the very low magnetic particle content in Lake Fert}o sediments, to the point that their presence is not re ected by , which is also sensitive tot he para- and diamagnetic particles. The most often obtained low and positive values of indicate the dominance of paramagnetic materials in sediments (such as iron-bearing clay minerals). Low and negative values of occasionally obtained Acta Geod. Geoph. Hung. 35, 2000
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Fig. 1. Map showing the area of investigation and locations of coring sites Acta Geod. Geoph. Hung. 35, 2000
Fig. 2. Down-core variation in lithofacies and magnetic susceptibility in studied sedimentary cores
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are due to the presence of diamagnetic materials. The sediment diamagnetic properties are usually accounted for by a high quartz, carbonate and/or organic matter content. In our study, the dia- and paramagnetic properties are not clearly related to one type of lithology, such as: clay | para, and sand and/or organic matter | diamagnetic properties. It means that di erent components are present in all lithological units. Therefore it is impossible to distinguish the periods of the lake evolution on the basis of magnetic susceptibility. As does not show variations in magnetic fraction, we have chosen samples for analyses of the thermomagnetic behaviour on the basis of lithological changes for each core. In studied bulk sediment samples, we observed four characteristic types of behaviour during Curie balance experiments, when heated in air atmosphere. 1. The rst thermomagnetic behaviour shows the stepwise decrease in magnetisation in the sample on heating to 650 C, and stepwise remagnetisation during cooling to room temperature, without a clearly dened Curie temperature of any mineral (Fig. 3a). This behaviour indicates that paramagnetic minerals dominate in the sample, and these minerals, as the process is reversible, do not undergo any chemical transformation. This behaviour was observed in sand (Tadten-A 70 cm depth), clay (Kiraly 262 cm Fert}orakos 139 cm, 199 cm depth), sequence sand-clay (Fert}orakos 113 cm depth) and in organic matter (Kiraly 36 cm depth). 2. The second thermomagnetic behaviour shows the stepwise decrease in magnetisation during heating, as in the previous case, but during cooling we observe an increase in the magnetisation from 580 C (Fig. 3b), which is the Curie temperature of magnetite. When the sample is heated once more, we observe the behaviour of magnetite. This indicates that the paramagnetic material present in the analysed samples undergoes chemical changes during the experiment (at high temperature), which favour the creation of magnetite. This behaviour was observed in organic matter (Tadten-A 13 cm, 23 cm depth) and in the sequence of the transition organic matter-clay (Lebeny 32 cm depth). 3. The next thermomagnetic behaviour is characteristic of paramagnetic materials as previously until about 450 C during heating. At about 450 C, the magnetisation increases with the maximum at 500 C and decreases to 0 at 580 C (Fig. 3c). It suggests that the paramagnetic material present in the sample was transformed into magnetite during heating. In order to conrm the formation of magnetite, we heated another fresh sample to 500 C and after cooling we repeated the Curie balance experiment on heating it to 650 C. This experiment shows the thermomagnetic behaviour expected for magnetite (Fig. 3d). The observed transformation of the paramagnetic material to the magnetite is similar to that known for siderite if heated to 500 C (Hus 1990). In order to verify this hypothesis we made XRD analyses of the bulk sediment material (i) before and (ii) after heating to 500 C. These experiments show clearly the presence of the siderite in the unheated sample and this siderite disappears in the heated one. We also made XRD analysis of the magnetic Acta Geod. Geoph. Hung. 35, 2000
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extract obtained from the bulk sediment sample heated to 500 C the presence of the magnetite is di cult to identify probably because it is poorly crystallised. This thermomagnetic behaviour, which permits identication of siderite in sediment samples, was observed in sand (Tadten-A 96.5 cm Kiraly 346 cm Lebeny 150 cm, 176 cm Fert}orakos 92 cm depth), clay (Tadten (a) 44 cm, 53 cm Kiraly 236 cm Fert}orakos 35 cm, 68 cm, 263 cm, 310 cm depth) and organic matter (Lebeny 72 cm depth). 4. The last thermomagnetic behaviour is characteristic of paramagnetic minerals until about 300 C. At about 300 C, an increase in magnetisation is seen (Fig. 3e), followed by a decrease at 400 C and a second decrease to zero at 580 C. During cooling the magnetisation increases from 580 C. If we repeat the experiment on this material, we observe clearly the characteristic behaviour of magnetite. Therefore we observed the transformation of the initially paramagnetic material to ferromagnetic material at about 300 C, which afterwards is transformed to magnetite. In order to identify the magnetic phase created at 300 C, we heated a sample to 300 C. The created mineral was subjected to a Curie balance experiment. This experiment indicates the presence of a mineral that undergoes a major decrease in magnetisation between 300 and 400 C and afterwards is transformed to magnetite. Such behaviour is similar to that observed for iron monosulphides: (i) greigite (Tric et al. 1991, Snowball 1991, Ho man 1992, Roberts and Turner 1993, Reynolds et al. 1994, Jelinowska et al. 1995) and (ii) smythite (Fe9 S11 ) (Krs et al. 1992, Ho man 1993). Analyses with microprobe of a magnetic extract obtained from the sample heated until 300 C shows iron and sulphur ions in a ratio which conrms the presence of the iron monosulphide. Identication of this iron monosulphide by XRD was not possible most probably because, as in the previous case for magnetite, of the non crystalline form of the created mineral. This experiment shows the transformation of the initially paramagnetic material into magnetic iron monosulphide with temperature (about 300 C), which is unstable and transforms afterwards into magnetite. At the present state of the study we can not identify this non-magnetic material which contains iron and sulphur ions. The thermomagnetic behaviour described here was observed in clay sediments (Tadten-A 36.5 cm depth), sand (Kiraly 103 cm depth) and organic matter (Tadten-A 29.5 cm, Lebeny 8 cm).
Discussion and conclusions
Sediment from Lake Fert}o are dominated by a paramagnetic fraction. With Curie balance experiments we identied 4 di erent types of thermomagnetic behaviour: (i) the paramagnetic material remains stable during heating and cooling (ii) the paramagnetic material is transformed into magnetite at high temperature (iii) the paramagnetic material undergoes transformation into magnetite during heating (at 500 C) and is identied as siderite (iv) the paramagnetic material undergoes transformation into magnetic iron monosulphide during heating (at 300 C) Acta Geod. Geoph. Hung. 35, 2000
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Fig. 3. Typical thermomagnetic behaviours of sediment from the Lake Fert}o area when heated in air in a magnetic eld of 0.4 T. a) Tadten-A 70 cm depth, b) Tadten-A 13 cm depth, c) Kir aly 346 cm depth, d) Kir aly 346 cm depth { sample heated to 500 C, cooled and heated the second time until 650 C, e) Tadten-A 36 cm depth, f) Tadten-A 36 cm depth { sample heated to 300 C, cooled and heated the second time until 650 C, d) and f) show thermomagnetic curves from the second heating until 650 C Acta Geod. Geoph. Hung. 35, 2000
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and is not precisely identied, but it contains sulphur and iron ions. The observed behaviours, especially the two last ones with more ore less precisely identied iron minerals which transform with temperature (iron carbonates and iron sulphides), are independent of lithology. The presence of these minerals and their independence of lithology suggest they have a post-depositional (early diagenetic) origin. During early diagenesis of sediments in anoxic conditions, an important pathway for the degradation of organic matter would be microbial sulphate reduction, detrital iron oxide reduction and methanogenesis. Bacterial reduction of detrital iron oxides (reduction of Fe+3 to Fe+2 ) and sulphates (SO;4 2 ), which are dissolved in water (saline and brackish water bodies) or originate from organic matter (fresh water bodies) increases the alkalinity of the sediment pore-waters and favours precipitation of iron sulphides with the most stable pyrite (FeS2 ). The pathway by which it is formed from the initially amorphous precipitate of FeS involves the formation of intermediate iron monosulphides such as greigite and mackinawite. In environments with a low organic matter content and/or low sulphate conditions, iron sulphide formation will be limited to monosulphide, which persists upon burial. When all the sulphate present in the pore-waters is reduced and used to form iron sulphides, the organic matter degradation continues in the zone of methanogenesis. In this zone, if an Fe+2 reactive is present, iron carbonates (siderite) and/or iron phosphates (vivianite) can precipitate. Thus, the presence of iron carbonates, sulphides and phosphates in the sediment suggests two steps of organic matter degradation: in the zone of microbial sulphate reduction, and in the zone of methanogenesis (for more details see Jones and Bowser 1978, Berner 1971, 1980, 1981, Berner et al. 1979, Curtis 1987, Roberts and Turner 1993, Jelinowska et al. 1997, 1998). 1. The presence of iron sulphides and carbonates in Lake Fert}o sediments suggests anaerobic early diagenetic conditions related to the bacterial degradation of the organic matter. 2. The presence of iron sulphides indicates the sulphate reducing zone in the sediment. Their transformation into magnetic monosulphides during thermomagnetic experiment suggests that it is monosulphide rather than pyrite such transformations do not a ect pyrite but is known for hexagonal pyrrhotite (Fe1;x S 0 < x > 0:13), which alters to monoclinic pyrrhotite (Fe7 S8 ) with a corresponding increase in magnetisation (Schwarz 1975, Dekkers 1988). If monosulphide persists in the sediment, it suggests that there is not enough sulphur ions to form pyrite, so possibly the basin was poor in salts or rich in iron ions. As this behaviour in the Lake Fert}o is not related to only one lithological unit, it is most probable, that this is due to in uence of the water salinity rather than to the iron abundance. 3. The presence of siderite suggests a zone of methanogenesis. In the samples with siderite we do not observe sulphides. This means that either they are not present, so the environment had no sulphate ions as in freshwater conditions or else that pyrite, which we did not observe with the methods we used, may be present in small quantities. In this case the salinity might have been higher. Acta Geod. Geoph. Hung. 35, 2000
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4. It is interesting to note that the presence of monosulphides of iron and/or carbonates of iron indicate signicant changes of the environment. Monosulphides suggest low salinity conditions while carbonates were formed either in freshwater or strongly salty water (if sulphur is absent or abundant respectively). Siderite in sandy sediments (related to uvial transport) in Lake Fert}o sequences would rather indicate a freshwater environment. This study shows the potential of magnetic methods to trace the development of the basin in terms of the hydrological system (relation between evaporation and alimentation) of the lake. A more detailed study is, however, required to obtain a full record of its evolution.
References
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