Mn Layers of

Environ. Sci. Technol. 2002, 36, 411-420

P, As, Sb, Mo, and Other Elements in Sedimentary Fe/Mn Layers of Lake Baikal BEAT MU ¨ L L E R , * ,† L I B A G R A N I N A , ‡ T O B I A S S C H A L L E R , †,§ A N D R E A U L R I C H , |,⊥ A N D BERNHARD WEHRLI† Limnological Research Center, Swiss Federal Institute of Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-6047 Kastanienbaum, Switzerland, Limnological Institute, Russian Academy of Science, Irkutsk, Russia, and Swiss Federal Institute of Environmental Science and Technology (EAWAG), CH-8600 Du ¨ bendorf, Switzerland

Distinct layers with accumulated iron and manganese oxyhydroxides are found in the recent sediments of Lake Baikal (Siberia). In the South and Central Basins, these concretions accumulate close to the sediment-water interface. In northern Lake Baikal and the area of Academician Ridge, however, massive Fe/Mn crusts are formed within several thousand years at redox fronts 10 to 15 cm below the sediment surface. In some places, precipitated iron and manganese oxyhydroxides are spatially separated. The patterns are a result of secondary iron and manganese oxide precipitation. This natural long-term experiment allows the analysis of competitive adsorption and coprecipitation of trace elements with iron and manganese oxides in sediments. Background concentrations in the sediment of oxoanions (P, As, Sb, Mo); of trace metals (Cr, V, Cu, Zn, Cd, Pb); and of Mg, Ca, Sr, La, Ce, Pr, Nd, and Sm were analyzed by inductively coupled plasma mass spectrometry. Despite the differences in catchment geology of the many tributaries, they are remarkably uniform in sediment cores from different basins of Lake Baikal. Enrichment factors of P and As within Fe crusts revealed concentrations up to 14 and 58 times higher than the background, respectively. No enrichment of P and As was found in the Mn layers. By contrast, Mo accumulated exclusively in the Mn layer with up to 35-fold enrichment. Sb was only slightly enriched in both the Fe and the Mn layers. Among the trace metals studied, only Cd was found at elevated concentrations with a preference for the Mn layer. Ca and Sr were correlated with both Fe and Mn accumulations. The study quantifies the well-known specific adsorption and coprecipitation of P and As at authigenic iron oxides and of Mo on manganese oxides. In addition, the enrichment of Cd at manganese oxides in contrast to the conservative behavior of Zn and Pb reveals highly selective accumulation processes. * Corresponding author e-mail: [email protected]; phone: +41 41 349 21 49; fax: +41 41 349 21 68. † EAWAG and ETH. ‡ Russian Academy of Science. § Present address: Chemin Gregor-Sickinger 11, CH-1722 Bourguillon, Switzerland. | EAWAG. ⊥ Present address: Andrea Ulrich, Aachstrasse 1a, D-78224 Singen, Germany.

10.1021/es010940z CCC: $22.00 Published on Web 01/04/2002

 2002 American Chemical Society

Introduction The aquatic redox chemistry of manganese has remained an intense field of research since the seminal work of James Morgan on the oxidation of aqueous Mn(II) almost 40 years ago (1). In the meantime, the geochemical and ecological importance of manganese cycling has been recognized in places as remote as the sediments of Lake Baikal in Siberias the deepest (1635 m) and largest freshwater lake by volume (∼23 000 km3) (2). The lake extends in a northeast direction over 680 km along a tectonic rift zone, and its sediment, around 7 km in depth, provides a climatic record of some 25-30 million years. Ongoing research on Lake Baikal is focused on the analysis of its unique ecosystem and the use of the long paleo-record of its sediments (3). In the present study, we address the key geochemical role of manganese and iron oxides accumulated at the sediment-water interface or at redox fronts within the sediment. Specifically, we investigate whether oxoanions (P, As, Sb and Mo), trace metals (Cr, V, Cu, Zn, Cd, Pb), and alkali ions (Mg, Ca, Sr) are accumulated in these iron/manganese oxide barriers. At present, trace metal concentrations in natural environments of the Lake Baikal region are at levels of natural background (4, 5). Only a few studies of trace elements in the water column of Lake Baikal have been published so far (e.g., refs 4 and 6). The vertical profiles of trace elements in Lake Baikal show homogeneous distribution with small variations in concentrations. Information on trace metal concentrations in sediments and porewaters of Lake Baikal is scarce and sometimes contradictory. Oxidized Baikal mud as well as Fe/Mn layers were analyzed for Co, Ni, Cu, Zn, Mo, Ag, Pb, and Ba (7-9). Pampoura et al. (10) documented the variations of Ca and trace metal contents in an overview of sediments from different basins of the lake. The different metal contents could be attributed to the composition of the bedrock in the catchment. Even in the sediments near the Baikalsk pulp plant (southern Baikal), no anthropogenic contamination with trace metals could be detected (11). Trace metal contents in recent sediments were measured by Flower and co-workers (12, 13). They found a slight increase in Pb and interpreted their results as possible atmospheric transport from anthropogenic sources. Boyle et al. (14) confirmed the increased Pb concentrations but found no evidence for recent changes in Cu or Zn in the recent sediments. They argued that the recent Pb increase came from more intense weathering in the catchment. In many locations, Lake Baikal features spectacular layers of Fe/Mn in near-surface sediments (7, 15, 16). They consist mainly of oxyhydroxides. In the South and Central Basins, these layers accumulate close to the sediment-water interface. In northern Lake Baikal, however, massive Fe/Mn crusts are found at redox fronts 10-15 cm deep within the sediments (2). The strong accumulation and, in many locations, clear spatial separation of Mn and Fe layers in Lake Baikal sediments offer a natural setting to study the affinities of a variety of trace metals toward either manganese or iron oxides. The present study was designed to analyze and discuss trace metal enrichment in Fe/Mn crusts. Background concentrations of oxoanions, trace metals, earth alkali, and selected rare earth elements were measured in characteristic sedimentary environments of Lake Baikal. In addition, the specific enrichment in manganese or iron oxides was studied in a core from Academician Ridge (the sill between the North and the Central Basins). At this site, the Mn and Fe layers VOL. 36, NO. 3, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Map of Lake Baikal and locations of sampling sites during cruises in 1994 and 1996. were clearly separated. The Fe/Mn crust at that site accumulated over several thousand years. This unique geochemical environment represents a natural long-term geochemical experiment with competitive uptake kinetics between slowly precipitating iron and manganese oxides (16).

Experimental Section General Characterization. Baikal is a coldwater oligotrophic lake characterized by relatively low biological productivity and regular deep-water renewal. Hence, the entire water column is enriched throughout with oxygen (17, 18). In the upper water layers, saturation ranges from 94 to 104%, whereas in near-bottom waters it is usually less than 80%. Oxygen consumption within the deep-water zones of the central and northern basins ranges from 1.1 to 3.4 µmol L-1 yr-1 (19). Average long-term primary production is estimated to be about 4 × 106 ton of Corg/yr. The concentration of dissolved organic carbon in Baikal surface waters does not exceed 1-1.5 mg/L. The pH of the lake water is generally 7.1-7.2, although photosynthetic activity may result in a larger range (7.1-8.6) (6). The total mineral content of the lake is relatively low (94-96 mg L-1), reflecting the chemistry of the tributaries, which is controlled predominantly by the weathering of igneous crystalline rocks (typical of the lake drainage basin). Generally, Ca2+ and HCO3- dominate the major ion composition. During spring and summer, nutrients tend to deplete up to 100 m depth. Nitrate may limit productivity as it becomes severely depleted, while phosphate levels rarely fall to limiting levels (17). Cyclonic water currents in each of the three lake’s basins are characterized by more intensive water movements on the periphery of a basin and relatively quiescent dynamic conditions in the center of circulating rings. Sampling Sites. Two expeditions to Lake Baikal took place in 1994 (July 14-August 5) and 1996 (June 21-July 16) in the context of the Baikal International Center of Ecological Research (BICER). The six coring sites of our investigation are shown in Figure 1. Detailed descriptions of the sampling sites are published elsewhere (16). Coordinates, core numbers, and water depths are listed in Table 1. Sites 1 and 6, near the Selenga River Delta (core 94-05) and Maloe More, the “Little Sea” (cores 94-12, 96-52, and peeper-1), are characterized by shallow water depths and 412

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high allochthonous deposition. Particulate Mn/Fe layers have been found at the very top of sediments. Sites 2 and 3, the Southern and Central Basins (cores 9402 and 94-09), are close to the deepest locations of Lake Baikal. The basins reach depths of 1425 and 1635 m, respectively. Their sediment composition is dominated by terrigenous material, and turbidites are common at certain locations. Enriched Fe/Mn layers are observed at the top 1-2 cm. The core from the Central Basin shows additional separated peaks of Fe and Mn at 13 and 18 cm depth, respectively. A sedimentation rate of 0.24 mm/yr was determined (20). Sites 4 and 5, Northern Basin (core 94-10) and Academician Ridge (cores 94-11, 96-60, 96-110, and peeper-2), are exposed to very small sedimentation rates. For the Northern Basin 0.14 mm/yr (21) and for Academician Ridge 0.026 mm/ yr (22) were determined. The top sections (10-20 cm) of the cores are oxic. Sediments contain colored layers of Mn and Fe accretions at depths of 10-25 cm. Porewater Sampling. On the 1996 cruise, two sites (Maloe More and Academician Ridge) were sampled with porewater dialysis plates (peepers). The dialysis plates consisted of a 50 × 15 cm Plexiglas sheet of 1 cm thickness. The sampling cells allowed a spatial resolution of 1 cm. Sheets of 0.2-µm membrane filter (Polysulfone, HT-Tuffryn, Gelman Sciences Inc.) covered the Plexiglas. The holes were filled with degassed, doubly distilled water (23, 24). Specially designed tripods kept the dialysis plates in a vertical position and were lowered to the sediment with a winch. Ground plates along the bottom end of the tripod assured the position on the sediment and controlled the penetration depth of the peeper into the sediment. Before the recovery of the devices after 5 days of exposure, a protecting cover was released with a messenger to slide over the peeper plates. The cover captured the water and sediment in the wells outside the membrane. Thus, the chemical environment at the peeper membrane did not change during retrieval. The pore-water sampler could thus be recovered without contamination or losses of dissolved ions. The position of the sediment-water interface was again checked when the protecting cover was taken off and the cells covered with mud were visible. Water samples were obtained without O2 contamination by applying plastic syringes and steel hypodermic needles (Microlance) through butyl rubber stoppers at the side of the plates. On average, 6-8 mL of sample could be retrieved and was acidified with 100 µL of 35% nitric acid (suprapure, Fluka, Switzerland) for metal analysis. Samples were stored in glass bottles for transport to Switzerland. Phosphorus concentrations were determined colorimetrically with the standard ascorbic acid molybdate blue technique (25). The sulfate content of the samples was determined by ion chromatography (Metrohm IC 690, Switzerland). Metal concentrations were determined with an ICP-MS (Perkin-Elmer Sciex 5000). Concentration measurements in all three cases were performed using standard calibration. For each subset of 10 samples, a blank, a diluted standard, and a spiked sample were analyzed to assess blank levels and precision, respectively. Precision levels were 4-7%. Sediment Sampling and Analysis. In 1994, sediments were sampled with a box corer, and several subcores were taken. The stainless steel box corer of the Limnological Institute of Irkutsk had a size of 50 × 50 × 50 cm. After retrieval of the box corer, the top lid was opened, and subcores were sampled with PVC core tubes of 7 cm diameter and 40-50 cm length. The 1996 sediments were sampled using a gravity corer (26). The sediment was extruded with a piston, and 0.5-2 cm slices were stored in polycarbonate boxes and sealed with tape. Water content was determined after freezedrying. Sediment samples were freeze-dried with a Leybold Heraeus Lyvac GTZ and homogenized in an agate mortar.

TABLE 1. Enrichment Factors Calculated for Zones of Peak Concentrations Relative to Background (see Table 2)a location

site no.

core no.

depth (m)

Selenga Delta

1

94-05

80

Southern Basin

2

94-02

1425

Central Basin

3

94-09

1630

Northern Basin

4

94-10

895

Academ. Ridge

5

Maloe More

6

94-11 96-60 96-110 94-12 96-52

380 280 280 280 125

a

coordinates

Fe

Mn

P

As

Sb

Mo

Cd

Ca

Sr

Fe/P

52°19′15′′ N 106°08′43′′ E 51°42′00′′ N 105°02′10′′ E 53°21′18′′ N 108°13′05′′ E 54°27′19′′ N 109°03′53′′ E 53°42′13′′ N 108°14′22′′ E (core 94-11) 53° 26′ 24′′ N 107°38′17′′ E (core 94-12)

1.2

3.1

1.6

2.5

1.4

1.7

2.1

1.1

1.2

17.3

1.9

18.6

7.0

6.1

5.5

7.4

2.2

1.3

1.9

7.2

1.7

33.6

3.0

9.7

1.9

16.7

1.6

1.3

1.6

6.6

5.4

33.5

8.5

58.1

3.6

9.2

4.3

1.5

5.0

4.8

3.2 2.1 2.0 2.2 1.9

50.4 19.8 7.4 1.7 6.8

9.7 13.1 13.7 1.7 1.9

4.5 6.7 9.3 3.7 3.6

1.6 nd nd 2.3 nd

17.7 3.1 34.8 4.8 9.2

3.9 1.0 2.0 2.1 1.7

2.6 1.6 1.1 1.7 1.2

6.6 2.8 1.6 2.1 1.2

3.0 2.5 2.7 24.9 11.2

The Fe/P ratio is given for the zone of highest accumulation.

TABLE 2. Average Sediment Contents of Major and Minor Elements in Lake Baikala element Fe Mn P As Sb Mo V Cr Cu Zn Cd Pb Na K Mg Ca Sr La Ce Pr Nd Sm

94-05

94-09

94-11

732 ( 41 15.0 ( 7.3 36.2 ( 7.0 0.10 ( 0.04 0.001 ( 0 0.019 ( 0.005 1.65 ( 0.007 1.01 ( 0.05 0.54 ( 0.07 1.27 ( 0.15 0.001 ( 0 0.084 ( 0.008 92.3 ( 9.6 136 ( 8 451 ( 19 223 ( 10 0.66 ( 0.11 0.32 ( 0.01 0.62 ( 0.02 0.075 ( 0.003 0.27 ( 0.01 0.049 ( 0.002

765 ( 84 26.5 ( 6.4 67.6 ( 17.5 0.12 ( 0.03 0.001 ( 0 0.026 ( 0.009 1.57 ( 0.10 0.93 ( 0.11 0.64 ( 0.04 1.07 ( 0.12 0.002 ( 0 0.081 ( 0.005 153 ( 27 150 ( 14 416 ( 61 190 ( 14 0.71 ( 0.14 0.29 ( 0.02 0.54 ( 0.05 0.066 ( 0.005 0.24 ( 0.02 0.043 ( 0.004

799 ( 49 21.3 ( 2.7 60.5( 5.8 0.11 ( 0.06 0.002 ( 0 0.017 ( 0.007 1.55 ( 0.14 1.01 ( 0.33 1.02 ( 0.13 1.15 ( 0.12 0.005 ( 0.001 0.089 ( 0.004 78.2 ( 6.2 118 ( 8 372 ( 34 201 ( 11 0.77 ( 0.05 0.39 ( 0.02 0.64 ( 0.03 0.084 ( 0.004 0.30 ( 0.01 0.051 ( 0.002

a Measurements from zones with Fe/Mn accumulations were excluded. Cores from Selenga River Delta (94-05), Central Basin (94-09), and Academician Ridge (94-11) were selected as representative for the sites 1 and 6, 2 and 3, and 4 and 5, respectively. All concentrations are in µmol/g.

A portion of 50 mg dry sediment was digested with 4 mL of 65% nitric acid (suprapur, Fluka, Switzerland) and 1 mL of 30% hydrogen peroxide (suprapur, Fluka, Switzerland) in pressurized PTFE bombs in a Milestone mls 12000 mega microwave oven. For every set of 20 samples, a blank sample (nitric acid and hydrogen peroxide) as well as 50 mg of a rock standard (AIOL lake sediment) were digested to assess the quality of the analytical procedure. Major cations (Mg and Ca) and Fe and Mn concentrations were determined by ICPOES (Spectro Analytical Instruments). Trace element concentrations (V, Cr, As, Mo, Sb, As) were determined by ICPMS (Perkin-Elmer, Sciex 5000). Accuracy levels for ICP-OES measurements, based on rock standard samples, were between 2% and 8% for all elements except Fe (10%) and V (12%). Average concentrations of elements were determined from all values analyzed in the sediment cores except the regions with a peak-shaped increase of Fe or Mn. To determine enrichment factors, we calculated the total amount of Fe and Mn in the respective region of enrichment of these elements and divided by the average basis content

TABLE 3. Statistic Correlations of Element Contents with Fe and Mn, Respectively, in the Sediments of Lake Baikala correlation with Fe

Fe Mn P As Mo Cd Sb Ca Sr Mg Na K V Cr Cu Zn Pb La Ce Pr Nd Sm

correlation with Mn

94-09

94-10

94-11

94-09

94-10

94-11

1 0.41 0.99 0.99 0.19 0.00 0.10 0.61 0.93 -0.69 -0.49 -0.83 -0.78 -0.69 0.83 -0.69 -0.92 -0.79 -0.75 -0.77 -0.75 -0.74

1 0.31 0.98 0.99 0.43 0.43 0.37 0.42 0.85 -0.74 -0.24 -0.19 -0.28 -0.41 -0.05 -0.30 -0.16 -0.67 -0.73 -0.68 -0.66 -0.63

1 -0.41 0.95 0.97 -0.64 -0.68 -0.34 0.84 0.89 -0.74 -0.12 -0.68 -0.82 -0.48 -0.79 -0.77 -0.51 -0.68 -0.70 -0.68 -0.68 -0.68

0.45 1 0.50 0.48 0.96 0.73 0.30 0.87 0.69 -0.56 -0.34 -0.48 -0.54 -0.56 -0.42 -0.56 -0.54 -0.56 -0.63 -0.64 -0.65 -0.64

0.06 1 0.36 0.25 0.98 0.95 0.77 0.44 0.73 -0.31 0.41 0.53 -0.05 -0.44 0.32 0.21 0.15 -0.14 -0.21 -0.18 -0.17 -0.16

-0.4 1 -0.26 -0.29 0.98 0.91 0.83 0.53 0.42 -0.56 -0.08 0.05 -0.01 0.07 0.20 0.04 -0.64 -0.54 -0.57 -0.57 -0.58 -0.59

a Sediment cores are from Central Basin with small accumulation of Fe and Mn between 2.5 and 5 cm sediment depth (94-09), North Basin (94-10), and Academician Ridge (94-11), which both show Fe and Mn accumulations below 8 cm depth. Correlations with Fe and Mn were calculated from the whole concentration profile of the Central Basin. Correlations in the sediments of North Basin and Academician Ridge were calculated in the enrichment zones in order to obtain a clearer distinction between interactions with Fe or Mn (Mn (8-17 cm) and Fe (10-15 cm) in North Basin and Mn (9-20 cm) and Fe (15-30 cm) at Academician Ridge.

of this zone as estimated from the sediment content above and below the accumulation region. Correlations of various elements were obvious from the concentration profiles but were also calculated statistically from profiles. Results from three characteristic cores are presented in Table 3. Concentration profiles from the Central Basin (94-09) showed Fe and Mn accumulations at 2.5-5 cm depth (correlations were calculated from the whole depth profile). Sediments from the Northern Basin (94-09) showed narrow Mn and Fe accumulation zones (7-20 cm), and at Academician Ridge, Mn (9-20 cm) and Fe (15-30 cm) accumulations extend over 20 cm and were separated spatially in the sediment. The relationship between each element with Fe and Mn was calculated within the zones given above and is expressed with the correlation coefficient r. VOL. 36, NO. 3, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Depth profiles of trace metal contents in a sediment core from Academician Ridge (site 5, core 94-11). Concentrations of Fe and Mn are given in all graphs with dotted lines and empty squares and circles, respectively. Porosity was calculated from the water content and the density of the dry sediment, which was determined from the Corg concentration using an empirical relation from Niessen (27, 28).

Results General Concentrations of Major Ions and Trace Metals in the Sediments of Lake Baikal. Average element concentrations in the sediments are given in Table 2. We list background concentrations of three representative cores from zones as different as Selenga River Delta (core 94-05), Central Basin (core 94-09), and Academician Ridge (core 94-11). The variation in general concentrations of some elements may have its origin in the riverine inputs from a wide variety of geological catchments (29). There are more than 300 rivers flowing into Lake Baikal, and more than 50 spots with hydrothermal activity are known in the Northern Basin (30). Despite the contrasting sedimentation regimes, the background concentrations for most parameters are quite similar in the different regions. The concentration of the trace metals approach within a factor of 2 the chemical composition of deep-sea clay; however, Baikal sediments contain less trace metals throughout (31). Notable exceptions are Mn, Mo, Pb, Cu, and Sm with concentrations 5-10 times smaller in the Lake Baikal sediments as compared to average deep-sea clay. Fe/Mn Layers in Lake Baikal Sediments. All sediment cores from Lake Baikal feature pronounced layers that are enriched in Fe and Mn at various levels as compared to the background above and below the accumulation zone (Table 1). In the area near the Selenga Delta and in the South and Central Basins, Fe/Mn layers are restricted to the top of sediment cores. The Fe peak is typically located several mm below the Mn peak. Concentrations of Fe and Mn are on the order of 4 × 10-4 and 4 × 10-5 mol g-1 dry weight, respectively. In contrast to these accumulation zones, sites 4 and 5 in the Northern Basin (Figure 1) have clear redox fronts within the sediment. Dark brown to black layers are found at Academician Ridge and in the Northern Basin at depths of 5-20 cm and occasionally even deeper downcore. Concentrations of Fe in these layers reach values between 4 × 10-4 and 2 × 10-3 mol g-1 dry weight. Mn concentrations range from 10-5 to 3 × 10-3 mol g-1. Fe concentrations in the layers at sites 4 and 5 are enriched by factors of 2-5.4, and Mn concentration are enriched by factors of 7-50 as compared to the background. Variations in enrichments in sediments from Academician Ridge (e.g., Mn in Table 1) may originate from the very diverse topography in this geographic region. Academician Ridge is

an underwater sill that separates the Central Basin from the Northern Basin. The lake floor rises from 1636 m in the Central Basin to less than 250 m in some places to descend again down to 900 m in the Northern Basin. The underwater currents at this sill vary widely. Sedimentation rates from close locations can be different and, at least temporarily, can be negative due to resuspension. Element Enrichments in Fe/Mn Crusts. Some elements show distinct enrichments in the sediments that correlate with the enrichment zones of Fe and Mn. Table 3 lists statistical correlations of all elements measured with both Fe and Mn from three sediment cores containing Fe and Mn accumulations. The strong positive correlations of P and As with Fe and of Mo and Cd with Mn are striking. Similar positive correlations are also observed for Sr with Fe and for Sb with Mn. Positive correlations with both Fe and Mn found for Ca and Sr are in contrast to negative correlations for Mg and all rare earth elements. These correlations are weak and not immediately obvious from the depth profiles. Correlation coefficients for other elements were less significant and not constant through all sediment samples. Representative concentration profiles of the oxoanions of P, As, Sb, and Mo; the trace metals Cu and Cd; and the two earth alkali ions Ca and Sr are compared in Figure 2 with the Fe and Mn profiles at Academician Ridge. Here the Fe and Mn peaks were well separated, with peak maxima at 25 and 13 cm depth, respectively. The same geochemical patterns as in Figure 2 were found in all other downcore profiles from the North Basin and the Academician Ridge. However, the Fe and Mn crusts in the other cores listed in Table 1 showed stronger overlap. Therefore, we will use core 94-11 as an illustrative example to compare trace element accumulation in authigenic Fe and Mn crusts. Among the oxoanions, the concentration profiles of P and As in Figure 2 show strong correlations with the Fe profile. On the other hand, the concentration of Sb is only slightly elevated in the Fe layer but also increases significantly in the Mn layer. An extreme accumulation in the Mn layer is observed for Mo. This element is present only at background concentrations in the Fe crust. The enrichment factors from other sediment cores are given in Table 1. The data reveal that P is always enriched in the iron oxide layers with factors of up to 14 for the iron crusts (Academician Ridge). The second element with specific enrichment in the Fe layers is As with enrichment factors of up to 58 in the Fe accumulation horizon in the Northern Basin. In some cores, enrichments of Fe (94-09, 96-110) and Mn (94-05, 94-09, 96-52, 96-60, 96-110) are found only in the topmost centimeters of the sediment. VOL. 36, NO. 3, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Such cores with surface enrichments of Fe exhibit the “typical” simultaneous enrichments for P and As. The molar ratio of Fe/P is around 10-20 in the surface layers near the Selenga Delta and in the South and Central Basins. However, it reaches values as low as 2.5 in the oldest Fe layers from Academician Ridge (site 5). Both P and As remain constant in the Mn accretion zones of those cores where Fe and Mn layers are clearly separated. The profiles of Sb and Mo from core 94-11 (Academician Ridge) in Figure 2 are in clear contrast to those of P and As. Mo is exclusively enriched in the Mn phase, and Sb also shows a preference for Mn in this case. In other cores, where the crusts are not as clearly separated (94-10, 96-60), it appears that Mo and certainly Sb also increase with the Fe peak. Strong enrichment factors of up to 35 for Mo and of 2-5 for Sb are listed in Table 1. Among the trace metal cations, only Cd is selectively enriched in the Mn layer (Figure 2). No Cd accumulation can be sensed in the Fe crust. Enrichment factors of Cd are similar to those of Sb (Table 1). No Cd accumulation is detected in cores where the enriched layer at the sediment-water interface consists predominantly of Mn (core 94-5 from the Selenga Delta, 96-60 from Academician Ridge, 96-52 from Maloe More). In contrast to the deeper layers, these Mn surface layers seem to accumulate some Mo but no Cd and Sb. The profile of Cu in Figure 2 represents a special case with a slight depletion within the Fe/Mn zone. Other trace metals (V, Cr, Zn, Pb) exhibit no significant trends in their vertical concentration profiles. Among alkali, earth alkali, and rare earth elements, only the concentrations of Ca and Sr were enhanced in the zones of Fe and Mn layers by moderate factors of 1.6-6.6 and 1.24, respectively. Figure 2 shows that Ca and Sr accumulate in both layers, but there is a preferential enrichment in the Fe accumulation zone. Porewater Profiles. A porewater profile sampled with dialysis plates at 1-cm resolution on Academician Ridge (site 5) is compared with the sediment composition of core 96-60 from a location some 100 m apart. Figure 3 compares both particulate and dissolved concentrations of Fe, Mn, P, and As. Porewater concentrations of Fe and Mn increase just below the Fe and Mn layers at 9 cm depth. Sulfate decreases from 60 µM at the sediment surface to below 20 µM at 20-cm depth. A steep decrease of SO42- concentration in the region just below the Fe layer occurs where dissolved Fe is detected. Phosphate concentration increases sharply just below the sediment surface to >1 µM and drops back to the detection limit at the depth of the Fe layer. Dissolved o-PO4 increases a second time together with the content of particulate P at the location of the second Fe layer. The high concentration of dissolved Fe at this depth indicates reducing conditions. The dissolution of the second Fe layer may therefore liberate PO4 at 18-cm depth. At the position of the first Fe/Mn layer at 9-cm depth, the typical maxima for particulate P, As, and Mo are found. At this depth, the porewater concentrations of dissolved P and As pass through a minimum.

Discussion At locations with sedimentation rates 50 mm at Academician Ridge. Impact of Fe/Mn Enrichments on Other Elements. The distribution patterns of several other major and minor elements are closely related to the redox dynamics of the Fe/Mn crusts. Our results complement and expand the previous reports of different researchers (2, 7-9, 14, 34). Up to now co-enrichments with Fe/Mn are not known with certainty. Leibovich (7) reported correlations of Co, Cu, Ni, Pb, and Zn. Granina (8) observed weak joint accumulation of Ni and Cu with Mn and of Pb with Fe. Boyle et al. (14) and Flower et al. (12), however, could not confirm correlation of Cu, Pb, or Zn with Fe or Mn neither in near-surface enrichments nor in buried layers. Our own measurements support the earlier results concerning Cu, Pb, and Zn. However, striking similarities in the sediment profiles of Fe, P, and As on one hand and Mn, Mo, and Cd (Figure 2) on the other hand strongly suggest that these distribution patterns must have co-evolved during sediment diagenesis. The correlation of the peaks suggests highly selective processes. The distribution of Sr, Ca, and Sb seem to be influenced by the formation of both manganese and iron oxide layers. A stratigraphic correlation of Ca with Fe and Mn was also observed by Flower et al. (12). None of the other elements measured in the particulate phase (V, Cr, Cu, Zn, Pb, Na, K, Mg, La, Ce, Pr, Nd, and Sm) showed patterns that correlated with depth or with the profiles of Fe and Mn as shown for Cu in Figure 2. While the concentration profiles of Ca and Sr correlated with both Fe and Mn distributions, no such correlation was found for Mg. Iron and Phosphate. The most striking correlation is observed between sedimentary iron and phosphate (Figures 2 and 3). The strong specific interaction between iron oxyhydroxides and phosphate is well-known (35-37). Figures 2 and 3 both show the accumulation of phosphate simultaneously with particulate Fe and are not influenced at all by the zone of Mn enrichment. The adsorption of phosphate to synthetic iron hydroxides has been studied in great detail (e.g., ref 38), and surface binding constants have been determined (39). Sorption experiments with Baikal sediments documented that sorption to iron oxides is responsible for high P concentration in Fe/Mn layers (40). Other conditions influence the binding of phosphate by iron oxides such as the precipitation of the hydroxide in the presence of phosphate or the presence of organic material during the authigenic formation of the oxide. Structural information on Fe-PO4 mineral phases formed during the oxidative coprecipitation process has recently been published (41). For eutrophic freshwater lakes, Buffle et al. (42) and Lienemann et al. (43) found that iron-rich particles always contained P and Ca and that organic matter is intimately bound to the surface. Taillefert et al. (44) documented that the morphology of iron oxides precipitated from dissolved Fe2+ was influenced by organic fibrils. This surface sorption of organic matter may prevent further coagulation of colloidal iron oxyhydroxides and thus increase the P/Fe ratio in the organic-rich sediments of Lake Lugano, Switzerland, where a constant ratio of P/Fe of 0.48 ( 0.11 was measured (43). Gunnars et

FIGURE 3. Profiles of total concentrations (line, filled circles) and porewater concentrations (dotted line, empty circles) of Fe, Mn, P, As, and SO42- in sediment core 96-60 from Academician Ridge (location 5). Split circles for dissolved Fe were considered as artifacts. al. (45) in their experiments of controlled oxidation of natural reduced bottom water found an upper limiting stoichiometry of P/Fe ) 0.5 above which no P could be removed even at higher free phosphate concentrations. Since stoichiometric ratios of P/Fe >0.25 cannot be reached by adsorption alone,

they propose coprecipitation as the major scavenging process and the formation of a compound (H2O)z(H)x[Fe2(OH)2PO4]yx-y. Both effects, organic coatings and coprecipitation, increase the P/Fe ratio to stoichiometries found in the accumulation zones in Lake Baikal sediments, where values VOL. 36, NO. 3, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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increase up to 0.4. This value was found in the large Fe layer at Academician Ridge, which is, according to measured sedimentation rates (16), over 8000 years old (Table 1). This suggests that the strong scavenging of P in the Baikal sediment is due to the formation of coprecipitates on the new mineral phase when Fe(II) is reoxidized. Figure 3 depicts an increase in dissolved phosphate across the sediment surface and approaching the Fe layer at 8-cm depth, a steep decrease. This oxic sediment zone provides a flux of phosphate down to the Fe layer as well as upward into the bottom water. The scavenging within the sediment is very efficient since dissolved P inside the Fe layer was at the detection limit despite the high total concentration. The removal processes in the crust create concentration gradients of the solutes in the porewater initiating fluxes toward the redox boundary. The flux of P to the layer estimated from the concentration gradient is 3.5 µmol m-2 d-1. On the basis of the amount of P in the Fe crust (4.3 mol m-2), the time for its accumulation is calculated to be more than 3300 years. Hence, the strong enrichment factors observed in the cores from Academician Ridge and North Basin are the results of long-term processes. Observations of Fe/Mn enrichments downcore below the “active” crusts suggest that these layers may remain when the redox boundary shifts higher up. Figure 3 shows a possible example of such a “buried” layer. The “old” Fe layer at 16-cm depth is embedded in the reductive zone and slowly dissolving. A small peak of dissolved P appears over the particulate P at 16-cm depth. The local concentration may become sufficiently high that new mineral phases, such as vivianite, may form. Deike et al. (46) have shown that a transformation of iron oxide with phosphate into vivianite occurs as layers are buried. The concentrations measured for dissolved Fe and P in the porewaters were often close to equilibrium with vivianite at many sites in Baikal sediments (47). As and Sb. The accumulation of As in the zone of Fe concretions was very pronounced in all sediment cores. As was enriched up to a factor of 58 in the Northern Basin (Table 1). It is notable that As content always increased together with Fe but never with Mn. The close interaction of As and Fe is well-documented in the literature (e.g., ref 48). Arsenic exhibits two redox states in natural waters, arsenate, As(V), and under slightly reducing conditions arsenite, As(III). Both are oxoanions. The structure of arsenate is similar to phosphate and also exhibits a very strong sorption behavior with iron oxide particles. The adsorption constant for arsenate is only 2 orders of magnitude lower than phosphate (39), and the protolysis constants are very similar to the ones of phosphoric acid (49). In addition, arsenate minerals are known to have isomorphic structure with phosphates. In contrast, the first protolysis constant of arsenous acid, As(OH)3, is pK1 ) 9.23; therefore, the anion is not readily available for adsorption at near-neutral pH. Moreover, in the presence of free sulfide, the solubility of As(III) is controlled by the precipitation of As2S3. The standard redox potential of the As(III)/As(V) couple is similar to that of Fe(II)/ Fe(III), and thermodynamic considerations suggest that As(V) is the dominant species in oxic waters. As(III) is known to be oxidized to As(V) in the presence of manganese oxides (50). Under redox conditions that allow iron oxyhydroxides, we have therefore oxidized the As(V) present, which shows strong affinity toward iron oxide surfaces similar to phosphate (48). The distribution of As between particulate phase and porewater, KD, was 8 × 103-13 × 103 L kg-1. In the accumulation zone, KD peaked to a maximum value of 6.8 × 104 L kg-1. Kuhn and Sigg (50) determined KD values in settling material of a freshwater lake of 2 × 103 L kg-1 (summer) and 19 × 103 L kg-1 (winter), thus covering the same range as in the Baikal sediment outside the As peak. 418

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Investigation of sediment trap material in a lake with temporarily anoxic hypolimnion showed high correlation of inorganic As with Fe but not with Mn (50). Thus, we assume that both HPO42- and HAsO42- are related to iron oxides by adsorption and coprecipitation and that the mechanisms of accumulation in the Fe/Mn layers are similar for P and As. Little is known about the sorption and precipitation of Sb in the sedimentary environment. Sb(V) at near-neutral pH occurs as an anion, SbO(OH)4-. Sb(V) can be reduced to Sb(III) at low redox potential and is known to form complexes and precipitates with sulfides (51). Its concentration in the sediment is very low (