Rare earth elements in the water column of Lake Vanda, McMurdo Dry Valleys, Antarctica

Geochimica et Cosmochimica Acta, Vol. 66, No. 8, pp. 1323–1333, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 ⫹ .00

Pergamon

PII S0016-7037(00)00861-4

Rare earth elements in the water column of Lake Vanda, McMurdo Dry Valleys, Antarctica ERIC HEINEN DE CARLO1,* and WILLIAM J. GREEN2 1

Department of Oceanography, 1000 Pope Road, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA 2 School of Interdisciplinary Studies, Miami University, Oxford, OH 45056, USA (Received January 8, 2001; accepted in revised form November 8, 2001)

Abstract—We present data on the composition of water from Lake Vanda, Antarctica. Vanda and other lakes in the McMurdo Dry Valleys of Antarctica are characterized by closed basins, permanent ice covers, and deep saline waters. The meromictic lakes provide model systems for the study of trace metal cycling owing to their pristine nature and the relative simplicity of their biogeochemical systems. Lake Vanda, in the Wright Valley, is supplied by a single input, the Onyx River, and has no output. Water input to the lake is balanced by sublimation of the nearly permanent ice cap that is broken only near the shoreline during the austral summer. The water column is characterized by an inverse thermal stratification of anoxic warm hypersaline water underlying cold oxic freshwater. Water collected under trace-element clean conditions was analyzed for its dissolved and total rare earth element (REE) concentrations by inductively coupled plasma mass spectrometry. Depth profiles are characterized by low dissolved REE concentrations (La, Ce, ⬍15 pM) in surface waters that increase slightly (La, 70 pM; Ce, 20 pM) with increasing depth to ⬃55 m, the limit of the fresh oxic waters. Below this depth, a sharp increase in the concentrations of strictly trivalent REE (e.g., La, 5 nM) is observed, and a submaximum in redox sensitive Ce (2.6 nM) is found at 60- to 62-m depth. At a slightly deeper depth, a sharper Ce maximum is observed with concentrations exceeding 11 nM at a 67-m depth, immediately above the anoxic zone. The aquatic concentrations of REE reported here are ⬃50-fold higher than previously reported for marine oxic/anoxic boundaries and are, to our knowledge, the highest ever observed at natural oxic/anoxic interfaces. REE maxima occur within stable and warm saline waters. All REE concentrations decrease sharply in the sulfidic bottom waters. The redox-cline in Lake Vanda is dominated by diffusional processes and vertical transport of dissolved species driven by concentration gradients. Furthermore, because the ultraoligotrophic nature of the lake limits the potential for organic phases to act as metal carriers, metal oxide coatings and sulfide phases appear to largely govern the distribution of trace elements. We discuss REE cycling in relation to the roles of redox reactions and competitive scavenging onto Mn- and Fe-oxides coatings on clay sized particles in the upper oxic water column and their release by reductive dissolution near the anoxic/oxic interface. Copyright © 2002 Elsevier Science Ltd 1984; De Carlo et al., 1987, 2000; De Carlo and McMurtry, 1992). Oxides of Fe and Mn are also involved, in conjunction with sulfidic phases, in metal cycling across oxic/anoxic interfaces (Bacon et al., 1980; Lewis and Landing, 1991, 1992; Schijf, 1992; Bau et al., 1997) and play important roles in trace metal cycling in lakes (e.g., Sigg, 1985; Green et al., 1986, 1989, 1993; Balistrieri et al., 1992a,b). Studies in Lake Vanda, Antarctica, have described how elements are distributed, scavenged, concentrated, transported, and eventually recycled in closed lake systems (Green et al., 1986, 1989, 1993; Canfield et al., 1995). These authors found the well-constrained biogeochemical system of Lake Vanda to be ideal for the examination of trace-element behavior. In this article, we expand earlier work and present data on the rare earth elements (REE) in the water column of Lake Vanda. We utilize the REE to examine the roles that Fe and Mn and their oxides play in scavenging, transporting, and recycling trace elements across the oxic/anoxic interface of the lake. Additionally, we use La and Ce to contrast simple sorptive processes from redox-driven processes and compare our results to those from studies conducted at marine oxic/anoxic interfaces (e.g., De Baar et al., 1988; German and Elderfield, 1989; German et al., 1991)

1. INTRODUCTION

Scavenging by particles in the water column largely controls the distribution of trace elements in the aquatic environment, and much research has been devoted to understanding reactions taking place at the aqueous/particle interface (e.g., Stumm and Morgan, 1981; Erel and Morgan, 1991; Stumm, 1992). Because natural systems are inherently complex, laboratory experiments with fewer and readily controllable variables have often been used to investigate reaction mechanisms (Hayes and Leckie, 1986; Koeppenkastrop and De Carlo, 1992, 1993; De Carlo et al., 1998). Iron and manganese oxides are two of the principal scavenging agents for trace elements in the natural environment and are thought to be responsible for the distribution of trace elements in the oceanic water column (Balistrieri et al., 1981; Li, 1991), hydrothermal plumes (Kadko et al., 1986 –1987; Trefry and Metz, 1989; Feely et al., 1991), and deep-sea sediments (Balistrieri and Murray, 1986), and for the enrichment of trace elements in deep-sea ferromanganese nodules (Piper, 1974) and seamount ferromanganese crusts (Aplin,

* Author to whom correspondence ([email protected]).

should

be

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E. H. De Carlo and W. J. Green

The chemically coherent REE can provide unique insights into geochemical processes in the aquatic environment. For example, the oceanic abundance of REE is regulated by a balance between solution and surface complexation through scavenging (e.g., Elderfield, 1988; Byrne and Kim, 1990) that generally involves coatings of Mn- and Fe-oxides and organic matter on particles (Hunter, 1991). Abundance patterns of the REE in the aquatic environment offer a useful means of studying how slight changes in chemical properties determine the behavior of trace elements in geochemical processes (Fleet, 1984; Elderfield and Pagett, 1986; Elderfield, 1988; Sholkovitz et al., 1994; De Carlo et al., 1998, 2000). To date, however, few published studies of REE cycling in lakes exist. Moller and Bau (1993) reported REE concentrations in two samples from Lake Van in Turkey. They observed subnanomalor concentrations of REE and extremely fractionated patterns in the oxic waters that result from solution complexation with carbonate, including a highly unusual positive Ce anomaly attributed to stabilization of Ce(IV) in solution by polycarbonato complexes. Lyons et al. (1994) utilized REE signatures in water to identify solute sources to the lakes of the McMurdo Dry Valleys, and Johannesson and Zhou (1999) evaluated the source of middle REE enrichments in waters of an acidic Arctic lake. 2. SITE DESCRIPTION

Lakes in the Dry Valleys of Antarctica are characterized by an internal drainage where no stream crosses the coastal threshold ridges to flow to the sea. These enclosed hydrologic systems are model systems for the study of trace metal behavior because they are pristine and uncontaminated, and because of their relative biogeochemical simplicity. Lake Vanda is located in Wright Valley, a remote, glacier-carved valley near the Ross Sea, and is fed nearly exclusively by the Onyx River, which flows about 6 weeks per year between late December and mid-February. Lakes Brownworth, Bull, and Vanda all form part of the Onyx River system (Fig. 1), although other, smaller ponds also exist. Lake Vanda is a small, enclosed, and permanently ice-capped lake ⬃5.6 km long, 1.5 km wide, and ⬃70 to 75 m deep in its western depression. The Onyx River runs from its source at Wright Lower Glacier, ⬃27 km to the east, with an annual discharge of ⬃2 ⫻ 109 L (Chinn, 1981). Although inflow volumes to the lake can vary widely from one summer to another owing to temperature differences, annual losses over both summer and winter remain constant. Meltwater runoff from surrounding glaciers during the short austral summers is the only significant source of water (to the Onyx River) that maintains the lake; year-round sublimation from the ice cover is the dominant loss. The lake level has been rising by slightly less than 1 m per year during the 1980’s and 1990’s (Stage, 1996). The 4-m-thick ice cap on Lake Vanda is permanent, but a nearshore moat forms during the summer as a result of riverine input and heating of shoreline rocks. Vanda possesses an inverse thermal stratification, with a minimum temperature of 0°C at the surface and a maximum of ⬃22°C at 74 m (Fig. 2; Bratina et al., 1998). This profile results from solar energy penetrating the ice cap; increases in both density and salinity with depth prevent deep, warm water from mixing with cooler surface water. Vanda is one of the clearest and least productive lakes in the

Fig. 1. Site location map showing Lake Vanda, Wright Valley, Antarctica. Modified from Green et al. (1998).

world. Low algal standing crops and low rates of primary productivity are observed at all depths except at 65 m, just above the anoxic zone, where a low-light–adapted active population of phytoplankton was observed (Goldman et al., 1967; Vincent and Vincent, 1982). The enhanced bioproductivity at

Fig. 2. Physical and chemical profiles of the Lake Vanda water column during the 1994 field season. (A) Temperature (solid triangles) and pH (open circles). (B) DO (open triangles), hydrogen sulfide (solid triangles), and Mn (open squares). Figure modified from Bratina et al. (1998).

Rare earth elements in Lake Vanda, Antarctica

this depth is thought to be due to a diffusional input of phosphorus from the anoxic zone below. Probably the most important feature of the lake is its long-standing (⬃1200 yr) and well-defined biologic, chemical, and physical stratification (Canfield and Green, 1985; Canfield et al., 1995). The unusually stable vertical structure in Vanda plays a key role in our ability to understand, from a fundamental standpoint, metal cycling in the water column of the lake. Little if any horizontal transport occurs because the permanent ice cover prevents wind-induced mixing, and the strong pycnocline limits convection caused by the Onyx River input to the upper portion of lake. A convection current of ⬃1 cm/s detected in the upper layer likely is responsible for the relative homogeneity of waters in the upper water column (Ragotzkie and Likens, 1964). Molecular diffusion driven by concentration gradients is assumed to be responsible for vertical transport of dissolved species. Lake Vanda is a simple meromictic two-layer system. Green and Canfield (1984) reported that water above 55 m (depth has been adjusted to account for the 7-m increase in lake level between that study and this work) is relatively fresh, whereas saline waters at depth exceed seawater chloride concentrations by nearly threefold. The warm Ca-Cl rich bottom waters below 55 m define a stable and dense diffusion zone in which salinity far overrides temperature in density considerations. Manganese oxides are important in the transport of metals in Lake Vanda. Canfield et al. (1995) reported redox cycling between 50 to 60 m, which they labeled the aerobic Mn reduction (AMR) zone. Although this depth range is supersaturated in dissolved oxygen (DO), redox-driven reduction and dissolution of Mn-oxides are thought to be responsible for peak Pb concentrations observed in this zone (Canfield et al., 1995). The depth range of the AMR zone had increased to 57 to 67 m by 1994, when samples for the current study were collected (Stage, 1996). Figure 2, taken from the concurrent (1994 field season) study of Bratina et al. (1998), presents profiles for Mn and DO in Lake Vanda. The Mn profile shows the presence of a characteristic submaximum at 61 m, and this occurs at roughly the same depth as the oxygen subminimum. This mirror-image relationship in the Mn and DO profiles was reported earlier by Green et al. (1986) and by Canfield et al. (1995) and is apparently a long-lived and stable feature of the water column. As in past studies (Green et al., 1986; Canfield et al., 1995), there is an 8- to 10-m depth interval between the onset of significant Mn reduction (58 m) on the one hand and iron reduction (67 m) on the other, with iron reduction occurring deeper in the lake (Stage, 1996). 3. METHODS Cleaning of all field gear was performed at the Crary Laboratory, McMurdo Station. Tygon tubing used for sample collection was flushed with 1 N Ultrex HCl for 1 d and rinsed with Milli-Q water. Nuclepore (0.2-␮m pore size) polycarbonate filters were soaked in Milli-Q water for 1 d, 1 N Ultrex HCl for 5 d, and rinsed in Milli-Q water for 1 d. Filters were individually packaged (in acid-washed plastic petri dishes) and transported to Lake Vanda. Cleaning procedures for high-density polyethylene (HDPE) trace metal bottles involved soaking in 1 N Ultrex HCl for 2 d, followed by three rinses with Milli-Q water, a subsequent acid wash in 1 N Ultrex HNO3 for 1 d, and finally triple rinsing with Milli-Q water.

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Field work was conducted between October and December 1994, during the austral spring. Samples were collected through the ice above the deep western basin of Lake Vanda. Samples were collected through a 10-inch sampling hole drilled through the ice and covered with a laboratory tent. Samples were obtained from the water column with a Barnstead Masterflex peristaltic pump and plastic tubing marked at 1-m intervals. The stability of the sampling platform (4-m-thick ice cover) permitted discrete sampling at high resolution with a depth uncertainty of less than 20 cm. Filtered samples were collected first followed by unfiltered samples. Because of the chemical stratification of the lake and increasing metal concentrations with depth, samples were collected in order of increasing depth. Water from the preceding depth interval was purged at each new depth by flushing the lines with ambient water. Water was filtered by use of a tripod apparatus equipped with a single 142-mm membrane filter. Each membrane was rinsed with 10 to 1500 mL of sample during sample collection. The amount flushed through the membrane varied as a function of the ease of filtration. Upper depth intervals allowed copious rinsing of the membrane and collection of up to 3.5 L of sample, whereas at greater depth, particularly below 60 m, the increasing abundance of particles and dissolved gases inhibited flow rates through the membranes. Multiple filters were necessary to collect sufficient volumes of sample at these depths (i.e., 750 mL at 70 m). HDPE bottles used for sample storage were triple rinsed with aliquots of the lake water before filling. Samples were preserved by acidifying to pH ⬍2 with Ultrex HNO3. The REE were determined by flow injection analysis (FIA) inductively coupled plasma mass spectrometry (ICP-MS). A VG-PQ2-S instrument equipped with a Gilson autosampler was used for REE analyses following the method described by De Carlo and Resing (1998). This FIA method involves on-line matrix elimination and preconcentration on an 8-hydroxyquinoline column prepared by a modification of the method described by Landing et al. (1986), followed by back elution with a small volume of HNO3 directly into the ICP-MS. The ICP-MS was tuned for maximum sensitivity in the mass range of 140 to 175. Calibration was performed with aqueous solutions of mixed REE standards diluted from commercially obtained SPEX stock solutions. These were passed through the FIA system in a manner analogous to samples. Calibrations were linear over the range of 0.005 to 1.00 ppb (r2 ⬎ 0.999). Slopes approached 1 Gcps/ppm for monoisotopic REE under optimal FIA and ICP-MS conditions. The standards used for calibration correspond to an aqueous concentration range of ⬃35 pM to 7 nM at mass 140 and ⬃ 29 pM to 5.7 nM at mass 175. A few samples were diluted twofold to bring Ce within the range defined by the calibration standards. The ICP-MS was monitored for drift throughout all analyses by using 115In and 209Bi as internal standards, and peak intensities automatically corrected appropriately for drift. Detection limits ranged from ⬍10 pM for multi-isotope REE to ⬍1 pM for monoisotopic REE. Lower detection limits are possible if longer sample loading times are used (i.e., increasing the volume loaded to greater than the ⬃7 mL used in this study), although instrumental blanks still limit the lower end of the range of REE concentrations that can be determined reliably. Accuracy of analysis was monitored by running separate REE standards as samples because no standard reference material certified for REE was available. A background seawater sample collected near Loihi Seamount to the south of the island of Hawaii was also analyzed (De Carlo et al., 1996) and REE concentrations compared with literature values (Piepgras and Jacobsen, 1992). Although seawater REE concentrations we measured near Hawaii were not identical to those reported by these authors for the North Pacific seawater they sampled, REE patterns from the two studies overlap significantly, suggesting our analyses are reliable. Analytical precision, based on replicate analysis of filtered samples at 45, 59, 60, 64, and 68 m, was generally better than 5% for (individual isotope) concentrations above 20 pM but typically deteriorated to 20 to 30% relative for REE concentrations of 5 to 10 pM. 4. RESULTS

4.1. Selected Chemical and Temperature Profiles Because discussion of our results requires reference to the hydrography and geochemistry of Lake Vanda, we briefly summarize previous findings shown in Figure 2.

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E. H. De Carlo and W. J. Green Table 1. REEs in filtered water collected from Lake Vanda, Antarctica, during the 1994 field season.a

Depth (m)

Y

5 15 25 35 45 57 58 59 60 61 62 63 64 65 66 67 68 70

0.02 0.087 0.109 0.11 0.117 0.395 0.765 1.62 3.26 4.68 5.59 3.11 6.46 2.90 7.06 7.66 0.084 0.092

a

La

Ce

0.005 0.004 0.023 0.018 0.041 0.017 0.064 0.022 0.069 0.015 0.454 0.123 1.06 0.42 2.64 1.11 4.73 2.52 4.52 3.8 3.18 3.53 1.09 1.49 2.84 2.81 0.69 0.65 2.39 1.64 2.91 11.2 0.099 0.279 0.088 0.227

Pr

Nd

0.002 0.002 0.005 0.01 0.01 0.051 0.112 0.234 0.404 0.625 0.733 0.387 0.772 0.311 0.797 0.954 0.015 0.022

0.01 0.021 0.022 0.031 0.027 0.144 0.357 0.762 1.37 2.01 2.83 1.57 2.96 1.30 3.13 3.63 0.059

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

0.005 0.012 0.042 0.078 0.144 0.237 0.322 0.243 0.415 0.259 0.438 0.507 0.004

0.006 0.013 0.013 0.023 0.032 0.053 0.047 0.075 0.047 0.078 0.077

0.007 0.022 0.055 0.114 0.214 0.380 0.373 0.252 0.495 0.255 0.623 0.756

0.010 0.016 0.029 0.034 0.054 0.038 0.069 0.041 0.079 0.081

0.017 0.039 0.079 0.136 0.195 0.334 0.234 0.435 0.259 0.420 0.559

0.01 0.022 0.026 0.041 0.059 0.046 0.085 0.058 0.110 0.119

0.017 0.024 0.046 0.083 0.139 0.182 0.146 0.234 0.180 0.338 0.361

0.009 0.009 0.014 0.023 0.019 0.032 0.025 0.046 0.046

0.015 0.020 0.029 0.050 0.073 0.111 0.115 0.171 0.151 0.238 0.250 0.003

0.012 0.011 0.013 0.017 0.016 0.028 0.027 0.039 0.036

CeANOM 0.31 0.55 0.27 0.21 0.17 0.18 0.28 0.31 0.39 0.53 0.57 0.55 0.47 0.33 0.29 1.63 1.73 1.27

Sm/ Yb

0.80 2.10 2.69 2.88 3.25 2.90 2.11 2.43 1.72 1.84 2.03 1.26

Concentrations are expressed in nM, except Ce anomaly and Sm/Yb ratio (unitless).

The water column of Lake Vanda is defined by three redox zones. Water down to the present 65-m depth is supersaturated (⬎0.4 mM) with respect to DO, with a maximum found between 35 and 45 m. DO decreases below this depth, but the photosynthetic maximum at 64 m causes a DO submaximum. A sharp decrease occurs between 65 and 67 m from 0.55 to 0.15 mM in a zone termed “suboxic.” DO concentrations remain significant within this zone, and the term “suboxic” is used in a relative sense here. Oxygen becomes fully depleted by 68 m, below which concentrations of sulfide increase, reaching ⬃0.4 mM by 70-m depth (Fig. 2). The pH of Lake Vanda is between 7.7 and 8.3 in the upper 45 m, with the maximum occurring at the latter depth. Below 45 m, pH drops steadily to a minimum of 6.3 at the oxic/anoxic interface (Stage, 1996), although it fluctuates in a narrow range between 62- and 64-m depth. The redox potential (Eh) of the water column remains between 0.43 and 0.55 V throughout the oxic water column but drops sharply to less than ⫺0.1 V in the anoxic zone (Stage, 1996). The pronounced temperature gradient between 50 and 70 m (⬃7 to 21°C; Fig. 2) coincides with an increase in Cl from below 20 mM in the upper 45 m to nearly 1.7 mol/L at 70 m; other major ions exhibit gradients that mimic Cl (Green and Canfield, 1984). Low dissolved Fe concentrations between 20 and 28 nM in the upper 60 m of the water column increase to near 80 nM by 65-m depth, then rise sharply to a maximum of nearly 30 ␮M at 68 m (Stage, 1996). Dissolved Mn, however, displays a more complex profile than Fe. Concentrations below 2.5 nM Mn in the upper 45 m increase to a submaximum of 6.4 ␮M at 64 m, then rise sharply to nearly 35 ␮M at 68 m, the oxic/anoxic interface (Fig. 2). Other trace metals reported by Green et al. (1993), Canfield et al. (1995), and Stage (1996) display rapidly changing profiles below 57 m (adjusted for changing lake levels), the depth range in which steep and stable chemical and thermal gradients exist. The depths at which dissolved concentration maxima are observed vary slightly and are thought to be controlled by the respective diffusion coefficients of the individual metals (Canfield et al., 1995).

4.2. REE Concentrations of REE in filtered and unfiltered water samples from Lake Vanda are presented in Tables 1 and 2, respectively. Depth profiles between 40 and 72 m are shown in Figures 3 and 4. As previously observed for other dissolved metals, concentrations of the REE (Table 1) are very low at the surface (La, Ce, ⬍15 pM) and only increase gradually with increasing depth to 55 m (La, 69 pM; Ce, 18 pM), the limit of the freshwaters. Because of low concentrations, not all REE could be determined reliably in samples from the uppermost and lowermost parts of the water column. Below 56 m, dissolved concentrations of REE increase sharply, with strictly trivalent REE displaying a dissolved maximum (e.g., La, 4.5 nM; Table 1) at 60 m, 1 m above the dissolved Mn submaximum. The redox-sensitive Ce, however, exhibits a profile that is different from the other REE. Its dissolved submaximum of 3.8 nM at 61 m is followed by a sharp maximum of ⬃11 nM at 67 m. The trivalent REE maxima occur within stable and warm waters and coincide with the most productive zone of the water column, where an active phytoplankton population occurs. It should also be noted from Figure 3 that filtered samples from 63 and 64 m appear reversed; although we have no evidence to support this contention, they may have been mislabeled. Dissolved REE concentrations account for an overwhelming majority of the measured REE in the upper oxic water column, except at 61 to 62 m, where they represent 60 to 80% of the total. The dissolved REE concentrations in the upper water column of Lake Vanda are similar to those measured in oxic upper ocean water (e.g., De Baar et al., 1988; German and Elderfield, 1989; German et al., 1991; Piepgras and Jacobsen, 1992). Concentrations of dissolved Ce in the oxic water column of Lake Vanda are enriched relative to those observed in seawater; however, the shale-normalized REE patterns (Fig. 5) still display negative Ce anomalies. At the anoxic boundary, however, the dissolved REE concentrations reported here are nearly two orders of magnitude higher than observed in other anoxic basins (e.g., De Baar et al., 1988; German and Elder-

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Table 2. REEs in unfiltered water collected from Lake Vanda, Antarctica, during the 1994 field season.a Depth (m) 25 35 55 57 58 59 60 61 62 63 64 65 66 67 68 69 70 72 a

Y 0.139 0.123 0.247 0.402 0.763 1.57 3.1 5.2 6.73 7.3 6.52 6.3

La

Ce

0.068 0.042 0.072 0.032 0.207 0.047 0.499 0.106 1.07 0.415 2.55 1.06 4.85 2.41 5.61 4.17 5.26 4.5 3.67 3.8 2.92 2.82 2.1 1.7 2.23 1.69 2.71 10 2.49 13.2 2.38 11.4 2.1 9.57 1.63 6.87

Pr

Nd

0.011 0.011 0.036 0.057 0.105 0.221 0.427 0.711 0.925 0.888 0.77 0.663 0.801 0.895 0.9 0.885 0.851 0.695

0.042 0.049 0.105 0.165 0.384 0.8 1.34 2.1 3.27 3.2 2.95 2.73

Sm

0.022 0.04 0.119 0.169 0.354 0.383 0.402 0.328

Eu

0.033 0.054 0.055 0.054 0.059

Gd

0.116 0.205 0.375 0.451 0.557 0.5 0.526

Tb

0.027 0.039 0.06 0.076 0.067 0.071

Dy

0.046 0.054 0.13 0.203 0.311 0.439 0.4 0.42

Ho

0.027 0 0.07 0.087 0.086 0.083

Er

0.029 0.017 0.025 0.037 0.083 0.137 0.21 0.24 0.254 0.27

Tm

0.027 0.022 0.024 0.031

Yb

0.028 0.036 0.076 0.109 0.151 0.173 0.154

Lu

0.024 0.03 0.032

CeANOM 1.06 0.77 0.13 0.14 0.28 0.3 0.36 0.49 0.49 0.52 0.46 0.35 0.31 1.57 2.13 1.89 1.7 1.52

Concentrations in nM except Ce anomaly (unitless). REE from Nd to Lu not measured in samples below 65 m.

field, 1989; German et al., 1991). Below the oxic/anoxic interface (67 m), dissolved REE concentrations decrease sharply, whereas particulate concentrations remain significant (Fig. 3). For example, between 68 and 72 m, total La and Ce decrease

from 2.5 to 1.6 nM and 13.2 to 6.9 nM, respectively, whereas dissolved concentrations of these elements at 68 m are less than 0.3 and 0.1 nM, respectively. Trends similar to those described for La are observed for other trivalent REE.

Fig. 3. Profiles of dissolved (filtered) La, Ce, Ce anomaly, Sm/Yb, Mn, and Fe in the water column of Lake Vanda during the 1994 sampling season. The Ce anomaly is defined by Eqn. 2. The three redox zones of the water column are indicated as oxic, AMR, and anoxic. The Sm/Yb ratio of 2.46 observed in shales is indicated by an arrow.

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Fig. 4. Profiles of total (unfiltered) La, Ce, Ce anomaly, Mn, and Fe in the water column of Lake Vanda during the 1994 sampling season. The Ce anomaly and redox zones are defined as in Figure 3.

5. DISCUSSION

The profiles of La and Ce in the water column of Lake Vanda are used throughout the discussion to exemplify the utility of the REE in evaluating metal cycling associated with scavenging by particles in the oxic part of the water column and release under suboxic to anoxic conditions. Furthermore, differences that exist between the profiles of La and Ce are helpful in distinguishing between uptake reactions that occur primarily through surface complexation and reactions that also involve oxidative scavenging (e.g., Sholkovitz and Elderfield, 1987; De Baar et al., 1988; German and Elderfield, 1989; German et al., 1991). Results of laboratory experiments to study interactions of dissolved REE with suspended pure mineral phases (e.g., Koeppenkastrop and De Carlo, 1992, 1993; De Carlo et al., 1998) have also been applied to elucidate how associations of REE with mineral phases in the marine environment, particularly Fe- and Mn-oxides, are controlled by competitive complexation and redox reactions (De Carlo et al., 2000). These authors observed variations in the Sm/Yb throughout strata of marine ferromanganese crusts that presumably result from competitive complexation during uptake of REE from solution

and utilized these to infer changes in oceanic chemistry through geologic time. Here, we apply the same principles to describe REE cycling near the oxic/anoxic boundary of Lake Vanda. In Lake Vanda, cycling of REE appears to be predominately associated with reactions of Mn-oxides, as is shown below, rather than Fe-oxides or organic matter. Our findings are consistent with observations made previously on the cycling of transition metals throughout the water column, although previous work suggested that organic particle formation between 65and 67-m depth may also play a role in metal scavenging (Green et al., 1993; Canfield et al., 1995; Stage, 1996). Examination of the depth profiles of dissolved La and Ce in the range of 57 to 70 m (Fig. 3) reveals several features. First is the near coincidence of the dissolved La maximum and a Ce submaximum with the Mn submaximum of ⬃4 ␮M. These features occur within the AMR zone, an interval in which dissolved concentrations of Fe range from ⬍20 nM to ⬃80 nM. Second, the dissolved La maximum is at 60 to 61 m with a smaller submaximum at 67 m, the oxic/anoxic interface, whereas Ce displays a subtle submaximum in the AMR zone and its pronounced maximum occurs at 67 m, just above the

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Fig. 5. Shale normalized patterns of REE in filtered water samples collected from Lake Vanda during the 1994 sampling season. Values were calculated by using REE concentrations in shales reported by Byrne and Sholkovitz (1996).

dissolved Mn maximum. The dissolved Fe maximum at 68 m coincides with that of Mn; however, whereas dissolved Fe concentrations nearly triple between 67 and 68 m, those of Mn only increase by ⬃10% (Fig. 3). The coincidence of the trends in the dissolved La and Ce profiles with that of Mn leads us to suggest that Mn and its oxides dominate REE cycling in the water column of Lake Vanda, although some contribution associated with scavenging by and dissolution of Fe-oxide coatings on particles cannot be excluded without particle analyses. Without prior knowledge of the existence of the AMR zone reported by Canfield et al. (1995), it would appear odd that dissolved concentrations of Mn increase in water supersaturated with respect to DO. Canfield et al. (1995) measured the Eh of the oxic waters at depths corresponding to 52 to 61 m (adjusted to 1994 lake level) between 453 and 496 mV, a range well below values expected for equilibrium with the O2/H2O couple. This is not an uncommon occurrence in aquatic environments (Stumm and Morgan, 1981). When the Eh and pH

values measured in Lake Vanda are placed on a stability diagram for Mn (Canfield et al., 1995), waters below 50 m fall increasingly within the field of Mn2⫹. Although redox calculations should always be interpreted cautiously, the measured Eh values in the AMR zone are consistent with those calculated ⫺ with the NO⫺ 3 /NO2 couple by using nutrient data. Thus, reductive dissolution of Mn below 50 m appears to be responsible for (partial) release of REE (and other metals) scavenged within the oxic and higher pH upper water column. Additionally, whereas the exact nature of the manganese oxide phase extant in Lake Vanda remains equivocal, thermodynamic calculations strongly suggest the existence of Mn3O4 in equilibrium with dissolved Mn within the AMR according to Mn3O4 ⫹ 6H⫹ ⫽ 3 Mn2⫹ ⫹ 1⁄2 O2 ⫹ 3H2O.

(1)

The above explanations, however, cannot account for the decrease in dissolved REE observed from 62 to 65 m. Although the Eh of waters in this depth interval falls well within the field

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of Mn2⫹, the concentration of dissolved Mn also decreases in this range, before rising sharply at the oxic/anoxic interface. Canfield et al. (1995) proposed that because water at 65-m depth (corrected to 1994 lake level) is supersaturated with respect to calcite, formation of this phase could lead to uptake of Mn2⫹ (e.g., Mucci, 1988). The latter is also consistent with a removal of REE within this depth range; work by Zhong and Mucci (1995) demonstrated that mixed Mn-carbonate phases such as kutnahorite can also scavenge REE during their formation. Alternatively, organic ligands derived from the active phytoplankton population in this part of the water column may also be involved in Mn and REE removal from solution. We now examine more closely the process by which release of REE from particles can occur within the AMR. We evaluate two possibilities: pH-induced release of surface complexes of the REE on Mn-oxides (or particles coated with Mn-oxides); and true reductive dissolution of REE oxide coatings from Mn-oxide particles. The first mechanism should result in the release of all trivalent REE as well as tetravalent Ce hydroxy complexes, whereas the second should preferentially affect Ce(IV) oxide coatings on falling particles. Because standard free energies for the reduction of Ce(IV) oxides and Mn(IV) oxides are similar (Elderfield and Sholkovitz, 1987), it has been previously suggested that the distribution of Ce in anoxic basins is controlled by oxidation–reduction cycles of the Ce (III)-Ce(IV) couple (De Baar et al., 1988; German and Elderfield, 1989). Thus, to examine the influence of redox reactions on removal from REE and their release to the water column and separate redox processes from surface complexation reactions, we compare and contrast the behavior of La and Ce. Additionally, we compare the differing tendencies of REE to form complexes and fractionate from one another (e.g., Cantrell and Byrne, 1987) during uptake and release reactions through examination of variations in the Sm/Yb in the water column. A simple means to examine redox processes is through use of the Ce anomaly (e.g., Elderfield, 1988), which we define here as follows: Ce anomaly ⫽ 2(Ce/Ce*)/(La/La*⫹ Pr/Pr*),

(2)

where Ce*, La*, and Pr* represent the concentration of the respective REE in the average shale reported in Byrne and Sholkovitz (1996). The Ce anomaly represents the deviation of the Ce concentration from that expected by linear interpolation between the concentrations of La and Pr in shale. It is thus a measure of the deviation (fractionation) of Ce from expected (trivalent) REE behavior that can be attributed exclusively to redox chemistry (Henderson, 1984; Elderfield, 1988). Values of Ce anomaly greater than 1 indicate a preferential enrichment of Ce, whereas values less than 1 indicate preferential removal of Ce with respect to the strictly trivalent La and Pr. Depth profiles of Ce anomalies (Fig. 3) can be compared with those of La and Ce to illustrate contrasts between these two REE. The Ce anomalies of the dissolved fraction at 5 and 15 m (data not shown) are 0.3 and 0.55, respectively, then decrease to a minimum of 0.17 at 45 m. Because concentrations of REE measured in the upper water column are near our detection limits, they are subject to significant uncertainties, and Ce anomalies calculated from them are also likely subject to large uncertainties. Thus, interpretations of differences in Ce

anomalies in the uppermost water column of Lake Vanda, unless they are quite large, should be made cautiously. Yet a decrease in the Ce anomaly of similar magnitude has also been observed in marine waters above anoxic basins (De Baar et al., 1988; German et al., 1991), suggesting our observations in waters of Lake Vanda are real and not analytical artifacts. At 25 m, the Ce anomaly of the unfiltered fraction is close to 1—that is, there is no apparent fractionation of Ce from strictly trivalent REE, whereas the Ce anomaly of the dissolved fraction is near 0.3. The latter indicates that Ce has been either strongly scavenged from the water column by particles at this depth or that the (dissolved) Onyx River input is itself strongly (negatively) fractionated with respect to Ce. Because dissolved REE account for more than 90% of the total in the upper oxic water column, particles carrying REE in this portion of the water column should have a value of Ce anomaly substantially greater than 1. The latter is consistent with our prior observations of Ce anomalies in marine Fe-Mn deposits formed under highly oxic conditions (e.g., De Carlo and McMurtry, 1992; Wen et al., 1997; De Carlo et al., 2000). Between 25 and 57 m, the Ce anomalies of the unfiltered and filtered fractions decrease steadily to approximately 0.14 and 0.18, respectively. Ce anomalies increase below 57 m, displaying a submaximum of 0.57 (i.e., still depleted in Ce relative to La and Pr) at 62 m. The sharpest rise in the dissolved Ce anomaly occurs across the anoxic boundary at 67 m, with the maximum of 1.73 observed at 68-m depth. Although the very low Ce anomalies observed in the filtered fraction immediately above the AMR zone of Lake Vanda might reflect a strong preferential scavenging of Ce by particles, the nearly identical Ce anomaly of the unfiltered phase, coupled with the fact that the dissolved fraction accounts for most of the total REE, implies that the few particles that do exist at 57-m depth also have a relatively low Ce anomaly. Therefore, these particles appear to have little Ce on their surface that can be attributed to oxidative scavenging in the upper water column. Below a 57-m depth, the Ce anomalies of the dissolved phase define a slightly greater positive gradient than that of the unfiltered samples, with the two reaching submaxima at 62 and 63 m, respectively. Interestingly, the Ce anomaly submaxima occur 1 to 2 m below the concentration submaxima of La and Ce. Although these differences in depth are small, we think they are real and may reflect a delay between the desorption of surface-bound (trivalent) REE from the sinking (and dissolving) Mn-oxide– coated particles and the reduction of Ce(IV) oxide from the surface of particles. Both Mn(IV) and Ce(IV) have similar free energies of reduction; thus, if reduction of Mn-oxides occurs, so should that of Ce(IV) phases. Yet we observe a delay and only limited release of Ce within the AMR that can be attributed to reductive dissolution. Perhaps this results from the absence of any organism that can catalyze the reduction Ce(IV), whereas the presence of Carnobacterium, isolated from these waters by Bratina (personal communication), catalyzes the reduction of Mn(IV) oxides. Thus, although thermodynamic considerations favor reduction of the both Mnand Ce-oxides, kinetics do not favor reduction of the latter. It is not until the oxic/anoxic boundary at 67 m is reached that a large contrast between the behavior of Ce and La is observed. The Ce anomaly peaks across the anoxic boundary with a value near 2 for both the dissolved (Ce anomaly ⫽ 1.7)

Rare earth elements in Lake Vanda, Antarctica

and unfiltered phase (Ce anomaly ⫽ 2.1), and, as observed in the upper water column, most of the Ce is found in the dissolved phase. Concentration maxima for La and Ce occur at this depth, with the concentration of Ce at 67 m exceeding that of La by a factor of nearly 4, whereas the concentration of La was equal to or greater than that of Ce within the AMR zone (Fig. 3). The large increases in both the concentration of Ce and the Ce anomaly suggest an overwhelming dominance of the reductive dissolution of Ce(IV) oxide coincident with reductive dissolution of Mn-oxides, whereas changes observed within the AMR zone are likely dominated by pH induced reactions of the REE (including a small extent of reduction of Ce(IV) oxides on particle surfaces that is kinetically limited by the absence of bacterially mediated reduction). This interpretation is consistent with thermodynamic calculations made by De Baar et al. (1988), who contend that pH rather than Eh controls the reduction of CeO2 (or oxidation of Ce3⫹) in most marine systems, but who propose that the sharp drops in Eh across marine anoxic boundaries (and the concomitant increase in sulfide concentrations) are the primary force driving reduction of Ce(IV) oxides and release of Ce3⫹ to solution. Eh changes also likely drive Ce release in Lake Vanda because the decrease in Eh across the anoxic boundary is accompanied by only slight changes in pH, and sulfide concentrations increase to near 0.4 mM, a concentration nearly 10 times higher than observed in the Cariaco Trench (De Baar et al., 1988). The structure of the water column in Lake Vanda allows us to separate processes associated only with Mn reduction (i.e., in the AMR zone), that are controlled by changes in pH, from Mn (and Fe) reduction driven by the sharp drop in Eh at the anoxic boundary. Thus, our data provide evidence that helps clarify previous discussions in the literature (e.g., German et al., 1991; Sholkovitz, 1995) where the separation of REE cycling associated with reduction of Mn- and Fe-oxides was difficult to resolve unequivocally. Furthermore our data support the suggestion by De Baar et al. (1985) that shifts in the Ce anomaly in the eastern equatorial Pacific Ocean are associated with reductive dissolution of Mn in the presence of low concentrations of oxygen (i.e., in the absence of H2S). Particles, high concentrations of complexing anions, and pH all influence REE cycling and fractionation (e.g., Cantrell and Byrne, 1987; Koeppenkastrop and De Carlo, 1992, 1993; Sholkovitz, 1995; De Carlo et al., 1998, 2000). High-pH natural waters with colloids typically display the most fractionated REE patterns and the lowest concentrations (Sholkovitz, 1995), largely because heavy REE (HREE) have a propensity to form stronger solution complexes than their light (LREE) counterparts (e.g., Cantrell and Byrne, 1987). De Carlo et al. (2000) proposed that variations in the Sm/Yb ratio of laminae of deep-sea ferromanganese crusts reflect carbonate complexation– dependent fractionation between LREE and HREE during their uptake from seawater. Fractionation of the REE should also be expected to occur in the water column of Lake Vanda in association with particle cycling in the chemocline, both in the AMR zone and across the anoxic boundary. Between 57 and 61 m in Lake Vanda, decreasing concentrations of DO and a decreasing pH lead to the release of REE from the surface of Mn-oxide– coated carrier particles. During this process, the Sm/Yb ratio of the dissolved phase (Fig. 3) increases rapidly with increasing depth as a result of preferential release

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of LREE. Just as LREE are preferentially removed from seawater over HREE because the latter form stronger solution complexes with carbonate (Cantrell and Byrne, 1987), the LREE are preferentially released to Lake Vanda during particle dissolution in the AMR because the HREE form stronger surface complexes on the Mn-oxide particle coatings, as demonstrated by Koeppenkastrop and De Carlo (1993) under conditions where carbonate complexation is not important. Below 61 m, dissolved REE could potentially be removed from solution by a mixed Ca-Mn carbonate phase that was hypothesized by Canfield et al. (1995) to form in the calcium carbonate–saturated waters found in this depth range. Because the concentrations of dissolved REE are in the sub- to nanomolar range, and PO3⫺ 4 increases from 420 to 660 ␮M between 63- and 66-m depth, the potential effect of complexation or coprecipitation of REE with PO3⫺ 4 (e.g., Byrne and Kim, 1993) on fractionation should also be considered. Processes that remove REE from solution typically lead to preferential uptake of LREE because HREE are more strongly complexed with many dissolved ligands such as carbonate (Cantrell and Byrne, 1987) and organic compounds (e.g., Stanley and Byrne, 1990). This preference is reflected in the waters of Lake Vanda by a decreasing Sm/Yb ratio of the dissolved phase (Fig. 3) between 61- and 65-m depth, although the identity of the solid phase responsible for REE removal in this part of the water column remains equivocal. A less pronounced increase in the Sm/Yb ratio is observed just above the anoxic boundary than in the upper portion of the AMR. Enhanced reductive dissolution of Mn-oxide driven by the precipitous Eh drops likely releases previously sorbed REE efficiently and limits fractionation. Fractionation of Sm from Yb also occurs during the subsequent removal of dissolved REE within the anoxic waters as evidenced by a decreasing Sm/Yb ratio between 67- and 68-m depth (Fig. 3). Although total concentrations of REE only decrease slightly between 67 and 70 m, dissolved concentrations drop precipitously. The latter is thought to result from scavenging by particles forming in the anoxic part of the water column, likely Fe sulfides, although precipitation of MnS may also play a role. The Ce anomalies of filtered and unfiltered phases only decrease slightly below 67 m, suggesting Ce behaves similarly to La and Pr during scavenging by particles forming in anoxic waters. Thus, whereas Mn-oxide (coatings) dominate cycling of REE in the oxic water column, including the AMR zone and at the oxic/anoxic interface, sulfide phases appear to play an important role in removing REE from the deeper anoxic waters of Lake Vanda. 6. CONCLUSIONS

REE are cycled in the water column of Lake Vanda primarily through processes associated with changes in redox conditions. Scavenging of REE by particles in the upper oxygenated water column of Lake Vanda is followed by a release in low-pH and anoxic waters. The release of REE to solution is facilitated by the reductive dissolution of metal oxides. The coincidence of dissolved REE maxima with those of dissolved Mn is consistent with desorption of the REE during reductive dissolution of manganese oxide coatings on particles driven by changes in pH and Eh. Changes in the Ce anomaly profile reflect fractionation

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of Ce from trivalent REE in the water column attributable to its redox properties and further exemplifies the utility of REE in resolving redox processes from surface complexation reactions. Redox cycling of Ce in Lake Vanda appears to be driven by pH in the AMR zone between 57 and 61 m and by Eh at the anoxic boundary. The large Ce anomaly peak at the anoxic interface, attributed to Eh driven reductive dissolution of Ce(IV) oxide, contrasts the more extensive desorption of strictly trivalent REE during pH driven dissolution of MnO2 in the AMR. Large changes in the (dissolved) Sm/Yb ratio within the AMR zone above the oxic/anoxic interface are consistent with fractionation of the trivalent REE during their release from the surface of Mn-oxide coatings on particles upon suboxic dissolution and during their subsequent removal from solution by precipitation of other (carbonate?) minerals. Below the oxic/anoxic interface, sulfides of iron and possibly Mn may be responsible for removal of dissolved REE from solution. Acknowledgments—We thank Brian Stage, from whose M.S. thesis background information used in this study was obtained. J. A. Resing is acknowledged for his initial development of the FIA system that greatly expanded the capabilities of the ICP-MS. We thank K. Johannesson, P. L. Smedley, and an anonymous reviewer for thoughtful critical reviews, which helped improve the manuscript. This research was funded through NSF award OPP93-19044. The SOEST ICP-MS facility was funded by NSF awards EAR94-01770, EAR97-06711, and OCE94-06711. This is SOEST contribution 5559. Associate editor: D. J. Burdige REFERENCES Aplin A. C. (1984) Rare earth element geochemistry of Central Pacific manganese encrustations. Earth Planet. Sci. Lett. 71, 13–22. Bacon M. P., Brewer P. G., Spencer D. W., Murray J. W., and Goddard J. (1980) Lead-210, polonium-210, manganese and iron in the Cariaco Trench. Deep-Sea Res. 27A, 119 –135. Balistrieri L., Brewer P. G., and Murray J. W. (1981) Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean. Deep-Sea Res. 28A, 101–121. Balistrieri L. S. and Murray J. W. (1986) The surface chemistry of sediments from the Panama Basin: The influence of Mn oxides on metal adsorption. Geochim. Cosmochim. Acta 50, 2235–2243. Balistrieri L. S., Murray J. W., and Paul B. (1992a) The cycling of iron and manganese in the water column of Lake Sammamish, Washington. Limnol. Oceanogr. 37, 510 –528. Balistrieri L. S., Murray J. W., and Paul B. (1992b) The biogeochemical cycling of trace metals in the water column of Lake Sammamish, Washington: Response to seasonally anoxic conditions. Limnol. Oceanogr. 37, 529 –538. Bau M., Moller P., and Dulski P. (1997) Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redoxcycling. Mar. Chem. 56, 123–131. Bratina B. J., Stevenson B. S., Green W. J., and Schmidt T. M. (1998) Manganese reduction by microbes from the oxic regions of the Lake Vanda (Antarctica) water column. Appl. Environ. Microbiol. 64, 3791–3797. Byrne R. H. and Kim K. H. (1990) Rare earth element scavenging in seawater. Geochim. Cosmochim. Acta 54, 2645–2656. Byrne R. H. and Kim K. H. (1993) Rare earth precipitation and coprecipitation behavior: The limiting role of PO3⫺ 4 on dissolved rare earth concentrations in seawater. Geochim. Cosmochim. Acta 57, 519 –526. Byrne R. H. and Sholkovitz E. R. (1996) Marine chemistry and geochemistry of the lanthanides. In Handbook on the Physics and Chemistry of Rare Earths (eds. K. A. Gschneidner Jr. and L. R. Eyring), Vol. 23, pp. 497–593. Elsevier. Canfield D. E., Green W. J., and Nixon P. (1995) 210Pb and stable lead

through the redox transition zone of an Antarctic lake. Geochim. Cosmochim. Acta 59, 2459 –2468. Cantrell J. and Byrne R. H. (1987) Rare earth element complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta 51, 597– 605. De Baar H. J. W., Bacon M. P Brewer P. G., and Bruland K. W. (1985) Rare earth elements in the Pacific and Atlantic Oceans. Geochim. Cosmochim. Acta 49, 1943–1959. De Baar H. J. W. German C. R., Elderfield H., and Van Gaans P. (1988) Rare earth element distribution in anoxic waters of the Cariaco Trench. Geochim. Cosmochim. Acta 52, 1203–1219. De Carlo E. H., McMurtry G. M., and Kim K. H. (1987) Geochemistry of Ferromanganese crusts from the Hawaiian Archipelago, I: Northern survey sites. Deep-Sea Res. 34, 441– 467. De Carlo E. H. and McMurtry G. M. (1992) Rare earth element geochemistry of ferromanganese crusts from the Hawaiian Archipelago. Chem. Geol. 95, 235–250. De Carlo E. H., Resing J. A., Wen X. Y., and Cowen J. P. (1996) Rare earth elements in water samples collected over Loihi seamount during the August 1996 seismic event. Eos 77, 398. De Carlo E. H. and Resing J. A. (1998) Determination of picomolar concentrations of trace elements in high salinity fluids by FIA-ICPMS. ICP Information Newsl. 23,82– 83. De Carlo E. H., Wen X. -Y., and Irving M. (1998) The influence of redox reactions on the uptake of dissolved Ce by suspended Fe and Mn oxide particles. Aquat. Geochem. 3, 357–389. De Carlo E. H., Wen X. -Y., and Cowen J. P. (2000) Rare earth element fractionation in hydrogenetic Fe-Mn crusts: The influence of carbonate complexation and phosphatization on Sm/Yb ratios. In Marine Authigenesis: From Global to Microbial (eds. C. R. Glenn, L. Prevot-Lucas, and J. Lucas), pp. 271–285. Special Volume No. 64. SEPM. Erel Y. and Morgan J. J. (1991) The effect of surface reactions on the relative abundance of trace metals in deep-ocean water. Geochim. Cosmochim. Acta 55, 1807–1813. Feely R. A., Trefry J. H., Massoth G. J., and Metz S. (1991) A comparison of the scavenging of phosphorus and arsenic from seawater by hydrothermal iron oxyhydroxides in the Atlantic and Pacific Oceans. Deep-Sea Res. 38, 617– 623. German C. R. and Elderfield H. (1989) Rare earth elements in Saanich Inlet, Br. Columbia, a seasonally anoxic basin. Geochim. Cosmochim. Acta 53, 2561–2571. German C. R., Holliday B. P., and Elderfield H. (1991) Redox cycling of rare earth elements in the suboxic zone of the Black Sea. Geochim. Cosmochim. Acta 55, 3553–3558. Goldman C. R., Mason D. T., and Hobbie J. E. (1967) Two Antarctic desert lakes. Limnol. Oceanogr. 12, 295–310. Green W. J. and Canfield D. E. (1984) Geochemistry of the Onyx River (Wright Valley, Antarctica) and its role in the chemical evolution of Lake Vanda. Geochim. Cosmochim. Acta 48, 2457–2467. Green W. J., Canfield D. E., Lee G. F., and Jones R. A. (1986) Mn, Fe, Cu, and Cd distributions and residence times in closed-basin Lake Vanda (Wright Valley, Antarctica). Hydrobiologia 134, 237–248. Green W. J., Ferdelman T. G., and Canfield D. E. (1989) Metal dynamics in Lake Vanda (Wright Valley, Antarctica). Chem. Geol. 76, 85–94. Green W. J., Canfield D. E., Shenshong Y., Chave K. E., Ferdelman T. G., and Delanois G. (1993) Metal transport and release processes in Lake Vanda: The role of oxide phases. In Physical and Biological Processes in Antarctic Lakes (eds. W. J. Green and I. E. Friedmann), pp. 145–163. AGU. Green W. J., Canfield D. E., and Nixon P. (1998) Cobalt cycling and fate in Lake Vanda. In The McMurdo Dry Valley of Antarctica: A Cold Desert Ecosystem,pp. 205–215.AGU Antarctic Research Series. Hayes K. F. and Leckie J. O. (1986) Mechanism of lead ion adsorption at the goethite–water interface. In Geochemical Processes at Mineral Interfaces(eds. J. A. Davis and K. F. Hayes), ACS Symposium Series 323, pp. 114 –141. American Chemical Society. Henderson P. (1984) General geochemical properties and abundances of the rare earth elements. In Rare Earth Element Geochemistry (ed. P. Henderson), pp. 1–32. Elsevier. Hunter K. A. (1991) Surface charge and size spectra of marine parti-

Rare earth elements in Lake Vanda, Antarctica cles. In Marine Particles: Analysis and Characterization (eds. D. C. Hurd and D. W. Spencer), Geophysical Monograph 63, pp. 259 –262. American Geophysical Union. Johannesson K. H. and Zhou X. (1999) Origin of middle rare earth element enrichments in acid waters of a Canadian High Arctic lake. Geochim. Cosmochim. Acta 63, 153–165. Kadko D., Bacon M. P., and Hudson A. (1986 –1987) Enhanced scavenging of 210Pb and 210Po by processes associated with the East Pacific Rise near 8°45 N. Earth Planet. Sci. Lett. 81, 349 –357. Koeppenkastrop D. and De Carlo E. H. (1992) Sorption of rare earth elements from seawater onto pure mineral phases: An experimental approach. Chem. Geol. 95, 251–263. Koeppenkastrop D. and De Carlo E. H. (1993) Uptake of rare earth elements from solution by metal oxides. Environ. Sci. Technol. 27, 1796 –1802. Landing W. M., Haraldsson D and Paxeus. N. (1986) Vinyl polymer agglomerate based transition metal cation chelating ion-exchange resin containing the 8-hydroxyquinoline functional group. Anal. Chem. 58, 3031–3035. Lewis B. L., and Landing W. M. (1991) The biogeochemistry of manganese and iron in the Black Sea. Deep-Sea Res. 38, S773–S803. Lewis B. L., and Landing W. M. (1992) The investigation of dissolved and suspended trace metal fractionation in the Black Sea. Mar Chem. 40, 104 –141. Li Y.-H. (1991) Distribution patterns in the ocean: A synthesis. Geochim. Cosmochim. Acta 55, 3223–3240. Moller P. and Bau M. (1993) Rare earth patterns with positive Ce anomaly in alkaline waters from Lake Van, Turkey. Earth Planet. Sci. Lett. 117, 671– 676. Piepgras D. J. and Jacobsen S. B. (1992) The behavior of rare earth elements in seawater: Precise determination of variations in the North Pacific water column. Geochim. Cosmochim. Acta 56, 1851– 1862.

1333

Piper D. Z. (1974) Rare earth elements in ferromanganese nodules and other marine phases. Geochim. Cosmochim. Acta 38, 1007–1022. Ragotzkie R. A. and Likens G. E. (1964) The heat balance of tow Antarctic lakes. Limnol. Oceanogr. 9, 412– 425. Schijf J. (1992) Aqueous geochemistry of the rare earth elements in marine anoxic basins. Ph.D. dissertation. University of Utrecht. Sholkovitz E. R. (1995) The aquatic chemistry of rare earth elements in rivers and seawater. Aquat. Geochem. 1, 1–34. Sholkovitz E. R., Landing W. M., and Lewis B. L. (1994) Ocean particle chemistry: The fractionation of rare earth elements between suspended particles and seawater. Geochim. Cosmochim. Acta 58, 1567–1579. Sigg L. (1985) Metal transfer mechanisms in lakes: The role of settling particles. In Chemical Processes in Lakes (ed. W. Stumm), pp. 283–310. Wiley. Stage B. R. (1996) Transport and recycling of copper, cadmium, and nickel in an Antarctic Lake. M.S. thesis. Miami University, Ohio. Stanley J. K. and Byrne R. H. (1990) The influence of solution chemistry on REE uptake by Ulva lactucaL. in seawater. Geochim. Cosmochim. Acta 54, 1587–1595. Stumm W. (1992) Chemistry of the Solid–Water Interface. Wiley. Stumm W. and Morgan J. J. (1981) Aquatic Chemistry: An Introduction Emphasising Chemical Equilibria in Natural Waters. Wiley. Trefry J. H. and Metz S. (1989) Role of hydrothermal particles in the geochemical cycling of vanadium. Nature 342, 531–533. Vincent W. F., and Vincent C. L. (1982) Factors controlling phytoplankton production in Lake Vanda (77°S). Can. J. Fish. Aquat. Sci. 39, 1602–1609. Zhong S. and Mucci A. (1995) Partitioning of rare-earth elements (REE) between calcite and seawater solutions at 25°C, 1 atm, and high dissolved REE concentrations. Geochim. Cosmochim. Acta 59, 443– 453.