Magnetic properties of sediment in the Ría de Arousa (Spain): dissolution of iron oxides and formation of iron sulphides

Physics and Chemistry of the Earth 29 (2004) 947–959 www.elsevier.com/locate/pce

Magnetic properties of sediment in the Rı´a de Arousa (Spain): dissolution of iron oxides and formation of iron sulphides Suzan Emiro~ glu a

a,b,*

, Daniel Rey c, Nikolai Petersen

a

Dept. fu¨r Geo- und Umweltwissenschaften, Sektion Geophysik, Universita¨t Mu¨nchen, Theresienstr. 41, 80333 Mu¨nchen, Germany b Sektion Medizinische Physik, Universita¨t Oldenburg, 26111 Oldenburg, Germany c Dpto. de Xeociencias Marin˜as e Ordenacio´n do Territorio, Universidade de Vigo, 36200 Vigo, Spain Received 2 July 2003; received in revised form 6 December 2003; accepted 3 March 2004

Abstract A detailed magnetic study of mineral dissolution has been carried out in order to better characterise the sedimentary environment in the Rı´a de Arousa, an estuarine-like system in western Spain. There the large number (>3000) of mussel rafts has resulted in a high influx of organic matter causing a change of the original chemical environment. Magnetic measurements of sediment cores (reaching 4.1 m depth) combined with electron microscopy show that there is a distinctive depth trend for the concentration of magnetic minerals. The different iron oxides dissolve with increasing time and depth and at different rates, so that their relative proportions change. At the same time, pyrite is forming, replacing the iron oxides at shallow depths. The magnetic properties of the sediments indicate a high dissolution rate of magnetite (half-life 65 year) and other iron oxides in a strongly reducing environment. Additionally, goethite and hematite in the studied system seem to be less reactive than magnetite.
2004 Elsevier Ltd. All rights reserved. Keywords: Dissolution; Iron oxides; Rı´a; Magnetic properties; Sulphide; Pyrite

1. Introduction The Galician rı´as in western Spain were originally fluvial valleys that were inundated by the sea level transgression that succeeded the last glacial maximum, approximately 20 000 years ago (von Richthofen, 1886; Perillo, 1989). At present, they function under estuarine-like dynamics and are susceptible to natural and human impacts, and support coastal ecosystems with high biological productivity and great economic importance (Rubio et al., 2001). This is for the most part, the result of seasonal upwelling, which feeds the rı´as with nutrients, combined with the shelter (of islands and * Corresponding author. Address: Sektion Medizinische Physik, Universita¨t Oldenburg, 26111 Oldenburg, Germany. E-mail address: [email protected] (S. Emiro~ glu).

1474-7065/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.03.012

banks) provided by natural barriers in the rı´a mouth (Rubio et al., 2001). An extensive mussel industry has developed in the past 35 years, in particular in the Rı´a de Arousa, where, at present, more than 3000 rafts are situated occupying an area of 100 ha (Calvo de Anta et al., 1999). As a consequence, the flux and sedimentation rates of organic matter are locally up to two orders of magnitude higher than in normal near-shore conditions (Cabanas et al., 1979; Nombela et al., 1995). Heavy metals from industrialisation and shipping, which are present in the water column and sediment, can accumulate in the cultured species and in other marine organisms, potentially affecting the entire ecosystem. Despite the moderate pollution level of the rı´as at present, decomposition of high amounts of organic matter may boost dissolution of heavy-metal binding compounds in the sediment above the safety levels of heavy metal concentration in water.

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When introduced into the food chain, these heavy metals may ultimately be harmful for human health in the medium and long term. In this context, the Galician government has implemented an ambitious plan of monitoring the water quality and water–sediment interaction in the area. Previous studies carried out in adjacent rı´as showed that the heavy metal distribution in the Rı´a de Vigo and the Rı´a de Pontevedra is controlled not only by the location of contaminant sources, but also by factors such as organic matter content and the nature of early diagenetic mineral transformations (Lo´pez-Rodrı´guz et al., 2000; Rey et al., 2001; Mohamed et al., 2001; Rubio et al., 2001). In the present study, magnetic and analytical measurements of sediment cores rich in organic matter have been carried out in order to better characterise the reducing sedimentary environment. We investigate the advantages of environmental magnetism for the study of these processes and determine the diagenetic pathway of iron compounds by means of magnetic properties. Most of the magnetic particles contained in the sediments sensitively react to chemical variations in the environment. Therefore, magnetic properties can be used as indicators of certain chemical conditions. Dissolution of magnetic mineral assemblages in marine environments is largely controlled by microbially induced processes (Karlin and Levi, 1985). In reducing marine environments, characterised by a high flux of organic matter, diagenetic dissolution of iron oxides and their transformation into iron sulphides are common phenomena. Canfield and Berner (1987), Karlin (1990a,b), Leslie et al. (1990) and Bloemendal et al. (1992) studied the effects on magnetic phases involved in the reactions related to the decomposition of organic matter and the resulting changes in the magnetic properties of the sediment. They showed that the extent of mineral dissolution and re-mineralisation critically depends on the availability and reactivity of organic matter and metabolites. Frederichs et al. (1999) in a similar study presented the advantages of using magnetic parameters for studies of diagenesis in organic-rich marine environments.

2. Location, sampling and material The oceanography of the Rı´a de Arousa, as well as of the ‘‘Rı´as Baixas’’ in Galicia, is guided by two different processes. The inner zones, which are near the river mouths, are typical of estuaries, where the fundamental process consists of the mixing of fresh water (which is contributed mainly by the rivers, Ulla, Umia and Beluso) with sea water. The dynamics of the middle and outer parts of the rı´a are controlled by water exchange with the Atlantic Ocean, which results in a circulation in two layers. The upper current flows from the

outer to the inner parts of the rı´a, and the compensating bottom current circulates toward the rı´a mouth (Fraga and Margalef, 1979; Roso´n et al., 1995; Vilas et al., 2001). Consequently, the sediment in the Galician rı´as originates mostly from two different sources. One fraction is transported by upwelling currents from the continental shelf and the rı´a mouth and the second is transported from riverine sources. Over the last 40 years, the sediment composition in the Rı´a de Arousa has been complemented by organic matter derived from the fisheries (Rubio et al., 2001). The organic matter content in surface sediments of the Rı´a de Arousa varies between 4% and 14%. The lateral distribution shows a progressive decrease from the inner zones (where the density of mussel raft polygons is high) to the external zones of the Rı´a de Arousa. Comparison of the distribution of organic matter with the sediment texture shows that the zones with maximum organic content coincide with the finest sediment textures (Vilas et al., 1999; Informe, 2001a,b). The carbonate content in the rı´a is mostly bioclastic in origin, which reflects the high productivity of shellfish in this system. Below the mussel rafts, the carbonate concentration reaches up to 7.5% by weight of the sediment (Informe, 2001b). The carbonate content increases from the inner parts, where the influx of detrital sediments from rivers is high, to the outer parts of the rı´a. In October 1999, sediment-cores were taken at seven different sites in the Rı´a de Arousa, in the inner, intermediate and external zones. All cores were taken below mussel rafts: a gravity core and a box core were collected at each site (Fig. 1). The box cores were taken from the uppermost 25 cm of sediment. The gravity cores reached down to 4.1 m below the sediment surface and were compacted during recovery to 70% of the original length. Earlier analysis on these cores (Informe, 2001a,b) provide lithological and chemical information (Fig. 2). The granulometry (96% of the material has a grain size <63 lm) does not vary significantly along core BDGCAr4, from which results are presented below. Also the carbonate content (2.1–3.6%) does not show significant changes (Informe, 2001b). The top few centimetres consist of black mud with a strong smell associated with organic matter decomposition, with gastropods and unbroken or fragmented mussel shells. Below 7 cm, the sediment becomes more greenish (5Y/2.5/2 on Munsell
s colour-table), and gastropods and minor abundant shell fragments appear (Informe, 2001a). At 11 cm depth, there is a transition from more natural sediments below to biogenic detritus above. Gastropods and unbroken and fragmented shells of small bivalves occur between 11 and 141.5 cm, whereas almost no bioclasts appear below 141.5 cm. From this description and from X-radiography (right-hand side of Fig. 2) of core BDGCAr4, the prob-

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Fig. 1. Rı´a de Arousa, which is located in western Spain, with coring locations and position of the areas (polygons) covered by mussel rafts. At each numbered site, a gravity and a box core were taken. The cores are labelled BDGCAr for gravity core and BDBCAr for box core, respectively with a site-number of 1–7 (Table 1).

Fig. 2. Core BDGCAr4: nitrogen, sulphur, iron and manganese contents (mass-percentages) versus depth (NB: the plotted x-axes do not start at zero) (Informe, 2001a). X-ray densitometry (right) with description of important lithological changes in the core (dark grey indicates low X-ray transmission and high density). The grain size and the carbonate content does not vary significantly (see text).

able boundary between the original sediment and deposits from mussel rafts appears to lie at 141.5 cm (i.e. at about 200 cm without coring-induced compaction). This depth can be correlated to the year 1964 when the first mussel rafts were established in the Rı´a de Arousa (Informe, 2001a). This gives an average sedimentation rate of 5.6 cm/year since 1964, which is of the same order that one would expect given the high biogenic production from the rafts (Cabanas et al., 1979). Each raft of

200 m2 base produces 130 kg dry ‘‘waste’’ material (Cabanas et al., 1979) per day (averaged over a year), which is deposited on the sea-floor. With a density of 1.5 · 103 kg/m3 and when amplifying the deposition area to 500 m2, this gives a maximum of 6.3 cm of deposition per year below the rafts. The redox potential (Eh) of sediment pore waters has been measured to a depth of 20 cm. The observed Eh values lie between 396 and 43 mV in the box cores, with most of the Eh values

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< 200 mV. Combined with a buffered pH value of 8.0 ± 0.2, these data indicate a reducing environment corresponding to the bacterial methanogenic zone. The lowest Eh values were measured in the deeper parts, generally between 15 and 20 cm, but in some cases reducing layers could also be found in the uppermost sediment (Informe, 2001b). Some elemental analyses of core BDGCAr4 are available from earlier studies (Fig. 2), which show that the sulphur content increases from 0.7% at the top to 1.6% at a depth of 65–85 cm, below which it decreases slowly. The iron content increases from 3.4% to 3.9% at 65 cm, then decreases down to a depth of 100 cm and remains at relatively low values (3.0–3.3%) at greater depths (Informe, 2001a).

Table 1 Cores and sample positions for sediments studied Core

Number (length) of sampled core-sections

Depth of u-channels [cm]

BDGCAr1

2/3 (136 cm) 3/3 (102 cm)

12.5–139 141–230

BDGCAr2

1/2 (136 cm) 2/2 (141 cm)

136–266

BDGCAr3

1/2 (149 cm) 2/2 (111.5 cm)

BDGCAr4

1/2 (135.5 cm) 2/2 (141.5 cm)

BDGCAr6

1/1 (71 cm)

BDGCAr7

1/1 (44.5 cm)

0–44.5

3. Experimental methods Before sampling the sediment for magnetic and analytical measurements, the cores were stored for 3 years in a refrigerated chamber at 3 C. Two years before sampling, the cores were split into halves, and were then sealed with plastic wrapping. Chemical changes, and with it changes of magnetic properties, may have occurred in the sediment as a result of storage. This may have caused differences between conditions for the early geochemical measurements and the magnetic analyses described in this study. 3.1. Sampling, instrumentation and magnetic measurements For magnetic measurements, standard 2.5 · 2.2 cm cylindrical samples and 2 · 2 cm square cross-section uchannels (Weeks et al., 1993) were taken from the cores. All seven box cores and six of the seven gravity cores were sampled for this study. Box cores were sampled at 3 cm and gravity cores at 3–10 cm stratigraphic intervals. The four sampled u-channels are listed in Table 1 along with their respective depths within the cores. The magnetic measurements, for which standard cylindrical samples were used, were carried out in the palaeomagnetic laboratory of the University of Vigo and at the laboratory for rock- and palaeomagnetism at the University of Munich. The low-field magnetic susceptibility (v) was measured with a Bartington Instruments MS2 susceptibility bridge with a MS2B dual frequency sensor (465 and 4650 Hz). An isothermal remanent magnetisation (IRM) was imparted using a MMPM9 pulse magnetiser up to fields of 8 T. The anhysteretic remanent magnetisation (ARM) was imparted using an AGICO LDA-3/AMU-1 magnetiser with an alternating field (AF) of 100 mT and a DC bias field of 100 lT. Remanent magnetisation was measured using an AGICO JR5A spinner magnetometer and a 2G-

Enterprises cryogenic magnetometer. Subsamples of 0.4–0.7 cm3 were taken from the cylindrical samples for hysteresis, thermomagnetic and strong-field IRM measurements. Hysteresis and thermomagnetic measurements were carried out with a variable field translation balance, and thermal demagnetisation was carried out with a Scho¨nstedt furnace. Additional measurements were conducted at the University of Bremen on u-channel samples. Hysteresis loops were measured up to maximum fields of 1 T. The IRM3T was AF demagnetised in stepwise increasing peak fields up to 100 mT. The approximate remanence coercivity (Hcr) was calculated with the Wohlfarth (1958) method. The backfield demagnetisation of IRM2.2T was measured on selected samples. Thermomagnetic measurements were carried out with 10 C/min heating to 630 C in an external 900 mT field. Stepwise thermal demagnetisation of IRM2.2T or ARM was performed with 50 C steps. Additional samples for thermomagnetic analysis were demagnetised in an AF of 100 mT after applying the IRM2.2T and were then thermally demagnetised. Measurements were normalised to the wet mass. Water content was approximately the same for all samples, which is probably due to compaction while coring (Emiro~glu, 2003). 3.2. Electron microscopy Magnetic particles were extracted from the sediment at selected depths for electron microscope observations. First, 2 g of sediment was dispersed in 200 ml of water with dish-washing soap and was maintained in suspension by ultrasonic agitation. The magnetic particles were then extracted with a ‘‘magnetic finger’’ (von Dobeneck, 1993). The extracted material was ‘‘washed’’ by repeating the procedure two times.

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Electron microscopy was carried out at the Centro de Apoyo Cientı´fico y Tecnolo´gico a la Investigacio´n (C.A.C.T.I.) at the University of Vigo. A Phillips XL 30 scanning electron microscope (SEM) with secondary electron resolution of 3.5 nm and acceleration voltage between 0.2 and 30 kV was used. Back-scattered electron and secondary electron modes were both used. A semi-quantitative element analysis was carried out by X-ray-microanalysis (EDX) with a detection of elements starting with boron. Magnetic extracts were then investigated from the selected depths. A Phillips CM 20 transmission electron microscope (TEM) with a resolution of 0.27 nm and acceleration voltage between 20 and 200 kV was also used. Semi-quantitative element analysis was performed with an EDX.

951

magnetic characteristics, but with variations in depths of the features of interest. Magnetic properties are described for the representative 2.77-m-long gravity core BDGCAr4, which was cored in the central zone of the rı´a at a water depth of 35 m (Fig. 1). 4.1. Magnetic susceptibility and remanence intensity v, IRM3T and ARM have similar down-core characteristics, with high values in the upper part and lower values at depth in all measured cores (Fig. 3a, b and c). Core BDGCAr4 shows the most detailed susceptibility profile, which allows detailed study. Because of the relatively large paramagnetic fraction (Fig. 3d) the commonly published ratio SIRM/v does not give further information about magnetic minerals (Emiro~glu, 2003) and was omitted here.

4. Rock magnetic results 4.2. Magnetic hysteresis Magnetic susceptibility was measured on all samples, whereas most other magnetic measurements were carried out on samples from at least three gravity cores and three box cores. All investigated cores show similar

Magnetic hysteresis loops change with depth, the area inside the loop decreases and there is a slight decrease of the paramagnetic slope (Fig. 4). Hysteresis loops down

χ [10-8m3/kg] 0

4

8

12

χ [10-8m3/kg] 16

0

20

4

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0

0

100 BDGCAr... 1 2 3 4 6 7

200

Depth [cm]

Depth [cm]

4 8 BDBCAr... 1 2 3 4 5 6 7

12 16 20

(a)

(b) Magnetisation [10-3 Am2/kg] 0

1

κ [10-5 m3/m3]

2

0

100

200

(c)

10

20

0

SIRM ARM

Depth [cm]

Depth [cm]

0

100 susceptibilities total paramagnetic "ferrimagnetic"

200

(d)

Fig. 3. Various magnetic parameters versus depth. (a) Low-field magnetic susceptibility of gravity cores. (b) Low-field magnetic susceptibility of box cores. For gravity core BDGCAr4: (c) depth trend of ARM and IRM3T and (d) total, paramagnetic and ‘‘ferrimagnetic’’ volume susceptibility.

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20

1

IRM / IRM8T

M/ρ [10-3 Am2/kg]

10

0 27 cm 45 cm 60 cm 91 cm 155 cm 220 cm 270 cm

-10

0.5 sediment depth 21 cm 65 cm 84 cm 230 cm 0 10

-20 -200

0

200

B [mT] Fig. 4. Gravity core BDGCAr4: magnetic hysteresis loops of samples from seven different depths (27–270 cm).

to 40 cm have nearly the same shape, whereas below 40 cm, the enclosed area decreases with depth, but never completely disappears. The paramagnetic contribution remains nearly constant with depth (Fig. 3d) and dominates the hysteresis loops at depths >60 cm (Fig. 4). The ‘‘ferrimagnetic susceptibility’’ was obtained by subtracting the paramagnetic susceptibility, from the linear portion of the hysteresis loop, from the total susceptibility, measured with the Bartington Instruments susceptibility bridge (Fig. 3d). 4.3. IRM acquisition and AF demagnetisation of IRM3T The shapes of the IRM acquisition curves (Fig. 5) indicate that a low-coercivity mineral dominates the magnetic properties in the upper part of the core, where 90% of the IRM8T is saturated in fields of 100 mT. The average coercivity rises with depth and reaches its maximum at 60 cm, where only 50% of the IRM8T can be saturated in a 100 mT field and the sample cannot be saturated in an 8 T field. At around 90 cm, 40% of the IRM8T is acquired by grains with a coercivity between 80 and 400 mT. The magnetisation acquired in fields between 2 and 3 T as well as the IRM1.2T–IRM0.8T show similar down-core trends as the susceptibility (Figs. 3a and 6), and the IRM3T–IRM2T has values <3 · 10 4 A/m for samples deeper than 91 cm. IRM acquisition up to 8 T applied on some samples verified that IRM8T–IRM3T follows the same depth trend as IRM3T–IRM2T (Fig. 6a). The median destructive field (MDF) of the IRM3T upon AF demagnetisation (Fig. 6c) follows the same relative depth trend as found in Fig. 5.

100

1000

10000

B [mT] Fig. 5. Core BDGCAr4: normalised IRM acquisition versus applied pulsed field up to 8 T. Every fifth measurement point is plotted with a symbol.

4.4. High-temperature thermomagnetic measurements The large paramagnetic fraction in the sediment, as indicated by the hysteresis loops, makes it difficult to detect temperature-dependent changes in the carriers of remanent magnetisation by in-field thermomagnetic measurements. The thermomagnetic curves have the same shape for all depths (Fig. 7). The increase in magnetisation followed by a decrease between 420 and 580 C is caused by a chemical change, which was demonstrated by stepwise heating. When cooling from maximum temperatures of 640 C (not shown in Fig. 7), the magnetisation increases by a factor of 10–25 compared to the heating cycle. The magnetisation acquired on the heating cycles between 420 and 480 C is small in samples from shallow depths and triples with depth down to 91 cm (Fig. 6d). From 91 to below 200 cm, the acquired magnetisation decreases to the same magnetisation as at shallow depths. The magnetisation acquired between 420 and 480 C therefore shows a different depth trend compared to susceptibility, remanence and partial IRMs. 4.5. Thermal demagnetisation of IRM2.2T Upon thermal demagnetisation (not shown here), the IRM2.2T and ARM decrease up to 580 C for samples down to 60 cm (Emiro~glu, 2003). Thermal demagnetisation of the IRM with coercivity between 100 and 2200 mT decrease 25% between room-temperature and 120 C for samples from shallow depths (Fig. 8), but is only some 10% decreased for samples below 91 cm. Only some <10% of the IRM remains undemagnetised at

S. Emiro~ glu et al. / Physics and Chemistry of the Earth 29 (2004) 947–959

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953

IRM3T - IRM2T IRM8T - IRM3T

200

100 IRM1.2T - IRM0.8T IRM0.8T - IRM0.4T

200

(a)

Magnetisation acquired from 420 to 480°C [Am2/kg]

MDF [mT] 0

10

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(b)

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0.01

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Depth [cm]

Depth [cm]

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(c)

(d)

Hcr [mT]

M lost from 650 to 700°C [10-3 A/m] 0

4

8

12

16

0

20

40

60

80

0

Depth [cm]

0

Depth [cm]

20

77 cm

100 91 cm 100 mT < Hcr < 2.2 T

100

Hcr < 2.2 T

200

200

(e)

(f)

Fig. 6. Various magnetic parameters for core BDGCAr4 versus depth. (a) Acquired IRM between 2 and 3 T and between 3 and 8 T. (b) Acquired IRM between 0.4 and 0.8 T and between 0.8 and 1.2 T. (c) MDF of IRM3T determined after AF demagnetisation. (d) Magnetisation acquired between 420 and 480 C during thermomagnetic measurements in a 900 mT DC field. (e) Magnetisation lost between 650 and 700 C during stepwise thermal demagnetisation. Crosses refer to the coercivity fraction up to 2.2 T, while open circles refer to the high-coercivity fraction above 100 mT. (f) Remanence coercivity (Hcr). Hcr determined by the Wohlfarth (1958) method plotted as triangles; Hcr from backfield curves as stars.

650 C, after which there is a noticeable drop between 650 and 700 C (Fig. 8). The magnetisation lost over this interval is in Fig. 6e. 4.6. Remanence coercivity The approximate remanence coercivity was calculated for some samples from the intersection point of the normalised IRM acquisition curve with the normalised AF demagnetisation of the IRM. Hcr is nearly constant at 30 mT for the samples above 50 cm (Fig. 6f). It rises sharply to maximum values of 63 mT at 60 cm and then decreases below 110 cm to a value of 35 mT. This

trend matches the MDF obtained by the IRM demagnetisation (Fig. 6c). The remanence coercivity obtained by the method of Wohlfarth is lower than the measured coercivity (Fig. 6f), which was obtained from selected samples by backfield curves, but it follows the measured coercivity trend.

5. Results of electron microscopy Magnetic extracts from the upper 60 cm of the studied sediment yielded sufficiently large amounts (>1 mm) of magnetic particles to be macroscopically visible.

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12 cm 24 cm 60 cm 91 cm 270 cm

Magnetisation [Am2/kg]

0.04

0.02

0 0

200

400

600

Temperature [°C] Fig. 7. Core BDGCAr4: thermomagnetic curves, measured in a 900 mT DC field with 10 C/min heating rate. Every 50th measurement point is plotted with a symbol. The cooling cycle is not shown so that the heating cycle is clearly evident. The magnetisation increase and subsequent decrease between 120 and 230 C, which is not mentioned in the text results from an irreversible phase transition of a mineral, probably of an iron (oxy-) hydroxide, which could not be categorised further.

6 cm 30 cm 50 cm 91 cm 141 cm 200 cm 260 cm

160

M[10-3A/m]

120

80

40

0 0

200

400

600

Temperature[°C] Fig. 8. Core BDGCAr4: stepwise thermal demagnetisation of a highcoercivity remanence, which was applied with a 2.2 T pulsed field, and then demagnetised in a 100 mT alternating field.

Sediments below 60 cm yielded small amounts of magnetic extract even after several hours. Although, the amount recovered proved sufficient for electron microscopy. The extracts obtained between 6 and 27 cm contain detrital and authigenic particles with grain sizes

from 0.1 to 50 lm with different shapes. In EDX they were seen to be mostly composed of oxygen and iron. At 6 cm, the mixture is dominated by spherules of flyash (Fig. 9a) that contain iron and oxygen (probably magnetite). (The concentrations of elements in the particles observed with electron microscopy are described in the caption for Fig. 9.) Also, some inferred magnetite octahedra are visible (Fig. 9b). At 27 cm, some iron oxide particles have convoluted and pitted surfaces and seem to be affected by the onset of dissolution or recrystallisation (Fig. 9c). Iron oxides are also found at 60 cm as fly-ash and other particles with different shapes and uneven, pitted surfaces. Several iron-titaniumoxides have been detected, most of which are tabular-shaped (Fig. 9d). These tabular-shaped irontitanium-oxides dominate the bulk extract from 90 cm down to 160 cm (Fig. 9d). At around 90 cm, some iron sulphides were detected in the form of agglomerates of tiny grains (Fig. 9e). TEM observation of samples from 27 cm reveals many tinyneedle-shaped particles with lengths less than 1 lm (Fig. 9f), which consist only of iron and oxygen. The morphology and composition indicate that it is probably goethite (Maher et al., 1999). Although iron oxides with sizes <1 lm were found, neither with TEM nor with SEM were magnetotactic bacteria observed.

6. Discussion 6.1. Magnetite The depth trends of v, IRM3T and ARM suggest that relatively high concentrations of magnetite (Fe3O4) are present in the sediments above 50 cm and that concentrations decrease with depth starting at 24 cm. IRM acquisition, AF demagnetisation of IRM3T, the Wohlfarth parameter and backfield curves, which indicate low coercivity for samples from shallow depths, suggest that magnetite dominates the magnetic properties at depths <50 cm. This is confirmed by thermal demagnetisation of IRM2.2T and ARM, which indicates a magnetisation decrease to 580 C for samples from shallow depths. SEM observations also indicate that iron oxides are dominant in the upper part of the core, with smaller amounts at 60 cm and little magnetite at depths below 60 cm. Most of the observed Fe-oxide particles are flyashes, presumably produced by various industrial combustion processes. Some other grains have the shape of euhedral magnetite octahedra of presumably detrital origin from the surrounding granitic basement (Vilas et al., 1999). IRM, ARM and susceptibility of magnetite are orders of magnitude larger than of other minerals apart from greigite, which has a higher remanence (Maher et al., 1999). Assuming that no greigite is present here,

S. Emiro~ glu et al. / Physics and Chemistry of the Earth 29 (2004) 947–959

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Fig. 9. Scanning (a–e) and transmission (f) electron micrographs. Percentages in parentheses are concentrations of the element measured with EDX in the centre of each particle, given in mole % (the number of moles of element as a fraction of the total number of moles in the particle). (a) Fly-ash particle from 6 cm, composed of O (66%) and Fe (32%); (b) magnetite octahedron (right) with O (73%) and Fe (25%) and other iron oxides; (c) magnetite octahedron with etched surface from 27 cm with composition of O (66%) and Fe (32%); (d) iron-titanium-oxide flakes from 98 cm with a variable composition of Fe (14–16%), Ti (6–30%) and O (43–68%); (e) agglomerate of small grains with a composition of S (40–46%), Fe (20–36%) and O (18–38%); (f) iron oxides from 27 cm surrounded by needle-shaped iron oxides interpreted as goethite.

all three parameters are good proxies for the relative magnetite content in the sediment. 6.2. Hematite IRM after thermal demagnetisation at 650 C has a depth trend that is similar to the susceptibility (Fig.

6e). Therefore, the unblocking of magnetisation between 650 and 700 C indicates the presence of hematite (aFe2O3) (Fig. 8). IRM acquisition (Fig. 5), MDF derived from AF demagnetisation of IRM (Fig. 6c) and Hcr derived by the Wohlfarth method and backfield curve (Fig. 6f) indicate that a Hcr of 65–90 mT is reached at about 60 cm, where magnetite is less abundant

S. Emiro~ glu et al. / Physics and Chemistry of the Earth 29 (2004) 947–959

relative to the upper sediments. These data are consistent with the presence of hematite and/or goethite at this depth. However, due to the presence of goethite above 90 cm (Section 6.3), the presence of iron-titanium-oxides below 90 cm (Section 6.5) and the resulting overlap in coercivity between 0.4 and 1.2 T, neither coercivity nor a partial IRM, such as IRM0.8T–IRM0.4T or IRM1.2T– IRM0.8T (Fig. 6b), can be used as proxy for hematite. Because of the uniqueness of hematite in unblocking above 650 C, the magnetisation lost between 650 and 700 C thermal demagnetisation seems to be the best proxy for the relative amount of hematite present in the sediment. Hematite seems to have its maximum concentration at shallow depths, diminishing between 50 and 77 cm, and seems to have dissolved almost completely below 77 cm (Fig. 6e). 6.3. Goethite The IRM acquired in pulsed fields of 2–8 T (Fig. 5) has a trend similar to that of susceptibility, with a maximum at 24 cm, and decays completely for samples deeper than 91 cm (Fig. 6a). Goethite (aFe Æ OH) is the only known natural magnetic mineral that can be increasingly magnetised by fields higher than 2 T (Maher et al., 1999), which suggests the presence of goethite at depths down to 91 cm. This singular property makes the IRM acquired between 2 and 3 T a good proxy for the relative amount of goethite. IRM acquisition in high fields indicates a magnetisation increase above 5 T for samples down to 65 cm (Fig. 5), which we therefore attribute to the presence of goethite. The IRM acquired in fields >2 T suggests that the amount of remanence carrying goethite has its maximum at 24 cm and that it has nearly completely dissolved in samples deeper than 91 cm (Fig. 6a). This interpretation was confirmed by thermal demagnetisation of the high-coercivity remanence, which decreased at 120 C, partly as a result of dehydration and partly as a result of demagnetisation of goethite (Fig. 8). The high coercivity at 60 cm (Fig. 6c and f) (where the magnetic properties are not dominated by magnetite) is consistent with the presence of goethite below 60 cm, which was also confirmed by TEM observations (Fig. 9f). If goethite is present below 91 cm, it must be in the form of superparamagnetic grains. 6.4. Dissolution of iron oxides Previous work has demonstrated that the studied sediment is typified by a high organic carbon input and, consequently, by reducing diagenetic conditions (Vilas et al., 1999; Informe, 2001b; Rubio et al., 2001). This, combined with the observed variation of magnetic properties and SEM observations (Fig. 9c), indicates that the detrital iron oxides are dissolving with increasing depth.

Magnetic mineral dissolution has also been suggested for similar settings in the adjoining rı´as of Vigo and Pontevedra (e.g. Rubio et al., 2001; Mohamed et al., 2001; Rey et al., 2001). The time span over which the original concentration of magnetite decays to half is called the ‘‘half-life’’ of magnetite (Canfield and Berner, 1987). Although remanent magnetisation and magnetic susceptibility also depend on the grain size, both parameters can be used as proxies for the amount of magnetite, if magnetite dominates the susceptibility. In core BDGCAr4, the susceptibility reaches its maximum of 20 · 10 8 m3/kg at a depth of 24 cm, and decreases to a value of 3.3 · 10 8 m3/kg at 60 cm, below which magnetite is only rarely detected. Fifty percentage of this decrease is reached between depths of 40 and 45 cm (Fig. 3a). Therefore, the depth over which the magnetite decays to half of its original concentration, is approximately 20 cm in the compacted sediment and 28 cm in true (uncompacted) sediment depth. With a sedimentation rate of 5.6 cm/year this corresponds to a half-life of approximately 5 years. Similar values are obtained when using the IRM3T (4 years) or ARM (4.5 years) instead of susceptibility. In Fig. 10, dissolution of the different iron oxides can be compared using the different magnetic parameters as proxies for the different minerals, as described above. Dissolution of the different oxides apparently happens Relative amount 0

0.2

0. 4

0.6

0.8

1

0

100 Depth [cm]

956

200

magnetite goethite hematite pyrite

Fig. 10. Core BDGCAr4: depth trend of relative amounts of detected magnetic mineral phases evaluated by proxy measurements. For magnetite, the ARM was used as a proxy. For goethite the IRM acquired between 2 and 3 T was used. For hematite the IRM lost between 650 and 700 C upon thermal demagnetisation was used. For pyrite the magnetisation acquired by heating-induced change from 420 to 480 C in thermomagnetic curves was used. Each curve was normalised to the maximum value for each proxy.

S. Emiro~ glu et al. / Physics and Chemistry of the Earth 29 (2004) 947–959

at slightly different depths. Dissolution of iron oxides with increasing depth in marine sediments has already been described by different authors; Canfield and Berner (1987) and Hilgenfeldt (2000) for magnetite, Pyzik and Sommer (1981) for goethite, and Rubio et al. (2001) in the Galician rı´as. According to Canfield and Berner (1987), the degree of dissolution is a function of the surface area of the oxide particle, a mineral specific reaction rate constant, the concentration of dissolved sulphide and the time over which the mineral is in contact with sulphidic pore fluids. In investigations by Canfield and Berner (1987) and Pyzik and Sommer (1981), the halflife of magnetite ranged from 50 to 1000 years. In contrast, the sediment described here contains larger amounts of organic matter, leading to a highly-reducing environment and a magnetite half-life of less than 10 years. Additionally, the goethite and hematite in this study appear to dissolve slower than the magnetite (Fig. 10). In contrast, a notably faster dissolution of goethite compared to magnetite was observed by Canfield and Berner (1987) and Pyzik and Sommer (1981); and Canfield et al. (1992) argued that hematite will be more reactive than magnetite, based on laboratory stability values. However, other investigations on reducing marine systems (Bloemendal et al., 1993; Hounslow and Maher, 1999; Liu et al., 2004; Yamazaki et al., 2003) agree with the findings of the present study. For instance, Bloemendal et al. (1993) suggested that the antiferromagnetic fraction (goethite/hematite) is less susceptible to reductive diagenesis than fine-grained magnetite and Yamazaki et al. (2003) estimated hematite to be more resistive to the reductive dissolution than magnetite. The Eh values measured for the present sediment suggest highly reducing conditions with only a few cm of free oxygen in the top-most sediment (Informe, 2001a,b). The chemical analyses indicate an iron distribution with depth that does not undergo major changes in comparison to the susceptibility changes (Figs. 3a and 2). This means that the iron is not eliminated by dissolution of iron oxides but that it is recycled by formation of iron sulphides. This work suggests that all iron oxides (magnetite, hematite and goethite) dissolve with depth and that their dissolution rates are slightly different in this setting. 6.5. Iron-titanium-oxides The iron-titanium-oxides (Fig. 9d) observed with the SEM at all depths and that dominate the magnetic extract below depths of 90 cm, are presumably detrital hemoilmenites (Fe2 xTixO3) imported by rivers from the hemoilmenite-rich granite in the surrounding catchment area (Vilas et al., 1999). In the strongly reducing environments of the studied sediments, iron-titaniumoxides seem to be more stable than the magnetite, hem-

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atite and goethite. This is consistent with the observation of many other studies (e.g. Bloemendal et al., 1993; Reynolds and Goldhaber, 1978; Canfield et al., 1992). 6.6. Pyrite The significant magnetisation increase between 420 and 480 C in the thermomagnetic curves (Fig. 7) cannot be recognised in any of the corresponding thermal demagnetisation curves. The mineral responsible for this alteration is therefore not a remanence carrier; presumably it is pyrite (FeS2) and/or iron-rich clays that transform(s) into magnetite on heating. Pyrite is known to oxidise in this temperature range to magnetite and maghemite (Passier et al., 2001), so the presence of pyrite is likely. Siderite forms authigenically in highly reducing environments and also transforms at these temperatures; however, it starts transforming at lower temperatures between 250 and 400 C (Pan et al., 2000). The magnetisation growth between 420 and 480 C, and with it the amount of the transforming mineral, attains its maximum at 91 cm depth, where it reaches three times the value as at the uppermost depths (Fig. 6d). The geochemical results (Fig. 2), which show a similar sulphur trend to the amount of this mineral and a maximum sulphur content at 85 cm, confirm that the transforming mineral is pyrite. This suggests that the magnetisation growth between 420 and 480 C can be used as proxy for the relative amount of pyrite in the sediment and that pyrite has a different distribution to that of the iron oxides. The amount of pyrite apparently increases to 90 cm and then decreases slowly below this depth (Fig. 10). Studies of other Galician rı´as have proven the presence of pyrite below mussel rafts (Rubio et al., 1999, 2001). Rubio et al. (2001) show profiles with similar pyritisation trends to the amount of pyrite shown in Fig. 10. SEM observations of the magnetic extracts indicate below 84 cm the presence of particles containing iron, sulphur and oxygen. They look like agglomerates of tiny sulphides, so they may be pyrite- or perhaps greigite-covered magnetite (Canfield and Berner, 1987). The presence of greigite cannot be observed in other measurements, therefore greigite is unlikely as the sulphide bearing phase. Nevertheless, pyritisation of magnetite should be ubiquitous in reducing environments (Canfield and Berner, 1987) and its presence in the Galician rı´as has been reported by Rubio et al. (1999, 2001). The formation of pyrite and other iron sulphides in lake and marine sediments has been reported and described by numerous authors (e.g. Canfield and Berner, 1987; Roberts and Turner, 1993; Liu et al., 2004). In reducing environments, the iron sulphides replace the dissolving iron oxides, but it often takes several hundred years for their formation (Canfield and Berner, 1987;

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Liu et al., 2004). In the present case, sulphide-covered magnetite was detected at 84 cm (Fig. 9e), which corresponds to a formation time of 20 years. We therefore suggest that sulphide formation occurs more quickly here in highly-reducing systems with high sedimentation rates. Few geological examples (e.g. Pye, 1981) with sulphide formation rates similar to the present study are published. 7. Conclusions Observed variations of magnetic properties combined with evidence for reducing diagenetic conditions indicate that the iron oxides, magnetite, hematite and goethite, in the uppermost part of the studied sediment have progressively dissolved with increasing depth. The different iron oxides dissolve at slightly different rates, so that their relative proportions change with depth. In core BDGCAr4, the amount of magnetite decays to the limit of resolution at 60 cm, hematite at 77 cm, and goethite at 91 cm. In contrast, the amount of pyrite increases within the first 100 cm. The iron sulphide replaces the dissolving iron oxides so that pyrite triples in its concentration within the uppermost 91 cm. We show that the dissolution rates of the detected iron oxides are high in the studied highly reducing environment; the half-life of magnetite was determined as being less than 10 years, which is a much shorter residence time compared to normal marine conditions. In the present case, the dissolution of oxides and the formation of sulphides seems to occur at much faster rates than has been documented in most previous reports. Additionally, goethite and hematite show more resistance to dissolution than magnetite in this system. Acknowledgments The authors thank the Department of Geoscience at the University of Bremen for their support, and Benito Rodrı´guez and Jesu´s Me´ndez for their assistance with the SEM and TEM. This work was financially supported by Paleostudies, an EU-project of the Human Potential––Access to Research Infrastructures (ARI) program (Ref. HPRI-CT-2001-00124). It was also funded by the EU-project MAG-NET (Ref. FMRXCT-980247) and partially by the Xunta de Galicia (Ref. PGIDT03-RMA30101) and Comisio´n Interministerial de Ciencia y Tecnologı´a (CICYT) (REN200312822MAR/03233) of Spain.

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