Evidence for Vivianite Formation and Its Contribution to Long-Term Phosphorus Retention in a Recent Lake Sediment: a Novel Analytical Approach

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Biogeosciences, 11, 5169–5180, 2014 www.biogeosciences.net/11/5169/2014/ doi:10.5194/bg-11-5169-2014 © Author(s) 2014. CC Attribution 3.0 License.

Evidence for vivianite formation and its contribution to long-term phosphorus retention in a recent lake sediment: a novel analytical approach

M. Rothe1,2, T. Frederichs3, M. Eder4, A. Kleeberg1,*, and M. Hupfer1 1Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany 2Department of Geography, Humboldt University of Berlin, Berlin, Germany 3Department of Geosciences, University of Bremen, Bremen, Germany 4Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, Potsdam, Germany *now at: State Laboratory Berlin Brandenburg, Kleinmachnow, Germany

Correspondence to: M. Rothe ([email protected])

Received: 25 April 2014 – Published in Biogeosciences Discuss.: 21 May 2014 Revised: 26 August 2014 – Accepted: 27 August 2014 – Published: 26 September 2014

Abstract. Vivianite, Fe3(PO4)2 · 8H2O, is a ferrous iron not serve as a reliable predictor for the in situ formation of phosphate mineral which forms in waterlogged soils and sed- vivianite. In Lake Groß-Glienicke, vivianite formation con- iments. The phosphorus (P) bound in its crystal lattice is tinues to be triggered by the artificial iron amendment more considered to be immobilised because vivianite is stable un- than 20 yr ago, significantly contributing to P retention in sur- der anoxic, reducing, sedimentary conditions. Thus, vivianite face sediments. formation can make a major contribution to P retention during early diagenesis. Much remains unknown about vivianite in sediments, because technical challenges have rendered direct identification and quantification difficult. To identify vi- 1 Introduction vianite and assess its significance for P burial during early diagenesis we studied the consequences of a 1992/1993 in-lake Understanding phosphorus (P) dynamics in aquatic ecosys- application of FeCl3 and Fe(OH)3 aimed at restoring Lake tems has been of particular interest among researchers for Groß-Glienicke (Berlin, Germany). In a novel approach, we almost a century. Since P is an important nutrient limiting firstly applied a heavy-liquid separation to the iron-rich sur- primary production, the cycling of P, and the role of the face sediments which allowed direct identification of vivian- sediments in regulating P availability in the water column ite by X-ray diffraction in the high-density (ρ > 2.3 g cm−3) have been studied intensively. Long-term burial of P in sedi- sediment fraction. Secondly, we assessed the contribution ments is the only mechanism able to shield P from continu- of vivianite to P retention, combining results from chemi- ous (re)-cycling, and therefore represents an output of P from cal digestion with magnetic susceptibility data derived from the internal nutrient cycle (Katsev et al., 2006; Hupfer and magnetic hysteresis measurements. Scanning electron mi- Lewandowski, 2008). croscopy revealed that the dark blue spherical vivianite nod- The burial of P depends on several processes (Ruttenberg, ules were 40–180 µm in diameter, and formed of platy- and 1992; Søndergaard et al., 2001) and fixation is of either or- needle-shaped crystal aggregates. Although equilibrium cal- ganic or inorganic phases. Organic P originates from set- culations indicated supersaturation of vivianite throughout tled organic material which is refractory or not yet miner- the upper 30 cm of the sediment, the vivianite deposits were alised by microorganisms. Inorganic phases include P sorbed homogeneously distributed within, and restricted to, the up- onto the surface of iron(oxy)hydroxides, aluminium hydrox- per 23 cm only. Thus, supersaturated pore water alone can- ides, clays and calcite (Moosmann et al., 2006). P associated with Fe(III) minerals have been found in anoxic marine

Published by Copernicus Publications on behalf of the European Geosciences Union. 5170 M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention and estuarine sediments, indicating a resistance towards re- tions, or electron microprobe analysis lacking direct identifi- ductive dissolution of these compounds, at least under non- cation. The difficulty in identifying vivianite in a sediment sulfidic conditions (Hyacinthe and Van Cappellen, 2004; matrix might be a reason why there is limited knowledge Lehtoranta et al., 2009). Thus, during sediment diagenesis about the quantitative importance of vivianite formation in there may be the formation of stable phosphate minerals P burial during early diagenesis. which bind P in their crystal lattice and contribute to per- To address this need, the aims of our present study were to: manent P burial (see e.g. Jilbert and Slomp, 2013). However, (1) identify vivianite crystal aggregates from recent lake sed- the processes controlling the P burial under anoxic conditions iment showing favourable formation conditions in the pore are still not fully understood (e.g. Stigebrandt et al., 2013). water, (2) determine the significance of vivianite formation One specific phosphate mineral which forms during for P burial during early diagenesis, and (3) assess the impact sediment diagenesis is the ferrous phosphate vivianite, of artificial addition of Fe during lake restoration as a trigger Fe3(PO4)2 · 8H2O. The mineral is a regularly found deposit for vivianite formation. For these purposes we developed and in sedimentary cores (e.g. Brauer et al., 1999; Fagel et al., applied a novel analytical approach, combining heavy-liquid 2005; Sapota et al., 2006; Minyuk et al., 2013). Since P is separation of surface sediments followed by mineral identifi- only a minor constituent in sediments, the direct identifica- cation by X-ray diffraction, elemental analyses and magnetic tion of vivianite is difficult. While a large fraction of total hysteresis measurements. P can be organic or sorbed P, vivianite may comprise only a small part of the inorganic sediment matrix (Lindsay et al., 1989). 2 Material and methods In natural systems, vivianite is stable under pH conditions from 6 to 9 (Nriagu, 1972) and can form in water- 2.1 Study site logged soils and sediments (Berner, 1981). The precipitation Lake Groß-Glienicke is a dimictic lake located southwest of of vivianite directly from pore water solution is favoured by Berlin, Germany (52◦2705700 N, 13◦0603900 E), with a mean high concentrations of ferrous iron (Fe(II)) and soluble re- depth of 6.8 m, a maximum depth of 11 m, and a surface area active P (SRP) often found in anoxic non-sulfidic environ- of 0.67 km2. Until 1992 the lake was highly eutrophic, and ments (Nriagu, 1972; Nriagu and Dell, 1974; Roden and Ed- had on average total P (TP) and chlorophyll a (Chl a) con- monds, 1997). By flux calculations Emerson and Widmer centrations of 485 µg L−1 and 41 µg L−1 (1989–1992) (Klee- (1978) demonstrated that mineral formation is surface- rather berg et al., 2013). To reduce the P concentration in the wa- than diffusion-controlled, and hence there is no equilibrium ter column, the lake was treated with solid ferric hydrox- control by vivianite with respect to pore water SRP and Fe2+ ide (Fe(OH) ) and dissolved ferric chloride (FeCl ) between concentrations (Postma, 1981). Recently, Walpersdorf et al. 3 3 December 1992 and February 1993 (Wolter, 2010). Due to (2013) confirmed that vivianite does not control phosphate decreased external P inputs and the in-lake P precipitation, solubility in an anoxic meadow soil. TP and Chl a decreased reaching today’s mesotrophic levels The literature has reported vivianite crystal aggregates to of TP 20 µg L−1 and Chl a 7 µg L−1. Sediment stratigraphy be needle-shaped or spherical with diameters ranging be- shows a distinct increase in Fe and P content corresponding tween a few micrometres to several centimetres. Upon ex- to the time of the in-lake treatment. Today, this shift is evi- posure to air, the crystals turn vivid blue due to partial oxi- dent at a sediment depth of 23 cm. Both Fe and TP contents dation of Fe2+ (Hush, 1967). As ferric iron (Fe(III))-phases in the sediment almost doubled after the treatment, reach- and organic matter serve as source material for Fe2+ and ing 33 mg g−1 dry weight and 2.6 mg g−1 dry weight (Klee- SRP, vivianite often occurs in the vicinity of these phases. berg et al., 2012). Due to the iron’s redox sensitivity iron and Laboratory studies show that vivianite appears as a sec- TP contents are higher throughout the newly formed sedi- ondary mineral product following Fe(III) reduction by dis- ment which has accumulated since the in-lake treatment in similatory iron-reducing bacteria (DIRB) if sufficient PO3− 4 1992/93. is present in the culture medium (e.g. Borch and Fendorf, 2007; O’Loughlin et al., 2013). 2.2 Sample collection and preparation All these findings indicate that the interplay between the availability of source materials (Fe(III)-phases, organic mat- 2.2.1 Porewater ter), the activity of DIRB and their cell-mediated microenvironments, as well as the pore structure of the sediment ma- To document the geochemical conditions within the upper trix, are important in vivianite formation. However, direct ev- 30 cm of the sediment, we used two in situ dialysis sam- idence of vivianite in recent sediments (e.g. Nembrini et al., plers (Hesslein, 1976) each with 14 chambers and a vertical 1983; Hearn et al., 1983; Manning et al., 1991; Taylor et al., resolution of 4 cm. The samplers were filled with deionised 2008; Nanzyo et al., 2013) appears to be scarce. Often the water and covered by a 0.2 µm membrane filter (Gelman® claim of vivianite findings is solely based upon indirect mea- HT 200 Tuffryn). To remove oxygen from the chamber wa- sures, such as equilibrium calculations, sequential P extrac- ter the samplers were degassed with nitrogen for 24 h and

Biogeosciences, 11, 5169–5180, 2014 www.biogeosciences.net/11/5169/2014/ M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention 5171 subsequently deployed at the deepest site in the lake for 20 d (Shimadzu). Subsamples for DIC were stored in nitrogen- in September 2013 during thermal stratification. flushed air-tight vessels and analysed on the day of collection using a carbon analyser (multi N/C 3100, Jena Analyt- 2.2.2 Sediment ics). To prevent oxidation of free sulfides subsamples from each chamber were immediately fixed with 0.2 M zinc ac- Sediment cores 35–40 cm long and 60 mm in diameter were etate solution. HS− was measured photometrically by apply- taken in September 2012 and May 2013 by a gravity corer ing the methylene blue method (Cline, 1969). Concentrations + (UWITEC) at the deepest site in the lake. Two hours af- of SRP and NH4 were determined photometrically by the ter sampling the cores were extruded, immediately sectioned molybdenum blue method (Murphy and Riley, 1962) and the into 20 mm slices, frozen, and freeze-dried for at least 72 h. indophenol method (Bolleter et al., 1961) respectively, using The freeze-dried sediment served as raw material for further segmented flow analysis (Skalar Scan++, Skalar Analytical analysis and is in the following text referred to as “bulk” sed- B.V.). All determinations were performed in duplicate. iment. Subsamples of freeze-dried sediment (about 2.3.2 Sediment 1 g dry weight) were homogenised by gentle pestling in an agate mortar, and subsequently sieved through 80 µm The concentrations of Fe, Ca, Al, Mg, Mn, S and P were and 63 µm stainless steel meshes. Only the largest size determined by ICP-OES after wet digestion (HCl 36 %, fraction (≥ 80 µm) was used and thereafter separated by HNO3 76 %, volumetric ratio 1 : 3) in a high-pressure mi- density using two different concentrations of a sodium crowave oven (Gigatherm). Mineral composition of sedi- polytungstate solution (3Na2WO4 · 9WO3 · H2O, ABCR). In ment was characterised by powder X-ray diffraction (XRD) step I, a sediment sample of approximately 0.3 g was mixed with a Bruker AXS D8 diffractometer equipped with Cu- in a centrifuge tube with 10 mL of sodium polytungstate Kα-radiation and a Sol-X solid state detector. Synthetic, solution (density ρ = 1.9 g cm−3) and placed in an ultrasonic slightly oxidised (blue appearance) vivianite powder (Dr bath for 20 min. After sonication the mixture was centrifuged Paul Lohmann GmbH KG) served as an internal XRD stan- for 10 min at 9.050 rcf. The heavy fraction was then trans- dard. The XRD patterns were measured between 5 and ferred into a new tube. The light fraction and the supernatant 60 ◦ 2θ with a step of 0.05 ◦ 2θ and an integration time of were again sonicated for 10 min and then centrifuged for 12 s. Scanning electron micrographs of sediment concretions 10 min at 9.050 rcf and the heavy fraction transferred to were obtained with a FEI Quanta 600FEG field emission en- a new tube. This process was repeated a third time and the vironmental scanning electron microscope (FE-ESEM). Sed- heavy fractions of all three runs were pooled. In step II, iment concretions were sputter coated with palladium prior the pooled heavy fractions from step I were mixed with to the analysis of elemental composition by energy disper- a sodium polytungstate solution (density ρ = 2.3 g cm−3) sive X-ray spectroscopy at 15 kV acceleration voltage (Jeol- and processed analogously to step I. Subsequently, each of 7500F scanning electron microscope) with an EDX detector the three separated sediment fractions (A: ρ > 2.3 g cm−3, (X-Max, Oxford Instruments). B: 2.3g cm−3 > ρ > 1.9 g cm−3, C: ρ < 1.9 g cm−3) were Magnetic hysteresis measurements (magnetisation vs. rinsed with deionised water until conductivity dropped magnetic field) of subsamples of bulk sediment and high- below 50 µS cm−1, and then freeze-dried. In the following, density samples (ρ > 2.3 g cm−3; sample mass ranging from sediment samples from fraction A (ρ > 2.3 g cm−3) were about 1 to 5 mg) were conducted on an Alternating Gradi- analysed and are referred to as “high-density” samples. ent Magnetometer (Princeton Measurement Corporation) at room temperature in peak fields of 0.3 T (Flanders, 1988). 2.3 Analysis Diamagnetic matter, such as calcium carbonate and quartz, demonstrates a weak negative linear dependency of induced 2.3.1 Porewater magnetisation on the ambient magnetic field. In contrast, paramagnetic substances, such as many Fe-bearing miner- Subsamples from each dialysis chamber were taken for the als, including vivianite, show a strong and positive linear de- analysis of 13 parameters: 6 cations (Na+,K+, Ca2+, Mg2+, pendency of induced magnetisation on the ambient magnetic 2+ 2+ − 2− − Fe , Mn ), 3 anions (Cl , SO4 , NO3 ), dissolved inor- field. Finally, ferrimagnetic minerals such as magnetite ex- ganic carbon (DIC), free sulfides (HS−), SRP and ammonia hibit a much stronger and non-linear field dependency. Their + (NH4 ). Temperature and pH were measured in each chamber magnetisation curve is called the hysteresis loop. using a pH electrode (Mettler Toledo). The proportional factor between magnetisation and mag- Quick sampling and fixation with hypochloric acid pre- netic field is named magnetic susceptibility. This character- vented oxidation of redox-sensitive species (Fe2+, Mn2+). istic is field independent, i.e. linear, for dia- and paramag- Cations were analysed by inductively coupled plasma atomic netic substances, and field dependent in the case of ferrimag- emission spectrometry (ICP-OES, iCAP 7000series, Thermo netic substances. Thus, paramagnetic susceptibility may be Scientific). Anions were analysed by ion chromatography deduced from hysteresis measurements, by calculating the www.biogeosciences.net/11/5169/2014/ Biogeosciences, 11, 5169–5180, 2014 5172 M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention

( a ) ( b ) 3 Results - 2 0 - 2 0 3.1 Chemical conditions in the pore water - 1 0 - 1 0 ]

m There were distinct gradients in pH and concentrations of

c 0 0 [ + − 2+ 2 h

t DIC, NH4 , SRP, Fe and SO4 across the sediment–water p e

D 1 0 1 0 interface (SWI) (Fig. 1). The pH increased in the water body above the SWI from pH = 7.60 at −23 cm to pH = 2 0 2 0 8.00 at the SWI, and decreased to pH = 7.15 at 29 cm sediment depth (Fig. 1a). The concentration of DIC was al- 3 0 3 0 −1 7 . 0 0 7 . 2 5 7 . 5 0 7 . 7 5 8 . 0 0 0 2 4 6 8 most constant at 3.9 mmol L in the overlying 23 cm of - 1 − p H D I C [ m m o l L ] the water body above the SWI, and increased to 6 mmol L 1 within the upper 4 cm of the sediment and continuously in- N H + [ m m o l L - 1 ] 2 + - 1 −1 4 F e [ m m o l L ] creased down-core to reach 7.9 mmol L at 29 cm sedi-

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 0 . 0 0 0 . 0 5 0 . 1 0 0 . 1 5 0 . 2 0 0 . 2 5 ment depth (Fig. 1b). SRP concentration increased in the − ( c ) ( d ) water body above the SWI from 0.15 mmol L 1 at −23 cm - 2 0 - 2 0 to 0.25 mmol L−1 at −3 cm. Directly at the SWI, SRP had −1 - 1 0 - 1 0 a local maximum concentration of 0.59 mmol L and in- −1

] creased further downcore to reach 0.87 mmol L at 29 cm

m + c

[ 0 0

sediment depth (Fig. 1c). Concentration proﬁles of NH and

h 4

t + p 2

e Fe had a similar course: in the water body above the SWI

D 1 0 1 0 concentrations were almost constant at 0.1 mmol L−1 and −1 2 0 N H + 2 0 2 + 0.08 mmol L , respectively, and increased by a factor of two 4 F e 2 - S R P S O 4 and five respectively within the uppermost 4 cm of the sedi- 3 0 3 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 0 . 1 5 0 . 3 0 0 . 4 5 0 . 6 0 0 . 7 5 ment (compared to their concentrations at the SWI). Further −1 S R P [ m m o l L - 1 ] S O 2 - [ m m o l L - 1 ] down-core the concentrations increased to 0.74 mmol L 4 −1 2− and 0.20 mmol L at 29 cm (Fig. 1c and d). The SO4 con- Figure 1. Vertical profiles of six parameters taken from an in situ centration showed a decreasing sigmoidal trend across the pore water sampler deployed at the deepest point of Lake Groß- SWI, and from 4 cm below the SWI and deeper, the con- Glienicke (positive downward sampling depth) (September 2013): centration was so low as to be close to the detection limit (a) pH, (b) dissolved inorganic carbon (DIC), (c) soluble reactive (0.001 mmol L−1) (Fig. 1d). The concentrations of HS− and + 2+ − phosphorus (SRP) and ammonia (NH4 ), (d) ferrous iron (Fe ) NO were not detectable. 2− 3 and sulfate (SO4 ). A depth of 0 cm indicates the sediment–water The pore water of the sediment was supersaturated with interface. respect to Fe(III) and mixed Fe(III)/Fe(II) phases, including magnetite, hematite, goethite and lepidocrocite. According to our equilibrium calculations, saturation indices (SI) were slope of the increasing linear part of the hysteresis loop after highest for magnetite and lowest for lepidocrocite (SI = 12.9 magnetic saturation of ferrimagnetic minerals in sufficiently and 2.7, respectively) and slightly decreased with sediment strong magnetic fields. Magnetic hysteresis measurements depth. For vivianite, in the pore water of the sediment, there therefore allow the determination of paramagnetic suscep- was continuous supersaturation of four orders of magnitude tibility even in the presence of magnetically much stronger at depths of 4–28 cm (Fig. 2). At the SWI, the supersaturation ferrimagnetic minerals. of vivianite was about one order of magnitude lower than in subjacent pore fluids (4–28 cm), and 4 cm above the SWI the 2.4 Thermodynamic calculations supersaturation was close to equilibrium. There was a sharp 2+ increase in the activity of Fe (aFe2+ ) with sediment depth, Geochemical equilibrium calculations were based on the re- as shown by plotting aFe2+ against that of a 3− (Fig. 2). PO4 sults of the pore water analysis using the computer pro- The thermodynamic calculations showed a strong subsatu- gram PhreeqC (version 2.18.5570) (Parkhurst et al., 1999) ration for manganese phosphate (SI ≤ −10), saturation close with minteq4 database. The database was extended to in- to equilibrium for siderite (FeCO3) and supersaturation for clude metal complexation with dissolved organic carbon the calcium phosphate hydroxylapatite (SI ranged from 8 to (DOC). The equilibrium constants (log K) for 1 : 1 metal- 10) in the upper 28 cm of the pore water of the sediment. ligand complexes (e.g. Ca-, Fe(II)-fulvic ligand complexes) To determine the effect of dissolved organic carbon (DOC) were taken from Steinmann and Shotyk (1997). The solubil- on the output of the equilibrium calculations, and in partic- −36 ity constant of vivianite (Kviv = 1 × 10 ) was taken from ular on aFe2+ (see e.g. Reuter and Perdue, 1977), we per- Nriagu (1972). formed equilibrium calculations with DOC concentrations

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- 9 . 0 0 S e d i m e n t d e p t h diameter) of platy- and needle-shaped crystals (Fig. 3b). Oc- F e ( P O ) 8 H O [ c m ] 3 4 2 2 B casionally the aggregates also included organic debris such A - 4 B 0 as diatom shells. - 9 . 2 5 C 4 The XRD patterns of high-density samples from sediment 5 D 8 4 E 1 2 layers L1, L2 and L3 showed reﬂexes characteristic for vi- F 1 6 A 3 G 2 0 vianite, and conﬁrmed the existence of the mineral in the sed-

- - 9 . 5 0 3 4 H O 2 4 P 2 I 2 8 iment (Fig. 4a). The abundance of blue nodules in samples a

g from L1 and L2 was similar to each other but higher than in o 1 l - 9 . 7 5 C samples from L3 (Fig. 3a). Iron, P and Ca were most abun- 0 D dant of the six elements in the high-density samples from - 1 E layers L1 and L2; S, Al and Mn were present at much lower - 1 0 . 0 0 G concentrations (Table 1). In L3, the relative amount of Fe, H I F e 2 + + P O 3 - F Ca, and Al did not signiﬁcantly differ from that in L1 and 4 - 1 0 . 2 5 L2. The relative amount of P of samples from L1 and L2 was - 6 . 0 - 5 . 5 - 5 . 0 - 4 . 5 - 4 . 0 - 3 . 5 similar, whereas the amount in L3 was signiﬁcantly lower.

l o g a 2 + F e For S, the relative amount was more than four times higher in samples from L3 than in samples from L1; for Mn, the rela- Figure 2. Degree of saturation of the pore water with respect to vi- tive amount in samples from L1 and L2 was similar, whereas vianite of Lake Groß-Glienicke in nine different sediment depths L3 had a significantly lower concentration. (A–I) (September 2013). Letters next to data points indicate sam- The SEM-EDX analysis on the dark blue aggregates re- pling depth (positive downward sampling depth). The thick line vealed the major elemental constituents to be Fe and P, with K = in the diagram represents the vivianite solubility constant viv a mean Fe : P ratio of 1.49±0.1 (determined from peak area, 1 × 10−36 given by Nriagu (1972). Fig. 4b), which is close to the Fe : P ratio of vivianite (Fe : P = 1.5). The Si content was high, reaching 8 mol%. Minor ranging from 10 to 500 mg L−1. The results showed that constituents of aggregates were Mn (4 mol%), Ca (3 mol%), S (2 mol%) as well as Mg and Al (both ≤ 1.5 mol%). there is a significant decrease in aFe2+ through the formation of Fe(II)–organic-ligand complexes only at DOC concentra- Magnetic hysteresis measurements showed that in com- tions above 200 mg L−1. As this threshold concentration is at parison to bulk sediment, the high-density samples were least one order of magnitude higher than those found in the enriched with paramagnetic material such as vivianite and water column of Lake Groß-Glienicke, we did not include iron sulfides (FeSx) (Fig. 5a). Both bulk sediment and high- DOC as input data for equilibrium modelling. density samples contained ferrimagnetic material, most prob- ably magnetite, recognised by the opening of the hysteresis 3.2 Structural and elemental composition of sediments loops. The vivianite content (cvivianite [weight as % of total]) in The XRD patterns from different sediment depths showed high-density samples was estimated by comparing the mea- quartz and calcium carbonate phases as well as several other sured paramagnetic mass-specific susceptibility of a high- 3 −1 minerals, e.g. dolomite, plagioclase and pyrite (for clarity not density sample (MSsample [m kg ]) with the value given −6 3 −1 indicated in Fig. 4a) to be present in the sediment. However, for vivianite, of MSvivianite = 1.05 × 10 m kg (Minyuk the XRD reflexes characteristic for vivianite could not be et al., 2013). The MSsample values were calculated from the identified from bulk sediment samples. The diffraction pat- slope of the linear increasing part of the hysteresis loops af- tern of synthetic vivianite powder (blue appearance) was in ter magnetic saturation of ferrimagnetic minerals. The rela- coincidence with the vivianite reference pattern which use tive vivianite content in a high-density sample is then given data from the Crystallography Open Database (COD). The as follows: synthetic vivianite powder changed its colour from dark blue MSsample to orange-brown and lost its characteristic diffraction pattern c = × 100. (1) vivianite MS upon heating at 60 ◦C for 24 h (Fig. 4a). vivianite Examination of sediment with a reflected-light microscope This calculation is valid assuming that: (1) the contribu- revealed that dark blue nodules were present throughout the tion of diamagnetic materials, such as calcium carbonate upper 23 cm of the sediment, but absent from subjacent sed- and quartz, to the overall measured magnetic susceptibility iment layers (23–40 cm). The heavy-liquid-separated, high- is small, and (2) vivianite is the only paramagnetic mate- −3 density sediment samples (fraction A, ρ > 2.3 g cm , from rial present in the high-density samples. Our MSsample val- sediment depth layers L1: 0–10 cm, L2: 11–20 cm, and ues ranged from 1.7 × 10−7 to 3.6 × 10−7 m3 kg−1, which L3: 21–30 cm) were enriched with the dark blue nodules is equivalent to a relative vivianite content of 22 to (Fig. 3a). Scanning electron micrographs of these blue nod- 48 weight as % of total. These values exceeded the maximum ules showed them to be spherical aggregates (40–180 µm in vivianite content based upon the amount of P present in www.biogeosciences.net/11/5169/2014/ Biogeosciences, 11, 5169–5180, 2014 5174 M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention

Figure 3. (a) Reﬂected-light microscope images of heavy-liquid separated, high-density, samples from three different sediment depths (L1: 0–10 cm, L2: 11–20 cm, L3: 21–30 cm) of Lake Groß-Glienicke. (b) Scanning electron micrographs of dark blue nodules enriched in high- density samples from L1–L3.

Table 1. Elemental composition [mol %] of high-density samples (ρ > 2.3 g cm−3) from three different sediment depth layers (L1–L3) of Lake Groß-Glienicke from September 2012 and May 2013. Data are mean ± standard deviation (SD, n = 5).

Fe Ca P S Al Mn Sediment depth L1 (0–10 cm); n = 5 43.0 ± 6.3 23.0 ± 11.0 23.5 ± 2.3 3.8 ± 1.5 3.4 ± 1.3 3.3 ± 0.4 L2 (11–20 cm); n = 5 51.2 ± 2.3 11.6 ± 2.5 23.0 ± 2.3 6.5 ± 2.1 5.1 ± 1.5 2.8 ± 0.6 L3 (21–30 cm); n = 5 55.9 ± 11.4 11.6 ± 4.5 13.9 ± 1.9 18.0 ± 2.5 7.4 ± 3.0 1.7 ± 0.3

3 −1 the high-density samples (Fig. 5b). To correct MSsample for where MSsample, FeS [m kg ] is the contribution of para- the contribution of paramagnetic FeSx, we assumed all sul- magnetic FeS to the measured mass-specific susceptibil- fur (Table 1) to be present as FeS in the high-density ity and cFeS is the content of FeS [weight as % of total] samples. Accordingly, high-density samples contained be- in a high-density sample. The FeS-corrected vivianite tween 4 and 21 weight as % of total FeS. Assuming a para- content in high-density samples ranged between 13 and magnetic mass specific susceptibility of FeS, of MSFeS = 33 weight as % of total. These values were, except for sample 1.54 × 10−7 m3 kg−1 (Lide, 2004), equation (1) can be re- H5, 1 to 15 % lower than the vivianite content derived from written as follows: chemical digestion (Fig. 5b). Relating the vivianite content in the high-density sam- (MSsample − MSsample, FeS) cvivianite = × 100 ples, calculated from chemical digestion, to the equivalent MSvivianite amount in bulk sediment, we were able to assess the sig- (MSsample − (MSFeS × cFeS)) = × 100, (2) nificance of vivianite formation for the retention of P in MSvivianite

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( a ) ( b )

α α α α β β V iv ia n it e α α β α α K K α α K K K L L K K

K K

g n a a a l i d d e e A S M S P F N P P M C F M e t a v iv ia n it e C ( I ) S y n t h e t ic v iv ia n it e ( I I ) S y n t h e t ic v iv ia n it e a f t e r o x id a t io n ( 2 4 h a t 6 0 ° C ) ] s t i n ] u s

t i y r n a u

t q y i ( I I I ) H ig h - d e n s it y s e d im e n t r

b q - q u a r t z a r

s a m p le r a t i [ v - v iv ia n it e

q b

y r

t v q i a

v [ s v v v v v v v n q

v s t e t n n u i

( I V ) B u lk s e d im e n t q o e c - c a lc it e C v i

t q - q u a r t z a l e c R q c c c c q c q q c

1 0 2 0 3 0 4 0 5 0 6 0 1 2 3 4 5 6 7 8 2 θ [ ° ] E n e r g y [ k e V ]

Figure 4. (a) XRD patterns of (I) synthetic vivianite, (II) synthetic vivianite after oxidation at 60◦C for 24 h, (III) high-density sample and (IV) bulk sediment. Characteristic reflexes of vivianite could only be observed after heavy-liquid separation in the high-density sample (III). The line patterns of vivianite (blue, file number 96-901-2899) and metavivianite (black, file number 96-100-1784) use data from the Crystallography Open Database (COD) REV 64680 (2012 edition). (b) SEM-EDX elemental spectrum obtained from a dark blue sediment concretion from Lake Groß-Glienicke. The corresponding atomic Fe:P ratio is 1.49. Elemental peaks of carbon and oxygen are omitted.

( a ) ( b ) 0 . 1 H i g h - d e n s i t y s e d i m e n t s a m p l e 5 0 ] ] 1 l - a g t k o

t 2

f 4 0 m o

A [

ؓ s a

t 3 0 h g i

B u l k s e d i m e n t s a m p l e e

w [

n 2 0

- 0 . 3 0 . 3 e B [ T ] t n o c

t 1 0 i n a i v i V 0 H 1 H 2 H 3 H 4 H 5 H 6

- 0 . 1 H i g h - d e n s i t y s e d i m e n t s a m p l e p a r a m a g n e t i c s u s c e p t i b i l i t y i r o n s u l f i d e - c o r r e c t e d p a r a m a g n e t i c s u s c e p t i b i l i t y c h e m i c a l d i g e s t i o n

Figure 5. (a) Magnetisation σ [A m2 kg−1] vs. magnetic field B [T], peak value B = 0.3 T of a high-density sample, and a bulk sediment sample from 10 cm sediment depth. The positive slope of the high-density sample at higher magnetic fields indicates enrichment with paramagnetic material after heavy-liquid separation. (b) Variations in vivianite content (expressed as % weight of the sample) in each of six high-density samples, based upon paramagnetic susceptibility, iron sulfide-corrected paramagnetic susceptibility and chemical digestion. the sediment. In samples from L1 and L2, vivianite for- cause depth layer L3 included sediment layers where no vi- mation accounted for 0.95 ± 0.21 mg g−1 dry weight P and vianite nodules were present (24–30 cm sediment depth). 0.96 ± 0.16 mg g−1 dry weight P, respectively (Fig. 6a). The The vivianite–nonvivianite transition at a sediment depth vivianite accounts for 20 % of TP concentration in the up- of 23 cm coincided with a peak in sedimentary Fe and per 20 cm of the sediment. In contrast, there was signifi- P content (Fe 80 mg g−1 dry weight, P 5 mg g−1 dry weight) cantly less P in the high-density samples from L3 (0.54 ± (Fig. 6a and b). Above this, in the upper 23 cm of the sed- 0.13 mg g−1 dry weight P). This lower concentration is be- iment, the Fe (and P) content was on average 2.4 (and 3.1) times higher than in the deeper, nonvivianite-bearing www.biogeosciences.net/11/5169/2014/ Biogeosciences, 11, 5169–5180, 2014 5176 M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention

P [ m g g - 1 d r y w e i g h t ] F e [ m g g - 1 d r y w e i g h t ] C a [ m g g - 1 d r y w e i g h t ] S [ m g g - 1 d r y w e i g h t ] 0 1 2 3 4 5 6 0 2 0 4 0 6 0 8 0 2 0 4 0 6 0 8 0 5 1 0 1 5 2 0 2 5 3 0 0 0 0 0 0

( a ) F e ( b ) ( c ) ( d ) -

L 0 M n 5 1 5 5 5

c : m 1 0 1 0 1 0 1 0 1

- ]

2 L m 0 1 5 2 1 5 1 5 1 5 c

: [ c

m h t

p 2 0 2 0 2 0 2 0 e 2

d 1

L t - 3 3 n 0 : e 2 5 2 5 2 5 2 5

c m i m d

e 3 0 3 0 3 0 3 0 S

3 5 3 5 3 5 3 5

4 0 4 0 4 0 4 0

0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 M n [ m g g - 1 d r y w e i g h t ] e q u i v a l e n t p h o s p h o r u s c o n t e n t f r o m h i g h - d e n s i t y s e d i m e n t s a m p l e s

Figure 6. Sediment stratigraphic chart of (a) phosphorus (P), (b) iron (Fe) and manganese (Mn), (c) calcium (Ca) and (d) sulfur (S) of Lake Groß-Glienicke from May 2013. Bar charts in graph (a) represent the equivalent P content analysed in the high-density samples from depth layers L1, L2 and L3. Error bars denote ± SD, n = 5. sediment zone (24–30 cm sediment depth). The course of not lead to a significant oxidation of vivianite and the min- manganese resembled that of Fe but showed an increasing eral is detectable by X-ray diffraction even after contact with trend above 12 cm sediment depth (Fig. 6b). Calcium content air; this is contrary to what has been reported by Olsson et al. showed a reverse trend, and decreased on average by a fac- (1997) and März et al. (2008). According to these studies, tor of 1.9 in the vivianite-bearing sediment zone compared vivianite is not expected to be detectable by X-ray diffrac- to the nonvivianite zone (Fig. 6c). The mean molar Fe : S ration after contact with air due to its high sensitivity towards tio increased from 0.9 in the 25–30 cm sediment depth, to oxidation. Aerobic handling of sediment and freeze-drying, 1.6 in the upper 23 cm of vivianite bearing sediment (Fig. 6b however, might have impacted the speciation of sulfur – i.e. and d). crystalline FeS2 and FeS might have been oxidised and al- tered to amorphous phases (Hjorth, 2004). EDX analysis and scanning electron microscopy reveal 4 Discussion fundamental characteristics of the mineral aggregates, and a conventional reflected-light microscope allows identifica- 4.1 Identification and quantification of vivianite tion of single vivianite nodules out of bulk sediments. Thus, a combination of techniques allowed a qualitative investi- The heavy-liquid separation and the enrichment of vivian- gation of vivianite deposits in different sediment layers and ite aggregates are crucial steps for the direct identification comparison with results from saturation calculations. of vivianite by X-ray diffraction. In our study, X-ray diffrac- The enrichment of high-density samples with vivianite tion was successful, even though the vivianite nodules were nodules further allowed us to assess the amount of vivianite partially oxidised due to contact with air (Figs. 3a and 4a) present in the sediment. To estimate the quantity of vivian- and already are an alteration product of unoxidised, pris- ite in high-density sediment samples, the vivianite content tine vivianite. The diffraction pattern of synthetic vivianite was calculated based upon two different methods: (1) chemi- powder, both in its slightly oxidised form (blue appearance) cal digestion, and (2) magnetic hysteresis measurements. The and after heating at 60 ◦C for 24 h supported that the vivian- latter method provides a different, and independent approach ite nodules detected in the sediment were slightly surface- to estimate the vivianite content, taking advantage of the oxidised only and did not lose their characteristic diffraction heavy-liquid separation of sediment samples and the para- pattern upon freeze-drying and exposure to air. Comparing magnetic nature of vivianite. the diffraction pattern of the high-density sample with the reference diffraction pattern of vivianite and metavivianite (which accounts for a partial oxidation of Fe(II)) there was even a higher conformity with the vivianite reference pattern than with the metavivianite reference pattern (Fig. 4a). This finding confirms that an aerobic handling of sediment does

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4.2 Contribution of vivianite to phosphorus retention ical digestion and the FeS-corrected paramagnetic susceptibility methods (Fig. 5). Considering the range of a factor of The presence of vivianite within the first centimetre of the la- two for the mass-specific magnetic susceptibility of vivianite custrine sediment of freshwater Lake Groß-Glienicke shows (Minyuk et al., 2013), the vivianite contents determined by that vivianite is currently forming. The incorporation of di- both the chemical digestion and the FeS-corrected paramag- atom shells and other organic debris within the nodules con- netic susceptibility methods match acceptably. firms their authigenic origin (Fig. 3b). The occurrence of Furthermore, a high-density sediment sample from 35 cm vivianite nodules in a recent and unlayered sediment hori- sediment depth (data not shown), containing no vivian- zon distinguishes the vivianite finding described here from ite nodules, demonstrated a negative susceptibility at Lake Groß-Glienicke from the vivianite deposits regularly higher magnetic fields, indicative of a majority of diamag- detected in palaeolimnetic core records (e.g. Brauer et al., netic material present in the high-density sediment sam- 1999; Sapota et al., 2006; Minyuk et al., 2013). ple. The P content of this sample accounted only for In the high-density samples from layers L1 (0–10 cm) and 0.05 mg g−1 dry weight P which is less than 6 % of the P con- L2 (11–20 cm) there was a higher amount of vivianite nod- tent detected in high-density samples from L1 and L2. The ules than in samples from layer L3 (21–30 cm). These ob- good agreement between results from magnetic susceptibil- servations are reflected in the P content of the high-density ity measurements and chemical digestion confirmed that the sediment samples. In samples from L1 and L2 the P content amount of P analysed in the high-density samples in sedi- was similar (to each other), whereas samples from L3 con- ment layers L1–L3 mainly represented vivianite-bound P. tained significantly less P. Although the sediment preparation method we used ne- Due to the high enrichment of vivianite nodules in the glected vivianite nodules smaller than 80 µm in diameter, high-density samples, the P content that we determined the amount of vivianite present in the high-density samples mainly represented the amount of P bound in vivianite. We serves as a reliable proxy for the overall vivianite content cannot exclude that part of the P analysed in the high-density in the sediment. The sieving step was applied in order to samples originated from compounds other than vivianite, maximise the enrichment of vivianite nodules in high-density such as P sorbed onto the surface of Fe(III) minerals which samples and minimise the relative amount of small-sized (< resisted reductive dissolution, Ca-associated P, or other Fe- 80 µm) Si and Ca compounds at the same time. phosphates. These compounds were expected to accumulate Our results suggest a homogeneous vivianite content in in the high-density samples as their density is higher than the upper 20 cm of the sediment. Vivianite formation signif- 2.3 g cm−3 and chemical digestion data confirmed the pres- icantly contributed to P retention in Lake Groß-Glienicke, ence of Ca and high content of Fe in these samples (Table 1). accounting for 20 % of the total sedimentary P in the upper However, our results from magnetic susceptibility measure- 20 cm of the sediment (Fig. 6). Assuming an increase in total ments support the assumption that the amount of P analysed sedimentary P due to the artificial application of Fe during in the high-density samples could mainly be attributed to lake restoration of 2.5 mg g−1 in the upper 20 cm of the sed- vivianite-bound P. iment in comparison to sediment layers deposited prior to Provided that the weakening of the measured positive the in-lake measure, vivianite formation could explain about magnetisation due to the presence of diamagnetic Si and Ca 40 % of this increase. The remaining 60 % of increase in total compounds is small, the vivianite content can be determined sedimentary P might be due to Fe(III)-phases which sorb P, from paramagnetic susceptibility. This assumption is valid, and mixed Fe(III)/Fe(II) phosphate minerals, which resisted since the values of the diamagnetic mass-specific susceptibil- reductive dissolution within the anoxic sediment matrix (Hy- ity of SiO2 and CaCO3 are more than two orders of magni- acinthe and Van Cappellen, 2004; Lehtoranta et al., 2009). tude smaller than that of vivianite (Lide, 2004). Even if high- Redox-sensitive Fe(III)-phases could be preserved by a pro- density samples contained 3–5 times more diamagnetic SiO2 tective coating of FeSx preventing further reduction (De Vitre and CaCO3 than vivianite, the correspondent weakening of et al., 1988). Moreover, P originating from preserved organic the positive magnetisation was negligible. The contribution material, which co-precipitated during iron oxidation, could of paramagnetic FeSx to the measured positive magnetisa- significantly contribute to the increase in P burial after the tion of high-density samples was significant (Fig. 5b) when addition of Fe during lake restoration (Lalonde et al., 2012). assuming all sulfur to be present as FeS. For the susceptibil- The presence of these compounds may explain the peak in P ity correction we used the MSFeS value given by Lide (2004). and Fe content at the sediment depth of 23 cm, even though The FeS correction could account for the overestimation of no elevated vivianite contents could be detected at this depth. the vivianite content derived from uncorrected magnetic susceptibility data. The FeS-corrected vivianite contents were, except for one sample, lower than values derived from chemical digestion. However, it is important to note that the relative changes in vivianite content between different high- density samples were determined similarly by both the chem- www.biogeosciences.net/11/5169/2014/ Biogeosciences, 11, 5169–5180, 2014 5178 M. Rothe et al.: Vivianite formation and its contribution to phosphorus retention

4.3 Prediction of vivianite formation by saturation 4.4 Processes influencing vivianite formation calculations We consider that the Fe application in 1992/93 was the stim- Changes in pH and dissolved substances across the SWI ulus for vivianite formation in Lake Groß-Glienicke. In 2003 (Fig. 1) are due to the deposition of organic and inorganic Gächter and Müller (2003) used a simple conceptual model matter, and its subsequent microbial decomposition. Along to explain that an increasing ratio of reactive Fe(II) to reac- with the liberation of SRP and Fe2+ there was moderate tive P leads to an increased permanent P burial through the vivianite supersaturation (SI = 3–4) throughout the upper formation of a solid ferrous iron phosphate, such as vivianite. 30 cm of the sediment (Fig. 2). The decrease in orthophos- It is exactly that shift in geochemical conditions which led phate activity with sediment depth is due to the declining to the favourable conditions for vivianite formation in Lake pH counteracting the concomitant increase of SRP concen- Groß-Glienicke. During restoration of Lake Groß-Glienicke tration with depth. While the activity of H+ and SRP varied Fe was provided in surplus. As a consequence, not all reac- by a similar factor, the variation in pH has a stronger influ- tive Fe(II) is precipitated as solid FeSx but can react with 3− ence on the saturation state of vivianite than does the change PO4 to form stable Fe(II)-phosphate minerals (Gächter and in SRP concentration. Since simultaneous with the decrease Müller, 2003; Kleeberg et al., 2013). The relative S content in orthophosphate activity, the activity of Fe2+ increased in the sediment decreased after the artificial application of with depth, the degree of vivianite supersaturation persisted. iron and explains why high-density samples from L1 and L2 The degree of supersaturation underlines that mineral for- contained less S than samples from L3 (Table 1). mation is slow, and not governed by the saturation state of The Fe and Mn content is elevated throughout the upper the pore water but rather by another process, as proposed by 23 cm of the sediment (Fig. 6), because there has been a con- Postma (1981) and Emerson and Widmer (1978). Accord- tinuous cycling of both elements between their dissolved and ing to those studies crystal growth is driven by surface re- particulate forms at the SWI after the Fe application. This actions and is not diffusion controlled. Formation of siderite feature reflects the change in redox conditions in the hy- 2+ (FeCO3) which potentially competes with vivianite for Fe polimnion and at the SWI after the Fe supplement, i.e. the is supposed to be controlled by very slow precipitation ki- sediment surface has become more oxidised. A high reactive netics (Postma, 1981). Therefore, it is unlikely that the near- Fe(II) concentration led in turn to the formation of vivian- equilibrium conditions we report for Lake Groß-Glienicke ite in freshly deposited sediments after the Fe application. indicate siderite precipitation. No crystalline siderite could The layer of formation of vivianite is the uppermost sedi- be identified in the high-density samples using the XRD pat- ment horizon close to the SWI. This explains the homoge- tern. neous vivianite content in the upper 20 cm of the sediment, According to these saturation calculations, and from and emphasises that supersaturated pore water alone is not a thermodynamic point of view, the precipitation of vivian- sufficient to cause vivianite formation. ite was favoured in the upper 30 cm of the sediment. The At the SWI, fresh organic matter is decomposed, and present geochemical conditions, with relatively high Fe2+ iron(oxy)hydroxides undergo reductive dissolution, lead- concentrations (0.02 to 0.2 mmol L−1) and high SRP con- ing to the release of Fe2+, SRP from organic matter, −1 3− centrations (0.04 to 0.09 mmol L ), at neutral to slightly and the liberation of previously adsorbed PO4 from alkaline pH in the absence of free sulfides, plot within the iron(oxy)hydroxides. The presence of vivianite aggregates stability field of vivianite proposed by Nriagu (1972) and in association with organic matter or organic-rich de- Nriagu and Dell (1974). However, to rely upon these calcu- posits has been reported by numerous authors (Mackereth, lations to predict in situ vivianite formation omits consider- 1966; Kjensmo, 1968; Rosenqvist, 1970; Postma, 1981; ation of a key feature, namely the sharp transition between Peretyazhko and Sposito, 2005; Taylor et al., 2008), sug- the vivianite-bearing and nonvivianite-bearing sediment lay- gesting a crucial function of this organic material for the ers. Vivianite was only present in the upper 23 cm of the formation of crystal aggregates. The gel-like pore structure sediment, not in subjacent sediment layers even though the of a sediment matrix rich in organic matter, in combination pore water was supersaturated at sediment depths of 24 and with an intensified release of SRP and Fe2+ due to micro- 28 cm. Assuming constant precipitation of vivianite from su- bial activity, may serve as necessary prerequisite for vivian- persaturated pore water, the highest concentrations of vivianite formation in sediments. Through the activity of bacteria, ite should be in sediment layers which have been supersatu- cell-mediated microenvironments within the sediment matrix rated the longest time, for example in the sediment deposited may evolve, sustaining vivianite-supersaturation during crys- shortly after the iron addition. However, this was not the case tal growth, even on a small scale (Cosmidis et al., 2014). Hy- for Lake Groß-Glienicke. From our findings, it is therefore drophic, negatively charged, carbon-rich fibres which build questionable whether supersaturated pore water alone is a re- up a structure of walls within the sediment may thereby catal- liable predictor of the in situ formation of vivianite. yse the growth of seed crystals (Zelibor et al., 1988).

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5 Conclusions (Massif Central, France), Geochim. Cosmochim. Ac., 126, 78– 96, doi:10.1016/j.gca.2013.10.037, 2014. Heavy-liquid separation of sediment leads to an enrichment De Vitre, R., Buffle, J., Perret, D., and Baudat, R.: A study of of vivianite nodules in high-density samples, and is key to iron and manganese transformations at the O2/S(-II) transition demonstrating the presence of vivianite by X-ray diffrac- layer in a eutrophic lake (Lake Bret, Switzerland): a multi- tion. The enrichment achieved after sediment preparation, method approach, Geochim. Cosmochim. Ac., 52, 1601–1613, and combination of the results from standard chemical di- doi:10.1016/0016-7037(88)90229-3, 1988. Emerson, S. and Widmer, G.: Early diagenesis in anaerobic lake gestion and magnetic hysteresis measurements, allow quan- sediments – II. Thermodynamic and kinetic factors controlling tification of the P bound in vivianite for different sediment the formation of iron phosphate, Geochim. Cosmochim. Ac., 42, depths. However, the absence of vivianite from layers of sed- 1307–1316, doi:10.1016/0016-7037(78)90035-2, 1978. iment that are supersaturated with pore water constrains the Fagel, N., Alleman, L., Granina, L., Hatert, F., Thamo-Bozso, E., validity of equilibrium calculations regarding the in situ pre- Cloots, R., and André, L.: Vivianite formation and distribution cipitation and occurrence of vivianite. At our study site at in Lake Baikal sediments, Global Planet. Change, 46, 315–336, Lake Groß-Glienicke, formation of vivianite was triggered 2005. by an artificial Fe supplement, and explains 20 % of total P Flanders, P.: An alternating-gradient magnetometer, J. Appl. Phys., in sediment layers deposited after the in-lake measure. 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