Comparison of arsenic and molybdenum geochemistry in meromictic lakes of the McMurdo Dry Valleys, Antarctica: Implications for oxyanion-forming trace element behavior in permanently stratified lakes

Chemical Geology 404 (2015) 110–125

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Comparison of arsenic and molybdenum geochemistry in meromictic lakes of the McMurdo Dry Valleys, Antarctica: Implications for oxyanion-forming trace element behavior in permanently stratified lakes Ningfang Yang a, Kathleen A. Welch b,d, T. Jade Mohajerin a, Katherine Telfeyan a, Darren A. Chevis a, Deborah A. Grimm c, W. Berry Lyons b,d, Christopher D. White a, Karen H. Johannesson a,⁎ a

Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA School of Earth Science, The Ohio State University, Columbus, OH, USA c Coordinated Instrumentation Facility, School of Science and Engineering, Tulane University, New Orleans, LA 70118, USA d Byrd Polar and Climate Research Center, The Ohio State University, Columbus, OH, USA b

a r t i c l e

i n f o

Article history: Received 3 December 2014 Received in revised form 30 March 2015 Accepted 31 March 2015 Available online 11 April 2015 Editor: Carla M Koretsky Keywords: Dry Valley Lakes Antarctica Arsenic Molybdenum Speciation Geochemical modeling

a b s t r a c t Water samples were collected for arsenic (As) and molybdenum (Mo) analysis from different depths in Lakes Hoare and Fryxell, both of which are located in the Taylor Valley within the McMurdo Dry Valleys of Antarctica. Sampling depths within each lake were chosen to capture variations in As and Mo concentrations and As speciation in the oxic mixolimnia and anoxic monimolimnia of these meromictic lakes. Arsenic concentrations ranged from 0.67 nmol kg−1 to 3.54 nmol kg−1 in Lake Hoare and from 1.69 nmol kg−1 to 17.5 nmol kg−1 in Lake Fryxell. Molybdenum concentrations varied between 5.05 nmol kg−1 and 43 nmol kg−1 in Lake Hoare, and between 3.52 nmol kg−1 and 25.5 nmol kg−1 in Lake Fryxell. Concentrations of As and Mo generally increased with depth in the mixolimnion of each lake, consistent with uptake near the ice–water interface by organic particles and/or Fe/Mn oxides/oxyhydroxides, followed by gravitational settling and regeneration/remineralization at depth in the vicinity of the redoxcline. Arsenic concentrations either remained constant (Hoare) or increased with depth (Fryxell) in the anoxic monimolimnia, whereas Mo exhibited dramatic decreases in concentrations across the redoxcline in both lakes. Geochemical modeling predicts that As and Mo occur as thioanions in the anoxic bottom waters of Lakes Hoare and Fryxell, and further that the contrasting behavior of both trace elements reflects the respective reactivity of their thioanions towards Fe-sulfide minerals such as mackinawite (FeS) and/or pyrite (FeS2). More specifically, the geochemical model suggests that Fe-sulfide mineral precipitation in the anoxic monimolimnia of both lakes regulates dissolved sulfide concentrations at levels that are too low for As-sulfide minerals (e.g., orpiment, realgar) to precipitate, whereas mackinawite and/or pyrite react(s) with particle reactive thiomolybdate anions, possibly forming an Fe–Mo–S mineral that precipitates and, hence, leads to Mo removal from solution. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Understanding the geochemical cycling of trace elements in the environment is critically important from a number of standpoints including the fact that many are highly toxic to organisms, including humans, whereas others are important trace nutrients, and hence, necessary in maintaining healthy metabolisms (Alloway, 2012; Hodson, 2012). For example, within the ocean a number of trace elements are important micronutrients for phytoplankton, and many of these organisms have evolved mechanisms to regulate trace element concentrations via release of strong metal-binding ligands, and/or facilitate redox reactions

⁎ Corresponding author. Tel.: +1 504 862 3193. E-mail address: [email protected] (K.H. Johannesson).

http://dx.doi.org/10.1016/j.chemgeo.2015.03.029 0009-2541/© 2015 Elsevier B.V. All rights reserved.

that promote trace element cycling in surface waters (e.g., Bruland et al., 1991; Morel and Price, 2003). Furthermore, study of the biogeochemical cycles of trace elements, as well as their isotopes (e.g., 143Nd/144Nd, δ98Mo), has contributed to society's understanding of climate change, carbon cycling, ocean anoxia, ecosystem functioning, and, of course, anthropogenic contamination, and it is generally agreed that trace elements are critical tracers of processes occurring in the oceans today, as well as in the past (GEOTRACES Planning Group, 2006). Nonetheless, uncertainties in the sources, sinks, chemical speciation, and cycling within the oceans and other water bodies limits our ability to fully exploit trace elements in studies of the environment. Moreover, quantitative investigations of trace element cycling in the environment are commonly hampered by difficulties in separating anthropogenic inputs from natural sources and the corresponding processes that promote mobilization of trace elements in the environment (e.g., Chappaz et al.,

N. Yang et al. / Chemical Geology 404 (2015) 110–125

2008). As the oceans are large scale, complex systems, investigating the chemical speciation and biogeochemical cycling of trace elements in smaller, less complex settings represents a promising approach for isolating and quantifying fundamental processes that play important roles in the biogeochemical cycling of trace elements in the environment. Similar issues apply to watersheds where complex hydrology and biology can further confound efforts to decipher trace element cycles owing to mixing of different water sources (e.g., mixing of surface water with groundwaters, overland flow, and interflow), lake turnover, losses from evapotranspiration, precipitation events, and bioturbation, among others. Because of these complications the McMurdo Dry Valleys in Antarctica are ideal settings to study the biogeochemical cycling of trace elements because their relatively simple biogeochemistry and hydrology, which consists of low amounts of annual precipitation (b 10 cm yr−1 water equivalent as snow), lack of higher, vascular plants (although mosses and lichen have been reported), low anthropogenic impact, and the fact that groundwater is essentially absent from these environments, substantially simplify “source-to-sink” investigations of trace elements (Chinn, 1993; Runkel et al., 1998; McKnight et al., 1999; Nezat et al., 2001; DeCarlo and Green, 2002; Doran et al., 2002; Gooseff et al., 2013). Previous research on trace metals in the McMurdo Dry Valley lakes involved studies of transition metals such as iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb), and mercury (Hg), with the majority of this work conducted during the late 1980s and into the 2000s by W. J. Green with his students and collaborators (e.g., Green et al., 1986a, b, 1989a, b, c, 1993, 1998, 2004, 2005). These previous investigations have proven extremely important in our understanding of the behavior of transition metals and Hg in these systems. For example, the Hg investigations indicate that the dissolved concentrations are extremely low and reflect little, if any, anthropogenic influence (Lyons et al., 1999a). However, all of these metals are predominately cations in solution and unless complexed or associated with dissolved organic matter, at circumneutral pH they are subject to adsorption onto particulate material, especially the oxides/ oxyhydroxides of Fe and Mn (Green et al., 1993, 2004; DeCarlo and Green, 2002). In contrast, trace elements that form oxyanions such as arsenic (As), antimony (Sb), selenium (Se), molybdenum (Mo), tungsten (W), and vanadium (V), typically occur as oxoacids or negatively charged oxyanions in solution, the form of which, and hence their reactivity, being directly related to the dissociation constants of the oxoacids (Hingston et al., 1967). Furthermore, oxyanion-forming trace elements such as Mo, Se, and V, play important roles in biological processes (e.g., Mo in N2 fixation), whereas others like As, Sb, and probably W, are toxic to many organisms including humans (e.g., see Fraústo da Silva and Williams, 1991; Kletzin and Adams, 1996; Hille, 2002; McEwan et al., 2002; Oremland and Stolz, 2003; Fordyce, 2005; Hughes et al., 2009; Kelly et al., 2013). Nevertheless, despite their toxicity many bacteria and some eukaryotic phytoplankton have developed strategies for coping with elevated concentrations of oxyanion-forming trace elements that include intracellular reduction and methylation, whereas others use oxidized and reduced forms as electron acceptors or donors, respectively, to gain energy for growth (Andreae and Klumpp, 1979; Sanders and Windom, 1980; Oremland and Stolz, 2003, 2005). To the best of our knowledge, other than Mo (e.g., Boswell et al., 1967; Weand et al., 1976), there have been no published values of oxyanionforming trace elements in the McMurdo Dry Valley lakes. Consequently, in this contribution we present concentration data for two oxyanionforming trace elements (i.e., As and Mo) from lakes within the McMurdo Dry Valleys. Specifically, we focus on Lakes Hoare and Fryxell within the Taylor Valley, and compare the concentrations and speciation data (As only) of these oxyanion-forming trace elements to geochemical features as well as measures of primary production, and hypothesize potential controls on these trace elements. More detailed investigations of As, Sb, Se, Mo, W, and V in these and other lakes of the dry valleys are planned for the future.

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2. Hydrologic setting The hydrologic cycle in the McMurdo Dry Valleys is driven by glacial melt during the short, austral summer (4–12 weeks) and transport in small streams and their associated hyporheic zones (Green and Canfield, 1984; Lyons et al., 1998a; Fountain et al., 1999; Nezat et al., 2001; Gooseff et al., 2002; Maurice et al., 2002). Water subsequently leaves the lakes via sublimation of ice that covers a number of perennially ice-covered, terminal lakes in the region (e.g., Lyons et al., 1998b). Many of these lakes, including Lakes Hoare and Fryxell in the Taylor Valley (Fig. 1), have been studied since the early 1960s (Angino et al., 1962a), and during the 1980s a number of extensive biogeochemical investigations by scientists from both the United States and New Zealand occurred (e.g. Vincent, 1981; Wharton et al., 1982; Green et al., 1988). The McMurdo Dry Valleys Long-Term Ecological Research (MCM-LTER) site project was established in 1992–1993 and with it a longer term perspective on these lakes, their evolutionary history, and how ecological and biogeochemical processes respond to climatic variations (i.e., Priscu, 1995; Lyons et al., 2000, 2005; Foreman et al., 2004). Most recently a series of review articles have summarized much of the geochemical work that is based on major cation/anion and nutrient analyses (Lyons et al., 2006; Lyons and Finlay, 2008; Green and Lyons, 2009). Yet in spite of all these years of investigation, little is still known about the biogeochemistry of many elements, particularly trace metals, and especially the oxyanion-forming metals and metalloids in these systems. Both Lake Hoare and Lake Fryxell are closed-basin and chiefly meromictic lakes. Lake Fryxell is brackish with a euxinic (i.e., anoxic and sulfidic) monimolimnion that begins at mid-depth (i.e., ~9 m depth; Green et al., 1989a), whereas Lake Hoare is relatively fresh and only exhibits anoxia below ~27 m depth (Green et al., 1986a). Using bathymetric data for Lakes Hoare and Fryxell (i.e., Priscu, 2015), we estimate that the monimolimnion accounts for between 2 and 5% of Lake Hoare's water volume, whereas the monimolimnion of Lake Fryxell represents ~47% of the lake water by volume (Table 1). The estimate for Lake Hoare is in close agreement with an earlier estimate of Green et al. (1986a). Lakes Hoare and Fryxell are young but the higher salinity in Lake Fryxell is due to the diffusion of brine into the lake from the subsurface (Lyons et al., 1998a). Another major difference is that input to Lake Fryxell is dominated by a number of glacial meltwater streams within the basin (Fig. 1). Most of these streams are of low gradient and have extensive hyporheic zone exchange with enhanced chemical weathering and microbial biogeochemical processes like nitrification and denitrification (Gooseff et al., 2002, 2003; Maurice et al., 2002; McKnight et al., 2004). In contrast, much of the water that flows into Lake Hoare comes directly from melting of the Canada Glacier (Fig. 1). A summary of the physical characteristics of these lakes is given in Table 1, but a much more detailed description can be found in Spigel and Priscu (1998). 3. Methods 3.1. Sample collection and analysis All sample bottles (HDPE), chromatography columns, and labware used for trace element sampling and analysis were rigorously cleaned prior to use by standard trace element cleaning methods (e.g., Johannesson et al., 2004). Water samples were collected from four depths in both Lake Hoare and Lake Fryxell to capture the oxic mixolimnion and anoxic monimolimnion of each lake. Water samples were collected from Lake Hoare on 10 December 2009, whereas the Lake Fryxell samples were collected on 7 December 2009. Lake water samples were filtered on-site through 0.4 μm Nuclepore® filters during collection by the MCM-LTER “limno team”. The filtered water samples were then transported by helicopter back to the Crary Laboratory at McMurdo Station where they were acidified with ultrapure HNO3 (Seastar Chemicals Inc., Baseline®) to pH b 2 within 12 h of collection.

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N. Yang et al. / Chemical Geology 404 (2015) 110–125

Fig. 1. Map of Taylor Valley within the McMurdo dry valley region of Antarctica showing locations of major features discussed within the text including Lakes Hoare and Fryxell, Anderson Creek, and Canada Stream. Map is after Fortner et al. (2005).

The sample bottles containing the filtered and acidified water samples were then double bagged in pre-cleaned polyethylene, Zip-lock®-style bags, and stored cold at ~4 °C until analysis. Filtered lake water samples for As species analysis were collected in amber HDPE bottles to prevent photocatalyzed oxidation of As(III) (e.g., McCleskey et al., 2004). Before acidification to pH b 2 with ultrapure HNO3 at the Crary Laboratory, a separate aliquot (~100 mL) of filtered lake water was acidified to pH ~ 4 with ultrapure HNO3 and then passed through an anion-exchange column (Bio-Rad Poly-Prep 0.8 × 4 cm chromatography columns) packed with Bio-Rad AG® 1 × 8, 50–100 mesh, anion-exchange resin converted to the acetate form (Ficklin, 1983; Wilkie and Hering, 1998; Leybourne et al., 2014). At pH ~ 4, As(V) is retained on the exchange resin owing to the negative charge of the 0 chief species, H2AsO− 4 , whereas As(III), which occurs as H3AsO3, passes through the column and is collected in a separate amber HDPE bottle (Wilkie and Hering, 1998; Haque and Johannesson, 2006; Datta et al., 2011). The eluted fraction was then acidified to pH b 2 with ultrapure HNO3, and stored cold, as described above, until analysis. Concentrations of As species [i.e., As(III), As(V)], Mo, Fe, and Mn were determined at Tulane University using a high resolution (magnetic sector) inductively coupled plasma mass spectrometer (Thermo Fisher Element II) after appropriate dilution with ultra-pure MilliQ water

(18.2 MΩ cm), and following procedures described previously (Datta et al., 2011; Willis et al., 2011; Johannesson et al., 2013; Mohajerin et al., 2014). Water samples were spiked with 4 μg kg−1 of In and Re as internal standards to monitor instrument drift during analysis. The instrument was calibrated and the sample concentrations verified by employing mixed As, Mo, Fe, and Mn calibration standards ranging from 1 ng kg−1 to 1000 ng kg−1 that were prepared from NIST traceable, High Purity (Charleston, SC, USA) single element standards. Check standards prepared from Perkin-Elmer multielement solutions were analyzed during all sample runs to certify accuracy. The National Research Council Canada (Ottawa, Ontario, Canada) Standard Reference Material for trace elements in river waters (SLRS-4) was also analyzed during all sample analysis runs to further ensure accuracy. In general, analytical precision for all trace elements was typically better than 5% RSD (relative standard deviation) for all sample analyses. Major ion chemistry, nutrient concentrations, and ancillary biogeochemical data (e.g., chlorophyll a) were obtained from the McMurdo Dry Valleys Long-Term Ecological Research web site (MCM-LTER; http://www.mcmlter.org). All of the data obtained from the MCM-LTER web site are for the sample collection periods that correspond to our specific lake sampling dates (i.e., 10 December 2009 for Lake Hoare, 7 December 2009 for Lake Fryxell).

Table 1 Physical characteristics of Lakes Fryxell and Hoare from Taylor Valley, Antarcticaa. Elevation (m) Fryxell 18 Hoare 73 a b c d

Maximum Distance Surface area Total Monimolimnion volumeb volume depth from sea (km2) (106 m3) (106 m3) (m) (km) 20 34

9 15

7.1 ~2

25.2 17.5

Spigel and Priscu (1998). Pugh et al. (2003) and Priscu (2015). Does not include ice. Computed from bathymetric data (Priscu, 2015). Based on estimate in Green et al. (1986a).

11.8c 0.44c, 0.88d

Maximum TDS Characteristics (mg kg−1)

History

6.2 0.7

Evaporated to playa Few thousand years Current glacier melt ~1000

Brackish monimolimnion Fresh

Relative age (a)

N. Yang et al. / Chemical Geology 404 (2015) 110–125

3.2. Geochemical modeling Geochemical modeling of As and Mo in waters from Lakes Hoare and Fryxell was conducted using the SpecE8 and React programs of the Geochemist's Workbench® (release 9.0; Bethke and Yeakel, 2013). The Lawrence Livermore National Laboratory database (Delaney and Lundeen, 1989) provided with the software (i.e., thermo.dat) was revised by recasting the arsenic species in terms of the dissociation of arsenous (H3AsIIIO3) and arsenic (H3AsVO4) acid using the dissociation constants from Nordstrom et al. (2014) as listed in Table 2. Solubility data for a number of As- and Fe-sulfide minerals were also added to the database,

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as well as a Fe–Mo–S mineral (i.e., Fe5Mo3S14) recently proposed by Helz et al. (2011). In addition, sulfidation reactions for Mo from Erickson and Helz (2000) and for As from Helz and Tossell (2008) were included in the modified database (Table 2). It is important to recognize that although the model of Helz and Tossell (2008) is preliminary and represents the results of an ab initio computational approach, these thermodynamic data are the most current data available for modeling As speciation in sulfidic waters (e.g., Kirk et al., 2010). To compute the speciation of As in the lake waters, our model assumes that the measured As(III) concentration represents the sum of all arsenite and thioarsenite species in a given lake water sample, and the measured

Table 2 Geochemical reactions added to the thermodynamic database for the Geochemist's Workbench®. Reaction type Redox Hydrolysis

Sulfidation

Complexation Ionization

Mineral solubility

a

Reactions H3AsO04

H3AsO03

↔ + 0.5O2(aq) Arsenous acid 0 − H3AsO3 ↔ H2AsO3 + H+ 2− + H+ H2AsO− 3 ↔ HAsO3 Arsenic acid + H3AsO04 ↔ H2AsO− 4 +H 2− + H+ H2AsO− 4 ↔ HAsO4 ↔ AsO3− + H+ HAsO2− 4 4 Molybdic acid + H2MoO04 ↔ HMoO− 4 +H 2− + H+ HMoO− 4 ↔ MoO4 As(III) H3AsO03 + H2S(aq) ↔ H3AsSO02 + H2O H3AsSO02 + H2S(aq) ↔ H2AsS2O0 + H2O H2AsS2O0 + H2S(aq) ↔ H2AsS03 + H2O As(V) H3AsO04 + H2S(aq) ↔ H3AsSO03 + H2O H3AsSO03 + H2S(aq) ↔ H3AsS2O02 + H2O H3AsS2O02 + H2S(aq) ↔ H3AsS3O0 + H2O H3AsS3O0 + H2S(aq) ↔ H3AsS04 + H2O Mo(VI) + H2S(aq) ↔ MoSO2− + H2O MoO2− 4 3 + H2S(aq) ↔ MoS2O2− + H2O MoSO2− 3 2 2− 2− MoS2O2 + H2S(aq) ↔ MoS3O + H2O 2− 2− MoS3O + H2S(aq) ↔ MoS4 + H2O + Fe2+ + 2H2O ↔ [FeO(OH)MoS4]3− + 3H+ MoS2− 4 + 2Fe2+ + 2H2S(aq) ↔ [(Fe2S2)(MoS4)2]4− + 4H+ 2MoS2− 4 As(III) + H3AsSO02 ↔ H2AsSO− 2 +H 2− + H+ H2AsSO− 2 ↔ HAsSO2 H3AsS2O0 ↔ H2AsS2O− + H+ H2AsS2O− ↔ HAsS2O2− + H+ + H3AsS03 ↔ H2AsS− 3 +H 2− + H+ H2AsS− 3 ↔ HAsS3 As(V) + H3AsSO03 ↔ H2AsSO− 3 +H 2− + H+ H2AsSO− 3 ↔ HAsSO3 ↔ AsSO3− + H+ HAsSO2− 3 3 + H3AsS2O02 ↔ H2AsS2O− 2 +H + 2− ↔ HAsS O + H H2AsS2O− 2 2 2 ↔ AsS2O3− + H+ HAsS2O2− 2 2 H3AsS3O0 ↔ H2AsS3O− + H+ H2AsS3O− ↔ HAsS3O2− + H+ HAsS3O2− ↔ AsS3O3− + H+ + H3AsS04 ↔ H2AsS− 4 +H 2− + H+ H2AsS− 4 ↔ HAsS4 ↔ AsS3− + H+ HAsS2− 4 4 Arsenic − + As2S3(orpiment) + 6H2O ↔ 2H2AsO− 3 + 3HS + 5H − + As2S3(amorphous) + 6H2O ↔ 2H2AsO− 3 + 3HS + 5H + − AsS(realgar) + 0.125SO2− + 2.5H2O ↔ H2AsO− 4 3 + H + 1.125HS + As(α) + 0.75O2(aq) + 1.5H2O ↔ H2AsO− 3 +H + As2O3(arsenolite) + 3H2O ↔ 2H2AsO− 3 + 2H + + 2H As2O3(claudetite) + 3H2O ↔ 2H2AsO− 3 + 6H+ As2O5(c) + 3H2O ↔ 2AsO3− 4 Iron FeS(mackinawite) + H+ ↔ Fe2+ + HS− Fe3S4(greigite) + 3H+ ↔ 3Fe2+ + 3HS− + S0 Molybdenum FeMo0.6S2.8(s) + 0.6H2S(aq) ↔ FeS(mackinawite) + 0.6MoS2− + 1.2H+ 4

298.15 K, ionic strength = 0 mol kg−1.

log Ka

Reference

−23.65

Nordstrom and Archer (2003), Kirk et al. (2010)

−9.24 −14.1

Nordstrom et al. (2014) Nordstrom and Archer (2003)

−2.25 −6.98 −11.58

Nordstrom et al. (2014) Nordstrom et al. (2014) Nordstrom et al. (2014)

−4.0 −4.24

Smith and Martell (2004) Smith and Martell (2004)

0.4 3.8 5.6

Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008)

11.0 0.1 3.5 2.6

Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008)

5.19 4.8 5.0 4.88 −16.9 −2.78

Erickson and Helz (2000) Erickson and Helz (2000) Erickson and Helz (2000) Erickson and Helz (2000) Helz et al. (2014) Helz et al. (2014)

−3.7 −14.1 −3.7 −8.6 −3.7 −8.6

Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008) Helz and Tossell (2008)

−3.3 −7.2 −11.0 2.4 −7.1 −10.8 1.7 −1.5 −10.8 2.3 −1.5 −5.2

Thilo et al. (1970) Thilo et al. (1970) Thilo et al. (1970) Helz and Tossell (2008) Thilo et al. (1970) Thilo et al. (1970) Helz and Tossell (2008) Helz and Tossell (2008) Thilo et al. (1970) Helz and Tossell (2008) Helz and Tossell (2008) Thilo et al. (1970)

−64.77 −63.35 −25.17 42.7 −19.84 −19.92 −28.23

Webster (1990) Eary (1992) Nordstrom and Archer (2003) Nordstrom and Archer (2003) Nordstrom et al. (2014) Nordstrom and Archer (2003) Nordstrom and Archer (2003)

−3.5 −12.84

Rickard (2006) Morse et al. (1987)

−11.7

Helz et al. (2011)

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N. Yang et al. / Chemical Geology 404 (2015) 110–125

As(V) concentration is the sum of all arsenate and thioarsenate species in the water sample. Although we acknowledge that this approach is not perfect (e.g., Jay et al., 2004), without measurements of zerovalent sulfur in the anoxic monimolimnia of these lakes, the model cannot rigorously compute the equilibrium As(III)/As(V) distribution in the lake bottom waters (Helz, 2014). Instead, the available data require us to predict the arsenite–thioarsenite and arsenate–thioarsenate speciation independent of each other (Helz, 2014, written comm.). Despite these limitations, the model can predict the speciation of As(III) and As(V) oxyanions and thioanions at the order of magnitude level in sulfidic waters, facilitating an improved understanding of these difficult to quantify species (Planer-Friedrich et al., 2007, 2010; Helz and Tossell, 2008; Kirk et al., 2010). Another drawback is that because the sulfidation, complexation, and ionization reactions listed in Table 2 lack enthalpy data, the geochemical modeling was conducted assuming a temperature of 25 °C. Nevertheless, we tested the impact of this assumption for the hydrolysis reactions listed in Table 2 (for which enthalpy data exist) by comparing the predicted distributions of As(III), As(V), and Mo(VI) species in the oxic lake water samples at 25 °C with those computed using the in situ lake water temperatures (Table 3). Predicted As(III) species distributions at 25 °C differed, on average, by ± 15% from the distributions predicted using the actual lake water temperatures, whereas the predicted As(V) species differed by ±5%, on average, between the model simulations conducted at 25 °C and those computed using the actual lake water temperatures. More specifically, at 25 °C the model slightly under predicts the importance of the H3AsO3 species (b 12% difference) and over predicts the importance of the H2AsO− 3 species (b30% difference). The model also slightly under predicts the importance of the 2− H2AsO− 4 species at 25 °C (b10% difference), whereas the HAsO4 species (b1% difference) is relatively insensitive to temperature differences. The results for MoO24 − distributions were also insensitive to temperature (i.e., MoO24 − always accounted for 100% of dissolved Mo) in the oxic waters of both lakes. Consequently, with these caveats in mind, we submit that the geochemical model is useful for predicting the possible speciation and solubility behavior of the various As and Mo species and minerals in these lakes, giving us a first approximation of As and Mo speciation in the anoxic monimolimnia of both lakes.

4. Results 4.1. Hydrography and geochemistry Major ion and nutrient concentrations for water samples corresponding to the trace element samples collected for this study are presented in Table 3. Again, these data were obtained from the MCM-LTER website (http://www.mcmlter.org). A number of key biogeochemical parameters for Lakes Hoare and Fryxell are plotted as a function of depth in Figs. 2 and 3, respectively. Because the major solute and nutrient geochemistry of the dry valley lakes has been the focus of many previous investigations, we only highlight some of the chief hydrographic and geochemical features of these lakes, and refer the reader to the numerous previous studies for more details (e.g., Angino et al., 1962a; Matsubaya et al., 1979; Wilson, 1979; Green et al., 1989a; McKnight et al., 1991, 2004; Lyons and Mayewski, 1993; Wharton et al., 1993; Priscu, 1995; Lyons et al., 1998b; Pugh et al., 2003; Barrett et al., 2007; Green and Lyons, 2009). Briefly, Figs. 2 and 3 illustrate the shallower depth and brackish nature of Lake Fryxell (~ 18 m) compared to Lake Hoare (~ 30 m). Specifically, the ionic strength of Lake Frxyell waters ranges from 0.004 mol kg − 1 at the ice–water interface to 0.17 mol kg− 1 in the euxinic monimolimnion. In comparison, the ionic strength of Lake Hoare water is generally lower than Lake Fryxell water (i.e., 0.003 ≤ I ≤ 0.013 mol kg−1). Both lakes exhibit anoxic bottom waters, but the oxic–anoxic boundary (i.e., redoxcline) occurs at shallower depth in Lake Fryxell (~ 9 m) compared to Lake Hoare (~27 m). Dissolved Fe and Mn concentrations generally increase with increasing depth in both lakes (Figs. 2, 3). The only exception is Mn in Lake Fryxell, which exhibits a water column maximum (3 μmol kg− 1) at 10 m depth, approximately 1 m below the redoxcline, and subsequently decreases by a factor of 1.7 by the bottom of the lake. Although both lakes have anoxic monimolimnia, dissolved sulfide concentrations reach 1.2 mmol kg−1 in the bottom waters of Lake Fryxell compared to only 32 μmol kg−1, or a factor of ~ 38 lower, at the bottom of Lake Hoare (Figs. 2, 3). Dissolved sulfide concentrations shown on Fig. 3 and presented in Table 4 for Lake Hoare are from Green et al. (1986a), whereas those for Lake Fryxell were taken from Green et al. (1989a). Dissolved S(-II) concentrations in the Lake Fryxell water column

Table 3 Major ions, nutrients, and chlorophyll-a concentrations in Lakes Hoare and Fryxella.

pH Temp (°C) Cond. (mS cm−1) mol kg−1 Ionic strength mmol kg−1 Ca2+ Mg2+ Na+ K+ Cl− DIC SO2− 4 μmol kg−1 DOC H4SiO4 SRP NO− 3 NO− 2 NH+ 4 −1 μg L Chl-a a

Hoare (5 m)

Hoare (12 m)

Hoare (20 m)

Hoare (30 m)

Fryxell (5 m)

Fryxell (8 m)

Fryxell (10 m)

Fryxell (18 m)

8.97 0.62 0.24

8 0.64 0.6

7.39 0.26 0.71

7.24 0.18 0.84

8.04 1.84 0.47

7.91 2.92 2.64

7.62 2.91 3.85

7.45 2.18 8.25

0.003

0.013

0.016

0.019

0.004

0.033

0.055

0.173

0.3 0.25 0.9 0.1 0.78 0.8 0.16

1.36 0.86 5.22 0.53 4.49 4.2 0.92

1.79 1.14 6.74 0.66 5.64 5.9 1.04

2.01 1.33 7.66 0.73 6.46 7.9 0.92

0.35 0.34 2 0.14 1.72 1.4 0.11

2.09 2.48 19.7 1.12 17.1 10.6 0.89

2.72 4.36 35.9 1.87 30.1 17.6 1.31

3.45 14.2 123 4.99 107 58.5 1.07

63 38.8 0.02 0.59 0.11 0.22

168 189 0.08 0.06 0.08 0.02

187 235 0.16 4.42 0.11 0.01

215 250 4.53 5.97 0.07 31.4

87 128 0.06 0.1 0.03 0.18

325 248 0.19 0.03 0.05 –

519 361 0.19 0.05 0.07 0.15

1910 562 44.7 3.19 0.09 526

0.85

1.87

1.16

0.49

2.15

3.79

14.1

0.58

All data from the McMurdo Dry Valley Long Term Ecological Research website (LTER: http://www.mcmlter.org) for the 2009–2010 field season. Lake Hoare was sampled on 10 December 2009, and Lake Fryxell was sampled on 7 December 2009.

N. Yang et al. / Chemical Geology 404 (2015) 110–125

115

Lake Hoare 0

0 ICE COVER

ICE COVER

ICE COVER

ICE COVER

5

5

Depth (m)

10

10

PPR Chl-a

15

15

pH Temp.

20

Depth (m)

S(-II) Fe Mn DO NH4

20

I H4SiO4

25

25 Oxic Anoxic

Oxic Anoxic

Oxic Anoxic

Oxic Anoxic

30

30

a 7.0

b 7.5

8.0

8.5

9.0

0.0

0.2

0.4

0.6

0.5

0.8

Temperature ( C)

1.0 0

10

20

30 -1

Fe and Mn (µmol kg )

0.01

0.02

0.0

Ionic Strength (mol kg ) 0

50

100

150

200 -1

H4SiO4 (µmol kg )

250

0.5

1.0

1.5

2.0

2.5

µg Chl-a L-1

-1

Dissolved O2 (mmol kg )

o

d

0.00

1.0 -1

pH 0.0

c

0.0

0.1

0.2

0.3

Primary Production Rate µg C L-1 day-1

Fig. 2. Lake Hoare profiles for a variety of geochemical and biological properties plotted as a function of depth. All data are from McMurdo Dry Valleys Long Term Ecological Research site (http://www.mcmlter.org; date of collection, 10 December 2009), except for Fe, Mn (this study) and dissolved sulfide (Green et al., 1986a). Ionic strength (I; mol kg−1) is shown on panel c. Dashed, blue horizontal line at 27 m depth delineates the approximate depth of the transition between the predominantly oxic mixolimnion (above) and the anoxic monimolimnion (below), and is based on Green et al. (1986a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

determined by Howes and Smith (1990) are in good agreement with those of Green et al. (1989a). Ammonium concentrations are also elevated in the bottom waters of both lakes, reaching 526 μmol kg−1 and 31.4 μmol kg−1 in Lakes Fryxell and Hoare, respectively (Table 3). Chlorophyll-a concentrations and primary production rates are markedly higher in Lake Fryxell compared to Lake Hoare (Figs. 2, 3). Lake Fryxell is recognized as one of the most productive lakes in the McMurdo Dry Valleys, whereas Lake Hoare is the least productive (e.g., Takacs and Priscu, 1998; Pugh et al., 2003). Both lakes exhibit deep chlorophyll maxima (DCM; Roberts et al., 2004a) that occur below the depth where the rate of primary production is greatest. Specifically, Lake Fryxell exhibited a pronounced DCM of 14.1 μg Chl-a L−1 at 10 m (7 December 2009), whereas the DCM in Lake Hoare (10 December 2009) was more diffuse and only reached 2.2 μg Chl-a L−1 at 14 m depth (Figs. 2, 3). Previous work suggests that the DCM in these lakes chiefly reflects phytoplankton that are dominantly cryptophytes (Roberts et al., 2004a, 2004b). The primary production rate in both lakes is highest within the first 5 m of the water column directly beneath the ice–water interface, and subsequently decreases with increasing depth in both lakes (Figs. 2, 3). For example, the mean (±σ) primary production rate in the top 5 m of the water column in Lake Hoare during our sampling campaign was 0.25 ± 0.05 μg C L−1 day−1, whereas it was 0.88 ± 0.11 μg C L−1 day−1 for the top 5 m in Lake Fryxell (Figs. 2, 3; http://www.mcmlter.org). Arsenic and Mo concentrations in waters from Lakes Hoare and Fryxell are presented in Table 4, and plotted as a function of lake depth

in Fig. 4. Total dissolved As concentrations range from 0.67 nmol kg−1 to 3.54 nmol kg−1 in Lake Hoare, and from 1.60 nmol kg−1 to 17.5 nmol kg−1 in Lake Fryxell (Table 4). The lowest As concentrations occur at the ice–water interface in both lakes (i.e., 5 m). Arsenic concentrations generally increase with depth in both lakes, although the As concentration for the bottom of Lake Hoare (30 m) is slightly lower (i.e., 1.2-fold) than in the overlying 20 m water sample. Arsenite [i.e., As(III)] is the predominant form of As detected in waters at the ice–water interface in Lake Hoare, whereas our measurements indicate that As(V) is more abundant in all of the deeper lake waters of Lake Hoare, including the anoxic bottom waters at 30 m depth (Table 4, Fig. 4). In contrast, in Lake Fryxell As(V) predominates in the oxic mixolimnion and As(III) is more abundant in the anoxic, sulfidic monimolimnion (Fig. 4). Molybdenum concentrations range between 5.05 nmol kg−1 and 43.1 nmol kg− 1 in Lake Hoare, and between 3.52 nmol kg− 1 and 25.5 nmol kg−1 in Lake Frxyell (Table 4, Fig. 4). Molybdenum concentrations are similar at the ice–water interface in both lakes (~5 nmol kg−1) and increase with increasing depth in the oxic water columns of the mixolimnion, reaching intermediate-depth maxima in both lakes (20 m in Lake Hoare and 8 m in Lake Fryxell; Fig. 4). Molybdenum concentrations decrease, however, in the anoxic bottom waters of both lakes such that the lowest Mo value in Lake Fryxell (3.52 nmol kg−1) occurs near the lake bottom. Specifically, Mo decreases by a factor of ~ 6 across the redoxcline in Lake Hoare and by more than 7-fold across the redoxcline in Lake Fryxell.

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Lake Fryxell 0

0 ICE ICE COVER COVER

ICE COVER

ICE COVER

ICE COVER

5

5

Oxic

Oxic

Anoxic

Anoxic

Oxic

Oxic

Anoxic

Anoxic

10

10

DO Mn S(-II) Fe

15

15 H4SiO4

PPR Chl-a

I

20

a

b

7.4

7.6

7.8

8.0

1.5

2.0

0.5

1.0

0.00

d 0.05

0.10

0.15

20

0

5

Dissolved O2 and HS- (mmol kg-1) Ionic Strength (mol kg-1)

pH 1.0

c

0.0

2.5

3.0

o

Temperature ( C)

0

1

Depth (m)

Depth (m)

pH Temp.

2

3

100 -1

Fe and Mn (µmol kg )

200

300

400

500 -1

H4SiO4 (µmol kg )

10

15

µg Chl-a L-1 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Primary Production Rate µg C L-1 day-1

Fig. 3. Lake Fryxell profiles for a variety of geochemical and biological properties plotted as a function of depth. All data are from McMurdo Dry Valleys Long Term Ecological Research site (http://www.mcmlter.org; date of collection, 7 December 2009), except for Fe, Mn (this study) and dissolved sulfide (Green et al., 1989a). Dashed, blue horizontal line at 9 m depth delineates the approximate depth of the transition between the oxic mixolimnion (above) and the anoxic and sulfidic (i.e., euxinic) monimolimnion (below), and is based on Green et al. (1989a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Arsenic and Mo concentrations for waters from other closed-basin lakes from around the world are presented in Table 5, along with mean concentrations of these trace elements in seawater. The outstanding feature of the closed-basin lake data presented in Table 5 is that As and Mo are orders of magnitude greater in waters from the majority of these lakes than in Lakes Hoare and Fryxell. Indeed, only Lake Awassa in the Ethiopian rift valley has broadly similar As and Mo concentrations to Lakes Hoare and Fryxell (Zinabu and Pearce, 2003). Sasyk-kul lake in central Asia also has similar Mo concentrations to the ice–water interface (i.e., 5 m) and bottom water samples from both Hoare and Fryxell (Lyons et al., 2001). The concentrations of As and Mo in Lakes Hoare

and Fryxell are more similar to those of seawater than the majority of the closed-basin lakes for which As and Mo have been reported. Nevertheless, As concentrations in Lakes Hoare and Fryxell are, on average, a factor of 9 and 2.7 times lower, respectively, than in seawater (Tables 4, 5). The chief exception is the brine at the bottom of Lake Fryxell where the total As concentration (17.5 nmol kg−1) approaches the seawater value (i.e., 23 nmol kg−1). The Mo concentrations in waters from both lakes are also lower than the seawater concentration (~105 nmol kg−1). Specifically, the average Mo concentrations of water from Lakes Hoare and Fryxell are close to 5- and 8-fold lower, respectively, than in seawater.

Table 4 Concentrations of trace elements and dissolved sulfide in Dry Valley lake watersa.

Hoare

Fryxell

a b c

Depth (m)

Fe (μmol kg−1)

Mn (μmol kg−1)

S(-II)b (μmol kg−1)

AsT (nmol kg−1)

As(III) (nmol kg−1)

As(V)c (nmol kg−1)

Mo (nmol kg−1)

5 12 20 30 5 8 10 18

0.36 1.97 1.79 3.4 0.54 0.36 0.9 3.04

0.09 0.49 0.46 3.09 0.27 0.82 3 1.75

− − − 32.4 − − 319 1210

0.67 3.17 3.54 2.91 1.69 6.15 8.24 17.5

0.53 1.33 1.13 0.8 0.39 − 6.38 11.4

0.14 1.84 2.41 2.11 1.3 6.15 1.86 6.1

5.05 35.2 43.1 7.13 5.89 25.5 18.1 3.52

Sampling dates are as in Table 1. From Green et al. (1986a, 1989a). As(V) = AsT − As(III).

N. Yang et al. / Chemical Geology 404 (2015) 110–125

Lake Hoare

117

Lake Fryxell

0

a

0

b ICE COVER

ICE COVER

5

5

Oxic

Depth (m)

15

10

15

20

Depth (m)

Anoxic

10

20 0

2

4

6

8

10

12

25

30

As (nmol kg-1) 0

25

5

10

15

20 -1

Mo (nmol kg )

Oxic Anoxic 30

As(V) As(III) Mo 0.0

0.5

1.0

1.5

2.0

2.5

As (nmol kg-1) 0

10

20

30

40

50

-1

Mo (nmol kg ) Fig. 4. Concentrations of As species [i.e., As(III), As(V)] and Mo as a function of depth in (a) Lake Hoare and (b) Lake Fryxell. Dashed, blue horizontal lines in panels (a) and (b) are as defined in Figs. 2 and 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

4.2. Geochemical modeling

5. Discussion

The results of the speciation modeling for As and Mo in Lakes Hoare and Fryxell are presented in Table 6. The outstanding feature of the speciation modeling is that although oxyanions of both As and Mo are predicted to predominate in the oxic, mixolimnion of both lakes, thioanions of As and Mo are predicted to prevail in the anoxic, monimolimnia of Lakes Hoare and Fryxell. The prevalence of thioanions is especially substantial in the euxinic bottom waters of Lake Fryxell were S(-II) reaches concentrations of 1.2 mmol kg−1 (Table 6). More is predicted to account for specifically, the thioarsenate species AsS3− 4 as much as 57% and ~ 35% of total dissolved As in bottom waters of Lakes Hoare and Fryxell, respectively, whereas the thioarsenite species 2− are predicted to account for ~ 51% and ~ 10% of H2AsS− 3 and HAsS3 total dissolved As, respectively, in the euxinic monimolimnion waters of Lake Fryxell (Table 6). Thiomolybdate ions are predicted to predominate in waters from the monimolimnia in both lakes, with the tetrathiomolybdate ion, MoS24 −, predicted to be the most abundant form of Mo in the euxinic bottom waters of Lake Fryxell (Table 6). The model also predicts that ferrous tetrathiomolybdate complexes are also likely to be important in the euxinic bottom waters of Lake Frxyell.

5.1. Sources of As and Mo to Lakes Hoare and Fryxell Two recent studies evaluated As and Mo concentrations in glacier surface snow as well as supraglacial, and proglacial streams in Taylor Valley (Fortner et al., 2011, 2013). Fortner et al. argue that the majority of As and Mo found on the glaciers of Taylor Valley reflect input of eolian material, and furthermore, when these elemental concentrations are normalized to Ca, they suggests that with the possible exception of As, the sources of Mo and other oxyanion-forming elements (e.g., V) are from upper continental crustal and/or carbonate enriched materials (Fortner et al., 2011). The source of the enrichment of As is unknown, but might be from Mt. Erebus, as local volcanic input. When the dissolved concentrations of these elements are compared to stream flow, their variation is primarily controlled by stream type. Specifically, in a lower gradient stream with more hyporheic exchange (i.e., Canada Stream; Fig. 1), the source of these trace elements appears to come from chemical weathering within the hyporheic zone of the stream (Fortner et al., 2013). In Andersen Creek, a steeper stream with less hyporheic exchange, the input of dissolved metals/metalloids may derive from the solubilization of eolian materials deposited within the

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Table 5 Arsenic and Mo concentrations in waters from close-basin lakes and seawater.

Western USA Pyramid Walker Mono Aberte Summer Goosee Salton Seae Great Salt Lake Ethiopiag Awassa Abaya Central Asia Issyk-kulh Sasyk-kuli Seawaterj a b c d e f g h i j

As nmol kg−1

Mo nmol kg−1

1411 ± 140a 15,200 ± 3014a 200,000d 17,351 787e 467 227 1108e

600b 3200b 806 ± 132c 1021 459f 1178 386 196 ± 113c

45.4 88–160

18.8 400–521

− 107 23

808 ± 67.8 5.94 105

Johannesson et al. (1997). Johannesson et al. (2000). Domagalski et al. (1990). Oremland et al. (2000). California Environmental Protection Agency (1990). Phillips and Van Denburgh (1971). Zinabu and Pearce (2003). Lyons et al. (2001). Volkova (1998). Bruland and Lohan (2006); Pilson (2013).

stream channels and/or the flushing of eolian derived materials on the glacier surface (Fortner et al., 2013). Normalization of As and Mo concentrations to the corresponding Cl concentrations of the lake waters also generally supports weathering reactions and/or dissolution of crustal materials deposited by eolian processes within each lake's watershed as the chief source of these trace elements to the lakes as opposed to a direct marine origin. For example, Fig. 5 presents mole ratios of As and Mo to Cl in Lakes Hoare and Fryxell as a function of depth, compared to the corresponding mole ratios for seawater. With the exception of Mo in the bottom waters from Lake Frxyell (i.e., 18 m depth), water from both lakes is enriched in As and Mo over the corresponding seawater ratios (Fig. 5). The Cl-normalized ratios are generally greater for Mo than As relative to

seawater, except again for bottom water from Lake Frxyell, which is depleted in Mo relative to seawater. These features can be more quantitatively characterized by computing enrichment factors using equations of the form:   E F i; j ¼ Mi =L j

sample

  = Mi =L j

standard

where Mi represents the concentration of As or Mo, Lj is the concentration of the reference element (i.e., Cl for comparison to seawater or SiO2 for comparison to upper continental crust [UCC]), sample refers to the lake water samples, and standard is either seawater or the upper continental crust (e.g., Kłos et al., 2011). Columns 3 and 4 in Table 7 list enrichment factors for As/Cl and Mo/Cl ratios of the lake waters relative to corresponding seawater ratios, whereas columns 5 and 6 present enrichments factors for As/SiO2(aq) and Mo/SiO2(aq) ratios of the lake water samples relative to the same ratios in the upper continental crust. Assuming that seawater is the chief source of Cl to the Taylor Valley lakes (e.g., Lyons et al., 2005), the computed enrichment factors relative to Cl further support that chemical weathering and/or dissolution of eolian deposits represents a major source of As and Mo to Lakes Hoare and Fryxell. Enrichment factors for As and Mo computed relative to their silica-normalized ratios in the upper continental crust can provide insights into possible weathering sources of these trace elements within the catchment basins of Lakes Hoare and Fryxell. Specifically, enrichment factors of lake waters computed with respect to silica, and relative to the upper continental crust, that are close to 1 suggest that the trace element is primarily sourced from chemical weathering of silicate minerals typical of the upper continental crust (e.g., feldspars, micas). However, As is enriched in the lake waters compared to upper crustal silicate sources by 2- to more than 5-fold, whereas Mo is enriched between a factor of 6 to as much as 180 over upper crustal silicates (Table 7). These large computed enrichment factors suggest that Mo, and to a lesser extent As, may be chiefly sourced from minerals specifically enriched in these trace elements, such as sulfide and/or carbonate minerals within the rocks of the McMurdo Dry Valleys (e.g., Keys and Williams, 1981). Pyrite is reported to occur in the Neoproterozoic to Cambrian metamorphic rocks of the Koettlitz Group that underlie portions of Taylor Valley near Lakes Hoare and Fryxell, as well as in the overlying Early Devonian to Late Triassic clastic sedimentary rocks of the Beacon Supergroup (Findlay et al., 1984; Faure and Mensing, 2010). The Koettlitz Group also includes metamorphosed carbonate

Table 6 Modeled As and Mo speciation in percent As(III), As(V), and Mo(VI) in waters from Lakes Hoare and Fryxell. Values within parentheses represent the percent of each As(III) and As(V) relative to the total dissolved As measured (i.e., AsT in Table 4).

As(III) H3AsO3 H2AsO− 3 H2AsSO− 2 H2AsS2OH2AsS− 3 HAsS2− 3 As(V) H2AsO− 4 HAsO2− 4 HAsS3O2− AsS3− 4 Mo(VI) MoO2− 4 MoO3S2− MoO2S2− 2 MoOS2− 3 2− MoS4 [FeO(OH)MoS4]3− [(Fe2S2)(MoS4)2]4−

Hoare (5 m)

Hoare (12 m)

Hoare (20 m)

Hoare (30 m)

Fryxell (5 m)

63.8 (50.5) 36.2 (28.7) – – – –

94 (39.5) 6.01 (2.54) – – – –

98.4 (31.4) 1.58 (0.5) – – – –

85.9 (23.6) 0.98 (0.3) 9.41 (2.59) 0.65 (0.18) 2.86 (0.79) –

93.7 (21.6) 6.64 (1.46) – – – –

0.86 (0.18) 98.8 (20.6) – –

6.37 (3.69) 93.6 (54.3) – –

21 (14.3) 79 (53.8) – –

– – 20.9 (15.2) 78.7 (57.1)

100 – – – – – –

100 – – – – – –

100 – – – – – –

18.9 28.3 18 18 13.1 3.11 –

Fryxell (8 m)

Fryxell (10 m)

Fryxell (18 m)

– – – – – –

7.35 (5.7) 0.22 (0.17) 9.77 (7.56) 3.2 (2.54) 65.9 (51) 13 (10)

– – 0.51 (0.34) 0.82 (0.54) 83.2 (54.2) 15.2 (9.88)

6.64 (5.1) 93.3 (71.8) – –

6.57 (6.57) 93.4 (93.4) – –

– – 1.58 (0.36) 98.3 (22.2)

– – 0.29 (0.1) 99.6 (34.7)

100 – – – – – –

100 – – – – – –

0.13 0.88 2.64 12.5 44.7 36.6 2.6

– – – 2.46 43.1 35.7 18.6

N. Yang et al. / Chemical Geology 404 (2015) 110–125

119

0 ICE COVER

ICE COVER 5

Hoare Fryxell

15

Seawater

20

Seawater

Depth (m)

10

25

30

a

0.01

b 0.1

1 6

AsT /Cl (10 )

10

0.1

1

10

6

Mo/Cl (10 )

Fig. 5. Molal (a) As/Cl (×106) and (b) Mo/Cl (×106) ratios as a function of depth in Lakes Hoare and Fryxell compared to the corresponding molar ratios for seawater. Seawater data are from Bruland and Lohan (2006) and Pilson (2013).

rocks such as the Salmon Marble Formation that contain pyrite, and which outcrops in the vicinity of the Nussbaum Riegel within Taylor Valley, and roughly 5 to 10 km southwest of these lakes (Angino et al., 1962b; Haskell et al., 1965; Findlay et al., 1984). To the best of our knowledge the As and Mo contents of pyrite and carbonate minerals from the local bedrock have not been measured. Consequently, we plan to sample bedrock, stream sediments, and local regolith in the future to further investigate the sources of As, Mo, and other oxyanion-forming trace elements to the waters of the dry valleys. 5.2. Controls on As and Mo within Lakes Hoare and Fryxell Our analyses of dissolved Fe and Mn in both lakes, which demonstrate increases in dissolved Fe and Mn concentrations with increasing depth (Figs. 2, 3), are supportive of previously advanced models of Fe Table 7 Enrichment factors (EFi,j) for oxyanion-forming trace elements (Mi) in Lakes Hoare and Fryxell relative to reference elements (Lj) Cla and SiO2b relative to seawater (SW) and the upper continental Crust (UCC).

Hoare

Fryxell

a

SWEFi,Cl

b

UCCEFi,SiO2

Depth (m)

SWEFAs,Cl

SWEFMo,Cl

UCCEFAs,Si

UCCEFMo,Si

5 12 20 30 5 8 10 18

20.5 16.8 14.9 10.7 23.4 8.54 6.5 3.89

33.6 40.8 49.8 5.74 27.8 7.75 3.12 0.17

3 2.91 2.61 2.01 2.29 4.29 3.95 5.4

126 180 177 27.6 44.5 99.4 48.4 6.06

= (Mi/Cl)lake/(Mi/Cl)seawater (data from Bruland and Lohan, 2006; Pilson, 2013). = (Mi/SiO2)lake/(Mi/SiO2)UCC, where UCC is the Upper Continental Crust (data from Rudnick and Gao, 2003; Hu and Gao, 2008).

and Mn cycling in these lakes whereby Fe(III)/Mn(IV,III) oxides/ oxyhydroxides, possibly associated with particulate or colloidal organic detritus, are transported downward through the mixolimnion, owing to gravitational settling, to the redoxcline where both Fe and Mn are solubilized by reductive dissolution (Green et al., 1986a, 1989a: Harnish et al., 1991). Specifically, dissolved Fe and Mn increase by 2- and 6-fold across the redoxcline in Lake Hoare, respectively, and by 2.5 and ~ 4 times in Lake Fryxell (Table 4). Some of this reduced Fe and Mn may subsequently diffuse upward into the oxic mixolimnion, where both are re-oxidized and reprecipitated (Davison, 1985; Harnish et al., 1991). The Mn maximum at 10 m depth in Lake Fryxell is followed by its decrease at greater depth, which is consistent with the findings of Green et al. (1989a) who reported a Mn maximum at 9 m depth. Moreover, these researchers noted that although the Mn maximum in Lake Fryxell occurred near the oxic–anoxic transition, dissolved Fe concentrations continued to increase in the water column with increasing depth below this boundary. Iron concentrations also continue to increase below the redoxcline in Lake Fryxell, reaching a value of 3.04 μmol kg−1 at 18 m depth (Fig. 3). Although the bottom water Fe concentration in Lake Fryxell is similar to the bottom water Fe concentration in Lake Hoare (i.e., 3.04 μmol kg−1 vs. 3.4 μmol kg−1, respectively), it is more than 8-fold higher than in the overlying oxic, mixolimnion. The different trends in Mn and Fe concentrations with depth in Lake Fryxell are consistent with the higher redox potential at which Mn(IV) is reduced compared to Fe(III) (e.g., Stumm and Morgan, 1996). The presence of dissolved S(-II) in bottom waters from both lakes indicates that sulfate reduction is occurring in the water columns, within the sediment porewaters, or both. Previous measurements of SO2− 4 and S(-II) within the lake bottom sediments suggested that sulfate reduction is important in the sediment porewaters of Lake Fryxell (Howes

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and Smith, 1990). These researchers argue that S(-II) produced in the sediment porewaters subsequently diffuses upward into the overlying anoxic lake waters at a rate that is sufficient to account for the consump(Howes and Smith, 1990). tion of the water column inventory of SO2− 4 Nonetheless, these first order calculations do not rule out the possibility that sulfate reduction also occurs in the water column. Indeed, a recent genomic investigation demonstrates that sulfate-reducing bacteria are common in the water column of Lake Frxyell, strongly suggesting that S(-II) is also produced in the water column in addition to the sediments (Karr et al., 2005). The presence of dissolved sulfide in the bottom waters of Lake Hoare demonstrates the importance of sulfate reduction in the lake's monimolimnion (Fig. 2). Although the existing data for As and Mo in Lakes Hoare and Fryxell are currently limited in total sample numbers (Fig. 4), these data do reveal interesting features from which important biogeochemical reactions can be inferred. We suggest that the lake profiles for As and Mo in Lakes Hoare and Fryxell can be explained by a combination of biological processes, redox cycling, as well as by the physical and climatological history of these lakes. For example, the lowest concentrations of As and Mo occur at the ice–water interface in each lake, with the only exception being the bottom water sample from Lake Fryxell (i.e., 18 m depth) where the Mo concentration is 1.7-fold lower than at the ice–water interface (Table 4). The low concentrations of As and Mo near the ice–water interface likely reflect uptake of these trace elements by either organisms, such as phytoplankton, and/or Fe/Mn oxides/ oxyhydroxides particles transported into the lakes via runoff or that form within the oxic water column of each lake's mixolimnion (e.g., Seyler and Martin, 1989; Sohrin et al., 1997; Glass et al., 2013). Biological uptake could occur by adsorption to cell walls, or more directly by transport across cell walls for incorporation into important enzymes (e.g., Mo), detoxification (As), or for respiratory processes (As) in the case of bacteria (e.g., Fraústo da Silva and Williams, 1991; Cole et al., 1993; Oremland and Stolz, 2003). Non-biological uptake of As and Mo by Fe/Mn oxides/oxyhydroxides in the oxic mixolimnion waters of both lakes likely occurs by both adsorption and coprecipitation (Bertine, 1972; Belzile and Tessier, 1990; De Vitre et al., 1991; Emerson and Huested, 1991; Waychunas et al., 1993; Dixit and Hering, 2003). Future study is planned to characterize the oxyanion-forming trace element contents of particulate matter in these lakes. It is important to point out that despite the 4–5 m of permanent icecover, sufficient sunlight penetrates the ice of both lakes to support photosynthetic microbes that include eukaryotic algae (e.g., diatoms) and cyanobacteria (Wharton et al., 1993; Vincent et al., 1998). In addition, diverse populations of heterotrophic bacteria and Archaea have also been identified from the water columns and sediments within these lakes (Voytek et al., 1999; Brambilla et al., 2001; Taton et al., 2003; Karr et al., 2005, 2006; Sattley and Madigan, 2006, 2007; Clocksin et al., 2007). Figs. 2 and 3 present the Chl-a concentrations and rates of primary production measured as a function of depth in Lakes Hoare and Fryxell, respectively, collected at the same time as the As and Mo samples presented herein. The rate of primary production is relatively high at the ice–water interface in each lake, and remains high until a depth of 10 m and 9 m in Lake Hoare and Lake Fryxell, respectively, before decreasing to values below detection near or below the redoxcline (Figs. 2, 3). Bacterial production, measured as thymidine uptake rates (TDR; Takacs and Priscu, 1998; Priscu, 2006), is also relatively high near the ice–water interface in each lake, although in highly productive Lake Fryxell the bacterial production rate (i.e., 3.02 × 10−11 mol TDR day−1) was roughly a factor of 3 higher at the ice–water interface than in Lake Hoare (1.02 × 10−11 mol TDR day−1) during our sampling campaigns (data not shown; see http://www.mcmlter.org). Furthermore, although bacterial production decreased with increasing depth in Lake Hoare, the bacterial production rate exhibited a mid-depth maximum in Lake Fryxell. Specifically, bacterial production rate initially decreased with depth in Lake Fryxell until the top of the redoxcline (i.e., 9 m), and subsequently increased by ~15-fold across the redoxcline, reaching a rate

of 2.5 × 10−10 mol TDR day−1 at 11 m depth. The bacterial production then dropped to low rates at depth (e.g., 3.0 × 10−13 mol TDR day−1 at 15 m; http://www.mcmlter.org). These data clearly demonstrate the existence of active photosynthetic phytoplankton/cyanobacteria and heterotrophic bacterial communities in Lakes Hoare and Fryxell, which could play important roles in the biogeochemical cycling of As and Mo in both lakes. Evaluating the likely importance of microbial cycling of these trace elements in each lake will, however, require more complete As and Mo sampling than we have conducted to date. Nevertheless, the As speciation data are in general agreement with the notion that biological processes play a role in As cycling in these lakes. For example, although As speciation changes with depth in both lakes, As species exhibit profound disequilibrium in Lake Hoare, such that As(III) predominates in the waters of the oxic mixolimnion at the ice–water interface, whereas As(V) is more prevalent at depth in the anoxic monimolimnion (Fig. 4). The predominance of As(III) in the surface waters of Lake Hoare suggests that phytoplankton processing (e.g., luxury uptake) and/or P-limiting conditions may have existed during our sampling (Hellweger et al., 2003; Haque et al., 2007; Wurl et al., 2013). Specifically, intracellular As(V) reduction by phytoplankton followed by excretion of As(III) to the water column could explain the predominance of As(III) in the oxic mixolimnion of Lake Hoare (e.g., Andreae, 1979; Andreae and Klumpp, 1979; Sanders, 1979; Sanders and Windom, 1980). Because of the similarities between the phosphate and arsenate oxyanions, phytoplankton and bacteria take up As(V) from the water column via their phosphate transport systems reflecting the inability of such microbes to distinguish arsenate from phosphate (e.g., Oremland and Stolz, 2003). Intracellular conversion of As(V) to As(III) and subsequent excretion from the cell represents a detoxification mechanism for these microbes (Maeda, 1994; Silver et al., 2002). In the case of Lake Fryxell, As concentrations are 2- to 6-fold higher than in Lake Hoare, and with the exception of As(V) below the redoxcline, As continues to increases with depth in Lake Fryxell, which may reflect release of As(III) from lake sediments owing to microbial respiratory processes occurring within sediment porewaters. Therefore, the measured speciation data (Fig. 4), and to a lesser extent the possible sediment source of As(III) to the water column in Lake Fryxell, both suggest, although do not prove, that active microbial processing of As by phytoplankton and/or bacteria may be important within both lakes (e.g., Oremland et al., 2000; Hollibaugh et al., 2005). Another striking feature of the As and Mo profiles of Lakes Hoare and Fryxell is the general increase in As concentrations with increasing depth in both lakes, especially in the case of Lake Fryxell, compared to the contrasting mid-depth Mo maxima that occurs within the mixolimnion of each lake, followed by dramatic decreases in Mo concentrations below the redoxclines (Fig. 4). As described above, the increases in As and Mo with depth in the mixolimnion of both lakes likely reflects a combination of remineralization of sinking biological particles at depth and the subsequent release of incorporated As and Mo back to the water column, as well as release of sorbed/co-precipitated As and Mo from sinking Fe/Mn oxide/oxyhydroxide particles in the vicinity of the redoxcline (Seyler and Martin, 1989; Sohrin et al., 1997; Glass et al., 2013). Some of the As and Mo released by reductive dissolution of Fe/Mn oxides/oxyhydroxides near the redoxcline would be expected to diffuse upwards into the overlying mixolimnion as occurs with Mn and Fe, accounting in part for the mid-depth Mo maxima (e.g., Davison, 1985; Harnish et al., 1991). Higher sampling resolution across the redoxcline in both lakes is necessary to quantitatively evaluate this hypothesis. The contrasting behavior of As and Mo in the anoxic monimolimnia of each lake, where As concentrations generally increase (or remain constant) and Mo concentrations decrease, can be explained by differences in the geochemistry of the respective thioanions. More specifically, thioarsenic species appear to stabilize As in solution within sulfidic waters, whereas the formation of thiomolybdate anions leads to the removal of Mo to the sediments (van der Weijden et al., 1990; O'Day

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et al., 2004; Hollibaugh et al., 2005; Lee et al., 2005; Kirk et al., 2010; Helz et al., 2011; Dahl et al., 2013; Chappaz et al., 2014). The contrasting behavior of the thioanions reflects a combination of their respective reactivity towards Fe-sulfide minerals, chiefly mackinawite, which is commonly the first Fe-sulfide mineral to precipitate in low-temperature, aqueous systems (Rickard, 1995, 2006; Wolthers et al., 2003), and in the case of As, the relative solubility of As-sulfide minerals compared to mackinawite. For example, the experiments of Kirk et al. (2010) suggest that precipitation of mackinawite buffers S(-II) concentrations at levels that are too low for orpiment or realgar precipitation to occur, except possibly in Fe depleted systems, and further that As exhibits a low affinity to adsorb onto mackinawite surfaces. Instead, As removal from solution may be largely controlled by the extent and rate of pyrite precipitation and the relative solubility of thioarsenic species (Kirk et al., 2010). We note that the bottom waters of Lakes Hoare and Fryxell are in equilibrium to slightly oversaturated with respect to mackinawite, suggesting that mackinawite precipitation is important in the anoxic bottom waters of both lakes (Fig. 6). Specifically, the saturation index (i.e., SI = log Q/K) for mackinawite is 0.284 and 1.32 ± 0.47 for Hoare (30 m) and Fryxell (10 m and 18 m), respectively. Geochemical modeling predicts that As chiefly occurs as the thioarsenate anions, AsS34 − and HAsS3O2 − (~ 15% and 57% of total dissolved As, respectively), in the anoxic monimolimnion waters of Lake Hoare (30 m), where these two species together are predicted to account for 72% of the As in solution (Table 6). The remaining ~28% is predicted to occur as the arsenite oxyanion (~ 24% as H3AsO3) and various thioarsenite species (e.g., 2.6% as H2AsSO− 2 ; Table 6). In the euxinic monimolimnion waters of Lake Fryxell, the geochemical model predicts that the majority of As occurs in solution as various 2− account for 51–54% thioarsenite anions (e.g., H 2AsS− 3 and HAsS 3

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and ~ 10% of total dissolved As, respectively) as well as the thioarsenate anion, AsS3− 4 (i.e., 22–35%; Table 6). Although they are difficult to quantify (e.g., Planer-Friedrich et al., 2007, 2010), future studies should focus on verifying the existence of thioanions of As in the anoxic monimolimnion waters of these lakes. The geochemical model also predicts that the anoxic monimolimnion waters of both lakes are highly undersaturated with respect to orpiment, realgar, and amorphous As2S3 (Fig. 6). For example, the saturation indices for orpiment, realgar, and amorphous As2S3 in the anoxic bottom water sample from Lake Hoare (i.e., 30 m) are −7.88, −4.44, and −9.3, respectively, whereas for the euxinic bottom water from Lake Fryxell (i.e., 18 m), the corresponding saturation indices are − 7.28, − 4.56, and − 8.7, respectively (Fig. 6). Consequently, the As profiles for both lakes, along with the results of the geochemical modeling, suggest that As is relatively soluble in these anoxic monimolimnion waters because: (1) mackinawite precipitation regulates dissolved sulfide concentrations at levels too low for As-sulfide minerals to precipitate; and (2) mackinawite is a poor substrate for As adsorption (Fig. 6; Kirk et al., 2010). Arsenic removal from solution may occur, however, as metastable mackinawite dissolves and stable pyrite precipitates in the lake bottom sediments (e.g., Rickard and Luther, 2007, and references therein). We note that the bottom waters, and presumably the sediment porewaters, from both lakes are highly oversaturated with respect to pyrite (Fig. 6), and further, that pyrite has been reported in sediments from the anoxic portions of each lake (Bishop et al., 2001; Wagner et al., 2006). Molybdenum is predicted to occur as the molybdate oxyanion (MoO24 −) in the oxic mixolimnion of both lakes but is converted to thiomolybdate anions (i.e., MoOx S 24 − − x , where x ranges from 0 to 4) in the anoxic monimolimnion of each lake (Table 6). The tetrathiomolybdate anion, MoS 24 −, is predicted to predominate in

Lake Hoare

Lake Fryxell

0

0

a

b

ICE COVER

ICE COVER

5

5

Oxic Anoxic

10

15

15

20

Depth (m)

Depth (m)

10

20 -5

0

5

10

log Q/K 25

Realgar Orpiment Fe5Mo3S14(c)

Oxic Anoxic

Pyrite Rhodochrosite Siderite Mackinawite

30

-5

0

5

10

log Q/K Fig. 6. Saturation indices (i.e., log Q/K) for various Fe, Mn, and As minerals as a function of depth in (a) Lake Hoare and (b) Lake Frxyell. Sources of the solubility product data for the various sulfide minerals are presented in Table 2. Dashed, blue horizontal lines in panels (a) and (b) are as defined in Figs. 2 and 3.

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the euxinic bottom waters of Lake Fryxell followed by an Fe(II) complex of tetrathiomolybdate, [FeO(OH)MoS 4] 3 − (e.g., Helz et al., 2014). Because thiomolybdate anions are particle reactive, sulfidation of the molybdate oxyanion to thiomolybdate anions is recognized as an important step leading to Mo removal from solution in sulfidic waters (e.g., Helz et al., 1996, 2004, 2011; Erickson and Helz, 2000; Zheng et al., 2000; Bostick et al., 2003; Vorlicek et al., 2004; Dahl et al., 2010, 2013, and references therein). Although the exact mechanism(s) by which oxythiomolybdate anions are removed from solution in sulfidic waters are incompletely understood, a number of viable processes have been identified. These include: (1) reaction of trithiomolybdate (MoOS2− 3 ) with rhombic sulfur (S8) forming polysulfide rings that are subsequently scavenged from solution by Fe-sulfide minerals such as mackinawite and pyrite (Helz et al., 2004; Vorlicek et al., 2004; Dahl et al., 2013); (2) reduction of Mo(VI) to Mo(V), which may adsorb to mineral surfaces or be further reduced to Mo(IV) by interaction with polysulfides and subsequently removed from solution (Wang et al., 2011); (3) complexation with natural organic matter followed by reduction to Mo(IV) and burial in bottom sediments (Tribovillard et al., 2006; Chappaz et al., 2014); and (4) precipitation of a Fe–Mo–S mineral (i.e., Fe5MoVI3S14) from the water column (Helz et al., 2011; Chappaz et al., 2014). Although all of these processes may be invoked to explain the removal of Mo from the anoxic monimolimnion waters of Lakes Hoare and Fryxell, we note that the bottom waters from both lakes, and especially Lake Fryxell, are highly oversaturated with respect to the hypothetical Fe– Mo–S mineral proposed by Helz et al. (2011). Specifically, the saturation index for Fe5Mo3S14 in the bottom water sample from Lake Hoare (i.e., 30 m) is 3.77, whereas for the two sulfidic monimolimnion waters we sampled in Lake Fryxell, the mean (± σ) log Q/K for Fe5Mo3S14 is 5.1 ± 0.4 (Fig. 6). Hence, the greater degree of oversaturation of the Lake Fryxell monimolimnion with respect to the proposed Fe–Mo–S mineral, as compared to Lake Hoare bottom waters, is consistent with greater removal of Mo from Lake Fryxell bottom waters via precipitation of Fe5Mo3S14. Indeed, the pH and dissolved sulfide concentrations of the 30 m deep water sample from Lake Hoare, as well as the 10 m and 18 m water samples from Lake Fryxell, plot within the stability field for the proposed Fe5Mo3S14 mineral (Helz et al., 2011) indicating that precipitation of a Fe–Mo–S mineral within the anoxic monimolimnion waters of these lakes can explain Mo removal from the water column (Fig. 7). Future work should focus on investigating

Mo concentrations in porewaters and conducting speciation work (e.g., X-ray absorption spectroscopy) in sediments from these lakes to better constrain the phase or phases that are responsible for Mo removal within their respective anoxic monimolimnia. 5.3. Legacy effects The strong chemical and density gradients that characterize many of the McMurdo Dry Valley lakes are thought to reflect the response of these systems to past climates changes (e.g., Wilson, 1964; Hendy et al., 1977; Matsubaya et al., 1979). More specifically, stable oxygen and hydrogen isotope data along with major solute concentrations suggest that Lake Fryxell lost its ice cover and evaporated down to a small, hypersaline pond by 1000 to 1200 years ago in response to colder and drier climate conditions (Lyons et al., 1998b, 1999b). As climate warmed, the lake was refilled, and the chemical stratification and upward diffusive mixing of solutes from the now brackish bottom waters and sediment porewaters into the fresh surface waters is a legacy of this past, cold and dry climate (Lyons et al., 1998b, 1999b, 2005). Lake Hoare, on the other hand, is either a new feature (i.e., younger than 1000 years) or was completely desiccated and the salts ablated during this past cold, dry event (Lyons et al., 1998b). Such legacy effects are an important aspect of these lakes as evidenced by the upward diffusion of nutrients and organic carbon from deep waters and/or lake bottom sediments, which helps to support phytoplankton productivity in Lake Fryxell and other lakes of the dry valleys (McKnight et al., 1991; Priscu, 1995). Accordingly, diffusion of As(III) from the anoxic and brackish monimolimnion, and/or the bottom sediments, of Lake Fryxell appears to be an important source of As to the oxic mixolimnion, where it is oxidized to As(V) (Fig. 4). A similar trend is not apparent for As in Lake Hoare, and Mo appears to be influenced more strongly by scavenging and removal from the water column in the anoxic bottom waters of these lakes as described above (Figs. 4, 6, 7). Modeling using the 1-D diffusion equation and the infinite dilution diffusion coefficient for As (Li and Gregory, 1974), suggests that the As profile in Lake Fryxell (Fig. 4b) could have formed by diffusion from the bottom waters or sediment over a period of ~1000 years, which is consistent with previous investigations (Lawrence and Hendy, 1985; Lyons et al., 1998b, 1999b). Nevertheless, more data are necessary to fully test the possible legacy effects on oxyanion-forming trace element cycling in these lakes. 6. Conclusions

Fig. 7. Predicted Mo speciation as a function of the activity of H2S(aq) and pH for sulfidic waters characteristic of the monimolimnia of Lakes Hoare and Fryxell. The speciation is calculated for [Mo] = 10−7.75 mol kg−1, [Fe(II)] = 10−5.8 mol kg−1, and [SO2− 4 ] = 10−3 mol kg−1, where [ ] indicate concentrations, and using the model presented in Table 2. These concentrations represent the mean values for Lakes Hoare and Fryxell.

Arsenic and Mo concentrations in Lakes Hoare and Fryxell can be explained by a combination of biological processes, redox cycling, and the physical and climatological history of these lakes. Chemical weathering within the hyporheic zone of the streams feeding both lakes, solubilization of eolian materials deposited within these stream channels, and/or the flushing of eolian derived materials on local glacier surfaces appear to be important potential sources of As and Mo to Lakes Hoare and Fryxell. It is possible that carbonate and/or sulfide minerals contained within local bedrock, regolith, and stream sediments may contribute As and Mo to these lakes. Geochemical modeling predicts that As and Mo occur as thioanions in the anoxic bottom waters of Lakes Hoare and Fryxell, and further that the contrasting behavior of both trace elements in the anoxic bottom waters reflects the different reactivity of As and Mo thioanions towards Fe-sulfide minerals such as mackinawite (FeS). The anoxic monimolimnion of both lakes is saturated to slightly oversaturated with respect to mackinawite, highly oversaturated with respect to a previously proposed Fe–Mo–S mineral, and highly undersaturated with respect to orpiment, realgar, and amorphous As2S3. These observations suggest that precipitation of mackinawite from bottom waters in both lakes keeps dissolved sulfide concentrations at levels too low for As-sulfide minerals to form (i.e., undersaturated with respect to orpiment, realgar, and amorphous

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As2S3), but facilitates Mo removal as particle reactive thiomolybdate anions react with mackinawite, forming an Fe–Mo–S mineral that is removed to the lake bottom sediments. Acknowledgments This project was supported by NSF grants EAR-0805332 and EAR1014946 to Johannesson, and the NSF McMurdo Dry Valley Long Term Ecological Research Program grants OPP ANT-0423595 and OPP ANT1115245 to Lyons. In addition, we wish to thank Michael and Mathilda Cochran for endowing the Cochran Family Professorship in Earth and Environmental Sciences at Tulane University, which also helped to support this study. Finally, we are grateful for the comments of three anonymous reviewers and the Editor-in-Chief, Dr. Carla Koretsky, whose suggestions substantially improved this manuscript. We appreciate J. C. Priscu and his "limno-team" with the help in sampling and data access. References Alloway, B.J., 2012. Heavy metals and metalloids as micronutrients for plants and animals. In: Alloway, B.J. 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